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Unorthodox Combination of Cation−π and Charge-Transfer Interactions within a Donor–Acceptor Pair

Pramanik, Bapan, Ahmed, Sahnawaz, Singha, Nilotpal, Das, Basab Kanti, Dowari, Payel, Das, Debapratim
Langmuir 2018 v.35 no.2 pp. 478-488
density functional theory, hydrogels, nuclear magnetic resonance spectroscopy, temperature
Cation−π and charge-transfer (CT) interactions are ubiquitous in nature and involved in several biological processes. Although the origin of both the interactions in isolated pairs has extensively been studied, CT interactions are more prominent in supramolecular chemistry. Involvement of cation−π interactions in the preparation of advanced functional soft materials is uncommon. Moreover, a combination of these two interactions within a pair of electron donor (D) and acceptor (A) is uncharted. Here, we present a rational design to incorporate a combination of these two interactions within a D–A pair. A pyrene–peptide conjugate exhibits a combination of cation−π and CT interactions with a cationic naphthalenediimide (NDI) molecule in water. Nuclear Overhauser effect spectroscopy NMR along with other techniques and density functional theory calculations reveal the involvement of these interactions. The π-planes of pyrene and NDI adopt an angle of 56° to satisfy both the interactions, whereas β-sheet formation by the peptide sequence facilitates self-assembly. Notably, the binary system forms a self-supporting hydrogel at a higher concentration. The hydrogel shows efficient self-healing and injectable property. The hydrogel retains its thixotropic nature even at an elevated temperature. Broadly, we demonstrate a pathway that should prove pertinent to various areas, ranging from understanding biological assembly to peptide-based functional soft materials.