Inter-ring Torsional Modulation in Molecular Lasers. Ultraviolet Lasing via Amplified Spontaneous Emission Spectroscopy of Phenylimidazoles

The intramolecularly torsion-capable molecules 2-phenylimidazole (I), 2-phenylbenzimidazole (II), 1-methyl-2-phenylimidazole (III), and 1-methyl-2-phenylbenzimidazole (IV) are shown to yield efficient UV lasing action. The amplified spontaneous emission (ASE) laser spikes wavelengths (and gain coefficients) are for I, 321 nm (α = 7 cm-1); for II, 341 nm (α = 10.5 cm-1); for III, 324 nm (α = 8 cm-1); and for IV, 345.5 nm (α = 9 cm-1) (ASE cell optical length of 0.8 cm). The laser spikes represent for each molecule the normal simple case of wavelength coincidence with the fluorescence maximum. Theoretical calculations are presented to correlate electronic structural changes with observed spectra and for theoretical torsional potential functions. In cases I and II, the torsional mode is active only in the excited-state S1, and serves merely to modulate by a large shift (∼3000 cm-1) the ASE laser spike position, driven by stretching mode vibronic excitation. In the cases of III and IV, the torsional mode is interpreted to be the driving mode, with an ASE laser spike Δν of 5000 cm-1 or more (measured as the Franck−Condon shift of λmax from absorption to fluorescence, and the ASE laser spike position), deduced from the theoretically calculated torsional potentials offering a four-level population inversion system.