Dr. Jake A. Tan

The anharmonic vibrational structures of XHX⁻ anions, X = {F, Cl, Br, and I}, were examined with a focus on the n₁v₁+v₃ combination bands, where
is the excitation quanta of the symmetric X-H stretch (v₁) and v₃is the asymmetric X-H stretch. Full-dimensional potential energy surfaces (PESs) were built by using a multilevel scheme based on CCSD(T) and MP2 levels of theories. These dual-level PESs were then used to simulate the anharmonic spectra and to assess the extent of vibrational coupling. The vibrational signatures of XHX⁻ are very similar to those of the previously studied proton-bound noble gas dimers. The combination bands become more visible as the halide becomes heavier. The quantum nature of these combination bands was examined using an adiabatic model. Furthermore, the halide binding energies for XHX⁻ , X = {F, Cl, Br, and I}, were studied using a modern fragmentation-based method, energy decomposition analysis based on absolutely localized molecular orbitals (ALMO-EDA). The halide binding energy decreases from that of FHF⁻ to that of IHI⁻. Both the geometric distortion and dispersion-free frozen components destabilize these complexes, weakening their binding energies. However, the dispersion, polarization effects, and charge transfer contributions stabilize these complexes.

We present a Grassmann extrapolation method (G-Ext) that combines the mathematical framework of the Grassmann manifold with the direct inversion in the iterative subspace (DIIS) technique to accurately and efficiently extrapolate density matrices in electronic structure calculations. By overcoming the challenges of direct extrapolation on the Grassmann manifold, this indirect G-Ext-DIIS approach successfully preserves the geometric structure and physical constraints of the density matrices. Unlike Tikhonov regularized G-Ext, G-Ext-DIIS requires no tuning of regularization parameters. Its DIIS subspace is compact, numerically stable, and independent of descriptor dimensionality, system size, and basis set, ensuring both robustness and computational efficiency. We evaluate G-Ext-DIIS using alanine dipeptide and its zwitterionic form along phi and psi torsional scans, employing Coulomb, overlap, and core Hamiltonian matrix descriptors with the diffuse 6-311++G(d,p) and aug-cc-pVTZ basis sets. When using overlap or core Hamiltonian descriptors, G-Ext-DIIS achieves sub-millihartree accuracy across angular extrapolation ranges that exceed typical geometry optimization step sizes. This indicates its potential for generating high quality initial density matrices in each optimization cycle. Compared to direct extrapolation methods with or without McWeeny purification, as well as the Lowdin extrapolation from nearby geometries, G-Ext-DIIS demonstrates superior accuracy, variational consistency, and reliability across basis sets. We also explore Fock matrix extrapolation using the same DIIS coefficients, although this strategy proves less reliable for distant geometries. Overall, G-Ext-DIIS offers a robust, efficient, and transferable framework for constructing accurate density matrices, with promising applications in geometry optimization and ab initio molecular dynamics simulations.

A greenish-blue zinc complex Zn(PhOBz)-PXZ with enhanced thermally activated delayed fluorescence (TADF) properties has been prepared from Zn(OAc)2 and 4PXZ2OHBz, a 2-(1H-benzimidazol-2-yl)phenol-based TADF ligand. The TADF phenomenon has been confirmed by time-resolved photoluminescence (TrPL) studies. The DFT calculations show spatially well-separated HOMO and LUMO in their ground states, along with a small energy splitting between the excited singlet (S1) and triplet (T1) states, in a good agreement with the TADF mechanism. Due to the high thermal stability of Zn(PhOBz)-PXZ, OLED devices can be fabricated by vacuum vapor deposition, and greenish-blue OLEDs with the maximum emission at 521 nm were successfully demonstrated. The maximum external quantum efficiency (EQEmax) of 10.6 %, with Commission Internationale de l’Eclairage (CIE) coordinates of (0.28, 0.47) were recorded. Zinc TADF complexes have the advantages of cost-effectiveness, greater abundance of natural resources, environmentally friendly metals, making them potential replacements for future precious metal emitters.

