Nonlinear optical responses and ultrafast dynamics of three soluble ladder-conjugated molecules: A study on the terminal group effect

Abstract

To study the structure-property relationship as well as the intramolecular charge transfer properties of typical ladder-conjugated molecules, three ladder-conjugated molecules named LT2, LT3 and BPTF-2 were systematically investigated by UV–vis absorption spectra and fluorescence spectra, the open-aperture Z-scan technique, two-photon excited fluorescence and femtosecond pump-probe experiments. These ladder-conjugated molecules employ the same fluorene group as the π-bridge and the central core, with substituent changes at the terminal position. These molecules exhibit a large two-photon absorption response and two molecules emit intense two-photon fluorescence at a wavelength of 800 nm, which is a commonly used wavelength in two-photon absorption/two-photon fluorescence applications. Our results indicate that the π-bridge and the terminal group play an important role in determining the two-photon absorption response. The dynamic results obtained from degenerated pump-probe experiments revealed that the evolution times of the intramolecular charge transfer state were >1000 ps, >1000 ps, and 40 ps for LT2, LT3 and BDTF-2, respectively. The results match those of the two-photon absorption responses and the fluorescence emission properties of these ladder-conjugated molecules. Our results also indicate that ladder-conjugated molecules have potential applications in two-photon absorption/two-photon fluorescence fields.