Photochromism of Amphiphilic Dithienylethenes as Langmuir

Article Cite This: Langmuir XXXX, XXX, XXX−XXX

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Photochromism of Amphiphilic Dithienylethenes as Langmuir− Schaefer Films Andreas K. Rossos,†,⊥ Maria Katsiaflaka,†,‡,⊥ Jianxin Cai,§,⊥ Sean M. Myers,§ Elena Koenig,† Robert Bücker,† Sercan Keskin,† Günther Kassier,† Reǵ is Y. N. Gengler,† R. J. Dwayne Miller,†,‡,∥ and R. Scott Murphy*,§

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†

Atomically Resolved Dynamics Division, Max Planck Institute for the Structure and Dynamics of Matter, Building 99 (CFEL), Luruper Chaussee 149, 22761 Hamburg, Germany ‡ The Hamburg Center for Ultrafast Imaging, University of Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany § Department of Chemistry and Biochemistry, University of Regina, 3737 Wascana Parkway, Regina, Saskatchewan S4S 0A2, Canada ∥ Departments of Chemistry and Physics, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada S Supporting Information *

ABSTRACT: Surface pressure−area isotherms were recorded under different irradiation conditions for single-component Langmuir films of three photochromic amphiphilic dithienylethenes. Nonirradiated films of these photochromic amphiphiles were mechanically stable. In addition, a shift of the isotherms to larger mean molecular areas was observed for films prepared from UV-light-irradiated dithienylethenes. Unexpectedly, a significant expansion was observed for a film prepared from visible-lightirradiated dithienylethene incorporating large branched alkyl chains. Upon further study, atomic force microscopy and transmission electron microscopy images of Langmuir−Schaefer films revealed that this pronged dialkyl derivative undergoes a photoinduced change in morphology, as circular aggregates coalesce into larger continuous aggregated structures. Nevertheless, its photoisomerization was completely reversible as singlecomponent multilayer thin films upon direct UV or visible light irradiation.

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mixtures to facilitate film formation, albeit at the expense of DTE density within the film. Specifically, a neutral DTE was combined with a triglyceride15,16 and a cationic DTE was combined with an anionic surfactant.14 In both cases, photochromism was observed, although film formation was relatively poor for the latter. We have recently reported on the inclusion of a series of amphiphilic DTEs in the bilayer membrane of lipid vesicles.17 We have shown that molecular photoswitches 1 and 2 (Chart 1) can be used to control ion permeation in vesicles. To further investigate the photochromism of amphiphilic DTEs in organized assemblies and to expand their application to 2D materials, the goals of this paper are to assess the spreading behavior and stability of Langmuir films of 1, 2 and a new amphiphilic DTE, 3, as a single component. In addition, we have examined the photochromism and nanostructure of multilayer LS films using absorption spectroscopy, and atomic force microscopy (AFM) and transmission electron microscopy (TEM), respectively.

INTRODUCTION Lower-dimensional systems have amassed a significant and growing interest due to advancements in technological fields that require these nanostructured materials. Furthermore, systems prepared from responsive molecules are emerging as examples of programmable soft matter. In particular, materials that can be switched between well-defined states with light have promising “smart” applications, such as the development of high-capacity optical data-storage devices. Photochromic materials based on the dithienylethene (DTE) molecular switch are especially well suited for such applications. DTEs are well known for their thermal irreversibility, high fatigue resistance, high quantum yields, and ultrafast reaction dynamics.1 The fabrication of Langmuir−Blodgett/Schaefer (LB/S) films is a common approach used for the preparation and evaluation of two-dimensional photoresponsive materials. LB films incorporating azobenzene2−6 and spiropyran7−13 have been extensively examined; however, these systems are thermally reversible, which limits their potential application in data-storage devices. Surprisingly, only a few examples of DTEs in LB films have been reported.14−16 Moreover, these films were not prepared from a single component of DTE. Instead, the LB films were comprised of two-component © XXXX American Chemical Society

Received: July 23, 2018 Revised: August 16, 2018 Published: August 19, 2018 A

DOI: 10.1021/acs.langmuir.8b02484 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir Chart 1. Structures of the Dithienylethene Photoisomers for 1−3

Scheme 1. Synthesis of 3

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For AFM and TEM studies, a single deposition was performed using 1 cm × 1 cm silicon wafers (Ultrasil Corporation) and 50 μm × 50 μm untreated, hydrophobic, amorphous silicon nitride wafers with 5 nm thin windows (Plano GmbH), respectively.

