Proton-Coupled Electron Transfer from Water to a Model Heptazine-Based Molecular Photocatalyst

Reverse Intersystem Crossing Dynamics in Vibronically Modulated Inverted Singlet-Triplet Gap System: A Wigner Phase Space Study

We inspect the origin of the inverted singlet-triplet gap (INVEST) and slow change in the reverse intersystem crossing (rISC) rate with temperature, as recently observed. A Wigner phase space study reveals that, though INVEST is found at equilibrium geometry, variation in the exchange interaction and the doubles-excitation for other geometries in the harmonic region leads to non-INVEST behavior. This highlights the importance of nuclear degrees of freedom for the INVEST phenomenon, and in this case, geometric puckering of the studied molecule determines INVEST and the associated rISC dynamics.

Experimental and computational studies of the optical properties of 2,5,8-tris(phenylthiolato)heptazine with an inverted singlet-triplet gap

Photophysical properties of the three-fold symmetric 2,5,8-tris(phenylthiolato)heptazine molecule (1) are studied from combined experimental and computational viewpoints. The intense blue photoemission of 1 in the solid state and in toluene solution is proposed to have a fluorescent origin on the basis of a relatively short emission lifetime and no detectable triplet decay. Calculations at correlated ab initio levels of theory also show that 1 has a large inverted singlet-triplet (IST) gap, a non-vanishing spin-orbit coupling matrix element between the first excited singlet and triplet states, and a fast intersystem crossing rate constant that leads to singlet population from the higher-lying triplet state. The IST gap implies that the first excited singlet state is the lowest excited one, agreeing with the measured fluorescent behaviour of 1. IST gaps are also obtained for the oxygen-containing (2) and selenium-containing (3) analogues of 1 at the ADC(2) level of theory, but not for the tellurium one (4). Calculations of the magnetically induced current density demonstrate that the heptazine core of 1 is globally non-aromatic due to the alternation of carbon and nitrogen atoms along its external rim.

Harnessing Proton-Coupled Electron Transfer for Hydrogenation of Aza-Arenes: Photochemistry of Quinoxaline Derivatives in Methanol

Three quinoxaline derivatives are investigated both experimentally and theoretically to assess their ability for the methanol oxidation and harvesting of hydrogen. In inert solvents, the nonplanar compounds exhibit very weak fluorescence from the lowest excited singlet state, whereas the planar and rigid chromophore emits non-Kasha fluorescence from the S2(ππ*) state despite the proximity of the S1(nπ*) state. In methanol, hydrogen-bonded complexes with solvent molecules are formed, and in all chromophores, the lowest singlet state is populated after excitation of the S2(ππ*) state. The switch from non-Kasha emission of the planar compound in inert solvents to regular emission in methanol is related to reduced symmetry of the hydrogen-bonded complex with methanol which results in effective mixing of ππ* and nπ* states and fast internal conversion to the lowest excited singlet state. The S1(nπ*) state of the hydrogen-bonded complex has charge-transfer character, and for all compounds in methanol, hydrogen transfer to the chromophore is observed. The chromophores retain the transferred hydrogen atoms, serving both as photocatalysts and as hydrogen storage materials. Undesired dark side reactions that occur are also discussed.

Novel Triangulenes: Computational Investigations of Energy Thresholds for Photocatalytic Water Splitting

Organic materials with Inverted Singlet-Triplet (INVEST) gaps are interesting for their potential use in photocatalytic small molecule transformations such as the entirely solar-driven water splitting reaction. However, only a few INVEST emitters are thermodynamically able to split water requiring a first singlet excited dark state, S1 , above 1.27 or 1.76 eV, and absorption near solar the maximum, 2.57 eV. These requirements and the INVEST character are key for achieving a long-lived photocatalyst for water splitting. The only known INVEST emitters that conform to these criteria are large triangular boron carbon nitrides with unknown synthesis pathways. Using ADC(2), a quantum-mechanical method, we describe three triangulenes. 3 a is a cyano azacyclopenta[cd]phenalene derivative while 3 b and 3 c are cycl[3.3.3]azine derivatives. 3 b has a previously undescribed disulfide bridge. Overall, 3 a fulfills requirements for photocatalytic four-electron reduction of water while the S1 states of 3 b and 3 c are likely slightly low for the two-electron reduction process. By analyzing impacts of ligands, we find that there are guidelines describing how S1 -S5 energies and oscillator strengths, T1 energies, and ΔES1T1 gaps are affected, requiring deep-learning algorithms for which studies will be presented by us in due time. The impact of ground-state geometries, solvation effects, as well as reduced-cost ADC(2) algorithms on our findings are also discussed.

