Position and Orientation Control of a Photo- and Electrochromic

Article pubs.acs.org/JPCC

Position and Orientation Control of a Photo- and Electrochromic Dithienylethene Using a Tripodal Anchor on Gold Surfaces Thomas C. Pijper,†,‡ Oleksii Ivashenko,† Martin Walko,‡ Petra Rudolf,† Wesley R. Browne,*,†,‡ and Ben L. Feringa*,†,‡ †

Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747AG Groningen, The Netherlands Centre for Systems Chemistry, Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747AG Groningen, The Netherlands

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S Supporting Information *

ABSTRACT: A tripodal system for anchoring photochromic dithienylethenes on gold surfaces is reported. The selfassembled monolayers of a tripod-functionalized dithienylethene were characterized by cyclic voltammetry, surfaceenhanced Raman spectroscopy (SERS), and X-ray photoelectron spectroscopy (XPS). These data are compared with solution studies, solid state Raman spectroscopy, and density functional theory (DFT) calculations. We show that the tripod-functionalized dithienylethene forms stable monolayers on gold in which all three legs of the tripod are adsorbed via the thiol units, thus providing a fixed position and orientation of the dithienylethene moiety with respect to the surface. Importantly, immobilization in this way allows for retention of both the photochemical and electrochemical functionality of the dithienylethene unit and reduces photochemical fatigue observed in solution.

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monolayer to lie flat on the surface, which can result in a disorganized SAM.14 Furthermore, proximity to a surface can result in loss of functionality through interactions between the surface and the monolayer, in which case the second requirement is not met. One example of such interactions is the quenching of the electronic excited states of a photochromic compound reducing or even blocking completely its photochemical responsiveness.15 Such unfavorable interactions with the surface were recently also observed in our study on the photochemical behavior of asymmetric dithienylethenes tethered to gold through a short thiol linker,16 and with surface bound light-driven molecular rotary motors.17 Our earlier findings prompted us to consider alternative strategies for tethering a photochromic switching unit to a gold surface that meet the requirements outlined above. Here, we utilize a tripodal structure reminiscent to that employed by Tour and co-workers.18,19 This anchoring unit consists of three thiol-substituted diphenyl ethyne units attached to a silicon atom (Scheme 2). This design was chosen over other tripodal structures reported previously,20−26 because the silicon atom should reduce potential long distance through-bond interactions with the gold substrate.27,28 In addition, the relatively large structure increases the separation of the switching unit from the surface by an estimated 7 Å (based on density

INTRODUCTION The design of solid-state molecular electronic and photonic devices incorporating photochromic or redox-active molecules is a topic of continuing interest.1−4 Several aspects pertaining to the photochromic or redox-switchable component in the material present challenges in the design of such devices. First, the switchable component should be incorporated with maximum control over its position and its orientation with respect to the surface it is interfaced with. Second, undesirable interactions between the photo/electro-switchable molecules and the bulk substrate should be minimized so that the responses to external stimuli observed in solution are retained when immobilized. An important method for incorporating organic molecules into devices is through the formation of selfassembled monolayers (SAMs). Responsive compounds can undergo self-assembly by introducing functional groups that show affinity toward (or can react with) the substrate on which the self-assembly is desired.5,6 The self-assembly of compounds on a solid surface typically involves the use of a linker that provides a single anchoring point to the substrate.5,6 Such an anchoring strategy has been used widely for the tethering of photochromic units to a range of substrates, such as gold,7−10 SiOx,11,12 and ITO.13 However, while effective in anchoring the compound to the substrate, such an approach does not meet the first requirement outlined above as it does not dictate the orientation of the switchable molecule with respect to the surface. This may allow a substantial part of the molecules in the self-assembled © XXXX American Chemical Society

Received: December 13, 2014 Revised: January 18, 2015

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DOI: 10.1021/jp512424d J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Scheme 1. Photochemical Switching of a Dithienylethene and the Irreversible Formation of a Photochemically Inert Isomer

