Conducting Silicone-Based Polymers and Their Application

molecules

Review Conducting Silicone-Based Polymers and Their Application

Jadwiga Sołoducho *, Dorota Zaj ˛ac,Kamila Spychalska, Sylwia Baluta and Joanna Cabaj

Department of Organic and Medical Chemistry, Faculty of Chemistry, Wroclaw University of Science and Technology, Wybrzeze˙ Wyspia´nskiego27, 50-370 Wrocław, Poland; [email protected] (D.Z.); [email protected] (K.S.); [email protected] (S.B.); [email protected] (J.C.) * Correspondence: [email protected]; Tel.: +48-71320-28-91

Abstract: Over the past two decades, both fundamental and applied research in conducting polymers have grown rapidly. Conducting polymers (CPs) are unique due to their ease of synthesis, environmental stability, and simple doping/dedoping chemistry. Electrically conductive silicone polymers are the current state-of-the-art for, e.g., optoelectronic materials. The combination of inorganic elements and organic polymers leads to a highly electrically conductive composite with improved thermal stability. Silicone-based materials have a set of extremely interesting properties, i.e., very low surface energy, excellent gas and moisture permeability, good heat stability, low-temperature ﬂexibility, and biocompatibility. The most effective parameters constructing the physical properties of CPs are conjugation length, degree of crystallinity, and intra- and inter-chain interactions. Conducting polymers, owing to their ease of synthesis, remarkable environmental stability, and high conductivity in the doped form, have remained thoroughly studied due to their varied applications in ﬁelds like biological activity, drug release systems, rechargeable batteries, and sensors. For this reason, this review provides an overview of organosilicon polymers that have been reported over the past

two decades.

Citation: Sołoducho, J.; Zaj ˛ac,D.; Keywords: silicon-containing polymers; silane; silole; organic light-emitting diode (OLED); organic Spychalska, K.; Baluta, S.; Cabaj, J. photovoltaic (OPV); sensors Conducting Silicone-Based Polymers and Their Application. Molecules 2021, 26, 2012. https://doi.org/10.3390/ molecules26072012 1. Introduction Silicon, as the second most common element in the earth’s crust, forms bonds mainly Academic Editor: Carlos Alemán with oxygen (siloxanes), hydrogen (silanes), and carbon (silanes and siloles). Due to their physicochemical properties, silicone structures have found wide application in materials Received: 9 March 2021 science [1], optoelectronic technology [2], medicine [3], and environmental protection [4]. Accepted: 29 March 2021 Compared to carbon and its bonds, e.g., with hydrogen, the Si–H bond is polarized in Published: 1 April 2021 such way that silicon has a positive character and hydrogen has a hydride character [5]. As a result, silanes undergo heterolytic reactions more easily than alkanes. Moreover, Publisher’s Note: MDPI stays neutral Si–H and Si–Si bonds are weaker than their carbon analogs, while Si–X bonds (X = N, with regard to jurisdictional claims in O, S, F, Cl, Br, and I) are much stronger than C–X bonds. Another difference is the published maps and institutional affil- instability of hydrosilicons, given the thermodynamic and kinetic results, compared to iations. hydrocarbons [6,7]. This stability can be increased by introducing organic groups (Si–C bonds instead of Si–H). Furthermore, the Si–Si bond, due to the presence of delocalized δ-electrons, can be compared to the C = C bond and delocalization of π-electrons [8]. The siloxanes have a Si–O bond with a length of 1.64 Å, which is significantly longer than that Copyright: © 2021 by the authors. of the C–C bond (1.53 Å). Moreover, the Si–O–Si bond angle is approximately 143◦, which is Licensee MDPI, Basel, Switzerland. much larger than the tetrahedral angle (~110◦). The significant fact is that the Si–O–Si bond This article is an open access article angle is so flexible that it can easily pass through the linear 180◦ state [9]. Furthermore, distributed under the terms and the torsional potential of Si–O bonds is significantly lower than that of C–C bonds. Due to conditions of the Creative Commons Attribution (CC BY) license (https:// these properties, siloxanes are flexible chains and have high thermal stability [10]. creativecommons.org/licenses/by/ Recently, silanes and siloles have been used as promising building blocks for functional 4.0/). organic materials, since silicone-containing π-conjugated compounds are endowed with

Molecules 2021, 26, 2012. https://doi.org/10.3390/molecules26072012 https://www.mdpi.com/journal/molecules Molecules 2021, 26, 2012 2 of 30

efficient electron-transporting properties and/or high quantum yields. Due to the direct relationship between the electronic structure of π-conjugated heteroaromatic systems and optoelectric properties, compounds with specific parameters can be designed. By adjusting the highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels, one can control the length of the π-coupling, the frequency of light emission, as well as the injection of the charge. The silicon-containing polymers with multifunctional properties and the ease of their modification by chemical synthesis, using the most common polycondensation reactions based on Suzuki-Miyaury, Stille coupling, Heck, and Sonogashira reactions, have a wide range of potential applications in optoelectronics [11], gas separation membranes [12], porous organic frameworks [13], surface coatings [14], and sensors [15]. This review provides an overview of organosilicon polymers that have been reported over the past two decades. For a review of work in this area in the earlier years, the reader is referred to other reviews [16–18]. For the purposes of this review, coverage will be restricted to arylsilanes and siloles, which are increasingly used in industry and health. The work focuses on polymers that have been used mainly in organic light-emitting diode (OLED), organic photovoltaic (OPV), and sensors.

2. Arylsilanes—The Most Commonly Used Syntheses Recently, a lot of attention has been paid to arylsilanes, because silicon can serve as a link between moieties in a molecule and also between organic and inorganic materials [19,20]. Such materials, due to the higher ionic character of the C–Si bond compared to the C– C bond, have greater thermal stability [21]. Moreover, molecules based on silane core have been proven to be building blocks for constructing various 3D porous metal-organic frameworks (MOFs), dynamic covalent frameworks (DCFs), element-organic framework (EOF), and dendrimer-like structures [22–24]. The next important group of silanes is the group of tetra-arylsilane (TPS) derivatives, due to their wide energy gap, triplet energy level, and high electrochemical, thermal, and morphological stability [25,26]. More importantly, with its higher solubility used in low-cost processing techniques, such as spin-coating and inkjet printing, desirable for large-area applications, TPS is a promising building material on an industrial scale [27–29]. In the synthesis of arylsilanes, palladium-catalyzed coupling reactions (e.g., Suzuki and Stille reactions) and Grignard reactions are most often used [26,30–32]. In order to obtain silane intermediates in the above reactions, the most common is the metalation reaction with the addition of chlorosilane additionally substituted with an aryl group or an alkyl chain [33–35]. This solution was used by R. Yang et al. [36]. The targeted compounds named TCzSi and DCzMzSi were synthesized by a palladium-catalyzed Suzuki–Miyaura coupling reaction (Scheme1). The first stage was based on a metalation reaction with the addition of phenyltrichlorosilane. The results indicate that the typical tetrahedral geometry owing to the coordination of silicon was effective in ensuring the separation of the three functional units in a molecule and lead to a high triplet energy level (ET) and relatively wide band gap (Eg). O. Bezvikonnyi and co-workers obtained 9,90-bis(triphenylsilyl)-9H,90H-3,30- 0 bicarbazole (BiCzSiPh3) in a two-step synthesis [37]. The first step, the synthesis of 3,3 - bicarbazole, was a Friedel–Crafts reaction with FeCl3 as Lewis acid. This method allows the synthesis of 3,30-bicarbazole without using column chromatography. Compound BiCzSiPh3 was synthesized by standard treatment of the intermediate with n-BuLi, and then the reaction was quenched with chlorotriphenylsilane (Scheme2). The thermal and thermomechanical properties of the polymer containing arylsilane, bis(4-(40-(40-phenoxy)phenyl)phenyl)dimethylsilane phthalonitrile (SiBPPN), was studied by J. Hu et al. [38]. They obtained SiBPPN in a three-step reaction. The first stage, as previously, was a metalation reaction with the addition of dimethyldichlorosilane. The next step was the Suzuki reaction, and in the end, the SiBPPN was achieved through base-catalyzed aromatic displacement (Scheme3). Molecules 2021, 26, x FOR PEER REVIEW 3 of 31

Molecules 2021, 26, 2012 3 of 30 Molecules 2021, 26, x FOR PEER REVIEW 3 of 31

Scheme 1. The synthetic route to TCzSi and DCzMzSi [36].

O. Bezvikonnyi and co-workers obtained 9,9′-bis(triphenylsilyl)-9H,9′H-3,3′-bicarbazole (BiCzSiPh3) in a two-step synthesis [37]. The first step, the synthesis of 3,3′-bicarbazole, was a Friedel–Crafts reaction with FeCl3 as Lewis acid. This method allows the synthesis of 3,3′-bicarbazole without using column chromatography. Compound BiCzSiPh 3 wasSchemeScheme synthesized 1. 1. TheThe synthetic syntheticby standard route treatment toto TCzSiTCzSi and andof the DCzMzSi DCzMzSi intermediate [36 [3].6]. with n-BuLi, and then the reaction was quenched with chlorotriphenylsilane (Scheme 2). O. Bezvikonnyi and co-workers obtained 9,9′-bis(triphenylsilyl)-9H,9′H-3,3′-bicarbazole (BiCzSiPh3) in a two-step synthesis [37]. The first step, the synthesis of 3,3′-bicarbazole, was a Friedel–Crafts reaction with FeCl3 as Lewis acid. This method allows the synthesis of 3,3′-bicarbazole without using column chromatography. Compound BiCzSiPh3 was synthesized by standard treatment of the intermediate with n-BuLi, and then the reaction was quenched with chlorotriphenylsilane (Scheme 2).

Scheme 2. SynthesisSynthesis and chemi chemicalcal structures of BiCzSiPh3 and BiCzTos [[3377].].

Scheme 2. Synthesis and chemical structures of BiCzSiPh3 and BiCzTos [37].

Molecules 2021, 26, x FOR PEER REVIEW 4 of 31

The thermal and thermomechanical properties of the polymer containing arylsilane, bis(4-(4′-(4′-phenoxy)phenyl)phenyl)dimethylsilane phthalonitrile (SiBPPN), was studied by J. Hu et al. [38]. They obtained SiBPPN in a three-step reaction. The first stage, as previously, was a metalation reaction with the addition of dimethyldichlorosilane. The Molecules 2021, 26, 2012 next step was the Suzuki reaction, and in the end, the SiBPPN was achieved4 of 30 through base- catalyzed aromatic displacement (Scheme 3).

SchemeScheme 3. Synthesis 3. Synthesis of bis(4-(4 of bis(40-(40-phenoxy)phenyl)phenyl)dimethylsilane-(4′-(4′-phenoxy)phenyl)phenyl)dimethylsilane phthalonitrile (SiBPPN) phthalonitrile [38]. (SiBPPN) [38]. L. H. Tagle et al. obtained poly(esters)—PEs derived from diacids containing bulky side groups, which have a halogenated (Cl, Br) imide ring, an aminoacidic residue (glycine, L-alanine,L. H.L-valine), Tagle and et anal. amideobtained group poly(esters) with a silicon-containing—PEs derived diphenol from (Scheme diacids4)[ 39 containing]. bulky PEsside were groups, obtained which with good have yields a halogenatedin a typical polymerization (Cl, Br) imide process ring, with pyridine, an aminoacidic 4- residue toluenesulfonyl(glycine, L-alanine, chloride, andL-valine) dimethylformamide, and an amide (DMF). group The solubility with was a silicon good in-containing polar diphenol aprotic(Scheme solvents 4) [39 and]. PEs for some were of obtained them in tetrahydrofurane with good yields (THF). in The a typical majority polymerization showed process m swellingwith pyridine, in THF and 4-toluenesulfonyl-cresol. chloride, and dimethylformamide (DMF). The solubility An example of the use of the Grignard reaction is a synthesis of the silicon-containing arylacetylenewas good in resins polar with aprotic oxadiazole solvents moieties and for [40 ].some In a of standard them in procedure tetrahydrofurane using (THF). The magnesiummajority showed powder, swelling bromoethane, in THF and and 2,5-bis(4-(4-ethynylphenoxy)phenyl)-[1,3,4] m-cresol. oxadiazole, followed by an appropriate silane derivative (methylphenyldichlorosilane, dimethyldichlorosilane, and diphenyldi-chlorosilane), PODSA-MP, PODSA-MM, and PODSA-PP resins were obtained (Scheme5). S. A. Milenin and co-workers also used the Grignard reaction, using commercially available and cheap alkoxy and chlorosilanes, as well as p-bromotoluene, as the starting reagents to obtain p-tolylalkoxy- and p-tolyl(hydrido)silanes (Scheme6)[41]. Further, based on p-tolylsilanes 1–6, they obtained a series of p-tolylalkoxysilanes 7–12 by a variety of synthetic approaches, e.g., by a condensation of the corresponding alkoxysilanes with excess trimethylsilanol in the presence of acetic acid or by reﬂuxing in dioxane with sodium hydroxide, followed by blocking the reaction with trimethylchlorosilane (Scheme7). This method, as the authors claim, will ﬁnd practical application in the future for synthesizing symmetric disiloxanes due to the inexpensive and commercially available reagents used and simplicity of the reaction conditions.

Molecules 2021, 26, 2012 5 of 30 Molecules 2021, 26, x FOR PEER REVIEW 5 of 31

SchemeScheme 4.4. SynthesisSynthesis ofof poly(esters) (PEs)(PEs) [ 39]39]..

An example of the use of the Grignard reaction is a synthesis of the silicon-containing arylacetylene resins with oxadiazole moieties [40]. In a standard procedure using magnesium powder, bromoethane, and 2,5-bis(4-(4-ethynylphenoxy)phenyl)- [1,3,4]oxadiazole, followed by an appropriate silane derivative (methylphenyldichlorosilane, dimethyldichlorosilane, and diphenyldi-chlorosilane), PODSA-MP, PODSA-MM, and PODSA-PP resins were obtained (Scheme 5).

Scheme 5. Synthesis route of silicon-containing arylacetylene resins with oxadiazole moieties, THF: tetrahydrofuran; subscript n: stands for the number of repeat units in the polymer chain [40].

Molecules 2021, 26, x FOR PEER REVIEW 6 of 31

Molecules 2021, 26, 2012 S. A. Milenin and co-workers also used the Grignard reaction, using commercially6 of 30 available and cheap alkoxy and chlorosilanes, as well as p-bromotoluene, as the starting reagents to obtain p-tolylalkoxy- and p-tolyl(hydrido)silanes (Scheme 6) [41].

Molecules 2021, 26, x FOR PEER REVIEW 7 of 31 SchemeScheme 6. Synthesis 6. Synthesis of ofp-tolylalkoxyp-tolylalkoxy-- and and pp--tolyl(hydrido)silanestolyl(hydrido)silanes1–6 1–by6 by the the Grignard Grignard reaction react [41ion]. [41].

Further, based on p-tolylsilanes 1–6, they obtained a series of p-tolylalkoxysilanes 7– 12 by a variety of synthetic approaches, e.g., by a condensation of the corresponding alkoxysilanes with excess trimethylsilanol in the presence of acetic acid or by refluxing in dioxane with sodium hydroxide, followed by blocking the reaction with trimethylchlorosilane (Scheme 7). This method, as the authors claim, will find practical application in the future for synthesizing symmetric disiloxanes due to the inexpensive and commercially available reagents used and simplicity of the reaction conditions.

SchemeScheme 7. Synthesis 7. Synthesis of ofp-ptolylalkoxysilanes-tolylalkoxysilanes 7–12;7–12;Trimethylsilanol Trimethylsilanol (TMS-OH) (TMS-OH) [41]. [41].

N. N. Makhmudiyarova et al. were the first to synthesize eight- and 11-membered cyclic silicon-containing di- and triperoxides with high yields and selectivity by the reaction of geminal bis-hydroperoxides with bis(methoxymethyl)-diphenylsilane in the presence of La(NO3)3·6H2O as a catalyst (Scheme 8) [42]. Eleven-membered diphenylhexaoxasiladispiro cycloalkanes were reduced to the corresponding diphenyltetraoxasiladispiroalkanes. According to the authors, cyclic peroxysilanes can also be used as hydroxylating, peroxidizing, and oxidizing agents and used for the preparation of polymers, similar to linear peroxides containing Si–O–O fragments.

Scheme 8. Synthesis of di(tri)peroxysilacycloalkanes by cyclization and reduction of 11-membered diphenylhexaoxasiladispirocycloalkanes [42].

3. Siloles—The Most Commonly Used Syntheses Siloles possess a low-lying LUMO level, due to the σ*–π* conjugation between the π* orbital of the butadiene moiety and the σ* orbital of the two exocyclic σ-bonds on the silicon atom [43]. Furthermore, the silicon atom stabilizes the highest occupied molecular

Molecules 2021, 26, x FOR PEER REVIEW 7 of 31

Molecules 2021, 26Scheme, 2012 7. Synthesis of p-tolylalkoxysilanes 7–12; Trimethylsilanol (TMS-OH) [41]. 7 of 30

N. N. Makhmudiyarova et al. were the first to synthesize eight- and 11-membered cyclic siliconN.-containing N. Makhmudiyarova di- and ettriperoxides al. were the with ﬁrst to high synthesize yields eight- and selectivityand 11-membered by the reactioncyclic of geminal silicon-containing bis-hydroperoxides di- and triperoxides with bis(methoxymethyl) with high yields and selectivity-diphenylsilane by the reaction in the presenceof geminal of La(NO bis-hydroperoxides3)3·6H2O as with a catalyst bis(methoxymethyl)-diphenylsilane (Scheme 8) [42]. Eleven in the-membered presence diphenylhexaoxasiladispiroof La(NO3)3·6H2O as a catalyst cycloalkanes (Scheme 8 were)[ 42]. Eleven-membered reduced to the diphenylhexaoxasi- corresponding diphenyltetraoxasiladispiroalkanes.ladispiro cycloalkanes were reduced According to the corresponding to the authors, diphenyltetraoxasiladispiroalka- cyclic peroxysilanes can also benes. used According as hydroxylating, to the authors, peroxidizing, cyclic peroxysilanes and oxidizing can also agents be used and as hydroxylating, used for the peroxidizing, and oxidizing agents and used for the preparation of polymers, similar to preparation of polymers, similar to linear peroxides containing Si–O–O fragments. linear peroxides containing Si–O–O fragments.

Scheme Scheme 8. Synthesis 8. Synthesis of of di(tri)peroxysilacycloalkanes di(tri)peroxysilacycloalkanes by cyclization by cyclization and reduction and of 11-membered reduction ofdiphenylhexaoxasi- 11-membered diphenylhexaoxasiladispirocycloladispirocycloalkanes [42alkanes]. [42].

