Fragment-Based Configuration Interaction: Towards a Unifying Description of Biexcitonic Processes in Molecular Aggregates

TL;DR

This paper introduces a fragment-based configuration interaction framework to systematically model biexcitonic states in molecular aggregates, aiding understanding of processes like singlet fission and exciton annihilation.

Contribution

It develops a unified, physically interpretable approach to construct diabatic Hamiltonians for biexcitonic processes from monomer units, enabling large-scale and benchmark-quality calculations.

Findings

Identifies CTX configurations as electronic gateways bridging excitonic states.

Reveals CT-mediated relaxation pathways compete with exciton annihilation.

Shows LECT admixture stabilizes bi-excimers in H-aggregates.

Abstract

Biexcitonic states govern singlet fission, triplet-triplet and exciton-exciton annihilation, yet a unified understanding of how these processes compete within a shared electronic manifold remains elusive. We outline a conceptual framework based on fragment-based configuration-interaction that systematically constructs diabatic Hamiltonians spanning the full one-particle (LE, CT) and two-particle (LELE, CTCT, TT, CTX with X = LE, CT, or T) manifolds from monomer-local building blocks, preserving physical interpretability throughout. SymbolicCI provides analytic Hamiltonian matrix elements for efficient large-scale calculations; NOCI-F delivers benchmark-quality couplings. The resulting diabatic Hamiltonians can be coupled to quantum dynamics simulations. Applications to ethylene aggregates and the anthracene crystal reveal CTX configurations as electronic gateways bridging excitonic manifolds, with CT-mediated relaxation pathways competing with conventional annihilation. In H-type aggregates, LECT admixture stabilizes a "bi-excimer" analogous to one-particle excimers. By providing first-principles access to biexciton formation, separation, and transport, we hope to stimulate further exchange between electronic structure and quantum dynamics communities toward a predictive understanding of multiexcitonic photophysics.