Group 14 Metalloles Condensed with Heteroaromatic Systems

Org. Photonics Photovolt. 2016; 4:52–59

Review Article Open Access

Joji Ohshita* Group 14 metalloles condensed with heteroaromatic systems

DOI: 10.1515/oph-2016-0006 deficient silole and electro-rich thiophene would cause Received December 6, 2016; accepted December 22, 2016 strong electronic interaction between these rings [9, 10]. Abstract: There has been keen interest in group 14 met- Although such intrasystem donor-acceptor (D-A) type in- alloles as building units of conjugated organic functional teraction cannot be clearly seen, DTS has enhanced conju- σ π materials. This short review summarizes our recent work gation ascribable to its high planarity and *- * conjuga- on group 14 metalloles condensed with heteroaromatic tion, similar to simple siloles [11]. This review summarizes systems, including thiophene and pyridine. These con- our recent work on the synthesis, properties and function- densed metalloles show interesting properties depending alities of DTS derivatives and related compounds. on both the group 14 metallole elements and the heteroaromatic systems. High planarity of the condensed systems and interaction between the element σ*-orbital and 2 Dithienosiloles the heteroaromatic π*-orbital enhance the conjugation in these systems leading to their potential applications as In 1998, we reported the synthesis of DTSs for the first functional materials, such as carrier transporting materi- time [9]. DTSs are typically prepared by the reactions of als and emissive materials. 3,3’-dilithio-4,4’-bis(trimethylsilyl)-2,2’-bithiophene with Keywords: fluorescence, phosphorescence, organic semi- dichloro and difluorosilanes (Scheme 1) [9, 10]. The two conductor, sensitizing dye trimethylsilyl groups are readily replaced with halo- gen atoms to allow further transformation, including oligomerization and polymerization. Simple DTS com- 1 Introduction pounds that have no conjugated substituents on the thiophene rings have UV absorption bands around 350 nm, Since Tamao and coworkers reported the use of silole longer than those of similarly substituted bithiophene derivatives as efficient electron-transporting materials for and dithienocyclopentadiene, DTC (cyclopentadithio- organic light emitting diodes (OLEDs) in 1996 [1], many pa- phene, CPDT), by approximately 50 and 30 nm, respec- pers have appeared to date that describe the synthesis and tively [10, 11]. Fig. 1 represents the HOMO and LUMO en- properties of silole derivatives [2–8]. Their applications as ergy levels and their profiles of DTS, together with those of functional materials, such as photo and electrolumines- DTC, as derived from density functional theory (DFT) cal- cent materials, carrier transporting materials and sensing culations. Both the HOMO and LUMO of DTS lie at lower dyes have been widely explored. The bonding interaction energy levels than those of DTC. The latter is ascribable between the silicon σ*-orbital and the cyclic butadiene π*- to the σ*-π* conjugation, as observed for simple non- orbital (σ*-π* conjugation) stabilizes the silole LUMO. This condensed siloles (vide supra) [2, 3]. The former arises interaction results in electro-accepting properties of silole from reduced anti-bonding interaction between the thio- and reduces the HOMO-LUMO energy gap. This is in con- phene ring systems by introducing longer Si–C bonds. trast to the carbon congener, cyclopentadiene, where the This is also a generally observed characteristic of siloles. presence of such orbital interaction is unclear [2, 3]. Consequently, DTS has a smaller HOMO-LUMO energy gap We initiated studies concerning the synthesis of than DTC, agreeing with the experimental data — that thiophene-fused silole, namely dithienosilole (DTS), as is, DTS usually has a UV absorption band at lower en- the first example of siloles condensed with heteroaromatic ergy and its cyclic voltammogram (CV) reveals the anodic systems, expecting that ring-condensation of electro- peak at slightly higher or similar potential in comparison with those of the DTC analogs. Based on the low-lying *Corresponding Author: Joji Ohshita: Department of Applied HOMO and LUMO, several DTS derivatives were applied to Chemistry, Graduate School of Engineering, Hiroshima University, OLED as electron-transporting materials with good hole- Higashi-Hiroshima 739-8527, Japan, E-mail: [email protected] © 2016 Joji Ohshita, published by De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License. Joji Ohshita, Group 14 metalloles condensed with heteroaromatic systems Ë 53 blocking properties [10, 12, 13]. In particular, a vapor de- phenylene (pDTS-T and pDTS-2T in Chart 1) as semi- posited film of DTS-Py (Chart 1) was introduced to OLED conducting materials of p-type organic thin film transis- as an electron-transporting layer, improving the device tors (TFT) with good mobility [14]. A little later, an excel- performance [12]. However, in highly conjugated systems, lent photovoltaic polymer was prepared by Yang et al., such as π-conjugated polymers, the σ*-π* conjugation is which was composed of bis(2-ethylhexyl)dithienosilole not efficiently operative in DTS, with minimal effect onthe and benzothiadiazole alternating units (pDTS-BT) [15]. A LUMO energy levels [11]. bulk hetero-junction polymer solar cell (BHJ-PSC) based on a blend film of pDTS-BT and PC71BM was reported to achieve a photo-current conversion efficiency of 5.1%, which was higher than that reported for a similar cell based on the DTC-based analogous polymer [16]. These two excellent findings triggered a wide variety of subse- quent studies concerning the use of DTS-based conjugated polymers as optoelectronic device materials [4, 7, 8, 11]. We have recently demonstrated that disilanobithiophene Scheme 1. Synthesis of DTS. (DSBT) is also applicable as a component of π-conjugated photovoltaic polymers, as shown in Chart 2 [17–20]. A blend film of a DSBT-containing conjugated polymer (pDSBT-BT) with PC71BM was applied to BHJ-PSC with higher voltage and thereby with higher photo-current conversion efficiency (6.38%) [17], compared to the similar pDTS-BT-based cell reported by Yang et al. [15]. This is primarily ascribable to appropriate twisting of the bithiophene unit of DSBT, which lowers the HOMO energy level of the polymer. It is well known that the lower-lying HOMO of the polymer basically leads to a higher voltage from the polymer-based cell.

