Excited-State Charge Transfer in Push-Pull Platinum(II) π-Extended Porphyrins Fused with Pentacenequinone

Platinum(II) π-extended porphyrins fused with pentacenequinone and dihydropentacene have been successfully synthesized. These porphyrins were investigated using various techniques including absorption, steady-state, and time-resolved phosphorescence spectroscopy and differential pulse voltammetry. UV-vis absorption spectra of pentacenequinone-fused porphyrins (SW-Pt1 and SW-Pt2) showed unusually broad and nontypical absorption patterns. Phosphorescence spectra of SW-Pt1, SW-Pt2, and SW-Pt3 displayed similar emissions in the 704-706 nm region indicating electronic transitions of similar origin; however, the triplet lifetimes were found to be quenched in the case of both SW-Pt1 and SW-Pt2, suggesting the occurrence of excited-state events. Facile reductions were obtained for both the pentacene-quinone-fused monomer (SW-Pt2) and dimer (SW-Pt1) and were identified to be located at the pentacenequinone components. The observed orbital segregations for SW-Pt2 and SW-Pt1 from DFT calculations supported the possibility of charge transfer in these push-pull systems. Interestingly, the established energy level diagram revealed that the charge transfer from the triplet excited Pt porphyrin is thermodynamically an uphill process. Systematic studies involving both femtosecond and nanosecond transient absorption techniques revealed that the singlet excited Pt porphyrins undergo an intermediate charge transfer state prior to populating the triplet state, providing a plausible explanation for phosphorescence quenching. The lifetime of the intermediate charge transfer states was found to be 25.9 and 5.68 ps, respectively, for SW-Pt1 and SW-Pt2.

Porphyrins and phthalocyanines as biomimetic tools for photocatalytic H2 production and CO2 reduction

The increasing energy demand and environmental issues caused by the over-exploitation of fossil fuels render the need for renewable, clean, and environmentally benign energy sources unquestionably urgent. The zero-emission energy carrier, H₂ is an ideal alternative to carbon-based fuels especially when it is generated photocatalytically from water. Additionally, the photocatalytic conversion of CO₂ into chemical fuels can reduce the CO₂ emissions and have a positive environmental and economic impact. Inspired by natural photosynthesis, plenty of artificial photocatalytic schemes based on porphyrinoids have been investigated. This review covers the recent advances in photocatalytic H₂ production and CO₂ reduction systems containing porphyrin or phthalocyanine derivatives. The unique properties of porphyrinoids enable their utilization both as chromophores and as catalysts. The homogeneous photocatalytic systems are initially described, presenting the various approaches for the improvement of photosensitizing activity and the enhancement of catalytic performance at the molecular level. On the other hand, for the development of the heterogeneous systems, numerous methods were employed such as self-assembled supramolecular porphyrinoid nanostructures, construction of organic frameworks, combination with 2D materials and adsorption onto semiconductors. The dye sensitization on semiconductors opened the way for molecular-based dye-sensitized photoelectrochemical cells (DSPECs) devices based on porphyrins and phthalocyanines. The research in photocatalytic systems as discussed herein remains challenging since there are still many limitations making them unfeasible to be used at a large scale application before finding a large-scale application.

Ruthenium Assemblies for CO2 Reduction and H2 Generation: Time Resolved Infrared Spectroscopy, Spectroelectrochemistry and a Photocatalysis Study in Solution and on NiO

Two novel supramolecular complexes RuRe ([Ru(dceb)₂(bpt)Re(CO)₃Cl](PF₆)) and RuPt ([Ru(dceb)₂(bpt)PtI(H₂O)](PF₆)₂) [dceb = diethyl(2,2′-bipyridine)-4,4′-dicarboxylate, bpt = 3,5-di(pyridine-2-yl)-1,2,4-triazolate] were synthesized as new catalysts for photocatalytic CO₂ reduction and H₂ evolution, respectively. The influence of the catalytic metal for successful catalysis in solution and on a NiO semiconductor was examined. IR-active handles in the form of carbonyl groups on the peripheral ligand on the photosensitiser were used to study the excited states populated, as well as the one-electron reduced intermediate species using infrared and UV-Vis spectroelectrochemistry, and time resolved infrared spectroscopy. Inclusion of ethyl-ester moieties led to a reduction in the LUMO energies on the peripheral bipyridine ligand, resulting in localization of the ³MLCT excited state on these peripheral ligands following excitation. RuPt generated hydrogen in solution and when immobilized on NiO in a photoelectrochemical (PEC) cell. RuRe was inactive as a CO₂ reduction catalyst in solution, and produced only trace amounts of CO when the photocatalyst was immobilized on NiO in a PEC cell saturated with CO₂.

A Robust Photocatalytic Hybrid Material Composed of Metal-Organic Cages and TiO2 for Efficient Visible-Light-Driven Hydrogen Evolution

The design of photochemical molecular devices (PMDs) for photocatalytic H₂ production from water is a meaningful but challenging subject currently. Herein, a Pd₂ L₄ type metal-organic cage (denoted as MOC-Q2) is designed as a PMD, which consists of two catalytic centers (Pd²⁺ ) and four photosensitive ligands (L-2) with four pyridine anchoring groups. Subsequently, the MOC-Q2 is combined with TiO₂ to form TiO₂ -MOC-Q2 hybrid materials with different MOC-Q2 contents by a facile sol-gel method, which have micro/mesoporous structures and large surface areas. The optimized TiO₂ -MOC-Q2 (6.5 wt%) exhibits high H₂ production activity (7.9 mmol g^-1  h^-1 within 5 h) and excellent durability, giving a TON value of 23477 or 11739 (based on MOC-Q2 or Pd moles) after recycling for 7 rounds. By contrast, the pure MOC-Q2 only shows an ordinary photocatalytic H₂ production rate (0.84 mmol g^-1  h^-1 within 5 h) in the homogeneous system. It can be deduced that TiO₂ drives the photocatalysis and simultaneously acts as the structure promoter. This study presents a meaningful and distinctive attempt of a new approach for the design and development of MOC-based heterogeneous photocatalysts.

