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 which 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 which unravels the complex photoinduced water-splitting reaction into simple sequential light-driven one-electron redox reactions followed by simple dark radical-radical recombination reactions.


Introduction
During the past decade, polymeric carbon nitride materials, consisting of s-triazine or heptazine (tri-striazine) molecular building blocks, emerged as promising photocatalysts for the evolution of molecular hydrogen. [1][2][3][4] These materials are attractive because they consist of truly earth-abundant elements, are easily prepared from cheap precursors by standard procedures and are surprisingly stable under ultraviolet (UV) irradiation. The most widely employed material, generally referred to as graphitic carbon nitride (g-C 3 N 4 ), 5 consists of imine-linked heptazine (Hz) units. Numerous modifications, exhibiting varying degrees of crystallinity or various kinds of morphology, chemical modification or doping with metal atoms, have been prepared and have been tested for hydrogen evolution efficiency under irradiation with UV light. [2][3][4] Unfortunately, the poorly constrained chemical compositions and atomic structures of these materials render the interpretation of the experimental data very difficult.
The vast literature on carbon nitrides mainly addresses materials properties, such as the band gap, the exciton dissociation efficiency, or the charge carrier mobility. [6][7][8][9][10][11][12][13][14][15][16][17] The photophysical properties of the Hz chromophore itself and the molecular aspects of the oxidation of water have received rather little attention so far. In this article, we describe an alternative perspective which focusses on the photochemical reactivity of molecular N-heterocycles with protic solvent molecules such as water or phenol. The research discussed herein has evolved from two originally independent developments, one in computational chemistry and the other in molecular spectroscopy and reaction kinetics. On the theoretical side, it was shown that photoexcited heterocycles, such as pyridine, triazine, or Hz, can oxidize water in hydrogen-bonded clusters via low-barrier excited-state proton-coupled electrontransfer (PCET) reactions and the detailed mechanisms of this photoreactivity were clarified. [18][19][20] On the experimental side, it was discovered that a functionalized derivative of Hz, tri-anisole-heptazine (TAHz), is chemically and photochemically remarkably stable and liberates OH radicals by photochemical hydrogen abstraction from liquid water. 21 TAHz also exhibits photocatalytic activity for H 2 evolution (with Pt as co-catalyst and with a sacrificial electron donor) which matches that of g-C 3 N 4 in aqueous suspensions. Moreover, the initial steps of the photoreaction could be kinetically resolved by the measurement of dynamic luminescence quenching and the PCET character of the photoreaction was confirmed by the detection of the kinetic isotope effect in H 2 O vs. D 2 O. 21 Taken together, these theoretical and experimental findings suggest that the basic mechanisms of the water photooxidation reaction can be explored by the investigation of precisely defined and chemically tunable molecular chromophores in neat solvents. The harrowing ambiguities of photochemistry in poorly defined complex polymeric materials can thereby be avoided.
While the solubility of TAHz in liquid water is low, the molecular photophysics of TAHz can more conveniently be investigated in organic solvents, for example in toluene. Replacing water by phenol (substituted water), which also is readily soluble in organic media, can serve as a model molecular testbed in which photoinduced PCET reactions between hydrogen-accepting chromophores and hydroxylic reaction partners can be studied under precisely controlled conditions. While phenol exhibits a significantly lower oxidation potential than water (and therefore is a sacrificial electron donor), its oxidation potential can be tuned over a wide range by functional groups. [22][23][24][25][26] The effect of the oxidation potential (as well as other properties) of the electron donor on the hydrogen evolution reaction can therefore systematically be investigated. This strategy apparently was not considered so far in the context of photocatalytic hydrogen evolution with solid-state organic or inorganic photocatalytic materials.
