Issues on DFT+U calculations of organic diradicals

The diradical state is an important electronic state for understanding molecular functions and should be elucidated for the in silico design of functional molecules and their application to molecular devices. The density functional theory calculation with plane-wave basis and correction of the on-site Coulomb parameter U (DFT+U/plane-wave calculation) is a good candidate of high-throughput calculations of diradical-band interactions. However, it has not been investigated in detail to what extent the DFT+U/plane-wave calculation can be used to calculate organic diradicals with a high degree of accuracy. In the present study, using typical organic diradical molecules (bisphenalenyl molecules) as model systems, the discrepancy in the optimum U values between the two electronic states (open-shell singlet and triplet) that compose the diradical state is detected. The calculated results show that the reason for this U value discrepancy is the difference in electronic delocalisation due to π-conjugation between the open-shell singlet and triplet states, and that the effect of U discrepancy becomes large as diradical character decreases. This indicates that it is necessary to investigate the U value discrepancy with reference to the calculated results by more accurate methods or to experimental values when calculating organic diradicals with low diradical character. For this investigation, the local magnetic moments, unpaired beta electron numbers, and effective magnetic exchange integral values can be used as reference values. For the effective magnetic exchange integral values, the effects of U discrepancy are partially cancelled out. However, because the effects may not be completely offset, care should be taken when using the effective magnetic exchange integral value as a reference. Furthermore, a comparison of DFT+U and hybrid-DFT calculations shows that the DFT+U underestimates the HOMO-LUMO gap of bisphenalenyls, although a qualitative discussion of the gap is possible.

Model calculations for the prediction of the diradical character of physisorbed molecules: p-benzyne/MgO and p-benzyne/SrO

The analysis of the diradical state of functional open-shell molecules is important for understanding their physical properties and chemical reactivity. The diradical character is an important factor in the functional elucidation and design of open-shell molecules. In recent years, attempts have been made to immobilise functional open-shell molecules on surfaces to form devices. However, the influence of surface interactions on the diradical state remains unclear. In this study, the physisorption structures of p-benzyne, which is a typical diradical molecule, on MgO(001) and SrO(001) surfaces are used as models to investigate how the diradical character is affected by physisorption. This is done using approximate spin-projected density functional theory calculations with dispersion correction and plane-wave basis (AP-DFT-D3/plane-wave calculations). The diradical character change (Δy) due to adsorption can be categorised into three factors, namely the change due to the distortion of the diradical molecule (Δydis), the interaction between neighbouring diradical molecules (Δycoh), and molecule-surface interactions (Δysurf). In all the calculated models, physisorption reduced the diradical character (Δy < 0), and the contribution of Δysurf was the largest among the three factors. The calculated results show that adsorption induces electron delocalisation to π-conjugated orbitals and intramolecular charge polarisation, both of which contribute to reducing the occupancy of singly occupied molecular orbitals. This indicates that the diradical character of p-benzyne is reduced by the stabilisation of the resonance structures. Furthermore, geometry optimisation of the surfaces shows that the chemical-soft surface (SrO) varies the diradical character more significantly than the chemical-hard surface (MgO). This study shows that the open-shell electronic state and stack structure of diradical molecules can be controlled through the analysis of the surface diradical state.

Can we enhance diradical character using interaction with stoichiometric surfaces of ionic oxides? A theoretical investigation using chemical indices

Chemical indices are effective tools for examining the functions and reactivities of stable radical species. In this study, we formulated an approximation to estimate chemical indices using electron density. Theoretical investigations using the developed scheme revealed that surface interactions can tune chemical indices and that the diradical character was enhanced by weak adsorption onto ionic solids with charge-dipole interactions.