Ultra-precise photothermal measurements reveal near unity photoluminescence quantum yields of molecular emitters in solution.

Molecules with a photoluminescence quantum yield (PLQY) approaching unity enable new applications such as efficient luminescent solar concentrators and spectral redistributors. Moreover, they have the potential for thermally assisted photon upconversion and optical refrigeration, for which the slightest amount of non-radiative loss is detrimental. However, when the PLQY is within a few percent of 100%, it cannot be precisely determined using standard techniques. Here, we combine spectroscopic measurements with photothermal techniques to determine the photothermal threshold energy, the minimum photon energy at which the chromophores produce heat upon excitation. The PLQY is directly related to this energy and is determined for six fluorescent molecules in low concentration solutions with an unprecedented precision down to ±0.003 within 95% confidence intervals. Independent measurements based on photothermal-deflection spectroscopy and thermal lensing spectroscopy generally provide values within the margin of error, demonstrating the reliability of this measurement concept. Solutions of perylene red in carbon tetrachloride are found to have the highest PLQY of the measured series, being 0.994 ± 0.003. In addition, we observe phonon-assisted, optical upconversion when exciting perylene red within its optical gap at photon energies below its photothermal threshold. Similar measurements on perylene orange in chloroform reveal the presence of low energy sub-gap impurities, preventing upconversion when exciting at the photothermal threshold.