2.2.1 Redox properties

In the type I compounds, exploration of the electron mobility across the polyyn-diyl chains is related to the investigation of the electronic coupling between two redox-active [Ru₂L₄] moieties, which was assessed on the basis of the comparison of their voltammetric behavior (CV/DPV) with that of the “half” molecule (monomer). For instance, the cyclic voltammogram of monomer [Ru₂]–C₄TMS ([Ru₂] = [Ru₂(DiMeOap)₄]) exhibits two reversible one-electron processes at ca. 0.5 V (oxidation) and ca. −0.7 V (reduction) [38]. On the other hand, the dimer molecule, [Ru₂]₂(μ-C₆) (A3) clearly exhibits more complex redox features including multiple reversible one-electron couples within the same potential window (Fig. 2): two stepwise oxidations (2+/1+) and (+1/0) at potentials close to 0.5 V and two stepwise reductions (0/−1) and (−1/−2) at potentials close to −0.7 V. Both oxidation couples and the first (least cathodic) two reduction couples are Ru₂ based. The pairwise appearance of one-electron couples instead of a two-electron wave is an indicative of significant electronic coupling between two [Ru₂] units. Moreover, the potential splitting within each pair (ΔE_1/2) is proportional to the coupling strength and can serve as a qualitative indictor, as well known in the mixed-valence chemistry [41–43]. In addition, the potential differences within the oxidations (ΔE_1/2(+1)) and reductions (ΔE_1/2(−1)) allow the estimation of comproportionation constants for the corresponding mixed-valence cation (K_com(+1))and anion (K_com(−1)), respectively [41–44], which are summarized in Table 1. Finally, [Ru₂L₄] monomers with L as both ap and DiMeOap exhibit almost identical redox potentials (differences within 30 mV), which indicates minimal effect of ligand L on the electronic structures of Ru₂ core [38,45].

The voltammetric plots (differential pulse voltammetry) in Fig. 3 and electrode potential data in Table 1 reveal similar redox behaviors within [Ru₂L₄]₂(μ-C_2n) type compounds and some noteworthy trends. First, for each dimer, the difference between E_1/2(0/−1) and E_1/2(−1/−2) (ΔE_1/2(−1)) is much larger than the difference between E_1/2(+2/+1) and E_1/2(+1/0) (ΔE_1/2(+1)), which suggests that the [Ru₂L₄]₂(μ-C_2n) type compounds are much better electron carrier than hole carrier, partly due to the electron-deficient nature of both the diruthenium core and polyyn-diyl bridge [36]. Second, as the polyyn-diyl chain elongates, both ΔE_1/2(+1) and ΔE_1/2(−1) gradually decrease (from resolved one-electron couples to merged pseudo two-electron waves) indicating the decrease of electronic coupling between two Ru₂ centers. This trend is consistent with the consensus that the degree of coupling between a pair of donor–acceptor centers decays exponentially as the distance increases [41,43,46]. Third, potentials for all redox couples display a significant anodic shift as the polyyne chain elongates, which is attributed to the electron-deficient nature of the added acetylene units. Additionally, the second pair of one-electron reductions at more negative potentials was tentatively assigned as polyyn-diyl based [36].

2.2.2 Spectroelectrochemistry

As established in the mixed-valence chemistry, the splitting in potentials of oxidation/reduction pairs (ΔE_1/2) and the resultant comproportionation constant K_com usually serve as qualitative indicators of the nature of mixed-valency. These parameters generated from the electrochemical measurements significantly depend on the nature of solvents and electrolytes [47], and hence may be inappropriate as quantitative measures. A more suitable approach is to examine spectroscopic characteristics of the mixed-valence species, namely the intervalence transitions between donor and acceptor Ru₂ moieties as mediated by the polyyn-diyl bridge, through spectroelectrochemical studies [41,43,44,48]. Fig. 4 shows electronic absorption spectra derived from spectroelectrochemical studies of [Ru₂(ap)₄]₂(μ-C₄) A2′ and its reference compound Ru₂(ap)₄(C₂TMS) [36]. Upon either one-electron reduction or one-electron oxidation, intervalence charge transfer (IVCT) bands appeared and half-widths (bandwidths at half-peak height) were estimated from spectral deconvolution assuming Gaussian profiles (data are summarized in Table 2). The band at ca. 8450 cm⁻¹ is clearly attributed to the CT between two Ru₂ moieties of {[Ru₂(ap)₄]₂(μ-C₄)}⁻¹. The lower energy band (5670 cm⁻¹) is likely due to the mixing of LMCT and IVCT states in such a strongly delocalized system, and a definitive answer awaits a high level computational study. Based on the comparison of the estimated half-widths with those predicted from the Hush model, which describes weakly coupled mixed-valence species (Robin-Day class II) [49,50], the mixed-valence cation and anion of compound A2′ are strongly coupled species due to the narrowness of the observed IVCT bands [36,44]. They may be regarded as the class III species close to the borderline of class II–III using the Γ parameter analysis [51].