Local Hydrogen Bonding Determines Branching Pathways in Intermolecular Heptazine Photochemistry

Heptazine is the molecular core of the widely studied photocatalyst carbon nitride. By analyzing the excited-state intermolecular proton-coupled electron-transfer (PCET) reaction between a heptazine derivative and a hydrogen-atom donor substrate, we are able to spectroscopically identify the resultant heptazinyl reactive radical species on a picosecond time scale. We provide detailed spectroscopic characterization of the tri-anisole heptazine:4-methoxyphenol hydrogen-bonded intermolecular complex (TAHz:MeOPhOH), using femtosecond transient absorption spectroscopy and global analysis, to reveal distinct product absorption signatures at ∼520, 1250, and 1600 nm. We assign these product peaks to the hydrogenated TAHz radical (TAHzH^•) based on control experiments utilizing 1,4-dimethoxybenzene (DMB), which initiates electron transfer without concomitant proton transfer, i.e., no excited-state PCET. Additional control experiments with radical quenchers, protonation agents, and UV-vis-NIR spectroelectrochemistry also corroborate our product peak assignments. These spectral assignments allowed us to monitor the influence of the local hydrogen-bonding environment on the resulting evolution of photochemical products from excited-state PCET of heptazines. We observe that the preassociation of heptazine with the substrate in solution is extremely sensitive to the hydrogen-bond-accepting character of the solvent. This sensitivity directly influences which product signatures we detect with time-resolved spectroscopy. The spectral signature of the TAHzH^• radical assigned in this work will facilitate future in-depth analysis of heptazine and carbon nitride photochemistry. Our results may also be utilized for designing improved PCET-based photochemical systems that will require precise control over local molecular environments. Examples include applications such as preparative synthesis involving organic photoredox catalysis, on-site solar water purification, as well as photocatalytic water splitting and artificial photosynthesis.

Inequivalent Solvation Effects on the N 1s Levels of Self-Associated Melamine Molecules in Aqueous Solution

This work shows how the N 1s photoemission (PE) spectrum of self-associated melamine molecules in aqueous solution has been successfully rationalized using an integrated computational approach encompassing classical metadynamics simulations and quantum calculations based on density functional theory (DFT). The first approach allowed us to describe interacting melamine molecules in explicit waters and to identify dimeric configurations based on π-π and/or H-bonding interactions. Then, N 1s binding energies (BEs) and PE spectra were computed at the DFT level for all structures both in the gas phase and in an implicit solvent. While pure π-stacked dimers show gas-phase PE spectra almost identical to that of the monomer, those of the H-bonded dimers are sensibly affected by NH···NH or NH···NC interactions. Interestingly, the solvation suppresses all of the non-equivalences due to the H-bonds yielding similar PE spectra for all dimers, matching very well our measurements.

Photochemical and Photophysical Dynamics of the Aqueous Ferrate(VI) Ion

Ferrate(VI) has the potential to play a key role in future water supplies. Its salts have been suggested as "green" alternatives to current advanced oxidation and disinfection methods in water treatment, especially when combined with ultraviolet light to stimulate generation of highly oxidizing Fe(V) and Fe(IV) species. However, the nature of these intermediates, the mechanisms by which they form, and their roles in downstream oxidation reactions remain unclear. Here, we use a combination of optical and X-ray transient absorption spectroscopies to study the formation, interconversion, and relaxation of several excited-state and metastable high-valent iron species following excitation of aqueous potassium ferrate(VI) by ultraviolet and visible light. Branching from the initially populated ligand-to-metal charge transfer state into independent photophysical and photochemical pathways occurs within tens of picoseconds, with the quantum yield for the generation of reactive Fe(V) species determined by relative rates of the competing intersystem crossing and reverse electron transfer processes. Relaxation of the metal-centered states then occurs within 4 ns, while the formation of metastable Fe(V) species occurs in several steps with time constants of 250 ps and 300 ns. Results here improve the mechanistic understanding of the formation and fate of Fe(V) and Fe(IV), which will accelerate the development of novel advanced oxidation processes for water treatment applications.