Fig. 1. HOMO and LUMO energy levels and profiles for DTC and DTS, as derived from DFT calculations at the B3LYP/6-31G(d,p) level of theory. The σ*-π* conjugation is seen in the LUMO of DTS. Reprinted with permission from ref 11. Copyright (2009) WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Chart 2. DSBT-containing photovoltaic polymers.

Chart 1. DTS derivatives. The interesting use of DTS-containing polymers as OLED materials has been explored As mentioned above, we have demonstrated that vapor-deposited films of DTS Currently, DTS has been extensively studied as a build- derivatives, including DTS-based conjugated oligomers, ing unit of conjugated polymers. In 2006, Marks and are useful as electron-transporting layers of OLEDs. How- coworkers reported the applications of alternating copoly- ever, the introduction of DTS units into polymeric systems mers of dihexyldithienosilole with thienylene and bithio- 54 Ë Joji Ohshita, Group 14 metalloles condensed with heteroaromatic systems enables their use as hole-transporting materials and emissive materials, as summarized in the previous review [11]. DTSs are types of bithiophene units and thus essentially possess hole-affinity and are usable as hole-transporting materials. The rigid tricyclic systems lead to highly photolumi- nescence (PL) properties of the DTS-based compounds. Phosphorous-substituted DTSs are highly emissive and Fig. 2. DTS-based compounds with donor/acceptor units and their one of those derivatives (DTS-P in Chart 3) shows aggrega- photos under UV-irradiation. The Photo of DTS-B is reproduced from Ref. [22] with permission from The Royal Society of Chemistry. tion enhanced emission with a higher PL efficiency (Φ = 0.79) in the solid state than that in the solution phase (Φ = 0.22) [20]. This is likely due to the sterically bulky phosphorous substituents that cover the DTS chromophore to avoid concentration quenching. DTS-PO was applied as a blue-green colored electro luminescent material in OLEDs. A gold complex DTS-PAu was prepared, showing efficient blue-colored PL in the solid state (Φ = 0.33). This complex has a higher PL efficiency in solution (Φ = 0.99) [21]. Boryl- substituted DTSs also show highly PL properties in solid states (Fig. 2) [22]. Bis(dimesitylboryl)-DTS (DTS-B) shows sensing properties, responding to fluoride anion in the solution phase. Its UV-vis absorption spectra change clearly upon contact with fluoride anions, likely ascribable to the Fig. 3. DTS-containing π-conjugated oligomers and a photo of their B-F complex formation. solutions under UV irradiation. Reprinted with permission from ref. [23]. Copyright (2006) American Chemical Society.

