Establishing Design Principles for Emissive Organic SWIR Chromophores from Energy Gap Laws

Rational design of bright near and shortwave infrared (NIR: 700–1000 SWIR: 1000–2000 nm) molecular and nanoscale emitters is a fundamental scientific question with applications ranging from deep tissue imaging to new photonic materials. However, all reported organic chromophores with energy gaps in the SWIR have very low quantum yields. Is this the result of a fundamental limit for the quantum yield of organic chromophores in the SWIR? Here we combine experiment and theory to derive an energy gap quantum yield master equation (EQME), which describes the fundamental limits in SWIR quantum yields for organic chromophores in terms of energy gap laws for radiative and nonradiative decay. We parametrize EQME using experimental data from time-correlated single photon counting in the SWIR acquired using superconducting nanowire single photon detectors operating beyond the bandgap of silicon. Evaluating the photophysics of 21 polymethine NIR/SWIR emissive chromophores, we explain the precipitous decline of past 900 nm as the result of decreased radiative rates and increased nonradiative deactivation via high frequency vibrations as a function of singlet energy gap. From EQME we can compare quantum yields among NIR/SWIR chromophores while accounting for changes in energy gaps. We find that electron donating character on polymethine heterocycles results in improvements of radiative parameters obscured by a simultaneous redshift. We correlate this improvement to changes in transition dipole moments across the chromenylium polymethine family. Finally, understanding energy gap laws reveals quantitative estimates of the effect of deuteration and molecular aggregation as strategies to increase in the SWIR. We experimentally demonstrate that partial deuteration of the chromophore Flav7 results in decreased nonradiative rates and concomitant increases in quantum yield. These insights will enable optimal chromophore designs for SWIR fluorescence.