# Efficient Visible-Light-Driven Hydrogen Production in Aqueous Media by the Association of the Triazatriangulenum (TATA+) Dye and the DuBois' Nickel Catalyst

Songling Lyu, Daniel H Cruz Neto, Juan Aguirre-Araque, Lucile Termeau, Fakourou Camara, Sawsen Cherraben, David Martin, Eric Brémond, Thomas Pino, Minh-Huong Ha-Thi, et al.

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Songling Lyu, Daniel H Cruz Neto, Juan Aguirre-Araque, Lucile Termeau, Fakourou Camara, et al. Efficient Visible-Light-Driven Hydrogen Production in Aqueous Media by the Association of the Triazatriangulenum (TATA+) Dye and the DuBois' Nickel Catalyst. Artificial Photosynthesis, 2025, 10.1021/aps.5c00013. [hal-05246639](https://hal.science/hal-05246639v1)

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# Efficient Visible-Light-Driven Hydrogen Production in Aqueous Media by the Association of the Triazatriangulenium (TATA⁺) Dye and the DuBois' Nickel Catalyst

Songling Lyu, Daniel H. Cruz Neto, Juan Aguirre-Araque, Lucile Termeau, Fakourou Camara, Sawsen Cherraben, David Martin, Eric Brémond, Thomas Pino, Minh-Huong Ha-Thi, Philippe P. Lainé,* Marie-Noëlle Collomb,* and Jérôme Fortage*

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Cite This: https://doi.org/10.1021/aps.5c00013

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Supporting Information

**ABSTRACT:** In 2018, we introduced a triazatriangulenium (TATA⁺) dye as an efficient photosensitizer for visible-light-driven hydrogen production in water. In an acidic aqueous medium (pH 4.5), combining TATA⁺ with a cobalt tetraaza macrocyclic catalyst and ascorbate as a sacrificial electron donor led to sustained H₂ evolution, outperforming the benchmark [Ru(bpy)₃]²⁺. Here we investigated the performance of TATA⁺ with a water-soluble derivative of the DuBois nickel H₂-evolving catalyst (NiP) under similar conditions. At pH 4.5, turnover numbers per catalyst ranged from 2800 to 19500 depending on catalyst concentration (50 to 1 µM), significantly higher than those previously reported with cobalt or NiP using other photosensitizers. A detailed photophysical study revealed that TATA⁺ exhibits thermally activated delayed fluorescence (TADF), involving an equilibrium between its singlet and triplet excited states. Only the singlet excited-state undergoes reductive quenching by ascorbate, forming the radical TATA*, which in turn reduces the NiP catalyst. The high photocatalytic efficiency is attributed to the stability of TATA*, due to delocalization of the radical across the dye's structure. This delocalization was confirmed by EPR, UV-visible spectroscopy, and DFT calculations. These findings underscore the promise of TATA⁺ dyes for photocatalytic applications, even under acidic aqueous conditions.

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**KEYWORDS:** triazatriangulenium, organic dye, nickel phosphine catalyst, hydrogen production, photocatalysis

## 1. INTRODUCTION

Molecular hydrogen (H₂) is still regarded as a viable alternative to fossil fuels, and its production via light-driven water-splitting reactions has attracted significant interest from the chemistry community.¹⁻⁴ The first homogeneous photocatalytic systems for H₂ production in water were introduced as early as the late 1970s by Shilov,⁵ Lehn/Sauvage,⁶,⁷ Amouyal/Kagan,⁸,⁹ Kalyanasundaram/Grätzel,¹⁰,¹¹ Meyer/Whitten,¹² and Harri- man.¹³ These systems combined a sacrificial electron donor (SD), a molecular photosensitizer (PS), a redox mediator, and a colloidal platinum-based catalyst (Cat). Most of these photocatalytic systems employed the ruthenium tris-bipyridine complex ([Ru(bpy)₃]²⁺, denoted **Rubpy**; see Scheme 1) as a photosensitizer owing to its remarkable properties.¹⁴,¹⁵ **Rubpy** exhibits strong absorption in the visible spectrum (ε ≈ 14,600 M⁻¹ cm⁻¹ at 452 nm in water), a long triplet excited state lifetime (³MLCT, τ ≈ 600 ns in deaerated water), and excellent solubility in both water and organic solvents. Moreover, its efficient electron transfer capability, enabling it to readily accept or donate electrons in the excited state, makes it an ideal intermediate in photoinduced processes. Since these

early studies, numerous photocatalytic systems for H₂ production have been reported and have shifted toward catalysts based on molecular complexes of non-noble metals (particularly cobalt, nickel, and iron) that operate without a redox mediator.⁴,¹⁶,¹⁷ Nevertheless, **Rubpy** remains the benchmark photosensitizer in these three-component PS/ Cat/SD systems, and this, despite its poor stability in acidic water, which constitutes the primary cause of system deactivation. Specifically, when the photocatalytic mechanism proceeds via the reductive quenching of the excited state [RuIII(bpy)₂(bpy•⁻)]²⁺ (denoted **Rubpy***) by the SD, the resulting reduced state [RuII(bpy)₂(bpy•⁻)]⁺ (denoted **Rubpy**•⁻) readily loses one bipyridine ligand. This loss results

Received: May 15, 2025

Revised: June 19, 2025

Accepted: June 27, 2025

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<figure><img src="image_10.png" /><figcaption>Scheme 1. Photosensitizers, Catalysts, and Sacrificial Donors Discussed in This Work</figcaption></figure>

in the formation of a bis-bipyridine complex that is inactive as a photosensitizer.<sup>18–25</sup>

Metal-free organic dyes, composed entirely of Earth-abundant elements, present a compelling alternative to noble metal-based photosensitizers like **Rubpy**.<sup>4,26,27</sup> Commercial dyes based on xanthene,<sup>28–30</sup> rhodamine,<sup>31</sup> and acridine/acridinium<sup>5,32</sup> have been predominantly used for photoinduced H<sub>2</sub> production in homogeneous systems, whereas only a few studies have employed synthetic dyes as Bodipy,<sup>33</sup> perylene,<sup>34,35</sup> fluorene,<sup>36</sup> or free-base porphyrin.<sup>37</sup> Nevertheless, the photocatalytic performance of these organic dye systems generally does not compete with those using **Rubpy**. Although these organic dyes exhibit strong absorption in the visible spectrum (ε > 30 000 M<sup>-1</sup> cm<sup>-1</sup>), their performance is often hindered by a short singlet excited state lifetime (τ ≤ 5 ns),

which limits efficient intermolecular electron transfer. Additionally, their relatively weak reducing power (E<sub>red</sub> between -0.54 and -1.22 V vs SCE) compared to that of **Rubpy** (E<sub>red</sub> = -1.5 V vs SCE) reduces the driving force necessary for catalyst reduction by the photosensitizer. Furthermore, most of these dyes rapidly decompose during photocatalysis, especially in acidic water, due to the poor stability of their radical reduced forms generated by a reductive quenching with a sacrificial donor, which undergo dehalogenation or hydrogenation of their organic backbones.<sup>26</sup>

In 2018, we introduced a novel class of organic photosensitizers from the triazatriangulenum family<sup>38–41</sup> for visible-light-driven H<sub>2</sub> production. The water-soluble tris(ethoxyethanol)-triazatriangulenum derivative<sup>26</sup> (denoted TATA<sup>+</sup>, Scheme 1) has proven to be an outstanding
photosensitizer when combined with the cobalt tetraazamacrocyclic complex [Co<sup>III</sup>(CR14)Cl<sub>2</sub>]Cl catalyst (denoted CoCR14, Scheme 1) and ascorbate (HA<sup>-</sup>) as a sacrificial electron donor in a purely aqueous acidic medium, outperforming in terms of stability the benchmark Rubpy. The high performance of this photocatalytic system was attributed not only to the high efficiency of this cobalt catalyst in water,<sup>24,42–49</sup> but also to the remarkable properties of this dye, which are particularly well-suited for photoinduced electron transfer reactions proceeding *via* a reductive quenching pathway to drive catalytic processes.<sup>41</sup> Indeed, TATA<sup>+</sup> combines a relatively long singlet excited-state lifetime of 14 ns in water, the robustness of the reduced radical form, TATA*, contrasting with the poor stability of the radical form of most organic dyes described above, as well as a relatively negative reduction potential of –1.18 V vs SCE.<sup>26</sup> Following this work, this triazatriangulenum dye was successfully used by other groups for photoinduced catalytic reduction of CO<sub>2</sub> to CO<sup>50</sup> as well as organic photoredox catalysis.<sup>51</sup>

Our objective of this study is to determine whether this water-soluble TATA⁺ dye can efficiently activate other H₂-evolving catalysts in aqueous media. We focused on the well-known DuBois' catalyst, a nickel complex featuring pendant amines incorporated in two cyclic diphosphine ligands that function as a proton relay. Owing to the optimal basicity of these amine groups and their strategic positioning relative to the metal center, this molecular complex efficiently catalyzes both the oxidation and production of H₂ at a low over-potential.<sup>52–55</sup> It is also regarded as one of the most effective molecular catalysts for proton reduction in both organic and acidic aqueous media.<sup>56</sup> In this study, we employed a commercially available water-soluble derivative functionalized with phosphonate groups (denoted NiP, Scheme 1), previously reported by Reisner's group, which has shown a relatively high performance for photoinduced catalytic H₂ production in acidic water when paired with Rubpy<sup>57</sup> or quantum dots<sup>58,59</sup> as the photosensitizer. The photocatalytic performance of the new TATA⁺/NiP/HA⁻ system was compared with that of several previously reported systems using the DuBois nickel catalyst<sup>35,57–63</sup> and with the TATA⁺/CoCR14/HA⁻ system.<sup>26</sup> Additionally, for the first time, we investigated the high stability of TATA*—a key factor contributing to the exceptional performance of TATA⁺-based photocatalytic systems—by combining X-band EPR and UV-visible spectroscopy with theoretical calculations to evaluate the degree of delocalization of the radical across the entire TATA platform. Finally, we also conducted a comprehensive photophysical study to assess the kinetics of electron transfer within this molecular system and the photocatalytic mechanism.

