Enhanced red-light-driven hydrogen evolution by a diplatinum photocatalyst by the larger wavefunction leakage of iodide coordinated to the platinum center

Abstract

The single-molecular photocatalyst Pt₂(bpia)Cl₃ (bpia = bis(2-pyridylimidoyl)amido), recently proven to promote red-light-driven hydrogen evolution reaction (HER) in water via singlet-to-triplet (S–T) transitions (Angew. Chem. Int. Ed., 2025, 64, e202418884), is shown to gain a significantly improved photocatalytic performance upon ligating iodide instead of chloride to give Pt₂(bpia)I₃. Upon iodide ligation, the absorption and emission energies both show a red shift basically due to destabilization of the HOMO by mixing of the iodide orbitals having essentially higher orbital energies compared to those of chloride. Consequently, the triplet lifetime and the luminescence quantum yield both decrease by obeying the energy gap law. The unique S–T transition features of Pt₂(bpia)Cl₃ are preserved in Pt₂(bpia)I₃ with the excited-state redox properties remaining unchanged. In spite of apparently disadvantageous photophysical features induced by the iodide ligation, Pt₂(bpia)I₃ is ascertained to promote the photocatalytic HER at a considerably higher rate in comparison with Pt₂(bpia)Cl₃, primarily attributed to the higher reductive quenching efficiency for the triplet excited state of Pt₂(bpia)I₃. The observations are rationalized due to the substantially more excellent acceptor characteristics of iodide, in which its larger wavefunction leakage significantly contributes to a larger electronic coupling factor in driving the outer-sphere electron transfer from the sacrificial electron donor.

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Introduction

The photochemical hydrogen evolution reaction (HER) promoted by homogeneous and heterogeneous catalysts (Cat) has received considerable attention to solve the problems arising from the shortage of fossil fuels and global warming.¹ In the past several decades, extensive efforts have been made to achieve molecular-based artificial photosynthetic systems promoting the photocatalytic HER with high efficiency.² In our earlier studies, a photochemical system made up of EDTA (ethylenediaminetetraacetic acid disodium salt) as a sacrificial electron donor (SED), [Ru(bpy)₃]²⁺ (bpy = 2,2′-bipyridine) as a photosensitizer (PS), and methylviologen (i.e., MV²⁺) as an electron relay was often adopted to evaluate the HER accelerated by various molecular Cat, such as mono- and dinuclear Pt(II) complexes,³ tetracarboxylate Rh(II) dimers,⁴ and N-heterocyclic carbene cobalt complexes.⁵ As an extended approach, we and others also attempted to develop the dyads installed with a photosensitizing [Ru(bpy)₂(phen)]²⁺ (phen = 1,10-phenantroline) chromophore and a catalytically active Pt(bpy)Cl₂ moiety (i.e., [RuPt]²⁺, Scheme 1a).^6,7 Through our investigations on a series of dyads with varying redox characteristics, we could gain an improved understanding of the mechanism of the photo-induced intramolecular electron transfer (ET) process from the PS moiety to the Cat moiety together with the mechanism of the HER by the Pt(II)-based molecular systems.⁸

Scheme 1 Structures of (a) [RuPt]²⁺,⁶ (b) [PtCl(tpy)]⁺,⁹ (c) [PtCl(tctpy)]²⁻,¹⁰ (d) [PtCl(Mepytpy)]²⁺,¹¹ and (e) [PtCl₂(bpyMV4)]⁸⁺.^12a

On the other hand, we have reported that several [PtCl(tpy)]⁺-type (tpy = 2,2′: 6′,2′′-terpyridine) complexes, such as [PtCl(tpy)]Cl,⁹ Na₂[PtCl(tctpy)]·5H₂O¹⁰ (tctpy = 2,2′: 6′,2′′-terpyridine-4,4′,4′′-tricarboxylate), [PtCl(Mepytpy)](ClO₄)₂·H₂O¹¹ (Mepytpy = 4′-(4-methyl-pyridinio)-2,2′: 6′,2′′-terpyridine), [PtCl₂(bpyMV4)]⁸⁺,^12a and [PtCl₂(dpbpyMV4)]⁸⁺ (dpbpy = 4,4′-diphenyl-2,2′-bipyridine),^12c shown in Scheme 1b–e, can serve as single-molecular photocatalysts for the HER in fully aqueous media in the presence of EDTA (abbreviated as H₂Na₂Y) as a SED. Our studies on these systems were mostly carried out in a weakly acidic aqueous pH 5.0 solution (0.1 M acetate buffer), in which the dianionic form of EDTA (i.e. H₂Y²⁻) has the highest abundance in solution (93%) and plays a key role in promoting the intra-ion-pair ET during the reductive quenching (Scheme 2).⁹ The transient absorption studies showed that such intra-ion-pair ET proceeds with time constants in the order of 10–100 ps after populating the ³MLCT states.^8g,12c

Scheme 2 Intra-ion-pair electron transfer from the dianionic form of EDTA (H₂Y²⁻) to either a dyad or single-molecular photocatalyst (PS(Cat)ⁿ⁺).

