Long-lived charge separation and applications in artificial photosynthesis

Zwitterionic Acridinium Amidate: A Nitrogen-Centered Radical Catalyst for Photoinduced Direct Hydrogen Atom Transfer

The development of small organic molecules that can convert light energy into chemical energy to directly promote molecular transformation is of fundamental importance in chemical science. Herein, we report a zwitterionic acridinium amidate as a catalyst for the direct functionalization of aliphatic C-H bonds. This organic zwitterion absorbs visible light to generate the corresponding amidyl radical in the form of excited-state triplet diradical with prominent reactivity for hydrogen atom transfer to facilitate C-H alkylation with a high turnover number. The experimental and theoretical investigations revealed that the noncovalent interactions between the anionic amidate nitrogen and a pertinent hydrogen-bond donor, such as hexafluoroisopropanol, are crucial for ensuring the efficient generation of catalytically active species, thereby fully eliciting the distinct reactivity of the acridinium amidate as a photoinduced direct hydrogen atom transfer catalyst.

Photoredox-catalyzed intramolecular nucleophilic amidation of alkenes with β-lactams

The direct nucleophilic addition of amides to unfunctionalized alkenes via photoredox catalysis represents a facile approach towards functionalized alkylamides. Unfortunately, the scarce nucleophilicity of amides and competitive side reactions limit the utility of this approach. Herein, we report an intramolecular photoredox cyclization of alkenes with β-lactams in the presence of an acridinium photocatalyst. The approach uses an intramolecular nucleophilic addition of the β-lactam nitrogen atom to the radical cation photogenerated in the linked alkene moiety, followed by hydrogen transfer from the hydrogen atom transfer (HAT) catalyst. This process was used to successfully prepare 2-alkylated clavam derivatives.

Functional molecular models of photosynthesis

This perspective focuses on functional models of photosynthesis to achieve molecular photocatalytic systems that mimic photosystems I and II (PSI and PSII). A long-lived and high-energy electron-transfer state of 9-mesityl-10-methylacridinium ion (Acr+-Mes) has been attained as a simple and useful model of the photosynthetic reaction center. Acr+-Mes has been used as an effective photoredox catalyst for photocatalytic hydrogen evolution and regioselective reduction of NAD(P)+ from plastoquinone analogs as a molecular functional model of PSI. A functional molecular model system to mimic the function of PSII has also been developed to oxidize water by plastoquinone analogs to produce O2 and plastoquinol analogs. The PSI molecular models have finally been integrated with the PSII molecular models to achieve production of a solar fuel (hydrogen) and NAD(P)H and its analogs from water by use of solar energy as a molecular artificial photosynthesis.

Theoretical description of photoinduced electron transfer in donor-acceptor supramolecular complexes based on carbon buckybowls

Herein, we explore, from a theoretical perspective, the nonradiative photoinduced processes (charge separation and energy transfer) within a family of donor-acceptor supramolecular complexes based on the electron-donor truxene-tetrathiafulvalene (truxTTF) derivative and a series of curved fullerene fragments (buckybowls) of different shapes and sizes (C30H12, C32H12, and C38H14) as electron acceptors that successfully combine with truxTTF via non-covalent interactions. The resulting supramolecular complexes (truxTTF·C30H12, truxTTF·C32H12, and truxTTF·C38H14) undergo charge-separation processes upon photoexcitation through charge-transfer states involving the donor and acceptor units. Despite the not so different size of the buckybowls, they present noticeable differences in the charge-separation efficiency owing to a complex decay post-photoexcitation mechanism involving several low-lying excited states of different natures (local and charge-transfer excitations), all closely spaced in energy. In this intricate scenario, we have adopted a theoretical approach combining electronic structure calculations at (time-dependent) density functional theory, a multistate multifragment diabatization method, the Marcus-Levitch-Jortner semiclassical rate expression, and a kinetic model to estimate the charge separation rate constants of the supramolecular heterodimers. Our outcomes highlight that the efficiency of the photoinduced charge-separation process increases with the extension of the buckybowl backbone. The supramolecular heterodimer with the largest buckybowl (truxTTF·C38H14) displays multiple and efficient electron-transfer pathways, providing a global photoinduced charge separation in the ultrafast time scale in line with the experimental findings. The study reported indicates that modifications in the shape and size of buckybowl systems can give rise to attractive novel acceptors for potential photovoltaic applications.

Aggregation promotes charge separation in fullerene-indacenodithiophene dyad

Fast photoinduced charge separation (CS) and long-lived charge-separated state (CSS) in small-molecules facilitate light-energy conversion, while simultaneous attainment of both remains challenging. Here we accomplish this through aggregation based on fullerene-indacenodithiophene dyads. Transient absorption spectroscopy reveals that, compared to solution, the CS time in aggregates is accelerated from 41.5 ps to 0.4 ps, and the CSS lifetime is prolonged from 311.4 ps to 40 μs, indicating that aggregation concomitantly promotes fast CS and long-lived CSS. Fast CS arises from the hot charge-transfer states dissociation, opening up additional resonant channels to free carriers (FCs); subsequently, charge recombination into intramolecular triplet CSS becomes favorable mediated by spin-uncorrelated FCs. Different from fullerene/indacenodithiophene blends, the unique CS mechanism in dyad aggregates reduces the long-lived CSS dependence on molecular order, resulting in a CSS lifetime 200 times longer than blends. This endows the dyad aggregates to exhibit both photoelectronic switch properties and superior photocatalytic capabilities.

Unveiling the critical role of TiO2-supported atomically dispersed Cu species for enhanced photofixation of N2 to nitrate

Nitrate products are widely used in manufacturing as crucial raw materials and fertilizers. The traditional nitrate synthesis process involves high energy consumption and emission, thereby opposing the goals of zero-carbon emission and green chemistry. Thus, a sustainable roadmap for nitrate synthesis that uses green energy input, clean N sources, and direct catalytic processes is urgently required (e.g., developing a novel photosynthesis system). Here, we synthesized TiO2-supported atomically dispersed Cu species for N2 photofixation to nitrate in a flow reactor. The optimized photocatalyst yielded a high nitrate photosynthesis rate of 0.93 μmol h-1 and selectivity of ∼90%, which is superior to most of the values reported thus far. Further, experimental results and in-situ investigations revealed that the atomically dispersed Cu sites in the as-designed sample significantly enhanced the separation and transfer efficiency of photogenerated carriers, adsorption and activation of reactants, and the formation of chemisorbed NOx intermediates, thereby realizing the excellent photofixation of N2 to nitrate.

