Near Infrared Dyes Based on Diradicaloids: Molecular Design, Photophysical Properties and Bioapplications

Organic near-infrared (NIR) dyes are essential in materials science and biotechnology. Conventional design of NIR dyes typically rely on extending π-conjugation and constructing electronic donor–acceptor motifs, however, these approaches often face significant challenges in modulating bandgaps and achieving optical features beyond 800 nm, particularly in the critical NIR-II region (1000–1700 nm). The design concept based on diradicaloids offers a promising alternative by leveraging the open-shell electronic nature, which can produce exceptionally narrow bandgaps through HOMO–LUMO admixing and symmetry-breaking charge transfer (SBCT), thereby enabling strong absorption or emission in long wavelength NIR region. This review systematically summarizes the molecular design principles, photophysical properties, and emerging biomedical applications of theses diradicaloid-based NIR dyes. We focus on three representative molecular families including extended para-quinodimethanes and their analogues, zethrenes and their analogues, and heteroatom doped diradicaloid systems. Furthermore, we illustrate how structural modification, such as conjugation extension, substituent engineering, and heteroatom incorporation, can be used to tune the diradical character index (y0), thereby modulating the stability and NIR photoresponse. Overall, this work provides a clear and actionable framework for the design of next-generation diradicaloid NIR dyes tailored for high-performance bioapplications.