Zur strahlungslosen Desaktivierung des tiefsten angeregten Singulett-Zustandes von 2-Alkyl-indazolen in neutraler und protonierter Form

On the Mechanism of Radiationless Deactivation of the First Excited Singlet State of 2-Alkyl-indazoles The temperature dependence of the quantum yields of fluorescence (Φ_F), intersystem crossing (Φ_T) and photoreactions (Φ_R) have been measured for several neutral and protonated 2-alkyl-indazoles. The experimental data (Fig. 1 and 3) can be interpreted in terms of the reaction schemes 1 and 2 with temperature independent constants for fluorescence emission and intersystem crossing and temperature dependent photochemical primary processes. Whereas at low temperatures the sum of the quantum yields of fluorescence and intersystem crossing equals 1, at higher temperatures a very efficient radiationless deactivation process takes place (Table). Based on kinetic and photochemical data it is concluded that this deactivation proceeds via a hypersurface crossing (Fig. 2) or hypersurface touching (Fig. 4) in the course of the photochemical rearrangements 1 → 2 and 4 → 5 respectively. Similar mechanisms are expected to be responsible for the unusual internal conversion in other heterocyclic compounds.