Linear and two-photon absorption properties of interacting polar chromophores: Standard and unconventional effects

Investigations into the effects of electrostatic interactions on linear and two-photon absorption (TPA) properties are probed using a reference polar chromophore and different related compounds. Compounds in which a passive charge is present near the active chromophore, as well as dimers of the reference molecule, were studied. The combined experimental and theoretical investigation demonstrates that strong modulation of the properties can be attained. In particular, the TPA response of dimers is heavily affected by unconventional excitonic effects, that is, beyond the standard Heitler-London approximation. These effects, which stem from purely electrostatic interactions, lower the nonlinear optical response in the investigated case by an additional amount (up to 20%), and are expected to enhance the TPA cross section in other supramolecular alignments. Electrostatic interactions cannot be overlooked when modelling or investigating highly concentrated and/or confined samples, as are usually needed in many applications. The correct knowledge of their effects can be exploited to guide engineering at the molecular and supramolecular level.