Architecturally diverse nanostructured foldamers reveal insightful photoinduced single-molecule dynamics

Single molecule fluorescence spectroscopy has been used to probe architecturally diverse and unique model oligomers containing exactly two or four perylene tetracarboxylic diimide (PTDI) units: linear foldamers lin2 and lin4, monocyclic complement cyc2, and concatenated foldable rings cat4. Linear, cyclic, and concatenated foldamers reveal that photoabsorption and excitation induces unfolding and refolding, generating colorful spectral switching from one spectral type to another. Foldamer architectures dictate the unfolding and refolding processes, and hence the spectral dynamics. As a result, linear tetramer exhibits active frame-to-frame spectral switching accompanying dramatic changes in colors, but a concatenated tetramer displays a multicolored composite spectrum with little or no spectral switching. Excited state dynamics causes spectral switching: an electronically decoupled PTDI monomer emits green fluorescence while electronically coupled PTDI pi-stacks emit red fluorescence, with longer pi-stacks emitting redder fluorescence. A key question we address is the excited-state delocalization length, or the exciton coherence length, in the pi-stacks, which has been proven difficult to measure directly. Using foldamers having controlled sequences, structures, and well-defined length and chromophore numbers, we have mapped out the exciton coherence length in pi-stacks. Single molecule fluorescence studies on chromophoric foldamers reveal that the maximum domain length is delocalized across just four pi-stacked PTDI dyes and no new pure color can be found for oligomers beyond the tetramer. Therefore, the range of fluorescent colors in pi-stacks is a function of the number of chromophores only up to the tetramer.