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Direct Observation of Photoinduced trans-cis Isomerization on Azobenzene Single Crystal Chia-Yun Lai, Gijo Raj, Ieva Liepuoniute, Matteo Chiesa, and Pance Naumov Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 01 May 2017 Downloaded from http://pubs.acs.org on May 1, 2017

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Crystal Growth & Design

Chia-Yun Lai,1,† Gijo Raj,2,† Ieva Liepuoniute,2 Matteo Chiesa1,* Panče Naumov2,* 1

Laboratory for Energy and NanoScience (LENS), Institute Center for Future Energy (iFES), Masdar Institute of Science and Technology, 54224 Abu Dhabi, United Arab Emirates 2 New York University Abu Dhabi, PO Box 129188, Abu Dhabi, United Arab Emirates

ABSTRACT: Photoexcitation can lead to either homogeneous or heterogeneous transformations of a reactive surface. Homogeneous transformations result in a statistical mixture of reactants and products, whereas the outcome of heterogeneous transformations is a coexistence of macroscopic reactant and product domains, separated by a phase boundary. Heterogeneous photoinduced changes are also typically restricted to the surface, have individual phase structures that are inaccessible with classical diffraction methods, and possess surface properties that cannot readily be measured by the traditional wetting (water contact angle) technique. In this study, we demonstrate application of the Atomic Force Microscopy (AFM) to obtain high spatial resolution surface energy distribution in the trans and cis domains on the surface of azobenzene single crystal. UV excitation of single crystals of 3′,4′-dimethyl-4-(dimethylamino)azobenzene results in domino-like trans-to-cis isomerization on their surface. In the AFM phase channel, this affords contrasting domains with different physicochemical properties. Small Amplitude Small Set point (SASS) method and bimodal AFM operated in the attractive regime provide maps of the tip-sample adhesion force and the Hamaker constant, respectively. The results show that the Hamaker constant of the cis domains (~1×10‒19 J) is higher than that of the trans domains (~7×10‒20 J). After UV irradiation, the calculated surface energies of the domains were ~40% higher based on the Hamaker constant. Within a broader context, the results presented here demonstrate the potency of AFM-based surface-sensitive techniques for probing of the dynamic changes in surface properties upon photoinduced isomerization of molecular switches.

INTRODUCTION Due to its reversibility, energy-tunability, robustness and synthetic accessibility of its photoswitchable unit, the azobenzene chromophore is widely applied in supplementing photoactivity to chemical structure. It has been incorporated in a plethora of photoresponsive systems consisting of small molecules, 1‒3 polymers,4‒8 metal-organic assemblies,9,10 and proteins11 to control properties such as optical properties, surface topography, 12 gas adsorption,13 and conductivity.14 Recently, the prominent structural change that accompanies the trans-cis isomerization of the azo bridge was adapted as a trigger for mechanical response from mesophasic and ordered materials, where the molecular motion following the isomerization is collectively expressed as macroscopic contraction or bending of the bulk material, in some cases leading to bending, twisting or curling of polymer strips.15,16 The most recent advances of these macroscopic mechanical effects are the use of UV light to rapidly bend single crystals of azobenzenes.8,17 Since termination of the excitation could restore the crystals to their original shape, mechanical coupling to systems that can be actuated could lead to the development of optical micro- or nano-transducers.18‒21 Unlike polymeric hosts, which offer sufficient space to accommodate azobenzene isomerization, the ordered solid state (crystals) is much more sterically restrictive. The crystal bulk imposes limitations for significant structural perturbation—

change in the relative disposition of two phenyl rings—required for trans-cis isomerization. As such, isomerization on crystals is restricted to their surface and the observed mechanical effects are usually a result of photochemical conversion yields on the order of only a few percent.22 The mechanism that makes this reorganization possible at the crystal surface remains poorly understood and there is still no quantitative information on the molecular layer thickness that has switched to the cis form, although there are indications that light can only penetrate a few microns into the crystal. A few models have attempted to scrutinize the mechanical effects in bendable crystals by assuming that the reactions occur homogeneously, and that the product molecules (the cis isomers) are statistically distributed in the matrix of the host.8,17 However, such claims are yet to be supported by experimental evidence for sterically demanding azobenzenes. Previous studies using IR spectroscopy on preirradiated crystals with X-ray or γ-rays also failed to provide evidence of trans-to-cis photoisomerization of azobenzene in the solid state.23 A more recent study using atomic force microscopy (AFM) revealed surface restructuring upon photoirradiation of azobenzene microcrystals on highly oriented pyrolytic graphite (HOPG) that appear as changes in the step height. 24 Although the measurements were performed in the intermittent contact mode, the phase channel, which shows contrast upon change in physiochemical properties of the material, was not explored.

