Engineered [FeFe]-hydrogenase mimics featuring heteroaryl linkers: molecular design and photocatalytic hydrogen evolution under visible light

Abstract

Inspired by the active site of [FeFe]-hydrogenase, we have developed synthetic mimics engineered from the reaction of heteroaryl thioketone derivatives ferrocenyl(5-(4-(diphenylamino)phenyl)thiophen-2-yl)methanethione (PS-Fc-1), ferrocenyl(5′-(4-(diphenylamino)phenyl)-[2,2′-bithiophen]-5-yl)methanethione (PS-Fc-2) and phenyl(5′-(4-(diphenylamino)phenyl)-[2,2′-bithiophen]-5-yl)methanethione (PS-Ph) as pro-ligands with Fe₃(CO)₁₂. The resulting complexes contain thiolato ligands, which enable a close linkage between heteroaryl chromophores and the catalytic center, thereby promoting efficient photocatalytic hydrogen evolution under visible light irradiation. These mimics incorporate a push–pull organic chromophore, consisting of triphenylamine and (bi)thiophene groups, designed to facilitate direct photoexcitation into a charge-separated state. Electrochemical properties were examined using cyclic voltammetry, and photophysical characteristics were determined by steady-state spectroscopy and nanosecond transient absorption supported by (TD-)DFT simulations. Whilst both catalytically active species revealed the formation of charge-separated states directly upon excitation, fast deactivation due to relaxation into low-lying ferrocene-located states prevents the formation of long-lived excited states in the ferrocene-linked dyad. This explains the reduced activity for hydrogen generation of the dyad containing the ferrocene moiety compared to the phenyl one.

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Introduction

Energy is considered the keystone for productivity improvement in agriculture and industrial sectors. However, the main energy source for most of the world's energy requirements is governed by fossil fuels, which are the primary drivers of greenhouse gas emissions. Therefore, the search for eco-friendly alternatives to reduce these emissions is progressively becoming a major concern for industrial developers and world governments.^1–3 It is important to consider that any alternative selected should be abundant, affordable, environmentally sustainable, and widely available across regions to effectively compete with traditional fuels. Among the sustainable energy sources, green hydrogen is considered the most viable long-term solution that meets these criteria. Green hydrogen can be produced via photocatalysis using visible sunlight as the sole energy source.^4,5 A photocatalytic system is typically achieved through the combination of a catalyst (CAT) and a photosensitizer (PS), with the support of a sacrificial electron donor (SED), to ensure the thermodynamic feasibility of catalytic reactions while inhibiting unproductive back-electron transfer. In the interest of economic and environmental impact, the most efficient photocatalytic system should be one in which both the CAT and the PS are designed entirely from earth-abundant elements and provide long-term stability and high capacity for hydrogen formation.^6,7 Such photocatalytic systems can be either intermolecular, in which the photoactive unit is separated from the catalyst, or intramolecular, where the photoactive unit is combined with the catalytic center as a single entity.⁸ The former is considered less efficient as it is limited by diffusion processes bringing the PS and the CAT close enough to facilitate the transfer of electrons from the excited PS to the CAT center.^8,9–11 A paradigm for a catalyst based on earth-abundant elements is provided by nature. The protein [FeFe]-hydrogenase is recognized as a highly efficient catalyst for hydrogen formation, with a reported turnover frequency (TOF) greater than 10⁴ s⁻¹ under optimal conditions.^12,13 The structure of the active site (the so-called H-cluster) of [FeFe]-hydrogenase revealed by X-ray crystallographic analysis is presented in Fig. 1.^14–17 As shown in Fig. 1, the topology of the H-cluster contains a butterfly [2Fe2S] subunit acting as the catalytic center for proton reduction and an [4Fe4S] ferredoxin cluster that is responsible for electron transfer to the H-cluster.¹⁴ Inspired by the natural archetype of [FeFe]-hydrogenase, a variety of its mimics have been developed to function as catalysts for the electrochemical reduction of protons.^18–25 Building on this existing knowledge, it is intuitive to explore the potential applications of these mimics as catalysts in photochemical hydrogen production processes. In this context, photochemically driven proton reduction systems have been achieved through the combination of a PS and an [FeFe]-hydrogenase mimic as a CAT.^26–31

Fig. 1 The active site of natural [FeFe]-hydrogenase.

Reviewing the literature on these photochemically driven proton reduction systems, one can find that the PS can be bonded to the CAT using two different strategies, both with concomitant advantages and disadvantages.³¹ In the first strategy, the PS is attached to the CAT via an organic dithiolato linker, while the second approach relies on introducing the PS directly to one of the iron centers through a ligand as an alternative to one of the terminal CO groups.^28,32–41 Following the first scenario, the group of Sun reported the first example of this strategy in which an acetylene-functionalized ruthenium bis(terpyridine) complex was selected as the PS, and the synthetic model of [FeFe]-hydrogenase, [Fe₂(CO)₆{μ-(SCH₂)₂NR}] (R = C₆H₅I), was used as the CAT.³² As a result, the spectroscopic and electrochemical data of their dyad system indicated that oxidative quenching of the photoexcited *[Ru(terpy)₂]²⁺ by the CAT was uphill by 0.59 eV.⁴² In subsequent years, further studies on such systems using dyads containing Ru- or Re-based photosensitizers or organic chromophores have been reported and they showed diversity in their turnover number (TON) for light-driven hydrogen evolution.³¹ However, the highest TON of 31.8 was observed in a study by Hou and co-workers, in which they synthesized a dyad system by covalently connecting two metal-free PSs and one [Fe₂S₂] cluster.⁴³ The ongoing development of these scenarios has led our group to design a new class of dyads that absorb light in the UV region. This includes a small, compact, heavy-metal-free photosensitizer–catalyst dyad by utilizing a silicon-containing heteroaromatic system.^44,45 In this study, a TON of 539 was reached after 7 hours of irradiation under optimal conditions, and hence it was considered the highest reported value at that time for such small, compact systems. More recently, we have designed another prototype dyad comprising π-conjugated oligothiophenes as light absorbers, which shows a remarkable long-term photocatalytic activity among the reported analogous complexes in the visible spectral range.⁴⁶ Considering these remarkable findings and our ongoing interest in the reaction of heteroaryl thioketones with Fe₃(CO)₁₂, we aimed to design photoactive thioketone derivatives that are good at absorbing visible light.^24,47,48 These derivatives can react with Fe₃(CO)₁₂ to form [FeFe]-hydrogenase-mimicking complexes that can be activated by visible light. The thioketones developed in this study are composed of three essential components: (i) the thiocarbonyl group, (ii) a ferrocenyl moiety, and (iii) a push–pull organic chromophore based on triphenylamine and (bi)thiophene aimed at direct photoexcitation into a charge-separated state. The thiocarbonyl group is regarded as the reactive site for coordination with the metal center and the good donor ability of the ferrocenyl group is expected to enhance the electron density in such a system, supporting the reactivity for coupling and increasing stability. Furthermore, the ferrocenyl moiety could act as an intrinsic electron donor, thereby supporting the formation of a charge-separated state and charge carrier accumulation at the [FeFe] center.^49–53 The resulting [FeFe]-hydrogenase-mimicking complexes are investigated for their photocatalytic hydrogen generation ability under visible light. Additionally, to evaluate the impact of the ferrocenyl moiety, which can act as an electron donor but also as an energy acceptor, a structural analogue featuring a phenyl ring in place of the ferrocenyl group was synthesized.⁵⁴ Moreover, the known compound 4-([2,2′-bithiophen]-5-yl)-N,N-diphenylaniline (PS) was synthesized for comparison, following the method previously reported in the literature.⁵⁵