The electrochemical reduction (ECR) of CO2 is a promising approach for CO2 removal and utilization, which is a critical component of the circular carbon economy. However, developing efficient and selective electrocatalysts is still challenging. Single-atom catalysts (SACs) have gained attention because they offer high metal atom utilization and uniform active sites. However, tuning the active metal centres to achieve high activity and selectivity in CO2 reduction remains a significant challenge. This study presents a novel electrocatalyst (Co-Ni-N-C) for CO2 ECR on the diatomic metal-nitrogen sites prepared through ion exchange using a zeolitic imidazolate framework (ZIF) as a precursor. During pyrolysis, nitrogen-doped graphitic carbon serves as the host material, anchoring the diatomic Co-Ni sites. The resulting bimetallic active sites demonstrate exceptional performance, achieving a high CO yield rate of 53.36 mA mg(cat.)(-1) and an impressive CO faradaic efficiency of 94.1% at an overpotential of -0.27 V. Spectroscopic, microscopic, and density functional theory (DFT) analyses collectively unveil the crucial synergistic role of the Co-Ni-N-6 moiety in promoting and sustaining exceptional electrocatalytic activities. The successful utilization of bimetallic sites in enhancing catalyst performance highlights the potential of this approach in developing efficient electrocatalysts for various other reactions.

The Lagrange-based Grassmann interpolation (G-Int) method has been extended for open-shell systems using restricted open-shell (RO) methods. The performance of this method was assessed in constructing potential energy surfaces (PESs) for vanadium(ii) oxide, benzyl radical, and methanesulfenyl chloride radical cation. The density matrices generated by G-Int when used as initial guesses for self-consistent field (SCF) calculations, exhibit superior performance compared to other traditional SCF initial guess schemes, such as SADMO, GWH, and CORE. Additionally, the energy obtained from the G-Int scheme satisfies the variational principle and outperforms the direct energy-based Lagrange interpolation approach. In the case of methanesulfenyl chloride radical cation, a unique example with a flat PES at the end region along the H-C-S-Cl dihedral angle, the use of an equally-spaced grid sampling leads to significant oscillations near the end of the interval due to the effects of Runge's phenomenon. Introducing an unequally-spaced grid sampling based on a scaled Gauss-Chebyshev quadrature effectively mitigated the Runge's phenomenon, making it suitable for combining with G-Int in constructing PESs for general applications. Thus, G-Int provides an efficient and robust strategy for building spin contamination-free PESs with consistent accuracy.
The Lagrange-based Grassmann interpolation (G-Int) method has been extended for open-shell systems using restricted open-shell methods in building spin contamination-free potential energy surfaces.

Thermally activated delayed fluorescence (TADF) is a promising approach to harvest triplet excitons and achieve high-performance organic light-emitting diodes (OLEDs) for displays. In this study, we synthesized two new TADF emitters, 4Ac25CzPy and 4Ac35CzPy, featuring acridan-pyrimidine-carbazole moieties. Remarkably, a slight modification in the carbazole group position enables precise control of luminous color, resulting in emissions at 483 nm and 494 nm for 4Ac25CzPy and 4Ac35CzPy, respectively, in the electroluminescent device. Both compounds exhibit small energy difference between their singlet and triplet states (Delta EST) of 0.14 eV and 0.15 eV, confirming their TADF characteristics. Notably, OLEDs utilizing 4Ac35CzPy achieve outstanding performance with the maximum external quantum efficiency (eta EQE) of 21.2% and a photoluminescence quantum yield of 65.1%. This high efficiency is attributed to efficient energy transfer from the host to the emitter. Moreover, the 4Ac35CzPy device exhibits a high light outcoupling efficiency of 0.3, further enhancing its remarkable performance.
Thermally activated delayed fluorescence (TADF) is a promising approach to harvest triplet excitons and achieve high-performance organic light-emitting diodes (OLEDs) for displays.

