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 {\AA}ngstr{\o}m 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 1{\pi}{\pi}* 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.


INTRODUCTION
The generation of hydrogen by electrolysis of water with renewable electricity is by now a mature technology, although the need of large amounts of precious metals is a serious bottleneck for global deployment. Solar water splitting, i.e. the direct decomposition of water into molecular hydrogen and molecular oxygen with photocatalysts, could be a more efficient way to convert solar energy into renewable chemical energy. Fifty years ago, the seminal paper of Fujishima and Honda stimulated the vision of solar-driven decomposition of water into molecular hydrogen in an electrochemical device with TiO 2 and Pt electrodes. 1 The experiment of Fujishima and Honda and its interpretation have intrigued researchers through the past five decades. Photoelectrochemical water splitting with TiO 2 or related transition-metal (TM) oxides has extensively been investigated and the basic mechanisms seem to be well understood. [2][3][4][5] However, the band gap of TM oxide semiconductors generally is too large for efficient harvesting of solar energy. By a major effort in materials science, semiconductors with significantly lower band gaps have been identified and explored as photocatalytic materials for water splitting. [6][7][8] Unfortunately, the chemical stability of most of these semiconductors under the operating conditions of photoelectrochemical devices is currently too low for technological applications. 9,10 The lack of photostability also is a serious unsolved problem with the organometallic TM-metal complexes which are used as co-catalysts for hydrogen evolution or oxygen evolution. 11,12 Unexpectedly and counterintuitively, a metal-free material, polymeric carbon nitride (PCN), has been found to be photocatalytically active for hydrogen evolution with near-UV light and to exhibit remarkable long-term photostability. 13 This material of nominal composition C 6 N 9 H 3 was first prepared by Berzelius in 1834 and termed "melon" by Justus von Liebig 14 . PCN materials, also known as graphitic carbon nitrides (g-C 3 N 4 ), 15 are readily obtained from nitrogen-rich small molecules by pyrolysis. 16 The decent efficiency of PCNs for hydrogen evolution from sacrificial electron donors and their high photostability triggered a vast research effort during the past decade. There exists an immense literature on photocatalytic hydrogen evolution with polymeric carbon nitrides. We refer to refs. [17][18][19] for representative reviews. The PCNs are considered as semiconducting materials by the community and the goal has been the improvement of the efficiency of exciton dissociation into charge carriers as well as the improvement of charge carrier mobility and lifetime. However, the poorly constrained chemical compositions and molecular structures of these polymeric materials have been detrimental to explorations of the precise energy conversion mechanisms. 20 In this perspective, we summarize recent insights which were obtained by taking a deliberately complementary approach toward water splitting which focusses on the spectroscopic and photochemical properties of the molecular building blocks of PCNs rather than on their semiconducting properties as solid materials. In particular, our analysis focusses on the heptazine (heptaazaphenalene) molecule which is the building block of g-C 3 N 4 . 15 A derivative of heptazine (Hz), trianisole-heptazine (TAHz), has recently been synthesized as a purified molecule and its photocatalytic activity was explored in homogeneous solution. 21 TAHz in organic (toluene) solution was found to be photochemically exceptionally stable and to exhibit remarkable spectroscopic properties. TAHz in aqueous suspension, on the other hand, was found to be able to photooxidize water (which was confirmed by the detection of OH radicals) and to evolve molecular hydrogen with an efficiency which is comparable to polymeric g-C 3 N 4 . 21 On the theoretical side, the excited electronic states of Hz and derivatives thereof were characterized with accurate ab initio calculations.
The photochemical reaction mechanisms in hydrogen-bonded complexes of Hz or TAHz with substrate molecules were explored by calculations of excited-state reaction paths and potential-energy (PE) surfaces as well as by simulations of the ultrafast photochemical dynamics with nonadiabatic trajectory calculations. [22][23][24] Based on this extensive set of experimental and computational data, we suggest a molecular scenario of water oxidation and hydrogen evolution which is fundamentally different from the established scenario based on semiconductor physics.