Since toluene is an inert solvent, its effect on the photochemistry of the chromophore-phenol complexes should be minor. The interpretation of the spectroscopic and kinetic studies can therefore be supported by ab initio electronic-structure calculations for the isolated chromophore-phenol complexes. The ab initio electronic-structure calculations referred to in this article were performed with the ADC(2) method, which is a computationally efficient wave-function-based single-reference propagator method. [27][28][29][30] It yields electronic excitation energies with an accuracy of a few tenths of an electron volt and provides, in particular, a balanced description of locally excited electronic states and electronic charge-transfer (CT) states in molecular complexes, which is an essential feature for accurately describing PCET reactions. In previous work, the ADC(2) method was benchmarked against multi-configuration multi-reference methods (CASSCF/CASPT2) for complexes of small heterocycles (pyridine, triazine) with water molecules. 18,19 For a more detailed description of the computational methods, the reader is referred to these earlier publications.
The structure of this perspective article is as follows. First, the unusual and rather unique spectroscopic properties of the isolated Hz molecule and of derivatives of Hz are analysed with ab initio electronic-structure calculations. The photochemical reactivity is explored with computational and experimental methods for hydrogen-bonded complexes of Hz-based chromophores with protic substrate molecules. The complexes form spontaneously by self-assembly in solution. In addition to the locally excited electronic states of the chromophore, there exists an intermolecular CT state which involves electron transfer from the substrate molecule to the Hz molecule. The calculations reveal that the energy of the CT state can be tuned over a wide range by functional groups on either the Hz molecule or on the substrate. The energy of the CT state, in turn, controls the barrier of the PCET reaction. It is shown by the interplay of theory and experiment that the PCET reaction can be chemically tuned from a barrier-dominated reaction to a barrierless reaction on the potential energy (PE) surface of the lowest excited singlet state. A specific feature of the Hz chromophore, the exceptionally long lifetime of the lowest excited singlet state, allows the demonstration of active laser control of the PCET reaction by pump-push-probe spectroscopy, which yields deep insight into the interplay of photophysical and photochemical reaction mechanisms. In addition to radical pair formation by PCET, radical recombination reactions have been explored with computational methods.
It is shown that under certain conditions Hz-based photocatalysts have the capability of spontaneous self-repair after undesired radical recombination reactions. Finally, the closure of the photocatalytic cycle is discussed. The reduced Hz radicals (HzH) generated by the sequential homolytic decomposition of two H 2 O molecules can recombine in thermal radical-radical recombination reactions, which yields molecular hydrogen and regenerates the Hz photocatalyst. Alternatively, the Hz chromophore can be recovered by photodetachment of the excess hydrogen atom from the HzH radical, resulting in the release of free H-atom radicals (in nonpolar solvents) or solvated electrons (in aqueous solution).

Excited states of Hz
Hz exhibits D 3h symmetry. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are nondegenerate π orbitals which are displayed in Fig. 1. The lowest excited singlet state of Hz, S 1 (A 2 ') with a calculated vertical excitation energy of 2.60 eV at the ADC(2)/cc-pVDZ level, is a nearly pure HOMO-LUMO excitation. The dipole transition moment from the electronic ground state is zero by symmetry. The next higher ππ* excited singlet state, S 4 (E') with an excitation energy of 4.43 eV at the ADC(2)/cc-pVDZ level, results from the excitation of an electron from the HOMO to the degenerate π orbital above the LUMO. This transition carries significant oscillator strength (f = 0.54). The S 4 state is responsible for the onset of strong absorption in Hz-based condensed materials near 400 nm. Between the S 1 (ππ*) and S 4 (ππ*) states there exist two nπ* excited singlet states, S 2 (A 1 '') and S 3 (E''), see Table 1. The 1 nπ* states are electronically dark, but may borrow intensity from the nearby bright 1 ππ* state by vibronic coupling. Experimental data on the electronic excitation spectrum of the isolated Hz molecule are not available because Hz hydrolyses rapidly under ambient conditions. 31, 32 Table 1. Vertical excitation energies (in eV) of the lowest four singlet and triplet excited states of Hz, calculated with the ADC(2) method with the cc-pVDZ basis set. Oscillator strength for singlet transitions is given in parentheses.
The triplet manifold exhibits the same pattern of nondegenerate and degenerate ππ* and nπ* excited states, see Table 1. Inspection of Table 1  The origin of the S 1 /T 1 inversion in Hz can be qualitatively understood by the inspection of Fig. 1.
