Polypyridyl transition metal complexes possess appealing electrochemical and photophysical properties. They are widely used in a range of optoelectronic applications, including solar cells [1-3], light-emitting devices [4-6], molecular electronics [7, 8], and ion sensing and bioimaging [9-11]. Among them, metal complexes functionalized with additional redox-active organic motifs show multistep redox processes and rich absorption and emission properties. They are particularly useful for electrochromism [12-14], information storage [15], and intelligent materials with multistimuli responsiveness [16].

Triarylamines are important molecular materials with good electron-donating and hole-transporting abilities [17-19]. The combination of triarylamine and transition metal complexes has been shown to afford molecular materials with rich redox and photophysical properties [20, 21]. Considering the presence of electronic coupling among different structural motifs in these hybrid compounds, it would be interesting to examine the mutual influence of individual components on their optoelectronic properties. We have recently reported a series of metal-amine conjugated complexes which exhibit metal-mediated amineamine electronic coupling [22, 23], amine-mediated metal-metal communication [24], and amine-modified emission properties of ruthenium complexes [25]. In this contribution, a N, N, N', N'-tetraphenylbenzidine-bridged bis-bpy (bpy = 2, 2'-bipyridine) ligand 3 and the corresponding diruthenium complex 4(PF₆)₄ were synthesized and studied (Scheme 1). In particular, the spectroelectrochemical measurements of these compounds were examined, which showed distinct potential-dependent absorption and emission spectral changes. It is known that, due to the presence of strong electronic coupling between two amine sites, N, N, N', N'-tetraphenylbenzidine displays two-step amine oxidation waves at low potentials and intense intervalence charge-transfer (IVCT) transitions in the singly-oxidized mixed-valent state [26, 27]. We expect the combination of bpy and Ru(bpy)₃ with N, N, N', N'-tetraphenylbenzidine would allow us to further modulate the optoelectronic properties of these components.

Two methods were used to synthesize the desired ligand 3 and the diruthenium complex 4(PF₆)₄ (Scheme 1). The Pd-catalyzed CN coupling of 4-(p-bromophenyl)-2, 2'-bipyridine [28] with diphenylamine afforded compound 1 in good yield. The reaction of 1 with cis-[Ru(bpy)₂Cl₂], followed by anion exchange using KPF₆, gave a monoruthenium complex 2(PF₆)₂. The N, N, N', N'-tetraphenylbenzidine-bridged bis-bpy ligand 3 was obtained via the Cu (ClO₄)₂-mediated oxidative coupling of 1 in 85% yield [27]. Alternatively, ligand 3 could be prepared by the twofold C-N coupling of N, N'-diphenylbenzidine with 4-(p-bromophenyl)-2, 2'-bipyridine. A similar Cu(ClO₄)₂-mediated oxidative coupling of the monoruthenium complex 2(PF₆)₂ afforded the diruthenium complex 4(PF₆)₄ in 54% yield, which could alternatively be prepared by the reaction of ligand 3 with cis-[Ru(bpy)₂Cl₂] in acceptable yield. We failed to obtain a single crystal of 3 or 4(PF₆)₄ for X-ray structural analysis to date. The use of two different methods leads to the same products ensures the identity of these compounds.

The anodic cyclic voltammograms (CVs) and differential pulse voltammogram (DPVs) of the above compounds are displayed in Fig. 1. Compound 1 shows an amine oxidation (N^0/+·) wave at +1.02 V vs. Ag/AgCl in CH₂Cl₂, while two N^0/+· waves at +0.80 and +1.10 V were observed due to the presence of strong amine-amine electronic coupling (Table 1) [26, 27]. The monoruthenium complex 2(PF₆)₂ and diruthenium complex 4(PF₆)₄ show similar one or two N^0/+· waves in CH₂Cl₂ in the similar potential region. Possibly due to the relatively low solubility of the ruthenium complexes in CH₂Cl₂, their Ru(Ⅱ/Ⅲ) processes appear as sharp and intense peaks during the reverse reduction scan. This is caused by the adsorption to and desorption from the electrode surface of the ruthenium complexes during the redox reaction. Because of the observation of this ill-defined Ru(Ⅱ/Ⅲ) peak in CH₂Cl₂, we performed the CV and DPV measurements of 2(PF₆)₂ and 4 (PF₆)₄ in CH₃CN with better solubility. Well-defined Ru(Ⅱ/Ⅲ) processes could be observed in CH₃CN for 2(PF₆)₂ and 4(PF₆)₄ at a more positive potential. In addition, reduction peaks of the bpy ligands of these complexes could be observed in the cathodic scan (Fig. S1 in Supporting information).

Ligands 1 and 3 both show two absorption bands in the UV region (Table 1 and Fig. S2 in Supporting information). They are brightly emissive, with an emission maximum (λ_{max, emi}) at 458 nm and 504 nm in CH₂Cl₂, respectively. The absorption spectrum of 3 is slightly dependent on the solvent polarity (Fig. S2). However, the structureless emission of 3 distinctly shifts to the lower-energy region with increasing solvent polarity. The emission of 3 in CH₂Cl₂ has a lifetime of 4.1 ns (Fig. S3 in Supporting information). These results suggest that the emission of 3 belongs to singlet charge transfer (¹CT) emission. The emission of 1 is believed to have the same ¹CT character. Very similar absorption pattern is observed for complexes 2(PF₆)₂ and 4(PF₆)₄. The molar absorptivity of 4(PF₆)₄ is almost twice that of 2(PF₆)₂. The visible absorptions of 2(PF₆)₂ and 4(PF₆)₄ are believed to be dominated by an admixture of metal-toligand charge transfer (MLCT), ligand-to-ligand charge transfer (LLCT), and intraligand charge transfer (ICT) transitions (see further discussions on TDDFT results). Both ruthenium complexes display an emission band at 606 nm, which is characteristic for triplet MLCTemissions of [Ru(bpy)₃] analogues. Compared to 1 and 3, the quantum yield of 2(PF₆)₂ and 4(PF₆)₄ significantly decreased (Table 1).

