Department of Chemistry, Graduate School of Engineering, Yokohama National University, Tokiwadai, Hodogaya-ku, Japan.
Photochem Photobiol Sci. 2011 Dec;10(12):1902-9. doi: 10.1039/c1pp05168g. Epub 2011 Oct 14.
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 (T(1)) 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 T(1) states of these new UV absorbers in ethanol at 77 K by using benzophenone as a triplet sensitizer. The observed T(1) lifetimes, zero-field splitting (ZFS) parameters and molecular orbital calculations of the ZFS parameters suggest that T(1) states of these new UV absorbers posses mainly (3)ππ* character. The deactivation processes of the lowest excited singlet (S(1)) states are predominantly fluorescence and internal conversion to the ground (G) states in MePMC and MePEC, while the main deactivation process of the S(1) 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.
在 77 K 的乙醇中，通过利用乙基碘的外部重原子效应，观察到非磷光或磷光很弱的新型 UV 吸收剂 2-甲基苯基肉桂酸酯（MePC）、2-甲基苯基 4-甲氧基肉桂酸酯（MePMC）和 2-甲基苯基 4-乙氧基肉桂酸酯（MePEC）的磷光光谱。这些新型 UV 吸收剂的最低激发三重态（T(1)）能量低于酮式和烯醇式两种形式的广泛使用的 UV-A 吸收剂 4-叔丁基-4'-甲氧基二苯甲酮（BM-DBM）。通过测量时间分辨磷光光谱，观察到光不稳定的 BM-DBM 与 MePMC 之间的分子间三重态-三重态能量转移。通过使用苯甲酮作为三重态敏化剂，在 77 K 的乙醇中观察到这些新型 UV 吸收剂的 T(1)态的电子顺磁共振光谱。观察到的 T(1)寿命、零场分裂（ZFS）参数和 ZFS 参数的分子轨道计算表明，这些新型 UV 吸收剂的 T(1)态主要具有（3）ππ*特征。MePMC 和 MePEC 中最低激发单线态（S(1)）态的失活过程主要是荧光和内部转换到基态（G）态，而 MePC 的 S(1)态的主要失活过程是内部转换到 G 态。MePMC 和 MePEC 在 UV-A 和 UV-B 区域的摩尔吸收系数大于最广泛使用的 UV-B 吸收剂辛基甲氧基肉桂酸酯。