## 2. RESULTS AND DISCUSSION

### 2.1. Photocatalytic Activity for Hydrogen Production with the TATA⁺/NiP/HA⁻ System.
The photocatalytic activity of the TATA⁺/NiP/HA⁻ system was evaluated in deaerated acidic water at pH 4.5 buffered with 1 M acetate and 0.1 M ascorbic acid/ascorbate as a sacrificial electron donor (H₂A/HA⁻). These conditions were previously identified as optimal for H₂ production with our system TATA⁺/CoCR14/HA⁻.<sup>26</sup> Similarly, Reisner's group also achieved the highest photocatalytic performance for H₂ production at this pH in water with NiP, in combination with [Ru(bpy)₂(bpy-(PO₃H₂))₂]<sup>2+</sup> (RubpyP, Scheme 1) photosensitizer and

ascorbate, either in homogeneous solution or adsorbed on TiO₂.<sup>57,64</sup>

We first accessed the stability of NiP in 1 M acetate buffer at pH 4.5 using UV-vis spectroscopy. While the redox properties of NiP have been studied in water and water/acetonitrile (see below),<sup>57</sup> its UV-vis absorption spectra have primarily been reported in organic solvents such as methanol<sup>57,65</sup> or acetonitrile with the derivative presenting phosphonate ethyl ester.<sup>66</sup> Figure 1 presents the UV-vis absorption spectra of NiP in a 1 M acetate buffer at pH 4.5 under argon, with concentrations ranging from 0.1 mM to 0.76 mM (Figure 1A). A concentration dependence is observed. At higher concentrations (0.5 and 0.76 mM), the spectra closely resemble those reported by Reisner's group in methanol at 0.1 mM, displaying a broad band at 496 nm along with a shoulder at 450 nm (Figure 1A).<sup>57</sup> As the concentration decreases to 0.1 mM, this visible band shifts to a lower wavelength by approximately 50 nm (maximum at 447 nm), while the molar absorption coefficient remains consistent (ε ≈ 1950 M⁻¹ cm⁻¹). The spectral evolution was also monitored over 7 days at 0.5 and 0.1 mM under both argon and air (Figure 1B,C). In the absence of oxygen, aqueous solutions of NiP remained stable regardless of concentration, with only a slight decrease in the intensity of the visible band—approximately 9% for 0.5 mM (Figure 1B) and 19% for 0.1 mM (Figure 1C) after 1 week. However, in the presence of oxygen, the solutions exhibited reduced stability over time with a progressive decrease in the visible band intensity, particularly at the lower concentration. At 0.5 mM, a 15% decrease is observed after 7 days (Figure 1B), whereas at 0.1 mM, the intensity decrease dropped by about 50% after 2 days and up to 85% after 7 days (Figure 1C). Based on these findings, stock solutions of NiP at 0.76 mM in 1 M acetate buffer were prepared and stored in a glovebox under argon for photocatalytic experiments. Solutions for photocatalytic experiments containing a mixture of TATA⁺, NiP, and HA⁻ were also prepared in a glovebox under argon (see the section Materials and Methods for further details).

The samples were irradiated using a xenon lamp (P = 74–78 mW cm⁻²) under visible irradiation, with two different filters: 400–700 nm and 420–720 nm. The TATA⁺ concentration was fixed at approximately 500 µM, while the NiP concentration varied from 50 to 1 µM, resulting in a PS/Cat ratio ranging from 10:1 to 500:1. Figure 2 presents the evolution of H₂ production over time in TONCat (Figure 2A) and in nH₂/vH₂ vs catalyst (Figure 2B) for catalyst concentrations of 50, 10, 5, and 1 µM. As previously observed in various photocatalytic molecular systems,<sup>48</sup> a decrease in catalyst concentration leads to an increase in both TONCat and TOFcCat. This trend is expected as a higher PS/Cat ratio enhances catalyst reduction, thereby promoting catalysis. Moreover, regardless of the catalyst concentration, the photocatalytic system demonstrates greater stability over time when irradiated in the 420–720 nm range compared to 400–700 nm, resulting in higher TONCat values at each concentration. Notably, the TONCat values measured here are remarkably high, ranging from approximately 2800 at 50 µM to 19500 at 1 µM, which correspond to 17.15 and 2.38 mL of H₂, respectively for a 5 mL solution (Figures 2 and Table S1). These values are more than twice those previously obtained with the TATA⁺/CoCR14/HA⁻ system, which ranged from 920 at 50 µM to 9000 at 1 µM (Table S1).<sup>26</sup> Although direct comparisons between catalyst activities should be made with
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<figure><img src="image_13.png" /><figcaption>**Figure 1.** UV-visible spectra of aqueous solutions of NiP at different concentrations (0.1, 0.5, and 0.76 mM) in 1 M acetate buffer at pH 4.5 under argon (A). Evolution over time (7 days) of the spectra of aqueous solutions of NiP at 0.5 (B) and 0.1 mM (C) under argon (solid trace) or air (dashed trace); optical path = 1 cm.</figcaption></figure>

caution, since experimental conditions are never strictly identical or conversely, the optimum operating conditions for

the catalysts may be different, the HER activity of NiP appears to be higher than that of CoCR14. Nonetheless, these results confirm that the NiP catalyst is highly efficient for the H₂ production in water. The quantum yields measured for the TATA⁺/NiP/HA⁻ system in both ranges of wavelength (400–700 and 420–720 nm, Figure S1), between 3.8 and 12.8% for catalyst concentrations ranging from 5 to 50 µM, are relatively high (Table S1). These yields are consistent with those reported in the literature for homogeneous molecular photocatalytic systems devoted to H₂ production and confirm the efficiency of our system.⁴

Control experiments have also been performed. Indeed, while the TATA⁺ and HA⁻/H₂A mixture does generate some H₂, the amount is minimal (0.027 mL), accounting for only 1.1 and 1.6% of the H₂ produced at the lowest NiP concentration of 1 µM (2.38 and 1.72 mL of H₂ produced, respectively) and can therefore be considered negligible (Table S1 and Figure 1B). Furthermore, in the absence of either HA⁻/H₂A or NiP, the system did not generate quantifiable amounts of H₂.

Another key characteristic of H₂-evolving photocatalytic systems using TATA⁺ is their high stability over time. Indeed, both the TATA⁺/NiP/HA⁻ and TATA⁺/CoCR14/HA⁻ systems remain stable even at very low catalyst concentrations of 1 µM. In other words, reducing the amount of catalyst does not compromise the system's stability, as H₂ production remains effective even after about 22 h. This stands in sharp contrast to similar systems in acidic water that use Rubpy as a photosensitizer.¹⁸⁻²⁵ In such systems, the poor stability of the reduced form of Rubpy significantly impacts performance, meaning that lowering the catalyst concentration typically leads to a marked decline in system stability, with H₂ production ceasing after only a few hours. Indeed, when the reduced state of the PS is not very stable in acidic water—as is the case for Rubpy—a higher concentration of the catalyst enhances the electron transfer from the reduced PS to the catalyst. This shortens the time that the reduced PS remains in solution, thereby limiting its degradation.

The stability of TATA⁺ was accessed by recording the UV-visible absorption spectra at the end of each experiment (Figure 3). The intensity of the TATA⁺ absorption band at 530 nm decreased by no more than 3% after 21 to 24 h of irradiation, regardless of the filter used and catalyst concentration, highlighting the excellent stability of TATA⁺ under photocatalytic conditions. Interestingly, a higher degradation of around 25% was previously observed for the TATA⁺/CoCR14/HA⁻ system in the same aqueous buffer after a similar irradiation period.²⁶ This difference could contribute to the higher performance of the TATA⁺/NiP/HA⁻ system as well as the remarkable catalytic activity of NiP for H₂ generation. Regardless of whether irradiation occurs between 400–700 nm or 420–720 nm, the degradation of TATA⁺ remains identical. This suggests that the reduced stability of the photocatalytic system under 400–700 nm irradiation is likely due to the poorer stability of the NiP catalyst under these conditions rather than that of TATA⁺.

