Effect of Molecular Diffusion on the Spin Dynamics of a Micellized Radical Pair in Low Magnetic Fields Studied by Monte Carlo Simulation

Magnetic field effect is a powerful tool to study dynamics and kinetics of radical pairs (RPs), which are one of the most important intermediates for organic photon-energy conversion reactions. However, quantitative discussion regarding the relationship between the modulation of interelectron interactions and spin dynamics at low magnetic fields (<10 mT) is still an open question. We have studied the spin dynamics of a long-lived RP in a micelle by newly developed Monte Carlo simulation, in which fluctuations of the exchange and magnetic dipolar interactions by in-cage diffusion are directly introduced to the time-domain spin dynamics calculation. State-dependent relaxation/dephasing times of a few to a few tens of nanoseconds are obtained by simulations without hyperfine interactions (HFIs) as a function of the mutual diffusion constant (∼10^–6 cm²/s). Simulations with the HFIs exhibit incoherent singlet–triplet (S–T) mixings resulting from interplay between the HFIs and the fluctuating spin–spin interactions. The experimentally observed incoherent S–T mixing of ∼20 ns at 3 mT for a singlet-born RP in a sodium dodecyl sulfate micelle is reproduced by the simulation with reasonable diffusion coefficients. The computational method developed here contributes to quantitative detection of molecular motion that governs the recombination efficiency of RPs

Real-Time Observation of the Spin-State Mixing Process of a Micellized Radical Pair in Weak Magnetic Fields by Nanosecond Fast Field Switching

The singlet−triplet spin-state mixing process of a singlet-born radical pair confined in a sodium dodecyl sulfate (SDS) micelle was studied by observing the nanosecond switched external magnetic field (SEMF) effect on the transient absorption signals. A long-lived singlet radical pair is generated by the photoinduced bond cleavage reaction of tetraphenylhydrazine in an SDS micelle. Application of off−on type SEMF results in the increase of the free radical yield contrary to the decrease produced by an applied static magnetic field. The S−T mixing process in low magnetic field was observed by means of a delay-shift SEMF experiment. Observed incoherent mixing processes are explained in terms of the interplay between coherent hyperfine interaction and fast dephasing processes caused by the fluctuation of electron-spin interactions. Singlet−triplet and triplet−triplet dephasing rate constants are determined independently to be 2 × 108 and 0.2 × 108 s-1, respectively, by a simulation based on a modified single-site Liouville equation. This is the first direct observation of the incoherent spin-state mixing process at magnetic fields comparable to the hyperfine interactions of the radical pair

Photoreaction of 9,10-Anthraquinone-2,6-disulfonic Acid and Ascorbic Acid Complexed at the Cationic Bilayer Interface and Consecutive Radical Dynamics

The antioxidant ability of substances such as vitamin C has attracted wide attention, and many antioxidant effects on our body are considered to act in the biomembrane interface region. However, the detailed dynamics of the radicals at the interface have not been deeply clarified. Here, we studied the reaction of photoexcited 9,10-anthraquinone-2,6-disulfonic acid dianion (AQDS2–) and ascorbic acid in the cationic vesicle solution of didodecyldimethyl ammonium bromide using time-resolved electron paramagnetic resonance (TR-EPR) and UV–vis absorption methods. The results were compared with those of micellar and aqueous systems. Analysis of UV–vis spectra revealed the formation of an intermolecular charge-transfer complex of ascorbic acid monoanion (AscH–) and AQDS2– at the cationic bilayer interface. The TR-EPR spectra revealed a hydrogen atom abstraction reaction by the excited triplet state of AQDS2– from AscH– to form an ascorbic acid monoanion radical (Asc•–) and an anthrasemiquinone dianion radical (AQDSH•2–). The spin relaxation of these radicals demonstrates that Asc•– is weakly bound at the interface, whereas AQDSH•2– is strongly anchored. These results suggest that the strong oxidative ability of the photoexcited quinone is quenched by complexed AscH– and stabilized at the cationic bilayer interface

Electron Spin Polarization Transfer to the Charge-Separated State from Locally Excited Triplet Configuration: Theory and Its Application to Characterization of Geometry and Electronic Coupling in the Electron Donor−Acceptor System

We present a theoretical model of analysis of the time-resolved electron paramagnetic resonance (TREPR) spectrum of the charge-separated (CS) state generated by the photoinduced electron transfer (ET) reaction via the locally excited triplet state in an electron donor−acceptor (D−A) system with a fixed molecular orientation. We show, by the stochastic−Liouville equation, that chemically induced dynamic electron polarization (CIDEP) of the triplet mechanism is explained by lack of transfer of quantum coherence terms in the primary triplet spin state, resulting in net emissive or absorptive electron spin polarization (ESP) which is dependent on anisotropy of the singlet−triplet intersystem crossing in the precursor excited state. This disappearance of the coherence is clearly shown to occur when the photoinduced ET rate is smaller than the angular frequency of the Zeeman splitting: the transferred coherence terms are averaged to be zero due to effective quantum oscillations during the time that the chemical reaction proceeds. The above theory has been applied to elucidate the molecular geometries and spin−spin exchange interactions (2J) of the CS states for both folded and extended conformers by computer simulations of TREPR spectra of the zinc porphyrin−fullerene dyad (ZnP−C60) bridged by diphenyldisilane. On the extended conformation, the electronic coupling is estimated from the 2J value. It has been revealed that the coupling term is smaller than the reported electronic interactions of the porphyrin−C60 systems bridged by diphenylamide spacers. The difference in the electronic couplings has been explained by the difference in the LUMO levels of the bridge moieties that mediate the superexchange coupling for the long-range ET reaction

