Principles of Photochemical Reactions

Abstract

Photochemical reactions of organic molecules are now becoming important tools to improve the quality of our lives through the search of ‘green’ solutions of modern life. Photochemical reactions occur in light-sensitive organic molecules, in which organic molecules are excited or activated by absorption of light or photon. In excited molecules, the vertical electronic transitions, known as Franck Condon transitions, occur in the excited molecules, where the atomic nuclei remain in stationary states (known as Franck Condon Principle). Usually spin allowed π → π* and spin forbidden n → π* electronic transitions occur in most cases of photo-excited organic molecules. A fraction of the excited molecules undergo photochemical reactions and the rest are deactivated to ground states through the intramolecular and intermolecular photophysical relaxation processes, depicted in Jablonski diagram. In intramolecular relaxation processes, the activated molecules are deactivated by emitting photons through spin allowed fluorescence (S1 → S0) and spin forbidden phosphorescence (T1 → S0) processes. In intermolecular relaxation processes, the activated molecules transfer their excess energy to the ground state molecules in quenching processes through formation of the exciplexes/excimers. The quenching rate can be determined by the Stern–Volmer equation. Intermolecular electronic energy transfer from activated molecules to the ground state molecules occurs through long-range singlet–singlet and short-range triplet–triplet energy transfer processes. The former process is known as FRET process and the latter is known as Dexter energy transfer process. The mechanism of these photo-induced electron transfer (PET) processes and their applications in tumor therapy and other fields are highlighted.