Biscoumarin-containing Acenes as Stable Organic Semiconductors for Photocatalytic Oxygen Reduction to Hydrogen Peroxide

X-ray diffraction Data collections were performed at the X-ray diffraction beamline (XRD1) of the Elettra Synchrotron, Trieste (Italy)[1S]. Complete datasets were collected at 100 K (nitrogen stream supplied through an Oxford Cryostream 700) with a monochromatic wavelength of 0.700 Å through the rotating crystal method. Images were acquired using a Pilatus 2M image plate detector. The crystals of compounds 8, 9 and 10 were dipped in N-paratone and mounted on the goniometer head with a nylon loop. The diffraction data were indexed, integrated and scaled using XDS.[2S] 8 and 10 crystallize with triclinic P -1 unit cells. Complete dataset was obtained by merging two data collections obtained from two different orientations of two different crystals. 9 crystallizes with a monoclinic P 21/c unit cell. The unit cell and space group have also been determined at room temperature and no phase change has been detected for the three compounds. The structures were solved by direct methods using SIR2014,[3S] Fourier analyzed and refined by the full-matrix least-squares based on F2 implemented in SHELXL-2014.[4S] The Coot program was used for modeling.[5S] Anisotropic thermal motion modeling was then applied to atoms with full occupancy. Hydrogen atoms were included at calculated positions with isotropic Ufactors = 1.2 Ueq. All the molecules show an inversion center that matches crystallographic inversion centers. The asymmetric units contain two crystallographically half independent moieties for 8 and 9 and one plus half molecules for 10. No disorder and no solvent molecules are present in these crystals forms. Essential crystal and refinement data (Table S1) are reported below.