Ab initio trajectory surface-hopping dynamics studies of excited-state proton-coupled electron transfer reactions in trianisoleheptazine-phenol complexes

The excited-state proton-coupled electron-transfer (PCET) reaction in hydrogen-bonded complexes of trianisoleheptazine (TAHz), a chromophore related to polymeric carbon nitrides widely used in hydrogen-evolution photocatalysis, with several phenol derivatives were recently studied by Schlenker and coworkers with time-resolved photoluminescence quenching and pump-probe experiments. A pronounced dependence of the PCET reactivity on the electron-donating/electron-withdrawing character of the substituents on phenol was found, with indications of a barrierless or nearly barrierless PCET reaction for the most strongly electron-donating substituent, methoxy. In the present work, the excited-state PCET dynamics was explored with first-principles nonadiabatic dynamics simulations using the TDDFT/ωB97X-D electronic-structure model for two selected complexes, TAHz-phenol and TAHz-methoxyphenol. The qualitative reliability of the TDDFT/ωB97X-D electronic-structure model was assessed by extensive benchmarking of excitation energies and potential-energy profiles against a wave-function-based ab initio method, the algebraic-diagrammatic construction of second order (ADC(2)). The nonadiabatic dynamics simulations provide temporally and structurally resolved insights into paradigmatic PCET reactions in TAHz-phenol complexes. The radiationless relaxation of the photoexcited bright ¹ππ* state to the long-lived dark S₁ state of TAHz occurs in less than 100 fs. The ensuing PCET reaction on the adiabatic S₁ surface is faster in TAHz-methoxyphenol complexes than in TAHz-phenol complexes due to a lower H-atom-transfer barrier, as observed in the experiments. The relaxation of the complexes to the electronic ground state is found to occur exclusively via PCET within the 250 fs time window covered by the present simulations, confirming the essential role of the hydrogen bond for the fluorescence quenching process. The absolute values of the computed PCET time constants are significantly shorter than those extracted from time-resolved photoluminescence measurements for mixtures of TAHz with phenolic substrates in toluene. The possible origins of this discrepancy are discussed.

Ab Initio Electronic Structure Study of the Photoinduced Reduction of Carbon Dioxide with the Heptazinyl Radical

The photocatalytic conversion of carbon dioxide to liquid fuels with electrons taken from water with solar photons is one of the grand goals of renewable energy research. Polymeric carbon nitrides recently emerged as metal-free materials with promising functionalities for hydrogen evolution from water as well as the activation of carbon dioxide. Molecular heptazine (Hz), the building block of polymeric carbon nitrides, is one the strongest known organic photo-oxidants and has been shown to be able to photo-oxidize water with near-visible light, resulting in reduced (hydrogenated) heptazine (HzH) and OH radicals. In the present work, we explored with ab initio computational methods whether the HzH chromophore is able to reduce carbon dioxide to the hydroxy-formyl (HOCO) radical in hydrogen-bonded HzH-CO₂ complexes by the absorption of a photon. In remarkable contrast to the high barrier for carbon dioxide activation in the electronic ground state, the excited-state proton-coupled electron transfer (PCET) reaction is nearly barrierless, but requires the diabatic passage of three conical intersections. The possibility of barrierless carbon dioxide activation by excited-state PCET has so far not been taken into consideration in the interpretation of photocatalytic carbon dioxide reduction on carbon nitride materials.

Water Oxidation and Hydrogen Evolution with Organic Photooxidants: A Theoretical Perspective

In this Perspective, we discuss a novel water-splitting scenario, namely the direct oxidation of water molecules by organic photooxidants in hydrogen-bonded chromophore-water complexes. In comparison with the established scenario of semiconductor-based water splitting, the distance of electron transfer processes is thereby reduced from mesoscopic scales to the Ångström scale, and the time scale is reduced from milliseconds to femtoseconds, which suppresses competing loss processes. The concept is illustrated by computational studies for the heptazine-H₂O complex. The excited-state landscape of this complex has been characterized with ab initio electronic-structure methods and the proton-coupled electron-transfer dynamics has been explored with nonadiabatic dynamics simulations. A unique feature of the heptazine chromophore is the existence of a low-lying and exceptionally long-lived ¹ππ* state in which a substantial part of the photon energy can be stored for hundreds of nanoseconds and is available for the oxidation of water molecules. The calculations reveal that the absorption spectra and the photochemical functionalities of heptazine chromophores can be systematically tailored by chemical substitution. The options of harvesting hydrogen and the problems posed by the high reactivity of OH radicals are discussed.