While the HOMO is exclusively localized on the peripheral nitrogen atoms, the LUMO is localized on the carbon atoms and the central nitrogen atom. This peculiar pattern results in zero spatial overlap of HOMO and LUMO. As a consequence, the exchange integral K, which primarily determines the singlet-triplet energy gap, is strongly suppressed. For Hz, K = 0.12 eV, which is very small compared with the Coulomb integral, J = 5.62 eV. Coupling with double excitations, which can be interpreted as spin polarization in the unrestricted orbital picture, 34 stabilizes the singlet state relative to the triplet state, resulting in the inverted S 1 /T 1 energy gap. 35 Exploratory computational results indicate that S 1 /T 1 inversion is a very robust property of Hz chromophores. Neither substitution of Hz nor oligomerization nor stacking has any significant effect on the S 1 /T 1 energy gap. 35 These results provide strong evidence that S 1 /T 1 energy inversion exists also in crystalline and polymeric Hz-based materials.
The theoretical prediction of S 1 /T 1 inversion was carefully scrutinized by spectroscopic investigations for the TAHz chromophore. 35 The photoluminescence decay of TAHz in toluene was measured with high precision on the microsecond timescale. The effects of molecular oxygen (a triplet quencher) and of ethyl iodide (a heavy atom carrier) was investigated. No evidence of triplet quenching was found, nor could phosphorescence be detected. Microsecond transient absorption experiments did not find evidence of the computationally predicted triplet-triplet absorption, whereas the predicted singletsinglet absorption was readily detected. 35 These data convincingly confirm the inversion of the S 1 and T 1 states in TAHz. The photophysics of several analogues of Hz, such as cycl [3.3.3]azine and triazacycl [3.3.3]azine, was explored earlier by Wirz and coworkers. 36,37 They found evidence for unusual luminescence properties and singlet-triplet near-degeneracy, but could not unequivocally establish singlet-triplet inversion.
The singlet-triplet inversion has important implications for the photochemistry of the molecular Hz chromophore and of Hz-based materials. The absence of a triplet state below the lowest singlet state obviously eliminates the decay channel of intersystem crossing (ISC) from the lowest singlet state.
The absence of quenching by ISC together with the symmetry-forbidden radiative decay endows the S 1 state of Hz with an exceptionally long lifetime. For TAHz in toluene, the S 1 lifetime has been established as 287 ns with a fluorescence quantum yield of 0.69. 21 As will be discussed below, the S 1 state plays the role of a reservoir state in the photophysics of Hz chromophores in which a significant part of the energy of the absorbed photon is stored in the excited state on time scales which are relevant for molecular diffusion and intermolecular reactions.
It is well established that aza-arenes generally possess low-lying long-lived triplet states which, upon irradiation, are populated with high quantum yields by ISC. These long-lived triplet states efficiently generate reactive singlet oxygen ( 1 O 2 ) from atmospheric triplet oxygen ( 3 O 2 ). The generation of reactive 1 O 2 is a long-standing problem in artificial photosynthesis with organometallic photocatalysts. 38,39 The S 1 /T 1 inversion in Hz chromophores completely eliminates this problem for Hz-based materials. Indeed, unexpected ultralow yields of singlet oxygen have been reported for irradiated g-C 3 N 4 [40][41][42] and may be one of the reasons for the exceptional photostability of these materials.

