Conical Intersections and the Mechanism of Singlet Photoreactions

Abstract

In contrast to the previously accepted theoretical model, the reaction funnel of a nonadiabatic photoreaction, which has similar significance as the transition state of a thermal reaction, is a real conical intersection (CI) at geometries of low symmetry. This is shown schematically on the right.

Inverted Substrate Preferences for Photochemical Heterolysis Arise from Conical Intersection Control

“…For example, the meta-donor-substituted systems have an excited singlet ion diradical form that is electronically analogous to the classic meta xylylene diradical, 49 with a radical at the "carbenium center" and a cation radical donor substituent. There are numerous examples of cations that fall within this type (9,12,13,17,18,19,22,23,24,25,26,27,31). Thus, while the donor group does not act to stabilize the ground-state cation via resonance, it leads to stabilized singlet diradical excited states.…”

“…24, 25 As a result, the role and importance of the conical intersection for nonadiabatic photoreactions has been likened to that of the transition state for thermal reactions in terms of governing the photoreaction. 26 For example, the propensity of many photoreactions to generate strained molecules has been attributed in part to conical intersection control, 16 wherein highly strained photoproducts are located at energetic spikes on the ground-state surfaces leading to nearby conical intersections with the excited state, providing a productive channel for the photoreaction to proceed from the excited-state to the strained ground-state minimum.…”

How the Conical Intersection Seam Controls Chemical Selectivity in the Photocycloaddition of Ethylene and Benzene

“…Ortho addition is preferred when there is a substantial difference between the electron-donor and electron-acceptor properties of the arene and the alkene, and meta when these differences are small. Finally, the [1,4] pathway takes place in a very few cases where the steric factors are important 16 or when the alkene is an allene 23 or a diene 24,25 . In the case of the unsubstituted reactants (benzene + ethylene), the wavelength of light used in experiments [26][27][28] suggests that the reaction proceeds via the lowest-lying singlet excited state of benzene 11,12,15,19,21,22,26 .…”

“…The two branches are connected at a point where the potential energy surfaces become degenerate known as a conical intersection (CI) [1][2][3][4][5][6][7] . However, a conical intersection is not an isolated point, but a collection of points, which we will refer to as a conical intersection "seam" 1,[8][9][10] .…”

“…The prototype example of such a photocycloaddition is the reaction of benzene with ethylene [11][12][13][14][15][16][17][18][19][20][21][22] . Three cycloaddition modes (regioselectivity) can be distinguished (See Scheme 1): ortho-cycloaddition [1,2], meta-cycloaddition [1,3], and para-cycloaddition [1,4], and many applications of these reactions in organic synthesis have been described, as they afford the possibility to obtain polycyclic compounds in one step, which is important in the design of more complex molecular frameworks. …”