Freezing Out Vavilov's Rule: Temperature‐Dependent Fluorescence of a Rhodanine Dye

This study addresses the anomalous fluorescence of a rhodanine‐based organic dye of interest for nonlinear optical applications. At room temperature, the dye exhibits weak fluorescence, while spectra collected in glassy 2Me‐THF at 77 K show a surprising fluorescence intensity increase by several orders of magnitude with respect to room temperature. Moreover, a pronounced dependence of the fluorescence quantum yield on the excitation wavelength is observed, indicating a breakdown of Vavilov's rule, a corollary of Kasha's rule, which states that the fluorescence quantum yield is independent of the excitation energy. Quantum chemical calculations demonstrate the presence of a bright ππ* excited state that lies very close in energy to a dark nπ* state. The subtle interplay between these two excited states with different natures is responsible for the intriguing spectral behavior of the dye. Specifically, experimental results are rationalized in terms of a population branching between the two excited states, which can be tuned upon varying the temperature or the excitation energy.