Weak intermolecular interactions promote blue luminescence of protonated 2,2′-dipyridylamine salts

In this work we demonstrate that protonation and pi-stacking can be exploited to convert non-luminescent 2,2'-dipyridylamine into blue-emitting derivatives. We have synthesized a 'series of luminescent 2,2'-dipyridylamine (dpa) salts, i.e., (dpaH)X center dot nSolv (dpa = 2,2'-dipyridylamine, X = HF2, n = 0.5, Solv = H2O 1; X = Cl, n = 2, Solv = H2O 2; X = Br, n = 2, Solv = H2O 3; X = I n = 1, Solv = H2O 4a; X = I n = 1, Solv = CHCl3 4b), (dpaH)(2)[SiF6]center dot H2O 5 and (dpaH)X (X = l(3) 6; SbF6 7; BF4 8) and characterized their emission properties, both in the solid-state and in solution. To rationalize our observations and relate the luminescence properties to the structure in the solid state and in solution, we have performed ab initio computations and showed that pi-stacking interactions facilitate emission by sterically hindering radiation-less deactivation via a conical intersection. Our results thus not only provide a synthetic approach to cheap blue-emitting materials, but also revealed the essential role of weak intra- and intermolecular interactions, in particular hydrogen bonding and pi-stacking, in the emission process. Our insights therefore provide new concepts for designing dpa-based systems with improved luminescence properties.