RC-03 — Quantum Dynamics & Molecular–Environment Design
D1 — Research Orientation
The electronic states of molecules play a central role in processes that unfold over time. Following photoexcitation, molecules can undergo changes in electronic states as they interact with neighboring molecules and their environments, accompanied by the transfer or conversion of energy.
We study these processes from the perspective of quantum dynamics. One example is singlet fission, in which one singlet excitation is converted into two triplet excitations. By investigating the roles of electronic-state interactions and the environment, we aim to understand the evolution of quantum states and connect this understanding with molecular and environmental design.
D2 — Scientific Explanation
Photoexcitation and Electronic-state Conversion
Upon absorbing light, a molecule can enter an excited state and subsequently undergo electronic-state conversion through molecular vibrations and interactions with its surroundings. Understanding these processes requires consideration of electronic-state energies, interactions between states, and their temporal evolution.
Singlet Fission and Correlated Triplet Pairs
Singlet fission (SF) is a process in which one singlet excitation is converted into two triplet excitations. An important step is the formation of a correlated triplet pair, in which two triplet excitations are quantum mechanically correlated. Understanding the conditions that favor efficient correlated triplet-pair formation is a central question in singlet-fission mechanisms and molecular design.
This process depends not only on electronic-state energies but also on couplings between states and molecular arrangement.
The Roles of Molecules and Their Environments
The evolution of excited states is also influenced by molecular vibrations and interactions with the environment. For example, fluctuations in molecular arrangement can modify couplings between electronic states, affecting state conversion, relaxation, and quantum coherence.
We combine electronic-structure calculations and theories of quantum dynamics to investigate how molecular structure, electronic states, and environmental interactions influence excited-state conversion.
D3 — Scientific Detail
1. Electronic-state Evolution and Singlet Fission
An important process in singlet fission is the conversion of an initial singlet excitation into a correlated triplet pair, in which two triplet excitations are coupled to form an overall singlet spin state.
This conversion depends on electronic-state energies and couplings between states. For example, the relationship between and provides a commonly used energetic guideline for molecular design. However, interactions between triplet excitations also affect the energy of the correlated triplet pair, and energetic conditions alone do not determine the rate or efficiency of its formation. Understanding how electronic couplings influence the time evolution of these states is therefore essential.
2. Effective Electronic-state Models and Couplings
To investigate the electronic mechanisms of singlet fission, we use effective electronic-state models involving Frenkel excitations (FE), intermolecular charge-transfer (CT) configurations, and correlated triplet-pair (TT) states or configurations. By describing their energies and couplings through an effective Hamiltonian, we examine the factors governing correlated triplet-pair formation.
CT configurations, for example, can play an important role in effective coupling between FE and TT. This does not necessarily imply a sequential FE→CT→TT process. Multiple couplings and pathways may contribute, with quantum interference influencing the resulting dynamics. Connecting these relationships with molecular structure and intermolecular arrangement is an important part of our research.
3. Open Quantum Systems and Molecular–Environment Interactions
The evolution of molecular excited states is influenced by interactions with the surrounding environment. In open quantum system theory, selected degrees of freedom are treated as the system, while others are represented as the environment. Methods such as quantum master equations allow us to investigate the evolution of electronic-state populations and quantum coherence while accounting for environmental interactions.
An important consideration is that the choice of system and environment, and the degrees of freedom treated explicitly, depend on the phenomenon and timescales of interest. For example, thermal structural fluctuations may be represented through an ensemble of molecular configurations or through the explicit evolution of selected nuclear degrees of freedom. The appropriate description depends on the purpose and approximations of the model.
4. Toward Molecular–Environment Design
Molecular design for singlet fission requires consideration of electronic-state energies, couplings between states, and competing relaxation processes. For example, structural modifications can change excited-state ordering and electronic couplings, potentially influencing correlated triplet-pair formation.
We seek to establish design principles connecting molecular structure, intermolecular arrangement, and environmental interactions through electronic-state analysis and quantum dynamics. In the longer term, we also aim to explore how the quantum nature of molecular dynamics may open new possibilities for controlling molecular processes and utilizing quantum phenomena.