Cyclodextrin Insulation Prevents Static Quenching of Conjugated Polymer Fluorescence at the Single Molecule Level

Abstract

Conjugated polymers (CPs) are promising materials for fluorescence imaging application. However, a significant problem in this field is the unexplained abnormally low fluorescence brightness (or number of fluorescence photons detected per one excitation photon) exhibited by most of CP single chains in solid polymer hosts. Here it is shown that this detrimental effect can be fully avoided for short chains of polyfluorene-bis-vinylphenylene (PFBV) embedded in a host polymer matrix of PMMA, if the conjugated backbone is insulated by cyclodextrin rings to form a polyrotaxane (PFBV-Rtx). Fluorescence kinetics and quantum yields are measured for the polymers in liquid solutions, pristine films, and solid PMMA blends. The fluorescence brightness of PFBV-Rtx single chains dispersed in a solid PMMA is very close to that expected for a chain with 100% fluorescence quantum yield, while the unprotected PFBV chains of the same length possess 4 times lower brightness. Despite this, the fluorescence decay kinetics are the same for both polymers, suggesting the presence of static or ultrafast fluorescence quenching in the unprotected polymer. About 80% of an unprotected PFBV chain is estimated to be completely quenched. The hypothesis is that the cyclodextrin rings prevent the quenching by working as 'bumpers' reducing the mechanical forces applied by the host polymer to the conjugated backbone and help retaining its conformational freedom. While providing a recipe for making CP fluorescence bright at the single-molecule level, these results identify a lack of fundamental understanding in the community of the influence of the environment on excited states in conjugated materials.

Improved Spectral Coverage and Fluorescence Quenching in Donor–acceptor Systems Involving Indolo[3‐2‐b]carbazole and Boron‐dipyrromethene or Diketopyrrolopyrrole

“…Furthermore, the values found for the shortest decay time, τ 1 , are subject to a large relative error as they are close to or beyond the time resolution of the setup. One cannot exclude that at short times the decay is faster than the time resolution of our setup (5.0 × 10 −2 ns) and that static quenching occurs . This could also explain the large discrepancy between the values of < k q τ > found for triad 1 and polymer 1 in the two solvents.…”

“…As the way the average fluorescence decay time < τ > is calculated emphasizes long decay times and hence the tail of the distribution of the quenching rate constants, the values of < k q τ > are probably underestimated. For triad 1 and polymer 1, the inability to measure accurately components of the decays with a decay time less than the time resolution of our set‐up for SPT (static quenching) can lead to a further underestimation of < k q τ >. This argument does, however, not hold for polymer 2 where the shortest decay times of 270 and 210 ps in toluene and chloroform, respectively, are clearly much larger than the time resolution of the SPT setup.…”

Single Molecule Studies of a Ladder Type Conjugated Polymer: Vibronic Spectra, Line Widths, and Energy Transfer

“…Obvious reasons are unfavorable transition dipole orientations or absence of ZPLs (due to strong linear electron‐phonon coupling or spectral diffusion), the latter at least hindering the detection of a chromophore. Moreover, in recent single molecule studies of CPs, fluorescence quenching of chromophores along the chain has been discussed . In order to explain these observations, the existence of so‐called “dark” regions within a CP chain has been proposed.…”

Unusually high fluorescence quantum yield of a homopolyfluorenylazomethine – towards a universal fluorophore

“…For example, polyazomethines having high F fl in polar protic solvents would be valuable for use as biological fluorescence probes. [33][34][35] Given the limited examples of highly fluorescent polyazomethines, the solvent dependent fluorescence of the intrinsically fluorescent homopolyazomethine 2 (Chart 1) is herein presented. 31,32 This is in contrast to their F fl E 50% in aprotic organic solvents.…”