New Model Unites Complex Molecular Energy Transfers

Scientists are increasingly focused on understanding biexcitonic states and their role in crucial photophysical processes such as singlet fission and exciton annihilation, yet a comprehensive theoretical description bridging these phenomena has proved challenging. Johannes E. Adelsperger, from the Center for Nanosystems Chemistry and Institute for Physical and Theoretical Chemistry at Julius-Maximilians University Würzburg, Coen de Graaf from the Departament de Química Física i Inorgànica, Universitat Rovira i Virgili and ICREA, and Merle I. S. Röhr, also of the Julius-Maximilians University Würzburg institutions, and colleagues present a novel fragment-based configuration-interaction approach to systematically describe these complex interactions. This work establishes a conceptual framework constructing diabatic Hamiltonians from monomer-local building blocks, offering both physical interpretability and computational efficiency through the SymbolicCI and NOCI-F methods. Their applications to ethylene aggregates and the anthracene crystal reveal previously overlooked electronic pathways and configurations, potentially revolutionising the predictive modelling of multiexcitonic photophysics and fostering greater collaboration between electronic structure and dynamics research communities.

Molecular energy transfer just gained a powerful new language for describing complex interactions. Understanding how light energy moves within materials is now possible with greater precision and detail, promising to accelerate the design of more efficient solar cells and light-harvesting technologies. Scientists are increasingly focused on understanding biexcitonic states, which govern several light-driven processes including singlet fission, exciton annihilation, and triplet-triplet annihilation within molecular aggregates.

A unified theoretical description of how these processes compete remains elusive, hindering progress in areas like high-energy charge generation and the design of efficient light-harvesting materials. Researchers have developed a fragment-based configuration-interaction framework offering a systematic way to construct diabatic Hamiltonians, effectively mapping the complex interaction of electronic states within these aggregates.

This approach spans the full one-particle and two-particle manifolds, starting from easily interpretable monomer building blocks and preserving physical meaning throughout the calculations. Describing biexcitons presents a significant theoretical challenge because their double-excitation character falls outside the scope of common, simpler computational methods.

The new framework treats Frenkel-type biexcitons, charge-transfer biexcitons, triplet-pair states, and mixed configurations on an equal footing, providing a more general and chemically insightful picture, unlike previous attempts that relied on system-specific models or incomplete descriptions. By systematically constructing all possible two-particle configurations from local building blocks, the method resolves key questions regarding the structure, energetics, and coupling mechanisms of biexcitons in extended systems.

Once formed, biexcitons exhibit diverse electronic structure dependent on molecular arrangement and intermolecular interactions. The resulting diabatic Hamiltonians, generated using SymbolicCI and benchmarked with NOCI-F, can be directly applied to quantum dynamics simulations. Applications to ethylene aggregates and the anthracene crystal reveal CTX configurations acting as electronic gateways between different excitonic manifolds, with charge-transfer mediated relaxation pathways offering an alternative to conventional annihilation processes.

The fragment-based configuration-interaction framework provides access to inter-biexciton couplings, enabling systematic analysis of both formation and migration pathways. Work focused on systematically comparing these methods across different aggregate geometries, H-type, J-type, Null-type, and Zero-Frenkel, each comprising 15 monomers with a fixed interplanar distance of 3.50Å.

A manually selected 72-state subspace was used for benchmarking, encompassing ground states, Frenkel excitations, charge-transfer, Frenkel combinations, and nearest-neighbour CTCT biexcitons. Results demonstrate that both approaches accurately capture the most significant couplings within the Hamiltonian, with the overall structure of the calculated Hamiltonians being highly comparable.

Discrepancies emerge when examining smaller couplings involving charge-transfer states, where NOCI-F calculations generally report larger values than those obtained using SymbolicCI. For the Zero-Frenkel aggregate, the coupling associated with parallel biexciton diffusion shows a noticeable difference between the two methods. Similarly, the coupling between a Frenkel-type biexciton and an LECT exciton in the H-type aggregate also exhibits some variation.

The study lies in the construction of a shared diabatic subspace, ensuring that any observed differences stem from the electronic-structure formulation rather than state selection. SymbolicCI systematically generates all single and double excitations within the active space, resulting in a Hamiltonian dimension of 25 651 × 25 651 for the ethylene aggregates.

While a full NOCI-F calculation at this scale is computationally prohibitive, the chosen 72-state subspace allows for a direct and meaningful comparison. Calculations for anthracene pentamers were performed with a smaller basis set, while ethylene calculations utilised a larger one. Detailed comparisons confirm the overall agreement between SymbolicCI and NOCI-F, with deviations primarily occurring in couplings involving charge-transfer states and specific biexciton configurations. These findings provide a systematic validation of the diabatic couplings and relative energetic trends described by SymbolicCI, using NOCI-F as a benchmark.

Fragment-based diabatic Hamiltonians for modelling biexciton electronic structure

SymbolicCI and NOCI-F methodologies underpin the work, establishing a first-principles foundation for understanding biexciton behaviour. Monomer-local building blocks representing local excited states, specifically, local excitation, charge transfer, and triplet states, were defined. These fragments account for both adjacent and spatially separated Frenkel biexcitons, charge-separated double excitons, triplet-pair states, and mixed CTX configurations.

Two complementary fragment-based approaches were implemented to construct diabatic Hamiltonians, which provide a static, interaction-picture representation of the electronic structure. SymbolicCI efficiently builds these Hamiltonians within fragment-local active spaces, reducing computational demands for larger aggregates, while NOCI-F delivers benchmark-quality biexciton couplings by fully relaxing multiconfigurational fragment states using a non-orthogonal CI formalism.

Once established, these diabatic Hamiltonians were prepared for coupling to dynamics simulations, allowing investigation of biexciton propagation. By systematically constructing the Hamiltonian spanning one-particle and two-particle manifolds, the research avoids empirical parameters and preserves physical interpretability. Further analysis focused on the connectivity between different exciton manifolds.

Applications to ethylene aggregates and the anthracene crystal revealed CTX configurations acting as electronic gateways, bridging excitonic manifolds and introducing CT-mediated relaxation pathways that compete with conventional annihilation processes. This approach moves beyond phenomenological Frenkel-exciton models, offering a more complete and accurate description of multiexcitonic photophysics in functional molecular materials.

Mapping energy flow in molecular systems with systematically linked electronic states

Understanding how energy dissipates in complex molecular systems is becoming ever more pressing. For years, scientists have struggled to model the fate of excited states in materials, particularly when multiple excitations interact. These interactions, including singlet fission and exciton-exciton annihilation, determine whether absorbed light becomes useful energy or simply lost as heat.

Previous approaches often relied on approximations that obscured the underlying physics, treating these processes as separate events rather than parts of a connected web. A new framework offers a way to map these interactions with greater precision, building a detailed picture of how energy flows within and between molecules. By constructing a system that systematically links different electronic states, researchers can begin to untangle the competition between energy transfer and loss pathways.

Unlike earlier models, this approach focuses on the fundamental building blocks of electronic structure, allowing for calculations that are both accurate and interpretable. Applications to relatively simple systems like ethylene and anthracene demonstrate the method’s potential, revealing previously hidden connections between excitonic states. Scaling these calculations to larger, more realistic materials remains a challenge.

A key limitation lies in accurately describing the active environment of these molecules, as crystal distortions and solvent effects can dramatically alter energy landscapes. Future work might focus on integrating this electronic structure framework with molecular dynamics simulations, creating a truly predictive model of multiexcitonic photophysics. This represents a move towards designing materials with tailored light-harvesting properties, with implications for solar energy, organic electronics, and even photodynamic therapy.