The IVCT bands are also useful in estimating the adiabatic electronic coupling element H_ad for the series of [Ru₂L₄]₂(μ-C_2n) through H_ad = (ΔG_rE_IVCT)^1/2, where ΔG_r is the free energy of resonance exchange and E_IVCT is related to the energy of IVCT band [52]. Furthermore, the electronic coupling attenuation constant γ, an efficient measure of the medium's ability to mediate electronic coupling, can be extracted using the equation H_ad = H₀ exp(−γR), where H_ad and H₀ are donor and acceptor wave function resonance exchanges at distance R and van der Waals contact distance, respectively, and R is the separation between the donor and acceptor from van der Waals contacts [41,43,44]. The success of extending the polyyn-diyl bridge to C₂₀ provides the possibility of probing the electronic coupling over a long distance. For the reduced mixed-valence ions of the [Ru₂L₄]₂(μ-C_2n) compounds, two estimated γ were derived from a ln(H_ad) versus dRu2−Ru2 plot¹ (Fig. 5): γ₁ = 0.100 Å⁻¹ for n = 1–3 and γ₂ = 0.015 Å⁻¹ for n = 4–10, which are comparable to that found for oligoene-bridged pentaamineruthenium(II,III) donor–acceptor pairs (γ = 0.070 Å⁻¹) [53] and Fc–Fc ⁺ donor–acceptor pairs (γ = 0.087 Å⁻¹) [54]. The appearance of dual attenuation parameters (bimodal) within a homologous series of molecules is relative rare [55], but has been frequently documented in electron transfer mediated by peptide chains [56]. The bimodal behavior in the latter case has been interpreted as superexchange (tunneling) at shorter distances and hopping at longer distances, although the precise mechanisms await more detailed studies.

3.2.1 Redox properties

Both the cyclic and differential pulse voltammograms of more soluble Ru₂(m-MeODMBA)₄-based compounds recorded in THF are shown in Fig. 7. For instance, compound B1′ exhibits three sequential one-electron oxidations (between +0.4 V and +1.0 V) and one one-electron reduction at ∼−1.2 V in the potential window of −1.5–+1.5 V (vs. Ag/AgCl). Spectroelectrochemical studies (details discussed below) identified the reduction and first oxidation as Ru₂ based, while the second and third oxidations as Fc based. While the butadiynyl spaced di-ferrocene (Fc(CC)₄Fc) to a pseudo two-electron oxidation [65], compound B1′ displays a remarkably large potential splitting of 310 mV, indicating a strong coupling between two ferrocenyl units (K_com ≈ 173 000). As the Fc…Fc distance increases, similar redox behaviors to those illustrated in the [Ru₂]–(μ-C_2n)–[Ru₂] are observed: (1) all redox couples are positively shifted, which is attributed to the strong electron-withdrawing nature of the added acetylene units; (2) Ru₂ oxidation couples drastically shift and overlap with the first Fc oxidation couples as n is increased to generate a two-electron wave.

3.2.2 Spectroelectrochemistry

Since the reduction couple is Ru₂ based, spectroelectrochemical measurements focused on oxidations only [57,58]. Spectroelectrochemical study of symmetric compound B1′ provided absorption spectra of its oxidation products (B1′)⁺, (B1′)²⁺ and (B1′)³⁺, which show clean isosbestic points with good reversibility (>95% recovery) (Fig. 8). The first oxidation to (B1′)⁺ resulted in appearance of a very broad low energy band (band I) in the spectrum extending from the near-IR to IR region, with the maximum absorption out of the spectral range of study. The spectrum of dication (B1′)²⁺ displays a new higher energy band (band II) around 6000 cm⁻¹ in addition to band I. Upon third oxidation to (B1′)³⁺, both bands disappeared. Bands I and II are assigned to IVCT bands; which vanished in (B1′)³⁺ since both Fe are in the 3+ state. Compared to the IVCT bands observed for biferrocene [65], band II has approximately the same energy and hence is assigned as Fc–[Ru₂]⁺–Fc⁺ based (Fc to Fc⁺ transition). The other two oxidized forms are Fc–[Ru₂]⁺–Fc ((B1′)⁺, mixed-valent, Fc to Ru₂⁷⁺ transition) and Fc⁺–[Ru₂]⁺–Fc⁺ ((B1′)³⁺, no transitions), respectively. Unsymmetrical compound B2′ and symmetric compound B3′ exhibit similar spectroelectrochemical behaviors and data related to their mixed-valent species are listed in Table 3. Bandwidths of Fc to Fc⁺ IVCT are generally narrower than those predicted by Hush model, even across a distance of 16.6 Å [23,49,57,58]. Obviously, the characteristics of the IVCT band are consistent with the magnitude of the comproportionation constants, indicating a valence delocalized state. The extent of delocaliztion is sufficient to classify (B1′)²⁺, (B2′)²⁺ and (B3′)²⁺ as Robin-Day class III, class II–III and II–III species, respectively [50,51]. In contrast, other Fc–X–Fc⁺ systems (with X = none, acetylene, butadiyne or metal-containing units) exhibit very weak electronic communication despite short Fc–Fc⁺ distances [65–67]. Thus, all these results evidence the proficiency of [Ru₂] core in mediating superexchange electronic coupling (especially hole transfer) over a long distance. The mechanism of [Ru₂] acting as an efficient charge transfer bridge is under investigation.