Low-Threshold Reversible Electron-Induced and Selective

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Low Threshold Reversible Electron Induced and Selective Photo Induced Switching of Azobenzene Derivatives at Ambient Conditions Khushboo Yadav, Sayantan Mahapatra, Thomas Halbritter, Alexander Heckel, and Thiruvancheril G. Gopakumar J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b02875 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 20, 2018

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The Journal of Physical Chemistry Letters

Low Threshold Reversible Electron Induced and Selective Photo Induced Switching of Azobenzene Derivatives at Ambient Conditions Khushboo Yadav,

†

Sayantan Mahapatra,

‡

Heckel,

†,¶

Thomas Halbritter,

‡,§

Alexander

∗,†

and Thiruvancheril G. Gopakumar

†Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India ‡Institute for Organic Chemistry and Chemical Biology, Goethe-University Frankfurt, Max-von-Laue-Str. 9, 60438 Frankfurt, Germany

¶Current Address: Department of Chemistry, University of Illinois at Chicago, Illinois 60607, United States

§Current Address: Department of Chemistry, University of Iceland, Dunhaga 3, 107 Reykjavik, Iceland E-mail: [email protected]

Phone: +91 5122596830. Fax: +91 5122596806

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Abstract Mono-Carboxyl functionalized azobenzene and arylazopyrazole have been employed for electron and photo induced switching at ambient conditions. The microscopic structure and the switching behavior is understood using scanning tunneling microscopy (STM). The carboxyl functional group in these molecules oers low threshold energy for the electron induced reversible switching compared to non functionalized azobenzene. The low threshold is understood using charged intermediate states during the switching. A selectivity has been observed for the photo induced switching. Due to strong hydrogen bonding only the free phenyl groups in the molecules are changing their conguration collectively.

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Molecules that possess interchangeable electronic states (associated to congurations of molecules) are possible switches for future molecular electronics. Azobenzene (AB) and its derivatives form a unique class of photo-active molecules and undergo reversible cis-

trans isomerization upon illumination of light 13 and hence have the ability to convert light into molecular mechanical motion. 4 AB derivatives have also been demonstrated as smart adsorbents with photoregulated gates. 5,6 The associated electronic structure of these two states (cis and trans ), particularly when adsorbed on surface, makes these type of molecules as molecular switches. 7,8 Several derivatives of AB have revealed molecular level switching on surfaces by light, 916 electric eld 17 and tunneling electrons. 8,1822 Under ultra-high vacuum (UHV) and using low temperature STM, switching (trans cis isomerism) of several AB derivatives have been understood at molecular level. 9,10,16,17,2229 Certain eorts have also been made for switching of AB derivatives at the solid/liquid interface 3032 and at the solid/air interface. 33,34 It would be interesting to achieve controllable switching of AB derivatives at ambient conditions from the point of view of its applicability at room temperature. For this, rigid adlayers of AB derivatives stabilized through strong intermolecular interactions may be of importance. Molecules with carboxyl functional groups are known for formation of stable adlayers through dimer hydrogen bonding. In contrast to AB, functionalization of AB with carboxyl group leads to well-dened growth of molecular islands on Highly Oriented Pyrolytic Graphite (HOPG) stabilized via carboxyl hydrogen bonding. 35 In this letter, we report the reversible electron and light induced switching of 4-(phenylazo) benzoic acid (PABA) and 4-((1,3,5-trimethyl-1H -pyrazol-4-yl)diazenyl) benzoic acid (PyABA) (cf. Figure 1a and 2a) on HOPG/air interface using STM. Compared to AB, PABA and PyABA show low threshold voltage for electron induced switching. This is attributed to a low lying charged electronic state of the molecules and is stabilized most likely due to the presence of the functional group in the molecules. While the photo induced switching of PABA shows a periodicity along a molecular row of its adlayer, PyABA shows switching of 3



