Synthesis of Unsymmetrically Condensed Benzo- and Thienotriazologermoles

Germoles and siloles unsymmetrically condensed with heteroaromatic units are attracting much interest. In this study, compounds containing a triazologermole core unit condensed with a benzene or thiophene ring were prepared. Thienotriazologermole was subjected to bromination to obtain the bromide, which underwent transformation via the palladium-catalyzed Stille coupling reaction to form triphenylamine-substituted thienotriazolegermole, with an effective extension of conjugation. The electronic states and properties of these triazologermole derivatives are discussed on the basis of optical and electrochemical measurements and density functional theory calculations. Triphenylamine-substituted thienotriazolegermole showed clear solvatochromic properties in photoluminescence measurements, suggesting that intramolecular charge transfer occurs at the photo-excited state. This clearly indicates that the triazologermole unit is useful as an acceptor of donor–acceptor compounds. The potential application of triphenylamine-substituted thienotriazolegermole as a sensing material was also explored.


Introduction
Triazoles are five-membered aromatic ring systems containing two double bonds and three sequentially arranged nitrogen atoms.In addition to their distinctive structural properties such as a high dipole moment, molecular rigidity, and promising photochemical characteristics, their excellent accessibility and ease of modification make triazoles highly appealing.Moreover, triazoles serve as Lewis bases to facilitate intermolecular interactions with Lewis acids.Triazole derivatives constitute an important class of heterocycles for applications in pharmaceuticals [1], biologically active agents that can interact with biologically active substances through such interactions as hydrogen bonding [2], and other functional materials such as mechanochromic, photoluminescent, and fluorosensing materials [3][4][5][6][7][8], and have been demonstrated to serve as building blocks for the synthesis of polymers, dendrimers, and other functional materials [9].Examples include triazolecontaining organic electronic materials that can be used for solar cells [10][11][12][13], field-effect transistors [14][15][16], and light-emitting diodes [17][18][19].Materials incorporating triazole units have been developed for gas storage [20], gas separation [21], and optical brightening [22], and the potential utilization of these materials in sensors [23] and catalysts [24] has been explored.Triazole units are regarded as promising electron-acceptor candidates for the construction of donor-acceptor (D-A) compounds.
Si/Ge-condensed compounds, such as siloles and germoles, have received considerable attention in recent decades for their potential applications in organic electronics [25,26].These structures exhibit a notable phenomenon known as σ*-π* conjugation, which is frequently observed between Si/Ge σ*-orbitals and endocyclic butadiene π*-orbitals.This interaction serves to stabilize the lowest unoccupied molecular orbital (LUMO), resulting in an enhanced electron affinity and reduced energy gaps between the highest occupied molecular orbital (HOMO) and LUMO [27][28][29].The high planarity of siloles and germoles is also advantageous for their use as building units of conjugated materials.These characteristics have significant implications on the electronic properties of siloles and germoles.Researchers have explored strategies to fine-tune the functionalities of these compounds to allow for tailored applications in various electronic devices via condensation with aromatic ring systems [30].Currently, dithienosilole and dithienogermole are used as building units for preparing conjugated functional materials for optoelectronic devices, such as OLEDs [31], organic solar cells [32], and organic transistors [33].However, most condensed siloles and germoles reported so far possess symmetrical tricyclic systems [34], and studies on unsymmetrical tricyclic systems have been limited to simple cases, such as combinations of benzene-pyridine [35,36], benzene-thiophene [37,38], and thiophene-pyridine ring systems [26].
In this work, we designed and synthesized two types of germoles unsymmetrically condensed with benzene-triazole and thiophene-triazole units (BTAG and TTAG), with the hope that D-A interactions between benzene/thiophene and triazole units linked by a germole system would allow for the development of building units of conjugated functional materials having finely tuned electronic states.The optical and electrochemical properties of these triazologermole derivatives were systematically investigated.We also prepared TTAG bromide as a precursor of various TTAG-containing conjugated materials.Indeed, TTAG bromide underwent the palladium-catalyzed Stille coupling reaction to furnish triphenylamine-substituted TTAG (TTAG-TPA) with the effective extension of conjugation.The electronic states of TTAG-TPA were investigated by optical and electrochemical measurements and density functional theory (DFT) calculations.Although BTAG and TTAG showed no evident intramolecular D-A interaction, TTAG-TPA exhibited solvatochromic properties attributable to the intramolecular D-A interaction between the triphenylamine and triazole units, with a shift of the photoluminescence (PL) band toward lower energies with increasing solvent polarity.These results suggest potential applications of triazologermole units as building units of conjugated materials.