WATER-SPLITTING PHOTOCATALYSIS WITH SEMICONDUCTORS
The standard free energy difference for the splitting of water into H 2 and O 2 is 113.5 kcal/mol, which corresponds to 1.23 eV per photon (four photons are needed to drive the four-electron O 2 evolution process). In semiconductor photocatalysis, a photon with an energy in excess of the band gap generates charge carriers (electrons in the conduction band and holes in the valence band). The charge carriers are presumed to separate and migrate to the solid-liquid interface, 25 where they react with adsorbed water molecules. 1 It has been shown that water molecules adsorbed on nano-scale TiO 2 particles are partially heterolytically dissociated into protons and OHanions. 26,27 Electrons arriving at the interface thus can neutralize the protons and holes can neutralize the OHanions.
For TiO 2 and related semiconductors, this scenario is supported by extensive experimental data for charge carrier concentrations, charge carrier mobilities as well as rate constants for redox reactions at interfaces. 2,4,5,[28][29][30] Charge carrier lifetimes have been determined to be in the range of picoseconds to nanoseconds, while the timescales of interfacial charge transfer and redox reactions were estimated to be in the range of milliseconds to seconds. 28,29 There is thus a mismatch of several orders of magnitude between the lifetime of the charge carriers and the timescales of productive redox reactions.
It has been estimated that enhancements of the charge carrier lifetimes by factors of 10 3 -10 9 would be required for efficient energy conversion in semiconductor photocatalysis. 2 The expectations that cocatalysts could speed up the surface redox reactions by orders of magnitude did not materialize in recent decades despite extensive research. 5,7,31 The main bottleneck is the oxygen evolution reaction, requiring the accumulation of four holes, which is kinetically extremely challenging. Most experiments actually consider only one of the two half-reactions (hydrogen evolution or oxygen evolution) and are performed with sacrificial substrates which serve as convenient electron or hole scavengers.
For these reasons, the realization of a technologically feasible, scalable, and economically viable process based on light-driven water splitting with semiconductor materials has remained elusive. 32 The yields of molecular hydrogen and molecular oxygen achieved with the photoelectrochemical watersplitting scheme have remained disappointingly low. Although the improvement of the efficiency of water splitting with semiconductor photocatalysts is generally perceived as a problem of materials science, the available data on charge carrier generation, transport, and reactivity in TiO 2 indicate that the core of the problem is the photoelectrochemical scenario itself, namely the "indirect" oxidation of water. In particular, the thermodynamically and kinetically most challenging reaction step, the oxidation of water, is spatially (by micrometers to millimeters) as well as temporally (by milliseconds to seconds) separated from the energy harvesting event (the absorption of the photon). While the initial steps (photon absorption and charge separation) are very fast, the final reaction steps (redox reactions with water) are slow. Large losses due to charge carrier recombination and trapping therefore appear inevitable. Moreover, the surviving charge carriers have to home in on rather dilute reaction partners, such as H 3 O + and OH -, at the solid-liquid interface, which implies a significant kinetic challenge.

PHOTOOXIDANTS
Although the basic mechanistic challenges of photoelectrochemical water splitting are well documented and well understood, there have been few attempts to explore alternative scenarios. The concept of artificial photosynthesis with metal-organic donor-chromophore-acceptor complexes [33][34][35] also invokes charge separation followed by water oxidation, albeit for shorter distances than in semiconductor photocatalysis. In this perspective, we discuss an alternative scenario: the direct lightdriven oxidation of water molecules with the strongest available organic photooxidants in preassociated hydrogen-bonded complexes of the substrate with the chromophore. We suggest that the direct oxidation of water via a photoinduced reaction in pre-assembled chromophore-water complexes may represent a mechanistically superior alternative to the indirect photoelectrochemical watersplitting scenario described above.
