Triplet-triplet energy transfer in artificial and natural photosynthetic antennas.

In photosynthetic organisms, protection against photooxidative stress due to singlet oxygen is provided by carotenoid molecules, which quench chlorophyll triplet species before they can sensitize singlet oxygen formation. In anoxygenic photosynthetic organisms, in which exposure to oxygen is low, chlorophyll-to-carotenoid triplet-triplet energy transfer (T-TET) is slow, in the tens of nanoseconds range, whereas it is ultrafast in the oxygen-rich chloroplasts of oxygen-evolving photosynthetic organisms. To better understand the structural features and resulting electronic coupling that leads to T-TET dynamics adapted to ambient oxygen activity, we have carried out experimental and theoretical studies of two isomeric carotenoporphyrin molecular dyads having different conformations and therefore different interchromophore electronic interactions. This pair of dyads reproduces the characteristics of fast and slow T-TET, including a resonance Raman-based spectroscopic marker of strong electronic coupling and fast T-TET that has been observed in photosynthesis. As identified by density functional theory (DFT) calculations, the spectroscopic marker associated with fast T-TET is due primarily to a geometrical perturbation of the carotenoid backbone in the triplet state induced by the interchromophore interaction. This is also the case for the natural systems, as demonstrated by the hybrid quantum mechanics/molecular mechanics (QM/MM) simulations of light-harvesting proteins from oxygenic (LHCII) and anoxygenic organisms (LH2). Both DFT and electron paramagnetic resonance (EPR) analyses further indicate that, upon T-TET, the triplet wave function is localized on the carotenoid in both dyads.

Metal-free organic sensitizers for use in water-splitting dye-sensitized photoelectrochemical cells.

Solar fuel generation requires the efficient capture and conversion of visible light. In both natural and artificial systems, molecular sensitizers can be tuned to capture, convert, and transfer visible light energy. We demonstrate that a series of metal-free porphyrins can drive photoelectrochemical water splitting under broadband and red light (λ > 590 nm) illumination in a dye-sensitized TiO2 solar cell. We report the synthesis, spectral, and electrochemical properties of the sensitizers. Despite slow recombination of photoinjected electrons with oxidized porphyrins, photocurrents are low because of low injection yields and slow electron self-exchange between oxidized porphyrins. The free-base porphyrins are stable under conditions of water photoelectrolysis and in some cases photovoltages in excess of 1 V are observed.

Understanding iridium oxide nanoparticle surface sites by their interaction with catechol.

Iridium oxide (IrOx) is one of the best water splitting electrocatalysts, but its active site details are not well known. As with all heterogeneous catalysts, a strategy for counting the number of active sites is not clear, and understanding their nature and structure is remarkably difficult. In this work, we performed a combined study using optical spectroscopy, magnetic resonance and electrochemistry to characterize the interaction of IrOx nanoparticles (NPs) with a probe molecule, catechol. The catalyst is heterogeneous given that the substrate is in a different phase, but behaves as a homogeneous catalyst from the point of view of electrochemistry since it remains in colloidal suspension. We find two types of binding sites: centers A which bind catechol irreversibly making up 21% of the surface, and centers B which bind catechol reversibly making up 79% of the surface. UV-vis absorption spectroscopy shows that the A sites are responsible for the characteristic blue color of the NPs. Electrochemical experiments indicate that the B sites are catalytically active and we give the number of active sites per nanoparticle. We conclude by performing a survey of ligands used in solar cell architectures and show which ones bind well to the surface and which ones inhibit the catalytic activity when doing so, presenting quantitative guidelines for the correct handling of IrOx nanoparticles during their incorporation into multifunctional solar energy harvesting architectures. We suggest ligands binding on the surface oxygen atoms allow for large bound ligand densities with no detrimental effect on the catalytic activity.

Preparation of Halogenated Fluorescent Diaminophenazine Building Blocks.

A short, convenient, and scalable protocol for the one-pot synthesis of a series of fluorescent 7,8-dihalo-2,3-diaminophenazines is introduced. The synthetic route is based on the oxidative condensation of 4,5-dihalo-1,2-diaminobenzenes in aqueous conditions. The resulting diaminophenazines could be attractive intermediates for the preparation of polyfunctional phenazines and extended polyheteroacenes. We find that the undesired hydroxylation byproducts, typically obtained in aqueous conditions, are completely suppressed by addition of a stoichiometric amount of acetone during the oxidation step allowing for selective formation of 7,8-dihalo-2,2-dimethyl-2,3-dihydro-1H-imidazo[4,5-b]phenazine derivatives with good to excellent yields. Under reductive conditions, the imidazolidine ring can be hydrolyzed into the desired 7,8-dihalo-2,3-diaminophenazines. Furthermore, we report a selective route under highly reducing conditions to monohydrodeaminate the 2,3-di(methylamino) phenazine derivatives, which allows for further structural variations of these phenazine building blocks. All of these derivatives are luminescent, with measured fluorescence quantum-yields of up to 80% in ethanol for the more rigid structures, highlighting the potential of such materials to provide new fluorophores.