Suppression of Charge Recombination by Auxiliary Atoms in Photoinduced Charge Separation Dynamics with Mn Oxides: A Theoretical Study

Charge separation is one of the most crucial processes in photochemical dynamics of energy conversion, widely observed ranging from water splitting in photosystem II (PSII) of plants to photoinduced oxidation reduction processes. Several basic principles, with respect to charge separation, are known, each of which suffers inherent charge recombination channels that suppress the separation efficiency. We found a charge separation mechanism in the photoinduced excited-state proton transfer dynamics from Mn oxides to organic acceptors. This mechanism is referred to as coupled proton and electron wave-packet transfer (CPEWT), which is essentially a synchronous transfer of electron wave-packets and protons through mutually different spatial channels to separated destinations passing through nonadiabatic regions, such as conical intersections, and avoided crossings. CPEWT also applies to collision-induced ground-state water splitting dynamics catalyzed by Mn₄CaO₅ cluster. For the present photoinduced charge separation dynamics by Mn oxides, we identified a dynamical mechanism of charge recombination. It takes place by passing across nonadiabatic regions, which are different from those for charge separations and lead to the excited states of the initial state before photoabsorption. This article is an overview of our work on photoinduced charge separation and associated charge recombination with an additional study. After reviewing the basic mechanisms of charge separation and recombination, we herein studied substituent effects on the suppression of such charge recombination by doping auxiliary atoms. Our illustrative systems are X–Mn(OH)₂ tied to N-methylformamidine, with X=OH, Be(OH)₃, Mg(OH)₃, Ca(OH)₃, Sr(OH)₃ along with Al(OH)₄ and Zn(OH)₃. We found that the competence of suppression of charge recombination depends significantly on the substituents. The present study should serve as a useful guiding principle in designing the relevant photocatalysts.

The mechanism of alkali promoting water splitting on g-C3N4

Alkali promotes H+ to obtain electrons and turn into H2.

Ab Initio Nonadiabatic Surface-Hopping Trajectory Simulations of Photocatalytic Water Oxidation and Hydrogen Evolution with the Heptazine Chromophore

In the past decade, polymeric carbon nitrides consisting of heptazine (Hz) building blocks emerged as highly promising materials for photocatalytic hydrogen evolution from water or sacrificial electron donors with near-ultraviolet light. However, the complexity of these materials and their poor characterization at the atomic level are detrimental to the unraveling of the detailed mechanisms of the reactions leading to hydrogen evolution. Recently, it has been shown that a derivative of the Hz molecule, trianisole-heptazine (TAHz), is a potent photobase, which readily oxidizes various derivatives of phenol and even water by an excited-state proton-coupled electron-transfer (PCET) reaction. Energy profiles along minimum-energy reaction paths and relaxed PCET potential-energy surfaces, which previously were computed with ab initio electronic-structure methods for complexes of Hz and TAHz with protic substrates, led to qualitative insights. To obtain more quantitative insight, reaction dynamics simulations are required. In the present work, the time scales of the electron and proton transfer processes and the branching ratios of competing channels were explored with ab initio on-the-fly quasiclassical surface-hopping trajectory simulations for the hydrogen-bonded Hz-H₂O complex. By the analysis of representative trajectories, detailed insight into the interplay of various nonadiabatic electronic transitions, electron transfer, proton transfer, and vibrational energy relaxation is obtained. The HzH radicals which are formed by the photoreduction of Hz can disproportionate to Hz and HzH₂ in an exothermic reaction with a low reaction barrier. The time scales and branching ratios of competing channels in H-atom photodetachment from the HzH₂ molecule also were explored with ab initio surface-hopping simulations. These results delineate for the first time a quantitatively supported scenario of water oxidation and hydrogen evolution with a molecular carbon nitride photocatalyst.

Donor-Acceptor Conjugated Heptazine Polymers with Highly Efficient Photocatalytic Degradations towards Tetracyclines

Photocatalysis is an efficient and green technology in the environmental protection. Due to the high charge separation and transfer, donor-acceptor (D-A) conjugated polymers attract much attention for their photocatalytic degradations towards organic pollutants. Herein, the authors reported three novel D-A conjugated polymers, named as HPBP, HPTP, and HPF, with heptazine moieties as electron acceptors, while biphenyl, terphenyl, or fluorene moieties as electron donors, respectively, which indeed exhibit a highly efficient photocatalytic degradation towards tetracyclines upon the visible-light irradiation. Among them, the photocatalytic performance of HPF is especially noticeable with the degradation rate up to 87% within 30 min, almost 11 times in comparison to those of pristine g-C₃ N₄ , which is mainly attributed to its high crystallinity and conjugation. For their photocatalytic mechanism, the •O₂ ^- radical anions are regarded as the active species.