The spectroelectrochemical measurements were performed using an ITO glass as the working electrode to examine the potential-dependent absorption and emission spectral change of 1-4(PF₆)₄ (Figs. 2 and 3). When ligand 1 was oxidized stepwisely, the absorption band at 355 nm slightly decreased and a new absorption band at around 450 nm appeared. This process is attributed to the oxidation of the amine unit to aminium radical ion, namely the N^0/+· process [27, 29]. When the monoruthenium complex 2(PF₆)₂ was subjected to the same oxidation process, in addition to the appearance of the absorption of aminium radical at around 700 nm, a broad absorption band between 900 and 2000 nm was observed. In accordance with the previous reports on related ruthenium complexes with amine substituents [22], this NIR band was assigned to the ruthenium to aminium charge transfer transitions. During the amine oxidation process, the emissions of 1 and 2(PF₆)₂ were both largely quenched (Fig. 2).

Because the two amine units of 3 and 4(PF₆)₄ could be stepwisely oxidized at relatively low potentials, two distinct stepwise spectral changes were recorded for these two compounds (Fig. 3). In the single-oxidation step, intense IVCT absorptions were observed in the NIR region, in addition to the N^0/+·-localized absorptions at around 500 and 750 nm. The IVCT absorptions gradually decreased in the double-oxidation step, with the further increase of the absorption band at 750 nm. Again the emissions of 3 and 4(PF₆)₄ were largely quenched during the amine oxidation process. When the potential was reversed, the absorption and emission of 3 and 4(PF₆)₄ could be essentially recovered. This suggests the good reversibility of the amine oxidation process and their potential applications in electrochromism and electrofluorochromism [25, 30]. The spectral changes accompanied the oxidation of the ruthenium center was not examined, due to the relatively high potential involved in this process.

The IVCT band of the above diamine compound in the singlyoxidized state was fitted using a Gaussian function on the wavenumber scale (Fig. S4 in Supporting information). On the basis of the simulated curve, the IVCT band of 3⁺ has an estimated ε_max of 21, 300 L mol^-1 cm^-1, of 6070 cm^-1, (the bandwidths at half-height) of 4160 cm^-1, respectively (Table S1 in Supporting information). In contrast, the IVCT band of 4⁵⁺ has an estimated ε_max of 17, 700 L mol^-1 cm^-1, of 6730 cm^-1, of 2800 cm^-1, respectively. According to the Hush formula V_ab = 0.0206 [31, 32], the electronic coupling parameter V_ab was determined to be 1510 and 1190 cm^-1 for 3⁺ and 4⁵⁺, respectively (Table S1 in Supporting information). The attachment of the Ru(bpy)₂ component to ligand 3 seems to slightly decrease the degree of the amine-amine electronic coupling of the bridge. Indeed, the above electrochemical results suggest the same trend. The potential splitting of the two amine oxidation waves of 3 (around 300 mV) is relatively larger with respect to that of 4(PF₆)₄ (around 200 mV, Table 1). We consider that the electron-withdrawing properties of the added Ru(bpy)₂ component may be responsible for this difference. However, further data are necessary to clarify this assertion.

DFT calculations of 3 and 4⁴⁺ show that the HOMO and HOMO-1 of both compounds are dominated by two amine units (Fig. 4). This is consistent with the above interpretations on the electrochemical and spectroelectrochemical results that the two amine units of 4 (PF₆)₄ are oxidized prior to the Ru(Ⅱ/Ⅲ) process. The LUMO of two compounds are dominated by bpy ligands. TDDFT results of 3 suggest that the S₁ vertical excitation is dominated by the HOMO → LUMO transition, which is of the ICT character (Figs. 4, S5 and S6 and Table S2 in Supporting information). In agreement with the above emission studies, this state is believed responsible for the observed ¹CT emission of 3. TDDFT results of 4⁴⁺ suggest that the visible absorptions are associated with an admixture of LLCT, ICT, and MLCT transitions (Figs. S7, S8 and Table S3 in Supporting information). The predicted S₁, S₃, S₈, S₁₉, and S₂₂ excitations are associated with the LLCT and ICT transitions from the bridge-dominated HOMO orbital. The higher-energy excitations (S₂₇, S₂₈, and S₃₀) have major contributions from the MLCT transitions from the metal-dominated HOMO-4, HOMO-6, and HOMO-7 orbitals.

In conclusion, a N, N, N', N'-tetraphenylbenzidine-bridged bisbpy ligand and the corresponding diruthenium complex were synthesized and characterized. Electrochemical studies show that the amine units of these compounds were stepwisely oxidized at a relatively low potential. IVCT analysis under the same measurement conditions suggest that the amine-amine electronic coupling slightly decreased when the diamine ligand was chelated with the Ru(bpy)₂ component. The emissions of these compounds were largely quenched upon amine oxidation. They are potentially useful as electrochromic and electrofluorochromic molecular materials.

Acknowledgments

We thank the National Natural Science Foundation of China (Nos. 21472196, 21501183, 21601194, and 21501062), the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB 12010400), and the Science and Technology Commission of Shanghai Municipality (No. 16DZ1100300) for financial support.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cclet.2017.11.014.