doi.org/10.26434/chemrxiv.6188654.v1

Effective Quenching and Excited-State Relaxation of a Cu(I)Photosensitizer Addressed by Time-Resolved Spectroscopy and TDDFTCalculationsAleksej Friedrich, Olga S. Bokareva, Shu-Ping Luo, Henrik Junge, Matthias Beller, Oliver Kühn, StefanLochbrunner

Submitted date: 26/04/2018 • Posted date: 26/04/2018Licence: CC BY-NC-ND 4.0Citation information: Friedrich, Aleksej; Bokareva, Olga S.; Luo, Shu-Ping; Junge, Henrik; Beller, Matthias;Kühn, Oliver; et al. (2018): Effective Quenching and Excited-State Relaxation of a Cu(I) PhotosensitizerAddressed by Time-Resolved Spectroscopy and TDDFT Calculations. ChemRxiv. Preprint.

Homogenous photocatalytic systems based on copper photosensitizers are promising candidates for noblemetal free approaches in solar hydrogen generation. To improve their performance a detailed understandingof the individual steps is needed. Here, we study the interaction of a heteroleptic copper (I) photosensitizerwith an iron catalyst by time-resolved spectroscopy and ab-initio calculations. The catalyst leads to ratherefficient quenching of the 3MLCT state of the copper complex, with a bimolecular rate being about three timessmaller than the collision rate. Using control experiments with methyl viologen an appearing absorption bandis assigned to the oxidized copper complex demonstrating that electron transfer from the sensitizer to the ironcatalyst occurs and the system reacts along an oxidative pathway. However, only about 60% of the quenchingevents result in an electron transfer while the other 40% experience deactivation indicating that thephotocatalytic performance could be improved by optimizing the intermolecular interaction.

File list (1)

download fileview on ChemRxivCuPS.pdf (1.10 MiB)

Effective quenching and excited-state relaxation of a Cu(I) photosensitizer addressed by time-resolved spectroscopy and TDDFT calculations

Aleksej Friedrich[a], Olga S. Bokareva[a],*, Shu-Ping Luo[b,c], Henrik Junge[c], Matthias

Beller[c], Oliver Kühn[a], Stefan Lochbrunner[a],*

[a] Institute of Physics, University of Rostock, Albert-Einstein-Str. 23-24, 18059 Rostock, Germany

[b] State Key Laboratory Breeding Base of Green Chemistry– Synthesis Technology, Zhejiang University of Technology, 310014 Hangzhou, China

[c] Leibniz Institute for Catalysis, Albert-Einstein-Str. 29a, 18059 Rostock, Germany

Abstract

Homogenous photocatalytic systems based on copper photosensitizers are promising candidates for noble metal free approaches in solar hydrogen generation. To improve their performance a detailed understanding of the individual steps is needed. Here, we study the interaction of a heteroleptic copper (I) photosensitizer with an iron catalyst by time-resolved spectroscopy and ab-initio calculations. The catalyst leads to rather efficient quenching of the 3MLCT state of the copper complex, with a bimolecular rate being about three times smaller than the collision rate. Using control experiments with methyl viologen an appearing absorption band is assigned to the oxidized copper complex demonstrating that electron transfer from the sensitizer to the iron catalyst occurs and the system reacts along an oxidative pathway. However, only about 60% of the quenching events result in an electron transfer while the other 40% experience deactivation indicating that the photocatalytic performance could be improved by optimizing the intermolecular interaction.

1. Introduction

The rapid technical progress and the increasing energy demand of modern human society leads to an accelerated depletion of exhaustible resources[1]. To cope with this situation, alternative energy sources must be explored. In the recent decades, substantial progress on wind and solar energy has been achieved. However, this energy has limited possibilities to be stored and reused on demand. A reasonable solution would be to switch to hydrogen as an energy storage reservoir[2–4] . In 1972, the principal capability to obtain hydrogen directly by light-driven decomposition of water was demonstrated[5]. Nowadays, there exist various photocatalytic water-splitting systems based on precious metals (for reviews, see[6–9], for other studies suggesting the use of simple redox-active chromophores such as quinones, see ref. [10] ). In contrast, noble-metal free systems are not as widely explored, for selected examples, see[11–13].

To make further progress in developing novel photocatalytic systems with good performance, high efficiency, and long-term stability, detailed investigations of the underlying elementary physical and chemical processes are necessary since the rational design of the systems calls for

a microscopic understanding[14]. Knowledge on peculiarities of the single steps in the entangled photo-activated catalytic cycle such as absorption of light, electron transfer, and oxidation/reduction reactions will facilitate targeted changes and optimizations of the existing systems. In multi-component systems, the efficiency depends primarily on the electron transfer between various components. Very often, the quenching rates reported for catalytic systems are quite high, but it should be taken into account that electron transfer is only one possible channel of quenching. Exemplarily, for catalytic systems, energy transfer and collisional deactivation might also be possible. For instance, in ref. 39 we found for the case of an Ir(III) photosensitizer that electron transfer is inhibited by the energetic mismatch between the relevant electron states. Despite this fact collisions with an iron catalyst result in efficient quenching. For this reason, it is of high importance to differentiate between electron transfer and other processes.