3. Siloles3. Siloles—The—The Most MostCommonly Commonly Used Used Syntheses Syntheses Siloles Silolespossess possess a low a-lying low-lying LUMO LUMO level, level, due due to the to the σ*–σπ**–π conjugation* conjugation between between thetheπ π** orbital orbitalof the ofbutadiene the butadiene moiety moiety and and the the σ*σ* orbitalorbital of of the the two two exocyclic exocyclicσ-bonds σ-bonds on the siliconon the silicon atom [[4343].]. Furthermore, Furthermore, the the silicon silicon atom atom stabilizes stabilizes the highest the highest occupied occupied molecular molecular orbital; thus, siloles exhibit high thermodynamical stability in the air [44]. Moreover, aggregation- induced emission (AIE) has been observed in silole [45]. Phenyl substituted siloles and polysilanes can be considered examples of mixed π-conjugation/cross-hyperconjugation and π-conjugation/σ-conjugation, respectively. The synthesis of dithiensiloles, as in the case of arylsiloles, is based on C–C coupling reactions, e.g., the Stille and Sonogashira reactions [46,47]. Similarly, silole-containing intermediates are synthesized by metalation in reaction with n-BuLi or reductive cyclization with lithium naphthalenide [46,48]. A. Pöcheim et al. attempt to show that the combination of siloles as the prototypical examples of cross-hyperconjugated molecules and oligosilanes, representative of σ- conjugation, forms systems of extended conjugation, leading to altered optical absorption properties [49]. Researchers decided to use di(phenylalkynyl)silane 37 as a starting material (Scheme9). Reductive cyclization with lithium naphthalenide was followed by a reaction with excess dichlorodimethylsilane to the 2,5-chlorodimethylsilyl substituted silole 38. If chlorotrimethylsilane was used instead of dichlorodimethylsilane, the 2,5- bis(trimethylsilyl) substituted silole 40 was obtained. The chlorodimethylsilyl substituents of 38 provide the possibility to introduce oligosilanyl groups to the 2,5-positions. A reaction with potassium tris(trimethylsilyl)silanide led to a formation of the neopentasilanyl substituted silole 39. Compound 38 was also reacted with 1,1,4,4,4-pentakis(trimethylsilyl)tetramethyltetrasilanylpotassium to yield compound 41 with attached oligosilanyl fragments containing linear hexasilane units. Molecules 2021, 26, x FOR PEER REVIEW 8 of 31

orbital; thus, siloles exhibit high thermodynamical stability in the air [44]. Moreover, aggregation-induced emission (AIE) has been observed in silole [45]. Phenyl substituted siloles and polysilanes can be considered examples of mixed π-conjugation/cross- hyperconjugation and π-conjugation/σ-conjugation, respectively. The synthesis of dithiensiloles, as in the case of arylsiloles, is based on C–C coupling reactions, e.g., the Stille and Sonogashira reactions [46,47]. Similarly, silole-containing intermediates are synthesized by metalation in reaction with n-BuLi or reductive cyclization with lithium naphthalenide [46,48]. A. Pöcheim et al. attempt to show that the combination of siloles as the prototypical examples of cross-hyperconjugated molecules and oligosilanes, representative of σ- conjugation, forms systems of extended conjugation, leading to altered optical absorption properties [49]. Researchers decided to use di(phenylalkynyl)silane 37 as a starting material (Scheme 9). Reductive cyclization with lithium naphthalenide was followed by a reaction with excess dichlorodimethylsilane to the 2,5-chlorodimethylsilyl substituted silole 38. If chlorotrimethylsilane was used instead of dichlorodimethylsilane, the 2,5- bis(trimethylsilyl) substituted silole 40 was obtained. The chlorodimethylsilyl substituents of 38 provide the possibility to introduce oligosilanyl groups to the 2,5- positions. A reaction with potassium tris(trimethylsilyl)silanide led to a formation of the neopentasilanyl substituted silole 39. Compound 38 was also reacted with 1,1,4,4,4- Molecules 2021, 26, 2012 8 of 30 pentakis(trime-thylsilyl)tetramethyltetrasilanylpotassium to yield compound 41 with attached oligosilanyl fragments containing linear hexasilane units.

SchemeScheme 9.9. SynthesisSynthesis of of a varietya variety of 2,5-disilylof 2,5-disilyl substituted substituted siloles siloles [49]. [49].

Researchers report that the redshift of the absorption bands clearly suggests some type Researchers report that the redshift of the absorption bands clearly suggests some of conjugation between the silole and the oligosilane units. The hypothesis of conjugation betweentype of the conjugation oligosilane between substituent the and silole the silole and core the in oligosilane compounds units.39 and The41 hypothesiswas of supportedconjugati byon thebetween results the of the oligosilane density functional substituent computations. and the silole core in compounds 39 and 41 wasAnother supported interesting by the example results of of silole the density is a work functional by Y. Cai computations. et al. [50]. They report the synthesis,Another characterization, interesting example and photophysical of silole is a work properties, by Y. including Cai et al. aggregation- [50]. They report the enhancedsynthesis, emission charact (AEE)erization, characteristics and photophysical of cyclotetrasiloxanes properties,42–44 containing including differ- aggregation- entenhanced silole-based emission fluorogenic (AEE) units characteristics (silafluorene, of 1,3-diphenyl-9-silafluorene, cyclotetrasiloxanes 42–44 tetraphenyl- containing different silole) (Scheme 10). 9,9-Dichloro-9-silafluorene, 9,9-dichloro-1,3-diphenyl-9-silafluorene, silole-based fluorogenic units (silafluorene, 1,3-diphenyl-9-silafluorene, tetraphenylsilole) dichloro(methyl)phenylsilane, and 1,1-dihydroxyl-2,3,4,5-tetraphenylsilole were synthesized by cohydrolysis and condensation reactions. Their optical properties in a solution and as crystals were studied. These compounds have low quantum yields in solution (Φfl = 0.01–0.18) with fluorescence maxima at 359– 375 nm for silafluorene-containing compounds 42 and 44 and at 491 nm for AEE-active tetraphenylsilole compound 43. However, 42–44 have high solid-state quantum yields (Φfl = 0.65–0.78) with fluorescence maxima at 377–390 nm for compounds 42 and 44 and at 517 nm for tetraphenylsilole- and silafluorene-containing compound 43. Recently, C. Gu et al. reported a novel conjugated polymer PDTSDTBT consisting of a donor unit with a tetrahedral Si (sp3) named DTS and an acceptor unit named DTBT with branched side chains [51]. The polymer was obtained by Stille coupling and then transformed into nanoparticles by nanoprecipitation (Scheme 11). Recently, L. Yang and co-workers reported two newly synthesized chromophores, i.e., 2-(2-thienyl)-benzo[b]silole (4T1) and 2-(2,20-bithiophen)-5-yl]-benzo[b]silole (4T2), obtained by a double trimethylsilylation of the bromobenzonitrile 46 (Scheme 12)[52]. Lithium halogen exchange, followed by treatment with trimethylborate, gave the boronic acid 47 (52% yield over three steps). The reaction of 47 with acetylene 48 (n = 1, 2) by rhodium(I)-catalyzed Chatani alkyne cycloaddition afforded the thienyl-appended benzosilole adducts 4T1 and 4T2 without any complication from the nitrile. Molecules 2021, 26, x FOR PEER REVIEW 9 of 31

(Scheme 10). 9,9-Dichloro-9-silafluorene, 9,9-dichloro-1,3-diphenyl-9-silafluorene, Molecules 2021, 26, 2012 9 of 30 dichloro(methyl)phenylsilane, and 1,1-dihydroxyl-2,3,4,5-tetraphenylsilole were synthesized by cohydrolysis and condensation reactions.

SchemeScheme 10. 10. CohydrolysisCohydrolysis and and condensation condensation reactions reactions among among 9,9-dichloro-9-silaﬂuorene, 9,9-dichloro-9-silafluorene, 9,9- 9,9- dichloro-1,3-diphenyl-9-silaﬂuorene,dichloro-1,3-diphenyl-9-silafluorene, dichloro(methyl)phenylsilane, dichloro(methyl)phenylsilane, and 1,1-dihydroxyl-2,3,4,5- and 1,1-dihydroxyl-2,3,4,5- Molecules 2021, 26, x FOR PEER REVIEW 10 of 31 tetraphenylsiloletetraphenylsilole [50 ].[50].

Their optical properties in a solution and as crystals were studied. These compounds have low quantum yields in solution (Φfl = 0.01–0.18) with fluorescence maxima at 359– 375 nm for silafluorene-containing compounds 42 and 44 and at 491 nm for AEE-active tetraphenylsilole compound 43. However, 42–44 have high solid-state quantum yields (Φfl = 0.65–0.78) with fluorescence maxima at 377–390 nm for compounds 42 and 44 and at 517 nm for tetraphenylsilole- and silafluorene-containing compound 43. Recently, C. Gu et al. reported a novel conjugated polymer PDTSDTBT consisting of 3 a donor unit with a tetrahedral Si (sp ) named DTS and an acceptor unit named DTBT with branched side chains [51]. The polymer was obtained by Stille coupling and then SchemeScheme 11.11. The synthesis routeroute ofof PDTSDTBTPDTSDTBT [[5511]].. transformed into nanoparticles by nanoprecipitation (Scheme 11). Recently, L. Yang and co-workers reported two newly synthesized chromophores, i.e., 2-(2-thienyl)-benzo[b]silole (4T1) and 2-(2,2′-bithiophen)-5-yl]-benzo[b]silole (4T2), obtained by a double trimethylsilylation of the bromobenzonitrile 46 (Scheme 12) [52]. Lithium halogen exchange, followed by treatment with trimethylborate, gave the boronic acid 47 (52% yield over three steps). The reaction of 47 with acetylene 48 (n = 1, 2) by rhodium(I)-catalyzed Chatani alkyne cycloaddition afforded the thienyl-appended benzosilole adducts 4T1 and 4T2 without any complication from the nitrile.

Scheme 12. Synthesis of benzosiloles 4T1 and 4T2. LDA = lithium diisopropylamide, cod = cyclooctadiene, DABCO = 1,4-diazabicyclo[2.2.2]octane [52].

R. Cao et al. synthesized a series of functional silole derivatives via the hydrosilylation reaction of 1-methyl-2,3,4,5-tetraphenylsilole and different constructing units that contain an ethynyl group (Scheme 13) [53]. Initially, ethynyl compounds were synthesized by the Sonogashira cross-coupling reaction of the corresponding bromides, followed by the subsequent hydrosilylation reaction with 1-methyl-2,3,4,5- tetraphenylsilole at a refluxing temperature in the presence of the Karstedt’s catalyst to produce the target silole-based derivatives with satisfying reaction yields. They report that the designed compounds demonstrate two emissive wavelengths in the solution and in the solid-state, and the dual emission is tuned by the aggregation state in the mixed solvent. The compound containing silole and pyrene units shows polymorphism- dependent solid-state emissive behavior. The crystal packing of the target compounds is influenced by the steric effects of the corresponding different molecular structures. In 2019, C. N. Scott and M. D. Bisen reported a series of 1,7-disubstituted 1,6-diynes by the Sonogashira reaction and converted them to the reactively functional 2,5-

Molecules 2021, 26, x FOR PEER REVIEW 10 of 31

Scheme 11. The synthesis route of PDTSDTBT [51].

Recently, L. Yang and co-workers reported two newly synthesized chromophores, i.e., 2-(2-thienyl)-benzo[b]silole (4T1) and 2-(2,2′-bithiophen)-5-yl]-benzo[b]silole (4T2), obtained by a double trimethylsilylation of the bromobenzonitrile 46 (Scheme 12) [52]. Lithium halogen exchange, followed by treatment with trimethylborate, gave the boronic Molecules 2021, 26, 2012 acid 47 (52% yield over three steps). The reaction of 47 with acetylene 48 (n = 1,10 2) of 30by rhodium(I)-catalyzed Chatani alkyne cycloaddition afforded the thienyl-appended benzosilole adducts 4T1 and 4T2 without any complication from the nitrile.

SchemeScheme 12. 12.Synthesis Synthesis of of benzosiloles benzosiloles 4T1 4T1 and and 4T2. 4T2. LDA LDA = = lithium lithium diisopropylamide, diisopropylamide, cod cod = = cyclooc- cyclooctadiene, DABCO = 1,4-diazabicyclo[2.2.2]octane [52]. tadiene, DABCO = 1,4-diazabicyclo[2.2.2]octane [52].

R.R. Cao Cao et al. et synthesized al. synthesized a series a of functionalseries of silole functional derivatives silole via derivatives the hydrosilylation via the Molecules 2021, 26, x FOR PEER REVIEWreactionhydrosilylation of 1-methyl-2,3,4,5-tetraphenylsilole reaction of 1-methyl-2,3,4,5- andtetraphenylsilole different constructing and different units thatconstructing contain11 of 31

anunits ethynyl that contain group (Schemean ethynyl 13 )[group53]. Initially,(Scheme ethynyl13) [53]. compoundsInitially, ethynyl were compounds synthesized were by thesynthesized Sonogashira by cross-couplingthe Sonogashira reaction cross-coupling of the corresponding reaction of the bromides, corresponding followed bromides, by the followed by the subsequent hydrosilylation reaction with 1-methyl-2,3,4,5- subsequentdisubstituted hydrosilylation siloles using reactionthe Kumada with-type 1-methyl-2,3,4,5-tetraphenylsilole nickel-mediated intramolecular at a cyclization reﬂuxing tetraphenylsilole at a refluxing temperature in the presence of the Karstedt’s catalyst to temperature(Scheme 14) in [5 the4]. presence Following of the Karstedt’s preparation catalyst of the to silole produce precursors, the target they silole-based pursued produce the target silole-based derivatives with satisfying reaction yields. They report derivativespolymerizations with satisfying of the reaction siloles yields. with They 3,6- reportbis(5-b thatromothiophene the designed-2-yl) compounds-N,N-bis(2- that the designed compounds demonstrate two emissive wavelengths in the solution and demonstratedecyltetradecyl) two-1,4 emissive-dioxopyrrolo[3,4 wavelengths-c]pyrrole in the solution (DPP-Br) and in using the solid-state, the appropriate and the in the solid-state, and the dual emission is tuned by the aggregation state in the mixed dualpolymerization emission is methods. tuned by All the polymers aggregation have statebroad in absorption the mixed spectra solvent. and The reduced compound optical solvent. The compound containing silole and pyrene units shows polymorphism- containingbandgaps silole (Eg < and 2.0 pyrene eV). The units desilylated shows polymorphism-dependent polymer had a lower absorption solid-state maximum emissive behavior.dependent The solid crystal-state packing emissive of thebehavior. target compoundsThe crystal ispacking inﬂuenced of the by target the steric compounds effects of is compared to the other polymers due to the absence of the silicon orbital interaction with theinfluenced corresponding by the differentsteric effects molecular of the corresponding structures. different molecular structures. the π-system. In 2019, C. N. Scott and M. D. Bisen reported a series of 1,7-disubstituted 1,6-diynes by the Sonogashira reaction and converted them to the reactively functional 2,5-

SchemeScheme 13. 13.The The synthesis synthesis route route of of the the target target compounds compounds [ 53[5].3].

Scheme 14. Representative reaction scheme for the preparation of siloles 54, RT: room temperature [54].

4. Application in Different Fields Typical optoelectronic devices are converters capable of converting the energy generated between light and electric current. Optoelectronic devices have a wide range of applications, ranging from light-emitting diodes through to photovoltaic technology, laser diodes, photoemission tubes for cameras, photoconductive cells, etc. Their extremely versatile applications require economical production on an industrial scale. Recent years have witnessed the dynamic development and enormous improvement in the efficiency of these devices and proved the enormous demand for new organic semiconductor materials, which are at the heart of such devices. In this chapter, examples of the use of silole-based compounds in optoelectronic devices will be presented.

Molecules 2021, 26, x FOR PEER REVIEW 11 of 31

disubstituted siloles using the Kumada-type nickel-mediated intramolecular cyclization (Scheme 14) [54]. Following the preparation of the silole precursors, they pursued polymerizations of the siloles with 3,6-bis(5-bromothiophene-2-yl)-N,N-bis(2- decyltetradecyl)-1,4-dioxopyrrolo[3,4-c]pyrrole (DPP-Br) using the appropriate polymerization methods. All polymers have broad absorption spectra and reduced optical bandgaps (Eg < 2.0 eV). The desilylated polymer had a lower absorption maximum compared to the other polymers due to the absence of the silicon orbital interaction with the π-system.

Molecules 2021, 26, 2012 11 of 30

In 2019, C. N. Scott and M. D. Bisen reported a series of 1,7-disubstituted 1,6-diynes by the Sonogashira reaction and converted them to the reactively functional 2,5-disubstituted siloles using the Kumada-type nickel-mediated intramolecular cyclization (Scheme 14)[54]. Following the preparation of the silole precursors, they pursued polymerizations of the siloles with 3,6-bis(5-bromothiophene-2-yl)-N,N-bis(2-decyltetradecyl)-1,4-dioxopyrrolo pyrrole (DPP-Br) using the appropriate polymerization methods. All polymers have broad absorption spectra and reduced optical bandgaps (Eg < 2.0 eV). The desilylated polymer had a lower absorption maximum compared to the other polymers due to the absence of Scheme 13. The synthesisthe silicon route orbital of the interaction target compounds with the π -system.[53].

