Symmetrical and unsymmetrical multibranched D-pi-A molecules based on 1,3,5-triazine unit: synthesis and photophysical properties

A series of V-shaped (D-pi-A-pi-D) and star-shaped ((D-pi-)(3)A) molecules based on the electron acceptor 1,3,5-triazine core (A) connected to different electron donor groups (D) by a carbon-carbon triple bond as a conjugation bridge (pi) has been synthesized. The studied molecules can be separated into symmetrically and unsymmetrically substituted molecules depending on the combination of the electron donating branches connected to the triazine core. Their photophysical properties are characterized experimentally and the structure-properties relationship is analysed with the aid of theoretical calculations. The symmetrically branched 1,3,5-triazine-based molecules exhibit similar UV-vis absorption to the corresponding linear molecules, but an obvious blue-shift in the emission is observed with increasing dimensionality. The absorption of the unsymmetrically branched 1,3,5-triazine-based molecules is clearly localized on a specific branch, suggesting a weak interbranch conjugation in the ground state. Emission is mainly controlled by the branch with the lowest energy excited state, which corresponds to the one with the largest intramolecular charge transfer (ICT) effect. The two-photon absorption properties of selected molecules are studied. They exhibit strong two-photon absorption activities and a modest interbranch conjugation effect, enhancing the TPA cross-section beyond the additive effect of increasing branch number.