Spectroscopic Investigations Provide a Rationale for the Hydrogen-Evolving Activity of Dye-Sensitized Photocathodes Based on a Cobalt Tetraazamacrocyclic Catalyst

Charge Transfer Dynamics in Organic-Inorganic Hybrid Heterostructures-Insights by Vibrational-Sum Frequency Generation Spectroscopy

Organic-inorganic heterostructures play a pivotal role in modern electronic and optoelectronic applications including photodetectors and field effect transistors, as well as in solar energy conversion such as photoelectrodes of dye-sensitized solar cells, photoelectrochemical cells, and in organic photovoltaics. To a large extent, performance of such devices is controlled by charge transfer dynamics at and across (inner) interfaces, e.g., between a wide band gap semiconductor and molecular sensitizers and/or catalysts. Hence, a detailed understanding of the structure-dynamics-function relationship of such functional interfaces is necessary to rationalize possible performance limitations of these materials and devices on a molecular level. Vibrational sum-frequency generation (VSFG) spectroscopy, as an interface-sensitive spectroscopic technique, allows to obtain chemically specific information from interfaces and combines such chemical insights with ultrafast time resolution, when integrated as a spectroscopic probe into a pump-probe scheme. Thus, this minireview discusses the advantages and potential of VSFG spectroscopy for investigating interfacial charge transfer dynamics and structural changes at inner interfaces. A critical perspective of the unique spectroscopic view of otherwise inaccessible interfaces is presented, which we hope opens new opportunities for an improved understanding of function-determining processes in complex materials, and brings together communities who are devoted to designing materials and devices with spectroscopists.

Investigation of the Excited-State Electron Transfer and Cage Escape Yields Between Halides and a Fe(III) Photosensitizer

Excited-state quenching and reduction of [Fe(phtmeimb)2]+, where phtmeimb is phenyl[tris(3-methyl-imidazolin-2-ylidene)]borate, with iodide, bromide, and chloride were studied in dichloromethane, acetonitrile, and acetonitrile/water 1:1 mixture by means of steady-state and time-resolved spectroscopic techniques. Quenching rate constants were almost diffusion-limited in dichloromethane and acetonitrile and followed the expected periodic trend, i.e., I- > Br- > Cl-. Confirmation of excited-state reductive electron transfer was only unambiguously obtained when iodide was used as a quencher. The cage escape yields, i.e., the separation of the geminate radical pair formed upon bimolecular excited-state electron transfer, were determined. These yields were larger in dichloromethane (0.079) than in acetonitrile (0.017), and no photoproduct could be observed in acetonitrile/water 1:1. This study further emphasizes that solvents with low dielectric constant are more suited for productive excited-state electron transfer using Fe(III) photosensitizers with 2LMCT excited state.

Embedding biocatalysts in a redox polymer enhances the performance of dye-sensitized photocathodes in bias-free photoelectrochemical water splitting

Dye-sensitized photoelectrodes consisting of photosensitizers and molecular catalysts with tunable structures and adjustable energy levels are attractive for low-cost and eco-friendly solar-assisted synthesis of energy rich products. Despite these advantages, dye-sensitized NiO photocathodes suffer from severe electron-hole recombination and facile molecule detachment, limiting photocurrent and stability in photoelectrochemical water-splitting devices. In this work, we develop an efficient and robust biohybrid dye-sensitized NiO photocathode, in which the intermolecular charge transfer is enhanced by a redox polymer. Owing to efficient assisted electron transfer from the dye to the catalyst, the biohybrid NiO photocathode showed a satisfactory photocurrent of 141±17 μA·cm-2 at neutral pH at 0 V versus reversible hydrogen electrode and a stable continuous output within 5 h. This photocathode is capable of driving overall water splitting in combination with a bismuth vanadate photoanode, showing distinguished solar-to-hydrogen efficiency among all reported water-splitting devices based on dye-sensitized photocathodes. These findings demonstrate the opportunity of building green biohybrid systems for artificial synthesis of solar fuels.

