The fundamental photophysics of conjugated oligomer herringbone aggregates

Abstract

The photophysical properties of defect-free herringbone aggregates of π-conjugated oligomers are investigated theoretically using a two-particle basis set consisting of vibronic excitons and coupled vibronic–vibrational excitons. Incorporation of periodic boundary conditions allows the treatment of aggregates containing up to 1000 molecules. The vibrational distortion fields for the optically allowed excitons, including those responsible for the upper and lower Davydov components, are evaluated. The herringbone lattice supports both vibrationally dressed, heavy excitons as well as nearly free, light excitons. The former are responsible for the b-polarized absorption origin as well as two ac-polarized peaks slightly higher in energy. The strongly blueshifted main absorption peak is due to an exciton which travels with almost no nuclear distortion. The main absorption features are studied as a function of aggregate size and exciton bandwidth. The vibronic replicas in the aggregate emission spectrum are found to be strongly dependent on a destructive interference between one and two particle emissions. The primarily ac polarized replica intensities initially decrease with the number of molecules comprising the aggregate, N, converging to a nonzero value in the large N limit. By contrast, the b-polarized 0–0 line intensity increases linearly with N, eventually dominating the rest of the vibronic progression when N surpasses approximately 10. Beyond this size the aggregate radiative decay rate, γagg, scales linearly with N, eventually driving the quantum yield to unity when γagg surpasses the nonradiative decay rate. The relative magnitude of the 0–0 emission line versus the rest of the progression generally increases with increasing excitonic interactions. The sum of the (dimensionless) replica intensities diminishes from 1−exp(−λ2) in the weak excitonic coupling regime to approximately zero in the strong coupling regime. By contrast, the 0–0 line intensity scales as N throughout, increasing by a factor of exp(λ2) in going from the weak to strong excitonic coupling regimes.