LASER PHOTOLYSIS STUDIES ON [2+2]-PHOTOCYCLOREVERSION AND PHOTOISOMERIZATION REACTIONS OF TRANS-1,2-DI(1-PYRENYL)CYCLOBUTANE AND CIS-1,2-DI(1-PYRENYL)CYCLOBUTANE DIMERS THROUGH RADICAL ION INTERMEDIATES

Transient radical ion intermediates of [2 + 2]-photocycloreversion and photoisomerization reactions of the 1-vinylpyrene (1) cyclobutane dimers, trans- and cis-1,2-di(1-pyrenyl)cyclobutanes (2t and 2c, respectively), were studied by nanosecond laser photolysis measurements; the reaction quantum yields were determined by photoproduct analysis. The excited singlet state of dimers 2t and 2c was quenched by an electron acceptor, 1,4-dicyanobenzene, with a diffusion-controlled rate in a polar acetonitrile solvent. The transient absorption spectra of the dimers in the system showed the formation of a dimer radical cation of pyrene chromophores, whose positive charge was delocalized in the two pyrene rings. The dimers were cleaved to form 1 through the dimer radical cation. By contrast, the formation of a dimer radical anion could not be observed in quenching by an electron donor, N,N-dimethyl-p-toluidine. The cleavage yield of the dimers from the anionic species was low. The decrease in the electron density in the cyclobutane bond in the cationic state promotes the cycloreversion reaction of dimers. In the absence of an electron donor or acceptor in the nonpolar benzene solvent, the direct photocleavage of 2t and 2c to 1 through the singlet excited state occurred efficiently, whereas in polar acetonitrile the cleavage was reduced to half by solvation of the excited states. A photocleavage of dimers also occurred in their triplet states.