Measuring the Total Photon Economy of Molecular Species through Fluorescent Optical Cycling (FOC)

The total photon economy of a molecule/material is a measure of how light input is converted to light and heat output across energies and timescales. We describe a technique, Fluorescent Optical Cycling (FOC), which allows for simultaneous observation of prompt and delayed emission during and after multiple pulsed excitation, ultimately granting access to multi-state photophysical rates. We exercise control over the excitation pulse train, which allows us to “optically shelve” long-lived intermediate states without the use of diode or flashlamp excitation. By recording all photon arrival times in the visible and shortwave infrared, we can simultaneously resolve fluorescence, phosphorescence, and singlet oxygen sensitization in a single experiment. We use FOC to examine the photophysics of dual emitting bis(di-R-phosphino)alkanethiophene-pyridine-platinum ([Pt(thpy)(dppm)]+) under different solvation conditions, revealing changes in intersystem crossing and phosphorescent rates induced by the external heavy atom effect. Coupling FOC with Decay Associated Fourier Spectroscopy (DAFS), we demonstrate simultaneous correlated spectral and lifetime data in this dual emitting complex. Finally, FOC combined with superconducting nanowire single photon detectors (SNSPDs) allows us to observe the shortwave infrared region (SWIR) phosphorescence of singlet oxygen sensitized by Rose Bengal. Overall, FOC provides a powerful tool to simultaneously study multiple photophysics across timescales, even in weakly populated electronic states.