A nitrogenase-inspired biomimetic chalcogel system comprising double-cubane [Mo 2 Fe 6 S 8 (SPh) 3 ] and single-cubane (Fe 4 S 4 ) biomimetic clusters demonstrates photocatalytic N 2 fixation and conversion to NH 3 in ambient temperature and pressure conditions. Replacing the Fe 4 S 4 clusters in this system with other inert ions such as Sb 3+ , Sn 4+ , Zn 2+ also gave chalcogels that were photocatalytically active. Finally, molybdenum-free chalcogels containing only Fe 4 S 4 clusters are also capable of accomplishing the N 2 fixation reaction with even higher efficiency than their Mo 2 Fe 6 S 8 (SPh) 3 -containing counterparts. Our results suggest that redox-active iron-sulfide-containing materials can activate the N 2 molecule upon visible light excitation, which can be reduced all of the way to NH 3 using protons and sacrificial electrons in aqueous solution. Evidently, whereas the Mo 2 Fe 6 S 8 (SPh) 3 is capable of N 2 fixation, Mo itself is not necessary to carry out this process. The initial binding of N 2 with chalcogels under illumination was observed with in situ diffuse-reflectance Fourier transform infrared spectroscopy (DRIFTS). 15 N 2 isotope experiments confirm that the generated NH 3 derives from N 2 . Density functional theory (DFT) electronic structure calculations suggest that the N 2 binding is thermodynamically favorable only with the highly reduced active clusters. The results reported herein contribute to ongoing efforts of mimicking nitrogenase in fixing nitrogen and point to a promising path in developing catalysts for the reduction of N 2 under ambient conditions. nitrogenase mimics | chalcogel | N 2 fixation | ammonia synthesis | photocatalytic T he reduction of atmospheric nitrogen to ammonia is one of the most essential processes for sustaining life. Currently, roughly half of the fixed nitrogen is supplied biologically by nitrogenase, while nearly the other half is from the industrial Haber-Bosch process, which operates under high temperature (400-500°C) and high pressure (200-250 bar) in the presence of a metallic iron catalyst (1). Nitrogenase, a two-component protein system comprising a MoFe protein and an associated Fe protein, carries out this "fixation" in nature under ambient temperature and pressure (2-4). N 2 substrate binding and activation take place at the ironmolybdenum-sulfur cofactor (FeMoco), and in some cases, Mofree iron-sulfur cofactor FeFeco and iron-vanadium-sulfur cofactor FeVco cofactors. Electron transfer during this catalytic process is believed to proceed from a [4Fe:4S] cluster located in the Fe protein to another Fe/S cluster (the P cluster) buried in the MoFe protein and finally to the FeMoco (Fig. 1A) (2, 5, 6). Whereas the role of Mo in the reactivity of nitrogenase has been the subject of long debate, iron is now well recognized as the only transition metal essential to all nitrogenases, and recent biochemical and spectroscopic data point to iron as the site of N 2 binding in the FeMoco (7-9). Naturally, understanding and mimicking how the nitrogenas...

The influence of molecular structure on excited-state properties and dynamics of a series of cyclometalated platinum dimers was investigated through a combined experimental and theoretical approach using femtosecond transient absorption (fs TA) spectroscopy and density functional theory (DFT) calculations. The molecules have the general formula [Pt(ppy)(μ-R2pz)]2, where ppy = 2-phenylpyridine, pz = pyrazolate, and R = H, Me, Ph, or (t)Bu, and are strongly photoluminescent at room temperature. The distance between the platinum centers in this A-frame geometry can be varied depending on the steric bulk of the bridging pyrazolate ligands that exert structural constraints and compress the Pt-Pt distance. At large Pt-Pt distances there is little interaction between the subunits, and the chromophore behaves similar to a monomer with excited states described as mixtures of ligand-centered and metal-to-ligand charge transfer (LC/MLCT) transitions. When the Pt(II) centers are brought closer together with bulky bridging ligands, they interact through their dz(2) orbitals and the S1 and T1 states are best characterized as metal-metal-to-ligand charge transfer (MMLCT) in character. The results of the femtoseconds TA experiments reveal that intersystem crossing (ISC) occurs on ultrafast time scales (τS1 < 200 fs), while there are two relaxation processes occurring within the triplet manifold, τ1 = 0.5-3.2 ps and τ2 = 20-70 ps; the longer time constants correspond to the presence of bulkier bridging ligands. DFT calculations illustrate that the Pt-Pt distances further contract in the T1 (3)MMLCT states; therefore, slower relaxation may be related to a larger structural reorganization. Subsequent investigations using faster time resolution are planned to measure the ISC process as well as to identify any potential coherent interaction(s) between the platinum centers that may occur.

