Isomerization of Orthogonal Molecular Switches Encapsulated within

Article pubs.acs.org/JPCC

Isomerization of Orthogonal Molecular Switches Encapsulated within Micelles Solubilizing Carbon Nanotubes Stefanie K. Kreft,*,†,∥ Michael Åxman Petersen,‡,⊥ Mogens Brøndsted Nielsen,§ Stephanie Reich,† and Antonio Setaro*,† †

Institut für Experimentalphysik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany Institut für Chemie, Technische Universität Berlin, Straße des 17. Juni 135, 10623 Berlin, Germany § Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 København, Denmark ∥ Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 1TN, United Kingdom ⊥ Department of Chemistry, Technical University of Denmark, 2800 Kongens Lyngby, Denmark ‡

ABSTRACT: We study the effects of the proximity of the orthogonal dipole-switching moiety dihydroazulene/vinylheptafulvene (DHA/VHF) to carbon nanotubes (CNTs). The switches are introduced into a micelle surrounding the CNTs, thereby achieving very close proximity between the molecules and the CNTs for the first time. The change of the molecules’ configuration is not hindered by its encapsulation: We report the reversible switching of molecules inside CNT surrounding micelles. The orthogonality of the switch also allows us for the first time to observe the effect of the molecule on the emission spectra of the CNTs and the resulting reversible redshift of the nanotubes’ emission by the change of the molecules’ conformation.

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INTRODUCTION Several systems combining the well-known properties of CNTs and of molecular switches have been investigated in recent years,3,7−9,12,16 aiming to both understand the interaction between molecular switches and surfaces, here represented by CNTs, and to obtain CTN-hybrids for exploitation in devices. So far, most of the molecular switches had the drawback of isomerization being initiated in the same wavelength region in which CNTs are probed, leaving only one of the molecule’s configurations stable enough for analysis.1 Recently, we have functionalized CNTs with a DHA/VHFbiphenyl compound.18 Because the back-isomerization of this compound is only thermally driven, properties of the CNTs in the UV−vis region can be probed without inducing isomerization. The configuration-dependent dipole moment of the molecular switch14 is expected to influence the CNTs, as predicted for analogous dipole-switching compounds.11 As reported in ref 18 and supported by the work in ref 2, the distance between the dipole and the CNT is essential for effective interaction. The system reported here therefore utilizes the micelle-swelling technique,15,19 introducing the DHA/VHF compound directly into micelles surrounding the CNTs, thereby securing close proximity between the two. The stability and accessibility of the ground state of the molecules, especially when these are in proximity to other molecular moieties, is of importance. Studies indicated that the energetic overlap between the pyrene anchor and the azobenzene moiety in ref 1 made the fast de-excitation pathway © 2015 American Chemical Society

through pyrene emission so efficient that no long-lived isomerization could take place and the azobenzene was found only in its trans form. Other mechanisms can hinder the isomerization of molecular switches, e.g., the polarity of the environment and/or substituents,4,5 steric hindrance, and exciton coupling between adjacent switching species.6 Here, we report the isomerization of molecular switches encapsulated within micelles containing CNTs. The effect of the molecular dipole moment on the electronic and optical properties of the CNTs was investigated. We observed a reversible redshift of the CNT emission upon switching of the molecular configuration compared to the emission of pure CNT micelles. We additionally investigated the effect of the presence of the CNTs on the kinetics of the molecularswitching events.

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EXPERIMENTAL METHODS We solubilized CoMoCAT-CNTs in water in a commonly used approach with SDS as surfactant.13 THF was used as a carrier for the non-water-soluble DHA/VHF compound and introduced into the CNT-containing solution. Wang et al. showed the introduction of organic solvent into a CNT-containing micelle.19 Here, THF was utilized as carrier for the non-watersoluble molecular switches. Using an approach similar to that of Received: April 23, 2015 Revised: June 5, 2015 Published: June 22, 2015 15731

DOI: 10.1021/acs.jpcc.5b03899 J. Phys. Chem. C 2015, 119, 15731−15734

Article

The Journal of Physical Chemistry C

absorption measurements (Figure 2). The absorption spectra confirm the presence of the CNTs as well as of DHA and VHF.

