Cooperative effects to enhance two-photon absorption efficiency: intra- versus inter-molecular approach

In the search of new materials characterized by high two-photon absorption (TPA) efficiency, many efforts have been devoted to design chromophores with enhanced TPA responses progressively moving from linear chromophores such as dipoles and quadrupoles toward multimeric complex molecular architectures. This approach is mainly based on the optimization of intra-molecular charge transfer interactions. In contrast to the extensive investigations based on this intramolecular approach, the effect of inter-molecular interactions on TPA has not been fully elucidated, although theoretical studies predict that the presence of such interaction could induce large size-scalable TPA enhancements. Despite these promising predictions, only few investigations have been devoted to understand how intermolecular interactions affect the TPA response of molecular aggregates. Even less are the experimental studies that indeed compare the TPA efficiency of molecules in their monomeric and aggregated form and a thorough rationalization of the results was missing. This perspective aims to fill this gap providing a unified view of the efforts and the results obtained following this strategy.