Magnetic Field Effect: A Tool for Identification of Spin State in a Photoinduced Electron-Transfer Reaction

Abstract

The present work addresses the investigation of the influence of substitution on the initial spin state in photoinduced electron-transfer (PET) reactions with a series of four exciplex systems i.e., N -ethylcarbazole (ECZ)−1,4-dicyanobenzene (DCB), 1,4,5,8,9-pentamethylcarbazole (PMC)−DCB, ECZ−1,2,4,5-tetracyanobenzene (TCNB), and PMC−TCNB by means of a low magnetic field (MF) (0.05 T). The two primary intermediates that play major roles in determining the efficiencies of bimolecular PET reactions are the contact ion pair (CIP), i.e., (A - D + ), and the solvent-separated ion pair (SSIP) (A - (S)D •+ ). The effect of MF of the order of hyperfine interaction present in the system on such reactions reflects the unique combination of spin dynamics, diffusion dynamics, and geminate recombination in the SSIPs. Thus MF can be successfully used to investigate the initial spin state of a SSIP where electronic coupling between acceptor (A) and donor (D) molecules is small indeed. The experimental techniques have used either laser flash photolysis to estimate the magnetic field effect (MFE) on triplet free ions or an improved phase-sensitive detection system to measure the enhancement in singlet CIP or exciplex luminescence. By the changes of the substituents in A/D molecules, the modifications in the production of either singlet or triplet SSIPs have been discussed. The observed MFEs have been correlated with the Marcus relation between free energy changes and redox potentials. Another novel finding is that MFE on exciplex luminescence is controlled not only by the dielectric of the medium but the extent of electronic coupling, i.e., the extent of charge transfer (δ), between D and A molecules also plays a major role in it. The deviation in ε max, the dielectric for maximum MFE, from the previously obtained values has been discussed on the basis of the modification in the potential energy surfaces between CIP and SSIP, which has been further supported by an analytical model.