Polycarbazole and Its Derivatives: Synthesis and Applications

polymers

Review Polycarbazole and Its Derivatives: Synthesis and Applications. A Review of the Last 10 Years

Fadila Bekkar 1, Faiza Bettahar 1, Isabel Moreno 2, Rachid Meghabar 1, Mohammed Hamadouche 3 , Estibaliz Hernáez 4, José Luis Vilas-Vilela 4,5 and Leire Ruiz-Rubio 4,5,*

1 Laboratoire de Chimie des Polymères, Université Oran1 Ahmed Ben Bella, El-Mnaouer, BP 1524, Oran 31000, Algerie; [email protected] (F.B.); [email protected] (F.B.); [email protected] (R.M.) 2 Macromolecular Chemistry Group (LQM), Organic Chemistry II Department, Faculty of Science and Technology, University of the Basque Country (UPV/EHU), 48940 Leioa, Spain; [email protected] 3 Laboratoire de Chimie Fine, Département de Chimie, Faculté des Sciences Exactes et Appliquées, Université Oran1 Ahmed Ben Bella, El-Mnaouer, BP 1524, Oran 31000, Algerie; [email protected] 4 Macromolecular Chemistry Group (LQM), Physical Chemistry Department, Faculty of Science and Technology, University of the Basque Country (UPV/EHU), 48940 Leioa, Spain; [email protected] (E.H.); [email protected] (J.L.V.-V.) 5 BCMaterials, Basque Center for Materials, Applications and Nanostructures, UPV/EHU Science Park, 48940 Leioa, Spain * Correspondence: [email protected]; Tel.: +34-94-6017-972

Received: 20 August 2020; Accepted: 26 September 2020; Published: 28 September 2020

Abstract: Polycarbazole and its derivatives have been extensively used for the last three decades, although the interest in these materials brieﬂy decreased. However, the increasing demand for conductive polymers for several applications such as light emitting diodes (OLEDs), capacitators or memory devices, among others, has renewed the interest in carbazole-based materials. In this review, the synthetic routes used for the development of carbazole-based polymers have been summarized, reviewing the main synthetic methodologies, namely chemical and electrochemical polymerization. In addition, the applications reported in the last decade for carbazole derivatives are analysed. The emergence of ﬂexible and wearable electronic devices as a part of the internet of the things could be an important driving force to renew the interest on carbazole-based materials, being conductive polymers capable to respond adequately to requirement of these devices.

Keywords: carbazole derivatives; conducting polymers; electropolymerization; chemical polymerization

1. Introduction Electrically conductive polymers are a highly demanded class of materials due to their extended uses in electronic and optical devices or sensors, among others. These materials present main advantageous properties of conventional polymers, such as solubility, mechanical flexibility, non-expensive fabrication or processing, with conductivity levels that could be compared to those of semiconductors or even metals. In 1977, Macdiarmid, Shirakawa and Heeger demonstrated that chemical doping of polyacetylene (PA) resulted in a highly conducting material, with its conductivity being eleven orders of magnitude higher than a pristine polymer. For this they were awarded with the Nobel Prize in 2000 for their research on conductive polymers [1,2]. These materials have induced the development of many applications, such as organic light emitting diodes (OLEDs) [3–6], organic field effect transistors (OFETs) [7–11], dye-sensitized solar cells [12–16], photochromic dyes [17–21], batteries [22–26], and electrocatalyst [27,28] or (bio)sensors [26,29–32]

Polymers 2020, 12, 2227; doi:10.3390/polym12102227 www.mdpi.com/journal/polymers Polymers 2020, 12, x FOR PEER REVIEW 2 of 33 Polymers 2020, 12, x FOR PEER REVIEW 2 of 33 Polymersfield effect2020 ,transistors12, 2227 (OFETs) [7–11], dye-sensitized solar cells [12–16], photochromic dyes [17–21],2 of 33 field effect transistors (OFETs) [7–11], dye-sensitized solar cells [12–16], photochromic dyes [17–21], batteries [22–26], and electrocatalyst [27,28] or (bio)sensors [26,29–32] batteries [22–26], and electrocatalyst [27,28] or (bio)sensors [26,29–32] The term conductive polymer encompasses both organic molecules containing of alternating The term conductive polymer encompasses both organic moleculesmolecules containing of alternatingalternating simple (type σ) and double (type π) carbon bonds and/or conjugated aromatic nuclei on their simple (type(typeσ σ) and) and double double (type (typeπ) carbonπ) carbon bonds bonds and/or and/or conjugated conjugated aromatic aromatic nuclei onnuclei their skeleton.on their skeleton. This particularity, π-conjugated structure, allows the transfer of charges (electrons or holes) Thisskeleton. particularity, This particularity,π-conjugated π-conjugated structure, structure, allows the allows transfer the transfer of charges of charges (electrons (electrons or holes) or holes) along along the macromolecular skeleton. The most commonly used conjugated polymers are thealong macromolecular the macromolecular skeleton. sk Theeleton. most commonlyThe most usedcommonly conjugated used polymers conjugated are poly(thiophene)polymers are poly(thiophene) (PT) [33,34], poly(pyrrole) (PPy) [35], poly(p-phenylene) (PPP) [36], poly (p- (PT)poly(thiophene) [33,34], poly(pyrrole) (PT) [33,34], (PPy) poly(pyrrole) [35], poly(p-phenylene) (PPy) [35], poly(p-phenylene) (PPP) [36], poly (p-phenylenevinylene) (PPP) [36], poly (p- phenylenevinylene) (PPV) [37], and polyfluorene (PF) [38–41], with their structures summarized in (PPV)phenylenevinylene) [37], and polyﬂuorene (PPV) [37], (PF) and [38 –polyfluorene41], with their (P structuresF) [38–41], summarized with their structures in Figure1 .summarized In addition toin Figure 1. In addition to the processability inherent of many polymers, it is important to notice their theFigure processability 1. In addition inherent to the ofprocessability many polymers, inherent it is importantof many polymers, to notice theirit is important ability to modulate to notice their ability to modulate their conductivity by doping, which could vary 10from the1 isolation state (<10−10 conductivityability to modulate by doping, their which conductivity could vary by from doping the isolation, which could state( 105 4 S.cm−11) stateS.cm− (~101), to− theS.cm semiconductor− ), and even state to the (~10 conductive−5 S.cm−1), material and even (> 10to theS.cm conductive− ) near copper material (5 (>1010 4S.cm S.cm−−1).) near copper (5 × 105 S.cm−1). Among electronic conductive polymers, polymers containing× a carbazole Amongnear copper electronic (5 × 10 conductive5 S.cm−1). Among polymers, electronic polymers conductive containing polymers, a carbazole polymers ring (PCz) containing also present a carbazole good ring (PCz) also present good electrical and optical properties. electricalring (PCz) and also optical present properties. good electrical and optical properties.

Figure 1. The structure of some of the main conjugated polymers. Figure 1. The structure of some of the main conjugated polymers.

Carbazole (C12HH99N),N), also also named named dibenzopyrrole or diphenylenimine,diphenylenimine, is an N-containigN-containig Carbazole (C12H9N), also named dibenzopyrrole or diphenylenimine, is an N-containig heterocyclic compound compound discovered discovered by by Graebe Graebe and and Gl Glaseraser in in1872 1872 [42]. [42 Its]. structure Its structure consists consists of two of heterocyclic compound discovered by Graebe and Glaser in 1872 [42]. Its structure consists of two twobenzene benzene ring ringfused fused on either on either side side of ofa central a central pyrro pyrrolele ring ring (Figure (Figure 2)2)[ [43].43]. Carbazole Carbazole represents represents an benzene ring fused on either side of a central pyrrole ring (Figure 2) [43]. Carbazole represents an important class of heterocycles withwith severalseveral advantages.advantages. For example, a large variety of substituents important class of heterocycles with several advantages. For example, a large variety of substituents can be easily introduced in the nitrogen atom andand the aromaticaromatic frameworkframework can be substituted in can be easily introduced in the nitrogen atom and the aromatic framework can be substituted in positions 3 andand 6,6, modifyingmodifying thethe physicochemicalphysicochemical properties.properties. Carbazol-basedCarbazol-based polymers (PCz) have positions 3 and 6, modifying the physicochemical properties. Carbazol-based polymers (PCz) have attracted increased attention over the last 50 years owing to their stability and higher redox potential attracted increased attention over the last 50 years owing to their stability and higher redox potential compared to other conducting polymers [[44].44]. Simila Similarly,rly, they present good electro-electro- andand photoactivephotoactive compared to other conducting polymers [44]. Similarly, they present good electro- and photoactive properties because of their high hole transporting mobility and strong absorption in the UV spectral properties because of their high hole transporting mobility and strong absorption in the UV spectral region [45[45].]. These These characteristics characteristics have have extended extended the use the of thisuse kindof this of polymer kind of in polymer several applications, in several region [45]. These characteristics have extended the use of this kind of polymer in several suchapplications, as transistors such as [ 46transistors,47], smart [46,47], windows smart [ 48wind], lightows [48], emitting light diodeemitting [49 diode–52], (bio)sensor[49–52], (bio)sensor [53–55], applications, such as transistors [46,47], smart windows [48], light emitting diode [49–52], (bio)sensor and[53–55], photovoltaic and photovoltaic devices [devices56,57]. [56,57]. [53–55], and photovoltaic devices [56,57]. H 8 NH 1 8 N9 1 7 9 2 7 2 6 5 4 3 6 4 3 5 Figure 2. Structure of carbazole. Figure 2. Structure of carbazole. The ﬁeld of the conductive polymers has exponentially increased since 1985. The growth of the last The field of the conductive polymers has exponentially increased since 1985. The growth of the decadeThe has field been of favouredthe conductive by the increasepolymers in has devices exponentially related to theincreased internet since of things 1985. (IoT). The Thesegrowth types of the of last decade has been favoured by the increase in devices related to the internet of things (IoT). These deviceslast decade require has newbeen materials favoured capable by the increase of satisfying in devic theires high related requirements, to the internet with of conductive things (IoT). polymers These types of devices require new materials capable of satisfying their high requirements, with conductive beingtypes oneof devices of the mostrequire widely new usedmaterials materials capable for theseof satisfying devices their [57–69 high]. Among requirements, the conductive with conductive polymers, polymers being one of the most widely used materials for these devices [57–69]. Among the polycarbazolepolymers being derivatives one of the have most also widely received used increasing materials attention. for these As devices can be observed[57–69]. Among in Figure the3, conductive polymers, polycarbazole derivatives have also received increasing attention. As can be thereconductive is a relation polymers, between polycarbazole the increase derivatives on the number have of also publications received increasing related to conductiveattention. As polymers can be observed in Figure 3, there is a relation between the increase on the number of publications related (green)observed and in the Figure increase 3, there of publications is a relation based between on polycarbazole the increase derivatives on the number (red). of However, publications this increase related to conductive polymers (green) and the increase of publications based on polycarbazole derivatives hasto conductive been signiﬁcantly polymers lower (green) for theand case the ofincrease poly(N-vinylcarbazole) of publications based (black). on polycarbazole derivatives (red). However, this increase has been significantly lower for the case of poly(N-vinylcarbazole) (red). However, this increase has been significantly lower for the case of poly(N-vinylcarbazole) (black). (black).

Polymers 2020, 12, x FOR PEER REVIEW 3 of 33 Polymers 2020, 12, 2227 3 of 33

200 2000 Poly(carbazole) 180 Poly(N-vinylcarbazole) 1800

160 Conductive polymers 1600

140 1400

120 1200

100 1000

80 800

60 600

40 400

20 200

Nº of publications on polycarbazole polycarbazole on publications of Nº

0 0 polymers conductive on publication of Nº 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020 Years Figure 3. The evolution on the number of publications of conductive polymers (green), polycarbazole Figurederivatives 3. The evolution (black) and on poly(N-vinylcarbazole) the number of publications (red) of (Source: conductive Scopus). polymers (green), polycarbazole derivatives (black) and poly(N-vinylcarbazole) (red) (Source: Scopus). In recent years, it is important to highlight the work of Mario Leclerc et al. with several reviews andIn book recent chapters years, focusedit is important on the to synthesis highlight of the poly(3,6-carbazole) work of Mario Leclerc and poly(2,7-carbazole) et al. with several derivatives reviews andfor book plastic chapters electronics focused and on solar the cellssynthesis [70–74 of]. poly(3,6 Considering-carbazole) the Scopus and poly(2,7 database,-carbazole) in a total derivatives of 17 reviews for(eight plastic in electronics the last 10 and years) solar and cells rive [70 chapters–74]. Considering (four in the the last Scopus 10 years) data polycarbazolebase, in a total derivativesof 17 review ares (eightmentioned. in the last Tables 10 years)1 and 2and summarize rive chapters this information.(four in the last However, 10 years) in polycarbazole many of them, derivatives polycarbazole are mentionedderivatives. Table are parts 1 and of the 2 publication,summarize butthis suchinformation publications. However, are not in completely many of devotedthem, polycarbazole to them, or the derivativespublication are is part focused of the only publication on one type, but of such derivative publications and/or are application. not completely devoted to them, or the publication is focused only on one type of derivative and/or application. Table 1. Summary of the reviews on poly(carbazole)derivatives extracted from Scopus.

