Electrochemical Remote Control for Dithienylethene-Ferrocene

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Electrochemical Remote Control for Dithienylethene-Ferrocene Switches Gonzalo Guirado,*,†,‡ Christophe Coudret,*,† and Jean-Pierre Launay† Centre d’Elaboration de Materiaux et d’Etudes Structurales (CEMES-CNRS), 29 rue Jeanne MarVig, BP 94347, 31055 Toulouse Cedex 4, France ReceiVed: October 6, 2006

The electrochromic properties of dithienylethene derivates is a field of great importance and interest. In this manuscript we describe a potential molecular remote control system, which can be electrochemically triggered. Diferrocenyl compounds containing a diethylenene photoelectrochromic core with perhydro- and perfluorocyclopentene ring were prepared in order to induce a change in the chromic properties via an intramolecular electron-transfer reaction from the redox group to the photochromic core. The electrochemical behavior of open and closed isomers was thoroughly studied using photochemical, electrochemical, and spectroelectrochemical techniques. The first redox couple was typically assigned to the ferrocene for the open isomer. However, for the closed isomer, an electrocatalytic ring-opening process was observed for the perhydrocyclopentene ring, while a slightly different electrochemical process was observed for the perfluorocyclopentene system. Mechanistic investigations revealed that an internal charge transfer is necessary to destabilize the closed bridge. Once opened, the bridge’s cation radical is a strong oxidant, and the charge eventually gets localized on the ferrocene. The charges then migrate throughout the solution by self-exchange reactions. Hence, the redox status of the Fc units triggers the photochrom’s reactivity playing the role of “antenna” that can temporarily store a charge and facilitate the transformation. This way to perform the transformation, i.e., by electrochemistry rather than photochemistry, presents the great advantage of being much more local and, thus, would permit the ultimate stage of miniaturization at the scale of just one molecule.

Introduction The design of molecular switching devices requires single molecular units which can be reversibly triggered by applying external stimuli such as light, heat or electricity.1 A molecule which can undergo different types of transformation depending on the type of the external stimulus may be used as a molecular device for multiple-mode information processing. In this sense, much effort has been focused on the study of optical memory systems which respond to electrical inputs.2 This is usually achieved for compounds that present high to low HOMOLUMO gap variation. A very promising family is the group of photochromic compounds, and especially the subgroup of the diarylethene chromophore,3 since it has very few conformations for both closed (colored) and open (colorless) states. It has been chosen to act as an electronic switch, the best results being obtained to date on switching ON and OFF an intervalence transition (i.e., a photoinduced electron-transfer process).4 Eventually, it was also envisioned to control an electrical current by such a compound.5 However, in recent reports we6 and others7 have found that such compounds do possess a very interesting electrochemical property: once they are oxidized, depending on the substituent pattern, the corresponding photoisomer undergoes a thermal isomerization to the cation radical of the other isomeric form. (Figure 1 left), thus unlocking the system for further transformations. * Corresponding author. † CEMES-CNRS. Phone: 33(0)5 62 25 78 59. Fax: 33(0)5 62 25 79 99. E-mails: [email protected]; [email protected]. ‡ Currently at Departament de Quimica, Universitat Autonoma de Barcelona 08193-Bellaterra, Barcelona, Spain. Phone: 00 34 93 581 28 59, Fax: 00 34 93 581 29 20. E-mail: [email protected].

The electrochemical oxidation mechanism of this type of compounds has been recently established,6 and three main trends have been identified, depending on the substituent pattern, and the “photoisomeric” state of the dithienylethene (DTE) unit (ring-opened, i.e., bis(thiophene)-like, or ring-closed, i.e., octatetraene-like) involving a transient cation radical: -bimolecular reaction as for classic thiophene chemistry, for some ring-opened compounds; -reversible oxidation for electron-rich, ring-closed species, giving rise to true electrochromism upon quantitative oxidation (i.e., an electrochemically reversible color change); -cation-radical-induced isomerization to the other photoisomeric form (in its oxidized state). When this process is a ring closure, the combination of the oxidation process and a structural modification can be qualified as “electrochromism with memory,” since a reduction step does not restore the starting material but its photoisomer. The possibility to unlock the thermal isomerization via a cation-radical intermediate occurs on a very simple molecules and can be considered as an intrinsic property of the chromophore, thus generalizing the previous published data.6-7 It is also under control of substituent’s electronic effects; however, it still requires the oxidation of the dithienylethene moiety which can occur at rather high potentials. A way to circumvent this drawback would be to introduce a substituent, the electronic behavior of which could be switched reversibly from electron donating to electron withdrawing, in order to trigger the ring-closure/opening processes at a lower potential than that required for performing this process.8 To take benefit of a possible “cation-radical path,” an intramolecular electron transfer is highly desirable; hence, a redox stimuli could be advantageous. For such a purpose, ferrocene is an attractive

