Anatomy of an exciton: vibrational distortion and exciton coherence in H- and J-aggregates.

In organic materials, coupling of electronic excitations to vibrational degrees of freedom results in polaronic excited states. Through numerical calculations, we demonstrate that the vibrational distortion field accompanying such a polaron scales as the product of the excitonic interaction field and the exciton coherence function. This scaling relation is derived analytically in the regime where excitonic interactions are weak, yet it is shown to remain valid for interaction strengths ranging up to physically relevant values. Moreover, it is not affected by the magnitude of exciton-vibrational coupling or the presence of disorder in the molecular transition energies, despite the dramatic changes observed in the excited state. An application to helical MOPV4 aggregates is presented, followed by a quantitative study of the vibrational distortion field when excitonic interactions are strong. Our findings allow for a straightforward interpretation of widely varying polaron profiles, thereby facilitating the characterization of organic excited states.