The widely investigated photobehaviors of 9-bromo and 9,10-di- bromoanthracenes have been revisited here to clarify the competition among different relaxation paths of their lowest two electronic excited states. The results obtained show that these two molecules exhibit a parallel photobehavior, which depends on the excited electronic and vibronic transition, the medium viscosity, and the temperature. The first electronic state of either of these does not exhibit photochemistry in fluid solution or rigid matrices (80 K). The fluorescence emission occurs with a very low quantum yield (∼10 -2) at room temperature but with a very high quantum yield (0.9 to ∼ 1) at 80 K. When exciting in the second electronic transition, the fluorescence intensity is lower than when exciting in S 1 at both room temperature and low temperature due to competition with the observed photocleavage of the C-Br bond. The reaction yield decreases as the temperature decreases and depends on the viscosity of the solvent; the higher the viscosity, the lower the observed yield of photochemistry. Temperature and viscosity effects are a consequence of the fact that the radicals produced by C-Br bond breakage cannot escape from the solvent cage and, moreover, quickly recombine within the cage giving the appearance that no photochemistry occurred. The presence of photochemistry in S 2 and its absence in S 1 is principally due to the fact that S 2 has a π,σ* character in the C-Br bond, whereas the S 1 state has its origin from a π,π* delocalized configuration
Effects of the exciting wavelength and viscosity on the photo-behavior of 9- and 9,10- bromoanthracenes
FAVARO, Giovanna;DI NUNZIO, MARIA ROSARIA;GENTILI, Pier Luigi;ROMANI, Aldo;
2007
Abstract
The widely investigated photobehaviors of 9-bromo and 9,10-di- bromoanthracenes have been revisited here to clarify the competition among different relaxation paths of their lowest two electronic excited states. The results obtained show that these two molecules exhibit a parallel photobehavior, which depends on the excited electronic and vibronic transition, the medium viscosity, and the temperature. The first electronic state of either of these does not exhibit photochemistry in fluid solution or rigid matrices (80 K). The fluorescence emission occurs with a very low quantum yield (∼10 -2) at room temperature but with a very high quantum yield (0.9 to ∼ 1) at 80 K. When exciting in the second electronic transition, the fluorescence intensity is lower than when exciting in S 1 at both room temperature and low temperature due to competition with the observed photocleavage of the C-Br bond. The reaction yield decreases as the temperature decreases and depends on the viscosity of the solvent; the higher the viscosity, the lower the observed yield of photochemistry. Temperature and viscosity effects are a consequence of the fact that the radicals produced by C-Br bond breakage cannot escape from the solvent cage and, moreover, quickly recombine within the cage giving the appearance that no photochemistry occurred. The presence of photochemistry in S 2 and its absence in S 1 is principally due to the fact that S 2 has a π,σ* character in the C-Br bond, whereas the S 1 state has its origin from a π,π* delocalized configurationI documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
