Molecular Switches

A Molecular Multiproperty Switching ArrayBased on the Redox Behavior of a FerrocenylPolychlorotriphenylmethyl Radical**

Christian Sporer, Imma Ratera, Daniel Ruiz-Molina,Yuxia Zhao, Jos� Vidal-Gancedo, Klaus Wurst,Peter Jaitner, Koen Clays, Andr� Persoons,Concepci% Rovira, and Jaume Veciana*

The preparation of molecular switches has attracted a greatdeal of attention in the last few years owing to their potentialuse in the future as key nanoscale components for digitalprocessing and communication.[1] Special interest exists indeveloping molecular switches showing an optical outputsignal at the near-IR (NIR) region which is important fortelecommunication purposes. To date, molecular switchesexhibiting changes in one property, such as color,[2] lumines-cence,[3] optical nonlinearity,[4] or magnetic properties,[5] havebeen reported. However, recently the number of usefulproperties being simultaneously modulated on a bistablemolecule-based material has been extended to three proper-ties (electrical, optical, and magnetic) although in this case thechanges in the properties has an intermolecular origin.[6]

Herein we report a new multifunctional redox-switchablemolecular array involving simultaneous changes of three

different outputs—the linear optical, nonlinear optical, andmagnetic properties—which is based on the rich electro-chemical behavior of compound 1 (Scheme 1).

Radical 1 is composed of two electroactive units linked byan CH=CH bridge, these are an acceptor unit, the poly-chlorinated triphenylmethyl (PTM) radical, and a donor unit,the nonamethylferrocene group. The reduction of 1 into itsdiamagnetic anionic form 1� or the oxidation into theferrocenium radical derivative 1+, gives rise to three differentredox states that should show distinct physical properties.Indeed, substituted PTM radicals not only possess unpairedelectrons contributing to the magnetic properties, but theyalso show nonlinear optical responses owing to their octupo-lar nature[7] and, in addition, their color is different from thatof the corresponding carbanions. The methylated ferrocenederivatives gave stable oxidized species with an open-shellcharacter. Moreover, donor–acceptor dyads properly linkedusually show nonlinear optical properties. Consequently,radical 1 appeared as an ideal candidate for developing amultiproperty three-state switching molecular device gov-erned by electrical or chemical inputs.

The synthesis of radical 1 was performed following amultistep synthetic methodology (see Supporting Informa-tion). A Wittig–Horner–Emmons reaction between the poly-chlorotriphenylmethane phosphonate derivative[8] and thenonamethylated ferrocene carboxaldehyde[9] gave the transisomer of 1-H. Then 1-H was deprotonated to the carbanion1� , which was isolated as the [K([18]crown-6)] salt in 89%yield. Finally radical 1 was obtained in a 57% overall yield byoxidation of the [K([18]crown-6)] salt. Treatment of 1 withAgBF4 yields the corresponding ferrocenium radical deriva-tive 1+, which was isolated as the 1+BF4

� salt in a 79% yield.X-ray crystal structure (see Supporting Information)[10] of

radical 1 shows that in each radical unit the three phenyl ringsare twisted in the same way adopting a propeller-likeconformation with either a D or L helicity. The transconfiguration of the double bond leads to small dihedralangles between the mean planes containing the CH=CHbridge and the phenyl and tetramethylcyclopentadienyl ringsattached to it, 31(2)8 and 35(3)8, respectively. These valuesindicate a large degree of conjugation between the twoelectroactive units.

As shown in Figure 1, the colors of the studied compoundsin CH2Cl2 solution are quite different. Thus, the[K([18]crown-6)]+1� salt is deep purple, radical 1 is brown,the 1+BF4

� salt is yellow, and the hydrocarbon 1-H is palepink.

In accordance with their distinct colors, the opticalabsorption spectra of the three species 1, 1+, 1� showstrikingly differences (Figure 2 and Supporting Information).Thus, radical 1 shows besides the characteristic absorptions ofconjugated PTM radicals[8] at 385 nm, 497, and 656 nm abroad intervalence charge-transfer (IVCT) absorption bandin the NIR region, centered at 1520 nm which is associatedwith an intramolecular electron transfer from the ferrocenedonor unit to the radical acceptor unit (the PTM unit). Themaximum of this IVCT band shifts from 1360 nm in n-hexaneto 1540 nm in THF indicating a positive solvatochromism withincreasing solvent polarity. This behavior indicates a positive

[*] Dr. C. Sporer, Dr. I. Ratera, Dr. D. Ruiz-Molina, Dr. J. Vidal-Gancedo,Prof. C. Rovira, Prof. J. VecianaInstitut de Ci!ncia de Materials de Barcelona (CSIC)Campus Universitari de Bellaterra08193 Cerdanyola (Spain)Fax: (+34)93-580-57-29E-mail: vecianaj@icmab.es

Dr. Y. Zhao, Prof. K. Clays, Prof. A. PersoonsLaboratorium voor Chemische en Biologische DynamicaKatholieke Universiteit LeuvenCelestijnenlaan 200D, 3001 Leuven (Belgium)

Dr. K. Wurst, Prof. P. JaitnerInstitut fAr Allgemeine Anorganische und Theoretische ChemieUniversitCt Innsbruck, Innrain 52a, 6020 Innsbruck (Austria)

[**] This work was supported by grants from the DGI (Spain), projectMAT2003-04699, DGR (Catalunya), project 2001SGR00362, CatalanNetwork CeRMAE, Flemish Fund for Scientific Research (FWO-VG.0297.04), COST Chemistry D14 Action “Functional Materials”and AcciJn Integrada Hispano-AustrLaca, project HU2002-0046.C.S. is grateful to the FWF Vienna for a postdoctoral grant (Erwin-SchrMdinger Stipendium J2103).