10.1021/jp066571g CCC: $37.00 © 2007 American Chemical Society Published on Web 01/23/2007

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Figure 1. (left) Electrochemistry opens access to new colors and a way to bypass photochemically inefficient transformations. The thermal processes are controlled by the intrinsic stability of the radicals, and the direction of transformation depends on the particular system. Further oxidation processes are possible, especially from the more easily oxidized closed isomer. (right) A new color, photochemically stable, is generated by oxidation (potentials vs SCE).

SCHEME 1

substituent. Indeed, it can be introduced by standard chemical reactions, and its one-electron redox couple lies at the bottom limit of the electrochemical redox potential window of the known DTE units explored to date. On the electronic point of view, the ferrocene undergoes a large “umpolung” with no bond modification upon oxidation, and thus if it is a mild donor in the +II state of iron (Fc), it becomes an acceptor in the +III state (Fc+). This “D-to-A” redox switch has been envisioned to trigger nonlinear optical (NLO) properties in the literature.9 Furthermore, depending on the amount of charge transfer from the DTE core in the +III state, it can also be considered as a oxidizer that could be used to destabilize a ring-closed photochromic isomer of the DTE chromophore. A pictorial analogy would be considering the ferrocenyl substituent as the remote control to operate the dithienylethene unit. Results and Discussion Synthesis. The synthesis of the target compounds 1,2-bis(5′-ethynylferrocene-2′-methylthien-3′-yl)cyclopentene (FcPCHopen-Fc) and 1,2-bis(5′-ethynylferrocene-2′-methylthien-3′yl)perfluorocyclopentene (Fc-PCFopen-Fc), outlined is Scheme 1, relies on the ethynylation of the photochromic iodinated precursors and by ethynylferrocene as described previously.6 Thus, the 1,2-bis(5′-iodo-2′-methylthien-3′-yl)-cyclopentene was treated with 5′-ferrocenylethynyl under standard Sonogashira

TABLE 1: UV-Vis Data Recorded in Acetonitrile (ACN) for Neutral Species and in the Presence of 0.1 M of Supporting Electrolyte (Tetrabutylammonium Hexafluorophosphate, N-Bu4NPF6) for Charged Species “in situ” while Controlled Potential Electrolysis Were Performed after the First Oxidation Wave compounds13

λmax (nm)

compounds

λmax (nm)

