Ground state photobiology

Because of this difference in electron-donorand electron-acceptor properties, excited states have very different redox properties from those of related ground states. The effect is so marked that many photochemical processes begin with a complete transfer of an electron from (or to) an excited state (1.2), and the subsequent chemistry is that of radical cations and radical anions, species that are regarded as unusual in ground-state organic reactions. The importance of photochemical electron transfer is underlined by its extensive involvement in photobiological processes such as photosynthesis. 

In the chapters of Part II of this book it has been demonstrated for a variety of examples that conical intersections can provide the mechanism for extremely fast chemical processes, e.g. photodissociation, photoisomerization and internal conversion to the electronic ground state. Time-dependent quantum wave-packet calculations have established that radiationless transitions between electronic states can take place on a time scale of the order of 10 fs, if a conical intersection is directly accessible after preparation of the wave packet in the excited state, see, e.g. Chapters 8-11 and 14 15. In view of these findings and the omnipresence of conical intersections in polyatomic molecules (cf. Chapter 6), it is now widely accepted that conical intersections are of fundamental importance for the understanding of the reaction mechanisms in photochemistry and photobiology. 

Proton transfer reactions between excited molecules and ground-state molecules are considered to be a model for the primary step of some photobiological processes such as bacterial photosynthesis and photoreception [129]. Proton transfer photoreactions of 1-hydoxypyrene with some bases in micellar solutions of CTAB were studied by the authors of the present review with co-workers [130]. Addition of bases (sodium acetate, hydroxylamine) increases the rate of formation of excited hydroxypyrene anions (Fig. 6). The ratio of the fluorescence quantum yields linearly depends on the concentration of and reverse values of the overall concentration [R]q of the base in... 

Fig. 10.9 Bond length differences between the ground and excited states of ZsZsZa BV, ZsZsZa PCB and ZaZsZa PB in the absence and presence of a chloride counterion. Reproduced from Strambi A, Durbeej B (2011) Initial excited-state relaxation of the bilin chromophores of phytochromes a computational study, Photochem Photobiol Sci 10 569-579, with permission of the Royal Society of Chemistry (RSC) on behalf of the European Society for Photobiology, the European Photochemistry Association, and RSC...

---