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.

Citation

Dias, F., Pollock, S., Hedley, G., Palsson, L., Monkman, A., Perepichka, I., …Bryce, M. (2006). Intramolecular charge transfer assisted by conformational changes in the excited state of fluorene-dibenzothiophene-S,S-dioxide co-oligomers. Journal of Physical Chemistry B (Soft Condensed Matter and Biophysical Chemistry), 110(39), 19329-19339. https://doi.org/10.1021/jp0643653

• 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