AuthorsTable 1. Summary of the reviews Title on poly(carbazole)derivatives Year extracted Journal from Scopus. Cited Ref. Authors Optically activeTitle vinyl polymers Year Journal Cited Ref. containingOptically active ﬂuorescent vinyl polymers groups: containing 5. fluorescent groups: 5. Fluorescence properties of Houben J.L. et al. Fluorescence properties of 1978 Polymer 35 [75] Houben J.L. et al. poly(9-vinyl carbazole) and optically active 1978 Polymer 35 [75] poly(9-vinylpolymers containing carbazole) carbazole and units Polymeroptically membranes active polymersin clinical sensor containing applications. Murphy S.M. et al. II. The designcarbazole and fabrication units of permselective 1992 Biomaterials 27 [76] hydrogels for electrochemical devices PolymerCarbazole-containing membranes polymers: in clinical synthesis, Progress in Polymer J.V. Grazulevicius et al. 2003 660 [77] sensor applications.properties and applications II. The design Science Murphy S.M. et al. and fabrication of permselective 1992 BiomaterialsMacromolecular 27 [76] Morin J.-F. et al. Polycarbazoles:hydrogels 25 years for of progress 2005 Rapid 552 [74] electrochemical devices Communications Highly efficient green-emitting phosphorescent Advanced Functional Ding J. et al. Carbazole-containing polymers: 2006 290 [78] J.V. iridium dendrimers based on carbazole dendrons ProgressMaterials in Polymer synthesis, properties 2003 660 [77] Grazulevicius et al. Developments in the field of conducting and non- Science Faridbod F. et al. conductingand polymer applications based potentiometric 2008 Sensors 120 [79] membrane sensors for ions over the past decade MacromolecularAdvances in Polymer Rapid BoudreaultMorin P. J.-F.-L.T. et et al.al. Polycarbazoles:Poly(2,7-carbazole)s 25 and years related of progress polymers 20052008 61552 [71] [74 ] CommunicationsScience A high-mobility low-bandgap poly(2,7-carbazole) Zou Y., Gendron D. et al. 2009 Macromolecules 220 [80] derivative for photovoltaic applications Conducting polymer coated carbon surfaces and Progress in Organic Ates M. et al. 2009 106 [81] biosensor applications Coatings Solar-energy production and energy-efficient lighting: Photovoltaic devices and white-light- Beaupré S. et al. 2010 Advanced Materials 187 [82] emitting diodes using poly(2,7-fluorene), poly(2,7- carbazole), and poly(2,7-dibenzosilole) derivatives Boudreault P.-L.T. et al. Polycarbazoles for plastic electronics 2010 Polymer Chemistry 149 [70]

Polymers 2020, 12, 2227 4 of 33

Table 1. Cont.

Authors Title Year Journal Cited Ref. Highly efficient green-emitting Advanced Functional Ding J. et al. phosphorescent iridium dendrimers 2006 290 [78] Materials based on carbazole dendrons Developments in the field of conducting and non-conducting Faridbod F. et al. polymer based potentiometric 2008 Sensors 120 [79] membrane sensors for ions over the past decade Boudreault Poly(2,7-carbazole)s and Advances in Polymer 2008 61 [71] P.-L.T. et al. related polymers Science A high-mobility low-bandgap Zou Y., Gendron poly(2,7-carbazole) derivative for 2009 Macromolecules 220 [80] D. et al. photovoltaic applications Conducting polymer coated carbon Progress in Organic Ates M. et al. 2009 106 [81] surfaces and biosensor applications Coatings Solar-energy production and energy-efficient lighting: Photovoltaic devices and Beaupré S. et al. white-light-emitting diodes using 2010 Advanced Materials 187 [82] poly(2,7-fluorene), poly(2,7-carbazole), and poly(2,7-dibenzosilole) derivatives Boudreault Polycarbazoles for 2010 Polymer Chemistry 149 [70] P.-L.T. et al. plastic electronics Journal of Polymer Dubey N., Leclerc Conducting polymers: Efficient 2011 Science, Part B: 252 [83] M. thermoelectric materials Polymer Physics Gendron D., New conjugated polymers for Energy and 2011 240 [84] Leclerc M. plastic solar cells Environmental Science International Journal A review on medical applications of of Polymeric Materials Grigoras A.G. poly(N-vinylcarbazole) and 2016 2 [85] and Polymeric its derivatives Biomaterials The recent development of Tan S.E., Sarjadi carbazole-, benzothiadiazole-, and Polymer 2017 7 [86] M.S. isoindigo-based copolymers for Science—Series B solar cells application: A review Stereoregular polymers with pendant carbazolyl groups: Liguori R. et al. 2018 Synthetic Metals 2 [87] Synthesis, properties and optoelectronic applications Current trends in the development Ghorbani Zamani TrAC—Trends in of conducting polymers-based 2019 18 [88] F. et al. Analytical Chemistry biosensors Polymers 2020, 12, 2227 5 of 33

Table 2. Summary of the book chapters on poly(carbazole)derivatives extracted from Scopus.

Title Author Year Book Ref. Perepichka, D.F., Organic Light-Emitting Light-emitting polymers Perepichka, I.F., 2006 [89] Materials and Devices Meng, H., Wudl, F. Synthesis of Poly(2,7-carbazole)s Boudreault, P.-L.T., Design and Synthesis of 2010 [71] and Derivatives Morin, J.-F., Leclerc, M. Conjugated Polymers Conducting polymer-based Yemata, T.A., Ye, Q., Hybrid Polymer thermoelectric composites: Zhou, H., (...), 2017 Composite Materials: [90] Principles, processing, Chin, W.S., Xu, J. Applications and applications Xun, S., Perepichka, D.F., Organic Light-Emitting Light-emitting polymers Perepichka, I.F., 2017 Materials and Devices, [91] Meng, H., Wudl, F. Second Edition Miscellaneous Vinyl Brydson’s Plastics Gilbert, M. 2017 [92] Thermoplastics Materials: Eighth Edition

Considering the previous mentioned lacks, in this review, a global overview of the main polycarbazole derivatives, focused in the last decade, and their applications are revised. The main synthetic routes used for the development of carbazole-based polymers have been summarized in two parts: chemical and electrochemical polymerization of carbazole and its derivatives. In addition, the applications reported in the last years for carbazole-based polymers are reviewed (2010–2020).

2. Synthesis of Polymers The synthesis and characterization of conducting polymers have become one of the most important areas of the research in polymer and materials science. In general, conductive polymers can be synthesized by different methods, with chemical polymerization (classical organic synthesis) and electrochemical polymerization (electrochemical synthesis) being the most commonly used processes. The following sections describe these two different synthesis techniques of polycarbazole and its derivatives. Carbazole presents several active positions (3,6-, 2,7-, and 1,8-positions), being the 3,6 postions easier to polymerize. Poly(N-vinylcarbazole) and its derivatives has been highly studied since decades, but in the last decade 3,6-carbazole derivatives have been intensively investigated. Nevertheless, these derivatives present several limitations in their application due to their low molecular weight and poor conjugation of the electrons in their structure. On the other hand, the development of 2,7-carbazoles present better properties and applicability than the 3,6 due to their extended conjugation and lower band gap [70]. Finally, poly(1,8-carbazole) derivatives are the less developed derivatives, and poly(1,8-cabazole) is less planar compared to 3,6 and 2,7 [93,94]. However, this property makes these derivatives more suitable for the electrets of photoresponsive organic field-effect transistor memory applications [46].

2.1. Chemical Polymerization of Carbazoles and Its Derivatives Chemical polymerization of carbazole has been carried out in the presence of oxidizing agents such as ammonium persulphate ((NH4)2S2O8) (APS), ferric chloride (FeCl3), and potassium dichromate (K2Cr2O7). The structure and properties of the obtained polymer are strongly dependent on the concentration, the catalyst (oxidizing agent), and the solvent [74]. The chemical synthesis takes place by oxidation-reduction reactions that are accompanied by a change in the number of electrons in the π system. The first studies on the oxidation of carbazole were published by Branch and Tucker [95–97]. Although the most commonly used dopants are FeCl3 or I2, these oxidants can promote polymer aggregation and, as consequence, important problems on the device production. To Polymersovercome2020, 12this, 2227 drawback Aoai et al. [98] described a photodoping method based on the use6 ofof 33 triaylsulfonium or diaryliodonium salts as PAG (photo acid generator). The mechanism proposed for the synthesis of PCz (or poly(3,6-cabazole)) is presented in Figure drawback Aoai et al. [98] described a photodoping method based on the use of triaylsulfonium or 4. As depicted in Figure 4, first, carbazole monomer is oxidized by a single electron transfer forming diaryliodonium salts as PAG (photo acid generator). the cation radical. Then more stable dicarbazyl dimer is produced as a result of the coupling of two The mechanism proposed for the synthesis of PCz (or poly(3,6-cabazole)) is presented in Figure4. cation radicals and the loss of two protons. Regarding the regiochemistry, the polymerization process As depicted in Figure4, ﬁrst, carbazole monomer is oxidized by a single electron transfer forming the takes place at 3 or 6 positions. The reactivity of 1 and 8 positions is probably prevented by the rigid cationstructure radical. of carbazole Then more heterocycle. stable dicarbazyl dimer is produced as a result of the coupling of two cation radicalsIn andorder the to loss guarantee of twoprotons. de oxidation Regarding of the the carb regiochemistry,azole core, and the so, polymerization polymerization process process, takes a placeminimum at 3 or 6electronic positions. density The reactivity has to ofbe1 ensured and 8 positions in the isstarting probably monomer. prevented Consequently, by the rigid structurestrong ofelectron-withdrawing carbazole heterocycle. substituents in the aromatic framework do not benefit the reaction.

FigureFigure 4.4. ProposedProposed mechanistic mechanistic scheme scheme for the for synthesis the synthesis of carbazol-based of carbazol-based polymers (PCz). polymers Reprinted (PCz). Reprintedwith permission with permission from Sangwan from Sangwanet al. [99]. etCopyright al. [99]. Copyright(2016) Wiley. (2016) Wiley.

InShakir order and to guaranteeco-workers de [100], oxidation reported of one the carbazoleof the first core, attempt and to so, obtain polymerization a new conductive process, ananocomposite minimum electronic of polycarbazole density has (PCz) to be with ensured titanium in thedioxide starting (TiO monomer.2) nanoparticles. Consequently, This composite strong electron-withdrawingwas successfully synthesized substituents by in-situ in the chemical aromatic polymerization framework do in not the benefit presence the of reaction. different amounts of nanosizedShakir and TiO co-workers2 using ammonium [100], reported persulfate one (APS) of theas oxidizing first attempt agent, to in obtain1:1 molar a new ratio conductive(Cz:APS), nanocompositein dichloromethane of polycarbazole at room temperature (PCz) with for a titanium period of dioxide 24 h (Figure (TiO2 5).) nanoparticles. It is important This to notice composite that, wasin a successfully first step, the synthesized different solutions by in-situ of chemical TiO2 nanoparticles polymerization were in added the presence dropwise of ditoff theerent monomer amounts ofsolution nanosized under TiO 2constantusing ammonium stirring, this persulfate step allows (APS) the as carbazole oxidizing absorption agent, in 1:1 on molar the surface ratio (Cz:APS), of the innanoparticles dichloromethane before at roomits polymerization. temperature for Creamy a period coloured of 24 h (Figure solutions5). It iswere important obtained to noticewhich that, later in a first step, the different solutions of TiO2 nanoparticles were added dropwise to the monomer solution under constant stirring, this step allows the carbazole absorption on the surface of the nanoparticles before its polymerization. Creamy coloured solutions were obtained which later transformed into greenish black sediments. The characterization results revealed that the polymerization of PCz had been achieved on the surface of the TiO2 nanoparticles indicating strong interaction between PCz and TiO2 nanoparticles. The same approach was also carried out by Baig et al. [101] for the synthesis of zirconium (IV) phosphate/polycarbazole nanocomposites. Polymers 2020, 12, x FOR PEER REVIEW 6 of 33 transformed into greenish black sediments. The characterization results revealed that the polymerization of PCz had been achieved on the surface of the TiO2 nanoparticles indicating strong Polymersinteraction2020 ,between12, 2227 PCz and TiO2 nanoparticles. The same approach was also carried out by Baig7 of et 33 al. [101] for the synthesis of zirconium (IV) phosphate/polycarbazole nanocomposites.

