How molecular functional materials respond to the environment: Solvation to cooperativity

-conjugated chromophores with low-lying charge-transfer degrees of freedom (CT-chromophores) represent an interesting family of molecules where the intrinsic non-linearity of ?-electrons combines with the sensitivity of CT excitations to electrical fields. Cooperativity emerges in these systems from the interaction of the molecular CT degrees of freedom with local electrical fields generated by the environment (either a solvent or nearby molecules) and/or by molecular vibrations. Essential state models are developed for different families of CT chromophores that, accouting for vibrations and for solvation degrees of freedom, accurately describe low-energy optical spectra of CT chromophores in solution. Particular emphasis is given to: symmetry-breaking in multipolar chromophores, inhomogeneous broadening and vibronic coupling in non-linear optical responses. Essential state models developed for CT chromophores in solutions offer powerful tools to investigate collective and cooperative phenomena in more complex environments. Models for resonant energy transfer are addressed to discuss the role of dark state in the process. Collective behavior in multichromophoric assemblies is recognized in non-additive linear and non-linear optical responses. Finally, cooperativity in clusters of CT chromophores is recognized in the occurrence of charge instabilities. The related phenomena of multistability and multiexciton generation in clusters of polar CT chromophores will be discussed as the most impressive examples of emergent behavior driven by interchromophore interactions in clusters of CT chromophores.