side products in the photocatalytic generation of hydrogen with copper photosensitizers by resonance Raman spectroelectrochemistry †

Institute of Physical Chemistry, Friedrich S 07743 Jena, Germany. E-mail: benjamin.die Leibniz Institute of Photonic Technology Jen Jena, Germany. E-mail: benjamin.dietzek@l University of Stuttgart, Institute of Organi Stuttgart, Germany. E-mail: michael.karnah Centre for Energy and Environmental Chem Lessingstrasse 10, 07743 Jena, Germany † Electronic supplementary information ( the setup, further electrochemical data a spectroelectrochemistry. See DOI: 10.1039 ‡ Both authors contributed equally to thi Cite this: RSC Adv., 2016, 6, 105801


Introduction
For decades, mankind has mainly relied on fossil fuels such as coal, oil and natural gas. 1,2However, concerns regarding the greenhouse effect, environmental pollution and resource depletion have driven the search for renewable and more sustainable energy carriers.In this respect, molecular hydrogen, which is generated by the photocatalytic splitting of water, is considered to be an efficient (high energy density of 119 kJ g À1 ) and clean energy carrier. 3Therefore, great efforts have been made to design suitable photosensitizers, which can be applied for the light-driven production of hydrogen from water by using the energy of the sun. 4,5n order to bring molecular photosensitizer into practice, conventional noble metal based systems need to be replaced by cheaper and more abundant ones. 4,6,7][10] In contrast to traditional homoleptic bisdiimine Cu(I)-complexes this class of photosensitizers enables emission lifetimes in the microsecond range and (together with Fe-based molecular catalysts) high catalytic activities, which can even exceed those of the commonly used Ru(II)and Ir(III)-complexes.
By systematically changing the nature, size and bulkiness of the ligand environment, the heteroleptic copper photosensitizers (CuPS) can be modied in a wide range. 8,9,11,12Most popular are variations of the diphosphine ligand backbone and of the substituents at the diimine ligand in order to tune the energetics and lifetimes of excited states as well as to increase the chemical stability of the complexes. 9,12As a result, [(Xant) Cu(Me 2 phenPh 2 )](PF 6 ) (with Xant ¼ xantphos and Me 2 phenPh 2 ¼ 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, abbr.as 2 within this report) was found as one of the most efficient CuPS so far. 9In combination with [Fe 3 (CO) 12 ] as water reduction catalyst (WRC) and triethylamine as sacricial reductant (SR) 2 achieved a turnover number of 862. 9 Furthermore, it represents the rst fully noble metal-free system which is capable to generate hydrogen by the reduction of protons aer an intermolecular electron transfer from the CuPS to the iron-based catalyst (Scheme 1).
Previous studies revealed, that the excited CuPS* is able to undergo electron transfer with both, the SR (reductive quenching) and the WRC (oxidative quenching, Scheme 1), resulting in CuPS À or CuPS + as intermediates of the catalytic cycle. 9However, oxidative quenching was identied as the dominant pathway for catalytic activity (Scheme 1).Nevertheless, upon oxidative quenching of the photosensitizer a slow decomposition of the initial CuPS during the course of the reaction was observed. 9,12,13Previously, a dissociation of the diphosphine ligand from the heteroleptic CuPS under catalytic conditions was suggested, forming a homoleptic bisdiimine Cu(I)-species, which contributes to the end of catalysis. 12,13So far, the existence of catalytic intermediates or side products arising from the CuPS has only been inferred by using UV-Vis spectroelectrochemistry (UV-Vis-SEC), mass spectrometry and 31 P-NMR spectroscopy. 12,13Therefore, to provide a deeper understanding of the photocatalytic system the present study aims for the verication of the respective side products.To this end and for the rst time resonance Raman spectroelectrochemistry (RR-SEC), which allows for an observation of detailed structurally sensitive vibrational modes that are coupled to an electronic transition, 14 was applied to directly identify different copper species.[17]

