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.

Kinetics of thermal-assisted delayed fluorescence in blue organic emitters with large singlet–triplet energy gap

“…there is no well-defined temperature above which the delayed fluorescence intensity suddenly increases, as is observed in TADF [8]. Moreover, the energy barrier determined by following the temperature dependence of the delayed fluorescence is 0.16 ± 0.02 eV and is in excellent agreement with the energy barrier for viscous flow in ethanol (0.15 eV) [21], clearly showing that the origin of the delayed luminescence is a diffusion-controlled mechanism. Interestingly, in (2) the delayed fluorescence spectrum does not match the steady-state fluorescence ( figure 6a,b).…”

“…These materials are required for emission in the blue region, where the TADF materials formed by relatively strong electron donor-electron acceptor units are more difficult to work, owing to the large Stokes-shift induced by the excited state charge transfer [21]. In fact, most TADF materials emit in the green region, and obtaining efficient TADF in the blue region is a very difficult task, which has not been fully achieved so far [20].…”

Exciton Diffusion in Polyfluorene Copolymer Thin Films: Kinetics, Energy Disorder and Thermally Assisted Hopping

“…The PFS 30 emission has a charge transfer (CT) character, [15,22] and is strongly affected by inhomogeneous broadening at 290 K. With decreasing temperature, small local molecular reorientations due to dipole-dipole interactions between the excited chromophore and surrounding molecules become increasingly more hindered and emission starts to show the typical vibrational modes of polyfluorene derivatives. in Figure 8 (middle), increases with decreasing temperature.…”

Conformational Relaxation of p‐Phenylenevinylene Trimers in Solution Studied by Picosecond Time‐Resolved Fluorescence

“…[4] A similar interpretation was given by other authors for the occurrence of fast decay/rise times observed with an oligo(p-phenylenevinylene), [5] and of fluorene-dibenzothiophene-S,S-dioxide co-oligomers. [6] A more common and alternative interpretation of similar observations in polymer decays has been based on resonant energy migration from higher energy (short length planar polymer segments) to lower energy polymer segments. [7] Alternative interpretations, also based on the biexponential nature of the decays, claim that Fçrster energy transfer dominates the dynamics of the decay process in MEH-PPV both in solution and solid state, but with interaction between different chains.…”