Unravelling the Impact of Porosity on Water Stability of Porous Bi(III) Halide Semiconductors and Their Potential for Red Tide Mitigation

We recently developed a family of materials, Porous Metal Halide Semiconductors (PMHS), that exhibit record water stability and tunable optoelectronic properties. However, the extent to which porosity influences these features remains unclear. Motivated by this, we report the synthesis of (DHT)(H3O)2Bi4I16·2(H2O) (DHT = [3.3.3] cryptand), where the custom-made DHT cage expands the pore size from 4.3 to 6.2 Å, compared to previously reported DHS-based analogs (DHS = [2.2.2] cryptand). The material has been water-stable for 24 months, featuring a single-crystal to single-crystal transformation, while gas and vapor sorption studies demonstrated that it can selectively adsorb and desorb H2O at 298 K. (DHT)(H3O)2Bi4I16·2(H2O) is a direct bandgap semiconductor that exhibits broad band-edge emission at room temperature. This combination of attributes prompted us to evaluate the material against Karenia brevis, an alga responsible for harmful algal blooms (HABs). Our studies revealed that (DHT)(H3O)2Bi4I16·2(H2O) suppressed K. brevis growth at concentrations as low as 10 mg/L, with no inhibition observed against nontarget marine algae strains. This work elucidates the influence of porosity on the optoelectronic properties and water stability of PMHS compounds and underscores their potential for impactful, previously unexplored applications such as HAB mitigation.