Introduction
Demand for robust small molecule semiconductors with improved and novel functionality has encouraged materials chemists to search for solutions in many branches of organic chemistry.2][3] Many classical chromophores such as indigos, 2 quinacridones, 4 diketopyrrolopyrroles, [5][6][7][8] epindolindiones, 9,10 bay-annulated indigos, 11 and indanthrones 12 have been repurposed as functional chromophores due to their exceptional photophysical properties and potential applications in organic electronics.As organic light-emitting diode technology has achieved large-scale commercial success and organic photovoltaic technologies edge closer to technological maturity, nding next generation applications for organic semiconductors becomes pertinent.6][17] For both of these application directions, stability in oxygenated and aqueous environments is critical.Many of the well-known organic semiconductors are unfortunately unstable under these conditions.
In our work, we have focused on catalytic oxygen reduction to H 2 O 2 .9][20][21] A number of semiconductor photocatalysts, most notably ZnO, are known to photochemically reduce O 2 to H 2 O 2 while oxidizing a range of sacricial electron donors, such as phenol, alcohols, etc. [22][23][24] On the other hand it was recently reported that an archetypical carbonyl pigment, perylenetetracarboxylic diimide, can achieve the oxygen evolution reaction in the presence of some sacricial electron acceptors. 25A structurally-related naphthalene diimide polymer was also shown to function as a photoanode for water oxidation. 26Several works, on the other hand have focused on researching sustainable photocatalytic systems for H 2 O 2 production via oxygen reduction. 27,28H 2 O 2 can be photochemically generated to some extent by TiO 2 via water oxidation. 29,30he overall photocatalytic process of oxygen reduction to H 2 O 2 accompanied by oxidation of water has been recently observed for the rst time by Hirai et al. using a graphitic carbon nitride (g-C 3 N 4 ) semiconductor, 31,32 and recently found also to be catalyzed by graphene oxide. 28In 2016, we showed that acridone pigments like epindolidione and quinacridone can photocathodically reduce O 2 to H 2 O 2 , with the results implicating carbonyl groups as the critical catalytic site. 33This motivated us to evaluate carbonyl pigments as potential catalysts: while searching for better photocatalysts we turned our attention to coumarins. 34oumarins have been extensively studied in the past as colorants, and have found application notably in dye lasers. 35Recently we discovered an oxidative ring closure reaction of biscoumarins that can lead to large fused-ring acene-like structures. 36Preliminary observations have suggested that these compounds possess high stability under various conditions.Their general structural formula suggests that strong p-p stacking in the solid state could lead to effective charge transport.The aim of this study was to synthesize such fused biscoumarins and to study their performance in photochemical generation of H 2 O 2 .Herein we would like to present the results of this study.
We have adapted this synthetic scheme (Fig. 1a) to yield pextended structures 8-10 (Fig. 1b).X-ray crystal structures reveal that 8 and 9 are relatively planar extended p systems, while the larger 10 is a propeller-shaped molecule (Fig. 1c).In this work, we nd that these large acene-like molecular semiconductors that are intrinsically stable and display impressive semiconductor catalytic properties in a wide pH range: 2-13.The two electron-withdrawing coumarin units ank the carbocyclic core of the molecule, resulting in effective stability against oxidative degradation.The electron-poor coumarin substituents also lead to favourable n-type conduction properties, and quasi-reversible reduction in electrochemistry, with irreversible oxidation.Increasing the size of the molecular core from a pentacene to a dibenzo[a,f]heptacene core raises the HOMO level and yields p-type behavior as well, leading to overall ambipolar transport and both quasi-reversible reduction and oxidation.We have discovered that visible light irradiation of thin lms of materials 8-10 in pure water leads to H 2 O 2 photosynthesis.Based on photoelectrochemical measurements we identied that H 2 O 2 evolution proceeds via photoreduction of dissolved O 2 , accompanied by water oxidation.In our work we have found that our biscoumarin compounds are the rst carbonyl pigments to achieve this photocatalytic transformation, only g-C 3 N 4 derivatives were previously reported to give this catalytic functionality. 31,32,37The catalytic efficiency afforded by 8-10 in terms of quantity of H 2 O 2 evolution rate per amount catalyst exceeds g-C 3 N 4 by at least a factor of 2-4Â.Aside from pure photocatalysis, materials 9 and 10 are suitable for fabrication of semiconductor photoelectrodes capable of anodic or cathodic photoelectrocatalysis.This way electrical power can be harvested alongside photosynthesis of H 2 O 2 .Depending on pH and applied potential, either photoanodic or photocathodic modes are accessible, and operation under continuous illumination for at least twelve days in a two-electrode photochemical cell is possible.From the point of view of H 2 O 2 photosynthesis, 8-10 have a signicant advantage over the known inorganic catalysts such as ZnO, namely the latter can only function at neutral pH.These ndings of novel catalytic and electrochemical functionality in aqueous conditions is enabling both for application at the interface with biology as well as catalysis, both new frontiers for small molecule semiconductors.

Results and discussion
2.1.Synthesis and solid-state structure Following the previously developed general strategy 36 we carried out double Knoevenagel condensation of compound 1 with esters 2, 3 and 4 to obtain biscoumarins 5-7 in good yields (Scheme 1).Subsequently, all three biscoumarins were subjected to UV irradiation, which causes double 6p-electrocyclization 38 combined with concomitant oxidation of the intermediates to aromatic compounds 8-10.This two-step strategy allowed us to prepare three structurally unique heterocyclic pigments 8-10 in overall yields in the range 29-38% (Scheme 1).It is noteworthy that in the case of biscoumarin 7 full regioselectivity was observed in the nal cyclization step and reaction occurred exclusively at position 1 of the naphthalene substituent.Very poor solubility of compounds 8-10 precluded performing NMR studies and the identity and purity were established based on high-resolution mass spectrometry and combustional analysis.Compounds 8-10 were puried by repeated temperature gradient sublimation in a vacuum of <1 Â 10 À6 mbar.The main impurities were found to be uncyclized precursor or singly-cyclized derivatives.As expected, these impurities were more volatile than the desired fully-cyclized molecules and traveled to lower temperature zones of the tube.Single crystals were grown at a pressure of 1 atm by sublimation of puried material in a stream of N 2 .The crystal growth zone had a temperature of 120-150 C. Experimental and crystallographic details can be found in the ESI.† Crystal packing shows pillars held together by p-p stacking interactions (distance between mean planes of packed molecules are comparable: 3.35(5) Å in 8, 3.38(9) Å in 9 and 3.80(1) Å in 10).8 and 9 pillars are built of alternating molecules that are ipped with respect to the mean molecular plane.Neighboring columns are kept packed through weak dipole-dipole contacts, all with the same orientation in 8 and 10 or with a herringbone layered pattern in 2. The functionalization with uorine atoms changes packing symmetry due to a different superimposition area between molecules stacked on the same pillar and a signicant tilt angle change between neighboring molecular columns (less than 1 in 8 and almost 18 in 9).In contrast with highly planar 8 and 9, peripheral benzene rings in 10 break planarity due to steric repulsion of proximal hydrogens and slightly bend the polyaromatic scaffold of the molecule.This leads to a longer p-p packing distance in 10 compared with the more planar molecules, though as it will be shown later this apparently does not have a detrimental effect on charge transport.

