Metalloporphyrinyl-Phosphonates with Serrated Chain Structures: Exfoliation and NIR Photothermal Effect

Near-infrared (NIR) photothermal materials have received
significant
attention due to their potential applications in photothermal therapy,
diagnosis, and imaging. In this work, we constructed three mixed-ligand
metal phosphonates, namely, Co(Ni–H6TPPP)(phen)(H2O)2 (CoNi), Ni(Ni–H6TPPP)(phen)(H2O)2 (NiNi), and
Co(Co–H6TPPP)(phen)(H2O)2 (CoCo), where H10TPPP is 5,10,15,20-tetrakis(p-phenylphosphonic acid)-porphyrin and phen is 1,10-phenanthroline.
These compounds are isostructural showing 1D serrated chain structures
in which the porphyrin rings are stacked between the chains with π–π
interactions. All displayed a broad absorption from the UV–vis
to NIR (ca. 1500 nm) regions in the solid state. Interestingly, the
chains in these compounds can be chemically exfoliated in water by
adjusting the pH, forming interwoven nanostrands/nanosheets with thicknesses
of one to several chains. The photothermal effect was observed for CoNi, NiNi, and CoCo in the solid
state and in HEPES solution (pH = 7.4) upon 808 nm laser irradiation,
among which CoCo showed the highest photothermal conversion
efficiency. The important role of the cobalt(II) ion in the porphyrin
ring and phenylphosphonic unit in extending π-electron delocalization
and enhancing the absorption in the NIR region in CoCo is revealed by theoretical calculations.