Figure 5.5.Schematic Schematic Diagram Diagram of the of Formation the Formation Mechanism Mechanism of (a) PCz of and ( (ab) ) PCzPCz/ TiOand2 Nanocomposite. (b) PCz/TiO2 Nanocomposite.Reprinted with permission Reprinted fromwith Shakir permission et al. [100from]. CopyrightShakir et (2014)al. [100]. American Copyright Chemical (2014) Society. American Chemical Society. It is important to notice that this nanocomposite presents antimicrobial properties. The antibacterial activityIt is was importantin vitro evaluated to notice against that Staphylococcusthis nanocomposite aureus, Staphylococcuspresents antimi epidermidis,crobial properties. Proteus mirabilis The, antibacterialand Escherichia activity coli. Shakir was in et al.vitro reported evaluated an improvement against Staphylococcus in the antimicrobial aureus, Staphylococcus activity for the epidermidis, PCz/TiO2 ProteusNanocomposite mirabilis, compared and Escherichia to TiO 2coli.. Shakir et al. reported an improvement in the antimicrobial activityThe for first the workPCz/TiO on2 the Nanocomposite chemical synthesis compared of unsubstituted to TiO2. polycarbazole and the formation of hollowThe first microspheres work on the based chemical on this synthesis polymer of wasunsubstituted reported by polycarbazole Gupta andPrakash and the information 2010 [102 of]. hollowInterfacial microspheres polymerization based of on carbazole this polymer was carried was reported out using by ammonium Gupta and peroxodisulfatePrakash in 2010 (1.2 [102]. M) Interfacialas oxidizing polymerization agent in dichloromethane of carbazole was at roomcarried temperature. out using ammonium After 12 hperoxodisulfate of polymerization, (1.2 M) dark as oxidizinggreen polycarbazole agent in dichloromethane films were obtained at room withtemper aature. yield After of 50% 12 h of2%. polymerization, During this dark interfacial green ± polymerization,polycarbazole films three-dimensional were obtained with hollow a yield spheres of 50% of ± polycarbazole 2%. During this of interfacial various diameters polymerization, in the three-dimensionalrange of a few micrometers hollow spheres were obtained. of polycarbazol The growthe of of various these spheres diameters was observed in the range using of scanning a few micrometerselectron microscopy were obtained. (SEM) and The atomic growth force of microscopy these spheres (AFM) was techniques observed atusing different scanning time intervals.electron Thesemicroscopy hollow (SEM) microspheres and atomic are force grown microscopy in the carbazole (AFM) techniques micelles formedat different in the time interface. intervals. That These is, hollowmonomer microspheres micelles are are formed grown in thein the reaction carbazole solution micelles at the formed interface in duethe interface. to the agitation That is, (mechanical monomer micellesor thermal) are [102formed,103]. in The the polymerization reaction solution begins at the inside interface the micelles due to andthe theyagitation act as (mechanical a template foror thermal)the polycarbazole, [102,103]. beingThe polymerization the size of the begins micelle inside dependant the micelles on the and reaction they conditionsact as a template (temperature, for the polycarbazole,concentration, stirringbeing the rate, size etc.). of the micelle dependant on the reaction conditions (temperature, concentration,A similar procedurestirring rate, was etc.). followed by Sangwan et al. [99], in their work the effects of surfactants and theirA concentration similar procedure on interfacial was followed polymerization by Sangwan of carbazole et al. [99], was in studied. their work Three the surfactants effects of surfactants of different andnature their such concentration as nonionic on Tween interfacial 20 (TW20), polymerization cationic hexadecyltrimethylammonium of carbazole was studied. Three bromide surfactants (CTAB), of differentand anionic nature sodium such dodecylas nonionic sulphate Tween (SDS) 20 (TW2 were0), used cationic for dihexadecyltrimethylammoniumfferent micelle formation. Ammonium bromide persulfate(CTAB), and (APS) anionic was used sodium as oxidizing dodecyl agent, sulphate being (SDS the) polymerization were used for performed different inmicelle dichloromethane formation. (DCM)Ammonium at 25 ◦persulfateC for 24 h. (APS) was used as oxidizing agent, being the polymerization performed in dichloromethaneThe reported (DCM) SEM images at 25 °C revealed for 24 h. several PCz morphologies depending on the surfactant type and concentration.The reported SEM As it images could be revealed observed several at Figure PCz6, morp macroporoushologies honeycomb depending (Figureon the surfactant6b), connected type andhollow concentration. spheres (Figure As6 c),it andcould smaller be observed hollow spheresat Figure (Figure 6, macroporous6d) when using honeycomb TW20, CTAB (Figure and SDS, 6b), connectedrespectively. hollow On the spheres other hand,(Figure for 6c), the and system smaller with hollow no surfactant, spheres the(Figure particle 6d) shapeswhen using are typically TW20, CTABhollow and sphere SDS, structures respectively. (Figure On 5thea). other In addition, hand, for the the electrical system conductivity with no surfactant, of thedi theff erentparticle PCz shapes were measured being 1.72 0.06 10 4, 2.62 0.79 10 3, 2.16 1.79 10 5 and 2.72 0.32 10 6 S.cm 1, ± × − ± × − ± × − ± × − − for PCz, PCz/TW20, PCz/CTAB and PCz/SDS, respectively. In this case, the maximum electrical Polymers 2020, 12, x FOR PEER REVIEW 7 of 33

are typically hollow sphere structures (Figure 5a). In addition, the electrical conductivity of the different PCz were measured being 1.72 ± 0.06 × 10−4, 2.62 ± 0.79 × 10−3, 2.16 ± 1.79 × 10−5 and 2.72 ± 0.32 Polymers 2020, 12, 2227 8 of 33 × 10−6 S.cm−1, for PCz, PCz/TW20, PCz/CTAB and PCz/SDS, respectively. In this case, the maximum electrical conductivity was achieved for PCz/CTAB formulation. They observed that the electrical conductivity wasdepends achieved of the for packaging PCz/CTAB capacity formulation. of the TheyPCz particles. observed The that particle the electrical size obtained conductivity was depends3213 ± 944, of 1182 the packaging ± 327, 2068 capacity ± 455, and of the2841 PCz ± 835 particles. nm for PCz, The particlePCz/TW20, size PCz/CTAB, obtained was and 3213 PCz/SDS,944, ± 1182respectively.327, 2068 That is,455, a smaller and 2841 particle835 nmsize forprovides PCz, PCz higher/TW20, packing PCz/ CTAB,that provides and PCz higher/SDS, respectively.surface area ± ± ± Thatfor electron is, a smaller transfer. particle PCz/CTAB size provides presented higher the highes packingt conductivity that provides due higher its high surface packaging. area for However, electron transfer.in the case PCz of/CTAB neutral presented surfactant, the highesteven if conductivitythe particle size due itswas high small, packaging. the surface However, seems in to the becase not ofadequate neutral surfactant,to obtain good even ifelectrical the particle conductivity size was small, compared the surface to other seems formulations. to be not adequate Moreover, to obtain the goodmaterials electrical were conductivity doped with comparedHClO4 at different to otherformulations. ratios. Overall, Moreover, an important the materials increase were on the doped electrical with HClOconductivity4 at diﬀ erentwas observed ratios. Overall, for all anthe important materials. increase Moreover, on theit is electrical important conductivity to notice the was maximum observed forvalue all obtained the materials. for PCz/CTAB Moreover, doped it is important at 1:50 PCz:HClO to notice4, the in which maximum the electrical value obtained conductivity for PCz reached/CTAB doped11.3 ± 0.36 at 1:50 S.cm PCz:HClO4,−1. in which the electrical conductivity reached 11.3 0.36 S.cm 1. ± −

Figure 6. SEMSEM images images of of the the different diﬀerent PCz PCz synthesi synthesizedzed with with different diﬀerent surfactant surfactant type: type: (a) PCz; (a) PCz; (b) (PCz/TW20;b) PCz/TW20; (c) PCz/CTAB; (c) PCz/CTAB; and (d and) PCz/SDS (d) PCz at /24h.SDS Reprinted at 24 h. with Reprinted permission with from permission Sangwan fromet al. Sangwan[99]. Copyright et al. [ 99(2016)]. Copyright Wiley. The (2016) scale Wiley. bar is The 30 μ scalem. bar is 30 µm.

In 2011, Gupta et al. [[104],104], reported the fabrication of PCz/gold PCz/gold nanoparticles nanocomposite by chemical synthesis using gold (III) chloridechloride hydratehydrate (HAuCl(HAuCl44) as an oxidizing agent by two different different techniques:techniques: emulsionemulsion and and interfacial interfacial polymerization. polymerization In. In the the proposed proposed mechanism, mechanism, the synthesisthe synthesis of both of PCzboth and PCz gold and nanoparticles gold nanoparticles undergoes undergoes in cooperation, in cooperation, Au+3 is reduced Au+3 is to reduced Au0, whereas, to Au the0, whereas, monomer the is oxidized,monomer that is is,oxidized, gold nanoparticles that is, aregold formed nanoparticles simultaneously are toformed the PCz simultaneously polymerization. to The the product PCz waspolymerization. obtained as aThe green product powder was with obtained 85% and as a 75% gree yieldsn powder for emulsion with 85% and and interfacial 75% yields polymerization, for emulsion respectively,and interfacial after polymerization, 24 h of polymerization respectively, in dichloromethane. after 24 h of polymerization It is interesting in dichloromethane. to notice that UV–Vis It is spectrainteresting and to Fourier-transform notice that UV–Vis infrared spectra and (FTIR) Four spectraier-transform revealed infrared the charge (FTIR) transfer spectra betweenrevealed thethe polymercharge transfer matrix andbetween nanoparticles the polymer and matrix interaction and (Figurenanoparticles7), indicating and interaction that this metal-polymer (Figure 7), indicating hybrid nanomaterialthat this metal-polymer had improved hybrid technologically nanomaterial useful had propertiesimprovedfor technologically molecular electronics useful properties system. for molecularIn addition, electronics others system. oxidizing agents, such as anhydrous ferric chloride (FeCl3), have been also used in the synthesis of PCz. Polycarbazole was synthesized with FeCl3 oxidation in chloroform at room temperature for 24 h. A green precipitate was collected, and washed in order to remove the Fe moieties, being the polymerization yield 80% 2% [105]. However, despite an intensive cleaning ± of the resulting product, iron moieties are still present in the polymer. The hypothetical interaction between iron and polymer could be based on two different assumptions. So, metallic moieties could be trapped in bulk polymer or cationic ions could be coordinated with the nitrogen of the carbazole. On the other hand, these moieties could affect positively to this polymer properties since this PCz presents higher affinity for proton that could be related to the presence of Fe ion. Due to this proton affinity, PCz with iron moieties could be used in sensors [105].

Polymers 2020, 12, 2227 9 of 33 Polymers 2020, 12, x FOR PEER REVIEW 8 of 33

FigureFigure 7. 7. (I(I) )FT-IR FT-IR spectra spectra obtained obtained of of (1) (1) pure pure PCz PCz (2 (2)) PCz–Au PCz–Au nanocomposite nanocomposite (by (by emulsion) emulsion) and and (3) (3) PCz–AuPCz–Au nanocomposite nanocomposite (by (by interfacial interfacial polymerization). ( (IIII)) UV–Visible UV–Visible spectra of (A)A) Pure Pure PCz, B) (B) PCz–AuPCz–Au nanocompositenanocomposite (by (by emulsion) emulsion) and (C) and PCz–Au C) nanocompositePCz–Au nanocomposite (by interfacial (by polymerization). interfacial polymerization).Inset: Zoom for smallInset: peak Zoom due for to thesmall gold peak particles due (600–620to the gold nm) particles and broad (600–620 absorption nm) band and onwardbroad absorption750 nm. Reprinted band onward with permission750 nm. Reprinted from Gupta with etpe al.rmission [104]. Copyrightfrom Gupta (2012) et al. [104]. Wiley. Copyright (2012) Wiley. A comparative study in terms of the structural, thermal, morphological, and electrochemical properties of polycarbazole (PCz) synthesized by controlled interfacial polymerization using two In addition, others oxidizing agents, such as anhydrous ferric chloride (FeCl3), have been also different oxidizing agents, ammonium persulfate (APS) and potassium permanganate (KMnO ) used in the synthesis of PCz. Polycarbazole was synthesized with FeCl3 oxidation in chloroform at4 roomhas been temperature reported for by 24 Kumar h. A green et al. precipitate [106]. The was polymerization collected, and was washed carried in outorder in theto remove dark at the room Fe moieties,temperature being in the dichloromethane polymerization for yield 24 h, 80% with ± good2% [1 yields05]. However, for PCz–APS despite and an PCz–KMnO intensive cleaning4, 82% and of the75%, resulting respectively. product, In this iron work, moieties electrochemical are still present impedance in the spectroscopic polymer. The studies hypothetical of both polymers interaction were betweencarried out iron in and order polymer to analyse could their be based charge-transfer on two different properties assumptions. in the vicinity So, metallic of modified moieties PCz/ couldglassy becarbon trapped (GC) in andbulk PCzpolymer/ Pt electrodes. or cationic Twoions supportingcould be coordinated electrolytes wi wereth the used nitrogen in this of the study, carbazole. namely On0.1 the M tetraethylammoniumtetrafluoroborateother hand, these moieties could affect (TEATFB) positively and to this 0.1 Mpolymer tetraethylammonium-p-toluene properties since this PCz presentssulfonic higher acid (TEA-p-TSA). affinity for proton Figure that8 shows could the be EIS rela responseted to the obtained presence for of thisFe ion. study, Due in to the this form proton of a Nyquist plot. The comparison between the synthesized polymers indicates that PCz–APS presents affinity,Polymers 2020PCz, 12 with, x FOR iron PEER moieties REVIEW could be used in sensors [105]. 9 of 33 betterA electroncomparative transfer study kinetics in terms compared of the tostructural PCz–KMnO4, thermal, at either morphological, electrode (GC) and or electrochemical (Pt). properties of polycarbazole (PCz) synthesized by controlled interfacial polymerization using two different oxidizing agents, ammonium persulfate (APS) and potassium permanganate (KMnO4) has been reported by Kumar et al. [106]. The polymerization was carried out in the dark at room temperature in dichloromethane for 24 h, with good yields for PCz–APS and PCz–KMnO4, 82% and 75%, respectively. In this work, electrochemical impedance spectroscopic studies of both polymers were carried out in order to analyse their charge-transfer properties in the vicinity of modified PCz/glassy carbon (GC) and PCz/ Pt electrodes. Two supporting electrolytes were used in this study, namely 0.1 M tetraethylammoniumtetrafluoroborate (TEATFB) and 0.1 M tetraethylammonium-p- toluene sulfonic acid (TEA-p-TSA). Figure 8 shows the EIS response obtained for this study, in the form of a Nyquist plot. The comparison between the synthesized polymers indicates that PCz–APS presents better electron transfer kinetics compared to PCz–KMnO4 at either electrode (GC) or (Pt).

Figure 8. EIS responses, in form of NyquistNyquist plots, for PCz–APSPCz–APS andand PCz–PCz– KMnOKMnO44 in the presence ofof 0.1 mm TEATFB TEATFB and and 0.1 0.1 m TEA-p-TSAm TEA-p-TSA in acetonitrile in acetonitrile at diﬀ erentat different electrodes: electrodes: (I) GC and (I) (IIGC) Pt. and Reprinted (II) Pt. withReprinted permission with permission from Kumar from et al. Kumar [106]. et Copyright al. [106]. Copyright (2015) Wiley. (2015) Wiley.