Roquelet et al., molecules suspended in THF were introduced into the micelles.15 The solvent evaporated, leaving the molecules encapsulated within the micelles. Additionally, we introduced the molecules in otherwise empty SDS micelles to evaluate the effect of the CNTs on the molecular properties. The specifications of the dihydroazulene (DHA)/vinylheptafulvene (VHF) compound have been described in one of our previous publications.18 In this work, we used the 2phenyl-substituted DHA/VHF moiety without the biphenyl linker used in our previous work. A sketch of the molecule can be found in the inset of Figure 5. The encapsulation within the surfactant micelle ensures close proximity to the CNTs. The switching of the molecule leads to a change of the dipole moment from 5.5 to 13.6 D.14 The conversion from DHA to VHF is achieved upon irradiation with λ = 366 nm; the backisomerization is thermally driven. This ensures that the absorption and emission bands of the nanotubes can be probed without altering the switching state of the molecules. CoMoCat-CNTs were purchased from SWeNT (batch SG76), and SDBS was purchased from Sigma-Aldrich. UV−vis measurements were performed with a Scinco S-3100 UV−vis spectrophotometer, with a range from 190 to 1100 nm. Photoluminescence excitation spectroscopy (PLE) was performed using a Horiba Jobin Yvon Fluorolog-3 spectrofluorometer. The excitation wavelength regime ranged from 240 to 1000 nm, and the detection wavelength regime ranged from 200 to 1580 nm. All measurements were performed at room temperature. High fluorescence indicated successful debundling of the CNTs.

Figure 2. Conversion of DHA to the corresponding VHF form upon irradiation after encapsulation of the molecule within a micelle formed by SDS that surrounds CNTs. The inset shows the absorption of the nanotubes in the S22 region, i.e., the absorptions from the second van Hove singularities, mostly of the semiconducting (7,6) and (9,5) tubes at 650 nm and of the (9,4), (8,6), and (8,7) tubes at 730 nm.

We estimated the filling of the micelles from the absorption measurements. The observed molecular absorption stems from encapsulated molecules because the compounds are not watersoluble. Because of the close proximity between the molecules and the nanotubes, caused by the limited space within the micelles, the filling of the micelle is assumed to be equal to the surface coverage of the CNTs. The intensity of the molecules’ absorption was obtained by subtraction of the absorption spectrum of only CNT-containing micelles from the spectrum of micelles containing both CNTs and molecular switches. The thus-obtained absorption hence merely stemmed from the molecular switches. Absorption intensities obtained for different initial concentrations of molecular switches were fitted with a Langmuir isothermal (Figure 3, i.e., θ(c) = (Kmaxβc)/(1 +

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RESULTS AND DISCUSSION The absorption spectrum of the compound, encapsulated in an SDS micelle without nanotubes, is shown in Figure 1. For the

Figure 1. Conversion of DHA to the corresponding VHF form upon irradiation after encapsulation of the molecule within a micelle formed by SDS.

Figure 3. Coverage of the CNTs with molecular switches for different starting concentrations of the molecule.

molecules encapsulated within the micelles, it shows the switching upon irradiation of the absorption band being centered at 366 nm (λDHA, with DHA) to the absorption band being centered at 490 nm (λVHF, with VHF). Both directions of isomerization could be observed after encapsulation of the molecules within the micelles. Effect of Encapsulation and Tube Proximity. Encapsulation of DHA/VHF into micelles surrounding CNTs preserved the switching ability of DHA, as proven by

βc)), where the coverage of the surface by adsorbed molecules θ(c) depends on the concentration of the molecule c, with constants for the maximum coverage Kmax, here Kmax = 1.26, and adsorption behavior β, here β = 0.04.17 Kmax is related to the enthalpy change of the system and allows one to relate the distribution of adsorbates between the CNTs’ surface to the 15732


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more randomized because of the presence of the CNTs. This theory is supported by the fact that the activation barrier for the molecules encapsulated together with CNTs is equal to the activation barrier of low-concentration molecules without additional CNTs. The slowed conversion times in dark and increased conversion times under irradiation could stem from an interaction of the molecular switches with the CNTs, i.e., a lower molecule concentration equals an effectively higher CNT concentration and thus increased interaction leading to the increased conversion time. Because no effect on the activation barrier for the DHA to VHF conversion within the CNT containing micelles was found, the change in conversion times could be caused by energy transfer or a similar transfer process from the nanotubes to the molecules. Effect of the Switching State on the CNTs. A redshift between the emission of CNTs in SDS micelles and that of in SDS−molecular-switch micelles is found after excitation at several wavelengths. The magnitude of the redshift depends on the concentration of the molecules as well as on their switching state. A higher concentration of the molecules leads to a larger redshift for both configurations, although the effect is more significant for the DHA configuration (Figure 5). This was

