Design of Clickable\nAromatic Donor–Acceptor Pairs for Easy Installation onto\nPolymers

New intrinsically unsymmetric aromatic\ndonors (D) and acceptors\n(A) were designed to simplify the incorporation of a single reactive\ngroup. Naphthalene diimide (NDI) was desymmetrized by replacing one\nof the imide units with two nitro groups to yield 3,6-dinitronaphthalene\nmonoimide (NMI(NO<sub>2</sub>)<sub>2</sub>); likewise, 3,6-dimethoxycarbazole\n(CBZ(OMe)<sub>2</sub>) was designed as a 3-ring aromatic donor. Different\nderivatives of naphthalene monoimide (NMI) and carbazole (CBZ) were\nprepared, and charge-transfer complex formation between them was studied\nusing NMR and UV–visible titrations; the estimated association\nconstants (<i>K</i><sub>a</sub>) were compared with the\ncommon pair, NDI, and 1,5-dialkoxy naphthalene (DAN). To rationalize\nthe variation of <i>K</i><sub>a</sub> values, the electrostatic\npotential maps and the energies of highest-occupied molecular orbital\n(HOMO) and lowest-unoccupied molecular orbital (LUMO) of different\nderivatives were computed by using density functional theory. Some\nof the new D–A pairs outperform the DAN–NDI pair; as\nexpected, the best pair was NMI(NO<sub>2</sub>)<sub>2</sub> and CBZ(OMe)<sub>2</sub>. The <i>K</i><sub>a</sub> for this pair was 50\nM<sup>–1</sup>, which is slightly higher than that of NDI–DAN\nbut with the advantage that a single reactive handle, like azide,\ncan be easily incorporated and used to click them onto supramolecular\nmotifs or polymers. To illustrate this feature, a low <i>T</i><sub>g</sub> copolymer of <i>n</i>-butyl acrylate and propargyl\nacrylate (10 mol %) was clicked with the azido-derivatives of the\nnew donors and acceptors; the influence of the charge-transfer interaction\non the mechanical properties of blends of these copolymers carrying\nD and A units was examined using nanoindentation studies. Additionally,\nthe propargyl acrylate copolymers were also coclicked with both D\nand A units, and their properties were examined as a function of the\ndensity of interacting sites. In summary, this study illustrates an\nalternate design strategy for aromatic donors and acceptors that are\neasier to synthesize, scale-up, and derivatize; importantly, they\ncan be readily installed onto different motifs for the efficient design\nof supramolecular materials and polymers.