We present experimental data illustrating that photochemical upconversion based on sensitized triplet–triplet annihilation can exhibit anti-Stokes emissions whose intensities with respect to the excitation power can vary between quadratic and linear using a noncoherent polychromatic light source. The benchmark upconverting composition consisting of Pd(II) octaethylporphyrin (PdOEP) sensitizers and 9,10-diphenylanthracene (DPA) acceptors/annihilators in toluene was selected to generate quadratic, intermediate, and linear behavior under both coherent and noncoherent excitation conditions. Each of these power laws was traversed in a single sample in one contiguous experiment through selective pumping of the sensitizer using an Ar+ laser. Wavelength-dependent responses ranging from quadratic to pseudolinear were also recorded from the identical sample composition when excited by Xe lamp/monochromator output in a conventional fluorimeter, where the optical density at λex dictates the observed incident power dependence. Finally, pure linear behavior was derived from noncoherent excitation for the first time at higher sensitizer concentrations.

Upconversion photochemistry occurring between palladium(II) octaethylporphyrin (PdOEP, 1) and 9,10-diphenylanthracene (DPA, 2) in toluene successfully sensitizes nanostructured WO₃ photoanodes (E_g = 2.7 eV) to sub-bandgap non-coherent green photons at low power density.

Homogeneous aqueous solutions of photocatalytic ensembles, consisting of [Ru(bpy)₃]²⁺ as a photosensitizer, ascorbic acid/ascorbate as the electron source, and 10 distinct Co²⁺-based molecular catalysts, were evaluated for visible-light induced hydrogen evolution using high-throughput screening. The combined results demonstrate that Co²⁺ complexes bearing tetradentate ligands yield more active photocatalytic compositions than their congeners with pentadentate ligands while operating with high catalyst stability. Additionally, molecular Co²⁺ catalysts with cis open coordination sites appear to be significantly more active for hydrogen evolution than those with trans open sites. As evidenced by mass spectrometric analysis of the reactor headspace and associated deuteration experiments, the H₂ gas generated in all instances was derived from aqueous protons. One of the most promising cis-disposed Co²⁺ species, [Co(bpyPY2Me)(CH₃CN)(CF₃SO₃)](CF₃SO₃) (1), engages in highly efficient hydrogen evolving photocatalysis, achieving a turnover number of 4200 (H₂/Co) and a turnover frequency of 3200 (H₂/Co per h) at pH 4 under simulated sunlight (AM 1.5G, 100 mW cm⁻²) at room temperature. At equimolar concentrations of photosensitizer and 1, the total hydrogen produced appears to be exclusively limited by the photostability of [Ru(bpy)₃]²⁺, which was observed to decompose into an Ru(bpy)₂–ascorbate adduct, as evidenced by HPLC and ESI-MS experiments. Lowering the operating temperature from 27 to 5 °C significantly attenuates bpy dissociation from the sensitizer, resulting in a net ∼two-fold increase in hydrogen production from this composition. The primary electron transfer steps of this photocatalytic ensemble were investigated by nanosecond transient absorption spectroscopy. Photoexcited [Ru(bpy)₃]²⁺ undergoes reductive quenching by ascorbic acid/ascorbate (k_q = 2.6 × 10⁷ M⁻¹ s⁻¹), releasing [Ru(bpy)₃]⁺ from the encounter solvent cage with an efficiency of 55 ± 5%. In the presence of catalyst 1, [Ru(bpy)₃]⁺ generated in the initial flash-quench experiment transfers an electron (k_et = 2 × 10⁹ M⁻¹ s⁻¹) at an efficiency of 85 ± 10% to the catalyst, which is believed to enter the hydrogen evolution cycle subsequently. Using a combinatorial approach, all ten Co²⁺ catalysts were evaluated for their potential to operate under neutral pH 7.0 conditions. Catalyst 7, [Co(PY4MeH₂)(CH₃CN)(CF₃SO₃)](CF₃SO₃), was revealed to be most promising, as its performance metrics were only marginally affected by pH and turnover numbers greater than 1000 were easily obtained in photocatalytic hydrogen generation. These comprehensive findings provide guidelines for the development of molecular compositions capable of evolving hydrogen from purely aqueous media.

A new pentadentate, redox-active ligand bpy2PYMe has been synthesized and its corresponding transition metal complexes of Fe²⁺ (1), Co²⁺ (2), Ni²⁺ (3), Cu²⁺ (4), and Zn²⁺ (5) have been investigated for electro- and photo-catalytic proton reduction in acetonitrile and water, respectively. Under weak acid conditions, the Co complex displays catalytic onset at potentials similar to those of the ligand centered reductions in the absence of acid. Related Co complexes devoid of ligand redox activity catalyze H₂ evolution under similar conditions at significantly higher overpotentials, showcasing the beneficial effect of combining ligand-centered redox activity with a redox-active Co center. Furthermore, turnover numbers as high as 1630 could be obtained under aqueous photocatalytic conditions using [Ru(bpy)₃]²⁺ as a photosensitizer. Under those conditions catalytic hydrogen production was solely limited by photosensitizer stability. Introduction of an electron withdrawing CF₃ group into the pyridine moiety of the ligand as in bpy2PYMe-CF₃ renders its corresponding Co complex 6 less active for proton reduction in electro- and photocatalytic experiments. This surprising effect of ligand substitution was investigated by means of density functional theory calculations which suggest the importance of electronic communication between Co¹⁺ and the redox-active ligand. Taken together, the results provide a path forward in the design of robust molecular catalysts in aqueous media with minimized overpotential by exploiting the synergy between redox-active metal and ligand components.

The Cu(I) metal-to-ligand charge-transfer complex, [Cu(dsbtmp)₂]⁺ (dsbtmp = 2,9-di(sec-butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline), exhibits outstanding stability as a visible-light-absorbing photosensitizer in hydrogen-evolving homogeneous photocatalysis. In concert with the Co(dmgH)₂(py)Cl water reduction catalyst and N,N-dimethyl-p-toluidine sacrificial donor in 1:1 H₂O:CH₃CN, this Cu(I) sensitizer remains active even after 5 days of visible-light-pumped (λ_ex = 452 ± 10 nm) hydrogen evolution catalysis. Deuteration studies illustrate that the hydrogen produced from this composition does indeed originate from aqueous protons.