ScheSchememe 14. 14. RepresentativeRepresentative reaction reaction scheme scheme for thefor preparationthe preparation of siloles of 54siloles, RT: room 54, RT: temperature room temperature [54]. [54]. 4. Application in Different Fields 4. Application in DifferentTypical optoelectronicFields devices are converters capable of converting the energy generated between light and electric current. Optoelectronic devices have a wide range of Typical optoelectronicapplications, ranging devices from are light-emitting converters diodes capable through of converting to photovoltaic the technology, energy laser generated betweendiodes, light photoemissionand electric curre tubesnt. for Optoelectronic cameras, photoconductive devices have cells, a wide etc. Their range extremely of applications, rangingversatile from applications light-emitting require economical diodes through production to onphotovoltaic an industrial technology, scale. Recent years laser diodes, photoemissionhave witnessed tubes the dynamicfor cameras, development photoconductive and enormous cells, improvement etc. Theirin extremely the efficiency of versatile applicationsthese devicesrequire and economical proved the production enormous demand on an for industrial new organic scale. semiconductor Recent years materials, which are at the heart of such devices. In this chapter, examples of the use of silole-based have witnessed thecompounds dynamic in development optoelectronic devicesand enormous will be presented. improvement in the efficiency of these devices and proved the enormous demand for new organic semiconductor materials, which 4.1.are Resinsat the heart of such devices. In this chapter, examples of the use of silole-based compoundsThe in research optoelectronic carried out devices shows will that thebe presented. existence of the silicon-containing units decreases the melting point and improves the solubility of synthesized resin. In addition, the cured resin exhibits excellent thermal properties at temperatures as high as 546 ◦C under N2 atmosphere. The SiBPPN-containing laminate possessed a high bending strength of 389 MPa and a high interlaminar shear strength of 30.44 MPa at room temperature (Scheme3)[ 38]. The resultant composite may be potentially applied as a high performance structural or functional material in aircraft, aerospace, and electronics fields. An example of materials modified by the addition of arylsilanes are poly(esters)— PEs (Scheme4)[ 39]. Despite excellent mechanical properties, chemical resistance, and thermal stability, PEs with high aromatic content are difficult to process due to high glass- transition temperature (Tg) values and insolubility in common organic solvents. The Tg and the thermal degradation temperatures values showed a tendency in the sense that when the size of the halogen atom is increased, the values decrease, due to the higher distance between the polymeric chains, which increases their flexibility and diminishes the interactions between them. PEs showed Ultraviolet/visible (UV-vis) transparence between approximately 500 and 600 nm, and those containing halogen atoms showed auto-extinction when put in the flame. Molecules 2021, 26, x FOR PEER REVIEW 12 of 31

Molecules 2021, 26, x FOR PEER REVIEW 12 of 31

4.1. Resins The research carried out shows that the existence of the silicon-containing units 4.1. Resins decreases the melting point and improves the solubility of synthesized resin. In addition, theThe cured research resin carried exhibits out excellent shows that thermal the existence properties of theat temperatures silicon-containing as high units as 546 °C decreasesunder the N melting2 atmosphere. point and The improve SiBPPNs the-containing solubility of laminate synthesized possessed resin. In aaddition, high bending the curedstrength resin of exhibits 389 MPa excellent and athermal high interlam propertiesinar at sheartemperatures strength as of high 30.44 as 546 MPa °C at room undertemperature N2 atmosphere. (Scheme The 3) [3 SiBPPN8]. The- containingresultant composite laminate may possessed be potentially a high applied bending as a high strengthperformance of 389 MPa structural and a or high functional interlam materialinar shear in aircraft, strength aerospace, of 30.44 MPaand electronics at room fields. temperature (Scheme 3) [38]. The resultant composite may be potentially applied as a high An example of materials modified by the addition of arylsilanes are poly(esters)— performance structural or functional material in aircraft, aerospace, and electronics fields. PEs (Scheme 4) [39]. Despite excellent mechanical properties, chemical resistance, and An example of materials modified by the addition of arylsilanes are poly(esters)— thermal stability, PEs with high aromatic content are difficult to process due to high glass- PEs (Scheme 4) [39]. Despite excellent mechanical properties, chemical resistance, and transition temperature (Tg) values and insolubility in common organic solvents. The Tg thermal stability, PEs with high aromatic content are difficult to process due to high glass- transitionand the temperature thermal degradation (Tg) values andtemperatures insolubility values in common showed organic a tendency solvents. in Thethe Tgsense that and whenthe thermal the size degradation of the halogen temperatures atom is valuesincreased, showed the avalues tendency decrease, in the duesense to that the higher whendistance the size between of the halogen the pol atomymeric is increased,chains, which the valuesincreases decrease, their flexibilitydue to the and higher diminishes distancethe interactionsbetween the pol betweenymeric them.chains, PEs which showed increases Ultraviolet their flexibility/visible and (UV diminishes-vis) transparence the interactionsbetween approximately between them. 500 PEs and showed 600 nm, Ultraviolet and those/v isible containing (UV-vis halogen) transparence atoms showed betweenauto - approximatelyextinction when 500 put and in 600the flame.nm, and those containing halogen atoms showed

Molecules 2021, 26, 2012 auto-extinctionAnother when interesting put in the flame. example was presented by S. Fan and12 of co30 -workers [55]. ResearchersAnother interesting obtained example an inherent was presentedflame-retardant by S. polyamide Fan and co 6 -workers (FR-PA6) [5 containing5]. Researcherspolydiphenylsiloxane obtained an inherent (PDPS) under flame- retardantthe action polyamideof ethylene 6 glycol (FR-PA6) (EG) containingvia a facile “two- polydiphenylsiloxanestepAnother” bulk interestingpolymerization. (PDPS) example under wasPrepared presentedthe action FR by- PA6of S. Fanethylene reached and co-workers glycol 28.3% (EG) [ 55of]. the Researchersvia limiting a facile “oxygentwo- index stepobtained”(LOI) bulk polymerization. and an inherent passed ﬂame-retardant the Prepared V-0 level polyamide FR -PA6 of UL94 reached 6 (FR-PA6) test 28.3% containing with of suppressed the polydiphenylsilox- limiting meltoxygen-dripping. index After (LOI)aneinvestigating and (PDPS) passed under the the char action V-0 residues levelof ethylene of and UL94 glycol pyrolysis test(EG) viawith avolatiles facile suppressed “two-step” of FR melt- bulkPA6-dripping. polymer- after combustion, After the investigatingization.formation Prepared the of chara FR-PA6silicon residues- reachedrich protectiveand 28.3% pyrolysis of thelayer limiting volatiles was oxygenprove of FRn index -toPA6 be (LOI) anafter essential and combustion, passed factor theto increase formathetion V-0 of level a silicon of UL94-rich test protective with suppressed layer was melt-dripping. proven to Afterbe an investigating essential factor the char to increase residuesthe flame and pyrolysis retardancy volatiles of of FR FR-PA6-PA6 after via combustion, reducing the the formation diffusion of a silicon-rich of pyrolysis volatiles, the flame retardancy of FR-PA6 via reducing the diffusion of pyrolysis volatiles, protectiverestricting layer the was heat proven transfer to be an and essential mass factor loss, to as increase well as the preventing ﬂame retardancy polymer of melts from restrictingFR-PA6dripping. viathe reducingThe heat novel transfer the diffusionroute and offered mass of pyrolysis loss,in the asvolatiles, work well is asrestricting easily preventing accessible the heat polymer transfer to large meltsand-scale from industrial dripping.massfabrication. loss, The as novel well Polydiphenylsiloxane asroute preventing offered polymer in the work melts (PDPS) fromis easily was dripping. preparedaccessible The novel to by large dehydration route-scale offered industrial condensation fabrication.inaccording the work Polydiphenylsiloxane is to easily the accessibleliterature to [5 large-scale 6 (PDPS)], and industrialthe was synthetic prepared fabrication. route by dehydrationPolydiphenylsiloxane is shown in condensation Scheme 15. FR-PA6 (PDPS) was prepared by dehydration condensation according to the literature [56], and the accordingwas prepared to the literature by two -[5step6], andpolymerization. the synthetic The route chemical is shown structure in Scheme of FR 15.-PA6 FR-PA6 is show n in wassynthetic prepared route by istwo shown-step in polymerization. Scheme 15. FR-PA6 The was chemical prepared structure by two-step of polymerization. FR-PA6 is shown in TheScheme chemical 16. structure of FR-PA6 is shown in Scheme 16. Scheme 16.

SchemeSchemeScheme 15. 15.Synthetic 15Synthetic. Synthetic route route forroute for preparing preparing for preparing PDPS PDPS via via PDPS dehydration dehydration via dehydration condensation condensation condensation [55]. [55]. [55].

SchemeSchemeScheme 16 16.. The The16 chemical. chemicalThe chemical structure structure of of FR-PA6 FR-PA6 of [56 FR ].[5-6PA6]. [56].

OtherOther compounds compounds that that use use thethe thermalthermal properties properties of silanes of silanes are arylacetylene are arylacetylene resins resins PODSA-MM,Other PODSA-MP,compounds and that PODSA-PP use the withthermal 2,5-diphenyl-[1,3,4]-oxadiazole properties of silanes are moieties, arylacetylene resins PODSA-MM, PODSA-MP, and PODSA-PP with 2,5-diphenyl-[1,3,4]-oxadiazole moieties, preparedPODSA by-MM, Grignard PODSA reactions-MP, in and THF PODSA (Scheme-5PP)[ 40 with]. The 2,5 resins-diphenyl have good-[1,3,4] processing-oxadiazole moieties, prepared by Grignard reactions in THF (Scheme 5) [40]. The resins have good processing fluidityprepared and areby solubleGrignard in common reactions solvents in THF such (Schem as THF, CHe 5)2Cl [420, and]. The toluene. resins The have cured good processing fluidityresins and have are good soluble mechanical in common properties solvents due tosuch the as existence THF, CH of rigid-rod2Cl2, and 2,5-diphenyl- toluene. The cured fluidity and are soluble in common solvents such as THF, CH2Cl2, and toluene. The cured [1,3,4]-oxadiazole and polar aromatic ether structures. The flexural strength and flexural resinsresins have havegood good mechanical mechanical properties properties due to due the toexistence the existence of rigid of-rod rigid 2,5--roddiphenyl 2,5-diphenyl- - [1,3,4]modulus-oxadiazole of the curedand polar PODSA-MP aromatic resin ether at room structures. temperature The are flexural 69.6 MPa strength and 2.5 and GPa, flexural respectively.[1,3,4]-oxadiazole All cured resinsand polar have good aromatic thermal e andther thermo-oxidative structures. The stabilities flexural attributed strength and flexural to a number of benzene rings and strong charge-transfer interactions of 2,5-diphenyl-[1,3,4]- oxadiazole. Cured resins have low water uptake due to crosslinked networks. The resins

possess potential application in the heat-resistant field. 4.2. Organic Solar Cells Due to the increasing energy demand, solar cells have become a promising way to harness the inexhaustible clean energy of the sun. Organic solar cells (OSC) consisting of conjugated polymers or small molecules (constituting a mixture of donors) with fullerene derivatives (acting as acceptors) attract special attention due to their unique properties, such as relatively light weight, low production cost, and the possibility of producing large elastic surface. As a result, in the last decade, a significant improvement in the power conversion efficiency (PCE) above 10% was achieved [57–61]. In order to achieve high photovoltaic efficiency, it is necessary to maintain the specific properties of conjugated polymers; in particular, their absorption spectra and the molecular energy levels must Molecules 2021, 26, 2012 13 of 30

be closely matched. The appropriate energy levels of the HOMO and the LUMO not only significantly facilitate exciton dissociation at the donor/acceptor interface, but also generate higher open-circuit voltage (Voc) of OSC devices [57,62]. Despite enormous advances, fullerene derivatives have some inherent disadvantages, such as poor absorption in the visible region, high synthesis cost, difficult modification of energy levels, and poor morphological stability, which makes the use of these compounds in the OSC construction problematic. Therefore, in recent years, a great deal of effort has gone into developing non-fullerene acceptors that combine good stability, easy synthesis, strong visible and near-infrared absorption, and the ability to adjust energy levels. The use of polymer donors having complementary absorption spectra with non-fullerene acceptors resulted in the PCE levels of 10–14% often reported [58,63,64]. Aromatic compounds containing silicon in their structure constitute an attractive building block due to their unique electronic structures, which consist of low LUMO energy levels and relatively small band gaps, due to the interaction between the silicon σ* orbital and the π* butadiene orbital. Because of all these properties, many conjugated polymers containing silicon in their structure were used as active substances in solar cells [57,65,66]. L. Liu et al. prepared a series of random dithienosilole-based terpolymers using two different dithienosilole monomers with a nonanoyl group and malononitrile as electron- withdrawing groups, using microwaves to aid in the polymerization of Stille coupling. Newly synthesized polymers were also tried as donor materials in polymer solar cells, all of which exhibited high open-circuit voltage above 1.0 V. The final power conversion efficiency was in the range of 1.16–3.29% with appropriately adjusted monomer proportions [57]. Dithienosilole copolymers have also been prepared by M. Li et al. A series of copolymers were designed and prepared from dithienosilole monomers with different acyl groups and benzothiophene as a comonomer (Scheme 17). A microwave-assisted Stille junction was used for polymerization. The individual polymers exhibited similar absorption behav- Molecules 2021, 26, x FOR PEER REVIEWior with slight differences. Newly synthesized polymers were assessed as donor materials 14 of 31 in OSC, all of which showed a high open-circuit voltage of about 1.0 V. The branched modified 3,7-dimethyloctane polymer showed the best performance at a PCE of 2.66% [67].

SchemeScheme 17. 17.Chemical Chemical structure structure of PBDTDTSi of PBDTDTSi [67]. [67].

Y.Y. Zhao Zhao and and co-workers co-workers designed designed and synthesized and synthesized two new copolymers two new combining copolymers combining dithieno[3,2-b:20,30-d]silole as the donor unit and thieno[3,4-c]pyrrole-4,6-dione as the acceptordithieno[3,2 unit (Scheme-b:2′,3′ -18d]silole). The group as the investigated donor unit the and effects thieno[3,4 of various-c]pyrr side chainsole-4,6 on-dione as the ac- opticalceptor properties, unit (Scheme energy 18). levels, The morphology, group investigated and photovoltaic the effects properties. of various As a result, side chains on opti- thecal individual properties, polymers energy exhibited levels, different morphology, levels of lightand andphotovoltaic energy absorption, properties. which As a result, the hadindividual a signiﬁcant polymers effect on exhibited the short circuit different current levels and open-circuit of light and voltage. energ Finally,y absorption, the which had maximum conversion efﬁciency was obtained at the level of 2.65% [64]. a significant effect on the short circuit current and open-circuit voltage. Finally, the maximum conversion efficiency was obtained at the level of 2.65% [64].

Scheme 18. Chemical structure of DTS derivatives [64].

X. Chen et al. designed and synthesized two new small molecules of the acceptor– donor–acceptor DINDTS and DINDCNDTS containing dithienosilole as the base unit and 1,3-indanedione (IN) or a malononitrile derivative as closed-end groups (Scheme 19). These molecules were synthesized for processing in a solar cell solution with a collective heterojunction (BHJ). Optimized solar cells based on DINDTS: PC71BM were characterized by a short-circuit current of 13.5 mA cm–2 and an energy conversion efficiency (PCE) of 6.60% [68].

Scheme 19. Chemical structure of DINDTS and DINCNDT [68].

Molecules 2021, 26, x FOR PEER REVIEW 14 of 31 Molecules 2021, 26, x FOR PEER REVIEW 14 of 31

Scheme 17. Chemical structure of PBDTDTSi [67]. Scheme 17. Chemical structure of PBDTDTSi [67]. Y. Zhao and co-workers designed and synthesized two new copolymers combining Y. Zhao and co-workers designed and synthesized two new copolymers combining dithieno[3,2-b:2′,3′-d]silole as the donor unit and thieno[3,4-c]pyrrole-4,6-dione as the dithieno[3,2-b:2′,3′-d]silole as the donor unit and thieno[3,4-c]pyrrole-4,6-dione as the acceptor unit (Scheme 18). The group investigated the effects of various side chains on acceptor unit (Scheme 18). The group investigated the effects of various side chains on optical properties, energy levels, morphology, and photovoltaic properties. As a result, optical properties, energy levels, morphology, and photovoltaic properties. As a result, Molecules 2021, 26, 2012 the individual polymers exhibited different levels of light and energy absorption, 14which of 30 the individual polymers exhibited different levels of light and energy absorption, which had a significant effect on the short circuit current and open-circuit voltage. Finally, the had a significant effect on the short circuit current and open-circuit voltage. Finally, the maximum conversion efficiency was obtained at the level of 2.65% [64]. maximum conversion efficiency was obtained at the level of 2.65% [64].

Scheme 18. Chemical structure of DTS derivatives [64]. SchemeScheme 18. ChemicalChemical structure structure of DTS derivatives [6 [644]. X. Chen et al. designed and synthesized two two new small molecules of of the acceptor acceptor–– X. Chen et al. designed and synthesized two new small molecules of the acceptor– donordonor–acceptor–acceptor DINDTS DINDTS and and DINDCNDTS DINDCNDTS containing containing dithienosilole dithienosilole as the as thebase base unit unitand donor–acceptor DINDTS and DINDCNDTS containing dithienosilole as the base unit and 1,3and-indanedione 1,3-indanedione (IN) (IN) or a or malononitrile a malononitrile derivative derivative as as closed closed-end-end groups groups (Scheme (Scheme 19).19). 1,3-indanedione (IN) or a malononitrile derivative as closed-end groups (Scheme 19). These molecules were synthesizedsynthesized for processing in a solar cell solution with a collective These molecules were synthesized for processing in a solar cell solution with a collective heterojunction (BHJ). (BHJ). Optimized Optimized solar solar cells based cells on based DINDTS: on PC71BM DINDTS: were PC71BM characterized were heterojunction (BHJ). Optimized solar–2 cells based on DINDTS: PC71BM were characterizedby a short-circuit by current a short- ofcircuit 13.5 mA current cm ofand 13.5 an energymA cm conversion–2 and an energyefﬁciency conversion (PCE) of characterized by a short-circuit current of 13.5 mA cm–2 and an energy conversion efficiency6.60% [68]. (PCE) of 6.60% [68]. efficiency (PCE) of 6.60% [68].

Molecules 2021, 26, x FOR PEER REVIEW 15 of 31

Scheme 19. Chemical structure of DINDTS and DINCNDT [68]. SchemeScheme 19. 19. ChemicalChemical structure structure of DINDTS DINDTS and DINCNDT [6 [688].

AA similar similar donor donor–acceptor–donor–acceptor–donor system system using using 2,6-(4,4 2,6-(4,4-bis-(2-ethylhexyl)-4-bis-(2-ethylhexyl)-4H-cyclo-H- 0 0 0 0 cyclopenta[2,1-b;3,4-bpenta[2,1-b;3,4-b′]dithiophene]dithiophene (DTC) (DTC) and and (4,4 (4,4′-bis(2-bis(2-ethylhexyl)dithieno[3,2-b:2-ethylhexyl)dithieno[3,2-b:2′,3′-d]si-,3 - d]silole)lole) (DTS) (DTS) as a as central a central building building unit unit and and 3-ethyl 3-ethyl-rhodanine-rhodanine as closed as closed-end-end groups groups was was de- designedsigned and and synthesized synthesized by byW. W. Ni Niand and co- co-workersworkers (Scheme (Scheme 20). 20 The). The group group investigated investigated the theinfluence inﬂuence of bridging of bridging atoms atoms on on optical, optical, electrochemical, electrochemical, and and morphological morphological properties. DifferencesDifferences ofof singlesingle atomsatoms inin thethe structurestructure ofof polymerspolymers resultedresulted inin changeschanges inin individualindividual properties.properties. AnAn optimizedoptimized solarsolar cellcell basedbased onon DR3TDTSDR3TDTS showedshowed aa PCEPCE aboveabove 8%8% [[6969].].

SchemeScheme 20.20. ChemicalChemical structurestructure ofof DR3TDTCDR3TDTC andand DR3TDTSDR3TDTS [[6969]]..

Examples of the use of polymers containing silicon in their structure for the construction of organic semiconductors (OSCs) are presented in Table 1.

Table 1. Examples of the use of polymers containing silicon in their structure for the construction of OSCs. PCE: the power conversion efficiency; Voc: the open-circuit voltage.