Excited states of Hz derivatives: tuning the absorption spectrum
TAHz is a multi-chromophoric system. The excitonic interactions between the excited states of the Hz core and the excited states of the anisole pendant groups have a substantial impact on the absorption spectrum and the absorption cross section. It is instructive to take a more systematic look at this phenomenon by considering substitutions of Hz with a selection of aromatic pendants. The vertical excitation energies and oscillator strengths of the five lowest singlet excited states of the Hz derivatives tri-phenyl-heptazine (TPhHz), tri-pyrazinyl-heptazine (TPyHz) and tri-anisole-heptazine (TAHz) are listed in Table 2. These data may be compared with the data for Hz in Remarkably, the energy of the low-lying S 1 (ππ*) state of Hz is marginally affected by the substituents, see Table 2. Table 2. Vertical excitation energies (in eV) and oscillator strengths (in parentheses) of the five lowest singlet excited states of tri-phenyl-heptazine (TPhHz), tri-pyrazinyl-heptazine (TPyHz) and tri-anisoleheptazine (TAHz).  state may drop below the energy of the 1 A state. When this happens, the singlet-triplet energy inversion of Hz is removed and the chromophore will possess a long-lived (phosphorescent) triplet state. It also has to be kept in mind that the 3 E state is spatially degenerate in D 3h symmetry and therefore exhibits, in contrast to the nondegenerate S 1 and T 1 states, a Jahn-Teller effect which may substantially lower the minimum energy of this state by the Jahn-Teller stabilization energy.  By the energy gap law, the lowering of the energy of the absorbing 1 ππ* state in aromatically substituted heptazines enhances the radiationless deactivation rate of this state to the long-lived S 1 (ππ*) state. The ligands thus affect the competition between radiationless decay and reactive PCET processes in complexes of these chromophores with protic substrate molecules.

The Hz-H 2 O complex
Hz has six peripheral nitrogen atoms which can serve as acceptor atoms for hydrogen bonding with a water molecule. The structure of the Hz···H 2 O complex with in-plane orientation of the water molecule is shown in Fig. 4. The length of the hydrogen bond is 2.046 Å at the MP2/cc-pVDZ computational level, which represents a relatively strong hydrogen bond. A closer analysis reveals that the nonbonding orbital of the accepting N-atom is partially delocalized over the p x,y orbitals of the oxygen atom of the water molecule, indicating a partially covalent "non-innocent" hydrogen bond. 20 The HOMO and LUMO of the complex are the π and π* orbitals of Hz. They are practically indistinguishable from those shown in Fig. 1. The electronic excitation spectrum of the complex is essentially identical with that of isolated Hz. The degeneracy of the 1 E'(ππ*) and 1 E''(nπ*) states of isolated Hz is lifted, but the energy splittings are less than 0.1 eV. 20 We define the bond length R OH of the OH group of the water molecule involved in the hydrogen bonding (see Fig. 4) as the reaction coordinate for the H-atom transfer reaction from the water molecule to Hz. For small displacements R OH , the potential-energy (PE) functions of the 1 ππ* and 1 nπ* excited states of the complex are essentially parallel to the PE function of the electronic ground state, as can be seen on the left hand side of Fig. 5, revealing that the OH bond of the water molecule is little affected by the local electronic excitation of Hz.  This is, however, no longer true when large changes of R OH or of the distance R ON of the donor oxygen atom and the acceptor nitrogen atom are considered. For extended R OH or reduced R ON , an additional electronic state becomes relevant which arises from the excitation of an electron from the p z orbital on the O-atom of water to the π* orbital of Hz. This excited state is located at high energy (5.40 eV) at the ground-state equilibrium geometry of the Hz···H 2 O complex. When the energy of this state is minimized by geometry optimization, this state not only becomes the lowest excited state of the complex, but its energy even drops below the energy of the closed-shell ground state. This phenomenon is illustrated on the right hand side of Fig This surface, which is displayed in Fig. 6, exhibits a topography which is characteristic for H-atom transfer reactions. [44][45][46][47][48][49] The well on the left hand side, corresponding to the locally excited 1 ππ* state of Hz, and the valley on the right hand side, corresponding to the HzH···OH biradical, are separated by a barrier of 0.75 eV relative to the S 1 (ππ*) minimum. While the S 1 state is extremely long-lived in the isolated Hz molecule, as discussed above, its population can be quenched in the Hz···H 2 O complex by H-atom transfer, either by over-the-barrier dynamics if sufficient vibrational excess energy is available, or by H-atom tunnelling through the barrier. Experimentally, the water photooxidation reaction was explored using suspensions of TAHz in water. 21 The extinction and stationary photoluminescence spectra of TAHz in water are shown in Fig.   7(a). The absorption of the bright 1 ππ* state exhibits an onset at 400 nm. Upon closer inspection, three weak peaks are visible near the red edge of the absorption spectrum, see Fig. 7(b). The two more intense peaks, which are absent for TAHz in toluene, arise from 1 nπ* states which are blue-shifted in water due to hydrogen bonding and acquire intensity due to their proximity to the bright 1 ππ* state.
