Perfluorinated gallium phthalocyanine axially grafted black phosphorus nanosheets for optical limiting

Abstract

Perfluorinated gallium phthalocyanine (F₁₆PcGa) axially grafted few-layer black phosphorus (BP) nanosheets (F₁₆PcGa–BP) were synthesized by reaction of F₁₆PcGaCl with 4-hydroxylbenzenediazonium tetrafluoroborate-functionalized BP (4-HBT-BP). Direct evidence for the formation of a P–C bond between phosphorus and 4-hydroxylphenyl in 4-HBT-BP has been observed. In contrast to few layer BP with a highly hydrophilic character, which will degrade rapidly within the scope of hours in the presence of oxygen and moisture (even very low concentration of air moisture), the LUMO energy level of F₁₆PcGa–BP (−4.76 eV) lies below the redox potential of O₂/O₂⁻ (∼−4.10 eV), implying that BP in F₁₆PcGa–BP is very hard to oxidize to P_xO_y that can react with water to produce phosphate compounds. This result demonstrates that covalent modification of BP with phthalocyanines can considerably improve the environmental stability of BP. Upon excitation with 400 nm laser light, the transient absorption (TA) spectrum of F₁₆PcGaCl shows a typical triplet–triplet absorption band centered at 510 nm in the range of 415–590 nm. Axial grafting of the F₁₆PcGa moieties onto the BP surface gives rise to the red-shift of the maximum TA peak by Δλ = 46 nm when compared to F₁₆PcGaCl. In comparison with BP and F₁₆PcGaCl, F₁₆PcGa–BP exhibits better nonlinear optical and optical limiting responses at 532 nm due to the increase of the intramolecular transition dipole moment between the excited states involved in the electronic transition responsible for the OL effect during the photoinduced electron transfer process. Its nonlinear coefficient (β_eff) and imaginary third-order susceptibility (Imχ⁽³⁾) at 400 μJ are 282.86 cm GW⁻¹ and 1.21 × 10⁻¹⁰ esu, respectively.