Effect of anchoring groups on the photocatalytic performance of iridium(III) complexes in hydrogen production and their toxicological analysis

Abstract

Three iridium(III) complexes (Ir1–Ir3) with different anchoring moieties, namely, 4,4′-dinitro-2,2′-bipyridine, tetraethyl [2,2′-bipyridine]-4,4′-diylbis(phosphonate) and diethyl [2,2′-bipyridine]-4,4′-dicarboxylate were designed, synthesised and used as photosensitisers for water-splitting hydrogen generation. The influence of these anchoring moieties on the photophysical and electrochemical characteristics of the Ir(III) complexes was investigated via density functional theory (DFT) simulations and experimental methods. The hydrogen production efficiency of the Ir1@Pt-TiO₂ system was as high as 4020.27 mol μg⁻¹ h⁻¹. Among the three anchoring moieties, tetraethyl [2,2′-bipyridine]-4,4′-diylbis(phosphonate) improved the performance of the complexes to the greatest extent. Toxicological investigation revealed that the toxicity of the Ir(III) complexes to luminous bacteria did not differ significantly from that of TiO₂, implying that the Ir(III) complexes synthesised in this study do not pose a significant threat to marine environments, similar to TiO₂. This finding has potential implications for the development of highly efficient Ir(III) photosensitisers to be used in the water-splitting process required for hydrogen production.

---

1. Introduction

The search for alternative energy sources is a crucial scientific endeavour to address the issue of growing global energy consumption coupled with a limited supply of petroleum and coal.¹ Solar energy is a viable solution to existing energy problems because it is a carbon-neutral and sustainable energy source,² and because its photocatalytic technology can efficiently harness this energy to generate power.^3–10 In particular, solar-powered photocatalytic water-splitting is an environmentally friendly method for producing hydrogen fuel,^11,12 first proposed by Fujishima and Honda in 1972 using a TiO₂ photoanode.¹³ The technology for splitting water to produce hydrogen has since advanced significantly.^14,15

Iridium(III) complexes are widely applied in studies of photocatalytic hydrogen.¹⁶ Such complexes include metal dyes synthesised from first- and second-row transition metals, and benefit from the outstanding ligand field stabilisation energies of these elements, which can be attributed to the presence of the 5d valence shell.^2,17 The potential of Ir(III) complexes of the type [Ir(C^N)₂(N^N)]⁺—where C^N is the cyclometalating ligand and N^N is the anchoring group—was investigated by Bernhard et al. for use in photochemical water-splitting required for hydrogen production.^2,18–20 A turnover number (TON) of 800 was obtained when 50 μM [Ir(ppy)₂(bpy)]⁺ was used as the photosensitiser in a 1 : 1 (v/v) water–acetonitrile solvent mixture.^18,21–24 Prospective uses of cyclometalated Ir(III) complexes as photosensitisers have since attracted considerable attention.^25–27

C^N ligands increase the stability of Ir(III) complexes by promoting σ-donation from carbon atoms attached to the metal core. Photocatalytic studies have shown that this method increases the energy density at the metal core, improving the endurance of these complexes.^2,28 Furthermore, physicochemical characteristics, such as the energy gap, can be modified by changing the ligands present.^29,30 Cyclometalated Ir(III) complexes are thus attractive candidates for dyes used in H₂ production via water-splitting owing to their favourable properties. However, their ability to absorb visible light, comprising approximately 40% of the solar spectrum, is limited.^4,6,30–33 Consequently, cyclometalated Ir(III) complexes must be structurally modified to overcome this drawback and optimise hydrogen production. Extensive research is also required to optimise the effectiveness of Ir-based photosensitisers in photocatalytic hydrogen production. Crucially, high efficiency Ir(III)-based photosensitisers must be developed to improve the kinetics of hydrogen production.

Current developments related to Ir(III)-based photosensitisers are focused on improving the photochemical and physical properties of the cyclometalating and auxiliary ligands by altering their structures.^3,16,34,35 Photosensitisers capable of absorbing light and transferring electrons more efficiently can be obtained by carefully selecting chromophores and precisely designing their molecular structures.³⁶ These photosensitisers can increase the photocatalytic hydrogen production efficiency.²⁵

Donor–π–acceptor (D–π–A) compounds have been recognised as target compounds for water-splitting hydrogen generation.³⁷ The photoinduced intramolecular charge-transfer (ICT) characteristics of the excited state of these D–π–A compounds are the key to their advantageous optical and electronic properties.^38,39 Triphenylamine (TPA) is a major electron donor exhibiting highly beneficial characteristics when modified.^40,41 It can be combined with other functional groups to obtain D–π–A structures with high ICT capabilities.^37,42 The demonstrated ability of TPA to efficiently transport holes increases the lifespan of the charge-separated state, which is typically achieved by spreading the produced cations throughout a flat amine unit, thereby improving the stability and effectiveness of photocatalytic processes.^43,44 The introduction of a heterocyclic ring spacer, such as furan, reduces the band gap and broadens the absorption spectra via ICT,⁴⁵ which can enhance the water-splitting hydrogen production ability. Furthermore, the introduction of pyridine functional groups causes the absorption spectrum to undergo a slight red shift,^46,47 which can increase the absorption of light in the visible region. The nitro functional group is an electron-withdrawing group capable of acting as an effective electron acceptor.⁴⁸ The molecular structures of the dyes are integrated with 4-(5-(5-nitropyridin-2-yl)furan-2-yl)-N,N-diphenylaniline to form an architectural framework known as the D–π–A framework.^{37,42,49–52}

