FLUORESCENCE AND PHOTODIMERIZATION STUDIES OF CYANO-SUBSTITUTED DIPHENYLBUTADIENES

Fluorescent emission and photodimerization of crystalline (1E,3E)-1,4- diphenylbutadiene (2, DPB), (IZ,3E)l-cyano-1 ,4-diphenylbutadiene (3, CDPB) and (1Z, 3E)-1-cyano-3-methyl-1,4-diphenplbutadiene (4, CMDPB) h ave been studied. Although crystals of all the three dienes exhibit re d-shifted fluorescence, only CDPB (3) crystals give [2 + 2] photoprodu ct on photolysis, despite having a rather long center-to-center distan ce of 5.04 Angstrom between cyano-substituted double bonds. Large diff erences in the absorption and fluorescence excitation spectra are obse rved for crystalline dienes. For example, absorption by a solution of CDPB (3) in n-hexane is at 339 mn, while its crystals show excitation maximum at 414 nm. However, in solution, the excitation spectra are qu ite similar. Further, the excitation spectra of crystalline DPB (2) an d CMDPB (4) are much broader and considerably blue shifted relative to what of CDPB (3). The difference in the photophysical behavior is rel ated to the crystal structure differences and reactivity differences. The results are discussed in terms of the possible involvement of eith er excimers or ground state molecular aggregates in the observed photo processes.