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.