Research focused on cyclometalated Ir(III) complexes has shown that many anchoring groups on N^N ligands can result in highly stable and effective water-splitting systems.^53,54 The addition of phosphonic acid or carboxylate to the bipyridine ligand structure of [Ir(C^N)₂(N^N)]⁺-type dyes can increase their stability of adhesion to semiconductors.⁵⁴ Strong interactions with semiconductors such as TiO₂ are particularly effective in complexes containing anchoring ligands with carboxyl functional groups. Consequently, electron transport and hydrogen production are enhanced. However, significant concerns exist regarding the stability of these dyes in photocatalytic processes.⁵⁵ For example, hydrolysing the carboxylate link limits the efficiency of electron transfer from the photosensitive agent to the TiO₂ surfaces.⁵⁶ Phosphonate linkages operate more steadily than carboxylate linkages when attached to TiO₂ surfaces.^26,56 NO₂ anchoring groups have been used in various fields, exhibiting satisfactory results.⁵⁷ However, very few studies have applied these groups to the domain of photocatalytic water splitting for hydrogen production.

In this study, three Ir(III) complexes, designated as Ir1–Ir3, were prepared (Fig. 1) for use as photosensitisers in the light-driven water-splitting process for hydrogen production. We comprehensively investigated the effect of anchoring groups on the characteristics of these complexes using photophysical and electrochemical techniques. In addition, the connection between the individual hydrogen-generating ability of the anchoring groups was examined. Furthermore, toxicity tests of these three Ir(III)-based dyes were performed to determine their impact on the environment.

Fig. 1 Chemical structures of the Ir(III) dyes (Ir1–Ir3); TPA = triphenylamine.

2. Experimental

All reactions were performed under N₂ atmospheres according to the standard Schlenk technique. All glassware was dried overnight in an oven before use. All solvents were dried via distillation using appropriate drying agents under N₂ atmospheres. All reagents for chemical synthesis were purchased from Sigma-Aldrich (USA) or Dieckmann (China). All chemicals were directly used as received unless otherwise stated. Reactions were monitored via thin-layer chromatography (TLC) using Merck silica gel pre-coated aluminium plates. Products were purified via column chromatography using silica gel (230–400 mesh) purchased from Dieckmann (China). Full details of the experiments are presented in the ESI.†

Schemes 1 and 2 show the synthesis processes for the C^N ligand and Ir(III)-based photosensitisers, respectively, both of which have a replaceable N^N auxiliary ligand and two C^N cyclometalating ligands (L1) surrounding the Ir(III) core. L1 was synthesised by combining 4-(diphenylamino)phenylboronic acid and 2-(5-bromofuran-2-yl)-5-nitropyridine via the Suzuki coupling process.

Scheme 1 Synthetic cyclometalation method for preparing the L1 ligand.

Scheme 2 Synthetic pathway for preparing the Ir(III) complexes (Ir1–Ir3).

All synthesised ligands were characterised using ¹H and ¹³C-nuclear magnetic resonance (NMR) spectroscopy. Furthermore, a comprehensive analysis of the Ir(III) complexes was performed using a combination of liquid chromatography–electrospray ionisation quadrupole time-of-flight mass spectrometry (LC-ESI-QTOF-MS) and ¹H-NMR spectroscopy. Fourier-transform infrared (FTIR) spectroscopy was employed to ascertain the structural composition of the Ir(III) complexes, and elemental mapping of Ir1@Pt-TiO₂ was performed to confirm its composition (Fig. 2). This combination of techniques provided important information regarding the structural characteristics and composition of the complexes. The spectroscopic data obtained were consistent with the predicted molecular structures of the complexes.

Fig. 2 Elemental mapping of Ir1@Pt-TiO₂.

3. Results and discussion

3.1. Photophysical properties

The photophysical characteristics of the Ir(III) complexes were investigated by acquiring their ultraviolet-visible (UV-vis) absorbance spectra in dichloromethane (DCM, CH₂Cl₂), as shown in Fig. 3; their UV-vis absorption parameters are listed in Table 1. Ligand-to-ligand charge transfer (LLCT) and metal-to-ligand charge transfer (MLCT) transitions produced a prominent band in the wavelength range of 300–400 nm.^35,58 The broad absorbance band in the visible region of the spectrum is possibly due to triplet excitations.^25,53,58Ir1 exhibited broader absorption peaks with higher ε values than Ir2 and Ir3, indicative of its superior light-harvesting capability. Notably, Ir3 exhibited the lowest absorption intensities, which may be attributed to the presence of the –NO₂ moiety (which is strongly electron-withdrawing) among its anchoring functional groups.⁵⁹ With increasing electron-withdrawing ability, the intramolecular charge transfer decreases and the intramolecular interactions become insufficient.⁶⁰ Electrochemical impedance spectroscopy (EIS) results also confirmed these observations.

