| Abstract
| - The widely investigated photobehaviors of 9-bromo and 9,10-di-bromoanthracenes have been revisited hereto clarify the competition among different relaxation paths of their lowest two electronic excited states. Theresults obtained show that these two molecules exhibit a parallel photobehavior, which depends on the excitedelectronic and vibronic transition, the medium viscosity, and the temperature. The first electronic state ofeither of these does not exhibit photochemistry in fluid solution or rigid matrices (80 K). The fluorescenceemission occurs with a very low quantum yield (∼10-2) at room temperature but with a very high quantumyield (0.9 to ∼ 1) at 80 K. When exciting in the second electronic transition, the fluorescence intensity islower than when exciting in S1 at both room temperature and low temperature due to competition with theobserved photocleavage of the C−Br bond. The reaction yield decreases as the temperature decreases anddepends 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 bondbreakage cannot escape from the solvent cage and, moreover, quickly recombine within the cage giving theappearance that no photochemistry occurred. The presence of photochemistry in S2 and its absence in S1 isprincipally due to the fact that S2 has a π,σ* character in the C−Br bond, whereas the S1 state has its originfrom a π,π* delocalized configuration.
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