Photophysics of Organic Compounds: Theoretical Studies

Organic molecules are not stable under persistent irradiation with ultraviolet (UV) light. UV photons can break covalent bonds and thus can induce a great variety of chemical transformations (isomerizations or fragmentations). In this view, it is amazing that life can thrive under full exposition to sunlight. Moreover, biogenesis took place long before the formation of the stratospheric ozone layer (which today filters out the most dangerous UV components of sunlight) and thus under conditions of extremely intense short-wavelength UV radiation. Thus, it must have resulted in an extreme selection pressure for UV protection. These considerations suggest that photostability may have been the decisive selection criterion which has determined the molecular architecture of life at the beginning of the biological evolution. Fortunately, during last two decades, new advancements in Quantum chemical methods and Laser spectroscopy, provided the opportunity to investigate several challenging subjects such as photostability and photophysics of life and organic systems in molecular scales. The non-Born Oppenhimer potential energy surfaces have been introduced as powerful tools for predicting the so-called Conical Intersections (CIs), being responsible for ultrafast deactivation pathways of excited molecules via internal conversions. Thus, in this lecture, a brief discussion on recent theoretical results, supporting the hypothesis that the fundamental building blocks as well as the supramolecular structures of life are optimized with respect to photostability, will be presented.