Enhanced Two-Photon Absorption of Cross-Conjugated Chalcone Derivatives: Modulation of the Effective π-Conjugated Structure.

Three cross-conjugated chalcone derivatives T3CT, T3CP2, and T3CP3 were designed and synthesized to develop excellent organic nonlinear optical (NLO) materials. In a Z-scan experiment, all compounds show good NLO absorption characteristics in the visible to near-infrared region. The photophysical mechanism is confirmed to be two-photon absorption (TPA)-induced excited-state absorption (ESA). Intramolecular charge transfer (ICT) observed in transient absorption spectra (TAS) significantly affects molecular NLO properties. We define the π-conjugated system that dominates the electron transition process in the cross-conjugated structure as the effective π-conjugated structure. Electron transition analysis shows a sufficiently strong ICT can effectively expand the effective π-conjugated structure in these cross-conjugated structures. The TPA cross sections of these compounds at 650 and 750 nm are only in the range of 17-97 GM. However, we achieve a significant enhancement of the TPA cross section at 580 nm (1737-2027 GM) by extending the effective π-conjugated structure. Excited by 580 nm femtosecond laser pulses, all compounds exhibit excellent OL performance and the minimum OL threshold is 4.71 × 10-3 J/cm2. The results show that these cross-conjugated chalcone derivatives have promising applications in OL, and their NLO performance can be effectively improved by modulating the effective π-conjugated structure.