Thienylene vinylene dimerization: from solution to self-assembled monolayer on gold

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Cite this:Nanoscale, 2018,10, 1613 Received 28th September 2017, Accepted 27th December 2017 DOI: 10.1039/c7nr07238d rsc.li/nanoscale

Thienylene vinylene dimerization: from solution to self-assembled monolayer on gold †

Sihame Bkhach, Olivier Alévêque, Philippe Blanchard, Christelle Gautier * and Eric Levillain *

The electrochemical and spectroelectrochemical studies of thienylene vinylene (TV) derivatives in the immobilized state are compared with the ones obtained in solution. The results highlight the exaltation of the dimerization process onto TV-based self- assembled monolayers, in which the π interaction is maintained even after 75% dilution.

The electrochemical and optical properties of a myriad of π-conjugated organic molecules have been analysed in solution giving access to a wide range of structure–property relationships.^1–4

However, application of these molecules often requires the passage from the solution to the solid-state. To this end, the elaboration of 2D modified surfaces can represent a useful intermediate situation to study the behaviour of molecules in a controlled and confined environment. In addition, the development of those 2D sophisticated platforms has been already used for recognition,⁵sensing,^6,7and catalysis applications.⁸

Among the variety of available immobilization strategies, the elaboration of self-assembled monolayers (SAMs) on gold oﬀers well organized and reproducible modified surfaces.⁹ Although they are not the most stable organic layers, their ease of implementation and their single molecule thickness make them prominent substrates for the study of interfacial reactions. In this context, we investigated the ability of a thienylenevinylene (TV) derivative to dimerize in the confined state imposed by a SAM. Indeed, TV dimerization under its oxi- dized form is well-known in solution¹⁰but its behavior after immobilization on gold remains unknown. In the present paper, we present comparative electrochemical and spectroelectrochemical studies of a TV derivative in solution and in the immobilized state.

For the study on a surface, we synthesized a 2,5-bis(methyl- thio)-thienylenevinylene dialkyl disulfide derivative, namely TV-SS (Scheme 1), which allows the formation of reproducible SAMs.

This seven-step synthesis is described in the ESI.†

Study of TV core in solution

Cyclic voltammetry

The redox properties of the TV core were probed by cyclic voltammetry on a TV-1 compound (Fig. 1A, see the ESI†for synthesis). At 293 K, in a wide range of concentration (C0), the cyclic voltammogram (CV) of TV-1 in dichloromethane exhibits two reversible one-electron oxidation processes corresponding to the successive generation of the cation radical and dication at apparent redox potentialsEPeak 1andEPeak 2.

As shown in Fig. 1B,EPeak 1andEPeak 2vary linearly with log(C0) with a slope of −29 and +29 mV per log unit, respectively. Such a variation, already observed for oligothiophenes,^10–12 is characteristic of a mechanism invol- Scheme 1 Synthesis of TV-SS.

†Electronic supplementary information (ESI) available. See DOI: 10.1039/

c7nr07238d

Université d’Angers, Laboratoire MOLTECH-Anjou, UMR CNRS 6200, 2 boulevard Lavoisier, 49045 Angers Cedex, France. E-mail: [email protected], [email protected]

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ving the reversible dimerization of two cation radicals according to an ECDimE scheme.^13,14 The variation of EPeak 1 and EPeak 2withC0and the equilibrium constantKis given by

KC01; EPeak 1¼E^°₁RT

2FlogðKC0Þ 0:22RT 2F EPeak 2¼E^°₂þRT

2FlogðKC0Þ þ0:22RT 2F 8>

<

>: ð1Þ

whereE^°₁andE^°₂are the normal potentials of the two successive oxidations of TV-1.

Note that the invariance of apparent potentials with scan rate between 0.10 and 100 V s⁻¹indicates that the dimerization equilibrium is fast. The peak currents scale linearly withv^1/2, as expected for a diﬀusion-controlled process (see the ESI†).

Absorption spectroelectrochemistry (A-SEC)

To go into more detail, the optical properties of the cation radical and dication of TV-1 were investigated by visible and near-IR spectroelectrochemistry at diﬀerent concentrations under thin layer conditions. Fig. 2 shows the evolution of the

absorption spectra of TV-1 recorded during two potential scans between 0.7 V and 1.45 Vvs. AgNO₃/Ag.

