Photoinduced Electron Transfer from Triplet Fullerene, 3C60, to Tetracyanoethylene. Fourier Transform Electron Paramagnetic Resonance Study

Charge-Transfer Controlled Exchange Interaction in Radical-Triplet Encounter Pairs as Studied by FT-EPR Spectroscopy

The exchange interaction, J, producing quartet and doublet energy separation in radical-triplet excited molecule encounter pairs, was investigated in solution by measuring chemically induced dynamic electron polarization (CIDEP) created through the radical-triplet pair mechanism. A time-resolved FT-EPR method was utilized to measure CIDEP of galvinoxyl radical by recording FID signals and an absolute magnitude of CIDEP, P(n), was determined for each radical-triplet system by detailed analysis of the time evolution curves of CIDEP. A transient FT-EPR signal phase remarkably depends on the triplet molecule. The signal phase is related to the sign of J value, which is responsible for the radical-triplet pair interaction. Most of galvinoxyl-triplet systems showed normal negative sign. An unusual positive sign was found in some systems characterized by a small energy gap, DeltaG, between the radical-triplet pair and intermolecular charge transfer (CT) states. A theoretical calculation of J value for radical-triplet encounter pairs was carried out by considering exchange integral and intermolecular CT interaction. According to the calculated J value and the diffusion theory for CIDEP magnitude, experimental Pn values were theoretically reproduced as a function of DeltaG. The present results confirm our previously reported CT model explaining the complicated nature of the sign of J value in the galvinoxyl-triplet encounter pairs. According to the proposed model for CT effect on J value and CIDEP results, nature of J value in radical-triplet pairs is discussed.

Time-Resolved EPR Study of the Photophysics and Photochemistry of 1-(3-(Methoxycarbonyl)propyl)-1-phenyl[6.6]C61

Time-resolved (TR) EPR was used to study the photophysics and photochemistry of 1-(3-(methoxycarbonyl)propyl)-1-phenyl[6.6]C61 (M1). The CW TREPR spectra of M1 in the photoexcited triplet state, frozen in a rigid matrix and in liquid solution at room temperature, were compared with those of 3C60. The introduction of the substituent on C60 has a striking effect on the spectra of the triplets, which is attributed to the lifting of the orbital degeneracy by the reduction in symmetry. Fourier transform (FT) EPR was used in an investigation of electron-transfer reactions in liquid solutions mediated by 3M1. Of particular interest was the system of M1/chloranil (CA)/perylene (Pe). Photoexcitation of M1 is found to lead to the formation of the chloranil anion radical and the perylene cation radical. From the chemically induced dynamic electron polarization (CIDEP) patterns in the FTEPR spectra and the dependence of the reaction kinetics on reactant concentrations, it was deduced that CA- is formed by two competing pathways following photoexcitation of M1: (1) direct electron transfer from 3M1 to CA followed by electron transfer from Pe to M1+ and (2) energy transfer from 3M1 to Pe followed by oxidative quenching of 3Pe by CA. In both pathways, M1 acts as a light-energy harvester and mediator of electron-transfer reactions from Pe to CA without itself being consumed in the process, that is, as a photocatalyst. It is found that the functionalization of C60 makes its triplet state a worse electron donor and acceptor, but it has no significant effect on the triplet energy transfer reaction.

Scandium ion-promoted photoinduced electron-transfer oxidation of fullerenes and derivatives by p-chloranil and p-benzoquinone

In the presence of scandium triflate, an efficient photoinduced electron transfer from the triplet excited state of C(60) to p-chloranil occurs to produce C(60) radical cation which has a diagnostic NIR (near-infrared) absorption band at 980 nm, whereas no photoinduced electron transfer occurs from the triplet excited state of C(60) (3C(60)) to p-chloranil in the absence of scandium ion in benzonitrile. The electron-transfer rate obeys pseudo-first-order kinetics and the pseudo-first-order rate constant increases linearly with increasing p-chloranil concentration. The observed second-order rate constant of electron transfer (k(et)) increases linearly with increasing scandium ion concentration. In contrast to the case of the C(60)/p-chloranil/Sc(3+) system, the k(et) value for electron transfer from 3C(60) to p-benzoquinone increases with an increase in Sc(3+) concentration ([Sc(3+)]) to exhibit a first-order dependence on [Sc(3+)], changing to a second-order dependence at the high concentrations. Such a mixture of first-order and second-order dependence on [Sc(3+)] is also observed for a Sc(3+)-promoted electron transfer from CoTPP (TPP(2-) = tetraphenylporphyrin dianion) to p-benzoquinone. This is ascribed to formation of 1:1 and 1:2 complexes between the generated semiquinone radical anion and Sc(3+) at the low and high concentrations of Sc(3+), respectively. The transient absorption spectra of the radical cations of various fullerene derivatives were detected by laser flash photolysis of the fullerene/p-chloranil/Sc(3+) systems. The ESR spectra of the fullerene radical cations were also detected in frozen PhCN at 193 K under photoirradiation of the fullerene/p-chloranil/Sc(3+) systems. The Sc(3+)-promoted electron-transfer rate constants were determined for photoinduced electron transfer from the triplet excited states of C(60), C(70), and their derivatives to p-chloranil and the values are compared with the HOMO (highest occupied molecular orbital) levels of the fullerenes and their derivatives.

NEW EPR METHODS FOR INVESTIGATING PHOTOPROCESSES WITH PARAMAGNETIC INTERMEDIATES

▪ Abstract  Some of the significant advances in time-resolved multifrequency electron paramagnetic resonance (EPR) methods are reviewed, with the explicit focus on studies of light-driven processes and photoreactions in real time. Prominent examples are excited state electron transfer reactions with transient charge-separated radical pairs playing a central role. Paramagnetic intermediates and products are key functional states; thus EPR is the method of choice for their characterization. Photogenerated spin polarization and coherences as process-inherent features add the practical advantage of compensation in the trade-off between sensitivity and time resolution. Additionally, they provide detailed structural and dynamic information on the photoreactive system. Significance and specificity of the results achieved for charge separation in photosynthetic reaction centers and donor-acceptor model complexes indicate highly promising perspectives in photochemical research.