Red aqueous room-temperature phosphorescence modulated by anion-pi and intermolecular electronic coupling interactions

Aqueous room temperature phosphorescence (aRTP) from purely organic materials has been intriguing but challenging. In this article, we demonstrated that the red aRTP emission of 2Br-NDI, a water-soluble 4,9-dibromonaphthalene diimide derivative as a chloride salt, could be modulated by anion-pi and intermolecular electronic coupling interactions in water. Specifically, the rarely reported stabilization of anion-pi interactions in water between Cl- and the 2Br-NDI core was experimentally evidenced by an anion-pi induced long-lived emission (lambda(Anion-pi)) of 2Br-NDI, acting as a competitive decay pathway against the intrinsic red aRTP emission (lambda(Phos)) of 2Br-NDI. In the initial expectation of enhancing the aRTP of 2Br-NDI by inclusion complexation with macrocyclic cucurbit[n]urils (CB[n]s, n = 7, 8, 10), we surprisingly found that the exclusion complexation between CB[8] and 2Br-NDI unconventionally endowed the complex with the strongest and longest-lived aRTP due to the strong intermolecular electronic coupling between the n pi* orbit on the carbonyl rims of CB[8] and the pi pi* orbit on 2Br-NDI in water. It is anticipated that these intriguing findings may inspire and expand the exploration of aqueous anion-pi recognition and CB[n]-based aRTP materials.