Dimerized π-Complexes in Self-Assembled Monolayers Containing Viologens:  An Origin of Unusual Wave Shapes in the Voltammetry of Monolayers

Abstract

A direct comparison of the self-assembly on Au and Ag of thiol and disulfide derivatives of viologens bearing long n-alkyl chains was made in order to ascertain the relative efficiency of monolayer formation for each type of functionality. The structures of the two derivatives that were studied can be written as CH3V2+(CH2)12SH and [CH3V2+(CH2)12S]2 for the thiol and disulfide, respectively, where V2+ represents the viologen (i.e. N,N‘-dialkyl-4,4‘-bipyridinium) redox group. In contrast to the behavior of n-alkane thiols and di-n-alkyl disulfides, which adsorb to give very nearly the same surface coverage and interfacial properties, these two viologen derivatives exhibit different saturation surface coverages of 1.8 × 10-10 mol cm-2 for the disulfide and 4.5 × 10-10 mol cm-2 for the thiol, as determined from the charge for exhaustive reduction and reoxidation of the viologen redox groups. In addition, monolayers of the thiol derivative that had very high 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 that does not occur in monolayers prepared from pure samples of the disulfide derivative. Vibrational spectroscopic examination of these monolayers 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 surface-enhanced resonance Raman) was used to demonstrate that the lateral interaction of the cation radical viologen redox groups in these monolayers results in the formation of π complex dimers. The presence of these dimers is correlated with the very sharply peaked cyclic voltammetric responses. The Raman bands due exclusively to the dimer are assigned to the out-of-phase coupling combination of the totally symmetric ring modes of the component cation radicals in the dimer.

Influence of the Hofmeister series of anions on the molecular organization of positively ionized monolayers of a viologen derivative

“…The properties and consequently applications of viologens are sensitive to a variety of factors, including the nature of anions X − [12]. In contrast to the large body of published work dealing with the electrochemistry of thin solid films of viologen derivatives [25][26][27][28][29][30][31][32][33] there is little data available for the detailed organization and properties of amphiphilic viologens arranged in thin solid films together with different counterions X − . This has motivated us to analyze the influence of the Hofmeister series of anions on the film forming behaviors of a viologen amphiphile, and determine how these anions may affect the organization of the viologen moieties within the Langmuir monolayers.…”

Reactions and Reactivity in Self-Assembled Monolayers

“…For instance, gold±thiol monolayers (vide infra) form well-packed, quasi-crystalline structures if the tail group is just a long alkane chain. [9] Incor- poration of bulky or charged (e.g., quaternary ammonium [14] ) groups in the monolayers disrupts the order due to steric/electrostatic repulsion. The presence of polar groups (e.g., sulfones [15] ) sometimes enhances order because of dipole±dipole interactions.…”

Tailor‐Made Modification of a Gold Surface for the Chemical Binding of a High‐Activity [FeFe] Hydrogenase

“…The one-electron reduction of MBBP 2+ induced a change of the bipyridyl moiety to the corresponding radical cation. [35] As a consequence, six bands arose at 1635, 1593, 1504, 1351, 1331, and 1029 cm -1 in the observed spectral range. [36] A positive band at 1635 cm -1 is assigned to the asymmetric in-plane ring vibration mode of the radical cation monomer, while other positive bands are assigned to the vibrational modes of the radical cation dimer with a plane-to-plane configuration linked by π-bonding.…”

“…This is in good agreement with reported values (1.8-4.5 ϫ 10 -10 mol cm -2 ) for related systems. [40,41] As v increases, the positions of the oxidative and reductive peaks shift to more positive and more negative potentials, respectively. The peak potentials are plotted as a function of log v in Figure 3c.…”