Scheme 2. (a) Structure of 1o and (b) Photochemical Switching between Open and Closed Forms of Dithienylethene 1 When Immobilized on a Gold Surface

functional theory calculations, vide inf ra), thus minimizing through-space interactions with the substrate. In contrast to the design by Tour and co-workers,29 which has methylene spacers connecting the thiol functionalities to the phenyl groups, we have chosen to attach the thiol functionalities directly to the phenyl groups. The rationale for this design choice is that aryl thiols adsorb on gold much more readily, without the need for an exogenous base to cleave the acetate protecting group prior to self-assembly. The switching unit chosen for attachment to the tripodal anchoring unit is a diphenyl-substituted dithienyl-perhydrocyclopentene. Dithienylethenes are a popular class of photochromic molecules that can switch reversibly between an closed and open isomer by irradiation with UV and visible light, respectively (Scheme 1).30−32 Important characteristics are that the two isomers can be addressed independently due to differences in their UV/vis absorption spectra, they are thermally stable,30 and display distinct electronic properties because of differences in their π-systems.7,33,34 In addition, it has been shown that switching can for certain systems also be induced electrochemically.35−41 A drawback of dithienylethenes, however, is that, as has been reported for a few specific designs, they can display a small amount of switching fatigue that becomes noticeable upon repeated switching.30,42 Irie and co-workers have shown that, for a related system, switching fatigue was the result of the conversion of the closed form isomer to a photoinactive isomer (Scheme 1).42 In the present study, the importance of this process will be addressed as well. Herein, we report the synthesis and characterization, and the electro- and photochemical properties of dithienylethene 1 (Scheme 2) in solution and as SAMs on gold substrates. We demonstrate that the tripod structure of 1 enables the formation of SAMs on gold surfaces in which all three thiol functionalities are adsorbed. The SAMs formed were found to be stable under the conditions applied for photochemical switching and, to a lesser extent, electrochemical switching.

Furthermore, we demonstrate that 1 displays reversible photochemical and electrochemical switching, both in solution and on gold substrates. Finally, although 1 displays photochemical switching fatigue in solution, this is not observed during photochemical switching of SAMs of 1 on gold surfaces.

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RESULTS AND DISCUSSION Synthesis of Dithienylethene 1. Dithienylethene 1 was synthesized using a convergent synthetic route (Scheme S1). The synthesis of the tripod started with the synthesis of thioether 4, which was prepared by first converting 1,3diiodobenzene to thioether 2 by treatment with t-butylthiol and Pd(PPh3)4 (Scheme S1). A Sonogashira cross-coupling43 of 2 with trimethylsilylacetylene (TMS-acetylene) was then used to obtain thioether 3 in 85% yield. Removal of the TMS group of 3 by treatment with TBAF provided 4 in 85% yield. It should be noted that introduction of the TMS-protected acetylene group first was attempted, however, subsequent introduction of the thioether group was found to result in undesirable reaction of the acetylene. The tripod was prepared starting with lithiation of 1,4diiodobenzene followed by treatment with tetraethoxysilane, providing triiodide 5. A Sonogashira cross-coupling of 5 with thioether 4 was used to prepare trithioether 6 in 87% yield. The phenyl spacer connecting the dithienylethene moiety to the tripod was introduced by the lithiation of 1,4-diiodobenzene and subsequent treatment with 6, which provided trithioether 7 in 68% yield (Scheme S1). The dithienylethene moiety was synthesized starting from a dichloro-substituted dithienylethene precursor.44 Successive treatment with n-BuLi and tributylborate provided the crude boronic ester, which was used in a Suzuki cross-coupling reaction with 4-bromotoluene in order to obtain the tolylsubstituted dithienylethene 8. After conversion of 8 to the corresponding boronic ester as above, a Suzuki cross-coupling reaction with trithioether 7 afforded dithienylethene 9 in 87% B