Results and discussion

Synthesis and characterization

The synthetic routes for obtaining the target systems are outlined in Scheme 1. Initially, the reaction of ferrocenecarboxylic acid with 2-bromothiophene or 5-bromo-2,2′-bithiophene in CH₂Cl₂ solution produced compounds 1 and 2, respectively, as shown in Scheme 1 (first step).

Scheme 1 Preparation of compounds 1–6 and PS-Fc-1, PS-Fc-2, and PS-Ph. First step: Friedel–Crafts acylation. Second step: Stille coupling. Third step: thionation using Lawesson's reagent.

In parallel, the reaction of benzoic acid with 5-bromo-2,2′-bithiophene under the same conditions afforded compound 3 (Scheme 1, first step). In the subsequent step, compounds 1–3 were reacted with N,N-diphenyl-4-(tributylstannyl)aniline, respectively, in the presence of Pd(PPh₃)₄ as a catalyst, resulting in the formation of the desired compounds 4–6, respectively, as depicted in Scheme 1 (second step). Compounds 1–6 were characterized by means of ¹H and ¹³C{¹H} NMR spectroscopic techniques as well as elemental analysis, mass spectrometry and single-crystal X-ray structure determination of compound 4 (Fig. S1, SI), which are described in detail in the SI. The next step involved thionating the carbonyl groups in compounds 4–6 using Lawesson's reagent in THF at 65 °C for 4 hours, resulting in compounds PS-Fc-1, PS-Fc-2, and PS-Ph with moderate yields, as shown in Scheme 1 (third step). The obtained compounds were characterized by spectroscopic techniques (¹H and ¹³C{¹H} NMR, Fig. S14–S19, SI), IR (Fig. S41–43, SI), mass spectrometry (Fig. S31–33, SI), and elemental analysis. The absence of signals corresponding to the ketone group in the ¹³C{¹H} NMR spectra of compounds PS-Fc-1, PS-Fc-2, and PS-Ph along with the appearance of a signal around 220 ppm for the carbon atoms of the thioketone group confirmed the postulated structures of these compounds.

Synthesis and characterization of diiron complexes

Treatment of equimolar amounts of Fe₃(CO)₁₂ and compounds PS-Fc-1, PS-Fc-2, and PS-Ph in boiling THF for 2 hours, followed by column chromatography, afforded complexes PS-Fc-CAT-1, PS-Fc-CAT-2, PS-Fc-CAT-3, and PS-Ph-CAT with isolated yields ranging from 27% to 46%, as depicted in Scheme 2. Notably, only compound PS-Fc-2 produced a mixture of ortho-metalated complexes PS-Fc-CAT-2 and PS-Fc-CAT-3 under similar conditions (Scheme 2). This behavior has been previously observed in similar reactions of thioketone derivatives with Fe₃(CO)₁₂, as reported by our group.^30,47 The new complexes were characterized by ¹H and ¹³C{¹H} NMR, IR, UV-Vis spectroscopy, mass spectrometry, elemental analysis, cyclic voltammetry, and X-ray crystallography for complex PS-Fc-CAT-2.

Scheme 2 Synthetic routes of complexes PS-Fc-CAT-1–3 and PS-Ph-CAT.

The IR spectra of complexes PS-Fc-CAT-1, PS-Fc-CAT-2, and PS-Ph-CAT (Fig. S44, 45 and 47, SI) reveal three absorption bands located at 2068, 2027, and 1987 cm⁻¹ (for PS-Fc-CAT-1); 2068, 2029, and 1990 cm⁻¹ (for PS-Fc-CAT-2); and 2070, 2031, and 1994 cm⁻¹ (for PS-Ph-CAT), corresponding to the terminal CO ligands. In contrast, the IR spectrum of complex PS-Fc-CAT-3 (Fig. S46, SI) shows four characteristic metal/carbonyl stretching bands located at 2043, 2035, 2000, and 1946 cm⁻¹. The obtained results are consistent with those of the corresponding iron complexes reported in the literature.^30,47 It is noteworthy that the frequencies of complex PS-Ph-CAT are slightly shifted to higher values compared to those of complexes PS-Fc-CAT-1 and PS-Fc-CAT-2, indicating a weaker electron-donating character of the phenyl ring to the FeFe center compared to the ferrocene one.