Otherwise, since the degradation of TATA⁺ after 22–23.5 h of irradiation can be considered as negligible, the decline in H₂ production over time could primarily be attributed to the accumulation of dehydroascorbic acid generated during photocatalysis (Scheme S1). This effect may be further exacerbated by the possible degradation of NiP. Dehydroascorbic acid (DHA), the oxidized and deprotonated form of HA⁻,⁶⁷ is a good electron acceptor. It can trap electrons from
<figure><img src="image_14.png" /><figcaption>**Figure 2.** Photocatalytic hydrogen production in TON vs catalyst (A) and in $n_{H_2}/v_{H_2}$ vs catalyst (B) as a function of time from a deaerated 1 M acetate buffer (5 mL) at pH 4.5 in the presence of TATA⁺ (500 µM), HA⁻/H₂A (0.1 M), and various concentrations of NiP (50, 10, 5, and 1 µM), under visible light irradiation using a filter 420–720 nm (solid trace) or 400–700 nm (dashed trace).</figcaption></figure>

the reduced state of the photosensitizer TATA⁺, preventing the reduction of NiP. As a result, the photocatalytic cycle is disrupted, ultimately hindering H₂ production.⁶⁸⁻⁷⁰

The photocatalytic activity of the TATA⁺/NiP/HA⁻ system was also compared to previously reported photocatalytic systems for H₂ production based on NiP or other derivatives of DuBois' nickel catalyst in association with molecular or inorganic photosensitizers (Scheme 1). The performances of these systems are summarized in Table 1.

At first glance, the TON<sub>Cat</sub> values achieved with TATA⁺/NiP/HA⁻ far exceed those reported in previous studies, regardless of the catalyst concentration. The catalytic activity of NiP in acidic water has been investigated by the Reisner group in the presence of [Ru(bpy)₂(bpy(PO₃H₂))₂]²⁺

(RubpyP),⁵⁷ carbon dot (CQD),⁵⁸,⁵⁹ and polymeric carbon nitride (CNₓ).⁶¹ However, highest reported performance reached only 1094 TON<sub>Cat</sub> after 24 h of irradiation using 3.3 µM NiP combined with 10 mg of CQD and a SD mixture of tris(2-carboxyethyl) phosphine (TCEP, 0.1M)/H₂A (0.1 M) (Table 1). Other groups, including those of Stupp,³⁴,³⁵ Huang,⁶² and Natali⁶³ have developed photocatalytic systems operating in acidic water with derivatives of the DuBois' nickel catalyst associated with perylene monoimide (PMI-COONa) or quantum dots (CdSe and CdTe/CdS). However, none of these systems has surpassed 800 TON<sub>Cat</sub>. Additionally, Eisenberg's group achieved 2700 TON<sub>Cat</sub> in a CH₃CN:H₂O (1:1; pH 2.25) mixture using a photocatalytic system combining the PS Rubpy (400 µM) with a nonsubstituted
<figure><img src="image_15.png" /><figcaption>**Figure 3.** UV–visible absorption spectra of 500 µM TATA (Cl) in 1 M acetate buffer (5 mL) at pH 4.5 and HA⁻/H₂A (0.1 M) obtained at the end of the photocatalytic experiments after about 21–24 h of irradiation using a filter 420–720 nm (A) or 400–700 nm (B) and with various concentrations of NiP (50, 10, 5, and 1 µM); optical path = 1 mm. The spectra of initial solutions (full line) correspond to a mixture of TATA⁺ and NiP in 1 M acetate buffer (pH 4.5), before the addition of 0.1 M HA⁻/H₂A.</figcaption></figure>

DuBois' nickel catalyst Ni-CH₃CN (140 µM) and 0.5 M H₂A.⁶⁰ However, this performance was obtained after 150 h of irradiation with regular additions of Rubpy and H₂A every 25 h in the photocatalytic solution. This demonstrates the great robustness of DuBois' nickel catalyst in a hydro-organic medium.

**2.2. Study of the Reduced State TATA• by EPR and UV-Visible Absorption Spectroscopies Coupled to TDDFT Calculation.** We previously correlated the great stability of TATA⁺ under photocatalytic conditions to the robustness of its reduced state TATA• generated via the reductive quenching of the singlet excited state of TATA⁺ (¹TATA⁺*) by HA⁻ occurring during photocatalysis (see below the discussion on mechanism).²⁶ We also hypothesized that the high stability of TATA• in solution could stem from the delocalization of the radical throughout the structure of the organic dye, which would make it less prone to react with protons present in solution. To evaluate the degree of delocalization of the radical, the electronic properties of TATA• were further assessed by UV-visible absorption and EPR spectroscopy in acetonitrile, together with computations performed with density-functional theory (DFT) and its time-dependent variant, using the PBE0 hybrid exchange-correlation functional⁷¹ and the polarizable continuum model (PCM) implicit solvation model.⁷²

We previously reported that the cyclic voltammogram of TATA⁺ in CH₃CN 0.1 M [Bu₄N]ClO₄ exhibits one reversible redox process at -1.48 V vs Ag/AgNO₃ (i.e., -1.18 V vs SCE) and one irreversible redox process at +0.96 V vs Ag/AgNO₃ (i.e., + 1.26 V vs SCE) assigned respectively to TATA⁺/TATA• and TATA²⁺•/TATA⁺.²⁶,⁷³ We have also shown that it is possible to generate quantitatively the one-electron-reduced form TATA•, by bulk electrolysis at -1.6 V vs Ag/AgNO₃ under an inert argon atmosphere at room temperature. The resulting solution is perfectly stable, at least for 1 h, demonstrating the excellent stability of TATA• in acetonitrile under anaerobic conditions. The electrogenerated solution was characterized by UV-visible and EPR spectroscopy. The EPR signature of the frozen solution of TATA• at 100 K is characteristic of an organic radical species, with a single isotropic signal centered at g = 2.²⁶ In this work, the isotropic X-band EPR spectrum of an electrogenerated TATA• solution was recorded at room temperature. The resulting spectrum features a complex hyperfine structure (Figure 4A), which could be fitted with a line-broadening parameter lw = 0.055

and the following set of hyperfine coupling constants: a(¹⁴N) = 4.1 MHz (2 nuclei), a(¹H) = 11.6 MHz (3 nuclei), a(¹H) = 2.9 MHz (6 nuclei) and a(¹H) = 1.9 MHz (6 nuclei).⁷⁴ In comparison, we assessed EPR parameters of TATA• by DFT at the PCM(acetonitrile)-PBE0/EPR-III level. Importantly, the computed hyperfine coupling constants were found very close to the experimental values: a(¹⁴N) = 4.2 MHz (2 nuclei), a(¹H<sub>ortho</sub>) = 10.7 MHz (3 nuclei), a(¹H<sub>meta</sub>) = 3.8 MHz (6 nuclei) and a(¹H<sub>CH₂</sub>) = 3.4 MHz (6 nuclei). As depicted in Figure 4B, the Mulliken spin density of TATA• is spread over the triazatriangulenium platform. The central atom bears only 38% of the Mulliken spin density. The expected delocalization of the radical certainly accounts for the great stability of the reduced state of TATA⁺ during photocatalysis.

TDDFT computations have also been carried out at the PCM(acetonitrile)-PBE0/6-31+G* level of theory in order to assign the UV-visible absorption spectra of TATA⁺ and TATA• in acetonitrile (Figure 5). The spectrum of TATA⁺ is featured by a strong and large absorption band in the visible region computed at 449 nm within the vertical TDDFT approximation. It is driven by a degenerated HOMO → LUMO transition of the π → π* type, both orbitals being delocalized over the whole TATA platform (Figure S2). The blueshift observed with respect to the experimental maximum of the band is explained by the strong vibronic character of the transition.⁷⁵ Indeed, the computation of the vibronic progression within the Franck-Condon-Hertzberg-Teller approximation⁷⁵,⁷⁶ shows a large and complex band, with a 0-0 transition lying at 496 nm. The UV-visible signature of TATA• differs from that of TATA⁺. It is characterized by a first band computed at 776 nm corresponding again to a π → π* transition (SOMO → LUMO) delocalized over the whole TATA⁺ platform (see Table 2 and Figure S3 in the Supporting Information). It is followed by a well reproduced maximum of absorption lying at 394 nm. TDDFT computations on TATA• also confirm the delocalized character of its molecular orbitals over the organic platform of the photosensitizer.