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weaker absorption with a maximum at 373 nm. These spectral changes are characteristic of the formation of the closed isomer 1c.30 Irradiation at >440 nm reversed these spectral changes although the broad absorption in the visible region did not disappear fully. Repeated switching of the sample by alternating between irradiation at 365 nm and >440 nm showed an increase in the intensity of this residual absorption with each cycle (Figure 1b). The photochemical switching fatigue observed for 1 is attributed to the conversion of 1c into its isomer 1byprod (Scheme 3). The visible absorption band of 1byprod has a maximum at 527 nm, which is shifted hypsochromically compared to the visible absorption of 1c. This is in agreement with dithienylethene switching fatigue reported by Irie and coworkers.42 The photochemical switching was also monitored by 1H NMR spectroscopy (Figure 2). Irradiation of 1o in CD2Cl2 at 365 nm resulted in the formation of a second isomer identified as 1c. The most significant spectral changes observed upon UV irradiation were the shifts in the signals of the hydrogens of the thiophene groups from 7.13 and 7.01 ppm to 6.54 and 6.42 ppm (due to loss of aromaticity of the thiophenes) and the shifts of the hydrogens of the central cyclopentene group from 2.84 and 2.13−2.05 ppm to 2.51−2.46 and 1.93−1.85 ppm (due to the rearrangement of the compound’s π-system). Continued irradiation at 365 nm resulted in the appearance of a second isomer identified as 1byprod. Compared to the spectrum of 1c, 1byprod exhibited the signals of the hydrogens of the thiophenes shifted slightly, from 6.54 and 6.42 ppm to 6.56 and 6.43 nm, indicating that the thiophenes in this species have not regained their aromaticity (which is consistent with the expected molecular structure). Furthermore, the signals from the two central methyl groups displayed a shift from 2.00 ppm to 2.55 and 2.50 ppm. This downfield shift of the methyls for 1byprod is in agreement with findings by Irie and co-workers.42 Subsequent irradiation at >440 nm resulted in the disappearance of 1c and reappearance of 1o but did not change the intensity of the signals of 1byprod in the spectrum.45 It has been shown previously that dithienylethenes can undergo opening and closing both photochemically and electrochemically.35,40,41 This was found to be the case for dithienylethene 1 also (Figure 3). As with structurally related dithienylethenes,35,40 an irreversible oxidation of the open form, 1o, was observed at 1.1−1.2 V (Figure 3b), and is assigned to a two-electron oxidation that is followed by cyclization of the dithienylethene unit to the closed form (Figure 3a).35,40 Two reduction waves at 0.72 and 0.38 V were observed on the return cycling from 1.1 to 0.0 V, which are assigned to the reduction of 1c2+ to 1c+ and, finally, to 1c. The two corresponding oxidation

yield. Finally, conversion of the thioether groups to thioester groups by treatment with BBr3 and acetyl chloride provided dithienylethene 1o in 45% yield (Scheme S1). For details of spectroscopic characterization, see ESI. Photochemical Switching of 1 in Solution. The photochemical behavior of dithienylethene 1 in solution was studied by UV/vis absorption and 1H NMR spectroscopy, and cyclic voltammetry. The UV/vis spectrum of the open form isomer of 1 (1o) in toluene displayed several strong absorption bands 370 nm (Figure 1a). Irradiation of the sample at 365 nm resulted in the appearance of a broad absorption with a maximum at 542 nm as well as a

Figure 1. Photochemical switching of dithienylethene 1 in toluene monitored by UV/vis absorption spectroscopy. (a) Absorption spectra of 1o (solid) and 1PSS365 (dashed) in toluene. (b) Absorbance at 542 nm measured over five switching cycles. Irradiation at 365 nm was carried out such that 99% of the absorbance at PSS365 nm was reached in the first cycle.

Scheme 3. Conversion of Dithienylethene 1c into the Photochemical Byproduct 1byproda

a The structure shown is inferred by comparison with the 1H NMR and UV/Vis absorption spectrum of an analogous compound reported by Irie and co-workers.42

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Figure 2. Photochemical switching of 1 in CD2Cl2 monitored with 1H NMR spectroscopy. 1H NMR spectra of (a) 1o before irradiation, (b) after irradiation at 365 nm, and (c) after irradiation at >440 nm.