Clicked BODIPY-Fullerene-Peptide Assemblies: Studies of Electron Transfer Processes in Self-Assembled Monolayers on Gold Surfaces

Two BODIPY-C60-peptide assemblies were synthesized by CuAAC reactions of BODIPY-C60 dyads and a helical peptide functionalized with a terminal alkyne group and an azide group, respectively. The helical peptide within these assemblies was functionalized at its other end by a disulfide group, allowing formation of self-assembled monolayers (SAMs) on gold surfaces. Characterizations of these SAMs, as well as those of reference molecules (BODIPY-C60-alkyl, C60-peptide and BODIPY-peptide), were carried out by PM-IRRAS and cyclic voltammetry. BODIPY-C60-peptide SAMs are more densely packed than BODIPY-C60-alkyl and BODIPY-peptide based SAMs. These findings were attributed to the rigid peptide helical conformation along with peptide-peptide and C60-C60 interactions within the monolayers. However, less dense monolayers were obtained with the target assemblies compared to the C60-peptide, as the BODIPY entity likely disrupts organization within the monolayers. Finally, electron transfer kinetics measurements by ultra-fast electrochemistry experiments demonstrated that the helical peptide is a better electron mediator in comparison to alkyl chains. This property was exploited along with those of the BODIPY-C60 dyads in a photo-current generation experiment by converting the resulting excited and/or charge separated states from photo-illumination of the dyad into electrical energy.

Intermolecular Proton-Coupled Electron Transfer Reactivity from a Persistent Charge-Transfer State for Reductive Photoelectrocatalysis

Interest in applying proton-coupled electron transfer (PCET) reagents in reductive electro- and photocatalysis requires strategies that mitigate the competing hydrogen evolution reaction. Photoexcitation of a PCET donor to a charge-separated state (CSS) can produce a powerful H-atom donor capable of being electrochemically recycled at a comparatively anodic potential corresponding to its ground state. However, the challenge is designing a mediator with a sufficiently long-lived excited state for bimolecular reactivity. Here, we describe a powerful ferrocene-derived photoelectrochemical PCET mediator exhibiting an unusually long-lived CSS (τ ∼ 0.9 μs). In addition to detailed photophysical studies, proof-of-concept stoichiometric and catalytic proton-coupled reductive transformations are presented, which illustrate the promise of this approach.

Artificial Photosynthesis for Regioselective Reduction of NAD(P)+ to NAD(P)H Using Water as an Electron and Proton Source

In photosynthesis, four electrons and four protons taken from water in photosystem II (PSII) are used to reduce NAD(P)+ to produce NAD(P)H in photosystem I (PSI), which is the most important reductant to reduce CO2. Despite extensive efforts to mimic photosynthesis, artificial photosynthesis to produce NAD(P)H using water electron and proton sources has yet to be achieved. Herein, we report the photocatalytic reduction of NAD(P)+ to NAD(P)H and its analogues in a molecular model of PSI, which is combined with water oxidation in a molecular model of PSII. Photoirradiation of a toluene/trifluoroethanol (TFE)/borate buffer aqueous solution of hydroquinone derivatives (X-QH2), 9-mesityl-10-methylacridinium ion, cobaloxime, and NAD(P)+ (PSI model) resulted in the quantitative and regioselective formation of NAD(P)H and p-benzoquinone derivatives (X-Q). X-Q was reduced to X-QH2, accompanied by the oxidation of water to dioxygen under the photoirradiation of a toluene/TFE/borate buffer aqueous solution of [(N4Py)FeII]2+ (PSII model). The PSI and PSII models were combined using two glass membranes and two liquid membranes to produce NAD(P)H using water as an electron and proton source with the turnover number (TON) of 54. To the best of our knowledge, this is the first time to achieve the stoichiometry of photosynthesis, photocatalytic reduction of NAD(P)+ by water to produce NAD(P)H and O2.

Photophysical Properties of Homo- and Hetero-Aggregate Assemblies Made of N-Annulated Perylene Derivatives

We report the absorption spectra and photophysical properties of homo and hetero-aggregate assemblies of a strongly emissive N-annulated perylene dye (P) and of a dyad made of P and a methyl viologen derivative (P-MV), in ethanol-water solutions. In homo-aggregate assemblies of P, the π-π* fluorescence of the isolated chromophore is replaced by excimer emission at lower energy, with a lifetime of 900 ps, due to excimer formation from the initially prepared excitons. In homo-aggregate assemblies of P-MV, photoinduced charge separation, with formation of P+ -MV- species, occurs in 3 ps with a charge recombination of 20 ps. In hetero-aggregate P/P-MV systems, the light energy absorbed by the P components delocalizes over various P subunits, and when a P-MV unit is reached, charge separation occurs; however, excimer emission is present for P/P-MV ratio larger than 3 : 1, indicating that delocalized excitons within the hetero-aggregate systems extend over a limited number of P chromophores.

Photoredox/Cobalt-Catalyzed Cascade Oxidative Synthesis of 2,5-Disubstituted 1,3,4-Oxadiazoles under Oxidant-Free Conditions

An efficient oxidant-free, photoredox-mediated cascade cyclization strategy for the synthesis of 1,3,4-oxadiazoles by using an organo acridinium photocatalyst and a cobaloxime catalyst has been developed. Various acylhydrazones have been transformed into the corresponding 1,3,4-oxadiazole products in up to 96% yield, and H2 is the only byproduct. Mechanistic experiments and density functional theory (DFT) calculation studies indicate carbon-centered radicals rather than oxygen-centered radicals as π-radicals produced by the oxidation of photoexcited Mes-Acr+* along with deprotonation, which is responsible for this transformation. The practical utility of this method is highlighted by the one-pot gram-scale synthesis starting directly from commercially available aldehydes and acylhydrazides.