Ultrafast Electron Transfer from CuInS2 Quantum Dots to a Molecular Catalyst for Hydrogen Production: Challenging Diffusion Limitations

Systems integrating quantum dots with molecular catalysts are attracting ever more attention, primarily owing to their tunability and notable photocatalytic activity in the context of the hydrogen evolution reaction (HER) and CO2 reduction reaction (CO2RR). CuInS2 (CIS) quantum dots (QDs) are effective photoreductants, having relatively high-energy conduction bands, but their electronic structure and defect states often lead to poor performance, prompting many researchers to employ them with a core-shell structure. Molecular cobalt HER catalysts, on the other hand, often suffer from poor stability. Here, we have combined CIS QDs, surface-passivated with l-cysteine and iodide from a water-based synthesis, with two tetraazamacrocyclic cobalt complexes to realize systems which demonstrate high turnover numbers for the HER (up to >8000 per catalyst), using ascorbate as the sacrificial electron donor at pH = 4.5. Photoluminescence intensity and lifetime quenching data indicated a large degree of binding of the catalysts to the QDs, even with only ca. 1 μM each of QDs and catalysts, linked to an entirely static quenching mechanism. The data was fitted with a Poissonian distribution of catalyst molecules over the QDs, from which the concentration of QDs could be evaluated. No important difference in either quenching or photocatalysis was observed between catalysts with and without the carboxylate as a potential anchoring group. Femtosecond transient absorption spectroscopy confirmed ultrafast interfacial electron transfer from the QDs and the formation of the singly reduced catalyst (CoII state) for both complexes, with an average electron transfer rate constant of ≈ (10 ps)-1. These favorable results confirm that the core tetraazamacrocyclic cobalt complex is remarkably stable under photocatalytic conditions and that CIS QDs without inorganic shell structures for passivation can act as effective photosensitizers, while their smaller size makes them suitable for application in the sensitization of, inter alia, mesoporous electrodes.

Photophysical Study on the Effect of the External Potential on NiO-Based Photocathodes

In the present study, we investigate the effects of the applied external potential on a dye-sensitized NiO photocathode by time-resolved photoluminescence and femtosecond transient absorption spectroscopy under operating conditions. Instead of the anticipated acceleration of photoinduced hole injection from dye into NiO at a more negative applied potential, we observe that both hole injection and charge recombination are slowed down. We cautiously assign this effect to a variation in OH- ion concentration in the inner Helmholtz plane of the electrochemical double layer with applied potential, warranting further investigation for the realization of efficient solar fuel devices.

A Multi-Pyridine-Anchored and -Linked Bilayer Photocathode for Water Reduction

The development of efficient photocathodes is of critical importance for the constructions of promising tandem photo-electrochemical cells. Most known dye-sensitized photocathodes are prepared with the conventional carboxylic or phosphonic acid anchors and require the presence of other terminal linking groups to connect catalysts; they suffer from high synthetic difficulty and low adsorption stability in aqueous media. Here, a compact bilayer photocathode has been prepared by using a pyrene-based photosensitizer with multiple terminal pyridine moieties as both the anchoring and linking groups to connect a Co hydrogen-evolution catalyst to the NiO substrate. The catalyst and dye molecule are assembled in a layer-by-layer manner on NiO through the metal-pyridine coordination. This photocathode exhibits good dye adsorption stability in aqueous media. A stable cathodic photocurrent of 70 μA cm-2 was achieved, with H2 being generated at the photocathode under the visible-light irradiation. The Faraday efficiency of H2 evolution was estimated to be 9.1 %. Transient absorption spectral studies suggest that the interfacial hole transfer occurs within a few picoseconds. The integration of the organic photosensitizer with pyridine anchoring and linking groups is expected to provide a simple method for the fabrication of stable and efficient photocathodes.

Chloride, Bromide, and Iodide Photooxidation in Acetonitrile/Water Mixtures Using Binuclear Iridium(III) Photosensitizers

Two iridium(III) binuclear photosensitizers, [Ir(dFCF3ppy)2(N-N)Ir(dFCF3ppy)2]2+, where N-N is tetrapyrido[3,2-a:2',3'-c:3″,2″-h:2‴,3‴-j]phenazine (Ir-TPPHZ) and 1,4,5,8-tetraazaphenanthrene[9,10-b]-1,4,5,8,9,12-hexaazatriphenylene (Ir-TAPHAT) are reported for iodide, bromide, and chloride photooxidation in acetonitrile and acetonitrile/water mixtures using blue-light irradiation. Excited-state reduction potentials Ered* of +2.02 and +2.09 V vs NHE were determined for Ir-TPPHZ and Ir-TAPHAT, respectively. Both photosensitizers' excited states were efficiently quenched by iodide, bromide, and chloride with quenching rate constants in the (3.5-9.2) × 1010 and (0.0036-2.9) × 1010 M-1 s-1 ranges in neat acetonitrile and acetonitrile/water mixtures, respectively. Nanosecond transient absorption spectroscopy provided unambiguous evidence of reductive excited-state electron transfer, with all halides in the solvent mixtures containing up to 50% water. Cage-escape yields were large (55-96%) in acetonitrile and dropped below 32% in 50:50 acetonitrile/water mixtures.

