title: Intramolecular charge transfer assisted by conformational changes in the excited state of fluorene-dibenzothiophene-S,S-dioxide co-oligomers.
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 Deltaf (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 (CT(unrelaxed) --> CT(Relaxed)) can be followed independently. The magnitude of the forward rate constant, k(1)(20 degrees C) approximately 20 x 10(9) s(-1), and the reaction energy barrier, E(a) approximately 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.
authors.affiliation: OEM Research Group, Department of Physics, University of Durham, Durham DH1 3LE, UK. f.m.b.dias@durham.ac.uk
authors.forename: Fernando B
authors.initials: FB
authors.lastname: Dias
authors.forename: Sam
authors.initials: S
authors.lastname: Pollock
authors.forename: Gordon
authors.initials: G
authors.lastname: Hedley
authors.forename: Lars-Olof
authors.initials: LO
authors.lastname: Pålsson
authors.forename: Andy
authors.initials: A
authors.lastname: Monkman
authors.forename: Irene I
authors.initials: II
authors.lastname: Perepichka
authors.forename: Igor F
authors.initials: IF
authors.lastname: Perepichka
authors.forename: Mustafa
authors.initials: M
authors.lastname: Tavasli
authors.forename: Martin R
authors.initials: MR
authors.lastname: Bryce