The steady‐state fluorescence spectra of 1‐methoxynaphthalene and 1‐naphthol were measured in pure organic solvents and in binary mixtures of water and several organic solvents. The 1‐isomers exhibited a much larger fluorescence Stokes‐shift than the corresponding 2‐isomers. The emission spectra of 1‐methoxynaphthalene and 1‐naphthol in pure nonpolar organic solvents exhibited roughly the same structured spectral features, while the fluorescence spectra in water and formamide exhibited much broader red‐shifted and less structured spectral features. In accord with previous observations, we attribute these spectral changes to two emitting states, 1Lb and 1La, whose relative fluorescence intensities are sensitive to solvent polarity. Our analysis of the fluorescence spectra of 1‐naphthol and 1‐methoxynaphthalene by Pekarian functions have demonstrated that the 1La state becomes the dominant emitting state in polar solvents. In addition, the 1La state was found to be further stabilized in hydrogen‐bond‐donating solvents. In contrast to previous suggestions, the onset of the excited‐state proton transfer reaction from 1‐naphthol occurred at higher solvent polarities than those required for the establishment of a dominantly 1La emitting state.

The structures of pyrrolylmethylidene derivatives of 2,3-dihydro-1H-inden-1-one (3), 3,4-dihydro-naphthalen-1(2H)-one (4), and cycloalkanones (5-7) were studied for the first time in the solid state and solution by NMR, IR, and UV spectroscopies supported by DFT quantum mechanical calculations. It was shown that all studied compounds except cycloheptanone derivative 7 both in crystal and in solution exist in the form of dimers where single E or E,E configuration with respect to the exocyclic C═C bond is stabilized by intermolecular hydrogen bonds N-H···O═C. UV irradiation at a wavelength of 365 nm of MeCN or DMSO solutions of 3-6 results, depending on the exposition time and solvent, partial to complete isomerization to the Z or Z,E isomers (in the case of 6, also the Z,Z isomer). The NMR and IR spectroscopy data show the existence of a strong intramolecular hydrogen bond N-H···O═C in the Z moieties of isomerized compounds. The studied compounds are protonated by trifluoroacetic acid at the carbonyl oxygen, in spite of the reverse order of basicity and nucleophilicity of the carbonyl group and the pyrrole ring. Investigation of the behavior of compound 6 with respect to acetate and fluoride anions allows one to consider it as a potential fluoride sensor.

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