Concerted Electron-Nuclear Motion in Proton-Coupled Electron Transfer-Driven Grotthuss-Type Proton Translocation

Photoinduced proton-coupled electron transfer and long-range two-proton transport via a Grotthuss-type mechanism are investigated in a biomimetic construct. The ultrafast, nonequilibrium dynamics are assessed via two-dimensional electronic vibrational spectroscopy, in concert with electrochemical and computational techniques. A low-frequency mode is identified experimentally and found to promote double proton and electron transfer, supported by recent theoretical simulations of a similar but abbreviated (non-photoactive) system. Excitation frequency peak evolution and center line slope dynamics show direct evidence of strongly coupled nuclear and electronic degrees of freedom, from which we can conclude that the double proton and electron transfer processes are concerted (up to an uncertainty of 24 fs). The nonequilibrium pathway from the photoexcited Franck-Condon region to the E2PT state is characterized by an ∼110 fs time scale. This study and the tools presented herein constitute a new window into hot charge transfer processes involving an electron and multiple protons.

Tuning the redox potential of tyrosine-histidine bioinspired assemblies

Photosynthesis powers our planet and is a source of inspiration for developing artificial constructs mimicking many aspects of the natural energy transducing process. In the complex machinery of photosystem II (PSII), the redox activity of the tyrosine Z (Tyr_z) hydrogen-bonded to histidine 190 (His190) is essential for its functions. For example, the Tyr_z-His190 pair provides a proton-coupled electron transfer (PCET) pathway that effectively competes against the back-electron transfer reaction and tunes the redox potential of the phenoxyl radical/phenol redox couple ensuring a high net quantum yield of photoinduced charge separation in PSII. Herein, artificial assemblies mimicking both the structural and redox properties of the Tyr_z-His190 pair are described. The bioinspired constructs contain a phenol (Tyr_z model) covalently linked to a benzimidazole (His190 model) featuring an intramolecular hydrogen bond which closely emulates the one observed in the natural counterpart. Incorporation of electron-withdrawing groups in the benzimidazole moiety systematically changes the intramolecular hydrogen bond strength and modifies the potential of the phenoxyl radical/phenol redox couple over a range of ~ 250 mV. Infrared spectroelectrochemistry (IRSEC) demonstrates the associated one-electron, one-proton transfer (E1PT) process upon electrochemical oxidation of the phenol. The present contribution provides insight regarding the factors controlling the redox potential of the phenol and highlights strategies for the design of futures constructs capable of transporting protons across longer distances while maintaining a high potential of the phenoxyl radical/phenol redox couple.

Multi PCET in symmetrically substituted benzimidazoles

Proton-coupled electron transfer (PCET) reactions depend on the hydrogen-bond connectivity between sites of proton donors and acceptors. The 2-(2′-hydroxyphenyl) benzimidazole (BIP) based systems, which mimic the natural Tyr_Z-His190 pair of Photosystem II, have been useful for understanding the associated PCET process triggered by one-electron oxidation of the phenol. Substitution of the benzimidazole by an appropriate terminal proton acceptor (TPA) group allows for two-proton translocations. However, the prototropic properties of substituted benzimidazole rings and rotation around the bond linking the phenol and the benzimidazole can lead to isomers that interrupt the intramolecular hydrogen-bonded network and thereby prevent a second proton translocation. Herein, a strategic symmetrization of a benzimidazole based system with two identical TPAs yields an uninterrupted network of intramolecular hydrogen bonds regardless of the isomeric form. NMR data confirms the presence of a single isomeric form in the disubstituted system but not in the monosubstituted system in certain solvents. Infrared spectroelectrochemistry demonstrates a two-proton transfer process associated with the oxidation of the phenol occurring at a lower redox potential in the disubstituted system relative to its monosubstituted analogue. Computational studies support these findings and show that the disubstituted system stabilizes the oxidized two-proton transfer product through the formation of a bifurcated hydrogen bond. Considering the prototropic properties of the benzimidazole heterocycle in the context of multiple PCET will improve the next generation of novel, bioinspired constructs built by concatenated units of benzimidazoles, thus allowing proton translocations at nanoscale length.

Proton-coupled electron transfer (PCET) reactions depend on the hydrogen-bond connectivity between sites of proton donors and acceptors.

Models to study photoinduced multiple proton coupled electron transfer processes

In water-oxidizing photosynthetic organisms, excitation of the reaction-center chlorophylls (P680) triggers a cascade of electron and proton transfer reactions that establish charge separation across the membrane and proton-motive force. An early oxidation step in this process involves proton-coupled electron transfer (PCET) via a tyrosine-histidine redox relay (Yz-H190). Herein, we report the synthesis and structural characterization of two isomeric dyads designed to model this PCET process. Both are based on the same high potential fluorinated porphyrin (model for P680), linked to isomeric pyridylbenzimidazole-phenols (models for Yz-H190). The two isomeric dyads have different hydrogen bond frameworks, which is expected to change the PCET photooxidation mechanism. In these dyads, 1H NMR evidence indicates that in one dyad the hydrogen bond network would support a Grotthuss-type proton transfer process, whereas in the other the hydrogen bond network is interrupted. Photoinduced one-electron, two-proton transfer is expected to occur in the fully hydrogen-bonded dyad upon oxidation of the phenol by the excited state of the porphyrin. In contrast for the isomer with the interrupted hydrogen bond network, an ultrafast photoinduced one-electron one-proton transfer process is anticipated, followed by a much slower proton transfer to the terminal proton acceptor. Understanding the nature of photoinduced PCET mechanisms in these biomimetic models will provide insights into the design of future generations of artificial constructs involved in energy conversion schemes.