2.2. Electropolymerization of Carbazole and Its Derivatives Conductive polymers could be directly synthesized in their doped conductive form from their monomer by an anodic or cathodic reaction. However, anodic polymerization is still the most widely used method. This method offers several advantages, it does not require the addition of catalyst in the electrolytic medium, so it could be considered a clean method and it does not require passage through a halogenated substrate (direct grafting of polymer on a substrate). Generally, this technique consists on the deposition of a polymer film by oxidation, that is, an anodic polymerization on the surface of an electrode of noble metal (gold, platinum) or other conductive materials such as glassy

carbon or ITO (indium tin oxide) (Figure 9) [77,107,108]. On the other hand, cathodic polymerization is less implemented than the anodic oxidation method. It consists of two successive electrochemical reactions, followed by a chemical reaction that requires a catalyst such as nickel [109]. The material deposited on the electrode is obtained in the neutral state, therefore non-conductive, which could inhibit the reaction and requires to regenerate the active surface by doping the polymer [110–112]. Films obtained by electrochemical polymerization are films with better-defined and controlled properties and structure. The electrochemical polymerization has been widely used in recent years for the synthesis of insulating or semi-conductive polymers. This technique presents several advantages, such us homogeneity, relative ease of processing, and obtaining films with controllable and reproducible thickness and structure. It is also important to notice that these polymer thin films are usually difficult to prepare due to their low solubility in solvents [113].

Polymers 2020, 12, x FOR PEER REVIEW 9 of 33

Figure 8. EIS responses, in form of Nyquist plots, for PCz–APS and PCz– KMnO4 in the presence of 0.1 m TEATFB and 0.1 m TEA-p-TSA in acetonitrile at different electrodes: (I) GC and (II) Pt. Reprinted with permission from Kumar et al. [106]. Copyright (2015) Wiley. Polymers 2020, 12, 2227 10 of 33 2.2. Electropolymerization of Carbazole and Its Derivatives

2.2.Conductive Electropolymerization polymers of could Carbazole be directly and Its synthesi Derivativeszed in their doped conductive form from their monomer by an anodic or cathodic reaction. However, anodic polymerization is still the most widely used method.Conductive This polymersmethod offers could several be directly advantages, synthesized it does in theirnot require doped the conductive addition form of catalyst from their in themonomer electrolytic by anmedium, anodic orso cathodicit could reaction.be considered However, a clean anodic method polymerization and it does isnot still require the most passage widely throughused method. a halogenated This method substrate offers (direct several grafting advantages, of polymer it does on not a substrate). require the Generally, addition ofthis catalyst technique in the consistselectrolytic on the medium, deposition so it couldof a polymer be considered film by a oxidation, clean method that and is, itan does anodic not polymerization require passage on through the surfacea halogenated of an electrode substrate of (directnoble metal grafting (gold, of polymer platinum) on or a substrate).other conductive Generally, materials this technique such as consistsglassy carbonon the or deposition ITO (indium of a polymertin oxide) film (Figure by oxidation, 9) [77,107,108]. that is, an anodic polymerization on the surface of an electrodeOn the of other noble hand, metal cathodic (gold, platinum) polymerization or other conductiveis less implemented materials suchthan asthe glassy anodic carbon oxidation or ITO method.(indium It tin consists oxide) of (Figure two successive9)[77,107 ,108electrochemica]. l reactions, followed by a chemical reaction that requiresOn a the catalyst other hand,such as cathodic nickel polymerization[109]. The materi is lessal deposited implemented on the than electrode the anodic is oxidationobtained method.in the neutralIt consists state, of therefore two successive non-conductive, electrochemical which reactions, could inhibit followed the reaction by a chemical and requires reaction to that regenerate requires a thecatalyst active suchsurface as nickelby doping [109 ].the The polymer material [110–112]. deposited on the electrode is obtained in the neutral state, thereforeFilms non-conductive,obtained by electrochemical which could polymerization inhibit the reaction are andfilms requires with better-defined to regenerate theand active controlled surface propertiesby doping and the structure. polymer [The110– electrochemical112]. polymerization has been widely used in recent years for theFilms synthesis obtained of byinsulating electrochemical or semi-conductive polymerization polymers. are films This with better-definedtechnique presents and controlled several advantages,properties andsuch structure. us homogeneity, The electrochemical relative ease polymerization of processing, hasand been obtaining widely films used with in recent controllable years for andthe reproducible synthesis of insulatingthickness and or semi-conductive structure. It is also polymers. important This to technique notice that presents these polymer several advantages,thin films aresuch usually us homogeneity, difficult to prepare relative easedue ofto processing,their low solubility and obtaining in solvents films with [113]. controllable and reproducible thickness and structure. It is also important to notice that these polymer thin films are usually difficult to prepare due to their low solubility in solvents [113].

Figure 9. A schematic of the electrochemical synthesis set-up. Reprinted with permission from Balint et al. [114] Creative Commons CC-BY.

Electrochemical oxidation of carbazole proceeds in a similar way to chemical oxidation but seems to be more selective. The first and most significant study on the electrochemical oxidation of carbazole was published by Ambrose and Nelsonin in 1968 [115]. Ates and Özyılmaz [49] conducted systematic study of corrosion performance of polycarbazole (PCz) and PCz derivatives. In their study, films of PCz, and two nanocomposites of nanoclay and zinc nanoparticles were developed. Films were chemically and electrochemically deposited on a stainless steel (SS304), and their anticorrosive properties were tested against 3.5% NaCl solution by EIS and potentiodynamic polarization curves. Carbazole was electropolymerized by chronoamperometric technique on an SS304 electrode for 3600 s in an oxalic acid/acetonitrile solution. In addition, the chemical polymerized carbazole was carried in acetonitrile using cerium ammonium nitrate (CAN) as initiator for 6–8 h at room temperature. This study showed that PCz, PCz/nanoclay and PCz/nanoZn films obtained using chemical method coated on SS304 electrodes displayed better corrosion protection Polymers 2020, 12, 2227 11 of 33 performance compared to the films obtained by the electrochemical method. For chemically technique, PCz films, the highest protection efficiency (PE = 99.81%) has been obtained. Srivastava et al. [51] reported PCz electropolymerization and deposition on ITO-coated glass. A study of polycarbazole films prepared on the different metal contacts, such as Aluminium, Copper, and Tungsten, was also carried out for the fabrication of Schottky diodes. PCz was synthesized by oxidative polymerization of carbazole in dichloromethane and as an oxidant tetrabutylammonium perchlorate (TBAP) in an electrochemical workstation. The polymerization of carbazole requires a low anodic potential (1.3 V) to be oxidized. The electrodeposition was prepared in a similar way. The metal contact was deposited in a previously fabricated PCz/Ito films by vacuum thermal evaporation deposition. Authors reported the fabricated diodes presented reasonably good performance rating parameters, showing the ITO/PCz/W device exceptionally good barrier height (0.95) and reverse saturation current density (J ) of 1.312 10 13 A/cm2. 0 × − To the best of our knowledge, only few samples of PCz and biopolymer composites have been reported until date. Kayan et al. [107] reported a study in which a polycarbazole/chitosan composite (PCz/Chi) films were successfully synthesized. The synthesis was carried out by using electrochemical polymerization by depositing on a Pt disk electrode by cyclic voltammetry after 5 cycles in the range of 0.0 V to +1.6 V in acetonitrile solution and 0.1 M lithium perchlorate as a supporting electrolyte. The composites were obtained by a similar procedure, adding solution of chitosan at different concentrations. Authors reported an increase the electrical conductivity of the films increase with the presence of chitosan. On the other hand, EIS measurements indicated that small amount of chitosan could enhance films conductivity by easing electron transfer.

2.3. Polymerization of N-Substitution Carbazoles Poly(N-vinylcarbazole) (PVK) is one of the most interesting polymers based on N-substituted of carbazole due to its wide applications and its excellent thermal stability, doping behaviour, and UV durable property [77,107,108]. However, PVK present poor processability due to the π π electron − system along its backbone reducing its stability versus oxidation, which reduces the conductivity of the polymers [116,117]. In order to reduce these disadvantages, carbazole-based monomers have been modified. The substitution at N-position with a wide variety of functional groups could provide carbazole derivatives with improved properties such as solubility, better thermal stability, electrical, photoelectric, ion exchange and other physicochemical properties [118,119]. The improvement on their properties wides the applicability of these materials [120,121]. The polymerization N-vinylcarbazole (NVK) was reported for the first time by Reppe and co-workers in 1934 [122]. N-vinyl carbazole polymerization (NVK) is extensively investigated and many methods have been used, such as free radical [123], cationic polymerization [124], anionic polymerization [125], atom transfer radical polymerization (ATRP) [126], reversible additional fragmentation chain transfer (RAFT) polymerization [127], nitroxide-mediated polymerization (NMP) [128], charging transfer [129], electrocuting [129], solid state polymerization [130], and organometallic-mediated radical polymerization [131]. More recently, Marimuthu and Murugesan [132] reported an efficient and facile polymerization of N-vinyl carbazole (NVK). 1,4-bis (triethyl methyl ammonium) benzene dibromide (TEMABDB) was used as multi-site phase transfer catalyst (MPTC) and potassium peroxydisulphate (PDS) as water soluble initiator at 40 2 C in two phase system (cyclohexane/water) with ultrasound condition ± ◦ (45 kHz/550 W) and silent. The polymerization rate for this system was significantly increased when ultrasound was used. Frau et al. [133] reported the development of a conjugated polymer network (CPN) based on PVK to fabricate anticorrosion coatings. Electrochemical deposition on steel and ITO substrates by both potentiostatic and potentiodynamic methods were used. Anodic oxidation of the carbazole functional groups was used to prepare a cross-linked macromolecular structure (Figure 10). The electrodeposition of a PVK in dichlorometane was carried out on a potentiostat with a Pt wire as a counter electrode, Polymers 2020, 12, x FOR PEER REVIEW 11 of 33

(NMP) [128], charging transfer [129], electrocuting [129], solid state polymerization [130], and organometallic-mediated radical polymerization [131]. More recently, Marimuthu and Murugesan [132] reported an efficient and facile polymerization of N-vinyl carbazole (NVK). 1,4-bis (triethyl methyl ammonium) benzene dibromide (TEMABDB) was used as multi-site phase transfer catalyst (MPTC) and potassium peroxydisulphate (PDS) as water soluble initiator at 40 ± 2 °C in two phase system (cyclohexane/water) with ultrasound condition (45 kHz/550 W) and silent. The polymerization rate for this system was significantly increased when ultrasound was used. Frau et al. [133] reported the development of a conjugated polymer network (CPN) based on PVK to fabricate anticorrosion coatings. Electrochemical deposition on steel and ITO substrates by both potentiostatic and potentiodynamic methods were used. Anodic oxidation of the carbazole functionalPolymers 2020 groups, 12, 2227 was used to prepare a cross-linked macromolecular structure (Figure 10).12 The of 33 electrodeposition of a PVK in dichlorometane was carried out on a potentiostat with a Pt wire as a counter electrode, nonaqueous Ag/AgCl electrode (0.1 M in acetonitrile) as a reference electrode, and, nonaqueous Ag/AgCl electrode (0.1 M in acetonitrile) as a reference electrode, and, finally, steel or ITO finally, steel or ITO films as working. A step of 1.2 V for 1000 s was induced for the potentiostatic films as working. A step of 1.2 V for 1000 s was induced for the potentiostatic deposition, whereas deposition, whereas potentiodynamic deposition was carried out by cycling the potential between 0 potentiodynamic deposition was carried out by cycling the potential between 0 and 1.4 V and a rate of −1 and 1.4 V and1 a rate of 50 mV s for 20 cycles. Morphological studies indicted a higher roughness on 50 mV s− for 20 cycles. Morphological studies indicted a higher roughness on the substrates after the substrates after a potentiostatic deposition compared to potentiodynamic deposition. In addition, a potentiostatic deposition compared to potentiodynamic deposition. In addition, the EIS results the EIS results demonstrated that the PVK coating present good ion transport blocking properties, demonstrated that the PVK coating present good ion transport blocking properties, according to according to accelerated corrosion tests. Moreover, they showed efficient corrosion resistance on steel accelerated corrosion tests. Moreover, they showed efficient corrosion resistance on steel coupons used coupons used as a model metal substrate. as a model metal substrate.

FigureFigure 10. 10. SchematicSchematic representation representation of of the the precursor precursor po polymerlymer approach approach to to CP CPNN films. ﬁlms. Reproduced Reproduced withwith permission permission from from Frau Frau et et al. al. [133]. [133]. Copyright Copyright (2010) (2010) American American Chemical Chemical Society. Society.