The photoluminescence in Fig. 7(a) originates from the nominally dark long-lived S 1 (ππ*) state. The lifetime of the S 1 state of TAHz is reduced from 287 ns in toluene to 16 ns in water. 21  These experimental data provide clear and reproducible evidence that the photoexcited TAHz molecule indeed can oxidize water, not just sacrificial electron donors with low oxidation potentials.
Moreover, it has thereby been shown that a Hz-based chromophore in solution can oxidize water in a homogenous photochemical reaction, providing evidence that water oxidation photocatalysis is not a priori dependent on charge separation and charge transport processes in solid-state materials.

Complexes of TAHz with functionalized phenols
Unlike water, phenol is soluble in organic solvents. Given the good solubility of TAHz in toluene, hydrogen-bonded complexes of TAHz with phenol in toluene solution represent ideal model systems for the exploration of the photochemistry of heptazine-based chromophores with hydroxylic partners.
Like water molecules, phenol molecules form a hydrogen bond with the peripheral nitrogen atoms of Hz or TAHz. The effect of hydrogen bonding can be observed in the absorption spectrum of TAHz, as is shown in Fig. 9. Addition of 100 mM PhOH to TAHz in toluene causes a distinct red-shift of the absorption threshold of TAHz near 400 nm. 50 On the other hand, weak peaks arising from nπ* states are blue-shifted due to hydrogen bonding and gain intensity, see the inset in Fig. 9. From the shift of the absorption spectrum as a function of PhOH concentration, the association constant of TAHz and PhOH can be determined. 50 The reactivity of phenol in hydrogen-bonded complexes can be tuned by functional groups R, for example in the para position. It has been shown that the oxidation potential of phenols is shifted cathodically (anodically) by electron donating (electron withdrawing) substituents. 22 Table 3), on the other hand, is highly sensitive to R. It drops by more than one electron volt from 3.86 eV for R = CN to 2.80 eV for R = MeO. The pronounced variation of the energy of the intermolecular CT state is reflected by a corresponding strong variation of the topography of the relaxed PE surface for PCET in Hz···R-PhOH complexes.
These surfaces were computed for R = CN, H, and MeO and are displayed in Fig. 10 PCET rate constants and KIEs (PhOH/D) were measured for TAHz···R-PhOH complexes 50 and more details can be found in this publication. The TAHz···CN-PhOH complex exhibits the lowest PCET rate. The rate measured for the TAHz···Me-PhOH complex is faster by a factor of about ten. For the TAHz···MeO-PhOH complex, the increase of the rate appears to saturate, which may indicate that other factors, such as the association constant, may become rate determining. As shown in Fig. 11, the rates measured for the TAHz···R-PhOH complexes exhibit a reasonable correlation with the barriers computed for the Hz···R-PhOH complexes.

Complexes of functionalized heptazines with water
The electronic-structure calculations and kinetic measurements on Hz-phenol complexes revealed that substituents on phenol can have a dramatic effect on the excited-state energy landscape of these complexes. In particular, the excitation energy of the intermolecular CT state and, as a consequence, the PCET barrier were found to be highly sensitive to substituents on phenol. These findings suggest that the PCET reactivity in complexes of Hz with hydroxylic substrate molecules may alternatively be

Laser control of PCET in TAHz-phenol complexes
As discussed in Section 3.  A schematic illustration of the principle of this experiment for Hz-solvent complexes is illustrated in Fig. 13. The excited state populated by the pump pulse is the bright 1 ππ* state of the Hz-based chromophore, which may be mixed with dark nπ* states and is designated as S n (ππ*/nπ*) in Fig. 13.

