Optical Performance of Hybrid Co-Networks of Poly(thioctic Acid) Coordinated with Fe3+: Optical Focusing and Photothermal Conversion

The hybrid co-networks (PTA-Fe) of poly(thioctic acid) (PTA) coordinated with Fe³⁺ having PTA polymers with the number-average molecular weight (M_n) of 1–60 kg mol⁻¹ and molar Fe³⁺ content (C_Fe) of 0%–31% have been successfully in situ synthesized. The PTA-Fe film shows high transmittance of 97% with 6.9% ≤ C_Fe ≤ 16.1% and M_n ≥ 12.5 kg mol⁻¹. The PTA-Fe film reveals self-focusing phenomenon especially under 808 nm irradiation, due to the nonlinear refraction caused by the polarized charge transfer complex (CTC) in hybrids excited photons. Meanwhile, the refractive index at 589 nm (n_D) of PTA-Fe hybrid increases from 1.54 to 1.68 with incremental C_Fe for the disulfides and CTC structures. The Abbe number (ν_D) of PTA-Fe hybrids slightly decreases (47–45) due to their self-focusing behavior. The PTA-Fe materials show adjustable photothermal conversion under 808 nm irradiation by mediating C_Fe or NIR intensity (I₈₀₈). The photothermal conversion efficiency (PCE) of PTA-Fe hybrid can reach 98% even at low I₈₀₈ of 50 mW cm⁻², for the PTA-Fe hybrid with self-focusing character can converge the light into a point inside the materials. The multifunctional PTA-Fe hybrid co-networks with high performance in optical transparency, focusing, and photothermal conversion would have potential applications in advanced optical materials.