One of the major drawbacks in the above single-molecular photocatalysts lies in their limited light-harvesting capability with the ¹MLCT bands only absorbing light below 450 nm.^9–12 Single-molecular photocatalysts capable of harvesting the whole range of visible light are indeed quite limited. An important approach to this issue involves the reports by Turro et al., in which paddlewheel-type dirhodium complexes were shown to serve as single-molecular photocatalysts promoting red-light-driven (i.e., 670-nm-light-driven) HER in organic media in the presence of relatively strong acid and a SED.¹³ Recently, we also demonstrated that a unique dinuclear Pt(II) complex (Pt₂(bpia)Cl₃,¹⁴ bpia = bis(2-pyridylimidoyl)amido,¹⁵Scheme 3a) serves as a single-molecular photocatalyst for the HER with a superior light harvesting ability up to 650 nm.¹⁴ In spite of its singlet-to-singlet transition absorption similarly located below 450 nm, Pt₂(bpia)Cl₃ was realized to be a rare example capable of harvesting light up to the red-light region due to the allowance of the singlet-to-triplet (i.e., S–T) transitions.¹⁴ In addition, Pt₂(bpia)Cl₃ shows an exceptionally long emission (105 ns) even in aqueous media at 20 °C.¹⁴ Due to its superior photophysical properties, Pt₂(bpia)Cl₃ exhibits photocatalytic performance significantly higher than that of [PtCl(tpy)]⁺-type single-molecular photocatalysts developed in our group.¹⁴ More importantly, Pt₂(bpia)Cl₃ is the first example of a single-molecular photocatalyst promoting the low-energy-light-driven HER in water via direct S–T transitions (λ > 580 nm).¹⁴ These results revealed the importance of raising the spin–orbit coupling constant derived from the diplatinum entity in order to allow the direct S–T transitions, as described elsewhere.¹⁶ However, further optimization of a diplatinum-type single-molecular photocatalyst promoting the low-energy-light-driven HER in water still remains unexplored.

Scheme 3 Structures of (a) Pt₂(bpia)Cl₃,¹⁴ and (b) Pt₂(bpia)I₃.

On the other hand, Rau and co-workers demonstrated that the [RuPt]²⁺-type dyad (i.e., [Ru(tbbpy)₂(tpphz)PtX₂](PF₆)₂, tbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine, tpphz = tetrapyrido[3,2-a:2′,3′-c:3′′,2′′-h:2′′′,3′′′-j]phenazine; X = Cl⁻ or I⁻) exhibits visible-light-driven HER with much higher activity upon replacement of chloride by iodide, which was attributed to the increased electron density at the Pt center by the iodide ligation.¹⁷ Inspired by the above report by Rau et al., a new dinuclear Pt(II) complex Pt₂(bpia)I₃ (Scheme 3b) has been synthesized and examined by focusing on the role of iodide. To our surprise, upon replacement of chloride by iodide, the initial rate of H₂ evolution under visible-light irradiation (λ > 400 nm) becomes 4 times faster. Moreover, the amount of H₂ evolved under lower-energy light (λ > 580 nm) also becomes 7 times larger. Our study here revealed for the first time that excellent acceptor characteristics of iodide are crucial to significantly improve the photocatalytic performance of a diplatinum-type single-molecular photocatalyst due to its larger wavefunction leakage facilitating the reductive quenching of the triplet excited state by EDTA.

Results and discussion

Pt₂(bpia)I₃ was synthesized by the reaction of bis(2-pyridylimidoyl)amine (Hbpia)¹⁵ and two equivalents of PtI₂(cod)¹⁸ (cod = 1,5-cyclooctadiene) in DMF, and was obtained as a brown powder (Fig. S1). As reported for Pt₂(bpia)Cl₃,¹⁴ Pt₂(bpia)I₃ only becomes well soluble when dissolved in a mixed solvent containing N-methyl-2-pyrrolidone (NMP). We therefore adopted a mixed solvent of either H₂O/NMP or DMF/NMP for all the experiments.

