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°−11° 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.