Chain Length Dependent Excited-State Decay Processes of Diluted PF2/6 Solutions

Abstract

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.

Effects of Charge Density on Photophysics and Aggregation Behavior of Anionic Fluorene-Arylene Conjugated Polyelectrolytes

“…This is attributed, at least in part, to conformational relaxation of the conjugated polymer backbone [50,51]. The excited singlet state decay of polyfluorenes depends on the molecular weight of the polymer [52], while the relative amplitude reflects the degree of conformational disordering. The lifetime of this short decay component only varies slightly on going from PBS-PFP to PBS-PFP3, which is in agreement with the three CPEs having similar molecular weights.…”

Unusual photophysical properties of conjugated, alternating indigo–fluorene copolymers

“…3). 48 In the case of the alternating indigo-uorene copolymers, the decay times (in the 1.13-1.30 ns range) are higher than those found for the long-lived component in PF2/6. This behavior should be associated to an initial, instantaneously formed (nonrelaxed) species (120 ps), which is then further transformed into a relaxed species (1.3 ns).…”

“…This behavior should be associated to an initial, instantaneously formed (nonrelaxed) species (120 ps), which is then further transformed into a relaxed species (1.3 ns). 48 In the case of the alternating indigo-uorene copolymers, the decay times (in the 1.13-1.30 ns range) are higher than those found for the long-lived component in PF2/6. Moreover and interesting to note, the increased decay time obtained for the uorene-related emission (relaxed state) remained constant (1.13 ns) upon increasing the copolymer length (going from n ¼ 8 to 11).…”

From molecular modelling to photophysics of neutral oligo- and polyfluorenes incorporated into phospholipid bilayers

“…The shortest component of around 55 ps in the organic solvents (Table 3), with a contribution of less than 5% to the total emission, appears as a rise time (negative amplitude) when the decay is collected in the emission tails (440 and 475 nm). This is the signature of fast conformation relaxations of the PFD backbone 86,87 and is not observed for PFD within the phospholipid bilayer, which indicates that PFD is in a more rigid environment in the liposomes. For most of the systems, the intermediate and the longest components show similar contributions to the total emission, except for PFD in cyclohexane which has a predominant contribution of the longest component (480.4 ps).…”