THE SIMILARITIES IN THE ELECTRONIC ABSORPTION SPECTRA OF AROMATIC FREE RADICALS, RADICAL CATIONS, AND MOLECULES IN THEIR TRIPLET STATES.

Author Institution: Department of Chemistry, Faculty of Engineering ScienceThe electronic spectra of various unstable aromatic species in solutions have been obtained either by the flash method or by the low-temperature matrix-isolation technique. By carefully analyzing the data, the carrier of the spectra were determined in most cases unambiguously. The unstable species are classified into three groups: [1]. The molecule AR in the lowest triplet state, $^{3}AR. [2]$, the cation $AR^{+}. [3]$. The $radical {\AA} or H{\AA}R$ (Here, R is H or $CH_{2}$). It has been found that all these three species show absorption spectra quite similar to each other. For instance, both the spectra of anilino radical $\phi NH$ and aniline cation $\phi NH_{2}^{+}$ consist of three bands in the region of $\lambda>2000 \AA$, with wavelengths slightly different from each other. The visible absorption spectra of $N,N,N^{1},N^{1}$-tetramethyl-p-phenylenediamine in the triplet state is almost identical to that of the cation of the same compound (Wurster's blue). Benzophenone $\begin{array}{c}\phi C \phi\\\|\\O\end{array}$ in the triplet state shows visible absorption bands at ca. 320 and $530 m\mu$, also very similar to those of benzo-ketyl radical $\phi C(OH)\phi$. Even the spectrum of diphenyl carbene $\phi \ddot{C}\phi$ shows two bands at the same wavelenghts. It is pointed out that, for the sake of these similarities in the spectra of different species, the determination of the unstable species by use of ultraviolet spectroscopy alone should be done carefully. It is also pointed out that all these similarities are not accidental, but arise from the fact that these species have common $\pi$-electronic configurations, in other words, they are iso-electronic with respect to the $\pi$-electrons and have common geometrical structures