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Here we report direct observation of nanoscopic effects of UV light on the surface of a single crystal of 3′,4′-dimethyl-4-(dimethylamino)azobenzene (DDAB) obtained by performing in situ steady-state and time-dependent AFM experiments. The phase contrast maps show that photoexcitation of DDAB crystals, which are originally in the trans form, results in heterogeneous nucleation and growth of the cis phase on the crystal surface. Evidence of trans-to-cis photoisomerization at the DDAB crystal surface are confirmed by means of the Hamaker constant and adhesion force maps obtained by exploiting the raw parameter space in bimodal atomic force microscopy (AFM). The Hamaker constants and adhesion force values for the cis and the trans domains are significantly different, confirming the inherent chemical heterogeneity of these isomers. Time-dependent AFM experiments reveal the growth kinetics of the cis domains. These results show that advanced AFM operation modes can be used as a very convenient tool to obtain spatially resolved surface energies and other quantitative information on the properties of individual phases from partially photoreacted crystals.

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as trans-DDAB, while the small darker color region is identified as the cis isomer (cis-DDAB), since no other polymorphs of trans-DDAB can be found by recrystallization or heating. Although direct structural information on the photoinduced form could not be obtained, the evidence provided by AFM confirms that the physicochemical properties of the photoinduced domains are different from those of the initial trans-form of the crystal. They also suggest that the product is the cis form, the most stable photoproduct reported for this class of photoactive compounds. The separation between the two phases with clearly detectable boundary highlights the structural incompatibility—due to their large conformal difference—of the trans and cis isomers.

RESULTS AND DISCUSSION Single crystals of DDAB were grown by recrystallization of commercially available material (details provided in the Experimental section). Figure 1a shows the optical micrograph of a typical crystal. The crystal surfaces are predominantly flat and correspond to the trans isomer of the molecule (trans-DDAB), as confirmed by means of single crystal X-ray diffraction (Figure S1, Supporting Information). When the sample is exposed to UV light, a trans-to-cis photoisomerization, mainly restricted to the surface, may take place. However, with 80 s of UV illumination on the single crystal DDAB, no significant differences between the pre- and post-illumination spectra are observable by IR spectroscopy as shown in Figure 1b. Both X-ray diffraction and IR spectroscopy results show the inaccessibility of the individual isomer structure by means of classical spectroscopy techniques. The fact that isomerization is merely a surface phenomenon, i.e. the bulk of the crystal remains unmodified, suggests that experimental techniques with higher sensitivity for surface properties could yield greater insights into the photoisomerization mechanisms of single DDAB crystals. Therefore, we turned towards various Atomic Force Microscopy (AFM) operation modes to investigate the trans-to-cis photoisomerization occurring on the DDAB surface upon exposure to UV irradiation. To investigate this effect, the {001} surface of a single crystal that has not been exposed to UV light was examined with Amplitude Modulation AFM (AM-AFM). The crystal surface is flat, with microscopic root mean square roughness of 3.87 nm measured over 30 × 30 µm2 scan area (Figure 1c). The phase contrast images generated by simultaneously monitoring the phase shift between the drive signal and the cantilever response, highlight the presence of two distinct domains (Figure 1d). It is known that the phase signal is sensitive to the energy dissipation at the tip‒surface contact, 25 and thus it reflects sample properties responsible for the energy dissipated by the tip-sample interaction. In addition, the topological examinations around the different domains depict surface uniformity, as shown in the inset of Figure 1c, shows that the contrast in the phase map is not caused by topography characteristics, but rather by chemical heterogeneity. Therefore, we could infer that two distinct domains correspond to different isomers of DDAB. The large lighter color region is identified

Figure 1. Optical microscopy and AFM images of 3′,4′-dimethyl4-(dimethylamino)azobenzene (DDAB) crystal. (a) Optical micrograph showing the AFM tip positioned over a single crystal of DDAB. The inset shows the chemical structure of DDAB. (b) Infrared spectra of DDAB surface before (orange line) and after (blue line) 80 s of UV irradiation. (c) AFM topography image of the crystal before exposure to UV light. The inset shows the cross-section across the region indicated by a red line. (d) AFM phase image showing phase-contrasting regions.

