Temperature and viscosity effects on fluorescence in 9-t-butylanthracene

Abstract

Temperature and viscosity effects on the fluorescence in 9-t-butylanthracene (TBA) have been investigated over a wide temperature range (77–365 K) by measuring fluorescence lifetimes with the time-correlated single-photon counting technique. In non-viscous solvents such as isopentane, ethanol and EPA τ_ƒ increases from ca. 0.2 ns at room temperature to a maximum value of 15.9 ns at ca. 123 K, but the fluorescence decays always remain single exponential. The activation energies and frequency factors associated with temperature-dependent radiationless processes are calculated to be ca. 21 kJ mol^–1 and 10¹³ s^–1, respectively. On the other hand, τ_f values in highly viscous solvents such as glycerol and poly(methyl methacrylate)(PMMA) are already rather long at room temperature (4–10 ns), and the fluorescence decays are not single exponential but are analysed as double or triple exponential. A detailed analysis of the multiple exponentials yields 7.8 ± 1.2 kJ mol^–1 and (2.8 ± 0.1)× 10⁹ s^–1 as the activation energy and frequency factor, respectively, for the radiationless process from S₁ in PMMA. A large difference in the frequency factors between the non-viscous solvents and in PMMA implies the existence of two different temperature-dependent radiationless processes from S₁. Very fast [graphic omitted] internal conversion is thought to be dominant in non-viscous solvents. However, in PMMA this internal conversion is totally suppressed and instead decay via an intermediate X, which may finally lead to 9-t-butyl-9,10-(Dewar anthracene), is proposed to be competitive with fluorescence, thus making the fluorescence decay double exponential.