The Electronic Spectra of Aromatic Molecular Crystals. I. Substitued Benzene Molecules

The electronic absorption spectra of some substituted benzene molecules p-nitroaniline, benzoic acid, acetanilide, trans-cinnamic acid, and durene have been measured in ethanol or n-heptane solutions and as single crystals, using polarized light. The measurement of the crystalline spectra reveals in many respects the properties of the transitions of substituted benzene molecules. The 277 mμ of durene, which is reasonably regarded as a band analogous to the 260 mμ of benzene, is shown to be polarized perpendicular to the two-fold symmetrical axis of the molecule. Bands of the same type occur at 264 mμ in p-nitroaniline (crystal), at 284 mμ in benzoic acid, at 282 mμ in acetanilide and at 299 mμ in trans-cinnamic acid. In addition, those molecules in which the benzene ring is substituted by the electron-accepting groups, show a strong band, associated with intramolecular charge transfer, in the near ultraviolet region. The 370 mμ band of p-nitroaniline is shown to be polarized parallel to the long axis of the molecule, and the 230 mμ band of benzoic acid has a polarization parallel to the line connecting the centers of the benzene ring and the carbonyl group. The directions of the transition moments of the 240 mμ band of acetanilide and the 276 mμ band of trans-cinnamic acid are also shown to be on the line connecting the substituents and the benzene ring. Davydov splitting is observed in the crystal of p-nitroaniline; this splitting is in good agreement with the values calculated by the exciton theory.