Pt₂(bpia)I₃ dissolved in a H₂O/NMP (9 : 1, v/v) mixture shows an absorption band at 409 nm (Fig. 1), revealing the overall shifts of the bands to the lower-energy region compared to those of Pt₂(bpia)Cl₃. Based on the TD-DFT results, the transition at 409 nm is predominantly assignable to the so-called metal/halide-to-ligand charge transfer (¹MX-LCT) transition from the diplatinum-centered (i.e., cis-PtI₂-/PtI-centered) orbital (HOMO) to the π* orbital localized over the bpia (LUMO+1) (Fig. S2, S3 and Table S1). On the other hand, the transition in the lower-energy region (444 nm) is ascribed to the ¹MX-LCT transition from cis-PtI₂ to bpia (HOMO-1 → LUMO) and clearly shows a bathochromic shift relative to the transition of Pt₂(bpia)Cl₃ at 378 nm¹⁴ (Fig. 1, S2, S3 and Table S1).

Fig. 1 Absorption spectra of Pt₂(bpia)Cl₃ (blue) and Pt₂(bpia)I₃ (red) in a mixed solvent of H₂O and NMP (9 : 1, v/v). Orange bars and green squares denote the oscillator strengths of S–S transitions and the calculated energies of S–T transitions, respectively. The inset shows the magnified figure.

On the other hand, Pt₂(bpia)I₃ also displays a broad absorption band at around 530 nm, which can be viewed as related to the spectral features shown by Pt₂(bpia)Cl₃ (Fig. 1). We previously attributed this low-energy band to the spin-forbidden direct S–T transitions for Pt₂(bpia)Cl₃.¹⁴ The TD-DFT results reveal that the low-energy band for Pt₂(bpia)I₃ is also assigned to the direct S–T transitions. In addition, the results further reveal that the transition energies together with the molar absorptivity ascribed to the S–T transitions are almost unaffected by the replacement of the halide ligands (Table S2), implying that iodide has a minor contribution to the spin–orbit coupling.

The concentration dependence of absorbance for Pt₂(bpia)I₃ reveals satisfaction of the Beer's law at all the maximum absorption wavelengths (350, 409, and 550 nm) at a concentration below 90 μM (Fig. S4), ruling out the presence of dimerization equilibrium in its ground state.

On the other hand, Pt₂(bpia)I₃ in a H₂O/NMP (9 : 1, v/v) mixture also exhibits weak luminescence at around 684 nm at 20 °C with a luminescence quantum yield of 0.2% (Fig. S5 and Table S3), slightly lower than the value of 0.3% reported for Pt₂(bpia)Cl₃.¹⁴ The emission band is red-shifted by 39 nm compared to that of Pt₂(bpia)Cl₃ (645 nm).¹⁴ The red shift observed in emission, together with that observed for the visible absorption (Fig. 1), can be rationalized by the cathodically shifted oxidation potential of the cis-PtX₂/PtX moieties in the iodide form compared to the chloride form as described below. The emission spectrum of Pt₂(bpia)I₃ upon excitation at 410 nm is almost consistent with that given by excitation at 500 or 585 nm (Fig. S6), confirming that all available excited states rapidly relax to the lowest-energy triplet state by satisfying Kasha's rule. On the other hand, the emission decay at 684 nm shows a good fit to a single exponential function, leading to an estimated emission lifetime of 20 ns (Table S4 and Fig. S7). This lifetime is only one fifth of that reported for Pt₂(bpia)Cl₃ (105 ns).¹⁴ The emission quantum yield and lifetime are both diminished upon ligating iodide in place of chloride, which can be rationalized by the enhanced non-radiative decay rate due to the decrease in the HOMO–LUMO gap (i.e., energy gap law). It must be noted here that almost all the Pt(II)-based single-molecular photocatalysts summarized in Scheme 1, except for Pt₂(bpia)Cl₃, are only emissive at 77 K but do not emit under such conditions due to the enhanced non-radiative deactivation.¹⁹ The excited-state quenching of such non-emissive photocatalysts by the sacrificial electron donor EDTA has primarily been explained to proceed via static quenching within the ground-state adduct of photocatalysts with anionic EDTA.^11,12 In this regard, Pt₂(bpia)X₃ (X = Cl⁻ or I⁻) must be regarded as an exceptional family of photocatalysts affording a long-lived excited state, thus permitting the dynamic quenching under aqueous conditions. As described elsewhere, non-radiative decay rates are often greatly increased by the interactions of water molecules in aqueous media.²⁰

