How Rigidity and Conjugation of Bidentate Ligands Affect the Geometry and Photophysics of Iron -Heterocyclic Complexes: A Comparative Study.

Two iron complexes featuring the bidentate, nonconjugated N-heterocyclic carbene (NHC) 1,1'-methylenebis(3-methylimidazol-2-ylidene) (mbmi) ligand, where the two NHC moieties are separated by a methylene bridge, have been synthesized to exploit the combined influence of geometric and electronic effects on the ground- and excited-state properties of homoleptic Fe-hexa-NHC [Fe(mbmi)](PF) and heteroleptic Fe-tetra-NHC [Fe(mbmi)(bpy)](PF) (bpy = 2,2'-bipyridine) complexes. They are compared to the reported Fe-hexa-NHC [Fe(btz)](PF) and Fe-tetra-NHC [Fe(btz)(bpy)](PF) complexes containing the conjugated, bidentate mesoionic NHC ligand 3,3'-dimethyl-1,1'-bis(-tolyl)-4,4'-bis(1,2,3-triazol-5-ylidene) (btz). The observed geometries of [Fe(mbmi)](PF) and [Fe(mbmi)(bpy)](PF) are evaluated through L-Fe-L bond angles and ligand planarity and compared to those of [Fe(btz)](PF) and [Fe(btz)(bpy)](PF).

Photoredox matching of earth-abundant photosensitizers with hydrogen evolving catalysts by first-principles predictions.

Photoredox properties of several earth-abundant light-harvesting transition metal complexes in combination with cobalt-based proton reduction catalysts have been investigated computationally to assess the fundamental viability of different photocatalytic systems of current experimental interest. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations using several GGA (BP86, BLYP), hybrid-GGA (B3LYP, B3LYP*), hybrid meta-GGA (M06, TPSSh), and range-separated hybrid (ωB97X, CAM-B3LYP) functionals were used to calculate relevant ground and excited state reduction potentials for photosensitizers, catalysts, and sacrificial electron donors. Linear energy correction factors for the DFT/TD-DFT results that provide the best agreement with available experimental reference results were determined in order to provide more accurate predictions.

Ligand-centered to metal-centered activation of a Rh(iii) photosensitizer revealed by molecular dynamics simulations.

Excited state evolution of the rhodium(iii) complex [Rh(iii)(phen)(NH)] (phen = 1,10-phenanthroline) has been investigated theoretically to gain a better understanding of light-driven activation of high-energy metal centered states. molecular dynamics (AIMD) simulations show the significance of asymmetric motion on a multidimensional potential energy landscape around the metal center for activated crossover from triplet ligand centered (LC) to triplet metal centered (MC) states on picosecond timescales. Significant entropic differences arising from the structural distributions of the LC and MC states revealed by the simulations are found to favor the forward crossover process.

Competing dynamics of intramolecular deactivation and bimolecular charge transfer processes in luminescent Fe(iii) N-heterocyclic carbene complexes.

Steady state and ultrafast spectroscopy on [Fe(phtmeimb)]PF (phtmeimb = phenyl(tris(3-methylimidazol-2-ylidene))borate) was performed over a broad range of temperatures. The intramolecular deactivation dynamics of the luminescent doublet ligand-to-metal charge-transfer (LMCT) state was established based on Arrhenius analysis, indicating the direct deactivation of the LMCT state to the doublet ground state as a key limitation to the lifetime. In selected solvent environments photoinduced disproportionation generating short-lived Fe(iv) and Fe(ii) complex pairs that subsequently undergo bimolecular recombination was observed.

Microsecond Photoluminescence and Photoreactivity of a Metal-Centered Excited State in a Hexacarbene-Co(III) Complex.

The photofunctionality of the cobalt-hexacarbene complex [Co(III)(PhB(MeIm))] (PhB(MeIm) = tris(3-methylimidazolin-2-ylidene)(phenyl)borate) has been investigated by time-resolved optical spectroscopy. The complex displays a weak (Φ ∼ 10) but remarkably long-lived (τ ∼ 1 μs) orange photoluminescence at 690 nm in solution at room temperature following excitation with wavelengths shorter than 350 nm. The strongly red-shifted emission is assigned from the spectroscopic evidence and quantum chemical calculations as a rare case of luminescence from a metal-centered state in a 3d complex.