Resacante Cars of Diphenylpolyenes in the Excited States: Vibrenic Thift of A 2Ag Scate Vibcatien

Aesonance ICAS and ISAS spectra of α,ω-diphenylpolyenes(butadiene—decapentaene) were recorded in the lowest excited singlet(S 1 ) and triplet (T 1 ) states. A symmetric stretching vibration was observed with unusually high frequency in the S 1 state of diphenyloctatetraene(1755 cm -1 ) and diphenyldecapentaene (1775 cm -1 ). This observation is most probably due to common origin with the similar frequency known in some other longer polyenes, and may certainly represent the characteristic features of polyene 2 1 A g state. The origin of the frequency shift was investigated by a perturbation calculation based on a simple valence bond formalism. It was shown that a syranetric chain mode in the 2 1 A g state is prone to cause effective vibronic coupling, predominantly between the ground and 2 1 A g states, resulting the observed large frequency shift.