Pt₂(bpia)I₃ in a DMF/NMP (4 : 1, v/v) mixture displays a broad oxidation peak above 0.7 V vs. SCE and a reversible reduction wave at −0.93 V vs. SCE with a peak-to-peak separation of 80 mV (Fig. S8–S10). The broad oxidation peak is assignable to the oxidation at the cis-PtI₂/PtI moiety as shown by the HOMO (Fig. S3). By analogy with our previous report on Pt₂(bpia)Cl₃,¹⁴ the reversible reduction wave is reasonably assigned to the π*(bpia)-based one-electron reduction, as indicated by the LUMO, which is dominantly spread over the entire bpia in our DFT results (Fig. S2). This assignment is further ascertained by the DFT calculations for the one-electron-reduced species of Pt₂(bpia)I₃ (i.e., Pt₂(bpia⁻˙)I₃), revealing that the spin density is indeed delocalized over the entire bpia moiety (Fig. S11a). Notably, the imine moiety of Pt₂(bpia⁻˙)I₃ exhibits a significant bend upon reduction (Fig. S11b), in contrast to the planar geometry stabilized for Pt₂(bpia⁻˙)Cl₃.¹⁴

On the other hand, the electrocatalytic activity of Pt₂(bpia)I₃ was evaluated in an aqueous acetate buffer (0.09 M, pH 5.0) containing NMP (9 : 1, v/v). The catalytic current for the HER starts to flow at around −1.05 V (Fig. 2). The overpotential for the HER (η(E_cat/2)) catalyzed by Pt₂(bpia)I₃ is determined to be 680 mV from the potential difference between the potentials giving the half-height of the maximum catalytic current (−1.22 V, i.e., E_cat/2) and the standard water reduction potential under these conditions (−0.54 V).²¹ The η(E_cat/2) for Pt₂(bpia)I₃ is substantially larger than that of Pt₂(bpia)Cl₃ (420 mV,¹⁴ Table S5), indicating that the driving force for the HER catalyzed by Pt₂(bpia)I₃ is larger than that of Pt₂(bpia)Cl₃. This is in line with the higher photocatalytic activity of Pt₂(bpia)I₃ compared to that of Pt₂(bpia)Cl₃, as shown below.

Fig. 2 LSVs of an aqueous acetate buffer solution (0.09 M, pH = 5.0) containing 10 vol% NMP in the presence (red) and absence (black) of 0.1 mM Pt₂(bpia)I₃. The scan rate for each measurement was 0.1 V s⁻¹. The diameter of the glassy carbon working electrode is 1 mm.

During 12 h of photolysis, Pt₂(bpia)I₃ evolves 10 μmol (1.1 mL) of H₂ from a 10 mL aqueous acetate buffer (0.09 M, pH 5.0) containing NMP (9 : 1, v/v) only in the presence of EDTA under visible light irradiation (400 < λ < 800 nm) (Fig. 3a), revealing that Pt₂(bpia)I₃ also serves as a diplatinum-type single-molecular photocatalyst. The TOF_max estimated for Pt₂(bpia)I₃ (4.4 h⁻¹, Table S6) is 4 times higher than that of Pt₂(bpia)Cl₃ (1.1 h⁻¹,¹⁴ Table S6), evidencing the rate-accelerating effect of iodide. As confirmed for Pt₂(bpia)Cl₃,¹⁴in situ dynamic light scattering measurements for the photolysis solution of Pt₂(bpia)I₃ were carried out to clearly rule out the formation of platinum nanoparticles during the initial 6 h of photolysis (Fig. S12 and S13), evidencing the homogeneous nature of catalysis by Pt₂(bpia)I₃. The reason for the higher activity of Pt₂(bpia)I₃ compared to Pt₂(bpia)Cl₃ can be discussed based on the following two key factors. One is the superior visible-light-harvesting ability of Pt₂(bpia)I₃ in the range of 400–500 nm. The other is the higher efficiency in the reductive quenching of ³*Pt₂(bpia)I₃ by EDTA, as discussed below. Thus, the obvious decrease in the rate of H₂ production for Pt₂(bpia)I₃ after 4 h of photolysis is likely due to the dissociation of iodide (Fig. 3a). However, our efforts to suppress the iodide dissociation by adding NaI failed due to the spontaneous reaction of NaI with NMP, leading to the formation of deep orange-colored species interfering with the photocatalysis (Fig. S14 and S15), as described elsewhere.²² In the presence of 0.1 M NaI, the triplet emission is totally quenched (Fig. S16) and H₂ production is substantially suppressed (Fig. S17).