Photochemistry of carbon nitrides and heptazine derivatives

We explore the photochemistry of polymeric carbon nitride (C₃N₄), an archetypal organic photocatalyst, and derivatives of its structural monomer unit, heptazine (Hz). Through spectroscopic studies and computational analysis, we have observed that Hz derivatives can engage in non-innocent hydrogen bonding interactions with hydroxylic species. The photochemistry of these complexes is influenced by intermolecular nπ*/ππ* mixing of non-bonding orbitals of each component and the relative energy of intermolecular charge-transfer (CT) states. Coupling of the former to the latter appears to facilitate proton-coupled electron transfer (PCET), resulting in biradical products. We have also observed that Hz derivatives exhibit an extremely rare inverted singlet/triplet energy splitting (ΔE_ST). In violation of Hund's multiplicity rules, the lowest energy singlet (S₁) is stabilized relative to the lowest triplet (T₁) electronic excited state. Exploiting this unique inverted ΔE_ST character has obvious implications for transformational discoveries in solid-state OLED lighting and photovoltaics. Harnessing this inverted ΔE_ST, paired with light-driven intermolecular PCET reactions, may enable molecular transformations relevant for applications ranging from solar energy storage to new classes of non-triplet photoredox catalysts for pharmaceutical development. To this end, we have explored the possibility of optically controlling the photochemistry of Hz derivatives using ultrafast pump-push-probe spectroscopy. In this case, the excited state branching ratios among locally excited states of the chromophore and the reactive intermolecular CT state can be manipulated with an appropriate secondary "push" excitation pulse. These results indicate that we can predictively redirect chemical reactivity with light in this system, which is an avidly sought achievement in the field of photochemistry. Looking forward, we anticipate future opportunities for controlling heptazine photochemistry, including manipulating PCET reactivity with a diverse array of substrates and optically delivering reducing equivalents with, for example, water as a partial source of electrons and protons. Furthermore, we wholly expect that, over the next decade, materials such as Hz derivatives, that exhibit inverted ΔE_ST character, will spawn a significant new research effort in the field of thin-film optoelectronics, where controlling recombination via triplet excitonic states can play a critical role in determining device performance.

Are Heptazine-Based Organic Light-Emitting Diode Chromophores Thermally Activated Delayed Fluorescence or Inverted Singlet-Triplet Systems?

Two chromophores derived from heptazine, HAP-3MF and HAP-3TPA, were synthesized and tested as emitters in light-emitting diodes (OLEDs) by Adachi and co-workers. Both emitters were shown to exhibit quantum efficiencies which exceed the theoretical maximum of conventional fluorescent OLEDs. The enhanced emission efficiency was explained by the mechanism of thermally activated delayed fluorescence (TADF). In the present work, the electronic excitation energies and essential features of the topography of the excited-state potential-energy surfaces of HAP-3MF and HAP-3TPA have been investigated with a wave function-based ab initio method (ADC(2)). It is found that HAP-3MF is an inverted singlet-triplet (IST) system; that is, the energies of the S₁ and T₁ states are robustly inverted in violation of Hund's multiplicity rule. Notably, HAP-3MF presumably is the first IST emitter which was implemented in an OLED device. In HAP-3TPA, on the other hand, the vertical excitation energies of the S₁ and T₁ states are essentially degenerate. The excited states exhibit vibrational stabilization energies of similar magnitude along different relaxation coordinates, resulting in adiabatic excitation energies which also are nearly degenerate. HAP-3TPA is found to be a chromophore at the borderline of TADF and IST systems. The spectroscopic data reported by Adachi and co-workers for HAP-3MF and HAP-3TPA are analyzed in light of these computational results.

Triangular boron carbon nitrides: an unexplored family of chromophores with unique properties for photocatalysis and optoelectronics

