Giant Compressed Mo Blue Wheel {Mo118}: A Molecular Photothermal Agent for High-Effective NIR-II Chemo-Photothermal Therapy.

The rational construction of well-defined polyoxometalate (POM)-based second near-infrared (NIR-II) molecular photothermal agents (PTAs) and the elucidation of their structure-activity relationships are crucial, but remain unresolved. Herein, we report a decameric molybdenum blue (MB) wheel {Mo118}, comprising a {Mo112} wheel capped by two {(SO4)Mo3} units, where compression of the giant ring arises from {Mo1*} units and distinct coordination modes of corner-sharing {Mo2} units. As a rare structurally well-defined POM-based molecular PTA that exhibit photothermal conversion performance under NIR-II region, {Mo118} features extensive delocalization of 28 Mo 4d electrons within its framework, which endows it with a photothermal conversion efficiency of 50.8%, substantially outperforming that of the classical MB-based analogue {Mo154} PTA. Femtosecond transient absorption spectroscopy further elucidates that a more rapid non-radiative relaxation process in {Mo118} underpins its enhanced photothermal performance. By encapsulating {Mo118} into a dual-responsive hydrogel via electrostatic interactions, a biocompatible system was constructed that affords photothermal-triggered chemo-photothermal therapy, demonstrating potent antitumor activity against 4T1 tumors both in vitro and in vivo. This work not only provides critical insights into the relationship between the electronic structure of POMs and their photothermal properties, but also establishes a new paradigm for the rational design of next-generation molecular cluster-based PTAs.