Optical and electrochemical properties
UV-Vis absorption and photoluminescence spectra were recorded for 8-10 in dilute solution in toluene as well as for sublimated thin lms (Fig. 2).All three materials in thin lms demonstrated bathochromic shis in absorption of roughly 50 nm, accompanied with broadening of the absorption features.Nevertheless a clear vibronic structure is apparent in both solution and solid-state spectra.While there is a very small Stokes shi in solution, in solid-state the luminescence is markedly red-shied, broad, and weak, signaling that luminescence may primarily originate from defect states in the polycrystalline lms rather than molecular excitons.The optical band gaps for each material, estimated from absorption onset, were very similar, around 2.1-2.3 eV (Table 1).The stronger absorption in the green region for 10 results in the apparent red colour of the pigment, in contrast to 1 and 2, which appear yellowish-orange.While the optical properties of the three p-extended biscoumarins are not remarkably different from each other, their electrochemical behaviour varies considerably (Fig. 3).We measured cyclic voltammetry (CV) of thin lms evaporated on indium tin oxide substrates in acetonitrile electrolyte under inert (N 2 ) conditions.8 and 9 demonstrated two quasi-reversible reduction peaks, with irreversible oxidation.The tetrauorinated 9, as can be expected from the electronegativity of the uorine atoms, affords a reduction onset 500 mV more positive than 8.In addition to this substantial lowering of the LUMO level, the four uorine atoms in 9 also lower the HOMO level by 200 mV with respect to 8. Compound 10, in contrast, gave not only quasi-reversible reduction but also quasi-reversible 2-electron oxidation.This can be rationalized by the presence of the larger extended electron-rich p-system in this compound.The Frontier molecular orbital energies are estimated (values shown in Fig. 3) assuming an NHE electrode value of À4.75 eV on the Fermi scale. 39

Semiconducting properties
In order to evaluate the electrical properties of 8-10, we fabricated eld-effect transistors (FETs) with the device structure shown in Fig. 4a.Compounds 8 and 9 demonstrated n-type transport, with electron mobility of 0.06 and 0.004 cm 2 V À1 s À1 , respectively (Fig. 4b and c).Hole transport was not measurable with these two materials under any conditions.In contrast, 10 afforded well-balanced n-and p-type mobility.With aluminium source-drain contacts, enhanced n-type behaviour was observed (Fig. 4d), while higher workfunction gold source-drain electrodes allowed the fabrication of a device showing ambipolar behavior (Fig. 4e).The observed transport polarity for the three materials corresponds logically to the electrochemical results: 8 and 9 show only reversible reduction, and thus only n-type behavior, while 10, with both reduction and oxidation accessible electrochemically, supports   ambipolar transport.The LUMO level of each material is too high-energy to observe air-stable electron transport 40 in OFETs, however p-type transport in 3 was found to be stable over 150 days of measurements, where the devices were stored and measured in ambient conditions.Mobility declined to 0.04 cm 2 V À1 s À1 aer a few days and remained stable at this value (ESI S2 †).