surrounding environment. The intensities obtained from the UV−vis measurements were normalized by Kmax and the “coverage” shown in Figure 3 is hence related to the percentage of the CNTs’ surface covered with molecular switches. The saturation limit for the filling of the micelle is reached for the initial molecule concentration of θ(c) = 130 μmol/L. After that, increasing the amount of added dipole switches causes precipitation of the excess molecules and partial disruption of the micelles. Absorption measurements were also used to determine the kinetics of the switching process. The isomerization under irradiation from DHA to VHF was slowed down considerably when CNTs were encapsulated inside the micelle with the molecular switches, i.e., from approximately 35 s for the pure compound to approximately 70 s for compound encapsulated in micelles containing CNTs. The thermally driven back-isomerization showed a strong dependence on the concentration of the molecules. The compound within an otherwise empty micelle showed a faster conversion rate for increasing concentration at low temperatures. This effect was accompanied by a decreasing activation barrier with increasing concentration of the molecules, which was determined by probing the back-isomerization at different temperatures and creating an Arrhenius plot. This could however not be observed for molecules stored in a micelle that also contained CNTs. We found no effect of the concentration of the molecules on the activation barrier, which is in magnitude comparable to the low concentration activation barrier of the pure compound (Figure 4). Note that the plot

Figure 5. Shift between the emission line of (7,6) CNT with DHA and VHF within the micelle. The inset shows a sketch of the molecule.

theoretically predicted by Malic et al. for CNTs functionalized with spiropyran molecular switches10 and is a consequence of the interference between neighboring dipole switches in the case of high coverage of the tubes. The decreased shift for the highest measured concentration is assumed to stem from a completely filled micelle, which shows increased interaction between the molecules themselves and thus reduced interaction with the CNTs. One would intuitively expect a larger shift for the molecule with the larger dipole moment, i.e., VHF, but calculations on the molecular switch spiropyran/merocyanin showed that the orientation of the dipole moment to the nanotubes’ axis is of importance as well10 indicating that a reorientation of the molecule within the SDS micelle takes place and thus leads to an effectively larger interaction between DHA and the nanotubes and results in a larger redshift of the emission lines. The shift could repeatedly be increased and decreased by changing the configuration of the molecules. To favor the parallel orientation of the molecule with respect to the CNT, leading to the greater effect on the nanotubes, we are considering attachment of a functional group to the

Figure 4. Activation barriers for the VHF to DHA conversion inside the micelles with (bound) and without (pure) CNTs.

shows the values for surface coverage of the CNTs and not the initial concentrations. These values are the actual values of surface coverage for the CNT-filled micelles, and in the absence of CNTs, a lower boundary of filling for the otherwise empty micelles, as the same, or a slighter higher filling of the micelles with molecular switches is expected. We did however find a dependence of the conversion times on the concentration. A lower concentration of the molecules leads to an increased conversion time from VHF to DHA. The effect on the activation barrier of the molecule indicates a change of its energy landscape because of interaction between the molecular switches. We could not observe an effect on the activation barrier once the molecules were inside a micelle that also contained CNTs. This indicates that the interaction between the molecular switches is reduced by the presence of the CNTs, i.e., the spacing between the molecules might be 15733


