Transition dipole orientation of linear polyenes: Semiempirical models andextrapolation to the infinite chain limit

Linear conjugated polyenes have an electronic transition dipole moment for the strongly allowed 1A(g) to 1B(u) electronic excitation that is not orien ted along the major axis of the polyene chain. In this paper, the experimen tal values of this off-axis angle are compared to semiempirical and ab init io theoretical predictions. The semiempirical computations are shown to be reliable and are extended to long chains (n = 20). The results are then ext rapolated to the infinite chain Limit. A nonzero value of 6 degrees-11 degr ees for the infinite polyene is suggested by this extrapolation but state o f the art ab initio results for a series of small polyenes yield a zero asy mptotic value.