Influence of multi-ring modifications on the nonlinear absorption properties of A-π-D-π-A type dyes in the near-infrared region.

Nonlinear optical (NLO) materials with strong multi-photon absorption (MPA) are of great importance for laser protection. In this work, three A-π-D-π-A type dyes, ITIC, ITIC-4F and IXIC, were systematically investigated to clarify the correlation between molecular structure, electronic properties and nonlinear absorption behavior in the near-infrared region. Z-scan and optical limiting (OL) measurements were performed under femtosecond excitation in the wavelength range of 800-1200 nm, revealing a wavelength dependent evolution of the dominant nonlinear absorption mechanism from two-photon absorption (2PA) to three-photon absorption (3PA) among the three molecules. The experimental results show that IXIC exhibits the strongest MPA and the lowest OL threshold among the studied dyes. Specifically, the maximum 2PA cross-section of IXIC reaches 91,722 GM at 900 nm, and its fifth-order nonlinear absorption coefficient attains 43.0 × 10-27 m3/W2 at 1200 nm, demonstrating its superior nonlinear response in the near-infrared region. Density functional theory calculations combined with the sum-over-states model were employed to analyze the underlying physical mechanism. Theoretical analysis indicates that replacing the central benzene unit with thieno[3,2-b]thiophene significantly narrows the energy gap and enhances intramolecular charge transfer by elevating the highest occupied molecular orbital level while maintaining a nearly unchanged lowest unoccupied molecular orbital level. This electronic modulation strengthens the dominant excited-state transition pathway in the 2PA process and leads to an enhanced MPA response. By integrating experimental characterization with excited-state theoretical analysis, this study establishes a clear molecular structure and NLO properties relationship for A-π-D-π-A type dyes. It further provides a rational molecular design strategy for developing high-performance NLO materials in the near-infrared region.