As further confirmation, the properties of trans- and cis-DDAB were investigated by means of bimodal AFM operated in the attractive regime (detailed description provided elsewhere26). In brief, with standard monomodal dynamic AFM, contrast in phase image show qualitative heterogeneity in the chemical properties of the sample. On the other hand, by exciting the second mode of the AFM probe and exploiting higher harmonics, we were able to associate experimental observables to physically meaningful parameters, i.e. the Hamaker constant (AH), providing quantitative understanding of sample chemical composition. The Hamaker map of a sample after exposure to 80 s of UV irradiation is shown in Figure 2. The different values for trans- and cis-DDAB indicate that two chemical substances are present on the sample surface, i.e. trans-DDAB with ~7×10‒20 J and cis-DDAB with ~1×10‒19 J. These observations are in agreement with the chemical transformation due to trans-to-cis photoisomerization of DDAB upon UV irradiation. The exact Hamaker constant values are shown in Table 1. Five images with different DDAB crystals and AFM probes result in reproducible Hamaker values.


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Crystal Growth & Design Where D is the interatomic separation between tip and sample surface in contact (0.165 nm),28 and R is the spherical radius of curvature of the AFM tip. For solids and liquid materials devoid of hydrogen bonds, the Hamaker constant is related to the surface energy, and hence to the chemistry of the material, through the following equation28 𝐴 ≈ 4𝜋𝑅γ ≈ 2.1 × 10−21 𝛾

(2)

where γ is the surface energy of the solid material (in mJ/m2). Figure 2. Chemical composition of DDAB crystal studied by attractive bimodal AFM. (a) Hamaker map showing two distinct values for trans- and cis-DDAB. (b) Histogram of the Hamaker map

Table 1. Hamaker values obtained from five different imagesa Phase 1 / 10‒20 (J)

Phase 2 / 10‒20 (J)

Image 1

7.13

10.1

Image 2

7.11

10.4

Image 3

6.84

91.8

Image 4

7.39

11.4

Image 5

-

10.4

a

Images 1 and 2 were recorded from the same crystal at different times. Image 3 was recorded from a second crystal. Images 4 and 5 were recorded from the second crystal at different time.

With the estimation of the respective Hamaker constants which confirm that the two domains of the AFM phase image belong to trans- and cis-DDAB, we proceeded to investigate their physical properties. It is known that the AFM tip-sample interaction force profiles provide access to two important properties—the minimum force, defined as the adhesion force (FAD), and the effective elastic modulus, which is obtained by invoking the DMT model.27 These two properties can provide insight into the surface energy and mechanical properties of the DDAB crystal. As such, the force-distance profiles between the AFM probe and the crystal were analyzed, and used to estimate these two parameters. The reproducibility of these measurements was first confirmed from the force-distance profiles of two single crystals (Figure 3a). Then, over thousands force-distance profiles were recorded in situ on the DDAB crystal before UV irradiation to 24 s of UV exposure. As shown in Figure 3b‒c, the temporal evolution of the aforementioned parameters reveals that the effective elastic modulus and the absolute value of FAD monotonously increase upon UV irradiation. Each point reported in Figure 3b‒c represents an average of 50 randomly selected force-distance curves, taken on the sample surface, showing the overall behavior of the DDAB surface. These changes suggest that the DDAB crystal may bend15,16 or alter its wettability under UV irradiation. By assuming the AFM tip end to be spherical and the DDAB surface as an infinite plane, the van der Waals force expression can be approximated as 𝐹𝑣𝑑𝑤 = −

𝐴H 𝑅 6𝐷2

(1)

Figure 3. Force-distance profiles of the cis-DDAB phase. (a) Force-distance profiles of two single crystals showing good reproducibility. (b,c) Evolution of the effective elastic modulus (b) and adhesion force (c) during exposure of the crystal to UV light.

