Photochemistry funnels

Figure 5.1. Various adiabatic photochemical reaction mechanisms (see text for details), (a) Simple case of dual fluorescence (b) illumination changes sample (i.e., photochemistry) (c) strong fluorescence quenching (photochemical funnel) (d) competitively coupled product species (e) consecutively coupled product species.

We hope the reader has been convinced that it is technically feasible to describe a photochemical reaction coordinate, from energy absorption to photoproduct formation, by means of methods that are available in standard quantum chemistry packages such as Gaussian (e.g., OPT = Conical). The conceptual problems that need to be understood in order to apply quantum chemistry to photochemistry problems relate mainly to the characterization of the conical intersection funnel. We hope that the theoretical discussion of these problems and the examples given in the last section can provide the information necessary for the reader to attempt such computations. 

Regions of surface touching or near touching are referred to jointly as funnels (Michl, 1972). As we shall discuss, they play a central role in photochemistry. 

BonaCiC-Kouteck, V., Kouteck, J., Michl, J. (1987), Neutral and Charged Biradicals, Zwitterions, Funnels in S, and Proton Translocation Their Role in Photochemistry, Photophysics and Vision, Angew. Chem. Int. Ed. Engl. 26, 170. 

A schematic representation of the surfaces for the carbon-carbon attack is shown in Figure 7.39. The very flat region of the S surface (barriers of the order of I kcal/mol) corresponds to the C.O-biradical. The biradical has a CC bond length of 156 pm and corresponds to a conical intersection geometry in the case of the singlet, and to a minimum in the case of the triplet. Thus for the singlet photochemistry the decay to So occurs close to the products, and the reaction appears to be concerted. Since, however, the formation of the singlet biradical is also possible from the same funnel, a certain fraction of photoexcited reactant can evolve via a noncon-certed route. 

P. Bruckmann, 1. Salem, Coexistence of two opossitely polarized zwitterionic forms on the lowest excited singlet surface of terminaUy twisted butadiene. Two-funnel photochemistry with dual stereochemistry, J. Am. Chem. Soc. 98 (1976) 5037. 

Bonacic-Koutecky V, Koutecky J, Michl J (1987) Neutral and charged Brradicals, Zwitterions, funnels in SI, and proton translocation their role in photochemistry, photophysics, and vision. Ang Chem 26 170... 

Schematic surface diagram for a diabatic photoreaction. Excitation is followed by a geometry change on Sj toward the funnel region, designated by a box in the figure. At the minimum on S, relaxation to So occurs. Depending on the precise nature of this jump from one surface to another, the photochemistry can be productive (producing product) or non-productive (reforming starting material).

Funnels or conical intersections play a crucial role in photochemistry. They provide an efficient exit point from the excited state to the ground state, and all photochemical reactions must end up back on the ground state surface. In addition, the precise geometry of the funnel determines whether the photochemistry is efficient—that is, whether it tends to produce product (exits to the "right"). What kind of geometries should be conducive to funnel formation That is, what structures have a very small gap between the So and Si surfaces This is... 

There are variations on the diabatic process. One important feature is that some reactions will have a small barrier on S that separates the initial excited state geometry from the funnel geometry. This can adversely affect photochemical efficiency and produce temperature dependent quantum yields. Still, the basic idea of finding geometries in which the excited state and ground state are close in energy is central to photochemistry. 

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