Chain folding of carbazole-donor containing polymers via a two-point interaction with naphthalene monoimide-based acceptors

Abstract

Linear polymers carrying electron-rich dialkoxy-carbazole (DACBZ) units linked by flexible oligooxyethylene segments were induced into a zigzag folded state by an external folding agent, which carries two subunits: an electron-deficient dinitro-naphthalene monoimide (NMI(NO₂)₂) acceptor and an ammonium perchlorate unit. The ammonium unit of the folding agent interacts with the backbone oxyethylene segment and, in turn, brings two adjacent CBZ moieties on either side of the electron-deficient NMI(NO₂)₂ unit of the folding agent to induce a charge-transfer (CT) interaction; this two-point interaction was shown to be crucial for the stability of the pleated chain structure. An interesting, and potentially useful, feature of this system is the possibility to incorporate a pendant unit at the carbazole nitrogen (N) site; a variety of segments, such as linear, branched, or chiral alkyl chains, were installed to examine if the folding of the chain is influenced by the nature of the pendant segment. The formation of the pleated structure was studied by ¹H-NMR and UV-visible titration experiments; both these clearly revealed the presence of strong charge transfer (CT) interactions between the donor (D) and acceptor (A) units. The length of the spacer segment linking the NMI(NO₂)₂ acceptor unit and the ammonium group in the folding agent was varied, and it was seen that a 3-carbon spacer yielded the strongest cooperative interaction. To translate the folded conformation into the solid state, a solution of the donor polymer and the most efficient acceptor, taken in a 1 : 1 (D:A) ratio, was cast on a quartz plate. UV-visible studies of the film revealed the retention of the CT band; more importantly, immersing the film into an aqueous NaHCO₃ solution neutralized the ammonium group to generate the free amine. This caused an unexpected deepening of the colour, along with a hypsochromic shift of the CT band, suggesting that the free amine acceptor readjusts within the film to improve the CT interaction, exploiting the newly garnered freedom after de-coordination with the oligooxyethylene segment.