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

Nanosecond flash photolysis of 1,2‐ and 1,8‐dinitronaphthalenes (1,2‐DNO 2 N; 1,8‐DNO 2 N) 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‐DNO 2 N and 550 nm and 2.5 μsec for 1,8‐DNO 2 N. In deaerated ethanol the corresponding values are 550 nm and 4.3 μsec for 1,2‐DNO 2 N and 590 nm and 5.3 μsec for 1,8‐DNO 2 N. The transient absorptions are attributed to the lowest triplet excited states T 1 of the 1,2‐ and 1,8‐DNO 2 N. The observed red shifts in the absorption maxima of the T 1 states are indicative of the extent to which electronic charge is transferred intramolecularly during the T 1 → T n transitions. Furthermore, the increased lifetime of the T 1 states with increasing solvent polarity indicates the intramolecular charge transfer character of the T 1 states. Changes of dipole moments accompanying the T 1 → T n transitions as well as rate constants for electron or proton transfer and hydrogen abstraction reactions involving the T 1 states of 1,2‐ and 1,8‐DNO 2 N and tributyl tin hydride (Bu 3 SnH) as the hydrogen donor were determined together with the activation energy of the hydrogen abstraction reaction for the case of 1,2‐DNO 2 N. The spectroscopic and kinetic data obtained in this work demonstrate that the triplet states of 1,2‐ and 1,8‐DNO 2 N 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.