It has recently been shown that cycl[3.3.3]azine and heptazine (1,3,4,6,7,9,9b-heptaazaphenalene) as well as related azaphenalenes exhibit inverted singlet and triplet states, that is, the energy of the lowest singlet excited state (S1) is below the energy of the lowest triplet excited state (T1). This feature is unique among all known aromatic chromophores and is of outstanding relevance for applications in photocatalysis and organic optoelectronics. Heptazine is the building block of the polymeric material graphitic carbon nitride which is an extensively explored photocatalyst in hydrogen evolution photocatalysis. Derivatives of heptazine have also been identified as efficient emitters in organic light emitting diodes (OLEDs). In both areas, the inverted singlet-triplet gap of heptazine is a highly beneficial feature. In photocatalysis, the absence of a long-lived triplet state eliminates the activation of atmospheric oxygen, which is favourable for long-term operational stability. In optoelectronics, singlet-triplet inversion implies the possibility of 100% fluorescence efficiency of electron-hole recombination. However, the absorption and luminescence wavelengths of heptazine and the S1-S0 transition dipole moment are difficult to tune for optimal functionality. In this work, we employed high-level ab initio electronic structure theory to devise and characterize a large family of novel heteroaromatic chromophores, the triangular boron carbon nitrides. These novel heterocycles inherit essential spectroscopic features from heptazine, in particular the inverted singlet-triplet gap, while their absorption and luminescence spectra and transition dipole moments are widely tuneable. For applications in photocatalysis, the wavelength of the absorption maximum can be tuned to improve the overlap with the solar spectrum at the surface of earth. For applications in OLEDs, the colour of emission can be adjusted and the fluorescence yield can be enhanced.

State of the Art in the Preparation and Properties of Molecular Monomeric s-Heptazines: Syntheses, Characteristics, and Functional Applications

This review gives an account on the fast expanding field of monomeric (or molecular) heptazines, at the exclusion of their various polymeric forms, often referred to as carbon nitrides. While examples of monomeric heptazines were extremely limited until the beginning of this century, the field has started expanding quickly since then, as has the number of reports on polymeric materials, though previous reviews did not separate these fields. We provide here a detailed report on the synthetic procedures for molecular heptazines. We also extensively report on the different achievements realized from these new molecules, in the fields of physical chemistry, spectroscopy, materials preparation, (photo)catalysis, and devices. After a comprehensive summary and discussion on heptazines syntheses and characteristics, we show that starting from well-defined molecules allows a versatility of approaches and a wide tunability of the expected properties. It comes out that the field of monomeric heptazines is now emerging and possibly heading toward maturity, while diverging from the one of polymeric carbon nitrides. It is likely that this area of research will quickly surge to the forefront of the search for active organic molecules, with special attention to the domains of catalysis and organic-based functional materials and devices.

Intermolecular Hydrogen Bonding Tunes Vibronic Coupling in Heptazine Complexes

To better understand how hydrogen bonding influences the excited-state landscapes of aza-aromatic materials, we studied hydrogen-bonded complexes of 2,5,8-tris (4-methoxyphenyl)-1,3,4,6,7,9,9b-heptaazaphenalene (TAHz), a molecular photocatalyst related to graphitic carbon nitride, with a variety of phenol derivatives (R-PhOH). By varying the electron-withdrawing character of the para-substituent on the phenol, we can modulate the strength of the hydrogen bond. Using time-resolved photoluminescence, we extract a spectral component associated with the R-PhOH-TAHz hydrogen-bonded complex. Surprisingly, we noticed a striking change in the relative amplitude of vibronic peaks in the TAHz-centered emission as a function of R-group on phenol. To gain a physical understanding of these spectral changes, we employed a displaced-oscillator model of molecular emission to fit these spectra. This fit assumes that two vibrational modes are dominantly coupled to the emissive electronic transition and extracts their frequencies and relative nuclear displacements (related to the Huang-Rhys factor). With the aid of quantum chemical calculations, we found that heptazine ring-breathing and ring-puckering modes are likely responsible for the observed vibronic progression, and both modes indicate decreasing molecular distortion in the excited state with increasing hydrogen bond strength. This finding offers new insights into intermolecular excited-state hydrogen bonding, which is a crucial step toward controlling excited-state proton-coupled electron transfer and proton transfer reactions.

Photobase effect for just-in-time delivery in photocatalytic hydrogen generation

Carbon dots (CDs) are a promising nanomaterial for photocatalytic applications. However, the mechanism of the photocatalytic processes remains the subject of a debate due to the complex internal structure of the CDs, comprising crystalline and molecular units embedded in an amorphous matrix, rendering the analysis of the charge and energy transfer pathways between the constituent parts very challenging. Here we propose that the photobasic effect, that is the abstraction of a proton from water upon excitation by light, facilitates the photoexcited electron transfer to the proton. We show that the controlled inclusion in CDs of a model photobase, acridine, resembling the molecular moieties found in photocatalytically active CDs, strongly increases hydrogen generation. Ultrafast spectroscopy measurements reveal proton transfer within 30 ps of the excitation. This way, we use a model system to show that the photobasic effect may be contributing to the photocatalytic H₂ generation of carbon nanomaterials and suggest that it may be tuned to achieve further improvements. The study demonstrates the critical role of the understanding the dynamics of the CDs in the design of next generation photocatalysts.