In this paper, we study a homogeneous noble-metal free system comprising of iron carbonyl [HFe3(CO)11][HNEt3] (Et = CH2CH3) (Fe-cat) as water-reduction catalyst[15] and a heteroleptic copper(I) photosensitizer (CuPS)[16,17] comprising of a xantphos and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline ligand[13,18] (for the structure, see Fig. 1). The electron transfer from the CuPS to the Fe-cat is addressed by means of time-resolved spectroscopy and quantum-chemical calculations. Methyl viologen was applied to identify the spectral signatures of the oxidized CuPS appearing in the quenching pathway.

The paper is organized as follows. First, we describe the applied experimental and theoretical methods and proceed then with the UV/Vis spectra and the results of the nanosecond transient absorption experiments. After that, we provide an analysis of the time-resolved spectra by ab-initio calculations and reference experiments with methyl viologen and estimate the efficiency of the quenching and electron transfer processes. Finally, the obtained results are summarized, and conclusions concerning the performance of the photocatalytic system are given.

Figure 1. Chemical structure of CuPS and Fe-cat.

2. Experimental Methods

Materials and sample preparation The copper (I) photosensitizer CuPS and the iron catalyst [HNEt3][HFe3(CO)11] (Fe-cat) were synthesized as described in previous work[15,16]. Acetonitrile was of spectroscopic grade (Merk) and the water was bidestilled received from Carl Roth GmbH. Methyl viologen (MV) dichloride hydrate (98%) was purchased from Sigma Aldrich. Oxygen was removed from the solvents by bubbling with argon for at least 10 minutes.

The samples were prepared applying the Schlenk air-free technique under argon atmosphere. All studies were performed at room temperature (20°C).

Spectroscopic measurements Steady-state UV/Vis-absorption spectra were recorded with a Specord 50 spectrophotometer (Analytik Jena). Time-resolved emission spectroscopy was performed by means of a streak camera system (Streakscope C10627, Hamamatsu). In doing so the samples were excited with 200 fs long laser pulses at 388 nm generated by frequency doubling the output of a Ti:sapphire laser system (CPA-2001, Clark MXR). Transient absorption measurements on the nano- and microsecond timescale were performed by means of a pump-probe setup based on a femtosecond white light continuum for probing and a YAG laser system for excitation. The probe was generated by focusing a small fraction of the Ti:sapphire output into a 5 mm thick CaF2-crystal which resulted in a continuum extending from 330 nm to 730 nm. Excitation pulses at 390 nm with a duration of 7 ns were generated by an optical parametric oscillator, which was pumped by a Nd:YAG laser (NT-240, Ekspla) and operated at 616 nm, and subsequent sum frequency mixing with the fundamental at 1064 nm. The delay between the pump and probe pulses was adjusted by means of an electronic delay generator (DG645, Stanford Research Systems). The pulses were focused onto the sample to overlapping spots with diameters of approximately 350 µm for the pump and 100 µm for the probe. The probe beam was thereafter recollimated and dispersed by a fused silica prism and its spectrum recorded by CCD array. The sample solutions were investigated in fused silica cells with an optical path length of 2 mm.

3. Computational details

Complex metal-organic systems with various intra- and intermolecular charge-transfer processes in the ground and excited electronic states require special computational approaches. The size of CuPS restricts the choice only to the popular density functional theory (DFT) approach. However, many standard functionals suffer from incorrect asymptotic behavior and are thus unable to adequately model long-range charge transfer. The remedy is to introduce a portion of exact exchange. A reliable description of frontier-orbitals dependent properties such as absorption spectra, ionization potentials, and fundamental and optical gaps can be achieved within the optimally-tuned long-range corrected density functional approach (LC-BLYP)[19–21]. In this method, a switching between short- and long-range parts of the exchange-correlation potential is implemented and thus the fulfillment of Koopmans’ theorem guaranteed. The switching is controlled by a system-specific non-empirical parameter ω, which is evaluated within the so-called ∆SCF technique[22–24]. The optimally-tuned LC-BLYP method has been previously used for studying a photocatalytic system based on IrPS [8,25–28], for computational issues see ref. [25]. For CuPS, the value ω = 0.15 bohr−1 has been determined and applied for all calculations in the present study. This value is of the same order as for IrPS[25] which can be rationalized by the fact that the inverse of ω correlates with the system size. The standard values of 0.33 and 0.47 bohr-1, deduced from test computations of small molecules and used as default parameters in most quantum-chemical programs are notably larger.

All calculations have been performed using the Gaussian09[29] program suite with the LANL2DZ ECP[30] basis set for Cu and Fe and the 6-31G(d)[31,32] basis for all other atoms.

In DFT/TDDFT computations, no symmetry restrictions were applied. The geometry has been optimized adapting the structure reported in Ref. [33].The environment effects due to the solvent acetonitrile were included via the Polarizible Continuum Model (IEFPCM)[34]. For the broadening of TDDFT stick spectra, a Gaussian line width of 0.1 eV was applied as universal parameter.

