Real-time observation of conformational switching in single conjugated polymer chains

Continuous Illumination of a Conjugated Polymer Causes Strong Enhancement of Photoluminescence

We present measurements of absorbance and photoluminescence (PL) for films of poly(3-(2,5-dioctylphenyl)thiophene) (PDOPT) as a function of temperature (T) and time (t) of illumination. While having no detectable influence on absorbance of this conjugated polymer, our experiments clearly revealed that illumination of PDOPT caused a significant increase in the PL intensity (I_PL(T,t)), that is, the emission probability of PDOPT. Without illumination, we always observed a decrease in I_PL with time. An increase in I_PL was only detectable when the sample was illuminated. Interestingly, while absorption and emission of photons occur on a time scale of nanoseconds, the here-reported changes in the emission probability were slow and occurred on a time scale of minutes to hours. The influence of illumination on changes in I_PL(T,t) was qualitatively similar for slowly and rapidly crystallized PDOPT, that is, the degree of crystallinity was not decisive for the observation. The rate of the increase in I_PL depended clearly on the power of the illumination light source. As a function of the illumination time, the change in I_PL(T,t) was nonmonotonic and depended on sample temperature. We speculate that changes in polymer interactions caused by excited electronic states might have induced slow changes in polymer conformations.

Real-time detection of single-molecule reaction by plasmon-enhanced spectroscopy

Determining structural transformations of single molecules (SMs) is an important fundamental scientific endeavor. Optical spectroscopies are the dominant tools used to unravel the physical and chemical features of individual molecules and have substantially contributed to surface science and biotechnology. In particular, Raman spectroscopy can identify reaction intermediates and reveal underlying reaction mechanisms; however, SM Raman experiments are subject to intrinsically weak signal intensities and considerable signal attenuation within the spectral dispersion systems of the spectrometer. Here, to monitor the structural transformation of an SM on the millisecond time scale, a plasmonic nanocavity substrate has been used to enable Raman vibrational and fluorescence spectral signals to be simultaneously collected and correlated, which thus allows a detection of photo-induced bond cleavage between the xanthene and phenyl group of a single rhodamine B isothiocyanate molecule in real time. This technique provides a novel method for investigating light-matter interactions and chemical reactions at the SM level.

Excitonic Processes in a Conjugated Polyelectrolyte Complex

In aqueous solution, a di-sulfonated phenylenevinylene polymer (DPS-PPV) forms a complex with non-ionic poly(vinyl alcohol) (PVA) leading to absorption spectroscopic shifts and a dramatic (6-fold) increase in DPS-PPV fluorescence intensity. Spectroscopic investigations demonstrate that the complexation with PVA and other neutral polymers results in conformational changes in the DPS-PPV chains that lead to the removal of non-fluorescent energy traps and results in the observed increase in fluorescence in the bulk solution. Single molecule fluorescence measurements of DPS-PPV chains dispersed on glass and in PVA films confirm that efficient exciton energy transfer occurs within each photo-excited DPS-PPV chain and that the observed increase in fluorescence intensity in the PVA film environment is also associated with fewer quenching sites. The results highlight the importance of conjugated polyelectrolyte conformation on exciton relaxation pathways.

Gel Trapping Enables Optical Spectroscopy of Single Solvated Conjugated Polymers in Equilibrium

Single-molecule studies have provided a wealth of insight into the photophysics of conjugated polymers in the solid and desolvated state. Desolvating conjugated chains, e.g., by their embedding in inert solid matrices, invariably leads to chain collapse and the formation of intermolecular aggregates, which have a pronounced effect on their properties. By contrast, the luminescent properties of individual semiconducting polymers in their solvated and thermodynamic state remain largely unexplored. In this paper, we demonstrate a versatile gel trapping technique that enables the chemistry-free immobilization and interrogation of individual conjugated macromolecules, which retain a fully equilibrated conformation by contrast to conventional solid-state immobilization methods. We show how the technique can be used to record full luminescence spectra of single chains, to evaluate their time-resolved fluorescence, and to probe their photodynamics. Finally, we explore how the photophysics of different conjugated polymers is strongly affected by desolvation and chain collapse.

Homo-FRET in π-Conjugated Polygons: Intermediate-Strength Dipole-Dipole Coupling Makes Energy Transfer Reversible

The concept of homo-FRET is often used to describe energy transfer between like chromophores of molecular aggregates such as in π-conjugated polymers. Homo-FRET is revealed by a dynamic depolarization in fluorescence but strictly only applies to the limit of weak dipole-dipole coupling, where energy transfer occurs on time scales much longer than those of nuclear relaxation. By considering the polarization anisotropy of photoluminescence emission and excitation of model multichromophoric aggregates on the single-molecule level, we demonstrate the transition of energy-transfer dynamics from the case of weak coupling to that of strong coupling, revealing the elusive regime of intermediate-strength coupling where energy transfer between degenerate donor and acceptor chromophores becomes reversible so that information on the excitation route of the emitting chromophore is lost.

