Synthesis, optical and electrochemical properties of 4,4′-bibenzo[c]thiophene derivatives

We designed and synthesized unsubstituted 4,4′-bibenzo[c]thiophene4,4′-BBTand its silyl-substituted derivatives1,1′-Si-4,4′-BBTand1,1′,3,3′-Si-4,4′-BBTwith one or twotert-butyldimethylsilyl groups on each thiophene ring, as new π-building blocks in emitters, photosensitizers and semiconductors for organic optoelectronic devices. The characterization of4,4′-BBT,1,1′-Si-4,4′-BBTand1,1′,3,3′-Si-4,4′-BBTwas successfully determined by FTIR,¹H and¹³C NMR measurements, high-resolution mass spectrometry (HRMS) analysis, photoabsorption and fluorescence spectroscopy, cyclic voltammetry (CV) and density functional theory (DFT) calculations. Moreover, a single-crystal X-ray structural analysis was successfully made for1,1′-Si-4,4′-BBTand1,1′,3,3′-Si-4,4′-BBT. The photoabsorption and fluorescence maxima (λ abs maxandλflmax) of the three 4,4′-bibenzo[c]thiophene derivatives in toluene exhibit bathochromic shifts in the order of4,4′-BBT(359 nm and 410 nm) <1,1′-Si-4,4′-BBT(366 nm and 420 nm) <1,1′,3,3′-Si-4,4′-BBT(371 nm and 451 nm). The HOMO and LUMO energy levels rise in the order of4,4′-BBT(−5.55 eV and −2.39 eV) <1,1′-Si-4,4′-BBT(−5.45 eV and −2.34 eV) <1,1′,3,3′-Si-4,4′-BBT(−5.34 eV and −2.30 eV), but the rise of the HOMO energy level is larger than that of the LUMO energy level, resulting in the bathochromic shift of the photoabsorption band from4,4′-BBTto1,1′,3,3′-Si-4,4′-BBT. The fluorescence quantum yields (Φ_fl) of4,4′-BBT,1,1′-Si-4,4′-BBTand1,1′,3,3′-Si-4,4′-BBTin toluene are 0.41, 0.41 and 0.36, respectively. It is worth mentioning that in the solid state1,1′-Si-4,4′-BBTand1,1′,3,3′-Si-4,4′-BBTshow relatively highΦ_fl-solidvalues of 0.22 and 0.25, respectively, whereas4,4′-BBTexhibits poor solid-state fluorescence properties (Φ_fl-solid< 0.02). This work provides an efficient synthetic method for the 4,4′-bibenzo[c]thiophene derivatives and their photophysical properties in the solution and solid state, electrochemical properties and X-ray crystal structures.