Fluorescence and Photodimerization Studies of Cyano-Substituted Diphenylbutadienes

Fluorescent emission and photodimerization of crystalline (1 E,3 E )-1,4-diphenylbutadiene ( 2, DPB), (1 Z,3 E )-1-cyano-1,4-diphenylbutadiene ( 3, CDPB) and (1 Z, 3 E )-1-cyano-3-methyl-1,4-diphenylbutadiene ( 4, CMDPB) have been studied. Although crystals of all the three dienes exhibit red-shifted fluorescence, only CDPB ( 3 ) crystals give [2 + 2] photoproduct on photolysis, despite having a rather long center-to-center distance of 5.04 Å between cyano-substituted double bonds. Large differences in the absorption and fluorescence excitation spectra are observed for crystalline dienes. For example, absorption by a solution of CDPB ( 3 ) in n -hexane is at 339 nm, while its crystals show excitation maximum at 414 nm. However, in solution, the excitation spectra are quite similar. Further, the excitation spectra of crystalline DPB ( 2 ) and CMDPB ( 4 ) are much broader and considerably blue shifted relative to that of CDPB ( 3 ). The difference in the photophysical behavior is related to the crystal structure differences and reactivity differences. The results are discussed in terms of the possible involvement of either excimers or ground state molecular aggregates in the observed photoprocesses.