Design of Clickable Aromatic Donor-Acceptor Pairs for Easy Installation onto Polymers

New intrinsically unsymmetric aromatic donors (D) and acceptors (A) were designed to simplify the incorporation of a single reactive group. Naphthalene diimide (NDI) was desymmetrized by replacing one of the imide units with two nitro groups to yield 3,6-dinitronaphthalene monoimide (NMI(NO₂)₂); likewise, 3,6-dimethoxycarbazole (CBZ(OMe)₂) was designed as a 3-ring aromatic donor. Different derivatives of naphthalene monoimide (NMI) and carbazole (CBZ) were prepared, and charge-transfer complex formation between them was studied using NMR and UV-visible titrations; the estimated association constants (K_a) were compared with the common pair, NDI, and 1,5-dialkoxy naphthalene (DAN). To rationalize the variation of K_a values, the electrostatic potential maps and the energies of highest-occupied molecular orbital (HOMO) and lowest-unoccupied molecular orbital (LUMO) of different derivatives were computed by using density functional theory. Some of the new D-A pairs outperform the DAN-NDI pair; as expected, the best pair was NMI(NO₂)₂ and CBZ(OMe)₂. The K_a for this pair was 50 M^-1, which is slightly higher than that of NDI-DAN but with the advantage that a single reactive handle, like azide, can be easily incorporated and used to click them onto supramolecular motifs or polymers. To illustrate this feature, a low T_g copolymer of n-butyl acrylate and propargyl acrylate (10 mol %) was clicked with the azido-derivatives of the new donors and acceptors; the influence of the charge-transfer interaction on the mechanical properties of blends of these copolymers carrying D and A units was examined using nanoindentation studies. Additionally, the propargyl acrylate copolymers were also coclicked with both D and A units, and their properties were examined as a function of the density of interacting sites. In summary, this study illustrates an alternate design strategy for aromatic donors and acceptors that are easier to synthesize, scale-up, and derivatize; importantly, they can be readily installed onto different motifs for the efficient design of supramolecular materials and polymers.