Molecular excitons are central to a variety of optoelectronic applications, including natural and artificial light harvesting, organic electronics, and nanoscale computing. Designing material systems optimized for these applications requires methods that permit control over their photophysical properties, which are governed by their underlying molecular packing. The most common packing arrangements occur when molecules orient in a face-to-face (H-type) or end-to-end (J-type) arrangement; it is very uncommon to pack in a perpendicular (oblique-type) arrangement, although the resulting photophysical properties are highly desirable for certain applications. Here, we demonstrate an approach based on mechanically interlocked molecules to consistently form oblique aggregates in a series of squaraine rotaxane dimers templated using DNA. Although squaraine rotaxanes were originally designed to prevent aggregation, we utilize DNA to bring the molecules into close proximity (i.e., on the nanometer scale) to form delocalized molecular excitons. Leveraging the unique sterics of squaraine rotaxanes, we demonstrate intermediate-level control of packing by consistently forming oblique aggregates rather than the most common packing types. The oblique dimers exhibit near-equal intensity excitonically-split optical transitions, which is desirable for panchromatic absorption and for fundamental studies of quantum phenomena. We demonstrate even finer control over packing by fluorinating the rotaxane macrocycle, which promotes denser packing and stronger excitonic interactions. Lastly, we show that excited-state quenching is still prevalent in the squaraine rotaxane dimer aggregates, and find that careful selection of the chemical substituents enables some control over their excited-state lifetimes.

DNA-templated aggregates of squaraine rotaxanes: Oblique packing and tunable excitonic properties

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