Ultrafast optical properties and two-photon absorption of new copolymers with linear or tri-branched structure

Femtosecond ultrafast spectroscopic techniques were used to study the optical properties and ultrafast dynamics of two novel copolymers, linear structure copolymer M2 and tri-branched copolymer M3 with triphenylamine and cyano groups in main chain. Their solutions in CHCl₃ have strong absorption band in the blue region with peak at 467 and 436 nm, and fluorescence band at 542 and 535 nm, respectively. Under excitation of femtosecond laser pulse with 1 kHz, 120 fs and 800 nm, two-photon absorption (TPA) induced intense frequency-unconverted emission was observed. M2 and M3 have large TPA cross-section measured by z-scan technique(3.3 and 7.1 × 10^-20 cm⁴/GW respectively). Two-photon excited states decaying dynamics studied by femtosecond pump-probe technique with 800 nm pulses show an ultrafast transient absorption followed by a fast and a long-lived depopulation processes of the excited states. The ultrafast absorption was ascribed to the transient two-photon absorption from both the pump and probe beams.