Ultrafast responses of two V-shaped compounds with a reverse conjugated structural configuration: an investigation of the reason for the enhanced two-photon absorption cross-section

To study the effect of the conjugated structural configuration on the two-photon absorption (TPA) properties of V-shaped compounds, two 1,3,5-triazine-based compounds with the same electron donor (D) and acceptor (A) connected in a reverse-conjugated structural configuration ( T02 : D- π -A- π -D; R02 : A- π -D- π -A) were systematically investigated using steady-state and transient absorption spectroscopy, open-aperture Z-scan measurements, and two-photon fluorescence measurements. The TPA cross-section of compound R02 connected in a A- π -D- π -A-conjugated structural configuration with triphenylamine as the central core was 203 GM, which showed a 2.3-fold enhancement compared with compound T02 connected in a reverse D- π -A- π -D-conjugated structural configuration (90 GM, with 1,3,5-triazine as the central core). This result indicates that the conjugated structural configuration plays an important role in the TPA properties. A two-color pump–probe experiment was used to investigate the effect of the conjugated structural configuration on the excited state and intra-molecular charge transfer (ICT) properties of these V-shaped compounds. The formation and relaxation lifetimes of the ICT state were determined. The results indicate that the electron-donating/accepting strength of the central group, which serves as a communal group for two D- π -A subunits, was confirmed to be a key role to the overall effect of the ICT for V-shaped compounds. These ultrafast dynamic results are in agreement with the TPA properties.