Figure 4. Raman spectra of dithienylethenes 1o (top), 1PSS365 (middle), and 1c (bottom, obtained by subtraction of the top and middle spectra).

the spectrum of 1PSS365. Comparing the spectra of 1o and 1c, it was found that the bands at 2224, 1586, and 1152 cm−1 are present in both spectra and that their relative intensities remain constant. As such, these are attributed to the nonphotoactive part of the molecule, i.e. the tripod moiety. The region 1520− 1200 cm−1 of the spectra of 1o and 1c shows several differences. Bands in this region are thus attributed to the dithienylethene moiety. The most pronounced of these bands is a band at 1506 cm−1 in the spectrum of 1c. This intense band was observed in earlier Raman studies of dithienylethenes16,46 and is assigned as a stretching vibration of the molecule’s conjugated polyene backbone. In order to confirm that the recorded spectra correspond to isomers of 1 as well as to elucidate their spectral features, Raman spectra of 1o and 1c were calculated using density functional theory (Figure 5). The B97-D3 functional47,48 and def2-SVP basis set49 were used for the calculation of the geometry and the vibrational modes. Raman activities were calculated using the B3LYP hybrid functional50−52 (using VWN

Figure 3. (a) Electrochemical and photochemical switching processes typically observed for dithienylethenes. (b) Cyclic voltammogram of dithienylethene 1o in DCM/0.1 M (TBA)PF6, recorded at 0.5 V s−1 with a glassy carbon working, Pt counter, and SCE reference electrode.

waves (1c to 1c+ to 1c2+) were observed in the second and subsequent cycles, at 0.44 and 0.78 V. Repeated cycling caused the intensity of the redox waves of 1c to increase until diffusion to and from the electrode was equilibrated. Solid State Raman Spectra of 1. Raman spectra (Figure 4) were recorded of dithienylethene 1o in the solid state as well as for a sample of 1o that was irradiated at 365 nm in solution to its photostationary state (PSS) and subsequently concentrated in vacuo to a solid (denoted 1PSS365). The spectrum of 1c was obtained by a scaled subtraction of the spectrum of 1o from D



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Figure 5. Experimentally obtained (red) and calculated (blue) Raman spectra of dithienylethenes 1o (top), 1c (middle), and 1byprod (bottom).

formula 1 RPA correlation52) and def2-SVPD basis set.53 A scaling factor of 0.998 was applied to the calculated wavenumbers in order to improve correspondence between the theoretical and experimental spectra. It was found that the calculated spectra of 1o and 1c resemble the experimental spectra closely. Analysis of the calculated vibrational modes supports the assignment of modes present in the recorded spectra. The Raman spectrum of 1byprod was also calculated (Figure 5, bottom spectrum). This isomer is predicted to show Raman scattering at 1559 and 1518 cm−1, both originating from the dithienylethene moiety.54 X-ray Photoelectron Spectroscopy of 1o in a SAM. SAMs of 1o on gold were prepared by immersing a freshly prepared gold on mica substrate into a solution of 1o (0.5 mM) in dichloromethane for 16 h and analyzed by X-ray photoelectron spectroscopy (XPS) to determine the elemental composition after SAM formation. In the overview spectra of 1o on gold/mica (see electronic Supporting Information) contributions from carbon (90.0 ± 0.3 atomic percent (at. %)), sulfur (6.0 ± 0.2 at. %), oxygen (4.5 ± 0.5 at. %), and Au were detected. The high resolution sulfur 2p XPS spectrum (Figure 6a) was fitted with two doublet components, centered at 162.0 eV (32%) and at 163.7 eV (68%). Contributions from oxidized species (i.e., SOx), typically observed above 167 eV,55 were not apparent. The absence of oxidized sulfur species indicates that deprotection of the thiols occurs via cleavage of the S−Ac bond with subsequent formation of a bond with the gold surface as opposed to undergoing oxidation. The doublet peak at 162.0 eV is characteristic for chemisorbed thiolates56 and its apparent intensity is consistent with a monolayer thickness. The components at 163.7 eV originate from the thiophene groups of 1o.57 The area ratio of the two sulfur components S−C/S− Au is 2.1 and is higher than expected (0.66) based on the stoichiometry in 1o, but is consistent with the attenuation of the emission from three S−Au at the surface with respect to the two S−C at the top of the monolayer. The calculated C/S ratio