The surface energies of trans-DDAB and cis-DDAB on the surface of the crystal calculated using Eq. 2 were 33.3 mJ m ‒2 and 47.6 mJ m‒2, respectively, corresponding to nearly 40% increase in surface energy upon photoisomerization. This magnitude is in good agreement with the reported changes in photoswitched wettability of azobenzene-functionalized monolayers29 and thin polymer films.30 It has also been reported that upon UV irradiation, the water contact angle of azobenzene-



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functionalized polyelectrolyte monolayers deposited on patterned silicon substrate decreases from 152.6° to 86.8°.31 This decrease in the contact angle was associated to changes in the dipole moment of the azobenzene units accompanying the trans-to-cis isomerization. Importantly, the determination of surface wettability based on water contact angle measurements is usually performed routinely for highly flat and homogeneous surfaces such as monolayers or thin polymer films. However, similar contact angle measurements are not feasible on azobenzene single crystals due to the large size of the water droplet (millimeters) relative to the single crystals (tens to hundreds of micrometers). The method for estimating surface energy of single crystals from the Hamaker constant maps generated in bimodal AFM mode, described in the present study, presents a valuable tool for quantifying photoinduced changes in surface wettability of micrometer-size single crystals. Although the force-distance profile is inherently a one-dimensional point-wise method, with the recently developed bimodal small amplitude small set point (SASS) method 26 the FAD map can be recorded while scanning the surface, i.e. it is a two-dimensional plane-wise method. This method is exercised, as seen in the Hamaker map, to show the trans-to-cis photoisomerization of DDAB on domains of the sample surface. Figure 4a shows two force-distance profiles specifically measured against trans- and cis-DDAB domains on the crystal surface. Transand cis-DDAB exhibit |FAD| value of ~1.1 nN and ~1.5 nN. This is consistent with the FAD map of the DDAB crystal surface that has been exposed to UV light for 80 s as shown in Figure 4b. Table 2 compares the statistics of the |FAD| measurements on the trans- and cis-DDAB domains recorded from four different single crystals. Table 2. The force of adhesion (FAD) determined by one-dimension point-wise and two-dimension plane-wise methods from four crystals

Crystal 1 (1D)

Phase 1 Mean (nN) ‒1.1

0.1

Phase 2 Mean Std (nN) ‒1.3 0.3

Crystal 2 (1D)

‒1.1

0.3

‒1.6

0.4

Crystal 3 (2D)

‒1.0

0.3

‒1.6

0.3

Crystal 4 (2D)

‒1.1

0.2

‒1.4

0.1

Std

Figure 4. Bimodal SASS AFM operation for FAD analysis. (a) Representative force profiles for trans- and cis-DDAB. (b) The FAD map showing two distinct regions with different FAD values.

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The adhesion forces obtained from local contact areas (one-dimensional point-wise method) are compared with those obtained from the two-dimensional plane-wise method. To this point, it is worth mentioning that the examination of specific trans- and cis-DDAB domain shows greater |FAD| values for cisDDAB, and the overall DDAB |FAD| increases monotonously with longer UV exposure time. It is then feasible to presume that with increasing UV irradiation time, the cis-DDAB domain will advance. The contrast in the AFM phase image was further investigated by time-dependent in situ experiments with a setup that allows direct irradiation of the sample. A 3 × 3 µm area on the {001} face of the crystal was analyzed before and after 80 s of exposure to UV light. Figure 5a show the DDAB crystal before UV irradiation. From the image, we observe that the surface was mostly trans-DDAB (99%). The time-lapse AFM images show that the phase-contrasting domains grow from a preexisting smaller domains. The phase identity of these domains with the photoinduced phase indicates that they are from a small amount of the cis form that exists due to the exposure of the crystal to light. Upon exposure to UV light, these smaller domains act as nucleation points for growth of the photoinduced product. After the sample is irradiated 80 s with UV light, the dark-contrasting domains grow and expand irregularly (Figure 5b) as transDDAB is gradually converted to domains of cis-DDAB. Scanning of the same irradiated area was repeated after the crystal was stored for 2, 4, 6, 8, 10, 12, and 14 hours in dark in the AFM environmental chamber at room temperature (Figure 5c‒i). The sequential AFM images are collated as shown in Movie S1, Supporting Information. The spatially progressive growth of cis-DDAB on the surface of trans-DDAB indicates that the photoinduced transformation is heterogeneous, and occurs via a domino-like mechanism. The converted domains spread by cooperative transformation of molecules at the phase front, until the entire surface is converted to cis-DDAB.

Figure 5. Time-dependent AFM of the surface of trans-DDAB single crystal. The crystal surface is shown before (a), and after 80 s (b), 2 hours (c), 4 hours (d), 6 hours (e), 8 hours (f), 10 hours (g), 12 hours (h), and 14 hours (i) of exposure to UV radiation.


