Photoexcited triplet states of new UV absorbers, cinnamic acid 2-methylphenyl esters

Abstract

Phosphorescence spectra of nonphosphorescent or very weakly phosphorescent new UV absorbers, 2-methylphenyl cinnamate (MePC), 2-methylphenyl 4-methoxycinnamate (MePMC) and 2-methylphenyl 4-ethoxycinnamate (MePEC) have been observed by using external heavy atom effects of ethyl iodide in ethanol at 77 K. The lowest excited triplet (T₁) energies of these new UV absorbers are lower than those of a widely used UV-A absorber, 4-tert-butyl-4′-methoxydibenzoylmethane (BM-DBM), in both keto and enol forms. The intermolecular triplet–triplet energy transfer from photolabile BM-DBM to MePMC was observed by measuring the time-resolved phosphorescence spectra. Electron paramagnetic resonance spectra have been observed for the T₁ states of these new UV absorbers in ethanol at 77 K by using benzophenone as a triplet sensitizer. The observed T₁ lifetimes, zero-field splitting (ZFS) parameters and molecular orbital calculations of the ZFS parameters suggest that T₁ states of these new UV absorbers posses mainly ³ππ* character. The deactivation processes of the lowest excited singlet (S₁) states are predominantly fluorescence and internal conversion to the ground (G) states in MePMC and MePEC, while the main deactivation process of the S₁ state of MePC is internal conversion to the G state. The molar absorption coefficients of MePMC and MePEC in the UV-A and UV-B regions are larger than that of most widely used UV-B absorber, octyl methoxycinnamate.