Figure 6. (a) S 2p core level photoelectron spectrum of a SAM of dithienylethene 1o on gold/mica. The peak was fitted with two doublet components related to the chemisorbed S−Au at 162.0 eV and thiophene S−C at 163.7 eV. (b) C 1s core level photoelectron spectrum of 1o on gold/mica fitted with two singlet components at 284.6 eV from C−C, CC, and C−Si, and at 285.7 eV from C−S and possibly C−O. (c) O 1s core level photoelectron spectrum of 1o on gold/mica showing contributions from C−O−C and/or C−OH at 532.5 eV due to contamination from air during sample transfer. Notably, signals assignable to CO groups (531.0−531.5) are absent.

of 15 is close to the stoichiometrically expected value of 14 and the mismatch is similarly rationalized by attenuation. The carbon 1s core level region of the XPS spectrum (Figure 6b) was fitted with two components: one at 284.6 eV accounting for C−C aromatic and aliphatic bonds and C−Si with a total 91% of the overall C intensity; and the second one at 285.7 eV originating from C−S and possibly C−O. Contributions from the carbonyl of the acetate protecting groups, which are observed typically around 287 eV, were not apparent in the C 1s spectrum of the SAM. This suggests that successful deprotection and the subsequent removal of the acetate group from the surface occurs upon SAM formation. In the oxygen 1s core level XPS spectrum (Figure 6c), a component at 532.5 eV was observed related to C−OH or C−O−C, and again contributions assignable to CO (expected typically at 531.0−531.5 eV)29 were absent, which is in agreement with the C 1s XPS spectrum. The stoichiometric ratio of C/S leads to the conclusion that 1 adsorbs on the surface without loss of molecular integrity. The attenuation of the S−Au component in the S 2p spectrum E



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and the absence of contributions from CO groups in the C 1s and O 1s spectral regions testify that chemisorption of 1 rather than physisorption (i.e., in the protected form, 1Sac) occurs upon self-assembly and confirm that binding of all three “legs” to the surface occurs. Exposure to UV light for 2 h within the XPS vacuum chamber did not result in detectable changes in S 2p core level spectrum, which suggests that the S−Au bond is stable toward UV irradiation. Surface-Enhanced Raman Spectroscopy. Surface enhanced Raman (SERS) spectra of 1o or 1PSS365 obtained by mixing with aggregated colloidal gold were recorded at 785 nm (Figure 7). The recorded spectra were found to display

Figure 8. SERS spectra of a SAM of dithienylethene 1o on a roughened gold bead before irradiation (top), after irradiation at 365 nm (middle), and after subsequent irradiation at 785 nm (bottom).

Figure 7. SERS spectra of dithienylethenes 1o (top) and 1PSS365 (bottom) mixed with aggregated colloidal gold.

significant differences compared to the Raman spectra of the solid compounds (Figure 4). In particular, the alkyne stretching band at 2224 cm−1 was broadened substantially and shifted to a lower wavenumber, while the band at 1152 cm−1 (identified as an acetylene−Ph stretching mode) was reduced in relative intensity. These spectral changes suggest a chemical reaction of the acetylene groups with the gold, which has been observed previously.58 Notably, the spectra of 1o and 1PSS365 showed significant differences in the region 1000 to 1650 cm−1, however, given that it is clear that the acetylene units have reacted with gold from the colloid a detailed further analysis of the spectra is not warranted. SERS spectra of SAMs of 1 on electrochemically roughened gold surfaces were more informative and indicated that under these conditions the acetylene units did not react with gold. SERS spectra of SAMs of 1o on roughened gold beads (Figure 8) were found to match the Raman spectra obtained from bulk solid samples closely. Bands originating from the acetylene groups (ca. 2000 cm−1) were essentially identical to those of the solid samples, confirming that they did not react with the surface. Irradiation of the SAM of 1o at 365 nm resulted in the appearance of an intense band at 1508 cm−1 as well as several additional bands in the region 1400−1200 cm−1, which is attributed to the presence of 1c. As was found in our earlier study on dithienylethene based SAMs on gold, 16 the observation of 1c was complicated by the fact that the laser (785 nm) used to obtain Raman spectra also induced ringopening, even when employing low intensities and short exposure times (440 nm also. A SERS spectrum of 1PSS365 self-assembled on gold (Figure 9, top spectrum) was found to differ from the SERS spectrum

