LIF spectroscopy of jet-cooled 1:1 complex between 7-azaindole and 2-pyrrolidinone

► LIF spectroscopy and photophysics of 7-azaindole⋯2-pyrrolidinone complex. ► The vibronic bands in the fluorescence excitation spectrum show puckering doublets. ► Puckering barrier is predicted ∼260 cm−1 and zero-point splitting 10 cm−1. ► Complex emits tautomer fluorescence in hydrocarbon solution but UV emission in jet. ► High barrier is suggested for inhibition of tautomerization in isolated complex.