Laser Spectroscopic Characterization of Supersonic Jet-Cooled 2,6-Diazaindole (26DAI)

The article presents a comprehensive laser spectroscopic characterization of a nitrogen-rich indole derivative, namely, 2,6-diazaindole (26DAI), in the gas phase. A supersonic jet-cooled molecular beam of 26DAI was characterized using two-color resonant two-photon ionization (2C-R2PI) and laser-induced fluorescence spectroscopy (LIF) to investigate the electronic excitation. The S₁ ← S₀ origin transition was obtained at 33915 cm^-1, which was red-shifted from that of one (indole) and two (7-azaindole) nitrogen-containing indole derivatives by 1317 and 713 cm^-1, respectively. The molecular orbital and energy analysis for the S₁ ← S₀ transition shows the significant stabilization of LUMO on subsequent N-insertion, resulting in the lowering of the S₁ ← S₀ (ππ*) transition energy. The single vibronic level fluorescence spectrum from the vibrationless S₁ state of the molecule was recorded. The spectrum displayed an extensive Franck-Condon activity until 2500 cm^-1 for the vibrational modes of the S₀ state of the 26DAI molecule. The experimental ground state vibrational frequencies were compared to the calculated ones obtained at three different levels of theories. More accurate results were found at DFT B3LYP-D4 than those at the wave function-based MP2 and CCSD levels of theories. Further, the N-H stretching frequency of 26DAI in the S₀ state was measured at 3524 cm^-1 using fluorescence-dip infrared (FDIR) spectroscopy. The stability of 26DAI against ionization radiation was probed by measuring the two-color photoionization energy (IE_2P) of 26DAI at 71866 cm^-1. The IE_2P value is significantly higher than those of N-poor counterparts (indole and 7-azaindole). The NBO charges and spin density (SD) values of the 26DAI molecule have shown that electronegative N(6) makes the cationic ground state less stable due to the position of the positive centers on the N atom. The results provided insights into the stability of N-rich biomolecules against photodamage. The current investigation can shed light on nature's way of stabilizing biomolecules with a possible N-insertion mechanism.