The ¹H NMR spectra of complexes PS-Fc-CAT-1, PS-Fc-CAT-2, and PS-Ph-CAT (Fig. S20, 22 and 26, SI) display a singlet resonance that is related to the methine moieties in the region of 5.40–5.57 ppm. However, this resonance is completely absent in the spectrum of PS-Fc-CAT-3 (Fig. S24, SI), indicative of a distinct alteration in its structure. Moreover, nine protons of the ferrocene moiety in complexes PS-Fc-CAT-1–3 were also detected in the area between 3.21 and 4.74 ppm. The signals of the other protons, which confirmed the proposed structures of the obtained complexes, were also detected within the expected range. The ¹³C{¹H} NMR spectrum of PS-Fc-CAT-3 (Fig. S25, SI) displays a distinct singlet resonance at approximately 179.0 ppm, which is absent in the spectra of PS-Fc-CAT-1, PS-Fc-CAT-2, and PS-Ph-CAT (Fig. S21, 23 and 27, SI). This resonance is attributed to the carbon atom adjacent to the ferrocene moiety and hence provides strong support for the proposed structure of PS-Fc-CAT-3. Additionally, all complexes show signals of the terminal carbonyl carbon atoms of the iron cores in the range of 209.0–213.0 ppm (Fig. S21, 23, 25 and 27, SI).

The molecular structure of complex PS-Fc-CAT-2 was unambiguously determined through single-crystal X-ray diffraction, and its molecular geometry is depicted in Fig. 2. A suitable single crystal of complex PS-Fc-CAT-2 was successfully obtained through the diffusion of pentane into a CH₂Cl₂ solution of complex PS-Fc-CAT-2 at a low temperature (−20 °C). It is evident from Fig. 2 that the thiolato ligand in complex PS-Fc-CAT-2 is bound to the two iron centers via the sulfur atom, with an average Fe–S bond length of 2.2552(7) Å, which is consistent with those of analogous complexes reported in the literature.^30,47

Fig. 2 Molecular structure of compound PS-Fc-CAT-2 in the crystal. Displacement ellipsoids are drawn at the 50% probability level, and H-atoms are omitted for clarity.

Moreover, complex PS-Fc-CAT-2 features a σ-bond to one iron atom (Fe1) through the β-carbon of the thiophene ring, directly bonded to the thiolato moiety, while the other iron atom (Fe2) is π-coordinated to the α- and β-C atoms of the same thiophene ring in an η²-mode. The Fe–Fe bond length in complex PS-Fc-CAT-2 (2.5311(6) Å) is comparable to those of similar complexes described in the literature.^30,47 The average Fe–CO bond length in complex PS-Fc-CAT-2 (1.7937(3) Å) aligns with those observed for analogous complexes reported in the literature.^30,47 The ferrocene moiety in complex PS-Fc-CAT-2 adopts an eclipsed conformation.

The electrochemical behavior of the resulting complexes was investigated by applying cyclic voltammetry in CH₂Cl₂-[nBu₄N][BF₄] solution at a scan rate of 0.2 V s⁻¹ (V referenced to the Fc⁺/Fc couple).

As indicated in Fig. 3, the cyclic voltammogram of each complex exhibited two reversible couples in the reduction region at E_1/2 = −1.49 and −1.78 V (for PS-Fc-CAT-1), E_1/2 = −1.48 and −1.77 V (for PS-Fc-CAT-2), E_1/2 = −1.28 and −1.79 V (for PS-Fc-CAT-3), and E_1/2 = −1.45 and −1.71 V (for PS-Ph-CAT). These two reversible couples can be assigned to the Fe^I–Fe^I → Fe^I–Fe⁰ couple and Fe^I–Fe⁰ → Fe⁰–Fe⁰ couple, respectively. This behavior is similar to that of its analog reported in the literature, and the reduction values of these complexes are shifted to lower values compared to those of the analog.³⁰

Fig. 3 Cyclic voltammetry of 1.0 mM of complexes PS-Fc-CAT-1–3 and PS-Ph-CAT in CH₂Cl₂-[nBu₄N][BF₄] (0.1 M) solutions at a 0.2 V s⁻¹ scan rate using a glassy carbon disk (d = 1.6 mm). The arrows indicate the scan direction. The potentials E are given in V and referenced to the Fc⁺/Fc couple.

Photocatalytic activities

The photocatalytic H₂ production activities of the molecular dyad (PS-CAT without Fc) as well as the molecular triad (PS-CAT with Fc) were investigated under visible light irradiation. Initially, in order to determine the excitation wavelength of PS-CATs, the UV-vis absorption of the complexes was recorded in CH₂Cl₂. PS-Fc-CAT-2, PS-Fc-CAT-3, and PS-Ph-CAT (Fig. S48, SI) showed strong absorption bands around 393, 651, and 390 nm, respectively. Contrarily, complex PS-Fc-CAT-1 absorbs radiation only in the UV region (Fig. S48, SI) in accordance with the size of the conjugated system of the chromophoric oligothiophene unit. Accordingly, PS-Fc-CAT-2, PS-Fc-CAT-3, and PS-Ph-CAT were tested for their hydrogen evolution activity. For this purpose, photocatalysis under visible light irradiation in the presence of BIH (1,3-dimethyl-2-phenylbenzimidazoline) as both the electron and proton source was performed. PS-Fc-CAT-2 and PS-Ph-CAT showed catalytic activity under 405 nm irradiation. In contrast, PS-Fc-CAT-3 was inactive for hydrogen evolution under 625 nm irradiation, which agrees with observations recently reported for a related system with a diene linker between the sensitizer and the catalyst unit.⁴⁷ Fig. 4 shows the time profile of H₂ evolution for PS-Fc-CAT-2 and PS-Ph-CAT. The turnover numbers (TONs) after 20 h of irradiation reached ∼294 and ∼112 for PS-Ph-CAT and PS-Fc-CAT-2, respectively. It is noteworthy that the turnover frequencies (TOF) for the initial 2 hours of irradiation were ∼55.4 h⁻¹ for the dyad and ∼2.6 h⁻¹ for the triad. However, the triad showed superior catalytic stability, i.e., PS-Ph-CAT lost its catalytic activity after 12 hours of irradiation, though PS-Fc-CAT kept producing H₂ beyond this period. The findings indicated that the Fc moiety provided catalytic stability; however, it exerted a negative influence on the catalytic efficiency of the triad. Nevertheless, compared to our previously reported [FeFe]-hydrogenase mimics with visible activity, the TOF of PS-Ph-CAT was significantly higher.^46,47 This phenomenon suggests that the thioketone structure provides PS-CAT with unique photoexcited properties. In order to gain insights into the photoexcited states of dyads and triads and the underlying reasons for their efficiency and stability, nanosecond transient absorption (ns-TA) spectroscopy of PS-Ph-CAT and PS-Fc-CAT-2 was performed.

