Spectroscopic Characterization of Hydrogen-Bonded 2,7-Diazaindole Water Complex Isolated in the Gas Phase

We present a systematic experimental analysis of the 1:1 complex of 2,7-diazaindole (27DAI) with water in the gas phase. The complex was characterized by using two-color-resonant two-photon ionization (R2PI), laser-induced fluorescence (LIF), single vibronic level fluorescence (SVLF), and photoionization efficiency (PIE) spectroscopic methods. The 0₀⁰ band of the S₁←S₀ electronic transition of the 27DAI-H₂O complex was observed at 33,074 cm^-1, largely red-shifted by 836 cm^-1 compared to that of the bare 27DAI. From the R2PI spectrum, the detected modes at 141 (ν'_Tx), 169 (ν'_Ty), and 194 (ν'_Ry) cm^-1 were identified as the internal motions of the H₂O molecule in the complex. However, these modes were detected at 115 (ν″_Tx), 152 (ν″_Ty), and 190 (ν″_Ry) cm^-1 in the ground state, which suggested a stronger hydrogen bonding interaction in the photo-excited state. The structural determination was aided by the detection of ν_NH and ν_OH values in the ground and excited state complexes using the FDIR and IDIR spectroscopies. The detection of ν_NH at 3414 and ν_OH at 3447 cm^-1 in 27DAI-H₂O has shown an excellent correlation with the most stable structure consisting of N(1)-H···O and OH···N(7) hydrogen-bonded bridging water molecule in the ground state. The structure of the complex in the electronic excited state (S₁) was confirmed by the corresponding bands at 3210 (ν_NH) and 3265 cm^-1 (ν_OH). The IR-UV hole-burning spectroscopy confirmed the presence of only one isomer in the molecular beam. The ionization energy (IE) of the 27DAI-H₂O complex was obtained as 8.789 ± 0.002 eV, which was significantly higher than the 7AI-H₂O complex. The higher IE values of N-rich molecules suggest a higher resistivity of such molecules against photodamage. The obtained structure of the 27DAI-H₂O complex has explicitly shown the formation of a cyclic one-solvent bridge incorporating N(1)-H···O and O-H···N(7) hydrogen bonds upon microsolvation. The lower excitation and higher ionization energies of the 27DAI-H₂O complex compared to 7AI-H₂O established higher stabilization of N-rich molecules. The solvent clusters forming a linear bridge between the hydrogen/proton acceptor and donor sites in the complex can be considered as a stepping stone to investigate the photoinduced deactivation mechanisms in nitrogen containing biologically relevant molecules.