A theoretical study of geometry and transition moment directions of flexible fluorescent probes--acetoxy derivatives of phenylanthracene

Abstract

Recently synthesized acetoxy derivatives of phenylanthracene are highly fluorescent compounds. They may be used as laser dyes and fluorescent probes of a biomolecular structure [J.R. Heldt, J. Heldt and K. Aleksandrzak, J. Photochem. Photobiol. A: Chem., 81 (1994) 79]. The rational application of these compounds in spectroscopic studies, e.g. fluorescence anisotropy measurements, requires detailed knowledge of the geometries and the transition moment directions. This contribution presents the first systematic INDO/S semiempirical study of the singlet absorption 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 all compounds were optimized with the Austin Model 1 (AM1) method and compared with the available X-ray data. Additionally, the barrier to rotation of the acetoxy group attached to the phenyl ring was calculated using the AM1 Hamiltonian. Our results indicate that spectra of derivatives studied do not depend on the location of the acetoxy groups, but the hydrodynamic shape of the C2FAC molecule may be different than the other phenylanthracenes. The calculated geometries are in reasonable agreement with the experimental ones.