We postulate a chromophore A which is a base (proton acceptor) in the electronic ground state and a strong oxidant (electron acceptor) in the photoexcited state. Being a base, A spontaneously associates with water molecules by hydrogen bonding, forming A•••HOH complexes (the dots denote hydrogen bonding). The site of the deposition of the photon energy is thus as close as possible to the water molecule to be oxidized. The distance of electron transfer is reduced from mesoscopic scales to the Ångstrøm scale. To be a good absorber in the visible or near-UV spectrum, A likely is an aromatic molecule, that is, an aromatic base. The initial step of the photoinduced reaction is where A* denotes the chromophore in a bright excited state, typically a 1 ππ* state. If A* is a sufficiently strong oxidant, it may be able to abstract an electron from the water molecule The electronic charge separation generates a strong driving force for the transfer of a proton from the water molecule to the chromophore (the proton wants to follow the electron), resulting in charge This process has been termed "electron-driven proton transfer" (EDPT) 36 and is more widely known as excited-state proton-coupled electron transfer (PCET). 37 Notably, the AH and OH radicals in Eq. (3) are formed in their electronic ground states. Part of the energy of the absorbed photon is thus stored as chemical energy in the radical pair. The simplest aromatic base is pyridine (Py), see Fig. 1a. Its lowest singlet and triplet excitation energies are listed in Table 1. It has been shown in a molecular beam experiment under very precisely defined conditions that Py excited to its lowest 1 ππ* excited state can oxidize water in Py-(H 2 O) n clusters with n > 3. 38 The photoinduced reaction yields PyH and OH(H 2 O) n-1 radicals. The PyH radical was detected via its characteristic ionization potential and its electronic absorption spectrum. The reaction mechanism was confirmed by ab initio calculations of excited-state PE surfaces and by nonadiabatic molecular dynamics simulations. [39][40][41] Analogous experiments and calculations were  42 These studies demonstrate that water molecules can be homolytically dissociated with photon energies which are significantly below the photodissociation threshold of water (6.66 eV or 186 nm). Because the absorption bands of Py and Pm (see Table 1) are far outside the solar spectrum at the surface of Earth, Py and Pm are, like TiO 2 , model systems for the exploration of the fundamental mechanisms of the photooxidation of water rather than practically useful photocatalysts. Table 1. Vertical excitation energies and oscillator strengths (in parentheses) of the three lowest singlet excited states and the three lowest triplet excited states of pyridine, pyrimidine, acridine, heptazine and trianisole-heptazine, calculated with the ADC(2) method and the cc-pVDZ basis set at MP2-optimized ground-state equilibrium geometries. Obviously, the absorption bands can be lowered in energy by extension of the size of the aromatic frame. An example is 5-methoxyquinoline, which according to a recent simulation is able to oxidize water by a photoinduced PCET reaction. 43 Another example is acridine (Ac, see Fig. 1c). Ac exhibits a 1 ππ* excitation energy in the near UV (3.5 eV), see Table 1. Calculations predict that the barrier for PCET in the 1 ππ* excited state of the Ac•••H 2 O complex is merely 0.40 eV, which is about twice the zero-point energy of the PT coordinate. 44 Such barriers can efficiently be tunnelled by H-atoms within the lifetime of the 1 ππ* state. It has recently been shown that the incorporation of Ac in carbon dots indeed leads to a strong increase of photocatalytic hydrogen evolution (methanol was used as sacrificial reagent). 45

PHOTOOXIDATION OF WATER WITH THE HEPTAZINE CHROMOPHORE