Synthesis
The synthetic strategy is shown in Scheme 1. Compound 1 was prepared following a previously reported method [39] and used for the subsequent synthesis of BTAG via compound 2. TT-TMS was obtained by the ethyl substitution of 4 prepared from 3. Compound 3 was prepared as previously reported [40].The dilithiation of TT-TMS using LDA, followed by a ring-forming reaction with dichlorodi(n-octyl)germane, resulted in a mixture containing TTAG-TMS, unsubstituted TT-TMS, and numerous unidentified products.From the mixture, TTAG-TMS was isolated in 58% yield by silica gel column chromatography.TTAG-TMS was treated with TBAF to obtain TTAG with a yield of 85%.The bromination of TTAG with NBS gave TTAG-Br, which was further subjected to palladium-catalyzed Stille coupling with 5 to obtain TTAG-TPA.Scheme 1. Synthesis of BTAG, TTAG, TTAG-TMS, and TTAG-TPA.

Optical and Electrochemical Properties
We examined the optical and electrochemical properties of the prepared triazologermole derivatives to clarify their electronic states.The UV-vis absorption and PL spectra of BTAG, TTAG, TTAG-TMS, and TTAG-TPA in THF are shown in Figure 1, and the corresponding photophysical data are summarized in Table 1.BTAG displayed a maximum absorption at 281 nm in THF with a shoulder at 290 nm.Replacing benzene with thiophene in TTAG caused a bathochromic shift of 13 nm to give an absorption maximum at 294 nm in THF.When compared with thienopyridinogermole, the absorption maximum of TTAG was shifted to the shorter wavelength region by 6 nm, reflecting the less extended conjugation of thienyltriazole than thienylpyridine [26].In fact, 5-trimethylsilyl-2-thienylpyridine possesses the absorption maximum at 324 nm, redshifted by approximately 40 nm compared to that of TT-TMS [26].Silyl substitution at the α-position of the thiophene ring in TTAG-TMS shifted the absorption maximum slightly to 286 nm.This finding differed from a previous report on DTS derivatives that found a shift of absorption bands to lower energies because of a similar silyl substitution [41].Germole condensation in TTAG-TMS resulted in a red shift of the PL band by 9 nm relative to that of TT-TMS.However, despite our expectation, their UV absorption bands were at nearly the same energies.Compounds BTAG, TTAG, and TTAG-TMS showed weak PL. Figure S1a-c shows the absorption spectra of BTAG, TTAG, and TTAG-TMS recorded in different solvents.Only negligible solvatochromic shifts were observed in both the UV-vis and PL spectra of these compounds, indicating the absence of evident D-A interactions.TTAG-TPA exhibited an absorption band and a PL band at 375 nm and 438 nm in THF, respectively, both of which were largely red-shifted compared with those of BTAG, TTAG, and TTAG-TMS.It is likely that the introduction of the triarylamine substituent on the thiophene ring efficiently enhanced conjugation.In the UV-vis absorption spectrum, a band at 300 nm was also observed in THF, which is possibly due to the local excitation regarding the TTAG unit.The PL bands have intramolecular charge transfer (ICT) characteristics, which can be verified by the solvatochromic effects, as shown in Figure 2.For example, the PL band of TTAG-TPA was observed at 423 nm in toluene, which was bathochromically shifted to 456 nm in more polar MeCN.In contrast, the PL spectra of BTAG, TTAG, and TTAG-TMS showed no clear solvatochromism.The red shift of the PL band with increasing solvent polarity clearly indicates that the excited states were more polar than the ground state [42].In fact, nearly no evident solvatochromic effects were observed in the UV-vis absorption spectra of TTAG-TPA.To further investigate the ICT characteristics of TTAG-TPA, the Stokes shift (νa − νf) in each solvent was plotted as a function of its orientational polarizability Δf to obtain Lippert-Mataga plots [43,44].A linear relationship between the Stokes shifts and orientational polarizabilities was observed (Figure 2 Scheme 1. Synthesis of BTAG, TTAG, TTAG-TMS, and TTAG-TPA.

