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

抄録

The analysis of the diradical state of functional open-shell molecules is important for understanding their physical properties and chemical reactivity [1]. 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 [1]. However, the influence of surface interactions on the diradical state remains unclear. In this study, approximate spin-projected density functional theory calculations with dispersion correction and plane-wave basis (AP-DFT-D3/plane-wave calculations) were performed using physisorption models (p-benzyne/MgO(001) and p-benzyne/SrO(001)) to investigate variations in the diradical character caused by physisorption. A decrease in the diradical character caused by molecular distortion, variation in the diradical character caused by intermolecular interaction, and decrease in the diradical character caused by interaction with the surface were identified, resulting in a decrease in the diradical character in all models. This is different from the s-electron systems reported in previous studies [2,3], where the diradical character increased by adsorption. This is because the singly occupied molecular orbitals of p-benzyne have a horizontal distribution with respect to the surface and p-benzyne is a through-bonded diradical. The energy change due to molecular cohesion in the adsorption model was small, whereas the change in the diradical character due to cohesion was significant. This implies that it is difficult to immobilise diradicals on surfaces in a completely isolated state. In other words, it is a manifestation of the ability to tailor molecular functions derived from the diradical state by changing the adsorption structure and self-assembly by the surfaces. Furthermore, the interaction with the surfaces induces electron delocalisation to π-conjugated orbitals and intramolecular charge polarisation. This indicates that the contribution of the electron configurations, which are too small in the gas phase, is amplified by the surface interactions. Hence, the stability between the phases of materials can be tuned through inert ionic surface interactions. The difficulty of studying excited states by DFT methods requires more accurate methods, such as multi-referenced approaches using cluster models, to investigate in detail the tuning of open-shell electronic states by adsorbing on a surface. The present study is regarded as an elementary demonstration. We explicitly showed that in a real molecule, the effects of surface interactions on the diradical state are not negligible and that the theoretical design of molecular devices with functional open-shell molecules immobilised on the surface requires a detailed investigation of the surface adsorption effects in terms of their diradical states.

[1] T. Stuyver, B. Chen, T. Zeng, P. Geerlings, F. De Proft and R. Hoffinann, Chem. Rev., 2019, 119, 11291–11351.

[2] K. Fujimaru, K. Tada, H. Ozaki, M. Okumura and S. Tanaka, Suf. Interfaces, 2022, 33, 102206.

[3] K. Tada, H. Ozaki, K. Fujimaru, Y. Kitagawa, T. Kawakami and M. Okumura, Phys. Chem. Chem. Phys., 2021, 23, 25024–25028.