Realistic vibronic modeling of H and J excitonically coupled dimers benchmarked with femtosecond stimulated Raman spectroscopy

Coupling between vibrations and electronic excitations in molecular aggregates is a critical issue in energy transfer and exciton diffusion dynamics in light-harvesting by photosynthetic proteins and molecular materials used in photovoltaics. In most theoretical treatments of this coupling, the vibrational space is dramatically truncated to just a few vibrational modes that approximate the relative frequencies and reorganization energies of the actual vibrations. In this work, we explicitly determine the vibrational coupling of all the normal modes of two common fluorescent dyes, Bodipy (20 coupled modes) and Rhodamine B (29 coupled modes), to their monomeric electronic absorption. The ground-state resonance Raman (RR) spectra of both monomers and their dimers were collected using femtosecond stimulated Raman spectroscopy. We developed a multi-mode excitonic Hamiltonian, which includes 9 vibrational modes on each molecule, to calculate the vibronic structure of the absorption and RR excitation profile of dimers. This Hamiltonian is used to determine the magnitude of exciton coupling between the two monomers in the end-to-end covalently linked "J" dimer of Bodipy and the non-covalently bound, π-stacked "H" dimer of Rhodamine B. The excitonic Hamiltonian successfully reproduces the spectra of both dimers and reveals that the Bodipy-dimer has a coupling of -267 cm-1 and that the RhB dimer has a coupling of +806 cm-1. The multi-mode excitonic Hamiltonian explains the RR intensity of the dimers, except for a small number of vibrational modes that have a significant enhancement of RR intensity that is unexplained by the exciton model.