Fig. 4 Time profile of photocatalytic H₂ production and the TONs of PS-Fc-CAT-2 and PS-Ph-CAT. The catalyst concentrations were 100 μM and the BIH concentrations were 10 mM in the acetonitrile/N-methyl-2-pyrrolidone = 1/3 mixture under 405 nm irradiation.

Properties of excited states in PS-CAT dyads

To explore the manifold of excited states and analyze the excitation conditions, the absorption spectra of the dyads PS-Ph-CAT and PS-Fc-CAT-2 are compared to a model sensitizer PS. Upon linking the catalysts to the sensitizer PS, slight characteristic changes in the absorption and emission properties are observed (Fig. 5). The absorption spectrum of the sensitizer unit PS shows two absorption features at 305 nm (ε = 4.4 × 10⁴ L mol⁻¹ cm⁻¹) and 379 nm (ε = 8.3 × 10⁴ L mol⁻¹ cm⁻¹) and a broad, intense emission band at 459 nm, which is independent of the excitation wavelength (Fig. S51, SI). The absorption spectra and, thus, the electronic structure of the modified sensitizer species PS-Fc-2 and PS-Ph acting as precursors for the dyads were revealed to be heavily influenced by the additionally introduced thioketone functionality. Although changes in the photophysical properties due to the linkage of either the phenyl or ferrocenyl groups are evident in the absorption spectra, the absorption features of the dyads are more comparable to the unsubstituted sensitizer PS. Hence, once the reaction center is linked to the sensitizers to form PS-Fc-CAT-2 and PS-Ph-CAT, the absorption spectra more closely resemble PS than the thioketone form of the sensitizers. The catalyst PS-Ph-CAT reveals a split UV absorption feature with peaks at 283 nm (ε = 2.9 × 10⁴ L mol⁻¹ cm⁻¹) and 301 nm (ε = 2.8 × 10⁴ L mol⁻¹ cm⁻¹) and a peak at 390 nm (ε = 2.8 × 10⁴ L mol⁻¹ cm⁻¹) in the UV region. Additionally, a pronounced shoulder in the visible absorption region develops at 482 nm (ε = 1.0 × 10⁴ L mol⁻¹ cm⁻¹). The absorption spectrum of the ferrocene-containing catalyst PS-Fc-CAT-2 exhibits two strong bands at 303 nm (ε = 3.1 × 10⁴ L mol⁻¹ cm⁻¹) and 393 nm (ε = 3.8 × 10⁴ L mol⁻¹ cm⁻¹) with the latter one, similar to PS-Ph-CAT, extending into a shoulder up to 600 nm. The emission intensity is strongly quenched in both dyads and both systems show only a weak emission band with a peak at around 490 nm and a second strongly red-shifted broad feature (Fig. S51, SI). This indicates that the presence of the catalyst quenches the emitting state and the residual emission in the dyad is further reduced by the presence of the ferrocene unit.

Fig. 5 Experimental (solid line) and calculated (dashed line) UV-vis spectra of PS (top), PS-Fc-CAT-2 (middle), and PS-Ph-CAT (bottom) with the calculated vertical transition energies and oscillator strengths indicated as red bars. Regions assigned to transitions isolated at certain functional groups are highlighted in grey, push–pull transitions from the triphenylamine unit to the thiophene unit in red, ferrocenyl-centered transitions in violet, and charge-transfer (CT) transition from the sensitizer to the [FeFe] center in green. The most important transitions of each spectrum are represented as NTOs (natural transition orbitals) on the right.

To explore the character of the electronic states responsible for light absorption, TD-DFT calculations (for details on the theoretical methods and calculations, see SI) were performed (Fig. 5). The calculated UV-vis absorption spectrum of PS allows the assignment of the strong absorption feature at 379 nm in the experimental spectrum of PS to a strongly dipole-allowed transition to the S₁ state (f = 1.08), which mainly contains contributions of the HOMO to LUMO transition with ¹ππ* character. Excitation to the S₁ state leads to a decreased electron density at the amine nitrogen atom and a subsequent increase at the thiophene units and thus shows slight push–pull character induced by the electron-rich triphenylamine unit, although PS does not contain a strong acceptor group.^56,57 The weaker absorption band at 305 nm is represented by the calculated transitions to the S₃ and S₄ states (f = 0.22 and f = 0.19, respectively) with contributions of both the thiophene and triphenylamine units in the initial as well as the final state with ¹ππ* character. In the dyad and triad, the character of these short wavelength transitions remains basically unchanged, while the push–pull transition appears red-shifted compared to PS in accordance with the red shift in the experimental absorption spectra and shows increased push–pull character with the catalyst unit acting as an acceptor. Hence, in the dyad and triad, these transitions possess direct charge-transfer character from the sensitizer unit to the catalyst (Fig. 5). However, the most prominent differences in the PS-Ph-CAT and PS-Fc-CAT-2 electronic transitions can be found in the energetically low-lying shoulders in the visible range of the absorption spectra. In the ferrocenyl-linked triad, PS-Fc-CAT-2, the extension of this shoulder can be assigned to the two energetically lowest weak transitions S₁ and S₂ at 550 nm (f = 1.00 × 10⁻⁴ and f = 2.55 × 10⁻⁵, respectively). Both transitions are exclusively located at the ferrocene moiety, being isolated from the [FeFe]-center and the sensitizing unit. The low oscillator strength of the energetically lower excited states S₁ and S₂ is in line with the observed decreased emission intensity compared to PS and is also evident in the emission spectra of ferrocenyl-linked sensitizer species PS-Fc-2. Compared to the phenyl-linked dyad PS-Ph-CAT, the transitions S₁ and S₂ (f = 0.03 and f = 0.10, respectively) are located at the [FeFe]-center with contributions of the sensitizer unit, indicating some shifting of electron density from the light-harvesting unit to the catalytically active center. Additionally, transition S₅ (f = 0.14) also assigned to the shoulder in the visible range of PS-Ph-CAT's experimental absorption spectrum, is of push–pull character with the [FeFe]-center as the electron density acceptor and the light-harvesting sensitizer unit acting as the donor. Also, the triad PS-Fc-CAT-2 facilitates these push–pull transitions, as resembled by excitation into the S₃, S₁₀ and S₁₂ states (f = 0.71, f = 0.14 and f = 0.21, respectively).

