ABINITIO CALCULATIONS ON THE LOWER EXCITED-STATES OF SHORT-CHAIN POLYENALS

The lower excited states of the planar all-trans isomers of crotonaldehyde, hexadienal, and octatrienal have been investigated by multi-reference single and double excitation configuration interaction (MRD-CI) calculations. For crotonaldehyde we find both the lowest excited singlet state S1 and the lowest triplet state T0 to be n-pi* in nature; for hexadienal and octatrienal S1 is n-pi* and T0 is pi-pi* in nature. Bond-order reversal stabilizes the lower excited states and destabilizes the ground state. The pi-pi* states decrease more in energy than the n-pi* states upon lengthening the polyenal. We calculate the radiative lifetime of the 3-pi-pi* state in the ground-state geometry to vary from 35 ms for crotonaldehyde to 75 s for octatrienal and explain the absence of phosphorescence for polyenals.