THEORETICAL AND EXPERIMENTAL STUDIES OF INDOANILINE DYES - A NOVEL RELATIONSHIP BETWEEN ABSORPTION-SPECTRA AND MOLECULAR-STRUCTURE

The longest absorption band of indoaniline dyes depends on the dihedral angle between the quinone and the aniline rings. The absorption shifts to longer wavelength and the absorption intensity decreases upon increasing the dihedral angle (18-degrees --> 90-degrees) with the loss of planarity. This wavelength change is not consistent with the generally accepted concept that the more planar the molecule, the larger the red shift because of the increased pi-conjugation. The novel spectrum change has been examined by INDO/S and AM1 methods on the basis of experimental results. The change cannot be explained by the HOMO-LUMO energy gap. The configuration interaction plays an essential role, and the imino N atom lone pair to LUMO transition is especially involved. The relationship between the absorption spectrum and the structure has been obtained. This reproduces well the observed spectra and X-ray structure of indoaniline dyes. The detailed features of the dihedral angle dependent wavelength shift have been discussed focusing on the essential role of configuration mixing induced by the structural change. On the basis of these findings, the altering of the dihedral angle for the tuning of the absorption spectrum can be achieved by adjusting substituents, such as the introduction of a bulky substituent or an intramolecular hydrogen bond. The current analysis extends the existing concept for long wavelength absorption (by large pi-conjugation) and provides a new guiding principle for the design of functional dyes. The principle is not limited to the present series of dyes but can be applicable to a wide range of dyes having a nonplanar structure.