Photodynamics of a PV Trimer in High‐Viscosity Solvents and in PMMA Films: A New Insight into Energy Transfer versus Conformational Relaxation in Conjugated Polymers

Abstract

Non-Stokes-Einstein relaxation: The rate constant of conformational relaxation of a phenylenevinylene trimer (see picture) in different solvents is proportional to eta(-alpha), with alpha values decreasing from close to unity (low viscosity) to zero at sufficiently high solvent viscosity. This behaviour is attributed to the flexible methylbutyl side chains of the trimer, which partially screen the solvent friction. The p-phenylenevinylene (PV) trimer (MBOPV3) was used to probe the effect of conformational relaxation on fluorescence decays of PV-based polymers in the high solvent viscosity regime, from n-hexadecane (3.45 cP at 293 K) to liquid paraffin (123 cP at 293 K), and in dilute poly(methyl methacrylate) (PMMA) solid films. The effect of intermolecular energy transfer and radiative transport on the fluorescence decays was also analysed by increasing the concentration of MBOPV3 in the PMMA films up to a pure MBOPV3 film. The rate constant of conformational relaxation was found to decrease with increasing solvent viscosity up to about 23 cP, but then becomes viscosity independent. The non-Stokes-Einstein behaviour of k(CR) versus eta over the whole viscosity range apparently results from the presence of the flexible methylbutyl side chains of MBOPV3, which partially screens the solvent friction. Conformational relaxation is not observed in very dilute PMMA solid films, where the fluorescence decay becomes single exponential. The decays become multi-exponential again with an increase of MBOPV3 concentration in the PMMA films, but in this case it is due to intermolecular (interchain) energy transfer from less planar to more planar conformations of MBOPV3. In the pure MBOPV3 film, interchain energy transfer (radiative and non-radiative) is the major process responsible for the observed (tetra-exponential) decays.

Photoluminescence of Solvent-Selected Fluorescent Moieties in MEH-PPV Solutions and Films

“…Shorter lifetimes are compatible to those expected for MEH-PPV S 1 -S 0 radiative transition, although longer ones are related to an energy migration that arises from the overlap of emissions from distinct aggregates that were sequentially excited, as stated by Di Paolo et al, 47 but this effect is less effective than the one observed in the chloroform-based films, which presents even longer lifetimes, as showed earlier.…”

“…Therefore, their behavior is bi-exponential. This result is also corroborated by Di Paolo et al 47 findings, in which decays for MEH-PPV trimer are either mono-exponential, bi-exponential or multi-exponential, depending on the emitting moieties that are accessed by selective excitation. Yet, the longer lifetimes they found corroborate our findings as well.…”

“…These short lifetimes, occurring at shorter emission wavelengths, were also observed by Di Paolo et al 47 They attributed it to an energy transfer between distinct aggregates in pure films that occur either by Förster or Dexter mechanisms, since both result in the excited state destabilization. They also performed their studies in a model molecule, the trimer of MEH-PPV repeat units, to show that there are geometrical aspects related to the bi-exponential behavior of the polymer lifetime.…”

“…Lifetimes obtained for chloroformbased films are too long, which could be a result of non-convergence between experimental and calculated decays, since χ 2 parameter is too large, although longer lifetimes of nanoseconds have been already registered by Di Paolo et al 47 In their work, they related the longer lifetimes to the generation of excited MEH-PPV emitting units promoted by energy migration. The authors assume that radiative energy migration triggers successive generations of new excited molecules or aggregates that have their emissions overlapped to such a degree that an apparent longer lifetime is achieved.…”

Distyrylbenzene-based segmented conjugated polymers: Synthesis, thin film morphology and chemosensing of hydrophobic and hydrophilic nitroaromatics in aqueous media

“…The DSB family of chromophores displays excellent photophysical properties and has been the subject of intense research both as molecular materials [29][30][31] and as SCPs with flexible spacers [12,[32][33][34]. However, the DSB groups tend to aggregate making their morphology at the molecular level highly dependent on subtle modifications of their structures.…”