Untersuchungen zum UV/VIS-Spektralverhalten von Azofarbstoffen. XV. Eine Analyse der Absorptionsspektren von 4,4′-Diaminoazobenzenen

Studies on UV/VIS Absorption Spectra of Azo Dyes. XV. An Analysis of the Absorption Spectra of 4,4′-Diaminoazobenzenes In the visible absorption spectrum 4,4′-bis-diethylaminoazobenzene shows two absorption maxima (λ_max = 426 nm and λ_max = 477 nm) with high intensity. Quantum chemical calculations and examination by peak separation of six 4-substituted azobenzenes and six 4′-substituted 4-diethylaminoazobenzenes ( 1a–k ) indicate that the longest wavelength band is due to a n-π* transition and the other band is due to a π-π*-transition. This is verified using an increment system for n-π* absorption maxima. A linear relation is found between intensity ratios of the two bands and the HAMMETT -σ-constants of the substituents. With increasing electron releasing tendency of the substituents the absorption intensity of the n-π*-band increases.