ORIGIN OF THE HIGH-ENERGY EMISSION OF 1,1'-DIETHYL-2,2'-CYANINE DYE

In addition to the yellow emission ascribed to the J aggregates of 1,1 '-diethyl-2,2'-cyanine, this compound also exhibits a high-energy emis sion around 400 nm in dilute aqueous solutions (c<10(-3) mol 1(-1)). I t was found using time-resolved emission measurements that protonation of 1,1'-diethyl-2,2'-cyanine is responsible for this emission. At pH< 4, the excited state protolytic reaction competes with ground state pr otonation. Both pathways give the same excited species, luminescing at 400 nm. The non-protonated form shows only weak fluorescence. The exc ited state lifetime of the protonated form is 4.5 ns.