Fc-PCHopen-Fc Fc-PCHclosed-Fc Fc-PCFopen-Fc Fc-PCFclosed-Fc

434 438, 560 450 615

Fc+-PCHopen-Fc + Fc+-PCHclosed-Fc+ Fc+-PCFopen-Fc+ Fc+-PCFclosed-Fc+

583, 898 569, 825 615, 569, 825

conditions [THF/ diisopropylamine CuI/Pd((PPh3)2Cl2)] at room temperature for 72 h after time optimization. To our surprise, the photochromic compound Fc-PCHopen-Fc was found to be always accompanied with large amount of the so-called “Glaser product”, namely, di-1,4-ferrocenylbutadiyne. Several reaction condition modifications, especially a greater care about the reaction vessel’s degassing, did not improve the yield, and the compound Fc-PCHopen-Fc required purification by repeated preparative thin-layer chromatography to be isolated eventually in a 32% yield. Moving to the diodo fluorinated precursor was even more difficult. Indeed, the Glaser compound coeluated with the coupled product Fc-PCFopen-Fc; thus, a catalytic system to avoid the oxidative dimerization of the alkyne was highly desirable. Recently, a similar system was used successfully to achieve per-ethynylation of polyiodinated compounds.10 The catalyst is generated in situ from Pd2dba3 but with the very bulky trimesitylphosphine ligand, and the coupling, involving the use of the Hu¨nig base (iPr2NEt) along with an excess of tetrabutylammonium iodide in a polar solvent (DMF), allows the reaction to start at temperature as low as -20 °C.11-12 Applied to the diodofluorinated photochromic precursor, the reaction proceed efficiently, and the desired coupled product Fc-PCFopen-Fc was isolated free of Glaser byproduct in a 95% over. An interesting by product, the 1-(5′-ethynylferrocene-2′methylthien-3′-yl)-(5′′-chloro-2′′-methylthien-3′′-yl perfluorocyclopentene (Fc-PCFopen-Cl), could also be isolated in c.a. 5% yield, resulting from an incomplete halogen exchange at the previous step when non-exhaustive purification processes were carried out (for synthetic details, see Supporting Information). Optical and Electrochemical Properties of DithienyletheneFerrocene Switches. In Tables 1 and 2 are gathered the main data obtained for all compounds for electronic absorption spectroscopy and cyclic voltammetry. For comparison purpose, the redox couples of a previously studied compound with almost the same conjugation, namely, 1,2-bis(5′-(para-anisylethynyl)2′-methylthien-3′-yl)-cyclopentene (An-PCHopen-An), as well as of its closed isomer, are also given.

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TABLE 2: Standard and Anodic Peak Potential Values Are Gathered for the Selected Compoundsa compound

E°(Fc + /Fc) (number of e-; ∆E0)b

An-PCHclosed-An

-

An-PCHopen-An Fc-PCHopen-Fc Fc-PCFopen-Fc Fc-PCHclosed-Fcc Fc-PCFclosed-Fc

0.522 (2 e-; 78mV) 0.555 (2 e-; 81mV) 0.549 (2 e-; 79mV)

E°(PCclosed+./PCclosed) 0.52 V (1 e-) 0.68 V (1 e-), 160 mV (∆E°) 1.36 irrev

E°(PCopen+./PCopen) 1.18 irrev 1.44 irrev 1.78 irrev

a All the potential were measured using a saturated calomel electrode, a glassy carbon electrode (0.5 mm in diameter), and a platinum wire as reference, working and counter electrodes, respectively. The scan rate used for investigating ACN solutions of electroactive compounds (ca 1 mM), which contained 0.1 M of n-Bu4NPF6, was 100 mV‚s-1. b Measured using a rotating disk electrode.14 c Not recordable.

Figure 2. UV-vis absorption spectrum of (a) Fc-PCHopen-Fc and Fc-PCHclosed-Fc and (b) Fc-PCFopen-Fc and Fc-PCFclosed-Fc.

The visible part of the electronic absorption spectrum is dominated by the absorption of the ferrocene chromophore: ring-closed compounds being only “darker” (Figure 2, Table 1 where λmax(nm) ) 434 and 450 nm for the perhydro- or perfluorophotochroms, respectively). The electronic interaction between the closed photochromic unit and the ferrocene is seen in the bathochromic shift observed in the visible region absorption maxima upon moving from Fc-PCHclosed-Fc to FcPCFclosed-Fc, for which the LUMO is expected to be lying lower than for the former. Total oxidation leads to the replacement of the Fc-based transition, by bands associated to the ferricinium chromophore. Once again, for the “open” series, the electronwithdrawing effect of the perfluoro chain lowering the bridges’ π orbitals explains the hypsochromic shift of the visible transitions. For the closed one for which only the fluorinated compound gives reliable data (vide infra), one can note the lowenergy transition at 825 nm. Electrochemical studies revealed a reversible oxidation wave c.a. 0.5 V versus SCE (Figure 3, Table 2). By comparison with the well-known one-electron oxidation of ferrocene and tris(4bromophenyl)amine, in the same medium, it was possible to show that this was a two-electrons wave, further confirmed by controlled potential electrolysis. Thus, this redox process was attributed to the oxidation of the two ferrocenyl moieties present in each compound, as expected for similar supramolecular systems containing ferrocene.8 Since these carry multiple redox centers, one can also analyze the comproportionation equilibrium between non-oxidized, mono-oxidized, and bi-oxidized forms, using pulsed linear voltammetry on a rotating disk electrode.14 All Kc’s are in the range of 20, thus not very different from similar compounds with comparable size.15 For both open isomers, a second oxidation wave, irreversible, was found at higher potential which is very reminiscent of that of photochromic (PCopen) unit. The comparison with the bis anisyl (An-PCHopen-An) compound’s redox potentials shows the that PCHopen redox couple is anodically shifted, which is