Fig. 3 UV-vis absorption spectra of the Ir1–Ir3 complexes recorded in CH₂Cl₂ at 293 K.

Table 1 UV-vis absorption parameters of the Ir1–Ir3 complexes

Dye	λ max/nm (ε/10^5 M^−1 cm^−1)
Ir1	302 (2.00), 346 (1.66), 501 (1.49), 547 (1.48)
Ir2	304 (1.79), 354 (1.34), 504 (1.06), 572 (1.21)
Ir3	306 (1.39), 353 (1.23), 505 (1.01), 576 (1.16)

---

The photoluminescence (PL) spectra of Ir1–Ir3 were acquired in DCM at 293 K (Fig. 4). All the dyes exhibited broad emission peaks in the wavelength range of 449–456 nm, which can be ascribed to their strong ³MLCT characteristics.^25,61–63 Similar peaks were observed in all emission spectra, suggesting a comparable highest occupied molecular orbital (HOMO) energy level for each dye.^20,25,64,65 This observation was further substantiated by subsequent cyclic voltammetry (CV) analysis and density functional theory (DFT) calculations. Fluorescence spectroscopy revealed that the emission peaks of Ir1 (449 nm), Ir2 (453 nm), and Ir3 (456 nm) exhibited a regular red shift, indicating that the enhanced electron-withdrawing ability of the ligands (nitro group > carboxylic acid > phosphonate) led to a significant reduction in the excited-state energy level of the MLCT.⁶⁶ In particular, the phosphonate group in Ir1, through the strong σ-donor character of the “P=O” bond, maintained an appropriate MLCT energy level, whereas its bidentate anchoring mode formed stable Ir–O–Ti chemical bridges with the TiO₂ surface.⁶⁷

Fig. 4 PL spectra of the Ir1–Ir3 complexes recorded in DCM at 293 K.

3.2. Electrochemical properties

In Ir(III) complexes, the energy gap between the TiO₂ semiconductor and expendable electron donor must be adjusted for efficient water-splitting hydrogen generation. To achieve electron transfer and charge segregation in these complexes, the HOMO energy level should be below the valence band (VB) of the electron donor, and the lowest unoccupied molecular orbital (LUMO) energy level should be above the conduction band (CB) of the semiconductor. Table 2 lists the electrochemical parameters and energy state values determined from these investigations.

Table 2 Electrochemical parameters and energy state values of the Ir1–Ir3 complexes

Dye	E ^MaxOx/V	E HOMO /eV	E g /eV	E LUMO /eV
a Calculated as −(E^MaxOx + 4.8). b Energy band gap calculated from the onset of absorption. c E LUMO = EHOMO + Eg.
Ir1	0.53	−5.33	1.72	−3.61
Ir2	0.53	−5.33	1.70	−3.63
Ir3	0.52	−5.32	1.67	−3.65

---

Relative to all Ir(III)-based dyes, the CB level of the TiO₂ semiconductor was low, i.e. −4.4 eV. This value was less than the E_LUMO values of the dyes, which ranged from −3.61 to −3.65 eV, according to CV data. The lower CB level of TiO₂ facilitated efficient electron injection throughout the photosynthetic hydrogen production process.⁶⁸ Because the E_HOMO values (ranging from −5.33 to −5.32 eV) of the Ir1–Ir3 complexes were significantly lower than the redox potential (−4.65 eV, pH ∼ 4) of the sacrificial electron donor (SED; ascorbic acid in this study), the charge transfer was considered to be efficiently handled.^69,70 The energy levels of all Ir(III)-based dyes met the necessary conditions for efficient electron injection and charge separation, indicating their potential for use in water-splitting hydrogen production.

The charge recombination characteristics of the Ir(III) complexes were investigated using EIS.^71,72 EIS Nyquist plots are shown in Fig. 5; these provide a visual representation of the impedance behaviour. It has been shown that a smaller arc radius in the EIS Nyquist plot corresponds to a lower resistance to electric charge transfer and a higher capacity for hydrogen generation.^71–74 Among all the as-synthesised dyes, Ir1 exhibited the shortest arc radius, indicating that it has the best charge carrier transport properties.⁷⁵ This observation is consistent with the ability of the dyes to produce hydrogen in photocatalytic water-splitting reactions.

Fig. 5 EIS Nyquist plots of the Ir1–Ir3 complexes.

