Excited state tautomerization of 7-azaindole catalyzed by pyrazole

Pyrazole, a five member cyclic azole, is reported here as an efficient catalyst for excited state tautomeric conversion of 7-azaindole. In hydrocarbon solution the two compounds efficiently form a doubly hydrogen-bonded 1:1 cyclic complex whose association constant value is found comparable with 7-azaindole dimerization constant, and according to B3LYP/6-311G++∗∗ calculation the binding energies of the complex and dimer are nearly same. In the excited state (S1), the TDDFT calculation predicts tautomer of the complex to be 13.4 kcal/mol more stable than normal form. Fluorescence spectra reveal that upon UV excitation the complex emits exclusively from the tautomeric form.