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

Abstract

Kanosecond flash photolysis of 1,2-and 1,8-dinitronaphthalenes (1, 1, 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 ( l / K ) are 490 nm and 1.0 psec for 1,2-L)XO& and 550 nm and 2.5 psec for l,&DNO,K. In deaerated ethanol the corresponding values are 550 nm and 4.3 psec for 1,2-DNOzN and 590 nm and 5.3 psec for l,%DNOzN. The transient absorptions are attributed to the lowest triplet excited states TI of the 1,2-and l&DNOZS. The observed red shifts in the absorption maxima of the TI states are indicative of the extent to which electronic charge is transferred intramolecularly during the 7 ' 1 + I', transitions. Furthermore, the increased lifetime of the TI states with increasing solvent polarity indicates the intramolecular charge transfer character of the Changes of dipole moments accompanying the TI -+ T,, transitions as well as rate constants for electron or proton transfer and hydrogen abstraction reactions involving the 7'1 states of 1,2-and 1,%1)N02N and tributyl tin hydride (ButSnH) as the hydrogen donor were determined together with the activation energy of the hydrogen abstraction reaction for the cme of 1,2-DNOzN. The spectroscopic and kinetic data obtained in this work demonstrate that the triplet states of 1,2-and 1,8DNOzN behave like n + T* states in nonpolar media while in polar solvents then -+ ?r* character of these states is reduced with a simultaneous increase in their intramolecular charge transfer character.states.