10170 | Chem. Commun., 2021, 57, 10170–10173 This journal is © The Royal Society of Chemistry 2021

Cite this: Chem. Commun., 2021,

57, 10170

Thiophene-fused boracycles as photoactiveanalogues of diboraanthracenes†

Shreenibasa Sa, Anna Chandrasekar Murali, Prakash Nayak andKrishnan Venkatasubbaiah *

The construction of thiophene-fused analogues of diboraanthracenes

with different aryl substituents through boron-mercury exchange

followed by the nucleophilic replacement of the chlorines of di-

chlorodiboradithiophene 2 with Grignard reagents is reported. These

diboradithiophenes exhibited unusual photophysical and electro-

chemical properties. They all undergo photoisomerisation, which can

be traced using photophysical and 1H NMR spectroscopy studies.

Organic p-conjugated materials have gained much attentionowing to their tuneable and diverse properties that are favour-able for applications as organic electronics, bio-imaging andsensing materials.1–3 Incorporation of non-carbon elements,especially main-group elements such as B, P, Si, S, Sn, Se, Teetc., into p-conjugated materials has emerged as an attractivemethod to fine tune the electronic properties and to overcomethe shortcomings experienced by purely organic materials.4

Among the different main-group element doped p-electronsystems, tri-coordinated boron compounds have gained over-whelming interest due to their potential applications in organiclight emitting diodes, non-linear optics, fluorescent probes andbiological probes.5–7 Introduction of a Lewis acidic boron atomperturbs the electronic structure of the p-conjugated system throughthe interaction of the p-electron clouds with the p-orbital at theboron center. Among the different types of p-conjugated boranes,fused boracycles and (or) boron embedded cyclic systems havegained much attention.8 Notable progress has been made in thedevelopment of borafluorene, and diboraanthracene derivatives.8,9

Diboron fused aromatic systems have been used to make sandwichcomplexes,10 perfluorinated derivatives have been studied as activa-tors in Ziegler–Natta olefin polymerization,11 and ferrocenyl deriva-tives12 have been explored as redox-responsive materials and

boron-doped polyaromatics as precursors to boron-dopedgraphene flakes or nano-ribbons.13

Thiophene is a five-membered aromatic system that is fre-quently incorporated as a building block to construct functionalmaterials. The coplanarity, quinoid character, and intermolecularp-stacking and sulphur–sulphur interactions help to attain uniquechemical and electronic characteristics.14 Incorporation of boron inbetween thiophene moieties has turned out to be an attractivemethod to tune the chemical and electronic properties of thethiophene system.15 Apart from boron-embedded oligo- and poly-thiophenes, boron incorporated cyclic systems have gained muchattention due to their attractive photophysical properties.16 Forexample, Yamaguchi and co-workers17 reported thiophene-fusedboroles with high antiaromaticity. Piers and co-workers18 reporteda benzothiophene-fused diborane which emits red light. Siebertand co-workers19 reported the reactivity of diiododiboradithio-phene. Recently, the Tovar and Ohshita groups19 explored thesynthesis of dithienoborepins, in which boron is fused in aseven-membered ring system. In spite of these exciting results,studies on thiophene-fused diboradithiophene derivatives are spar-sely explored. Here we report the synthesis of diboradithiophenesas analogues of diboraanthracenes, and discuss the unique electro-nic structure and the unexpected discovery of photochemicalprocesses upon UV irradiation.

The synthesis of diboradithiophenes is shown in Scheme 1.Reaction of compound 120 with BCl3 in toluene gave a moderateyield of dihalodiboradithiophene. The presence of B–Cl bondsallows compound 2 to be used as a precursor for the synthesisof other diboradithiophenes. The reaction of compound 2 withArMgBr provides compounds 3–5 in moderate yields. Com-pounds 3–5 were purified by column chromatography on silicagel. Compounds 2–5 were fully characterised by 1H, 13C and11B NMR spectroscopy. The 11B NMR spectra of compounds 3–5display a broad signal at around B56 ppm which falls in theregion of tri-coordinated organoboron compounds. Further-more, the molecular structure of compounds 3 and 4 wasdetermined by single crystal X-ray diffraction analysis (Fig. 1and Table S2, ESI†). Compounds 3 and 4 crystallized in the

School of Chemical Sciences, National Institute of Science Education and Research