Aqueous photo(electro)catalysis
Applications of organic semiconductors have oen been plagued with stability problems, and considerable attention has been devoted to producing materials that are stable with respect to oxygen and water. 34It is therefore not surprising that organic semiconductors are not oen explored for aqueous catalytic applications, in stark contrast to inorganic semiconductors, which are a major focus of renewable energy research. 41,42We observed that thin lms of 8-10 in pure 18 MU water under ambient conditions, irradiated with a commercial white LED (30 mW cm À2 ), produced H 2 O 2 , with evolution rates of 1-3 mg H 2 O 2 per mg of compound per hour (Table 1).The amount of H 2 O 2 was quantied using the tetramethylbenzidine/ horseradish peroxidase assay (TMB/HRP).Since the only available reagents are water and dissolved atmospheric oxygen, the apparent reaction is 2-electron reduction of O 2 to H 2 O 2 , with simultaneous oxidation of water.The oxidation of the semiconductor itself, as a sacricial donor, can be excluded on the basis that the amount of peroxide produced exceeds by at least a factor of ten the total molar quantity of semiconducting molecules in the lm.The only precedent from the literature is a g-C 3 N 4 derivative that was recently reported to achieve this catalytic transformation. 31In terms of catalytic performance, 8-10 are all at least twice as efficient as this previous report, which is shown for comparison in Table 1.In these reports and our experiments only pure water is used, though in our case we perform the experiment in ambient air instead of under 1 atm of pure oxygen, further underscoring the better performance of the biscoumarin molecules.The question of the identity of the anodic process of water oxidation will be elucidated by photoelectrochemical measurements discussed in the following.Aer verifying the photocatalytic potential for 8-10 and determining that 9 and 10 had the best performance in terms of H 2 O 2 photosynthesis, we moved on to measure the photoelectrocatalytic properties of these two materials (Fig. 5).To fabricate photoelectrodes, we evaporated 220 nm thick lms of 9 or 10 on uorine-doped tin oxide (FTO).Measuring linear sweep voltammetry with modulated on/off illumination (Fig. 5a) reveals clearly enhanced photoanodic performance for 9 versus a better photocathodic performance for 10.At low pH, both materials give photocathodic currents at voltages #0 V vs. Ag/ AgCl.Increasing pH leads to larger photoanodic currents.10 gives the highest photocathodic currents, cyclic voltammetry in the dark and light at different pH shows the crossover from photocathodic to photoanodic regimes as the pH is increased (Fig. 5b) We thus speculate that the dominant photoanodic process present in 8-10 must be peroxide formation followed by hydroxyl radical generation.The photoanodic behaviour is clearly complex and should be a topic of more detailed future studies.

H 2 O 2 -evolving photoelectrochemical cell
We found that over the course of repeated measurements, photoelectrode samples of 9 and 10 on FTO did not degrade or change in performance.Encouraged by the apparent photoelectrochemical stability of the materials, we fabricated a simple photoelectrochemical cell comprising 220 nm lm of 3 on FTO and a platinum counter electrode (Fig. 5c).The device was measured in pH 2 electrolyte using a source meter while being continuously illuminated at 200 mW cm À2 over 12 days.In this conguration, a consistent closed-circuit photocurrent of $3 mA cm À2 was generated.The open circuit potential of this photocell was 200 mV.The anodic reaction on Pt is chloride oxidation to Cl 2 , which bubbles out of solution.Fresh HCl is replenished to the cell every 24 h to ensure continuous operation.This arrangement yields a photoelectrochemical cell that produces photovoltaic power while accumulating hydrogen peroxide and chlorine as chemical products.Not only did the device function without any apparent degradation, the photocathodic current was actually found to increase over the course of 4-5 days before levelling off.This behaviour we attribute to exposure of more catalytically-active sites at the surface, a phenomenon found to occur recently for acridone pigments. 33

