The Isomer Dependence of Oscillator Strengths in Retinal and Related Molecules. Spectroscopic Assignments

The spectroscopic properties of the higher excited states of retinal isomers and related molecules are considered in detail. A simple graphical description is used to characterize the transition moments to various excited states and it is shown that the isomer dependence of oscillator strengths is a particularly reliable parameter for making spectroscopic assignments. We show that the 280-nm (β) band of retinal isomers is the cis band corresponding to a transition to the lowest A_g⁺ state. It derives intensity from the s-cis ring chain conformation in all retinal isomers and is further enhanced by cis conformations along the polyene chain. It is particularly intense for conformations with central cis double bonds in keeping with the classical behavior expected for cis bands. Our assignment of the cis band allows us to explain a large number of experimental observations including the appearance of vibrational structure for this band alone at low temperature. The 250-nm (γ) band in retinais is not assigned; however, we show that previous assignments for this transition are incorrect. Based on our results for retinais we are able to identify the cis (β) and γ bands in the spectrum of both rhodopsin and bacteriorhodopsin.