As a result, such strong optically allowed transitions with push–pull nature could be a source of efficient charge separation and, thus, high catalytic activity. Besides the presence of transitions with strong direct charge transfer, additional [FeFe]-centered low-lying states are present and could be a possible deactivation pathway. Furthermore, the ferrocenyl-linked triad revealed the lowest states to be ferrocene-centered, which could be a source of additional deactivation of photocatalytically active excited states via relaxation of these states by energy transfer to the Fc unit.^58–61 This hypothesis is in line with the observed strong quenching of the singlet excited state in the ferrocene-linked compounds and the slightly quenched emission in the [FeFe]-linked dyads.

Previous studies on thiophene-based hydrogenase mimics indicate that long-lived triplet states of oligothiophenes play a crucial role in the activation mechanism leading to catalytic activity.^46,62 To elucidate whether this is also the case in the newly developed dyads, nanosecond transient absorption spectroscopy was performed to probe the triplet states formed in oligothiophene chromophores via intersystem crossing. As expected, the sensitizer PS exhibits broad excited state absorption (ESA) with an absorption maximum at 600 nm (Fig. 6). The transient absorption signal decays mono-exponentially with a lifetime of τ = 29.0 ± 0.13 μs. This feature can be assigned to the thiophene-located ³ππ* state, also in good agreement with TD-DFT calculations (Fig. S53, SI) and with previous results reported in the literature.^46,62 Unfortunately, due to the instability of PS-Ph-CAT under the conditions of the ns-TA measurements, the excited state lifetime could only be estimated from a measurement at a single probe wavelength of 600 nm (Fig. 6). At 600 nm, a potentially residual signal from the ³ππ* sensitizer-centered excited state, as observed for PS, can be detected with a lifetime estimated to be 38.3 ± 2.27 μs. Nevertheless, the influence of photoproducts caused by degradation of the dyad cannot be excluded. For the triad PS-Fc-CAT-2, which is stable under the measurement conditions, no long-lived sensitizer triplet state can be detected at all (Fig. 6). These findings for PS-Ph-CAT and PS, together with the strong quenching of the singlet state emission in both species, indicate that the formation of a PS-localized ³ππ* triplet state probably does not take place in this case and that a fast alternative channel already occurring from the singlet state is active in these systems. This could be related to the changed character of the excited singlet states, changing the character of the visible absorption from ππ* to a charge transfer state, directly inducing charge separation between the sensitizer and the catalyst. Furthermore, the presence of [FeFe]-centered low-lying states opens the possibility for the population of these states, providing a general loss channel. In PS-Fc-CAT-2, additionally, the population and formal excitation energy transfer to the Fc unit increase this effect.⁶³ All four processes can be the source of the absence of any long-lived triplet signature, either preventing the formation of the triplet in the first place or quickly quenching this state. The process transferring excitation to the Fc unit, only present in PS-Fc-CAT-2, could be responsible for the decreased activity compared to PS-Ph-CAT. This interpretation is supported by the location of the spin density of the lowest excited triplet state, being centered at the catalytically active [FeFe]-center in the PS-Ph-CAT dyad (Fig. S70, SI), but being located at the inactive ferrocene (Fig. S74, SI) in the PS-CAT-Fc-2 species. Also, measurements in the presence of the electron donor BIH were conducted and delivered no sign of any long-lived species for the PS-Fc-CAT-2 species with a reduced [Fe⁰Fe^I]-center. This indicates that both the charge-separated state and doublet-state ESA of PS-Fc-CAT-2 occur on time scales below the measurement range, potentially further influenced by the proton-donating ability of BIH forming protonated intermediates.

Fig. 6 Nanosecond transient absorption spectra of 21 μM PS solution in CH₂Cl₂ at different time delays (left) and kinetic decays at 600 nm of 21 μM PS, 27 μM PS-Fc-CAT-2 and 20 μM PS-Ph-CAT normalized by the absorbance of PS at λ_exc = 440 nm (right).

Activation, reduction and hydrogen evolution pathway of PS-Ph-CAT and PS-Fc-CAT-2

In the light of the data presented, we suggest the following mechanism to explain the activity of the system. The results from the analysis of the singlet excited state manifold indicate a drastic change in the character of the visible absorption features, revealing a unique charge density shifting transition of CT character, with the triphenylamine group acting as an electron-rich donor unit and the [FeFe]-hydrogenase mimic as an acceptor. This results in PS-Ph-CAT undergoing direct charge separation already upon excitation, transferring electron density towards the catalyst. This could be the reason for the observed increased catalytic activity compared to previously reported systems, where ππ* excitation of the sensitizer's thiophene unit is followed by ISC, and charge transfer occurring from the triplet-state manifold is a necessary step. This process is bypassed in the reported systems via a direct push–pull transition, which transfers charge carriers from the sensitizer to the catalyst directly. This is in agreement with the observation of a strong quenching of the emission from the singlet sensitizer state. Nevertheless, the singlet excited charge transfer state is not the lowest state in the system, and [FeFe]-centered states are also present and could be a source of radiationless deactivation. If the population of such states could be prevented, a further increase in catalytic activity is expected to occur. The ferrocene unit in PS-Fc-CAT-2 causes further quenching of emission and additionally reduces the catalytic activity compared to PS-Ph-CAT. This indicates a significant interference of the ferrocene unit. There are two known pathways for the quenching mechanism: one including an energy transfer step and a second one with electron transfer from the donor ferrocene to an acceptor.^46,62 Energy transfer to the ferrocene and subsequent thermal relaxation populates a catalytically inactive state, while ferrocene acting as an electron donor would be expected to activate the system either via reductive quenching of the PS unit in PS-Fc-CAT-2, supporting subsequent electron transfer to the catalyst from the reduced sensitizer, or in the case of direct charge transfer as observed in our system, preventing recombination by reduction of the formally oxidized PS.⁶¹ The decreased activity for hydrogen evolution of the Fc-containing triad suggests that the energy transfer pathway is active and although the triad PS-Fc-CAT-2 shows increased stability, the activity for hydrogen evolution is decreased.

