The visible absorption spectra of a series of Z and E hemi-indigo photochromic compounds have been evaluated by using a time-dependent density functional theory approach, explicitly taking into account bulk solvent effects. The theory reproduces the major experimental features, with, in particular a mean absolute error limited to 13.2 nm for the longest wavelength of maximal absorption (lambda(max)). In order to maximize the difference of lambda(max) between the two conformers, as well as the absorption probabilities, new structures are proposed. (c) 2007 Elsevier B.V. All rights reserved.