Conclusion
Based on combining observations herein with evidence from previous works, we believe we can make some conclusions concerning organic catalysts for H 2 O 2 production: it appears that the presence of duel carbonyl functional groups is the crucial catalytic site that allows the selective 2e À /2H + reduction of oxygen to H 2 O 2 .In our previous ndings 27 on acridones (epindolidione and quinacridone) as H 2 O 2 -evolving photoelectrocatalysts, which contain two aromatic amines and two carbonyl functions, we postulated that the two-electron twoproton photoreduction of the pigment generated a reduced leuco species at the surface, which initiates nucleophilic attack on O 2 in the same manner as is known for the industrial anthraquinone synthesis 43 of H 2 O 2 .We found that N-methylation of epindolidione did not preclude O 2 reduction to H 2 O 2 , suggestion that the NH group is not critical for this reaction, but rather only the carbonyl groups.In the work outlined here, we found that biscoumarin compounds containing two carbonyl functions are photocatalysts for producing H 2 O 2 .In the work of Hirai et al., the authors found that only when g-C 3 N 4 was functionalized with pyromellitic diimide, a unit with 4 carbonyl groups, was H 2 O 2 evolution observedother g-C 3 N 4 materials did not accomplish this reaction. 25This strongly suggests that the well-known quinone-like 2e À /2H + electrochemistry of carbonyl pigments is at the heart of all these observations of H 2 O 2 photosynthesis.Based on this, we outline our proposed mechanism, illustrated for compound 10, in Fig. 6.In conclusion, we have reported a synthetic scheme to a family of organic small molecule semiconductors with competitive semiconductor benchmark performance and novel catalytic ability.
The application of organic semiconductors as catalytic materials in their own right is a nascent eld where the body of knowledge in dye and pigment chemistry can be used to achieve next generation catalytic technologies.We believe the work we have presented will be inspiring to move this concept forward.was added under an argon atmosphere and the resulting suspension was stirred at 100 C for 1 hour.The reaction mixture was then poured into an aqueous solution of 0.2% acetic acid (300 ml), followed by addition of 200 ml of DCM.The resulting mixture was separated and the water phase was extracted 3 times with DCM.Next the organic extracts were combined and dried with anhydrous Na 2 SO 4 .Aer drying with anhydrous Na 2 SO 4 the organic phase was ltered through a short DCVC column and eluted with DCM.Fractions containing biscoumarin were combined and aer concentration, ethanol (30-40 ml) was added.Aer evaporating DCM the remaining organic residue was cooled down.The precipitate was ltered off and washed with ethanol yielding 7 as a yellow solid (293 mg, 54%). 1  General photocyclization of biscoumarins.2 mmol of biscoumarin was dissolved in THF (1 L) and photoirradiated (365 nm) in photoreactor (specially assembled for these reactions, see ESI † for details) for 24 h at room temperature.The reaction volume was reduced to around 200 ml and resulting precipitate was ltered off and washed with THF to give fused derivatives.

Experimental section
Pentacene[5,6,7-cdef:12,13,14-c 0 d 0 e 0 f 0 ]dichromene-2,10dione (8)  mbar) in a custom-built organic thin lm evaporator from resistively-heated alumina crucibles.Rate was controlled by quartz crystal microbalance.For thin lm transistor devices, the semiconductor material was evaporated at a rate of 0.2-0.3Å s À1 to a total thickness of 80 nm.The gate structures comprised aluminum with an anodically-grown 32 nm thick alumina layer passivated with a 20 nm thick tetratetracontane layer prepared as reported previously. 8Aluminium or gold source-drain electrodes were evaporated at a rate of 5-10 Å s À1 through a stainless steel shadow mask to give W/L ¼ 2 mm/60 mm.

