High-Performance Photothermal Conversion of a 3D Mn(IV)-Polyoxometalate Architecture

In this work, we present a stable three-dimensional compound, [Na(μ-H2O)2Na(H2O)Cd(β-Mo8O26)0.5(Mn(L))] (1) (L = (1s,3s,8s,10s,12s,17s)-1,3,4,7,8,10,12,13,16,17,19,22-dodecaazatetracyclo[8.8.4.13,17.18,12]tetracosane-5,6,14,15,20,21-hexaone), possessing a {32·4·52·62·73}2{32·6·73} topology, synthesized via the hydrothermal method, engaging a mononuclear manganese(IV) compound as the foundational building unit. Within compound 1, β-[Mo8O26]4– serves as a template, facilitating the utilization of three pairs of carbonyl groups by [MnIV(L)]2– in conjunction with Cd2+ to assemble a two-dimensional structure that features a 4-membered metal ring Mn2Cd2 and an 8-membered metal ring Mn4Cd4. The coordination of β-[Mo8O26]4– with Cd2+ through Mo–Ot is accommodated in an 8-membered ring, which drastically changes the three pairs of carbonyl angles of [MnIV(L)]2–. Concurrently, sodium ions expand the two-dimensional network into a three-dimensional architecture by the carbonyl O and N atoms of [MnIV(L)]2–, thereby augmenting the water solubility and stability of 1. Compound 1 exhibits a broad light absorption range and a high extinction coefficient. Under laser irradiation, the powder reaches 163.4 °C, while a 400 μM solution of compound 1 heats up by 30.5 °C under near-infrared exposure. Furthermore, a poly(vinyl alcohol) (PVA) film containing compound 1 enables high-resolution imaging. Compound 1 achieves a photothermal conversion efficiency of 70.59%, outperforming most previously reported materials. It also demonstrates exceptional stability with consistent performance through repeated heating and cooling cycles under the same laser conditions. Consequently, we anticipate that the three-dimensional polyoxometalate compound, anchored by Mn(IV) blocks, may offer a promising avenue for the development of photothermal conversion materials.