Revealing the Electron–Phonon Coupling in a Conjugated Polymer by Single‐Molecule Spectroscopy

In recent years p-conjugated polymers have attracted considerable attention because features such as fast and efficient energy-transfer, nonlinear optical behavior, and chemical modification of their spectral characteristics make conjugated polymers promising candidates for the design and development of novel optoelectronic devices with tailored properties, such as organic light-emitting diodes, organic photovoltaic cells, and “plastic” lasers. Their electronic and optical properties are determined to a large extend by the electron– phonon coupling strength, which is a measure for the rigidity of the polymer backbone upon electronic excitation or injection of charge carriers into the polymer chain. This key parameter strongly influences the efficiency of chargeand energy-transfer processes in conjugated polymers, and is therefore of great practical interest for possible applications. However, the pronounced structural disorder, which is an inherent feature in this type of functional materials, as well as strong spectral diffusion processes of the optical transitions have prevented a direct determination of this parameter thus far. Here, we employ single-molecule spectroscopy in combination with pattern recognition techniques, which allows us to retrieve the profile of the electronic spectrum and concomitantly the electron–phonon coupling strength in a methyl-substituted ladder-type poly(para-phenylene) (MeLPPP, see inset Fig. 1). The results indicate a weak electron–phonon coupling at low temperatures, consistent with the fast excitation energy transfer processes that have been observed for this polymer. Conjugated polymers are organic macromolecules that consist of tens to hundreds of covalently bound molecular building blocks that are arranged in a chainlike structure. Their photophysical properties are governed by an extended p-electron system that determines the character of the lowest electronically excited states. However, owing to conformational disorder, kinks, and coiling of the polymer chain as well as the presence of chemical impurities the electronic excitations are not delocalized along the entire polymer backbone. At present the widely accepted picture is that a conjugated polymer can be considered as a chain of segments, each comprising typically 5–10 chemical repeat units. The electronic excitation energy is localized within a segment, which is commonly referred to as chromophore or site, and transferred efficiently between these segments by hopping processes. As a consequence of this, the emission stems only from very few chromophores on a chain. Experimental evidence for this model has been obtained mainly from site-selective fluorescence spectroscopy. Whereas it is consensus that the electronic properties are determined to a large extend by the conformation of the polymer chain, the prediction of these properties for a random chain conformation in a disordered matrix is difficult; however, prediction of such properties is of key importance for C O M M U N IC A TI O N