Spontaneous Emission Study of the Femtosecond Isomerization Dynamics of Rhodopsin

The spontaneous emission spectra and yields of the visual pigment rhodopsin and its 9- cis retinal analog isorhodopsin have been measured following excitation at 472.7, 514.5, and 568.2 nm. The fluorescence quantum yields are (0.9 ± 0.3) × 10 -5 for rhodopsin and (1.8 ± 0.7) × 10 -5 for isorhodopsin. By use of a Strickler−Berg analysis, these quantum yields correspond to excited electronic state lifetimes of 50 and 100 fs for rhodopsin and isorhodopsin, respectively. The fluorescence spectra also undergo a blue-shift of ∼1800 cm -1 upon shifting the excitation wavelength from 568.2 to 472.7 nm. This is consistent with the idea that the emission arises from unrelaxed, nonstationary vibrational states. For rhodopsins, there is a correlation among these rapid initial nuclear dynamics out of the Franck−Condon region, the rapid photoisomerization rates, and the high isomerization quantum yields. These results support a new mechanism for the isomerization reaction dynamics in visual pigments where the reaction efficiency is mechanistically linked to the reaction rate through a dynamic Landau−Zener tunneling process.