SelectionSelection of of the the appropriate appropriate monomer monomer and and the the right right polymerisation polymerisation techniques techniques are are crucial crucial considerationsconsiderations for for materials materials with with excellent gas-uptakegas-uptake capacities. In In this this context, context, conjugated conjugated microporousmicroporous polymers polymers (CMP) (CMP) have have arisen arisen as as very very promising promising type type of of microporous microporous organic organic polymers polymers (MOP).(MOP). Huang Huang etet al.al. [ 134[134]] reported reported two two simple simple strategies strategies (name (name in their in their work work as path as (1)path and (1) path and (2)) path for (2))the for preparation the preparation of CMPs of based CMPs on based N-vinyl on N-vinyl carbazole carbazole derivatives. derivatives. Four diff Fourerent different derivatives derivatives (P1 to P4) were synthesised by combining both free radical polymerization and oxidative FeCl polymerization (P1 to P4) were synthesised by combining both free radical polymerization and 3oxidative FeCl3 polymerization(Figure 11). The (Figure oxidative 11). polymerization The oxidative waspolymerization performed atwas ambient performed temperature at ambient in chloroform temperature using in FeCl as oxidizing agent for 24 h. On the other hand, the free radical polymerization was carried chloroform3 using FeCl3 as oxidizing agent for 24 h. On the other hand, the free radical polymerization wasout bycarried using out 2, 2by0-azobisisobutyronitrile using 2, 2′-azobisisobutyronitrile (AIBN) as initiator (AIBN) in as toluene initiator at in 70 toluene◦C for 6at h. 70 The °C eforffects 6 h. of Thesynthetic effects methods of synthetic and methods sequences and on sequences the performance on the asperformance CMP were evaluated.as CMP were The evaluated. BET surface The area BET of 2 1 surfacethe polymers area of wasthe polymers determined. was In determined. path 1, the In BET path obtained 1, the BET for P2obtained (878.46 for m P2g− (878.46) was significantlym2 g−1) was 2 1 significantlyhigher than thathigher of P1than (68.65 that m of gP1− ).(68.65 However, m2 g1 in). However, path 2, the in values path obtained2, the values for both obtained polymers for both were 2 1 2 1 polymerssimilar, being were 621.18similar, m beingg− and 621.18 660.62 m2 g m−1 andg− for660.62 P3 andm2 g P4,−1 for respectively. P3 and P4, Therespectively. gas uptake The results gas evaluated for the absorption of carbon dioxide (CO ), methane (CH ), and hydrogen suggested that uptake results evaluated for the absorption of carbon2 dioxide (CO2),4 methane (CH4), and hydrogen P2 presents the best performance, that is, path 1 was the most appropriate method to obtain N-vinyl Polymerssuggested 2020 , that12, x P2FOR presents PEER REVIEW the best performance, that is, path 1 was the most appropriate method12 of 33to obtaincarbazole-based N-vinyl carbazole-based CMPs. CMPs.

Figure 11. Scheme of the two synthetic routes described by Huang et al.: path 1 and path 2. Reproduced Figure 11. Scheme of the two synthetic routes described by Huang et al.: path 1 and path 2. with permission from Huang et al. [134]. Copyright (2014) Royal Society of Chemistry. Reproduced with permission from Huang et al. [134]. Copyright (2014) Royal Society of Chemistry.

In recent years, graphene has attracted tremendous attention due to its properties and versatility, being used in ON and OFF for memory devices or conducting nanocomposites, among others [135]. Santos et al. [136] reported the preparation thin films of poly(N-vinylcarbazole)-GO (PVK-GO) nanocomposites via electrodeposition by cyclic voltammetry (CV) on bare ITO by repeatedly scanning the potential between 0 to 1500 mV at a scan rate of 10 mVs−1 for 50 cycles. The nanocoposite was crosslinked due to the electropolymerization process of the carbazole side groups of PVK. This improved the stability of the nanocomposites to several solvents such as 2-pyrrolidone and N- methylpyrrolidone, close to 30 days. Similarly, Wang et al. [137] synthesized poly(9-vinylcarbazole)/silver nanocomposites by in situ formation of silver nanotubes and networks formed at the air–water interface via the reduction of Ag+ ions. The structures of the silver nanotubes were strongly dependent of the experimental conditions such as temperature. The stability of the redox states is the most suitable property for an electro-active polymer to be useful in building new electrochromic device [138–141]. Furthermore, the ability of a material to demonstrate a significant colour shift is important to electrochromic applications. Kocaeren [142] reported the synthesis of carbazole derivatives with electrochemical and electrochromic properties to be used electrochromic devices (ECDs). Firstly, bis-4-(9H-carbazol-9-yl) phenyl-3,4-diyloxy thiophene (B1) was synthesized from the reaction of 4-(9H-carbazol-9-yl) phenol and 3,4-dibromo thiophene in the presence of potassium carbonate (K2CO3) in tetrahydrofuran (THF). After that, the polymer of B1 was deposited onto an ITO-glass surface by oxidative electrochemical polymerization (Figure 12). The electropolymerization was performed on a potentiostat in acetonitrile, using Pt wire as counter electrode, Ag wire as reference electrode and ITO as working electrode, scanned from +0.3 to +1.4 V. The presence of the polymer was evidence due to the increase of a peak in a cyclic voltammetry at 0.95 V. It is important to notice that the polymer film presents a blue colour between 1.0 and 1.4 V due to its oxidation, whereas its colour turns into a light yellow between 0.5 and 0.9 V owning to its reduction. The maximum absorbance wavelengths were 320 and 670 nm. This carbazole derivative, which presents high stability, could be used in electrochromic devices (ECDs) according to redox stability measurements.

Polymers 2020, 12, 2227 13 of 33

In recent years, graphene has attracted tremendous attention due to its properties and versatility, being used in ON and OFF for memory devices or conducting nanocomposites, among others [135]. Santos et al. [136] reported the preparation thin films of poly(N-vinylcarbazole)-GO (PVK-GO) nanocomposites via electrodeposition by cyclic voltammetry (CV) on bare ITO by repeatedly scanning 1 the potential between 0 to 1500 mV at a scan rate of 10 mVs− for 50 cycles. The nanocoposite was crosslinked due to the electropolymerization process of the carbazole side groups of PVK. This improved the stability of the nanocomposites to several solvents such as 2-pyrrolidone and N-methylpyrrolidone, close to 30 days. Similarly, Wang et al. [137] synthesized poly(9-vinylcarbazole)/silver nanocomposites by in situ formation of silver nanotubes and networks formed at the air–water interface via the reduction of Ag+ ions. The structures of the silver nanotubes were strongly dependent of the experimental conditions such as temperature. The stability of the redox states is the most suitable property for an electro-active polymer to be useful in building new electrochromic device [138–141]. Furthermore, the ability of a material to demonstrate a significant colour shift is important to electrochromic applications. Kocaeren [142] reported the synthesis of carbazole derivatives with electrochemical and electrochromic properties to be used electrochromic devices (ECDs). Firstly, bis-4-(9H-carbazol-9-yl) phenyl-3,4-diyloxy thiophene (B1) was synthesized from the reaction of 4-(9H-carbazol-9-yl) phenol and 3,4-dibromo thiophene in the presence of potassium carbonate (K2CO3) in tetrahydrofuran (THF). After that, the polymer of B1 was deposited onto an ITO-glass surface by oxidative electrochemical polymerization (Figure 12). The electropolymerization was performed on a potentiostat in acetonitrile, using Pt wire as counter electrode, Ag wire as reference electrode and ITO as working electrode, scanned from +0.3 to +1.4 V. The presence of the polymer was evidence due to the increase of a peak in a cyclic voltammetry at 0.95 V. It is important to notice that the polymer film presents a blue colour between 1.0 and 1.4 V due to its oxidation, whereas its colour turns into a light yellow between 0.5 and 0.9 V owning to its reduction. The maximum absorbance wavelengths were 320 and 670 nm. This carbazole derivative, which presents high stability, could be used in electrochromic devices (ECDs) according to redox Polymersstability 2020 measurements., 12, x FOR PEER REVIEW 13 of 33

FigureFigure 12. 12.The performedThe performed polymer polymer synthesis: synthesis: (a) Synthesis (a )of Synthesis bis-4-(9H-carbazol-9-yl) of bis-4-(9H-carbazol-9-yl) phenyl-3,4- diyloxyphenyl-3,4-diyloxy thiophene (B1). thiophene (b) Electrochemical (B1). (b) Electrochemical synthesis of synthesis P(B1). Reproduced of P(B1). Reproduced with permission with permission from [142].from Copyright [142]. Copyright (2015) Elsevier. (2015) Elsevier.

Another example of electrochromic carbazole derivative was synthetized by Hsiao and co- worker [143]. In their study, two previously synthesized carbazole-based monomers were successfully electrodeposited and polymerized onto the ITO electrode by electropolymerization. 4,4′- di(carbazol-9-yl)-4″-methoxytriphenylamine (TPA-2Cz) and 3,6-di(carbazol-9-yl)-N-(4- methoxyphenyl)carbazole (PhCz-2Cz). The electropolymerization onto an ITO (working electrode) of the monomers was carried out with a polymer and 0.1 M Bu4NClO4 solution in dichloromethane and ITO as working electrode by several cycles between 0 and 1.4 V at a scan rate of 50 mV s−1. P(PhCz-2Cz) (Figure 13a) present a blue-green colour in its maximum oxidation state around 1.28 V. This film changes to yellow (1.07 V) and finally, colourless at 0.0 V. Similarly, P(TPA-2Cz) presents a brown colour at its fully oxidized state and dark green colour at semi-oxidized states.

Polymers 2020, 12, 2227 14 of 33

Another example of electrochromic carbazole derivative was synthetized by Hsiao and co-worker [143]. In their study, two previously synthesized carbazole-based monomers were successfully electrodeposited and polymerized onto the ITO electrode by electropolymerization. 4,40-di(carbazol- 9-yl)-4”-methoxytriphenylamine (TPA-2Cz) and 3,6-di(carbazol-9-yl)-N-(4-methoxyphenyl)carbazole (PhCz-2Cz). The electropolymerization onto an ITO (working electrode) of the monomers was carried out with a polymer and 0.1 M Bu4NClO4 solution in dichloromethane and ITO as working electrode 1 by several cycles between 0 and 1.4 V at a scan rate of 50 mV s− . P(PhCz-2Cz) (Figure 13a) present a blue-green colour in its maximum oxidation state around 1.28 V. This ﬁlm changes to yellow (1.07 V) and ﬁnally, colourless at 0.0 V. Similarly, P(TPA-2Cz) presents a brown colour at its fully oxidized state andPolymers dark 2020 green, 12, x colourFOR PEER at semi-oxidizedREVIEW states. 14 of 33

Figure 13.13. ElectrochromicElectrochromic study of the ((aa)) P(PhCz-2Cz)P(PhCz-2Cz) andand ((bb)) P(TPA-2Cz).P(TPA-2Cz). ReproducedReproduced withwith permission fromfrom [[143].143]. Copyright (2015) Elsevier.

The samesame strategy strategy was was used used in otherin other work work of Hsiao of Hsiao and Lin and [141 Lin] in [141] which in poly(amide-carbazole) which poly(amide- andcarbazole) poly(imide-carbazole) and poly(imide-carbazole) were used were for the used development for the development of two series of two of diamide-cored series of diamide-cored carbazole dendronscarbazole dendrons (6CzR-DA) (6CzR-DA) and diimide-cored and diimide-cored carbazole carbazole dendrons dendrons (6CzR-DI). (6CzR-DI). These monomersThese monomers series wereseries synthesized were synthesized from condensation from reactionscondensation of 3,6-di(carbazol-9-yl)-N-(4-aminophenyl)carbazole reactions of 3,6-di(carbazol-9-yl)-N-(4- (NHaminophenyl)carbazole2-3Cz) with aromatic dicarboxylic(NH2-3Cz) acidswith andaromatic tetracarboxylic dicarboxylic dianhydrides, acids respectively.and tetracarboxylic Similar to theirdianhydrides, previous work,respectively. these polymer Similar ﬁlms to showedtheir previous good electrochemical work, these polymer and electrochromic films showed properties. good electrochemical and electrochromic properties. Yang and co-workers [144] performed a study on induced oxidative polymerization of 1,3,6,8- tetrakis(4-(9H-carbazol-9-yl) phenyl)pyrene (L) with FeCl3 as oxidant in anhydrous chloroform for 24 h at 60 °C. This process resulted in the formation of the bulk polymer (Figure 14), being a highly luminescent conjugated microporous polycarbazole derivative (CK-CMP).

Polymers 2020, 12, x FOR PEER REVIEW 14 of 33

Figure 13. Electrochromic study of the (a) P(PhCz-2Cz) and (b) P(TPA-2Cz). Reproduced with permission from [143]. Copyright (2015) Elsevier.

The same strategy was used in other work of Hsiao and Lin [141] in which poly(amide- carbazole) and poly(imide-carbazole) were used for the development of two series of diamide-cored carbazole dendrons (6CzR-DA) and diimide-cored carbazole dendrons (6CzR-DI). These monomers series were synthesized from condensation reactions of 3,6-di(carbazol-9-yl)-N-(4- aminophenyl)carbazole (NH2-3Cz) with aromatic dicarboxylic acids and tetracarboxylic Polymers 2020, 12, 2227 15 of 33 dianhydrides, respectively. Similar to their previous work, these polymer films showed good electrochemical and electrochromic properties. YangYang and and co-workers co-workers [144] [144 performed] performed a study a study on induced on induced oxidative oxidative polymerization polymerization of 1,3,6,8- of 3 tetrakis(4-(9H-carbazol-9-yl)1,3,6,8-tetrakis(4-(9H-carbazol-9-yl) phenyl)pyrene phenyl)pyrene (L) with (L) FeCl with as FeCl oxidant3 as oxidant in anhydrous in anhydrous chloroform chloroform for 24 hfor at 2460 h°C. at 60This◦C. process This process resulted resulted in the in formation the formation of the of thebulk bulk polymer polymer (Figure (Figure 14), 14 ),being being a ahighly highly luminescentluminescent conjugated conjugated microporous microporous po polycarbazolelycarbazole derivative derivative (CK-CMP). (CK-CMP).

