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

Abstract

Rational design of bright near and shortwave infrared\n(NIR: 700–1000 SWIR: 1000–2000 nm) molecular and nanoscale emitters is a\nfundamental scientific question with applications ranging from deep tissue\nimaging to new photonic materials. However, all reported organic chromophores\nwith energy gaps in the SWIR have very low quantum yields. Is this the result\nof a fundamental limit for the quantum yield of organic chromophores in the\nSWIR? Here we combine experiment and theory to derive an energy gap quantum\nyield master equation (EQME), which describes the fundamental limits in SWIR\nquantum yields for organic chromophores in terms of energy gap laws for\nradiative and nonradiative decay. We parametrize EQME using experimental data\nfrom time-correlated single photon counting in the SWIR acquired using\nsuperconducting nanowire single photon detectors operating beyond the bandgap\nof silicon. Evaluating the photophysics of 21 polymethine NIR/SWIR emissive\nchromophores, we explain the precipitous decline of \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n\n \n past 900 nm\nas the result of decreased radiative rates and increased nonradiative deactivation\nvia high frequency vibrations as a function of singlet energy gap. From EQME we\ncan compare quantum yields among NIR/SWIR chromophores while accounting for\nchanges in energy gaps. We find that electron donating character on polymethine\nheterocycles results in improvements of radiative parameters obscured by a\nsimultaneous redshift. We correlate this improvement to changes in transition\ndipole moments across the chromenylium polymethine family. Finally, understanding\nenergy gap laws reveals quantitative estimates of the effect of deuteration and\nmolecular aggregation as strategies to increase \n \n in the\nSWIR. We experimentally demonstrate that partial deuteration of the chromophore\nFlav7 results in decreased nonradiative rates and concomitant increases in\nquantum yield. These insights will enable optimal chromophore designs for SWIR\nfluorescence.