“…However, the electronic structure of the MCs derivatives was found to be extremely sensitive to influence of substituents in both indoline and pyran rings in these photochromes. By varying the acceptor or donor properties, steric hidrances and positions of the substituents, it is possible to polarize the π-system of the MC form sufficiently to reach a zwitterionic-like structure and observe negative solvatochromism in the push-pull-type dyes, containing electron-donating groups in the indoline and electron-withdrawing ones in the pyran moieties [4,5,10,14,23,26,27,[54][55][56].…”