Near-Infrared Self-Assembled Hydroxyl Radical Generator Based on Photoinduced Cascade Electron Transfer for Hypoxic Tumor Phototherapy

The hydroxyl radical (•OH) is an extremely potent reactive oxygen species that plays a crucial role in photooxidations within the realm of hypoxic tumor therapy. However, the current methods for •OH photogeneration typically rely on inorganic materials that require UV/vis light excitation. Consequently, photogenerators based on organic molecules, especially those utilizing near-infrared (NIR) light excitation, are rare. In this study, the concept of photoinduced cascade charge transfer (PICET), which utilizes NIR heavy-atom-free photosensitizers (ANOR-Cy5) to generate •OH is introduced. The ANOR-Cy5 photosensitizer, with its flexible hydrophobic structure, enables the formation of nanoparticles in aqueous solutions through molecular assembly. PICET involves a symmetry-breaking charge separation-induced localized charge-separated state, transitioning to a delocalized charge-separated state, which governs the efficiency of •OH generation. Thanks to the oxygen-independent nature of •OH generation and its robust oxidative properties, the ANOR-Cy5-based photosensitizer demonstrates highly effective photoinduced anti-cancer effects, even under severely hypoxic conditions. This discovery emphasizes the potential for achieving •OH photogeneration using a single organic molecule through the engineering of molecular self-assembly, thereby opening up new possibilities for phototherapy and beyond.

Control of electronic polarization via charge ordering and electron transfer: electronic ferroelectrics and electronic pyroelectrics

Ferroelectric, pyroelectric, and piezoelectric compounds whose electric polarization properties can be controlled by external stimuli such as electric field, temperature, and pressure have various applications, including ferroelectric memory materials, sensors, and thermal energy-conversion devices. Numerous polarization switching compounds, particularly molecular ferroelectrics and pyroelectrics, have been developed. In these materials, the polarization switching usually proceeds via ion displacement and reorientation of polar molecules, which are responsible for the change in ionic polarization and orientational polarization, respectively. Recently, the development of electronic ferroelectrics, in which the mechanism of polarization change is charge ordering and electron transfer, has attracted great attention. In this article, representative examples of electronic ferroelectrics are summarized, including (TMTTF)2X (TMTTF = tetramethyl-tetrathiafulvalene, X = anion), α-(BEDT-TTF)2I3 (BEDT-TTF = bis(ethylenedithio)-tetrathiafulvalene), TTF-CA (TTF = tetrathiafulvalene, CA = p-chloranil), and [(n-C3H7)4N][FeIIIFeII(dto)3] (dto = 1,2-dithiooxalate = C2O2S2). Furthermore, polarization switching materials using directional electron transfer in nonferroelectrics, the so-called electronic pyroelectrics, such as [(Cr(SS-cth))(Co(RR-cth))(μ-dhbq)](PF6)3 (dhbq = deprotonated 2,5-dihydroxy-1,4-benzoquinone, cth = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraaza-cyclotetradecane), are introduced. Future prospects are also discussed, particularly the development of new properties in polarization switching through the manipulation of electronic polarization in electronic ferroelectrics and electronic pyroelectrics by taking advantage of the inherent properties of electrons.

Far-Red Excitation Induced Electron Transfer in Bis Donor-AzaBODIPY Push-Pull Systems; Role of Nitrogenous Donors in Promoting Charge Separation

A far-red absorbing sensitizer, BF2 -chelated azadipyrromethane (azaBODIPY) has been employed as an electron acceptor to synthesize a series of push-pull systems linked with different nitrogenous electron donors, viz., N,N-dimethylaniline (NND), triphenylamine (TPA), and phenothiazine (PTZ) via an acetylene linker. The structural integrity of the newly synthesized push-pull systems was established by spectroscopic, electrochemical, spectroelectrochemical, and DFT computational methods. Cyclic and differential pulse voltammetry studies revealed different redox states and helped in the estimation of the energies of the charge-separated states. Further, spectroelectrochemical studies performed in a thin-layer optical cell revealed diagnostic peaks of azaBODIPY⋅- in the visible and near-IR regions. Free-energy calculations revealed the charge separation from one of the covalently linked donors to the 1 azaBODIPY* to yield Donor⋅+ -azaBODIPY⋅- to be energetically favorable in a polar solvent, benzonitrile, and the frontier orbitals generated on the optimized structures helped in assessing such a conclusion. Consequently, the steady-state emission studies revealed quenching of the azaBODIPY fluorescence in all of the investigated push-pull systems in benzonitrile and to a lesser extent in mildly polar dichlorobenzene, and nonpolar toluene. The femtosecond pump-probe studies revealed the occurrence of excited charge transfer (CT) in nonpolar toluene while a complete charge separation (CS) for all three push-pull systems in polar benzonitrile. The CT/CS products populated the low-lying 3 azaBODIPY* prior to returning to the ground state. Global target (GloTarAn) analysis of the transient data revealed the lifetime of the final charge-separated states (CSS) to be 195 ps for NND-derived, 50 ps for TPA-derived, and 85 ps for PTZ-derived push-pull systems in benzonitrile.

π-Extended Pyrazinepyrene-Fused Zinc Phthalocyanines: Synthesis and Excited-State Charge Separation Involving Coordinated C60

A series of pyrazinepyrene-fused zinc phthalocyanines (ZnPc-Pyr_n) have been newly synthesized by reacting quinoxaline and the corresponding diamino-functionalized phthalocyanines as a new class of π-extended phthalocyanine systems. Bathochromically shifted absorption as a function of the number of pyrazinepyrene entities due to extended π-conjugation and quenched fluorescence due to the presence of fused pyrazinepyrene were witnessed. The electronic structures of these phthalocyanines were probed by systematic computational and electrochemical studies, while the excited-state properties were examined by pump-probe spectroscopies operating at the femto- and nanosecond time scales. Similar to the excited singlet lifetimes, the excited triplet states also revealed diminished lifetimes with an increased number of pyrazinepyrene entities. Further, the coordinatively unsaturated zinc in these molecules was coordinated with phenyl imidazole-functionalized fullerene, ImC₆₀, to form a new series of donor-acceptor conjugates. Upon full characterization of these conjugates, the occurrence of excited-state charge separation was established by transient pump-probe spectroscopy, covering wide temporal and spatial regions. The lifetime of the final charge-separated states was ∼2 ns and decreased with an increase in the number of fused pyrazinepyrene units.

