Structural Effects on Photoinduced Electron Transfer in Carotenoid−Porphyrin−Quinone Triads

Abstract

meso-Polyarylporphyrins are often used as components of molecules that mimic photosynthetic reaction centers by carrying out photoinduced electron-transfer reactions. Studies of these systems have raised questions concerning the role of alkyl substituents at the "β-pyrrolic" positions on the porphyrin periphery in limiting π-π overlap between the macrocycle and the aryl rings. The degree of overlap affects electronic coupling and, therefore, the rates of electron-transfer reactions. There is also evidence that when the linkages joining porphyrins to electron-acceptor or -donor moieties contain amide bonds, the sense of the amide linkage may strongly affect electron-transfer rate constants. In this study, three carotenoid-porphyrin-quinone molecular triads and various model compounds have been prepared, and electron-transfer has been studied using timeresolved emission and absorption techniques. The results show that steric hindrance due to methyl groups at the β-pyrrolic positions reduces electron-transfer rate constants by a factor of ∼ 1 / 5 . In addition, amidecontaining donor-acceptor linkages having the nitrogen atom attached to the porphyrin meso-aryl ring demonstrate electron-transfer rate constants ∼30 times larger than those for similar linkages with the amide reversed, after correction for thermodynamic effects.

Increasing the Yield of Photoinduced Charge Separation through Parallel Electron Transfer Pathways

“…À for triad 1 in benzonitrile is only 0.044 [8]. The low yield results from inefficient competition of step 4 with charge recombination by step 3.…”

“…Although dyads consisting of porphyrins linked to quinones or other electron acceptors can undergo photoinduced electron transfer to produce chargeseparated states in high yield, generating such states wherein charge recombination is slow, and thus facilitating the harvesting of the stored energy by diffusional or other processes, has in general required molecular triads or more complex supermolecular constructs which employ multistep electron transfer pathways conceptually related to those found in natural reaction centers. Carotene (C)-porphyrin (P)-quinone (Q) triad 1 exemplifies this strategy [7,8]. Excitation of the porphyrin moiety generates C-1 P-Q, which decays mainly by photoinduced electron transfer to the quinone, giving C-P .…”

Pathways for photoinduced electron transfer in meso-nitro-phenyl-octaethylporphyrins and their chemical dimers

“…Correspondingly, steric interactions of the phenyl spacer with the connected subunits as well as the p-electronic nature of the linkage may influence both the deactivation of porphyrin excited states and the electronrenergy transfer efficiency. The specific role of steric interactions of meso-phenyls with b-CH pyrrole substituents was mentioned for ET 3 w x processes in carotenoid-porphyrin-quinone triads 7 .…”

Competition between electron transfer and energy migration in self-assembled porphyrin triads

“…H P fluorescence intensity the population r as well as 2 3 3 a character of the excited states dynamics. Our simulations display that the time dependence of the population r as well as its temperature dependence at 33 low acetone concentration qualitatively differs from the behaviour at high acetone concentration. It means that under various experimental conditions, different reaction mechanisms may be realized.…”