Electrochemistry and molecular recognition

Non-Faradiac process: Process such as absorption or desorption, in which charge does not across the interface, but external current can flow because of the surface potential change etc. (e.g. C d )

 s =  i +  o Total current density IHP OHP Electrical Double Layer

300A (10 -2 M) Diffuse layer

Cyclic Voltammetry E = E i + vt (forward sweep) E = E s – vt (reverse sweep) I: initial S: Switching

Cell Design should exclude oxygen and moisture. Glove boxes or glove bags are necessary sometimes.

Solvents Should be stable to the redox conditions Electrolytes Bu 4 NBF 4 Bu 4 NPF 6 Bu 4 NClO 4 e.g. Bu 4 NPF 6 in CH 3 CN: +3.4 to –2.9 v

Working electrodes The WE should have a facile electron transfer with electroactive species. The factors behind are not well understood. Pretreatment or polishing of the electrode-surface is necessary. Pt, glassy carbon, gold, silver, amalgams are common materials for the WE.

Reference electrodes Electrode potential does not change when current increases.

Faradiac process: Electron transfer causes oxidation and reduction to occur. This process is governed by Faraday’s law (e.g. R)

E 1/2 = (E pc +E pa )/2 = E o + (RT/nF)ln  where  = (D(O)/D(R)) 1/2

CV for absorbed monolayer CV curve for reduction of absorbed O and sweep reversal.  e p.1/2 = 3.53RT/nF

Schematic configuration of photoactivated molecular switches:

(A) Chronoamperometric responses of the assembly consisting of the ferrocene- functionalized  -cyclodextrin threaded onto the azotranslocation benzene-alkyl chain and blocked by the anthracene barrier. (a) Monolayer in trans-azobenzene state. (b) Monolayer in cis- azobenzene state. (B) Cyclic variation of the electron transfer rates of trans the ferrocene- functionalized b-cyclodextrin in the molecular ‘train- shuttle’ upon reversible photoisomerization of the monolayer: (  )monolayer in trans-azobenzene state, (  ) monolayer in cis-azobenzene state.

(A) Cyclic voltammetric response of cytochrome c (0.1mM) at a mixed 6a/b and pyridine monolayer-modified electrode in (a) the 6a and (b) the 6b states. Inset: switching behavior of the system; circles and squares represent the photoisomerizable units in the spiropyran and the merocyanine states, respectively. (B) The same system with COx (1  M) and O2 and in (a) the 6a and (b) the 6b states. Inset: Switching behavior (as for part A).

From Enzyme to Molecular Device. Exploring the Interdependence of Redox and Molecular Recognition

Phenyl receptor 6a, which cannot form appreciable aromatic - aromatic contacts with bound flavin 1, causes an 18 mV shift in the E1/2 of flavin 1 to less negative values, accompanied by an enhancement of the reversible reoxidation wave. These two effects are indicative of the stabilization of Fl(rad) - by hydrogen bonding, as observed in the flavin 1 - receptor 2 complex. The  -stacking receptors 6b,c, however, shift the flavin 1 E1/2 to more negative values, making flavin 1 harder to reduce.

Size and Polarizability