Polymers 2020, 12, x FOR PEER REVIEW 15 of 33

Figure 14.14. The synthesis of CK-CMP proposedproposed byby YangYang etet al.al. inin theirtheir work.work. Reproduced with

permission fromfrom [144 ].[144]. Copyright Copyright (2019) Elsevier.(2019) L:Elsevier. 1,3,6,8-tetrakis(4-(9H-carbazol-9-yl) L: 1,3,6,8-tetrakis(4-(9H-carbazol-9-yl) phenyl)pyrene, CK-CMP:phenyl)pyrene, conjugated CK-CMP: microporous conjugated polycarbazole microporous derivative. polycarbazole derivative.

Soganci et al. [[48]48] reported a novel method for electropolymerization a disulﬁde-linkeddisulfide-linked N-alkyl substituted carbazolecarbazole derivative.derivative. 1,2-bis[6-(9-carbozol-9-yl)hexyl]disulfane 1,2-bis[6-(9-carbozol-9-yl)hexyl]disulfane (CS) (CS) monomer was synthetized inin thisthis workwork (Figure(Figure 15 15)) and and then, then, electropolymerized electropolymerized using using cyclic cyclic voltammetry voltammetry and and ITO ITO as workingas working electrode. electrode. The electropolymerizationThe electropolymerization process ofprocess CS monomer of CS was monomer performed was comparatively performed incomparatively the BFEE (Boron in the triﬂuoride BFEE (Boron diethyl trifluoride etherate) containing diethyl etherate) solution containing and BFEE-free solution electrolytic and BFEE-free solutions. Thiselectrolytic material solutions. presents This interesting material electrochromic presents interesting properties electrochromic that could be potentiallyproperties that used could in smart be windowpotentially applications, used in smart due window to the high applications, optical contrast due to value the andhigh stability optical obtainedcontrast value in BFEE and compared stability toobtained other N-alkyl in BFEE substituted compared carbazoleto other N-alkyl appeared substituted in literature. carbazole appeared in literature.

Figure 15.15. Electrochemical polymerization of 1,2-bis[6-(9-carbozol-9-yl)hexyl]disulfane1,2-bis[6-(9-carbozol-9-yl)hexyl]disulfane proposed by Soganci et al. Reproduced with permissionpermission from [[48].48]. Copyright (2018) Elsevier.

Duran et al. [[145]145] successfullysuccessfully depositeddeposited aa poly(N-methylpoly(N-methyl carbazole)carbazole) (PNMeCz)(PNMeCz) coating on stainless steel typetype 304.304. The The fi filmlm was deposited by electropolymerizationelectropolymerization of N-methylN-methyl carbazolecarbazole (NMeCz)(NMeCz) monomermonomer in acetonitrile solution containingcontaining tetrabutylammonium perchlorateperchlorate using cyclic voltammetry and stainless steel asas workingworking electrode.electrode. The film film was electrodeposited applying a potential between ++0.50.5 and ++1.71.7 V with a raterate ofof 5050mV/s mV/s during 15 cycles.cycles. The resistance to the corrosion was evaluated, demonstratingdemonstrating thesethese filmsfilms presentedpresented goodgood anticorrosionanticorrosion properties.properties. Elkhidr and co-workers [146], synthesized and studied different electrochemical and optical properties for three carbazole derivatives with different substitution at N-positions, methanol (carbazol-9-yl-methanol), carboxylic acid (carbazol-9-yl- carboxylic acid) and cyanoethyl (carbazol-9- yl-cyanoethyl). Polymeric films of these derivatives were obtained (PCz−OH, PCz−COOH, and PCz- CN, respectively) by the electropolymerization on ITO substrate by repetitive cyclic voltammetry. The electropolymerization was carried out on a potentiostat-galvanostat system, with tree electrodes; the working electrode was ITO, whereas platinum and silver wires were used as counter and pseodoreference electrodes, respectively. A potentiodynamic electropolymerization was performed between +0.0 to +1.6 V with a scan rate of 100 mV/s. The monomer solutions containing NaClO4- LiClO4 electrolyte dissolved in acetonitrile. Polycarbazole presents high solubility problems, being almost insoluble in most of inorganic solvent and soluble in only few organic solvents. However, the polycarbazole derivatives reported in this work showed a good solubility in common organic solvents such as dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), tetrahydrofuran (THF), and dimethylacetamide (DMAC). The colour variations induced by the redox switching of the carbazole derivatives during the electrochemical process are summarized in Figure 16, where L is the luminance or brightness, a is hue and b is the saturation using the International Commission on Illumination (CIE) system.

Polymers 2020, 12, 2227 16 of 33

Elkhidr and co-workers [146], synthesized and studied different electrochemical and optical properties for three carbazole derivatives with different substitution at N-positions, methanol (carbazol-9-yl-methanol), carboxylic acid (carbazol-9-yl- carboxylic acid) and cyanoethyl (carbazol-9-yl-cyanoethyl). Polymeric films of these derivatives were obtained (PCz OH, PCz COOH, − − and PCz-CN, respectively) by the electropolymerization on ITO substrate by repetitive cyclic voltammetry. The electropolymerization was carried out on a potentiostat-galvanostat system, with tree electrodes; the working electrode was ITO, whereas platinum and silver wires were used as counter and pseodoreference electrodes, respectively. A potentiodynamic electropolymerization was performed between +0.0 to +1.6 V with a scan rate of 100 mV/s. The monomer solutions containing NaClO4-LiClO4 electrolyte dissolved in acetonitrile. Polycarbazole presents high solubility problems, being almost insoluble in most of inorganic solvent and soluble in only few organic solvents. However, the polycarbazole derivatives reported in this work showed a good solubility in common organic solvents such as dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), tetrahydrofuran (THF), and dimethylacetamide (DMAC). The colour variations induced by the redox switching of the carbazole derivatives during the electrochemical process are summarized in Figure 16, where L is the luminance or brightness, a is hue and b is the saturation using the International Commission on PolymersIllumination 2020, 12 (CIE), x FOR system. PEER REVIEW 16 of 33

Figure 16. The colour variations induced by redox process on PCz OH, PCz COOH and PCz-CN. Figure 16. The colour variations induced by redox process on PCz−OH, PCz−−COOH and PCz-CN. Reproduced with permission from [146]. Copyright (2020) Elsevier. Reproduced with permission from [146]. Copyright (2020) Elsevier. Qin et al. [147] reported the successful synthesis of conjugated network based on poly(ethylenoxide) Qin et al. [147] reported the successful synthesis of conjugated network based on grafted carbazole. In their study, ﬁrst poly(N-epoxypropyl carbazole) (PEPC) was obtained by poly(ethylenoxide) grafted carbazole. In their study, first poly(N-epoxypropyl carbazole) (PEPC) was anionic ring-open polymerization of N-epoxypropyl carbazole (EPC) using potassium hydroxide and obtained by anionic ring-open polymerization of N-epoxypropyl carbazole (EPC) using potassium 18-crown-6 in toluene at 90 C for 12 h. After obtaining PEPC, a conjugated network was fabricated by hydroxide and 18-crown-6 in◦ toluene at 90 °C for 12 h. After obtaining PEPC, a conjugated network electrodeposition of poly[poly(N-epoxypropyl carbazole)] (PPEPC). The electropolymerization was was fabricated by electrodeposition of poly[poly(N-epoxypropyl carbazole)] (PPEPC). The carried put in a potentiostat using Pt wire as counter electrode and stainless steel and Pt sheets as electropolymerization was carried put in a potentiostat using Pt wire as counter electrode and working electrodes on which the polymer was deposited. A scheme of the complete synthetic process stainless steel and Pt sheets as working electrodes on which the polymer was deposited. A scheme of carried out in this work is depictured in Figure 17. The synthesized PPEPC showed favourable thermal the complete synthetic process carried out in this work is depictured in Figure 17. The synthesized stability and strong mechanical properties, and can be easily bent or cut into diﬀerent forms. PPEPC showed favourable thermal stability and strong mechanical properties, and can be easily bent or cut into different forms.

Figure 17. Synthesis route and scheme for PEPC and PPEPC. Reproduced from [147]. (CC by 4.0).

3. Application The carbazole based molecules have emerged as important core fragments with highly interesting biological activity. Its derivatives present several advantageous properties such as antimicrobial, antitumor, antioxidant, anti-inflammatory, and pancreatic lipase inhibition properties

Polymers 2020, 12, x FOR PEER REVIEW 16 of 33

Figure 16. The colour variations induced by redox process on PCz−OH, PCz−COOH and PCz-CN. Reproduced with permission from [146]. Copyright (2020) Elsevier.

Qin et al. [147] reported the successful synthesis of conjugated network based on poly(ethylenoxide) grafted carbazole. In their study, first poly(N-epoxypropyl carbazole) (PEPC) was obtained by anionic ring-open polymerization of N-epoxypropyl carbazole (EPC) using potassium hydroxide and 18-crown-6 in toluene at 90 °C for 12 h. After obtaining PEPC, a conjugated network was fabricated by electrodeposition of poly[poly(N-epoxypropyl carbazole)] (PPEPC). The electropolymerization was carried put in a potentiostat using Pt wire as counter electrode and stainless steel and Pt sheets as working electrodes on which the polymer was deposited. A scheme of the complete synthetic process carried out in this work is depictured in Figure 17. The synthesized PolymersPPEPC 2020showed, 12, 2227 favourable thermal stability and strong mechanical properties, and can be easily17 bent of 33 or cut into different forms.

Figure 17. Synthesis route and scheme for PEPC and PPEPC.PPEPC. Reproduced from [[147].147]. (CC by 4.0). 3. Application 3. Application The carbazole based molecules have emerged as important core fragments with highly The carbazole based molecules have emerged as important core fragments with highly interesting biological activity. Its derivatives present several advantageous properties such as interesting biological activity. Its derivatives present several advantageous properties such as antimicrobial, antitumor, antioxidant, anti-inflammatory, and pancreatic lipase inhibition properties [43]. antimicrobial, antitumor, antioxidant, anti-inflammatory, and pancreatic lipase inhibition properties In addition to biological activities, these molecules also present opto-electronic, electrochemical, and electrical properties when polymerized, being their main applications such as photovoltaic devices, electroluminescent displays, batteries, bio(sensors), etc. PVK presents adequate levels of photoconductivity to be used is electrophotography, being used by IBM in its Copier I series in 1970. In addition, poly(N-vinylcarbazole) has been widely used as a host material in OLEDs. PVK presents a high energy blue-emissive singlet excited state and could acts as electron donor and hole transporting material. However, since it is not a conjugated polymer its transport is carried out by radical cation hopping among the discrete carbazole units [77]. Since them, many research have been focus on the development of 3,6 disubstitued carbazoles, taking advantage of the high reactivity of 3,6 positions [148]. In this context, linear and hyperbranched 3,6-carbazole derivatives present excellent redox activity, photorefractive and nonlinear optical properties [106,149]. These properties make them good candidates as materials for organic light-emitting diodes (OLEDs). However, as has been previously mentioned, they present several limitations such as their reduced conjugation of electrons and low molecular weight. On the other hand, 2,7-carbazole-based polymers are usually more linear, this improves their organization and the extension of the conjugation length, construing to a lower band gap [150]. These properties are especially interesting for the development of organic field effect transistors (OFETs), bulk-heterojunction (BHJ) solar cells, thermoelectric [84,149]. It is important to notice that synthetic pathway could condition the potential applications. That is, chemical synthesis is more interesting for those applications in which bulk materials are needed, or a control over the morphology is required. In addition, the electropolymerization is more suitable to planar applications in which a coating or a thin film is required [102,151]. In this section, some of the last applications of above-mentioned potential fields of N-carbazole containing polymers are described. Polymers 2020, 12, x FOR PEER REVIEW 17 of 33

[43]. In addition to biological activities, these molecules also present opto-electronic, electrochemical, and electrical properties when polymerized, being their main applications such as photovoltaic devices, electroluminescent displays, batteries, bio(sensors), etc. PVK presents adequate levels of photoconductivity to be used is electrophotography, being used by IBM in its Copier I series in 1970. In addition, poly(N-vinylcarbazole) has been widely used as a host material in OLEDs. PVK presents a high energy blue-emissive singlet excited state and could acts as electron donor and hole transporting material. However, since it is not a conjugated polymer its transport is carried out by radical cation hopping among the discrete carbazole units [77]. Since them, many research have been focus on the development of 3,6 disubstitued carbazoles, taking advantage of the high reactivity of 3,6 positions [148]. In this context, linear and hyperbranched 3,6- carbazole derivatives present excellent redox activity, photorefractive and nonlinear optical properties [106,149]. These properties make them good candidates as materials for organic light- emitting diodes (OLEDs). However, as has been previously mentioned, they present several limitations such as their reduced conjugation of electrons and low molecular weight. On the other hand, 2,7-carbazole-based polymers are usually more linear, this improves their organization and the extension of the conjugation length, construing to a lower band gap [150]. These properties are especially interesting for the development of organic field effect transistors (OFETs), bulk- heterojunction (BHJ) solar cells, thermoelectric [84,149]. It is important to notice that synthetic pathway could condition the potential applications. That is, chemical synthesis is more interesting for those applications in which bulk materials are needed, or a control over the morphology is required. In addition, the electropolymerization is more suitable to planar applications in which a coating or a thin film is required [102,151]. In this section, some of the last applications of above-mentioned potential fields of N-carbazole containing polymers are described

3.1. Light Emitting Diode Application Organic light emitting diodes (OLEDs) made from polymers have attracted increasing interest since the group led by Richard Friend in 1992 [37] reported the emission of light from a semiconducting polymer sandwiched between two contacts and connected to a battery. In the Figure Polymers18, an 2020example, 12, 2227 of an OLED system could be observed [152]. OLEDs are arising as one of the18 most of 33 used technologies in full-colour displays and as an environmentally friendly lighting source. The 3.1.present Light excellent Emitting colour Diode Application quality, and they are cost-effective and mercury-free [153]. One to the first examples of PVK based OLED was reported by Hebner et al. [154]. They fabricatedOrganic OLEDs lightemitting with low diodes turn-on (OLEDs) voltages made by ink-jet from printing polymers using have PVK attracted luminescence increasing films interest with sincedyes the coumarin group led 6. Low-molar-mass by Richard Friend carbazole-based in 1992 [37] reported derivatives the emission for organic of light light from emitting a semiconducting diodes have polymerbeen reviewed sandwiched by Krucaite between and two Grigalevicius contacts and [155] connected in a recent to a battery. study. InThey the concluded Figure 18, anthat example carbazole of anbased OLED materials system could could be be used observed as hole [152 transporting]. OLEDs are materials, arising as emitting one of thematerials, most used thermally technologies activated in full-colourdelayed fluorescence displays and emitters as an environmentally and host material friendlys for lightingphosphorescent source. The dopants present and excellent organic colour light quality,emitting and diodes. they are cost-eﬀective and mercury-free [153].

