Synthesis of fluorene-thiophene-benzothiadiazole (D-π-A) molecules by direct arylation reactions: Formation of nanoparticles and their fluorescence study by one- and two-photon absorption

This work presents the synthesis of Donor-pi-Acceptor (D-pi-A) systems including the monomer (M1), dimer (O1b), oligomer (O1aH, O1aT) and polymers (P1bT, P1cT), which are derived from fluorene-thiophene and benzothiadiazole (BT) using direct arylation reactions under different conditions. These compounds have a structure with fluorene moieties as D groups, benzothiadiazole derivatives as A groups and the thiophene moiety acting as a polarizable pi bridge. Their structures and properties were spectroscopically and physicochemically characterized and analyzed. These D-pi-A molecules were encapsulated into silica nanoparticles (NPs) to provide possible biocompatibility. Doped silica NPs were synthesized by the microemulsion method and their morphology was analyzed by Field Emission Scanning Electron Microscopy (FESEM). The optical properties of the solution, suspension of NPs and solid-state thin films of the title compounds were investigated by UV-vis absorption and photoluminescence spectroscopies. Fluorescence quantum yields of monomer (M1) and dimer (O1b) in THF solutions were 92% and 75%, respectively, while for the higher molecular weight polymer (P1cT) was 38%. Two-photon absorption (TPA) cross-sections were measured for O1b NPs by two-photon excited fluorescence (TPEF) method, which gave a maximum value of 55 GM (1 GM = 10(-50) cm(4)s) at 750 nm.