Chain length-dependent luminescence in acceptor-doped conjugated polymers

Semiconducting polymers doped with a minority fraction of energy transfer acceptors feature a sensitive coupling between chain conformation and fluorescence emission, that can be harnessed for advanced solution-based molecular sensing and diagnostics. While it is known that chain length strongly affects chain conformation, and its response to external cues, the effects of chain length on the emission patterns in chromophore-doped conjugated polymers remains incompletely understood. In this paper, we explore chain-length dependent emission in two different acceptor-doped polyfluorenes. We show how the binomial distribution of acceptor incorporation, during the probabilistic polycondensation reaction, creates a strong chain-length dependency in the optical properties of this class of luminescent polymers. In addition, we also find that the intrachain exciton migration rate is chain-length dependent, giving rise to additional complexity. Both effects combined, make for the need to develop sensoric conjugated polymers of improved monodispersity and chemical homogeneity, to improve the accuracy of conjugated polymer based diagnostic approaches.

Self-assembly of donor-acceptor conjugated polymers induced by miscible 'poor' solvents

The solution-phase self-assembly of donor-acceptor conjugated polymer (DACP) poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl))thieno[3,2b]thiophene] (DPPDTT), is demonstrated and investigated from binary solvent mixtures. It is found that the polarity of a miscible 'poor' solvent (e.g. methanol, dimethyl sulfoxide), which is added to a stable polymer solution in chloroform (i.e. 'good' solvent), strongly affects the resulting nanostructure. Nanoribbons are formed by the addition of certain polar (e.g. methanol) 'poor' solvents to the mixture, while amorphous aggregates are formed upon addition of non-polar 'poor' solvent, such as n-hexane. Atomic force microscopy (AFM), scanning transmission electron microscopy (sTEM) and small angle neutron scattering (SANS) are used to characterize the shape and size of the nanostructures. Experiments show complex self-assembly in solution occurs for DACPs when compared to conjugated homopolymers. SANS results also provide quantitative analysis of DACP conformations in solution before self-assembly occurs. The addition of different polar 'poor' solvents could also alter the size of the assembled nanostructures, as well as the fraction of polymers that self-assemble. The surface orientation and the crystal structure of the nanostructures is also probed by grazing-incidence wide-angle X-ray scattering (GIWAXS). Organic field effect transistors (OFETs) are used to characterize charge transport properties for nanoribbons where enhancement of the average hole mobility is observed.

Fluctuations in the Emission Polarization and Spectrum in Single Chains of a Common Conjugated Polymer for Organic Photovoltaics

Measuring the nanoscale organization of conjugated polymer chains used in organic photovoltaic (OPV) blends is vital if one wants to understand the materials. This is made very difficult with high efficiency OPV polymers such as PTB7 that form aggregates, as a lack of periodicity and a high degree of disorder make understanding of the nanoscale organization challenging. Here, single molecule spectroscopy is used to observe single chains and aggregates of PTB7. Using four detectors the photoluminescence intensity, wavelength, polarization, and lifetime are simultaneously monitored. Fast (milliseconds) and slow (seconds) fluctuations are observed over a time window of 30 s in all of these observables from single aggregates and chains as individual chromophores activate and deactivate, leading to dynamical changes in the emission spectrum and dipole orientation. This information can be used to help reconstruct the spatial and spectral organization of disordered aggregates of PTB7, thereby adding valuable new information on how the chains are arranged in space.

Effect of Spacer Length and Solvent on the Concentration-Driven Aggregation of Cationic Hydrogen-Bonding Donor Polythiophenes

Aggregation of cationic isothiouronium polythiophenes with alkoxy-spacers of different lengths at the 3-position of the thiophene ring was studied in solvents of different polarities. Hydrogen-bonding capacity was assessed by steady-state absorption and fluorescence spectroscopy, whereas the aggregation in aqueous solutions was studied by electron paramagnetic resonance spectroscopy, using paramagnetic probes of different polarities. The two polymers displayed similar features in respect to conformation, effect of cosolvents on aggregation, unstructured absorption-fluorescence spectra, Stokes shifts when aggregated, solvatochromic effect, and self-quenching concentration. However, these polymers also showed different specific interactions with water, Stokes shifts in water, effect of the solvent on the extent of dominant state of the S1 level, and also different inner cavities and hydrophobic-hydrophilic surface area in aqueous solution aggregates. Water maximized the difference between the polymers concerning the effect of specific increases in concentration, whereas the presence of 1,4-dioxane generated almost identical effects on both polymers.