Naked-eye repeatable off-on-off and on-off-on switching luminescence of copper(i)-1H-imidazo[4,5-f][1,10]phenanthroline complexes with reversible acid-base responses.

Considerable attention has been focused on Cu(i)-diimine-phosphine complexes because of their good room-temperature phosphorescence since 1979, but no attention has been focused on the exploration and exploitation by using them as smart acid-base responsive switching luminescent materials. Herein, the reaction of [Cu(MeCN)4]PF6, 2-(2-naphthyl)-1H-imidazo[4,5-f][1,10]phenanthroline (nimpH) and chelating diphosphine ligands bis[(2-diphenylphosphino)phenyl] ether (POP) and 9,9-dimethyl-4,5-bis(diphenylphosphino)-9H-xanthene (xantphos) or (±)-2,2'-bis(diphenylphosphino)-1,1'-binaphthalene (BINAP) in a 1 : 1 : 1 ratio afforded three ionic [Cu(nimpH)(PP)]PF6 complexes (PP = POP, 1a; xantphos, 2a; BINAP, 3a) and three neutral [Cu(nimp)(PP)] complexes (PP = POP, 1b; xantphos, 2b; BINAP, 3b) in neutral and strong alkaline media, respectively. The non-emissive complexes 1a, 2a and 3a exhibit turn-on luminescence upon exposure to the strong base NaOH and tetrabutylammonium fluoride (TBAF), and the intensely emissive complexes 1b, 2b and 3b display turn-off luminescence upon exposure to trifluoroacetic acid (TFA) in CH2Cl2 solutions. Naked-eye repeatable off-on-off or on-off-on luminescence behaviors of these complexes are observed in CH2Cl2 with reversible base-acid or acid-base responses. On the other hand, the addition of TBAF to neutral complexes 1b, 2b and 3b results in obvious luminescence enhancement, especially for the diluted solutions and complexes with the flexible diphosphine ligand POP. The plausible mechanisms of the above results are discussed.