Excited state tautomerization of 7-azaindole in a 1:1 complex with δ-valerolactam: a comparative study with the homodimer.

A comparative analysis for relative stability between normal and tautomeric forms in the excited electronic states of 7-azaindole···δ-valerolactam 1:1 complex and 7-azaindole homodimer has been presented. The tautomeric configuration of the complex is estimated to be ~6 kcal/mol more stable than normal form, and the same for homodimer appears to be ~10 kcal/mol. Consistent with these estimates both the complex and homodimer undergo facile double proton transfer tautomerization upon UV excitation in hydrocarbon solutions (Chou; et al. J. Am. Chem. Soc. 1995, 117, 7259). However, we notice that such similarity in photophysical behavior of the two hydrogen-bonded systems is lost completely in a cold supersonic jet expansion. The jet-cooled homodimer emits only the tautomer fluorescence in the visible spectral region, but the complex emits exclusively from the locally excited state in ultraviolet. We have interpreted this contrast by arguing that the effective barrier for excited state double proton exchange tautomerization of the complex is larger compared to that of the homodimer, and the difference originates because of asymmetric nature of the two hydrogen bonds of the complex.