Optical and Electrochemical Properties
We examined the optical and electrochemical properties of the prepared triazologermole derivatives to clarify their electronic states.The UV-vis absorption and PL spectra of BTAG, TTAG, TTAG-TMS, and TTAG-TPA in THF are shown in Figure 1, and the corresponding photophysical data are summarized in Table 1.BTAG displayed a maximum absorption at 281 nm in THF with a shoulder at 290 nm.Replacing benzene with thiophene in TTAG caused a bathochromic shift of 13 nm to give an absorption maximum at 294 nm in THF.When compared with thienopyridinogermole, the absorption maximum of TTAG was shifted to the shorter wavelength region by 6 nm, reflecting the less extended conjugation of thienyltriazole than thienylpyridine [26].In fact, 5-trimethylsilyl-2-thienylpyridine possesses the absorption maximum at 324 nm, redshifted by approximately 40 nm compared to that of TT-TMS [26].Silyl substitution at the α-position of the thiophene ring in TTAG-TMS shifted the absorption maximum slightly to 286 nm.This finding differed from a previous report on DTS derivatives that found a shift of absorption bands to lower energies because of a similar silyl substitution [41].Germole condensation in TTAG-TMS resulted in a red shift of the PL band by 9 nm relative to that of TT-TMS.However, despite our expectation, their UV absorption bands were at nearly the same energies.Compounds BTAG, TTAG, and TTAG-TMS showed weak PL. Figure S1a-c shows the absorption spectra of BTAG, TTAG, and TTAG-TMS recorded in different solvents.Only negligible solvatochromic shifts were observed in both the UV-vis and PL spectra of these compounds, indicating the absence of evident D-A interactions.TTAG-TPA exhibited an absorption band and a PL band at 375 nm and 438 nm in THF, respectively, both of which were largely red-shifted compared with those of BTAG, TTAG, and TTAG-TMS.It is likely that the introduction of the triarylamine substituent on the thiophene ring efficiently enhanced conjugation.In the UV-vis absorption spectrum, a band at 300 nm was also observed in THF, which is possibly due to the local excitation regarding the TTAG unit.The PL bands have intramolecular charge transfer (ICT) characteristics, which can be verified by the solvatochromic effects, as shown in Figure 2.For example, the PL band of TTAG-TPA was observed at 423 nm in toluene, which was bathochromically shifted to 456 nm in more polar MeCN.In contrast, the PL spectra of BTAG, TTAG, and TTAG-TMS showed no clear solvatochromism.The red shift of the PL band with increasing solvent polarity clearly indicates that the excited states were more polar than the ground state [42].In fact, nearly no evident solvatochromic effects were observed in the UV-vis absorption spectra of TTAG-TPA.To further investigate the ICT characteristics of TTAG-TPA, the Stokes shift (ν a − ν f ) in each solvent was plotted as a function of its orientational polarizability ∆f to obtain Lippert-Mataga plots [43,44].A linear relationship between the Stokes shifts and orientational polarizabilities was observed (Figure 2 and Table S1).Using the slope of the Lippert-Mataga plot, the dipole moment change (∆µ) upon photoexcitation was calculated to be 18.4 D (Figure S2) [26].This was smaller than that calculated for thienopyridinogermole with the same substituents (19.9 D), suggesting that fine tuning of the D-A interaction is possible by changing the acceptor unit.
and Table S1).Using the slope of the Lippert-Mataga plot, the dipole moment change (Δμ) upon photoexcitation was calculated to be 18.4 D (Figure S2) [26].This was smaller than that calculated for thienopyridinogermole with the same substituents (19.9 D), suggesting that fine tuning of the D-A interaction is possible by changing the acceptor unit.TTAG-TPA had a higher PL quantum yield than the other triazologermole derivatives, likely reflecting the extended conjugation.TTAG-TPA exhibited blue PL in all   and Table S1).Using the slope of the Lippert-Mataga plot, the dipole moment change (Δμ) upon photoexcitation was calculated to be 18.4 D (Figure S2) [26].This was smaller than that calculated for thienopyridinogermole with the same substituents (19.9 D), suggesting that fine tuning of the D-A interaction is possible by changing the acceptor unit.
(a) (b)    TTAG-TPA had a higher PL quantum yield than the other triazologermole derivatives, likely reflecting the extended conjugation.TTAG-TPA exhibited blue PL in all TTAG-TPA had a higher PL quantum yield than the other triazologermole derivatives, likely reflecting the extended conjugation.TTAG-TPA exhibited blue PL in all solvents with maxima in the range of 410-490 nm, except in trifluoroethanol in which green emission was observed (Figure S3).Interestingly, the Stokes shift of TTAG-TPA in trifluoroethanol deviated from that predicted from the Lippert-Mataga plot of TTAG-TPA in different solvents, and the largely red-shifted PL band at 492 nm suggested a specific interaction of TTAG-TPA with trifluoroethanol.For instance, hydrogen bonding might have resulted in the complex formation of TTAG-TPA with trifluoroethanol, as previously reported for triphenylamine-substituted thienopyridinogermole [26].
Anodic cyclic voltammograms (CVs) were examined to investigate the redox characteristics and HOMO energy levels of triazologermole derivatives (Table 1 and Figure S4).The oxidation peak of TTAG-TMS was found at 1.62 V (vs.Fc + /Fc), lower than that of TT-TMS (1.84 eV), indicating that the introduction of the germole unit raised the HOMO energy level.The oxidation peak of BTAG was observed at 1.96 V, higher than that of TTAG (1.57V), further supporting the less extended conjugation in BTAG.The first oxidation peak (E0ox1) of TTAG-TPA was much lower than that of TTAG, at 1.04 V.The HOMO energy levels determined from the onsets of the first oxidation were −6.12 eV for BTAG, −5.80 eV for TTAG, −5.50 eV for TTAG-TMS, and −5.23 for TTAG-TPA.It should be noted that the introduction of the triphenylamine unit had a significant effect on the elevation of the HOMO energy level.This can be attributed to the contribution of the strong electron-donating triphenylamine group.In contrast, silyl substitution at the α-position of the thiophene ring in TTAG-TMS lowered the HOMO energy level.