all molecules. In addition, the strong intermolecular interaction (due to the carboxyl group) oers a selectivity in the part of the molecules, which is switching. Figure 1b depicts constant current STM topography of ultra-thin lm (prepared from solution by drop-casting) of trans -PABA molecules on HOPG basal plane. A mesh averaged image (according to Horcas et al. 36 ) is shown in the inset of Figure 1b. Scaled trans PABA molecules are superimposed on the averaged image in Figure 1c. The unit cell of the molecular assembly of PABA is marked with an oblique. The corresponding unit cell vectors → − → − are A (0.75 ± 0.05 nm) and B (3.2 ± 0.1 nm) and the angle between the vectors is θ (83 ± → − 2◦ ). The adsorption geometry of the molecules shows that the adjacent molecules along B direction interact through the carboxyl groups and form hydrogen bonded dimers. 35 PABA → − dimers in the molecular adlayer are further aligned and form dimer rows along A as indicated → − using double-headed arrows in Figure 1b and c. The intermolecular interaction along A is a weak hydrogen bond between the N of the azo group and the H of the neighboring phenyl group. 35,37,38 Figure 2b depicts constant current STM topography of ultra-thin lm (prepared from solution by drop-casting) of trans -PyABA molecule on HOPG basal plane. A mesh averaged image is shown in the inset of Figure 2b. In Figure 2c, scaled trans -PyABA molecules are overlaid over a mesh averaged STM image and the unit cell is marked with an oblique. Unit → − → − cell parameters are, A (1.6 ± 0.1 nm), B (3.0 ± 0.1 nm) and θ (85 ± 2◦ ). Similar as in PABA adlayer, PyABA adlayer forms hydrogen bonded dimers between adjacent molecules along → − → − B and dimer rows along A . Two adjacent dimer rows are indicated using double-headed → − arrows. The adjacent molecules show dierent adsorption geometry along A . This leads to → − nearly double the magnitude of A in PyABA adlayer compared to that of PABA. Intermolec→ − ular interaction along A direction is mediated through steric packing and weak hydrogen bonding between the carboxyl oxygen and phenyl CH. The growth at the global level is investigated using AFM and observed that PABA (previous publication 35 ) and PyABA (details are provided in SI) form highly crystalline adlayers leaving no amorphous regions in 4


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Figure 1: DFT-optimized ball and stick model of PABA (a). Constant current STM topography (200 pA, 0.86 V) of PABA on HOPG (b). The inset of (b) is a mesh averaged STM topography. Scaled PABA molecules are superimposed over the mesh averaged STM image (c). The oblique indicates the unit cell of the adlayer of PABA. A, B and θ are unit cell parameters. Double-headed arrows indicate the dimer rows in PABA. Scale bar of (b) is 6 nm .

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Figure 2: DFT-optimized ball and stick model of PyABA (a). Constant current STM topography (110 pA, 0.84 V) of PyABA on HOPG (b). The inset in (b) is a mesh averaged STM topography. Scaled molecules are superimposed over the mesh averaged STM image (c). The oblique indicates the unit cell of the adlayer of PyABA. A, B and θ are unit cell parameters. Double-headed arrows indicate the dimer rows in PyABA. Scale bar of (b) is 6 nm.

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comparison to non-functionalized AB. 35 To induce the switching, tunneling current is injected to molecular islands at either variable sample voltage (IV measurements) or at xed sample voltage (time trace of current) using STM tip. The feedback loop is switched o and the voltage is applied with respect to the sample. These measurements are completed within 2 seconds (previously calibrated) in order to avoid STM tip drifting away from the original locations of the measurements. Figure 3a and 3b show typical IV measurements recorded on PABA and PyABA adlayers, respectively. IV measurements show abrupt reversible changes (increase and decrease) in the current as the sample voltage is ramped. A small section of IV measurements (marked by dashed rectangles) is shown in the inset where the changes (increase/decrease) in the current is clearly visible. The increase in the current (blue arrow) indicates switching of molecules from trans to cis and the decrease in the current (green arrow) indicates switching from cis to trans. We start the switching experiments on a completely trans adlayer of the molecules, which is planar and adsorbed at on the surface as shown above. As the feedback loop is o during the measurements, the switching of planar trans to non-planar cis conguration leads to a decrease in the tip-sample distance and therefore the current increases. The similar observation has been shown previously for AB 8 and AB derivative 22 adsorbed on the metallic surface at low temperature. A statistics of sample voltage at which the current increase/decrease is obtained to understand further the switching behavior. The histograms of the switching events of PABA and PyABA are shown in the lower panel of Figures 3a and 3b, respectively. More than three hundred IV measurements are used for the statistics (additional IV measurements are shown in the SI). The statistics shows that the switching events increases drastically as the voltage increases. The nature of the statistics is similar for both molecules and for both sample voltage polarities. Strikingly, there is an onset for the switching voltage (threshold voltage) for PABA and PyABA at both polarities at ≈ 0.5 and +0.5 V. The thresholds (sample voltages at both polarities) indicate that the switching process is induced by elec7