Vibrational spectra in the acetylenic and aromatic C-H stretching regions of phenylacetylene and fluorophenylacetylenes, viz., 2-fluorophenylacetylene, 3-fluorophenylacetylene, and 4-fluorophenylacetylene, were measured using the IR-UV double resonance spectroscopic method. The spectra, in both acetylenic and aromatic C-H stretching regions, were complex exhibiting multiple bands. Ab-initio anharmonic calculations with quartic potential using B97D3/6-311++G(d,p) and vibrational configuration interaction were able to capture all important spectral features in both the regions of the experimentally observed spectra for all four molecules considered in the present work. Interestingly, for phenylacetylene, the spectrum in the acetylenic C-H stretching region emerges due to anharmonic coupling of modes localized on the acetylenic moiety along with the other ring modes, which also involve displacements on the acetylenic group, which is in contrast to what has been proposed and propagated in the literature. In general, this coupling scheme is invariant to the fluorine atom substitution. For the aromatic C-H stretching region, the observed spectrum emerges due to the coupling of the C-H stretching with C-C stretching and C-H in-plane bending modes.

With relevant chemical space growing larger and larger by the day, the ability to extend computational tractability over that larger space is of paramount importance in virtually all fields of science. The solution we aim to provide here for this issue is in the form of the generalized many-body expansion for building density matrices (GMBE-DM) based on the set-theoretical derivation with overlapping fragments, through which the energy can be obtained by a single Fock build. In combination with the purification scheme and the truncation at the one-body level, the DM-based GMBE(1)-DM-P approach shows both highly accurate absolute and relative energies for medium-to-large size water clusters with about an order of magnitude better than the corresponding energy-based GMBE(1) scheme. Simultaneously, GMBE(1)-DM-P is about an order of magnitude faster than the previously proposed MBE-DM scheme [F. Ballesteros and K. U. Lao, J. Chem. Theory Comput. 18, 179 (2022)] and is even faster than a supersystem calculation without significant parallelization to rescue the fragmentation method. For even more challenging systems including ion-water and ion-pair clusters, GMBE(1)-DM-P also performs about 3 and 30 times better than the energy-based GMBE(1) approach, respectively. In addition, this work provides the first overlapping fragmentation algorithm with a robust and effective binning scheme implemented internally in a popular quantum chemistry software package. Thus, GMBE(1)-DM-P opens a new door to accurately and efficiently describe noncovalent clusters using quantum mechanics.

The diazenylium ion (N2H+) is a ubiquitous ion in dense molecular clouds. This ion is often used as a dense gas tracer in outer space. Most of the previous works on diazenylium ion have focused on the shared-proton stretch band, νH+ . In this work, we have performed reduced-dimensional calculations to investigate the vibrational structure of N2H+Ng, Ng = {He, Ne, Ar, Kr, Xe, and Rn}. We demonstrate a few interesting things about this system. First, the vibrational coupling in N2H+ can be tuned to switch on interesting anharmonic effects such as Fermi resonance or combination bands by tagging it with different noble gases. Second, a comparison of the vibrational spectrum from N2H+He to N2H+Rn shows that the νH+ can be swept from an “Eigen-like” to a “Zundel-like” limiting case. Anharmonic calculations were performed using a multilevel approach, which utilized the MP2 and CCSD­(T) levels of theories. Binding energies for the elimination of Ng in N2H+Ng are also reported.

The recently reported Grassmann interpolation (G-Int) method [J. A. Tan and K. U. Lao, J. Chem. Phys. 158, 051101 (2023)] has been extended to spin-unrestricted open-shell systems. In contrast to closed-shell systems, where G-Int has to be performed only once since the alpha and beta density matrices are the same, spin-unrestricted open-shell systems require G-Int to be performed twice-one for the alpha spin and another for the beta spin density matrix. In this work, we tested the performance of G-Int to the carbon monoxide radical cation CO center dot+ and nickelocene complex, which have the doublet and triple ground states, respectively. We found that the Frobenius norm errors associated with the interpolations for the a and beta spin density matrices are comparable for a given molecular geometry. These G-Int density matrices, when used as an initial guess for a self-consistent field (SCF) calculation, outperform the conventional SCF guess schemes, such as the superposition of atomic densities, purified superposition of atomic densities, core Hamiltonian, and generalized Wolfsberg-Helmholtz approximation. Depending on the desired accuracy, these G-Int density matrices can be used to directly evaluate the SCF energy without performing SCF iterations. In addition, the spin-unrestricted G-Int density matrices have been used for the first time to directly calculate the atomic charges using the Mulliken and ChElPG population analysis.