Effect of the Tris(trimethylsilyl)silyl Group on the Fluorescence and Triplet Yields of Oligothiophenes

The origin of the large difference of room-temperature fluorescence yields (Φf(RT)) among tris(trimethylsilyl)silylated oligothiophene derivatives was investigated. Tris(trimethylsilyl)silylated thiophene (1) and tris(trimethylsilyl)silylated terthiophene (3) show low fluorescence yields while that of tris(trimethylsilyl)silylated bithiophene (2) is high. Nanosecond transient absorption measurements for 2 and 3 verified that the large difference between their intersystem crossing (ISC) rates from the lowest singlet excited state (S₁) causes the large difference in Φf(RT). Quantum calculations indicated that the Si–Si σ bond of (Me₃Si)₃Si, corresponding to the highest occupied molecular orbital (HOMO), is closely involved in the ISC from S₁. The planar conjugated core having much higher or comparable HOMO energy relative to the (Me₃Si)₃Si substituent, such as 1 and 3, induces large spin–orbit coupling (SOC) between S₁ and the second-order triplet excited state (T₂), resulting in fast ISC from S₁ leading to a small Φf(RT). However, a planar conjugated core having slightly higher HOMO energy than that of the (Me₃Si)₃Si substituent, such as 2, minimizes SOC between S₁ and T₂, resulting in slow ISC from S₁ leading to a large Φf(RT). Thus, the relationship between the HOMO level of the (Me₃Si)₃Si substituent and that of the planar conjugated core is key to controlling the ISC from S₁.