Temporal fluctuations in excimer-like interactions between pi-conjugated chromophores

TL;DR

This study investigates how fluctuations in molecular spacing and chromophore positioning affect excimer-like interactions in conjugated polymers, revealing intrinsic dynamic variations at the single-molecule level.

Contribution

It demonstrates that excimer-like interactions are highly sensitive to sub-Angstrom fluctuations, providing new insights into the dynamic nature of chromophore coupling in molecular solids.

Findings

Spectral shifts and decay rates correlate with coupling strength.

Significant scatter indicates a wide distribution of interaction strengths.

Intrinsic dynamics cause time-dependent spectral variations.

Abstract

Inter- or intramolecular coupling processes between chromophores such as excimer formation or H- and J-aggregation are crucial to describing the photophysics of closely packed films of conjugated polymers. Such coupling is highly distance dependent, and should be sensitive to both fluctuations in the spacing between chromophores as well as the actual position on the chromophore where the exciton localizes. Single-molecule spectroscopy reveals these intrinsic fluctuations in well-defined bi-chromophoric model systems of cofacial oligomers. Signatures of interchromophoric interactions in the excited state - spectral red-shifting and broadening, and a slowing of photoluminescence decay - correlate with each other but scatter strongly between single molecules, implying an extraordinary distribution in coupling strengths. Furthermore, these excimer-like spectral fingerprints vary with time, revealing intrinsic dynamics in the coupling strength within one single dimer molecule, which constitutes the starting point for describing a molecular solid. Such spectral sensitivity to sub-Angstrom molecular dynamics could prove complementary to conventional FRET-based molecular rulers.