Magnetic Field Effects on Photochemical Reactions

The mechanism and dynamics regulating photoinduced electron transfer reactions in liquid solutions, rigid matrices and biological systems are the most fundamental problems in photophysical and photochemical primary processes. Conventional spectroscopic studies using steady-state absorption, fluorescence, etc yield useful information about the ground-state and excited-state phenomena of most of the photoinduced reactions; however the occurrence of excited-state electron transfer might go unnoticed if the radical ions, which are produced as transient intermediates, cannot be identified. Laser flash photolysis has been proved to be one of the effective techniques for identification of non-fluorescent transients. It is obvious that electrons play an essential role in such reactions. Therefore, an external magnetic field might affect the reaction pathways by perturbing the spin magnetic moments of free electrons. Studies on magnetic field effect and its dynamics have been carried out for various kinds of photochemical and photobiological reactions. The importance of the effect lies in its capability to envisage the initial spin state, one of the deciding factors for ultimate products of the reactions and the intermediate distance of geminate spin-correlated radical ion pairs/radical pairs produced as transients. The effect becomes maximum for an optimum distance between radical ions/radicals in a geminate pair, which depends on structure and physical properties of participating molecules and solvent. Some of the systems would be discussed focusing the importance of the above features.