Molecular Structure and Photoinduced Intramolecular Hydrogen Bonding in 2‑Pyrrolylmethylidene Cycloalkanones

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.