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 εmax, 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.

Time-Resolved Magnetic Field Effects Distinguish Loose Ion Pairs from Exciplexes

“…29,30,54,55,58,63−65 In contrast to that, many studies focusing predominantly on photochemical aspects can be found, suggesting that the exciplex is the initial quenching product even at fairly high permittivities. 28,57,66,67 …”

Associated electron and proton transfer between Acridine and Triethylamine in AOT reverse micelles probed by laser flash photolysis with magnetic field

“…It is well established that the photochemical reactions which involve formation of geminate radical-ion-pairs (RIPs, A ÅÀ /D Å+ ) and radical-pairs (RPs) containing a pair of spin-correlated free electrons formed as primary transient intermediates through electron transfer between D and A and bond cleavage/hydrogen abstraction, respectively are susceptible to a magnetic field, either internal or external [3][4][5][6][7]. The initial spin-correlation between the electrons of geminate RIPs/RPs will be either singlet (S) or triplet (T) depending on the spin state of the photo-sensitizer which induces such reactions.…”

Interactions between 9,10-anthraquinone and aromatic amines in homogeneous and micellar media: A laser flash photolysis and magnetic field effect study

“…The overall effect is the increase in the population of the initial spin state of the RPs or RIPs. The micellar cage provides sufficient volume so that the component radicals of a pair are allowed to separate to such a distance where exchange interaction becomes negligible as well as spin correlation is still retained; therefore, spin flipping and subsequent geminate recombination are possible, which are the essential criteria for magnetic field effect (MFE) [12]. The fluorescence, laser flash photolysis and MFE studies indicate that ET occurs in organic homogeneous medium and it becomes predominant over H abstraction between AQ and micelle itself in micellar medium.…”