There exists a huge number of N-heterocyclic dyes with absorption bands in the visible range of the spectrum which could be scrutinized as potential chromophores for the photooxidation of water as described by Eqs. (1)(2)(3). However, organic dyes typically are not photostable under irradiation with visible or UV light. N-heterocycles, in particular, generally possess low-lying 3 ππ* excited states below the lowest singlet excited state. Transfer of population from the singlet excited states to the lower-lying triplet states by intersystem crossing (ISC) typically initiates destructive photochemistry such as isomerization or fragmentation. Moreover, the population in long-lived triplet states may activate atmospheric triplet oxygen to highly reactive singlet oxygen which readily destroys organic materials. The activation of oxygen by long-lived triplet states is a long-standing problem in photocatalytic water splitting with TM complexes. 46,47 As shown by Antonietti, Maeda and coworkers, g-C 3 N 4 , consisting of Hz building blocks linked by imine groups, exhibits photocatalytic activity for hydrogen evolution with near-UV light in the presence of sacrificial electron donors and also exhibits high photostability under long-term irradiation. 13 As a purified molecule, heptazine (Hz) was first synthesized and structurally characterized by Leonard and coworkers in 1982 48 and the photoelectron spectrum was recorded. 49 Because the Hz molecule hydrolyses rapidly in the presence of traces of water, it attracted little interest in synthetic organic chemistry and only few experimental data on the photophysics and photochemistry of the Hz molecule are available. Two derivatives of Hz were synthesized and spectroscopically characterized by the Adachi group and were shown to be promising chromophores for organic light-emitting diodes (OLEDs). 50,51 Recently. Schlenker and coworkers synthesized another Hz derivative, trianisole-heptazine (TAHz, see Fig. 1e), and showed that this molecular chromophore is, in contrast to Hz itself, chemically as well as photochemically highly stable. 21 Insights into the mechanisms of the photoreactivity of TAHz with water molecules were obtained by time-resolved luminescence measurements. 21 The exceptional photostability and photocatalytic activity of Hz-based polymers as well as molecular species derived from Hz suggest that this chromophore may have unique spectroscopic and photochemical properties among N-heterocycles. Recent ab initio electronic-structure calculations for the excited states of Hz, TAHz and several other derivatives indeed revealed that the Hz chromophore exhibits highly unusual features which distinguish this chromophore from typical N-heterocycles. 22 Besides the very bright 1 ππ* state (calculated at 4.43 eV in Hz and 3.62 eV in TAHz with oscillator strengths of 0. 54 and 2.38, respectively) Hz chromophores possess a characteristic low-lying ( ≈ 2.6 eV) 1 ππ* state (S 1 ) which is dark in absorption and emission, see Table 1. The transition dipole moment of the S 1 state with the electronic ground state is suppressed by the specific non-overlapping structures of HOMO and LUMO and additionally by symmetry selection rules in trigonal symmetry. 22 A low-lying dark 1 ππ* state is so unusual in N-heterocycles that the S 1 (ππ*) state of Hz derivatives occasionally has incorrectly been assigned as 1 nπ* state. 52 Moreover, the S 1 and T 1 states of Hz and its derivatives exhibit the very rare phenomenon of singlet-triplet inversion, that is, the vertical (and adiabatic) excitation energy of the T 1 state is higher than the vertical (and adiabatic) excitation energy of the S 1 state, 53-55 see Table 1. This unusual ordering of S 1 and T 1 states eliminates the decay channel of ISC for the S 1 state (in addition, S 1 -T 1 SO coupling is forbidden by symmetry in first order in Hz and TAHz 55 ). Together, these features of the S 1 state result in an exceptionally long lifetime of the S 1 state (measured as 287 ns for TAHz in toluene) and the remarkable fact that TAHz is strongly luminescent (quantum yield of 0.7 in toluene) despite the symmetry-forbidden transition dipole moment. 21 The exceptionally long-lived S 1 state of Hz chromophores is of great significance for photocatalysis.
The S 1 state plays the role of a reservoir state which allows the storage of a substantial part of the photon energy for timescales which are sufficient for productive photochemical transformations. The absorption of a photon by the strongly allowed second 1 ππ* state is followed by radiationless relaxation to the dark S 1 state on sub-picosecond timescales. This constellation resolves the problem that a large absorption cross section of a given state necessarily implies a short fluorescence lifetime.