Charge Recombination Deceleration by Lateral Transfer of Electrons in Dye-Sensitized NiO Photocathode

Control of charge separation and recombination is critical for dye-sensitized solar cells and photoelectrochemical cells, and for p-type cells, the latter process limits their photovoltaic performance. We speculated that the lateral electron hopping between dyes on a p-type semiconductor surface can effectively separate electrons and holes in space and retard recombination. Thus, device designs where lateral electron hopping is promoted can lead to enhanced cell performance. Herein, we present an indirect proof by involving a second dye to monitor the effect of electron hopping after hole injection into the semiconductor. In mesoporous NiO films sensitized with peryleneimide (PMI) or naphthalene diimide (NDI) dyes, dye excitation led to ultrafast hole injection into NiO from either excited PMI* (τ < 200 fs) or NDI* (τ = 1.2 ps). In cosensitized films, surface electron transfer from PMI^- to NDI was rapid (τ = 24 ps). Interestingly, the subsequent charge recombination (ps-μs) with NiO holes was much slower when NDI^- was generated by electron transfer from PMI^- than when NDI was excited directly. We therefore indicate that the charge recombination is slowed down after the charge hopping from the original PMI sites to the NDI sites. The experimental results supported our hypothesis and revealed important information on the charge carrier kinetics for the dye-sensitized NiO photoelectrode system.

Optimized H2-evolving dye-sensitized LaFeO3 photocathodes prepared via the layer-by-layer assembly of dyes and catalysts

A molecular dye and a molecular catalyst were loaded onto the surface of a mesoporous LaFeO₃ (LFO) film via layer-by-layer assembly relying on the coordination of phosphates and Zr⁴⁺. After assembling six layers of the dye and four layers of the catalyst, the (NiP-4 + PQA-6)@LFO photocathode exhibited a significant photocurrent for light-driven H₂ generation.

D-A-D organic small molecules with AIE effect for fluorescence imaging guided photothermal therapy

Near infrared (NIR) fluorescent organic molecules as fluorescent probes accurately guide photothermal therapy as a potential antitumor method. However, the aggregation and quenching of organic fluorescent molecules and poor tissue permeability greatly limit their therapeutic effect and clinical transformation. In this paper, with a D-A-D structure as the molecular skeleton, cyclopentadithiophene (CPDT) as the donor (D), diketopyrrolopyrrole (DPP) as the acceptor (A), and long-chain isooctane as the shielding unit, organic fluorescent small molecules with a strong absorption band and bright NIR-II emission were synthesized. Then, tetraphenylethylene (TPE) molecules with typical AIE structure characteristics were introduced on both sides of the organic fluorescent small molecules, and an organic small molecular fluorophore (TDA) with AIE characteristics and the photothermal effect was designed. Through a series of experimental characterization techniques, it is proved that TDA NPs have good biocompatibility and tissue permeability, and can accurately locate the tumor location through NIR-II fluorescence imaging to achieve local photothermal treatment of tumors.

Push-pull organic dyes and dye-catalyst assembly featuring a benzothiadiazole unit for photoelectrochemical hydrogen production

In this work, we report the design and the preparation of two new dyes and a molecular dyad for the photoelectrochemical hydrogen production from water in a dye-sensitized photoelectrochemical cell (DSPEC). We designed dyes that include a benzothiadiazole (BTD) and an indacenodithiophene (IDT) units, and we obtained a new molecular dyad by covalent coupling with the cobalt diimine-dioxime catalyst. The introduction of the benzothiadiazole core in the structure improves the absorption properties and leads to an extension of the spectrum in the visible range up to 650 nm. The photoelectrochemical properties of the new dyad were evaluated on pristine and lithium-doped NiO electrodes. We demonstrate that increasing the light harvesting efficiency of the dyad by introducing a IDT-BTD chromophore is clearly beneficial for the photoelectrochemical activity. We also demonstrate that lithium doping of NiO, which improves the electronic conductivity of the mesoporous film, leads to a significant increase in performance, in terms of TON and F.E., more than doubled with our new dyad. This BTD-based molecular system outperforms the results of previously reported dyads using the same catalyst.

