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

Abstract

Two p-phenylenevinylene (PV) trimers, containing 3'-methylbutyloxyl (in MBOPV3) and 2'-ethylhexyloxyl (in EHOPV3) side chains, are used as model compounds of PV-based conjugated polymers (PPV) with the purpose of clarifying the origin of fast (picosecond time) components observed in the fluorescence decays of poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV). The fluorescence decays of MBOPV3 and EHOPV3 reveal the presence of similar fast components, which are assigned to excited-state conformational relaxation of the initial population of non-planar trimer conformers to lower-energy, more planar conformers. The rate constant of conformational relaxation k(CR) is dependent on solvent viscosity and temperature, according to the empirical relationship k(CR)=aeta(o) (-alpha)exp(-alphaE(eta)/RT), where aeta(o) (-alpha) is the frequency factor, eta(o) is the pre-exponential coefficient of viscosity, E(eta) is the activation energy of viscous flow. The empirical parameter alpha, relating the solvent microscopic friction involved in the conformational change to the macroscopic solvent friction (alpha=1), depends on the side chain. The fast component in the fluorescence decays of MEH-PPV polymers (PPVs), is assigned to resonance energy transfer from short to longer polymer segments. The present results call for revising this assignment/interpretation to account for the occurrence of conformational relaxation, concurrently with energy transfer, in PPVs.

Picosecond Structural Relaxation of Abietic Acid Based Amine End Capped Para‐Phenylenevinylene Trimers in Solution

“…In the case of the corresponding non-endcapped oligomers (MBOPV3 and EHOPV3), the fast components (t 3 ) also appear as decay-time at the onset of the emission spectrum and as rise time at longer wavelength. [11] The intermediate component (t 2 ) has similar values for all the AECPV3 studied in this work (760-790 ps), and represents the largest contribution to the fluorescence decay at all emis- Table 1. Absorption (l abs ) and emission A C H T U N G T R E N N U N G (l em ) wavelength maxima, fluorescence quantum yields (f F ), lifetimes (t F ) and radiative (k F ) and radiationless (k NR ) rate constants of the AECPV3s in toluene at 293 K.…”

“…MBOPV3 438 [11] 493 [11] 0.72 [14] 1.54 [11] 0 [11] 496 [11] 0.71 [14] 1.54 [11] 0 [a] Major component (> 90 %) from triexponential decays (see Figure 3 and Table 2) Figure 2. Normalized absorption spectra of the amines used as end-groups in the AECPV3 studied, in MCH at 293 K, and the fluorescence spectrum of the styryl-phenyl amine, with excitation at 370 nm.…”

“…The photophysical study of MBOPV3 and EHOPV3 [11] has shown that fast conformational relaxation occurs during the early times after excitation (10-100 ps), leading to an increase in the backbone planarization and consequently to a higher…”

Excited State Properties of Oligophenyl and Oligothienyl Swivel Cruciforms

“…By analogy with previously studied p-phenylenevinylene oligomers, this would be consistent with the occurrence of conformational relaxation within the oligomer skeleton. 12 Moreover, an additional observation, which seems to favor this hypothesis, is that with BPH; the rotational change is made within the C-C bond. BPH displays the longest decay time (1704 ps in MCH and ∼1540 ps in 3MP), while comparison with the literature data for p-terphenyl with a decay time of 1.3 ns (1300 ps) 15 strongly suggests that we are now observing the fluorescence lifetime relative to the p-terphenyl structure.…”

Exciton Formation, Relaxation, and Decay in PCDTBT

“…82,83 Those have previously been investigated with oligomeric model systems in solvents of varying viscosity. For example, oligomeric phenylene(ethynylenes) undergo planarization in the excited state.…”