The first wave observed on the voltammogram, corresponding to the oxidation of the TV core in TV^•+, involved the appearance of two main peaks, located at 647 nm and 1017 nm. The second subsequent oxidation in TV²⁺led to the disappearance of these two bands and the appearance of a new large one located between 500 and 550 nm.

To assign the absorption bands, spectroelectrochemical experiments were performed at diﬀerent concentrations (i.e.

from 0.05 to 2 mM). Assuming that the absorbances of the dication are only concentration-dependent, Fig. 3A and B show the evolution of the normalized optical spectravs.C₀, extracted atE_{Peak 1}andE_{Peak 2}from spectroelectrochemical experiments and corresponding to the cation radical and dication oxidation states of the TV core, respectively. These results provide evi- dence of the development of the characteristic spectral fea- tures of the π-dimer in the visible region (475–575 and 750–850 nm) at the expense of the cation radical, localized around 600–700 nm,¹⁵when the concentration increases.

Furthermore, the band located around 1100 nm seems to correspond both to the radical cation and to theπ-dimer since its intensity is quasi-independent of the concentration.

Study of TV core in immobilized state

Once each absorption band was assigned to a TV state (i.e.

TV^•+, dimer and TV²⁺), the TV core was immobilized on a surface and the obtained material was spectroelectrochemi- cally studied.^16–18To reach this goal, TV-SS was immobilized on a gold PVD electrode after 15 min of immersion in a 1 mM solution of TV-SS in dichloromethane (Fig. 4).

By comparison with the data obtained in solution, the vol- tabsorptogram recorded on an SAM exclusively composed of TV moieties clearly highlights the absence of absorption bands of TV^•+ in their monomeric form at 647 nm and the unique presence of absorption bands of the pi-dimer in the Fig. 1 (A) CVs of a 2 mM solution of TV-1 in 0.1 M Bu4NPF6/CH2Cl2at

100 mV s⁻¹on a Pt working electrode (Φ= 0.15 cm). (B) Apparent peak of the two successive oxidations (EPeak 1andEPeak 2)vs. log(C0). The slope is in agreement with an ECDIME mechanism (dEPeak/dlog(C0) =

±29 mV at 293 K).

Fig. 2 3D representation of A-SEC of 1 × 10⁻⁴mol L⁻¹of TV-1 in 0.1 M Bu₄NPF₆/CH₂Cl₂at 10 mV s⁻¹and 293 K over two redox cycles under thin-layer conditions (ca.60 µm).

Fig. 3 2D-cut atEPeak 1(A) andEPeak 2(B) extracted from spectroelectrochemical experiments performed at diﬀerent concentrations of TV.

Absorbance values are normalized with the normalization coeﬃcient obtained for the band located at 525 nm atEPeak 2, assuming that the absorption bands of dication are dependent only on the concentration of TV-1.

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visible region (475 nm–575 nm). This important change in the spectroelectrochemical behavior between TV in solution and in its immobilized state points out the eﬀect of the species confinement on their reactivity.

To validate this aﬃrmation, the TV-based SAM was diluted with dodecanethiol to increase the distance between TV enti- ties and thus make the dimerization less favorable. These mixed SAMs were performed by the immersion of TV-based SAMs in a 1 mM solution of dodecanethiol in dichloromethane to achieve the exchange (from 1 min to 5 min, according to the expected surface coverage).

Fig. 5A shows the CVs recorded on mixed SAMs with diﬀerent TV surface coverages.

The exchange between immobilized TV moieties and dodecanethiol in solution allowed an important decrease of TV voltammetric signals. The decrease of TV surface coverage is

accompanied by a potential positive shift of the first oxidation and a negative shift of the second oxidation.

