Effect of intramolecular charge transfer on nonlinear optical properties of chalcone derivatives: a visual description of the charge transfer process

Intramolecular charge transfer (ICT) is an important factor in the nonlinear optical (NLO) properties of organic molecules. In order to study the effect of ICT on two-photon absorption (TPA) and excited-state absorption (ESA), three chalcone derivatives (1, 2 and 3) with different electron push-pull systems were designed and synthesized. The ICT performance of these chalcone derivatives depends on the electron push-pull systems and mainly includes ultrafast ICT in the femtosecond time domain and long-lived charge transfer state (CTS) in the picosecond time domain, which dominate the performance of molecular TPA and ESA respectively. Hole-electron analysis and femtosecond Z-scan experiment indicate that the TPA cross section of these chalcone derivatives can be effectively enhanced by introducing stronger ultra-fast ICT in the case of little difference in ground-state absorption and expanding the molecular π-conjugated structure. Transient absorption spectrum (TAS) experiments of these compounds in solvents of varying polarities were conducted to visualize the establishment of CTS. The local excited state (LES) and charge transfer state (CTS)-based ESA of these chalcone derivatives are extremely dependent on the strength of ICT. Our experimental results show that the superposition of LES and CTS by enhancing ICT performance can effectively improve the ESA, which offers us a practical method to improve the long-impulse response of organic materials.