Intermolecular Charge Transfer: A Design Motif for the Study of Organic Ferroelectricity, Semiconductivity and Exchange in Supramolecular AssembliesPublic Deposited
Intermolecular charge transfer between electron-rich donor and electron-poor acceptor molecules offer great promise in the development of novel, low-cost electronic materials. It is hypothesized that control over the intermolecular interactions and supramolecular self-assembly of these systems could tune electronic properties and discover new functions. To that end, a series of co-crystals were designed, incorporating a naphthalene electron donor and pyromellitic diimide acceptor molecules, following the paradigm of lock-arm supramolecular ordering (LASO). While these co-crystals traditionally grow with 1:1 association between acceptor and donor molecules, the co-crystal of 1-amino-5-naphthol and diethylene glycol-functionalized pyromellitic diimide grew with a 2:1 acceptor to donor ratio, where the molecules orient themselves for charge transfer in nearly orthogonal face-to-face and edge-to-face mixed stacks. The co-crystals, while crystallographically centrosymmetric gave rise to second harmonic generation, which indicates noncentrosymmetric structure. In addition, they displayed room-temperature ferroelectric polarization, a consequence of electron transfer and hydrogen bonding, along two distinct crystallographic axes. Building on the LASO paradigm, a second series of co-crystals were developed, where the diethylene glycol "arms" were functionalized on the donor instead of the acceptor. While changing the donor chemistry did not change the crystallographic refinement of the co-crystals, it did influence the measured nonlinear optical and electronic properties. Specifically, the co-crystal with 1,5-diaminonaphthalene displayed second harmonic activity and room-temperature ferroelectricity. Reducing the nucleophilicity of the electron donor by switching the functional groups on the donor from amines to ester, on the other hand, neither generated ferroelectric not second harmonic activity. This observation links the strength of the electron donor molecule to the breaking of inversion symmetry and observation of stable ferroelectric polarization. Attempts to make charge transfer complexes more amenable to solution processing saw the development of two series of donor and acceptor molecules, one with complimentary urea and sulfonamide hydrogen bonding groups, and the other with added amino acids. The choice of urea-sulfonamide chemistry promoted heterodimeric charge transfer association between the donor and acceptor molecules upon co-assembly in organic solvents. Casting the co-assemblies onto a film, however, resulted in the phase separation of the two molecules into their respective supramolecular assemblies. Residual charge transfer in the phase separated films, likely due to dopant molecules within bulk supramolecular assemblies, led to an increase in thin-film conductivity by two orders of magnitude when compared against that of separate donor and acceptor films. In addition, the importance of hydrogen bonding in driving long-range supramolecular order and forming conducting pathways between electrodes was established by the lower thin-film conductivities of molecules without urea and sulfonamide functionalities. The second set of solution-based charge transfer complexes, where naphthalene and naphthalene diimide aromatic cores were functionalized with short tetrapeptide sequences, revealed pH-sensitive self-assembly and co-assembly in aqueous media. Having established that fluorescence emission quenching of the self-assembled donor is a consequence of intermolecular charge transfer between the donor and acceptor moieties, quenching was used as a metric for supramolecular exchange between assemblies of donor and acceptor molecules. Despite modulating the molar fraction of donor and acceptor moieties within the supramolecular nanostructures, inter-fiber exchange occurred far faster than that observed between non-interacting supramolecular assemblies. This implies that the driving force for exchange is dominated by charge transfer interactions. In addition, the placement of the aromatic moieties within hydrophobic segments of the amphiphilic molecules implies that charge transfer-mediated exchange requires the dissolution and re-assembly of the supramolecular assemblies to facilitate intermolecular association and inter-fiber exchange.