Photoinduced Electron Transfer in Carotenoporphyrin−Fullerene Triads:  Temperature and Solvent Effects

Abstract

A carotenoid (C) porphyrin (P) fullerene (C 60 ) molecular triad (C-P-C 60 ) has been synthesized and found to undergo photoinduced electron transfer from the porphyrin first excited singlet state or to the fullerene first excited singlet state to yield C-P •+ -C 60 •-. Electron transfer from the carotenoid then gives a C •+ -P-C 60 •final charge-separated state. This state is formed with quantum yields up to 0.88 and has a lifetime of up to 1 µs, depending upon the conditions. The various electron transfer rate constants are relatively insensitive to solvent and temperature. The quantum yield of C •+ -P-C 60 •is relatively constant under conditions ranging from fluid solutions at ambient temperatures to a rigid organic glass at 8 K. In most solvents, recombination of C •+ -P-C 60 •yields the carotenoid triplet state, rather than the ground state. The results suggest that the energies of the charge-separated states of fullerene-based systems are only about half as sensitive to changes in solvent dielectric constant as are those for similar molecules with quinone electron acceptors, and that total reorganization energies for electron transfer are also smaller.

Impaired protein conformational landscapes as revealed in anomalous Arrhenius prefactors

“…Reports of values for an Arrhenius prefactor in excess of 10 17 s −1 are exceedingly rare, and we are not aware of an instance where a value as large as 10 25 s −1 has been measured previously. For reactions proceeding by a tunneling mechanism, a tunnel correction predicts reduced values of Arrhenius prefactor (45), while the Marcus formalism for electron tunneling predicts Arrhenius behavior (46), with values for A int not expected to greatly exceed 10 13 s −1 ; experimental data for intramolecular electron transfer confirm these expectations (47)(48)(49)(50)(51).…”

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

“…5,7 The understanding of the fundamental photosynthetic electronic transition pathways, which include singlet-singlet energy transfer, triplet-triplet energy transfer, and photo-initiated electron transfer, has provided a firm platform for the development of chemical systems which duplicate such efficient energy conversion processes. 3,7, The molecular building blocks employed in many artificial reaction centers for the successful mimicry of photosynthetic energy conversion usually consist of organic/inorganic pigments covalently linked to electron donor and/or acceptor moieties 17,18,21,22,24,25,[34][35][36][37][38][39][40][41][42][43] .…”

“…The effect of the solvent is significant as the experimental CT energy of the + CPC 60 -state is significantly smaller than the gas-phase calculated value. 25,65 The magnitude of the excited state dipole moment for the triad is 153 D, where experimental estimates were made in deriving the dipole value. 62 The approximations entail a simplification of the molecular shape (ellipsoid), underestimation of the Coulomb attraction between the polarizable chromophores as well as the oppositely charged carotenoid-fullerene components, and conformational changes for the charge separated state in solution.…”

Primary charge-recombination in an artificial photosynthetic reaction center

“…In addition, characterizations of V will be useful for designing molecular electronic devices (35). Key to efficient photoinduced CS is prevention of the energywasting charge recombination (CR) characterized in part by V. Although V and have been investigated for various ET steps by experimentally measuring k ET s in systems such as D 2 -D 1 -A triad (7,9,36) and protein complexes (24,25,34), primary CR, D 2 Rhodobacter sphaeroides, which contains as cofactors a special pair (P), two bacteriopheopheophytins (H A and H B ), and two quinones (Q A and Q B ), the CR kinetics of the primary CS state of P ؉• H A Ϫ• Q A cannot easily be observed, because H A Ϫ• 3 Q A forward ET (Ϸ200 ps) is much faster than the primary CR (10-20 ns, which has been reported for Q A -depleted complex) in the CS state (37).…”