Figure 9. SERS spectra of a SAM of (top) 1PSS365 and (bottom) a mixture of 1o and 1byprod (prepared by extended irradiation of a solution of 1o at 365 nm followed by irradiation with visible light to revert any remaining 1c to 1o) on a roughened gold bead.

obtained from a SAM of 1o irradiated at 365 nm; the largest difference being the presence of two intense signals at 1508 and 1478 cm−1. These signals did not reduce in intensity upon irradiation at >440 nm and are assigned to the presence of 1byprod, which is formed to a small extent during irradiation to the PSS365 nm in solution (vide supra). A sample of 1byprod generated in solution (by irradiation at 365 nm for extended periods and subsequently irradiated at >400 nm to revert any remaining 1c to 1o) and then self-assembled on gold (Figure 9, bottom spectrum) was found to give the same spectrum. It should be noted that 1byprod is not observed after 1o on gold is irradiated successively at 365 and >440 nm (Figure 8), which indicates that the photochemical switching fatigue observed in solution is absent when the molecules were self-assembled on gold. This was corroborated by the observation that irradiation of 1o on gold at 355 nm (10 mW) for 1 h did not result in the appearance of Raman bands corresponding to 1byprod. Cyclic Voltammetry of SAMs of 1o on Gold Electrodes. As observed for 1o in solution (vide supra), oxidatively induced switching was observed for a SAM of 1o on gold. Cycling between 0.0 and 1.1 V59 at 2 V s−1 effected the formation of 1c manifested in the appearance of reversible redox waves at 0.46 and 0.73 V (Figure 10). The peak-to-peak separation (after compensation for solution resistance) for the first and second redox process were found to be 11 mV and 14 mV, respectively, F



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monolayer occurs predominantly rather than electrochemically driven ring-opening. Cyclic voltammetry of SAMs of 1o recorded on a roughened gold bead allowed for SERS spectra of the SAM to be obtained between cycles. It was found that upon full depletion of 1c, the SAM consisted mainly of 1o with a minor amount of 1byprod observable (Figure 12). As such, the lower amount of

Figure 12. SERS spectrum of a SAM of dithienylethene 1o on a roughened gold bead after electrochemically induced ring-closing followed by electrochemically induced ring-opening.

Figure 10. Cyclic voltammograms of a SAM of dithienylethene 1o on a gold bead electrode, recorded at 2 V s−1 in DCM/0.1 M (TBA)PF6 with a Pt counter electrode and SCE reference electrode. Repeated cycling results in the formation of 1c. The inset shows the scan rate dependence of the non-Faradaic (0.125 V vs SCE, squares) and Faradaic (0.70 V vs SCE, circles) currents of dithienylethene 1c. R2 for the linear fits is >0.99.

detectable 1c upon the second electrochemical closing/opening cycle is attributed primarily to the partial desorption of the SAM. Such desorption has been observed in other instances where slow scan rates were used and is attributed to oxidation of the gold substrate.41 Photochemically induced switching of 1 on smooth gold beads was monitored by cycling between 0.0 and 0.8 at 0.5 V s−1 (Figure 13). The number of cycles was limited in order to

which is close to ideal behavior for a surface bound redox couple and indicates that the rate of heterogeneous electron transfer is fast on the cyclic voltammetry time scale (>0.05 s−1). Similarly a linear relation between current and scan rate was observed, consistent with surface confinement of the redox active species (Figure 10, inset). A surface coverage Γ of ca. 5 × 10−11 mol cm−2, calculated from the first redox wave of the closed form, corresponds to that typically observed for a monolayer of a small molecule.41 Cycling between 0.0 and 0.8 at 0.25 V s−1 effected the depletion of the redox response of 1c (Figure 11) with a complete loss of current after ca. 100 cycles. Subsequent cycling between 0.0 and 1.1 V resulted in the reappearance of the redox waves of 1c, albeit with peak areas approximately 90% less than those observed initially, which indicates desorption of the

Figure 13. Cyclic voltammogram of a SAM of dithienylethene 1o on a smooth gold bead electrode before irradiation (thin line), after irradiation at 365 nm (thick line), and after subsequent irradiation at >440 nm (dashed line). Recorded at 0.5 V s−1 in DCM/0.1 M (TBA)PF6 with a Pt counter electrode and an SCE reference electrode.

minimize electrochemical switching. It was found that irradiation of the bead at 365 nm resulted in the formation of 1c. Subsequent irradiation at >440 nm resulted in the depletion of the signal of 1c.

