Intramolecular charge transfer assisted by conformational changes in the excited state of fluorene-dibenzothiophene-S,S-dioxide co-oligomers

Dias, F.B.; Pollock, S.; Hedley, G.; Palsson, L.O.; Monkman, A.; Perepichka, I.I.; Perepichka, I.F.; Tavasli, M.; Bryce, M.R.

Abstract

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, k1(20 °C) ≈ 20 × 109 s-1, and the reaction energy barrier, Ea ≈ 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.

• Journal Article Type: Article
• Publication Date: 2006-10
• Journal: Journal of Physical Chemistry B (Soft Condensed Matter and Biophysical Chemistry)
• Print ISSN: 1520-6106
• Electronic ISSN: 1520-5207
• Publisher: American Chemical Society
• Peer Reviewed: Peer Reviewed
• Volume: 110
• Issue: 39
• Pages: 19329-19339
• DOI: https://doi.org/10.1021/jp0643653
• Keywords: FLUORENE-BASED COPOLYMERS; LIGHT-EMITTING DEVICES; WAVELENGTH ABSORPTION-EDGE; ELECTRON-ACCEPTING UNIT; CONJUGATED POLYMERS; OPTICAL-PROPERTIES; DUAL FLUORESCENCE; POLYFLUORENE COPOLYMERS; TEMPERATURE-DEPENDENCE; MOBILITY ENHANCEMENT
• Public URL: https://durham-repository.worktribe.com/output/1536392