Figure 18. Example of a cross section of a polymer LED. Modiﬁed from [152] MDPI 2014 (CC by 3.0).

One to the first examples of PVK based OLED was reported by Hebner et al. [154]. They fabricated OLEDs with low turn-on voltages by ink-jet printing using PVK luminescence films with dyes coumarin 6. Low-molar-mass carbazole-based derivatives for organic light emitting diodes have been reviewed by Krucaite and Grigalevicius [155] in a recent study. They concluded that carbazole based materials could be used as hole transporting materials, emitting materials, thermally activated delayed fluorescence emitters and host materials for phosphorescent dopants and organic light emitting diodes. A polymer used as light-emission material must meet two basic characteristics, electrical conductivity (semiconductive polymer) and high photoluminescence (PL) efficiency [156]. Among these materials, carbazol-based materials benefit from the wide bandgap of carbazole as well as its remarkable thermal, photochemical, and chemical stability. Especially, the relatively high triplet energy level of carbazole makes it an appealing candidate to design hosts for wide bandgap triplet emitters such as blue dopants [157]. In addition, the substituted carbazoles could cover the range of visible lights from blue to green colour due to the modification of the carbazole backbone with different moieties. For example, the introduction of 3(9)-aryl carbazoles and 3,6-diaryl substituted derivatives were very effective as the host materials for blue (EQE < 24%), green (EQE < 20%) and red (EQE < 19%) phosphorescent organic light emitting diodes [155]. Syutkin et al. [158] reported the synthesis of chalcones with carbazole substituents, being some of them capable to react with guanidine sulphate to produce 2-amino-4,6-diarylpyrimidines. This compound could form a stable coloured conjugated polymer films on the surface of a working electrode under conditions of cyclic voltammetry, being promising materials for the design of light emitting diode. Polymers 2020, 12, x FOR PEER REVIEW 18 of 33

Figure 18. Example of a cross section of a polymer LED. Modified from [152] MDPI 2014 (CC by 3.0).

A polymer used as light-emission material must meet two basic characteristics, electrical conductivity (semiconductive polymer) and high photoluminescence (PL) efficiency [156]. Among these materials, carbazol-based materials benefit from the wide bandgap of carbazole as well as its remarkable thermal, photochemical, and chemical stability. Especially, the relatively high triplet energy level of carbazole makes it an appealing candidate to design hosts for wide bandgap triplet emitters such as blue dopants [157]. In addition, the substituted carbazoles could cover the range of visible lights from blue to green colour due to the modification of the carbazole backbone with different moieties. For example, the introduction of 3(9)-aryl carbazoles and 3,6-diaryl substituted derivatives were very effective as the host materials for blue (EQE < 24%), green (EQE < 20%) and red (EQE < 19%) phosphorescent organic light emitting diodes [155]. Syutkin et al. [158] reported the synthesis of chalcones with carbazole substituents, being some of them capable to react with guanidine sulphate to produce 2-amino-4,6-diarylpyrimidines. This Polymerscompound2020, 12could, 2227 form a stable coloured conjugated polymer films on the surface of a working19 of 33 electrode under conditions of cyclic voltammetry, being promising materials for the design of light emitting diode. Hsiao etet al. al. [141 [141]] reported reported the constructionthe construction of electrochromic of electrochromic devices baseddevices on carbazolebased on derivatives carbazole (poly(3,6-di(carbazol-9-yl)-N-(4-nitrophenyl)-carbazole) (P(NO -3Cz) and poly(3,6-di(carbazol-9-yl)- derivatives (poly(3,6-di(carbazol-9-yl)-N-(4-nitrophenyl)-carbazole)2 (P(NO2-3Cz) and poly(3,6- N-(4-aminophenyl)carbazole) (P(NH -3Cz)) electrodeposited on ITO substrates. In their study, di(carbazol-9-yl)-N-(4-aminophenyl)carbazole)2 (P(NH2-3Cz)) electrodeposited on ITO substrates. In single-layertheir study, single-layer electrochromic electrochromic cells were fabricated,cells were fabricated, sandwich-type sandwich-type device (Figure device 19 (Figurea), to evaluate 19a), to theevaluate potential the potential application application of this films of this in films electrochromic in electrochromic devices. devices. The sandwich The sandwich type devicetype device was fabricatedwas fabricated with with a gel a gel electrolyte electrolyte spread spread on on polymer-coated polymer-coated ITO ITO substrate substrate on on side side ofof thethe electrode, being closed by electrodes. In addition, the possible leakages were prevented by applying epoxy resin seal. The P(NO -3Cz)-based device presents a slightly yellowish colour in its neutral form (0.0 V), seal. The P(NO2-3Cz)-based2 device presents a slightly yellowish colour in its neutral form (0.0 V), and andthe intensity the intensity of the of colour the colour increased increased until untildarker darker yellow yellow when when the potential the potential vas increased vas increased until 2.3 until V 2.3(Figure V (Figure 19b). 19However,b). However, the films the filmsturned turned in to ina blue to a bluecolour colour when when the potential the potential reached reached +3.0 +V.3.0 The V. Thereversibility reversibility of this of this behaviour behaviour was was successfully successfully demonstrated, demonstrated, achieving achieving the the initial light-yellowlight-yellow colourcolour whenwhen thethe potentialpotential waswas removed.removed. On the otherother hand, the colouring and the potential peaks of thethe P(NHP(NH22-3Cz) filmfilm were slightly didifferentfferent dependingdepending onon thethe substitutionsubstitution (Figure(Figure 1919c).c).

Figure 19. (a) Schematic illustration of the structure of the electrochromic devices. Sandwich-type devices, using (b) P(NO2-3Cz) and (c) P(NH2-3Cz). Reproduced with permission from [141]. Copyright (2016) Royal Society of Chemistry.

3.2. Electrochemical Capacitors Supercapacitors are considered as an energy storage system, electrochemical energy in this case, with an important future as an alternative to other classical methods. Considering their power and energy density, these can be located between batteries and traditional dielectric capacitors [159]. Polymers 2020, 12, 2227 20 of 33

According to energy storage mechanism and electrode materials, the main electrochemical capacitors (EC) could be categorized in three families: carbon-based ultracapacitors, metaloxides-based supercapacitors, and conductive polymers based supercapacitors [160]. Polymer-based capacitators could be considered as crucial materials for the development of many high-power electrical systems [161]. Conductive polymers present good electrical conductivity, large specific power, simple synthesis and low cost. This properties have increase the interest on these materials as a promising materials for capacitors compared to expensive and toxic metal oxides, and carbon-based electrode materials [162–164]. Among these conductive polymers, carbazol-based polymers are often complementary parts of the active electrode material in capacitors. That is, when an electric field is applied ions are transfer in and out of the polymer backbone from the electrolyte over the course of the redox process [165]. This process occurs due to excellent attributes of hole transport, relatively high specific capacity, excellent atmospheric stability, in addition to their physical and electronic properties, such as surface morphology, thickness, electrical conductivity, internal resistance, and durability, which directly affect the performance of super capacitors [166]. However, the redox reaction could reduce the stability of the polymer inducing its degradation [167]. Ates and Uludag [168] reported the synthesis and study of the properties of a capacitator based on poly(9H-Carbazole-9-Carbothioic Dithioperoxyanhydride) (P(2CS2Cz)). P(2CS2Cz) was electrodeposited on a glassy carbon electrode (GCE) by cyclicvoltamentry. After the deposition, the capacitance of the films at different concentrations were evaluated, being the low frequency 2 capacitance 0.52 mF cm− ([2CS2Cz] = 0.25 mM) and double layer capacitance 571 µF([2CS2Cz] = 1 mM). Wang et al. [169], have synthesized three porous polycarbazole networks from poly(4,6- tri(9H-carbazol-9-yl)-1,3,5-triazine) (PTCT), poly(4,40,4”-tri-9-carbazolyltriphenylamine) (PTCA), and poly(4,40-bis(9H-carbazol-9-yl)biphenyl) (PBCP) by a chemical oxidative polymerization. The capacitive properties and their capacity for CO2 storage were also studied. These polymers present a charge/discharge rate of 8 s that could be considered as a fast, and a electrochemical capacity 1 of 558 F g− . In addition, the porous network synthesized in this study presents a nanometric pore size, 2 1 around 1nm, and large surface area, 1280 m g− . These characteristics, added to their nitrogen-rich structure, enables an efficient CO2 capture, being of 20.4 wt% at 1 bar and 0 ◦C. The use of carbazole based materials as conjugated microporous polymer was also reported by Huang et al. [134]. Recently, Duran et al. [170] studied the supercapacitive performance poly(carbazole) films electrodeposited on stainless steel (SS) by using 0.1 M supporting electrolyte of lithium perchlorate (LP), sodium perchlorate(SP) and tetrabutylammonium perchlorate (TBAP) in acetonitrile to form PCz(LP), PCz(SP) and PCz(TBAP), respectively. The specific capacitance values obtained for the SS/PCz(LP), 1 SS/PCz(SP) and SS/PCz(TBAP) systems were 133, 64, and 9 F g− , respectively. Authors claimed that PCz(LP) could be considered as adequate material for supercapacitor applications.

3.3. Biosensor Applications Conducting polymers have also successfully employed in biosensor applications [171]. Pernites et al. [53] developed a novel chemosensitive ultrathin films based on carbazole derivatives. The sensoring devices were based on electropolymerized molecular imprinted polymers (E-MIP) capable to detect three drugs, naproxen, paracetamol, and theophylline. The molecular imprinting enables a tailor-made specificity. In this case, the sensor is capable to recognize the selected drugs with a high selectivity, being in situ evaluated by surface plasmon resonance (SPR). Among, the different monomers used in this study author reported that bifunctional monomers containing –COOH and –OH functional groups were most effective for the imprinting process. Moreover, authors claimed that this method could be a promising approach for sensors fabrication. Figure 20 summarizes the E-MIP based sensor fabrication reported in this study. Polymers 2020, 12, x FOR PEER REVIEW 20 of 33

monomers used in this study author reported that bifunctional monomers containing –COOH and – OH functional groups were most effective for the imprinting process. Moreover, authors claimed that this method could be a promising approach for sensors fabrication. Figure 20 summarizes the E-MIP Polymersbased 2020sensor, 12, 2227fabrication reported in this study. 21 of 33

FigureFigure 20. 20. (a(a)) SensorSensor ﬁlmfilm fabrication fabrication by by molecular molecular imprinting imprinting and and template template removal removal by by constant constant potentialpotential wash wash atat 0.40.4 V (versus Ag/AgCl). Ag/AgCl). (b (b) )ESPR ESPR in in situ situ set-up set-up for for electropolymerization electropolymerization and and and and(c) (SPRc) SPR sensing sensing of ofthe the imprinted imprinted guest guest molecule molecule us using.ing. Reproduced Reproduced with with permissionpermission fromfrom [53[53].]. CopyrightCopyright (2011) (2011) Elsevier. Elsevier.

NovelNovel fluorescentfluorescent andand conductiveconductive hollowhollow microspheresmicrospheres basedbased onon anilineaniline andand carbazolecarbazole derivativesderivatives werewere successfullysuccessfully synthesizedsynthesized byby ChengaChenga and co-workers [172]. [172]. The The poly(aniline-co-3- poly(aniline-co- 3-amino-9-ethyl-carbazole)amino-9-ethyl-carbazole) (PAC) (PAC) obtained obtained by by oxid oxidativeative copolymerization with APSAPS waswas usedused asas eeffectiveffective fluorophore. fluorophore. The The sensoring sensoring capacity capacity of of the the materials materials were were evaluated evaluated for for two two types types of of analyte, analyte, 2+ namelynamely nitro-based nitro-based explosives explosives and and Cu Cu2+ cations,cations, withwith thethe reportedreported detectiondetection limitslimitsof of the the material material beingbeing 5 5µ MμM and and 5 nM,5 nM, respectively. respectively. Tested Tested analytes analytes decreased decreased the fluorescencethe fluorescence with thewith increase the increase of their of concentration,their concentration, as can beas observedcan be observed in Figure in 21 Figu wherere 21 the where results the obtained results forobtained nitrobenzene for nitrobenzene detection Polymersaredetection shown. 2020 are, 12, xshown. FOR PEER REVIEW 21 of 33

Figure 21. ((aa)) Study Study of of fluorescence ﬂuorescence emission emission quenching quenching of of PAC/ethanol PAC/ethanol dispersion at different diﬀerent

nitrobenzene/ethanolnitrobenzene/ethanol solutions; ( b)) Stern-Volmer plot between F/F F/F0 andand nitrobenzene nitrobenzene concentration. concentration. Reproduced with permission from [[172].172]. Copyright (2019) Elsevier.