Donor Acceptor Voc (V) PCE (%) Ref. DTS BDT 1.07 3.29 [57] DINTTDTS/DINDTS IDT-C8 0.89 4.52 [58] dithieno[3,2-b:2′,3′-d]silole thieno[3,4-c]pyrrole-4,6-dione 0.63 2.65 [64] dithieno[3,2-b:2′,3′-d]silole naphtho[2,3-c]thiophene-4,9-dione 0.90 5.21 [64] DTS BDT 1.01 2.66 [67] DTC/DTS 3-ethyl-rhodanine 0.82 8.0 [69] DINDTS/DINCNDTS IN/INCN 0.94 6.79 [70]

The above studies confirm that single changes in the polymer structure (different side chains or core groups) can have a huge impact on the properties of the obtained polymers in terms of their application in OSCs. The use of different alkyl side chains in the synthesis of polymers means that the obtained compounds exhibit different levels of light and energy absorption, which affects the short-circuit current and open-circuit voltage, respectively. The introduction of the alkyl chain into the π-bridge causes a significant increase in the dihedral angle between the mote and the donor unit, which may ultimately affect the value of the maximum energy conversion efficiency. The parameters of individual polymers are varied, which proves that the selection of appropriate side chains is of great importance when designing high-performance OSCs. Similar properties are exhibited by polymers with different groups in the core. Despite slight differences in the structure of the central core units (e.g., the silicon atom in the DTS block and the carbon atom in the DTC block [69]), individual polymers showed very different properties, including membrane absorption, molecular packing, and charge transport. The silicon atom showed much better absorption and packing properties than the carbon atom.

Molecules 2021, 26, 2012 15 of 30

Examples of the use of polymers containing silicon in their structure for the construction of organic semiconductors (OSCs) are presented in Table1.

Table 1. Examples of the use of polymers containing silicon in their structure for the construction of OSCs. PCE: the power conversion efﬁciency; Voc: the open-circuit voltage.

Donor Acceptor Voc (V) PCE (%) Ref. DTS BDT 1.07 3.29 [57] DINTTDTS/DINDTS IDT-C8 0.89 4.52 [58] dithieno[3,2-b:20,30-d]silole thieno[3,4-c]pyrrole-4,6-dione 0.63 2.65 [64] dithieno[3,2-b:20,30-d]silole naphtho[2,3-c]thiophene-4,9-dione 0.90 5.21 [64] DTS BDT 1.01 2.66 [67] DTC/DTS 3-ethyl-rhodanine 0.82 8.0 [69] DINDTS/DINCNDTS IN/INCN 0.94 6.79 [70]

The above studies confirm that single changes in the polymer structure (different side chains or core groups) can have a huge impact on the properties of the obtained polymers in terms of their application in OSCs. The use of different alkyl side chains in the synthesis of polymers means that the obtained compounds exhibit different levels of light and energy absorption, which affects the short-circuit current and open-circuit voltage, respectively. The introduction of the alkyl chain into the π-bridge causes a significant increase in the dihedral angle between the mote and the donor unit, which may ultimately affect the value of the maximum energy conversion efficiency. The parameters of individual polymers are varied, which proves that the selection of appropriate side chains is of great importance when designing high-performance OSCs. Similar properties are exhibited by polymers with different groups in the core. Despite slight differences in the structure of the central core units (e.g., the silicon atom in the DTS block and the carbon atom in the DTC block [69]), individual polymers showed very different properties, including membrane absorption, molecular packing, and charge transport. The silicon atom showed much better absorption and packing properties than the carbon atom.

4.3. Organic Light-Emitting Diodes Over the last three decades, organic light-emitting diodes (OLEDs) have enjoyed great interest in the industry due to their high potential for use in flat panel displays and semiconductor lighting [71,72]. OLED is a type of semiconductor lighting system that converts electric current into light. The emitting layer in semiconductors used in OLEDs is based on fluorescent, phosphorescent materials or TADF (thermally activated delayed fluorescence) [73]. OLEDs can be divided into two basic groups. The first group consists of small molecule-based OLEDs (MOLEDs), while the second group is polymer-based OLEDs (PLEDs). It should be mentioned that organic light-emitting materials significantly affect the efficiency of devices. Equipment performance is strongly dependent on the inherent physicochemical properties of molecular aggregates. The efficiency of emission and the ability to carry the medium in solid films are particularly important [71,74,75]. When comparing molecules or polymers with ACQ (aggregation-caused quenching) properties, active AIE materials show numerous advantages, such as high external quantum efficiency (EQE), relatively easy synthesis, unadulterated production, and simplified device configuration [75]. Among all materials belonging to AIE, silole-based compounds are readily used in OLEDs due to their high thermal stability, good electronic mobility, and affinity, and moreover, they exhibit high solid-state fluorescence quantum efficiency—some compounds achieve efficiency up to 100% [76,77]. R. Yang et al. synthesized tetraphenylsilane derivatives (TCzSi and DCzMzSi) (Scheme1)[36] . A TCzSi-based blue phosphorescent organic light-emitting diode (PHOLED) exhibited a maximum current efficiency (CE) of 25.5 cd A−1, maximum power Molecules 2021, 26, x FOR PEER REVIEW 16 of 31

4.3. Organic Light-Emitting Diodes Over the last three decades, organic light-emitting diodes (OLEDs) have enjoyed great interest in the industry due to their high potential for use in flat panel displays and semiconductor lighting [71,72]. OLED is a type of semiconductor lighting system that converts electric current into light. The emitting layer in semiconductors used in OLEDs is based on fluorescent, phosphorescent materials or TADF (thermally activated delayed fluorescence) [73]. OLEDs can be divided into two basic groups. The first group consists of small molecule-based OLEDs (MOLEDs), while the second group is polymer-based OLEDs (PLEDs). It should be mentioned that organic light-emitting materials significantly affect the efficiency of devices. Equipment performance is strongly dependent on the inherent physicochemical properties of molecular aggregates. The efficiency of emission and the ability to carry the medium in solid films are particularly important [71,74,75]. When comparing molecules or polymers with ACQ (aggregation-caused quenching) properties, active AIE materials show numerous advantages, such as high external quantum efficiency (EQE), relatively easy synthesis, unadulterated production, and simplified device configuration [75]. Among all materials belonging to AIE, silole-based compounds are readily used in OLEDs due to their high thermal stability, good electronic mobility, and affinity, and moreover, they exhibit high solid-state fluorescence quantum efficiency—some compounds achieve efficiency up to 100% [76,77].

Molecules 2021, 26, 2012 R. Yang et al. synthesized tetraphenylsilane derivatives (TCzSi and DCzMzSi)16 of 30 (Scheme 1) [36]. A TCzSi-based blue phosphorescent organic light-emitting diode (PHOLED) exhibited a maximum current efficiency (CE) of 25.5 cd A–1, maximum power efficiency (PE) of 18.15 lm W–1, maximum luminance of 6285 cd m–2, and maximum exter- − − nalefficiency quantum (PE) efficiency of 18.15 lm of W13.32%.1, maximum These results luminance reveal of that 6285 highly cd m twisted2, and maximum tetraarylsilane exter- compoundsnal quantum have efficiency great potential of 13.32%. for These highly results efficient reveal PHOLEDs that highly as host twisted materials. tetraarylsilane compoundsOn the other have hand, great potentialO. Bezvikonnyi for highly and efficientco-workers PHOLEDs synthesized as host two materials. new 3,3′-bicarba- 0 zole On derivatives the other functionalized hand, O. Bezvikonnyi with triphenylsilyl and co-workers and with synthesized tosyl moieties two att newached 3,3 to- 9,9‘positionsbicarbazole derivatives of 3,3′-bicarbazole functionalized moiety, with and triphenylsilyl they measured and their with thermal, tosyl moieties optical, attached photo- 0 physical,to 9,9‘positions electrochemical of 3,3 -bicarbazole, and charge moiety,-transporting and they measured properties their (Scheme thermal, 2) [3 optical,7]. Electron pho- mobilitytophysical, of electrochemical,5.6 × 10–4 cm2/(Vs) and was charge-transporting observed for the layer properties of BiCzSiPh (Scheme3 at 2the)[ 37electric]. Electron field −4 2 ofmobility 5.4 × 10 of5 V/cm, 5.6 × 10whilecm hole/(Vs) mobility was observedwas recorded for the to be layer 1.4 of× 10 BiCzSiPh–4 cm2/(Vs)3 at at the the electric same field of 5.4 × 105 V/cm, while hole mobility was recorded to be 1.4 × 10−4 cm2/(Vs) at the electric field. BiCzSiPh3-based phosphorescent OLED exhibited power and external quan- tumsame efficiencies electric field. as high BiCzSiPh as 253 -basedlm/W and phosphorescent 13.8%, respective OLEDly.exhibited power and external quantumH. Park efficiencies et al. synthesized as high as a 25 group lm/W of anddithiensiloles 13.8%, respectively. based on fluorene and carbazoles (DTS),H. which Park etwere al. synthesizedthen tested for a group their thermal, of dithiensiloles photophysical, based on and fluorene electrochemical and carbazoles properties(DTS), (Scheme which were 21). thenAll properties tested for theirconfirmed thermal, that photophysical, the compounds and could electrochemical be used as prop-basic erties (Scheme 21). All properties confirmed that the compounds could be used as basic materials for red emitters in OLED. All compounds showed reversible oxidation and had materials for red emitters in OLED. All compounds showed reversible oxidation and had low localized LUMO energy due to the conjugated fluorene/carbazole substituents in low localized LUMO energy due to the conjugated fluorene/carbazole substituents in DTS. DTS. This, together with high fluorescent quantum efficiency, confirms the possibility of This, together with high fluorescent quantum efficiency, confirms the possibility of using using these compounds in OLEDs as emitters, carriers, and materials for electron these compounds in OLEDs as emitters, carriers, and materials for electron transport [78]. transport [78].

Molecules 2021, 26, x FOR PEER REVIEW 17 of 31

Scheme 21. Chemical structure of DTS derivatives [[7788]]..

Another series series of carbazole of carbazole-substituted-substituted dithienosiloles dithienosiloles was synthesized was synthesized by Y. Xiong by etY. Xiongal. (Scheme et al. 22). (Scheme A thorough 22). A thoroughanalysis of analysis the crystal of the and crystal electronic and electronic structure, structure, thermal stability,thermal stability,and electrochemical and electrochemical and photophysical and photophysical properties properties confirmed confirmed the significant the significant properties of AIE with high emission efficiency from solid layers, thanks to which properties of AIE with high emission efficiency from solid layers, thanks to which these compoundsthese compounds can be can used be in used the indesign the design of light of-emitting light-emitting layers layersin undoped in undoped OLEDs OLEDs (electroluminescence(electroluminescence efficiency efficiency at the at level the level of 17.59 of 17.59 cd/A, cd/A, 12.55 12.55 lm/W lm/W,, and 5.63%) and 5.63%) [79]. [79].

Scheme 22. Chemical structures of DTS derivatives, synthesized by Xiong et al. [7979]]..

2,3,4,52,3,4,5-tetraarylsiloles-tetraarylsiloles are a class of very important important luminogenic luminogenic materials materials character- character- izedized by efficientefficient solid-statesolid-state emission emission and and excellent excellent electron electron transportability. transportability. B. ChenB. Chen et al.et in-al. introducedtroduced the the bulky bulky 9,9-dimethylfluorenyl, 9,9-dimethylfluorenyl, 9,9-diphenylfluorenyl, 9,9-diphenylfluorenyl, and and 9,90-spirobifluorenyl 9,9′-spirobifluo- renylsubstituents substituents at the 2,5at the positions 2,5 positions of the silole of the rings silole (Scheme rings (Scheme23). Compounds 23). Compounds modified inmodi- this fiedway inshowed this way higher showed fluorescence higher quantum fluorescence yields quantum (2.5–5.4%) yields than (2.5 2,3,4,5-tetraarylsiloles.–5.4%) than 2,3,4,5- tetraarylsiloles.The group also constructed The group efficient also constructed OLEDs using efficient synthesized OLEDs compounds using synthes as mainized com-emit- poundsters, which as main ensured emitters, high luminance, which ensured current high efficiency, luminance, and energycurrent efficiency efficiency, at and the levelenergy of − − − efficiency44,100 cdm at the2, 18.3 level cdA of 44,1001, and cdm 15.7– lmW2, 18.3 1cdA, respectively–1, and 15.7 [ 80lmW]. –1, respectively [80].

Scheme 23. Chemical structure of 2,5-difluorenyl-substituted silole [80].

H. Nie together with the group designed and synthesized four silole derivatives: (PBI) 2DMTPS, (PBI) 2MPPS, (PPI) 2MTPS, and (PPI) 2MPPS (Scheme 24), which are characterized by high AIE activity and emission in solid films, and also show good thermal stability and low energy LUMO. Three-layer OLED without doping (PPI) 2DMTPS has excellent electroluminescence (EL) parameters with an efficiency of 15.06 cd A–1, 16.24 lm W–1, and 4.84%. Importantly, similar parameters to the three-layer OLED (13.30 cd A–1, 14.51 lm W–1, and 4.25%) were obtained by an efficient two-layer OLED based on (PBI) 2DMTPS. The obtained results confirmed the effectiveness of the designed compounds in using them as n-type light emitters for efficient, simple, and inexpensive OLEDs [71].

Molecules 2021, 26, x FOR PEER REVIEW 17 of 31

Another series of carbazole-substituted dithienosiloles was synthesized by Y. Xiong et al. (Scheme 22). A thorough analysis of the crystal and electronic structure, thermal stability, and electrochemical and photophysical properties confirmed the significant properties of AIE with high emission efficiency from solid layers, thanks to which these compounds can be used in the design of light-emitting layers in undoped OLEDs (electroluminescence efficiency at the level of 17.59 cd/A, 12.55 lm/W, and 5.63%) [79].

Scheme 22. Chemical structures of DTS derivatives, synthesized by Xiong et al. [79].

2,3,4,5-tetraarylsiloles are a class of very important luminogenic materials characterized by efficient solid-state emission and excellent electron transportability. B. Chen et al. introduced the bulky 9,9-dimethylfluorenyl, 9,9-diphenylfluorenyl, and 9,9′-spirobifluorenyl substituents at the 2,5 positions of the silole rings (Scheme 23). Compounds modified in this way showed higher fluorescence quantum yields (2.5–5.4%) than 2,3,4,5- tetraarylsiloles. The group also constructed efficient OLEDs using synthesized com- Molecules 2021, 26, 2012 17 of 30 pounds as main emitters, which ensured high luminance, current efficiency, and energy efficiency at the level of 44,100 cdm–2, 18.3 cdA–1, and 15.7 lmW–1, respectively [80].

SchemeScheme 23. 23.Chemical Chemical structure structure of 2,5-diﬂuorenyl-substituted of 2,5-difluorenyl-substituted silole [80]. silole [80].

H.H. Nie Nie together together with thewith group the designed group designed and synthesized and foursynthesized silole derivatives: four silole (PBI) derivatives: 2DMTPS, (PBI) 2MPPS, (PPI) 2MTPS, and (PPI) 2MPPS (Scheme 24), which are character- (PBI) 2DMTPS, (PBI) 2MPPS, (PPI) 2MTPS, and (PPI) 2MPPS (Scheme 24), which are char- ized by high AIE activity and emission in solid films, and also show good thermal stability andacterized low energy by high LUMO. AIE Three-layer activity and OLED emission without dopingin solid (PPI) films, 2DMTPS and also has excellentshow good thermal − − Molecules 2021, 26, x FOR PEER REVIEWelectroluminescencestability and low (EL)energy parameters LUMO. with Three an efficiency-layer OLED of 15.06 without cd A 1, 16.24 doping lm W (PPI)1, and 2DMTPS18 of 31 has 4.84%.excellent Importantly, electroluminescence similar parameters (EL) to theparameters three-layer with OLED an (13.30 efficiency cd A−1, 14.51of 15.06 lm W cd− 1A, –1, 16.24 lm andW–1 4.25%), and 4.84%. were obtained Importantly, by an efficient similar two-layer parameters OLED to based the onthree (PBI)-layer 2DMTPS. OLED The (13.30 cd A–1, Molecules 2021, 26, x FOR PEER REVIEWobtained results confirmed the effectiveness of the designed compounds in using them18 of as 31 14.51 lm W–1, and 4.25%) were obtained by an efficient two-layer OLED based on (PBI) n-type light emitters for efficient, simple, and inexpensive OLEDs [71]. 2DMTPS. The obtained results confirmed the effectiveness of the designed compounds in using them as n-type light emitters for efficient, simple, and inexpensive OLEDs [71].

Scheme 24. Chemical structures of silole derivatives [71]. SchemeScheme 24. 24.Chemical Chemical structures structures of of silole silole derivatives derivatives [ 71[71].]. M. Ju et al. compared three dimethyltetraphenylsiloles with symmetrically substi- M.M. Ju Ju et et al. al. compared compared three three dimethyltetraphenylsiloles dimethyltetraphenylsiloles with with symmetrically symmetrically substituted substituted acceptor or donor moieties (Scheme 25). Of all the compounds tested, the multi- acceptortuted acceptor or donor or moietiesdonor moieties (Scheme (Scheme 25). Of 25). all Of the all compounds the compounds tested, tested, the multi-layer the multi- layer organic light emitting diode using DMTPS-DPA showed the highest efficiency, lu- organiclayer organic light emitting light emitting diode using diode DMTPS-DPA using DMTPS showed-DPA showed the highest the efﬁciency,highest efficiency, luminance, luminance, intensity, and internal efficiency: 3.1−2 V, 13,405− 1cd m–2, 8.28− 1cd A–1, 7.88 lm W–1, intensity,minance, and intensity internal, and efﬁciency: internal 3.1 efficiency V, 13,405: 3.1 cd mV, 13,405, 8.28 cdcd Am–2, 7.888.28 lmcd WA–1, ,7.88 and lm 2.42%, W–1, respectively.and 22..42%,42%, r espectively.r DMTPS-DPAespectively. DMTPS canDMTPS also-DPA- beDPA can used canalso in also holebe used be transporting usedin hole in transporting hole layers transporting due layers to the due highlayers to due to mobilitythethe highhigh ofmobility mobility holes [81 of ].of holes holes [8 1[].8 1].

N N N N Si Si Me Me Me Me

Scheme 25. Chemical structure of DMTPS-DPA [81]. SchemeScheme 25. 25.Chemical Chemical structure structure of DMTPS-DPA of DMTPS-DPA [81]. [81]. Examples of the use of polymers containing silicon in their structure for the construction ofExamples OLED are of presented the use of in polymersTable 2. containing silicon in their structure for the construction of OLED are presented in Table 2. Table 2. Examples of the use of polymers containing silicon in their structure for the construction of OLED. Table 2. Examples of the use of polymers containing silicon in their structure for the construction of OLED. Turn on Maximum Current Power External Efficiencies OLED Voltage Luminance Ref. Turn on Maximum (cd A–1) (lm W–1) (%) –2 Current Power External Efficiencies OLED Voltage(V) (Luminancecd m ) Ref. (cd A–1) (lm W–1) (%) Carbazole-substituted dithienosiloles 5.3(V ) 91(,cd920 m –2) 17.59 12.55 5.63 [79] MFMPS 3.2 31,900 16.00 13.50 4.80 [80] Carbazole-substituted dithienosiloles 5.3 91,920 17.59 12.55 5.63 [79] (PBI)2DMTPS 2.5 14,155 13.30 14.51 4.25 [71] DMTPSMFMPS-DPA 3.13.2 13,31405,900 8.2816.00 7.88 13.50 2.42 4.80 [81] [80] (PBI)PyDMS2DMTPS 3.52.5 49,14000,155 9.1013.30 7.10 14.51 3.00 4.25 [82] [71] DMTPS-DPA 3.1 13,405 8.28 7.88 2.42 [81] PyDMS Studies of individual3.5 electrochemical,49,000 9.10 thermal, and7.10 photophysical properties3.00 of [82] newly synthesized compounds based on DTS have shown that the introduction of conju- gatedStudies fluorene of/carbazole individual substituents electrochemical, into the systems thermal, has an and impact photophysical on obtaining properties high of newlythermal synthesized stability, intense compounds green emission based, onand DTS reversible have shownelectrochemical that the behavior. introduction More- of conju- gatedover, the fluorene differences/carbazole in the connect substituentsions between into the the silolesystems and hascarbazole an impact groups on have obtaining little high effect on the changes in optical properties, making it possible to obtain efficient OLEDs thermal stability, intense green emission, and reversible electrochemical behavior. More- using different combinations of silole and carbazole derivatives. over, the differences in the connections between the silole and carbazole groups have little effect on the changes in optical properties, making it possible to obtain efficient OLEDs using different combinations of silole and carbazole derivatives.