Photocurrent measurements were utilized to assess the stability and efficiency of charge separation in the metal complexes.⁷⁶ Consistent and rapid photocurrent response in light-on/light-off tests signifies steady photocatalytic activity⁷⁷ whereas heightened photocurrent density indicates efficient charge separation.⁷⁸ Photocurrent testing followed a previously established methodology.⁷⁹Fig. 6 shows the photocurrent behaviour of Ir1–Ir3 under cycles of visible light exposure, presenting the behaviour of these systems during six on–off cycles. This clearly demonstrates efficient electron transfer,⁷⁹ indicative of their consistent photocatalytic performance.⁷⁷Ir1 demonstrated a substantially increased photocurrent intensity when illuminated and a pronounced setback in its decrease upon light-off. This indicates greater efficiency in charge separation and an enhanced ability for hydrogen production compared to other dyes.^77,79–81

Fig. 6 Photocurrent of Ir1–Ir3.

3.3. DFT calculations

DFT simulations were performed to determine the electrical characteristics and charge-transfer behaviour of the three Ir(III) complexes. The corresponding data are shown in Table 3 and Fig. 7. The singlet ground state of the compounds was corrected for molecular geometry using the 6-31G(d)/LANL2DZ(Ir) basis set and the DFT/B3LYP technique. DCM was used as the solvent, and the polarizable continuum model (PCM) was employed to account for solvent effects. Computations were performed using the Gaussian 16 program.^82–84

Table 3 Computational analysis results (HOMO, LUMO, and energy gap) of the Ir(III)-based dyes. All values are in eV

	HOMO	E g	LUMO
Ir1	−5.25	2.05	−3.20
Ir2	−5.25	1.85	−3.40
Ir3	−5.28	1.28	−4.00

---

Fig. 7 Computed contour plots showing the HOMO and LUMO energy levels of the Ir(III) complexes.

The circular orbital ratios of the HOMOs and LUMOs for the three complexes were found to be similar. Analysis of orbital diagrams showed that LUMOs were delocalised over the anchoring groups, whereas HOMOs mostly comprised the π orbitals of the C^N ligands and the Ir metal system. From Ir1 to Ir3, the ability of the anchoring groups to remove electrons progressively increased, indicating an increase in the degree of electron delocalisation, which in turn resulted in an equal decrease in the energy gap. Taken together, the entire electrochemical dataset supports these computer-generated results. The HOMO and LUMO energy levels obtained from DFT calculations showed the same trend as the experimental test results.

3.4. Light-driven hydrogen production

The as-synthesised Ir(III)-based dyes were used as photosensitisers in the water-splitting hydrogen production process. We investigated the entire process of platinising TiO₂, conjugating the Ir(III) complexes to TiO₂, and the photocatalytic water-splitting process. The hydrogen production curves for all samples as a function of time are shown in Fig. 8, and the corresponding data are presented in Table 4. During the photo-catalytic process, the photosensitiser undergoes photoexcitation upon exposure to light, leading to the transfer of electrons to the CB of TiO₂. These electrons are subsequently transferred to the Pt nanoparticles deposited on the surface of TiO₂, where they facilitate proton reduction, stimulating hydrogen evolution. The oxidised photosensitiser is then regenerated to its ground state via electron donation from ascorbic acid.^35,85

Fig. 8 Photocatalytic H₂ production curves of Ir1@Pt-TiO₂, Ir2@Pt-TiO₂ and Ir3@Pt-TiO₂ in the presence of radiation.

Table 4 Photocatalytic H₂ production using the different Ir(III)-based dyes bonded to platinised TiO₂ (Ir1@Pt-TiO₂, Ir2@Pt-TiO₂ and Ir3@Pt-TiO₂) in the presence of radiation

	Time (h)	H2 (μmol)	H2/time (μmol h^−1)	H2 evolution (mol μg^−1 h^−1)
Ir1@Pt-TiO2	5	32.73	6.55	4020.27
Ir2@Pt-TiO2	5	11.02	2.20	1079.92
Ir3@Pt-TiO2	5	1.96	0.39	772.94

---

The Ir(III) dyes synthesised in this study effectively augmented hydrogen production via water-splitting, generating promising outcomes under illumination. Among all the systems investigated, Ir1@Pt-TiO₂ was found to be the most efficient hydrogen-generating system, with a remarkable H₂ evolution of 4020.27 mol μg⁻¹ h⁻¹. The second most efficient system was Ir2@Pt-TiO₂, with a H₂ evolution of 1079.92 mol μg⁻¹ h⁻¹. Relative to previously reported Ir(III)-based dyes, the as-synthesised Ir(III) complexes with D–π–A structures showed significantly increased hydrogen generation.^25,70,86 The Ir(III) dye with the phosphonate anchoring groups generated more H₂ than those with the carboxylate and nitro anchoring groups. The addition of a phosphonate group enhanced the adhesion of the Ir(III)-based dye to TiO₂, thereby increasing its H₂ production efficiency.^53,87–89

3.5. Toxicological analysis

Luminescent bacteria represent a distinctive class of microorganisms capable of emitting visible light as a result of their metabolic activities.⁹⁰ Their luminescence, however, is susceptible to disruption upon exposure to toxic substances, leading to a measurable decrease in light intensity. Leveraging this phenomenon, we evaluated the toxicity of various as-prepared photocatalytic materials by monitoring luminescence intensity changes in a water sample inoculated with the luminescent T3 strain.^91,92