Results and discussions
A series of four different copper photosensitizers, consisting of two heteroleptic CuPS and their respective homoleptic counterparts were investigated (Fig. 1).All compounds bear methyl substituents in the 2,9-position of the 1,10-phenanthroline (phen) ligand, as it was found that alkyl substituents at this position are benecial for long excited state lifetimes. 9In contrast, the complexes differ with respect to their substitution at the 4,7-position of the phen ligand (Fig. 1), where additional phenyl groups effect the emission intensity. 18he respective absorption spectra of 1/1 0 and 2/2 0 dissolved in acetonitrile are depicted in Fig. 2 and summarised in Table 1.The strong features between 250 and 350 nm originate from p-p* ligand-centred (LC) transitions within the phen ligands.Furthermore, at wavelengths above 350 nm metal-to-ligand charge transfer (MLCT) transitions cause moderately intense absorption bands.For the heteroleptic complexes 1 and 2 the MLCT transitions involve mainly acceptor orbitals of the phen ligand. 19omparison of 1 and 2 revealed a bathochromic shi of the MLCT and LC bands for 2 due to the increased p-system upon introduction of two additional phenyl substituents at the 4,7-position (+M-effect).Signicant differences are observed for the homoleptic complexes 1 0 and 2 0 , where two diimine ligands contribute to the MLCT transitions causing an additional bathochromic and hyperchromic shi of the respective absorption bands.Moreover, the absorption maxima are considerably red-shied by 78 nm (1 0 vs. 1, 4525 cm À1 ) and 89 nm (2 0 vs. 2, 4831 cm À1 ), respectively, with much higher extinction coefficients.
Emission is observed with maxima at 545 nm for 1 and 552 nm for 2 (Fig. 2).In accordance with literature, 21 the emission of 1 0 and 2 0 is completely quenched in acetonitrile solution.This quenching is explained by the interaction of the solvent with the Cu(II)* metal centre forming an exciplex due to the donor properties of the acetonitrile and offering a pathway for efficient radiationless deactivation. 6,21,22This quenching mechanism appears to be much less pronounced for the heteroleptic complexes 1 and 2, which reects well the inhibition of attening distortion upon photoexcitation and the effective shielding of the copper centre against nucleophilic attack by the bulky and rigid xantphos ligand.
For 1/1 0 and 2/2 0 cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were carried out in acetonitrile solution (Fig. 3A and S1 †).The CVs of 1 0 and 2 0 exhibit a fully reversible oxidation peak at 0.92 and 0.91 V vs. NHE, while the oxidation of the heteroleptic complexes 1 and 2 is irreversible and occurs at higher potentials (Table 1).Combining the electrochemical results with optical spectroscopy the excited state  Table 1 Summary of the photophysical and electrochemical properties of the complexes 1/1 0 and 2/2 0 in acetonitrile solution at room temperature redox potentials can be predicted (Fig. 3B and S1 †). 6,23For this purpose, the energy of the lowest lying excited states is estimated by the intersection of the normalised absorption and emission spectra, i.e. 2.64 (for 1) and 1.87 eV (for 1 0 ).It should be noted that the energy of the excited state determined as described above presents only an estimate as structural reorganization appears in the excited state upon photoexcitation. 19,22,24Nevertheless, both 1* and 1 0 * appear to be potent photoreductants with redox potentials of À1.19 V and À0.95 V vs. NHE, respectively.Similarly, 2* and 2 0 * possess a strong reducing ability as well (À1.17 and À0.90 V vs. NHE, Fig. S1 †).In consequence, the heteroleptic CuPS 1 and 2 appear to be stronger reducing agents in their excited state compared to their homoleptic analogues 1 0 and 2 0 .To structurally characterise the oxidised species and to check for potential side products in the photocatalytic cycle (Scheme 1) 9,12 we aim for resonance Raman spectroelectrochemistry.Therefore, an initial characterisation of 1 0 and 2 0 by UV-Vis-SEC, where comparable spectral features could be observed for both complexes, is mandatory.Upon electrochemical oxidation the intensity of the MLCT absorption band is signicantly decreased (Fig. S2 †).The reversibility of this process was also checked by measuring UV-Vis spectra during a CV (Fig. S3 †).For both complexes the UV-Vis spectra are completely recovered aer a full oxidative cycle, i.e. oxidation followed by rereduction.
The heteroleptic complexes 1 and 2 show decreasing MLCT bands and enhanced as well as red-shied LC bands upon electrochemical oxidation (Fig. 4).However, as expected from our previous work the MLCT bands of 1 and 2 do not fully recover aer the oxidative cycle. 13At the same time new bands centred at 456 (for 1) and 476 nm (for 2) are appearing.Notably, the position and shape of the new bands exactly match with the MLCT bands of the respective homoleptic complexes (Fig. 4, insets).Furthermore, the spectra obtained aer the oxidative cycle both for 1 and 2, can be quantitatively explained as a linear superposition of the spectra of the heteroleptic complexes (1 and 2) and their respective homoleptic counter parts (1 0 and 2 0 ).This is a strong indication that oxidative conditions lead to a ligand exchange reaction, which results in the dissociation of the heteroleptic complexes and the formation of the homoleptic bisdiimine Cu(I)-complexes and generalises the previously obtained results from UV-Vis-SEC of complex 2. 13 Furthermore, when running multiple oxidative CV cycles on the respective heteroleptic complexes the appearance of the redox signals, which can be assigned to the homoleptic compounds, also indicates the formation of the homoleptic species (Fig. S4 and  S5 †).In the meantime, the anodic peak of heteroleptic complexes decreases, signifying the decomposition of 1 and 2 under oxidative condition.
To gain detailed structural insights into the formed reaction products, nally resonance Raman spectroscopy was utilised.Therefore, an excitation wavelength of 473 nm was chosen to selectively obtain the resonance Raman signals of the electrochemically formed side products of 1 and 2. At this wavelength the absorption of 1 and 2 is rather low and, hence, almost no resonance Raman signals are recorded from these starting compounds.As a result, the resonance Raman spectra of 1 and 2 aer electrochemically oxidation closely resemble the corresponding features of the homoleptic counterparts 1 0 and 2 0 (Fig. 5 and S6 †).For both the homoleptic complex 1 0 and the oxidation product of 1 the same resonance Raman bands are observed, i.e. at 1589, 1502, 1439, 1420, 1304, 1293, 1214 and 1146 cm À1 .An electrochemically oxidative cycle of 2 yields Raman bands at 1604, 1571, 1427, 1285, 1269, 1172 and 1101 cm À1 , which coincide with the resonance Raman bands of the homoleptic complex 2 0 .These bands are exclusively associated  with vibrations of the phen ligand, e.g. the breathing modes of the phenanthroline rings contribute to the bands at 1439 and 1420 cm À1 for 1 0 , and 1427 cm À1 for 2 0 . 25Similarly, the methyl deformation relates to the Raman bands at 1017 and 1022 cm À1 as observed for 1 0 and 2 0 , respectively.Furthermore, excitation with 405 nm, which is in the blue-edge of the MLCT band for 1 0 and 2 0 , probes Raman active vibrations, which are coupled to electronic transitions to higher lying excited states.However, the Raman band positions observed with an excitation wavelength of 405 nm are identical to those obtained upon excitation at 473 nm (Fig. S7 and S8 †).Also for 1 and 2 an excitation wavelength of 405 nm falls into the red ank of the MLCT absorption band, and the Raman vibration modes are assigned to phen ligands, which is in agreement with results from DFT calculations, i.e. that the charge transfer occurs from the central Cu(I) to the phen ligands. 19When performing RR-SEC upon 405 nm excitation, only minor spectral changes are observed upon oxidation (Fig. S7 and S8 †), which is in accordance with the UV-Vis-SEC spectral features, that only a decrease of the absorption band occurs but no new features emerge at 405 nm.
Therefore, the structural sensitivity of RR-SEC proves the prevalent degradation pathway of the CuPS 1 and 2 under oxidative conditions.It reveals that dissociation of the xantphos ligand and subsequent formation of the homoleptic complexes 1 0 and 2 0 likely cause the partial deactivation of 1 and 2 under catalytic conditions. 12,13