4. Results and Discussion

4.1 UV/Vis steady state spectra

The absorption spectrum of the copper photosensitizer, CuPS, dissolved in acetonitrile (ACN) exhibits a strong absorption band with a maximum at 285 nm and a weaker band with a maximum at 390 nm, see Fig. 2a. According to the theoretical calculations (Fig. 2b), both bands consist of numerous transitions of different nature. Ligand-centered (LC) transitions are clearly dominant compared to the metal-to-ligand charge-transfer (MLCT) transitions. The luminescence spectrum consists of only one unstructured band with a maximum at 575 nm. For comparison, the theoretical 𝑻𝟏 → 𝑺𝟎 transition energy corresponds to 511 nm. Due to the complexity of the molecule, vibronic progressions have not been considered and only the 0-0 transitions were calculated.

Figure 2. a) Experimental absorption and fluorescence spectrum of CuPS in ACN. b) Comparison between experimental and simulated absorption spectrum of CuPS in ACN. The calculated stick spectrum with assignment of the transitions is also provided.

4.2 Nanosecond transient absorption

Transient absorption spectra of CuPS, dissolved in ACN with a concentration of 3.5×10-4 M, were recorded after optical excitation at 390 nm on the nano- to microsecond timescale (see Fig. 3a). They show a broad absorption band with a maximum at 530 nm and no negative signatures. This implies that in the considered spectral range the excited state absorption (ESA) is stronger than the ground state bleach. The spectrum is identical to the one observed in femtosecond absorption experiments for delay times longer than 20 ps and which was assigned

to the relaxed 3MLCT state of the CuPS.[35] The present experiments show that the signal decreases with an exponential decay time of 280 ns to zero.

Figure 3. Transient absorption spectra at selected delay times. a) CuPS in ACN, b) CuPS and Fe-cat in ACN, c) CuPS und MV dissolved in a mixture of ACN and H2O. The sharp signatures at 390 nm and 420 nm, respectively, originate from scattered excitation light.

The time dependence of the transient absorption measurements is globally analyzed by fitting the model function 𝐹 𝜆, 𝑡 = 𝐷𝐴𝑆. 𝜆 ⋅ exp(−t/𝜏.) + 𝐷𝐴𝑆; 𝜆<

.=> to the transient spectra. Here, λ is the probe wavelength and t the delay time. Further, ti represent exponential decay times and DASi decay associated spectra (DASs), which reflect the spectral shape of the corresponding exponential components. DAS¥ is a residual, long living contribution which has to be included in the case of the measurements presented below. The number n of fit

components was chosen as the minimal number to obtain satisfactory agreement between measurements and fits.

For the pure CuPS solution one exponentially decaying component with the constant given above is sufficient to accurately reproduce the data (see Fig. 4a). Time resolved emission measurements with the streak camera revealed within the experimental accuracy the same time constant for the decay of the CuPS phosphorescence. These findings indicate that the 3MLCT state of CuPS exhibits in ACN a lifetime of 280 ns and relaxes directly back to the electronic ground state.

Figure 4. Time traces according to Fig. 3 at the indicated probe wavelengths. The circles represent the measured data and the lines the fits from the global analysis. a) Monoexponential fit for pure CuPS in ACN; b) biexponential fit for CuPS and Fe-cat in ACN; c) biexponential fit for CuPS and MV in an ACN-water mixture.

To investigate the interaction between CuPS and the catalyst a second set of measurements was performed with 5×10-4 M of Fe-cat added to the CuPS solution (CuPS/Fe-cat solution, see Fig. 3b). After optical excitation, the mixed solution shows the same ESA signatures as the pure CuPS solution except at wavelengths below 380 nm (cf. Fig 3a and Fig 3b). The differences are caused by the absorption of the Fe-cat in this spectral region which suppresses the probe light. The ESA signal decreases in the solution with the Fe-cat faster than in the pure CuPS solution indicating that a quenching process is at work. In addition, the transient absorption of the mixture does not drop to zero in contrast to the pure CuPS solution. The residual absorption exhibits a local maximum at 443 nm and a broad band around 600 nm. Both persist for several microseconds and should result from products of the quenching process, e.g. from CuPS2+ if electron transfer contributes to the process.

To characterize the signal evolution by the global fit two exponentially decaying components had to be included (see Fig. 4b). The dominant fraction of the transient absorption and in particular the ESA band characteristic for the 3MLCT state decay with a time constant of 100 ns, which reflects the 3MLCT lifetime in the presence of the Fe-cat. In measurements with the streak camera the same time constant is observed for the decay of the phosphorescence supporting the assignment. The fit to the absorption data revealed in addition a second decay component with a time constant of 2.2 µs. It is related to the depopulation or decomposition of the quenching products but the process itself is not considered in more detail in the following.

To identify the origin of the product bands, additional measurements were performed with solutions containing beside of CuPS, methyl viologen (MV) which is known as a strong electron acceptor.[36–40] The change from MV2+ to MV+ due to an intermolecular electron transfer process modifies the absorption spectrum of MV and leads to characteristic bands in the visible. In Fig. 3c, transient absorption spectra are shown for a solution of 3.5×10-4 M CuPS and 3.3×10-

2 M MV in a ACN-water mixture with a volume ratio of 5:1 (CuPS/MV solution). The water fraction is needed to dissolve the applied MV salt [MV][Cl2]. The broad ESA of the optically excited CuPS decreases now with a decay time of 40 ns. Thereafter a rather long living signal is visible for several microseconds, which is assigned to the products of the fast process and exhibits a similar behavior as in the solution with the Fe-cat. The time resolved absorption measurements of the CuPS/MV solution were again analyzed by the global fitting procedure (see Fig. 4c). The data can be well reproduced by two exponentially decaying components with time constants of 40 ns and 2.4 µs and a residual long living contribution.