Accelerated Intramolecular Charge Transfer in Tetracyanobutadiene- and Expanded Tetracyanobutadiene-Incorporated Asymmetric Triphenylamine-Quinoxaline Push-Pull Conjugates

The excited-state properties of an asymmetric triphenylamine-quinoxaline push-pull system wherein triphenylamine and quinoxaline take up the roles of an electron donor and acceptor, respectively, are initially investigated. Further, in order to improve the push-pull effect, powerful electron acceptors, viz., 1,1,4,4-tetracyanobutadiene (TCBD) and cyclohexa-2,5-diene-1,4-diylidene-expanded tetracyanobutadiene (also known as expanded-TCBD or exTCBD), have been introduced into the triphenylamine-quinoxaline molecular framework using a catalyst-free [2 + 2] cycloaddition-retroelectrocyclization reaction. The presence of these electron acceptors caused strong ground-state polarization extending the absorption well into the near-IR region accompanied by strong fluorescence quenching due to intramolecular charge transfer (CT). Systematic studies were performed using a suite of spectral, electrochemical, computational, and pump-probe spectroscopic techniques to unravel the intramolecular CT mechanism and to probe the role of TCBD and exTCBD in promoting excited-state CT and separation events. Faster CT in exTCBD-derived compared to that in TCBD-derived push-pull systems has been witnessed in polar benzonitrile.

Excitation Wavelength-Dependent Charge Stabilization in Highly Interacting Phenothiazine Sulfone-Derived Donor-Acceptor Constructs

Prolonging the lifetime of charge-separated states (CSS) is of paramount importance in artificial photosynthetic donor-acceptor (DA) constructs to build the next generation of light-energy-harvesting devices. This becomes especially important when the DA constructs are closely spaced and highly interacting. In the present study, we demonstrate extending the lifetime of the CSS in highly interacting DA constructs by making use of the triplet excited state of the electron donor and with the help of excitation wavelength selectivity. To demonstrate this, π-conjugated phenothiazine sulfone-based push-pull systems, PTS2-PTS6 have been newly designed and synthesized via the Pd-catalyzed Sonogashira cross-coupling followed by [2 + 2] cycloaddition-retroelectrocyclization reactions. Modulation of the spectral and photophysical properties of phenothiazine sulfones (PTZSO₂) and terminal phenothiazines (PTZ) was possible by incorporating powerful electron acceptors, 1,1,4,4-tetracyanobutadiene (TCBD) and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD (exTCBD). The quadrupolar PTS2 displayed solvatochromism, aggregation-induced emission, and mechanochromic behaviors. From the energy calculations, excitation wavelength-dependent charge stabilization was envisioned in PTS2-PTS6, and the subsequent pump-probe spectroscopic studies revealed charge stabilization when the systems were excited at the locally excited peak positions, while such effect was minimal when the samples were excited at wavelengths corresponding to the CT transitions. This work reveals the impact of wavelength selectivity to induce charge separation from the triplet excited state in ultimately prolonging the lifetime of CCS in highly interacting push-pull systems.

Efficient Charge-Transfer Studies for Selective Detection of Bilirubin Biomolecules Using CsPbBr3 as the Fluorescent Probe

Bright luminescence hybrid halide perovskite nanocrystals (PNCs) as a novel fluorophore class have not been broadly explored in biological sensing. Herein, we synthesized highly fluorescent CsPbBr₃ PNCs through the LARP method using oleic acid and oleyl amine as capping ligands. Morphological and optical properties of as-synthesized PNCs were studied using transmission electron microscopy, X-ray diffraction, UV-vis, and emission spectroscopic analysis. Oleic acid- and oleyl amine-capped PNCs are employed for sensitive and selective detection of bilirubin (BR). A panel of characterizations (time-correlated single-photon count spectroscopy and photoluminescence (PL)) was carried out to investigate the detailed sensing study of PNCs-BR composite for quenching the PL emission of CsPbBr₃ with BR. It has been noticed that the synthesized nanoparticles are highly capable of detecting BR and thus act as a biological material sensor.

Self-assembled systems for artificial photosynthesis

The last few decades have seen an impressive development in molecular-based artificial photosynthesis, thanks to the design of integrated light-harvesting antennae, charge separation systems, and catalysts for water oxidation or hydrogen production based on covalently linked subunits. However, in recent years, self-assembly and spontaneous aggregation of components emerged - sometimes also through serendipity - for the preparation of multicomponent systems aimed to perform the basic processes needed for artificial photosynthesis. Here we critically discuss some key articles that have recently shown the potential of self-assembly for artificial photosynthesis, ranging from self-assembly of antennae and charge separation systems to integrated antenna/catalyst assemblies, to planned co-localization of various components into restricted environments. It is evident that self-assembly can generate emerging properties with respect to the non-aggregated species, and such emerging properties can be quite convenient for designing efficient photocatalytic systems.

Solvent dependent triplet state delocalization in a co-facial porphyrin heterodimer

The excited triplet state of a cofacial aluminum(III) porphyrin-phosphorus(V) porphyrin heterodimer is investigated using transient EPR spectroscopy and quantum chemical calculations. In the dimer, the two porphyrins are bound covalently to each other via a μ-oxo bond between the Al and P centres, which results in strong electronic interaction between the porphyrin rings. The spin polarized transient EPR spectrum of the dimer is narrower than the spectra of the constituent monomers and the magnitude of the zero-field splitting parameter D is solvent dependent, decreasing as the polarity of the solvent increases. The quantum chemical calculations show that the spin density of the triplet state is delocalized over both porphyrins, while magnetophotoselection measurements reveal that, in contrast to the value of D, the relative orientation of the ZFS axes and the excitation transition dipole moments are not solvent dependent. Together the results indicate that triplet state wavefunction is delocalized over both porphyrins and has a modest degree of charge-transfer character that increases with increasing solvent polarity. The sign of the spin polarization pattern of the dimer triplet state is opposite to that of the monomers. The positive sign of D predicted for the monomers and dimer by the quantum chemical calculations implies that the different signs of the spin polarization patterns is a result of a difference in the spin selectivity of the intersystem crossing.

