Effect of a cyclic fragment in a polymethine chain on absorption region and intensity

Effects of a centrally disposed symmetric cyclic bridge on the absorption s pectra of polymethine compounds are discussed using Green's functions in th e vicinity of the Fermi level. Simple expressions are derived which relate bridge-induced changes in the absorption maximum position and the transitio n dipole moment to topological parameters additively contributed by all con stituents of the molecules concerned. As shown, the polymethine compounds c ontaining four-membered cyclic bridge with oxygen substituents (squaraines) should have slightly changed spectral characteristics of the first electro nic transitions as compared to parent unbridged molecules; in contrast, com pounds with the analogous five-membered cycle incorporated into the polymet hine chain (crocoines) are expected to manifest drastically red-shifted abs orption. The trends revealed are corroborated by the experimental data avai lable in the literature. (C) 1999 Elsevier Science B.V. All rights reserved .