The reaction of DTS with tetracyanoethylene in DMF provides DTS-TCNE, as presented in Scheme 2 [24]. The DTS unit is involved in this system as an electro- donor, forming a D-A system with the strongly electro- Chart 3. Highly PL DTS-based compounds. accepting tricyanoetheny group. DTS-TCNE exhibits excellent film forming properties and its spin-coating and Efficient conjugation in DTS allows the use of the DTS- vapor-deposition provide fine thin solid films. Interest- 2,6-diyl system as an effective linker (π) of the donor- ingly, the films undergo quick color change upon expo- π-acceptor (D-π-A) type compounds. The D-DTS-A com- sure to an organic solvent vapor — such as ethanol, ace- pounds with methylthio as the donor and perfluorotolyl tone, and hexane, as illustrated in Fig. 4 — and thus is or dimesitylboryl as the acceptor (S-DTS-Ft and S-TDS- potentially useful for sensing volatile organic compounds B) have red-shifted emission maxima as compared with (VOCs). This process is reversible and a short contact with those of the corresponding symmetric A-DTS-A (DTS-Ft a chloroform vapor recovers the original color. The mech- and DTS-B) compounds. However, the absorption bands anism of the color changes is not clear yet, but XRD anal- of D-DTS-A compounds are at slightly higher energies than ysis of the film showed a slight shift of the diffraction those of the A-DTS-A compounds (Fig. 2) [22]. The ob- peak position accompanying the color change. This seems served larger Stocks-shifts for the D-DTS-A compounds to indicate that the packing mode of the molecules in are likely ascribable to the photo-induced intramolecu- the film changes when exposed to organic solvent va- lar charge transfer (ICT). We also prepared several DTS- pors. Attachment of a MeS or thienyl donor unit on DTS DTS-TCNE containing conjugated oligomers [23]. They show PL prop- of red-shifts the UV absorption maxima, indi- erties where the PL color is finely controllable by tuning cating possible color tuning of this type of VOC-sensing S-DTS- the conjugation length (Fig. 3). materials by modifying the chemical structure ( TCNE and T-DTS-TCNE) [25]. They also show clear sol- vatochromism, as presented in Fig. 5. DTS-linked D-A Joji Ohshita, Group 14 metalloles condensed with heteroaromatic systems Ë 55 type systems have also attracted much attention as excellent photo-sensitizing dyes for dye-sensitized solar cells (DSSCs). Indeed, a high photo-current conversion of 10.3% was achieved based on a DTS-containing dye by Wang and coworkers (Chart 4) [26].

Fig. 5. Photos of solutions of DTS-TCNE (Ar = p-tol) in ethyl acetate and chloroform, and T-DTS-TCNE in ethyl acetate and chloroform, from left to right.

Scheme 2. Synthesis of DTS-TCNE.

Chart 4. DTS-based sensitizing dye for DSSC.

vapor-deposited film, and is potentially usable as an ac- tive material for TFT. Although the mobility of the sDTS film was not yet very high (1.4 × 10−6 cm2V−1s−1), it seems interesting that such a bithiophene derivative with limited π-conjugation exhibits TFT activity [28]. Spiro- condensed dithienothiasiline derivatives (sDTTS) were also prepared [29].

