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.
Biexcitonic states play a central role in light-driven processes such as singlet fission, 1 excitonexciton annihilation, 2 triplet-triplet annihilation, 3 and high-energy charge generation 4 in electronically coupled molecular aggregates. They arise either from the correlation of two independently formed excitons 5 or through direct coupling from the optically accessible single-exciton manifold. 6 Once formed, biexcitons exhibit rich and highly system-dependent electronic structure: depending on molecular packing and intermolecular coupling, they may display long-range configuration mixing, pronounced charge-transfer character, or strong spin correlations. As a result, the biexcitonic manifold comprises a diverse set of correlated two-particle states, including Frenkel-type biexcitons (LELE), 7 charge-transfer biexcitons (CTCT), 4 singlet-coupled triplet pairs ( 1 (TT) or here TT), 8 and mixed CTX configurations. 9 Coexisting with single excitons in a shared electronic manifold, these states compete with them in shaping photophysical function and loss pathways.
Despite their importance, biexcitons remain challenging to describe theoretically. Their intrinsic double-excitation character places them beyond the scope of linear-response methods, such as time-dependent density functional theory, which are restricted to the singleexciton manifold. 10 Phenomenological exciton models have been extended to include twoparticle terms or annihilation operators, 2,11 but these descriptions are typically system specific and often neglect entire classes of configurations, in particular those involving charge transfer. Even within ab initio electronic-structure theory, most studies have focused on selected biexcitonic species-most prominently the TT state relevant for singlet fission 1,8,12,13 while a general, chemically interpretable framework that treats LELE, CTCT, and mixed CTX configurations on equal footing is still lacking. As a consequence, key questions regarding the structure, energetics, and coupling mechanisms of biexcitons in extended systems remain unresolved, particularly beyond the dimer limit.
In this Perspective, we outline a configuration-interaction framework that provides a unified ab initio description of the full biexcitonic manifold. Rather than focusing on individual biexciton species, the approach systematically constructs all two-particle configurations arising from monomer-local LE, cation D + , anion D -and triplet T building blocks, including adjacent and spatially separated Frenkel biexcitons, charge-separated double excitons, triplet-pair states, and mixed CTX configurations. We discuss two complementary fragment-based realizations of this framework. The SymbolicCI approach enables an efficient construction of diabatic Hamiltonians in fragment-local active spaces, allowing large aggregates to be treated with modest computational cost, while the NOCI-F methodology provides benchmark-quality biexciton couplings by combining fully relaxed multiconfigu-rational fragment states within a nonorthogonal CI formalism. Together, these methods establish a coherent first-principles foundation for biexciton theory. Beyond the question of how correlated two-particle states are accessed from the single-exciton manifold, a complete understanding of biexcitonic photophysics requires knowledge of how these correlated states propagate through the aggregate. In analogy to exciton transport in the one-particle picture, biexciton ’transport’ is governed by the electronic couplings between spatially distinct biexcitonic configurations. The fragment-based CI framework provides direct access to these inter-biexciton couplings, enabling a systematic analysis of both formation and migration pathways. After briefly summarizing experimental observations that motivate the explicit inclusion of the full biexcitonic manifold in photophysical models, 4,5,14 we introduce the conceptual principles underlying the fragment-based CI approach and illustrate its implications using representative ethylene and anthracene aggregates. These examples demonstrate how a unified electronic-structure framework clarifies multiexciton connectivity in extended molecular systems and opens new avenues for the predictive modeling of correlated excited-state phenomena in functional molecular materials.
Frenkel-Frenkel biexcitons arise from two local electronic excitations residing on different chromophores within a molecular aggregate. 11 The two excitons interact primarily through Coulomb coupling between transition densities and may become partially delocalized, depending on intermolecular distance, relative orientation, and aggregate packing. As a consequence, the LELE manifold comprises both nearby and spatially separated exciton pairs, with binding energies and splittings that are highly sensitive to aggregate geometry and exciton-exciton interactions.
Despite their conceptual simplicity, LELE biexcitons are rarely treated explicitly wit
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