Scaling law for Kasha's rule in photoexcited molecular aggregates

Abstract: We study the photophysics of molecular aggregates from a quantum optics perspective, with emphasis on deriving scaling laws for the fast non-radiative relaxation of collective electronic excitations, referred to as Kasha's rule. Aggregates exhibit an energetically broad manifold of collective states with delocalized electronic excitations originating from near field dipole-dipole exchanges between neighboring monomers. Photo-excitation at optical wavelengths, much larger than the monomer-monomer average separation, addresses almost exclusively symmetric collective states, which for an arrangement known as H-aggregate, show an upward hypsochromic shift. The extremely fast subsequent non-radiative relaxation via intramolecular vibrational modes populates lower energy, subradiant states, resulting in an effective inhibition of fluorescence. Our analytical treatment allows for the derivation of an approximate scaling law of this relaxation process, linear in the number of available low energy vibrational modes and directly proportional to the dipole-dipole interaction strength between neighbouring monomers.