In this study, TiO₂ was employed as the control to assess the potential cytotoxicity of Ir1@Pt-TiO₂, Ir2@Pt-TiO₂ and Ir3@Pt-TiO₂. Following 5 hours of irradiation, the photocatalytic samples were collected and incubated with luminescent bacteria for 15 minutes at room temperature. The relative luminescence intensity of bacteria exposed to Ir1@Pt-TiO₂, Ir2@Pt-TiO₂ and Ir3@Pt-TiO₂ remained at approximately 100% (Fig. 9), indicating no statistically significant differences relative to the TiO₂ control group. These results indicate that Ir1@Pt-TiO₂, Ir2@Pt-TiO₂ and Ir3@Pt-TiO₂ do not induce toxic effects, even after prolonged photocatalytic activity. These findings align with previous reports by Yao et al., confirming the biocompatibility of similar Ir-based materials.⁹³ Ir-based compounds have garnered considerable attention due to their potent antioxidant properties. Ma et al.⁹⁴ demonstrated that Ir(III) complexes exhibit remarkable mitochondria-targeted anti-cancer activity, underscoring their biomedical potential. In the context of environmental safety, the photocatalytic investigated in this study appear to pose minimal ecological risks to aquatic environments.

Fig. 9 Relative luminescence intensity of luminescent bacteria exposed to the as-prepared photocatalytic materials. Data are expressed as the mean ± standard error (n = 3). Relative to the control group (TiO₂), no significant difference (p > 0.05) was observed in the toxicity of the experimental groups (Ir1@Pt-TiO₂, Ir2@Pt-TiO₂ and Ir3@Pt-TiO₂).

4. Conclusions

This study reports the synthesis of novel Ir(III)-based photosensitisers with different anchoring groups. The as-synthesised Ir(III)-based dyes were thoroughly characterised using several analytical techniques, and water-splitting tests were conducted to measure their hydrogen generation rates. The UV-vis absorption spectra of the dyes showed noticeably broader and more intense absorption bands for Ir1 containing the phosphonate functional group than for Ir2 and Ir3, indicating its increased capacity to absorb light and, thus, its improved hydrogen generation. Under irradiation, the Ir1@Pt-TiO₂ device was the most efficient hydrogen-generating system, with a H₂ evolution of 4020.27 mol μg⁻¹ h⁻¹. Therefore, a phosphonate anchoring group must be included in Ir(III) complexes to significantly increase their stability and effectiveness when used as photosensitisers. Toxicological analysis of Ir(III) complexes and TiO₂ revealed that the luminescence intensity of luminescent bacteria exposed to these complexes did not significantly differ from that of the bacteria exposed to TiO₂. Notably, the results obtained suggest that the Ir(III) complexes do not pose a major risk to aquatic environments, similar to TiO₂.

Author contributions

Xiao Yao contributed to the synthesis and methodology of the iridium complexes, writing the original draft, and revising the article. Linyu Fan contributed to characterising the iridium complexes. Zhuwu Jiang contributed to the DFT calculations and revised the manuscript. Chaoqun Zheng contributed to toxicity detection. Jinfeng Chen contributed to the formal analysis and discussion. Yachen Jiang contributed to toxicity detection. Yisang Lu contributed to the DFT calculations. Cheuk-Lam Ho contributed to the formal analysis, discussion, and manuscript revision. Yuanmei Chen contributed to the methodology and writing and revising the article.

Data availability

The data presented in this study are available upon request from the corresponding author.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

This research was funded by the Fujian University of Technology, grant numbers E0600591 and GY-Z220180. We offer our thanks for the financial support of the Fujian Provincial Department of Finance (GY-Z23273), the Central Government Guides Local Funds for Science and Technology Development (2023), grant number: 2023L3015 and the Natural Science Foundation of Fujian Province (Grant No. 2024J08211).