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Further information on the mechanical properties of the molecular layers beneath the surface of the crystal was provided by nanoindentation. The total thickness of the crystal was ~19 µm (see Figure S2, Supporting Information). The mechanical properties were probed up to a depth of 300 nm (cracks appeared on the crystal surface at indentation depths > 300 nm). Figure S3, Supporting Information, shows the modulus and load vs displacement curves for DDAB crystals before and after UV irradiation. The modulus of the crystal increases from 1.7 ± 0.1 GPa, before exposure to UV light, to 2.6 ± 0.2 GPa after exposure. The increased elastic modulus of the crystal surface after photoisomerization is in line with the AFM observations, and suggests that the isomerization occurred up to depths of at least 300 nm.

Center (CCDC) and can be obtained as reference CCDDC 1531106).

CONCLUSIONS

Atomic Force Microscopy. The topography images of single crystals were acquired using the Alpha 300 atomic force microscope (Witec Gmbh, Germany). Single crystals were fixed on a carbon tape mounted on a steel disc sample holder. The AFM images were acquired under ambient conditions using arrowshaped cantilevers (Witec Gmbh, model Arrow FM). The characteristics of the probe were: nominal tip radius < 10 nm; resonance frequency, 75 kHz; spring constant, 2.8 N m ‒1. The images were recorded with a resolution of 512 lines per image and 512 points per line. For the in situ experiments, the radiation was performed with a commercial UV lamp (SP7, Ushio, Japan) with power 4 W cm‒2 with the main peak maximum at 365 nm.34 Time-lapse AFM imaging, force-distance acquisition, and bimodal AFM operation were performed with Olympus AC240TS standard cantilever with resonance frequency ~70 kHz and spring constant ~2 N m‒1 on a Cypher AFM equipped with an environmental control chamber with slots for introducing optical leads for direct irradiation of the sample on the AFM stage.

In summary, we have demonstrated that DDAB single crystals show the trans-to-cis isomerization of azobenzene units that occurs on the crystal surface by gradual and irregular growth of the product domains (cis-DDAB) in the matrix of the reactant. The significant difference in molecular structure causes separation of the two phases which have very different surface energies supported by the |FAD| and AH analysis. This heterogeneous nucleation restricts the formation of the cis form to the surface of the crystal. This is also the reason why the structure of the cis form has never been observed with diffraction methods in the crystal of the reactant, despite the fact that azobenzenes are ubiquitously used as switching units. Indeed, the crystal of the pure cis form can only be obtained by recrystallization, and it always has a very different molecular and crystal structure from that of the respective trans form.32,33 The force-distance profiles show that the domains of the cis product, expanding by a mechanism similar to a domino effect, have different adhesion properties from the reactant. The |FAD| maps generated using the recently developed SASS method, and Hamaker maps obtained with bimodal AFM operated in the attractive regime, provide quantitative information of trans- and cis-DDAB. The surface energy of the domains calculated from the Hamaker constant shows an increase of ~40% upon isomerization, in agreement with the expected changes in the dielectric constant of the azobenzene units upon trans-to-cis isomerization. Within a more general context, it is also demonstrated that the AFM-based surface sensitive techniques are a powerful tool to understand the dynamic changes in viscoelastic properties of photoisomerizable molecular switches upon photoirradiation. Although due to thickness the azobenzene crystal reported here does not undergo photomechanical effect, it helps to explain the generally small conversion yields observed with photomechanical effects in azobenzene crystals. Moreover, this approach provides a robust method to estimate, with high spatial resolution, the surface energy of the reactant and product domains that can hardly be accomplished with classical wetting measurements.

Infrared spectroscopy. The IR spectra were acquired by using the Cary 600 series FTIR microscope (Agilent) in single-point reflection mode. An azobenzene single crystal was placed on a gold-coated silicon wafer. The gold-coated silicon wafers were made by electron-beam deposition (model: Proline PVD 75, Kurt J. Lesker Co). The number of scans was set to 32 and the background spectra were obtained on the gold surface close to the crystal. A small area on the crystal was then focused using the microscope objective and the IR spectra were recorded on the crystal before and after photoirradiation under conditions similar to the ones applied in the AFM measurements.