consistent with the expected “umpolung” of the ferrocenyl groups’ electronic effect as they become once-oxidized, electronwithdrawing substituents. This effect also explains the relatively high potential value for the fluorinated open analogue. Dynamic Behavior of the Oxidized Closed Forms. The electrochemical and spectroelectrochemical behavior of the closed photochromic compounds were found to be more complex than the behavior of the corresponding open ones, since chemical transformations following the oxidation were always found to occur. Starting with the fluorinated compound Fc-PCFclosed-Fc, it was found that voltammograms were strongly dependent on the scan range. Contrary to its open isomer for which the ∆Ep ()Epa - Epc) remains constant whatever the scan range chosen, a new behavior is observed for the closed compound for the first ferrocene centered redox process, at 0.555V. Indeed, at a scan rate of 1 V/s, the ∆Ep increases from 83 to 115 mV when the scan range is expanded from a 0-to-1 V window to a 0.25-to-2 V one, and a careful comparison of the voltammograms shows that only the cathodic peak is affected (Figure 3b, green and light blue scans). This unusual feature could not be attributed to the modification of the electrode surface since identical results were obtained after careful cleaning of the glassy carbon disk electrode with known protocols. A closer look to the cyclic voltammogram run with the largest window also reveals the presence of a second redox process at 1.36 V, but only a very small anodic peak can be recorded. This potential value is very reminiscent of the photochromic core oxidation in itself and should correspond to the Fc+(PCFclosed)+.-Fc+/Fc+-PCFclosed-Fc+ couple.6 However, the anodic peak intensity relative to the ferrocene wave is only of Ipa(this one)/Ipa(Fc) ) 0.2 and not the expected two to one ratio (Figure 3b, inset). According to previous studies reported in the literature7b for related compounds, the low-intensity value as well as the irreversibility of this oxidation wave indicate that the chemical

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Figure 3. (a) Cyclic voltammetry (CV) of Fc-PCHopen-Fc (1.10 mM) in ACN+0.1 M n-Bu4NPF6. Scan range: 0.12/0.80/0.12 V (light blue line) and 0.12/1.60/0.12 V (dark blue line). CV of Fc-PCHclosed-Fc (0.90 mM) in ACN+0.1 M n-Bu4NPF6. Scan range: 0.12/1.00/0.12 V (green line) and 0.12/1.65 /0.12 (red line). (b) CV of Fc-PCFopen-Fc (0.75 and 0.70 mM) in ACN+0.1 M n-Bu4NPF6. Scan range: 0.00/1.00/0.00 (dark line) and 0.25/2.00/0.25 V (dark blue line). Cyclic voltammetry (CV) of Fc-PCFclosed-Fc (0.35 and 0.33 mM) in ACN+0.1 M n-Bu4NPF6. Scan range: 0.25/0.80/0.25 V (green line) and 0.25/2.00/0.25 V (light blue line). Inset: Zoom of the selected scan range used to observe the photochrom core cation-radical oxidation. Scan rate ) 1 V.s-1. Working electrode: glassy carbon electrode (0.5 mm diameter).

reaction, a first order one (EC mechanism) is characteristic of a catalytic oxidation process. The most likely process in this case should be the ring-opening one, since the presence of electron-withdrawing groups (oxidized ferrocene) will facilitate it,6 as it was previously mentioned. It can be thought that during the cyclic voltammetry experiments, the ring-closed form of the Fc+-PCFclosed-Fc+ is oxidized at 1.36 V, leading to the corresponding triply charged cation radical of the photochromic unit, Fc+-(PCFclosed)+.-Fc+. Before a significant change in current can be recorded, under these experimental conditions, the radical cation undergoes a rapid ring-opening reaction to produce the radical cation of the open isomer, Fc+-(PCFopen)+.Fc+, in agreement with the observed behavior of a PCFclosed cation radical destabilized by two electron withdrawing groups. The resulting species Fc+-(PCFopen)+.-Fc+ belong to a very oxidizing redox (1.78 V), hence it immediately oxidizes a neighboring molecule of Fc+-PCFclosed-Fc+. Consequently, this intermolecular electron exchange lead to the depletion of local concentration of the Fc+-PCFclosed-Fc+ thus affecting the peak intensity. This transformation provides an explanation of the first redox process’wave broadening upon expansion of the scanned potential window. Indeed the cathodic peak potential’s lowest value observed (i.e., for the largest scan window) coincides with the value obtained for the pure open isomer, which is consistent with the local build up of the Fc+-PCFopenFc+ species at the electrode. At the scan rate used the transformation has to be very efficient in order to achieve complete conversion of the initial species