DFT Calculations
To clarify the electronic states of triazologermole derivatives, we carried out DFT calculations using the Gaussian 16 suite of quantum chemical simulation programs for condensed germoles.First, structures were optimized in vacuo using DFT calculations at the B3LYP/6-31G(d,p) level [45].Figure 3 shows the results of DFT calculations.The HOMO of BTAG is based on π-conjugation.The higher HOMO of TTAG than that of BTAG by 0.27 eV is likely due to electron donation by the thiophene unit of TTAG, which suggests the higher Lewis basicity of TTAG than BTAG.At the same time, the LUMO is lowered by 0.23 eV relative to that of BTAG.The LUMOs of triazologermole derivatives show σ*-π* interaction, particularly between the thiophene β-carbon and the germanium atom.However, orbital overlapping is limited despite our expectations, with the LUMO of TTAG-TMS being at a higher energy than that of TT-TMS.The reduced HOMO-LUMO energy gap of TTAG-TMS compared with that of TT-TMS appears to be primarily due to the electron-donating properties of the germanium unit, raising the HOMO energy level.
solvents with maxima in the range of 410-490 nm, except in trifluoroethanol in which green emission was observed (Figure S3).Interestingly, the Stokes shift of TTAG-TPA in trifluoroethanol deviated from that predicted from the Lippert-Mataga plot of TTAG-TPA in different solvents, and the largely red-shifted PL band at 492 nm suggested a specific interaction of TTAG-TPA with trifluoroethanol.For instance, hydrogen bonding might have resulted in the complex formation of TTAG-TPA with trifluoroethanol, as previously reported for triphenylamine-substituted thienopyridinogermole [26].
Anodic cyclic voltammograms (CVs) were examined to investigate the redox characteristics and HOMO energy levels of triazologermole derivatives (Table 1 and Figure S4).The oxidation peak of TTAG-TMS was found at 1.62 V (vs.Fc + /Fc), lower than that of TT-TMS (1.84 eV), indicating that the introduction of the germole unit raised the HOMO energy level.The oxidation peak of BTAG was observed at 1.96 V, higher than that of TTAG (1.57V), further supporting the less extended conjugation in BTAG.The first oxidation peak (E0ox1) of TTAG-TPA was much lower than that of TTAG, at 1.04 V.The HOMO energy levels determined from the onsets of the first oxidation were -6.12 eV for BTAG, -5.80 eV for TTAG, -5.50 eV for TTAG-TMS, and -5.23 for TTAG-TPA.It should be noted that the introduction of the triphenylamine unit had a significant effect on the elevation of the HOMO energy level.This can be attributed to the contribution of the strong electrondonating triphenylamine group.In contrast, silyl substitution at the α-position of the thiophene ring in TTAG-TMS lowered the HOMO energy level.

