Studies on U.V./vis Absorption Spectra of Azo Dyes. V. Quantitative Prediction of the Absorption Maxima of Polysubstituted Azobenzenes with an Increment System
The effect of substituents on the long‐wave band of azobenzenes can be described by using an increment system. Absorption increments which specifically characterize the substituents are derived from the difference in excitation energy between azobenzene and monosubstituted azobenzenes. Analysis of about 350 polysubstituted azobenzenes with absorption maxima between 320 nm and 650 nm showed that the differences between observed and calculated values are generally ±5 nm.
Therefore, this increment system permits absorption maxima of azobenzenes to be deduced not as yet prepared or spectroscopically investigated.
Interpretation of the UV/vis Spectral Behaviour of Substituted Benzenediazonium Ions
In order to synthesize new, potent, wide‐range photosensitive diazonium compounds, an explanation of the various influences on the position of the longest wavelength absorption band (λmax) of the diazonium ions should be given.
For this purpose, SCF‐LCI calculations according to the Pariser, Parr, Pople method were made. The absorption behaviour of the diazonium ions can be explained on the basis of simple perturbation‐theoretical MO considerations. According to the regularities mentioned, it is possible to consider 4‐aminosubstituted benzenediazonium ions as meropolymethines. With the perturbation theory it is also possible to define the limit of λmax attainable with benzenediazonium compounds.
Studies on UV/VIS Absorption Spectra of Azo Dyes. III. Empirical Absorption Increments Derived from Azobenzenes and their Relationship to Reactivity Constants, Spectroscopic Substituent Constants and Ionisation Potentials
Absorption increments ϵ which specifically characterize the substituents are derived from the difference in exitation energy between azobenzene and monosubstituted azobenzenes. These increments have been found to predict accurately the effect of substituents of the long wave π → π* band of polysubstituted azobenzenes.
Comparison of the ϵ values with HAMMETT‐σ reactivity constants reveals direct proportionality between the two sets of constants. There is also a direct proportionality between the increments and spectroscopic substituent constants introduced by BLOOR and COPLEY. Furthermore, a correlation of the ϵ values with the ionization potentials of substituents was found.
Nitrogen‐15 and Carbon‐13 Chemical Shifts and 13C‐1H, 13C‐15N and 15N‐15N Coupling Constants of Parasubstituted Arene Diazonium Salts
Nitrogen‐15 and carbon‐13 chemical shifts and 13C‐1H,13C‐15N and 15N‐15N coupling constants of 15N‐enriched p‐substituted arene diazonium salts are reported and within the ΔE‐approximation investigated. It is shown that changes of ΔE should be taken into account when considering variation of the nitrogen‐15 chemical shifts. The influence of substituent effects was discussed within the DSP (dual substituent parameter)‐approach.
Studies on UV/VIS Absorption Spectra of Azo Dyes. IV. Solvent Effects on the Visible Absorption Band of Azobenzenes
Solvent effects on \documentclass{article}\pagestyle{empty}\begin{document}$ \begin{array}{*{20}c} \sim \\ {\nu _{\max } } \\ \end{array} $\end{document} values of substituted azobenzenes are studied and correlated with solvent polarity parameters. Poor correlations of \documentclass{article}\pagestyle{empty}\begin{document}$ \begin{array}{*{20}c} \sim \\ {\nu _{\max } } \\ \end{array} $\end{document} values with functions of ϵ and n of solvents are found. A comparison of spectral data with the ET scale shows a much poorer correlation. The correlation with the ET values are significantly better if the protic solvents are excluded. The best of the tested relationships was found with the empirical solvent parameters π*. Values of S in the equation \documentclass{article}\pagestyle{empty}\begin{document}$\begin{array}{*{20}c} \sim \\ {\nu _{\max } } \\\end{array} = {\rm S}\pi ^{\rm *} {\rm + }\begin{array}{*{20}c} \sim \\ {\nu 0} \\\end{array}, $\end{document} depend on the dye structure. It is found a relationship between S and the difference of quantum chemical calculated dipol moments in the first exited singlet (μ(S1)) and in the ground state (μ(S0)) of the dyes.
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