ABSTRACT
Visual perception is one of the most fascinating processes devised by nature. Through ultrafast (200 fs) photoinduced stereoselective isomerisation of its 11-cis retinal chromophore, the visual protein rhodopsin (Rh) converts the energy of a single photon into chemical energy with remarkable efficiency (65% quantum yield). This paradoxically fast and efficient but one-way photoactivated reaction calls for a primary event that is mediated by a strongly “peaked” conical intersection (CI) [1-2]. However, isorhodopsin (isoRh), a visual pigment analog containing a 9-cis retinal chromophore, isomerizes around the 9=10 double bond with a surprisingly lower quantum yield (22%) and slower rate [3], suggesting a different topography in its excited state and/or S1/S0 crossing seam. Thus, Rh and isoRh appear as the ideal systems to study and address how the excited state topography affects the photoisomerization efficiency and the dynamics of transitions between electronic states at CIs.
In this talk we compare the photophysical and photochemical behaviour of retinal chromophores in the gas phase and within the visual rhodopsin proteins (Rh and isoRh), providing a review of the latest achievements in this field and next forthcoming challenges. A hybrid QM(CASSCF)/MM(Amber) approach to compare the CI dynamics of Rh with that of isoRh is employed. In agreement with the experiments, we find a longer excited state lifetime in isoRh with pulsed stimulated emission decay and photoproduct formation. A mechanistic interpretation is provided.

[1] D. Polli, et al. Nature 467, 440-443 (2010). [2] O. Weingart, et al. PCCP 13, 3645-3648 (2011). [3] R. W. Schoenlein, et al. Phys. Chem. 97, 12087-12092 (1993)