Photoisomerization Dynamics in a Densely Packed Optically

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Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage

Photoisomerization Dynamics in a Densely Packed Optically Transformable Azobenzene Monolayer Kyle M McElhinny, Joonkyu Park, Youngjun Ahn, Peishen Huang, Yongho Joo, Arunee Lakkham, Anastasios Pateras, Haidan Wen, Padma Gopalan, and Paul G. Evans Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01524 • Publication Date (Web): 25 Aug 2018 Downloaded from http://pubs.acs.org on August 31, 2018

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Photoisomerization Dynamics in a Densely Packed Optically Transformable Azobenzene Monolayer Kyle M. McElhinny1, Joonkyu Park1, Youngjun Ahn1, Peishen Huang1, Yongho Joo1, Arunee Lakkham1, Anastasios Pateras1, Haidan Wen2, Padma Gopalan1, and Paul G. Evans1* *

Electronic mail: [email protected]

Affiliations 1

Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.

2

Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA.

Keywords time-resolved synchrotron x-ray reflectivity, photoisomerization, monolayer, azobenzene, donorsemiconductor Abstract Molecular monolayers that can be reconfigured through the use of external stimuli promise to enable the creation of interfaces with precisely selected dynamically adjustable physical and electronic properties with potential impact ranging from electronics to energy storage. Azobenzene-containing molecular monolayers have multiple stable molecular conformations but face a challenging nanoscale problem associated with understanding the basic mechanisms of reconfiguration. Time-resolved x-ray reflectivity studies show that the reconfiguration of a rhenium-azobenzene monolayer occurs in a period of many seconds. The degree of reconfiguration from trans to cis forms depends on the integrated UV fluence and has kinetics that are consistent with a mechanism in which the transformation occurs through the nucleation and growth of nanoscale two-dimensional regions of the cis isomer.

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1. Introduction Optically reconfigurable organic/inorganic hybrid materials have a variety of exciting applications including molecular machines,1-2 organic electronics,3 solar energy harvesting,4-5 high-density information storage,6 smart surfaces with applications in photoswitchable wetting,79

and catalysis.10 In these optically reconfigurable systems, the introduction of a surface is often

used to instill a directionality to the structural changes and magnify the effects of the reconfiguration.1 Often the reconfigurable functionality is provided by azobenzene groups incorporated into molecular monolayers.11-17 The creation of self-assembled or ordered monolayers of such optically reconfigurable molecules leads to surface densities on the order of 1014 molecules cm-2, corresponding to approximately 1 molecule/nm2. This high density facilitates the manipulation of physical, chemical, and electronic properties from nm to cm length scales with potentially large interfacial elecstrostatic potential differences, high energy storage densities, and dynamically selected differences in surface energy.1 The reconfigurable optical control of these properties enables novel structures, such as those in which liquid crystal alignment is controlled via interface reconfiguration.18 In electronic devices, a reconfigurable azobenzene monolayer at the gate/semiconductor interface of a field-effect transistor can produce optically induced changes in the threshold voltage that are equivalent to the full range of the device gate voltage.19 Further advances in the manipulation of interfacial properties via the incorporation of reconfigurable functionalities thus promise to enable a range of applications in electronic, optical, and fluidic devices. The molecular-scale mechanisms of the transformation of dense monolayers, particularly in the 100-nm-to-micron scale associated with optical and electronic functionalities, are not yet known. The reconfiguration dynamics can be expected to be highly complex because of the


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interactions between adjacent molecules, leading to the correlation of the reconfiguration events of individual molecules in the same nanoscale environment. Other challenges in understanding the reconfigurability of optically reconfigurable electronic monolayers are in evaluating structural variation arising from the immobilization of the molecules at the surface, unknown molecule-substrate and molecule-molecule interactions, and the anisotropic distribution of molecules on the surface. The combination of these effects changes the quantum yields and rates of photoisomerization and the overall reversibility of the reconfiguration.3, 11 Here we report a structural study that reveals the role of large-scale correlations in the switching of a monolayer of azobenzene-containing molecules. A structural study can be employed to probe dynamics because the change in molecular conformation during photoisomerization leads to a change in the effective thicknesses of azobenzene-containing monolayers that can be accurately measured using x-ray reflectivity. Ultraviolet (UV) photons induce a trans to cis photoisomerization of the azo group in an azobenzene-containing molecule. The transformation arises from a direct photoexcitation of orbitals localized on the azo group. As a result, the wavelength range of UV light inducing the trans-to-cis transformation only weakly depends on the remaining molecular structure and falls within the range spanning approximately 320 to 350 nm. The reverse, cis-to-trans reconfiguration can be induced by exposure to blue light in the wavelength range of 400 to 450 nm.20 The recovery from cis to trans can also be induced by heating to approximately 40 °C.20 In solution, the molecular isomerization from trans to cis and the reverse process typically occur on picosecond timescales. The transformation mechanisms of azobenzene-containing molecules in bulk solids, thin films, and monolayers are more complicated due to morphological and electronic effects. In


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solids and monolayers, steric constraints can in principle hinder the conversion from the trans to cis isomers. Two morphological effects are linked to the dependence of the rate of photoisomerization on molecular packing: (i) steric hindrance depends on the molecular environment and can be weaker for trans molecules near molecules in the cis isomeric state and (ii) electronic coupling and exciton delocalization are affected by the separation and mutual orientation of molecules within the monolayer. The possible role of exciton delocalization in hindering photoisomerization has been probed in a series of theoretical studies.21-22 Substratemolecule interactions can also affect the transformation, including via quenching of the excitation by the substrate, which is often metallic in previous studies.11, 15, 23 In the limit of weak intermolecular interaction, a specifically designed azobenzene monolayer exhibits fast switching when molecules have sufficient free volume and the azo group is electronically decoupled from the surface.16 In this case, the trans-to-cis transformation occurs randomly with an overall rate that can be fit to give an effective cross section of 4 × 10-18 cm2.16 Similarly, when molecules are deposited on spherical nanoparticles rather than planar surfaces, the free volume is also increased, leading to higher isomerized fractions.24 The emergence of molecule-molecule interactions due to close packing on the surface complicates the process of isomerization. The degree of transformation from trans to cis within a molecular layer depends on the total number of azobenzene-containing molecules per unit area.11, 23-24 The photoisomerization rates increase with decreasing azobenzene fraction in the monolayer, such that monolayers of 100% azobenzene-containing molecules linked by a thiol group to an Au surface showed no evidence of photoisomerization.23 When azobenzenecontaining molecules are co-assembled with non-photoisomerizing molecules or assembled in a partially completed monolayer, the transformation dynamics exhibit similar constraints even at


