Stable radicals in bacteria composites hybridized by a doubly-strapped perylene diimide for near-infrared photothermal conversion

Radical anions of electron-deficient perylene diimides (PDI) are attractive near-infrared (NIR) absorbers for photothermal conversion; however, their stability is often compromised by strong aggregation and reoxidation in air. Herein, we present a class of bacterial composites hybridized with a newly synthesized doubly-strapped PDI cyclophane, termed “Gemini Box” (GBox-3⁴⁺), which features air-stable PDI radicals for NIR photothermal conversion. The effective spatial isolation provided by the double-sided cationic molecular straps allows GBox-3⁴⁺ to completely suppress chromophore aggregation, even in concentrated aqueous solutions up to 2 mmol/L, thereby preserving its characteristic fluorescence for live-cell imaging. After incubation of bacteria with GBox-3⁴⁺, the radical species PDI^•– have been found to stably exist in the bacterial composites under ambient conditions, both in aqueous suspension and solid forms. Further experiments demonstrate that the air stability of the radical species relies on the simultaneous presence of the doubly-strapped PDI dye and the bacteria. Moreover, the dye-bacterial composites exhibited an high-efficiency NIR photothermal effect with high durability, enabling their application as photothermal agents for seawater desalination. This work provides a new access to the in situ fabrication of photothermal materials from biomass, relying on the rational molecular design and the unique microenvironment of bacteria.