Exploiting potential inversion for photoinduced charge-accumulation in molecular systems

In view of the steadily growing global population and concomitant increasing energy demand, the use of alternative energy sources becomes more and more important. Although the energy demand of the next few decades could in principle be met from fossil fuels, their supply progressively decreases. Moreover, the use of fossil fuels should be decreased because their combustion has devastating consequences for the environment. Aside from threatening human heath, the release of CO2 and other greenhouse gases contributes to global warming. For these reasons, the conversion of sunlight into useful and environmentally friendly energy has received significant attention in recent years and many research groups investigate the formation of so called solar fuels, especially from water. A major challenge of the light-driven water oxidation, or CO2 reduction, is the transfer and accumulation of multiple redox equivalents, which are required for successful conversion. The focus of this thesis is the development of a purely molecular system, in which multiple charges can be accumulated without the use of sacrificial agents. Dibenzo[c,e][1,2]dithiin was used as a central two-electron acceptor, the reduction of which with potential inversion should facilitate electron-accumulation. This acceptor was covalently linked to two ruthenium trisbipyridyl photosensitizers and the electron-accumulation investigated. Furthermore, this triad was extended to a pentad by attaching triarylamine groups to the bipyridine ligands of the sensitizers, which act as internal electron donors. Upon irradiation with visible light, two electrons were successfully accumulated in the triad in the presence of a sacrificial electron donor, whereas the twofold charge-separated state of the pentad is formed without the use of external reductants. The lifetime of the charge-accumulated pentad was determined to be ∼ 100 ns. Investigations of the pentad in the presence of acid revealed that upon protonation of the thiolate groups, a stable photoproduct is formed, which does not undergo charge-recombination to the ground state. Furthermore, it was shown that both the triad and pentad can be used as multi-electron photoredox catalysts. Twofold reduction of the substrate disulfide occurs via thiol-disulfide interchange with the catalyst, and can be performed catalytically in the presence of a sacrificial agent

Exploiting Potential Inversion for Photoinduced Multielectron Transfer and Accumulation of Redox Equivalents in a Molecular Heptad

Photoinduced multielectron transfer and reversible accumulation of redox equivalents is accomplished in a fully integrated molecular heptad composed of four donors, two photosensitizers, and one acceptor. The second reduction of the dibenzo[1,2]dithiin acceptor occurs more easily than the first by 1.3 V, and this potential inversion facilitates the light-driven formation of a two-electron reduced state with a lifetime of 66 ns in deaerated CH3CN. The quantum yield for formation of this doubly charge-separated photoproduct is 0.5%. In acidic oxygen-free solution, the reduction product is a stable dithiol. Under steady-state photoirradiation, our heptad catalyzes the two-electron reduction of an aliphatic disulfide via thiolate-disulfide interchange. Exploitation of potential inversion for the reversible light-driven accumulation of redox equivalents in artificial systems is unprecedented and the use of such a charge-accumulated state for multielectron photoredox catalysis represents an important proof-of-concept

Paramagnetic Molecular Grippers: The Elements of Six-State Redox Switches

The development of semiquinone-based resorcin[4]arene cavitands expands the toolbox of switchable molecular grippers by introducing the first paramagnetic representatives. The semiquinone (SQ) states were generated electrochemically, chemically, and photochemically. We analyzed their electronic, conformational, and binding properties by cyclic voltammetry, ultraviolet/visible (UV/vis) spectroelectrochemistry, electron paramagnetic resonance (EPR) and transient absorption spectroscopy, in conjunction with density functional theory (DFT) calculations. The utility of UV/vis spectroelectrochemistry and EPR spectroscopy in evaluating the conformational features of resorcin[4]arene cavitands is demonstrated. Guest binding properties were found to be enhanced in the SQ state as compared to the quinone (Q) or the hydroquinone (HQ) states of the cavitands. Thus, these paramagnetic SQ intermediates open the way to six-state redox switches provided by two conformations (open and closed) in three redox states (Q, SQ, and HQ) possessing distinct binding ability. The switchable magnetic properties of these molecular grippers and their responsiveness to electrical stimuli has the potential for development of efficient molecular devices

Photoinduced PCET in Ruthenium−Phenol Systems : Thermodynamic Equivalence of Uni- and Bidirectional Reactions

Six termolecular reaction systems comprised of Ru(4,4′-bis(trifluoromethyl)-2,2′-bipyridine)32+, phenols with different para substituents, and pyridine in acetonitrile undergo proton-coupled electron transfer (PCET) upon photoexcitation of the metal complex. Five of these six phenols are found to release in concerted fashion an electron to the ruthenium photooxidant and a proton to the pyridine base. The kinetics for this concerted bidirectional PCET process and its relationship to the reaction free energy were compared to the driving-force dependence of reaction kinetics for unidirectional concerted proton–electron transfer (CPET) between the same phenols and Ru(2,2′-bipyrazine)32+, a combined electron/proton acceptor. The results strongly support the concept of thermodynamic equivalence between separated electron/proton acceptors and single-reagent hydrogen-atom acceptors. A key feature of the explored systems is the similarity between molecules employed for bi- and unidirectional CPET

Light-actuated resorcin[4]arene cavitands

ISSN:0040-402