Chain length dependent excited-state decay processes of diluted PF2/6 solutions

The excited-state dynamics of a series of four poly[2,7-(9,9-bis(2-ethylhexyl)fluorene] fractions, PF2/6, with different chain length (degrees of polymerization DP: 5, 10, 39, and 205) was investigated in dilute solutions by steady-state and time-resolved fluorescence techniques. Two decay components are extracted from time-resolved fluorescence experiments in the picosecond time domain: a chain length dependent, fast decay time (τ(2)) for shorter emission wavelengths (ranging from 30 to 41 ps), which is associated with a rising component at longer wavelengths, and a longer decay time, τ(1) (ranging from 387 to 452 ps). The system was investigated with kinetic formalisms involving (i) a two-state system (A and B) involving conformational relaxation of the initially excited PF2/6 segment (A) under formation of a more planar (B) relaxed state and (ii) a time-dependent red shift of the emission spectrum using the Stokes shift correlation function (SSCF). In the case of (i), the kinetic scheme was solved considering the simultaneous excitation of A and B or only of A, and the rate constants for formation [k′(CR) or k′(CR)(α)], dissociation (k(–CR)), and deactivation (k(B)(*)) were obtained together with the fraction of species A and B present in the ground state. The use of the SSCF in (ii) was found to be more adequate leading to a decay law with a 3.4 ps component (associated with the slow part of the solvation dynamics process) and a longer decay (43.3 ps) associated with the conformational/torsional relaxation process with a rate constant k(CR). This longer component of the SSCF was found to be identical to the short-living decay (τ(2)) component of the biexponential decays, displaying an Arrhenius-type behavior with activation energy values in the range 5.8–8.9 kJ mol(–1) in toluene and 6.5–10.7 kJ mol(–1) in decalin. From the dependence of the fast decay component (k(CR) ≡ 1/τ(2)) on solvent viscosity and temperature, the activation energy for the conformational relaxation process was found to be distinctly dependent on the chain length, with the relaxation rate dependence with the solvent viscosity (k(CR) ≈ η(– γ)) displaying γ = 1 for the oligomer fraction with DP = 5 (i.e., k(CR) is associated with a pure diffusion-controlled process) and γ < 1 for the higher molecular weight PF2/6 fractions (with DP = 10, 39, 205). This happens because of a decreased conformational barrier between nonrelaxed and relaxed states promoted by the polymer skeleton.