Fluorescence excitation spectra of jet-cooled carbazole complexes with monohydric alcohols

Abstract

We have analyzed the fluorescence excitation spectra of carbazole complexes with a single molecule of methyl, deuterated methyl, ethyl, and propyl (1-propanol and 2-propanol) alcohols, cooled in a supersonic jet. We have determined the shifts in the fluorescence excitation spectra of the complexes relative to the frequency of the purely electronic transition of unbound carbazole. They occur as a result of formation of hydrogen bonds between the N-H group of the carbazole and the OH group of the alcohols. The frequencies of stretching vibrations of the hydrogen bonds with different alcohols vary within the range 150-157 cm -1 , while the frequencies of the bending vibrations vary in the range 21-22.9 cm -1 . From the shape of the rotational contours of the bands for the purely electronic and vibronic transitions of the complexes, we determined that they belonged to rotational conformers. We calculated the equilibrium configurations of the complexes in the ground state.Introduction. Complexes formed as a result of hydrogen bonding in a supersonic jet have been studied in a state isolated from the surroundings, and have a controllable composition and a certain equilibrium configuration. Jet cooling of complexes down to a few kelvins ensures resolution of the vibrational structure in the vibronic spectra, which makes it possible to obtain information about the magnitudes of the shifts of the vibrational-rotational absorption and emission bands of the complexes relative to the spectra of the components and about the characteristic lowfrequency vibrations of the hydrogen bond, and to identify conformers of the complexes from the shape of their rotational contours. Carbazole complexes with various gas molecules have been studied to date [1][2][3][4][5].Carbazole complexes with monohydric saturated alcohols have not been studied. Making the alcohol molecule successively more complicated makes it possible to determine the magnitude of the shift in the spectra per methylene group. On the other hand, several rotational conformers of the complex may belong to a given isomer. In a number of cases, they can be identified from the shape of the rotational contours.Experimental Procedure and Calculations. The fluorescence excitation spectra were measured on the automatic spectrometric system described in [2]. The helium carrier gas at a pressure of 2 atm was passed over the surface of the alcohol, which was at a temperature of 233 K to 273 K; the mixture was fed to a prechamber heated to 423 K and containing saturated carbazole vapor. The pulsed supersonic jet was formed by discharge of the ternary gas mixture into the vacuum chamber through a circular nozzle of diameter 0.7 mm. The complexes, cooled down to a few kelvins, were excited perpendicular to the jet axis, at a distance of 19 mm from the nozzle, by the unfocused second-harmonic emission from a frequency-tunable dye laser pumped by the second harmonic of a neodymium-doped yttrium aluminum garnet laser. In recording the rotational contours of the bands, the spectral widt...

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