Synthesis, cyclic voltammetric studies, and electrogenerated chemiluminescence of a new donor-acceptor molecule: 3,7-[bis[4-phenyl-2-quinolyl]]-10-methylphenothiazine

We report here the synthesis of a novel compound that contains two electron-accepting phenylquinoline groups covalently attached to the 3,7-positions of a light-emitting electron donor, 10-methylphenothiazine. The optimized geometry as determined from semiempirical MNDO calculations shows that the phenylquinoline moieties are twisted similar to 77.5 degrees from the phenothiazine central ring. As a result, no molecular orbital overlap between these two groups exists, inhibiting any delocalization of the charge upon electrochemical oxidation or reduction. Comparison between cyclic voltammograms obtained of this compound as well as of the individual compounds, 10-methylphenothiazine and 2-phenylquinonine, did indeed show no change in the electrochemical behavior of these two groups upon the covalent attachment, confirming the results obtained from the semiempirical calculations. A shift to lower energy wavelengths of phenothiazine was observed upon the addition of the electron-deficient phenylquinoline moieties. Overall, this unique geometry allows us to electrochemically produce the stable radical ions needed to generate the light-emitting excited state of phenothiazine within a potential window not obtainable with just 10-methylphenothiazine. ECL spectrum produced by annihilation between the radical cation of phenothiazine and the radical anion of phenylquinoline shows good agreement with the fluorescence emission of 10-methylphenothiazine.