PHOTOISOMERIZATION OF CYANINES - A COMPARATIVE-STUDY OF OXYGEN-CONTAINING AND SULFUR-CONTAINING SPECIES

We have compared the photoisomerization properties of two cyanine anal ogs, 3,3'-diethyloxadicarbocyanine iodide (DODCI) and 3,3'-diethylthia dicarbocyanine iodide (DTDCI). These two molecules are structurally si milar, differing only in the presence of oxygen or sulfur at two heter oatom sites. Measurement of the radiative and nonradiative population relaxation kinetics of these molecules reveals a difference in their e quilibrium geometries despite their outward similarities. We relate th is difference to the steric constraints imposed by the oxygen and sulf ur heteroatoms and to the occurrence of an excited state barrier predi cted by semiempirical calculations of the ground- and excited-state is omerization surfaces for these molecules.