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low concentrations due to segregation of the azobenzene monolayers to a separate surface phase.8, 24 The second effect limiting the transformation of closely packed monolayers is associated with the electronic environment of the molecules. In dense trans monolayers, intermolecular coupling and local-field effects weaken the exciton binding strength, resulting in changes in the optical absorption spectrum and potentially limiting the degree of transformation.25 In order to prevent delocalization on the timescales relevant for switching, the average distance between neighboring molecules should be increased by at least a factor of four from the separations of 6-8 Å observed in packed monolayers.21, 26 Microscopy studies indicate that the combination of physical and electronic effects leads to a spatial correlation of switching events. Scanning tunneling microscopy (STM) experiments show that azobenzene-containing molecules assembled in one-dimensional (1D) chains transformed more slowly than isolated molecules.11 A transformation of only a fraction of the molecules was observed when molecules were assembled in 1D chains with widths of several molecules or in two-dimensional (2D) islands.11 Similarly, domains of cis isomers with dimensions of tens of nanometers, corresponding to many molecular spacings, were observed in STM images acquired after exposure to UV light.13 The locations of domains can in principle result from local heterogeneity in packing density or possibly from local differences in the effective absorption cross section due to the relative orientation of the molecular transition dipole and the UV polarization.23 Together, these previous observations suggest that the dynamics of the transformation can be described with a domain model in which the transformation from trans to cis occurs through the expansion of areas of the cis isomer across the surface. We show here that the


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photoisomerization of a closely packed monolayer of the azobenzene-containing molecule ReAzoC (Fig. 1(a)) exhibits dynamics consistent with this domain model of the transformation. In this case, the transformation proceeds in a series of steps beginning from the initial trans state of the monolayer, as shown in Fig. 1(b)(i). The initial stage of photoisomerization occurs in a small fraction of molecules either near defects in the monolayer, as in Fig. 1(b)(ii) or at a small number of molecules deposited in the cis conformation. Point defects such as vacancies in the azobenzene monolayer or areas with locally larger free volume or line defects between domains of trans molecule with slightly different tilt orientation serve as nucleation sites for the cis domains.27 The distortion in the local ordering caused by the cis isomer reduces the steric hindrance of the twist of the N=N double bond that is crucial to the photoisomerization mechanism.27 The photoisomerization of molecules at the boundary between the trans and cis conformations, as seen in Fig. 1(b)(iii), results in the formation of large domains. The dynamics of the trans-to-cis transformation were probed using time-resolved x-ray reflectivity (XRR), a surface sensitive x-ray scattering technique in which x-rays are reflected from gradients in the electron density profile along the surface normal of an interface. The difference in the molecular heights of the two isomers leads to a significant difference in the distribution of intensity in the XRR curve and thus the structural difference between trans and cis isomers can be readily resolved in reflectivity studies. XRR also provides statistical information regarding the variation in molecular heights and surface roughness over the area probed by the x-ray beam, on the order of 1 mm2. The XRR studies were conducted during UV illumination in order to reduce effects associated with possible thermal relaxation to the trans state. In the range of UV intensities probed here, the rate of the structural change depends on the total integrated UV fluence rather than on the instantaneous UV intensity. The transformation


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occurs over a period of approximately 103 s, depending on the intensity (and thus the time required to reach a given total fluence). The rate of isomerization is consistent with a statistical description of the transformation based on a modification of the Johnson-Mehl-AvramiKolmogorov (JMAK) model in which the independent variable is the total integrated UV fluence.

2. Experimental 2.1 ReAzoC assembly and structure The ReAzoC molecule considered in this study consists of a Re-bipyridine group, an azobenzene group, and a carboxylic acid linker, as illustrated in Fig. 1(a). The synthesis of ReAzoC and the ordered assembly of the molecular monolayer using the Langmuir-Blodgett (LB) method have been previously described.28 The use of LB assembly allows the monolayer deposition conditions, particularly the surface pressure during assembly, to be selected to allow a closely packed single monolayer to be reproducibly deposited.28-29 The LB assembly of ReAzoC exhibits surface pressure isotherms similar to methyl 4-[(E)-2-[4-(nonyloxy)phenyl]diazen-1yl]benzoate (LCA), an azobenzene-containing liquid crystal that also assembles in trans state under ambient conditions.30 For this study, monolayers were assembled at a surface pressure of 25 mN m-1, corresponding to a surface coverage of 1.84 molecules nm-2, a density consistent with a closely packed single monolayer.28 The LB monolayers were transferred onto quartz or SiO2/Si substrates for the structural studies. XRR measurements, described below, indicate that the initial configuration of the monolayer is in the trans isomer on both substrates and that the structural parameters are in agreement with our previous study of the assembly of ReAzoC.28 2.2 Time-resolved x-ray reflectivity


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The evolution of the structure of the ReAzoC monolayer was studied during UV and blue optical illumination using time-resolved synchrotron XRR experiments conducted at station 7ID-C of the Advanced Photon Source at Argonne National Laboratory. A photograph and schematic diagram of the experimental arrangement are shown in Figs. 1(c) and 1(d), respectively. The x-ray photon energy was 10 keV with an x-ray beam size of 200 µm × 200 µm. The x-ray incident flux was 107 photons s-1, a value selected following a study of x-ray beam damage effects discussed below. The reflected x-ray intensity was recorded by an avalanche photodiode operated in a photon-counting mode. The XRR data consists of measurements of the intensity of the reflected x-ray beam as a function of wavevector Q, which is defined as Q=4 π sin(θ)/λ, where θ is the angle of incidence of the x-ray beam with respect to the sample surface, as shown in Fig. 1(d). The reflectivity measurements were conducted using values of θ ranging from 0 to 5°. Two different modes of operation were employed to collect the XRR measurements. Complete XRR curves were acquired by varying the incident angle across a wide angular range and recording the intensity of the reflected x-ray beam at a series of incident angles. The acquisition time per point for these complete angular scans was 1 s at each angular setting. The dynamics of the molecular transformation were probed by measuring the time dependence of the reflected x-ray intensity at a fixed incident angle, corresponding to a selected value of Q. As discussed below, the incident angle was selected to be at the local minimum of the XRR curve of the trans state in order to obtain the highest possible contrast and sensitivity to the trans-to-cis transformation. The counting time for each point was 1 s. Chemical and physical degradation during x-ray exposure is a significant concern in x-ray scattering experiments probing thin organic layers.31-33 A series of calibration