Figure 3: Typical I-V measurements performed on adlayer of PABA (a) and PyABA (b). The inset shows the enlarged view of the small section of I-V measurements marked with a dashed rectangle. Blue and green arrows indicate an increase and decrease in current due to switching of molecules. The lower panels of (a) and (b) show histograms of switching events. Time series of current obtained form adlayers of PABA (c) at 1.7 V and PyABA (d) at 1.7 V. The distinct states of molecules induced by electrons are marked as T, T1 (trans states), C0 , C1 , C2 and C3 (dierent cis states). The geometric models for possible electron induced switching states of PABA (e) and PyABA (f). T, C1 and C2 are DFT-optimized geometries. 8 ACS Paragon Plus Environment

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trons or holes. The injection of electrons or holes to the unoccupied or occupied states, respectively, excites the molecules to its charged excited states and triggers the switching. It has been shown that electron injection to frontier orbitals induces transcis isomerization in AB and its derivatives. 8,22,23 This also suggests that the threshold voltage of switching at negative and positive sample voltage polarities must be related to the energies of HOMO and LUMO, respectively, of the molecules. Surprisingly, the observed thresholds give rise to a HOMO-LUMO gap of 1 eV, which is lower than the theoretically obtained gap of PABA (2.484 eV) and PyABA (2.667 eV) in the gas phase. The details of the calculation are provided in the SI. Next, we understand the low threshold energy for switching. The electron/hole induced switching of molecules between trans and cis state is occurring through an intermediate charged state. Upon injection of electron/hole, molecules get excited to a charged excited state and from this excited state they relax to either trans or cis state statistically. Light induced transcis isomerization of AB and its derivatives occurs through neutral excited electronic states (S2 , excitation energy ≈ 370 nm or 3.35 eV). 4,9 Low threshold voltage for the electron induced switching has been previously shown for AB derivatives on metallic surfaces (due to strong molecule-substrate interaction). 23 Barrier-less transcis isomerism has been observed when electron is attached to dicyanoethylene. 39 We propose that charged excited state with long life time must be the rationale for the low threshold voltage of switching in PABA and PyABA. To understand this further we have investigated AB on graphite and analyzed the switching events as described earlier (data and details in SI). Strikingly, we observe high threshold voltages of switching at both polarities for AB on graphite. The threshold voltage gives rise to a HOMO-LUMO gap of ≈ 2.2 eV for AB, whereas the theoretical HOMO-LUMO gap in the gas phase is 2.717 eV. For AB the dierence in the HOMO-LUMO gap is not as strong as that observed for PABA and PyABA. This suggests that the charged excited state of PABA and PyABA is most likely long lived and directly contributing to the statistics. In other words, the observed switching events in the I 9



V measurements include both charged and uncharged molecules on the surface. Calculations indicate that the HOMO-LUMO gap of positively charged PABA and PyABA can be as low as ≈ 1.3 eV (see details in SI). Charging of molecules on graphite and NaCl/Cu(100) surfaces is reported earlier. 40,41 We also propose that the strong intermolecular interaction oered through the functional group of PABA and PyABA, compared to AB, 35 is most likely stabilizing the excited charged electronic state. We also propose that other eects as follows may also be related to the observed low threshold for switching. (1) Reduction in the HOMO-LUMO of molecules due to strong electronic coupling between the molecules and between molecule-substrate and (2) electrical eld induced switching. Reduction in the HOMO-LUMO gap due to strong electronic coupling between metallic surface and AB derivatives have been observed previously. 18,22,23 However, the weak van der Waals interaction at PABA/PyABA-graphite interface and the hydrogen bonding between PABA/PyABA molecules are not expected to cause the reduction in the HOMO-LUMO gap in the observed order. Electrical eld induced switching has been previously observed for AB derivative, 17 nevertheless, this type of switching occurs without threshold voltage. To understand the possible congurations of molecules during the switching we have performed time series of current for PABA and PyABA. The STM tip (feedback loop o) is placed over the top of the molecular assembly and time series of the current are measured at a xed voltage. Figure 3c and 3d show typical time series of the current for PABA (at 1.7 V) and PyABA (at 1.6 V), respectively at ≈ 0.9 nA (set current). Time series of the current measured at other voltages for PABA and PyABA are shown in the SI. Time series of current show abrupt increase and decrease in the current. For PABA we observe typically ve levels at which the current increases/decreases. We name these levels as T, T1 , C1 , C2 and C3 (marked in Figure 3c). We assign T to the trans conguration of PABA; experiments are initiated on an all trans adlayer. C1 , C2 and C3 are assigned to three possible cis states; since no feedback is applied the tip-molecule distance will decrease if the molecules are switching 10