References

1. S. Chu and A. Majumdar, Nature, 2012, 488, 294–303.
2. M. Yang, J. E. Yarnell, K. El Roz and F. N. Castellano, ACS Appl. Energy Mater., 2020, 3, 1842–1853.
3. M. Watanabe, Sci. Technol. Adv. Mater., 2017, 18, 705–723.
4. C. H. Siu, C. L. Ho, J. He, T. Chen, X. N. Cui, J. Z. Zhao and W. Y. Wong, J. Organomet. Chem., 2013, 748, 75–83.
5. W. Y. Wong and C. L. Ho, Coord. Chem. Rev., 2009, 253, 1709–1758.
6. W. Y. Wong and C. L. Ho, J. Mater. Chem., 2009, 19, 4457–4482.
7. Y. Chi and P. T. Chou, Chem. Soc. Rev., 2010, 39, 638–655.
8. G. Vilaça, B. Jousseaume, C. Mahieux, C. Belin, H. Cachet, M. C. Bernard, V. Vivier and T. Toupance, Adv. Mater., 2006, 18, 1073–1077.
9. X. Sun, S. Jiang, H. Huang, H. Li, B. Jia and T. Ma, Angew. Chem., Int. Ed., 2022, 61, e202204880.
10. H. S. Waheed, H. Ullah, M. W. Iqbal and Y.-H. Shin, Optik, 2022, 271, 170071.
11. C. J. Wang, Y. Chen and W. F. Fu, Dalton Trans., 2015, 44, 14483–14493.
12. K. U. Sahar, K. Rafiq, M. Z. Abid, A. Rauf, U. Ur Rehman, M. A. Nadeem, R. Jin and E. Hussain, Colloids Surf., A, 2023, 674, 131942.
13. A. Fujishima and K. Honda, Nature, 1972, 238, 37–38.
14. R. Singh and S. Dutta, Fuel, 2018, 220, 607–620.
15. A. Gupta, B. Likozar, R. Jana, W. C. Chanu and M. K. Singh, Int. J. Hydrogen Energy, 2022, 47, 33282–33307.
16. D. N. Tritton, F.-K. Tang, G. B. Bodedla, F.-W. Lee, C.-S. Kwan, K. C.-F. Leung, X. Zhu and W.-Y. Wong, Coord. Chem. Rev., 2022, 459, 214390.
17. J. C. Deaton and F. N. Castellano, in Iridium(III) in optoelectronic and photonics applications, ed. E. Zysman-Colman, Wiley-VCH, Weinheim, 2017, pp. 1–69.
18. J. I. Goldsmith, W. R. Hudson, M. S. Lowry, T. H. Anderson and S. Bernhard, J. Am. Chem. Soc., 2005, 127, 7502–7510.
19. M. S. Lowry and S. Bernhard, Chem. – Eur. J., 2006, 12, 7970–7977.
20. M. S. Lowry, J. I. Goldsmith, J. D. Slinker, R. Rohl, R. A. Pascal, G. G. Malliaras and S. Bernhard, Chem. Mater., 2005, 17, 5712–5719.
21. P. N. Curtin, L. L. Tinker, C. M. Burgess, E. D. Cline and S. Bernhard, Inorg. Chem., 2009, 48, 10498–10506.
22. L. L. Tinker, N. D. Mcdaniel, P. N. Curtin, C. K. Smith, M. J. Ireland and S. Bernhard, Chem. – Eur. J., 2007, 13, 8726–8732.
23. L. L. Tinker and S. Bernhard, Inorg. Chem., 2009, 48, 10507–10511.
24. S. Metz and S. Bernhard, Chem. Commun., 2010, 46, 7551–7553.
25. G. B. Bodedla, D. N. Tritton, X. Chen, J. Zhao, Z. Guo, K. C.-F. Leung, W.-Y. Wong and X. Zhu, ACS Appl. Energy Mater., 2021, 4, 3945–3951.
26. X. Yao, Q. Zhang, P.-Y. Ho, S.-C. Yiu, S. Suramitr, S. Hannongbua and C.-L. Ho, Inorganics, 2023, 11, 110.
27. M. R. Schreier, X. Guo, B. R. Pfund, Y. Okamoto, T. R. Ward, C. Kerzig and O. S. Wenger, Acc. Chem. Res., 2022, 55, 1290–1300.
28. E. A. Juban, A. L. Smeigh, J. E. Monat and J. K. Mccusker, Coord. Chem. Rev., 2006, 250, 1783–1791.
29. P. Wang, S. Guo, H. J. Wang, K. K. Chen, N. Zhang, Z. M. Zhang and T. B. Lu, Nat. Commun., 2019, 10, 1–12.
30. Y. Wang, X. Zhao, Y. Zhao, T. Yang, X. Liu, J. Xie, G. Li, D. Zhu, H. Tan and Z. Su, Dyes Pigm., 2019, 170, 107547.
31. S. Takizawa, C. Pérez-Bolívar, P. Anzenbacher Jr and S. Murata, Eur. J. Inorg. Chem., 2012, 3975–3979.
32. J. Zhao, W. Wu, J. Sun and S. Guo, Chem. Soc. Rev., 2013, 42, 5323–5351.
33. L. P. R. Pala and N. R. Peela, Renewable Energy, 2022, 197, 902–910.
34. A. O. Adeloye and P. A. Ajibade, Molecules, 2014, 19, 12421–12460.