Controlling Electronic Events Through Rational Structural Design in Subphthalocyanine-Corrole Dyads: Synthesis, Characterization, and Photophysical Properties

Porphyrinoids are considered perfect candidates for their incorporation into electron donor-acceptor (D-A) arrays due to their remarkable optoelectronic properties and low reorganization energies. For the first time, a series of subphthalocyanine (SubPc) and corrole (Cor) were covalently connected through a short-range linkage. SubPc axial substitution strategies were employed, which allowed the synthesis of the target molecules in decent yields. In this context, a qualitative synthetic approach was performed to reverse the expected direction of the different electronic events. Consequently, in-depth absorption, fluorescence, and electrochemical assays enabled the study of electronic and photophysical properties. Charge separation was observed in cases of electron-donating Cors, whereas a quantitative energy transfer from the Cor to the SubPc was detected in the case of electron accepting Cors.

Three-Dimensional Covalent Organic Frameworks with Topology

Reticular chemistry allows the control of crystalline frameworks at atomic precision according to the predesigned topological structures. However, only a limited number of topological structures of three-dimensional (3D) covalent organic frameworks (COFs) have been established. In this work, we developed a series of 3D COFs with an unprecedented she topology, which were constructed with D_3d- and D_4h-symmetric building blocks. The resulting COFs crystallize in a space group of Im3̅m, in which each D_3d unit connects with six D_4h units to form a noninterpenetrated network with a uniform pore size of 2.0 nm. In addition, these COFs exhibited high crystallinity, excellent porosity, and good chemical and thermal stability. The crystalline structures, composition, and physicochemical properties of these networks were unambiguously characterized. Notably, the inbuilt porphyrin units render these COFs as efficient catalysts for photoredox C-C bond forming and photocatalytic carbon dioxide reduction reactions. Thus, this work constitutes a new approach for the construction of 3D she-net COFs and also enhances the structural diversity and complexity of COFs.

Intermediate Complex-Mediated Interfacial Electron Transfer in a Radical Dianion/TiO2 Dye-Sensitized Photocatalytic System

We present a mechanistic study of a PTCDA^2-/TiO₂ dye-sensitized photocatalytic system, in which the stable radical dianion PTCDA^2- is formed via a two-step consecutive photoinduced electron transfer from its neutral precursor PTCDA (i.e., perylenetetracarboxylic dianhydride). Photoexcitation of PTCDA^2- brings forth an interesting behavior known as vibrationally excited-state-selective, visible-light photocatalytic hydrogen evolution reaction (HER). In conjunction with the information gleaned from optical spectroscopy and ultrafast dynamics, we reveal that an intermediate complex (IC) state with a lifetime of ∼12 ps exists in the vicinity of a certain vibrationally excited state of PTCDA^2-. Such a unique IC state mediates the interfacial electron transfer (IET) channel from the specific excited state of PTCDA^2- to the conduction band continuum of TiO₂. As an outcome, the effective IC-mediated IET process in this photocatalytic system leads to a remarkable HER rate that reaches ∼4660 μmol g^-1 h^-1.

The Hunter Falls Prey: Photoinduced Oxidation of C60 in Inclusion Complex with Perfluorocycloparaphenylene

Perfluorocycloparaphenylenes (PFCPPs) are cycloparaphenylenes (CPPs) in which all hydrogen atoms have been replaced by fluorine atoms. Like CPPs, PFCPPs are highly strained, hoop-shaped π-conjugated molecules. In this article, we report a computational modeling of photoinduced electron transfer processes in the inclusion complex of PF[10]CPP with C60 fullerene. Its unique feature is the favorable electron transfer from C60 to the host molecule. The photooxidation of C60 is predicted to occur on a sub-nanosecond timescale. The PF[10]CPP⊃C60 dyad is the first nanoring-fullerene complex in which C60 acts as an electron donor in the photoinduced charge separation.

Light-induced electron transfer/phase migration of a redox mediator for photocatalytic C-C coupling in a biphasic solution

Photocatalytic molecular conversions that lead to value-added chemicals are of considerable interest. To achieve highly efficient photocatalytic reactions, it is equally important as it is challenging to construct systems that enable effective charge separation. Here, we demonstrate that the rational construction of a biphasic solution system with a ferrocenium/ferrocene (Fc⁺/Fc) redox couple enables efficient photocatalysis by spatial charge separation using the liquid-liquid interface. In a single-phase system, exposure of a 1,2-dichloroethane (DCE) solution containing a Ru(II)- or Ir(III)-based photosensitizer, Fc, and benzyl bromide (Bn-Br) to visible-light irradiation failed to generate any product. However, the photolysis in a H₂O/DCE biphasic solution, where the compounds are initially distributed in the DCE phase, facilitated the reductive coupling of Bn-Br to dibenzyl (Bn₂) using Fc as an electron donor. The key result of this study is that Fc⁺, generated by photooxidation of Fc in the DCE phase, migrates to the aqueous phase due to the drastic change in its partition coefficient compared to that of Fc. This liquid-liquid phase migration of the mediator is essential for facilitating the reduction of Bn-Br in the DCE phase as it suppresses backward charge recombination. The co-existence of anions can further modify the driving force of phase migration of Fc⁺ depending on their hydrophilicity; the best photocatalytic activity was obtained with a turnover frequency of 79.5 h^-1 and a quantum efficiency of 0.2% for the formation of Bn₂ by adding NBu₄⁺Br^- to the biphasic solution. This study showcases a potential approach for rectifying electron transfer with suppressed charge recombination to achieve efficient photocatalysis.

Control of Photoinduced Electron Transfer Using Complex Formation of Water-Soluble Porphyrin and Polyvinylpyrrolidone

Inspired by the natural photosynthetic system in which proteins control the electron transfer from electron donors to acceptors, in this research, artificial polymers were tried to achieve this control effect. Polyvinylpyrrolidone (PVP) was found to form complex with pigments 5,10,15,20-tetrakis-(4-sulfonatophenyl) porphyrin (TPPS) and its zinc complex (ZnTPPS) quantitatively through different interactions (hydrogen bonds and coordination bonds, respectively). These complex formations hinder the interaction between ground-state TPPS or ZnTPPS and an electron acceptor (methyl viologen, MV2+) and could control the photoinduced electron transfer from TPPS or ZnTPPS to MV2+, giving more electron transfer products methyl viologen cationic radical (MV+•). Other polymers such as PEG did not show similar results, indicating that PVP plays an important role in controlling the photoinduced electron transfer.

