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The absorption spectra of 23 indoaniline dyes were calculated on the AM1-optimized structures by means of the INDO/S method. The observed and calculated absorption wavelengths showed good correlation for the main (longest wavelength) absorption bands (observed at 560-670 nm), and there was fair correlation between the observed molar extinction coefficients and the calculated oscillator strengths. The observed absorption wavelengths could be reproduced by the calculation for the subabsorption bands near the visible region (observed at 320-380 nm). A qualitative correlation of the solvatochromic shift with the difference between ground and excited state dipole moments was obtained.

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When the π-system at the quinone moiety in indoaniline dyes is expanded, the maximum absorption wavelength (λmax) of the main band in the absorption spectrum shows a shift to shorter wavelength (hypsochromic shift). This behavior stands in contrast to the generally accepted rule that expansion of a π-system leads to a bathochromic shift in the absorption spectrum, and the behavior was therefore investigated using the INDO/S method. It could be shown that the present systems are multicomponent systems, in which highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are located on different parts of the molecule. Therefore, only the LUMO rises in energy when the quinone system expands, thus leading to a widening of the HOMO−LUMO energy gap.

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