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measurements were conducted to find incident x-ray intensities and exposure times that did not lead to x-ray induced changes in the sample during the XRR study of photoisomerization. Repeated XRR measurements were conducted at one position on the sample using a series of two different molybdenum filters to attenuate the incident x-ray beam by factors of 8 and 86, respectively. X-ray radiation damage was observed after exposure durations of 30 s and 480 s, with attenuation factors of 8 and 86, respectively. Following these calibration measurements, both filters were employed simultaneously to ensure that degradation was not observed during the period of approximately 1000 s required for each optical experiment. Following the degradation, the contrast of x-ray intensity fringes in the reflectivity pattern was reduced and the total reflected intensity at higher angles decreased, both of these features are consistent with an x-ray induced roughening of the molecular monolayer. The total incident x-ray flux after attenuating using the final set of filters was 107 x-ray photons s-1. The x-ray reflectivity of an ReAzoC monolayer deposited under the conditions described above is shown in Fig. 2(a). The key feature of the XRR from the monolayer sample is a series of oscillations in the reflected intensity at high Q that results from x-ray interference due to the ReAzoC monolayer. The very high reflectivity below Q = 0.032 Å-1 and the rising in intensity as a function of Q at very low values of Q arise from total external reflection and an x-ray beamfootprint effect, respectively. XRR fitting procedures were used to construct a model of the thickness, electron density, and interface roughness of the molecular monolayer in its starting trans configuration.34-36 The Re-bipyridine group of the ReAzoC molecule has a relatively high electron density in comparison with purely organic molecular layers, which simplifies the collection and analysis of the XRR data.28-29, 37 The structural parameters of the monolayer quantities depend mainly on the


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periodicity and amplitude of the reflectivity fringes apparent at high values of Q in Fig. 2(a). The parameters of the structural model are ρsub, the electron density of the substrate, σsub, the roughness of the substrate-monolayer interface, and Lmonolayer, ρmonolayer, and σmonolayer, which are the thickness of the monolayer, the average electron density of the monolayer, and roughness of the monolayer-air interface, respectively. The fit of a model of the ReAzoC monolayer to the reflectivity is shown with the reflectivity data in Fig. 2(a). The parameters expected from the molecular model in Fig. 1(a) and the results of the fit to the ReAzoC/quartz reflectivity are given in columns (i) and (ii) of Table 1, respectively. The root-mean-square (rms) substrate roughness determined from Fig. 2(a) is consistent with the value of 8 Å rms provided by the quartz substrate manufacturer. The fit gives a thickness of the monolayer that is slightly larger than expected, an effect that we have previously connected to the correlation between the roughness of the substrate and roughness of the molecular surfaces.28 The x-ray reflectivity of an ReAzoC monolayer on an SiO2/Si substrate is shown as the curve labeled starting structure in Fig. 2(b). The parameters extracted from a fit of the structural model to the starting structure are shown in column (iii) of Table 1. The fit yields a thickness and density of the trans monolayer deposited on SiO2/Si that differ only slightly from structural parameters extracted from the monolayer on quartz, indicating that both monolayers are transferred in the trans configuration. The intensity contrast between the relative minimum near Q = 0.1 Å-1 and the adjacent intensity maximum varied for different locations across the sample, an effect which may arise from stick-slip instabilities in the contact line during the transfer of the LB monolayer to the substrate. The measurements reported here used regions in which there was maximum contrast, as shown in Fig. 2(c). The UV illumination used to induce the trans-to-cis reconfiguration was provided by


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pulsed laser producing 11 ps-duration pulses at a repetition rate of 54 kHz and wavelength of 355 nm (Duetto, Time-Bandwidth Products, Inc.). The output power of the laser was varied using a rotational half-wave plate positioned before a polarizing beam splitter. The optical polarization of the laser was in the horizontal plane at the sample, independent of the setting of the waveplate attenuator. This optical illumination corresponds to s-polarization at glancing incidence in the vertical scattering geometry employed for the x-ray experiments. The laser beam had fullwidth at half maxima of 350 µm and 450 µm in the horizontal and vertical directions, respectively, measured normal to the beam direction. The laser beam illuminated the sample at grazing incidence, in an arrangement in which the UV focusing optics did not rotate as the sample was scanned to vary the x-ray incident angle. The UV incident angle was 4.92º with respect to zero x-ray incidence angle, leading to a footprint in this orientation with area of 0.02 cm2. This footprint at zero angle was used to calculate the values of the UV photon flux and integrated UV fluence given below. The UV incidence angle, and thus the footprint and the UV flux, varied slightly as a function of x-ray incidence angle, leading to variation by 30% in the UV flux in the range from θ = 0 to 1.6° for which data is shown below. The horizontal and vertical sizes of the UV illumination were sufficiently large that the x-ray beam spot was contained within the optically exposed area at the x-ray angles of incidence relevant to the study of the transformation reported below. The blue illumination for the studies of the reversibility of the transformation was provided at normal incidence by a light emitting diode (LED) with a peak wavelength of 460 nm (LZ1-00DB00, Engin, Inc.).