Molecular Photocatalytic Water Splitting by Mimicking Photosystems I and II

In nature, water is oxidized by plastoquinone to evolve O₂ and form plastoquinol in Photosystem II (PSII), whereas NADP⁺ is reduced by plastoquinol to produce NADPH and regenerate plastoquinone in Photosystem I (PSI), using homogeneous molecular photocatalysts. However, water splitting to evolve H₂ and O₂ in a 2:1 stoichiometric ratio has yet to be achieved using homogeneous molecular photocatalysts, remaining as one of the biggest challenges in science. Herein, we demonstrate overall water splitting to evolve H₂ and O₂ in a 2:1 ratio using a two liquid membranes system composed of two toluene phases, which are separated by a solvent mixture of water and trifluoroethanol (H₂O/TFE, 3:1 v/v), with a glass membrane to combine PSI and PSII molecular models. A PSII model contains plastoquinone analogs [p-benzoquinone derivatives (X-Q)] in toluene and an iron(II) complex as a molecular oxidation catalyst in H₂O/TFE (3:1 v/v), which evolves a stoichiometric amount of O₂ and forms plastoquinol analogs (X-QH₂) under photoirradiation. On the other hand, a PSI model contains nothing in toluene but contains X-QH₂, 9-mesityl-10-methylacridinium ion (Acr⁺-Mes) as a photocatalyst, and a cobalt(III) complex as an H₂ evolution catalyst in H₂O/TFE (3:1 v/v), which evolves a stoichiometric amount of H₂ and forms X-Q under photoirradiation. When a PSII model system is combined with a PSI model system with two glass membranes and two liquid membranes, photocatalytic water splitting with homogeneous molecular photocatalysts is achieved to evolve hydrogen and oxygen with the turnover number (TON) of >100.

Acridinium salts as photoredox organocatalysts for photomediated cationic RAFT and DT polymerizations of vinyl ethers

A series of acridinium salts with high excited-state oxidative power are employed as photoredox organocatalysts for photomediated cationic RAFT and DT polymerizations under visible light.

Photoinduced host-to-guest electron transfer in a self-assembled coordination cage

Light–powered host–guest charge transfer (HGCT) is shown for a coordination cage based on electron-rich phenothiazines, containing an anthraquinone acceptor as guest. Transient absorption spectroscopy and spectroelectrochemistry data is presented.

Bridge-Length- and Solvent-Dependent Charge Separation and Recombination Processes in Donor-Bridge-Acceptor Molecules

The photoinduced intramolecular charge separation (CS) and charge recombination (CR) phenomena in a series of donor-bridge-acceptor (D-B-A) molecules are intensively investigated as a means of understanding electron transport through the π-B. Pyrene (Pyr) and triarylamine (TAA) moieties connected via phenylene Bs of various lengths are studied because their CS and CR behaviors can be readily monitored in real time by femtosecond transient absorption (fs-TA) spectroscopy. By combining the steady-state and fs-TA spectroscopic measurements in a variety of solvents together with chemical calculations, the parameters that govern the CS behaviors of these dyads were obtained, such as the solvent effects on free energy and the B-length-dependent electronic coupling (V_DA) between D and A. We observed the sharp switch of the CS behavior with the increase of the solvent polarity and B-linker lengths. Furthermore, in the case of the shortest distance between D and A when the electron coupling is sufficiently large, we observed that the CS phenomenon occurs even in low-polar solvents. Upon increasing the length of B, CS occurs only in strong polar solvents. The distance-dependent decay constant of the CS rate is determined as ∼0.53 Å^-1, indicating that CS is governed by superexchange tunneling interactions. The CS rate constants are also approximately estimated using Marcus electron transfer theory, and the results imply that the V_DA value is the key factor dominating the CS rate, while the facile rotation of the phenylene B is important for modulating the lifetime of the charge-separated state in these D-B-A dyads. These results shed light on the practical strategy for obtaining a high CS efficiency with a long-lived CS state in TAA-B-Pyr derivatives.

Rational Design and Synthesis of OEP and TPP Centered Phosphorus(V) Porphyrin-Naphthalene Conjugates: Triplet Formation via Rapid Charge Recombination

Six new "axial-bonding" type "phosphorus(V) porphyrin-naphthalene" conjugates have been prepared consisting of octaethylporphyrinatophosphorus(V) (POEP⁺)/tetraphenylporphyrinatophosphorus(V) (PTPP⁺) and naphthalene (NP). The distance between the porphyrin and NP was systematically varied using polyether bridges. The unique structural topology of the octaethylporphyrinatophosphorus(V) (POEP⁺) and tetraphenylporphyrinatophosphorus(V) (PTPP⁺) enabled construction of mono- and disubstituted phosphorus(V) porphyrin-naphthalene conjugates, respectively. The steady-state and transient spectral properties were investigated as a function of redox properties, distance, and molecular topology. Strong electronic interactions between the phosphorus(V) porphyrin and NP in directly bound conjugates were observed. The established energy diagrams predicted reductive electron transfer involving singlet excited phosphorus(V) porphyrin and NP to generate high-energy (∼1.83-2.11 eV) charge-separated states (POEP/PTPP)^•-(NP)^•+. Femtosecond transient absorption spectral studies revealed rapid deactivation of singlet excited phosphorus(V) porphyrin due to charge separation wherein the estimated forward rate constants were in the range of 10⁹-10¹⁰ s^-1 and were dependent on the distance between the NP and porphyrins units, as well as the redox potentials of the type of the phosphorus(V) porphyrin. Additionally, due to high exothermicity and low-lying triplet states, the charge recombination process was found to be rapid, leading to populating the triplet states of phosphorus(V) porphyrins.

