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1. Photocatalysis of Adsorbed Catechol on Degussa P25 TiO ₂ at the Air–Solid Interface

   https://doi.org/10.1021/acs.jpcc.4c05777  

   Hoque, Md Ariful; Barrios_Cossio, Josiel; Guzman, Marcelo I (October 2024, The Journal of Physical Chemistry C)

   Semiconductor photocatalysis with commercial TiO2 (Degussa P25) has shown significant potential in water treatment of organic pollutants. However, the photoinduced reactions of adsorbed catechol, a phenolic air pollutant from biomass burning and combustion emissions, at the air–solid interface of TiO2 remain unexplored. Herein we examine the photocatalytic decay of catechol in the presence of water vapor, which acts as an electron acceptor. Experiments under variable cut-off wavelengths of irradiation (λcut-off ≥ 320, 400, and 515 nm) distinguish the mechanistic contribution of a ligand-to-metal charge-transfer (LMCT) complex of surface chemisorbed catechol on TiO2. The LMCT complex injects electrons into the conduction band of TiO2 from the highest occupied molecular orbital of catechol by visible light (≥2.11 eV) excitation. The deconvolution of diffuse reflectance UV–visible spectral bands from LMCT complexes of TiO2 with catechol, o-semiquinone radical, and quinone and the quantification of the evolving gaseous products follow a consecutive kinetic model. CO2(g) and CO(g) final oxidation products are monitored by gas chromatography and Fourier-transform infrared spectroscopy. The apparent quantum efficiency at variable λcut-off are determined for reactant loss (Φ−TiO2/catechol = 0.79 ± 0.19) and product growth ΦCO2 = 0.76 ± 0.08). Spectroscopic and electrochemical measurements reveal the energy band diagram for the LMCT of TiO2/catechol. Two photocatalytic mechanisms are analyzed based on chemical transformations and environmental relevance. 

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2. Symmetry and substituent electronics dictate electronic structure of low spin, mixed-ring rhenocene complexes

   https://doi.org/10.1039/D4DT03309D  

   Asempa, Eyram; Curtin, Gregory M; May, Ann Marie; Dempsey, Jillian L; Jakubikova, Elena (January 2026, Dalton Transactions)

   Identifying the impact of molecular structure and symmetry on excited state character and energetics is vital to enable and control photochemical reactions. In this work, density functional theory (DFT) and time-dependent DFT (TD-DFT) were used to determine the electronic structure and excited state reduction potentials of six heteroleptic rhenocene complexes, each bearing a cyclopentadienyl ligand and a functionalized tetramethylated cyclopentadienyl ligand (ReCp(CpMe4R), where Me = CH3 and R = Me, CF3, tBu, CHCH2, CHO, or OMe). Calculations were performed using the B3LYP and BP86 functionals, utilizing SDD+f effective core potential and its associated basis set for Re, and 6-311G* basis set for all other atoms. All six complexes exhibit eclipsed geometries with similar Re-ring bond distances and angles. Despite their structural similarities, the electronic structure of these complexes varies with ligand functionalization. ReCp(Cp*), 1, (where Cp* = pentamethylcyclopentadienyl) has a 2A1 ground state with a dz2-based LUMOβ orbital, whereas functionalized tetramethylated derivatives, 2-6, have 2A′′ ground states with dx2-y2-based LUMOβ orbitals due to their lower molecular symmetries (C5v vs. Cs, respectively). Regardless, complementary TD-DFT and fragment orbital analyses show that low-energy LMCT excited states are retained across all six complexes (where LMCT character > 90%). Furthermore, hypsochromic shifts and higher oscillator strengths are observed for 2-6 compared to 1, resulting from the changes in HOMO–LUMO gaps and excitation into the dx2-y2 orbital for 2-6 rather than dz2 orbital for 1, which increases the orbital overlap between the hole-particle pairs that describe the lowest-energy LMCT excitations. Appending electron donating and withdrawing groups to these mixed-ring rhenocene derivatives also tunes ground state and excited state reduction potentials over a 400 mV and 700 mV range, respectively, enabling access to more oxidizing LMCT excited states. Collectively, these results showcase design strategies to control acceptor orbital character, orbital energetics, excited state energies, and reduction potentials, while simultaneously retaining low-energy LMCT excited states across a series of rhenocene derivatives. In result, this work establishes approaches to design and tailor next-generation mixed-ring rhenocenes with low-energy LMCT excited states for photochemical applications. 

