Triplet excited states of nitronaphthalenes. IV. 1,2‐ and 1,8‐dinitronaphthalenes

Abstract

AbstractNanosecond flash photolysis of 1,2‐ and 1,8‐dinitronaphthalenes (1,2‐DNO2N; 1,8‐DNO2N) in nonpolar and polar solvents shows transient species with absorption maxima and lifetimes dependent on solvent polarity. In deaerated n‐hexane the absorption maxima and lifetimes (1/K) are 490 nm and 1.0 μsec for 1,2‐DNO2N and 550 nm and 2.5 μsec for 1,8‐DNO2N. In deaerated ethanol the corresponding values are 550 nm and 4.3 μsec for 1,2‐DNO2N and 590 nm and 5.3 μsec for 1,8‐DNO2N. The transient absorptions are attributed to the lowest triplet excited states T1 of the 1,2‐ and 1,8‐DNO2N. The observed red shifts in the absorption maxima of the T1 states are indicative of the extent to which electronic charge is transferred intramolecularly during the T1 → Tn transitions. Furthermore, the increased lifetime of the T1 states with increasing solvent polarity indicates the intramolecular charge transfer character of the T1 states. Changes of dipole moments accompanying the T1 → Tn transitions as well as rate constants for electron or proton transfer and hydrogen abstraction reactions involving the T1 states of 1,2‐ and 1,8‐DNO2N and tributyl tin hydride (Bu3SnH) as the hydrogen donor were determined together with the activation energy of the hydrogen abstraction reaction for the case of 1,2‐DNO2N. The spectroscopic and kinetic data obtained in this work demonstrate that the triplet states of 1,2‐ and 1,8‐DNO2N behave like n → π* states in nonpolar media while in polar solvents the n → π* character of these states is reduced with a simultaneous increase in their intramolecular charge transfer character.