Dynamics of photoconversion processes: the energetic cost of lifetime gain in photosynthetic and photovoltaic systems

The continued development of solar energy conversion technologies relies on an improved understanding of their limitations. In this review, we focus on a comparison of the charge carrier dynamics underlying the function of photovoltaic devices with those of both natural and artificial photosynthetic systems. The solar energy conversion efficiency is determined by the product of the rate of generation of high energy species (charges for solar cells, chemical fuels for photosynthesis) and the energy contained in these species. It is known that the underlying kinetics of the photophysical and charge transfer processes affect the production yield of high energy species. Comparatively little attention has been paid to how these kinetics are linked to the energy contained in the high energy species or the energy lost in driving the forward reactions. Here we review the operational parameters of both photovoltaic and photosynthetic systems to highlight the energy cost of extending the lifetime of charge carriers to levels that enable function. We show a strong correlation between the energy lost within the device and the necessary lifetime gain, even when considering natural photosynthesis alongside artificial systems. From consideration of experimental data across all these systems, the emprical energetic cost of each 10-fold increase in lifetime is 87 meV. This energetic cost of lifetime gain is approx. 50% greater than the 59 meV predicted from a simple kinetic model. Broadly speaking, photovoltaic devices show smaller energy losses compared to photosynthetic devices due to the smaller lifetime gains needed. This is because of faster charge extraction processes in photovoltaic devices compared to the complex multi-electron, multi-proton redox reactions that produce fuels in photosynthetic devices. The result is that in photosynthetic systems, larger energetic costs are paid to overcome unfavorable kinetic competition between the excited state lifetime and the rate of interfacial reactions. We apply this framework to leading examples of photovoltaic and photosynthetic devices to identify kinetic sources of energy loss and identify possible strategies to reduce this energy loss. The kinetic and energetic analyses undertaken are applicable to both photovoltaic and photosynthetic systems allowing for a holistic comparison of both types of solar energy conversion approaches.

Unravelling the role of charge transfer state during ultrafast intersystem crossing in compact organic chromophores

When organic electron donor (D) and acceptor (A) chromophores are linked together, an electron transfer (ET) state can take place. When a short bridge such as one Sigma bond is used to link the donor and the acceptor, complete charge separation is difficult to access and one usually observes an intramolecular charge transfer (CT) state instead. Due to the inevitable coupling between the donor and the acceptor in compact organic chromophores, the most common decay pathway for the CT state is charge recombination, which may lead to a distinct longer wavelength fluorescence emission or non-radiative dissipation of the excited state energy. However, recent studies have shown that unique excited state dynamics can be observed when the CT state is involved during both forward and backward intersystem crossing (ISC) from singlet excited states to triplet excited states in organic chromophores. Analysis of the mechanism for ISC involving the CT state has received much attention over the last decade. In this perspective, we present a collection of molecular design rationales, spectroscopy and theoretical investigations that provide insights into the mechanism of the ISC involving the CT state in compact organic chromophores. We hope that this perspective will prove beneficial for researchers to design novel compact organic chromophores with a predictable ISC property for future biochemical and optoelectronic applications.

Near-Unity Triplet Generation Promoted via Spiro-Conjugation

An intersystem crossing (ISC) rate constant of 1.0×10¹¹  s^-1 was previously registered with a spiro-bis-benzophenone scaffold. Triplet generation efficiency could be further enhanced by stabilizing the spiro-charge-transfer (CT) state and rationally designing spiro-compounds (SCTs) that consist of electron-rich diphenyl ether as the spiro-CT donor and electron-deficient dinaphthyl ketone as the spiro-CT acceptor. Through fine-tuning of the energy level between the CT and high energy triplet states, near-unity triplet generation quantum yield was achieved and the underlying ISC mechanism is revealed by using ultrafast spectroscopy and quantum chemical calculations. Potential triplet sensitizing application was demonstrated in SCTs. Our findings suggest that a spiro-bichromophoric molecular system with an enhanced spiro-charge transfer warrants efficient triplet generation and is a powerful strategy of heavy-atom-free triplet sensitizers with predictable ISC properties.

Self-Assembly-Directed Organization of a Fullerene-Bisporphyrin into Supramolecular Giant Donut Structures for Excited-State Charge Stabilization

Functional materials composed of spontaneously self-assembled electron donor and acceptor entities capable of generating long-lived charge-separated states upon photoillumination are in great demand as they are key in building the next generation of light energy harvesting devices. However, creating such well-defined architectures is challenging due to the intricate molecular design, multistep synthesis, and issues associated in demonstrating long-lived electron transfer. In this study, we have accomplished these tasks and report the synthesis of a new fullerene-bis-Zn-porphyrin e-bisadduct by tether-directed functionalization of C60 via a multistep synthetic protocol. Supramolecular oligomers were subsequently formed involving the two porphyrin-bearing arms embracing a fullerene cage of the vicinal molecule as confirmed by MALDI-TOF spectrometry and variable temperature NMR. In addition, the initially formed worm-like oligomers are shown to evolve to generate donut-like aggregates by AFM monitoring that was also supported by theoretical calculations. The final supramolecular donuts revealed an inner cavity size estimated as 23 nm, close to that observed in photosynthetic antenna systems. Upon systematic spectral, computational, and electrochemical studies, an energy level diagram was established to visualize the thermodynamic feasibility of electron transfer in these donor-acceptor constructs. Subsequently, transient pump-probe spectral studies covering the wide femtosecond-to-millisecond time scale were performed to confirm the formation of long-lived charge-separated states. The lifetime of the final charge-separated state was about 40 μs, thus highlighting the significance of the current approach of building giant self-organized donor-acceptor assemblies for light energy harvesting applications.

Free Charge Carriers in Homo-Sorted π-Stacks of Donor-Acceptor Conjugates

Inspired by the high photoconversion efficiency observed in natural light-harvesting systems, the hierarchical organization of molecular building blocks has gained impetus in the past few decades. Particularly, the molecular arrangement and packing in the active layer of organic solar cells (OSCs) have garnered significant attention due to the decisive role of the nature of donor/acceptor (D/A) heterojunctions in charge carrier generation and ultimately the power conversion efficiency. This review focuses on the recent developments in emergent optoelectronic properties exhibited by self-sorted donor-on-donor/acceptor-on-acceptor arrangement of covalently linked D-A systems, highlighting the ultrafast excited state dynamics of charge transfer and transport. Segregated organization of donors and acceptors promotes the delocalization of photoinduced charges among the stacks, engendering an enhanced charge separation lifetime and percolation pathways with ambipolar conductivity and charge carrier yield. Covalently linking donors and acceptors ensure a sufficient D-A interface and interchromophoric electronic coupling as required for faster charge separation while providing better control over their supramolecular assemblies. The design strategies to attain D-A conjugate assemblies with optimal charge carrier generation efficiency, the scope of their application compared to state-of-the-art OSCs, current challenges, and future opportunities are discussed in the review. An integrated overview of rational design approaches derived from the comprehension of underlying photoinduced processes can pave the way toward superior optoelectronic devices and bring in new possibilities to the avenue of functional supramolecular architectures.

