Theoretical Study on Acylacetanilide Azomethine Dyes : A Relationship between Electronic Absorption Properties and Molecular Structures

The relationship between visual absorption properties and molecular structures was investigated for a series of yellow azomethine dyes derived from 4-diethylamino-2-methylaniline and 2-acyl-2'-chloroacetanilides, in whichthe acyl groups are 1-indolinylcarbonyl (IA), 1-methylcyclopropylcarbonyl (CA), acetyl, benzoyl, and pivaloyl (PA). DFT calculations predicted major conformational isomers attributed to deviations from the planarity of the p-aminophenyl-azomethine fragment. Observed wavelengths were well reproduced by TD-DFT calculations at global minimum geometries, whereas the intensities correlate with the Boltzmann-weighted oscillator strengths. The observed intensities varied by 1.5 times from minimum to maximum without significant shifts in the peak wavelengths, demonstrating the largest absorption of IA and the distinct increase of absorption by changing the acyl group from open- (PA) to closed-ring (CA). The different acyl groups nearly perpendicular to the azomethine fragment in global minima alter frontier orbital energies, while leaving HOMO-LUMO energy gaps almost unchanged. The acyl groups, which are twisted largely in local minima, exert mesomeric effects to lower the LUMO energies. The planarity of the p-aminophenyl-azomethine fragment affects HOMO-LUMO overlapping, but hardly changes HOMO and LUMO energies. These electronic and steric effects rationalize the absorption intensities and peak wavelengths observed for the dyes and also the sharp-cut curves of spectra in the long-wavelength sides for IA and CA.