3. Results 3.1 Evolution of x-ray reflectivity during UV illumination


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The change in the x-ray reflectivity of a ReAzoC monolayer was studied systematically during exposure to UV illumination using monolayers deposited on SiO2/Si substrates. A series of x-ray reflectivity curves at different integrated UV fluences is shown in Fig. 2(b). Following the acquisition of the reflectivity curve for the starting structure, the monolayer was exposed to a UV intensity of 0.85 mW cm-2 for a duration reaching an integrated UV fluence of 5 × 1019 photons cm-2. Then the UV illumination was stopped and the reflectivity was re-measured. This process was repeated with higher UV intensity of 1.28 W cm-2 until an integrated UV fluence of 1 × 1020, and with a laser power of 1.77 W cm-2 until integrated UV fluences of 2 × 1020 photons cm-2, 1 x 1020 photons cm-2, and 1 x 1022 photons cm-2. The reflectivity curves measured after each subsequent UV exposure are shown in Fig. 2(b). The UV exposure results in (i) an increase in Qmin, the observed wavevector of the first local minimum in scattered x-ray intensity, from its initial value Qmin,initial=0.1 Å-1 and (ii) an increase in the reflected intensity within the wavevector range near the minimum. At very high integrated UV fluence, on the order of 1022 photons cm-2, the x-ray reflectivity curve ceases to evolve in response to additional optical illumination. The parameters determined from a structural fit of a structural model to the XRR curve acquired following the highest integrated UV fluence of Fig. 2(b) are shown in column (iv) of Table 1. The XRR fit indicates that the monolayer is reduced in thickness by 6 Å after UV illumination. The increase in the XRR fitting monolayer roughness parameter, from approximately 4 Å rms before the transformation to 10 Å rms after the transformation, may indicate that the final state consists of a mixture of molecular conformations of different heights. For the purposes of the analysis described below, we have assumed that the structure reached after the long exposure reported in column (iv) of Table 1 represents the cis form of the molecular layer. The structural parameters reported in column (iv)


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are consistent with this assumption, but as indicated below in the incomplete saturation of the transformed fraction and by the increased roughness parameter after the transformation, the final state may include a population of molecules remaining in the trans state. In comparison with previously reported XRR studies, in which the transformed fraction was estimated to be approximately 15%, the XRR curves shown Fig. 2 are more completely evolved towards a cis configuration and we thus expect the transformed layer to consist of at least tens of percent of the cis isomer.28 Partially transformed reconfigurable monolayers present a significant challenge to x-ray reflectivity methods because the mixture of trans and cis monolayers results in lateral heterogeneity in layer thickness and density. The lateral heterogeneity can lead to the redistribution of the scattered x-ray intensity from the specular reflectivity to a diffuse background.38 In order to describe the reflectivity of the intermediate state quantitatively, we have adopted an approximation in which the measured x-ray reflectivity is the sum of intensities from transformed and untransformed regions. With this approximation, the x-ray reflectivity of the partially transformed monolayer can be predicted by summing the intensities of x-rays scattered from areas of cis and trans conformations. A key limitation of this interpretation is that each domain of either cis or trans must be large enough to be in the x-ray scattering limit where adding intensities is appropriate. Adopting this approach neglects effects associated with diffuse scattering, which may be relevant to future studies of the spatial distribution of the transformation. With the approximation we have adopted to describe the reflectivity of a partially transformed monolayer, the reflected intensity for a partially transformed monolayer is the weighted average of the intensities expected from trans and cis monolayers. When a fraction f of


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the ReAzoC molecules are in the cis state the weighted average of the trans and cis contributions is I(Q,f) is: (, ) = (1 − ) () + ()

(1)

Here ITrans(Q) and ICis(Q) are the reflected intensities of the trans and cis forms. The model given in Eq. (1) was used to obtain a series of predictions of the x-ray reflectivity of partially transformed monolayers as a function of the transformed fraction f. The trans intensity ITrans(Q) used in predictions was obtained using the parameters shown in column (iii) of Table 1. The model cis intensity ICis(Q) was computed using the model parameters obtained after extended UV illumination, column (iv) of Table 1. The reflectivity predicted using Eq. (1) is shown in Fig. 3(a). The key features of the evolution of the experimental reflectivity shown in Fig. 2(b) are reproduced by the prediction in Fig. 3(a): the continuous increase in intensity at the Q vector of the intensity minimum, and a shift of the wavevector of the intensity minimum to a higher Q. The model of the reflected intensity from the partially transformed monolayer can be used to determine the transformed fraction f of the monolayer from the experimental XRR measurements. The key feature of the model is that the predicted intensity in the range of Q near the intensity minimum increases monotonically as a function of the total exposure to UV photons. The predicted change in the reflected intensity at the initial intensity minimum Qmin,initial is shown as a function of f in Fig. 3(b). The change in the reflected intensity has been multiplied by an arbitrary constant in order to provide a prediction that matches the measured intensity of the reflected beam at Q=Qmin,initial, expressed in observed counts s-1. The change in the predicted intensity is proportional to the fraction of the monolayer isomerized and is accurately fit by I(f) =


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Itrans + a f, with a=1.5 × 103 photons sec-1. 3.2 Monolayer photoisomerization dynamics To evaluate the dynamics of the structural changes with better time resolution, the change in reflected intensity at wavevector Qmin,initial = 0.1 Å-1, the wavevector of maximum contrast between the trans and cis isomeric monolayers, was recorded continuously during UV illumination. The reflected intensity at Qmin,initial was measured before illumination for 10 s, and then subtracted from the measured intensity during illumination. The experiment was repeated for UV intensities ranging from 0.3 W cm-2 to 4.3 W cm-2, beginning with a previously unexposed area of the monolayer sample for each intensity. In terms of numbers of UV photons, these intensities were from 5.4 × 1017 photons cm-2 s-1 to 7.7 × 1018 photons cm-2 s-1. The total UV exposure time at each intensity was approximately 2000 s. The time dependence of the reflected intensity during UV illumination is shown in Fig. 4(a). The key feature of the variation of the reflected x-ray intensity in Fig. 4(a) is that the rate of change of the reflected x-ray intensity depends on the UV intensity, with a faster increase in reflected x-ray intensity for higher UV intensities. In addition to the overall trend of increasing reflected x-ray intensity during UV illumination, there is also a reproducible decrease in the reflected intensity during the first stages of the transformation, corresponding to integrated UV fluence less than 1 ×1020 photons cm-2. The initial decrease in intensity is apparent in the time dependence of the reflected x-ray intensity with the lowest UV intensity in Fig. 4(a). The decrease is not accounted for in the structural model we have presented and may indicate that the ReAzoC molecular structure is distorted by UV illumination before the transformation. We also expect that the absorption of the optical pulse leads to a transient temperature in the temperature


