The excited electronic states of linear conjugated polyenes related to the visual pigments will be investigated with emphasis on (1) the ordering of the low lying states of very short chains (butadiene and hexatriene) (2) the location and properties of the higher states of these systems including those involving sigma excitations and (3) the determination of the equilibrium conformation of these molecules in their low lying excited states. The long range goal is an understanding of the mechanism of the visual photochemical process. Small polyenes will be studied in detail because these molecules can be treated by nearly state of the art quantum mechanical methods. Comparison of theory with experiment will determine the adequacy of these theoretical methods and aid in the determination of possible simplifications which will permit calculations to be performed on the visual pigment chromophores themselves. It is clear that in the long run theoretical models will be of great utility in the determination of the mechanism of vision. What is needed is an adequate test of theory before the actual biochemical system can be treated reliably. The short polyenes, butadiene and hextriene, are non-fluorescent precluding the use of emission methods in the study of the lowest states. This presents a severe technical restriction because it eliminates the use of this highly sensitive technique. We here propose to determine the location and properties of the excited states of these systems using resonance Raman scattering in the ultraviolet region, thermal detection of two-photon excitation and the use of fluorescent derivatives of these species. (We have recently shown that 2,4,6-octatriene is fluorescent but is otherwise spectrally identical to hextriene.) Both low temperature cryogenic methods and supersonic nozzle beam methods will be used.
