Molecular Engineering Of Water-Soluble Oligomers To Elucidate Radical Pi-Anion Interactions In N-Doped Nanoscale Objects

Modulating, by design, the electronic properties of nanoscale objects that are water-compatible is at the forefront to develop redox-responsive materials capable of interfacing and probing living organisms. In the present work, we report the synthesis and structural and electronic characterizations of two novel water-soluble naphthalene diimide (NDI) oligomers that differ in the nature of the flexible chemical linkages tethering the pi-conjugated cores. Ground-state electronic absorption spectroscopy experiments, performed as a function of oligomer concentration in an aqueous medium, reveal negligible interactions between oligomer chains but significant interactions among NDI units that comprise a single oligomer chain. Contrasting the monomer controls, which demonstrate well-resolved first and second reduction processes, the NDI oligomers exhibit complex redox processes suggesting the screening of injected charges in the oligomer chains. We show that the flexible chemical linkages offer a synthetic handle to stabilize the first reduction potentials by more than 325 mV when compared to monomer controls. Because the electronic structures of these water-soluble oligomers are characterized by low-lying reduction potentials (E0/- = -0.025 V vs SCE), biologically relevant sacrificial electron donors, sodium ascorbate and sodium hydrosulfide, possess sufficient driving force to incrementally n-dope the NDI oligomers. Further supported by spectroelectrochemical measurements and time-dependent density functional theory (TD-DFT) calculations, the spectroscopic properties of the reduced NDI units indicate the emergence of novel spectroscopic states diagnostic of radical pi-anion interactions. The existence of delocalized electron spin density is unraveled by electron paramagnetic resonance spectroscopy and is shown to be dependent on the strength of the sacrificial electron donors used to n-dope the NDI oligomer chains. Our results not only deliver new insights into the spectroscopic signatures that allow for tracking pi-anion interactions in NDI-derived superstructures but also provide new avenues to tune electron spin properties in water-soluble nanoscale objects.