**2.3. Mechanistic Insight for the TATA⁺/NiP/HA⁻ System from Photophysical Measurements.** **2.3.1. Possible Mechanisms and Thermodynamics of Electron Transfers.** The possible mechanisms of the photocatalytic H₂ production with the TATA⁺/NiP/HA⁻ system are exhibited in Figure 6A. The first photoinduced step could be either an oxidative quenching of the singlet excited state of TATA⁺ (denoted ¹TATA⁺•) by Ni¹¹P (denoted Ni¹¹-L in Figure 6A, L
<table><caption>**Table 1. Performance of Photocatalytic Systems Reported in the Literature Employing NiP and Derivatives of DuBois' Nickel Catalyst for H₂ Production**</caption><thead><tr><th>components (concentration or mass)</th><th>photocatalytic conditions</th><th>TON<sub>Cat</sub> irradiation time</th><th>group, year<sup>ref</sup></th></tr></thead><tbody><tr><td>Rubpy (400 µM)</td><td>H<sub>2</sub>O/MeCN (1/1)</td><td>2700<sup>a</sup></td><td>Eisenberg, 2011<sup>60</sup></td></tr><tr><td>Ni-MeCN (140 µM)</td><td>pH 2.25</td><td>150 h</td><td></td></tr><tr><td>H<sub>2</sub>A (0.5 M)</td><td>λ > 410 nm, Hg lamp 200 W</td><td></td><td></td></tr><tr><td>RubpyP (22 µM)</td><td>H<sub>2</sub>O pH 4.5</td><td>651 (44 µM)</td><td>Reisner, 2014<sup>57</sup></td></tr><tr><td>NiP (44 or 9 µM)</td><td>λ > 420 nm</td><td>30 h</td><td></td></tr><tr><td>H<sub>2</sub>A (0.1 M)</td><td>100 mW/cm<sup>2</sup></td><td>723 (9 µM)</td><td></td></tr><tr><td>CN<sub>x</sub> (5 mg)<sup>b</sup></td><td>H<sub>2</sub>O pH 4.5</td><td>2 h</td><td>Reisner, 2014<sup>61</sup></td></tr><tr><td>NiP (6.6 µM)</td><td>λ > 300 nm</td><td>166</td><td></td></tr><tr><td>EDTA (0.1 M)</td><td>100 mW/cm<sup>2</sup></td><td>4 h</td><td></td></tr><tr><td>Hydrogel PMI-COONa (9.6 mM)</td><td>H<sub>2</sub>O pH 4.0</td><td>400</td><td>Wasielewski-Stupp, 2014<sup>34</sup> and 2015<sup>35</sup></td></tr><tr><td>+ PDDA (0.85 wt %)</td><td>250 W/cm<sup>2</sup></td><td>18 h</td><td></td></tr><tr><td>Ni-NH<sub>3</sub><sup>+</sup> (19.5 µM)</td><td>Halogen bulb</td><td></td><td></td></tr><tr><td>H<sub>2</sub>A/HA<sup>-</sup> (0.85 M)</td><td></td><td></td><td></td></tr><tr><td>CQD-COOH (10 mg)<sup>b</sup></td><td>H<sub>2</sub>O pH 6.0</td><td>64</td><td>Reisner, 2015<sup>58</sup></td></tr><tr><td>NiP (3.3 µM)</td><td>λ > 300 nm</td><td>4 h</td><td></td></tr><tr><td>EDTA (0.1 M)</td><td>100 mW/cm<sup>2</sup></td><td></td><td></td></tr><tr><td>CQD-COOH (10 mg)<sup>b</sup></td><td>H<sub>2</sub>O pH 5.0</td><td>1094</td><td>Reisner-Alberto, 2016<sup>59</sup></td></tr><tr><td>NiP (3.33 µM)</td><td>λ > 300 nm</td><td>24 h</td><td></td></tr><tr><td>TCEP/H<sub>2</sub>A (0.1M/0.1 M)</td><td>100 mW/cm<sup>2</sup></td><td></td><td></td></tr><tr><td>CdSe QD-MPA (13.7 µM)</td><td>H<sub>2</sub>O pH 3.8</td><td>511</td><td>Huang, 2017<sup>62</sup></td></tr><tr><td>Ni-SO<sub>3</sub><sup>-</sup> (20 µM)</td><td>LED 505 nm</td><td>12 h</td><td></td></tr><tr><td>H<sub>2</sub>A (0.1 M)</td><td>22 mW</td><td></td><td></td></tr><tr><td>CdTe/CdS QD-MPA (2.6 µM)</td><td>H<sub>2</sub>O pH 5.0</td><td>780</td><td>Natali, 2020<sup>63</sup></td></tr><tr><td>Ni-COOH (16 µM)</td><td>400 < λ < 700 nm</td><td>8 h</td><td></td></tr><tr><td>HA<sup>-</sup> (1.68 M)</td><td>Xe lamp</td><td></td><td></td></tr></tbody></table>

<sup>a</sup>Performance obtained after six successive additions of Rubpy (400 µmol) and H<sub>2</sub>A (320 nmol) in photocatalytic solution (4 mL) over 150 h of irradiation. <sup>b</sup>Photocatalytic solution of 3 mL. PDDA is poly(diallyl dimethylammonium) chloride. The other components of this table are defined in Scheme 1.

being the diphosphine ligand with two pendant amines), leading to the reduced state Ni¹-L (step 1') or a reductive quenching of ¹TATA⁺* by HA⁻, generating HA⁺ and TATA⁺ (step 1). Then, TATA⁺ is able to reduce the Ni¹-L catalyst into Ni¹-L (step 2, denoted ET). Photophysical experiments (see below) and reported earlier by us²⁶ clearly show that the reductive quenching by HA⁻ is the main deactivation pathway of ¹TATA⁺*, since the oxidative quenching by NiP does not occur (see discussion below). If TATA⁺ does not transfer an electron to Ni¹-L, a back electron transfer (BET) occurs with the oxidized forms of ascorbate, HA⁺, or dehydroascorbic acid (DHA) (Scheme S1), to regenerate the ground state TATA⁺.²⁶,⁴⁷,⁴⁸

In our previous study of the photocatalytic system for H₂ production combining TATA⁺, the CoCR14 catalyst with HA⁻ (Scheme 1), the lifetime of the excited state ¹TATA⁺*

was measured to be 14 ns.²⁶ This lifetime was long enough to allow an efficient and fast intermolecular electron transfer from HA⁻ to ¹TATA⁺* (i.e., the reductive quenching), which occurs with a time constant evaluated at ca. 3.7 ns.²⁶,⁷⁸ Otherwise, a recent study by Manoj's group⁷⁹ revealed that a TATA-EtOH⁺ dye (Scheme 1), a close analogue of TATA⁺ with ethyl alcohol groups linked to the nitrogen atoms, exhibits a multiresonance thermally activated delayed fluorescence (MR-TADF) in deaerated water with an emission lifetime of 15 µs, which is close to the lifetime of its triplet excited state (i.e., 21.6 µs).⁷⁹ The MR-TADF properties of TATA-EtOH⁺ stem from the weak energy difference of 0.14 eV between the singlet and triplet excited states of this dye, which enables an equilibrium between the two states and delays the fluorescence. Considering the strong structural and photophysical similarities between TATA⁺ and TATA-EtOH⁺,⁸⁰ it is not surprising to observe that TATA⁺ presents similar MR-TADF properties with an equilibrium at the excited state between ¹TATA⁺* and ³TATA⁺* (Figure 6A). Nevertheless, transient absorption spectroscopy experiments reveal that only ¹TATA⁺* is quenched by HA⁻, while ³TATA⁺* does not participate in the photoinduced electron transfer for H₂ production (see below). Thus, the MR-TADF properties of TATA⁺ have no direct effect on the photocatalytic performance.

Once the Ni¹-L species is generated after the Ni¹¹-L reduction by TATA⁺ (step 2, Figure 6A) two main catalytic mechanisms can occur. Inspired by the previous studies of the groups of DuBois,⁵⁴,⁸² Bullock,⁵⁶ and van Gastel⁶⁶ on the catalytic mechanism of H₂ release in organic solvents with nickel phosphine catalysts, Ni¹-L can undergo a protonation on the pendant amine of the ligand followed by a reduction by TATA⁺ into Ni⁰-LH (Step 3). The protonated amine acting as proton relay transfers its proton to the metal, giving the Ni¹¹H-L hydride species, for which the pendant amine is protonated again to generate the intermediate Ni¹¹H-LH. The close proximity of a highly reactive hydride species and a protonated amine allows us to release H₂ and regenerate the initial Ni¹¹-L catalyst. Another catalytic mechanism with a water-soluble DuBois' nickel catalyst, bearing an arginine residue on the amine of the L ligand, was recently proposed by the groups of Artero and Costentin.⁸³ This mechanism does not involve a Ni⁰-LH species but a Ni¹¹H-L hydride intermediate. With this derivative, the proton on the amine-arginine moiety within the Ni¹-LH species is acidic enough to be transferred on the Ni¹ center to form Ni¹¹H-L hydride. Then the latter can undergo a proton coupled electron transfer (PCET) via TATA⁺ (step 3') to give the key intermediate Ni¹¹H-LH, that releases H₂.

