Bioexcimers as Precursors of Charge Transfer and Reactivity in Photobiology

Accurate CASPT2//CASSCF calculations show that π‐stacked interactions in different biochromophores such as DNA nucleobases or porphyrin‐quinone pairs yield excimer‐like situations which behave as precursors of processes like charge transfer or photoreactivity. Examples are the transfer of charge between a reduced pheophytin and an accepting quinone molecule, process that trigger the sequence of electron transfer phenomena in photosynthetic photosystem II, the electron transfer between adjacent DNA nucleobases in a strand of oligonucleotides, and the photodimerization taking place in cytosine pairs leading to cyclobutanecytosine mutants. These processes take place through nonadiabatic photochemical mechanisms whose evolution is determined by the presence and accessibility of conical intersections and other surface crossings between different electronic states.