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

Abstract

<jats:p> A strategy for increasing the yield of long-lived photoinduced charge separation in artificial photosynthetic reaction centers which is based on multiple electron transfer pathways operating in parallel has been investigated. Excitation of the porphyrin moiety of a carotenoid ( C )–porphyrin ( P )–naphthoquinone (Q) molecular triad leads to the formation of a charge-separated state C <jats:sup>·+</jats:sup>– P – Q <jats:sup>·−</jats:sup> with an overall quantum yield of 0.044 in benzonitrile solution. Photoinduced electron transfer from the porphyrin first excited singlet state gives C – P <jats:sup>·+</jats:sup>– Q <jats:sup>·−</jats:sup> with a quantum yield of ~1.0. However, electron transfer from the carotenoid to the porphyrin radical cation to form the final state does not compete well with charge recombination of C – P <jats:sup>·+</jats:sup>– Q <jats:sup>·−</jats:sup>, reducing the yield. The related pentad C <jats:sub>3</jats:sub>– P – Q features carotenoid, porphyrin and quinone moieties closely related to those in the triad. Excitation of this molecule gives a C <jats:sup>·+</jats:sup>– P ( C <jats:sub>2</jats:sub>)– Q <jats:sup>·−</jats:sup> state with a quantum yield of 0.073. The enhanced yield is ascribed to the fact that three electron donation pathways operating in parallel compete with charge recombination. The yield does not increase by the statistically predicted factor of three owing to small differences in thermodynamic driving force between the two compounds. </jats:p>

Mechanism of Efficient Viologen Radical Generation by Ultrafast Electron Transfer from CdS Quantum Dots

“…, ET from the QD CB edge (at −3.40 eV vs vacuum) to PDQ 2+ , MV 2+ , and BV 2+ are ∼0.55, ∼0.65, and ∼0.73 eV, respectively. ,− Details of energy level calculation are shown in the Supporting Information (SI2). The photogenerated radical can undergo CR with the holes in the QD (VB or trapped below the band edge) or oxidized MPA molecules. − Thus, effective suppression of the CR process is essential for achieving steady-state radical generation.…”

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

“…3,12,[16][17][18]20,31,34,36,37,40,42,[44][45][46][47][48][49] Modeling natural photosynthesis led to the inclusion of quinones as the electron acceptor among some of the earliest synthesized photosynthetic mimics. 19,26,27,29,[50][51][52] Subsequently, fullerenes were found to possess ideal electron acceptor qualities in artificial photosynthetic systems due to their large electron affinity, large charge accumulation capacity, and a small reorganization energy upon electron transfer. [53][54][55][56][57][58][59][60] These considerations led to the synthesis of a CPC 60 molecular triad consisting of a diarylporphyrin (P) covalently linked to a carotenoid polyene (C) and a C 60 fullerene.…”

Photophysical Characterization of Free-Base N-Confused Tetraphenylporphyrins

“…Porphyrin photochemistry provides insight into the dynamics of related biomolecules, such as the photosynthetic reaction centers in purple bacteria and green plants and heme-based metalloproteins such as hemoglobin and myoglobin. Much of this work has recently been focused on free-base and metalloporphyrin assemblies for light-harvesting purposes, porphyrin-containing mimics of the photosynthetic reaction center, and electronic devices . Although the impetus for research in this area is the creation of molecular candidates for understanding and duplicating the photoinduced processes involved in solar energy conversion, these systems also help advance our knowledge of fundamental photochemical processes such as electronic and vibrational relaxation, energy transfer, and solvent dynamics …”