3 Dithienogermole

Dithienogermole (DTG) was first prepared and polymer- ized to form photovoltaic polymers in three independent Fig. 4. UV spectra of a vapor deposited film of DTS-TCNE (Ar = p-tol) groups at similar times (Chart 6) [30–33]. Following these on a quartz plate, (red line) before and (blue line) after a short con- original works, DTG has been extensively studied as a tact with ethanol vapor. Inset is a photo of the film. The bottom half π was treated with ethanol. Reprinted with permission from ref. [23]. building unit of photovoltaic -conjugated polymers, sim- Copyright (2002) American Chemical Society. ilarly to DTS. Quantum chemical calculations of DTG sug- gest that DTG should have essentially the same electronic structure as that of DTS. However, replacing DTS units As derivatives of DTS, dithienothiasilines (DTTS) and of photovoltaic polymers by DTG sometimes improves the a DTTS-based polymer (pDTTS-S) were prepared and cell performance. Enhanced intermolecular interaction for their hole-transporting properties have been studied in the DTG-containing polymers in film states raises the pho- an OLED system (Chart 5) [27]. Spiro-condensed DTS tocurrent [34]. (sDTS) shows p-type semi-conducting properties in its 56 Ë Joji Ohshita, Group 14 metalloles condensed with heteroaromatic systems

mophores in the DTG-PSQ1 polymeric system appears to cause concentration quenching. In contrast, copolymer- ization of the DTG monomer with methyltrimethoxysilane successfully provides highly emissive films of DTG-PSQ2. The PL efficiencies of the copolymers depend strongly on the monomer ratios; a maximal value of Φ = 0.33 was achieved with DTG monomer/methyltrimethoxysilane = 1/99. Incorporation of carrier conducting carbazole units in the polysilsesquioxane provides a film (DTG-PSQ3) that exhibits electroluminescence properties in OLEDs.

Chart 5. DTTS, and spiro-condensed DTS and DTTS.

Scheme 3. Preparation of DTG-containing oligo and polysilsesquiox- Chart 6. Examples of DTG-containing photovoltaic π-conjugated anes. polymers.

Generally, DTGs can be prepared by the reactions Another advantage in the use DTG in the place of DTS of dichlorogermanes with dilithiobithiophenes in higher owes to its chemical stability. DTS is less stable and treat- yields than the DTS congeners. This is ascribable to longer ment of DTS with an alkali solution in THF/water leads and thus more flexible Ge-C bonds than Si-C bonds, which to the cleavage of Si–C (thiophene) bonds, forming Si– makes the ring formation easier by reducing the ring OH and H–C (thiophene) bonds. In contrast, DTG remains strain. In addition, Ge-Cl bonds are less polar and less unchanged under the same conditions [35]. This chem- reactive than Si-Cl bonds and the ring forming reactions ical stability permits the preparation of DTG-containing proceed in a milder fashion for DTGs than that for DTSs. oligo and polysilsesquioxanes under alkali conditions, as Reflecting the lower reactivity of Ge-Cl bonds, 4,4-DTG shown in Scheme 3 [36]. The polyhedral oligosilsesquiox- dichlorides (DTG-Cl) are readily prepared from the reac- anes with DTG units at the corners (DTG-POSS) have mod- tions of tetrachlorogermane with dilithiobithiophenes as erately high PL properties (Φ = 0.54), similar to the corre- shown in Scheme 4 [37]. This contrasts with similar responding DTG monomer. Each DTG chromophores of DTG- actions of tetrachlorosilane that always provide the re- POSS are oriented to dispersed directions from the POSS spective spiro-compounds as the major products. Dichlo- core and are thus electronically separated from each other, rides DTG-Cl undergo numerous transformation by react- despite the accumulation of eight DTG units in a molecule. ing with nucleophiles and reducing agents, as shown in The random polysilsesquioxane (DTG-PSQ1) is obtained Scheme 4 [38]. Wurtz type coupling of DTG-Cl with sodium as a solid film. This film has a low PL efficiency (Φ = 0.02), gives the corresponding polygemanes pDTG. in contrast to DTG-POSS. The interaction of DTG chro- Joji Ohshita, Group 14 metalloles condensed with heteroaromatic systems Ë 57