Shakir et al. reported the development of PCz/TiO2 nanocomposite with good antibacterial activity, which presents also good capability for sensing ammonia. The evaluation of its capability for ammonia sensing was carried out by measuring resistivity changes on exposure to ammonia vapours. The nanocomposite showed a relatively fast response toward aqueous ammonia in the range of 0.25–1 M at room temperature (Figure 22).

Figure 22. Effect on the resistivity of PCz/TiO2 nanocomposite on exposure to different concentrations of ammonia with respect to time. Reprinted with permission from Shakir et al. [100]. Copyright (2014) American Chemical Society.

3.4. Photovoltaic Devices Applications Solar cells or photovoltaic devices capable to convert the solar energy into electric energy are under continuous development pushes by the need of greener electric sources. In this context, Li et al. [54], in addition to other authors [173–177], described in their review that carbazole-based polymers and copolymers could be considered among the most promising materials for highly efficient organic solar cells. Fujita and Michinobu [178] synthesized a carbazole-based conjugated polymer as donor– acceptor type alternating copolymers. Different type of poly(1,8-diethynylcarbazole) were synthesized and their fluorescence and electrochemical properties were evaluated being adequate to

Polymers 2020, 12, x FOR PEER REVIEW 21 of 33

Figure 21. (a) Study of fluorescence emission quenching of PAC/ethanol dispersion at different Polymersnitrobenzene/ethanol2020, 12, 2227 solutions; (b) Stern-Volmer plot between F/F0 and nitrobenzene concentration.22 of 33 Reproduced with permission from [172]. Copyright (2019) Elsevier.

Shakir et al.al. reported reported th thee development of PCzPCz/TiO/TiO22 nanocomposite with good antibacterialantibacterial activity, whichwhich presentspresents alsoalso goodgood capabilitycapability forfor sensingsensing ammonia.ammonia. The evaluation of its capability for ammonia sensing was carried out by measuringmeasuring resistivity changes on exposure to ammoniaammonia vapours. TheThe nanocompositenanocomposite showed showed a relativelya relatively fast fast response response toward toward aqueous aqueous ammonia ammonia in the in range the ofrange 0.25–1 of 0.25–1 M at room M at temperatureroom temperature (Figure (Figure 22). 22).

Figure 22. EffectEffect on the the resistivity resistivity of PCz/TiO PCz/TiO2 nanocompositenanocomposite on on exposure exposure to to different different concentrations of ammonia with respect to time. Reprinted with pe permissionrmission from Shakir et al. [100]. [100]. Copyright Copyright (2014) (2014) American Chemical Society. 3.4. Photovoltaic Devices Applications 3.4. Photovoltaic Devices Applications Solar cells or photovoltaic devices capable to convert the solar energy into electric energy are under Solar cells or photovoltaic devices capable to convert the solar energy into electric energy are continuous development pushes by the need of greener electric sources. In this context, Li et al. [54], under continuous development pushes by the need of greener electric sources. In this context, Li et in addition to other authors [173–177], described in their review that carbazole-based polymers and al. [54], in addition to other authors [173–177], described in their review that carbazole-based copolymers could be considered among the most promising materials for highly efficient organic polymers and copolymers could be considered among the most promising materials for highly solar cells. efficient organic solar cells. Fujita and Michinobu [178] synthesized a carbazole-based conjugated polymer as donor–acceptor Fujita and Michinobu [178] synthesized a carbazole-based conjugated polymer as donor– type alternating copolymers. Different type of poly(1,8-diethynylcarbazole) were synthesized and acceptor type alternating copolymers. Different type of poly(1,8-diethynylcarbazole) were their fluorescence and electrochemical properties were evaluated being adequate to be used as a synthesized and their fluorescence and electrochemical properties were evaluated being adequate to p-type semiconductor in solar cells. Synthesized poly(1,8-carbazole)s were used in the preparation of bulk-heterojunction photovoltaic cells, photoconversion efficiency (PCE) of the cells was between 0.05% and 0.24%. Similarly, Qin and co-workers [179] synthesized a series of conjugated polymers with carbazole as the donor unit or benzothiazole as the acceptor unit to be used in solar cells presenting a PCE between 5.8% and 0.43%.

3.5. Memory Device Applications In addition to the other possible applications, conducting polymers could be also used in the development of advanced memory devices due to their donor-acceptor (D-A) properties [136,180]. These D-A polymers present an electrically bistable behavior, this characteristic being highly desirable for memory devices. In addition, they present other characteristics that make them highly interesting, tailor-made structures, namely low-cost, processability (mainly in solution), and three- dimensional stacking capability [181]. In donor−acceptor (D-A) conjugated polymer systems, the charge transfer between donor and acceptor moieties could be controlled by the strength and ordering of donor and acceptor groups. These parameters play a critical role in enhancing electrically bistable switching behavior [182,183]. The most efficient way to enhance the intrinsic local packing of materials is to increase the planarity of the polymer main chain, which in turn can generate tight p–pPolymers stacking,2020, 12 because, 2227 the fused ring structures are much flat than the conjugated ‘‘single’’ bond 23[184]. of 33 In general, the incorporation of different electron acceptors into conjugated polymeric donors significantlythe incorporation affects of dithefferent memory electron properties acceptors into[185]. conjugated The p-type polymeric conductive donors polymers significantly could aff ectsbe employedthe memory as propertiesthe donor [materials185]. The and p-type then conductive they are mixed polymers with could a kind be of employed commercial as the acceptor donor materialmaterials [176]. and thenAmong they them, are mixed the poly(N-alkyl-2,7-c with a kind of commercialarbazole)s acceptorbased D-A material copolymers [176]. presents Among them,wide potentialthe poly(N-alkyl-2,7-carbazole)s owning to their excellent based hole-transporting D-A copolymers property presents and widegood potentialstability of owning the carbazole to their unitsexcellent [186]. hole-transporting property and good stability of the carbazole units [186]. Hahm etet al. al. [187 [187]] synthesized synthesized several several poly(2,7-carbazole) poly(2,7-carbazole) derivatives derivatives (Figure 23 (Figure) and successfully 23) and successfullystudied their electricalstudied memorytheir electrical properties. memory In order pr tooperties. evaluate In their order memory to evaluate characteristics, their amemory memory characteristics,device was fabricated a memory (Figure device 23). was First, fabricated aluminium (Figure was deposited 23). First, onaluminium glass substrates was deposited by electron on beamglass substratessputtering, by then electron the di ffbeamerent spu polymerttering, substrates then the weredifferent deposited polymer and, substrates finally, additional were deposited aluminium and, finally,electrodes additional were deposited aluminium on theelectrodes top thermal were evaporation. deposited on All the tested top thermal polymers evaporation./aluminum All sandwich tested polymers/aluminumtype devices presented sandwich similar dynamictype devices random-access presented similar memory dynamic (DRAM). random-access They present ONmemory/OFF (DRAM).current ratios They between present 10ON/OFF5 and 10 current9. Considering ratios between the obtained 105 and results 109. Considering these materials the couldobtained be suitable results thesefor the materials production could of low-costbe suitable programmable for the producti DRAMon of devices low-cost with programmable a high-performance DRAM and devices capable with to aoperate high-performance under very lowand powercapable consumption. to operate under very low power consumption.

Figure 23. (a) Structure of poly(2,7-carbazole) derivate polymers obtained by Hahm et al.; (b) Scheme diagram of the memory devices fabricated and (c) obtained electron density proﬁles. Reproduced with permission from [187]. Copyright (2011) American Chemical Society.

Similar system was reported by Zhang and co-workers [188]. In their study, they described a write-once, read-many-times (WORM) memory effect in sandwich type devices fabricated by using different carbazole derivatives, aluminum, and ITO. These poly(carbazole)-based polymers present a characteristic donor–trap–acceptor (D-T-A) structure and an electrical bistability due to the field-induced charge-transfer interactions. The results indicated that the devices could present a non-volatile non-erasable memory behaviour. Structural and electrical characterization of a block copolymers based unipolar non-volatile memory device were fabricated by Kang et al. [189]. In this study, a crucial factor was to adjust the block ratio of poly(9-(4-vinylphenyl)carbazole)-b-poly(2-vinylpyridne) (PVPCz-b-P2VP) copolymer in order to present a lamellar structure that could induce a unipolar switching behavior. This switching property was highly interesting for the fabrication Aluminum/PVPCz-b-P2VP/indium tin oxide (ITO) based memory devices (Figure 24). The memory behavior studies of these devices revels a good endurance cycling, around to 190 cycles, and a high ON/OFF ratio, higher than 104. Polymers 2020, 12, x FOR PEER REVIEW 23 of 33

Figure 23. (a) Structure of poly(2,7-carbazole) derivate polymers obtained by Hahm et al.; (b) Scheme diagram of the memory devices fabricated and (c) obtained electron density profiles. Reproduced with permission from [187]. Copyright (2011) American Chemical Society.

Similar system was reported by Zhang and co-workers [188]. In their study, they described a write-once, read-many-times (WORM) memory effect in sandwich type devices fabricated by using different carbazole derivatives, aluminum, and ITO. These poly(carbazole)-based polymers present a characteristic donor–trap–acceptor (D-T-A) structure and an electrical bistability due to the field- induced charge-transfer interactions. The results indicated that the devices could present a nonvolatile non-erasable memory behaviour. Structural and electrical characterization of a block copolymers based unipolar non-volatile memory device were fabricated by Kang et al. [189]. In this study, a crucial factor was to adjust the block ratio of poly(9-(4-vinylphenyl)carbazole)-b-poly(2-vinylpyridne) (PVPCz-b-P2VP) copolymer in order to present a lamellar structure that could induce a unipolar switching behavior. This switching property was highly interesting for the fabrication Aluminum/PVPCz-b-P2VP/indium tin oxide (ITO) based memory devices (Figure 24). The memory behavior studies of these devices revels Polymers 2020, 12, 2227 24 of 33 a good endurance cycling, around to 190 cycles, and a high ON/OFF ratio, higher than 104.

FigureFigure 24.24. ((aa)) AA SchemeScheme of of the the fabricated fabricated organic organic memory memory device device (Al (Al/polymer/ITO);/polymer/ITO); (b) ( Structureb) Structure of the of blockthe block copolymer copolymer of PVPCz- of PVPCzb-P2VP.-b-P2VP. Reproduced Reproduced with permissionwith permission from [ 189from]. Copyright[189]. Copyright (2011) Wiley.(2011) Wiley. 4. Conclusions and Future Perspective 4. ConclusionsPolycarbazole and and Future its derivatives Perspective have been extensively used for long time. This review describes the synthesis and applications of carbazole-based materials on the last 10 years. However, as can be Polycarbazole and its derivatives have been extensively used for long time. This review observed by the increase in the number of publications in the field of conductive polymers, there is describes the synthesis and applications of carbazole-based materials on the last 10 years. However, an increasing demand for conductive polymers in several applications, such as light emitting diodes as can be observed by the increase in the number of publications in the field of conductive polymers, (OLEDs), capacitators, or memory devices, among others. This demand has renewed the interest on there is an increasing demand for conductive polymers in several applications, such as light emitting carbazole-based materials. diodes (OLEDs), capacitators, or memory devices, among others. This demand has renewed the Considering the synthesis of carbazole-based polymers, the electrochemical polymerization has interest on carbazole-based materials. revealed the most employed methodology since many applications requires of a thin film of conductive Considering the synthesis of carbazole-based polymers, the electrochemical polymerization has polymer and this technique is highly suitable for film development. On the other hand, several revealed the most employed methodology since many applications requires of a thin film of research works have focused their attention to the development of new carbazole-based monomers conductive polymer and this technique is highly suitable for film development. On the other hand, with complex structures in order to improve or tailor the properties of these materials, and thereby several research works have focused their attention to the development of new carbazole-based increasing their potential applications. The variation in the structure of the carbazole derivatives could monomers with complex structures in order to improve or tailor the properties of these materials, improve their processability and solubility, but also increase the conjugation length of the electrons and thereby increasing their potential applications. The variation in the structure of the carbazole through their structure, or vary their opto-electrical properties. In this context, it is important to derivatives could improve their processability and solubility, but also increase the conjugation length highlight the emergence of flexible and wearable electronic devices as a part of the internet of the things of the electrons through their structure, or vary their opto-electrical properties. In this context, it is (IoT). In fact, the carbazole-based materials could have a promising future. The need for conductive important to highlight the emergence of flexible and wearable electronic devices as a part of the polymers capable to respond adequately to highly demanding requirements (mechanical or conductive, internet of the things (IoT). In fact, the carbazole-based materials could have a promising future. The among others) could encourage the development of new carbazole-based materials. Induced by the emergence of the internet of the things, flexible and wearable electronic devices have attracted great interest in the last years. These devices have gradually emerged in daily life due to their lightweight, the ability to attach onto clothes, or easy skin attachment potential, and their ability to withstand mechanical deformation. Among their applications, one could highlight portable displays, human activity monitoring sensors, and self-powered devices. The high physicochemical requirements, such as high electrical conductivity, good tensile strength, high flexibility, and light weight, fast oxidation/reduction reaction kinetics have distinguished CPs as excellent candidates. Carbazole based polymers are promising for constructing flexible energy harvesting and storage devices.

Author Contributions: E.H., I.M., R.M., M.H., L.R.-R. and J.L.V.-V. contributed the conception of the presented work. F.B. (Fadila Bekkar), F.B. (Faiza Bettahar), and L.R.-R. wrote the first draft of the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The F.B. (Fadila Bekkar) and F.B. (Faiza Bettahar) gratefully acknowledge Algerian Government for their PhD grants and travel grants (N◦ 128/PNE and N◦ 151/PNE). Conflicts of Interest: The authors declare no conflict of interest. Polymers 2020, 12, 2227 25 of 33

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