Inverted Substrate Preferences for Photochemical Heterolysis Arise from Conical Intersection Control

Abstract

Heterolytic bond scission is a staple of chemical reactions. While qualitative and quantitative models exist for understanding the thermal heterolysis of carbon-leaving group (C-LG) bonds, no general models connect structure to reactivity for heterolysis in the excited state. CASSCF conical intersection searches were performed to investigate representative systems that undergo photoheterolysis to generate carbocations. Certain classes of unstabilized cations are found to have structurally nearby, low-energy conical intersections, whereas stabilized cations are found to have high-energy, unfavorable conical intersections. The former systems are often favored from photochemical heterolysis, whereas the latter are favored from thermal heterolysis. These results suggest that the frequent inversion of the substrate preferences for nonadiabatic photoheterolysis reactions arises from switching from transition-state control in thermal heterolysis reactions to conical intersection control for photochemical heterolysis reactions. The elevated ground-state surfaces resulting from generating unstabilized or destabilized cations, in conjunction with stabilized excited-state surfaces, can lead to productive conical intersections along the heterolysis reaction coordinate. Disciplines Materials Chemistry | Other Chemistry | Physical Chemistry CommentsReprinted (adapted) with permission from J. Am. Chem. Soc., 2014, 136 (25) ABSTRACT: Heterolytic bond scission is a staple of chemical reactions. While qualitative and quantitative models exist for understanding the thermal heterolysis of carbon−leaving group (C−LG) bonds, no general models connect structure to reactivity for heterolysis in the excited state. CASSCF conical intersection searches were performed to investigate representative systems that undergo photoheterolysis to generate carbocations. Certain classes of unstabilized cations are found to have structurally nearby, low-energy conical intersections, whereas stabilized cations are found to have high-energy, unfavorable conical intersections. The former systems are often favored from photochemical heterolysis, whereas the latter are favored from thermal heterolysis. These results suggest that the frequent inversion of the substrate preferences for nonadiabatic photoheterolysis reactions arises from switching from transition-state control in thermal heterolysis reactions to conical intersection control for photochemical heterolysis reactions. The elevated ground-state surfaces resulting from generating unstabilized or destabilized cations, in conjunction with stabilized excited-state surfaces, can lead to productive conical intersections along the heterolysis reaction coordinate.

A New Photocage Derived from Fluorene

“…[39,40] As ar esult,t he fluorenyl carbocation has al ow-energy excited state and ah igh-energy ground state. The corresponding potential-energy surfaces are located in close proximity and this can lead to productivep hotoheterolysis through conicali ntersection, as recently postulated by Winter et al [41] Lukemana nd co-workers showed uncaging of 2,7-disubstituted-9-fluorenol acetate by irradiation at l = 300 nm. [42] Fluorene quaternary ammonium derivatives have also been used as ap hotobase to releasef ree tertiary amines.…”

BODIPY-Derived Photoremovable Protecting Groups Unmasked with Green Light

“…52 A recent computational study performed in our lab suggested the hypothesis that photoheterolysis reactions may be under conical intersection control. 53 That is, photoheterolysis of C-LG (carbon-leaving group) bonds to generate ion pairs 54 may be favored if the ion pair has access to a nearby productive conical intersection that provides an efficient channel for the excited state of the photoprecursor to decay to the ground-state ion pair. Because conical intersections are challenging to compute, we further suggested using the vertical energy gap of the carbocation to its first excited state as a simple predictor of a nearby conical intersection (CI).…”

Photoconversion of β-Lapachone to α-Lapachone via a Protonation-Assisted Singlet Excited State Pathway in Aqueous Solution: A Time-Resolved Spectroscopic Study

“…Winter and co-workers reported that some unstabilized carbocations have been found to have structurally nearby, low-energy conical intersections that are often favored for photochemical heterolysis, whereas some stabilized carbocations have been found to have high-energy, unfavorable conical intersections, which may be favored for thermal heterolysis. 50 For the formation of the carbocation intermediates in Scheme 2, the carbocation intermediate formed by the cleavage of the O 5 −C 6 bond could be considered unstabilized carbocations, so the cleavage of the O 5 −C 6 bond may be a photo-heterolysis control process rather than a thermal heterolysis process. 50 Therefore, the protonation of excited state β-LA may directly help induce the cleavage of the O 5 −C 6 bond and is hence proposed as being the predominant pathway (as shown in Scheme 2).…”

“…50 For the formation of the carbocation intermediates in Scheme 2, the carbocation intermediate formed by the cleavage of the O 5 −C 6 bond could be considered unstabilized carbocations, so the cleavage of the O 5 −C 6 bond may be a photo-heterolysis control process rather than a thermal heterolysis process. 50 Therefore, the protonation of excited state β-LA may directly help induce the cleavage of the O 5 −C 6 bond and is hence proposed as being the predominant pathway (as shown in Scheme 2).…”