Ultrafast Electron Transfer Dynamics in a Series of Porphyrin/Viologen Complexes: Involvement of Electronically Excited Radical Pair Products

Ultrafast electron transfer was studied for a series of metalloporphyrin/bipyridinium complexes in aqueous solution, using laser excitation in the Soret or Q-bands of the porphyrin. Electron transfer occurred before electronic and vibrational relaxation of the initial excited state. This allowed for a thorough investigation of the dependence of electron transfer rate constants on the driving force and the nature of the product state. The driving force dependence showed that electron transfer from the S₂ state occurred to an electronically excited radical pair state, and the present results provide the most direct evidence to date for the formation of such states in photoinduced electron transfer reactions. We also found that subsequent recombination of the radical pair produced vibrationally excited ground states; the excess energy of the radical pair generated from the initial state is not completely dissipated during the lifetime of the radical pair. The porphyrin/bipyridinium complexes where recombination lies deeper in the Marcus inverted region show <i>less</i> formation of unrelaxed ground states, contrary to what is expected from equilibrium electron transfer theories. Instead, the rate of the electron transfer, which competes with vibrational relaxation, was the main parameter controlling the relative yield of unrelaxed ground states within this series of complexes

Direct Evidence of a Tryptophan Analogue Radical Formed in a Concerted Electron−Proton Transfer Reaction in Water

Proton-coupled electron transfer (PCET) is a fundamental reaction step of many chemical and biological processes. Well-defined biomimetic systems are promising tools for investigating the PCET mechanisms relevant to natural proteins. Of particular interest is the possibility to distinguish between stepwise and concerted transfer of the electron and proton, and how PCET is controlled by a proton acceptor such as water. Thus, many tyrosine and phenolic derivatives have been shown to undergo either stepwise or concerted PCET, where the latter process is defined by simultaneous tunneling of the electron and proton from the same transition state. For tryptophan instead, it is theoretically predicted that a concerted pathway can never compete with the stepwise electron-first mechanism (ETPT) when neat water is the primary proton acceptor. The argument is based on the radical p<i>K</i>_a (∼4.5) that is much higher than that for water (p<i>K</i>_a(H₃O⁺) = 0), which thermodynamically disfavors a concerted proton transfer to H₂O. This is in contrast to the very acidic radical cation of tyrosine (p<i>K</i>_a ∼ −2). However, in this study we show, by direct time-resolved absorption spectroscopy on two [Ru­(bpy)₃]²⁺−tryptophan (bpy = 2,2′-bipyridine) analogue complexes, that also tryptophan oxidation with water as a proton acceptor can occur via a concerted pathway, provided that the oxidant has weak enough driving force. This rivals the theoretical predictions and suggests that our current understanding of PCET reactions in water is incomplete

Competitive Hole Transfer from CdSe Quantum Dots to Thiol Ligands in CdSe-Cobaloxime Sensitized NiO Films Used as Photocathodes for H₂ Evolution

Quantum dot (QD) sensitized NiO photocathodes rely on efficient photoinduced hole injection into the NiO valence band. A system of a mesoporous NiO film co-sensitized with CdSe QDs and a molecular proton-reduction catalyst was studied. While successful electron transfer from the excited QDs to the catalyst is observed, most of the photogenerated holes are instead quenched very rapidly (ps) by hole trapping at the surface thiols of the capping agent used as linker molecules. We confirmed our conclusion by first using a thiol free capping agent and second varying the thiol concentration on the QD’s surface. The later resulted in faster hole trapping as the thiol concentration increased. We suggest that this hole trapping by the linker limits the H₂ yield for this photocathode in a device

Competitive Hole Transfer from CdSe Quantum Dots to Thiol Ligands in CdSe-Cobaloxime Sensitized NiO Films Used as Photocathodes for H₂ Evolution

Quantum dot (QD) sensitized NiO photocathodes rely on efficient photoinduced hole injection into the NiO valence band. A system of a mesoporous NiO film co-sensitized with CdSe QDs and a molecular proton-reduction catalyst was studied. While successful electron transfer from the excited QDs to the catalyst is observed, most of the photogenerated holes are instead quenched very rapidly (ps) by hole trapping at the surface thiols of the capping agent used as linker molecules. We confirmed our conclusion by first using a thiol free capping agent and second varying the thiol concentration on the QD’s surface. The later resulted in faster hole trapping as the thiol concentration increased. We suggest that this hole trapping by the linker limits the H₂ yield for this photocathode in a device

State-Selective Electron Transfer in an Unsymmetric Acceptor−Zn(II)porphyrin−Acceptor Triad: Toward a Controlled Directionality of Electron Transfer from the Porphyrin S 2 and S 1 States as a Basis for a Molecular Switch

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Tuning of Conductivity and Density of States of NiO Mesoporous Films Used in p‑Type DSSCs

Nickel oxide has been used as the mesoporous electrode material for p-type dye sensitized solar cell (DSSC) for many years, but no high efficiency cells have been obtained yet. The poor results are commonly attributed to the lack of conductivity of the NiO film. In this paper we studied the electrical conduction of NiO mesoporous film with cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). We used unsensitized NiO on FTO as an electrode with no dye adsorbed on the surface. Tests made with a DSSC device-like cell (FTO-Pt-I^–/I₃^–-NiO-FTO) showed a surprisingly high Faradaic current (20 mA/cm^–2 at 1 V), proving a good electrical conductivity of mesoporous NiO. We also used lithium as dopant to improve the electrical properties of the film. The Li-doping resulted in widening the inert (not conductive) window in the CV plot. The EIS analysis clarified that this behavior is due to a strong dependence of the valence band shape and position with respect to the Li-doping concentration. Our results show that DSSC performance does not need to be limited by the conductivity of mesoporous NiO, which encourages more effort in p-type DSSC research based on this material

Accumulative Charge Separation Inspired by Photosynthesis

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