Fig. 3 Photocatalytic H₂ production from an aqueous acetate buffer solution (0.09 M, pH = 5.0) containing 10 vol% NMP and 30 mM EDTA in the presence of 0.05 mM Pt₂(bpia)I₃ (A) or 0.05 mM Pt₂(bpia)Cl₃ (B) under photoirradiation at 400 < λ < 800 nm (a), 500 < λ < 800 nm (b), and 580 < λ < 800 nm (c). Photolysis solutions for (B) in all three measurements also contain 0.1 M NaCl to suppress the ligand replacement of chloride.

Even under the visible-light irradiation conditions (λ > 500 nm), Pt₂(bpia)I₃ also evolves 922 nmol of H₂ (Fig. 3b), proving that Pt₂(bpia)I₃ promotes the lower-energy-light-driven HER via the S–T transitions since only direct S–T transitions take place under these conditions (Fig. 1, Tables S1 and S2). To our surprise, Pt₂(bpia)I₃ also shows the photocatalytic activity for the HER higher than that of Pt₂(bpia)Cl₃ under photoirradiation above 500 nm even though the observed difference is not ascribable to the difference in their light-harvesting ability within this wavelength domain, as shown by their similarly leveled molar absorption coefficients (Fig. 1 and S18). Moreover, even under the red-light irradiation conditions (λ > 580 nm), Pt₂(bpia)I₃ evolves 134 nmol of H₂ over 6 h of photolysis (Fig. 3c), which is 7 times larger than H₂ evolution photocatalyzed by Pt₂(bpia)Cl₃. These results clearly indicate that Pt₂(bpia)I₃ is much superior to Pt₂(bpia)Cl₃ in promoting red-light-driven HER in water via the S–T transitions.

To better understand the behavior of Pt₂(bpia)I₃ during the photocatalysis, several additional experiments were performed as follows. First, emission quenching behavior was investigated at various EDTA concentrations (0–60 mM), revealing a decrease in emission intensity as the EDTA concentration increases (Fig. S19). The Stern–Volmer plot made by these results shows a linear correlation consistent with a dynamic quenching of the triplet (³*Pt₂(bpia)I₃) by EDTA (Fig. 4a). By dividing the Stern–Volmer coefficient (K_SV = 6.2 M⁻¹) by the emission lifetime in the absence of a quencher (τ₀ = 20 ns), the quenching rate constant can be determined as k_q = 3.1 × 10⁸ M⁻¹ s⁻¹. It should be noted that the K_SV together with the k_q for Pt₂(bpia)Cl₃ were realized to be too small and could not be estimated.¹⁴ In other words, Pt₂(bpia)I₃ has a substantially superior efficiency relative to Pt₂(bpia)Cl₃ in promoting the reductive quenching required for the photocatalytic HER. Nevertheless, the reduction potential of ³*Pt₂(bpia)I₃ (ca. 1.08 V), calculated from the first reduction potential in water (−0.92 V, Table S7) and the E_0–0 (ca. 2.0 eV, Fig. S20), is almost identical to that of ³*Pt₂(bpia)Cl₃ (ca. 1.05 V),¹⁴ suggesting that the driving force for the quenching is not significantly different between them.

Fig. 4 (a) Stern–Volmer plot for the emission intensity of photolysis solution of Pt₂(bpia)I₃ against EDTA concentration. (b) Initial rate of H₂ production as a function of EDTA concentration, estimated from the amount of H₂ produced from the initial 30 to 60 min of photolysis under visible light irradiation (400 < λ < 800 nm). The calculated line (dotted line) is a least-squares fit to Michaelis–Menten equation. (c) Initial rate of H₂ production as a function of the square of Pt₂(bpia)I₃ concentration, estimated from the amount of H₂ produced from the initial 30 to 60 min of photolysis under visible light irradiation (400 < λ < 800 nm).

