Zum Mechanismus der photochemischen Umwandlung von 2-Alkyl-indazolen in 1-Alkyl-benzimidazole. II. Photophysikalische und photochemische Primärprozesse

Mechanistic studies on the photoisomerization of 2-alkyl-indazoles into 1-alkyl-benzimidazoles. II. Primary photochemical processes and photophysical deactivation. In the previous paper 1] the structure of the intermediate in the photochemical indazole-benzimidazole-isomerization was discussed ( 3 in Scheme 1). In this communication experiments concerning the photochemical primary processes and photophysical deactivation of 2-alkyl-indazoles ( 1 ) are described. The quantum yield of the rearrangement 1 → 2 (Φ_R) decreases with decreasing temperature while the fluorescence quantum yield (Φ_F) increases and finally reaches a constant value ( ≠ 1) (Fig.10). This behaviour is inconsistent with the mechanism shown in Scheme 2. Photoreaction and fluorescence are both quenched, but not to the same extent, by freon 113 (Fig. 2). In addition the Stern-Volmer-plots are not linear. These observations are best explained by assuming the existence of two excited states in equilibrium (Scheme 3). The mechanism in Scheme 3 correctly explains the quenching experiments and the temperature dependence of Φ_R and Φ_F if the Arrhenius law holds for the two rate constants k_sx and k_R. However, for a quantitative calculation of Φ_R, an additional branching of the reaction pathway must be postulated (Scheme 4). Two-dimensional drawings of hypothetical potential energy surfaces of the ground state and the first excited singlet state yielding a qualitative picture of the reaction and deactivation pathways of the discussed molecule are given in Fig. 15 a and b.