(NISER), HBNI, Bhubaneswar-752050, Odisha, India. E-mail: krishv@niser.ac.in

† Electronic supplementary information (ESI) available: Details of synthesis andcharacterization. CCDC 2091448 and 2091449. For ESI and crystallographic datain CIF or other electronic format see DOI: 10.1039/d1cc03323a

Received 22nd June 2021,Accepted 6th September 2021

DOI: 10.1039/d1cc03323a

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centrosymmetric P21/c and P21/n space group, respectively. Asshown in Fig. 1, the aryl group (‘xylyl’ in 3 and ‘mesityl’ in 4)attached to the boron atom is almost perpendicular to the ring/plane C4B2. The dihedral angle between the C4B2 plane and thearyl group is 89.9 and 89.31 in 3 and 4 respectively. The centralC4B2 planes of compound 3 and 4 are almost coplanar withrespect to the thiophene units (0.91 for 3 and 1.91 for 4). TheC–B–C bond angles and B–C bond lengths in 3 and 4 are similarto those observed in other diboracycles.9b,12 The B–C bonds tothe thiophene rings (1.550(3) and 1.553(3) for compound 3;1.552(2) and 1.551(2) for compound 4) are considerably shorter

than the B–C bonds to the exocyclic ‘xylyl’ and ‘mesityl’groups (1.584(3) for compound 3; 1.581(2) for compound 4),which suggests considerable carbon–boron p-bonding in theC4B2 plane.

To further examine the structural features and aromaticityof the boracycles, we performed density functional theory (DFT)and nucleus-independent chemical shift (NICS) calculationsusing Gaussian 03 (B3LYP/6-31g(d,p)). All three moleculesexhibit a planar geometry (thiophene rings and C4B2 plane)as observed using single crystal X-ray analysis of compounds 3and 4. The six-membered C4B2 rings of all three compounds3–5 have NICS values of +6.80, +6.81 and +6.84, respec-tively, which indicates that they have antiaromatic character(Fig. S9, ESI†).

The photophysical and electrochemical properties of thediboracycles were investigated by UV-vis absorption and emis-sion spectroscopy and cyclic voltammetry. All three compoundsexhibit a strong absorption band at around 275 nm and acomparatively weaker band at B355 nm (Fig. 2 and Table 1).The slight hypsochromic shift of compound 5 relative tocompounds 3 and 4 is due to the strong electron-donatingnature of the isopropyl groups. This is consistent with theincreased HOMO–LUMO gap observed for compound 5. Allthree boracycles in THF exhibited a bimodal emission spec-trum. The emission bands remain unaltered by changingthe solvent polarity (cyclohexane, THF and dichloromethane).They emit in the blue region with quantum yields of B5%(4.7 for 3; 5.2 for 4 and 6.2 for 5) in the solution state and

Scheme 1 Synthesis of diboradithiophene derivaties.

Fig. 1 (Top) Molecular structures of 3 and 4 (thermal ellipsoids at 30%probability). (Bottom) Side view of 3 and 4. Hydrogen atoms are omittedfor clarity. Selected interatomic distances (Å) and angles (1): for compound3, B1–C7 1.584(3), B1–C3 1.553(3), B1*–C2 1.550(3), C3–B1–C7 121.9(2),C2*–B–C3 116.7(2), C2*–B1–C7 121.3(2), Ar//C4B2 89.9(7), Thio//C4B2

0.9(5); for compound 4, B1–C7 1.581(2), B1–C2 1.552(2), B1–C4 1.551(2),C4–B1–C7 120.8(1), C4–B1–C2 116.7(1), C2–B1–C7 122.4(1), Ar//C4B2

89.3(3), Thio//C4B2 1.9(5).Fig. 2 (Top) Absorption spectra of compounds 3–5 in 10�5 M THF.(Bottom) Normalized emission spectra of compounds 3–5 in 10�5 M THF.

