Assignment of Electronic Transitions in Azo Dye Prototypes

Abstract

Quantitative calculations of the absorption frequency, transition moment length, and polarization of the first strong absorption band in the series of compounds C₆H₅N=N(C₆H₄N=N)_xC₆H₅ have been successfully carried out using a model employing delocalization of electronic excitation energy rather than delocalization of electrons. It is found that the compounds can be treated as azo‐perturbed phenyl groups interacting with both an electrostatic force and through conjugation. Because of the electrostatic part of the interaction, there results a demonstrable heat of single bond isomerization in phenylazo‐β‐naphthalene, its conformational isomers possessing different colorband spectra. The n—π^* transition in the azo dyes is shown to be, in all probability, the low‐frequency component of two previously degenerate n—π^* bands which have split. The intensity and polarization of the formally forbidden, low‐frequency n—π^* band can be explained by assuming a mixing of the n—π^* band with the strong, low‐frequency band responsible for the color in azo dyes. A high‐frequency band found at 44 kk (the symbol kk used throughout this paper stands for kilokayser or 10³ cm^—1) having a constant intensity throughout the series of dyes investigated has been shown to be isolated within the end phenyl groups.