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to cis states and therefore a large change in the magnitude of the current is expected. The change in current corresponding to T1 (with respect to T) is low compared to C1 , C2 and C3 . Therefore we assign this as a trans state of the molecule with slight variation in the adsorption geometry compared to T. We suggest that the strong carboxyl mediated hydrogen bonding could be the reason for the additional cis states. For AB and tert -butyl derivative of AB, only two states on surface are reported and are assigned to trans and cis states. 8,9,26 The DFT-optimized trans-trans, trans-cis and cis-cis dimers of PABA are shown in Figure 3e. We assign trans-trans, trans-cis and cis-cis dimers to T, C1 and C2 , respectively. The strong hydrogen bonding between PABA molecules keep them bound as dimers even after changing the conguration to trans-cis or cis-cis states. The two phenyl rings (connected to the carboxyl group) in the trans-cis or cis-cis dimer is planar with respect to the basal plane of the surface (side view of dimers in SI). That is, the geometry of molecule(s) in cis state within the dimer allows them to interact with the surface through π − π interaction. This emphasizes the role of hydrogen bonded dimers in the formation of stable electron induced cis state on HOPG surface. C3 possibly occurs when the hydrogen bonds between PABA molecules break and both molecules in a dimer separately reach cis conguration (possible conguration is indicated as C3 in Figure 3e). It is noted that the occurrence of C3 is relatively rare compared to C1 and C2 (refer to time traces), most likely due to the requirement of excess energy to break hydrogen bonds to form the absolute cis conguration. Six dierent states namely, T, T1 , C0 , C1 , C2 and C3 (marked in Figure 3d) are observed for PyABA. T and T1 are attributed to trans congurations of dimers. The cis states (C1 , C2 and C3 ) are attributed to dierent type of cis dimers similar as in the case of PABA. The DFT-optimized trans-trans, trans-cis and cis-cis dimers of PyABA are shown in Figure 3f. We assign trans-trans, trans-cis and cis-cis dimers to T, C1 and C2 , respectively, similar as assigned for PABA. We also note at very high set voltage additional states are observed for both PABA and PyABA. At this stage we are unable to predict the absolute geometry of intermediate states and experiments are underway to understand this. We also note 11



from time series of the current that the intermediate states in both PABA and PyABA are independently achievable from trans (T) directly. We have performed light induced switching of PABA and PyABA in order to account the feasibility and stability of transcis isomerization on the surface at ambient conditions. The ultra-thin lms of trans PABA and trans PyABA are illuminated with UV light (370 nm, ≈ 5 mW) to induce isomerization. Figure 4a shows the constant current STM topography of adlayers of PABA after irradiation with UV light. The inset of Figure 4a shows a mesh averaged STM image. The molecules are appearing relatively brighter (bright lobes) compared to trans PABA indicating that they have switched to the cis conguration. The sub-molecular resolution was not possible due to regular uctuations of the bright lobes. We attribute these uctuations during the scanning (alternate dark/bright contrast) to reversible switching (cistrans and transcis isomerization) induced by electrons. Magenta → − oblique corresponds to the unit cell of adlayer after switching with unit cell parameters, A0 → − (1.5±0.1 nm), B (3.1±0.1 nm) and θ (89±2◦ ). The unit cell parameters are identical to those observed for the trans adlayer except the unit vector along the dimer rows (double-headed → − → − arrows, A0 ). The magnitude of A0 is double the value of lattice parameter along dimer row → − direction ( A ) of the trans adlayer. This is striking and indicates that only alternate dimers along the dimer rows are switching upon irradiation with light. Scaled cis and trans PABA dimers (optimized using DFT calculations) are overlaid alternatively over the averaged STM image. We are assigning the trans and cis dimer to T and C2 , respectively, as suggested in the electron induced switching. The periodicity in the light induced switching is attributed to the adsorption geometry of alternate dimers along the dimer row. Similar periodicity in switching has been observed previously for AB derivative on metallic surfaces. 27 Unlike the switching in AB, 8 only one of the phenyl rings are switched away from the surface and the other is bound via intermolecular interaction of the carboxyl group. Thus, the functional group provides an additional selectivity in the part of the molecule, which is switching. It is noted that when molecules are switching while imaging there are two possible cis con12