4.3 Estimation of quenching efficiency

In order to quantify the yield of the electron transfer step we start out with analyzing the quenching efficiency 𝜃 which results from the measured decay times as follows:

𝜃 = 1 − ABAC= DB

DBEDC (1)

The intrinsic relaxation rate 𝑘G =>AC

of the CuPS 3MLCT state is the inverse of its lifetime 𝜏G

in the ACN solution without quencher. The lifetime 𝜏H in the presence of a quencher

corresponds to a quenching rate 𝑘H =>AB− >

AC which describes the depopulation of the CuPS

3MLCT state by interaction with the quencher, i.e. Fe-cat or MV, respectively. The 3MLCT lifetimes were, as already mentioned, found to be 280 ns and 100 ns, respectively, resulting in a quenching rate due to the Fe-cat of 𝑘H = 6.4 ∙ 10N𝑠P> and a quenching efficiency of 𝜃 =64%. With the Fe-cat concentration a bimolecular interaction rate of 𝑘R. = 𝑘H Fe-cat = 1.2 ∙10>G 𝑀𝑠 P> is obtained.

Previously, in the case of a homoleptic Cu(I) photosensitizer a rate of 𝑘H = 3.6 ∙ 10N s-1 was observed for quenching by the catalytic complex Co(dmgH)2(py)Cl at a concentration of 5×10-4 M which corresponds to a bimolecular rate of 0.7 ∙ 10>G(Ms)P>.[41] This value is of same order of magnitude as the quenching rate found in our study. Neubauer et al. investigated the interaction of the Fe-cat with a homoleptic Ir(III) photosensitizer in a tetrahydrofuran solution and measured a bimolecular quenching rate of 𝑘R. = 6.6 ∙ 10>G(Ms)P>.[42] The value is about a factor of five higher than in the present case. This results from the fact that the dielectric constant of tetrahydrofuran is smaller than that of ACN leading to a weaker screening and stronger attraction of the oppositely charged ions. Despite the large quenching rate, electron transfer from the Ir photosensitizer to the catalyst has not been observed, which was attributed to the energy mismatch between donor and acceptor states.39 This demonstrates that quenching by the catalyst can be caused by various processes.

The collision rate 𝑘]^__ between CuPS (c = 3.5×10-4 M) and Fe-cat (c = 5×10-4 M) was estimated by applying the Debye equation and taking the ionic charges of the compounds into accout:[43]

𝑘]^__ =`abDcd∙Ref ghP>

with𝑏 = mnompqrgCs

tu(vnoEvpqr)wxwCDcd (2)

𝑁z denotes the Avogadro and 𝑘{ the Boltzmann constant, T the temperature that was taken as 293 K, 𝑒G the elementary charge, and 𝜀G the permittivity of the vacuum. The viscosity 𝜂 and the dielectric constant 𝜀� of the solvent ACN amount to 3.68×10-4 Pa×s and 36, respectively. The charge numbers of the CuPS 𝑍�� and of the Fe-cat 𝑍]�� are +1 and -1. The radii 𝑅�� and 𝑅]�� of the sensitizer and the catalyst were determined to 5.7 Å and 3.0 Å, respectively, by calculating the radii of gyration. With these quantities and the given concentrations a collision rate of 𝑘]^__ = 1.9 ∙ 10� s-1 results from Eq. 2. Comparing this value to the quenching rate 𝑘H indicates that only every third collision leads to a quenching event of the excited CuPS by the catalyst.

4.4 Verification of the oxidative quenching pathway

In the following reference experiments using MV2+ as electron acceptor are discussed since they allow to assign the absorption bands observed after the quenching step and to verify that oxidative electron transfer from the CuPS+ to the Fe-cat is involved. In this and the following section, we indicate explicitly the charge state of the CuPS to discriminate between the original CuPS+ cation and the electron transfer product CuPS2+. As mentioned above the time evolution of the transient absorption of the CuPS/MV solution can be described by a 40 ns and 2.4 µs decay component as well as a long living contribution. Fig. 5a shows the sum of the DASs of the 2.4 µs and the long-lasting component (blue curve) which should represent the spectra of the species resulting from the electron transfer of the CuPS+ to MV2+. It is compared to the well-known spectrum of MV+ (red curve)[36–39] and to the 2.2 µs component of the CuPS/Fe-

cat solution (black curve). In the spectral region between 400 nm and 600 nm, the spectra of MV+ and the long-lasting components of the CuPS/MV solution are very similar. The only pronounced difference appears around 443 nm where the MV+ spectrum has a minimum. Here, the long living absorption of the CuPS/MV solutions exhibits a weak but clearly visible band. Since obviously MV+ and thereby also CuPS2+ are formed by the interaction of MV2+ with the optically excited complex CuPS+* the band at 443 nm has to be assigned to the electron transfer product CuPS2+. This is in good agreement with the predicted difference spectrum between the calculated ground state absorptions of CuPS+ and CuPS2+ (see Fig. 5b). The band is only present in the spectrum of the oxidized CuPS2+ species but not in the spectrum of the original CuPS+. The inset of Fig. 5b) shows the transition density difference associated with this band. It corresponds to a MLCT state with electron density transferred from the d-orbitals of the central copper ion to the π-orbital of the N-ligand.

