Unusually large coupling constants in diradicals obtained from excitation of mixed radical centers: a theoretical study on potential photomagnets.

Three sets of heterosubstituted, interconvertible, cyclophanediene (CPD), and dihydropyrenes (DDPs) and one such set involving dinitrilepyrenes were examined by UB3LYP broken-symmetry methodology with 6-311++g(d,p) bases. Nitronyl nitroxide and oxoverdazyl (with both N and C terminals) are monoradical centers, whereas CPD and DDP moieties serve as couplers. The photoexcited CPD converts to DDP. The calculated exchange coupling constant (J) for o-VER(N)-DDP-NN is surprisingly high, 6412 cm(-1), and much larger than 28.9 cm(-1) for the CPD species, but the unsubstituted DDP is known to transform readily into pyrene, with the loss of reversibility. Nevertheless, o-VER(N)-(15,16-dinitrile)DDP-NN also has a large J value, 589.4 cm(-1). The corresponding CPD species has J = 53.3 cm(-1). We predict that the latter CPD and DDP diradicals are potential molecules to synthesize photomagnetic materials. The o-VER(N)-DDP-NN can also be an excellent photomagnetic switch at a considerably low temperature.