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Figure 4: Typical constant current STM topograph of adlayers of PABA (a) (0.88 V, 96 pA) on HOPG after illumination of UV light (370 nm). The inset of (a) is a mesh averaged STM image. (b) An averaged STM image overlaid with scaled cis and trans dimers. The oblique marks the unit cell with unit cell parameters A, B and θ. trans-trans (T) and cis-cis (C2 ) dimers of PABA (c). The shaded part of C2 dimer shows other possible conguration with respect to surface plane after switching. Scale bar of (a) is 10 nm.

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gurations as shown in Figure 4c (shaded part of PABA). These congurations might be contributing to the large size of the bright lobe. We also observe regions, where long sections of dimer rows are appearing bright, indicating that all molecules are switched to the cis conguration in a dimer row (STM topography in the SI). Figure 5a shows the constant current STM topography of adlayers of PyABA after irradiation with UV light. Similar as in PABA, the dimers of PyABA are switching to the cis conguration along the dimer row upon irradiation with UV light as shown in Figure 5a and b. This is evident from the spacing between the bright rows indicated using double-headed → − arrows, which is comparable to the dimer row spacing ( B = 3.1 ± 0.1 nm). Unlike PABA, no → − periodicity for the switching was observed along A direction indicating that all dimers are switched to the cis conguration upon irradiation of light. This is clear from the fussy con→ − trast (no ordering or sub-molecular features along A direction) in the smoothed topography shown in Figure 5b. Molecular adlayer of cis dimers are superimposed in Figure 5b, which are DFT optimized (details in the SI). Possible cis congurations of dimers with respect to the surface plane are shown in Figure 5c (shaded part). We are assigning the trans and cis dimers to T and C2 , respectively, as suggested in the electron induced switching. We have successfully induced trans-cis isomerization in PABA and PyABA by electrons and photons on HOPG at ambient conditions. In comparison to AB, PABA and PyABA show low threshold voltage for electron induced reversible switching. This is attributed to a low energy charged excited state through which the switching is occurring. We suggest that the carboxyl functional group plays a crucial role in stabilizing the charged excited state. Both PABA and PyABA molecules form adlayers stabilized by hydrogen bonded dimers. The photo-induced switching shows a selectivity in the part of the molecule, which is switching. Due to strong hydrogen bonding the phenyl group which is not linked via hydrogen bonding in the molecules is changing the conguration and it occurs collectively through a molecular row. The study shows the importance of the functional group in inducing electron and optical switching in AB derivatives at ambient conditions. 14


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Figure 5: Typical constant current STM topography of adlayers of PyABA (b) (0.87 V, 90 pA) on HOPG after illumination of UV light (370 nm). (b) A smoothed (Gaussian) section of (a) with overlaid cis PyABA molecules. trans-trans (T) and cis-cis (C2 ) dimers of PyABA (c). The shaded part of C2 dimer shows other possible conguration with respect to surface plane after switching. Scale bar of (a) is 10 nm.

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Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Synthetic details of PyABA; Experimental details; AFM image of ultra-thin lm of PyABA on HOPG (0001); IV characteristics of PABA, PyABA and AB; Calculated HOMOLUMO gap and molecular orbital diagrams of PABA, PyABA and AB; Optimized cis monomers and dimers of PABA and PyABA; Time series of current at dierent sample voltages for PABA and PyABA; STM topography of UV irradiated ultra-thin lm of PABA and PyABA;

Acknowledgements The authors would like to thank Science and Engineering Research Board (SERB) for providing research grant under EMR/2014/000409. Furthermore, we thank the Deutsche Forschungsgemeinschaft for support through SFB 902. K.Y. would like to thank Indian Institute of Technology Kanpur for research fellowship (GATE).

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