Figure 5. a) Red curve: absorption spectrum of MV+ calculated for a concentration of 2.5×10-4

M with the data provided in ref. [38]; blue and black curves: spectra of the long living components for the CuPS/Fe-cat and CuPS/MV solution obtained by global fits to the transient absorption data. b) Difference absorption spectrum for oxidation of CuPS (CuPS2+−CuPS+) as predicted by theory. The inset shows the transition density difference for the marked feature at around 450 nm. Red and blue colors correspond to electron and hole density, respectively.

The absorption band at 443 nm is found as well in the corresponding spectrum of the CuPS/Fe-cat solution implying that electron transfer from the CuPS+ to the Fe-cat takes place after optical excitation of the CuPS+.

4.5 Electron transfer efficiency and molar extinction coefficient of CuPS2+

The efficiency ϑ of the electron transfer can be calculated as the ratio between the concentrations of the transfer product CuPS2+ and optically excited CuPS+*:

𝜗 = [����s�][�����⋇]

(3)

In order to obtain the concentration of CuPS2+ in the transient absorption measurements on the CuPS/Fe-cat solution, first the molar extinction coefficient of CuPS2+ has to be determined. In

the experiments on the CuPS/MV solution the concentration of the generated CuPS2+ can be obtained by making use of the known extinction coefficient of MV+. The MV+ ions result from the electron transfer reaction CuPS+*+MV2+ → CuPS2++MV+ and the concentrations of CuPS2+ and MV+ have to be the same. By comparing the spectrum of the long living component in the case of the CuPS/MV solution with the absorption spectrum of MV+, as it is reported in reference[38], the concentrations were found to be [MV+] = [CuPS2+] = 2.5·10-5 M (see Fig. 5a). The maximum of the absorption band at 443 nm, which is due to CuPS2+, was measured to be 11.8 mOD. This results in a molar extinction coefficient of CuPS2+ of 𝜀CuPS2+ = 2360 (M·cm)-1 in the band maximum. With this extinction coefficient, the CuPS2+ concentration in the CuPS/Fe-cat experiments was then determined from the long living absorption signal at 443 nm to [CuPS2+] = 4.2·10-6 M.

The concentration of optical excited CuPS+* which is needed to evaluate Eq. 3 was determined from the parameters of the excitation pulses. From the excitation wavelength 𝜆 = 390𝑛𝑚, the pulse energy 𝐸 = 7.2 ∙ 10P�𝐽, and the focus diameter 𝑑 = 350𝜇𝑚 an area density of incident photons of 𝐼��^�^< = 4𝐸𝜆 𝜋𝑑�ℎ𝑐 = 1.47 ∙ 10>�𝑐𝑚P� results. With the molar extinction coefficient 𝜀 390𝑛𝑚 = 5700 𝑀 ∙ 𝑐𝑚 P> of CuPS+ at 390 nm and the corresponding absorption cross section 𝜎 390𝑛𝑚 = 𝜀 390𝑛𝑚 ∙ 1000 ∙ ln 10 𝑁z = 2.18 ∙ 10P>�𝑐𝑚� an excitation probability of: 𝑃 = 𝜎���� ∙ 𝐼��^�^< = 3.2% is obtained. From this value and the CuPS concentration of [𝐶𝑢𝑃𝑆E]= 3.5 ∙ 10Pt𝑀 a concentration of optically excited CuPS+* of 𝐶𝑢𝑃𝑆E∗ = 𝑃 ∙ 𝐶𝑢𝑃𝑆E = 1.12 ∙ 10P�𝑀 is found.

Thus the electron transfer efficiency can be estimated as:

𝜗 = [𝐶𝑢𝑃𝑆�E] [𝐶𝑢𝑃𝑆E∗] = 4.2 ∙ 10PN𝑀 1.12 ∙ 10P�𝑀 = 38%

which has to be compared with the total quenching efficiency of the Fe-cat of 64%. It follows that 26% of the excited CuPS are deactivated by collisions with the Fe-cat without electron transfer. Another fraction of 36% decays back to the electronic ground state by intramolecular relaxation processes. In summary, only 38% of the excited CuPS contribute to the photocatalytic reaction.

The found electron transfer efficiency seems to be in a range which is also encountered in other electron transfer studies on metal complexes. However, publications providing explicitly the efficiency per quenching event are rare. In the case of a series of heteroleptic CuPSs interacting with MV transfer efficiencies between 4% and 21% were observed as one can deduce from the reported rates.[40] Khnayser et al. present a system consisting of [Ru(bpy)3]2+ as PS, ascorbic acid as electron donor, and [Co(bpyPY2Me)(CH3CN)(CF3SO3)](CF3SO3) (bpyPY2Me = 2-bis(2-pyridyl)(methyl)-methyl-6-pyridylpyridine) as catalyst and report an overall electron transfer efficiency from the PS to the catalyst of 47%.42 In this case the PS is reductively quenched by the electron donor and the electron is then transferred to the catalyst in a second step.