Attempts were made to characterize the intermediate, twiceoxidized, Fc+-PCFclosed-Fc+ species. It was generated by controlled-potential electrolysis at 0.9 V and followed by UVvis absorption spectroscopy, and the results were compared to the open isomer’s behavior (panels a (open isomer) and b (closed isomer) of Figure 4). Figure 4b shows that for the closed isomer, upon oxidation at 0.9 V (0.02 F (20 mC), 0.05F (50 mC), 0.75 F)), the absorption band at 615 nm associated with the Fc-PCFclosed-Fc remained mainly constant, and two new absorption bands appeared at 569 and 825 nm. These two bands are related to the formation of ferrocinium containing species such as Fc+-PCFclosed-Fc+ (see Figure 4a). However, upon the passage of 1.00 and 1.50 F, the absorption bands related with the ferrocinium formation were still growing,16 whereas the absorption band at 615 nm (closed isomer) was disappearing.2 Thus, it seems that the reopening process is already slowly operating at this redox stage. Hence, the oxidation of the lateral ferrocene destabilizes efficiently the ring-closed fluorinated photochromic unit. As expected from our previous studies, the two ferrocenium greatly enhance the reactivity of the photochromic unit’s closed cation radical. Less expected was the fact that these substituents also destabilize the neutral photochromic unit’s, lowering the global activation energy for the thermal ring opening. This is in agreement with data we obtained in the past for cyclometallatedruthenium perfluorodithienylethene-bridged dinuclear complexes for which even the mixed valent state was unstable.4 It was also shown in earlier work that thermal reopening of neutral

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Figure 4. UV-vis spectroelectrochemistry of (a) Fc-PCFopen-Fc and its oxidized species and (b) Fc-PCFclosed-Fc and its oxidized species.

Figure 5. UV-vis absorption spectrum upon controlled potential electrolysis of (a) Fc-PCHclosed-Fc and (b) Fc-PCHopen-Fc.

SCHEME 2

while the new color of the oxidized ring-opened compound is mainly due to the absorption properties of the oxidized redox sites (Scheme 2). An even more complex but interesting situation was encountered for the PCH closed system. Surprisingly, both closed and opened isomers showed in cyclic voltammetry experiments exactly the same electrochemical behavior: a first two-electron oxidation wave at 0.522 V versus SCE and a second oxidation wave at 1.436 V versus SCE, whatever the scan rate or the reversal potential. Attempts to generate the fully oxidized species Fc+-PCHclosed-Fc+ by electrolysis lead invariably to solutions the spectra of which was found identical to the Fc+-PCHopenFc+ compound one. Only a very fast ring-opening process could explain such results, since a quantitative isomerization is needed to explain that the last oxidation wave is identical with the one of the opened photochromic unit. Electrochemical studies at high scan rates from 50 to 100.000 V/s performed with ultramicroelectrodes indicate that the first oxidation wave associated to the ferrocene groups remains a reversible wave in the entire range of scan rate studied. The facts that (a) the number of electron remains constant and (b) no peak potential variation with the scan rate was detected, indicating that a EC stepwise mechanism is operating in this case. It is also possible to estimate the rate constant of the ring-opening process being in the 107-109 s-1 range.19 Spectroelectrochemical experiments revealed that the transformation occurred as soon as the controlled potential electrolysis after the first oxidation wave was started, and the disappearance of the PCHclosed transition at 560 nm was quantitative in a few seconds (Figure 5a). This change was also accompanied by the restoration of the PCHopen spectroscopic features (band at 434 nm): After the passage of 3 mC (0.01%