In addition, the singlet-triplet inversion in Hz chromophores implies the absence of a long-lived population in the lowest triplet state. The singlet-triplet inversion in Hz-based materials likely is one of the reasons for the exceptional photostability of these materials.    2). The timescale of this tunnelling process is expected to be in the picosecond to nanosecond range which is beyond the range which can be covered with ab initio on-the-fly dynamics simulations. The excitation energy of the bright 1 ππ* state (the "optical band gap") can be tuned by substituents on the Hz core, TAHz being an example. This is illustrated by Table 2, which shows the excitation energies of the 1 ππ* states and the CT state in the complexes of Hz, TAHz, trichloroheptazine (TClHz), and tricyanoheptazine (TCNHz) with a water molecule (the 1 nπ* states are omitted for clarity in Table 2). While the excitation energy of the S 1 (ππ*) state is little affected by substituents, the energy of the bright 1 ππ* state is stabilized by about 0.9 eV in TAHz relative to Hz.  23 Since the H-atom tunnelling rate depends exponentially on the barrier height, these data imply that the PCET rate can be substantially tuned by functionalization of the Hz core.    4 In contrast to the extensive data on carrier dynamics in metal-oxide semiconductors, quantitative data on transport coefficients are lacking for g-C 3 N 4 and related polymeric organic materials. 20 Excited-state lifetime data have been obtained for carbon nitrides with femtosecond to nanosecond transient absorption spectroscopy, [63][64][65][66][67] but these data can alternatively be interpreted in a localized molecular picture. 63 No data seem to exist on the rates and yields of redox reactions with water or sacrificial reagents at the polymer-solvent interface. Considering that the ionization potential of Hz is 9.6 eV 49 and the ionization potential of H 2 O is 12.6 eV in the gas phase 68 and 11.2 eV in a liquid jet, 69 it is unlikely that a relaxed hole in a carbon nitride material can oxidize water. Overall, there is no evidence supported by quantitative experimental data that the indirect water-splitting paradigm established for inorganic semiconductors can be transferred one-to-one to water oxidation and hydrogen evolution with organic polymers.
On the theoretical side, band structures of periodic models of g-C 3 N 4 have been computed with DFT. [70][71][72] Simulations of the dynamics of photoinduced reaction steps were performed at the DFT level for extended models of g-C 3 N 4 with periodic boundary conditions by Ma and coworkers 73,74 and Meng and coworkers. 75 In both simulations, the water oxidation step occurs locally on a Hz building block by H-atom transfer from a hydrogen-bonded water molecule to a peripheral N-atom of a Hz unit as found in the molecular simulations discussed above. Ma and coworkers additionally discussed followup dark reactions of the nascent OH radical. 74 Interestingly, Muuronen et al. found evidence for a similar spatially localized water oxidation mechanism in nonadiabatic dynamics simulations of photoinduced water oxidation on small TiO 2 nanoparticles. 76 An alternative extensively investigated class of metal-free organic materials for hydrogen evolution photocatalysis are conjugated linear polymers. Already in the early 1990s, Yanagida and coworkers reported hydrogen generation with poly(p-phenylene) and poly(pyridine-diyl) polymers in the presence of a dispersed noble-metal co-catalyst and a sacrificial electron donor. 77,78 More recently, the hydrogen evolution reaction with organic polymer photocatalysts was systematically explored by Cooper and coworkers. 20,79,80 Like in the polymeric carbon nitride materials, the photocatalytic mechanisms in these materials are currently poorly understood. Prentice and Zwijnenburg recently discussed the pros and cons of the charge-carrier mechanism and the radical-pair mechanism in conjugated polymers. 81,82 Evidence has been reported that conjugated polymers containing Nheterocycles are superior photocatalysts and that the hydrogen evolution yield seems to increase with the nitrogen content. [83][84][85] Moreover, it has been observed that small oligomers seem to be more efficient photocatalysts than extended polymers of the same material. 86 These findings are hints that photooxidation of the substrate at local nitrogen centers may play a role in the photocatalysis. The observations are more easily explained in the radical-pair mechanisms than in the charge-carrier mechanism.