Photoreactions and Molecular Dynamics of Radical Pairs in a Reversed Micelle Studied by Time-Resolved Measurements of EPR and Magnetic Field Effect

Photoreaction of N,N,N′,N′-tetramethyl-1,4-phenylenediamine (TMPD) in an aerosol OT (AOT) reversed micelle (RM) is studied by time-resolved EPR (tr-EPR) and the transient absorption detected magnetic field effect (MFE). Tr-EPR and transient absorption spectra indicate electron transfer from a highly excited triplet state of TMPD to the AOT headgroup regardless of W = [H2O]/[AOT] values from 0 to 40. Noticeable MFEs on the yield of TMPD cation radical (TMPD+) are observed at W > 0 and maximized at W ∼ 10. The dynamics of TMPD+ in the bound water region of the RM has been precisely analyzed by theoretical analysis of time-resolved magnetically affected reaction yield (MARY) spectra. The simulation of the MARY spectra indicates that two kinds of radical pairs exist, both of which are composed of an AOT alkyl radical and TMPD+. One system has TMPD+ strongly bound to the anionic interface, where the radical pair shows very slow relaxation and recombination. Another system has TMPD+ diffusing in the bound water, which shows a smaller diffusion coefficient than that in bulk water by 1 order of magnitude. In the larger water pool (W > 15), the spin correlated radical pair of the hydrated electron and TMPD+ generated by photoionization is observed by tr-EPR. The ionization reaction is followed by electron attachment to the AOT headgroup and generation of the sulfite radical. However, these radical pairs are not thought to contribute significantly to the observed MFEs. Spin multiplicities of the precursor state and recombination products have been discussed from the different sign of J values for the radical pairs at larger W

Time Resolved EPR Study on the Photoinduced Long-Range Charge-Separated State in Protein: Electron Tunneling Mediated by Arginine Residue in Human Serum Albumin

To elucidate how local molecular conformations play a role on electronic couplings for the long-range photoinduced charge-separated (CS) states in protein systems, we have analyzed time-resolved electron paramagnetic resonance (TREPR) spectra by polarized laser irradiations of 9,10-anthraquinone-1-sulfonate (AQ1S^–) bound to human serum albumin (HSA). Analyses of the magnetophotoselection effects on the EPR spectra and a docking simulation clarified the molecular geometry and the electronic coupling of the long-range CS states of AQ1S^•2–-tryptophan214 radical cation (W214^•+) separated by 1.2 nm. The ligand of AQ1S^– has been demonstrated to be bound to the drug site I in HSA. Molecular conformations of the binding region were estimated by the docking simulations, indicating that an arginine218 (R218⁺) residue bound to AQ1S^•2– mediates the long-range electron-transfer. The energetics of triad states of AQ1S^•2––R218⁺–W214^•+ and AQ1S^––R218^•–W214^•+ have been computed on the basis of the density functional molecular orbital calculations, providing the clear evidence for the long-range electronic couplings of the CS states in terms of the superexchange tunneling model through the arginine residue

Long-Distance Sequential Charge Separation at Micellar Interface Mediated by Dynamic Charge Transporter: A Magnetic Field Effect Study

Construction of photogenerated long-lived charge-separated states is crucial for light-energy conversion using organic molecules. For realization of cheap and easy-to-make long-distance electron transfer (ET) systems, we have developed a supramolecular donor­(D)–chromophore­(C)–acceptor­(A) triad utilizing a micellar interface. Alkyl viologen (A²⁺) is adsorbed on the hydrophilic interface of Triton X-100 micelle, which bears D units in the hydrophobic core. Excited triplet state of a hydrophobic flavin C entrapped in the supercage gives rise to primary ET from D, which is followed by the secondary ET from C^–• to A²⁺ to give the long-lived (>10 μs) charge-separated state with negligible yield of escaped C^–•. Analysis of magnetic field effect reveals that diffusion of C^–• from the core to the hydrophilic interface leads to long-distance ET with a low charge recombination yield of ∼20%. This novel concept of “dynamic charge transporter” has important implications for development of photon-energy conversion systems in solution phase

First Direct Evidence of Radical Intermediates in Samarium Diiodide Induced Cyclization by ESR Spectra

The mechanism of samarium diiodide (SmI2)-induced cyclization of α,β-unsaturated esters and ketones to bicyclic compounds was examined using ESR spectroscopy. This is the first report of direct evidence of the radical intermediates in the SmI2 reaction. The preferential reduction of the α,β-unsaturated carbonyl part in some substrates should be recognized as a main route

Time-Resolved EPR Characterization of a Folded Conformation of Photoinduced Charge-Separated State in Porphyrin−Fullerene Dyad Bridged by Diphenyldisilane

Time-Resolved EPR Characterization of a Folded Conformation of Photoinduced Charge-Separated State in Porphyrin−Fullerene Dyad Bridged by Diphenyldisilan