Photoinduced charge-separated molecular probe for ultrasensitive spectrum analysis and rapid colorimetric detection of platinum ions

Increased utilization of platinum ions in chemicals and drugs escalates environmental pollution and toxicity associated with Pt ions. However, current analysis and detection strategies of Pt ions display limited sensitivity due to the similar inert metal nature of platinum to gold. Herein, a photoinduced charge-separated molecule (MTPA)₂Ab was synthesized as a probe for enhanced sensitive selection of Pt ions. Long-lived charge-separated states generated upon exposure to 365 nm light lead to a stable complex between (MTPA)₂Ab and PtCl₂/PtCl₄ with highly-selectivity via sequential photoinduced electron transfers. Owing to the linear relationship of complex characteristic absorption and fluorescence emission intensities to Pt²⁺/Pt⁴⁺ concentrations, ultrasensitive spectrum analysis of Pt ions is achieved with a detection limit of 14.2 nM (2.8 ppb) for Pt²⁺ and 12.6 nM (2.5 ppb) for Pt⁴⁺ by an absorption spectrometer and 9.8 nM (1.9 ppb) for Pt ions (Pt²⁺/Pt⁴⁺) by a fluorescence spectrometer, far less than the reported values. Furthermore, a portable test box is developed based on (MTPA)₂Ab test strips due to distinguishable color change with Pt²⁺/Pt⁴⁺ concentrations for rapid colorimetric detection of Pt ions. The results highlight the promise of photoinduced charge-separated molecular probe in ultrasensitive and rapid detection of Pt ions to overcome current limitations of detection strategies.

Excited State Anions in Organic Transformations

Utilizing light is a smart way to fuel chemical transformations as it allows the energy to be selectively focused on certain molecules. Many reactions involving electronically excited species proceed via open-shell intermediates, which offer novel and unique routes to expand the hitherto used synthetic toolbox in organic chemistry. The direct conversion of non-prefunctionalized, less activated compounds is a highly desirable goal to pave the way towards more sustainable and atom-economic chemical processes. Photoexcited closed-shell anions have been shown to reach extreme potentials in single electron transfer reactions and reveal unusual excited-state reactivity. It is, therefore, surprising that their use as a reagent or photocatalyst is limited to a few examples. In this Review, we briefly discuss the characteristics of anionic photochemistry, highlight pioneering work, and show recent progress which has been made by utilizing photoexcited anionic species in organic synthesis.

Anionic Fluorinated Zn-porphyrin Combined with Cationic Endohedral Li-fullerene for Long-Lived Photoinduced Charge Separation with Low Energy Loss

Here we report an anionic meso-tetrakis(4-carboxymethylthio-2,3,5,6-tetrafluorophenyl) zinc porphyrin (ZnTF₄PPTC^4-) to form a supramolecular complex with a cationic lithium endohedral [60]fullerene (Li⁺@C₆₀). The supramolecular ZnTF₄PPTC^4-/Li⁺@C₆₀ complex formed by strong electrostatic attraction with a large binding constant generates a long-lived charge-separated (CS) state with low energy loss by photoinduced electron transfer from ZnTF₄PPTC^4- to Li⁺@C₆₀. The anionic fluorinated zinc porphyrin with high oxidation potential reduces the energy loss associated with the charge separation and enhances the energy level of the CS state. The energy level of the CS state determined by electrochemical measurements is at 0.94 eV, which is much higher than that of a similar supramolecular complex using an anionic meso-tetrakis(sulfonatophenyl) zinc porphyrin (ZnTPPS^4-) at 0.55 eV. Time-resolved transient absorption spectroscopy demonstrates that ZnTF₄PPTC^4-/Li⁺@C₆₀ generates a long-lived CS state with a lifetime of 0.29 ms in a binary solvent of acetonitrile and chlorobenzene. The lifetime of the CS state is comparable to that of ZnTPPS^4-/Li⁺@C₆₀ in benzonitrile.

A Mononuclear Non-Heme Manganese(III)-Aqua Complex in Oxygen Atom Transfer Reactions via Electron Transfer

Metal-oxygen complexes, such as metal-oxo [M(O^2-)], -hydroxo [M(OH^-)], -peroxo [M(O₂^2-)], -hydroperoxo [M(OOH^-)], and -superoxo [M(O₂^•-)] species, are capable of conducting oxygen atom transfer (OAT) reactions with organic substrates, such as thioanisole (PhSMe) and triphenylphosphine (Ph₃P). However, OAT of metal-aqua complexes, [M(OH₂)]ⁿ⁺, has yet to be reported. We report herein OAT of a mononuclear non-heme Mn(III)-aqua complex, [(dpaq)Mn^III(OH₂)]²⁺ (1, dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate), to PhSMe and Ph₃P derivatives for the first time; it is noted that no OAT occurs from the corresponding Mn(III)-hydroxo complex, [(dpaq)Mn^III(OH)]⁺ (2), to the substrates. Mechanistic studies reveal that OAT reaction of 1 occurs via electron transfer from 4-methoxythioanisole to 1 to produce the 4-methoxythioanisole radical cation and [(dpaq)Mn^II(OH₂)]⁺, followed by nucleophilic attack of H₂O in [(dpaq)Mn^II(OH₂)]⁺ to the 4-methoxythioanisole radical cation to produce an OH adduct radical, 2,4-(MeO)₂C₆H₃S^•(OH)Me, which disproportionates or undergoes electron transfer to 1 to yield methyl 4-methoxyphenyl sulfoxide. Formation of the thioanisole radical cation derivatives is detected by the stopped-flow transient absorption measurements in OAT from 1 to 2,4-dimethoxythioanisole and 3,4-dimethoxythioanisole, being compared with that in the photoinduced electron transfer oxidation of PhSMe derivatives, which are detected by laser-induced transient absorption measurements. Similarly, OAT from 1 to Ph₃P occurs via electron transfer from Ph₃P to 1, and the proton effect on the reaction rate has been discussed. The rate constants of electron transfer from electron donors, including PhSMe and Ph₃P derivatives, to 1 are fitted well by the electron transfer driving force dependence of the rate constants predicted by the Marcus theory of outer-sphere electron transfer.