5. Summary and Conclusions

The behavior of a heteroleptic copper (I) photosensitizer in a homogenous photocatalytic system for hydrogen generation was investigated by time-resolved spectroscopy on the nanosecond timescale and by ab-initio calculations. It was found that in the presence of the iron

catalyst with a concentration of 5×10-4 M the 3MLCT lifetime of CuPS+ reduces from 280 ns in pure ACN to 100 ns. The corresponding quenching rate is thereby about three times smaller than the collision rate. With the help of control experiments applying MV as electron acceptor it is shown that an absorption band at 443 nm appearing in the transient spectra at longer times is caused by CuPS2+. This is a clear signature for an oxidative quenching pathway and an electron transfer from CuPS+ to the Fe-cat. However, the quantitative analysis reveals that only about 60% of the quenching events are associated with an electron transfer, while the others just deactivate the excited CuPS.

The results indicate that the performance of this photocatalytic system can be improved by optimizing the electronic interaction and energetic matching of donor and acceptor levels in the collision complex of the CuPS and the Fe-cat by ligand modification. This affects the overal efficiency for electron transfer twofold: First, the number of collisions resulting in deactivation is reduced. Second, the bimolecular electron transfer rate is increased compared with the intramolecular relaxation rate.

Acknowledgements

This work was financially supported by the European Union (European Social Funds, ESF) via the project "PS4H", by the Federal Ministry of Education and Research of Germany via the project "Light to hydrogen" (L2H) which was part of the program "Spitzenforschung und Innovation in den Neuen Ländern", and by the German Research Foundation via the collaborative research center SFB 652 "Strong correlations and collective effects in radiation fields: Coulomb systems, clusters and particles".

References

[1] Q. Schiermeier, J. Tollefson, T. Scully, A. Witze, O. Morton, Energy alternatives: Electricity without carbon, Nature. 454 (2008) 816–823. doi:10.1038/454816a.

[2] A.J. Esswein, D.G. Nocera, Hydrogen production by molecular photocatalysis, Chem. Rev. 107 (2007) 4022–4047. doi:10.1021/cr050193e.

[3] R.E. Blankenship, D.M. Tiede, J. Barber, G.W. Brudvig, G. Fleming, M. Ghirardi, M.R. Gunner, W. Junge, D.M. Kramer, A. Melis, T.A. Moore, C.C. Moser, D.G. Nocera, A.J. Nozik, D.R. Ort, W.W. Parson, R.C. Prince, R.T. Sayre, Comparing Photosynthetic and Photovoltaic Efficiencies and Recognizing the Potential for Improvement, Science. 332 (2011) 805–809. doi:10.1126/science.1200165.

[4] M. Hambourger, G.F. Moore, D.M. Kramer, D. Gust, A.L. Moore, T.A. Moore, Biology and technology for photochemical fuel production, Chem. Soc. Rev. 38 (2009) 25–35. doi:10.1039/B800582F.

[5] A. Fujishima, K. Honda, Electrochemical Photolysis of Water at a Semiconductor Electrode, Nature. 238 (1972) 37–38. doi:10.1038/238037a0.

[6] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Dye-sensitized solar cells, Chem. Rev. 110 (2010) 6595–6663. doi:10.1021/cr900356p.

[7] W.T. Eckenhoff, R. Eisenberg, Molecular systems for light driven hydrogen production, Dalt. Trans. 41 (2012) 13004–13021. doi:10.1039/c2dt30823a.

[8] S.I. Bokarev, O.S. Bokareva, O. Kühn, A theoretical perspective on charge transfer in

photocatalysis. The example of Ir-based systems, Coord. Chem. Rev. 304–305 (2015) 133–145. doi:https://doi.org/10.1016/j.ccr.2014.12.016.

[9] H. Junge, N. Rockstroh, S. Fischer, A. Brückner, R. Ludwig, S. Lochbrunner, O. Kühn, M. Beller, Light to hydrogen: Photocatalytic hydrogen generation from water with molecularly-defined iron complexes, Inorganics. 5 (2017) 14. doi:10.3390/inorganics5010014.

[10] T.N. V. Karsili, D. Tuna, J. Ehrmaier, W. Domcke, Photoinduced water splitting via benzoquinone and semiquinone sensitisation, Phys. Chem. Chem. Phys. 17 (2015) 32183–32193. doi:10.1039/C5CP03831F.

[11] Y. Umena, K. Kawakami, J.R. Shen, N. Kamiya, Crystal structure of oxygen-evolving photosystem II at a resolution of 1.9 Å, Nature. 473 (2011) 55. http://www.nature.com/nature/journal/vaop/ncurrent/full/nature09913.html?WT.ec_id=NATURE-20110421%5Cnpapers2://publication/uuid/9BFD6749-0062-4664-9D3F-6201CB8541A5.

