Abstract

The Protonated Schiff Base of Retinal (RPSB) energizes activity in all Retinal Proteins (RPs) by absorbing light and isomerizing rapidly about a specific C=C bond. Understanding this crucial step is a prerequisite for appreciating photoinduced dynamics in this important family of photoreceptors. Accordingly, RPSB photochemistry in solution and within the RPs has been probed with ultrafast resolution from the UV to the far IR [1,2]. Despite this spectral coverage, the essential number of electronic surfaces necessary for describing isomerization in these systems remains unclear, with suggestions for involvement of more than one electronic excited state [3,4]. In the related Carotenoid (CARs) family of natural chromophores, involvement of multiple excited states is a cornerstone of photoreactivity, due to the optical “darkness” of the lowest excited singlet. This aspect of CAR photochemistry has been verified and enriched in recent years by ultrafast spectral probing in the Near IR (NIR) where absorption between the lowest excited states is directly accessible, providing lifetimes and energetics of the reactive potential surfaces [5]. In particular, a sub-ps shift from intense photoinduced absorption assigned to S₂→S_N transition, to weaker S₁→S₂ activity, is the hallmark of multiple stages of internal conversion in CARs. In contrast, the ultrafast photochemistry of RPs and their chromophore RPSB, has not been systematically explored in the NIR. Here we report the first femtosecond visible pump – NIR probe study of internal conversion in RPSB and in bacteriorhodopsin in the spectral range from 1-2 μm. Comparison of the results with those from similar experiments in CARs supports the notion that multiple excited states are involved in the photoisomerization of RPSB and RPs as well.

© 2010 Optical Society of America