DTE unit was facilitated by electron withdrawing substituents such as gem-dicyanovinyl,17 but the time scale was still much longer. In the present case, it would be tempting to put forward a charge transfer between the ferrocinium and the PCFclosed moieties, generating some Fc-(PCFclosed)+.-Fc+ species, although it is extremely unfavorable on an energetic point of view (by comparing the redox couple value for each partners). The “true” mechanism may just sit in-between, and one could analyze further the interplay between bonds and substituent as it was proposed for the ruthenated species.18 The entire process could be called a “photochromism with memory” for the ring-opening process, because the ring stays open even after back-reduction,

Dithienylethene-Ferrocene Switches SCHEME 3

SCHEME 4

J. Phys. Chem. C, Vol. 111, No. 6, 2007 2775 cidental degeneracy of the PCHclosed and the ferrocene redox couples allowing the second ferrocene unit to be then oxidized at the very same potential as the first one and leading to Fc+PCHclosed - Fc+. The reactive transient intermediate should be Fc+-[PCH.+ closed] -Fc, generated by intramolecular electron transfer from the PCHclosed core to one of the ferrocinum substituents. We believe that this is only at this stage that the ring opening can take place because at least one of the ferricinium groups will act as an electron withdrawing group thus destabilizing the PCH cation-radical.20 Once ring-opened, the PCHopen cation-radical now behaves as a strong oxidizer (1.43 V, Table 2) for the lateral ferrocene, and an exergonic back electron transfer should generate the fully oxidized opened Fc+-PCHclosed-Fc+. An intermolecular self-exchange reaction between ferrocene/ ferrocinium units can then take place to propagate the reactive center (i.e., the ferrocinium) through the medium, in good agreement with the observed catalytic ring-opening mechanism (see Scheme 3). However, while it was possible to show that the heterogeneous electron transfer and the chemical isomerization were two distinct processes, it is difficult at this point to prove the “full charge transfer” leading to a transient species with an oxidized PCHclosed bridge. Concluding Remarks

SCHEME 5

of 1F) at 0.7 V versus SCE (first oxidation wave), the characteristic colored solution became yellow-orange very quickly. Thus, it is possible to conclude that the ring-opening process is catalytic in electrons. Further oxidation of the solution (0.5, 1, or 2 F) leads to the fully oxidized open isomer Fc+-PCHopenFc+ (brown-green solution, λ ) 583 and 898 nm, Figure 5b).16 Upon reduction of the sample at 0.00 V, the spectrum of FcPCHopen-Fc is obtained (yellow solution). From our previous studies6 ( see also Table 2) on similar compounds, it is known that electron-donating substituents lower the first PCHclosed oxidation redox potential down to the “ferrocene zone”; hence, it is hard to say if the photochromic unit is oxidized prior to the ferrocenyl groups. However, if the PCHclosed moiety was oxidized first, the Fc-[PCHclosed].+-Fc radical cation would be surrounded by electrodonating groups so the ring-opening mechanism should be unlikely, while in the other way around, the first transient would then be the mixed valent compound Fc+-PCHclosed-Fc in which the PCHclosed central chromophore remains neutral, but is destabilized in part by the electron withdrawing ferricinium group. The presence of this electron-withdrawing substituent should lift the possible ac-

Thus, in the present paper, we have shown that the presence of ferrocenyl substituent group grafted on the photochromic core makes possible a quantitative thermal reopening upon partial or complete oxidation of this redox active without the need of oxidizing the photochromic unit (Scheme 4). Furthermore, it can be made catalytic in electrons by a proper choice of the central cyclopentene’s substituents: this is the case for the perhydro-DTE compound and seems then to be limited only by the rate of the bimolecular electron self-exchange between ferrocenyl groups.21 On the other hand, for the case of the perfluoro-DTE compound, the reopening process is much slower, and the compound can be fully oxidized. Our previous results, indicating that thermal isomerization processes can be greatly favored if transient DTE cation-radicals are generated, can help to propose a possible mechanism based on an internal charge transfer between the oxidized ancillary electrophore and the DTE unit. For the fluorinated family, despite the large energy difference, reflected in the difference of redox couple between each components, a certain mixing of the Fc+-(PCFclosed)-Fc+ and Fc-(PCFclosed)+.-Fc+ forms should occurs in the ground state, giving a new point of view of the general “A-PCFclosed-A” scheme: while the charge remains mainly localized on the Fc+ unit which then behaves as an electron withdrawing group (“A”), a partial charge-transfer destabilizes the neutral PCFclosed core. However in the perhydro series a more important charge delocalisation can be operating. In terms of valence bond description, the weight of the PCHclosed+ charge localized transient seems to be more important. In other words, the accidental quasidegeneracy of the redox potentials boosts the remote control effect. The question now is: Are the charge-localized species nuclearly relaxed hence being reaction intermediates or should they be considered as mesomeric forms? (Scheme 5) Up to now, no evidence of a transient oxidized photochrom has been observed. Hence, the isomerization reaction could also be due to a partial charge transfer (intramolecular “concerted” oxidation and bond breakdown, Scheme 5). We have thus shown that the breakdown of an internal bond is linked to the redox status of the peripheral ferrocene, which