Next, the initial rate of H₂ evolution photocatalyzed by Pt₂(bpia)I₃ was investigated at various EDTA concentrations (0–100 mM), revealing clear saturation kinetics with the maximum rate of ca. 1.0 μmol h⁻¹ (Fig. 4b and S21). In comparison, the initial rate of H₂ evolution by Pt₂(bpia)Cl₃ roughly exhibits saturation kinetics in the same EDTA concentration range with the maximum rate of only ca. 0.2 μmol h⁻¹.¹⁴ Thus, these observations also indicate that Pt₂(bpia)I₃ shows a photocatalytic performance much higher than that of Pt₂(bpia)Cl₃ due to the higher efficiency in reductive quenching of ³*Pt₂(bpia)I₃ by EDTA. Note that our previous studies emphasized the importance of ion-pair adduct formation between the positively charged single-molecular photocatalysts and the divalent EDTA anion as shown in Scheme 2.^11,12 However, it is important to pay attention to the fact that Pt₂(bpia)I₃ does not rely on ion-pair formation with the dianionic form of EDTA under the photolysis conditions. Thus, the observed higher efficiency in reductive quenching of ³*Pt₂(bpia)I₃ can be correlated with the higher probability of ET between the EDTA donor and the acceptor orbitals of ³*Pt₂(bpia)I₃. The donor orbital is an amine donor of EDTA, while the acceptor site in the triplet can be computed by DFT. As depicted in Fig. 5a, the oxidized part at the ³MX-LCT state can be recognized as the lower SOMO computed for ³*Pt₂(bpia)I₃, revealing that it is largely localized over the iodide singly coordinated to bis(pyridyl)amidoplatinum. In contrast, the SOMO of ³*Pt₂(bpia)Cl₃ consists of the dichlorodiimineplatinum- and bis(pyridyl)amidochloroplatinum-derived orbitals (Fig. 5b). More importantly, ³*Pt₂(bpia)I₃ shows a substantially larger spatial distribution at the accepting iodide orbital, indicative of the higher probability to accelerate the long-range ET (Fig. 5). As described elsewhere, the probability of ET between the donor (D) and acceptor (A) can be correlated with the relevant electronic coupling factor (H_DA).²³ Moreover, this factor is essentially governed by the degree of overlap between the donor and acceptor orbitals. In principle, the iodide has much larger wavefunction leakage relative to the chloride, and thus affords a larger H_DA resulting in higher reductive quenching efficiency in spite of bearing inferior excited-state properties. In our previous report on Pt₂(bpia)Cl₃,¹⁴ the relatively slow H₂ evolution rate was primarily attributed to the lower probability of reductive quenching because the imine moieties, considered as the electron acceptor site in ³*Pt₂(bpia)CI₃, are sterically blocked by the solvated NMP molecules.¹⁴ In contrast, ³*Pt₂(bpia)I₃ possesses the spatially larger electron-accepting iodide orbital even if the imine moieties are similarly solvated by the NMP molecules (Fig. 5a), which rationalizes the higher probability of ET to promote the reductive quenching of ³*Pt₂(bpia)I₃ relative to ³*Pt₂(bpia)Cl₃.

Fig. 5 Lower energy SOMO of ³*Pt₂(bpia)I₃ (a) and ³*Pt₂(bpia)Cl₃ (b).

Finally, the initial rate of H₂ evolution was further investigated at various Pt₂(bpia)I₃ concentrations (0–60 μM). The initial rate of H₂ evolution, estimated from the initial 2 h of photolysis, shows a second-order dependence on the Pt₂(bpia)I₃ concentration (Fig. 4c and S22), indicating that the H₂ evolution rate is limited by a step that is second order in either photocatalyst or photocatalyst-intermediate concentration. Nevertheless, dimerization of Pt₂(bpia)I₃ in its ground state is negligible because the absorbance change of Pt₂(bpia)I₃ completely obeys Beer's law at concentrations below 90 μM (Fig. S4). By analogy with the mechanism previously clarified for Pt₂(bpia)Cl₃,¹⁴ the rate-limiting step can be regarded as a bimolecular coupling of Pt₂(bpia⁻˙)I₃ to yield H₂ and two equivalents of Pt₂(bpia)I₃.

