Temporal Fluctuations in Excimer-Like Interactions between π-Conjugated Chromophores.

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 bichromophoric 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-Ångström molecular dynamics could prove complementary to conventional FRET-based molecular rulers.