Conclusion

In summary, we have developed a series of new photoactive thioketones, PS-Fc-1, PS-Fc-2, and PS-Ph, which react with Fe₃(CO)₁₂ to form [FeFe]-hydrogenase mimicking complexes, namely PS-Fc-CAT-1, PS-Fc-CAT-2, PS-Fc-CAT-3, and PS-Ph-CAT. These complexes were thoroughly characterized using various analytical techniques, including ¹H and ¹³C{¹H} NMR, IR spectroscopy, mass spectrometry, and elemental analysis, with X-ray crystallographic analysis performed on PS-Fc-CAT-2. By integrating a thiocarbonyl group, a ferrocenyl or phenyl moiety, and a push–pull organic chromophore, these systems were strategically engineered for direct photoexcitation into a charge-separated state. Furthermore, their potential for photocatalytic hydrogen evolution under visible light irradiation was explored. Among the synthesized complexes, only PS-Fc-CAT-2 and PS-Ph-CAT exhibited catalytic activity under 405 nm irradiation. Furthermore, the ferrocenyl-linked dyad revealed decreased activity compared to PS-Ph-CAT, indicating a deactivating rather than activating influence of the ferrocene. This is in line with the observed quenched emission and absent ns-TA signal upon linkage of the ferrocene to the dyad, which is due to low-lying Fc-centered excited states causing fast deactivation of the catalyst. However, the incorporation of the ferrocenyl unit can be justified by its ability to enhance the overall stability of the system, thereby contributing to the development of a more durable catalyst for future applications. In contrast, the increased catalytic activity of PS-Ph-CAT can be explained by the population of a charge transfer state that transfers electron density from the sensitizer to the catalyst directly upon light excitation.

The present work once again demonstrates the practical usefulness of ferrocenyl thioketones for the synthesis of diverse ferrocenyl-functionalized compounds, which are important for the preparation of medicinally relevant compounds, e.g. ferrocifenes, as well as for materials chemistry.^64–68

Experimental part

Materials and techniques

All manipulations concerning the preparation of all complexes were performed using standard Schlenk and vacuum-line techniques under an inert gas (Ar). The ¹H and ¹³C{¹H} NMR spectra were recorded with a Bruker Avance 400 or 600 MHz spectrometer. Chemical shifts are given in parts per million with references to internal SiMe₄ (¹H, ¹³C{¹H}). The mass spectrum was recorded with a Finnigan MAT SSQ 710 instrument. Elemental analysis was performed with Leco CHNS-932 apparatus. TLC was performed using Merck TLC aluminum sheets (Silica gel 60 F254). Solvents from Fisher Scientific and other chemicals from Acros and Aldrich were used without further purification. All solvents were dried and distilled prior to use according to standard methods. Compound PS was synthesized according to the procedure reported in the literature.⁵⁵ The synthetic procedures and characterization of compounds 1–6 are described in detail in the SI.

Electrochemistry

Corrections for the iR drop were performed for all experiments. CV measurements were conducted using a three-electrode technique [glassy carbon disk (diameter = 1.6 mm) as the working electrode, Ag/Ag⁺ in MeCN as the reference electrode, and a Pt wire as the counter electrode] using a Reference 600 Potentiostat (Gamry Instruments). All experiments were performed in CH₂Cl₂ solutions (concentration of the complexes: 1.0 mM) containing 0.1 M [nBu₄N][BF₄] at room temperature. The solutions were purged with N₂ and a stream of N₂ was maintained over them during the measurements. The vitreous carbon disk was polished on a felt tissue with alumina before each measurement. All potential values reported in this work are referenced to the potential of the ferrocenium/ferrocene (Fc⁺/Fc) couple.

Crystal structure determination

The single-crystal X-ray intensity data for the reported compounds were collected on a Bruker-Nonius Kappa-CCD diffractometer equipped with a Mo-Kα IμS microfocus source and an Apex2 CCD detector at T = 120(2) K. The crystal structures were solved with SHELXT-2018/3 and refined by full-matrix least-squares methods on F² with SHELXL-2018/3 using the Olex2 1.3 environment.^69–71 Multi-scan absorption correction was applied to the intensity data.⁷² Restraints on interatomic distances and anisotropic displacement parameters were used for the disordered C₄S ring in compound 4 (SADI and SIMU commands in SHELXL).⁷⁰ CCDC 2445996 (for PS-Fc-CAT-2) and 2445997 (for 4) contain the supplementary crystallographic data for this paper (see Table S1).

Steady-state spectroscopy

UV-vis absorption spectroscopy was carried out with a SPECORD S600 (Analytik Jena) and a Lambda 900 double-beam spectrometer (PerkinElmer). Emission spectroscopy was performed with a Fluorolog 3-22τ spectrometer (Horiba Jobin Yvon).

Nanosecond transient absorption spectroscopy

The time-resolved transient absorption spectra on a nanosecond timescale were recorded on a custom-built pump–probe transient absorption setup. A 10 Hz Nd:YAG laser (Continuum Surelite) with a 5 ns pulse width was used to pump the excited state. The fundamental (1064 nm) passes a second-harmonic generation step (532 nm) followed by a third-harmonic generation step to gain the frequency-tripled wavelength (355 nm). This beam is transmitted to an optical parametric oscillator (Continuum Surelite) generating tunable wavelengths from 400 nm to 650 nm with energies ranging from 15 mJ to 80 mJ. If not stated otherwise, an excitation wavelength of 440 nm and a pulse energy of 20 mJ were used. To record the ground state's transmission and probe the excited state, a 75 W xenon arc lamp (pulsed or CW) was used. By focusing the beam path with a concave mirror onto the sample, the transmitted light is spectrally dispersed (Acton Princeton Instrument 2300), detected by a photomultiplier tube (Hamamatsu R928) and processed (Pascher Instruments AB).

