Structures and optical properties of tris(trimethylsilyl)silylated oligothiophene derivatives

The structures and optical properties of tris(trimethylsilyl)silylated oligothiophenes were examined by spectroscopies, theoretical calculations, and single-crystal X-ray measurements. Bathochromic shift from the original oligothiophenes was observed in the tris(trimethylsilyl)silylated ones, confirming the σ-π conjugation between Si-Si σ bonds and π-orbital. 5,5'-Bis(tris(trimethylsilyl)silyl)-2,2'-bithiophene (Si-T2) showed the highest fluorescence quantum yield (ΦF) both in solution (0.67, excited at 350 nm) and the solid state (0.74, excited at 371 nm). The introduction of tris(trimethylsilyl)silyl groups led to the small nonradiative rate constant of Si-T2, resulting in the high ΦF in the solution state. Si-T2 also exhibited effective σ-π conjugation and poor molecular interaction, which reflected its high ΦF in the solid state. On the contrary, lower ΦF (0.13, excited at 331 nm) in the solid state was observed in the longest oligothiophene examined, 5,5‴-bis(1,1,1,3,3,3-hexamethyl-2-(trimethylsilyl)trisilan-2-yl)-2,2':5',2″:5″,2‴-quaterthiophene (Si-T4). Single-crystal X-ray measurement clarified that this compound adopted a zigzag packing structure and a rare syn-anti-syn conformation, which led to the poor σ-π conjugation and the decrease of π-orbital overlap in the solid state.