University of Cambridge > Talks.cam > Optoelectronics Group > Conical Intersection Dynamics in Rhodopsin and its 9-cis Isorhodopsin analog studied with femtosecond pump-probe spectroscopy.

Conical Intersection Dynamics in Rhodopsin and its 9-cis Isorhodopsin analog studied with femtosecond pump-probe spectroscopy.

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Light perception is based on a cascade of complex biochemical reactions whose first step is the ultrafast (completed in about 200 fs) and efficient (0.65 quantum yield) isomerisation of the retinal chromophore of rhodopsin around the 11=12 double bond from the 11-cis to the all-trans form. It is now well established that rhodopsin’s unique reactivity is mediated by conical intersections (CIs), i.e. singularities on the potential energy surfaces (PES) that form a multi-dimensional ‘seam’ connecting the ground and excited states at isoenergetic points. CIs are ubiquitous features in theoretical descriptions of organic photochemistry and are responsible for triggering radiationless decay and efficient and ultrafast conversion of photon energy into chemical energy.

An important open question is understanding how the topography around a seam affects the dynamics of transitions between electronic states at CIs. This can be addressed by studying analogs of the visual pigment. In this work, using broadband sub-20-fs ultrafast spectroscopy, we compare the CI dynamics of rhodopsin with that of isorhodopsin, containing a 9-cis retinal which isomerizes around the 9=10 double bond and has a significantly lower quantum yield of only 0.22.

We find that in rhodopsin the wavepacket reaches the CI in about 75 fs, after which it leaves the excited state of the reactant. This suggests that the CI seam has a strongly “peaked” topography. In isorhodopsin the wavepacket stays significantly longer in the excited state of the reactant, and observe oscillations both in the reactant stimulated emission (SE) decay and in the product photoinduced absorption (PA) build-up, which suggest multiple crossings of the CI.

This talk is part of the Optoelectronics Group series.

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