Assessing Excited State Energy Gaps with Time-Dependent Density Functional Theory on Ru(II) Complexes

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Materials Science

Background:

• Accurate prediction of electronic excited states is crucial for understanding photophysical processes.
• Density Functional Theory (DFT) is a widely used method, but its performance varies depending on the functional used.
• Evaluating excited state energy gaps and state character is vital for predicting properties like radiationless transition probabilities.

Purpose of the Study:

• To assess the performance of various density functionals across different rungs of Jacob's ladder for electronic excited states of Ru(II) complexes.
• To compare the accuracy of energy gaps between electronic excited states, and their character and ordering, against a high-level reference method.
• To determine which types of density functionals (pure vs. hybrid) are more suitable for specific excited-state properties.

Main Methods:

• Employing a set of density functionals from different rungs of Jacob's ladder.
• Calculating vertical excitation energies and energy gaps between electronic excited states.
• Utilizing wave function overlaps to systematically evaluate the effect of functionals on excited state character.
• Using multistate second-order perturbation theory complete active space (MS-CASPT2) as a reference method.

Main Results:

• Hybrid functionals generally yield better vertical excitation energies.
• Pure functionals demonstrate higher accuracy for excited state energy gaps.
• Pure functionals better reproduce the excited state character and ordering compared to hybrid functionals, aligning more closely with MS-CASPT2 results.

Conclusions:

• Pure density functionals are recommended for accurately calculating excited state energy gaps and characterizing electronic excited states in Ru(II) complexes.
• The choice of density functional significantly impacts the prediction of excited state properties, necessitating careful selection based on the desired property.
• This study provides valuable insights for computational chemists selecting DFT functionals for excited-state calculations in transition metal complexes.