Fig. 13. Cartoon potential energy diagram showing a photochemical proton transfer reaction driven by an electronic charge transfer (CT) state which is strongly stabilized by the proton transfer, resulting in the formation of a biradical (BR
The reservoir state is the unique long-lived lowest 1 ππ* state (S 1 ) of Hz. As indicated in Fig. 13, the interaction of the locally excited states with the reactive CT state generates barriers for PCET such that the barrier in the upper S n (ππ*/nπ*) states is much lower than the barrier in the S 1 state. As a result, excitation of the S 1 population by the push pulse (red arrow in Fig. 13) leads to rapid PCET which can be observed by the persistent loss of chromophores in the S 1 state.  Fig. 14(a) shows the calculated transition energies from the electronic ground state to the two locally excited 1 ππ* states and the intermolecular CT state for the four TAHz···R-PhOH complexes in graphical form. As discussed in the preceding section, the energies of the locally excited states are insensitive to the substituent R, while the energy of the CT state is significantly lowered by electron donating groups (Me, MeO). Fig.14(b) gives the corresponding information for the electronic transitions from the minimum of the long-lived S 1 state. Again, the excitation energies of the locally excited states are insensitive to R, whereas the excitation energy of the CT state is strongly reduced by the electron withdrawing groups Me and MeO. Based on these data, the push pulse was tuned to 1150 nm (1.08 eV) in the experiment.  56 The pump-push delay was 6 ps. As shown in Fig. 15

Mechanism of self-repair of Hz-based photocatalysts
While the decomposition of H 2 O molecules into H and OH radicals with visible or near infrared light has been a long-standing challenge in photochemistry, an even bigger challenge is the taming of the in situ generated OH radicals. OH radicals are highly reactive and exhibit an exceptionally high diffusion rate. To avoid or at least reduce detrimental reactions of OH radicals with (photo)catalysts, the OH radicals generated by hydrogen abstraction from water are scavenged in current experiments on molecular hydrogen evolution by sacrificial hole scavengers. In the future, the OH radicals should be captured and recombined in a controlled manner to the closed-shell products H 2 O 2 or H 2 O plus O 2 by suitable catalysts.
Once recombination with OH radicals is suppressed, the photogenerated reduced Hz radicals can be harvested and recombined to form H 2 , which regenerates the photocatalyst. The most likely undesired side reaction of photochemically generated radical pairs is in situ geminate radical recombination, in the present case the recombination of HzH and OH radicals. In particular, the CH group in the immediate vicinity of the reduced N-atom in the HzH radical is prone to be attacked by OH radicals, because the CN bond is weakened by the reduction of the N-atom. Computational studies have shown that the OH radical can attach to this CH group, forming a covalent bond. 61 The structure of the resulting closed-shell product, which formally is a hydrate of Hz, is shown in Fig. 16(b). A moderately endothermic ring-opening reaction can lead to the hydroxy-imino isomer displayed in Fig. 16 The existence of a thermochemically stable hydrate of Hz which is accessible by a low-barrier photochemical reaction explains the irreversible hydrolysis of the Hz molecule in the presence of water and light. 31,32 The predicted lowest excitation energy of the photohydrate is 3.5 eV (355 nm) at the ADC(2)/cc-pVDZ level. 61 The photohydrate therefore is colorless, which excludes the backconversion of the photohydrate to Hz and water under visible light. Table 5 lists the energy of the most stable form of the photohydrate relative to the energy of the chromophore-water complex for a series of substituted heptazines. A negative entry indicates thermochemical stability of the photohydrate, a positive entry indicates metastability. The data exhibit a clear trend: electron withdrawing substituents on Hz tend to stabilize the photohydrate, while electron donating substituents tend to destabilize the photohydrate. Anisole is an electron donating group and the photohydrate of TAHz indeed is found to be metastable by 5.85 kcal/mol, see Table 5. It therefore can convert back to TAHz and H 2 O by a thermal reaction in the dark. The prediction that TAHz has self-repairing capability is in agreement with the observations in the laboratory. It has been found that TAHz exhibits excellent long-term stability under UV irradiation, in sharp contrast to Hz. In H 2 evolution experiments driven at 365 nm in an aqueous suspension of TAHz with 2% Pt loading and TEOA as sacrificial electron donor, the H 2 evolution did not decrease over 25 days of continuous illumination. 21 The photostability of TAHz is comparable to that of polymeric carbon nitride materials. 1,5 It is remarkable that the celebrated operando stability of polymeric carbon nitrides can be matched by a molecular chromophore in solution/suspension. It is likely that self-repair after radical recombination reactions plays an essential role for the exceptional durability of molecular TAHz in aqueous environments under UV irradiation.