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of the monolayer that relaxes in the interval between pulses as well as to a steady-state increase in the temperature. In both cases the temperature increase is proportional to the optical fluence. We have observed here similar transformation kinetics at all optical fluences and we thus conclude that the structural effects we observe are not linked to the temperature increase. The intensity model described in the previous section provides the means to determine the fraction f of the layer that has been isomerized. The fraction of the monolayer isomerized as a function of the integrated UV fluence per unit area is shown in Fig. 4(b). The change in the reflected intensity was converted to the fraction of the monolayer photoisomerized using the model in Eq. (1) combined with a calibration based on the predicted intensity as a function of isomerized fraction in Fig. 3(b). Fig. 4(b) shows that the isomerized fraction f depends on the integrated UV fluence and that for a given integrated UV fluence the transformed fraction is independent of the UV intensity that had been used to reach that integrated fluence. The fluence dependence of the transformed fraction can be used to obtain insight into the mechanism of the trans-to-cis transformation in the ReAzoC monolayer. It is immediately apparent that the integrated UV fluence dependence of the transformed fraction in Fig. 4(b) is not exponential. An exponential dependence, as is found in isolated azobenzene molecules, occurs when the transformation occurs with a fixed probability per molecule.16 In the case of isolated molecules without intermolecular interaction, the evolution of the transformed fraction as a function of the integrated UV fluence would be expected to evolve with the following exponential dependence on the integrated UV fluence F following = 1 − , with a constant value of Kexp. This exponential integrated UV fluence dependence does not fit the data in Fig. 4(b). Rather than the non-interacting exponential fluence dependence, the transformed fraction


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in Fig. 4(b) shows a characteristic s-shaped dependence on the integrated UV fluence consistent with a JMAK model describing the kinetics of the transformation from one phase of matter to another via the formation of discrete spatially localized domains or phases. The independent variable of the JMAK model is often replaced by a particle fluence in problems such as such as ion-beam-induced transformations and ion-implantation damage that driven by a flux of particles.39-42 We take a similar approach here and use the integrated UV photon fluence as the JMAK model independent variable. In this case, the JMAK model describes the nucleation and growth of cis domains in a starting structure of primarily trans isomers and proceeding with a constant incremental displacement of the domain boundary per unit UV photon fluence. An analysis of the data in Fig. 4(b) in terms of the JMAK model can be used to obtain more information about the mechanism through which the isomeric domains form in the monolayer. The transformation is characterized by slow rates at the beginning and end of the process. Three assumptions underpin the JMAK model. First, nucleation occurs randomly and homogenously over the entire untransformed region of the layer. In the case of the ReAzoC monolayers, this translates to a model where the first molecules to photoisomerize from trans to cis, do so with a random spatial distribution within the monolayer. This random distribution is supported by previous measurements at low optical intensities in which partial photoisomerization was observed in UV-Vis spectroscopy measurements, but not in XRR measurements.28 The second and third assumptions are that the growth rate of cis domains from the nucleation sites does not depend on the extent of the transformation and that the growth rate per unit of UV photon fluence is the constant and isotropic with respect to in-plane directions on the sample surface. The JMAK equation predicts that the transformed fraction fJMAK varies as a function of


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!

elapsed time t, as = 1 − . The exponent n depends on the dimensionality of the transformation. Avrami curves for transformations that occur through a single mechanism, e.g. through the growth of large domains, are characterized by a single value of n and K. Here K is the parameter that describes the nucleation and growth rate. Since the monolayer is a 2D system, the maximum expected value of n is 3, which would correspond to 2D domain growth with continuing nucleation. The values of n have a straightforward geometric interpretation in the limit after nucleation has stopped. In this limit values of n=2 and n=1 indicate 2D and 1D growth, respectively. In the case of the photoisomerization of the ReAzoC monolayer corresponds to the fraction of the monolayer in the cis isomeric state. We have modified this model by replacing the time t with the integrated UV fluence, F, given by the product of the optical intensity and the elapsed time after starting the exposure. The modified JMAK equation for the case of photoisomerization is thus: !

= 1 − .

(2)

The parameters n and K can be determined by converting equation (2) to a logarithmic form. log%−ln(1 − )' = log ( + ) log *

(3)

In a plot of the relationship given in equation (3), the slope at each point is the dimensionality of the transformation and the intercept of the vertical axis gives the value of log K. A plot of the transformation data in the form suggested by equation (3) is shown in Fig. 5. There is not a single set of values of n and K that provide a reasonable fit to the data in Fig. 5. The data are noisy at low integrated UV fluence, below approximately 1020 photons cm-2


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as a result of the small change in intensity and small fraction isomerized at those times. The locus of points has a decreasing slope as the integrated UV fluence increases, with two general regimes of different n: i. Intermediate integrated UV fluence regime. At an integrated UV fluence between approximately 3 × 1020 photons cm-2 and 2 × 1021 photons cm-2, corresponding to isomerized fractions of 0.1 to 0.3, the transformation is characterized by n=1.6. This value suggests that, in the intermediate regime of transformed fractions, nucleation has greatly slowed or stopped all together and that the domains are growing via 2D growth. ii. High integrated UV fluence regime. At values of the total photon dose greater than 2 × 1021 photons cm-2, which corresponds to a total photoisomerized fraction of approximately 0.3, the transformation is characterized by n=0.76. Computational studies of nucleation and growth in two dimensions with constant growth rates have JMAK exponents consistent with 2D growth even at high transformed fraction.43 We thus suspect that the photoisomerization rate is reduced at high integrated UV fluence, perhaps due to inhomogeneities in thin film sample resulting in areas of different molecular environments. 3.3 Irreversibility of transformation at high optical exposures After exposure to UV light the samples were exposed to 460 nm illumination for periods of 4000 s at an intensity of 9.5 mW cm-2. However, no change in the reflected intensity or structure was observed. The lack of reversibility indicates an enhanced stability of the cis molecules within these large domains. The irreversibility has been observed previously in optical absorption spectroscopy studies and attributed to stabilization from enhanced π-π orbital overlap within the cis domains in comparison to isolated cis monomers.28 A long-timescale UV-vis


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spectroscopy study following an initial UV illumination for 360 s reveals an increase in the absorbance at π−π* electronic transition of trans-azobenzene over a period of tens of days due to the relaxation of the ReAzoC to the trans state. Approximately 20% of the initially transformed molecules relaxed to after 40 days, indicating that the trans-to-cis reconfiguration does not lead to a chemical transformation of the ReAzoC monolayer.

