DIMERIZED PI-COMPLEXES IN SELF-ASSEMBLED MONOLAYERS CONTAINING VIOLOGENS - AN ORIGIN OF UNUSUAL WAVE SHAPES IN THE VOLTAMMETRY OF MONOLAYERS

A direct comparison of the self-assembly on Au and Ag of thiol and dis ulfide derivatives of viologens bearing long n-alkyl chains was made i n order to ascertain the relative efficiency of monolayer formation fo r each type of functionality. The structures of the two derivatives th at were studied can be written as CH3V2+(CH2)(12)SH and [CH3V2+(CH2)(1 2)S](2) for the thiol and disulfide, respectively, where V2+ represent s the viologen (i.e. N.N'-dialkyl-4,4'-bipyridinium) redox group. In c ontrast to the behavior of n-alkane thiols and di-n-alkyl disulfides, which adsorb to give very nearly the same surface coverage and interfa cial properties, these two viologen derivatives exhibit different satu ration surface coverages of 1.8 x 10(-10) mol cm(-2) for the disulfide and 4.5 x 10(-10) mol cm(-2) for the thiol, as determined from the ch arge for exhaustive reduction and reoxidation of the viologen redox gr oups. In addition, monolayers of the thiol derivative that had very hi gh surface coverages exhibited very sharply peaked cyclic voltammetric responses that are attributed to very strong interactions between the one-electron-reduced cation radicals in the monolayer, a phenomenon t hat does not occur in monolayers prepared from pure samples of the dis ulfide derivative. Vibrational spectroscopic examination of these mono layers under conditions in which this unique voltammetric response is observed revealed the presence of vibrational spectroscopic signatures of viologen dimer formation. Specifically, surface Raman spectroscopy (including surface-enhanced Raman, surface resonance Raman, and surfa ce-enhanced resonance Raman) was used to demonstrate that the lateral interaction of the cation radical viologen redox groups in these monol ayers results in the formation of pi complex dimers. The presence of t hese dimers is correlated with the very sharply peaked cyclic voltamme tric responses. The Raman bands due exclusively to the dimer are assig ned to the out-of-phase coupling combination of the totally symmetric ring modes of the component cation radicals in the dimer.