Study of Photoselectivity\nin Linear Conjugated Chromophores\nUsing the XMS-CASPT2 Method

Photoisomerization, the structural alteration of molecules\nupon\nabsorption of light, is crucial for the function of biological chromophores\nsuch as retinal in opsins, proteins vital for vision and other light-sensitive\nprocesses. The intrinsic selectivity of this isomerization process\n(i.e., which double bond in the chromophore is isomerized) is governed\nby both the inherent properties of the chromophore and its surrounding\nenvironment. In this study, we employ the extended multistate complete\nactive space second-order perturbation theory (XMS-CASPT2) method\nto investigate photoisomerization selectivity in linear conjugated\nchromophores, focusing on two simple molecular models resembling retinal.\nBy analyzing electronic energies, intramolecular charge separation,\nand conical intersection topographies in the gas phase, we show that\nthe photoproduct formed by rotation around the double bond near the\nSchiff base is energetically favored. The topographic differences\nat the conical intersections leading to different photoproducts reveal\ndifferences in photodynamics. In multiphoton excitation, the primary\nphotoproduct typically reverts to the initial configuration rather\nthan rotating around a different double bond. Our study offers new\ninsights into the photodynamics of photoisomerizing double bonds in\nπ-conjugated chromophores. We anticipate that our findings will\nprovide valuable perspectives for advancing the understanding of biological\nchromophores and for designing efficient photochemical switches with\napplications in molecular electronics and phototherapy.