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 (Φ f (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 (S 1 ) causes the large difference in Φ f (RT). Quantum calculations indicated that the Si–Si σ bond of (Me 3 Si) 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 (Me 3 Si) 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 S 1 leading to a small Φ f (RT). However, a planar conjugated core having slightly higher HOMO energy than that of the (Me 3 Si) 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 Φ f (RT). Thus, the relationship between the HOMO level of the (Me 3 Si) 3 Si substituent and that of the planar conjugated core is key to controlling the ISC from S 1 .