Supporting information for this article is available on the WWWunder http://www.angewandte.org or from the author.

Communications

5266 � 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/anie.200454150 Angew. Chem. Int. Ed. 2004, 43, 5266 –5268

dipole-moment change between the ground and the excitedstate, which is characteristic of a system with a moderateelectronic coupling between the donor and acceptor units.[11]

The ferrocenium radical derivative 1+, shows besides thetypical bands of the ferrocenium at 280, 300, and 805 nm, anenhancement of the intensity of the radical band at 385 nm,which accounts for the yellow color of this species. On otherhand the IVCT band is absent, in agreement with the lack ofelectron-donor character of the organometallic unit when it isoxidized. In carbanion 1� the typical radical absorptions arenot observed while an intense absorption at 534 nm, charac-teristic of PTM anions, is present.[12] As for 1+, the broadIVCT absorption in the NIR region of 1 vanishes in 1�

confirming the absence of an electron-acceptor capability inthe reduced triphenylmethyl unit in 1� .

The dynamic hyperpolarizabilities of these three specieswere measured by hyper-Rayleigh scattering (HRS) experi-ments with a laser fundamental of 800 nm.[13] As suggested bythe intense IVCT band, radical 1 gives a large nonlinearoptical response with a dynamic hyperpolarizability valueb(800) of 545(� 30) A 10�30 esu. This value is reduced almostninefold to 66(� 7) A 10�30 esu for 1+, and even more for thecarbanion 1� which has a b(800) value of 30(� 3) A10�30 esu.[14]

Magnetic properties of the three studied species are alsodifferent. Thus, while [K([18]crown-6)]+1� is diamagnetic, theparamagnetic susceptibility of compounds 1 and 1+BF4

� in thesolid state between 4–300 K showed quasi-ideal paramagneticbehavior with effective magnetic moments of 1.72 for 1 and2.50 mB for 1+BF4

� at 300 K; as expected for systems with S=1/2 and S= 2 A 1/2 units. Indeed, the value of 2.50 mB indicatesthat the magnetic interaction between the open-shell ferro-cenium moiety and the triphenylmethyl radical unit in 1+ isvery weak with both spins being apparently uncoupled above4 K.

Analogous differences in the behavior of the threecomplexes were found in solution with EPR spectroscopy(see Supporting Information) since 1� is EPR silent while 1and 1+ show different complex signals that exhibit a complexbehavior when the temperature is lowered. The origin of thisintricate behavior is still not well understood although itmight be related to a spin-density redistribution associated toconformational changes or to a valence tautomerism phe-nomena.[15]

Scheme 1. The three states of the molecular switch: 1� , 1, and 1+.

Figure 1. Colors shown by CH2Cl2 solutions of the synthesized com-pounds. From left to right: 1-H, [K([18]crown-6)]+1� , 1, and 1+BF4

� .

Figure 2. Spectroelectrochemical experiments showing the evolution ofUV/Vis-NIR spectra during the oxidation of 1� in CH2Cl2 to radical 1.Trends in intensity changes are shown for selected bands by arrows.

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Reduction and oxidation processes of radical 1[16] to 1�

and 1+ are completely reversible as ascertained by thepresence of various isosbestic points in the spectroelectro-chemical experiments at the UV/Vis-NIR region (Figure 2and Supporting Information).

To demonstrate the complete reversibility of the redoxreactions of radical 1, several oxidation and reduction cycleswere performed. As shown in Figure 3 (see also the Support-ing Information), after each step, the optical spectrum was

recorded and could be fully recovered after each cycle.Analogous switching behavior was also detected with EPRspectroscopy by studying the magnetic response in solution ofthe redox-switchable molecular array.

The three species of this molecular array may beinterconverted with a variety of chemical reagents. Suchchemical transformations along with the conversions byelectrical stimuli let us expect a plethora of signal trans-duction mechanisms should this molecular switchable arraybe used as a molecular logic gate. Studies in this direction arein progress.

In summary, we have demonstrated that the new open-shell donor–acceptor dyad 1 can exist in three stable oxidationstates exhibiting different linear and nonlinear opticalresponses as well as distinct magnetic properties. Owing tofully reversible redox processes, this radical can act as a three-state redox-switchable molecular device combining chromic,magnetic, and nonlinear optical outputs in the same molecule.

Remarkable is the output signal at the NIR region shown bythis electrochromic switching molecular array.

Received: March 1, 2004Revised: June 11, 2004 [Z54150]

.Keywords: hyper-Rayleigh scattering · magnetic properties ·nonlinear optics · radicals · sandwich complexes

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[10] X-ray structure figure, data, and analysis are given as SupportingInformation. CCDC-191558 contains the supplementary crystal-lographic data for this paper. These data can be obtained free ofcharge via www.ccdc.cam.ac.uk/conts/retrieving.html (or fromthe Cambridge Crystallographic Data Centre, 12, Union Road,Cambridge CB21EZ, UK; fax: (+ 44)1223-336-033; or deposit@ccdc.cam.ac.uk).

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[16] Cyclic voltammetry of 1 in CH2Cl2 shows two reversibleprocesses, at potentials of E1/2=�238 and + 86 mV, versus Ag/AgCl, corresponding to its reduction to 1� and oxidation to 1+,respectively.

Figure 3. Cyclic stepwise oxidations and reductions carried out in THFwith a chronoamperometric technique monitoring the changes in thevisible spectrum. Top: Changes observed at a wavelength of 385 nmwhere 1 (*) and 1+ (~) exhibit the strongest absorption and 1� (&)shows a very weak absorption. Bottom: Fixed potentials E used in thedifferent steps of cyclic redox experiments.

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5268 � 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org Angew. Chem. Int. Ed. 2004, 43, 5266 –5268