Unveiling the Twisted Aromatic Donor Effect on the Nonlinear Response of D-π-A Type Malononitrile-Derived Chromophores.

This study presents the design, synthesis, and comprehensive characterization of a novel series of D-π-A type malononitrile-derived chromophores, BTC-1 to BTC-4. Combining various spectroscopic techniques, nonlinear Z-scan measurements, and quantum chemical calculations, we revealed the intricate relationship between nonlinear optical properties and the interplay of molecular structure, intramolecular charge transfer (ICT), and dipole moments (µ). Our experimental and computational findings corroborate that the polarization degree in the ground state, the charge separation in the excited state and ICT collectively dictate the nonlinear optical properties of the compounds. Notably, BTC-1 exhibits an exceptional nonlinear absorption coefficient ß value (2 × 10-8 m W-1), attributed to its optimized charge transfer efficiency and pronounced degree of charge separation. Our findings provide actionable insights for the rational design of high-performance organic NLO materials with potential applications in advanced photonic devices.

An investigation of broadband optical nonlinear absorption and transient nonlinear refraction in a fluorenone-based compound.

A novel fluorenone derivative, FO52, is designed and synthesized. The fluorenone group is introduced to provide the central π-conjugated system in the molecule and triphenylamine is substituted at both sides. Intramolecular Charge Transfer (ICT) from the terminal groups to the molecular center is confirmed via DFT calculations. Ultrafast optical nonlinearities are investigated via Z-scan and transient absorption spectroscopy (TAS) studies with a 190 fs laser. Reverse saturable absorption, two-photon induced excited-state absorption, and pure two-photon absorption are observed at 532 nm, 650 nm, and 800 nm, respectively. The different mechanisms at these wavelengths are discussed and interpreted with assistance from the results from TAS. Furthermore, strong excited-state refraction and ultrafast negative refraction from the bound electron response are resolved and discussed in phase object pump probe (POPP) experiments. The results suggest that the ICT-enhanced optical nonlinearities provide FO52 with strong optical limiting capabilities at visible wavelengths and ultrafast refraction with tiny attenuation in the near infrared region. The combination of these properties in one compound could be attractive for applications like laser protection and low-loss all-optical switching.