Guest-dependent folding and tunable charge transfer of an NDI-pentamer

Abstract

We present a covalently linked, electroactive, conformationally flexible naphthalene diimide (NDI) pentamer exhibiting unprecedented 10-electron reduction, solvent-dependent folding, and guest-induced tunable charge-transfer (CT). In dichloromethane and chloroform, the oligomer adopts a partially folded state, displaying bright excimer emission at 500 nm. The addition of methyl cyclohexane promotes ordered, long-range folding, thereby enhancing excimer emission. Conversely, the addition of toluene causes folding but quenches the excimer emission while inducing a distinct CT emission. Furthermore, various polyaromatic donors such as anthracene, triphenylene, pyrene, perylene, etc., promote folding by intercalating between NDI layers, forming a long-range alternate donor-acceptor stack with tunable CT absorption. The estimation of the association constant (K_a) revealed that the strength of CT complexation depends on the donor ability of the guests, with perylene showing the highest association constant. Interestingly, sequential addition of two different donors produces dual CT bands, resulting in broad visible light absorption. This study demonstrates a rare example of guest-dependent tunable CT-driven folding of a flexible multidecker NDI system, offering new insights into the guest-mediated conformational control of foldamer, and an approach towards ordered π-stacked materials.