Light-driven hydrogen evolution reactions

All photocatalytic reactions were performed with a 3D-printed photoreactor utilizing a 405 nm LED from LEDENGIN (LZ4-00UB0R).⁴⁶ The samples were prepared in LABSOLUTE clear glass screw neck vials (ND13) and screw seals (ND13, butyl red/PTFE grey) (total volume: 5 mL) under a nitrogen atmosphere. PS-CAT and BIH were dissolved in NMP and CH₃CN, which were degassed by freeze–pump–thaw. Photocatalytically produced hydrogen was quantified using a GC-2030 (column: SH-Rt-Msieve 5A, detector: BID (Dielectric-Barrier Discharge Ionization Detector)).

General procedure for the preparation of thioketone derivatives

A magnetically stirred solution of the corresponding ketones 4–6 (1.0 mmol) in 10 mL of THF was heated to 65 °C, followed by the addition of Lawesson's reagent (0.6 mmol) in a single portion. The mixture was then heated for 4 hours under an inert nitrogen gas atmosphere. Subsequently, the solvent was removed under reduced pressure, and the residue was purified by column chromatography using an n-hexane : CH₂Cl₂ (7 : 3) eluent, yielding the desired thioketones PS-Fc-1, PS-Fc-2, and PS-Ph as dark violet solids.

Compound PS-Fc-1: 69% yield. ¹H-NMR (600 MHz, CD₂Cl₂): δ7.81 (d, J_H–H = 3.6 Hz, 1H), 7.59 (d, J_H–H = 8.4 Hz, 2H), 7.32–7.30 (m, 5H), 7.15–7.04 (m, 8H), 5.14 (s, 2H), 4.78 (s, 2H), 4.24 (s, 5H). ¹³C{¹H}-NMR (150.9 MHz, CD₂Cl₂): δ220.6, 154.3, 152.4, 148.8, 147.1, 129.8, 129.4, 126.7, 125.1, 123.8, 123.5, 122.4, 89.1, 73.5, 72.8, 72.0. DEI-MS: m/z = 556 [M + H]⁺. Anal. calcd for C₃₃H₂₅FeNS₂: C, 71.35; H, 4.54; S, 11.54. Found C, 71.44; H, 4.68; S, 11.64.

Compound PS-Fc-2: 65% yield. ¹H-NMR (600 MHz, CD₂Cl₂): δ7.76 (d, J_H–H = 3.6 Hz, 1H), 7.50 (d, J_H–H = 8.4 Hz, 2H), 7.38 (d, J_H–H = 4.2 Hz, 1H), 7.31–7.28 (m, 4H), 7.25 (d, J_H–H = 4.2 Hz, 1H), 7.21 (d, J_H–H = 4.2 Hz, 1H), 7.13–7.05 (m, 9H), 5.14 (t, J_H–H = 1.8 Hz), 4.79 (t, J_H–H = 2.1 Hz), 4.23 (s, 5H). ¹³C{¹H}-NMR (150.9 MHz, CD₂Cl₂): δ220.2, 152.5, 147.9, 147.4, 147.3, 145.4, 135.0, 129.4, 129.3, 127.2, 126.7, 126.4, 124.8, 124.5, 123.4, 123.1, 89.1, 73.7, 72.9,72.1. DEI-MS: m/z = 638 [M + H]⁺. Anal. calcd for C₃₇H₂₇FeNS₃: C, 69.69; H, 4.27; S, 15.08. Found C, 69.81; H, 4.34; S, 15.22.

Compound PS-Ph: 79% yield. ¹H-NMR (600 MHz, CD₂Cl₂): δ7.67 (d, J_H–H = 7.2 Hz, 2H), 7.53 (t, J_H–H = 7.2 Hz, 1H), 7.49 (d, J_H–H = 8.4 Hz, 2H), 7.43–7.41 (m, 3H), 7.31–7.28 (m, 5H), 7.25 (d, J_H–H = 4.2 Hz, 1H), 7.22 (d, J_H–H = 4.2 Hz, 1H), 7.12 (d, J_H–H = 7.8 Hz, 4H), 7.09 (t, J_H–H = 7.2 Hz, 1H), 7.05 (d, J_H–H = 8.4 Hz, 2H). ¹³C{¹H}-NMR (150.9 MHz, CD₂Cl₂): δ220.58, 153.0, 151.4, 148.2, 147.2, 146.9, 134.6, 132.9, 130.9, 129.4, 128.5, 128.0, 127.6, 127.0, 126.5, 125.0, 124.9, 123.5, 122.9. DEI-MS: m/z = 530 [M + H]⁺. Anal. calcd for C₃₃H₂₃NS₃: C, 74.82; H, 4.38; S, 18.16. Found C, 74.98; H, 4.44; S, 18.07.

General procedure for the preparation of diiron complexes

A 100 mL Schlenk flask was loaded with Fe₃(CO)₁₂ (0.52 mmol) along with compounds PS-Fc-1, PS-Fc-2, and PS-Ph (0.47 mmol), followed by the addition of 30 mL of dry THF. The resulting green solution was refluxed for 2 hours under an inert nitrogen atmosphere, during which the color gradually changed to deep red. Afterward, the solvent was evaporated under vacuum, and the crude products were purified using silica column chromatography with n-hexane : CH₂Cl₂ (5 : 1) as the eluent. The target products PS-Fc-CAT-1, PS-Fc-CAT-2, and PS-Ph-CAT were obtained from the first red-orange fractions, while PS-Fc-CAT-3 was isolated from the second green fraction.