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(6) Gahl, C.; Schmidt, R.; Brete, D.; McNellis, E. R.; Freyer, W.; Carley, R.; Reuter, K.; Weinelt, M. Structure and excitonic coupling in self-assembled monolayers of azobenzene-functionalized alkanethiols. J. Am. Chem. Soc. 2010, 132, 1831−1838. (7) Guo, X.; Huang, L.; O’Brien, S.; Kim, P.; Nuckolls, C. Directing and sensing changes in molecular conformation on individual carbon nanotube field effect transistors. J. Am. Chem. Soc. 2005, 127, 15045− 15047. (8) Khairutdinov, R. F.; Itkis, M. E.; Haddon, R. C. Light modulation of electronic transitions in semiconducting single wall carbon nanotubes. Nano Lett. 2004, 4, 1529−1533. (9) Kördel, C.; Setaro, A.; Bluemmel, P.; Popeney, C. S.; Reich, S.; Haag, R. Controlled reversible debundling of single-walled carbon nanotubes by photo-switchable dendritic surfactants. Nanoscale 2012, 4, 3029−3031. (10) Malic, E.; Weber, C.; Richter, M.; Atalla, V.; Klamroth, T.; Saalfrank, P.; Reich, S.; Knorr, A. Microscopic model of the optical absorption of carbon nanotubes functionalized with molecular spiropyran photoswitches. Phys. Rev. Lett. 2011, 106, 097401. (11) Malic, E.; Setaro, A.; Bluemmel, P.; Sanz-Navarro, C. F.; Ordejón, P.; Reich, S.; Knorr, A. Carbon nanotubes as substrates for molecular spiropyran-based switches. J. Phys.: Condens. Matter 2012, 24, 394006. (12) Matsuzawa, Y.; Kato, H.; Ohyama, H.; Nishide, D.; Kataura, H.; Yoshida, M. Photoinduced dispersibility tuning of carbon nanotubes by a water-soluble stilbene as a dispersant. Adv. Mater. 2011, 23, 3922−3925. (13) O’Connell, M. J.; Bachilo, S. M.; Huffman, C. B.; Moore, V. C.; Strano, M. S.; Haroz, E. H.; Rialon, K. L.; Boul, P. J.; Noon, W. H.; Smalley, R. E. Band gap fluorescence from individual single-walled carbon nanotubes. Science 2002, 593, 593−596. (14) Perrier, A.; Maurel, F.; Jacquemin, D. Diarylethene-dihydroazulene multimode photochrome: a theoretical spectroscopic investigation. Phys. Chem. Chem. Phys. 2011, 13, 13791−13799. (15) Roquelet, C.; Lauret, J.; Alain-Rizzo, V.; Voisin, C.; Fleurier, R.; Delarue, M.; Garrot, D.; Loiseau, A.; Roussignol, P.; Delaire, J. A.; Deleporte, E. Pi-stacking functionalization of carbon nanotubes through micelle swelling. ChemPhysChem 2010, 11, 1667−1672. (16) Setaro, A.; Bluemmel, P.; Maity, C.; Hecht, S.; Reich, S. Noncovalent functionalization of individual nanotubes with spiropyranbased molecular switches. Adv. Funct. Mater. 2012, 22, 2425−2431. (17) Setaro, A.; Lettieri, S.; Diamare, D.; Maddalena, P.; Malagù, C.; Carotta, M. C.; Martinelli, G. Nanograined anatase titania-based optochemical gas detection. New J. Phys. 2008, 10, 053030. (18) Setaro, A.; Kreft, S. K.; Petersen, M. Å.; Brøndsted Nielsen, M.; Reich, S. Optical properties of carbon nanotubes coated with orthogonal dipole switches. Phys. Status Solidi B 2014, 251, 2356− 2359. (19) Wang, R. K.; Chen, W.; Campos, D. K.; Ziegler, K. J. Swelling the micelle core surrounding single-walled carbon nanotubes with water-immiscible organic solvents. J. Am. Chem. Soc. 2008, 130, 16330−16337.

molecule that increases long-range ordering between the switch and the CNTs via, e.g., hydrogen bonding.

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CONCLUSIONS We showed the encapsulation of molecular switches in otherwise empty SDS micelles and in SDS micelles surrounding CNTs. The encapsulation preserves the switching of the molecules; repeatedly reversible isomerization between the different configurations of the molecules is possible. Micelle swelling ensures close proximity between the molecular switches and the carbon nanotubes, enabling us to observe increased interaction between the nanotubes and molecules. The presence of CNTs in the micelles changes the kinetics of the switching process but does not suppress it completely. The limitation of effects to conversion times and the lack of evidence of an effect on the activation barrier indicates a form of energy transfer between the CNTs and switches. The emission bands of the CNTs are shifted by the presence of the molecular switch. The shift is stronger for DHA than for the VHF configuration. This indicates a reorientation of the dipole moment toward the CNT axis when the configuration is changed and thus a reduction of the effective dipole moment acting on the CNTs. The orthogonal switch allows us to probe the emission lines of the CNTs without changing the molecules’ configuration. Repeated cycles of isomerization were performed and proved the reversibility of the complexes’ configuration.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Phone: +49 30 838 56156. Fax: +49 30 838 56081. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the DFG under the SfB 658, subproject A6. A.S. gratefully thanks the FU Focus Area NanoScale for financial support. M.Å.P. thanks the Danish Research Council for Independent Research|Natural Sciences, The Carlsberg Foundation.

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REFERENCES

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Isomerization of Orthogonal Molecular Switches Encapsulated within

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