Closing the photocatalytic cycle
The products of the photochemical hydrogen abstraction reaction from protic substrates by Hz-based chromophores are substrate-derived radicals (e.g, hydroxyl or phenoxyl radicals) and reduced chromophore radicals (e.g., HzH or TAHzH). The substrate-derived radicals are waste products and ideally should be recombined to stable closed-shell products (e.g, 2  OH → H 2 O 2 ). The reduced chromophore, on the other hand, carries part of the energy of the absorbed photon as chemical energy.
To render the photochemical H-atom abstraction reaction catalytic, the chromophore has to be recovered and the chemical energy stored in the reduced chromophore radical has to be converted to a stable and useful chemical product. Here, we briefly discuss two scenarios for the closure of the photocatalytic cycle.
The HzH radical is a chemically stable species. Its dissociation energy has been estimated as 2.0 eV (46 kcal/mol). 20 Since the dissociation energy D 0 of H 2 is 4.48 eV, 62  In the second scenario, the chromophore, say Hz, is recovered by photodetachment of the excess Hatom from the HzH radical, see Scheme 1(c). It has been shown that the HzH radical as well as substituted HzH radicals possess so-called πσ* excited states which play an essential role for their photochemistry. 20,52 The characteristic photochemistry of πσ* excited states is well established for protic aromatic chromophores such as indole, aniline or phenol. 63 The vertical electronic excitation spectra of the HzH radical and a few HzH derivatives were computed with the unrestricted ADC(2) method. 69  intermediately formed which relax to the so-called hydrated electron by a spontaneous charge separation process. 70 Recombination of these H-atoms or hydrated electrons to H 2 is undesirable, since H-atoms or hydrated electrons are more powerful reducing agents than molecular hydrogen.
There should be a wide range of opportunities for the production of high-value chemicals from cheap precursors by exploitation of the exceptional reduction potential of atomic hydrogen or hydrated electrons.

Discussion
At the dawn of the research field of artificial photosynthesis it has been argued that radical intermediates have to be avoided in water-splitting scenarios owing to their high energy and high reactivity. 71 The research field has followed this paradigm since the 1970s. The overwhelming part of the literature on photocatalytic water splitting is concerned with two-electron (e.g., 2H + + 2e -→ H 2 ) or four-electron (e.g., 2H 2 O → 2H 2 + O 2 ) redox reactions between closed-shell species excluding radical intermediates. [72][73][74][75] However, despite decades of intensive research, an unequivocal demonstration that strongly endothermic multi-electron redox reactions can be driven purely by photons (without additional support by overpotentials in electrochemical devices) is lacking. It is now widely recognized that the bottleneck in water-splitting photocatalysis is the four-electron redox reaction which is required for the evolution of O 2 . [76][77][78][79][80] Photochemical redox reactions driven by low-intensity irradiation invariably are one-electron transfer reactions because photons couple to single electrons and electron-electron correlation in atoms and molecules is too weak to enable multi-electron processes driven by a single photon. One-electron transfer reactions starting from closed-shell educts only can yield radical intermediates. The typical rate for sequential absorption of photons under solar irradiation is 10 -3 s -1 , which is slow compared to characteristic rates of photoinduced intramolecular processes as well as molecular diffusion and intermolecular reaction processes. Therefore, multi-electron redox reactions can realistically only be realized as sequential absorption processes of several photons. This calls for a reconsideration of the strict dogma of exclusion of radical intermediates in water oxidation photocatalysis.
The energetic feasibility of the recombination reactions (2) and (3)   (4). In reaction (4), H 2 O 2 is the waste product rather than the primary product.
The advantage of the scenario described by Eqs. (1)(2)(3)(4) is the unravelling of the water-splitting problem into simple sequential partial reactions. The thermodynamically most difficult step, the breaking of the nevertheless represent an attractive goal for solar energy harvesting due to its simplicity.
