Relationship between π-conjugation size and electronic absorption spectrum:: Novel π-conjugation size dependence of indoaniline dyes

When the pi-system at the quinone moiety in indoaniline dyes is expanded, the maximum absorption wavelength (lambda(max)) 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 pi-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.