Non-covalent assemblies derived from π-conjugated chromophores that stack along a one-dimensional (1D) axis, also named π-conjugated supramolecular polymers (π-SPs), are fragile compositions. Minor perturbations in experimental conditions such as temperature, solvent dielectric, and ionic strength dramatically impact the ground- and excited-state properties of π-SPs. Consequently, the photophysical properties engineered in solution often bear little resemblance to those evidenced in the solid state. This limitation hampers the reliable construction of hierarchical organic superstructures that support long-range exciton transport properties.<br/>Covalent stapling of initially formed π-SPs opens entirely new avenues to rigidify the electro-active cores and enforce strong interactions between repeating units. We will show that stapling π-SPs built from water-soluble perylene diimide units deliver structurally well-defined nanoscale objects equipped with electronic properties that can be tailored via the length of the covalent tether. The use of excitonic coupling as a spectroscopic marker provides a handle to probe the formation of robust tethered assemblies whose ground-state electronic structures are insensitive to changing temperatures (275 to 370 K). Electrochemistry and spectroelectrochemistry experiments that interrogate the potentiometric properties and spectroscopic signatures of n-doped assemblies further confirm the formation of structurally and electronically well-defined nanoscale objects. Additionally, we will show that the excited state properties of stapled π-SPs can be tuned using the structure of the molecular stapler as a synthetic proxy. Photoexcitation of stapled π-SPs generates excited states with mixed Frenkel exciton and charge-transfer character. The charge transfer contributions to these states can be increased by enhancing the structural rigidity of the π-SPs by varying the length of the molecular tethers. This approach provides a potential pathway for separating charges for light-harvesting applications. Furthermore, examining exciton transport properties in tethered assemblies that form hierarchical 1D superstructures in the solid state reveals exciton diffusion coefficients that rival those reported in bulk organic crystals.

How to regulate the excitonic properties of light-harvesting superstructures by tethering π-conjugated supramolecular polymers

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ACS Fall 2022

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