Dual Singlet Excited-State Quenching Mechanisms in an Artificial Caroteno-Phthalocyanine Light Harvesting Antenna.

Under excess illumination, photosystem II of plants dissipates excess energy through the quenching of chlorophyll fluorescence in the light harvesting antenna. Various models involving chlorophyll quenching by carotenoids have been proposed, including (i) direct energy transfer from chlorophyll to the low-lying optically forbidden carotenoid S1 state, (ii) formation of a collective quenched chlorophyll-carotenoid S1 excitonic state, (iii) chlorophyll-carotenoid charge separation and recombination, and (iv) chlorophyll-chlorophyll charge separation and recombination. In previous work, the first three processes were mimicked in model systems: in a Zn-phthalocyanine-carotenoid dyad with an amide linker, direct energy transfer was observed by femtosecond transient absorption spectroscopy, whereas in a Zn-phthalocyanine-carotenoid dyad with an amine linker excitonic quenching was demonstrated. Here, we present a transient absorption spectroscopic study on a Zn-phthalocyanine-carotenoid dyad with a phenylene linker. We observe that two quenching phases of the phthalocyanine excited state exist at 77 and 213 ps in addition to an unquenched phase at 2.7 ns. Within our instrument response of ∼100 fs, carotenoid S1 features rise which point at an excitonic quenching mechanism. Strikingly, we observe an additional rise of carotenoid S1 features at 3.6 ps, which shows that a direct energy transfer mechanism in an inverted kinetics regime is also in effect. We assign the 77 ps decay component to excitonic quenching and the 3.6 ps/213 ps rise and decay components to direct energy transfer. Our results indicate that dual quenching mechanisms may be active in the same molecular system, in addition to an unquenched fraction. Computational chemistry results indicate the presence of multiple conformers where one of the dihedral angles of the phenylene linker assumes distinct values. We propose that the parallel quenching pathways and the unquenched fraction result from such conformational subpopulations. Our results suggest that it is possible to switch between different regimes of quenching and nonquenching through a conformational change on the same molecule, offering insights into potential mechanisms used in biological photosynthesis to adapt to light intensity changes on fast time scales.

Linear correlation models for the redox potential of organic molecules in aqueous solutions.

In this study, we use the molecular orbital energy approximation (MOEA) and the energy difference approximation (EDA) to build linear correlation models for the redox potentials of 53 organic compounds in aqueous solutions. The molecules evaluated include nitroxides, phenols, and amines. Both the MOEA and EDA methods yield similar correlation models, however, the MOEA method is less computationally expensive. Correlation coefficients (R2) below 0.3 and mean absolute errors above 0.25 V were found for correlation models built without solvent effects. When explicit water molecules and a continuum solvent model are added to the calculations, correlation coefficients close to 0.8 are reached, and mean absolute errors below 0.18 V are obtained. The incorporation of solvent effects is necessary for good correlation models, particularly for redox processes of charged molecules in aqueous solutions. A comparison of the correlation models from different methodologies is provided. Graphical abstract.

HYSCORE and DFT Studies of Proton-Coupled Electron Transfer in a Bioinspired Artificial Photosynthetic Reaction Center.

The photosynthetic water-oxidation reaction is catalyzed by the oxygen-evolving complex in photosystem II (PSII) that comprises the Mn4CaO5 cluster, with participation of the redox-active tyrosine residue (YZ) and a hydrogen-bonded network of amino acids and water molecules. It has been proposed that the strong hydrogen bond between YZ and D1-His190 likely renders YZ kinetically and thermodynamically competent leading to highly efficient water oxidation. However, a detailed understanding of the proton-coupled electron transfer (PCET) at YZ remains elusive owing to the transient nature of its intermediate states involving YZ⋅. Herein, we employ a combination of high-resolution two-dimensional 14N hyperfine sublevel correlation spectroscopy and density functional theory methods to investigate a bioinspired artificial photosynthetic reaction center that mimics the PCET process involving the YZ residue of PSII. Our results underscore the importance of proximal water molecules and charge delocalization on the electronic structure of the artificial reaction center.

Electron Transfer Assisted by Vibronic Coupling from Multiple Modes.

Understanding the effect of vibronic coupling on electron transfer (ET) rates is a challenge common to a wide range of applications, from electrochemical synthesis and catalysis to biochemical reactions and solar energy conversion. The Marcus-Jortner-Levich (MJL) theory offers a model of ET rates based on a simple analytic expression with a few adjustable parameters. However, the MJL equation in conjunction with density functional theory (DFT) has yet to be established as a predictive first-principles methodology. A framework is presented for calculating transfer rates modulated by molecular vibrations, that circumvents the steep computational cost which has previously necessitated approximations such as condensing the vibrational manifold into a single empirical frequency. Our DFT-MJL approach provides robust and accurate predictions of ET rates spanning over 4 orders of magnitude in the 106-1010 s-1 range. We evaluate the full MJL equation with a Monte Carlo sampling of the entire active space of thermally accessible vibrational modes, while using no empirical parameters. The contribution to the rate of individual modes is illustrated, providing insight into the interplay between vibrational degrees of freedom and changes in electronic state. The reported findings are valuable for understanding ET rates modulated by multiple vibrational modes, relevant to a broad range of systems within the chemical sciences.

