Search for: All records

Many transition metal coordination complexes are known to undergo a structural change in response to a stimulus, like light, which can have a debilitating impact on properties of interest (e.g., quantum yield, stability, reactivity, etc.). This is particularly true for Cu(I) coordination complexes that suffer from short, excited-state lifetimes due to D2d to D2 distortion and solvent coordination. Here, we investigate the impact of strategic surface binding and the role of the surface binding motif on the excited state lifetime of Cu(I) complexes with carboxylate-functionalized N-phenylpyridin-2-ylmethanimine ligands. Relative to the solution, the excited state lifetime for the ZrO2-bound complexes increases 7-fold when either one ligand is bound or both ligands are bound through a flexible linker but 17-fold when both ligands are rigidly bound to the surface. With support from theoretical calculations, we attribute the dramatic increase in lifetime for the latter to the rigid binding strategy inhibiting the planarizing distortion and possible quenching via solvent coordination. These results lend further support to the idea that molecular immobilization via strategic surface binding is an effective strategy for inhibiting undesired molecular distortion. 

Understanding the landscape of molecular photocatalysis is vital to enable efficient conversion of feedstock molecules to targeted products and inhibit off-cycle reactivity. In this study, the light-promoted reactivity of [RuCp*2]+ was explored via electronic structure, photophysical, and photostability studies and the reactivity of [RuCp*2]+ within a photocatalytic hydrogen evolution cycle was assessed. TD-DFT calculations support the assignment of a low-energy ligand-to-metal charge transfer transition (LMCT) centered at 500 nm, where an electron from a ligand-based orbital delocalized across both Cp* ligands is promoted to a dx2–y2-based β-LUMO orbital. Upon irradiating the LMCT absorption feature, ultrafast transient absorption spectroscopy measurements show that an initial excited state (τ1 = 1.3 ± 0.1 ps) is populated, which undergoes fast relaxation to a longer-lived state (τ2 = 12.0 ± 0.9 ps), either via internal conversion or vibrational relaxation. Despite the short-lived nature of these excited states, bulk photolysis of [RuCp*2]+ demonstrates that photochemical conversion to decomposition products is possible upon prolonged illumination. Collectively, these studies reveal that [RuCp*2]+ undergoes light-driven decomposition, highlighting the necessity to construct molecular photocatalytic systems resistant to off-cycle reactivity in both the ground and excited states. 

Molecules undergo a structural change to minimize the energy of excited states generated via external stimuli such as light. This is particularly problematic for Cu(I) coordination complexes which are an intriguing alternative to the rare and expensive transition metal containing complexes (e.g., Pt, Ir, Ru, etc.) but suffer from short excited state lifetimes due to D2d to D2 distortion and solvent coordination. Here we investigate strategic surface binding as an approach to hinder this distortion and increase the excited state lifetime of Cu(I) polypyridyl complexes. Using transient absorption spectroscopy, we observe a more than 20-fold increase in excited state lifetime, relative to solution, for a Cu(I) complex that can coordinate to the ZrO2 via both carboxylated ligands. In contrast, the Cu(I) complex that coordinates via only one ligand has a less pronounced enhancement upon surface binding and exhibits greater sensitivity to coordinating solvents. A combination of ATR-IR and polarized visible ATR measurements as well as theoretical calculations suggest that the increased lifetime is due to surface binding which decreases the degrees of freedom for molecular distortion (e.g., D2d to D2), with the doubly bound complex exhibiting the most pronounced enhancement. 

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. 

Electronic structure calculations on small systems such as H2, H2O, LiH, and BeH2 with chemical accuracy are still a challenge for the current generation of the noisy intermediate-scale quantum (NISQ) devices. One of the reasons is that due to the device limitations, only minimal basis sets are commonly applied in quantum chemical calculations, which allows one to keep the number of qubits employed in the calculations at a minimum. However, the use of minimal basis sets leads to very large errors in the computed molecular energies as well as potential energy surface shapes. One way to increase the accuracy of electronic structure calculations is through the development of small basis sets better suited for quantum computing. In this work, we show that the use of adaptive basis sets, in which exponents and contraction coefficients depend on molecular structure, provides an easy way to dramatically improve the accuracy of quantum chemical calculations without the need to increase the basis set size and thus the number of qubits utilized in quantum circuits. As a proof of principle, we optimize an adaptive minimal basis set for quantum computing calculations on an H2 molecule, in which exponents and contraction coefficients depend on the H-H distance, and apply it to the generation of H2 potential energy surface on IBM-Q quantum devices. The adaptive minimal basis set reaches the accuracy of the double-zeta basis sets, thus allowing one to perform double-zeta quality calculations on quantum devices without the need to utilize twice as many qubits in simulations. This approach can be extended to other molecular systems and larger basis sets in a straightforward manner. 

MLCT state is nested within metal-centered states leading to a long excited state lifetime. 

This work reports a combined experimental and computational study of the activation of an otherwise catalytically inactive cobalt complex, [Co(TIM)Br2]+, for aqueous nitrite reduction. The presence of phosphate buffer leads to efficient electrocatalysis, with rapid reduction to ammonium occurring close to the thermodynamic potential and with high Faradaic efficiency. At neutral pH, increasing buffer concentrations increase catalytic current while simultaneously decreasing overpotential, although high concentrations have an inhibitory effect. Controlled potential electrolysis and rotating ring-disk electrode experiments indicate that ammonium is directly produced from nitrite by [Co(TIM)Br2]+, along with hydroxylamine. Mechanistic investigations implicate a vital role for the phosphate buffer, specifically as a proton shuttle, although high buffer concentrations inhibit catalysis. These results indicate a role for buffer in the design of electrocatalysts for nitrogen oxide conversion.