EXPERIMENTAL SECTION

Details on instrumentation, materials, synthetic procedures for 3, including NMR and high-resolution mass spectra, and procedures for the quantum yield, photoconversion and photostability studies are described in the Supporting Information. Langmuir Film and LS Film Preparation. Prior to the preparation of Langmuir films, the trough was thoroughly cleaned with chloroform and filled with ultrapure water. Stock solutions of 1− 3 (0.15 mg mL−1) were prepared in chloroform. For UV-irradiated solutions, the nonirradiated stock solutions were exposed to a UV light source (6 W UVA lamp; λc = 365 nm) for 6 min. For visibleirradiated solutions, the UV-irradiated stock solutions were exposed to a visible light source (53 W halogen lamp) for 2 min. An aliquot of these solutions was transferred onto the subphase surface using a Hamilton syringe, and the chloroform was allowed to evaporate for a minimum of 15 min before compression. To facilitate these transfers, a previously reported procedure18 was used in which a piece of quartz glass was partially submerged in the trough, forming a ca. 50° angle with the air−water interface (Chart S1). This technique provides better control over sample transfer. All films were compressed at a rate of 10 cm2 min−1 and were deposited onto solid substrates by LS deposition (i.e., horizontal deposition) at a surface pressure of 30 mN m−1. For absorption studies, multiple layers were sequentially deposited onto Eppendorf UVette cells. Each time the substrate was lowered toward the trough, it contacted the air−water interface at ca. 0.5 mm below the subphase surface.

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RESULTS AND DISCUSSION Synthesis. We have previously reported on the procedures for the synthesis of the asymmetrical amphiphilic DTEs 1 and 2, which contain rigid phenylethynyl substituents at the reactive carbons and a peripheral, cationic alkyl ammonium substituent.17 DTE 3 was purposely designed for Langmuir film and LS film studies. The amphiphilicity of the DTE core was enhanced by incorporating a large, branched alkyl substituent and a cationic methylpyridinium substituent. In addition, methyl groups were incorporated at the reactive carbons as similar derivatives often display high fatigue resistance, high quantum yields, high photoconversions, and efficient photochromism as crystalline solids.1 The synthesis of 3 began with the conversion of an alkyl aryl ketone to a tertiary alcohol 4 via the Grignard reaction (Scheme 1). Dehydration of 4 and the in situ reduction of the alkene intermediate using borane-dimethyl sulfide gave 5, installing the branched alkyl substituent. Thereafter, 5 was conveniently converted to the corresponding boronic acid, which was used without further purification in the Suzuki− B


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ethyl acetate. Although an equilibrium mixture of two conformers (i.e., parallel and antiparallel) exists for DTEs in solution, the conrotatory photocyclization reaction can only proceed from the antiparallel conformation. Consequently, the cyclization quantum yield is dependent on the conformer ratio and generally does not exceed 0.5. Surprisingly, cyclization and cycloreversion quantum yields have not been reported for DTEs containing one or two pyridinium group.19,20,23,24 Qin et al. did report a cyclization quantum yield of 0.155 for a symmetrical DTE containing neutral pyridine groups; however, no cycloreversion quantum yield was presented. Nonetheless, the cyclization quantum yield for 3o was 3-fold lower than this derivative and 5-fold lower than a DTE with the same photochromic core, but no aryl substituents on the thiophene rings reported by Irie and et al. (i.e., 0.2125). It is apparent that the pyridinium substituent and/or branched alkyl substituent play a role, but the exact nature of this effect on the cyclization quantum yield will require further study. In addition, the cycloreversion quantum yield for 3c is significantly lower than the cyclization quantum yield for 3o, which is common for DTEs with a similar permethylated core.25 The photostability of 3 was measured by UV−vis absorption spectroscopy over 10 ring-closing/ring-opening cycles in ethyl acetate in the presence of air (inset of Figure 1). The absorbance was monitored at 605 nm, the λmax of the ring-closed isomer in the visible region. The fatigue resistance of 3 was high with only 3% degradation after 10 cycles. This is similar to an amphiphilic DTE previously prepared in our group that also contained methyl groups at the reactive carbons.17 Langmuir Films. Surface pressure−area isotherm measurements were performed on a Langmuir trough to characterize Langmuir films of 1−3 at the air−water interface. The primary reason for using this technique is to qualitatively assess differences in the packing arrangements of DTE isomers (i.e., open-ring and closed-ring forms), as Langmuir films. We hypothesize that the assembly of DTE isomers depends on their molecular structure (i.e., mean molecular area). Subsequently, differences in structure are expressed by relative differences in the surface pressure−area isotherms. Prior to these measurements, the mechanical stability of the Langmuir films was assessed by performing five consecutive compression−decompression cycles (Figure S1). In all cases, no significant loss of material was observed during these cycles, which suggests that these Langmuir films are reversible and stable. In Figure 2, three sets of isotherms are shown for 1−3 on an ultrapure water subphase surface. Each set represents Langmuir films prepared from the same DTE stock solution under three different irradiation conditions (i.e., no irradiation, UV light irradiation, and visible light irradiation). Within each set, shifts of an isotherm to larger or smaller mean molecular areas were qualitatively correlated to the relative area that a