The reactions of ethanol (CH 3 CH 2 OD) over cyclic (MO 3 ) 3 (M = Mo, W) clusters were studied experimentally and computationally. The cyclic clusters were prepared by sublimation of MoO 3 and WO 3 powders in a vacuum. To evaluate the cluster activity in dehydration, dehydrogenation, and condensation reactions, they were suspended in an ethanol matrix on an inert substrate, graphene monolayer on Pt(111). The reaction products formed upon heating were followed and quantified using temperature-programmed desorption. The experimental results were corroborated using coupled cluster CCSD(T) calculations at DFT optimized geometries that provide quantitative molecular-scale information on the reaction mechanisms. The dehydration and dehydrogenation of ethanol probe both the Lewis/Brønsted acid/ base and redox properties of the metal centers. The overall conversion of the alcohol is governed by the Lewis acidity of the metal center, and product selectivities, as determined by the relative weights of dehydrogenation and dehydration, are governed by the reducibility of the metal center.

The reactions of deuterated methanol, ethanol, 1-propanol, 1-butanol, 2-propanol, 2-butanol, and tert-butanol over cyclic (MO3)3 (M = Mo, W) clusters were studied experimentally with temperature-programmed desorption and theoretically with coupled cluster CCSD(T) theory and density functional theory. The reactions of two alcohols per M3O9 cluster are required to provide agreement with experiment for D2O release, dehydrogenation, and dehydration. The reaction begins with the elimination of water by proton transfers and forms an intermediate dialkoxy species that can undergo further reaction. Dehydration proceeds by a β-hydrogen transfer to a terminal MO. Dehydrogenation takes place via an α-hydrogen transfer to an adjacent MoVIO atom or a WVI metal center with redox involved for M = Mo and no redox for M = W. The two channels have comparable activation energies. H/D exchange to produce alcohols can take place after olefin is released or via the dialkoxy species, depending on the alcohol and the cluster. The Lewis acidity of the metal center with WVI being larger than MoVI results in the increased reactivity of W3O9 over Mo3O9 for dehydrogenation and dehydration. However, the product selection of aldehyde or ketone and olefin is determined by the reducibility of the metal center. Our calculations are consistent with the experiment in terms of the dehydrogenation, dehydration, and H/D exchange reactions. The condensation reaction requires a third alcohol with the sacrifice of an alcohol to form a metal hydroalkoxide, a strong gas-phase Brønsted acid. This Brønsted acid-driven reaction is different from the dehydrogenation and dehydration reactions that are governed by the Lewis acidity of the metal center.

We present an efficient theory and algorithm for computing four-component relativistic Dirac-Fock wave functions using the Coulomb, Gaunt, and full Breit interactions. Our implementation is based on density fitting, and is routinely applicable to systems with 100 atoms and a few heavy elements. The small components are expanded using 2-spinor basis functions. We show that the factorization of 3-index half-transformed integrals before building Coulomb and exchange matrices is essential for efficient evaluation of the Fock matrix. With the Coulomb interaction, the computational cost for evaluating the Fock operator has been found to be only 70-90 times that in the non-relativistic density-fitted Hartree-Fock method. The prefactors have been 170 and 350-450 for the Gaunt and Breit interactions, respectively. The largest molecule to which we have applied the Dirac-Fock-Coulomb method is an Ac(III) motexafin complex (130 atoms, 556 electrons, 1289 basis functions), for which one self-consistent iteration takes around 1100 s using 1024 CPU cores. In addition, we have found that, while the standard fitting basis sets are accurate for Dirac-Fock-Coulomb calculations, their accuracy is very poor for Dirac-Fock-Gaunt and Breit calculations. We report a prototype of accurate fitting basis sets for these cases.

