A universal picture of chromophores in π-conjugated polymers derived from single-molecule spectroscopy

Abstract

Single-molecule spectroscopy can provide insight into the fundamental photophysics of large macromolecules containing tens of thousands of carbon atoms by circumventing disorder broadening. We apply this technique to comparatively ordered ladder-type poly(para-phenylene) and highly disordered poly(phenylenevinylene) (PPV), both of which are materials of substantial technological interest. Identical spectroscopic features are observed on the single-chromophore level, independent of the chemical structure or the chain morphology. Both materials exhibit narrow fluorescence lines down to 0.5 nm wide, which we attribute to the single-chromophore zero-phonon line, accompanied by a discrete vibronic progression providing a signature of the chemical structure. The chromophore units display spectral diffusion, giving rise to dynamic disorder on the scale of the linewidth. Although the energetic range of spectral diffusion is small, it can influence intramolecular excitation energy transfer and thus the overall molecular emission. The spectral diffusion dynamics of single chromophores are identical in both material systems and follow a universal Gaussian distribution. In the case of emission from multiple chromophores situated on the molecule, which we observe for PPV, spectral diffusion follows Lorentzian-like statistics. The fundamental difference between the two materials is the possibility of coherent interchromophoric coupling in PPV, resulting in strong spectral broadening caused by aggregation or superradiance. Such behavior is absent in the ladder-type polymers, where the linewidth of the emissive species is identical for all molecules. Our results demonstrate that structure-property correlations in conjugated polymers derive mainly from chain morphology rather than chromophoric properties and should be considered extrinsic in nature.T he diversity of synthetic organic chemistry has opened up effectively endless options for the creation of organic semiconducting materials, revealing a rich spectrum of correlations between chemical structure and physical properties (1). A fundamental understanding of these correlations is vital to the design of novel materials for optoelectronic applications such as light-emitting diodes, solar cells, and photodiodes. Structureproperty relationships can be particularly complex in conjugated polymers (CPs), because the chemical structure not only controls the electronic properties of the -electron systems, but also strongly influences the chain morphology (2, 3). As in proteins, secondary and tertiary structure such as chain folding can be equally if not even more important than the primary chemical structure of the polymer. The chain conformation can have dramatic consequences on the overall electronic and photophysical properties of the molecule as a whole (4, 5). CPs are generally thought of as long chains of more or less isolated conjugated segments, which are referred to as chromophores (6-8). There has been much debate in the literature about the effective conjugation length o...

Excitation Migration along Oligophenylenevinylene-Based Chiral Stacks: Delocalization Effects on Transport Dynamics

“…The causes for this are found to be both intrinsic, i.e., self-localization due to coupling to vibrations, and extrinsic, i.e., the presence of structural or chemical defects. 7,8 Second, on top of this energetic disorder, most conjugated polymers are characterized by a relatively large polydispersity, i.e., the presence of a distribution of physical lengths, leading to the formation of unstructured amorphous films.…”

How Chromophore Shape Determines the Spectroscopy of Phenylene−Vinylenes: Origin of Spectral Broadening in the Absence of Aggregation

“…A wide range of spectroscopic properties, previously associated with interchromophoric interactions, are observed in the isolated chromophore. 9,20,21 Remarkably, the fluorescence linewidth correlates with polarization anisotropy, which provides a direct measure of chromophore conformation. In agreement with recent investigations of the twisting of single biphenyl units, 22 we find that even the shortest oligomers (7mers) can support a curvature of up to ∼80°without loss of conjugation.…”

“…The samples were prepared in a dry nitrogen glovebox to minimize any potential influence of oxygen or moisture on the molecular conformation [7][8]11 and subsequently mounted in a helium cold finger microscope cryostat under a vacuum of 10 -6 mbar and studied using an epifluorescence microscope described elsewhere. 9 Luminescence was excited by an Ar + laser (488 nm, up to 200 W/cm 2 intensity) and detected in 10 s integration windows. The spectra of single chains were recorded from almost diffraction-limited spots of approximately 1 µm in diameter.…”

“…25 The spectral similarity of oligomers and polymers demonstrates that the heptamer is indeed a suitable model system to study the conformation of chromophores in MEH-PPV. We previously linked the broadened spectral features to self-aggregation of multiple chromophores on a single chain, 9,26 following wellestablished assignments from ensemble spectroscopy. 20, 21 The fact that equal spectral signatures are observed in short oligomers and long polymer chains, however, illustrates that spectral broadening in the single chain emission cannot be due to interchromophoric contacts.…”

“…30 One way to test this origin of spectral broadening is to study spectral fluctuations on longer time scales and extrapolate to shorter, experimentally inaccessible timescales. 9 Room-temperature single-molecule imaging of OPVs recently demonstrated that blinking increases in bent molecules with low excitation polarization anisotropy. 33 We confirm this observation and generalize it to the overall spectral dynamics.…”