The proposed photocatalytic mechanism can be supported by estimating the driving forces (ΔG⁰) related to the electron transfers involving the TATA⁺ dye. For the calculation of these driving forces, the redox potentials at the ground and excited states of TATA⁺, NiP and HA⁻ have been reported in Figure 6B and Table S2. As discussed above, the redox potentials of TATA⁺ in acetonitrile have been determined at +1.26 and -1.18 V vs SCE,²⁶ and we hypothesize that these values are close to those in water. In addition, the energy level E₀₀ of ¹TATA⁺* has been estimated to be 2.24 eV in water,⁸⁴ so the oxidation and reduction potentials of the singlet excited state can be evaluated at -0.98 and +1.06 V vs SCE, respectively ascribed to the couples TATA²⁺/¹TATA⁺* and ¹TATA⁺*/TATA⁺ (see Table S2 for more details). Considering the potentials of redox couples HA⁺/HA⁻ (+0.11 V vs SCE),⁸¹ Ni¹¹L/Ni¹¹L (-0.59 V vs SCE),⁵⁷ TATA²⁺/¹TATA⁺* and
<figure><img src="image_16.png" /><figcaption>**Figure 4.** (A) Experimental (top) and simulated (bottom) isotropic X-band EPR spectra of TATA• radical generated by electrolysis of a solution of TATA(PF₆) in CH₃CN + 0.1 M [Bu₄N]BF₄ at -1.60 V vs Ag/AgNO₃. (B) Representation of the spin density isosurface of TATA• at the PCM(acetonitrile)-PBE0/EPR-III level of theory (0.003 isovalue).</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>**Figure 5.** Experimental and TDDFT computed UV-visible absorption spectra of (left) TATA⁺ and (right) TATA• in acetonitrile. Modeled spectra are obtained from Gaussian convolution (fwhm = 0.3 eV) centered on vertical electronic transitions computed at TD-PCM(acetonitrile)-PBE0/6-31+G* level of theory (see methodology from ref 77). Right inset plots the vibronic progression of the first singlet-singlet transition computed within the Franck-Condon/Hertzberg-Teller model at room temperature (see methodology from ref 75). Experimental UV-visible absorption spectra were obtained from a 0.5 mM solution of TATA(PF₆) in CH₃CN + 0.1 M [Bu₄N]BF₄ (left) before and (right) after an exhaustive reduction at -1.60 V vs Ag/AgNO₃ (formation of TATA•), optical path = 1 mm (reproduced from ref 26, with the permission from the American Chemical Society, Copyright 2018).</figcaption></figure>

<table><caption>**Table 2.** UV/vis Absorption Spectra of TATA• Computed at the PCM(acetonitrile)-TD-PBE0/6-31+G* Levelᵃ</caption><thead><tr><th>energy (eV)</th><th>wavelength (nm)</th><th>f</th><th>type</th></tr></thead><tbody><tr><td>1.60 (deg.)</td><td>776</td><td>0.02</td><td>π → π*</td></tr><tr><td>3.15 (deg.)</td><td>395</td><td>0.12</td><td>π → π*</td></tr><tr><td>3.38 (deg.)</td><td>367</td><td>0.05</td><td>π → π*</td></tr></tbody></table>

ᵃ(deg.) stands for degenerated electronic transition.

¹TATA⁺*/TATA•, the reductive quenching of ¹TATA⁺* by HA⁻ and the oxidative one by Ni¹¹-L appear clearly exergonic with ΔG⁰ values of -0.95 and -0.39 eV respectively (steps 1 and 1' in Figure 6B). Nevertheless, only reductive quenching occurs according to the photophysical experiments presented below. Then the reduction of Ni¹¹-L to Ni¹⁻L by TATA• (generated by the reductive quenching) is also exergonic with -0.59 eV (step 2, Figure 6B) corroborating the efficient and rapid activation of catalyst by the reduced state of the dye observed by transient absorption spectroscopy (see below). In *fine*, the second reduction of nickel catalyst ensured by TATA•, corresponding to the reduction of Ni¹⁻LH to Ni⁰-LH (step 3) or the PCET reaction of Ni¹¹H-L to Ni¹¹H-LH (step

3'), is thermodynamically feasible with a significant ΔG⁰ value of -0.46 eV (Figure 6A and B).

2.3.2. Stern–Volmer Plots of the First Photoinduced Electron Transfers. The rate constant for the reductive quenching of ¹TATA⁺* by HA⁻ (k<sub>red</sub>) was previously determined to be 3.6 × 10⁹ M⁻¹ s⁻¹ using a Stern–Volmer analysis in aqueous solution buffered at pH 4.5 with 1 M acetate.²⁶ In contrast, the oxidative quenching of ¹TATA⁺* by Ni¹¹–L, as measured in this study (Figure S4), appears negligible despite a favorable driving force (ΔG⁰ = -0.36 eV), indicating an exergonic process. The absence of oxidative quenching herein is surprising, since a very fast quenching of ¹TATA⁺* by Co¹¹CR14 (k<sub>ox</sub> = 1.23 × 10⁹ M⁻¹ s⁻¹) has been previously determined, although the reduction of CoCR14, occurring at a lower potential (E<sub>1/2</sub>(Co¹¹/1) = -0.85 V vs SCE)<sup>42,85,86</sup> than that of NiP (E<sub>1/2</sub>(Ni¹¹/1) = -0.59 V vs SCE),⁵⁷ is more difficult. Thus, only reductive quenching can occur within the photocatalytic system TATA⁺/NiP/HA⁻. This was confirmed by transient absorption spectroscopy (see below).

2.3.3. Transient Absorption Spectroscopy. We further investigated the photocatalytic mechanism by means of nanosecond optical transient absorption spectroscopy. The
<figure><img src="image_18.png" /><figcaption>**Figure 6.** (A) Possible photocatalytic mechanisms for the light-driven H₂ production with the system TATA⁺/NiP/HA⁻; NiP is denoted here Ni-L where L is the ligand protonated (LH) or not on the amino function; note that Ni-L has two L ligands, although for simplicity only one L is mentioned in the name; in the catalyst drawings, the phenyl groups on phosphine are omitted for clarity. (B) Diagrams of potentials referred versus SCE for the redox couples of TATA⁺ photosensitizer at the ground and excited states,²⁶ NiP catalyst,⁵⁷ and HA⁻;⁸¹ the singlet excited state of TATA⁺ is denoted ¹TATA⁺*; ³onset of the catalytic wave for proton reduction with NiP in water at pH 4.5 in the presence of H₂A (0.1 M), that could involve the redox couple Ni¹⁻LH/Ni⁰-LH (step 3) or Ni³³H·L/Ni³²H·LH (step 3'').⁵⁷</figcaption></figure>

experimental setup used for such measurements has been described in detail in previous reports.⁸⁷,⁸⁸ The mechanistic studies were carried out by preparing solutions of (i) TATA⁺ alone (40 µM), (ii) TATA⁺ (40 µM), and HA⁻/H₂A (0.1 M), and (iii) TATA⁺ (40 µM) and HA⁻/H₂A (0.1 M) in the presence of different concentrations of NiP (10 and 80 µM) in the deaerated acetate buffer at pH 4.5. This approach allowed us to scrutinize all photoinduced electron transfer reactions step by step. In all measurements, the dye was excited at 530 nm with an energy of about 1.3 mJ/pulse.

Excitation of TATA⁺ alone (Figure 7) leads to the
immediate detection of a singlet excited state (denoted
¹TATA⁺*), which is converted into a triplet excited state
(³TATA⁺*) for up to 100 ns. Broad excited state absorption
bands at 418 and beyond 580 nm provide evidence of the
dynamics of this triplet state, which persists for up to several

milliseconds (Figure 7). The transient decays were fitted using
a three-exponential function, yielding three-time constants of
16(±3) ns, 47(±2) µs, and 340 ± 20 µs (Figure S5). The first
decay time, which is consistent with the time-resolved
fluorescence measurements, corresponds to the lifetime of
the singlet excited state ¹TATA⁺*. The second and the third
decay times, both within the microsecond range, can be
attributed to the decay times of the high-energy (T₂) and low-
energy (T₁) triplet states, as previously observed for MR-
TADF molecular systems.⁸⁹,⁹⁰ Similar spectral features and
lifetime of the triplet state of an analogous triazatriangulenium
compound (TATA-EtOH⁺) have previously been reported by
Manoj.⁷⁹ In the presence of HA⁻/H₂A (0.1 M), reductive
quenching of ¹TATA⁺* by HA⁻ is observed, and a character-
istic transient band of the reduced photosensitizer, TATA*, is
detected at 406 nm for up to 40 ns (Figure 8A). Efficient
<figure><img src="image_19.png" /><figcaption>**Figure 7.** Transient absorption spectra of a solution containing TATA⁺ (40 µM) in deaerated 1 M acetate buffer at pH 4.5. Data set divided into time panels of 0 to 100 ns (top) and 250 ns to 3 ms (bottom). λ<sub>pump</sub> = 530 nm, 1.3 mJ/pulse.</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>**Figure 8.** Transient absorption spectra of a solution containing TATA⁺ (40 µM) and NaHA/H₂A (0.1 M) in deaerated 1 M acetate buffer at pH 4.5. The spectra are divided into time panels of (A) 0 to 40 ns, and (B) 500 ns to 7 ms. λ<sub>pump</sub> = 530 nm, 1.3 mJ/pulse.</figcaption></figure>

quenching of the excited singlet state is also confirmed with the fluorescence quenching measurements previously reported by us.²⁶ Of note, ³TATA⁺* is not observed in the presence of HA⁻ in Figure 8A, attesting to the very fast reductive quenching of ¹TATA⁺* by HA⁻. Subsequently, this band decays as a result of charge recombination between TATA· and the oxidized forms of electron donor, HA· and DHA (Figure 8B),²⁶ highlighting the reversibility of the electron transfer reaction. We further show that charge recombination follows a second order kinetics with k<sub>BET</sub> = 1.2 × 10⁹ M⁻¹ s⁻¹ (Figure S6), which is comparable with the rate constant that has been previously reported.²⁶