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Vibrational coherence in the metal–metal-to-ligand-charge transfer (MMLCT) excited state of cyclometalated platinum dimers with a pseudo C 2 symmetry was investigated where two nearly degenerate transitions from the highest occupied molecular orbital (metal–metal σ* orbital) to higher energy ligand π* orbitals could be simultaneously induced. We observed oscillatory features in femtosecond degenerate transient absorption (TA) signals from complexes [(ppy)­Pt­(μ-tBu2pz)]2 (1) and anti-[(ppy)­Pt­(μ-pyt)]2 (2), which are attributed to coherent nuclear motions that modulate the HOMO (antibonding σ*) energy level, and hence, the energy for the MMLCT transition. The characteristics of such coherent nuclear motions, such as the oscillatory frequency and the dephasing time, differ between 1 and 2 and are explained by mainly two structural factors that could influence the vibrational coherence: the Pt–Pt distance (2.97 Å for 1 vs 2.85 Å for 2) and molecular shape (1 in an “A” frame vs 2 in an “H” frame). Because the electronic coupling between the two Pt atoms determines the energy splitting of the bonding σ and antibonding σ* orbital, the Pt–Pt stretching mode coupled to the MMLCT transition changes the inter Pt distance from that of the ground state. Interestingly, while 1 shows a single Pt–Pt stretching frequency of 120 cm–1 in the MMLCT state, 2 exhibits multiple downshifted frequencies (80 and 105 cm–1) in the MMLCT state along with a shorter vibrational dephasing time than 1. Based on the ground state optimized structures and Raman calculations, the changes evident in the vibrational wavepacket dynamics in 2 are closely correlated with the “H” framed geometry in 2 compared to the “A” frame in 1, leading to the sharp increase in π–π interaction between ppy ligands. Although the TA experiments do not directly reveal the ultrafast intersystem crossing (ISC) because of a strong coherent spike at early time scales, the dependence of the vibrational wavepacket dynamics on molecular geometry can be understood based on previously proposed potential energy surfaces as a function of Pt–Pt distance, suggesting that the interaction between the cyclometalating ligands can be a key factor in determining the Pt–Pt shortening and the related energy relaxation dynamics in the Pt­(II) dimers. Further experiments using femtosecond broadband TA and X-ray scattering spectroscopy are planned to investigate directly the ISC and Pt–Pt contraction to support the relationship between ground state molecular geometry and photoinduced structural changes in the Pt­(II) dimers.

The theoretical description of the time-evolution of excitons requires, as an initial step, the calculation of their spectra, which has been inaccessible to most users due to the high computational scaling of conventional algorithms and accuracy issues caused by common density functionals. Previously (J. Chem. Phys. 2016, 144, 204105), we developed a simple method that resolves these issues. Our scheme is based on a two-step calculation in which a linear-response TDDFT calculation is used to generate orbitals perturbed by the excitonic state, and then a second linear-response TDDFT calculation is used to determine the spectrum of excitations relative to the excitonic state. Herein, we apply this theory to study near-infrared absorption spectra of excitons in oligomers of the ubiquitous conjugated polymers poly(3-hexylthiophene) (P3HT), poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV), and poly(benzodithiophene-thieno[3,4-b]thiophene) (PTB7). For P3HT and MEH-PPV oligomers, the calculated intense absorption bands converge at the longest wavelengths for 10 monomer units, and show strong consistency with experimental measurements. The calculations confirm that the exciton spectral features in MEH-PPV overlap with those of the bipolaron formation. In addition, our calculations identify the exciton absorption bands in transient absorption spectra measured by our group for oligomers (1, 2, and 3 units) of PTB7. For all of the cases studied, we report the dominant orbital excitations contributing to the optically active excited state-excited state transitions, and suggest a simple rule to identify absorption peaks at the longest wavelengths. We suggest our methodology could be considered for further developments in theoretical transient spectroscopy to include nonadiabatic effects, coherences, and to describe the formation of species such as charge-transfer states and polaron pairs.