The photophysical properties of defect-free herringbone aggregates of pi-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 alloed excitons, including those responsible for the upper and lower Davydov components, are evaluated. The herringenbone lattice supports both vibrationally dressed, heavy exciton as well as nearly free, light excitons. The former are responsible for the b-polarized absorption origin as well as two ac-polarized peaks sightly higher in energy. The strongly blueshifted main absorption peak is due to an exciton which travels with almost no nuclear distortion. The main adsorption 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 interferrence between one and two particle emissions. The primarily ac polarized replica intensities initially decrease with the number of molecules comprising the agreegate, 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, gamma_agg, scales linearly with N, eventually diriving the quantum yield to unity when gamma_agg surpasses the nonradiative decay rate. The relative magnitude of the 0-0 emission line versus the rest of the progession generally increases with increasing excitonic interactions. The sum of the (dimensionless) replica intensities diminishes from 1-exp(-lambda~2) in the weak excitonic coupling regime to approximately zero in the strong coupling regime. By contrast, the 0-0 ine intensity scales as N throghout, increasingly by a factor of exp(lambda~2) in going from the weak to strong excitonic coupling regimes.
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