Ionic Organic Microcrystals with Bright Deep-Blue Luminescence and Acid/Base Vapor Response

A new class of high-quality ionic organic microcrystals with bright deep-blue luminescence (with up to 60% quantum yield) have been prepared in situ by the protonation of 2,2′-di(pyrid-2-yl)biphenyl ( 1 ) with different proton acids (HCl, HClO 4, CH 3 SO 3 H, or CF 3 SO 3 H). The obtained rod-shaped microcrystals are characterized by SEM and PXRD analyses. As a result of the deprotonation and reprotonation of the pyridine groups, the microcrystals of 1 (HCl) 2 show reversible blue luminescence switching in response to NEt 3 /HCl vapor. In addition, single crystals of the ionic salts of 1 (HCl) 2, 1 (HClO 4 ) 2, 1 (CH 3 SO 3 H) 2, and 1 (CF 3 SO 3 H) 2 have been obtained. X-ray analyses show that the counteranions are connected to the pyridinium protons by hydrogen bonding.