The local temperature dependence of fluorescent centres in PVA films on the excitation wavenumber

The temperature difference Delta T = T* - T (where T* is the local temperat ure and T the ambient temperature) is studied as a function of the excitati on wavenumber <(nu)over tilde>(exc) in the longwave region of the absorptio n band of 4-amino-4'-nitrostilbene (ANS) and 4-dimethylamino-4'-nitrostilbe ne (DNS) as well as rhodamine S, rhodamine 6G and Na-fluorescein in poly(vi nyl alcohol) (PVA) films heated up to 403 K. Local temperatures T* are dete rmined by a universal Kennard-Stepanov relation. A linear dependence of Del ta T on <(nu)over tilde>(exc) has been found for all investigated luminesce nt compounds. For dyes, due to the big spectral overlaps between the absorp tion and fluorescence bands, it was possible to excite also in the anti-Sto kes region. In this case for Delta T = 0, i.e. when T* = T, one has <(nu)ov er tilde>(exc) = <(nu)over tilde>(0-0)'. In view of the slow geometrical re laxation of excitation energy excess in PVA polymers, the relaxation time t au(R) for thermal equilibration between the luminescent molecule and the vi cinal polymer exceeds distinctly the mean fluorescence lifetime tau(F).