Emission Properties of p-Hydroxybenzaldehyde with Particular Regard to Proton Transfer

The emission properties of p-hydroxybenzaldehyde have been studied in a hydrocarbon solvent in the presence of ether or triethylamine as a proton acceptor. At room temperature no emission is detected, and the absorption spectra indicate that a simple hydrogen-bonded complex is formed between p-hydroxybenzaldehyde and either of the two acceptors. The n→π* transition is shifted to the blue by the formation of the hydrogen bond, whereas the π→π* transition is shifted to the red. At liquid-nitrogen temperature, the p-hydroxybenzaldehyde–ether system shows a single phosphorescence spectrum with a vibrational fine structure, the spectrum being ascribable to the simple hydrogen-bonded complex. The p-hydroxybenzaldehyde–triethylamine system exhibits either a similar phosphorescence spectrum or a structureless phosphorescence spectrum accompanied by a broad fluorescence spectrum, depending on the region of excitation. It is concluded that, at liquid-nitrogen temperature, the latter system involves a proton-transferred complex (ion pair) as well as the simple hydrogen-bonded complex. The structureless phosphorescence and fluorescence spectra are assigned to the ion pair. In contrast to the case of the usual phenolic compounds, the proton transfer does not take place from p-hydroxybenzaldehyde to the amine during the lifetime of the excited state. This is attributed to the fact that the lowest singlet-excited state of the p-hydroxybenzaldehyde molecule is of an (n, π*) type, where the proton-donating power of the molecule is relatively weak.