Laser Spectroscopic Characterization of Supersonic Jet Cooled 2,7-Diazaindole (27DAI)

This article presents an extensively curated rich set of gas phase spectroscopic data of 2,7-diazaindole in the ground and excited states. Single vibronic level fluorescence spectroscopy (SVLF) was performed to determine the ground state vibrations of the molecule, which depicted a large Franck-Condon activity of the bands beyond 2600 cm-1. For the excited state, laser-induced fluorescence (LIF) and resonant two-colour two-photon ionization spectroscopy (R2PI) were performed. The band origin 〖(0〗_0^0) for S1←S0 transition appeared at 33910±1 cm-1 . The Frank-Condon active vibrational modes in the spectra were seen till 〖(0〗_0^0)+ 1600 cm-1 region, which suggested the similar ground and excited state geometries. The lower energy asymmetric vibrational modes at 207, 252 and 358 cm-1, observed in the excited state were absent in the SVLF spectrum. IR-UV double Hole Burning spectroscopy confirmed the absence of any other isomeric species in the molecular beam. Ionization potential (I.P) of the molecule was found to be 8.9310.001 eV, recorded using photoionization efficiency spectroscopy. The above value is significantly higher than the related azaindole derivatives. The ground and excited state N-H stretching frequencies of the molecule were determined using fluorescence-dip infrared spectra (FDIR) and resonant ion-dip infrared spectroscopy (IDIR), obtained at 3523 and 3467 cm-1, respectively. The lower value of NH in the electronic excited state implies the higher acidity of the group compared to the ground state. Moreover, to understand the excited state properties of the molecule, a comparative analysis of the experimental LIF/2C-R2PI spectra was done against Franck-Condon simulated spectra at three different levels of theories. The vibrational frequencies calculated at B3LYP-D4/def2-TZVPP showed the most accurate prediction on comparison with the experimentally detected symmetric modes in the ground state. However, in the excited state, the low energy asymmetric modes were correctly determined at B3LYP/def-SVP level of theory. This is most probably due to the distortion observed at the pyrazolyl ring leading to the appearance of asymmetric vibrational modes. However, all the three methods have shown nearly similar correlation with the experimental frequencies in the excited state, which was evident from their similar scaling factors.