4. Conclusions The transformation of a monolayer of azobenzene-containing molecules on inorganic substrates has significant implications. The reconfigurability has the potential to modify the rate of interfacial charge transfer in applications including dye-sensitized solar cells,44 photo- and electro-catalysis,45-46 38,39 and molecular electronics and sensing applications.47-48 XRR results indicate that the reconfiguration from the trans to the cis state is expected to change the distance between the Re-bipyridine group and the donor surface by 8 Å. As the distance between the charge transfer center of the molecule and the semiconducting substrate increases in these applications, the rate of charge transfer decreases due to the reduction in electronic coupling between the donor and semiconductor.49 The XRR study of the transformation shows that the transformed fraction depends on the total UV flux and that the dynamics are consistent with the growth of large domains of cis isomers within the trans isomeric monolayer. An important consequence of the domain model is the indication that intermolecular interactions between and among the trans and cis isomers have a key role in determining the rate of the transformation. There is thus the potential to yield more rapidly transforming monolayer materials by tailoring the headgroup and packing of monolayers, taking advantage of recent demonstrations that intermolecular interactions can be modified in that way.50


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Acknowledgements P.E., P. G., Y.J., and K.M acknowledge support from the National Science Foundation through the University of Wisconsin Materials Research Science and Engineering Center, through grant numbers DMR-1121288 and DMR-1720415. K.M. also acknowledges support from 3M Corporation through 3M Science and Technology Fellowship Program and from the NSF EAPSI program through grant number OISE-1515273. P.G. and P.H. acknowledge support by the Division of Materials Sciences and Engineering, Office of Basic Energy Science, U.S. Department of Energy under Award No. ER46590 for synthesis of the chromophore. Development of the x-ray scattering models was supported by the Division of Materials Sciences and Engineering, Office of Basic Energy Science, U.S. Department of Energy through grant number DE-FG02-04ER46147. Work at Argonne was supported by the U.S Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0206CH11357.


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References (1) Abendroth, J. M.; Bushuyev, O. S.; Weiss, P. S.; Barrett, C. J. Controlling Motion at the Nanoscale: Rise of the Molecular Machines. ACS Nano 2015, 9, 7746-7768. (2) Browne, W. R.; Feringa, B. L. Making molecular machines work. Nature Nanotechnol. 2006, 1, 25-35. (3) Browne, W. R.; Feringa, B. L. Light Switching of Molecules on Surfaces. Ann. Rev. Phys. Chem. 2009, 60, 407-428. (4) Zhitomirsky, D.; Cho, E.; Grossman, J. C. Solid-State Solar Thermal Fuels for Heat Release Applications. Adv. Energy Mater. 2016, 6, 8. (5) Luo, W.; Feng, Y. Y.; Cao, C.; Li, M.; Liu, E. Z.; Li, S. P.; Qin, C. Q.; Hu, W. P.; Feng, W. A high energy density azobenzene/graphene hybrid: a nano-templated platform for solar thermal storage. J. Mater. Chem. A 2015, 3, 11787-11795. (6) Xia, X.; Yu, H. J.; Wang, L.; ul-Abdin, Z. Recent progress in ferrocene- and azobenzenebased photoelectric responsive materials. RSC Adv. 2016, 6, 105296-105316. (7) Wang, S. T.; Song, Y. L.; Jiang, L. Photoresponsive surfaces with controllable wettability. J. Photochem. Photobiol. C-Photochem. Rev. 2007, 8, 18-29. (8) Russew, M. M.; Hecht, S. Photoswitches: From Molecules to Materials. Adv. Mater. 2010, 22, 3348-3360. (9) Jiang, W. H.; Wang, G. J.; He, Y. N.; Wang, X. G.; An, Y. L.; Song, Y. L.; Jiang, L. Photoswitched wettability on an electrostatic self-assembly azobenzene monolayer. Chem. Comm. 2005, 3550-3552. (10) Leonard, E.; Mangin, F.; Villette, C.; Billamboz, M.; Len, C. Azobenzenes and catalysis. Catal. Sci. Technol. 2016, 6, 379-398. (11) Zheng, Y. B.; Pathem, B. K.; Hohman, J. N.; Thomas, J. C.; Kim, M.; Weiss, P. S.


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Photoresponsive Molecules in Well-Defined Nanoscale Environments. Adv. Mater. 2013, 25, 302-312. (12) Wen, Y. Q.; Yi, W. H.; Meng, L. J.; Feng, M.; Jiang, G. Y.; Yuan, W. F.; Zhang, Y. Q.; Gao, H. J.; Jiang, L.; Song, Y. L. Photochemical-controlled switching based on azobenzene monolayer modified silicon (111) surface. J. Phys. Chem. B 2005, 109, 14465-14468. (13) Pace, G.; Ferri, V.; Grave, C.; Elbing, M.; von Hanisch, C.; Zharnikov, M.; Mayor, M.; Rampi, M. A.; Samori, P. Cooperative light-induced molecular movements of highly ordered azobenzene self-assembled monolayers. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 9937-9942. (14) Sortino, S.; Petralia, S.; Conoci, S.; Di Bella, S. Monitoring photoswitching of azobenzenebased self-assembled monolayers on ultrathin platinum films by UV/Vis spectroscopy in the transmission mode. J. Mater. Chem. 2004, 14, 811-813. (15) Krekiehn, N. R.; Muller, M.; Jung, U.; Ulrich, S.; Herges, R.; Magnussen, O. M. UV/Vis Spectroscopy Studies of the Photoisomerization Kinetics in Self-Assembled AzobenzeneContaining Adlayers. Langmuir 2015, 31, 8362-8370. (16) Wagner, S.; Leyssner, F.; Kordel, C.; Zarwell, S.; Schmidt, R.; Weinelt, M.; Ruck-Braun, K.; Wolf, M.; Tegeder, P. Reversible photoisomerization of an azobenzene-functionalized selfassembled monolayer probed by sum-frequency generation vibrational spectroscopy. Phys. Chem. Chem. Phys. 2009, 11, 6242-6248. (17) Delorme, N.; Bardeau, J. F.; Bulou, A.; Poncin-Epaillard, F. Azobenzene-containing monolayer with photoswitchable wettability. Langmuir 2005, 21, 12278-12282. (18) Seki, T. New strategies and implications for the photoalignment of liquid crystalline polymers. Polym. J. 2014, 46, 751-768. (19) Paoprasert, P.; Park, B.; Kim, H.; Colavita, P.; Hamers, R. J.; Evans, P. G.; Gopalan, P.


