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

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Abstract
Studies on UV/VIS Absorption Spectra of Azo Dyes. XV. An Analysis of the Absorption Spectra of 4,4′‐DiaminoazobenzenesIn the visible absorption spectrum 4,4′‐bis‐diethylaminoazobenzene shows two absorption maxima (λmax = 426 nm and λmax = 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.