Driving Force and Electronic Coupling Effects on Photoinduced Electron Transfer in a Fullerene-based Molecular Triad¶

Abstract

Tuning thermodynamic driving force and electronic coupling through structural modifications of a carotene (C) porphyrin (P) fullerene (C60) molecular triad has permitted control of five electron and energy transfer rate constants and two excited state lifetimes in order to prepare a high-energy charge-separated state by photoinduced electron transfer with a quantum yield of essentially unity (> or = 96%). Excitation of the porphyrin moiety of C-P-C60 is followed by a combination of photoinduced electron transfer to give C-P(.+)-C60.- and singlet-singlet energy transfer to yield C-P-1C60. The fullerene excited state accepts an electron from the porphyrin to also generate C-P(.+)-C60.-. Overall, this initial state is formed with a quantum yield of 0.97. Charge shift from the carotenoid to yield C(.+)-P-C60.- is at least 60 times faster than recombination of C-P(.+)-C60.-, leading to the overall quantum yield near unity for the final state. Formation of a similar charge-separate species from the zinc analog of the triad with a yield of 40% is also observed. Charge recombination of C(.+)-P-C60.- in 2-methyltetrahydrofuran yields the carotenoid triplet state, rather than the ground state. Comparison of the results for this triad with those for related triads with different structural features provides information concerning the effects of driving force and electronic coupling on each of the electron transfer steps.

Quantum coherence controls the charge separation in a prototypical artificial light harvesting system

“…S5). Transient photoinduced absorption bands in the near-infrared region have also been observed in earlier studies of caroteneporphyrin-fullerene triads 11,28 and have been assigned to transient absorption from either the C-P þ -C 60 À or the C þ -P-C 60 À charge-separated state. The lifetime of the biradical C þ -P-C 60 À is typically on the order of tens of nanoseconds, whereas C-P þ -C 60 À decays on a nanosecond scale.…”

“…It shows a broad absorption peak at B950 nm. Based on previous reports 28 , this peak is mostly likely dominated by the carotenoid radical anion of the C þ -P-C 60 À biradical, as the extinction coefficient of the carotenoid radical exceeds that of the fullerene anion by about one order of magnitude. Irrespective of the relative contribution of the C-P þ -C 60 À and the C þ -P-C 60 À transient absorption to this peak, its observation is taken as a clear signature for an ultrafast electron transfer process in the investigated triad in toluene solution upon pulsed excitation at 550 nm.…”

“…In earlier work, evidence for the formation of a charge-separated state in porphyrin and fullerene-containing dyads and triads has been given by probing transient absorption spectra in the near-infrared region 11,28 . To obtain independent evidence for a photoinduced electron transfer process in the investigated CPF triad and to connect our results to earlier work, we excited a triad solution in toluene with pump pulses centred at 550 nm and with a duration of B70 fs.…”

The effect of structural changes on charge transfer states in a light-harvesting carotenoid-diaryl-porphyrin-C60 molecular triad

“…Consequently, extended conjugation between the pi-system of the aryl ring and the pi-system of the porphyrin macrocycle may influence the donor-acceptor electronic interaction, which in turn affects the electron-transfer rate. 13 Studies have addressed this issue by attempting to disrupt extended conjugative interactions by placing alkyl substituents at beta-pyrrolic positions.…”

Quantum modeling of ultrafast photoinduced charge separation

“…(see figure 4) was observed by means of transient absorption spectroscopy and fluorescence measurements [48,61,62]. Quantum yields, transfer rates and the energetics of the intermediate and final products were measured with ps time resolution, indicating that electron transfer from the carotene to the porphyrin radical cation occurs with an overall yield of 0.95 in a time scale of 125 ps [63].…”