We also prepared dithienostannnole (DTSn) deriva- reveal much less efficiency (Φ < 0.02) than those of DTS tives DTSn1 and DTSn2 (Chart 7) [39]. Interestingly, and DTG derivatives (vide supra). However, they are phos- DTSn1 shows crystallization-enhanced emission, whereas phorescent at low temperature even as solids. Quantum DTSn2 does not. DTSn1 has a much higher PL quantum efficiency of the solid state phosphorescence of DPyG1 at yield as crystals (Φ = 0.556), emitting blue light, than 77K is Φ = 0.22, whereas those of DPyS1 and DPyS2 are those in the solution and amorphous phases with Φ = less than 0.01. It is speculated that the heavy atom effects 0.009 and 0.028, respectively. It should be also noted that of germanium of DPyG1 enhance the phosphorescent the σ*-π* interaction is operative in DTSn, although the properties. degree of the σ*-π* interaction seems to be minimized to an extent in comparison to DTS and DTG.

Scheme 5. Synthesis and reactions of dipyridinosilole and dipyridinogermole.

Viologen derivatives are known as electro-deficient compounds that exhibit reversible electrochromic properties on cathodes [41]. DPyG1 was treated with methyl Scheme 4. Preparation and reactions of DTG dichlorides. iodide to form a viologen derivative (DPyG1-Me) with pronounced electro-deficient properties, as compared with the corresponding germole-free simple viologen. DPyG is useful also as a ligand and its copper complex (DPyG1-Cu) shows clear room temperature phosphorescence in the solid state as well as in solution [42]. Spiro(dipyridinogermole)(dithienogermole)s, sDPyDTG, sDPyDTG-Br, and sDPyDTG-2T were also prepared from DTG-Cl [43]. Of these, sDPyDTG-2T shows excellent Chart 7. Dithienostannnoles. photo-sensitizing properties for singlet oxygen generation [44]. Generation of singlet oxygen is of current interest because of potential applications to photo catalytic systems and photo dynamic therapies. Spiro-condensed dipyridinogermole-dithienogermole is a new system for 4 Dipyridinosilole and germole the singlet oxygen generation. The germanium heavy atom effects, enhancing the intersystem crossing between the Silole and germole (DPyS1, DPyS2, and DPyG1) con- photo-excited singlet state and the triplet state, seem to be densed with electro deficient bipyridyl were prepared responsible for the sensitizing properties. (Scheme 5) [40]. These dipyridinosilole and germole Siloles and germoles condensed with benzothio- exhibit pronounced electro deficiency compared to phene (BBTS) [45], indole (DIS) [45] and benzofuran bipyridyl, which arises from the σ*-π* conjugation. Their (BBFS and BBFG) [46] have been synthesized (Chart 8). PL properties were first studied at room temperature, to Bis(benzofurano)metalloles BBFS and BBFG undergo in- 58 Ë Joji Ohshita, Group 14 metalloles condensed with heteroaromatic systems teresting reversible [2+2] dimerization upon crystalliza- [2] Yamaguchi S., Tamao K., Silole-Containing Sigma- and π- tion from a certain solvent. The dimers revert to the Conjugated Compounds, J. Chem. Soc., Dalton Trans., (22), 1998, 3693. monomers quantitatively when dissolved in organic sol- [3] Yamaguchi S., Tamao K., A Key Role of Orbital Interaction vents, as shown in Scheme 6. in the Main Group Element-Containing π-Electron Systems, Chem. Lett. 34 (1), 2005, 2. [4] Ponomarenko S. A., Kirchmeyer S., Conjugated Organosilicon Materials for Organic Electronics and Photonics, Adv. Polym. Sci., 235, 2011, 33. [5] Chen J., Cao Y., Development of Novel Conjugated Donor Poly- mers for High-Eflciency Bulk-Heterojunction Photovoltaic Devices, Acc. Chem. Res., 42 (11), 2009, 1709. 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