Diffusion-model analysis of effective CIDEP distance in solvent-separated radical-ion pair

Abstract

The radical pair mechanism (RPM) of chemically induced dynamic electron polarization (CIDEP) is theoretically analyzed to determine what intermolecular separations (reŸ effectively cont¡ to the CIDEP generated from diflhsive, separated radical-ion pairs (RIP) in terms of the chargetransfer interaction in the singlet-triplet energy splitting (J) by taking into aceount the distance-dependent electronic coupling and reorganization energy. The diffusion-model analysis reveals that the hyperŸ237 RPM polarization (PwM) phase is varied with the driving force (-AGc~) for the charge-recombination (CR) process and that the boundary -AGcR between the opposite phases coincides well with the total reorganization energy around the diffusible separation distante, rct r = 1.2 nm, between the ion radicals. For the first time, the roe r is well described by the exponent parameter (13) in the distance-dependent electronic coupling, suggesting that the RPM CIDEP detection can be applied to characterize the electronic coupling in individual solvent-separated RIP systems. It has been concluded that, in contrast to the spin exchange interaction of the neutral radical pairs, the characteristic long-range charge-transfer interaction enables us to utilize the simple diffusion-model analysis to successfully evaluate the rc~ and the P~M in homogeneous liquid polar solvents.

Superexchange Electron Tunneling Mediated by Solvent Molecules: Pulsed Electron Paramagnetic Resonance Study on Electronic Coupling in Solvent-Separated Radical Ion Pairs

“…Concerning solvent-separated radical ion pairs (RIP) produced via intermolecular, photoinduced charge separation (CS) reactions, the energy gap (2 J = E S − E T ) between the singlet (S) and triplet (T 0 ) states has been shown to be dominated by charge-transfer interactions ( J CT ); J CT is determined by V DA via the charge-recombination configurations in accordance with the Marcus theory. ,− Since the β determines the distance dependence of 2 J , ,− the β strongly influences chemically induced dynamic electron polarization (CIDEP) − created through the spin interactions within transient RIPs. During diffusional motion on the S and T 0 potential surfaces, radical pair mechanism (RPM) − electron spin polarization is created on ion radicals due to the S−T 0 state mixing taking place around the center-to-center intermolecular distance of 12 Å (in the case of β = 1 Å -1 ), − where the magnitude of 2 J is comparable to that of the hyperfine coupling (typically ca. 10 8 rad/s).…”

Reorganization Energy Induced by Noncovalent Bonding Interaction in Electron Transfer Reactions

“…Recently, we have developed a novel method to investigate the λ for photoinduced ET systems using time-resolved EPR (TREPR) spectroscopy in polar solvents. ,, In our previous papers, the λ values for the charge recombination reactions were determined with the good accuracy from the observation of the chemically induced dynamic electron polarization (CIDEP) generated in nanometer-separated radical ion pairs (RIPs). It was found that the solvent reorganization energies are larger by ca.…”

Control of the sign of exchange interactions in solvent-separated radical ion pairs

“…Recently, Kobori et al [15,16] showed that the distance dependence of the JcT value is affected by the exponential decay constant q of the squares of electronic coupling matrix elements in SSRIPs. The effective polarization distance of 1.2 nm can be applied under the condition of ti= 10 nm -1 [19,26]. Since the effective polarization distance dramatically changes with the /3 value [26], the large deviation of more than 0.1 eV may be due to a deviation of the distante of 1.2 nm.…”

“…The effective polarization distance of 1.2 nm can be applied under the condition of ti= 10 nm -1 [19,26]. Since the effective polarization distance dramatically changes with the /3 value [26], the large deviation of more than 0.1 eV may be due to a deviation of the distante of 1.2 nm. According to the solvent-mediated superexchange model [29,30], the distance dependence of the electronic coupling is predicted to be affected by the redox properties of solvent and solute molecules.…”

“…Recently, we verified that the charge transfer interaction Jcr dominates in RIPs and succeeded in applying time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy to determine the individual 2. for the bimolecular photoinduced ET reaction systems [18][19][20]26]. The mechanism of the JCT is quite different from the well-known electron spin exchange interaction in neutral radical pairs.…”