The Energy Gap Law at Work: Emission Yield and Rate Fluctuations of Single NIR Emitters.

Internal conversion (IC) often is the dominating relaxation pathway in NIR emitters, lowering their fluorescence quantum yield. Here, we investigate dibenzoterrylene (DBT) by bulk and single molecule spectroscopy. With increasing solvent polarity, the S 1 -S 0 energy gap decreases leading to a decrease of the fluorescence quantum yield and an increase of the IC rate in full accordance with the energy gap law. Making use of the unexpectedly strong fluorescence solvatochromism of this aromatic hydrocarbon, the validity of the energy gap law could also be demonstrated at the single molecule level. The S 1 -S 0 energy gap not only controls the fluorescence lifetime and quantum yield of single molecules but also dictates how these quantities develop during spectral fluctuations. Our results open new avenues into unexplored single molecule photophysics and appear as a promising tool for nanoscale probing of dynamic heterogeneities.