Fluorinated aluminum(III) porphyrins: Synthesis, spectroscopy, electrochemistry and photochemistry

A series of fluorinated free-base porphyrins (H2TPPF[Formula: see text], [Formula: see text] = 0, 8, 12, 20, 24) and the corresponding aluminum(III) porphyrin (AlTPPF[Formula: see text]-Ph, [Formula: see text] = 0, 8, 12, 20, 24) derivatives have been synthesized and their spectroscopic, redox and optical properties were investigated. The absorption studies show that the spectral shapes of investigated porphyrins are sensitive to the degree of fluorination on the meso-phenyl units. Analogously, the fluorescence quantum yields and singlet-state lifetimes depend on the number of fluorine atoms, and decrease by increasing the number of fluorine atoms. The H2TPPF[Formula: see text] and AlTPPF[Formula: see text]-Ph ([Formula: see text] = 8, 12, 20, 24) derivatives exhibited lower fluorescence intensities compared to the H2TPP and AlTPP, respectively. However, the AlTPPF[Formula: see text]-Ph ([Formula: see text] = 0, 8, 12, 20, 24) derivatives yield relatively a strong fluorescence compared to the well-known ZnTPP. As predicted, the redox potentials are shifted to the more positive side by increasing the fluorine atoms. The Lewis acidity of AlTPPF[Formula: see text]-Ph was quantified by using the absorption and fluorescence titrations with the Lewis base [Formula: see text]-methylimidazole (Me-Im). The titration data suggests that the Lewis acidity of the Al center rises when increasing the number of fluorine atoms on the porphyrin. Together, the high fluorescence quantum yields, high-potentials, unique optical and redox properties suggest that the investigated porphyrins could be potential sensitizers to mimic various components of artificial photosynthetic systems for the production of solar fuels.

Electron-Nuclear Dynamics Accompanying Proton-Coupled Electron Transfer

Although photoinduced proton-coupled electron transfer (PCET) plays an essential role in photosynthesis, a full understanding of the mechanism is still lacking due to the complex nonequilibrium dynamics arising from the strongly coupled electronic and nuclear degrees of freedom. Here we report the photoinduced PCET dynamics of a biomimetic model system investigated by means of transient IR and two-dimensional electronic-vibrational (2DEV) spectroscopies, IR spectroelectrochemistry (IRSEC), and calculations utilizing long-range-corrected hybrid density functionals. This collective experimental and theoretical effort provides a nuanced picture of the complicated dynamics and synergistic motions involved in photoinduced PCET. In particular, the evolution of the 2DEV line shape, which is highly sensitive to the mixing of vibronic states, is interpreted by accurate computational modeling of the charge separated state and is shown to represent a gradual change in electron density distribution associated with a dihedral twist that occurs on a 120 fs time scale.

Nonequilibrium Dynamics of Proton-Coupled Electron Transfer in Proton Wires: Concerted but Asynchronous Mechanisms

The coupling between electrons and protons and the long-range transport of protons play important roles throughout biology. Biomimetic systems derived from benzimidazole-phenol (BIP) constructs have been designed to undergo proton-coupled electron transfer (PCET) upon electrochemical or photochemical oxidation. Moreover, these systems can transport protons along hydrogen-bonded networks or proton wires through multiproton PCET. Herein, the nonequilibrium dynamics of both single and double proton transfer in BIP molecules initiated by oxidation are investigated with first-principles molecular dynamics simulations. Although these processes are concerted in that no thermodynamically stable intermediate is observed, the simulations predict that they are predominantly asynchronous on the ultrafast time scale. For both systems, the first proton transfer typically occurs ∼100 fs after electron transfer. For the double proton transfer system, typically the second proton transfer occurs hundreds of femtoseconds after the initial proton transfer. A machine learning algorithm was used to identify the key molecular vibrational modes essential for proton transfer: a slow, in-plane bending mode that dominates the overall inner-sphere reorganization, the proton donor-acceptor motion that leads to vibrational coherence, and the faster donor-hydrogen stretching mode. The asynchronous double proton transfer mechanism can be understood in terms of a significant mode corresponding to the two anticorrelated proton donor-acceptor motions, typically decreasing only one donor-acceptor distance at a time. Although these PCET processes appear concerted on the time scale of typical electrochemical experiments, attaching these BIP constructs to photosensitizers may enable the detection of the asynchronicity of the electron and multiple proton transfers with ultrafast two-dimensional spectroscopy. Understanding the fundamental PCET mechanisms at this level will guide the design of PCET systems for catalysis and energy conversion processes.