Naphthalene Diimide-Embedded Donor-Acceptor Carbon Nanohoops: Photophysical, Photoconductive and Charge Transport Properties

Designing the architecture of donor−acceptor (D−A) pairs is an effective strategy to tailor the electronic structure of conjugated macrocycles for optoelectronic devices. Herein, we present the synthesis of three D−A nanohoops NDI-[n]CPP (n=7, 8, 9) containing naphthalene diimide (NDI) unit as an acceptor and [n]cycloparaphenylenes ([n]CPPs) moieties as donors. The D−A characteristics of NDI-[7~9]CPPs were substantiated through absorption and fluorescence spectroscopic studies, electrochemical investigations and computational analysis. The device investigations demonstrated that the D-A nanohoops NDI-[7~9]CPPs can serve as the photoconductive layer and demonstrate a significant generation of photocurrent with fast response upon exposure to light. The magnitude of photocurrent shows highly dependence on the size of their rings, with an increasing trendency as the ring size decreases. The generation of photocurrent in free acceptor-based CPP has rarely been reported in the previous literatures. Significantly, the C60 complexes of NDI-[7~9]CPPs exhibited a marked enhancement in photocurrent under identical conditions, in particular, the photocurrent of C60NDI-[7]CPP is ca 3.5 times greater than that of NDI-[7]CPP alone. Furthermore, the potential applications of NDI-[7~9]CPPs in electron- and hole-transport devices have been also explored, revealing the clear evidence of their bipolar behavior as an active charge transport layer.