Molecules 2021, 26, 2012 18 of 30

Examples of the use of polymers containing silicon in their structure for the construction of OLED are presented in Table2.

Table 2. Examples of the use of polymers containing silicon in their structure for the construction of OLED.

Turn on Maximum External Current Power OLED Voltage Luminance Efﬁciencies Ref. (cd A−1) (lm W−1) (V) (cd m−2) (%) Carbazole-substituted dithienosiloles 5.3 91,920 17.59 12.55 5.63 [79] MFMPS 3.2 31,900 16.00 13.50 4.80 [80] (PBI)2DMTPS 2.5 14,155 13.30 14.51 4.25 [71] DMTPS-DPA 3.1 13,405 8.28 7.88 2.42 [81] PyDMS 3.5 49,000 9.10 7.10 3.00 [82]

Studies of individual electrochemical, thermal, and photophysical properties of newly synthesized compounds based on DTS have shown that the introduction of conjugated ﬂuorene/carbazole substituents into the systems has an impact on obtaining high thermal stability, intense green emission, and reversible electrochemical behavior. Moreover, the differences in the connections between the silole and carbazole groups have little effect on the changes in optical properties, making it possible to obtain efﬁcient OLEDs using different combinations of silole and carbazole derivatives.

4.4. Organic Field-Effect Transistors Work on improving and tuning the performance of organic field-effect transistors (OFETs) has attracted a lot of attention over the last decade. The key elements in constructing OFET are low production costs, long service life, and high mobility of the load. Compared to the small molecules used to construct OFET by vacuum deposition, high molecular weight conjugated polymers are a promising alternative for the cheap and convenient production of OFET. Hence, conjugated polymers have become promising candidates for OFET materials [83–85]. The necessary condition for the construction of a long-life OFET device is the use of small particles or polymers characterized by high thermal, photo-air, and morphological stability [83]. To date, many types of OFET based on polymer semiconductors have been developed and described. Due to the type of main charge carriers, polymer semiconductors can be divided into p- and n-type semiconductors and bipolar semiconductors, capable of transporting holes, electrons, and both, respectively. When selecting the organic semiconductor used in OFET construction, the HOMO and the LUMO energy levels of the organic semiconductor molecule in relation to the work function of the contact electrode material are important. So far, the techniques of producing p-type materials have been the most popular; however, more and more attention has been paid to modern techniques of n-type materials synthesis [86–88]. H. Zhang et al. designed two conjugated donor–acceptor π polymers with 4,40-bis(2- ethylhexyl)-5,50-bis(trimethyltin)dithieno[3,2-b:20,30-d]silole as donor and isodiketopyrrolo- pyrrole or isodithioketopyrrol-pyrrole as acceptor moiety (Scheme 26). Newly synthesized polymers were examined for their optical and electrochemical properties and their potential use in the construction of OFET. The work showed that isodithioketopyrrolopy- rroles are promising building blocks for efficient organic field-effect transistors (hole up to 0.49 cm2V−1s−1 and electron mobility up to 0.26 cm2V−1s1)[83]. K. Kim et al. used the Stille coupling reaction to synthesize two copolymers consisting of diketopyrrolopyrrole (DPP) and silole derivatives (Scheme 27). The newly synthesized polymers were tested for their electrical properties in OFET and circuits. After spinning, both OFETs showed quite low values of hole mobility, while in the case of their annealing at a temperature of 150 ◦C, typical properties of bipolar transport were observed with Molecules 2021, 26, x FOR PEER REVIEW 19 of 31

4.4. Organic Field-Effect Transistors Work on improving and tuning the performance of organic field-effect transistors (OFETs) has attracted a lot of attention over the last decade. The key elements in constructing OFET are low production costs, long service life, and high mobility of the load. Com- pared to the small molecules used to construct OFET by vacuum deposition, high molecular weight conjugated polymers are a promising alternative for the cheap and convenient production of OFET. Hence, conjugated polymers have become promising candidates for OFET materials [83–85]. The necessary condition for the construction of a long-life OFET device is the use of small particles or polymers characterized by high thermal, photo-air, and morphological stability [83]. To date, many types of OFET based on polymer semiconductors have been developed and described. Due to the type of main charge carriers, polymer semiconductors can be divided into p- and n-type semiconductors and bipolar semiconductors, capable of transporting holes, electrons, and both, respectively. When selecting the organic semiconductor used in OFET construction, the HOMO and the LUMO energy levels of the organic semiconductor molecule in relation to the work function of the contact electrode material are important. So far, the techniques of producing p-type materials have been the most popular; however, more and more attention has been paid to modern techniques of n-type materials synthesis [86–88]. H. Zhang et al. designed two conjugated donor–acceptor π polymers with 4,4′-bis(2- Molecules 2021, 26, 2012 ethylhexyl)-5,5′-bis(trimethyltin)dithieno[3,2-b:2′,3′-d]silole as donor and isodiketo-19 of 30 pyrrolo-pyrrole or isodithioketopyrrol-pyrrole as acceptor moiety (Scheme 26). Newly synthesized polymers were examined for their optical and electrochemical properties and their potential use in the construction of OFET. The work showed that isodithioketo- −1 −3 2 averagepyrrolopyrroles values of are hole promising and electron building mobility blocks of for 1 efficient× 10 andorganic 2 × field10 -effectcm /(Vs). transistors The above(hole up parameters to 0.49 cm conﬁrm2V–1s–1 and that electron the synthesized mobility compoundsup to 0.26 cm can2V be–1s1 used) [83]. in OFET [86].

MoleculesMolecules 20212021,, 2626,, xx FORFOR PEERPEER REVIEWREVIEWScheme 26. Chemical structure of DTS derivatives [83]. 2020 ofof 3131 Scheme 26. Chemical structure of DTS derivatives [83]. K. Kim et al. used the Stille coupling reaction to synthesize two copolymers consisting of diketopyrrolopyrrole (DPP) and silole derivatives (Scheme 27). The newly synthesized polymers were tested for their electrical properties in OFET and circuits. After spinning, both OFETs showed quite low values of hole mobility, while in the case of their annealing at a temperature of 150 °C, typical properties of bipolar transport were observed with average values of hole and electron mobility of 1 × 10–1 and 2 × 10–3 cm2/(Vs). The above parameters confirm that the synthesized compounds can be used in OFET [86].

SchemeSchemeScheme 27. 27.27. Chemical ChemicalChemical structure structurestructure of ofof diketopyrrolopyrrole diketopyrrolopyrrolediketopyrrolopyrrole and andand silole silolesilolederivatives derivativesderivatives [ 86[8[866].].].

0 0 Y.Y.Y. C. C.C. Pao PaoPao et etet al. al.al. designed designeddesigned and andand synthesized synthesizedsynthesized a tricyclic aa tricyclictricyclic diseleno[3,2-b:2 diseleno[3,2diseleno[3,2--b:2b:23 -d]silole′′33′′--d]siloled]silole (DSS) (DSS)(DSS) in 0 whichinin whichwhich the thethe 3,3 3,33,3position′′positionposition is bridged isis bridgedbridged by dioctylsilolebyby dioctylsiloledioctylsilole moieties moietiesmoieties (Scheme (Scheme(Scheme 28). 28).28). The TheThe DSS DSSDSS modiﬁed modi-modi- infiedfied this inin way thisthis waswayway copolymerized waswas copolymerizedcopolymerized with awithwith diketopyrrolopyrrole aa diketopyrrolopyrrodiketopyrrolopyrro acceptorlele acceptoracceptor (DPP), (DPP),(DPP), as a result asas aa re- ofre- whichsultsult ofof a whichwhich donor–acceptor aa donordonor––acceptoracceptor copolymer copolymercopolymer was obtained. waswas obtained.obtained. Selenophene SelenopheneSelenophene based PDSSDPP basedbased PDSSDPPPDSSDPP shows −2 2 –2−1 −2 1 –1 –1 theshowsshows mobility thethe mobilitymobility of 2.47 × ofof10 2.472.47 ×cm× 1010V–2 cmcms2VV–,1 suggestingss–1,, suggestingsuggesting that thatthat the DSSthethe DSSDSS unit unitunit and andand its polymers itsits polymerspolymers are promisingareare promisingpromising for use forfor inuseuse OFET inin OFETOFET [89]. [[8989].].

SchemeSchemeScheme 28. 28.28. Chemical ChemicalChemical structure structurestructure of ofof PDTSDPP PDTSDPPPDTSDPP and andand PDSSDPP PDSSDPPPDSSDPP [ [89[8989].].].

ExamplesExamplesExamples of ofof the thethe use useuse ofofof polymerspolymerspolymers containingcontaining siliconsilicon ininin theirtheir structurestructure forfor thethe construc-construction of OFET are presented in Table3. tiontion ofof OFETOFET areare presentedpresented inin TableTable 3.3.

TableTableTable 3. 3.3.Examples ExamplesExamples of ofof the thethe useuseuse ofof polymerspolymers containing containing silicon silicon inin theirtheir structurestructure structure forfor for thethe the constconst constructionructionruction ofofof OFET. OFET.OFET. HoleHole Mobility Mobility ElectronElectron Mobility Mobility CompoundsCompounds Hole Mobility Electron Mobility Ref.Ref. Compounds 2 2 –1−1–1 −1 2 2−1 –1−1–1 Ref. ((cmcm(cm2 VVV–1 ss–1s)) ) (cm((cmcmV2 VV–s1 ss–1))) DPPDPPDPP andand and silolesilole silole derivativesderivatives derivatives 0.490.49 0.49 0.260.260.26 [83[8[8] 33]] DPPDPP and and silole silole derivatives derivatives 0.10 0.10 0.200.20 [86[8] 6] DPP and silole derivatives 0.10 −2 0.20 [86] PDSSDPP 2.47 × 10 –2 [89] PDSSDPP 2.47 × 10–2 [89] PDSSDPP × −3 2.47 × 10 × −4 [89] Si1TDPP 3.7 10–3 5.1 10 –4 [90] Si1TDPPSi1TDPP 3.73.7 ×× 1010–3 5.15.1 ×× 1010–4 [9[900]]

TheThe aboveabove exampexamplesles confirmconfirm thatthat aa small,small, singlesingle differencedifference ofof atomsatoms oror functionalfunctional groupgroup in in the the structure structure of of polymerspolymers can can cause cause drastic drastic changes changes in in optical optical and and electronic electronic properties.properties. InIn thethe exampleexample reportedreported byby H.H. ZhangZhang etet al.al.,, thionationthionation ofof thethe productproduct notnot onlyonly significantsignificantlyly improved improved the the mobility mobility of of the the charge, charge, but but also also made made the the polymer polymer bipolar bipolar transporttransport propertiesproperties [8[833].]. Moreover,Moreover, quantumquantum chemicalchemical calculationscalculations andand electrochemicalelectrochemical analysisanalysis havehave confirmedconfirmed thatthat thionationthionation hashas anan additionaladditional effecteffect onon optimizingoptimizing molecularmolecular boundaboundaryry orbitalsorbitals andand facilitatingfacilitating chargecharge introduction.introduction.

4.4.55.. SiliconeSilicone--BasedBased MaterialsMaterials inin SensorsSensors andand BiosensorsBiosensors BiosensorsBiosensors are are devices devices that that help help to to assess assess the the levels levels of of biological biological markers markers or or any any chemicalchemical reactionreaction byby producingproducing thethe signalssignals thatthat areare mainlymainly relatedrelated toto thethe concentrationconcentration ofof

Molecules 2021, 26, 2012 20 of 30

The above examples confirm that a small, single difference of atoms or functional group in the structure of polymers can cause drastic changes in optical and electronic properties. In the example reported by H. Zhang et al., thionation of the product not only significantly improved the mobility of the charge, but also made the polymer bipolar transport properties [83]. Moreover, quantum chemical calculations and electrochemical analysis have confirmed that thionation has an additional effect on optimizing molecular boundary orbitals and facilitating charge introduction.

Molecules 2021, 26, x FOR PEER REVIEW 21 of 31 4.5. Silicone-Based Materials in Sensors and Biosensors Biosensors are devices that help to assess the levels of biological markers or any chemical reaction by producing the signals that are mainly related to the concentration of anan analyte analyte in in the the chemical chemical reaction. reaction. This This kind kind of of device device is is usually usually used used to to monitor monitor diseases diseases andand treatment treatment progress, progress, and and to to detect detect disease-causing disease-causing bacteria bacteria and and markers, markers, based based on on the the measurementmeasurement fromfrom bodybody ﬂuidsfluids (e.g.,(e.g., saliva,saliva, blood, and urine) urine) [9 [911].]. The The receptor receptor layer layer of ofeach each biosensor biosensor is isbuilt built from from a biorecognition a biorecognition material material (e.g. (e.g.,, enzyme enzyme or DNA), or DNA), which which rec- recognizesognizes the the desired desired analyte, analyte, whereas whereas the the transducer transducer converts converts the the (bio)chemical (bio)chemical signal signal re- resultingsulting from from the the interaction interaction of the analy analytete with thethe receptorreceptor intointo aadigital digital electronic electronic signal signal (Figure(Figure1)[ 1)92 [9].2].

FigureFigure 1. 1.A A schematic schematic description description of of the the operation operation of of a a biosensor. biosensor.

TheThe keykey stepstep inin the biosensors design design is is the the proper proper process process of ofimmobilization immobilization of ofthe thebiological biological material material on ona suitab a suitablele matrix, matrix, which which should should not notonly only effectively effectively bind bind the thebio- biomoleculemolecule on onits itssurface, surface, but butalso also prevent prevent the weakening the weakening of its ofcatalytic its catalytic activity activity and ensure and ensurestability stability during during measurements measurements as long as long as possible. as possible. The The manipulation manipulation of nanostructureof nanostruc- turematerials materials in conjunct in conjunctionion with with biological biological molecules molecules has hasled ledto the to thedevelopment development of a ofnew a newclass class of hybrid of hybrid modified modiﬁed sensors, sensors, in which in which both both enhancement enhancement of ofcharge charge transport transport and and bi- biologicalological activity activity preservation preservation may may be be preserved. preserved. Conductive Conductive polymer’s polymer’s charge charge transfer trans- fercapacity capacity acts acts as a asn anexcellent excellent matrix matrix for forbiomolecules, biomolecules, providing providing an enzyme an enzyme mimetic mimetic envi- environmentronment [93] [.93 Due].Due to the to presence the presence of aromatic of aromatic units unitsin the in polymer the polymer backbone, backbone, the immo- the immobilizationbilization is performed is performed with with the help the help of π of–ππ stacking–π stacking interactions interactions of the of the polymer polymer and and en- enzymaticzymatic protein. protein. These These strong strong interactions interactions effectively effectively stabilize stabilize the the tertiary tertiary structure structure of pro- of proteinsteins [94 [].94 ]. ConjugatedConjugated heterocyclesheterocycles areare oftenoften used used in in the the fabrication fabrication of of layered layered biosensors biosensors as as electronicelectronic mediators mediators to to improve improve the the contact contact between between the the active active center center of of the the enzyme enzyme and and thethe electrode electrode surface. surface. The The silane-based silane-based compounds compounds represent represent new new promising promising building building blocksblocks for for modifying modifying sensing sensing systems systems [ 95[9].5]. Dithienosilole Dithienosilole derivatives, derivatives, compared compared to to many many buildingbuilding units units of ofπ π-conjugated-conjugated polymers, polymers, such such as as furan, furan, pyrrole, pyrrole, or or pyridine, pyridine, show show the the lowestlowest level level of of LUMO LUMO energy energy and and a a relatively relatively high high level level of of HOMO HOMO orbitals, orbitals, which which means means thatthat compounds compounds based based on on dithienosilole dithienosilole formform stablestable ﬁlms films on on solid solid substrates substrates and and they they improveimprove the the charge charge transfer transfer (CT) (CT) [46 [4,966].,96 CT-type]. CT-type reactions, reactions, thanks thanks to electron to electron holes holes (h) and (h) − electronsand electrons (e ), are(e−), the are basic the basic element element of the of work the work of biosensor of biosensor devices, devices, using enzymesusing enzymes from thefrom oxidoreductase the oxidoreductase class, catalyzing class, catalyzing reactions reactions based on based the transport on the transport of the hopping of the hopping charge, and/orcharge, tunnelling and/or tunnelling of charge of carrierscharge carriers [97]. Depending [97]. Depending on the type on the of type redox of reaction redox reaction being catalyzed,being catalyzed charge, transportcharge transport can take can place take to (reduction)place to (reduction) or from (oxidation) or from (oxidation) the enzyme. the enzyme. According to the theory, electron holes and electrons are transported through the HOMO and LUMO orbitals. Tetraphenylsilane derivatives have a tetrahedral geometry—they are built from phenyl rings connected to a silicon atom in the sp3 configuration, which decides the possibility of their use in the construction of three-dimensional, porous organic materials [24,98]. Tetraphenylsilane as the core of the monomer unit with high triplet energy and wide band gap represents a group of charge-transferable compounds [99–101]. In this context, the silicon atom present in the structure of organosilicon compounds breaks the coupling between various building blocks so that the donor molecular orbitals do not overlap with the acceptor molecular orbitals (charge separation), while ensuring targeted charge transport between the donor and acceptor moieties [102,103].

