Temperature effects on the phosphorescence spectra and lifetimes of 2,4-, 2,5-, and 3,4-dimethylbenzaldehydes dispersed in durene single crystals

Abstract

The phosphorescence spectra and lifetimes of 2,4-, 2,5-, and 3,4-dimethylbenzaldehydes dispersed in durene single crystals have been measured as a function of temperature between 10 and 200 K. For all the guests involved, the vibrational structures of the spectra are found to be temperature dependent. This is interpreted in terms of two emissions that proceed from a triplet state having predominantly a ππ* character at low temperatures and from a thermally populated triplet state having essentially a nπ* character at higher temperatures. The energy gaps Δ E T between 3ππ* and 3nπ* states evaluated spectroscopically are found to be 100, 70, and 340 cm −1, respectively for 2,4-, 2,5- and 3,4-dimethylbenzaldehydes. Activation energies Δ E* determined from the Arrhenius plots of the phosphorescence decay rate constants are in good agreement with the Δ E T for the first two guests. In contrast, the Δ E* are higher than the Δ E T for 3,4-dimethylbenzaldehydes as well as for 2,4,5-trimethylbenzaldehyde (where Δ E T ≈ 400 cm −1) because of the rapid increase of radiationless transitions in the temperature range where thermal population of the upper 3nπ* state is efficient. In the low and high temperature ranges, the phosphorescence decays for all these guests are exponential. In the intermediate range, these decays are non-exponential. The origin of these non-exponential decays is discussed.