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Abstract
Bright, low-energy emission is desirable for various imaging and security applications due to the minimized attenuation in these regions. In particular, the near-infrared (NIR, 700-1700 nm) region is promising for biological imaging applications as it falls in the tissue-transparent range, allowing for informative, noninvasive imaging; however, most dyes emit poorly in this region due to a phenomenon known as the energy gap law. This “law” is the empirical observation that photoluminescence quantum yields (PLQYs) decrease exponentially with decreasing energy gaps. Thus, the closer you get to these desirable regions, the dimmer the emission. One strategy for circumventing this behavior is through heteroatom substitution. The introduction of heavier heteroatoms into organic scaffolds allows us to enable low energy transitions without the need for extended conjugation typically needed to promote this behavior. The comparatively weaker C–E (where E = S, Se, Te) bonding compresses the π manifold, and enables these unique properties on compact scaffolds. Furthermore, this substitution eliminates C–H moieties and other extraneous vibrational modes, which have been theorized to contribute to nonradiative decay, enabling brighter imaging agents. This strategy is also applicable to molecular qubit, or quantum bit, design. Typically, stable organic radicals also require extended conjugation or bulky groups to promote stability and prevent dimerization. This typically introduces more protons or other spin-active nuclei into the scaffold and decreases rigidity, contributing to decoherence and poor high temperature operation; however, a C–S-based scaffold allows for stable organic radical character on a compact, rigid scaffold, and eliminates deleterious protons. Thus, these scaffolds are much more robust, ideal for room temperature and biological quantum sensing.