Reversible Copper(II)/(I) Electrochemical Potential Switching Driven by Visible Light-Induced Coordinated Ring Rotation

We here describe the first metal complex system in which
electronic
signals can be repeatedly extracted by converting bistable states
related to an intramolecular ligand rotational motion, which is fueled
by visible light. The molecular structure for relating an electron
transfer and a motion consists of a copper center and a coordinated
unsymmetrically substituted pyrimidine derivative, whose rotational
isomerization causes an electrochemical potential shift. To harness
light energy effectively through metal-to-ligand charge transfer (MLCT)
excitation, we prepared a simple copper­(I) complex coordinated by
a 4-methyl-2-(6′-methyl-2′-pyridyl)­pyrimidine and a
bulky diimine. The thermodynamic and kinetic parameters of redox and
rotational reactions were analyzed by cyclic voltammograms at variable
temperatures, by considering four stable isomers related to copper­(II)/(I)
states and rotational isomeric states. The key feature of this compound
is that the rotation is frozen in the copper­(I) state (rate constant
for the rotation, kIi→o = 10–4 s–1) but is active
in the copper­(II) state (kIIi→o = 10–1 s–1) at 203 K.
The compound makes a bypass route to the isomeric metastable copper­(I)
state, via a tentative copper­(II) state formed by photoelectron transfer
(PET) in the presence of a redox mediator, decamethylferrocenium ion
(DMFc+), or upon a partial oxidation of the complex. Light-
and heat-driven rotation in the copper­(I) state with a potential shift
(ΔE°′ = 0.14 V) was analyzed by
electrochemical measurements of the complex in the solution state.
The rotor could be reset to the initial state by heating, thereby
completing the cycle and enabling repeated operation fueled by light
energy. A significant redox potential shift associated with the copper­(II)/(I)
transition accompanied the rotation, thereby providing a new type
of molecular signaling system.