Electronic Absorption Fluorescence and Phosphorescence of Naphthaldehydes and Acetonaphthones

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Abstract
The electronic spectra of the isomeric naphthaldehydes and acetonaphthones (naphthyl methyl ketones) have been studied in aqueous and hexane media. In the 2-isomers fluorescence originates from the 1Lb state. In the 1-isomers, solvent interactions in the excited state invert the order of states observed in the absorption spectrum so that the 1La state is lowest. The 2-isomers show normal excited state ionization behavior indicating that these compounds are about 7-8 orders of magnitude more basic in the 1Lb state than in the ground state. The 1-isomers appear to form stoi-chiometric complexes with the protonating species (H3O+) and are truly protonated in the excited state at higher acidities than would be expected. The reasons for this are not understood but the phenomenon appears to have a kinetic rather than a thermodynamic origin. The naphthaldehydes and acetonaphthones are of analytical interest because of the fluorogenic properties of the naphthyl group and the high reactivity of the carbonyl group, both of which may be useful in the preparation of fluorescent derivatives of non-fluorescent analytes. Although aromatic aldehydes and ketones are better known for their phosphorescences than for their fluorescences, a circumstance which supposedly arises from the occurrence of low lying nπ∗ transitions associated with the carbonyl group and favoring high yields of singlet-triplet intersystem crossing, some aromatic aldehydes with medium sized aromatic systems have been found to fluoresce in hydroxylic, highly polar solvents (but not in hydrocarbon solvents) at room temperature (1).

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