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The solid-state photophysical properties of semiconducting conjugated polymers result from the interplay between the intrinsic features of the individual polymer chains and interchain interactions. [1] These properties constitute a key ingredient to define the role that semiconducting polymers can play in their envisioned applications involving, for example, their incorporation as the active element in new generations of light-emitting diodes or photovoltaic devices.Exciton coupling, well-known in molecular solids, [2,3] is one of the major interchain interactions that affect the optical properties of semiconducting polymers. This coupling originates from the interaction between the transition dipole moments on neighboring chains and results in a splitting of the electronic excited states. In crystalline molecular solids, this (Davydov) splitting is equal to the number of (non-equivalent) interacting molecules in the unit cell. Exciton coupling theory predicts that, for a parallel orientation of chromophores, the electronic transition from the ground state to the lowest excited state is formally dipole forbidden. Hence, in a thin film of predominantly parallel polymer chains, a low quantum yield for photoluminescence or electroluminescence is expected, even when a strong optically allowed transition along the (isolated) polymer chain is present. [4] Such interchain effects have recently been invoked to explain the time-resolved photoluminescence decay and photoluminescence quantum yield of polythiophenes [5] and poly(p-phenylenevinylenes) [6,7] under conditions where interchain interactions play a significant role. [4] Adv.