Molecules 2021, 26, 2012 21 of 30

According to the theory, electron holes and electrons are transported through the HOMO and LUMO orbitals. Tetraphenylsilane derivatives have a tetrahedral geometry—they are built from phenyl rings connected to a silicon atom in the sp3 configuration, which decides the possibility of their use in the construction of three-dimensional, porous organic materials [24,98]. Tetraphenylsilane as the core of the monomer unit with high triplet energy and wide band gap represents a group of charge-transferable compounds [99–101]. In this context, the silicon atom present in the structure of organosilicon compounds breaks the coupling between various building blocks so that the donor molecular orbitals do not overlap with the acceptor molecular orbitals (charge separation), while ensuring targeted charge transport between the donor and acceptor moieties [102,103]. The C–Si bond (σ bond) in tetraphenylsilane is parallel to the π bonding orbitals; therefore, an interaction between σ–π bonds is possible. In addition, tetraphenylsilane has a high energy of the first excited state, up to 3.0 eV, resulting from an interruption of conjugation in the multiple bond system by the silicon atom. This leads to the inhibition of electron interactions between the unit made from tetraphenylsilane and neighboring groups [30]. The width of the energy gap (Eg = 4.2 eV) and the possibility of attaching a coupled silyl–aryl core and electron–donor and electron–acceptor systems to σ–π allow obtaining a material with ambipolar properties, which can be easily modified [104]. Until now, tertaphenylsilane derivatives have been mainly studied for their use in light-emitting diodes, but the above-mentioned optoelectronic and physicochemical properties make them an excellent candidate for use in medicine and biosensors. Y. Shang et al. reported that the star-shaped amphiphilic poly[2-(diethylamino)ethylmethacrylate]-b-poly[2-hydroxyethyl methacrylate]s copolymers, using tetraphenylsilane as a core, exhibit merits in comparison with the linear counterpart, including enhanced micelle stability, high drug-loading, and entrapment efficiency [105]. The π–π interaction between tetraphenylsilane cores results in excellent stability of micelles assembled from the star-shaped copolymers with critical micelle concentration (CMC) values in the range from 1.49 to 3.93 mg L−1. The in vitro experiments demonstrate that the drug-loaded micelles possess pH-response function and can effectively release a drug in a simulated cancer cell environment. The blank copolymers micelles are nearly non-cytotoxic, while the drug-loaded micelles have significant anticancer effects. Polymers based on tetraphenylsilane and dithiensiloles structures show the ability of oxidative polymerization, creating stable films on solid substrates [34,46]. Introduction of aryl groups, including thiophene, to organosilicon derivatives can improve the stability of the films based on these compounds, which is extremely important from the point of view of the construction of receptor matrices in biosensors. The studies performed earlier showed that applying tetraphenylsilane in an optical bio-sensing system, as well as dithienosilole derivatives in electrochemical biosensors, significantly improved working parameters of designed bio-devices. The microfluidic optical biosensor described by S. Baluta et al. [106] was based on the fluorescence sensing strategy for dopamine (DA) detection. The idea of measurement was based on forming the polydopamine (poly(DA)) film on the surface of graphene quantum dots (GQDs). Prepared GQDs were highly luminescent due to the aromatic planar structure. The ceramic-based miniature biosensor was designed and constructed through the immobilization of laccase on an electrochemically synthesized polymer-poly(bis(4-(thiophen-2-yl)phenyl)diphenylsilane), based on low temperature co- fired ceramics technology (LTCC), prepared as presented by K. Malecha [107]. This sensing system utilized the catalytic oxidation of DA to dopamine-o-quinone (DOQ), and then to poly(DA) (in alkaline conditions), which can selectively quench the strong luminescence of GQDs due to Förster Resonance Energy Transfer (FRET). The detection limit of the biosensor constructed this way was equal to 80 nM, and the system showed a broad linear range (1–200 µM) with insignificant influence of interference species on the detection (Figure2A,B). Molecules Molecules2021, 26,2021 x FOR, 26 ,PEER 2012 REVIEW 2223 of 30of 31

(A) (B)

Figure 2.Figure (A): A 2. scheme(A): A schemeof an external of an external platinum platinum electrode electrode made madewith withlow temperature low temperature co-fired co-ﬁred ceramincs ceramincs Pt- Pt-LTCCLTCC electrode coveredelectrode with covered poly- with(bis(4 poly-(bis(4-(thiophen-2-yl)phenyl)diphenylsilane)-(thiophen-2-yl)phenyl)diphenylsilane) and laccase; and laccase; (B): (microflowB): microﬂow module module housing housing for for dopamine dopaminedetection. detection.

OtherOther results results presented presented by by S. S.Baluta Baluta et et al. al. [1 [10808]] concern concern thethe electrochemicalelectrochemical enzymatic enzymatic platformsplatforms based based on dithienosilole on dithienosilole derivatives derivatives (Scheme (Scheme 29), 29 which), which allow allow monitoring monitoring of of ser- serotonin (5-HT) and DA. Electrochemical determination of serotonin and dopamine otonin (5-HT) and DA. Electrochemical determination of serotonin and dopamine was Molecules 2021, 26, x FOR PEER REVIEWwas achieved using cyclic voltammetry (CV) (to present a whole redox process, applied 24 of 31 achievedpotential using in rangecyclic −voltammetry0.2–0.8 V) and (CV) differential (to present pulse voltammetrya whole redox (DPV) process, (used forapplied linear po- tentialrange in range determination, −0.2–0.8 applied V) and potential differential in range pulse−0.2–0.8 voltammetry V) techniques. (DPV) (used for linear range determination, applied potential in range −0.2–0.8 V) techniques. A platinum electrode modified with poly(2,6-bis(3,4-ethylenedioxythiophen-5-yl)-4- methyl-4-octyl-dithienosilole) (Scheme 29A) film and laccase was used to selectively de- termine serotonin. The dithienosilole derivative had been polymerized onto platinum electrode electrochemically (via CV in the potential range 0–1.4 V). Laccase was immobilized onto the modified electrode with a thin film of semiconducting polymer. This investigation confirmed that serotonin undergoes a catalytic redox reaction by laccase. This was crucial, as the electrochemical oxidation of serotonin by laccase has never been described in detail in the scientific literature. The electrochemical nature of 5-HT was examined in a wide range of concentrations (0.1–200 µM), employing the CV method (applied potential −0.2–0.8 V, scan rate = 50 mV/s) in oxygen-saturated conditions. The bio-platform pre- paredScheme this way 29. 29. Chemicalpresented Chemical structures good structures working of (A ):of 2,6-bis(3,4-ethylenedioxythiophen-5-yl)-4-methyl-4-octyl- (A parameters,): 2,6-bis(3,4 such-ethylenedioxythiophen as working in a wide-5-yl) range-4-methyl of -4-octyl- concentrationdithienosiloles (0.1 and –and200 (B): ( 2,6-bis(selenophen-2-yl)-4-methyl-4-octyl-dithienosilole.µM)B): 2,6 and-bis(selenophen showing a very-2-yl) good-4-methyl detection-4-octyl limit-dithienosilole. of −48 nM. The sAecond platinum bio- electrodeplatform, modified a gold electrode with poly(2,6-bis(3,4-ethylenedioxythiophen-5-yl)- modified with a thin poly(2,6-bis(seleno- phen4-methyl-4-octyl-dithienosilole)-2-yl)The-4-methyl large- 4diversity-octyl-dithienosilole) of (Schemefunctional (Scheme29A) groups film 29B) and that film laccase can and wasbe horseradish attached used to selectively peroxidaseor obtained in self-as- (HRP)),determinesembled made for serotonin.monolayers dopamine The dithienosilole(SAMs)detection, represent also derivative presented a hadnew a been verymeans polymerized good of detectionsurface onto chemistry platinumlimit of −73 preparation. nM inelectrodeCurrently, a wide concentration electrochemically a high interest range (via in (0.1 CVSAMs– in200 the isµM). potentialobserved Both rangesystems due 0–1.4 to presentedtheir V). Laccaseability high wasto selectivity. alter immo- the surface prop- Investigationsbilizederties. ontoThey the are of modified obtained supramolecular electrode polymer with filmsaggregates a thinbased film on that of silicone semiconducting can -createbased structuresultrathin polymer. ontoorganic This sol- film on the ids, performedinvestigationsolids; the with most confirmed AFM investigated and that SEM serotonin combinationstechniques, undergoes showed ainclude catalytic no thiols visible redox on reaction surface gold by anddefects laccase. alkylsilanes and on sili- This was crucial, as the electrochemical oxidation of serotonin by laccase has never been significantly regular surface of both polymers. The diameter of the resulting grains of both describedcon [109– in11 detail1]. in the scientific literature. The electrochemical nature of 5-HT was polymersexamined wasX. Zhong in in a a range wide et al. rangeof described 1.5– of8 concentrationsµm. Thesea novel diameters glucose (0.1–200 allow µbiosensorM), employingenzyme designed molecules the CV with method to anchorself-assembling of freely(applieda intodouble the potential -polymericlayer 2D−0.2–0.8- networkfilms V, while scan of rate (3maintaining-merc = 50 mV/s)aptopropyl) their in oxygen-saturated catalytic-trimethoxysilane activity conditions. and permit(MPS), The the gold nanopar- rotation of the direct active site of the enzyme towards the substrate. Provided investiga- bio-platformticles, and glucose prepared oxidase this way presented(GOD), which good working was studied parameters, on a suchgold as substrate working [112]. The re- tions inshowed a wide rangethat silole of concentrations derivatives (0.1–200can be usedµM) andas a showing matrix afor very anchoring good detection the biological limit searchers prepared a bare gold electrode to obtain a self-assembled monolayer in the first elementsof − 48and nM. can be applied in biosensors. step and a 2D-network by immersing it in a special solution containing MPS dissolved in ethanol and by adding NaOH in the second step. As the final step, the gold nanoparticles were chemisorbed onto the thiol groups present in the second silane layer. In the end, the enzyme-GOD was physically immobilized on the surface of the gold nanoparticles. This prepared bio-platform was used for glucose detection with electrochemical impedance spectroscopy (EIS) and CV techniques. The results showed that the constructed biosensor system exhibits a good stability and sensitivity and can detect glucose in a wide concentration range of 0.0004–0.528 µM.

Silane-based compounds are not only used as matrices for the production of bio-platforms for biosensor applications. C. Hoffman and G. Tovar presented a different approach to the research of this group of compounds [113]. They focused on non-specific adsorption of proteins to the surface for better understanding of the exact quantity and activity of a protein—the control over the adsorption behavior of proteins in contact with solid surfaces. Researchers described monolayers from the newly synthesized compounds based on methoxy-tri(ethyleneglycol)-undecenyldimethylchlorosilane and dodecyldimethylchlorosilane (DDMS) (Scheme 30A,B), both mixed. Obtained SAMs were investigated for the possibility of enhanced protein repellent properties, e.g., Ras Binding Domain (RBD) was used, a protein with high relevance for cancer diagnostics. This protein was physically adsorbed onto the silicon oxide surfaces silanized with DDMS or non-silanized silicon wafers; however, no RBD protein was linked to surfaces silanized with created compounds (methoxy-tri(ethyleneglycol)-undecenyldimethylchlorosilane and dodecyldimethylchlorosilane (DDMS)).

Molecules 2021, 26, 2012 23 of 30

The second bio-platform, a gold electrode modified with a thin poly(2,6-bis(selenophen- 2-yl)-4-methyl-4-octyl-dithienosilole) (Scheme 29B) film and horseradish peroxidase (HRP)), made for dopamine detection, also presented a very good detection limit of −73 nM in a wide concentration range (0.1–200 µM). Both systems presented high selectivity. Investigations of obtained polymer films based on silicone-based structures onto solids, performed with AFM and SEM techniques, showed no visible surface defects and significantly regular surface of both polymers. The diameter of the resulting grains of both polymers was in a range of 1.5–8 µm. These diameters allow enzyme molecules to anchor freely into the polymeric films while maintaining their catalytic activity and permit the rotation of the direct active site of the enzyme towards the substrate. Provided investigations showed that silole derivatives can be used as a matrix for anchoring the biological elements and can be applied in biosensors. The large diversity of functional groups that can be attached or obtained in self- assembled monolayers (SAMs) represent a new means of surface chemistry preparation. Currently, a high interest in SAMs is observed due to their ability to alter the surface properties. They are supramolecular aggregates that can create ultrathin organic film on the solids; the most investigated combinations include thiols on gold and alkylsilanes on silicon [109–111]. X. Zhong et al. described a novel glucose biosensor designed with self-assembling of a double-layer 2D-network of (3-mercaptopropyl)-trimethoxysilane (MPS), gold nanoparticles, and glucose oxidase (GOD), which was studied on a gold substrate [112]. The researchers prepared a bare gold electrode to obtain a self-assembled monolayer in the first step and a 2D-network by immersing it in a special solution containing MPS dissolved in ethanol and by adding NaOH in the second step. As the final step, the gold nanoparticles were chemisorbed onto the thiol groups present in the second silane layer. In the end, the enzyme-GOD was physically immobilized on the surface of the gold nanoparticles. This prepared bio-platform was used for glucose detection with electrochemical impedance spectroscopy (EIS) and CV techniques. The results showed that the constructed biosensor system exhibits a good stability and sensitivity and can detect glucose in a wide concentration range of 0.0004–0.528 µM. Silane-based compounds are not only used as matrices for the production of bio- platforms for biosensor applications. C. Hoffman and G. Tovar presented a different approach to the research of this group of compounds [113]. They focused on non-specific adsorption of proteins to the surface for better understanding of the exact quantity and activity of a protein—the control over the adsorption behavior of proteins in contact with solid surfaces. Researchers described monolayers from the newly synthesized compounds based on methoxy-tri(ethyleneglycol)-undecenyldimethylchlorosilane and dodecyldimethylchlorosilane (DDMS) (Scheme 30A,B), both mixed. Obtained SAMs were investigated for the possibility of enhanced protein repellent properties, e.g., Ras Bind- ing Domain (RBD) was used, a protein with high relevance for cancer diagnostics. This protein was physically adsorbed onto the silicon oxide surfaces silanized with DDMS or non-silanized silicon wafers; however, no RBD protein was linked to surfaces silanized with created compounds (methoxy-tri(ethyleneglycol)-undecenyldimethylchlorosilane and dodecyldimethylchlorosilane (DDMS)). Generally, the organosilanes start to be widely used in biosensors as hetero-bifunctional cross-linkers. Mostly, these compounds are composed of two distinct reactive moieties: the silyl head group and the organic reactive group classically carried at the end of an aliphatic chain (tail group). The head group containing Si can react with the solid surface (-OH groups) and create a strong attachment of the organosilane molecule to the surface. The second group (tail chain), which is an organic functional group, e.g., carboxyl-, amino-, and epoxy-, allows the biologically active material’s immobilization, which allows the biomolecular recognition of the desired analyte [114]. MoleculesMolecules 20212021, 26, 26, x, 2012FOR PEER REVIEW 24 of 30 25 of 31

Scheme 30. 30.Chemical Chemical structure structure of (ofA): (A methoxy-tri(ethyleneglycol)-undecenyldimethylchlorosilane): methoxy-tri(ethyleneglycol)- andundecenyldimethylchlorosilane (B): dodecyldimethylchlorosilane. and (B): dodecyldimethylchlorosilane.

S.Generally, Corrie et al. the confirmed organosilanes the advantage start of to organosilane be widely bifunctionality used in biosensors by reported as hetero- spectroscopicbifunctional cross study-linkers involving. Mostly, the these chemical compounds and structural are composed modification of two ofdistinct thiol- reactive functionalized organosilica particles with aminosilane to produce a bifunctional silica moieties: the silyl head group and the organic reactive group classically carried at the end hybrid [115]. They presented a multiplexed model bioassay that shows bifunctionality, enablingof an aliphatic separate chain covalent (tail attachment group). The strategies head group for both containing homogeneous Si can incorporation react with of the solid fluorescentsurface (-OH dyes groups) and surface-specific and create a biomolecule strong attachment attachment. of the organosilane molecule to the surface.Another The s study,econd performedgroup (tail by chain), C. Corso which et is al., an was organic based functional on using group, two different e.g., carboxyl- silane, amino molecules,-, and epoxy (3-glycidyloxypropyl)trimethoxysilane-, allows the biologically active (GPS) material’s and (3-mercaptopropyl) immobilization, which trimethoxysilaneallows the biomol (MTS),ecular for recognition the immobilization of the desired of fluorescently analyte [11 labeled4]. IgG antibodies onto planarS. Corrie ZnO et surfacesal. confirmed [116]. According the advantage to the obtainedof organosilane results, thebifunctionality immobilization by of reported antibodiesspectroscopic onto a study film based involving on MTS gave the 21% chemical higher fluorescence and structural response modification in comparison of thiol- to GPS. Due to this, an MTS-based bio-platform was successfully used in the acoustic functionalized organosilica particles with aminosilane to produce a bifunctional silica biosensor tests. hybridThe [11 results5]. They presented presented by S. Baluta a multiplexed et al., described model in this bioassay section, that show shows that biosensors bifunctionality, constructedenabling separate with silicone-based covalent attachment materials represent strategies very for promising both homogeneous building blocks incorporation in the of casefluorescent of creating dyes a properand surface bio-platform-specific for biomolecule monitoring andattachment. detecting various biologically activeAnother species. Thisstudy, group performed of compounds by C. stronglyCorso et improve al., was working based on parameters using two of different biosen- silane sors,molecules such as, detection(3-glycidyloxypropyl)trimethoxysilane limit, selectivity, sensitivity, and lasting, in comparison (GPS) with and other (3- detectionmercaptopropyl)trimethoxysilane bio-systems, where a wide range (MTS) of, differentfor the immobilization conducting compounds, of fluorescently including labeled nanomaterials,IgG antibodies were onto used planar [117– ZnO120]. Obtaining surfaces [11 better6]. parameters According is to strongly the obtained linked with results, the the structure of silicone-based materials. In the case of electrochemical biosensors, the immobilization of antibodies onto a film based on MTS gave 21% higher fluorescence matrix based on conducting derivatives is expected to facilitate electron transfer, thereby enhancingresponse in the comparison sensor sensitivity. to GPS. Charge Due to carrier this, transferan MTS reactions-based bio are-platform vital in biosensors. was successfully Theused redox-active in the acoustic proteins biosensor are based tests. on the transfer of charge carriers by hopping and/or long-rangeThe results tunneling presented [97]. Due byto the S. potential Baluta et for al., oxidation described or reduction in this processes section, that show that arebiosensors suitable inconstructed a biosensor, with both silicone types of- CTbased can bematerials transferred represent to/from very the enzyme.promising Due building toblocks this, presentedin the case compounds of creating improve a proper an bio electron-platform transfer for in monitoring the case of electrochemicaland detecting various biosensors,biologically which active is the species. basis of This working group of such of bio-tools. compounds Silicone-based strongly improvederivatives working representparameters an excellentof biosensors, matrix forsuch application as detection in biosensors limit, selectivity, because they sensitivity, can act, due and to the lasting, in structure, as semiconductors that can transfer electrons from oxidized species, which are comparison with other detection bio-systems, where a wide range of different conducting detected by the long-range direct tunneling mechanism. compounds, including nanomaterials, were used [117–120]. Obtaining better parameters is strongly linked with the structure of silicone-based materials. In the case of electrochemical biosensors, the matrix based on conducting derivatives is expected to facilitate electron transfer, thereby enhancing the sensor sensitivity. Charge carrier transfer reactions are vital in biosensors. The redox-active proteins are based on the transfer of charge carriers by hopping and/or long-range tunneling [97]. Due to the potential for oxidation or reduction processes that are suitable in a biosensor, both types of CT can be transferred to/from the enzyme. Due to this, presented compounds improve

Molecules 2021, 26, 2012 25 of 30

5. Conclusions Organosilicon polymers are a group of high-molecular inorganic–organic compounds of high technological importance. This group of compounds with multifunctional properties has a wide range of potential applications. Organic materials deposited on a silane core are characterized by good mechanical and thermal properties. These compounds have a number of applications, including microelectronics, organic optoelectronics, biomedicine, surface coatings, and sensors. The article systematically reviews the synthesis methods and the important physical properties of silanes and siloles that have been developed over the last two decades. The most important synthetic methods in this field are based on C–C coupling and metalation reactions. Silicone-containing polymers create stable films on solid substrates and improve charge transfer. In addition, the sp3 silicon atom present in the structure of the silane compounds breaks the coupling between the different building blocks, while ensuring targeted charge transport between the donor and acceptor moieties. Product thioning (DTS derivatives [83]) not only significantly improved the mobility of the charge, but also made the polymer a bipolar transport property. Moreover, the introduction of the alkyl chain into the π bridge causes a significant increase in the dihedral angle between them and the donor unit, which may ultimately affect the value of the maximum energy conversion efficiency in the OPV. The parameters of individual polymers are varied, which proves that the selection of appropriate side chains is of great importance when designing high-performance OSCs. It should be emphasized that silicon-containing materials are building blocks that can be produced on an industrial scale. In addition, thanks to the use of a wide range of polymers containing silanes with different properties in the design, biosensors can be used in diagnostics, for the detection of single molecules as well as a large group of compounds. Such systems (e.g., dithienosilole and tetraphenylsilane) act as an ideal matrix for anchoring biological material, because they not only effectively bind to its structure, but also improve the performance parameters of bio-recognition elements. Due to good mechanical, thermal, and conductive properties, as well as application properties, systems based on silanes arouse more and more interest of scientific teams.