This phenomenon was supported by electrochemical simulation with Kissa-1D©¹⁹(Fig. 5B, KISSA Group web site address http://www.kissagroup.com/) from the following mechanism:

TVAdsÐTV^þ_Adsþe ðk01;α;E^°₁Þ 2TV^þ_AdsÐTV^2þ_{2 Ads} ðkf;krÞ TV^þ_AdsÐTV^2þ_Adsþe ðk02;α;E^°₂Þ

ð2Þ

Note that KISSA-1D© assumes chemical reactions between adsorbed species according to a Langmuir isotherm, excluding therefore a Frumkin isotherm with lateral interactions between species. Thus, the voltammetric wave widening/thin- ning, resulting from these interactions, are not taken into account.

Electrochemical simulations are consistent with previous works^20,21and endorse the presence of a positive and negative potential shift when an EC_DimE process occurs exclusively on the electrode surface.

The impact of TV dilution on the spectroelectrochemical properties of the SAM was subsequently evaluated by A-SEC. A drastic decrease of immobilized TV on the surface (25% TV) led to a decrease of the band attributable to the dimer but didn’t drive the reappearance of a TV^•+ absorption band expected around 650 nm, probably because the confined state really promotes dimerization, even if 75% of the initially immobilized TV species were replaced by dodecanethiols.

Lower TV surface coverages were achievable but the accuracy of the spectroelectrochemical bench was limiting at so low surface coverages, under cyclic voltammetry conditions, i.e.

low integration times.

To highlight the partial appearance of a TV^•+ absorption band at very low surface coverage, we need to perform A-SEC under chronoamperometry conditions, i.e. high integration times, in order to improve the signal-to-noise ratio.

Chronoamperometry was achieved at 1.0 V in order to place Fig. 4 2D representation of A-SEC on a TV based SAM (Γ= 2.5 × 10⁻¹⁰

mol cm⁻²) in 0.1 M Bu₄NPF₆/CH₂Cl₂at 10 mV s⁻¹and 293 K over one redox cycle.i vs.tis indicated in the right side.

Fig. 5 (A) CVs recorded on mixed TV-based SAMs in 0.1 M Bu₄NPF₆/ CH₂Cl₂ at 100 mV s⁻¹ (Γ100% = 2.5 × 10⁻¹⁰ mol cm⁻²). (B) Electrochemical simulation of an EC_DIME process when all the reactants are adsorbed onto the surface according to a Langmuir isotherm:k01= 10¹⁰⁺s⁻¹,α= 0.5,E^°1¼1:125 V,kf= 10²⁰⁺cm²s⁻¹mol⁻¹,kr= 10⁷⁺s⁻¹, k02= 10¹⁰⁺s⁻¹,α= 0.5,E2^°¼1:135 V,v= 0.1 V s⁻¹,T= 293 K andΓ100%= 2.5 × 10⁻¹⁰mol cm⁻².

Fig. 6 Normalized absorption spectra recorded on TV-based SAMs (100% TV, and mixed 75% and 25%) in 0.1 M Bu₄NPF₆/CH₂Cl₂at 1 V (Γ100%= 10⁻¹⁰mol cm⁻²).

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the TV moieties at a potential just before the first oxidation peak, and the absorption signal was integrated during 60 s at this potential.

Fig. 6 shows the results obtained on 100%, 75% and 25%

TV-based SAMs.

The representation of normalized variation of absorbance vs. wavelength, recorded for each SAM during the first oxidation step, makes the observation of the TV^•+ absorption band possible for lower TV coverages. This result corroborates the voltammetric data that showed the impact of the surface coverage on positive and negative potential shifts.

Conclusions

The electrochemical and spectroelectrochemical studies of thienylene vinylene (TV) derivatives in solution and in the immobilized state were compared.

In solution, the electrochemical data highlighted the occur- rence of an ECDimE mechanism and the optical bands of the products arising from TV-1 redox reactions (i.e.cation radical, dimer and dication) were assignedviaA-SEC experiments.

Voltammetric data performed onto TV-based SAMs showed that the dimerization also occurs on the confined state. The accuracy of spectroelectrochemical experiments revealed the presence of dimers only, which is maintained even after a 75% decrease of the TV coverage. Nevertheless, a very weak absorbance of the cation radical has been detected at low surface coverage by chronoamperometry coupled to absorption spectroscopy.

Con ﬂ icts of interest

There are no conflicts to declare.

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