NANOSECOND LASER PHOTOLYSIS OF POLYENES AND VISUAL PIGMENTS

The techniques involved in nanosecond flash photolysis along with two studies in which these techniques are used are described. First, excited state absorption spectroscopy is used with PPP-SCF-SDCI calculations to assign the symmetry of the lowest excited state in polyenes. Calculations for 1,3,5,7 octatetraene predict different spectra for the ('1)A(,g) or the ('1)B(,u) as the lowest excited state. The experimental S(,n) (<---) S(,1) spectrum is used in conjunction with these calculations to assign the lowest excited state as the ('1)A(,g). Similar calculations are presented for butadiene and hexatriene although no excited state absorptions were observed in the smaller polyenes. Excited state absorption measurements of 1,5 diphenylhexahydronaphthalene (HHN), a rigid analog of diphenylbutadiene (DPB), indicate that the lowest excited state in DPB is nearly planar.; Second, transient spectra corresponding to the bathrohodopsin intermediate are obtained 50 ns following room temperature excitation at 457 and 532 nm (6 ns FWHM) for detergent suspensions of rhodopsin and isorhodopsin. Similar transient lifetimes and spectra around 560 nm are observed in the two pigments. The batho yields for rhodopsin and isorhodopsin are similar under the experimental conditions used here. In addition, the transient spectrum for rhodopsin 50 ns following excitation shows a weak absorption at 440 nm which is not seen in isorhodopsin.; Transient spectra in the photolysis of rhodopsin and isorhodopsin are also obtained as a function of laser power. These results indicate secondary photochemistry under high flux conditions. A model is proposed using the known photochemical parameters for photolysis of rhodopsin, isorhodopsin and bathrohodopsin and is used to predict the bleaching yield of rhodopsin and isorhodopsin as a function of laser pulse energy. Good agreement is obtained if it is assumed that rotational diffusion of rhodopsin is incomplete during the laser pulse. Since no modification of the observed steady state photochemical parameters was necessary to account for the bleaching as a function of pulse energy, it is concluded that the batho products of rhodopsin and isorhodopsin are photochemically very similar.