X-ray diffraction. Single crystal XRD data were collected on a Bruker APEX DUO diffractometer using monochromated MoKα radiation (λ = 0.71069 Å) and CCD area detector. The diffraction data were collected with the crystallographic software APEX3.35 Absorption correction was applied with SADABS.36 The structure was solved by SHELXT 37 by using direct methods and refined by the least-square method embedded in Olex2.38

Supporting Information. Single crystal used to determine the crystal structure with face indices and plot of the molecular structure in the single crystal determined by single crystal X-ray diffraction (Figure S1), optical images of the surface of the crystal (Figure S2), results from the nanoindentation measurements (Figure S3), and AFM movie showing the conversion on the surface of the crystal (Movie S1). This material is available free of charge via the Internet at http://pubs.acs.org.

EXPERIMENTAL SEC TION Sample. Single crystals were obtained by recrystallization of commercially available 3′,4′-dimethyl-4-(dimethylamino)azobenzene (Sigma-Aldrich). 0.05 g of the compound was dissolved in 5 mL 1,4-dioxane/methanol mixture (1:1). The solution was left for crystallization in dark, and single crystals were obtained after a week. The identity of the compound was confirmed by single crystal X-ray diffraction (the crystal structure of DDAB is deposited at the Cambridge Crystallographic Data

[email protected] (M.C.) [email protected] (P.N.)



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The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

This work was financially supported by the National Research Foundation of the UAE (UIRCA 2014 - 683) and with a Research Enhancement Fund (“Smart Biomimetic Superhydrophobic SelfCleaning Materials”) from NYU Abu Dhabi.

†These

authors have contributed equally to this study.

This work was financially supported by the National Research Foundation of the UAE (UIRCA 2014 - 683) and with a Research Enhancement Fund from NYU Abu Dhabi, and was partially carried out using Core Technology Platform resources at New York University Abu Dhabi. We thank Dr. James Weston for the technical support with nanoindentation measurements, and Dr. Liang Li for the single crystal X-ray diffraction experiments. We also thank Israel Desta, Dr. Alain Lesimple, Dr. Patrick Commins and Dr. Matthew O’Connor for their support with the experiments and for the useful discussions on sample preparation. We thank Dr. Philip P. Rodenbough for reading the manuscript and language corrections.

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For Table of Contents Use Only Direct Observation of Photoinduced trans-cis Isomerization on Azobenzene Single Crystal Chia-Yun Lai, Gijo Raj, Ieva Liepuoniute, Matteo Chiesa,* Panče Naumov*

Synopsis: Atomic Force Microscopy (AFM) was applied to obtain high spatial resolution surface energy distribution in the trans and cis domains on the surface of an azobenzene single crystal. The results demonstrate that AFMbased surface-sensitive techniques can be used to probe dynamic changes in surface properties that occur upon photoinduced isomerization.


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Figure 1. Optical microscopy and AFM images of 3′,4′-dimethyl-4-(dimethylamino)azobenzene (DDAB) crystal. (a) Optical micrograph showing the AFM tip positioned over a single crystal of DDAB. The inset shows the chemical structure of DDAB. (b) Infrared spectra of DDAB surface before (orange line) and after (blue line) 80 s of UV irradiation. (c) AFM topography image of the crystal before exposure to UV light. The inset shows the cross-section across the region indicated by a red line. (d) AFM phase image showing phasecontrasting regions. 174x157mm (300 x 300 DPI)


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Figure 2. Chemical composition of DDAB crystal studied by attractive bimodal AFM. (a) Hamaker map showing two distinct values for trans- and cis-DDAB. (b) Histogram of the Hamaker map. 177x79mm (300 x 300 DPI)



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Figure 3. Force-distance profiles of the cis-DDAB phase. (a) Force-distance profiles of two single crystals showing good reproducibility. (b,c) Evolution of the effective elastic modulus (b) and adhesion force (c) during exposure of the crystal to UV light. 116x276mm (300 x 300 DPI)


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Figure 4. Bimodal SASS AFM operation for FAD analysis. (a) Representative force profiles for trans- and cisDDAB. (b) The FAD map showing two distinct regions with different FAD values. 206x95mm (300 x 300 DPI)



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Figure 5. Time-dependent AFM of the surface of trans-DDAB single crystal. The crystal surface is shown before (a), and after 80 s (b), 2 hours (c), 4 hours (d), 6 hours (e), 8 hours (f), 10 hours (g), 12 hours (h), and 14 hours (i) of exposure to UV radiation. 112x109mm (300 x 300 DPI)


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Table of contents graphics 173x72mm (300 x 300 DPI)


Direct Observation of Photoinduced - ACS Publications - American

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