Exploring Organic Photochemistry with Computational Methods: An Introduction

Resumen

Research in photochemistry has substantially changed in the last decades. Some time ago, the focus of research on photochemistry was mainly the effect of light on molecules and the photochemical reactivity or photophysical properties caused by light excitation. More recently, while interest in photochemistry at the molecular level still remains, new and more complex applications to exploit light energy have been developed: photocatalysis, sensing and signaling, photoactivated molecular machines, photoprotection, biomedical and environmental uses and others. The rational design of molecules capable of performing such tasks depends on the knowledge of the mechanisms operating at the molecular level. Thus, much effort has been devoted lately to gather a deeper insight into the way these photoreactions occur. In recent years, computational chemistry has emerged has an important tool for the detailed investigation of photochemical reactions. With the exponential increase of computer power, together with fundamental breakthroughs in the theoretical descriptions of photochemical reactions the scientific community has now the tools to explore and understand those organic photoreactions. In this contribution, a brief introduction to the computational methods currently available to study photoreactions of organic substrates will be presented. The main target will be to introduce a non-specialist reader into the features of computational organic photochemistry. This research field has their own methods, concepts and tools, advantages and drawbacks, usually different from those found in relatively closer topics such as ground state computational chemistry or experimental photochemistry. Thus, theoreticians trained to study thermal reactions or experimental photochemists trying to complement their research will find here a starting point. However, it is far beyond the scope of this chapter to comprehensively explore the state-of-the-art in computational photochemistry. Instead, a general overview will be presented and the reader will be invited to follow the references for an in-depth treatment of some sections. Due to the optimum balance between accuracy and computational cost for medium-sized organic compounds, the strategy based in the CASPT2//CASSCF level of theory will be especially explained. Some case studies will also be discussed in order to provide the reader with a general idea of both the importance and capacity of these tools to get a comprehensive picture of relevant processes for photochemical reactions at the molecular-level. Limitations as well as some perspectives into the near future will also be discussed. © 2011 by Nova Science Publishers, Inc. All rights reserved.