Elucidating Nonradiative Decay in Deep Near-Infrared Lumiphores: Leveraging New Design Principles to Develop a Telecom Band Organic Dye Laser

Near-infrared (NIR) lumiphores have been targeted for biological and technological applications; however, these dyes suffer from exponentially decreasing quantum yields as their energy gaps decrease. The accepted model of this phenomenon invokes excited-state coupling to C–H modes. Reduction of these modes has been a popular design strategy to design bright NIR lumiphores, but definitive experimental validation of this strategy is lacking. Here we systematically evaluate the role of C–H modes in nonradiative relaxation through isotopic labeling of a NIR-emitting complex. Perdeuteration of the emissive molecule indicates that C–H modes do not contribute significantly to nonradiative relaxation, in direct contrast with accepted models. Our results instead suggest that skeletal modes may play a larger role. This suggests that minimizing scaffold size is a more promising route for bright NIR lumiphores. We demonstrate the promise of such strategies through the development of the reddest organic-based laser dye yet reported.