The major goal of "Theoretical and Experimental Investigation of Photoheterolysis Reactions" is to understand the structure-reactivity relationships of photoheterolysis reactions.  Such photoheterolysis reactions are important in the development of photo cages (molecules that can release bioactive substrates using light), in understanding the photo degradation channels of pollutants and aerosols, and in chemical synthesis for metal free coupling reactions.  We have made a number of breakthroughs in this regard:

1) We have evaluated that the strategy of looking for carbocations with low-lying diradical states is an effective one for identifying new photoprecursors.  To this end, we have performed a computational screen of molecules with low-lying diradicals and shown that most photoactive substrates lead to carbocations with low-lying diradical states. Evidence for the importance of post transition-state conical intersections also come from static models. To provide a basic feasibility test of whether it is possible for post-transition-state dynamical considerations to govern the outcomes of a photoheterolysis reaction, we computed the ground state minimum (S0), the excited-state minimum (S1), and the conical intersection between S0 and S1 for several representative cations formed from photoheterolysis reactions. Our computations support the possibility that the photoreactions could be under conical intersection control (hypothesis 2). The unstable cations favored from photoheterolysis have access to a nearby low-energy conical intersection compared to the excited state minimum, whereas stable cations have energetically inaccessible conical intersections relative to the excited state minimum.

2) We have discovered an intimate connection between successful direct photoheterolysis reactions and generation of carbocation diradicals. Surprisingly, during our computational investigations, we discovered that carbocations that are formed from direct photoheterolysis reactions feature low-lying diradical states. In many cases, the diradical form of the carbocation is the computed ground state, not the expected closed-shell singlet carbocation configuration. Consistent with our computational predictions, Givens and Ramamurthy and coworkers recently reported experimental support that the carbocation rom the coumarin photocage has a diradical ground state,  in accord with our predictions. We have identified new classes of carbocations and other reactive intermediates with low-lying diradical states and in some cases diradical ground states.  These are candidate structures for novel photo cages. 

3) We have acquired femtosecond laser flash photolysis data on representative photoheterolysis substrates.  Early results indicate that the ortho-methoxy benzylic cation is an exception that proves the rule, since we discovered that its anomalous photoreactivity arises as a consequence of it going through an unusual adiabatic photoreaction mechanism.

4) We have synthesized a suite of photoheterolysis chromophores that can release substrates with visible light based on BODIPY dyes.  By tuning the structure, we have identified derivatives with exceptionally high quantum yields of photorelease.  We have collaborated with a colleague (Prof Emily Smith) to show that these work in living cells without phototoxicity.

This work has broader impact because understanding how photoheterolysis reactions work contributes to the bigger problem of understanding the mechanistic principles of how structure impacts photochemical reactivity.  From a human resources perspective, my lab has trained numerous female and under-represented students and undergraduates, and developed a website featuring video interviews with successful women academics.

Last Modified: 04/17/2019
Modified by: Arthur Winter