Photochemistry of Artificial Photosynthetic Reaction Centers in Liquid Crystals Probed by Multifrequency EPR (9.5 and 95 GHz)

Abstract

Photoinduced charge separation and recombination in a carotenoid-porphyrin-fullerene triad C-P-C(60)(1) have been followed by multifrequency time-resolved electron paramagnetic resonance (TREPR) at intermediate magnetic field and microwave frequency (X-band) and high field and frequency (W-band). The electron-transfer process has been characterized in the different phases of two uniaxial liquid crystals (E-7 and ZLI-1167). The triad undergoes photoinduced electron transfer, with the generation of a long-lived charge-separated state, and charge recombination to the triplet state, localized in the carotene moiety, mimicking different aspects of the photosynthetic electron-transfer process. Both the photoinduced spin-correlated radical pair and the spin-polarized recombination triplet are observed starting from the crystalline up to the isotropic phase of the liquid crystals. The W-band TREPR radical pair spectrum has allowed unambiguous assignment of the spin-correlated radical pair spectrum to the charge-separated state C(.+)-P-C(60)(.-). The magnetic interaction parameters have been evaluated by simulation of the spin-polarized radical pair spectrum and the spin-selective recombination rates have been derived from the time dependence of the spectrum. The weak exchange interaction parameter (J = +0.5 +/- 0.2 G) provides a direct measure of the dominant electronic coupling matrix element V between the C(.+)-P-C(60)(.-) radical pair state and the recombination triplet state (3)C-P-C(60). The kinetic parameters have been analyzed in terms of the effect of the liquid crystal medium on the electron-transfer process. Effects of orientation of the molecular triad in the liquid crystal are evidenced by simulations of the carotenoid triplet state EPR spectra at different orientations of the external magnetic field with respect to the director of the mesophase. The order parameter (S = 0.5 +/- 0.05) has been evaluated.

EPR-detected photoinduced electron transfer in three structurally related molecular triads

“…For weakly coupled donor-acceptor systems as the triads under investigation, as already demonstrated for the C-P-C60 triad bearing the tetraalkylporphy¡ bridge [18], the electronic coupling element VDA is notable to contrast the retardation effect produced by the nematic potential on the longitudinal relaxation time r L. The adiabacity factor ~c, which describes the transition from the nonadiabatic to the solvent-controlled adiabatic limit is a function of both V2A and r L [52]:…”

“…The radical pair spectral analysis was performed using programs written in MatLab, based on the theoretical description of nondiffusing spin-correlated radical pairs, in the high field approximation [18,34].…”

“…Among the numerous covalently-linked fullerene-based molecular devices reported, starting from the first example of a fullerene-porphyrin dyad [12], a se¡ of carotenoid (C), porphyrin (P) and fullerene (C60) molecular triads, differing one from the other in terms of small but significant structural changes, have shown interesting properties and excellent capabilities as artificial model compounds [7,10,[13][14][15][16][17][18][19]. They all feature several key properties of the reaction center primary photochemistry, which have been successfully mimicked by a few artificial model compounds.…”

Steady-state and time-resolved spectroscopic investigations on intramolecular electron transfer processes within a synthesized methoxynaphthalene dyad by using a nematic liquid crystal medium

“…In this context it is to be mentioned that in the cases of some triad systems of carotenoidporphyrin-fullerene and porphyrin-tetrathiafulvalene-fullerene, Valentin et al [50,51] observed that when these are embedded in LC medium, all the important features of photochemistry observed before in the isotropic solvents remain conserved. Similar investigations in the pseudo-ordered domain of 5CB with the other organic dyad systems are under progress.…”

Nature of charge separation and recombination processes within an organic dyad having short spacer

“…In recent years, various linked organic electron donor-acceptor dyad and multichromophoric systems have been synthesized [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Within the dyad systems, multi-step electron transfer reactions, which mimic the charge-separations in natural photosynthesis, occur.…”