Intramolecular charge delocalization in the singlet excited state of blocked pyrylium ions

The dual fluorescence emission of the pyrylium ion 3 and of the partly blocked 4 has been studied extensively under various conditions. The short-wavelength emitting species N^* of 3 is short-lived (≤200 ps at room temperature) while the long-wavelength emitting species A^* is long-lived (>3 ns, except in acetic acid). This long-wavelength fluorescence undergoes an important solvatochromic shift and the difference Δ between the absorption and fluorescence maxima versus Lippert’s solvent polarity function Δf is linear. Increasing the viscosity of the medium, or decreasing the temperature, decreases the long-wavelength emission quantum yield while that of the short-wavelength fluorescence and its lifetime (from <100 ps to >4 ns) both increase, indicating that A^* is formed from N^*. Introducing an ortho methyl group on the paraanisyl substituent (compound 4) blocks its rotation and reduces the fluorescence I_A./I_N. ratio, but it does not suppress completely the long-wavelength emission. This favors a ground state configuration where the phenyl substituent would be orthogonal to the xanthylium moiety. A strong interaction of 3 and 4 with aliphatic nitriles is characterized from the quenching of the fluorescence emission (with rate constants of ca. 2×10⁸ M⁻¹ s⁻¹). A static quenching process also occurs indicating a ground state interaction with the solvent. In pure aliphatic nitriles, this interaction is the main deactivation pathway of the singlet excited state, and practically no fluorescence nor triplet formation can be observed.