The solvent-mediated excited-state dynamics of the COOH-functionalized Fe-carbene photosensitizer [Fe(bmicp) 2 ] 2+ (bmicp = 2,6-bis(3-methyl-imidazole-1-ylidine)-4-carboxypyridine) is studied by time-dependent density functional theory (TD-DFT), as well as classical and quantum dynamics simulations. We demonstrate the crucial role of the polar acetonitrile solvent in stabilizing the MLCT states of the investigated molecule using the conductor polarizable continuum (CPCM) model. This leads to dynamics that avoid sub-ps back electron transfer to the metal, and an exceptionally long-lived 1 MLCT state that does not undergo sub-ps 1 MLCT → 3 MLCT intersystem crossing, as it is energetically isolated. We identify two components of the excited-state solvent reorganization process: an initial rotation (∼300 fs) and diffusional dynamics within the local cage surrounding the rotated solvent molecule (∼2 ps). Finally, it is found that the relaxation of the solvent only slightly affects the excited-state population dynamics of [Fe(bmicp) 2 ] 2+ . IntroductionExcited-state dynamics in transition metal complexes 1-3 (TMCs) are ubiquitous and a key to develop advanced technologies, solar energy conversion, 4 photocatalysis, 5,6 molecular data storage, 7,8 just to name a few. Among these intriguing molecular systems, Fe-N-heterocyclic carbenes 9 (NHCs) received recently special attention owing to their great potential as cheap, Earth-abundant photosensitizers. This is due to their relatively long-lived (ps) photoactive metal-to-ligand charge transfer (MLCT) states that can be exploited, for instance, to inject the photoexcited electron into the conduction band of a semiconductor. This injection process into TiO 2 was indeed observed with 92% yield in the [Fe(bmicp) 2 ] 2+ (bmicp = 2,6-bis(3-methyl-imidazole-1-ylidine)-4-carboxy-pyridine) complex, however the majority of these electrons were found to undergo fast (<10 ns) recombination with cations, thus preventing efficient photocurrent generation.
In this work, we investigate the excited-state solute and solvation structure of [Ru(bpy) 3 ] 2+ , [Fe(bpy) 3 ] 2+ , [Fe(bmip) 2 ] 2+ and [Cu(phen) 2 ] + (bpy=2,2'-pyridine; bmip=2,6-bis(3-methylimidazole-1-ylidine)-pyridine; phen=1,10-phenanthroline) transition metal complexes (TMCs) in terms of solute-solvent radial distribution functions (RDFs) and evaluate the performance of some of the most popular partial atomic charge (PAC) methods for obtaining these RDFs by molecular dynamics (MD) simulations. To this end, we compare classical MD of a frozen solute in water and acetonitrile (ACN) with quantum mechanics/molecular mechanics Born-Oppenheimer molecular dynamics (QM/MM BOMD) simulations. The calculated RDFs show that the choice of a suitable PAC method is dependent on the coordination number of the metal, denticity of the ligands, and type of solvent. It is found that this selection is less sensitive for water than ACN. Furthermore, a careful choice of the PAC method should be considered for TMCs that exhibit a free direct coordination site, such as [Cu(phen) 2 ] + . The results of this work show that fast classical MD simulations with ChelpG/RESP or CM5 PACs can produce RDFs close to those obtained by QM/MM MD and thus, provide reliable solvation structures of TMCs to be used, e.g. in the analysis of scattering data.
The calculations of the acidity constants (pKa) of a series of sulfur oxoacids including H2SOn (n = 1-5) and H2S2On (n = 1, 3, 4, 6 and 7) are presented for the first time. The calculations were performed using two expensive correlated levels of theory including MP2/6-311++G(3df,3pd) and CCSD/6-311++G(d,p) in both gas and aqueous phases. The new continuum solvation model, SMD, based on the quantum mechanical charge density of a solute molecule interacting with a continuum description of the solvent, used to account the solvent effects. The calculated pKas were corrected using the different correlation equations (Zimmermann and Tossell, J. Phys. Chem. A, 2009, 113, 5105-5111) to improve the accuracy of results. Also, the calculated results showed the effect of the intramolecular hydrogen bonding on the acidity strength.
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