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Dipolar Chromophore Functional Layers in Organic Field Effect Transistors. Adv. Mater. 2008, 20, 4180-4184. (20) Merino, E.; Ribagorda, M. Control over molecular motion using the cis-trans photoisomerization of the azo group. Beilstein J. Org. Chem. 2012, 8, 1071-1090. (21) 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 AzobenzeneFunctionalized Alkanethiols. J. Am. Chem. Soc. 2010, 132, 1831-1838. (22) Benassi, E.; Corni, S. Exciton Transfer of Azobenzene Derivatives in Self-Assembled Monolayers. J. Phys. Chem. C 2013, 117, 25026-25041. (23) Valley, D. T.; Onstott, M.; Malyk, S.; Benderskii, A. V. Steric Hindrance of Photoswitching in Self-Assembled Mono layers of Azobenzene and Alkane Thiols. Langmuir 2013, 29, 1162311631. (24) Moldt, T.; Brete, D.; Przyrembel, D.; Das, S.; Goldman, J. R.; Kundu, P. K.; Gahl, C.; Klajn, R.; Weinelt, M. Tailoring the Properties of Surface-Immobilized Azobenzenes by Monolayer Dilution and Surface Curvature. Langmuir 2015, 31, 1048-1057. (25) Cocchi, C.; Moldt, T.; Gahl, C.; Weinelt, M.; Draxl, C. Optical properties of azobenzenefunctionalized self-assembled monolayers: Intermolecular coupling and many-body interactions. J. Chem. Phys. 2016, 145, 11. (26) Utecht, M.; Klamroth, T.; Saalfrank, P. Optical absorption and excitonic coupling in azobenzenes forming self-assembled monolayers: a study based on density functional theory. Phys. Chem. Chem. Phys. 2011, 13, 21608-21614. (27) Cantatore, V.; Granucci, G.; Rousseau, G.; Padula, G.; Persico, M. Photoisomerization of Self-Assembled Monolayers of Azobiphenyls: Simulations Highlight the Role of Packing and


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Defects. J. Phys. Chem. Lett. 2016, 7, 4027-4031. (28) McElhinny, K. M.; Huang, P.; Joo, Y.; Kanimozhi, C.; Lakkham, A.; Sakurai, K.; Evans, P. G.; Gopalan, P. Optically Reconfigurable Monolayer of Azobenzene Donor Molecules on Oxide Surfaces. Langmuir 2017, 33, 2157-2168. (29) Joo, Y.; Spalenka, J. W.; McElhinny, K. M.; Schmitt, S. K.; Evans, P. G.; Gopalan, P. Structured Layer of Rhenium Dye on SiO2 and TiO2 Surfaces by Langmuir-Blodgett Technique. Langmuir 2014, 30, 6104-6113. (30) Piosik, E.; Kotkowiak, M.; Korbecka, I.; Galewski, Z.; Martynski, T. Photo-switching of a non-ionic azobenzene amphiphile in Langmuir and Langmuir-Blodgett films. Phys. Chem. Chem. Phys. 2017, 19, 23386-23396. (31) Mannebach, E. M.; Spalenka, J. W.; Johnson, P. S.; Cai, Z. H.; Himpsel, F. J.; Evans, P. G. High Hole Mobility and Thickness-Dependent Crystal Structure in alpha,omegaDihexylsexithiophene Single-Monolayer Field-Effect Transistors. Adv. Funct. Mater. 2013, 23, 554-564. (32) Fritz, S. E.; Martin, S. M.; Frisbie, C. D.; Ward, M. D.; Toney, M. F. Structural characterization of a pentacene monolayer on an amorphous SiO2 substrate with grazing incidence X-ray diffraction. J. Am. Chem. Soc. 2004, 126, 4084-4085. (33) Mannsfeld, S. C. B.; Virkar, A.; Reese, C.; Toney, M. F.; Bao, Z. N. Precise Structure of Pentacene Monolayers on Amorphous Silicon Oxide and Relation to Charge Transport. Adv. Mater. 2009, 21, 2294. (34) Daillant, J.; Gibaud, A., X-ray and Neutron Reflectivity. Springer: Berlin, 2009. (35) Sakurai, K., Introduction to X-ray Reflectivity. Kodansya: 2008. (36) Als-Nielsen, J.; McMorrow, D., Elements of Modern X-ray Scattering. Wiley: New York,