A straightforward synthesis of a new family of molecules: 2,5,8-trialkoxyheptazines. Application to photoredox catalyzed transformations

We have prepared several 2,5,8-trialkoxyheptazines starting from the soluble precursor 2,5,8-tris(3,5-diethylpyrazolyl)-heptazine. We present their synthesis along with their promising spectroscopic and electrochemical properties, which demonstrate large band gaps and high reduction potentials altogether. Subsequently, we provide a short assessment of the promising ability of one of these molecules to perform catalytic oxidation test-reactions.

Photooxidation of water with heptazine-based molecular photocatalysts: Insights from spectroscopy and computational chemistry

We present a conspectus of recent joint spectroscopic and computational studies that provided novel insight into the photochemistry of hydrogen-bonded complexes of the heptazine (Hz) chromophore with hydroxylic substrate molecules (water and phenol). It was found that a functionalized derivative of Hz, tri-anisole-heptazine (TAHz), can photooxidize water and phenol in a homogeneous photochemical reaction. This allows the exploration of the basic mechanisms of the proton-coupled electron-transfer (PCET) process involved in the water photooxidation reaction in well-defined complexes of chemically tunable molecular chromophores with chemically tunable substrate molecules. The unique properties of the excited electronic states of the Hz molecule and derivatives thereof are highlighted. The potential energy landscape relevant for the PCET reaction has been characterized by judicious computational studies. These data provided the basis for the demonstration of rational laser control of PCET reactions in TAHz-phenol complexes by pump-push-probe spectroscopy, which sheds light on the branching mechanisms occurring by the interaction of nonreactive locally excited states of the chromophore with reactive intermolecular charge-transfer states. Extrapolating from these results, we propose a general scenario that unravels the complex photoinduced water-splitting reaction into simple sequential light-driven one-electron redox reactions followed by simple dark radical-radical recombination reactions.

Laser-Induced Electron Transfer in the Dissociative Multiple Ionization of Argon Dimers

We report on an experimental and theoretical study of the ionization-fragmentation dynamics of argon dimers in intense few-cycle laser pulses with a tagged carrier-envelope phase. We find that a field-driven electron transfer process from one argon atom across the system boundary to the other argon atom triggers subcycle electron-electron interaction dynamics in the neighboring atom. This attosecond electron-transfer process between distant entities and its implications manifests itself as a distinct phase-shift between the measured asymmetry of electron emission curves of the Ar^{+}+Ar^{2+} and Ar^{2+}+Ar^{2+} fragmentation channels. This letter discloses a strong-field route to controlling the dynamics in molecular compounds through the excitation of electronic dynamics on a distant molecule by driving intermolecular electron-transfer processes.

Photoinduced water oxidation in pyrimidine-water clusters: a combined experimental and theoretical study

The photocatalytic oxidation of water with molecular or polymeric N-heterocyclic chromophores is a topic of high current interest in the context of artificial photosynthesis, that is, the conversion of solar energy to clean fuels. Hydrogen-bonded clusters of N-heterocycles with water molecules in a molecular beam are simple model systems for which the basic mechanisms of photochemical water oxidation can be studied under well-defined conditions. In this work, we explored the photoinduced H-atom transfer reaction in pyrimidine-water clusters yielding pyrimidinyl and hydroxyl radicals with laser spectroscopy, mass spectrometry and trajectory-based ab initio molecular dynamics simulations. The oxidation of water by photoexcited pyrimidine is unequivocally confirmed by the detection of the pyrimidinyl radical. The dynamics simulations provide information on the time scales and branching ratios of the reaction. While relaxation to local minima of the S₁ potential-energy surface is the dominant reaction channel, the H-atom transfer reaction occurs on ultrafast time scales (faster than about 100 fs) with a branching ratio of a few percent.

Charge separation and successive reconfigurations of electronic and protonic states in a water-splitting catalytic cycle with the Mn4CaO5 cluster. On the mechanism of water splitting in PSII

Charge separation, reloading of electrons and protons, and O₂ generation in a catalytic cycle for water splitting with Mn₄CaO₅ in PSII.