Spectroscopic Analysis of a Biomimetic Model of Tyr(Z) Function in PSII.

Using natural photosynthesis as a model, bio-inspired constructs for fuel generation from sunlight are being developed. Here we report the synthesis and time-resolved spectroscopic analysis of a molecular triad in which a porphyrin electron donor is covalently linked to both a cyanoporphyrin electron acceptor and a benzimidazole-phenol model for the TyrZ-D1His190 pair of PSII. A dual-laser setup enabled us to record the ultrafast kinetics and long-living species in a single experiment. From this data, the photophysical relaxation pathways were elucidated for the triad and reference compounds. For the triad, quenching of the cyanoporphyrin singlet excited state lifetime was interpreted as photoinduced electron transfer from the porphyrin to the excited cyanoporphyrin. In contrast to a previous study of a related molecule, we were unable to observe subsequent formation of a long-lived charge separated state involving the benzimidazole-phenol moiety. The lack of detection of a long-lived charge separated state is attributed to a change in energetic landscape for charge separation/recombination due to small differences in structure and solvation of the new triad.

A bioinspired redox relay that mimics radical interactions of the Tyr-His pairs of photosystem II.

In water-oxidizing photosynthetic organisms, light absorption generates a powerfully oxidizing chlorophyll complex (P680(•+)) in the photosystem II reaction centre. This is reduced via an electron transfer pathway from the manganese-containing water-oxidizing catalyst, which includes an electron transfer relay comprising a tyrosine (Tyr)-histidine (His) pair that features a hydrogen bond between a phenol group and an imidazole group. By rapidly reducing P680(•+), the relay is thought to mitigate recombination reactions, thereby ensuring a high quantum yield of water oxidation. Here, we show that an artificial reaction centre that features a benzimidazole-phenol model of the Tyr-His pair mimics both the short-internal hydrogen bond in photosystem II and, using electron paramagnetic resonance spectroscopy, the thermal relaxation that accompanies proton-coupled electron transfer. Although this artificial system is much less complex than the natural one, theory suggests that it captures the essential features that are important in the function of the relay.

Simple and accurate correlation of experimental redox potentials and DFT-calculated HOMO/LUMO energies of polycyclic aromatic hydrocarbons.

The ability to accurately predict the oxidation and reduction potentials of molecules is very useful in various fields and applications. Quantum mechanical calculations can be used to access this information, yet sometimes the usefulness of these calculations can be limited because of the computational requirements for large systems. Methodologies that yield strong linear correlations between calculations and experimental data have been reported, however the balance between accuracy and computational cost is always a major issue. In this work, linear correlations (with an R(2) value of up to 0.9990) between DFT-calculated HOMO/LUMO energies and 70 redox potentials from a series of 51 polycyclic aromatic hydrocarbons (obtained from the literature) are presented. The results are compared to previously reported linear correlations that were obtained with a more expensive computational methodology based on a Born-Haber thermodynamic cycle. It is shown in this article that similar or better correlations can be obtained with a simple and cheaper calculation.

Spectral characteristics and photosensitization of TiO2 nanoparticles in reverse micelles by perylenes.

We report on the photosensitization of titanium dioxide nanoparticles (TiO2 NPs) synthesized inside AOT (bis(2-ethylhexyl) sulfosuccinate sodium salt) reverse micelles following photoexcitation of perylene derivatives with dicarboxylate anchoring groups. The dyes, 1,7-dibromoperylene-3,4,9,10-tetracarboxy dianhydride (1), 1,7-dipyrrolidinylperylene-3,4,9,10-tetracarboxy dianhydride (2), and 1,7-bis(4-tert-butylphenyloxy)perylene-3,4,9,10-tetracarboxy dianhydride (3), have considerably different driving forces for photoinduced electron injection into the TiO2 conduction band, as estimated by electrochemical measurements and quantum mechanical calculations. Fluorescence anisotropy measurements indicate that dyes 1 and 2 are preferentially solubilized in the micellar structure, creating a relatively large local concentration that favors the attachment of the dye to the TiO2 surface. The binding process was followed by monitoring the hypsochromic shift of the dye absorption spectra over time for 1 and 2. Photoinduced electron transfer from the singlet excited state of 1 and 2 to the TiO2 conduction band (CB) is indicated by emission quenching of the TiO2-bound form of the dyes and confirmed by transient absorption measurements of the radical cation of the dyes and free carriers (injected electrons) in the TiO2 semiconductor. Steady state and transient spectroscopy indicate that dye 3 does not bind to the TiO2 NPs and does not photosensitize the semiconductor. This observation was rationalized as a consequence of the bulky t-butylphenyloxy groups which create a strong steric impediment for deep access of the dye within the micelle structure to reach the semiconductor oxide surface.