Computational Photochemistry and Photobiology

Authored by: Patrick Z. El-Khoury , Igor Schapiro , Mark Huntress , Federico Melaccio , Samer Gozem , Luis Manuel Frutos , Massimo Olivucci

CRC Handbook of Organic Photochemistry and Photobiology

Print publication date:  March  2012
Online publication date:  March  2012

Print ISBN: 9781439899335
eBook ISBN: 9781466561250
Adobe ISBN:

10.1201/b12252-42

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Abstract

When a thermally equilibrated chromophore is promoted to an excited electronic state by light absorption, nuclear motion is triggered. The evolution of the original molecular and electronic structures determines the fate of the absorbed light energy that can either be released (via internal conversion/ luminescence) or exploited to drive a change in the original chromophore structure (i.e., a photochemical reaction). Throughout these processes, the initially populated Franck–Condon (FC) active modes couple to other available normal modes, this coupling rendered even more effective by the anharmonicity of molecular vibrations. 1,2 Computational chemists view such events in terms of the evolution of the chromophore’s nuclear coordinates on different 3N-6 dimensional potential energy surfaces, where N is the number of atoms. These are rigorously defined on the basis of the Born–Oppenheimer (BO) approximation, 3,4 which states that the relatively large mass of a nucleus, as compared to that of an electron, permits the separation of electronic and nuclear motions. A consequence of the BO approximation is that for any molecular system, there exists a different potential energy surface for each electronic state. Since in these processes, the initial excited state electronic structure ultimately evolves into a ground state electronic structure, the computational description of light-energy wastage and exploitation at the molecular level requires the mapping of different potential energy surfaces. Most importantly, one has to attain an understanding of how and where potential energy surfaces connect.

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