Nonlocal Coupling Drives Excimer Formation in Molecular Dimers: The Case for J-Aggregate Excimers

The impact of nonlocal coupling on molecular H- and J-dimer photophysics is studied theoretically, based on a Frenkel-CT-Holstein-Peierls (FCTHP) Hamiltonian, with the goal of identifying favorable conditions for excimer formation. The Hamiltonian includes electronic coupling as well as the exciton-vibrational coupling involving (1) the “fast” progression-forming intramolecular aromatic-quinoidal mode and (2) a “slow” intermolecular (excimer-forming) mode, which can involve simple stretching or twisting. Nonlocal coupling derives from first-order changes in the electron (t_e) and hole (t_h) transfer integrals as the dimer complex evolves along the slow-mode coordinate, q_s, and is therefore described by two parameters, g_e ≡ dt_e/dq_s and g_h ≡ dt_h/dq_s, evaluated at the ground-state geometry. Generally, nonlocal coupling leads to shifted and relaxed potential energy surfaces (PESs) that depend on an interference between g_e and g_h. Excimers are found to be favored in H-dimers when the nonlocal coupling parameters have opposite signs (g_eg_h < 0) and in J-dimers when they have like signs (g_eg_h > 0). The interference between the nonlocal coupling parameters can also result in unusual J- and H-dimers where the k = 0 and k = π PES minima are inverted, resulting in weakly emitting J-dimers and super-emissive H-dimers. It is found that for perylene-based bichromophore complexes, excimers are far more favorable in H-dimers, as identified by vibronic spectral signatures in the absorption spectrum. Understanding the factors which influence excimer formation constitutes an additional step toward the more efficient design of organic materials for device applications.