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MOLECULAR SWITCHES
J.-P. Launay, C. Coudret, C. Patoux, S. Fraysse

Molecular Switches based on mixed-valence compounds have been developped in our group since 10 years. The principle is to insert in the structure of a mixed valence compound a molecular bridge with two possible states : "ON" (i.e. a non-zero electronic coupling between extremities), and "OFF" (i.e. no coupling).

The electronic coupling, denoted as Vab, is determined from the intensity of the intervalence transition through Hush Equation. Thus we monitor the photoinduced electron transfer process where an electron moves from one end to the other of the molecule.


Principle of the Molecular Switch. The switching action is usually performed in the homovalent form, while the spectroscopic determination of the coupling is made on the mixed-valent form.

Two recent examples are given below : one with a chemical control of the switch , the other one with a photochemical control.
For a chemical control of the Vab parameter, we have prepared 2,6-diferrocenylanthracene and investigated its intervalence transition before and after reaction with tetracyanoethylene (TCNE) giving a Diels-Alder adduct.

Since the adduct formation changes drastically the electronic structure with a loss of the conjugated character, one would expect a large decrease in Vab.
Actually the change is only from 0.032 to 0.019eV, i.e. a lowering by 40%.


2,6-diferrocenylanthracene and its Diels-Alder adduct with TCNE

A molecular orbital analysis based on Extended Hückel calculations explains the relative weakness of this change by a detailed analysis of the orbitals involved in the coupling.

The photochemical control has been achieved through the synthesis of a binuclear cyclometallated ruthenium complex where the bridge contains a bisthienylperfluorocyclopentene photochromic unit. Upon photoisomerization, this bridge is converted from a non-conjugated form to a conjugated one.


The photochromic Molecular Switch made of two metal sites grafted around a bisthienylperfluorocyclopentene unit.

In the ruthenium(II) - ruthenium(II) state, this molecule can be reversibly photoisomerized to give a closed form (with 254nm UV light) or an open form (with 600nm light). Intervalence band measurements on ruthenium(II) - ruthenium(III) states of both forms show no detectable coupling in the open form ("OFF"), and a noticeable coupling (0.025 eV) in the closed form ("ON").

This is the first example where reversible photochromism has been associated with an intervalence electron transfer process.
Here also the switching process can be explained by Extended Hückel calculations.


References :

  • J.-P. Launay, C. Coudret, in "Molecular Electronics-Science and Technology" A. Aviram and M. Ratner (eds), Annals NY Acad Sciences, 1998, 852, 116
  • V. Gonzalo, C. Coudret, C. Patoux, J.-P. Launay, Compt. Rend. Acad. Sci. Paris, t.2, Série II c, 1999, 321
  • S. Fraysse, C. Coudret, J.-P. Launay, Eur. J. Inorg. Chem. 2000, 1581

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