DFT Calculations
To clarify the electronic states of triazologermole derivatives, we carried out DFT calculations using the Gaussian 16 suite of quantum chemical simulation programs for condensed germoles.First, structures were optimized in vacuo using DFT calculations at the B3LYP/6-31G(d,p) level [45].Figure 3 shows the results of DFT calculations.The HOMO of BTAG is based on π-conjugation.The higher HOMO of TTAG than that of BTAG by 0.27 eV is likely due to electron donation by the thiophene unit of TTAG, which suggests the higher Lewis basicity of TTAG than BTAG.At the same time, the LUMO is lowered by 0.23 eV relative to that of BTAG.The LUMOs of triazologermole derivatives show σ*π* interaction, particularly between the thiophene β-carbon and the germanium atom.However, orbital overlapping is limited despite our expectations, with the LUMO of TTAG-TMS being at a higher energy than that of TT-TMS.The reduced HOMO-LUMO energy gap of TTAG-TMS compared with that of TT-TMS appears to be primarily due to the electron-donating properties of the germanium unit, raising the HOMO energy level.The HOMO-LUMO energy gaps decreased in the order of BTAG > TT-TMS > TTAG > TTAG-TMS > TTAG-TPA, in accordance with their optical properties, except for TTAG-TMS (TTAG-TMS showed a smaller optical band gap than TTAG).The CVs showed a higher anodic potential for TTAG-TMS than for TTAG, indicating a shallower HOMO energy level.These findings are inconsistent with the results of computation, suggesting the contribution of trimethylsilyl substitution to both the HOMO and LUMO to decrease the energy gap, although the degree of contribution seemed small.Unfortunately, we have no direct data to explain this discrepancy between the experimental and computational data for TTAG-TMS.The introduction of the triphenylamine group to the TTAG unit affected the HOMO−LUMO energy gap in TTAG-TPA mainly by raising the HOMO energy level, rather than lowering the LUMO energy level.Indeed, the LUMO of TTAG-TPA was lower only by 0.37 eV than that of TTAG, while the HOMO was higher by 0.77 eV.In TTAG-TPA, the HOMO is mainly on the (diphenylaminophenyl)thiophene unit, whereas the LUMO is distributed more on the triazologermole unit, suggesting that charge separation may occur in the photoexcited state.However, the separated distribution of the HOMO and LUMO was not observed in BTAG, TTAG, and TTAG-TMS.This finding is in line with the fact that the UV absorption and PL bands of these compounds exhibited no evident solvatochromic effects.

General Preparations
All reactions were carried out under a dry argon atmosphere.N,N-dimethylformamide (DMF) and tetrahydrofuran (THF) were distilled from CaH 2 and stored over activated molecular sieves in the dark until use as reaction solvents.Synthetic procedures for compounds 2 and 4 are presented in the Supporting Information.Compound 5 was prepared as reported in study [46].Nuclear magnetic resonance (NMR) spectra were recorded on a Varian 400-MR spectrometer (Palo Alto, CA, USA).NMR spectra of newly prepared compounds are presented in the Supporting Information.UV-vis absorption and PL spectra in various solvents were recorded at room temperature using Hitachi U-2910 (Tokyo, Japan) and HORIBA FluoroMax-4 spectrophotometers (Kyoto, Japan), respectively.APCI-mass spectra were obtained with a Thermo Fisher Scientific LTQ Orbitrap XL spectrometer (Waltham, MA, USA) at N-BARD, Hiroshima University.Electrochemical redox potentials were obtained by cyclic voltammetry measurements performed with a computer-controlled Autolab analyzer (Kyoto, Japan) using a typical three-electrode electrochemical cell in a solution of tetrabutylammonium hexafluorophosphate (0.1 M) in anhydrous acetonitrile at a scan rate of 50 mV s −1 at room temperature under nitrogen.A Pt disk was used as the working electrode, a Pt wire as the counter electrode, and an Ag/Ag + electrode as the reference electrode.The potential of the reference electrode was calibrated with ferrocene.

Synthesis of BTAG
To a solution of 2 (0.507 g, 1.532 mmol) in 20 mL of THF, 1.915 mL of n-BuLi (1.6 M) in hexane at −78 • C was added, and the mixture was stirred at this temperature for 2 h.Dichlorodi(n-octyl)germane (0.623 g, 1.685 mmol) was added to the mixture at −78 • C.

3 . 97 a
In THF at room temperature.b Excited at the absorption maximum.c Derived from CV anodic onset potential.d E (LUMO) = E (HOMO) + Eg. e Too low to determine.

a
In THF at room temperature.b Excited at the absorption maximum.c Derived from CV anodic onset potential.d E (LUMO) = E (HOMO) + Eg. e Too low to determine.

Figure 3 .
Figure 3. Energy levels and profiles of BTAG, TTAG, TTAG-TMS, TTAG-TPA, and TT-TMS derived from DFT calculations at the B3LYP/6-31G(d,p) level of theory in the gas phase.

Figure 3 .
Figure 3. Energy levels and profiles of BTAG, TTAG, TTAG-TMS, TTAG-TPA, and TT-TMS derived from DFT calculations at the B3LYP/6-31G(d,p) level of theory in the gas phase.

Table 1 .
Optical and electrochemical properties of triazologermoles.

Table 1 .
Optical and electrochemical properties of triazologermoles.

Table 1 .
Optical and electrochemical properties of triazologermoles.