Dual Role of Surface Hydroxyl Groups in the Photodynamics and Performance of NiO-Based Photocathodes

Photoelectrochemical (PEC) cells containing photocathodes based on functionalized NiO show a promising solar-to-hydrogen conversion efficiency. Here, we present mechanistic understanding of the photoinduced charge transfer processes occurring at the photocathode/electrolyte interface. We demonstrate via advanced photophysical characterization that surface hydroxyl groups formed at the NiO/water interface not only promote photoinduced hole transfer from the dye into NiO, but also enhance the rate of charge recombination. Both processes are significantly slower when the photocathode is exposed to dry acetonitrile, while in air an intermediate behavior is observed. These data suggest that highly efficient devices can be developed by balancing the quantity of surface hydroxyl groups of NiO, and presumably of other p-type metal oxide semiconductors.

Influence of the Linker Chemistry on the Photoinduced Charge‐Transfer Dynamics of Hetero‐dinuclear Photocatalysts

To optimize light‐driven catalytic processes, light‐mediated multi‐electron transfer dynamics in molecular dyads need to be studied and correlated with structural changes focusing on the catalytically active metastable intermediates. Here, spectro‐electrochemistry has been employed to investigate the structure‐dependent photoelectron transfer kinetics in catalytically active intermediates of two Ru−Rh catalysts for light‐driven NAD⁺ reduction. The excited‐state reactivity of short‐lived intermediates was studied along different photoreaction pathways by resonance Raman and time‐resolved transient absorption spectro‐electrochemistry with sub‐picosecond time resolution under operando conditions. The results demonstrate, for the first time, how the bridging ligand serves as a (multi‐)electron storage structure, mediates the strength of the electronic coupling of catalytic and photocenter and impacts the targeted electron transfer as well as parasitic electron‐transfer kinetics.

Electrocatalytic reduction of protons to dihydrogen by the cobalt tetraazamacrocyclic complex [Co(N4H)Cl2]+: mechanism and benchmarking of performances

The cobalt tetraazamacrocyclic [Co(N₄H)Cl₂]⁺ complex is becoming a popular and versatile catalyst for the electrocatalytic evolution of hydrogen, because of its stability and superior activity in aqueous conditions. We present here a benchmarking of its performances based on the thorough analysis of cyclic voltammograms recorded under various catalytic regimes in non-aqueous conditions allowing control of the proton concentration. This allowed a detailed mechanism to be proposed with quantitative determination of the rate-constants for the various protonation steps, as well as identification of the amine function of the tetraazamacrocyclic ligand to act as a proton relay during H₂ evolution.

Hydrogen Production at a NiO Photocathode Based on a Ruthenium Dye-Cobalt Diimine Dioxime Catalyst Assembly: Insights from Advanced Spectroscopy and Post-operando Characterization

The production of hydrogen by efficient, low-cost, and integrated photoelectrochemical water splitting processes represents an important target for the ecological transition. This challenge can be addressed thanks to bioinspired chemistry and artificial photosynthesis approaches by designing dye-sensitized photocathodes for hydrogen production, incorporating bioinspired first-row transition metal-based catalysts. The present work describes the preparation and photoelectrochemical characterization of a NiO photocathode sensitized with a phosphonate-derivatized ruthenium tris-diimine photosensitizer covalently linked to a cobalt diimine dioxime hydrogen-evolving catalyst. Under simulated AM 1.5G irradiation, hydrogen is produced with photocurrent densities reaching 84 ± 7 μA·cm^-2, which is among the highest values reported so far for dye-sensitized photocathodes with surface-immobilized catalysts. Thanks to the unique combination of advanced spectroscopy and surface characterization techniques, the fast desorption of the dyad from the NiO electrode and the low yield of electron transfer to the catalyst, resulting in the Co demetallation from the diimine dioxime framework, were identified as the main barriers limiting the performances and the stability of the system. This work therefore paves the way for a more rational design of molecular photocathodes for solar fuel production and represents a further step toward the development of sustainable processes for the production of hydrogen from sunlight and water.