HARVESTING OF HYDROGEN
To close the photocatalytic cycle after radical pair formation in a chromophore-water complex, the excess hydrogen atom has to be detached or abstracted from the reduced chromophore. Several reaction mechanisms are conceivable, both photoinduced (excited-state) reactions as well as thermal (ground-state) reactions. The most direct recovery of the chromophore is by the photodetachment of the excess H-atom from the AH radical AH + hv → A + H. (4) Calculations for the PyH, AcH, HzH and related radicals have shown that these species possess lowlying moderately absorbing 2 ππ* states as well as a low-lying dark 2 πσ* state. 22,39,44 While the 2 ππ* states are bound with respect to the NH bond distance, the potential-energy function of the 2 πσ* state is dissociative. When the 2 πσ* state is populated by vibronic intensity borrowing or by radiationless relaxation from higher 2 ππ* states, it can drive an ultrafast (non-statistical) photodetachment reaction, resulting in the recovery of the chromophore and a free hydrogen atom. For the PyH radical, the photodetachment reaction was detected in a molecular beam experiment 38  Hz, TAHz and related photooxidants possess several peripheral nitrogen atoms. These radicals can disproportionate according to AH + AH → AH 2 + A.
Calculations predict that the disproportionation reaction is exothermic by 1.5 eV for HzH. 24 The doubly reduced AH 2 molecule is a stable closed-shell species and therefore suitable for long-term chemical storage of hydrogen atoms. The hydrogen atoms can be recovered by photodissociation of AH 2 AH 2 + hv → AH + H.
The AH radicals formed by the reaction (6) will disproportionate again such that eventually A is regenerated and H radicals are produced and can be used for reduction reactions. The mechanism of the photodetachment reaction from HzH 2 is illustrated by Fig. 6. HzH 2 possesses two low-lying 1 πσ* states, one for each NH group. When one of the two N-H bond lengths is stretched, the PE function of the corresponding 1 πσ* state develops a dissociative character, see the light blue curve in Fig. 6. The dissociative 1 πσ* state can be populated either directly via a vibronically induced transition or indirectly by excitation of higher 1 ππ* state followed by rapid internal conversion to the 1 πσ* state. Fig. 6 illustrates that three conical intersections (with two low-lying 1 ππ* state and with the S 0 state) have to be passed diabatically in the H-atom detachment reaction. The nonadiabatic dynamics of this photodissociation process was investigated with ab initio surface-hopping trajectory simulations analogous to those for the Hz•••H 2 O complex discussed above, see ref. 24 for details. To illustrate the essential aspects of the reaction dynamics, the electronic states of HzH 2 are grouped as ground state (S 0 ), low 1 ππ* states (S 1 , S 2 ), 1 πσ* state (S 3 ), and high-lying 1 ππ*/ 1 nπ* states (S n , n > 3). Alternatively to the light-driven hydrogen harvesting reactions of Eqs. (4) and (6), the chromophore A and molecular hydrogen can be recovered by a thermal radical-radical recombination reaction AH + AH → 2 A + H 2 .