Boosting Photocatalytic Hydrogen Production by Modulating Recombination Modes and Proton Adsorption Energy

Solar-driven production of renewable energy (e.g., H₂) has been investigated for decades. To date, the applications are limited by low efficiency due to rapid charge recombination (both radiative and nonradiative modes) and slow reaction rates. Tremendous efforts have been focused on reducing the radiative recombination and enhancing the interfacial charge transfer by engineering the geometric and electronic structure of the photocatalysts. However, fine-tuning of nonradiative recombination processes and optimization of target reaction paths still lack effective control. Here we show that minimizing the nonradiative relaxation and the adsorption energy of photogenerated surface-adsorbed hydrogen atoms are essential to achieve a longer lifetime of the charge carriers and a faster reaction rate, respectively. Such control results in a 16-fold enhancement in photocatalytic H₂ evolution and a 15-fold increase in photocurrent of the crystalline g-C₃N₄ compared to that of the amorphous g-C₃N₄.

Photoinduced Water-Heptazine Electron-Driven Proton Transfer: Perspective for Water Splitting with g-C3N4

Heptazine-assembled polymeric carbon nitride (CN) materials have fascinated the research community as a photocatalyst for hydrogen evolution while less attention has been devoted to the mechanistic features of the host materials. Using excited-state nonadiabatic dynamics simulations, the molecular-level picture of the decomposition of heptazine hydrogen bonded to water molecule(s) (heptazine-water complex) into heptazinyl and hydroxyl biradical products is revealed. Dynamics simulations show that hydrogen detachment from the water molecule to the heptazine occurs within tens of femtoseconds and suggest that excited-state deactivation via N-H······O-H electron-driven proton transfer (EDPT) is the dominant and most relevant excited-state deactivation process in heptazine-water complexes leading to conical intersection. The observation of photorelaxation-induced water splitting by heptazine is proof of the water-splitting reaction principle, which presents further challenges for computational and experimental investigations of the deactivation of heptazinyl and OH biradical products for efficient hydrogen evolution.

On the Elementary Chemical Mechanisms of Unidirectional Proton Transfers: A Nonadiabatic Electron-Wavepacket Dynamics Study

We propose a set of chemical reaction mechanisms of unidirectional proton transfers, which may possibly work as an elementary process in chemical and biological systems. Being theoretically derived based on our series of studies on charge separation dynamics in water splitting by Mn oxides, the present mechanisms have been constructed after careful exploration over the accumulated biological studies on cytochrome c oxidase (CcO) and bacteriorhodopsin. In particular, we have focused on the biochemical findings in the literature that unidirectional transfers of approximately two protons are driven by one electron passage through the reaction center (binuclear center) in CcO, whereas no such dissipative electron transfer is believed to be demanded in the proton transport in bacteriorhodopsin. The proposed basic mechanisms of unidirectional proton transfers are further reduced to two elementary dynamical processes, namely, what we call the coupled proton and electron-wavepacket transfer (CPEWT) and the inverse CPEWT. To show that the proposed mechanisms can indeed be materialized in a molecular level, we construct model systems with possible molecules that are rather familiar in biological chemistry, for which we perform the ab initio calculations of full-dimensional nonadiabatic electron-wavepacket dynamics coupled with all nuclear motions including proton transfers.

Renewing accessible heptazine chemistry: 2,5,8-tris(3,5-diethyl-pyrazolyl)-heptazine, a new highly soluble heptazine derivative with exchangeable groups, and examples of newly derived heptazines and their physical chemistry

We have prepared 2,5,8-tris(3,5-diethyl-pyrazolyl)-heptazine, the first highly soluble heptazine derivative possessing easily exchangeable leaving groups. We present its original synthesis employing mechanochemistry, along with a few examples of its versatile reactivity. It is, in particular, demonstrated that the pyrazolyl leaving groups can be replaced by several secondary or primary amino substituents or by three aryl- or benzyl-thiol substituents. In addition to being a synthetic platform, 2,5,8-tris(3,5-diethyl-pyrazolyl)-heptazine is fluorescent and electroactive, and its attractive properties, as well as those of the derived heptazines, are briefly presented.

Photoinduced electron-driven proton transfer from water to an N-heterocyclic chromophore: nonadiabatic dynamics studies for pyridine–water clusters

We performed the excited-state dynamics simulations for pyridine–water clusters and found the more water molecules involved in the cluster, the higher efficiency the water-splitting reaction has, which is qualitatively in consistent with a recent gas-phase experimental observations.

Excited state structural dynamics of 4-cyano-4′-hydroxystilbene: deciphering the signatures of proton-coupled electron transfer using ultrafast Raman loss spectroscopy

The photo-initiated proton-coupled electron transfer process in the 4-cyano-4′-hydroxystilbene–tert-butylamine adduct strongly affects the excited-state structural dynamics of CHSB.