Excited State Photoisomerization Paths

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. 

Any mechanistic study undertaken using quantum chemistry methods requires considerable physical and chemical insight. Thus for a thermal reaction, there is no method that will generate automatically all the possible mechanistic pathways that might be relevant. Rather, one still needs to apply skills of chemical intuition, and it is necessary to make sensible hypotheses that can then be explored computationally. In excited state chemistry, these problems are even more difficult, and we hope the examples given in the last section provide a bit of this required insight. However, the DBH example shows just how complex these problems can become when many electronic excited states are involved. 

For the future, it is clear that dynamics methods are almost essential if one is going to examine the interesting results that are coming from femtosecond spectroscopy and to study quantum yields. These methods are just beginning to be exploited, and this is an exciting new direction for quantum chemistry. We have not commented on the role of the solvent or the role of the environment provided by a biochemical system. There are no special problems related to excited state chemistry for the former, and one can look forward to applications to biochemical systems to appear in the near future. 

This chapter is based (in part) on lecture courses given by M. Olivucci at the EPA Summer School in Noorwick, Holland, June 16-20, 1998, and by M. A. Robb at the Jyvaskyla Summer School, Jyvaskyla, Finland, August 3-21, 1998. We are grateful to the organizers of these schools for the opportunity to present this material and especially to the participants for their searching questions. 

We are also pleased to acknowledge coworkers Michael Bearpark, Paolo Celani, Stephane Klein, Naoko Yamamoto, Barry Smith, and Thom Vreven, whose work we have used in this tutorial and who have commented on various parts of the manuscript. 

---

Photolysis of [Rh(tfacac)3] (tfacac is the unsymmetrically substituted 1,1,1-trifluoromethyl-acac) reveals the existence of two photoinduced reaction paths the relative efficiency of the two paths is dramatically solvent dependent.1140 In cyclohexane, mer- cis isomerization is the only observed photoreaction, but if ethanol or 2-propanol is added to the solvent, the photoisomerization efficiency decreases, and photodecomposition occurs. The nature of the photodecomposition products is not specified, but the enhanced photoreactivity in the presence of tri(n-butyl)stannane, a hydrogen atom donor, and flash and continuous photolysis studies in mixed-solvent systems strongly implicate hydrogen atom abstraction from the solvent as a key step in the photodecomposition of wer-[Rh(tfacac)3] and suggests that the photo reactive states have considerable radical character .1140 Analysis of quantum efficiencies implies that at least two distinct photoproduced excited states must be involved. 

---