Coordination chemistry of NIR-II chromophore-metal hybrids for deep-tissue cancer theranostics

Coordination of near infrared (NIR) responsive chromophores to transition-metal centers provides a strategy for combining long-wavelength light harvesting with metal-mediated excited-state control. Cyanine dyes, boron dipyrromethenes (BODIPYs), diketopyrrolopyrroles, and other π-extended analogues show intense NIR absorption and fluorescence but frequently suffer from insufficient photostability and low reactive oxygen species (ROS) generation. Alternatively, Ru(II), Ir(III), Pt(II), and related coordination compounds can provide spin-orbit coupling, charge-transfer states, redox activity and access to triplet states, although most conventional complexes absorb inefficiently at long wavelengths. To overcome these drawbacks, covalent conjugation, encapsulation strategies, and supramolecular assembly have enabled hybrid systems that couple the light harvesting capacity of organic chromophores with the photophysical characteristics of metal complexes. Depending on the structural framework, energy or electron transfer through Förster resonance energy transfer (FRET), Dexter transfer, and photoinduced electron transfer (PET), and metal-to-ligand charge transfer (MLCT) or ligand-to-metal charge transfer (LMCT) pathways can redistribute excitation energy towards emission, triplet formation, ROS generation, and stimuli-responsive therapeutic activation. This review particularly examines how metal identity, coordination geometry and metal-ligand electronic coupling alter excited-state ordering, intersystem crossing, charge transfer, fluorescence, ROS generation and photothermal relaxation. Despite significant progress, challenges remain in fine-tuning energy-transfer efficiency, maintaining photostability over long time scales, and ensuring precise tumor selectivity. Systems directly absorbing or activated above 1000 nm are distinguished from NIR-I-excited systems whose emission extends into the NIR-II region. This review discusses principles for the molecular design of NIR-II light-absorbing organic probes, mechanistic insights into NIR-II chromophore-metal hybrids, and future directions for activatable, multifunctional, and translatable platforms for next-generation cancer theranostics.