35. S. C. Yiu, P. Y. Ho, Y. Y. Kwok, X. He, Y. Wang, W. H. Yu, C. L. Ho and S. Huang, Chem. – Eur. J., 2022, 28, e202104575.
36. F. Kausar, A. Varghese, D. Pinheiro and S. D. Kr, Int. J. Hydrogen Energy, 2022, 47, 22371–22402.
37. R. K. Chen, G. J. Zhao, X. C. Yang, X. Jiang, J. F. Liu, H. N. Tian, Y. Gao, X. E. Liu, K. L. Han and M. T. Sun, J. Mol. Struct., 2008, 876, 102–109.
38. S. Delmond, J.-F. Létard, R. Lapouyade and W. Rettig, J. Photochem. Photobiol., A, 1997, 105, 135–148.
39. W. Ma, Y. Wu, J. Han, D. Gu and F. Gan, J. Mol. Struct., 2005, 752, 9–13.
40. S. Haid, M. Marszalek, A. Mishra, M. Wielopolski, J. Teuscher, J. E. Moser, R. Humphry-Baker, S. M. Zakeeruddin, M. Grätzel and P. Bäuerle, Adv. Funct. Mater., 2012, 22, 1291–1302.
41. L. L. Tan, H. Y. Chen, L. F. Hao, Y. Shen, L. M. Xiao, J. M. Liu, D. B. Kuang and C. Y. Su, Phys. Chem. Chem. Phys., 2013, 15, 11909–11917.
42. X. L. Tang, W. M. Liu, J. S. Wu, C. S. Lee, J. J. You and P. F. Wang, J. Org. Chem., 2010, 75, 7273–7278.
43. I. M. Abdellah, T. H. Chowdhury, J.-J. Lee, A. Islam and A. El-Shafei, Sol. Energy, 2020, 206, 279–286.
44. C. H. Teh, R. Daik, E. L. Lim, C. C. Yap, M. A. Ibrahim, N. A. Ludin, K. Sopian and M. A. M. Teridi, J. Mater. Chem. A, 2016, 4, 15788–15822.
45. A. Kathiravan, V. Srinivasan, T. Khamrang, M. Velusamy, M. Jaccob, N. Pavithra, S. Anandan and K. Velappan, Phys. Chem. Chem. Phys., 2017, 19, 3125–3135.
46. M. Pagliai, G. Mancini, I. Carnimeo, N. De Mitri and V. Barone, J. Comput. Chem., 2017, 38, 319–335.
47. Y. K. Law and A. A. Hassanali, J. Chem. Phys., 2018, 148, 102331.
48. J. G. Geisler, Cells, 2019, 8, 280.
49. W. X. Zhang, B. Li, H. P. Ma, L. M. Zhang, Y. L. Guan, Y. H. Zhang, X. D. Zhang, P. T. Jing and S. M. Yue, ACS Appl. Mater. Interfaces, 2016, 8, 21465–21471.
50. S. P. Pitre, C. D. Mctiernan, W. Vine, R. Dipucchio, M. Grenier and J. C. Scaiano, Sci. Rep., 2015, 5, 1–10.
51. O. Filevich, L. Zayat, L. M. Baraldo and R. Etchenique, in Luminescent and Photoactive Transition Metal Complexes as Biomolecular Probes and Cellular Reagents, ed. K. Lo, Springer, Heidelberg, 2014, pp. 47–68.
52. G. C. Li, K. Hu, K. C. Robson, S. I. Gorelsky, G. J. Meyer, C. P. Berlinguette and M. Shatruk, Chem. – Eur. J., 2015, 21, 2173–2181.
53. A. Kobayashi, S. Watanabe, M. Yoshida and M. Kato, ACS Appl. Energy Mater., 2018, 1, 2882–2890.
54. R. A. Wahyuono, S. Amthor, C. Müller, S. Rau and B. Dietzek, ChemPhotoChem, 2020, 4, 618–629.
55. T. N. Murakami, E. Yoshida and N. Koumura, Electrochim. Acta, 2014, 131, 174–183.
56. E. Bae, W. Choi, J. Park, H. S. Shin, S. B. Kim and J. S. Lee, J. Phys. Chem. B, 2004, 108, 14093–14101.
57. S. Dery, S. Kim, G. Tomaschun, I. Berg, D. Feferman, A. Cossaro, A. Verdini, L. Floreano, T. KlüNer and F. D. Toste, J. Phys. Chem. Lett., 2019, 10, 5099–5104.
58. Z. Ning, Q. Zhang, W. Wu and H. Tian, J. Organomet. Chem., 2009, 694, 2705–2711.
59. S. A. Badawy, E. Abdel-Latif, A. A. Fadda and M. R. Elmorsy, Sci. Rep., 2022, 12, 12885.
60. M.-J. Kim, M. Ahn and K.-R. Wee, Mater. Adv., 2021, 2, 5371–5380.
61. Y. Wu and W. Zhu, Chem. Soc. Rev., 2013, 42, 2039–2058.
62. A. Tsuboyama, H. Iwawaki, M. Furugori, T. Mukaide, J. Kamatani, S. Igawa, T. Moriyama, S. Miura, T. Takiguchi and S. Okada, J. Am. Chem. Soc., 2003, 125, 12971–12979.