Conclusions

In summary, we have successfully synthesized Pt₂(bpia)l₃ and characterized its photochemical and photophysical properties together with the photocatalytic activity for the HER from water with particular attention paid to its low-energy-light-harvesting ability. Upon replacing chloride with iodide, the absorption and emission energies both show red shifts basically due to the destabilization of the HOMO by the mixing of iodide orbitals which possess essentially higher orbital energies compared to those of chloride. The HOMO–LUMO gap thus exhibits a little shrinkage upon iodide ligation, leading to a lower luminescence quantum yield together with a shorter triplet lifetime by obeying the energy gap law. The unique S–T transition features of Pt₂(bpia)Cl₃ are preserved by Pt₂(bpia)I₃ without fundamental changes observed in the excited-state redox properties relevant to the photocatalytic HER. In spite of apparently disadvantageous photophysical features induced by the introduction of iodides, Pt₂(bpia)I₃ is ascertained to promote the photocatalytic HER at a considerably higher rate compared with Pt₂(bpia)Cl₃. This enhancement is rationally interpreted as due to the substantially more excellent acceptor characteristics of iodide based on the larger spatial distribution of the acceptor orbital participating in the outer-sphere ET from EDTA. Our study thus, for the first time, demonstrates an outstanding role of a heavier halide ion ligated to the catalytically active platinum center in order to have a substantially enhanced photoinduced ET rate in such photocatalytic reactions.

Experimental section

Materials

All solvents and reagents were of the highest quality available and were used as received. K₂PtCl₄ was purchased from TANAKA Kikinzoku Kogyo K. K. All other reagents were purchased from Tokyo Chemical Industry Co., Ltd, and were used as received. Pt₂(bpia)Cl₃,¹⁴ Hbpia,¹⁵ and PtI₂(cod)¹⁸ were synthesized according to the literature methods. Purification of water was performed with an Advantec RFD250RB water distillation apparatus.

Synthesis of [I₂Pt(bpia)PtI]·0.5NMP (Pt₂(bpia)I₃)

Hbpia (33 mg, 0.15 mmol) and PtI₂(cod) (167 mg, 0.3 mmol) were dissolved in 4 mL dry DMF. The solution was stirred for 48 h at 110 °C followed by cooling it down to room temperature. The brown precipitate formed was collected by filtration, washed with chloroform (ca. 20 mL), acetone (ca. 20 mL), and diethyl ether (ca. 20 mL). Further purification was performed by recrystallization from a Pt₂(bpia)I₃-saturated NMP solution by adding excess acetone. Yield (brown powder): 15.1 mg, 10%. Anal. Calcd for C_14.5H_14.5I₃N_5.5O_0.5Pt₂ ([I₂Pt(bpia)PtI]·0.5NMP): H, 1.40; C, 16.67; N, 7.37. Found: H, 1.53; C, 16.76; N, 7.75. ESI-TOF mass (CH₃CN containing 5 vol% DMSO, positive mode) (m/z): 946.0 [M–I⁻ + DMSO]⁺ ([C₁₄H₁₆I₂N₅OPt₂S]⁺, calcd 946.0). The ¹H NMR spectrum could not be obtained due to the extremely low solubility in common organic solvents.

Measurements

¹H NMR spectra were acquired on a JEOL JNM-ESA 600 MHz spectrometer. ESI-TOF mass spectra were recorded on a JEOL JMS-T100LC mass spectrometer in positive ion mode. Infrared spectra were recorded on a PerkinElmer Spectrum One FT-IR spectrometer equipped with a Diamond ATR Crystal. UV-visible absorption spectra were recorded on a Shimadzu UV-3600 spectrophotometer, where all the sample solutions were thermostated at 20 °C. Transmittance spectra for two optical cutoff filters (Y-52 and R-60) were also recorded on a Shimadzu UV-3600 spectrophotometer. Luminescence spectra were recorded on a Shimadzu RF5300PC spectrofluorophotometer, equipped with a Hamamatsu R928 photomultiplier tube. Luminescence quantum yields were determined using a Hamamatsu C9920-02 absolute photoluminescence quantum yield measurement system equipped with a 150 W Xe lamp coupled to a monochromator and a Hamamatsu C10027-01 multichannel detector. Emission decays were recorded on a HORIBA FluoroCube 3000USKU, in which a diode laser (λ = 374 nm) (HORIBA N-470L) was used as the excitation source. Cyclic voltammograms (CVs) and square wave voltammograms (SWVs) were measured on a BAS ALS Model 602DKM electrochemical analyzer using a glassy carbon working electrode (Φ = 3 mm), a carbon rod counter electrode, and an Ag/Ag⁺ reference electrode. The electrolyte solution was a mixed solvent of DMF and NMP (4 : 1, v/v) containing 0.1 M TBAPF₆ (tetra(n-butyl)ammonium hexafluorophosphate). The potentials were referenced relative to the internal ferrocene/ferrocenium couple (0.45 V vs. SCE in DMF).²⁴ Linear sweep voltammograms (LSVs) were measured on a BAS ALS Model 602DKM electrochemical analyzer using a glassy carbon working electrode (Φ = 1 mm), a carbon rod counter electrode, and a saturated calomel reference electrode (SCE). The electrolyte solution was an aqueous acetate buffer solution (0.09 M, pH = 5.0) containing 10 vol% NMP. All solutions used in these measurements were bubbled with Ar at least 20 min prior to the measurements. The pH values of the buffer solutions were measured by using a DKK-TOA HM-25R pH meter. Dynamic light scattering (DLS) experiments were carried out using an Otsuka Electronics ELSZ-2PS particle analyzer equipped with a diode laser (660 nm). Sample solutions used for the DLS measurements were filtered via a Whatman syringe filter (0.2 μm) twice to remove possible insoluble impurities and particles in solution before initiating each photolysis experiment.