Author Contributions: Conceptualization: J.S. and J.C.; methodology: D.Z., K.S., and S.B.; software: D.Z., K.S., and S.B.; validation: D.Z., K.S., and S.B.; formal analysis: J.S. and J.C.; investigation: J.S.; resources: J.S.; data curation: J.S.; writing—original draft preparation: J.S., D.Z., K.S., and S.B.; writing—review and editing: J.S. and J.C.; visualization: J.S.; supervision: J.S. and J.C.; project administration: J.S.; funding acquisition: J.C. and D.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Wroclaw University of Science and Technology (No. K20/ 8211104160). Institutional Review Board Statement: Not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. Arukalam, I.O.; Oguzie, E.E.; Li, Y. Nanostructured superhydrophobic polysiloxane coating for high barrier and anticorrosion applications in marine environment. J. Colloid Interface Sci. 2018, 512, 674–685. [CrossRef][PubMed] 2. Tang, X.; Yao, L.; Liu, H.; Shen, F.; Zhang, S.; Zhang, H.; Lu, P.; Ma, Y. An Efficient AIE-Active Blue-Emitting Molecule by Incorporating Multifunctional Groups into Tetraphenylsilane. Chem. Eur. J. 2014, 20, 7589–7592. [CrossRef][PubMed] 3. Shang, Y.; Zheng, N.; Wang, Z. Tetraphenylsilane-Cored Star-Shaped Polymer Micelles with pH/Redox Dual Response and Active Targeting Function for Drug-Controlled Release. Biomacromolecules 2019, 20, 4602–4610. [CrossRef] 4. Ma, J.; Xia, W.; Zhang, R.; Ding, L.; Kong, Y.; Zhang, H.; Fu, K. Flocculation of emulsified oily wastewater by using functional grafting modified chitosan: The effect of cationic and hydrophobic structure. J. Hazard. Mater. 2021, 403, 123690. [CrossRef] 5. Koda, S. Kinetic aspects of oxidation and combustion of silane and related compounds. Prog. Energy Combust. Sci. 1992, 18, 513–528. [CrossRef] 6. Walsh, R. Bond dissociation energy values in silicon-containing compounds and some of their implications. Acc. Chem. Res. 1981, 14, 246–252. [CrossRef] 7. Walsh, R. Thermochemistry of silicon-containing compounds. Part 1—Silicon–halogen compounds, an evaluation. J. Chem. Soc. Faraday Trans. 1 1983, 79, 2233–2248. [CrossRef] Molecules 2021, 26, 2012 26 of 30

8. Semenov, V.V. Alkanes and silanes: Similarities and differences. Her. Russ. Acad. Sci. 2016, 86, 466–472. [CrossRef] 9. Mark, J.E.; Allcock, H.R.; West, R. Inorganic Polymers, 2nd ed.; Oxford University Press: New York, NY, USA, 2004. 10. Mark, J.E. Some Interesting Things about Polysiloxanes. Acc. Chem. Res. 2004, 37, 946–953. [CrossRef] 11. Lee, S.H.; Jang, B.B.; Kafafi, Z.H. Highly Fluorescent Solid-State Asymmetric Spirosilabifluorene Derivatives. J. Am. Chem. Soc. 2005, 127, 9071–9078. [CrossRef] 12. Ahmad, N.A.; Leo, C.P.; Ahmad, A.L. Amine wetting evaluation on hydrophobic silane modified polyvinylidene fluo- ride/silicoaluminophosphate zeolite membrane for membrane gas absorption. J. Nat. Gas. Sci. Eng. 2018, 58, 115–125. [CrossRef] 13. Zar˛eba,J.K. Tetraphenylmethane and tetraphenylsilane as building units of coordination polymers and supramolecular networks— A focus on tetraphosphonates. Inorg. Chem. Commun. 2017, 86, 172–186. [CrossRef] 14. Guan, W.W.; Shi, X.Y.; Xu, T.T.; Wan, K.; Zhang, B.W.; Liu, W.; Su, H.L.; Zou, Z.Q.; Du, Y.W. Synthesis of well-insulated Fe–Si–Al soft magnetic composites via a silane-assisted organic/inorganic composite coating route. J. Phys. Chem. Solids 2021, 150, 109841. [CrossRef] 15. Bomina, S.; Honglae, S. 1,3,5-Trinitrotoluene Sensor Based on Silole Nanoaggregates. J. Nanosci. Nanotechnol. 2019, 19, 991–995. [CrossRef] 16. Thames, S.F.; Panjnani, K.G. Organosilane polymer chemistry: A review. J. Inorg. Organomet. Polym. 1996, 6, 59–94. [CrossRef] 17. Corey, J.Y. Siloles: Part 2: Silaindenes (Benzosiloles) and Silafluorenes (Dibenzosiloles): Synthesis, Characterization, and Applications. In Advances in Organometallic Chemistry; Hill, A.F., Fink, M.J., Eds.; Academic Press: Cambridge, MA, USA, 2011; Volume 59, pp. 181–328. [CrossRef] 18. Chru´sciel,J.J.; Le´sniak,E. Modification of epoxy resins with functional silanes, polysiloxanes, silsesquioxanes, silica and silicates. Prog. Polym. Sci. 2015, 41, 67–121. [CrossRef] 19. Sasi Kumar, R.; Padmanathan, N.; Alagar, M. Design of hydrophobic polydimethylsiloxane and polybenzoxazine hybrids for interlayer low k dielectrics. New J. Chem. 2015, 39, 3995–4008. [CrossRef] 20. Liu, H.; Fu, Z.; Song, F.; Qingquan, L.; Chen, L. The controllable construction and properties characterization of organic–inorganic hybrid materials based on benzoxazine-bridged polysilsesquioxanes. RSC Adv. 2017, 7, 3136–3144. [CrossRef] 21. Hsieh, C.Y.; Su, W.C.; Wu, C.S.; Lin, L.K.; Hsu, K.Y.; Liu, Y.L. Benzoxazine-containing branched polysiloxanes: Highly efficient reactive-type flame retardants and property enhancement agents for polymers. Polymer 2013, 54, 2945–2951. [CrossRef] 22. Feng, D.; Wang, K.; Wei, Z.; Chen, Y.-P.; Simon, C.M.; Arvapally, R.K.; Martin, R.L.; Bosch, M.; Liu, T.-F.; Fordham, S.; et al. Kinetically tuned dimensional augmentation as a versatile synthetic route towards robust metal-organic frameworks. Nat. Commun. 2014, 5, 5723. [CrossRef] 23. Duncan, N.C.; Hay, B.P.; Hagaman, E.W.; Custelcean, R. Thermodynamic, Kinetic, and Structural Factors in the Synthesis of Imine-Linked Dynamic Covalent Frameworks. Tetrahedron 2012, 68, 53–64. [CrossRef] 24. Rose, M.; Böhlmann, W.; Sabo, M.; Kaskel, S. Element–organic frameworks with high permanent porosity. Chem. Commun. 2008, 2462–2464. [CrossRef] 25. Sun, D.; Zhou, X.; Li, H.; Sun, X.; Zheng, Y.; Ren, Z.; Ma, D.; Bryce, M.R.; Yan, S. A versatile hybrid polyphenylsilane host for highly efficient solution-processed blue and deep blue electrophosphorescence. J. Mater. Chem. C 2014, 2, 8277–8284. [CrossRef] 26. Li, X.; Bai, Q.; Li, J.; Lu, F.; Sun, X.; Lu, P. Synthesis and properties of wide bandgap polymers based on tetraphenylsilane and their applications as hosts in electrophosphorescent devices. New J. Chem. 2018, 42, 3344–3349. [CrossRef] 27. Moncada, J.; Terraza, C.A.; Tagle, L.H.; Coll, D.; Ortiz, P.; Pérez, G.; de la Campa, J.G.; Álvarez, C.; Tundidor-Camba, A. Synthesis, characterization and studies of properties of six polyimides derived from two new aromatic diamines containing a central silicon atom. Eur. Polym. J. 2017, 91, 354–367. [CrossRef] 28. Tundidor-Camba, A.; González-Henríquez, C.M.; Sarabia-Vallejos, M.A.; Tagle, L.H.; Hauyón, R.A.; Sobarzo, P.A.; González, A.; Ortiz, P.A.; Maya, E.M.; Terraza, C.A. Silylated oligomeric poly(ether-azomethine)s from monomers containing biphenyl moieties: Synthesis and characterization. RSC Adv. 2018, 8, 1296–1312. [CrossRef] 29. Cai, M.; Xiao, T.; Hellerich, E.; Chen, Y.; Shinar, R.; Shinar, J. High-Efficiency Solution-Processed Small Molecule Electrophospho- rescent Organic Light-Emitting Diodes. Adv. Mater. 2011, 23, 3590–3596. [CrossRef] 30. Oner, S.; Oner, I.; Akda, H.; Varlikli, G.C. Tetraphenylsilane group containing carbazoles as high triplet energy host materials for solution-processable PhOLEDs. Turk. J. Chem. 2015, 39, 917–929. [CrossRef] 31. Arsenyan, P.; Pudova, O.; Popelis, J.; Lukevics, E. Novel radial oligothienyl silanes. Tetrahedron Lett. 2004, 45, 3109–3111. [CrossRef] 32. Wang, Y.; Tan, D. Synthesis, thermal degradation and dielectric properties of poly[octyl(triphenylethynyl)]silane resin. Chem. Res. Chin. Univ. 2019, 35, 1076–1081. [CrossRef] 33. Kai, Z.; Li, H.; Shi, H.; Wu, J.; Xu, J.; Li, Y.; Zhao, C. Synthesis and Thermal Properties of Silicon-Containing Benzoxazine. High Perform. Polym. 2020, 32, 59–64. [CrossRef] 34. Zaj ˛ac,D.; Sołoducho, J.; Jarosz, T.; Łapkowski, M.; Roszak, S. Conjugated silane-based arylenes as luminescent materials. Electrochim. Acta 2015, 173, 105–116. [CrossRef] 35. Zaj ˛ac,D.; Sołoducho, J.; Jarosz, T.; Łapkowski, M.; Roszak, S. Push-pull structures of symmetric silane derivatives as a novel hosting materials. Indian J. Appl. Res. 2017, 7, 58–66. Molecules 2021, 26, 2012 27 of 30

36. Yang, R.; Li, D.; Bai, Y.; Zhang, L.; Liu, Z.; Hao, J.; Wei, Q.; Liao, L.; Peng, R.; Ge, Z. Novel tetraarylsilane-based hosts for blue phosphorescent organic light-emitting diodes. Org. Electron. 2018, 55, 117–125. [CrossRef] 37. Bezvikonnyi, O.; Grybauskaite-Kaminskiene, G.; Volyniuk, D.; Simokaitiene, J.; Danyliv, Y.; Durgaryan, R.; Bucinskas, A.; Jatautiene,˙ E.; Hladka, I.; Scholz, J.; et al. 3,30-Bicarbazole-based compounds as bipolar hosts for green and red phosphorescent organic light-emitting devices. Mater. Sci. Eng. B 2020, 261, 114662. [CrossRef] 38. Junjie, H.; Hu, Y.; Deng, S.F.; Zhou, J.L.; Jiang, N.; Zhu, Y.; Sun, M. Synthesis and Properties of a Novel Silicon-Containing Phthalonitrile Resin and Its Quartz-Fiber-Reinforced Composites. High Perform. Polym. 2020, 32, 1112–1121. [CrossRef] 39. Tagle, L.H.; Terraza, C.; Tundidor-Camba, A.; Coll, D. Silicon-containing poly(esters) with halogenated bulky side groups. Synthesis, characterization and thermal studies. RSC Adv. 2015, 5, 49132–49142. [CrossRef] 40. Ma, M.; Gong, C.; Li, C.; Yuan, Q.; Huang, F. The synthesis and properties of silicon-containing arylacetylene resins with rigid-rod 2,5-diphenyl-[1,3,4]-oxadiazole moieties. Eur. Polym. J. 2021, 143, 110192. [CrossRef] 41. Milenin, S.A.; Ardabevskaia, S.N.; Novikov, R.A.; Solyev, P.N.; Tkachev, Y.V.; Volodin, A.D.; Korlyukov, A.A.; Muzafarov, A.M. Construction of siloxane structures with P-Tolyl substituents at the silicon atom. J. Organomet. Chem. 2020, 926, 121497. [CrossRef] 42. Makhmudiyarova, N.N.; Ishmukhametova, I.R.; Ibragimov, A.G. Lanthanide-Catalyzed Synthesis of Cyclic Silicon-Containing Di- and Triperoxides. Russ. J. Org. Chem. 2020, 56, 1685–1690. [CrossRef] 43. Kim, D.H.; Ohshita, J.; Lee, K.H.; Kunugi, Y.; Kunai, A. Synthesis of π-Conjugated Oligomers Containing Dithienole Units. Organometallics 2006, 25, 1511–1516. [CrossRef] 44. Grisorio, R.; Suranna, G.P.; Mastrorilli, P.; Allegretta, G.; Loiudice, A.; Rizzo, A.; Gigli, G.; Manoli, K.; Magliulo, M.; Torsi, L. All-donor poly(arylene-ethynylene)s containing anthracene and silole-based units: Synthesis, electronic, and photovoltaic properties. J. Polym. Sci. Part A Polym. Chem. 2013, 51, 4860–4872. [CrossRef] 45. Chen, J.; Xie, Z.; Lam, J.W.Y.; Law, C.C.W.; Tang, B.Z. Silole-Containing Polyacetylenes. Synthesis, Thermal Stability, Light Emission, Nanodimensional Aggregation, and Restricted Intramolecular Rotation. Macromolecules 2003, 36, 1108–1117. [CrossRef] 46. Zaj ˛ac,D.; Honisz, D.; Łapkowski, M.; Sołoducho, J. Synthesis and Properties of New Dithienosilole Derivatives as Luminescent Materials. Molecules 2019, 24, 2259. [CrossRef] 47. Feng, Y.; Dai, C.; Lei, J.; Ju, H.; Cheng, Y. Silole-Containing Polymer Nanodot: An Aqueous Low-Potential Electrochemilumines- cence Emitter for Biosensing. Anal. Chem. 2016, 88, 845–850. [CrossRef] 48. Horst, S.; Evans, N.R.; Bronstein, H.A.; Williams, C.K. Synthesis of fluoro-substituted silole-containing conjugated materials. J. Polym. Sci. A Polym. Chem. 2009, 47, 5116–5125. [CrossRef] 49. Pöcheim, A.; Özpınar, G.A.; Müller, T.; Baumgartner, J.; Marschner, C. The Combination of Cross-Hyperconjugation and σ-Conjugation in 2,5-Oligosilanyl Substituted Siloles. Chem. Eur. J. 2020, 26, 17252–17260. [CrossRef][PubMed] 50. Cai, Y.; Samedov, K.; Dolinar, B.S.; Song, Z.; Tang, B.Z.; Zhang, C.; West, R. Synthesis and High Solid-State Fluorescence of Cyclic Silole Derivatives. Organometallics 2015, 34, 78–85. [CrossRef] 51. Chuantao, G.; Chunying, Z.; Bing, L.; Tingyu, F.; Jiping, M.; Haofen, S. Synthesis of a dithieno[3,2-b:20,30-d]silole-based conjugated polymer and characterization of its short wave near-infrared fluorescence properties. J. Innov. Opt. Health Sci. 2020, 13, 2041002. [CrossRef] 52. Yang, L.; Koo, D.; Wu, J.; Wong, J.M.; Day, T.; Zhang, R.; Kolongoda, H.; Liu, K.; Wang, J.; Ding, Z.; et al. Benzosiloles with Crystallization-Induced Emission Enhancement of Electrochemiluminescence: Synthesis, Electrochemistry, and Crystallography. Chem. Eur. J. 2020, 26, 11715–11721. [CrossRef] 53. Ruiqi, C.; Liqing, A.; Hongxia, Y.; Shuhong, L.; Caihong, X. Aggregation-tuned dual emission of silole derivatives: Synthesis, crystal structure, and photophysical properties. New J. Chem. 2020, 44, 5049–5055. [CrossRef] 54. Scott, C.N.; Bisen, M.D. Synthesis of reactively functionalized 2,5-siloles using kumada-type nickel-mediated intramolecular cyclization and their utilization in polymer synthesis. Polymer 2019, 170, 204–210. [CrossRef] 55. Fan, S.; Zhu, C.; Wu, D.; Wang, X.; Yu, J.; Li, F. Silicon-containing inherent flame-retardant polyamide 6 with anti-dripping via introducing ethylene glycol as the chain-linker and charring agent. Polym. Degrad. Stab. 2020, 173, 109080. [CrossRef] 56. Yuan, D.; Yin, H.; Cai, X. Synergistic effects between silicon-containing flame retardant and potassium-4-(phenylsulfonyl) benzenesulfonate (KSS) on flame retardancy and thermal degradation of PC. J. Therm. Anal. Calorim. 2012, 114, 19–25. [CrossRef] 57. Liu, L.; Zhang, L.; Li, M.; Guo, Y.; Song, J.; Wang, H. Random dithienosilole-based terpolymers: Synthesis and application in polymer solar cells. Dyes Pigment. 2016, 130, 63–69. [CrossRef] 58. Chena, X.; Sunb, Y.; Wanga, Z.; Gaob, H.; Lina, Z.; Keb, X.; Hea, T.; Yina, S.; Chenb, Y.; Zhanga, Q.; et al. Dithienosilole-based small molecule donors for efficient all-small-moleculeorganic solar cells. Dyes Pigment. 2018, 158, 445–450. [CrossRef] 59. Zhou, H.; Zhang, Y.; Mai, C.K.; Collins, S.D.; Bazan, G.C.; Nguyen, T.Q.; Heeger, A.J. Polymer homo-tandem solar cells with best efficiency of 11.3%. Adv. Mater. 2015, 27, 1767–1773. [CrossRef] 60. Zhang, J.; Zhang, Y.; Fang, J.; Lu, K.; Wang, Z.; Ma, W.; Wei, Z. Conjugated polymer-small molecule alloy leads to high efficient ternary organic solar cells. J. Am. Chem. Soc. 2015, 137, 8176–8183. [CrossRef] 61. You, J.; Dou, L.; Yoshimura, K.; Kato, T.; Ohya, K.; Moriarty, T.; Emery, K.; Chen, C.-C.; Gao, J.; Li, G.; et al. A polymer tandem solar cell with 10.6% power conversion efficiency. Nat. Commun. 2013, 4, 1446. [CrossRef] 62. Li, Y. Molecular design of photovoltaic materials for polymer solar cells: Electronic energy levels and broad absorption. Acc. Chem. Res. 2012, 45, 723–733. [CrossRef] Molecules 2021, 26, 2012 28 of 30

