Photolyses of Tetraphenylcyclobutanes at 254 nm

Photolyses of 1,2,3,4-tetraphenylcyclobutanes (TPCB) at 254 nm in hydrocarbon solvents have been studied by means of fluorometry, nanosecond time-resolved spectroscopy, and measurements of the reaction quantum yields. The photolysis of TPCB originates from the S1(π,π*) state to give trans-stilbene nearly concertedly at higher temperatures (>200 K). Significant temperature and cis-effects were observed. The radiationless rate constants for the dissociation (kr) and the other radiationless (kn) processes decreased with lowering temperature. The value of kr for γ-1,t-2,t-3,c-4-TPCB at 300 K, for example, was 4.7 times greater than that for γ-1,t-2,c-3,t-4-TPCB. The Arrhenius parameters of the relaxation processes were also determined. No intramolecular excimer emission was observed.