Pulse radiolytic and electrochemical investigations of intramolecular electron transfer in carotenoporphyrins and carotenoporphyrin-quinone triads

Abstract

Thermodynamic and kinetic aspects of intramolecular electron-transfer reactions in carotenoporphyrin dyads and carotenoid-porphyrin-quinone triads have been studied by using pulse radiolysis and cyclic voltammetry. Rapid (<1 µß) electron transfer from carotenoid radical anions to attached porphyrins has been inferred. Carotenoid cations, on the other hand, do not readily accept electrons from attached porphyrins or pyropheophorbides. Electrochemical studies provide the

Probing the interplay between factors determining reaction rates on silica gel using termolecular systems

“…This experiment was repeated using a tenfold increase in naphthalene loading. A 50% decrease in the initial reflectance change of the benzophenone triplet state was seen, consistent with efficient triplet state quenching by 50 naphthalene. The naphthalene triplet state is produced efficiently even at the lower loading with a ∆R 0 of 0.41 versus 0.33 for high and low loadings respectively.…”

“…At longer times after the laser pulse, the reflectance spectrum 50 was found to be very similar to the transient absorption spectrum. We chose to measure the kinetics of formation of this species at 515 nm.…”

“…Several bands can be seen following excitation and subsequent 30 electron transfer, which are not present in the absence of azulene. Given the short lifetime of the β-carotene excited triplet state we attribute these bands to the β-carotene radical cation [49][50][51] . A depletion of the ground state at 470 nm is observed with kinetics similar to the formation of the band above 800 nm.…”

“…In this case we expect that azulene will be the only mobile species on the silica 50 gel surface, and in electron transfer from such a large molecule as β-carotene one might expect that any steric influence to be considerably greater than that observed in the systems referred to above. It should be noted that whilst the β-carotene is relatively large, its size (estimated at 2.8nm using HyperChem) does not 55 preclude access to the porous structure of the silica gel.…”

“…We have also established that the fitting parameters are not dependent on the analysing wavelength over the absorption band of the species of interest in these systems. Additionally, we have successfully applied this model in the 50 analysis of data in other termolecular systems 3,4 , and hence the data presented here can be directly compared with our previously published work.…”