Intramolecular Charge Transfer Assisted by Conformational Changes in the Excited State of Fluorene-dibenzothiophene-S,S-dioxide Co-oligomers

• Published in: The journal of physical chemistry. B Vol. 110; no. 39; pp. 19329 - 19339
• Main Authors: Dias, Fernando B, Pollock, Sam, Hedley, Gordon, Pålsson, Lars-Olof, Monkman, Andy, Perepichka, Irene I, Perepichka, Igor F, Tavasli, Mustafa, Bryce, Martin R
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 05.10.2006
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp0643653

The strong solvatochromism observed for two fluorene-dibenzothiophene-S,S-dioxide oligomers in polar solvents has been investigated using steady-state and time-resolved fluorescence techniques. A low-energy absorption band, attributed to a charge-transfer (CT) state, is identified by its red shift with increasing solvent polarity. In nonpolar solvents, the emission of these conjugated luminescent oligomers shows narrow and well-resolved features, suggesting that the emission comes from a local excited state (LE), by analogy to their conjugated fluorene-based polymer counterparts. However, in polar solvents, only a featureless broad emission is observed at longer wavelengths (CT emission). A linear correlation between the energy maximum of the fluorescence emission and the solvent orientation polarizability factor Δf (Lippert−Mataga equation) is observed through a large range of solvents. In ethanol, below 230 K, the emission spectra of both oligomers show dual fluorescence (LE-like and CT) with the observation of a red-edge excitation effect. The stabilization of the CT emissive state by solvent polarity is accompanied/followed by structural changes to adapt the molecular structure to the new electronic density distribution. In ethanol, above 220 K, the solvent reorganization occurs on a faster time scale (less than 10 ps at 290 K), and the structural relaxation of the molecule (CTunrelaxed → CTRelaxed) can be followed independently. The magnitude of the forward rate constant, k 1(20 °C) ≈ 20 × 109 s-1, and the reaction energy barrier, E a ≈ 3.9 kcal mol-1, close to the energy barrier for viscous flow in ethanol (3.54 kcal mol-1), show that large-amplitude molecular motions are present in the stabilization of the CT state.