Theoretical study of the relations between structure and photophysical properties of model oligofluorenes with central keto defect.

A systematic study of fluorenone and model oligofluorenes (trimer, pentamer, and heptamer) with a central keto defect was performed at ab initio Hartree-Fock (HF), density functional theory (DFT), configuration interaction singles (CIS), and time-dependent density functional theory (TD-DFT) levels. The main aim of this work was the investigation of the direct influence of the central keto defect on the optimal geometry, torsional potentials, and photophysical properties. From the structural point of view, the optimal all-trans electronic ground state geometries of studied oligomers exhibit a uniform torsion of ca. 44-45 degrees (HF) or 37-38 degrees (DFT). The optical excitation leads to the planarization of the fluorenone and fluorene fragments in the central part of the molecule (approximately 34 degrees for CIS and approximately 29 degrees for TD-DFT). The computed excitation and fluorescence energies show a good agreement with the experiment. These presented theoretical results can be useful in designing novel fluorene-fluorenone optical materials as well as understanding of excitation-relaxation phenomena which may occur in various time-dependent optical experiments.