To investigate the fate of reduced TATA• in the photo-
catalytic solution, we have performed the same transient
absorption measurements in the presence of 10 µM (Figure
S7) and 80 µM (Figure S8) of the NiP catalyst. In both cases,

the second electron transfer reaction from TATA• to NiP
(step 2, Figure 6A) results in a significantly faster decay for the
reduced dye (Figure 9). No optical signatures of any reduced
form of NiP were detected in our experiments. Notably, the
groups of Reisner⁶⁴ and van Gastel⁶⁶ have reported the spectral
signatures of the reduced forms of NiP and its analogue
bearing phosphonate ethyl ester groups. These species exhibit
relatively weak absorption bands around 400 nm, a region that
overlaps with the strong absorbance of TATA• in our transient
absorption spectra (Figures S7, S8). The transient decays of the
reduced TATA•, measured at 400 nm for different catalyst
concentrations, are presented in Figure 9A. Fitting the decays
with an exponential function allowed the extraction of the
apparent rate constant, which changes with the concentration
of the catalyst under pseudo-first-order conditions. Linear
fitting of the dependence of these rate constants on the catalyst
concentration gives a straight line with a slope of 1.1 × 10⁹
M⁻¹ s⁻¹, corresponding to the electron transfer rate from the
reduced TATA• to the catalyst (Figure 9B, and step 2 in
Figure 6A). As shown in Table 3, the electron transfer reaction
from TATA• to NiP occurs more rapidly than previously
observed with CoCR14 as the catalyst, which is another
indication of the superior efficiency of the photocatalytic
system containing NiP.

The possibility of the reaction proceeding via direct
oxidative quenching of TATA⁺* by NiP at this concentration
range was also considered, and our control experiment mixing
only TATA⁺ and NiP showed a decrease in the lifetime of
³TATA⁺*, even though no spectral feature of the oxidized dye
was observed (Figure S9). It is important to recall that, in the
mixture containing also NaHA/H₂A (Figures S7 and S8), the
singlet ¹TATA⁺* is fully quenched by the electron donor, and
so the triplet excited state is not even formed, ruling out the
possibility of triplet quenching by NiP in the photocatalytic
mixture. Therefore, it is reasonable to conclude that, under
photocatalytic conditions, the mechanism proceeds via
reductive quenching of ¹TATA⁺* by HA⁻, producing the
reactive TATA· species that engages in an efficient electron
transfer reaction with NiP, thereby activating it toward the
substrate.

**3. CONCLUSIONS**

In this study, we employed a commercially available water-
soluble DuBois' nickel catalyst functionalized with phospho-
nate groups (denoted NiP), previously reported by Reisner's
group, which has shown relatively high performance for
photoinduced catalytic H₂ production in acidic water when
associated with Rubpy or quantum dots as the photosensitizer.
Herein, we further enhanced the catalytic activity of NiP—
now ranging between 2,600 and 19,500 TONCat—by
combining it with the organic dye TATA⁺ and HA⁻ as a
sacrificial electron donor. Such a high catalytic activity has
never been achieved with DuBois' nickel catalyst under
photocatalytic conditions. Additionally, we have for the first
time evaluated the high stability of TATA·, another key factor
contributing to the exceptional performance of TATA⁺-based
photocatalytic systems, through a combined study involving
EPR spectroscopy and theoretical calculations. Simulation of
the complex hyperfine structure of the room-temperature X-
band EPR spectrum of TATA· confirms the anticipated
delocalization of the radical across the entire TATA platform
with the central atom bearing only 38% of the Mulliken spin
density. The delocalized nature of the molecular orbitals over
<figure><img src="image_21.png" /><figcaption>**Figure 9.** (A) Transient kinetics of TATA• at 400 nm, for samples containing TATA⁺ (40 µM) and HA⁻/H₂A (0.1 M) alone (black, fitted with τ = 144 µs) and in the presence of 10 µM (red, τ = 69 µs) and 80 µM (blue, τ = 11 µs) of NiP in deaerated 1 M acetate buffer at pH 4.5. λ<sub>pump</sub> = 530 nm, 1.3 mJ/pulse. (B) Dependence of the pseudo-first-order rate constant (k<sub>app</sub>) on the NiP concentration, with a linear fit generating the second-order constant k<sub>ET</sub> = 1.1 × 10⁹ M⁻¹ s⁻¹.</figcaption></figure>

<table><caption>**Table 3.** Time and Rate Constants Measured by Steady State and Time-Resolved Emission and Transient Absorption Spectroscopies in Deaerated Aqueous Solutions Containing TATA⁺ in the Presence of the NaHA/H₂A Couple and/or the NiP Catalyst under the Experimental Conditions Described below<sup>a</sup></caption><thead><tr><th></th><th>this work</th><th>previously reported<sup>26</sup></th></tr></thead><tbody><tr><td>PS</td><td>TATA<sup>+</sup> (10 µM)<br>0.1 M acetate buffer at pH 4.5</td><td>TATA<sup>+</sup> (10 µM)</td></tr><tr><td>τ (*TATA<sup>+</sup>)<br>without HA<sup>-</sup>/<br>Cat</td><td>13.6 ns</td><td>14 ns</td></tr><tr><td>k<sub>red</sub></td><td>n.d.</td><td>3.6 × 10<sup>9</sup> M<sup>-1</sup> s<sup>-1</sup></td></tr><tr><td>k<sub>ox</sub></td><td>≈0 M<sup>-1</sup> s<sup>-1</sup> with NiP</td><td>1.23 × 10<sup>10</sup> M<sup>-1</sup> s<sup>-1</sup><br>with Co<sup>II</sup>CR14<sup>b</sup></td></tr><tr><td>PS</td><td>TATA<sup>+</sup> (40 µM)<br>0.1 M HA<sup>-</sup>/H<sub>2</sub>A + 0.1 M acetate buffer pH 4.5</td><td>TATA<sup>+</sup> (100 µM)</td></tr><tr><td>k<sub>BET</sub></td><td>1.2 × 10<sup>9</sup> M<sup>-1</sup> s<sup>-1</sup></td><td>3.26 × 10<sup>9</sup> M<sup>-1</sup> s<sup>-1</sup></td></tr><tr><td>k<sub>ET</sub></td><td>1.1 × 10<sup>9</sup> M<sup>-1</sup> s<sup>-1</sup><br>[NiP] = 10–80 µM</td><td>7.35 × 10<sup>8</sup> M<sup>-1</sup> s<sup>-1</sup><br>[CoCR14] = 200 µM</td></tr><tr><td>k<sub>dis</sub><sup>c</sup> with HA<sup>-</sup>/Cat</td><td>n.d.</td><td>7.1 × 10<sup>3</sup> s<sup>-1</sup><br>[CoCR14]<br>= 200 µM</td></tr></tbody></table>

<sup>a</sup>Optical path = 1 cm. <sup>b</sup>The Co<sup>II</sup>CR14 species (i.e. [Co<sup>II</sup>(CR14)-(H₂O)₂]<sup>2+</sup>) used for the Stern–Volmer plot was obtained by an exhaustive electrolysis at –0.20 V vs Ag/AgCl of an aqueous solution (0.1 M NaClO₄, pH 4.5) of 0.5 mM [Co<sup>III</sup>(CR14)Cl₂]<sup>+</sup>. <sup>c</sup>k<sub>dis</sub> is the kinetic constant related to the disappearance of Co<sup>1</sup> in solution. n.d.: not determined.

the dye's platform is further corroborated by TDDFT computations of the UV-visible signature of TATA•. A comprehensive photophysical study using Stern–Volmer plots and transient absorption spectroscopy elucidated the reaction mechanism, revealing a rapid electron transfer (3.6 × 10⁹ M⁻¹ s⁻¹) between the singlet excited state of TATA⁺ and HA⁻. Although a singlet–triplet equilibrium exists in the excited state of TATA⁺, only the singlet state participates in the photocatalytic mechanism due to the very rapid electron transfer with HA⁻. This study suggests that the high photocatalytic efficiency of the TATA⁺/NiP/HA⁻ system is likely attributed to two key factors: (1) the ability of TATA• to

rapidly reduce the NiP catalyst, with a remarkably high rate constant (1.1 × 10⁹ M⁻¹ s⁻¹), and (2) the efficiency of NiP as a proton reduction catalyst. This study further underscores the intrinsic advantages of the TATA⁺ organic dye—its robustness and optimal redox potentials—establishing it as a photosensitizer of choice for efficiently activating and enhancing the performance of catalysts like NiP in proton reduction to H₂. This new family of triangul-eniums opens exciting avenues for designing next-generation photosensitizers to drive redox photocatalysis, not only for H₂ production but also for CO₂ reduction and the redox transformation of organic substrates. In any case, advancing such systems will require replacing the oxidation of sacrificial electron donors with processes that oxidize organic compounds into value-added products for industrial use.