63. M. Bandini, M. Bianchi, G. Valenti, F. Piccinelli, F. Paolucci, M. Monari, A. Umani-Ronchi and M. Marcaccio, Inorg. Chem., 2010, 49, 1439–1448.
64. P. Tao, Y. Zhang, J. Wang, L. Wei, H. Li, X. Li, Q. Zhao, X. Zhang, S. Liu and H. Wang, J. Mater. Chem. C, 2017, 5, 9306–9314.
65. H.-W. Ham, K.-Y. Jung and Y.-S. Kim, J. Nanosci. Nanotechnol., 2012, 12, 1265–1270.
66. N. Sinha, C. Wegeberg, D. Häussinger, A. Prescimone and O. S. Wenger, Nat. Chem., 2023, 15, 1730–1736.
67. Z. P. Yan, M. X. Mao, Q. M. Liu, L. Yuan, X. F. Luo, X. J. Liao, W. Cai and Y. X. Zheng, Adv. Funct. Mater., 2024, 34, 2402906.
68. T. Bessho, E. Yoneda, J.-H. Yum, M. Guglielmi, I. Tavernelli, H. Imai, U. Rothlisberger, M. K. Nazeeruddin and M. GräTzel, J. Am. Chem. Soc., 2009, 131, 5930–5934.
69. Z. Ji, G. Natu and Y. Wu, ACS Appl. Mater. Interfaces, 2013, 5, 8641–8648.
70. S. Sebata, S.-Y. Takizawa, N. Ikuta and S. Murata, Dalton Trans., 2019, 48, 14914–14925.
71. P. Tian, X. He, L. Zhao, W. Li, W. Fang, H. Chen, F. Zhang, Z. Huang and H. Wang, Int. J. Hydrogen Energy, 2019, 44, 788–800.
72. P. Tian, X. He, L. Zhao, W. Li, W. Fang, H. Chen, F. Zhang, Z. Huang and H. Wang, Sol. Energy, 2019, 188, 750–759.
73. H. Wang, C. Gao, R. Li, Z. Peng, J. Yang, J. Gao, Y. Yang, S. Li, B. Li and Z. Liu, ACS Sustainable Chem. Eng., 2019, 7, 18744–18752.
74. Y. Wang, P. Zheng, M. Li, Y. Li, X. Zhang, J. Chen, X. Fang, Y. Liu, X. Yuan and X. Dai, Nanoscale, 2020, 12, 9669–9679.
75. R. Acharya, B. Naik and K. Parida, Beilstein J. Nanotechnol., 2018, 9, 1448–1470.
76. Y. Zhang, Z. Liu, C. Guo, T. Chen, C. Guo, Y. Lu and J. Wang, Appl. Surf. Sci., 2022, 571, 151284.
77. D. Jiang, W. Wang, S. Sun, L. Zhang and Y. Zheng, ACS Catal., 2015, 5, 613–621.
78. P. Peerakiatkhajohn, J. H. Yun, S. Wang and L. Wang, J. Photonics Energy, 2017, 7, 012006.
79. J. Y. Tang, R. T. Guo, W. G. Zhou, C. Y. Huang and W. G. Pan, Appl. Catal., B, 2018, 237, 802–810.
80. S. Cao, Q. Huang, B. Zhu and J. Yu, J. Power Sources, 2017, 351, 151–159.
81. F. Leng, H. Liu, M. Ding, Q. Lin and H. Jiang, ACS Catal., 2018, 8, 4583–4590.
82. P. J. Stephens, F. J. Devlin, C. F. Chabalowski and M. J. Frisch, J. Phys. Chem., 1994, 98, 11623–11627.
83. W. J. Hehre, R. Ditchfield and J. A. Pople, J. Chem. Phys., 1972, 56, 2257–2261.
84. S. Miertuš, E. Scrocco and J. Tomasi, Chem. Phys., 1981, 55, 117–129.
85. X. Yao, P.-Y. Ho, S.-C. Yiu, S. Suramitr, W.-B. Li, C.-L. Ho and S. Hannongbua, Dyes Pigm., 2022, 205, 110508.
86. S.-Y. Takizawa, N. Ikuta, F. Zeng, S. Komaru, S. Sebata and S. Murata, Inorg. Chem., 2016, 55, 8723–8735.
87. F. Légalité, D. Escudero, Y. Pellegrin, E. Blart, D. Jacquemin and O. Fabrice, Dyes Pigm., 2019, 171, 107693.
88. M. W. Ha, M.-H. Park, J. Y. Hwang, J. Kim, D.-H. Kim, T.-W. Lee and Y.-H. Kim, Dyes Pigm., 2021, 185, 108880.
89. K. L. Materna, R. H. Crabtree and G. W. Brudvig, Chem. Soc. Rev., 2017, 46, 6099–6110.
90. G. Hong, A. L. Antaris and H. Dai, Nat. Biomed. Eng., 2017, 1, 0010.
91. M. J. Halsey and E. B. Smith, Nature, 1970, 227, 1363–1365.
92. A. Chaturvedi, B. N. Rai, R. S. Singh and R. P. Jaiswal, J. Environ. Chem. Eng., 2021, 9, 105466.
93. X. Yao, L. Fan, Q. Zhang, C. Zheng, X. Yang, Y. Lu and Y. Jiang, Molecules, 2024, 29, 2564.
94. D. L. Ma, C. Wu, K. J. Wu and C. H. Leung, Molecules, 2019, 24, 2739.

---

Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cp04828h

‡ These authors contributed equally to this work.

---