RC-02 — Molecular Interaction & Collective Structure
D1 — Research Orientation
When molecules come together, interactions between them can reshape their electronic structures and give rise to new properties that depend on how the molecules interact.
We study how interactions between molecules influence electronic states, chemical bonding, and collective behavior. One example is stacked-ring aromaticity, in which closely stacked antiaromatic molecules exhibit unexpected aromatic characteristics. Through quantum chemical analysis, we seek to understand how these phenomena emerge and explore new ways of designing molecular assemblies through intermolecular interactions.
D2 — Scientific Explanation
Intermolecular Interactions and Electronic Structure
The properties of molecular assemblies depend not only on the electronic structures of their constituent molecules but also on how those molecules are arranged and interact. For example, when π-conjugated molecules approach each other, intermolecular orbital interactions can modulate their electronic states and influence optical and magnetic properties.
Understanding such changes requires connecting the electronic structures of individual molecules with the interactions between them.
Stacked-ring Aromaticity and Electronic States
Aromaticity is an important chemical concept for understanding cyclic conjugated molecules. Within the conventional Hückel framework, planar, cyclic, fully conjugated systems containing 4n π-electrons are classified as antiaromatic.
Interestingly, when such antiaromatic molecules are stacked sufficiently close together, intermolecular interactions can give rise to aromatic characteristics. Orbital overlap between molecules is important in this phenomenon. At suitable intermolecular distances and relative orientations, configurations in which the constituent molecules have triplet-like electronic structures coupled to form an overall singlet may contribute to the ground-state wavefunction. We refer to these as triplet-pair (TT) configurations.
Intermolecular charge-transfer (CT) configurations may also play an important role. By examining the contributions of these electronic configurations, we seek to develop chemical interpretations of the aromatic characteristics arising from intermolecular interactions.
D3 — Scientific Detail
1. Intermolecular Interactions and Configuration Mixing
The electronic states of molecular assemblies can be described by starting from the electronic states of individual molecules and introducing interactions between them. For example, states of the constituent molecules can be used to construct a basis for the assembly. By accounting for intermolecular couplings and charge-transfer configurations within an appropriate electronic-state description, we can investigate the electronic structure of the interacting system.
This approach connects our understanding of individual molecules with the electronic properties that emerge through intermolecular interactions.
2. Chemical Interpretation of Stacked-ring Aromaticity
Multiconfigurational wavefunction analyses of closely stacked antiaromatic dimers have revealed important contributions from triplet-pair (TT) and intermolecular charge-transfer (CT) configurations to their ground-state electronic structures.
The triplet-like electronic character associated with TT configurations provides a basis for interpreting aromatic characteristics through Baird's rule and effective π-electron counts. CT configurations, meanwhile, can be important for understanding intermolecular electronic bonding and stabilization. TT and CT are different components of the ground-state wavefunction, in which multiple electronic configurations are mixed.
Interestingly, these TT configurations share electronic-structure features with the correlated triplet pairs involved in singlet fission. This connection offers a way to consider ground-state intermolecular interactions and excited-state quantum dynamics in terms of related electronic-state concepts.
3. Molecular Identity and Assembly Design
Intermolecular interactions depend on distance, relative orientation, aggregate size, and the positions of molecules within an assembly. However, molecular arrangement cannot be considered independently of the shapes and electronic structures of the constituent molecules.
Understanding and designing molecular assemblies therefore requires us to connect the characteristics of individual molecules with how they interact and form collective structures. We investigate this relationship to understand the emergence of electronic properties in molecular assemblies and explore new possibilities for their design.