63. Xia, B.; Lu, K.; Yuan, L.; Zhang, J.; Zhu, L.; Zhu, X.; Deng, D.; Li, H.; Wei, Z. A conformational locking strategy in linked-acceptor type polymers for organic solar cells. Polym. Chem. 2016, 7, 1323. [CrossRef] 64. Zhao, Y.; Zhang, L.; Liu, S.; Yang, C.; Yi, J.; Yang, C. Thieno [3,2-b]thiophene-Bridged Conjugated Polymers Based on Dithieno [3,2-b:20,30-d] silole and Thieno m[3¨C-c]pyrrole-4,6-dione for Polymer Solar Cells: Influence of Side Chains on Optoelectronic Properties. Macromol. Chem. Phys. 2018, 219, 1800297. [CrossRef] 65. Song, J.S.; Du, C.; Li, C.H.; Bo, Z.S. Silole-containing polymers for high-efficiency polymer solar cells. J. Polym. Sci. Pol. Chem. 2011, 49, 4267–4274. [CrossRef] 66. Beaujuge, P.M.; Pisula, W.; Tsao, H.N.; Ellinger, S.; Mullen, K.; Reynolds, J.R. Tailoring structure-property relationships in dithienosilole-benzothiadiazole donoracceptor copolymers. J. Am. Chem. Soc. 2009, 131, 7514. [CrossRef] 67. Li, M.; Liu, L.; Zhao, C.; Zhou, Y.; Guo, Y.; Song, J.; Wang, H. Side chain engineering of dithienosilole-based polymers for application in polymer solar cells. Dyes Pigment. 2016, 134, 480–486. [CrossRef] 68. Chen, X.; Feng, H.; Lin, Z.; Jiang, Z.; He, T.; Yin, S.; Wan, X.; Chen, Y.; Zhang, Q.; Qiu, H. Impact of end-capped groups on the properties of dithienosilole-based small molecules for solution-processed organic solar cells. Dyes Pigment. 2017, 147, 183–189. [CrossRef] 69. Ni, W.; Li, M.; Liu, F.; Wan, X.; Feng, H.; Kan, B.; Zhang, Q.; Zhang, H.; Chen, Y. Dithienosilole-Based Small-Molecule Organic Solar Cells with an Efficiency over 8%: Investigation of the Relationship between the Molecular Structure and Photovoltaic Performance. Chem. Mater. 2015, 27, 6077–6084. [CrossRef] 70. Heo, H.; Kim, H.; Nam, G.; Lee, D.; Lee, Y. Multi-Donor Random Terpolymers Based on Benzodithiophene and Dithienosilole Segments with Different Monomer Compositions for High-Performance Polymer Solar Cells. Macromol. Res. 2018, 26, 238–245. [CrossRef] 71. Nie, H.; Chen, B.; Zeng, J.; Xiong, Y.; Zhao, Z.; Tang, B.Z. Excellent n-type light emitters based on AIE-active silole derivatives for efficient simplified organic light-emitting diodes. J. Mater. Chem. C 2018, 6, 3690–3698. [CrossRef] 72. Jou, J.H.; Kumar, S.; Agrawal, A.; Li, T.H.; Sahoo, S. Approaches for fabricating high efficiency organic light emitting diodes. J. Mater. Chem. C 2015, 3, 2974–3002. [CrossRef] 73. Furue, R.; Nishimoto, T.; Park, I.S.; Lee, J.; Yasuda, T. Angew, Aggregation-Induced Delayed Fluorescence Based on Donor/Acceptor-Tethered Janus Carborane Triads: Unique Photophysical Properties of Nondoped OLEDs. Chem. Int. Ed. 2016, 55, 7171. [CrossRef] 74. Cai, Y.; Qin, A.; Tang, B.Z. Siloles in optoelectronic devices. J. Mater. Chem. C 2017, 5, 7375–7389. [CrossRef] 75. Cai, Y.; Samedov, K.; Dolinar, B.S.; Albright, H.; Song, Z.; Zhang, C.; Tang, B.Z.; West, R. AEE-active cyclic tetraphenylsilole derivatives with ∼100% solid-state fluorescence quantum efficiency. Dalton Trans. 2015, 44, 12970. [CrossRef][PubMed] 76. Dou, D.; You, J.; Hong, Z.; Xu, Z.; Li, G.; Street, R.A.; Yang, Y. 25th anniversary article: A decade of organic/polymeric photovoltaic research. Adv. Mater. 2013, 25, 6642–6671. [CrossRef][PubMed] 77. Zhao, W.; Li, S.; Yao, H.; Zhang, S.; Zhan, Y.; Yang, B.; Hou, J. Molecular Optimization Enables over 13% Efficiency in Organic Solar Cells. Am. Chem. Soc. 2017, 139, 7148–7151. [CrossRef][PubMed] 78. Park, H.; Rao, Y.; Varlan, M.; Kim, J.; Ko, S.B.; Wang, S.; Kang, Y. Synthesis and characterization of fluorene and carbazole dithienosilole derivatives for potential applications in organic light-emitting diodes. Tetrahedron 2012, 68, 9278–9283. [CrossRef] 79. Xiong, Y.; Zeng, J.; Chen, B.; Lam, J.W.Y.; Zhao, Z.; Chen, S.; Tang, B.Z. New carbazole-substituted siloles for the fabrication of efficient non-doped OLEDs. Chin. Chem. Lett. 2019, 30, 592. [CrossRef] 80. Chen, B.; Jiang, Y.; Chen, L.; Nie, H.; He, B.; Lu, P.; Sung, H.H.Y.; Williams, I.D.; Kwok, H.S.; Qin, A.; et al. 2,5-Difluorenyl- Substituted Siloles for the Fabrication of High-Performance Yellow Organic Light-Emitting Diodes. Chem. Eur. J. 2014, 20, 1931–1939. [CrossRef] 81. Ju, M.; Wang, J.; Sun, J.Z.; Zhao, H.; Yuan, W.; Deng, C.; Chen, S.; Sung, H.H.Y.; Lu, P.; Qin, A.; et al. Siloles symmetrically substituted on their 2,5-positions with electron-accepting and donating moieties: Facile synthesis, aggregation-enhanced emission, solvatochromism, and device application. Chem. Sci. 2012, 3, 549–558. [CrossRef] 82. Chen, B.; Jiang, Y.; He, B.; Zhou, J.; Sung, H.H.Y.; Williams, I.D.; Lu, P.; Kwok, H.S.; Qiu, H.; Zhao, Z.; et al. Synthesis, Structure, Photoluminescence, and Electroluminescence of Siloles that Contain Planar Fluorescent Chromophores. Chem. Asian J. 2014, 9, 2937–2945. [CrossRef] 83. Zhang, H.; Yang, K.; Zhang, K.; Zhang, Z.; Sun, Q.; Yang, W. Thionating Iso-diketopyrrolopyrrole-based Polymers: From P-type to Ambipolar Field Effect Transistors with Enhanced Charge Mobility. Polym. Chem. 2018, 9, 1807–1814. [CrossRef] 84. Zhang, H.; Deng, R.; Wang, J.; Li, X.; Chen, Y.M.; Liu, K.; Taubert, C.J.; Cheng, S.Z.; Zhu, Y. Crystalline Organic Pigment-Based Field-Effect Transistors. ACS Appl. Mater. Interfaces 2017, 9, 21891–21899. [CrossRef] 85. Wang, Y.; Nakano, M.; Michinobu, T.; Kiyota, Y.; Mori, T.; Takimiya, K. Naphthodithiophenediimide–Benzobisthiadiazole-Based Polymers: Versatile n-Type Materials for Field-Effect Transistors and Thermoelectric Devices. Macromolecules 2017, 50, 857–864. [CrossRef] 86. Kim, K.; Jeon, J.; Ha, Y.H.; Cha, H.; Park, C.E.; Kang, M.; Jhon, H.; Kwon, S.K.; Kim, Y.H.; An, T.K. Ambipolar charge transport of diketopyrrolepyrrole-silole-based copolymers and effect of side chain engineering: Compact model parameter extraction strategy for high-voltage logic applications. Org. Electron. 2018, 54, 1–8. [CrossRef] 87. Lee, J.; Han, A.R.; Yu, H.; Shin, T.J.; Yang, C.; Oh, B.H. Boosting the ambipolar performance of solution-processable polymer semiconductors via hybrid side-chain engineering. J. Am. Chem. Soc. 2013, 135, 9540–9547. [CrossRef] Molecules 2021, 26, 2012 29 of 30

88. Hu, J.Y.; Nakano, M.; Osaka, I.; Takimiya, K. Naphthodithiophenediimide (NDTI)-based triads for high-performance air-stable, solution-processed ambipolar organic field-effect transistors. J. Mater. Chem. C 2015, 3, 4244–4249. [CrossRef] 89. Pao, Y.C.; Yang, C.T.; Lai, Y.Y.; Huang, W.C.; Hsu, C.S.; Cheng, Y.J. Synthesis and Field-Effect Transistor Properties of a Diseleno[3,2- b:2030-d]silole-Based Donor-Acceptor Copolymer: Investigation of Chalcogen Effect. Polym. Chem. 2016, 7, 4654–4660. [CrossRef] 90. Kang, W.; Jung, M.; Cha, W.; Jang, S.; Yoon, Y.; Kim, H.; Son, H.J.; Lee, D.-K.; Kim, B.; Cho, J.H. High Crystalline Dithienosilole- Cored Small Molecule Semiconductor for Ambipolar Transistor and Nonvolatile Memory. ACS Appl. Mater. Interfaces 2014, 6, 6589–6597. [CrossRef] 91. Barnett, N.W.; Hindson, B.J.; Lewis, S.W. Determination of 5-hydroxytryptamine (serotonin) and related indoles by flow injection analysis with acidic potassium permanganate chemiluminescence detection. Anal. Chim. Acta 1998, 362, 131–139. [CrossRef] 92. Ngoepe, M.; Choonara, Y.E.; Tygai, C.; Tomar, L.K.; du Toit, L.C.; Kumar, P.; Ndesendo, V.M.K.; Pillay, V. Integration of Biosensors and Drug Delivery Technologies for Early Detection and Chronic Management of Illness. Sensors 2013, 13, 7680–7713. [CrossRef] [PubMed] 93. Gokoglan, T.C.; Soylemez, S.; Kesik, M.; Toksabay, S.; Toppare, L. Selenium containing conducting polymer based pyranose oxidase biosensor for glucose detection. Food Chem. 2015, 172, 219. [CrossRef] 94. De Taxi du Poet, P.; Miyamoto, S.; Murakami, T.; Kiura, J.; Karube, I. Direct electron transfer with glucose oxidase immobilized in an electropolymerized poly(N-methylpyrrole) film on a gold microelectrode. Anal. Chim. Acta 1990, 235, 255–264. [CrossRef] 95. Han, X.; Wang, Z.; Cheng, Q. Mitochondria-dependent benzothiadiazole-based molecule probe for quantitatively intracellular pH imaging. Dyes Pigment. 2017, 145, 576–583. [CrossRef] 96. Liang, L.; Liming, D. Photovoltaic-Active Dithienosilole-Containing Polymers. Macromolecules 2007, 40, 9406–9412. [CrossRef] 97. Cordes, M.; Giese, B. Electron transfer in peptides and proteins. Chem. Soc. Rev. 2009, 38, 892–901. [CrossRef][PubMed] 98. Davies, R.P.; Less, R.; Lickiss, P.D.; Robertson, K.; White, A.J.P. Structural Diversity in Metal−Organic Frameworks Built from Rigid Tetrahedral [Si(p-C6H4CO2)4]4− Struts. Cryst. Growth Des. 2010, 10, 4571–4581. [CrossRef] 99. Ren, X.; Li, J.; Holmes, R.J.; Djurovich, P.I.; Forrest, S.R.; Thompson, M.E. Ultrahigh energy gap hosts in deep blue organic electrophosphorescent devices. Chem. Mater. 2016, 16, 4743–4747. [CrossRef] 100. Gong, S.; Chen, Y.; Yang, C.; Zhong, C.; Qin, J.; Ma, D. De Novo Design of Silicon-Bridged Molecule Towards a Bipolar Host: All-Phosphor White Organic Light-Emitting Devices Exhibiting High Efficiency and Low Efficiency Roll-Off. Adv. Mater. 2010, 22, 5370–5373. [CrossRef] 101. Kwak, G.; Masuda, T. Poly(silyleneethynylenephenylene) and Poly(silylenephenyleneethynylenephenylene)s: Synthesis and Photophysical Properties Related to Charge Transfer. Macromolecules 2002, 10, 4138–4142. [CrossRef] 102. Lee, C.W.; Lee, J.Y. Comparison of Tetraphenylmethane and Tetraphenylsilane as Core Structures of High-Triplet-Energy Hole- and Electron-Transport Materials. Chem. Eur. J. 2012, 18, 6457–6461. [CrossRef] 103. Tang, X.; Yao, L.; Liu, H.; Shen, F.; Zhang, S.; Zhang, Y.; Zhang, H.; Lu, P.; Ma, Y. Novel violet emitting material synthesized by stepwise chemical reactions. J. Mater. Chem. 2014, 2, 5019–5027. [CrossRef] 104. Hu, D.; Shen, F.; Liu, H.; Lu, P.; Liu, D.; Ma, Y. Separation of electrical and optical energy gaps for constructing bipolar organic wide bandgap materials. Chem. Commun. 2012, 48, 3015–3017. [CrossRef] 105. Shang, Y.; Guo, L.; Wang, Z. Tetraphenylsilane-Cored Star-Shaped Amphiphilic Block Copolymers for pH-Responsive Anticancer Drug Delivery. Macromol. Chem. Phys. 2019, 220, 1900248. [CrossRef] 106. Baluta, S.; Malecha, K.; Zaj ˛ac,D.; Sołoducho, J.; Cabaj, J. Dopamine sensing with fluorescence strategy based on low temperature co-fired ceramic technology modified with conducting polymers. Sens. Actuators B 2017, 252, 803–812. [CrossRef] 107. Malecha, K. The Implementation of Fluorescence-Based Detection in LTCC (Low-Temperature-Co-Fired-Ceramics) Microfluidic Modules. Int. J. Appl. Ceram. Technol. 2015, 13, 69–77. [CrossRef] 108. Baluta, S.; Zaj ˛ac,D.; Szyszka, A.; Malecha, K.; Cabaj, J. Enzymatic platforms for sensitive neurotransmitter detection. Sensors 2020, 20, 423. [CrossRef] 109. Ulman, A. Formation and Structure of Self-Assembled Monolayers. Chem. Rev. 1996, 96, 1533–1554. [CrossRef] 110. Xia, Y.; Whitesides, G.M. Soft Lithography. Angew. Chem. Int. Ed. 1998, 37, 550–575. [CrossRef] 111. Schreiber, F. Structure and growth of self-assembling monolayers. Prog. Surf. Sci. 2000, 65, 151–257. [CrossRef] 112. Zhong, X.; Yuan, R.; Chai, Y.; Liu, Y.; Dai, J.; Tang, D. Glucose biosensor based on self-assembled gold nanoparticles and double-layer 2d-network (3-mercaptopropyl)-trimethoxysilane polymer onto gold substrate. Sens. Actuators B 2005, 104, 191–198. [CrossRef] 113. Hoffmann, C.; Tovar, G.E.M. Mixed self-assembled monolayers (SAMs) consisting of methoxy-tri(ethylene glycol)-terminated and alkyl-terminated dimethylchlorosilanes control the non-specific adsorption of proteins at oxidic surfaces. J. Colloid Interface Sci. 2006, 295, 427–435. [CrossRef] 114. Dugas, V.; Demesmay, C.; Chevolot, Y.; Souteyrand, E. Use of Organosilanes in Biosensors; Nova Science Publishing Inc.: New York, NY, USA, 2010. 115. Corrie, S.R.; Vogel, R.; Keen, I.; Jack, K.; Kozak, D.; Lawrie, G.A.; Battersby, B.; Fredericks Trau, M. A structural study of hybrid organosilica materials for colloid-based DNA biosensors. J. Mater. Chem. 2008, 18, 523–529. [CrossRef] 116. Corso, C.D.; Dickherber, A.; Hunt, W.D. An investigation of antibody immobilization methods employing organosilanes on planar ZnO surfaces for biosensor applications. Biosens. Bioelectron. 2008, 24, 805–811. [CrossRef] Molecules 2021, 26, 2012 30 of 30

117. Rand, E.; Periyakaruppan, A.; Tanaka, Z.; Zhang, D.A.; Marsh, M.P.; Andrews, R.J.; Lee, K.H.; Chen, B.; Meyyappan, M.; Koehne, J.E. A carbon nanofiber based biosensor for simultaneous detection of dopamin˛eand serotonin in the presence of ascorbic acid. Biosens. Bioelectron. 2013, 42, 434–438. [CrossRef] 118. Ran, G.; Chen, X.; Xia, Y. Electrochemical detection of serotonin based on a poly (bromocresol green) film and Fe3O4 nanoparticles in a chitosan matrix. RSC Adv. 2017, 7, 1847–1851. [CrossRef] 119. Dinesh, B.; Veeramani, V.; Chen, S.M.; Saraswathi, R. In situ electrochemical synthesis of reduced Graphene oxide-cobalt oxide nanocomposite modified electrode for selective sensing of depression biomarker in the presence of ascorbic acid and dopamine. J. Electroanal. Chem. 2017, 786, 169–176. [CrossRef] 120. Young, L.; Lin, D.; Huang, J.; You, T. Simultaneous determination of dopamine, ascorbic acid and uric acid at electrochemically reduced graphene oxide modified electrode. Sens. Actuators B 2014, 193, 166–172. [CrossRef]