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 (Phi(f)-(RT)) among tris(trimethylsilyl)silylated oligothiophene derivatives was investigated. Tris(trimethylsilyOsilylated 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 (S-1) causes the large difference in Phi(f)(DOT). Quantum calculations indicated that the Si-Si sigma bond of (Me3Si)(3)Si, corresponding to the highest occupied molecular orbital (HOMO), is closely involved in the ISC from S-1. The planar conjugated core having much higher or comparable HOMO energy relative to the (Me3Si)(3)Si substituent, such as 1 and 3, induces large spin-orbit coupling (SOC) between S-1 and the second-order triplet excited state (T-2), resulting in fast ISC from 5, leading to a small Phi(f)(RT). However, a planar conjugated core having slightly higher HOMO energy than that of the (Me3Si)(3)Si substituent, such as 2, minimizes SOC between S-1 and T-2, resulting in slow ISC from S-1 leading to a large Phi(f)(DOT). Thus, the relationship between the HOMO level of the (Me3Si)(3)Si substituent and that of the planar conjugated core is key to controlling the ISC from S-1.