RC-01 — Open-Shell Electronic Structure & Molecular Properties
D1 — Research Orientation
Molecular properties are strongly influenced by how electrons are distributed within molecules, what electronic states they form, and how they interact with one another.
We study open-shell electronic structures, in which electrons are not necessarily paired, and investigate how their characteristics are connected to molecular properties, including optical and magnetic responses.
D2 — Scientific Explanation
Electron Pairing and Open-shell Electronic Structure
Electrons in molecules have spatial distributions described by molecular orbitals and interact with one another. Many stable molecules can be understood by starting from a closed-shell electronic structure, in which electrons are paired.
However, not every molecule can be adequately described by a single closed-shell electron configuration. When multiple configurations become important, open-shell character can arise even in an overall singlet state. Understanding such electronic states requires consideration of electron distributions, interactions, and electron correlation.
Electronic Structure and Molecular Properties
Open-shell electronic structures are closely related to molecular optical and magnetic properties, as well as chemical reactivity. However, a greater degree of open-shell character does not necessarily lead to a stronger response or a more pronounced property. For example, theoretical studies of the relationship between open-shell character and nonlinear optical responses have shown that particularly large responses can occur in molecular systems with intermediate diradical character.
Understanding such relationships requires consideration of multiple factors, including the electronic configurations associated with open-shell character, electronic-state energies, and interactions between states. We combine theoretical models and quantum chemical calculations to investigate how open-shell electronic structure influences molecular properties and to identify the underlying physical and chemical factors.
D3 — Scientific Detail
1. Understanding Open-shell Character and Electron Correlation
Open-shell character emerges from competing factors such as electron localization and delocalization, electron–electron repulsion, and orbital energies. Models involving a limited number of electrons and orbitals provide a useful starting point for understanding these relationships. In real molecules, however, additional electronic configurations and electron correlation can become important. We therefore combine simplified models that identify underlying physical factors with detailed electronic-structure calculations.
Quantitative descriptors of open-shell character also have their own ranges of applicability. Rather than relying on a single measure, we interpret these descriptors in relation to the electronic states and properties under investigation.
2. Open-shell Electronic Structure and Nonlinear Optical Responses
Third-order nonlinear optical responses provide an example of how open-shell electronic structure can influence molecular properties. The molecular quantity characterizing the contribution to the induced polarization that is cubic in the electric field is the second hyperpolarizability.
Theoretical studies of singlet diradical systems have shown that, in certain models, the second hyperpolarizability can become particularly large at intermediate diradical character. This behavior depends not only on open-shell character but also on the energies of relevant electronic states and the transition moments connecting them. Understanding nonlinear optical responses therefore requires consideration of how these factors are related.
3. From Electronic-structure Understanding to Molecular Design
A central challenge is to connect knowledge from theoretical models with the behavior and design of real molecules. For example, closely related regioisomers can exhibit pronounced differences in open-shell character and magnetic properties. This demonstrates the importance of how electrons interact through molecular connectivity, beyond molecular size alone.
We seek to connect theoretical analysis with experimental findings and interpret electronic structures through chemical concepts such as bonding and conjugation. This understanding provides a basis for exploring new possibilities in molecular design.