2776 J. Phys. Chem. C, Vol. 111, No. 6, 2007 thus acts as a remote control for the thermal isomerization process. This process is certainly related to the possibility of an intramolecular charge transfer. In other words the ferrocenyl unit play the role of a “charge garage,” storing and transferring it (totally or in part) to the DTEclosed unit, until its opening process is achieved, then recovering it by an irreversible back electron transfer (Schemes 3 and 4). This allows the electrochemical isomerization processes to occur at lower potentials than when a direct DTE oxidation is required. A further and interesting development would be to deposit such molecules inside a STM junction, in order to expose them to a flux of tunneling electrons. Under such conditions, it may happen that some electrons interact inelastically with the adsorbed molecule, so that the molecules are temporarily, but virtually, oxidized (or reduced).22 This would be a way to detect chemically inelastic events, but also to have a precise view of the vibronic couplings involved in the chemical process. Acknowledgment. We gratefully acknowledge Prof. I. Gallardo, Universitat Auto´noma de Barcelona, for some electrochemical instrumentation, Mrs. C. Viala for recording the NMR spectra, and Dr F. Thiemann for fruitful discussions. We also thank the “Ministerio de Educacion, Cultura y Deporte” of Spain for the award of a postdoctoral grant (G.G.) and the ZEON Corporation for a generous gift of perfluorocyclopentene. Supporting Information Available: Experimental section containing methods and synthesis. This material is available free of charge via the Internet at http//pubs.acs.org. References and Notes (1) Feringa, B. L., Ed. In Molecular Switches; Wiley-VCH: Weinheim, Germany, 2001. (2) Launay, J. P. Joachim, C.; Coudret, C. Molecular Switches. In Dekker Encyclopedia of Nanoscience and Nanotechnology; Marcel Dekker: New York, 2004; p 2145. (3) (a) Irie, M. Chem. ReV. 2000, 100, 1685 (b) Tian, H.; Yang, S. Chem. Soc. ReV. 2004, 33, 85. (4) Fraysse, S.; Coudret, C.; Launay, J.-P. Eur. J. Inorg. Chem. 2000, 1581. (5) (a) Dulic, C.; van der Molen, S. J.; Kudernal, T.; Jonkman, H. T.; De Jong, J. J. D.; Bowden, T. N.; van Esch, J.; Feringa, B. L.; van Wees, B. J. Phys. ReV. Letter. 2003, 91, 207402. (b) He, J.; Chen, F.; Liddell, P. A.; Andr’easson, J.; Straight, S. D.; Gust, D.; Moore, T. A.; Moore, A. L.; Li, J.; Sankey, O. F.; Lindsay, S. M. Nanotech. 2005, 16, 695. (6) Guirado, G.; Coudret, C.; Hliwa, M.; Launay, J.-P. J. Phys. Chem. B 2005, 109 (37), 1744. (7) (a) Gorodetsky, B.; Samachetty, H. D.; Donkers, R. L.; Workentin, M. S.; Branda, N. R. Angew. Chem., Int. Ed. Engl. 2004, 43, 2812 (b) Peters,

