Amlan K. Pal

We report here the synthesis of a new heteroleptic ruthenium–terpyridine complex that exhibits both high stability and enhanced photocatalytic activity with an increase of approximately one order of magnitude for the hydrogen evolution reaction coming from a strong push–pull effect. The complex demonstrates high performance under blue light (445 nm), achieving a turnover number (TON) above 3500 molH2 molPS–1 with sustained hydrogen production for over 6 days. That TON value is more than twice as high as that of the reference [Ru(bpy)3]2+ complex under the same conditions. Under red light (630 nm), this complex also shows high efficiency and stability with no significant decrease in the turnover frequency over 6 days, achieving a TON over 960 molH2 molPS-1, a value competitive with reference [Ru(bpy)3]2+ complex under blue light (this last showing no hydrogen evolution under red-light). We identified a kinetic competition between the main H2 production process and degradation reactions from the reduced complex species and could further enhance the sustainability of the system (under blue light) by increasing the cobalt catalyst concentration. A TON above 4100 molH2 molPS-1 could be reached after 8 days.

The persistent global energy crisis can be addressed by focusing on the advancement of cleaner energy sources. As a clean, efficient, and renewable option, hydrogen has long been regarded as a promising alternative to fossil fuels. Light-driven hydrogen evolution reaction (HER) offers a viable substitute for nonrenewable energy sources, since hydrogen is an energy-dense (143 MJ·kg–1) carbon-free fuel. In this respect, a handful of Pt(II) square-planar complexes, such as [Pt(II)(tpy)Cl]+ (where tpy = 2,2′:6′,2′′-terpyridine) have been reported in the literature for proton reduction, albeit with low turnover numbers (TONs) and a tendency to decompose to Pt nanoparticles. With these facts in mind, two Pt(II) terpyridyl complexes with varying electron densities were synthesized, structurally characterized, and successfully used for photocatalytic hydrogen production under different conditions (Pt catalyst + Ru photosensitizer, Co catalyst + Pt photosensitizer, and Pt photosensitizer + Pt catalyst) under varying light illumination. These complexes were found to be active for hydrogen production without the formation of nanoparticles, and a maximum turnover number of 1340 and a turnover frequency above 10000 for the combination of a Pt-complex as a catalyst and [Ru(bpy)3]2+ as a photosensitizer was achieved. Mechanistic studies revealed a reduced hydride dimer active for photocatalytic HER.

An easily oxidizable Ru(II) complex, 2, of strongly σ-donating ligand has been synthesized, structurally characterized, and successfully used for the photocatalytic hydrogen evolution reaction (HER) as a photosensitizer (PS) under light irradiation of different wavelengths. This complex was found to be active for HER in combination with the classic [Co(dmgH)₂(py)Cl] as a catalyst with a TOF_max ranging from 8 (λ_irr = 630 nm) to 183 (λ_irr = 445 nm) mmol_H2·mol_PS^-1·min^-1 and TON_last ranging from 9 (λ_irr = 630 nm) to 349 (λ_irr = 445 nm) mol_H2·mol_PS^-1. The study presents the first example of photoproduction of H₂ using a Ru(II)-heteroleptic terpyridine PS with an oxidation potential below 0.6 V vs the SCE.

The surge of organofluorine compounds, particularly incorporation of a -CF₃ group in organic molecules for pharmaceutical and materials sectors demands development of a mild and regioselective trifluoromethylation route. In this work, procarbenic benzo[d]imidazole based organic photocatalysts C1 and C2 enabled mild, regioselective trifluoromethylation, addressing the demand of -CF₃ incorporation in polyaromatic hydrocarbons (PAHs). These photocatalysts showed intense ligand-to-ligand charge transfer absorption in the UV (λ_abs ∼ 280 nm) and low-negative quasi-reversible reductions (E_red = -0.91 and -1.11 V vs SCE for C1 and C2). Both emitted deep-blue ¹LC fluorescence at RT (375-390 nm) and acted as strong photo-oxidants (E_C(II)*/C(I) ∼ 2.70 V vs SCE), enabling efficient metal- and additive-free mono(trifluoromethylation) of polyaromatic hydrocarbons.

We report herein the synthesis of a tridentate ligand (tris-4,4',4''-(4'''-pyridyl)-2,2':6',2''-terpyridine) and its corresponding homoleptic ruthenium(II) complex, which shows stable hydrogen photoproduction under red light (TON of 174 mol_H2·mol_PS ^-1 after 210 hours). A sustained activity under blue and green lights is also observed with TONs above 360 mol_H2·mol_PS ^-1 after 168 hours.

Effect of ancillary ligand with extended π surface on the photophysical properties and photocatalytic hydrogen production activity of bis(quinolinyl)isothiocyanate Pt( ii ) complexes were studied. The complexes were found to be sustainable for HER.

The development of a gentle and regioselective trifluoromethylation process is necessary due to the rise in organofluorine compounds, notably the inclusion of a −CF ₃ group in organic molecules for the pharmaceutical and materials sectors. This work presents novel examples of luminescent procarbenic purely organic photocatalysts ( C1 – C3 ) that exhibited low‐negative irreversible reduction events (E _Red =−0.86 V, −0.85 V and −0.68 V vs SCE for C1 , C2 & C3 , respectively) and strong ligand‐to‐ligand charge transfer absorption in the UV region (λ _abs ~280 nm). At room temperature (RT), both photocatalysts displayed deep‐blue ¹ LC emission in the 353–373 nm region. These photocatalysts were found to be strong photo‐oxidants (E _C(II)*/C(I) =2.54 V, 2.65 V and 2.63 V vs SCE for C1 , C2 and C3 , respectively) and were employed as low‐cost, metal and additive‐free photocatalysts for the regioselective mono(trifluoromethylation) of polyaromatic hydrocarbons (PAHs) and small organic molecules in good yields (~42–82 %).

Photocatalytic HER using hydrogenases and their mimics in conjugation with artificial photosensitizers like metal complexes, dyes, and semiconductors is showcased. These systems exhibit robust and sustainable solar energy-to-H₂ conversion.

We report square planar mononuclear Pt(II)-complexes of terpyridines in the form of [PtCl(L1/L2)]PF₆ as phosphorescent emitters (where L1 = 4-(3-pyridine)2,2':6',2''-terpyridine and L2 = 4'-(3-pyridinyl)-4,4''-di(tert-butyl)-2,2':6'2''-terpyridine). Complex 2 showed emission at 534 nm in the DCM solution with photoluminescence quantum efficiency (Φ_PL) = 14%, while in the mCBP host (5-wt % doped), the emission shifted to 584 nm with Φ_PL = 37.8% and a phosphorescence lifetime (τ_phos) of 37.8 μs. Complex 2 in mCBP was used to fabricate a solution-processed phosphorescent organic light-emitting diode (PhOLED) which showed maximum external quantum efficiency (EQE_max) = 7.4% with yellow emission at λ_EL = 570 nm and exhibited a low efficiency roll-off with an EQE drop to 7.0% at 1000 cd/m².