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2001. (37) Griffith, M. J.; James, M.; Triani, G.; Wagner, P.; Wallace, G. G.; Officer, D. L. Determining the Orientation and Molecular Packing of Organic Dyes on a TiO2 Surface Using X-ray Reflectometry. Langmuir 2011, 27, 12944-12950. (38) Sinha, S. K.; Sirota, E. B.; Garoff, S.; Stanley, H. B. X-ray and neutron scattering from rough surfaces. Phys. Rev. B 1988, 38, 2297-2311. (39) Campisano, S. U.; Coffa, S.; Raineri, V.; Priolo, F.; Rimini, E. Mechanisms of Amorphization in Ion-Implanted Crystalline Silicon. Nucl. Instrum. Methods Phys. Res. Sect. BBeam Interact. Mater. Atoms 1993, 80-81, 514-518. (40) Carter, G. The effects of flux, fluence and temperature on amorphization in ion implanted semiconductors. J. Appl. Phys. 1996, 79, 8285-8289. (41) Ramos, S. M. M.; Canut, B.; Ambri, M.; Bonardi, N.; Pitaval, M.; Bernas, H.; Chaumont, J. Defect creation in LiNbO3 irradiated by medium masses ions in the electronic stopping power regime. Radiat. Eff. Defects Solids 1998, 143, 299-309. (42) Bentini, G. G.; Bianconi, M.; Correra, L.; Chiarini, M.; Mazzoldi, P.; Sada, C.; Argiolas, N.; Bazzan, M.; Guzzi, R. Damage effects produced in the near-surface region of x-cut LiNbO3 by low dose, high energy implantation of nitrogen, oxygen, and fluorine ions. J. Appl. Phys. 2004, 96, 242-247. (43) Pang, E. L.; Vo, N. Q.; Philippe, T.; Voorhees, P. W. Modeling interface-controlled phase transformation kinetics in thin films. J. Appl. Phys. 2015, 117, 175304. (44) Hagfeldt, A.; Boschloo, G.; Sun, L. C.; Kloo, L.; Pettersson, H. Dye-Sensitized Solar Cells. Chem. Rev. 2010, 110, 6595-6663. (45) Anfuso, C. L.; Snoeberger, R. C.; Ricks, A. M.; Liu, W. M.; Xiao, D. Q.; Batista, V. S.;


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Lian, T. Q. Covalent Attachment of a Rhenium Bipyridyl CO2 Reduction Catalyst to Rutile TiO2. J. Am. Chem. Soc. 2011, 133, 6922-6925. (46) Ashford, D. L.; Song, W. J.; Concepcion, J. J.; Glasson, C. R. K.; Brennaman, M. K.; Norris, M. R.; Fang, Z.; Templeton, J. L.; Meyer, T. J. Photoinduced Electron Transfer in a Chromophore-Catalyst Assembly Anchored to TiO2. J. Am. Chem. Soc. 2012, 134, 19189-19198. (47) Albert, K. J.; Lewis, N. S.; Schauer, C. L.; Sotzing, G. A.; Stitzel, S. E.; Vaid, T. P.; Walt, D. R. Cross-reactive chemical sensor arrays. Chem. Rev. 2000, 100, 2595-2626. (48) Heller, A. Electrical connection of enzyime redox centers to electrodes. J. Phys. Chem. 1992, 96, 3579-3587. (49) Asbury, J. B.; Hao, E. C.; Wang, Y. Q.; Lian, T. Q. Bridge length-dependent ultrafast electron transfer from Re polypyridyl complexes to nanocrystalline TiO2 thin films studied by femtosecond infrared spectroscopy. J. Phys. Chem. B 2000, 104, 11957-11964. (50) Cocchi, C.; Draxl, C. Understanding the effects of packing and chemical terminations on the optical excitations of azobenzene-functionalized self-assembled monolayers. J. Phys.-Condes. Matter 2017, 29, 394005.


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McElhinny et al. Figure 1

Figure 1: (a) Solution-phase structures of the ReAzoC molecule in trans and cis isomeric conformations and conceptual structures of surface-bound molecules. (b) Domain model for the isomerization of ReAzoC monolayers: (i) initial monolayer of molecules in the trans conformation (blue) with a nucleation site indicated using a white circle. (ii) Photoisomerization of molecules near the nucleation site to the cis confirmation (purple) following UV exposure. (iii) Growth of domains of the cis conformation. (c) Photograph of time-resolved synchrotron x-ray reflectivity experiment, with the directions of the incident and reflected x-ray beams, UV, and blue illumination. (d) Schematic diagram of the time-resolved x-ray reflectivity experiment.


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Figure 2: (a) X-ray reflectivity of an ReAzoC monolayer assembled at a surface pressure of 25 mN m-1 monolayer and transferred to a quartz substrate (circles). The solid line shows the intensity distribution for the structural model given in column (ii) of Table 1. The wavevector of the first local minimum in the reflected intensity is indicated is Qmin,initial. (b) X-ray reflectivity of an ReAzoC monolayer assembled at a surface pressure of 25 mN m-1 monolayer and transferred to an SiO2/Si substrate, for the starting structure and following exposure to UV photons with an integrated UV fluence ranging from 5 × 1019 to 1 × 1022 photons cm-2.


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McElhinny et al. Table 1

24

(ii) Starting Structure (quartz) 32

(iii) Starting Structure (SiO2/Si) 31

(iv) 1.0 × 1022 UV photons cm-2 25

0.40

0.44

0.40

0.48

N/A

3.7

4.5

10

N/A

3

6.1

4.2

Parameter

(i) Model

Monolayer Thickness (Å) Lmonolayer Monolayer Electron Density (e-/Å3) ρmonolayer Substrate Roughness (Å) σsub Monolayer Roughness (Å) σmonolayer

Table 1. Structural parameters extracted from the x-ray reflectivity patterns in Figure 2. The fit parameters of monolayer thickness, density, roughness, and substrate roughness are shown for the (i) parameters based on the molecular structure, (ii) a 25 mN m-1 ReAzoC monolayer on a quartz substrate, (iii) a 25 mN m-1 ReAzoC monolayer on a SiO2/Si substrate, (v) the 25 mN m-1 ReAzoC monolayer on a silicon substrate after exposure to a 365 nm-wavelength integrated UV fluence of 1.0 × 1022 photons cm-2.


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Figure 3. (a) Predicted x-ray reflectivity for varying degrees of isomerization using the model in equation 1. (b) Predicted change in the observed count rate at Qmin=0.1 Å-1 as a function of the monolayer isomerized into the cis conformation.


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Figure 4. (a) X-ray intensity at Qmin =0.1 Å-1 as a function of time during UV exposure. (b) Fraction of the monolayer isomerized as a function of the integrated UV fluence. The data are converted from (a) using the structural model described in the text.


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Figure 5. Avrami plot of the cis fraction ReAzoC monolayer as a function of the integrated UV fluence. Fits giving Avrami constants of n=1.6 in the regime of intermediate integrated UV fluence and n=0.8 in the regime of high integrated UV fluence are shown as dashed and solid lines, respectively.


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Table of Contents Graphic


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Photoisomerization Dynamics in a Densely Packed Optically

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