PHOTOPHYSICAL TRIPLET-STATE PROCESSES OF 4-H-1-BENZOPYRANE-4-THIONE IN A PERFLUOROALKANE .1. TEMPERATURE-DEPENDENCE OF UNIMOLECULAR TRIPLETDECAY

The rate constant k(T)(0) for the unimolecular decay of the lowest tri plet state, T-1, of 4-H-1-benzopyrane-4-thione (BPT) in perfluoro-1,3- dimethylcyclohexane was determined between 230 and 300 K. By assuming a very fast and virtually complete thermal equilibration T-1 reversibl e arrow S-1 of the relative population of T-1 and the lowest excited s inglet state S,, three parameters can be calculated from the temperatu re dependence of k(T)(0): the individual rate constants beta(s) = (4.0 +/- 0.2) x 10(6) s(-1) for the decay of S-1 to S-0 and beta(T)(z) = ( 8.2 +/- 2) x 10(4) s(-1) for the decay of the strongly phosphorescent triplet substate T-1(z) to S-0, and the energy difference Delta E(S-1, T-1(z)) = hc x (757 +/- 13) cm(-1) between T-1(z) and S-1. The value o f Delta E(S-1,T-1(z)) is in agreement with the positions of T-1,T-0, a nd S-1,S-0 in the absorption spectrum. The spectrum of the S-0 --> S-1 transition is estimated. The assignment of the hot-band range of the delayed luminescence above the energy hc<(nu)over tilde>(0,9) T-1,T-0 is facilitated by an inverse Boltzmann weighting (multiplication with exp[hc(<(nu)over tilde> - <(nu)over tilde>(0,9))/k(B)T]). At the posit ion of S-1,S-0, a contribution of E-type delayed S-1 --> S-0 fluoresce nce can be discerned. The contribution of diffusion-controlled concent ration-quenching (rate constant k,) to the triplet decay was eliminate d by extrapolation to infinite dilution; k(Q) is proportional to the r atio T/eta of temperature T and viscosity eta. The temperature depende nce of the viscosity and density of perfluoro-1,3-dimethylcyclohexane were measured.