π-Topology and spin alignment in the photo-excited states of phenylanthracene- t -butylnitroxide radicals

We have studied the relationship between the π-topology and the photo-excited high-spin states of π-conjugated spin systems, 9-anthracen-(3-phenyl-t-butylnitroxide) radical (1m) and 9-anthracen-(4-phenyl-t-butylnitroxide) radical (1p) systems, by time-resolved ESR and transient absorption spectroscopies. For the meta-isomer, 1m, the excited quartet high-spin state (S = 3/2) was observed, while for the para-isomer, 1p, only a weak signal of the doublet state (S = 1/2) was detected. For the quartet state of 1m, the g value and fine-structure parameters have been determined to be g = 2.005, D = 0.0250 cm(-1), and E = ∼0.0 cm(-1). The mechanism of intramolecular spin alignment and the role of spin polarization in the excited states have been discussed based on the spin density distribution calculated by ab initio molecular orbital calculations.