Porphyrin-Cored H-Bonded Organic Polymer Hermetically Decorated MXene toward Broadband Optical Nonlinearities

Two-dimensional MXene (Ti₃C₂T_<i>x</i>) nanosheets suffer from surface oxidation and limited optical tunability. Herein, we report an aqueous-phase functionalization strategy using porphyrin-cored dopamine-type hydrogen-bonded organic polymers (ppd-HOPs) to simultaneously etch, exfoliate, and encapsulate Ti₃C₂T_<i>x</i>. Inspired by mussel adhesion mechanisms, ppd-HOPs undergo spontaneous self-polymerization on MXene surfaces, effectively passivating terminal groups (-OH, ═O, -F) against ambient moisture/oxygen while expanding interlayer spacing. Comprehensive characterization (X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, and scanning electron microscopy) confirms the formation of hermetic ppd-HOP/Ti₃C₂T_<i>x</i> nanocomposites (Ti₃C₂T_<i>x</i>(Por₁)_<i>y</i> and Ti₃C₂T_<i>x</i>(Por₂)_<i>y</i>) with covalently anchored different porphyrin units (Por₁ and Por₂). The hybrids exhibit exceptional broadband nonlinear optical (NLO) responses spanning 400-1600 nm, attributed to synergistic effects between MXene's plasmonic absorption and porphyrin's excited-state dynamics. Structure-dependent NLO modulation is demonstrated: Ti₃C₂T_<i>x</i>(Por₁)_<i>y</i> functionalized with polymerizable ppd-HOP1 (Por₁-cored dopamine-type hydrogen-bonded organic polymer) achieves a record reverse saturable absorption (RSA) coefficient (<i>β</i>_eff = 587 ± 24.1 cm GW^-1 at 800 nm), outperforming Ti₃C₂T_<i>x</i>(Por₂)_<i>y</i> (<i>β</i>_eff = 396 ± 20.0 cm GW^-1 at 650 nm). Ultraviolet-visible-near-infrared (UV-vis-NIR) spectroscopy indicates that significant red-shifted and broadened linear absorption induced by extended π-conjugation in ppd-HOP1 provides more ladders for two-photon absorption processes, enabling wavelength-selective RSA behavior. This bioinspired surface engineering approach not only improves MXene's environmental stability but also pioneers a general paradigm for tailoring MXene-based optoelectronic materials through rational organic-inorganic hybridization.