1. Introduction Ferrocene and its derivatives are widely synthesized and applied in electrochemistry,electron transfer,electrocatalysis, and molecular recognition due to their high thermal stability,good solubility in organic media,and reversible redox property ^{[1, 2, 3]}. Their electrochemical redox behavior is affected significantly by intramolecular electronic communication and the ion-pairing effect ^{[3, 4, 5, 6]}.

The ion-pairing effect of ferrocene and its derivatives has been well studied in one-electron electrochemical processes ^{[4, 5, 6]}, however,the ion-pairing effect in more complicated electrochemical process has not been reported yet,so the first study of multilevel ion-pairing effect in this report is of necessity and importance. Triferrocenylmethane (TriFcM),a typical three-center complex,was employed to investigate the multilevel ion-pairing effect through ‘‘thin-layer electrochemistry’’ approach. 2. Experimental 2.1. Chemicals The tetraethylammonium fluroperchlorate (TEAFP) and perchloric acid were all purchased from Alfa Aesar China Co.,Ltd.,and nitrobenzene (NB) from Alfa Aesar China Co.,Ltd. was purified consecutively with 0.1 mol/L HCl,0.1 mol/L NaOH,and deionized water. The sodium hydroxide,hydrochloric acid,and trichloromethane were all purchased from Beijing Chemical Works. Ferrocene (Fc) was from Aldrich and triferrocenylmethane (TriFcM) was provided by professor Hua-Zhong Yu at Simon Fraser University in Canada. 2.2. Apparatus and procedure Conventional electrochemical cells and instrumentation were employed for the electrochemical measurements. A cylindrical edge-plane graphite (EPG) rod sealed with heat-shrinkable tubing was employed as the working electrode,with an exposure area of 0.34 cm ² at the edge of the graphitic planes. A platinum wire was employed as the counter electrode while Ag/AgCl/3 mol/L KCl was used for the reference electrode. The working electrode was polished with a piece of 600-grit sand paper,sonicated,and washed in deionized water,then dried with a heat gun before 1.0 mL drop of NB containing TriFcM was applied onto the surface of the working electrode. The electrochemical experiments and UV-vis spectroscopy were carried out with a CHI610D instrument and UV-5300 PC spectrophotometer,respectively. All measurements were performed at room temperature. 3. Results and discussion 3.1. Characterization of TriFcM The UV-vis spectrum of TriFcM and Fc was obtained and shown in Fig. 1(a),obviously,two distinct bands exist. The peak around 245 nm corresponding to the cyclopentadienyl ring of TriFcM is smaller than that of Fc at 260 nm,which was caused by the electron-withdrawing effect of methenyl in TriFcM ^[3]. The single peak around 450 nm is contributed by the characteristic d-d transition in the ferrocenyl nucleus of TriFcM,which is almost the same with Fc molecule. These indicate the virtually unchanged electronic structure for each ferrocenyl moiety of TriFcM ^[3].

The TriFcM was investigated through thin-layer electrochemistry with 0.5 mol/L HClO₄ as the supporting electrolyte in aqueous solution. Interestingly,three pairs of reversible peaks of TriFcM appeared,and the potential separations between two adjacent peaks are =0.202 V and =0.134 V (shown in Fig. 1(b)),which was thought to be caused by two factors: one is the intramolecular electronic communication resulting from repulsive electrostatic forces among three ferroceniums during electrochemical process,the other is the multilevel ion-pairing effect between ferrocenium and perchlorate in the organic phase ^[4]. The whole reaction process could be demonstrated as follows:

3.2. Multilevel ion-pairing effect of TriFcM in organic phase In order to further study the multilevel ion-pairing effect quantitatively,electrochemical responses of 1.1 mmol/L TriFcM in thin NB film was obtained while different concentrations of perchloric acid (HClO₄) were employed as the supporting electrolyte in aqueous solution. As shown in Fig. 2(a),the negative potential shift reached as high as 125 mV (±5%) with the increase of concentration of HClO₄ from 0.1 mol/L to 2.0 mol/L in aqueous solution,which was mainly dictated by the formation of ion-pairs between ferroceniums produced in the process of electrochemical oxidation and the counter perchlorate in the organic phase ^[4].

Based on the confirmation of ion concentration in NB phase and the liquid/liquid junction potential in our previous work ^[4],the multilevel ion-pairing effect of TriFcM could be evaluated through Eq.(4) ^[4]:

where E_pis the peak potential of the symmetric oxidative wave.

To our surprise,three linear dependences of E_p(corrected for liquid junction potential Ej) for three ferrocenyl moieties of TriFcM on ln[ClO₄^- ] were obtained in Fig. 2(b). According to Eq. (4),three formation constants of the ion-pairs for three ferrocenyl moieties of TriFcM (K₁= 1.0 × 10 ⁷ ,K₂= 1.3 × 10 ⁷ ,K₃= 1.9 × 10 ⁷ ) were obtained through individual intercepts of Fig. 2(b). The order of K₁,K₂,K₃ was considered to be mainly caused by multilevel electrostatic interactions between ferrocenium and perchlorate, the bigger the electrostatic interaction,the bigger the formation constant of ion-pair ^[4].

The intramolecular electronic communication of TriFcM during successive processes of electrochemical oxidation results in different positive charge numbers for the three ferroceniums [2, 3],which follows the order of qFc+ 1 < qFc+ 2 < qFc+ 3 : Since anion(ClO4- ) is identical in each reaction,the comparative value of electrostatic interaction between ferrocenium and perchlorate was determined by the positive charge number of ferrocenium,the more positive the charge number,the bigger the electrostatic interaction. Therefore,the multilevel formation constant of ionpair follows the above order of K1,K2,K3. 4. Conclusion TriFcM was characterized with UV-vis spectroscopy and thinlayer electrochemistry,in which the multilevel ion-pairing effect was first studied. The three one-electron electrochemical reaction process of TriFcM was determined by the intramolecular electronic communication among ferrocenyl moieties and the multilevel ionpairing effect. The multilevel ion-pairing effect of TriFcM was studied quantitatively,which conforms to the order of K1< K2< K3 resulting from different positive charge numbers of ferrocenium.

Acknowledgment The authors gratefully acknowledge the NSFC (No. 21173023), the 111 Project (No. B07012) in China and the TriFcM provided from professor Hua-Zhong Yu at Simon Fraser University in Canada.