Complex PS-Fc-CAT-1: 46% yield. ¹H-NMR (400 MHz, CD₂Cl₂): δ7.58 (s, 1H), 7.50 (d, J_H–H = 8.8 Hz, 2H), 7.30 (dd, J_H–H = 8.4 Hz, 7.6 Hz, 4H), 7.13–7.04 (m, 8H), 5.41 (s, 1H), 4.20 (s, 5H), 4.09 (m, 2H), 3.97 (m, 1H), 3.21 (m, 1H). ¹³C{¹H}-NMR (100.6 MHz, CD₂Cl₂): δ209.7, 209.6, 165.8, 149.6, 149.0, 147.1, 135.1, 129.4, 127.5, 126.1, 125.1, 123.7, 122.5, 114.4, 91.1, 69.4, 68.5, 67.6, 67.3, 65.9, 58.2. IR (νCO): 2068, 2027, 1987 cm⁻¹. DEI-MS: m/z = 834 [M]⁺, 778 [M − 2CO]⁺. Anal. calcd for C₃₉H₂₅Fe₃NO₆S₂: C, 56.08; H, 3.02; S, 7.68. Found C, 56.19; H, 3.15; S, 7.76.

Complex PS-Fc-CAT-2: 41% yield. ¹H-NMR (600 MHz, CD₂Cl₂): δ7.52 (s, 1H), 7.49 (d, J_H–H = 9.0 Hz, 2H), 7.31–7.27 (m, 5H), 7.21 (d, J_H–H = 4.2 Hz, 1H), 7.13–7.05 (m, 8H), 5.40 (s, 1H), 4.22 (s, 5H), 4.10 (s, 2H), 3.99 (s, 1H), 3.26 (s, 1H). ¹³C{¹H}-NMR (150.9 MHz, CD₂Cl₂): δ209.6, 209.4, 165.8, 148.0, 147.3, 145.8, 142.7, 135.7, 133.6, 129.4, 127.1, 126.5, 124.8, 123.4, 123.0, 113.3, 90.9, 69.4, 68.6, 67.6, 67.4, 65.9, 58.2. IR (νCO): 2068, 2029, 1990 cm⁻¹. DEI-MS: m/z = 918 [M + H]⁺. Anal. calcd for C₄₃H₂₇Fe₃NO₆S₃: C, 56.30; H, 2.97; S, 10.48. Found C, 56.44; H, 3.05; S, 10.62.

Complex PS-Fc-CAT-3: 27% yield. ¹H-NMR (600 MHz, CD₂Cl₂): δ8.55 (d, J_H–H = 10.2 Hz, 1H), 7.97 (d, J_H–H = 10.2 Hz, 1H), 7.57–7.54 (m, 3H), 7.31 (t, J_H–H = 7.8 Hz, 5H), 7.26 (d, J_H–H = 10.2 Hz, 1H), 7.14–7.09 (m, 6H), 7.04 (d, J_H–H = 9.0 Hz, 1H), 4.74 (s, 1H), 4.44 (d, J_H–H = 6.6 Hz, 2H), 4.39 (s, 1H), 4.29 (s, 5H). ¹³C{¹H}-NMR (150.9 MHz, CD₂Cl₂): δ213.6, 211.5, 209.1, 178.5, 151.9, 149.7, 148.6, 147.1, 140.0, 138.0, 129.4, 128.2, 126.8, 126.5, 125.2, 125.1, 123.8, 123.7, 122.5, 113.1, 84.5, 70.7, 69.9, 69.4, 68.8, 68.6, 66.0. IR (νCO): 2043, 2035, 2000, 1946 cm⁻¹. DEI-MS: m/z = 918 [M]⁺, 890 [M − CO]⁺, 862 [M − 2CO]⁺, 834 [M − 3CO]⁺, 806 [M − 4CO]⁺, 778 [M − 5CO]⁺. Anal. calcd for C₄₃H₂₈Fe₃NO₆S₃: C, 56.23; H, 3.07; S, 10.47. Found C, 56.35; H, 3.18; S, 10.53.

Complex PS-Ph-CAT: 38% yield. ¹H-NMR (400 MHz, CD₂Cl₂): δ7.64 (s, 1H), 7.48 (d, J_H–H = 8.8 Hz, 2H), 7.29 (t, J_H–H = 7.8 Hz, 4H), 7.22–7.17 (m, 5H), 7.12–7.03 (m, 8H), 6.60 (dd, J_H–H = 2.8, 6.4 Hz, 2H), 5.57 (s, 1H). ¹³C{¹H}-NMR (100.6 MHz, CD₂Cl₂): δ209.4, 209.2, 168.8, 148.0, 147.3, 146.1, 141.9, 135.6, 129.4, 128.3, 128.1, 127.7, 127.0, 126.5, 126.2, 124.8, 123.5, 123.0,110.8, 60.06. APCI-MS (negative mode): m/z = 724 [M − 3CO]⁻, 640 [M − 6CO]⁻. Anal. calcd for C₃₉H₂₃Fe₂NO₆S₃: C, 57.87; H, 2.86; S, 11.88. Found C, 57.95; H, 2.92; S, 11.82.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

The data supporting this article have been included as part of the SI. Supplementary information is available. See DOI: https://doi.org/10.1039/d5qi01191d.

CCDC 2445996 and 2445997 contain the supplementary crystallographic data for this paper.^73a,b

Acknowledgements

I. B. is thankful to the Deutsche Akademischer Austauschdienst (DAAD) for a scholarship. M. W. acknowledges the Deutsche Forschungsgemeinschaft (German Research Foundation) via TRR234 CATALIGHT, Projektnummer 364549901 (TP: Z2). K. R. greatfully acknowledges the financial support by the Fonds der Chemischen Industrie (FCI) in the form of a Kekulé Scholarship (115571). We thank Dr. Stephan Kupfer (Uni Jena) for the fruitful discussions on the theoretical approaches and Prof. Christoph van Wüllen (RPTU) for the support in using the computation resources at RPTU. H. A.-F. acknowledges the financial support provided by the Deanship of Scientific Research at the Hashemite University (38/2024).

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Footnotes

† These authors contributed equally to this work.

‡ Current address: Institute for Technical Chemistry and Environmental Chemistry, Friedrich-Schiller University Jena, Philosophenweg 7a, 07743 Jena, Germany.

§ Current address: Center for Energy and Environmental Chemistry Jena (CEEC Jena), Friedrich Schiller University Jena, Philosophenweg 7a, 07743 Jena, Germany.

¶ Current address: Institute of Physical Chemistry and Kiel Nano, Surface and Interface Science (KiNSIS), Kiel University, Max-Eyth-Str. 1, 24118 Kiel, Germany. Email: waechtler@phc.uni-kiel.de

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