Synthesis of Molecular Wires of Linear and Branched Bis(terpyridine)‐Complex Oligomers and Electrochemical Observation of Through‐Bond Redox Conduction

Abstract

Films of linear and branched oligomer wires of Fe(tpy)2 (tpy = 2,2':6',2''-terpyridine) were constructed on a gold-electrode surface by the interfacial stepwise coordination method, in which a surface-anchoring ligand, (tpy-C6H4N=NC6H4-S)2 (1), two bridging ligands, 1,4-(tpy)2C6H4 (3) and 1,3,5-(C[triple bond]C-tpy)3C6H3 (4), and metal ions were used. The quantitative complexation of the ligands and Fe(II) ions was monitored by electrochemical measurements in up to eight complexation cycles for linear oligomers of 3 and in up to four cycles for branched oligomers of 4. STM observation of branched oligomers at low surface coverage showed an even distribution of nanodots of uniform size and shape, which suggests the quantitative formation of dendritic structures. The electron-transport mechanism and kinetics for the redox reaction of the films of linear and branched oligomer wires were analyzed by potential-step chronoamperometry (PSCA). The unique current-versus-time behavior observed under all conditions indicates that electron conduction occurs not by diffusional motion but by successive electron hopping between neighboring redox sites within a molecular wire. Redox conduction in a single molecular wire in a redox-polymer film has not been reported previously. The analysis provided the rate constant for electron transfer between the electrode and the nearest redox-complex moiety, k1 (s(-1)), as well as that for intrawire electron transfer between neighboring redox-complex moieties, k2 (cm2 mol(-1) s(-1)). The strong effect of the electrolyte concentration on both k1 and k2 indicates that the counterion motion limits the electron-hopping rate at lower electrolyte concentrations. Analysis of the dependence of k1 and k2 on the potential gave intrinsic kinetic parameters without overpotential effects: (k1(0) = 110 s(-1), k2(0) = 2.6x10(12) cm2 mol(-1) s(-1) for [n Fe3], and k1(0) = 100 s(-1), k2(0) = 4.1x10(11) cm2 mol(-1) s(-1) for [n Fe4] (n = number of complexation cycles).

Terpyridine‐Functionalized Surfaces: Redox‐Active, Switchable, and Electroactive Nanoarchitecturesgland

“…[ 43 ] A different type of molecular wire was reported by Ng et al [ 41 ] in which thiol-functionalized oligo(phenylene-ethylene) s (OPEs) that were end-capped with a pyridine moiety were assembled onto a gold surface. Subsequently, [Ru(tpy)(bpy)] 2 + or cyclometalated [Pt(pbpy)] + (pbpy: 6-phenyl-2,2'-bipyridine) were coordinated onto the organic SAM via its terminal pyridine units.…”

“…[ 41 ] At low surface coverage, scanning tunneling microscopy (STM) measurements revealed an even distribution of nanodots of uniform size and shape (for the linear nanowires, based on bis-terpyridine 4 , near closepacking into circular domains (diameter of 6 nm) could be observed indicating a π -stacking of the molecular chains [ 42 ] ). Extensive PSCA experiments were carried out and the currenttime characteristics suggested a through-bond electron transport involving two kinetic factors in which electron transfer between the nearest redox site and electrode ( k 1 in s − 1 ) as well as electron transfer between neighboring redox centers along the molecular wire ( k 2 in cm 2 mol − 1 s − 1 ) ("electron-hopping …”

Synthesis of an Anthraquinone‐Bridged bis(terpyridine) Ligand and its Use in the Stepwise Fabrication of Complex Oligomer Wires on Gold

“…[14] Preparation layer formation was carried out under the same condition as that for the 1st layer, that is the immersion time in a solution of L 1 was 2 h, little increase in the peak current for the [Fe(tpy) 2 ] 3þ/2þ redox couple, and, accordingly, little surface coverage were observed ( Figure 6). We then examined the dependency of the surface coverage of electroactive sites in [1FeL 2 ] on the immersion time in a L 1 solution for the second layer formation, and the results are given in Figure 6 (for 48 h) and Figure 7.…”

“…Figure 7 indicates that saturation of Fe(tpy) 2 complex formation in the 2nd layer is slower than in the 1st layer, taking more than 16 h. This period is much longer compared with cases using another bridging ligand such as tpy-C 6 H 4 -N¼N-C 6 H 4 -tpy [12] or tpy-C 6 H 4 -tpy. [14] This difference is likely due to the electron-withdrawing effect and the bulkiness of the anthraquinone moiety in L 1 , leading to slower kinetics in complex formation. Based on these results, we carried out stepwise complex layer formation using 1 day as the immersion time in a L 1 solution after the 2nd layer.…”

“…[12][13][14] In addition, based on observation of the electron transfer of the complex wire, intrachain stepwise electron transfer between redox sites has been determined. [14] The electron transfer kinetics between redox sites is thought to depend on the structure and properties of bridging ligands; in particular, the structure of the bridging moiety is thought to influence the electron transfer ability of the complex wire. Herein, I describe preparation of new complex wire using bridging ligand, L 1 , with two terpyridine moieties being bound to the 1,4-position of anthraquinone through a triple bond.…”