The reaction (7) is exothermic if the dissociation energy of the AH radical is less than half of the dissociation energy (4.48 eV) of H 2 . According to calculations, this condition is fulfilled for most of the AH radicals mentioned above. After disproportionation of AH (Eq. (5)), H 2 also may be recovered by a thermal decomposition reaction with a noble-metal catalyst

MANAGING THE PHOTOGENERATED OH RADICALS
There is no free lunch in water splitting photocatalysis. The long-standing carrier recombination problem in the indirect water oxidation mechanism is mirrored by the radical recombination problem in the direct water oxidation mechanism discussed herein. OH radicals are highly mobile and extremely reactive. Clearly, recombination of the photogenerated OH and H radicals is unproductive and has to be avoided. In addition, recombination of the photogenerated OH radicals with the reduced chromophore (AH) is undesirable, since this will destroy the precious photocatalyst. The thermochemistry of the latter reaction has been studied for the example of Hz and several derivatives of Hz. 23 The structures and relative energies of the Hz•••H 2 O complex, the HzH•••OH biradical, and possible products of the radical recombination reaction are displayed in Fig. 8a To avoid deleterious reactions of the OH radicals with the photocatalyst, efficient scavenging and disposal of OH radicals is essential. This problem seems to have found relatively little attention so far.
Noting that OH radicals are central to any combustion process, we propose that OH radicals may be disposed of by low-temperature combustion of a suitable sacrificial reactant, for example methane.
The oxidation of methane to methanol yields a valuable chemical feedstock and a liquid fuel. Methane is one of the few molecules with an ionization potential (12.7 eV) which is higher than the ionization potential of water (12.6 eV). 68 Moreover, methane does not form hydrogen bonds with N-heterocycles.
The presence of methane therefore does not interfere with the oxidation of water on Hz-based photocatalysts. In addition, the bond energy of methane (4.41 eV or 425 kcal/mol) is significantly lower than the bond energy of water (5.11 eV or 493 kcal/mol). The oxidation of CH 4 to CH 3 OH by OH radicals is thus strongly exothermic. Two OH radicals are consumed by the formation of one CH 3 OH molecule.
Imagining an optically transparent porous sieve decorated with a Hz-based photocatalyst, the photocatalytic water-oxidation and methane-oxidation reactions could take place jointly in the gas phase. The partial pressure of CH 4 could be adjusted to minimize the recombination of OH radicals with the Hz-based photocatalyst which accumulates and stores hydrogen atoms from water. It has been shown that the reaction of CH 4 with OH radicals which were photocatalytically generated with a TiO 2supported iron catalyst can yield methanol with a conversion rate of 90%. 88 When methane is recovered from biomass, this process yields green methanol and green H 2 . Notably, the hydrogen atoms evolved in this process are 100% taken from water.

CONCLUSIONS
In the current literature, PCNs are perceived as semiconducting materials and the emphasis of the current research efforts is on the improvement of the efficiency of charge carrier generation and charge migration in these materials. The suppression of the intrinsically high fluorescence quantum yield of PCNs by structural modifications or doping is considered desirable, based on the assumption that detrimental trapping of free charge carriers at defects and charge recombination are the origin of the fluorescence. If, however, excitons in PCNs never dissociate and the oxidation of water (or sacrificial electron donors) occurs via an excited-state photochemical reaction, then a high fluorescence quantum yield is a desirable property indicating a long excited-state lifetime. These considerations underline that a full understanding of the precise mechanisms of the photoinduced transformations is essential for the rational development of substantially improved photocatalytic materials.
In this perspective, we put the spectroscopic and photochemical properties of molecular Hz into the spot light. Albeit Hz was synthesized and structurally characterized already in 1982, 48  Ideally, the photocatalytic properties of organic materials should be optimized in two (or more) steps.
In the first step, the properties should be optimized at the molecular level, that is, for individual chromophores, by judicious comparisons of experiment data with high-level ab initio theory data. In a second step, structurally controlled materials (such as, e.g., covalent organic frameworks or molecular sieves) should be assembled from the pre-optimized building blocks and the properties of the assemblies should be optimized in a systematic manner with support from condensed-phase theory.
New levels of functionality of photocatalytic materials may thus be achieved in the future by synergistic efforts of synthesis, spectroscopy, photochemistry, and theory.