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.