## 4. MATERIALS AND METHODS

The NiP catalyst bis{P,P'-1,5-diphenyl-3,7-bis[(4-hydrogenphosphonate)phenyl]-1,5,3,7-diazadiphosphocine}nickel-(II) bromide (hydrogen bromide adduct) (C₆₀H₆₉Br₃N₄NiO₁₂P₈, Strem Chemicals), acetonitrile (CH₃CN, Fisher, HPLC grade), tetra-n-butyl-ammonium perchlorate ([Bu₄N]ClO₄, >99%, Fluka), L-ascorbic acid (H₂A, 99%, Acros), sodium L-ascorbate (NaHA, 99%, Acros), and reference gas (1.00% and 5.38% H₂ in Ar, Air Liquide) were purchased from commercial suppliers. The organic dye 4,8,12-tris(2-(2-hydroxyethoxy)ethyl)-8,12-dihydrobenzo[8,1][2,7]-naphthyridino[3,4,5,6-klmn]acridin-3a²(4H)-ylium chloride or 4,8,12-tri-(2-(2-hydroxyethoxy)ethyl)-4,8,12-triazatriangulenum chloride ([TATA(EE)₃]Cl) was prepared according to the literature procedure.²⁶ Purification of water (15.0 MΩ cm, 24 °C) was performed with a milli-Q system (Purelab option, Elga). The UV-vis absorption spectra were recorded with a PerkinElmer Lambda 650 UV-vis spectrometer.

### 4.1. General Procedure for Photoinduced Hydrogen Generation. 4.1.1. Sample Preparation.
The following procedure was performed inside a glovebox under argon: an aqueous solution of 1 M acetate buffer at pH 4.5 was prepared by mixing AcOH (15.75 mL, 0.55 M) and NaOAc (18.5 g, 0.45 M) in 500 mL of degassed water, and the pH was adjusted to 4.5 by adding few drops of HCl (6 M in aqueous solution). A 762 µM mother solution of NiP catalyst (5 mL) was prepared by solubilizing 6.0 mg of NiP (1584.41 g mol⁻¹) in 5 mL of 1 M acetate buffer at pH 4.5 (see above for the buffer preparation). The initial concentration of NiP solution (761 µM) was checked by UV-vis absorption spectroscopy, and at such concentration of NiP we used an ε value of 1950 M⁻¹ cm⁻¹ at 496 nm (an absorbance of 1.48 with an optical path length of 1 cm). Then
a 5 mL graduated flask was charged with 1.46 mg of the photosensitizer [TATA(EE)₃]Cl (582.09 g mol⁻¹) weighted as a solid, and an appropriate volume of the NiP mother solution (for 50 µM NiP, 328 µL mother solution of NiP was added; for 10 µM, 66 µL mother solution; for 5 µM, 33 µL mother solution, for 1 µM, 6.6 µL mother solution). After completing the volume to 5 mL with the 1 M acetate buffer at pH 4.5, the concentration of [TATA(EE)₃]Cl was checked by UV-vis absorption spectroscopy, considering an ε value of 8800 m⁻¹ cm⁻¹ at 530 nm (the final PS concentrations are comprised between 504 to 529 µM with absorbances between 0.44 and 0.46 with an optical path length of 0.1 cm). Then, for this pH, ascorbic acid (H₂A, 21 mg, 24 mM) and sodium ascorbate (NaHA, 75 mg, 76 mM) were added to the solution to give a total concentration of 0.1 M. The solution was then transferred to a homemade glass tube with a diameter of 2 cm fused with a round-bottom flask (head space volume of 199 mL) containing a magnetic stirrer (volume < 0.1 mL). The tube was sealed with a rubber septum, protected from light with aluminum foil before irradiation experiments, and removed from the glovebox for the photocatalysis experiment.

**4.1.2. Irradiation Conditions.** Continuous irradiation was performed at 298 K under stirring with a xenon lamp (150 W, Hamamatsu L8253, type LC8-03) equipped with a 400–700 nm or 420–720 nm large band filter which was placed 4 cm from the sample. The irradiation power received by the sample (P = 78 mW·cm⁻² with the filter 420–720 nm, and P = 74 mW·cm⁻² with the filter 400–700 nm) was measured with a calibrated Avantes spectrometer AvaSpec-ULS2048CL-EVORS-UA, with a slit of 20 µM, equipped with a fiber cable FC-UVIR400-1 and a Cosine corrector CC-UV/vis/ NIR-8MM.

**4.1.3. Quantification of H₂.** The amount of hydrogen evolved during photocatalysis was quantified from an analysis of the gas mixture in the headspace of the glass tube (sampling of 100 µL of gas) by gas chromatography (PerkinElmer Autosystem XL Gas Chromatograph equipped with a 5 Å molecular sieve column (oven temperature = 303 K) and a thermal conductivity detector (TCD)), which uses argon as a carrier gas. Prior to each experiment, GC/TCD calibration was carried out by using two samples of the reference gas (1.00% and 5.38% H₂ in Ar, from Air Liquid company). The turnover number (TON) and turnover frequency (TOF) for H₂ production were determined as previously described.²⁵,⁴²

**4.1.4. Calculation for Quantum Yield of H₂ Production.** The quantum yield of H₂ production (QYH₂) was calculated with the following formula:

$$QY_{H_2} = \frac{2 \times (\text{moles of H}_2 \text{ produced in 1 h})}{\text{moles of photons absorbed in 1 h}}$$

The moles of H₂ produced were quantified by GC (see above) after the first hour of irradiation, considering that H₂ production is linear in this range of time. These values are listed in Table S1.

The moles of photons absorbed by the photocatalytic solution (i.e., containing TATA⁺ at 500 µM) during 1 h of irradiation were estimated by integrating the absorbed fraction of the incident photon flux (Φabs, Figure S1A,B, black traces), giving the values of 1761.87 µmol s⁻¹ m⁻² with the filter 420–720 nm, and 1819.14 µmol s⁻¹ m⁻² with the filter 400–700 nm. Considering an irradiated surface of 1.13 × 10⁻⁴ m² on the photocatalytic reactor, the absorbed fraction of the incident photon flux per hour is 7.17 × 10⁻⁴ mol h⁻¹ with the filter 420–720 nm and 7.40 × 10⁻⁴ mol h⁻¹ with the filter 400–700 nm.

The absorbed fraction of the incident photon flux (Φabs) was obtained by normalizing the spectrum of the Xe irradiation lamp (Φp, Figure S1A,B, blue traces) and the absorption spectrum of TATA⁺ recorded in 1 M acetate aqueous buffer (pH 4.5) with an optical path of 0.1 cm (Figure S1A,B, red traces) using the following equation (see Figure S1 below):

$$\Phi_{\text{abs}} = \Phi_{\text{p}} \times (1 - 10^{-\text{abs}_{0.1 \text{ cm}}}) = \Phi_{\text{p}} \times (1 - 10^{-\text{abs}_{0.1 \text{ cm}} \times 18})$$

The spectrum of the Xe irradiation lamp, equipped with a filter 400–700 nm or 420–720 nm, is expressed as incident photons flux (Φp, µmol s⁻¹ m⁻² nm⁻¹) in the function of wavelength and was measured with the Avantes spectrometer AvaSpec-ULS2048CL-EVORS-UA (Figure S1A,B, blue traces). Since the photocatalytic reactor has a diameter of 1.8 cm, the absorbance of the TATA⁺ solution with an optical path of 1.8 cm (abs1.8 cm), used for photocatalysis, can be calculated for each wavelength from the TATA⁺ absorption spectrum recorded with an optical path of 0.1 cm (Figure S1A,B, red traces), by multiplying the absorbance (abs0.1 cm) by a factor of 18. The factor of 18 was obtained handling the Beer–Lambert law:

$$\text{abs}_{1.8 \text{ cm}} = \varepsilon \times c \times 1.8 = \frac{\text{abs}_{0.1 \text{ cm}}}{0.1} \times 1.8 = \text{abs}_{0.1 \text{ cm}} \times 18$$

where ε is extinction coefficient of TATA⁺ at each wavelength of its absorption spectrum, and c is the concentration of TATA⁺ (500 µM) in the photocatalytic reaction and the solution used to record the absorption spectrum (Figure S1A,B, red traces).