[12] M. Sandroni, M. Kayanuma, A. Planchat, N. Szuwarski, E. Blart, Y. Pellegrin, C. Daniel, M. Boujtita, F. Odobel, First application of the HETPHEN concept to new heteroleptic bis(diimine) copper(I) complexes as sensitizers in dye sensitized solar cells, Dalt. Trans. 42 (2013) 10818. doi:10.1039/c3dt50852h.

[13] S. Fischer, D. Hollmann, S. Tschierlei, M. Karnahl, N. Rockstroh, E. Barsch, P. Schwarzbach, S.-P. Luo, H. Junge, M. Beller, S. Lochbrunner, R. Ludwig, A. Brückner, Death and rebirth: Photocatalytic hydrogen production by a self-organizing copper–iron system, ACS Catal. 4 (2014) 1845–1849. doi:10.1021/cs500387e.

[14] Y. You, W. Nam, Photofunctional triplet excited states of cyclometalated Ir(III) complexes: Beyond electroluminescence, Chem. Soc. Rev. 41 (2012) 7061. doi:10.1039/c2cs35171d.

[15] F. Gärtner, B. Sundararaju, A.E. Surkus, A. Boddien, B. Loges, H. Junge, P.H. Dixneuf, M. Beller, Light-driven hydrogen generation: Efficient iron-based water reduction catalysts, Angew. Chemie - Int. Ed. 48 (2009) 9962–9965. doi:10.1002/anie.200905115.

[16] S.P. Luo, Esteban Mejía, A. Friedrich, A. Pazidis, H. Junge, A.E. Surkus, R. Jackstell, S. Denurra, S. Gladiali, S. Lochbrunner, M. Beller, Photocatalytic water reduction with copper-based photosensitizers: A noble-metal-free system, Angew. Chemie - Int. Ed. 52 (2013) 419–423. doi:10.1002/anie.201205915.

[17] E. Mejía, S.P. Luo, M. Karnahl, A. Friedrich, S. Tschierlei, A.-E. Surkus, H. Junge, S. Gladiali, S. Lochbrunner, M. Beller, A noble-metal-free system for photocatalytic hydrogen production from water., Chem. - A Eur. J. 19 (2013) 15972–15978. doi:10.1002/chem.201302091.

[18] M. Karnahl, E. Mejía, N. Rockstroh, S. Tschierlei, S.P. Luo, K. Grabow, A. Kruth, V. Brüser, H. Junge, S. Lochbrunner, M. Beller, Photocatalytic hydrogen production with copper photosensitizer-titanium dioxide composites, ChemCatChem. 6 (2014) 82–86. doi:10.1002/cctc.201300459.

[19] M. Chiba, T. Tsuneda, K. Hirao, Excited state geometry optimizations by analytical energy gradient of long-range corrected time-dependent density functional theory, J. Chem. Phys. 124 (2006) 144106. doi:10.1063/1.2186995.

[20] H. Iikura, T. Tsuneda, T. Yanai, K. Hirao, A long-range correction scheme for generalized-gradient-approximation exchange functionals, J. Chem. Phys. 115 (2001) 3540. doi:10.1063/1.1383587.

[21] Y. Tawada, T. Tsuneda, S. Yanagisawa, T. Yanai, K. Hirao, A long-range-corrected time-dependent density functional theory, J. Chem. Phys. 120 (2004) 8425–8433.

[22] E. Livshits, R. Baer, A well-tempered density functional theory of electrons in molecules, Phys. Chem. Chem. Phys. 9 (2007) 2932. doi:10.1039/b617919c.

[23] T. Stein, L. Kronik, R. Baer, Reliable Prediction of Charge Transfer Excitations in Molecular Complexes Using Time-Dependent Density Functional Theory, J. Am. Chem. Soc. 131 (2009) 2818–2820. doi:10.1021/ja8087482.

[24] T. Stein, L. Kronik, R. Baer, Prediction of charge-transfer excitations in coumarin-based dyes using a range-separated functional tuned from first principles, J. Chem. Phys. 131 (2009) 244119. doi:10.1063/1.3269029.

[25] O.S. Bokareva, G. Grell, S.I. Bokarev, O. Kühn, Tuning range-separated density functional theory for photocatalytic water splitting systems, J. Chem. Theory Comput. 11 (2015) 1700–1709. doi:10.1021/acs.jctc.5b00068.

[26] O.S. Bokareva, O. Kühn, Quantum chemical study of the electronic properties of an iridium-based photosensitizer bound to medium-sized silver clusters, Chem. Phys. 457 (2015) 1–6. doi:https://doi.org/10.1016/j.chemphys.2015.05.004.

[27] S. Fischer, O.S. Bokareva, E. Barsch, S.I. Bokarev, O. Kühn, R. Ludwig, Mechanistic study of photocatalytic hydrogen generation with simple iron carbonyls as water reduction catalysts, ChemCatChem. 8 (2016) 404–411. doi:10.1002/cctc.201500872.

[28] O. Bokareva, T. Möhle, A. Neubauer, S. Bokarev, S. Lochbrunner, O. Kühn, Chemical tuning and absorption properties of iridium photosensitizers for photocatalytic applications, Inorganics. 5 (2017) 23. doi:10.3390/inorganics5020023.