Photocatalytic experiments

The photolysis solution (0.09 M acetate buffer solution (0.027 M CH₃COOH and 0.063 M CH₃COONa), pH = 5.0) contains 10 vol% NMP, 30 mM EDTA, and 0.05 mM Pt₂(bpia)I₃. Each photolysis solution was bubbled with Ar for 30 min before the photolysis experiment and kept at 20 °C during the photolysis. Photoirradiation was carried out using an ILC Technology CERMAX LX-300 Xe lamp equipped with a CM-1 cold mirror (400 < λ < 800 nm, 300 W). The photolysis vial (21 mL) was immersed in a 20 °C water bath to remove IR radiation and to eliminate the temperature effects. The quantitative analysis of the H₂ evolved during the photocatalysis was carried out by using an automatic H₂ monitoring system developed by our group.²⁵ In this system, a continuous flow of Ar (10.0 mL min⁻¹, controlled by using an STEC SEC-E40/PAC-D2 digital mass flow controller) was bubbled through each photolysis solution (10 mL) contained in a Pyrex vial (ca. 20 mL). The vent gas from the vial was introduced into a 6-way valve, which allowed the automatic injection of the sample gas onto a gas chromatograph (Shimadzu GC-8A gas chromatograph equipped a Molecular Sieve 5A column of 2 m × 3 mm i.d. at 30 °C). The injection of sample gas was made by a control software operating on a Windows system. The signal output from the thermal conductivity detector was monitored by using a Shimadzu C-R8A integrator. Photocatalytic measurements under lower energy light irradiation conditions were conducted by using a Y-52 or R-60 cutoff filter (HOYA corporation), and the quantification of the amounts of H₂ evolved during the photocatalysis was manually performed by using a gastight syringe. The output signal from the thermal conductivity detector was recorded on a Shimadzu C-R8A integrator.

DFT calculations

Density functional theory (DFT) calculations were performed using the Gaussian 16 packages²⁶ to better understand the structural and spin-state candidates. The structures were fully optimized using the M06 hybrid functional²⁷ with the effect of solvation in water taken into consideration using the conductor-like polarizable continuum model (C-PCM) method.²⁸ The SDD basis set was adopted for the Cl, I and Pt atoms, while the 6-31+G* basis set was applied to the rest of atoms. Electronic excited states were calculated by the time dependent density functional theory (TD-DFT) calculation method as implemented in Gaussian 16 with the use of the same functional and basis sets described above.

Author contributions

T. K. performed the experiments, analyzed the data, and wrote the original draft. All the authors discussed the results and improved the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the Supplementary information (SI). Supplementary information: IR, absorption and emission spectra, DFT calculations, and results of electrochemical and photocatalytic measurements. See DOI: https://doi.org/10.1039/d5sc06457k.

Acknowledgements

This work was supported by JSPS KAKENHI grant numbers JP21H01952, JP22KK0074, JP23K04779, JP23H03831, JP24K01500, and JP24K01611. This work was also supported by JST PRESTO Grant Number JPMJPR22QA. H. O. acknowledges financial support from the ENEOS Tonengeneral Research/Development Encouragement & Scholarship Foundation and the Iketani Science and Technology Foundation. T. K. acknowledges financial support from the Kyushu University Q-PIT Support Program for Young Researchers and Doctoral Students, and JST SPRING, Japan Grant Number JPMJSP2136.

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