Controlling Solar Hydrogen Production by Organizing Porphyrins

In this study, a highly efficient photocatalytic H₂ production system is developed by employing porphyrins as photocatalysts. Palladium and platinum tetracarboxyporphyrins (PdTCP and PtTCP) are adsorbed or coadsorbed onto TiO₂ nanoparticles (NPs), which act as the electron transport medium and as a scaffold that promotes the self-organization of the porphyrinoids. The self-organization of PdTCP and PtTCP, forming H- and J-aggregates, respectively, is the key element for H₂ evolution, as in the absence of TiO₂ NPs no catalytic activity is detected. Notably, J-aggregated PtTCPs are more efficient for H₂ production than H-aggregated PdTCPs. In this approach, a single porphyrin, which self-organizes onto TiO₂ NPs, acts as the light harvester and simultaneously as the catalyst, whereas TiO₂ serves as the electron transport medium. Importantly, the concurrent adsorption of PdTCP and PtTCP onto TiO₂ NPs results in the most efficient catalytic system, giving a turnover number of 22,733 and 30.2 mmol(H₂ ) g(cat)^-1 .

Stabilization of a molecular water oxidation catalyst on a dye-sensitized photoanode by a pyridyl anchor

Understanding and controlling the properties of water-splitting assemblies in dye-sensitized photoelectrosynthesis cells is a key to the exploitation of their properties. We demonstrate here that, following surface loading of a [Ru(bpy)₃]²⁺ (bpy = 2,2′-bipyridine) chromophore on nanoparticle electrodes, addition of the molecular catalysts, Ru(bda)(L)₂ (bda  =  2,2′-bipyridine-6,6′-dicarboxylate) with phosphonate or pyridyl sites for water oxidation, gives surfaces with a 5:1 chromophore to catalyst ratio. Addition of the surface-bound phosphonate derivatives with L = 4-pyridyl phosphonic acid or diethyl 3-(pyridin-4-yloxy)decyl-phosphonic acid, leads to well-defined surfaces but, following oxidation to Ru(III), they undergo facile, on-surface dimerization to give surface-bound, oxo-bridged dimers. The dimers have a diminished reactivity toward water oxidation compared to related monomers in solution. By contrast, immobilization of the Ru-bda catalyst on TiO₂ with the 4,4′-dipyridyl anchoring ligand can maintain the monomeric structure of catalyst and gives relatively stable photoanodes with photocurrents that reach to 1.7 mA cm⁻² with an optimized, applied bias photon-to-current efficiency of 1.5%.

Understanding the properties of water-splitting assemblies in dye-sensitized photoelectrochemical cells is a key challenge in artificial photosynthesis. Here, the authors report the importance of anchoring groups on a water oxidation catalyst in determining active species on metal oxide surfaces.

Dynamic Role of Cluster Cocatalysts on Molecular Photoanodes for Water Oxidation

While loading of cocatalysts is one of the most widely investigated strategies to promote the efficiency of photoelectrodes, the understanding of their functionality remains controversial. We established new hybrid molecular photoanodes with cobalt-based molecular cubane cocatalysts on hematite as a model system. Photoelectrochemical and rate law analyses revealed an interesting functionality transition of the {Co(II)₄O₄}-type cocatalysts. Their role changed from predominant hole reservoirs to catalytic centers upon modulation of the applied bias. Kinetic analysis of the photoelectrochemical processes indicated that this observed transition arises from the dynamic equilibria of photogenerated surface charge carriers. Most importantly, we confirmed this functional transition of the cocatalysts and the related kinetic properties for several cobalt-based molecular and heterogeneous catalysts, indicating wide applicability of the derived trends. Additionally, complementary analytical characterizations show that a transformation of the applied molecular species occurs at higher applied bias, pointing to a dynamic interplay connecting molecular and heterogeneous catalysis. Our insights promote the essential understanding of efficient (molecular) cocatalyzed photoelectrode systems to design tailor-made hybrid devices for a wide range of catalytic applications.

Factors influencing the photoelectrochemical device performance sensitized by ruthenium polypyridyl dyes

The subtle variation in the alkyl substituents of the ruthenium photosensitizers controls the photo-electrochemical performance of the dye-only DSPECs (do-DSPECs) and those with sacrificial agent (sa-DSPECs).

The Electronic Structure and Optical Properties of Anatase TiO₂ with Rare Earth Metal Dopants from First-Principles Calculations

The electronic and optical properties of the rare earth metal atom-doped anatase TiO₂ have been investigated systematically via density functional theory calculations. The results show that TiO₂ doped by Ce or Pr is the optimal choice because of its small band gap and strong optical absorption. Rare earth metal atom doping induces several impurity states that tune the location of valence and conduction bands and an obvious lattice distortion that should reduce the probability of electron–hole recombination. This effect of band change originates from the 4f electrons of the rare earth metal atoms, which leads to an improved visible light absorption. This finding indicates that the electronic structure of anatase TiO₂ is tuned by the introduction of impurity atoms.