[29] M.J. Frisch, G.W. Trucks, M. Caricato, H.P.H. X. Li, M. Ishida, H. Nakai, T. Vreven, J. J. A. Montgomery, J.E. Peralta, J. Cioslowski, D.J. Fox, Gaussian 09, Revision C.1, Gaussian, Inc., Wallingford CT, 2009.

[30] P.J. Hay, W.R. Wadt, Ab initio effective core potentials for molecular calculations. Potentials for the transition metal atoms Sc to Hg, J. Chem. Phys. 82 (1985) 270–283.

[31] W.J. Hehre, R. Ditchfield, J.A. Pople, Self-consistent molecular orbital methods. XII. Further extensions of gaussian-type basis sets for use in molecular orbital studies of organic molecules, J. Chem. Phys. 56 (1972) 2257–2261. doi:10.1063/1.1677527.

[32] P.C. Hariharan, J.A. Pople, The influence of polarization functions on molecular orbital hydrogenation energies, Theor. Chim. Acta. 28 (1973) 213–222.

[33] N. Armaroli, G. Accorsi, M. Holler, O. Moudam, J.-F. Nierengarten, Z. Zhou, R.T. Wegh, R. Welter, Highly luminescent Cu(I) complexes for light-emitting electrochemical cells, Adv. Mater. 18 (2006) 1313–1316. doi:10.1002/adma.200502365.

[34] J. Tomasi, B. Mennucci, R. Cammi, Quantum mechanical continuum solvation models, Chem. Rev. 105 (2005) 2999–3093. doi:10.1021/cr9904009.

[35] S. Tschierlei, M. Karnahl, N. Rockstroh, H. Junge, M. Beller, S. Lochbrunner, Substitution-controlled excited state processes in heteroleptic copper(I) photosensitizers used in hydrogen evolving systems, ChemPhysChem. 15 (2014) 3709–3713. doi:10.1002/cphc.201402585.

[36] E.M. Kosower, J.L. Cotter, Stable free radicals. II. The reduction of 1-methyl-4-cyanopyridinium ion to methylviologen cation radical, J. Am. Chem. Soc. 86 (1964) 5524–5527. doi:10.1021/ja01078a026.

[37] C.R. Bock, T.J. Meyer, D.G. Whitten, Electron transfer quenching of the luminescent excited state of tris(2,2’-bipyridine)ruthenium(II). Flash photolysis relaxation technique for measuring the rates of very rapid electron transfer reactions, J. Am. Chem. Soc. 96 (1974) 4710–4712. doi:10.1021/ja00821a078.

[38] R. Lomoth, T. Häupl, O. Johansson, L. Hammarström, Redox-switchable direction of photoinduced electron transfer in an Ru(bpy)₃ 2+-viologen dyad, Chem. - A Eur. J. 8 (2002) 102–110. doi:10.1002/1521-3765(20020104)8:1<102::AID-CHEM102>3.0.CO;2-S.

[39] J.L. Dempsey, J.R. Winkler, H.B. Gray, Kinetics of electron transfer reactions of H₂ -evolving cobalt diglyoxime catalysts, J. Am. Chem. Soc. 132 (2010) 1060–1065.

doi:10.1021/ja9080259. [40] S. Sakaki, H. Mizutani, Y. Kase, K. Inokuchi, T. Arai, T. Hamada, Photoinduced

electron transfer between [Cu(dmphen)L₂ ]+ [dmphen = 2,9-dimethyl-1,10-phenanthroline, L = PPh�(C₆ H₄ OMe-p)₃ ₋ � (n= 0-3)] and methyl viologen, J. Chem. Soc. Dalt. Trans. (1996) 1909–1914. doi:10.1039/DT9960001909.

[41] R.S. Khnayzer, C.E. McCusker, B.S. Olaiya, F.N. Castellano, Robust cuprous phenanthroline sensitizer for solar hydrogen photocatalysis, J. Am. Chem. Soc. 135 (2013) 14068–14070. doi:10.1021/ja407816f.

[42] A. Neubauer, G. Grell, A. Friedrich, S.I. Bokarev, P. Schwarzbach, F. Gärtner, A.E. Surkus, H. Junge, M. Beller, O. Kühn, S. Lochbrunner, Electron- and energy-transfer processes in a photocatalytic system based on an Ir(III)-photosensitizer and an iron catalyst, J. Phys. Chem. Lett. 5 (2014) 1355–1360. doi:10.1021/jz5004318.

[43] P. Debye, Reaction rates in ionic solutions, Trans. Electrochem. Soc. 82 (1942) 265–272. doi:10.1149/1.3071413.

[44] R.S. Khnayzer, V.S. Thoi, M. Nippe, A.E. King, J.W. Jurss, K.A. El Roz, J.R. Long, C.J. Chang, F.N. Castellano, Towards a comprehensive understanding of visible-light photogeneration of hydrogen from water using cobalt(II) polypyridyl catalysts, Energy Environ. Sci. 7 (2014) 1477–1488. doi:10.1039/C3EE43982H.

download fileview on ChemRxivCuPS.pdf (1.10 MiB)