π Topology and Spin Alignment in Unique Photoexcited Triplet and Quintet States Arising from Four Unpaired Electrons of an Organic Spin System

Syntheses, electronic structures in the ground state, unique photoexcited states, and spin alignment are reported for novel biradical 1, which was designed as an ideal model compound to investigate photoinduced spin alignment in the excited state. Electron spin resonance (ESR), time‐resolved ESR (TRESR), and laser‐excitation pulsed ESR experiments were carried out. The magnetic properties were examined with a SQUID magnetometer. In the electronic ground state, two radical moieties interact very weakly (almost no interaction) with each other through the closed‐shell diphenylanthracene spin coupler. On photoirradiation, a novel lowest photoexcited state with the intermediate spin (S=1) arising from four unpaired electrons with low‐lying quintet (S=2) photoexcited state was detected. The unique triplet state has an interesting electronic structure, the D value of which is reduced by antiferromagnetic spin alignment between two radical spins through the excited triplet spin coupler. The general theoretical predictions of the spin alignment and the reduction of the fine‐structure splitting of the triplet bis(radical) systems are presented. The fine‐structure splitting of the unique photoexcited triplet state of 1, as well as the existence of the low‐lying quintet state, is interpreted well on the basis of theoretical predictions. Details of the spin alignment in the photoexcited states are discussed. Photoinduced spin alignment and unique photoexcited states of 1 are reported. The two radical moieties interact very weakly in the ground state. By photoirradiation, a novel lowest photoexcited triplet (S=1) state arising from four unpaired electrons with a low‐lying quintet (S=2) photoexcited state were detected. The D value of the triplet state is reduced by antiferromagnetic spin alignment between two radical spins through the excited triplet spin coupler.