Multiple Intramolecular Charge Transfers in Multimodular Donor-Acceptor Chromophores with Large Two-Photon Absorption

New multimodular donor-acceptor chromophores combining phenothiazine (D'), benzothiadiazole (A), and diphenylamine/carbazole (D) units in complex configurations (D-pi-A-pi-D'-pi-A-pi-D and D-A-pi-D'-pi-A-D) were designed, synthesized, and investigated for their peculiar intramolecular charge transfer (ICT) behavior. In these organic materials, the modification of the end-capping donor units and the extension of p-conjugation are key factors in tuning their optical, photophysical and electrochemical properties, as revealed by the density functional theory calculations, ultrafast spectroscopy experiments and cyclic voltammetry measurements. The femtosecond transient absorption and broadband fluorescence upconversion results surprisingly show two distinct ICT processes. The first ICT is assigned to the charge displacement from the lateral donors to the acceptors and the second ICT (ICT') to the electron donation from the central phenothiazine. The detection of two ICT states accounts for the peculiar dual emission observed for these molecules. In addition, large two-photon absorption cross sections were revealed for these symmetrical and quadrupolar chromophores. These features make the investigated molecules appealing as new organic materials for possible light-to-energy conversion applications.