Photo-induced spin alignment utilizing the excited molecular field between the excited triplet state of phenyl- or diphenylanthracene and the dangling nitroxide radicals: theoretical investigation of the mechanism for the intramolecular spin alignment

In the previous paper [J. Am. Chem. Soc., 122 (2000) 984], we reported the first observation of the photo-excited quartet (S = 3/2) and quintet (S = 2) states arising from the radical-triplet pairs on the purely organic pi -conjugated spin systems. 9-Anthracen(4-phenyliminonitroxide) (1) and 9,10-anthracen-bis(4-phenyliminonitroxide) (2) were designed and synthesized. The time-resolved ESR (TRESR) experiments were carried out in order to study the photo-induced spin alignments on the excited states. In this paper, we report the ab initio molecular orbital calculations based on the density functional theory for the photo-excited quarter (S = 3/2) state of 1 and the quintet (S = 2) state of 2 as well as the brief summary of our TRESR experiments. The mechanism of the photo-induced spin alignment has been clarified based on the spin distribution obtained by the MO calculations. (C) 2001 Elsevier Science Ltd. All rights reserved.