Intramolecular Charge Transfer, Studied by Electrochromism of Organic Molecules in Polymer Matrices

Abstract

The ground state dipole moment. , and the change in dipole moment upon excitation, , of various dyes have been determined from electrochromic measurements on solutions in polymer matrices. The merits of this method are discussed. High values of , much in excess of the sum of local group moments, are found for compounds in which a polar ground state is stabilized by large differences in electronegativity of the constituting groups (“Zwitterions”) and/or by a gain in resonance energy accompanying a charge transfer in π-electron ring systems. High values of , indicating a large charge transfer upon excitation, are connected with a loss of resonance energy. The amount of charge, transferred upon excitation from the donor group of the dye to the acceptor group, does not decrease significantly if the geometric distance between these groups is increased by inserting enthynylenic groups.