Electrogenerated Chemiluminescence. 61. Near-IR Electrogenerated Chemiluminescence, Electrochemistry, and Spectroscopic Properties of a Heptamethine Cyanine Dye in MeCN

The oxidation of the heptamethine cyanine dye 2-[4‘-chloro-7‘-(1‘‘-methyl-3‘‘,3‘‘-dimethylindolin-2‘‘-ylidene)-3‘,5‘-(propane-1‘‘‘,3‘‘‘-diyl)-1‘,3‘,5‘-heptatrien-1‘-yl]-1-methyl-3,3-dimethylindolinium cation (HMC + ) in the presence of tri- n -propylamine (TPrA) at +0.7 V vs SCE in MeCN produces near-IR electrogenerated chemiluminescence (ECL) with an emission maximum at about 805 nm and a relative ECL quantum efficiency of 0.21 vs Ru(bpy) 3 2+ (bpy = 2,2‘-bipyridine) with TPrA in MeCN as a standard. The ECL emission spectra are identical to the photoluminescence spectra. The concentration dependence of the ECL is attributed to both self-absorption and competition by radical dimerization reactions. An oxidative-reduction mechanism for excited state formation is proposed and is compared to the Ru(bpy) 3 2+ /TPrA reference system. Digital simulation was employed to examine the kinetics of the homogeneous redox reaction between TPrA and HMC + . The rate constant for the catalytic reaction is >2000 M -1 s -1, and the lower limit for proton transfer is 10 7 M -1 s -1 . Molecular mechanics calculations were used to determine the most stable configuration for the HMC + molecule.