Conclusions
In summary, the combination of UV-Vis and resonance Raman spectroscopy in conjunction with electrochemistry enabled the decisive proof that the original heteroleptic copper photosensitizer [(P^P)Cu(N^N)] + is transformed to a homoleptic bisdiimine species [Cu(N^N) 2 ] + under oxidative conditions, which are also present during photocatalysis.In particular, resonance Raman spectroscopy in combination with electrochemistry served as a sensitive tool for the identication of the spectral signatures of complexes and side products in catalytic systems.Thereby, a deactivation pathway for the light-driven production of hydrogen by using a heteroleptic copper photosensitizer along with an iron-based water reduction catalyst could be conrmed.

Fig. 1
Fig.1General structure of the heteroleptic Cu(I)-complexes 1 and 2 and their homoleptic analogues 1 0 and 2 0 applied in this study.

Fig. 3 (
Fig. 3 (A) Electrochemical potentials of ground state and excited state of 1 and 1 0 in acetonitrile, vs. NHE.(B) Cyclic voltammetry curves (black) of the complexes 1 and 1 0 in 0.1 M TBABF 4 /acetonitrile.Differential pulse voltammetry (red) were applied for better analysis.

Fig. 4
Fig. 4 UV-Vis spectra of 1 (upper panel) and 2 (lower panel) under open circuit potential (black solid lines), upon electrochemical oxidation (blue solid lines) and after an oxidative cycle (red dashed lines) in 0.1 M TBABF 4 /acetonitrile.The insets show an enlarged view on the spectral region of the MLCT absorption bands.The simulated spectra (green solid lines) are linear combinations of the absorption spectra of the heteroleptic complex and its corresponding homoleptic analogue.The applied RR excitation wavelengths (405 and 473 nm) are displayed as grey vertical lines.