The pursuit of advanced optical materials with ultrafast and tunable nonlinear optical (NLO) responses across a wide spectral range has long been a central goal in photonics and optoelectronics. In this work, we report the synthesis and comprehensive characterization of a novel ether-linked porphyrin covalent organic framework (COF-Pors), which exhibits exceptional broadband NLO switching behavior from the visible to the near-infrared region. By integrating highly conjugated porphyrin units via robust ether linkages, we successfully constructed a two-dimensional crystalline framework with a well-defined porous architecture and high structural order. This material demonstrates a unique intensity-dependent transition between saturable absorption (SA) and reverse saturable absorption (RSA), enabling dynamic control over light transmission—key for applications in optical limiting, signal processing, and intelligent photonic devices.
Structural analysis confirmed the successful formation of ether bonds through a nucleophilic aromatic substitution reaction between fluorinated and hydroxylated porphyrin precursors. Fourier-transform infrared (FT-IR) spectroscopy showed the disappearance of the C–F stretch at 1091 cm⁻¹ and the appearance of a new peak at ~1011 cm⁻¹ attributed to C–O stretching, indicating covalent bond formation. X-ray photoelectron spectroscopy (XPS) revealed no detectable F1s signal in COF-Pors, confirming complete removal of fluorine atoms during polymerization. Solid-state ¹³C CP-MAS NMR displayed distinct peaks corresponding to carbon atoms in ether linkages, further supporting the proposed molecular structure. Powder X-ray diffraction (PXRD) and 2D small-angle X-ray scattering (SAXS) patterns exhibited sharp, symmetric diffraction rings, consistent with a highly ordered crystalline phase with eclipsed stacking. High-resolution transmission electron microscopy (HR-TEM) images revealed a square-lattice arrangement with lattice distances of approximately 0.24 nm and 0.28 nm, matching the predicted interlayer spacing.
Nitrogen adsorption-desorption measurements at 77 K demonstrated microporous characteristics, with a BET surface area of ~101 m²/g.DCK Antibody site The pore size distribution curve displayed a dominant peak at 1.8 nm, closely aligning with the theoretical pore aperture derived from the crystal model. Thermogravimetric analysis (TGA) indicated good thermal stability, with decomposition onset at ~382 °C in air and a weight loss of 13.86 wt.% at that temperature. Electron paramagnetic resonance (EPR) spectra revealed a g-value of 2.0034 under ambient conditions, which shifted slightly to 2.0037 under 532 nm laser irradiation, suggesting light-induced generation of unpaired electrons—consistent with excited-state population and radical formation mechanisms.
UV-Vis absorption spectroscopy revealed strong B-band absorption at 410–430 nm and Q-bands in the 500–700 nm range. The B-band maximum shifted to 430 nm in COF-Pors compared to its precursors, reflecting enhanced π-conjugation due to extended planar stacking. Ultraviolet photoelectron spectroscopy (UPS) enabled precise determination of electronic energy levels: the HOMO was located at −6.02 eV and the LUMO at −3.23 eV, yielding an optical bandgap of ~2.79 eV. These values are comparable to those of g-C₃N₄, suggesting similar potential for photocatalytic and optoelectronic applications. The calculated electrochemical potentials indicate favorable thermodynamic conditions for both hydrogen evolution and CO₂ reduction reactions.
Nonlinear optical properties were evaluated using open-aperture Z-scan measurements with 6 ns pulses at 532 nm and 1064 nm. At low excitation energies (40 mJ), COF-Pors exhibited symmetric transmittance minima with Tmin values of ~18.4% (532 nm) and ~12.6% (1064 nm), characteristic of SA behavior. As incident pulse energy increased beyond 70 mJ (532 nm) or 150 mJ (1064 nm), RSA became dominant, and Tmin dropped significantly—reaching as low as 0.64 at 300 mJ (532 nm) and 0.83 at 300 mJ (1064 nm). This reversible SA-to-RSA transition was fully reproducible and stable over multiple cycles. Notably, neither the fluorinated nor hydroxylated porphyrin precursor showed any measurable NLO response at 1064 nm, underscoring the critical role of the extended covalent network in enabling broadband functionality.
At 532 nm, where photon energy (~2.33 eV) is below the bandgap (~2.79 eV), ground-state bleaching dominates at low intensities, leading to SA. With increasing intensity, multi-photon absorption populates higher-energy excited states, resulting in RSA.PTPRF Antibody custom synthesis At 1064 nm, the photon energy (~1.PMID:34212433 17 eV) is insufficient for direct bandgap excitation, yet strong NLO response persists due to thermally induced nonlinear scattering (NLS). Heat transfer from COF-Pors to DMF solvent generates transient microbubbles and plasma domains, causing significant optical scattering and absorption. The high heat capacity and low thermal conductivity of DMF amplify this effect, enhancing optical limiting performance.
This study establishes COF-Pors as a pioneering example of ether-linked COFs with superior broadband NLO switching capabilities. Its ability to switch dynamically between SA and RSA modes enables versatile applications in all-optical logic circuits, ultrafast optical switches, and protective coatings for sensitive optical systems. The design strategy—leveraging ether linkages to stabilize large conjugated frameworks—opens a new frontier in COF chemistry. Future research should focus on solid-state integration, device fabrication, and fine-tuning of linker geometry to optimize charge transport and NLO efficiency. Ultimately, this work paves the way for next-generation smart photonic materials based on tailored covalent organic frameworks.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com