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CONCLUSIONS The photochemical behavior of dithienylethene 1 was studied in solution as well as on gold substrates. In solution, 1 was shown to display reversible photochemical switching albeit with switching fatigue that was observed over multiple photochemical switching cycles due to irreversible isomerization. Self-

Figure 11. Cyclic voltammograms of a SAM of dithienylethene 1c on a gold bead electrode, recorded at 0.25 V s−1 in DCM/0.1 M (TBA)PF6 with a Pt counter electrode and an SCE reference electrode. Repeated cycling results in the depletion of the redox waves of 1c. Only every 10th cycle is shown. G



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Funding

assembly of 1 on gold substrates was confirmed by XPS and cyclic voltammetry to result in the formation of monolayers. XPS indicates that the three thiol groups of 1 are chemisorbed to the surface, which suggests that the orientation of 1 on the surface is as anticipated (Scheme 2). Furthermore, spectral characteristics of the acetate protecting groups were not detected by XPS. XPS and SERS confirmed adsorption of the tripod and the stability of the monolayer upon irradiation with UV and visible light. SAMs of 1 on gold are shown by SERS spectroscopy and cyclic voltammetry to display reversible photochemical switching. Importantly, self-assembly of the fatigue product (1byprod), generated by prolonged irradiation of 1o in solution, onto a gold surface allowed for observation of the SERS spectrum of 1byprod (Figure 9). In contrast to 1c, 1byprod is not subject to photoreversion upon irradiation at 785 nm and, together with its visible absorption which leads to preresonant enhancement of the spectrum, means that even if present in minor amounts in a SAM, it can be observed readily. Hence, the absence of bands from 1byprod in the spectra of SAMs of 1o, even after extended irradiation at 365 nm, confirms that this fatigue process is suppressed substantially when immobilized. The origin of the quenching of one chemical pathway and not another is unclear at present but given that the orientation of the switch is relatively well controlled, it may be that the dipole moments of the transitions that access the state leading to irreversible isomerization and reversible cyclization. As such, we conclude that, despite the design of the tripodal anchoring unit, there remains sufficient interaction between the switching unit and the gold surface to quench, albeit in this case usefully, certain photochemical processes (i.e., the irreversible rearrangement, Scheme 2). In conclusion, the tripodal structure has been shown to successfully link a photoswitchable unit to a gold surface while retaining the photochemical and redox switching properties. As the tripod structure has all of its thiols adsorbed, there is full control over position and orientation of the photoswitchable unit with respect to the gold surface. This provides a substantial improvement over the use of surface attachment units that offer only a single point of attachment and is an important step forward in the ongoing design of switchable SAM-based organic light responsive devices.

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The Foundation for Fundamental Research on Matter (FOM, a subsidiary of NWO) and the “Zernike Top Research School” (Bonus Incentive Scheme) and the Gravitation program (024.001.035, B.L.F. and W.R.B.) of The Netherlands’ Ministry of Education, Science, and Culture (T.C.P., O.I.), NWO (VIDI Grant No. 700.57.428, W.R.B.) and the European Research Council (ERC Advanced Grant No. 227897, B.L.F.) are acknowledged for funding. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Dr Apparao Draksharapu is thanked for assistance with preparation of the aqueous colloidal gold dispersion.

ASSOCIATED CONTENT

* Supporting Information S

Details of the synthesis and characterization of all compounds, additional spectroscopic data, and coordinates for DFT calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

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*(W.R.B.) E-mail: [email protected]. Telephone: +31-503634428. *(B.L.F.) E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. The authors declare no competing financial interests. H



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