The calculated QYH₂ values are listed in Table S1 and range from 3.8 to 12.8%.

**4.2. EPR Measurements.** EPR spectra were obtained at room temperature using an X-band Bruker EMX Plus spectrometer and fitted with the EasySpin simulation package.⁹¹

**4.3. Computational Details.** The ground-state structures of TATA⁺ and TATA* were fully optimized at the DFT level using the PBE0 global-hybrid density-functional approximation⁷¹ and the 6-31+G* double-ζ basis set. Solvent effects are taken into account by using the polarization continuum model, setting the dielectric constant to that of acetonitrile.⁷² EPR parameters are computed at ground-state-optimized structure of TATA* using this time the very large EPR-III triple-ζ basis set to ensure a good convergence of the property. UV-visible spectra of both systems were computed within the vertical approximations at the TDDFT level using the same methodology (PCM-PBE0/6-31+G*). The first vibrationally resolved absorption band of the spectrum of TATA⁺ was obtained at the same level of theory within the Franck–Condon–Herzberg–Teller (FCHT) approximation using the adiabatic Hessian model.⁹² All the computations were performed with the release C.01 of Gaussian 16.⁹³

**4.4. Stern Volmer Plots.** Stern–Volmer experiments were carried out with several degassed aqueous solutions (1 M acetate buffer at pH 4.5) containing TATA⁺ (10 µM), in the presence of NiP at different concentrations (0, 0.01, 0.02, 0.03, 0.04, 0.06, 0.08, and 0.10 mM) for oxidative quenching of PS. All these solutions were prepared in a glovebox under an argon atmosphere inside quartz cells (optical path = 1 × 1 cm²).

The Stern–Volmer plot was obtained from τ₀/τ values, and emission decays (τ) of the singlet excited state of TATA⁺ (¹TATA⁺*) were recorded at room temperature after excitation at 409 nm with a picosecond pulsed laser diode (PDL 800D PicoQuant, laser diode: LDH-P-C-405, entry and exit slits of 1 mm) and using time-correlated single photon counting detection (picoHarp 300). The lifetimes of TATA⁺* in the absence (τ₀) and presence of catalyst (τ), estimated from the luminescence decay correlation curve of TATA⁺* at 570 nm, were used to calculate the τ₀/τ value to trace the Stern–Volmer plots. The correlation curves are multiexponential functions of the type $I_{\text{em}}(t) = \sum_{i=1}^{n} A_i e^{-t/\tau_i}$ (where n = 1 or 2 or 3) where $I_{\text{em}}(t)$ is the emission intensity over time of the solution containing PS and Cat, and τᵢ is the lifetime of the luminescence decay with Aᵢ its percentage contribution.

The lifetimes of TATA⁺* alone (τ₀) were determined in degassed water at a rate of 13.62 ns. The oxidative quenching kinetic constants (kox) of ¹TATA⁺* by NiP was estimated to be close to zero, at pH 4.5.

**4.5. Nanosecond Transient Absorption Spectroscopy.** The occurrence of the reductive quenching was also supported by nanosecond laser flash photolysis experiments which were performed by exciting at 530 nm deaerated aqueous buffer solutions at pH 4.5 containing TATA⁺ (40 µM), in the presence or absence of NaHA/
H₂A (0.1 M), and in the presence or absence of NiP (10 and 80 µM). Samples were prepared in a glovebox under an argon atmosphere within a quartz cell (cell width = 10 × 10 mm²). Transient absorption spectra were acquired using an experimental setup previously described in detail.⁸⁷ In brief, a tunable optical parametric oscillator (OPO) laser operating at 10 Hz was employed as the excitation (pump) source, while a supercontinuum white light laser running at 20 Hz was used as an absorption probe. The probe beam was split using a beam splitter, directing one portion toward the sample cell and the other toward a reference path to compensate for energy fluctuations. The transmitted light was collected by a round-to-linear optical fiber bundle and directed to a spectrograph for spectral analysis. The dispersed light from the spectrograph was then detected by using an intensified charge-coupled device (ICCD) camera (PIMAX-4, Princeton Instruments). The pump-probe time delays were electronically controlled, and the transient absorption spectra were obtained by recording the spectra with and without the pump excitation.

## ASSOCIATED CONTENT

### Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/aps.5c00013.

Tables of photocatalytic performance of the systems TATA⁺/NiP/HA⁻ and TATA⁺/CoCR14/HA⁻ (Table S1), and of redox potentials of TATA⁺/NiP/HA⁻ (Table S2), oxidation processes of ascorbate (Scheme 1), spectrum of the incident photons flux of the Xe lamp (Figure S1), DFT and TDDFT calculation on TATA⁺ and TATA* (Figures S2–S3), Stern Volmer plots (Figure S3) and nanosecond transient absorption spectroscopy, transient decays of TATA⁺ alone (Figure S4), kinetic fit of charge recombination between TATA* and the oxidized electron donor (HA* or DHA) (Figure S5), transient absorption spectra of TATA⁺/NiP/HA⁻ with 10 and 80 µM NiP (Figures S7–S8), and transient absorption spectra of TATA⁺/NiP (Figure S9) (PDF)

## AUTHOR INFORMATION

### Corresponding Authors

Philippe P. Lainé – Université Paris Cité, ITODYS, CNRS, F-75013 Paris, France; ● orcid.org/0000-0001-7053-1833; Email: philippe.laine@u-paris.fr

Marie-Noëlle Collomb – Université Grenoble Alpes, CNRS, DCM, F-38000 Grenoble, France; ● orcid.org/0000-0002-6641-771X; Email: marie-noelle.collomb@univ-grenoble-alpes.fr

Jérôme Fortage – Université Grenoble Alpes, CNRS, DCM, F-38000 Grenoble, France; ● orcid.org/0000-0003-2673-0610; Email: jerome.fortage@univ-grenoble-alpes.fr

### Authors

Songling Lyu – Université Grenoble Alpes, CNRS, DCM, F-38000 Grenoble, France

Daniel H. Cruz Neto – Université Paris-Saclay, CNRS, Institut des Sciences Moléculaires d'Orsay, 91405 Orsay, France

Juan Aguirre-Araque – Université Grenoble Alpes, CNRS, DCM, F-38000 Grenoble, France

Lucile Termeau – Université Grenoble Alpes, CNRS, DCM, F-38000 Grenoble, France

Fakourou Camara – Université Grenoble Alpes, CNRS, DCM, F-38000 Grenoble, France

Sawsen Cherraben – Université Paris Cité, ITODYS, CNRS, F-75013 Paris, France

David Martin – Université Grenoble Alpes, CNRS, DCM, F-38000 Grenoble, France; ● orcid.org/0000-0002-3257-9876

Eric Brémond – Université Paris Cité, ITODYS, CNRS, F-75013 Paris, France; ● orcid.org/0000-0002-8646-9365

Thomas Pino – Université Paris-Saclay, CNRS, Institut des Sciences Moléculaires d'Orsay, 91405 Orsay, France

Minh-Huong Ha-Thi – Université Paris-Saclay, CNRS, Institut des Sciences Moléculaires d'Orsay, 91405 Orsay, France; ● orcid.org/0000-0003-3276-1843

Complete contact information is available at: https://pubs.acs.org/10.1021/aps.5c00013

## Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

### Funding

ANR TATADyes (ANR-20-CE05-0041); LABEX ARCANE (ANR-17-EURE-0003); ANR MoMoPlasm (ANR-23-CE29-0003).

### Notes

The authors declare no competing financial interest.

## ACKNOWLEDGMENTS

F.C. thanks the University Grenoble Alpes for his PhD grant. D.H.C.N. thanks MESRI for his PhD grant. L.T., S.C., and J.A.-A. thank the French National Research Agency through the project TATADyes (ANR-20-CE05-0041) for their postdoctoral grants. We are grateful to the University Grenoble Alpes, CNRS and the French National Research Agency through the project TATADyes (ANR-20-CE05-0041), Labex ARCANE (ANR-17-EURE-0003), and the ICMG Chemistry Nanobio Platform (FR2067) for their financial support. E.B. acknowledges the GENCI-TGCC Center for HPC Resources (Projects AD010810359R2 and AD010810359R3) as well as the French National Research Agency through the MoMo-Plasm project (ANR-21-CE29-0003).

## ABBREVIATIONS

PS, photosensitizer; Cat, H₂-evolving catalyst; SD, sacrificial electron donor; TATA⁺, triazatriangulenium; NiP, nickel phosphine complex also called DuBois' catalyst; H₂A, ascorbic acid; TON, turnover number

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