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. , Pålsson, L.-O., Monkman, A., Perepichka, I. I., Perepichka, I. F., Tavasli, M. and Bryce, M. R. (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, 110(39), pp. 19329-19339. (doi: 10.1021/jp0643653) (PMID:17004789)

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.

• Item Type: Articles
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Hedley, Dr Gordon
• Authors: Dias, F. B., Pollock, S., Hedley, G., Pålsson, L.-O., Monkman, A., Perepichka, I. I., Perepichka, I. F., Tavasli, M., and Bryce, M. R.
• College/School: College of Science and Engineering > School of Chemistry
• Journal Name: Journal of Physical Chemistry B
• Publisher: American Chemical Society
• ISSN: 1520-6106
• ISSN (Online): 1520-5207
• Published Online: 02 September 2006