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10172 | Chem. Commun., 2021, 57, 10170–10173 This journal is © The Royal Society of Chemistry 2021

B2% (2.0 for 3; 1.9 for 4 and 2.0 for 5) in the solid state. TheDFT studies reveal that the HOMOs of all three boracycles havecontributions from the thiophene units and also the ‘xylyl’ and‘mesityl’ groups in the case of compounds 3 and 4. Interest-ingly, the LUMOs of all boracycles have contributions from theheteroatom (boron) and thiophene units. Time-dependentdensity functional theory computations (TD-DFT) show thatthe longest wavelength absorptions have low oscillatorstrengths B0.15, which suggests that they are weakly absorbingmaterials (Table S4, ESI†). Cyclic voltammograms of the bora-cycles were acquired in 1,2-dimethoxyethane (DME) solutionusing [nBu4N][PF6] as a supporting electrolyte. All three bora-cycles exhibit two quasi reversible redox processes. The redoxpotentials (Table 1) are highly negative in comparison with mostsubstituted diboraanthracene8,21 and diboradiferrocene12 deriva-tives, which suggests that the diboradithiophenes are consider-ably more electron-rich. The HOMO–LUMO energy gaps werecalculated from the onset of absorption and onset of reductionpotentials, which are in broad agreement with the values obtainedfrom the DFT studies (Table S3, ESI†).

Diboraanthracene derivatives have shown excellent photo-physical properties9b and have also been studied as thermallyactivated delayed fluorescence materials in OLEDs.22a To oursurprise, the diboradithiophenes (3–5) were found to be verysensitive towards light. In order to investigate the photochro-mic properties, compound 4 was irradiated using UV light

(365 nm) in THF solution and the response was monitoredusing absorption and emission spectroscopy. Upon sequentialirradiation, the bands at 275, 351 and 367 nm in the absorptionand the bands at 377 nm and 394 nm in the emission steadilydecreased with time and completely disappeared after 35 minutesof UV exposure (Fig. 3 and Fig. S3 and S4, ESI†). A similarphenomenon was also observed when we tested compounds 3and 5 (Fig. S1, S2, S5 and S6, ESI†). It is worth noting thatcompound 5 showed a faster response relative to 3 and 4;although the reasons for this quick response are not known,we postulate that the stronger electron donating nature of the‘isopropyl’ groups makes 5 more reactive under the conditionsmentioned above.

To investigate the species formed upon irradiation at365 nm, the 1H NMR of compound 4 in C6D6 was recordedafter 5 h irradiation. Formation of a doublet (Fig. S23–S25,CH3 (a), ESI†) at 1.26 ppm and quartet at 3.38 ppm (H(b))was observed. The observed pattern is similar to that of theproduct reported by Yamaguchi and co-workers22b for thephotochemical intramolecular 1,6-sigmatropic rearrangementreaction of dimesityl(2,5-dimethyl-3-thienyl)borane. In additionto that, the presence of two carbons (–CH2–) with differentenvironments was also observed using a 13C DEPT-135 experi-ment (Fig. S27, ESI†), which suggests that more than onespecies is formed during the photoirradiation (ESI†). However,after 24 h photoirradiation, the presence of a symmetrical productwas observed. Further studies are needed to reveal the pathwaysinvolved in this process and also to know more details about thespecies involved in the process (ESI†). A similar phenomenon wasobserved when the photoirradiation was performed using com-pounds 3 and 5. Our attempts to revert the reaction by heating orpassing visible light were not successful.

In conclusion, we established a synthetic route to thiophene-fused analogues of diboraanthracenes starting from 3,4-bis(chloromercurio)2,5-dimethylthiophene. The halogenateddichlorodiboradithiophene (2) serves as a precursor to deriva-tives 3–5. The antiaromatic diboradithiophenes 3–5 are stablein air, and showed interesting optical and electrochemicalproperties. Upon photoirradiation, the absorption and theemission intensity of compounds 3–5 decrease gradually, whichwas exploited for the detection of UV radiation.

Conflicts of interest

There are no conflicts to declare.

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Table 1 Optical (THF) and electrochemical (DME) data for compounds3–5

Compound labs (nm)(log e)lem

(nm) FF

E1/2a

(V)

3 275(4.838), 351(4.021), 367(4.166) 377, 394 4.7 �2.53,�3.21

4 275(4.966), 351(4.149), 367(4.287) 376, 394 5.2 �2.55,�3.20

5 275(4.777), 349(3.859), 363(3.954) 375, 392 6.2 �2.59,�3.30

a [Bu4N][PF6] (0.1 M) in DME, scan rate 50 mV s�1; referenced withrespect to Fc/Fc+.

Fig. 3 Emission spectra of compound 4 in THF (10�5 M) after exposure toUV light (365 nm).

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