Guirado et al. A.; Branda, N. R. J. Am. Chem. Soc. 2003, 125, 3404 (c) Peters, A.; Branda, N. R.; Chem. Commun. 2003, 954 (d) : Zhou, X. H.; Zhang, F. S.; Yuang, P.; Sun, F.; Pu, S. Z.; Zhao, F. Q.; Tung, C. H. Chem. Lett. 2004, 33, 1006 (e) Moriyama, Y.; Matsuda, K.; Tanafuji, N.; Iries, S.; Irie, M. Org. Lett. 2005, 15, 3315 (f) Baron, R.; Onopriyenko, A.; Katz, E.; Lioubashevski, O.; Willner, I.; Wang, S.; Tian, H. Chem Comm. 2006, 2147. (8) The ferrocene groups incorporated to related systems have the same oxidation potential regardless of the structure. For more information on ferrocene in supramolecular systems, see Kaifer, A. E.; G’omez-Kaifer, M. Supramolecular Electrochemistry; Wiley-VCH: Weinheim, Germany, 1999. (9) Malaun, M.; Reeves, Z. R.; Paul, R. L.; Jeffery, J. C.; McCleverty, J. A.; Ward, M. D.; Asselberghs, I.; Clays, K.; Persoons, A. Chem. Commun. 2001, 49. (10) (a) Lu¨tzen, A.; Hapke, M.; Meyer, S. Synthesis, 2002, 15, 2289 (b) Lu¨tzen, A.; Thiemann, F.; Meyer, S. Synthesis 2002, 15, 2771. (11) Justin Thomas, K. R.; Lin, J. T.; Wen, S. Y. Organometallics 2002, 19, 1008. (12) Nakamura, K.; Okubo, H.; Yamaguchi, M. Synlett 1999, 5, 549. (13) For analytical investigations, photochemical irradiations to carry out the ring-closing reactions were performed in 1cm quartz cells placed in front of a standard TLC-visualization UV lamp. Irradiation of the samples was performed until detection of degradation products by 1H NMR, which allows determining the compositions of all photostationary states. The closed-open isomers were purified prior to perform electrochemical and spectroelectrochemical experiments using TLC-semipreparative techniques. (14) ∆E° was obtained by differential pulse voltammetry with a rotating glassy carbon electrode (1000 rev/min) with pulses of 70 ms duration and 25 mV amplitude, see D.E. Richardson, H. Taube, Inorg. Chem. 1981, 20, 1278. (15) Ribou, C.; Launay, J.-P.; Sachtleben, M. L.; Li, H.; Spangler, C. W. Inorg. Chem. 1996, 35, 3735 (16) a) Das, N.; Arif, A. M.; Stang, P. J.; Sieger, M.; Sarkar, B.; Kaim, W.; Fiedler, J. Inorg. Chem. 2005, 44, 5798. b) Mukherjee, P. S.; Das, N.; Kryschenko, Y. K.; Arif, A. M.; Stang, P. J. J. Am. Chem. Soc. 2004, 126, 2464. c) Prins, R. J. Chem. Soc. Chem. Commun. 1970, 280. d) Sohn, Y. S.; Hendrickson, D. N.; Gray, H. B. J. Am. Chem. Soc. 1971, 93, 3603. (17) Gilat, S. L.; Kawai, S. H.; Lehn, J.-M. Chem. Eur. J. 1995, 1, 275. (18) Launay, J.-P.; Fraysse, S.; Coudret, C. Mol. Cryst. Liquid Cryst. 2000, 344, 125. (19) (a) Nadjo, L.; Save´ant, J.-M. J. Electonanal. Chem. 1973, 48, 113. (b) Andrieux C. P.; Save´ant, J.-M. Electrochemical Reactions. In InVestigation of Rates and Mechanism of Reactions, Techniques of Chemistry; Bernasconi, C. F., Ed.; Wiley: New York, 1986; p 305. (20) If we assume that the anodic shift due to the ferrocenium groups on PCH’s waves is the same for both closed and opened isomers, then by comparison with the bis anisyl compound (table 2), one can estimate the [PCHclosed)+./PCHclosed to be only 0.1 V higher than the ferrocenium/ ferrocene one. Thus, an internal electron transfer could take place, leading to a Fc-[PCHclosed).+-Fc+ intermediate in which the radical cation of the PCHclosed unit is partly destabilized by one ferrocenium site. (21) Similar results were obtained by R.H. Mitchell et. al. for similar compounds, see Mitchell, R. H.; Brkic, Z.; Sauro, V. A.; Berg, D. J. J. Am. Chem. Soc. 2003, 125, 7581. (22) Wu, S. W.; Nazin, G. V.; Chen, X.; Qiu, X. H. Phys. ReV. Lett. 2004, 93, 236802. Repp, J.; Meyer, G.; Stojkovic, S.; Gourdon, A.; Joachim, C. Phys. ReV. Lett. 2005, 94, 026803.