Quenching Mechanisms and Diffusional Pathways in Micellar Systems Unravelled by Time‐Resolved Magnetic‐Field Effects

Abstract

Magnetic-field effects (MFEs) are used to investigate the photoreaction of xanthone (A) and DABCO (D) in anionic (SDS) or cationic (DTAC) micelles at high pH (DABCO = 1,4-diazabicyclo[2.2.2]octane, SDS = sodium dodecyl sulfate, DTAC = dodecyl trimethyl ammonium chloride). From MFE experiments with nanosecond time resolution, the radical anion A(.)(-) can be observed without any interference from the much more strongly absorbing triplet (3)A*, the different quenching processes can be separated and their rates can be measured. Triplet (3)A* is quenched dynamically both by the SDS micelle (k(1) = 5.0x10(5) s(-1)) and by DABCO approaching from the aqueous phase (k(2) = 2.0x10(9) M(-1) s(-1)). Static quenching by solubilised DABCO (association constant with the SDS micelles, 1.5 M(-1)) also participates at high DABCO concentrations, but is chemically nonproductive and does not lead to MFE generation. The MFEs stemming from the radical ion pairs A(.)(-) D(.)(+) are about 40 times larger in the anionic micelles than in the cationic ones despite a higher yield of free radicals in the latter case. This can be rationalised by different diffusional dynamics: Because of the location of their precursors, A(.)(-) and D(.)(+) are formed at opposite sides of the micelle boundary. Subsequently, the negatively charged Stern layer of the SDS micelle traps the radical cation, which then undergoes surface diffusion, so both the recombination probability and the spin mixing are high; in contrast, the positive surface charge of the DTAC micelle forces the radical cation into the bulk of the solution, thus efficiently blocking a recombination.

Millitesla magnetic field effects on the photocycle of an animal cryptochrome

“…10%) fraction of the Δ A ( λ ) signal decays within the first 10 μs leaving a substantial long-lived component. The effect of the magnetic field, as expected, is to suppress the formation of long-lived radicals, consistent with the formation of RP1 in a singlet state 15 16 . The Δ A (510 nm) response is qualitatively similar to that exhibited by At Cry1 and Ec PL ( Fig.…”

“…In the first 100 μs following excitation, the major change in the Δ A ( λ ) spectrum occurs in the wavelength range 520−650 nm and is assigned to the TrpH •+ → Trp • + H + deprotonation reaction 15 16 23 . The kinetics of this change are bi-phasic (see Fig.…”

“…Light-dependent magnetic field effects in vitro have been reported for cryptochrome-1 from the plant Arabidopsis thaliana (At Cry1) and the closely related DNA photolyase from E. coli (Ec PL) 15 16 . The magnetic responses of both molecules are explained by the radical pair mechanism 3 17 and the photocycle in Fig.…”

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

“…Change in the reaction yield by an applied magnetic field is quite sensitive to the degree of confinement of the RPs because the coherent singlet–triplet (S–T) mixing process, which occurs in a time scale of ∼10 ns, usually competes with diffusive dissociation of the RPs in a solution. For this reason, large MFEs are observable for long-lived RPs (≫ 10 ns) confined in chemical cages such as micelles, , reversed micelles, , ionic liquids, vesicles, and proteins . MFE studies in those systems have contributed greatly to the clarification of complex dynamics and kinetics of the RPs confined in the nanostructures.…”

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

“…Entrapment of D and A molecules in the supercage enhances the CS efficiency. Electron transfer (ET) mechanisms in such systems have been extensively studied by focusing on spin dynamics of the in-cage charge-separated states, or radical pairs (RPs), by means of optically detected magnetic field effect (MFE) or time-resolved EPR. − These studies indicate that the charge recombination (CR) and escape of the generated RP usually occur within less than a few microseconds, which is not sufficient for the subsequent conversion reactions to occur. It is widely accepted from these studies that the dynamic diffusion of the guest molecules is the cause of efficient CR and escape.…”