Photo(electro)catalysis
Photocatalysis in pure H 2 O (18 MU) was carried out by evaporating 220 nm of 1-3 with a fast rate (2-6 Å s À1 ) onto PET foil.Following coating, 1 Â 1 cm 2 squares were cut out of the foil and placed into a polystyrene 24-well culture plate.Each cell was then lled with a 2 ml volume of water, and the 24-well plate was placed onto an array of white LEDs, with one LED illuminating each well with a light intensity of 30 mW cm À2 .H 2 O 2 concentration was measured using the tetramethylbenzidine/ horseradish peroxidase assay, according to the exact procedure we have previously reported. 33Photoelectrocatalysis experiments were performed using conductive uorine-doped tin oxide (FTO) as an optically transparent substrate, with 220 nm thick evaporated lms prepared in the identical way as for photocatalytic experiments.An IPS Elektroniklabor GmbH PGU 10 V -1 A potentiostat was used for all three-electrode measurements.The FTO/pigment served always as the working electrode, while a graphite rod and Ag/AgCl wire were used as counter and quasireference electrodes, respectively.In two-electrode photoelectrochemical cell experiments, the counter electrode was replaced with a platinum foil and a Keithley 2400 source-meter was used to record current at short circuit conditions (0 V applied).In all aqueous experiments ionic strength of electrolytes was kept constant at 0.1 M by using Na 2 SO 4 with suitable addition of HCl or NaOH to adjust pH.As indicated in the text, three types of light sources were used for different experiments: white LEDs as described before, a tungsten halogen lamp with an irradiance of 60 mW cm À2 and nally for the long-term twoelectrode photoelectrochemical cell we used a 500 W Xe lamp, with a substantial UV component in order to subject our samples to harsh conditions to test their resistance to degradation.

Fig. 1
Fig. 1 (a) Synthesis scheme of p-extended biscoumarins.(b) Molecular structures of the three pigments evaluated in this work, with the acene p system highlighted in green.(c) Molecular structures in the solid state as measured by XRD.(d) Thin-films of the three pigments deposited on plastic foils photocatalytically reduce oxygen to hydrogen peroxide while water is oxidized to hydrogen peroxide, which apparently is further converted to hydroxyl radicals.

Fig. 2
Fig. 2 UV-visible absorption and photoluminescence spectra of 8-10.Black continuous line: absorption of solutions, 0.1 mM in toluene.Black dotted line: photoluminescence of the same solutions.Red continuous line: absorption of evaporated thin films.Red dotted line: photoluminescence of the same films.

Fig. 3
Fig. 3 Cyclic voltammograms of 8-10 deposited on ITO, functioning as the working electrode.Platinum and an Ag/AgCl wire functioned as the counter and quasi-reference electrodes, respectively.0.1 M tetrabutylammonium hexafluorophosphate in CH 3 CN was used as the electrolyte solution.

Fig. 4
Fig.4Thin film field-effect transistors.(a) Schematic of the device configuration used.Aluminum with anodically-grown aluminum oxide was used (oxide thickness: 32 nm), which was passivated with a 20 nm thick layer of tetratetracontane (TTC).Materials 8-10 were evaporated to form 80 nm thick films.Source-drain contacts were then evaporated through a shadow mask to give W/L ¼ 2 mm/60 mm.(b) nFET with material 8, with aluminum source-drain contacts.(c) nFET with material 9, using aluminum source-drain contacts.(d) nFET with material 10, using aluminum source-drain contacts.(e) Ambipolar FET with material 10, using gold source-drain contacts.

Fig. 5
Fig. 5 (a) Photoelectrodes comprising FTO coated with 9 or 10 measured with linear sweep voltammetry with a scan rate of 25 mV s À1 and a white LED (10 mW cm À2 ) modulated at 0.125 Hz.Both photocathodic and photoanodic regimes are present in each material, the former corresponding to oxygen reduction to hydrogen peroxide, the latter to oxidation of hydroxyl ions to peroxide followed by further oxidation to hydroxyl radicals.(b) Cyclic voltammograms of 10 on FTO measured in the dark and with 60 mW cm À2 illumination.(c) 10 on FTO measured in a two-electrode photoelectrochemical cell at pH 2 over 12 days of continuous illumination.The primary anodic reaction on Pt is chloride oxidation to Cl 2 .

Fig. 6
Fig. 6 Proposed mechanism for selective oxygen reduction to H 2 O 2 .