Vibronic and Excitonic Structure of a Template-Engineered Molecular-Graphene Heterostructure

Strong electron-phonon coupling described by the Holstein Hamiltonian governs the optical response of many molecular quantum materials. Here, we investigate epitaxial overlayers of 2,3,6,7,10,11-hexamethoxytriphenylene grown on graphene/SiC using Fourier transform photocurrent spectroscopy, photoluminescence, Raman spectroscopy, angle-resolved photoemission spectroscopy, and surface-sensitive microscopy. We resolve a vibronic manifold consistent with Davydov splitting arising from the P63/m crystal symmetry, which lifts the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) degeneracy into bright and dark excitonic branches. Using a tight-binding model parametrized by experiment, we determine the intermolecular coupling, polarization energy, Huang-Rhys factor, and Herzberg-Teller corrections to the Franck-Condon model. The results indicate polaron-mediated relaxation into the lower-energy branch, consistent with Kasha's rule. The developed graphene-supported molecular heterostructure provides a scalable platform for studying dark excitons and vibronic coupling in organic quantum materials.