Photophysical triplet state processes of 4-H-1-benzopyrane-4-thione in a perfluoroalkane. Part 1.—Temperature dependence of unimolecular triplet decay

Abstract

The rate constant k⁰_T for the unimolecular decay of the lowest triplet state, T₁, 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 T₁⇌ S₁ of the relative population of T₁ and the lowest excited singlet state S₁, three parameters can be calculated from the temperature dependence of k⁰_T: the individual rate constants β_s=(4.0 ± 0.2)× 10⁶ s^–1 for the decay of S₁ to S₀ and β^z_T=(8.2 ± 2)× 10⁴ s^–1 for the decay of the strongly phosphorescent triplet substate T^z₁ to S₀, and the energy difference ΔE(S₁,T^z₁)=hc×(757 ± 13) cm^–1 between T^z₁ and S₁. The value of ΔE(S₁, T^z₁) is in agreement with the positions of T_1,0 and S_1,0 in the absorption spectrum. The spectrum of the S₀→ S₁ transition is estimated.

The assignment of the hot-band range of the delayed luminescence above the energy hc[graphic omitted]_0,0 of T_1,0 is facilitated by an inverse Boltzmann weighting (multiplication with exp[hc([graphic omitted] – [graphic omitted]_0,0)/k_BT]). At the position of S_1,0, a contribution of E-type delayed S₁→ S₀ fluorescence can be discerned.

The contribution of diffusion-controlled concentration-quenching (rate constant k_Q) to the triplet decay was eliminated by extrapolation to infinite dilution; k_Q is proportional to the ratio T/η of temperature T and viscosity η. The temperature dependence of the viscosity and density of perfluoro-1,3-dimethylcyclohexane were measured.