Above the Energy Gap Law: Heavy Chalcogenide Substitution in NIR II-Emissive Diradicaloid Qubits

Near-infrared (NIR, 700–1700 nm)- and telecom
(∼1260–1625
nm)-emissive molecules are good candidates for biological imaging
and quantum sensing applications, respectively; however, bright low
energy emission is rare due to exponentially increasing nonradiative
decay rates in these regions, a phenomenon known as the energy gap
law. Recent literature has emphasized the importance of minimizing
skeletal modes to prevent increased nonradiative decay rates, but
most organic lumiphores in these regions utilize large, conjugated
scaffolds containing many CC modes. Here we report a compact,
telecom-emissive scaffold, tetrathiafulvalene-2,3,6,7-tetraselenate,
or TTFts, that displays remarkable air, water, and acid stability,
exhibits record quantum yields and brightness values, and retains
quantum coherence under ambient conditions. These properties are enabled
through methodical selenium substitution, which bathochromically shifts
emission while shifting skeletal vibrations to lower energies. This
new scaffold validates heavy heteroatom substitution strategies and
establishes a new class of bright telecom emitters and robust qubits.