Probing ultrafast excited state dynamics and nonlinear absorption properties of three star-shaped conjugated oligomers with 1,3,5-triazine core

The effects of different bridging fluorene units on the two-photon absorption (TPA) properties and ultrafast response of organic conjugated oligomers were investigated for three star-shaped oligomers named TFT-1, TFT-2 and TFT-3. These three oligomers are composed of the same central core 1,3,5-triazine, with three arms that consist of specific numbers of bridging fluorene groups and with triphenylamine as electron-donating group at the terminal end of each arm. The studies on these oligomers were carried out by using two-photon excited fluorescence (TPF), degenerate pump-probe techniques, transient absorption spectroscopy and time-resolved photoluminescence (TRPL) methods. The TPA cross-sections were determined to be 1509 GM, 1260 GM and 789 GM for TFT-1, TFT-2 and TFT-3, respectively, decreasing with the increase of bridging fluorene number. Ultrafast dynamics results show that there is a fast intramolecular charge transfer (ICT) formation time of about several ps and a relatively long decay process of the ICT state. The formation time of ICT was found to increase from TFT-1 (1.9 ps), to TFT-2 (3.0 ps) and TFT-3 (6.3 ps), with the increased number of the fluorene bridge, which may explain the action of fluorene bridge on the electron transmission properties, the ICT properties and the TPA behavior.