In this perspective article, we focussed on the light-driven part of the reaction and the optimization of the photosensitizer, ignoring the stabilization of OH radicals by co-catalysts. The relevance of preexisting hydrogen bonds for efficient excited-state PCET reactions has been pointed out previously. 25,[87][88][89][90][91][92] For the Hz-phenol system in toluene solution, we have found clear spectroscopic evidence for the formation of hydrogen bonds and for the relevance of hydrogen bonding for the intra-complex PCET reaction. 50 The essential role of hydrogen bonding for excited-state PCET is also confirmed by the ab In principle, the energy of the absorption maximum of the bright S n (ππ)* state can be lowered by substitution to an extent that it approaches the energy of the S 1 (ππ*) state (typically 2.60 eV). This way, the overlap of the absorption spectrum with the spectral profile of solar radiation can be improved and energy dissipation due to the S n (ππ)* → S 1 (ππ*) radiationless transition can be reduced.
TAHz with an absorption threshold at 400 nm currently appears to be a good compromise for initial mechanistic studies. In future studies, the possibility of lowering of the absorption threshold toward 500 nm or below should be explored.
Perhaps the most valuable data provided by the ab initio electronic-structure calculations are the vertical excitation energies of the dark intermolecular CT state in Hz-substrate complexes and their dependence on functional groups on Hz or on the substrate, because this information is not directly accessible experimentally. The energy of the CT state directly controls the barrier for the H-atom transfer reaction on the S 1 PE surface of the complex. The lowering of the energy of the CT state by electron donating groups on phenol reduces the barrier for the PCET reaction and thereby enhances the reaction rate dramatically, which is confirmed by the experimental data. In particular, it has been demonstrated that the PCET barrier can be completely eliminated by strongly electron donating substituents on phenol. The calculations predict that the energy of the intermolecular CT state in Hz-H 2 O complexes can likewise be modulated over a wide range by substituents on the Hz core. Electron withdrawing groups on Hz stabilize the energy of the HzH radical, which lowers the energy of the CT state and of the H-atom transfer barrier. In hydrogen-bonded complexes of functionalized heptazines with functionalized protic substrates, the dynamics of PCET reactions can be studied in unprecedented detail due the long-lived population in the S 1 state, which can be promoted in a controlled manner by frequency-tuned impulsive push pulses to the reactive region in the vicinity of the PCET barrier. 56 The ubiquitous photosensitizers in artificial photosynthesis are transition-metal complexes with organic ligands because of their long lifetimes of hundreds of nanoseconds up to microseconds in the lowest triplet state, the archetypical photosensitizer being Ru(bpy) 3 2+ . These long lifetimes, being competitive with molecular diffusion, are essential for productive photochemical transformations.
However, these long-lived triplet states are known to readily activate atmospheric molecular oxygen to highly reactive singlet oxygen which in turn destroys organic materials. The resulting lack of durability of organometallic (photo)catalysts under irradiation is one of the major unsolved problems of artificial photosynthesis. 38,39,77,100 The fact that Hz-based chromophores do not possess long-lived triplet states due to robust S 1 /T 1 energy inversion offers an unexpectedly simple resolution of this long-standing problem. The absence of singlet oxygen activation was confirmed both for molecular Hz-based chromophores (TAHz) 35 as well as for polymeric carbon nitride materials. [40][41][42] This feature of carbon nitride materials most likely is an important factor contributing to their exceptional operando stability.
A second property of carbon nitrides contributing to their high photostability may be the ability of self-regeneration after detrimental radical recombination reactions. Depending on the functionalizing groups on the Hz chromophore, the photohydrates formed by undesired radical recombination reactions can be metastable with respect to the chromophore-water complex, which allows recovery of the chromophore by a spontaneous thermal reaction. Electron donating groups on Hz enhance the capability of self-repair. This interesting feature of Hz-based materials is so far just a theoretical prediction which requires experimental confirmation. It is also predicted that the photohydrates of Hz chromophores exhibit characteristic near-UV absorption bands by which the formation and the decay of the photohydrates could be spectroscopically detected. 52,61 OH radicals are meanwhile routinely detected as by-products in photocatalytic hydrogen evolution experiments, for example with g-C 3