Presentation: 2025 ND EPSCoR Annual conference 

October 21, 2025, NDSU Memorial Union, Fargo, North Dakota

Vanadium Complexes: Predicted Excited States for Molecular Qubit Applications

Vanadium complexes supported by tris(2-aminoethyl)amine (tren) are promising candidates for molecular spin qubits, where ligand field symmetry and charge-transfer excitations may enable optical addressability. We computationally investigated a series of tren–vanadium complexes with different substituents to evaluate ground- and excited-state properties relevant to initialization, manipulation, and readout. Geometry optimizations and time-dependent density-functional theory (TDDFT) calculations were carried out with Gaussian16 at the B3LYP level using implicit solvation (CPCM, acetonitrile). Comparative analysis of total energies shows that triplet states are consistently stabilized relative to singlets, with substituent-dependent gaps ranging from ~1.6 eV (tBu, NH₂) to ~1.9 eV (NO₂). Average bond length comparisons further reveal systematic elongation of V–ligand distances in triplet geometries, highlighting a structural signature of spin-state preference. Simulated absorption spectra across substituents display clear differences between singlet and triplet manifolds: bright ligand-involved states appear in the visible region, while oscillator-weak d–d transitions dominate at longer wavelengths. Substituent effects modulate oscillator strengths and spectral peak positions, with electron-donating groups lowering excitation energies and electron-withdrawing groups enhancing triplet stabilization. Natural transition orbital (NTO) analysis confirms the distinct d–d versus MLCT/LMCT character of these states. Together, these results establish clear substituent-dependent trends linking structure, energetics, and photophysics. This study provides a computational blueprint for guiding experimental realization of vanadium-based molecular qubits with robust triplet stabilization,