EFFECTS OF HYDROGEN BONDING AND PROTON TRANSFER ON THE ELECTRONIC SPECTRA OF p-HYDROXYBENZALDEHYDE.

Supported by the National Science Foundation.Author Institution: Whitmore Chemical Laboratory, The Pennsylvania State UniversityThe absorption and emission spectra of p-hydroxybenzaldehyde (p-HBA) have been measured in hydrocarbon solution in the presence of ether or triethylamine as a proton acceptor, and the results have been interpreted in terms of the nature of the $n \rightarrow \pi^{\ast}$ excited state. At $\sim 300^{\circ}K$, practically no emission was observed, and the absorption spectra indicate that a simple hydrogen-bonded complex is formed between p-HBA and either of the two acceptors. p-HBA in hydrocarbon solution shows $n \rightarrow \pi^{\ast}$ and $\pi \rightarrow \pi^{\ast}$ transitions in the near ultraviolet. The $n \rightarrow \pi^{\ast}$ transition is shifted to the blue by the formation of the the hydrogen bond, whereas the $\pi \rightarrow \pi^{\ast}$ transition is shifted to the red. At $77^{\circ}K$, the p-HBA-ether system shows a single structured phosphorescence band ascribable to a simple hydrogen-bonded complex. The p-HBA-triethylamine system exhibits either a similar phosphorescence band or a structureless phosphorescence band accompanied by a broad fluorescence band, depending on the region of excitation. It is concluded that the latter system gives a protontransferred complex (ion-pair) as well as a hydrogen-bonded complex at the low temperature: and the structureless phosphorescence and fluorescence bands are assigned to the proton-transferred complex. The proton transfer and the electronic energy transfer are discussed in relation to the charge distribution in the $n \rightarrow \pi^{\ast}$ excited state