Effects of annulation on absorption and fluorescence characteristics of fluorescein derivatives: a computational study

Abstract

Semiempirical molecular orbital calculations (AM1 for structures, INDO/S for electronic excitation energies) for fluorescein and various annulated (linear and angular) derivatives are presented and a strong dependence on the mode of condensation is obtained. While the benzo derivatives 7–9 display a small bathochromic shift only, both the linearly and angularly annullated derivatives 10–18 show a rather pronounced bathochromic shift of absorption and fluorescence in both the neutral and the dissociated forms. The longwave absorptions and emissions make these derivatives useful as dyes for measuring intracellular pH because the spectra are distinctly outside the background emission of cells. The longest wavelength absorptions and emissions are predicted for compounds 10–15. The angularly annulated derivatives 7–12 are found to deviate substantially from planarity. For all anions symmetrical structures are calculated to be more stable than unsymmetrical ones. The solvent effect is treated by the SCRF and also by the supermolecule approximation. Calculated structures as well as electronic transition energies (absorption and emission) for the solvated molecules are completely in line with results obtained for the gas phase.