Transition dipole orientation of linear polyenes: Semiempirical models and extrapolation 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 oriented along the major axis of the polyene chain. In this paper, the experimental values of this off-axis angle are compared to semiempirical and ab initio theoretical predictions. The semiempirical computations are shown to be reliable and are extended to long chains (n = 20). The results are then extrapolated to the infinite chain Limit. A nonzero value of 6 degrees-11 degrees for the infinite polyene is suggested by this extrapolation but state of the art ab initio results for a series of small polyenes yield a zero asymptotic value.