ELECTRONIC RELAXATION IN ISOMERS OF 9,10-DIMETHOXYANTHRACENE AND THEIR RARE-GAS COMPLEXES

Two types of red shift observed upon complexation of the title compound (DMEOA) with the rare-gas atoms (Ar, Kr, and Xe) have revealed the existence of two stereo isomers (cis and trans) of DMEOA. The main vibronic bands of the uncomplexed DMEOA in the range 25 790-28 630 cm-1 can be assigned to either isomer. The fluorescence lifetime of the uncomplexed trans isomer is 28.1 ± 0.1 ns in the electronic origin and is rather insensitive to the excess energy, being 15.9 ± 0.1 ns even at the excess energy of 2835 cm-1. For the 1201 or 1202 band DMEOA exhibits isomer-dependent fluorescence lifetimes or intensities, but, for the other main vibronic levels, there is no significant difference in τf. Although both isomers form 1:1 complexes with rare-gas atoms in comparable yields, the formation of higher coordination complexes is less feasible for the cis isomer. In contrast to 9-methoxyanthracene (MEOA), all the rare-gas complexes have fluorescence lifetimes lying between 25 and 33 ns and no external heavy atom effect is found at any main vibronic level examined. The photodissociation of the vdW complexes and energy redistribution after dissociation will also be discussed in relation to the greatly shortened fluorescence decays in thermalized states of DMEOA and the rather insensitive excess-energy dependence of the fluorescence lifetime of the supercooled bare molecule. © 1990 American Chemical Society.