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3. Photophysical Studies of a Zr(IV) Complex with Two Pyrrolide-Based Tetradentate Schiff Base Ligands

   https://doi.org/10.1021/acs.inorgchem.4c00365  

   Zhang, Yu; Lee, Tia S; Petersen, Jeffrey L; Milsmann, Carsten (May 2024, Inorganic Chemistry)

   The reaction of two equivalents of N,N’-bis(2- pyrrolylmethylidene)-1,2-phenylenediamine (H2bppda) with tetra- benzylzirconium provided the air- and moisture-stable eight- coordinate complex Zr(bppda)2. Temperature-dependent steady- state and time-resolved emission spectroscopy established weak photoluminescence (ΦPL = 0.4% at 293 K) by a combination of prompt fluorescence and thermally activated delayed fluorescence (TADF) upon visible light excitation at and around room temperature. TADF emission is strongly quenched by 3O2 and shows highly temperature-sensitive emission lifetimes of hundreds of microseconds. The lifetime of the lowest energy singlet excited state, S1, was established by transient absorption spectroscopy and shows rapid deactivation (τ = 142 ps) by prompt fluorescence and intersystem crossing to the triplet state, T1. Time-dependent density functional theory (TD-DFT) calculations predict moderate ligand-to-metal charge transfer (LMCT) contributions of 25−30% for the S1 and T1 states. A comparison of Zr(bppda)2 to related zirconium pyridine dipyrrolide complexes, Zr(PDP)2, revealed important electronic structure changes due to the eight-coordinate ligand environment in Zr(bppda)2, which were correlated to differences in the photophysical properties between the two compound classes. ■ 

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4. Getting excited about cycloadditions

   https://doi.org/10.1126/science.abg7834  

   Schmidt, Valerie A. (March 2021, Science)

   Synthetic chemists use photochemistry to achieve challenging or unusual chemical transformations, but not all compounds are photoactive. Photosensitization is a process by which a molecule that is incapable of efficiently absorbing a particular wavelength of light directly is promoted to its triplet excited state (T 1 ) by an intermolecular triplet energy transfer from a photosensitizer, which is a compound that ideally has a large extinction coefficient, rapid rate of intersystem crossing, and a long-lived T 1 . A particular advantage of photosensitization is that distinctive reactivity profiles not accessible through ground states become facile from corresponding excited states ( 1 ). The use of photosensitizers in chemical synthesis has paralleled the rise in popularity and use of various photoredox catalysts ( 2 ). On page 1338 of this issue, Ma et al. ( 3 ) report a photosensitized dearomative [4 + 2] cycloaddition that converts simple, unsaturated building blocks into products of increased molecular complexity using visible light. 

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5. Ultrafast Study of Excited State Dynamics of Amino Metal Halide Molecular Clusters

   https://doi.org/10.1021/acs.jpclett.3c01952  

   Zhang, Heng; Zeitz, David C; Zhang, Jin Z (September 2023, The Journal of Physical Chemistry Letters)

   The excited state dynamics of ligand-passivated PbBr2 molecular clusters (MCs) in solution have been investigated for the first time using femtosecond transient absorption spectroscopy. The results uncover a transient bleach (TB) feature peaked around 404 nm, matching the ground state electronic absorption band peaked at 404 nm. The TB recovery signal can be fitted with a triple exponential with fast (10 ps), medium (350 ps), and long (1.8 ns) time constants. The medium and long time constants are very similar to those observed in the timeresolved photoluminescence (TRPL) decay monitored at 412 nm. The TB fast component is attributed to vibrational relaxation in the excited electronic state while the medium component with dominant amplitude is attributed to recombination between the relaxed electron and hole. The small amplitude slow component is assigned to electrons in a relatively long-lived excited electronic state, e.g., triplet state, or shallow trap state due to defects. This study provides new insights into the excited state dynamics of metal halide MCs. 

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