A THEORETICAL-STUDY OF GEOMETRY AND TRANSITION-MOMENT DIRECTIONS OF FLEXIBLE FLUORESCENT-PROBES - ACETOXY DERIVATIVES OF PHENYLANTHRACENE

Recently synthesized acetoxy derivatives of phenylanthracene are highl y fluorescent compounds. They may be used as laser dyes and fluorescen t probes of a biomolecular structure [J.R. Heldt, J. Heldt and K. Alek sandrzak, J. Photochem. Photobiol. A: Chem., 81 (1994) 79]. The ration al application of these compounds in spectroscopic studies, e.g. fluor escence anisotropy measurements, requires detailed knowledge of the ge ometries and the transition moment directions. This contribution prese nts the first systematic INDO/S semiempirical study of the singlet abs orption spectra of the series of the following molecules: anthracene, phenylanthracene, 9-acetoxy-10-(4'-phenyl)anthracene, 9-acetoxy-10-(4' -acetoxyphenyl)anthracene, 9-acetoxy-10-(2'-acetoxyphenyl)anthracene ( C2FAC). We put special emphasis on the calculations of the directions of absorption transition moment to all relevant singlet exited states and their relations to the geometries of the dyes. The geometries of a ll compounds were optimized with the Austin Model 1 (AM1) method and c ompared with the available X-ray data. Additionally, the barrier to ro tation of the acetoxy group attached to the phenyl ring was calculated using the AM1 Hamiltonian. Our results indicate that spectra of deriv atives studied do not depend on the location of the acetoxy groups, bu t the hydrodynamic shape of the C2FAC molecule may be different than t he other phenylanthracenes. The calculated geometries are in reasonabl e agreement with the experimental ones.