Unraveling the fluorescence features of individual corrole NH tautomers.

The fluorescence spectra of 10-(4,6-dichloropyrimidin-5-yl)-5,15-dimesitylcorrole have been studied in the temperature range from 4.2 to 332 K. For the first time, the individual fluorescence profiles of the two corrole NH tautomers have been assigned over the whole temperature range. The pronounced temperature dependence of the fluorescence spectra of the meso-pyrimidinylcorrole under study was found to originate from switching between the fluorescence emissions of the two tautomers due to a reduced NH tautomerization rate with decreasing temperature. As a result, the long wavelength tautomer dominates the total emission spectrum at room temperature, whereas at low temperatures, the majority of the emission comes from the short wavelength tautomer. Energy level diagrams (involving the two NH tautomers) explaining the excitation energy deactivation channels in the meso-pyrimidinylcorrole at room temperature and below are presented. A significant H/D isotope effect on the NH tautomerization rate has been observed, resulting in an enhanced contribution of the short wavelength tautomer to the total fluorescence spectrum at the expense of that of the long wavelength tautomer. Substantially different fluorescence quantum yields have been determined for the individual NH tautomers, leading to a pronounced temperature dependence of the overall fluorescence quantum yield. The obtained results allow the unambiguous statement that the two NH tautomers of corroles coexist in fluid and solid solutions in a wide range of temperatures, with the proportion depending on the corrole substitution pattern. Moreover, this study shows that the (future) interpretation of the fluorescence properties of meso-pyrimidinylcorroles and all other corrole materials should be done (more) carefully, taking into account the coexistence of NH tautomers with individual spectral signatures.