“…In the third group, sensing is based on monomer/excimer emission switching (28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38)(39)(40)(41)(42)(43) induced by changes in spatial distances between two pyrene moieties. Moreover, the pyrene and crown ether groups in these types of sensors are normally linked by flexible chains, such as methylene (17)(18)(19)(20)(21)28,29,(44)(45)(46), ether (30,47,48), oligoethylene glycol (31)(32)(33)(34), ester (22), amine (49), amide (25)(26)(27)(35)(36)(37)(38)(39)(40)(41)50), imine (23), triazole (51), sulfoxide (24) and DNA…”