Miyaura reaction with 2,4-dibromo-3,5-dimethylthiophene. Subsequently, 6 was coupled with 4-(2,3,3,4,4,5,5-heptafluoro-1-cyclopenten-1-yl)-3,5-dimethyl-2-(4-pyridyl)thiophene to give 7. Finally, methylation of 7 with methyl triflate produced the amphiphilic DTE 3. Photochromic Properties. The photochromisms of 1 and 2 were previously examined in ethyl acetate.17 Similarly, 3 was observed to undergo reversible photoisomerization. Upon irradiation with UV light (6 W UVA lamp; λc = 365 nm), a pale-yellow solution of 3o turned royal blue, and an increase in absorbance was observed at 408 and 605 nm with a concomitant decrease at 276 and 342 nm (Figure 1). These

Figure 1. Normalized absorption spectra of 3 in ethyl acetate prior to irradiation (black) and after irradiation with UV light (6 W UVA lamp; λc = 365 nm) or visible light (300 W halogen lamp; λc = 495 ± 6 nm) in the following sequence: 1 min of UV (orange), 2 min of UV (green), 3 min of UV (blue), 3.5 min of UV (purple; photostationary state), and 30 s of visible (red). The inset shows the change in normalized absorbance at 605 nm upon cycling 3 with UV and visible light.

new absorption bands in the visible region represent the formation of the closed-ring isomer 3c. The wavelengths of maximum absorption (λmax) for 3 were bathochromically shifted compared with 1 and 2 (Table 1). Similar shifts have been observed for other DTEs that contain pyridinium moieties.19−22 Further irradiation of 3 with UV light brought the system to a photostationary state (i.e., photochemical equilibrium) with a conversion similar to that of 1 and 2. Isosbestic points were clearly observed at 295, 315, and 367 nm, indicating the presence of only two interconverting isomers. In addition, the photoisomerization of 3c was completely reversible upon irradiation with visible light (300 W halogen lamp; λc = 495 ± 6 nm). The cyclization and cycloreversion quantum yields of 3 were also determined in

Table 1. Absorption Properties and Photochromic Reactivity of Dithienylethenes in Ethyl Acetatea,b λmax (nm)c 1o 2o 3o

310 312 276 342

εmax (104 M−1 cm−1)

Φo→cd

± ± ± ±

0.28 ± 0.03 0.10 ± 0.02 0.044 ± 0.011

1.93 4.17 2.17 2.03

0.09 0.50 0.02 0.02

λmax (nm)c 585 585 408 605

1c 2c 3c

εmax (103 M−1 cm−1)

Φc→oe

conversion (%)

± ± ± ±

0.38 ± 0.08 0.15 ± 0.01 0.0093 ± 0.0014

51.7 ± 0.4 48.1 ± 0.5 53.2 ± 0.9

4.72 9.80 6.56 15.6

0.05 0.82 0.06 0.1

a

The data for 1 and 2 were adapted with permission from ref 17. Copyright 2016 Royal Society of Chemistry. bThe error is the standard deviation for the mean taken from a minimum of three independent measurements. cThe error is ± 1 nm. dCyclization quantum yield. eCycloreversion quantum yield. C


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open-ring isomer.26 These isotherm shifts were also reversible, as the isotherms of Langmuir films prepared from solutions of 1 and 2 irradiated with visible light shifted to smaller mean molecular areas compared with films prepared from UVirradiated samples. For 1, reversibility was almost quantitative; however, for 2 the isotherm was still shifted to larger mean molecular areas than the nonirradiated sample. This difference in the extent of reversibility is most likely due to the difference in cycloreversion quantum yields, as 1 is more than 2-fold greater than 2. In addition, direct UV or visible irradiation of Langmuir films of 1 and 2 on the trough did not result in a change in surface pressure. This suggests that these close-packed structures do not allow for large changes in molecular volume that accompany the photoisomerization of 1 and 2. Moreover, the parent nonamphiphilic derivatives of 1 and 2 do not undergo photoisomerization in the crystalline state.26 For 3, a similar shift of the isotherm to larger mean molecular areas was observed for Langmuir films prepared from UV-irradiated samples, which also suggests that the assembly of closed-ring isomers requires a larger surface area than the open-ring isomers. Interestingly, the relative magnitude of the final surface pressures was significantly higher for 3 than those observed for 1 and 2. This implies that 3 occupies a larger effective molecular area than 1 and 2, owing to the branched alkyl chains. Surprisingly, direct UV irradiation of Langmuir films of 3 on the trough did not result in a change in surface pressure, even though the parent nonamphiphilic derivative of 3 undergoes photoisomerization in the crystalline state.27 Like 1 and 2, this may suggest that the photoisomerization of 3 as a Langmuir film is supressed due to a lack of free volume. In addition, the conformational mobility of 3 may be somewhat restricted at the air−water interface, inhibiting its ability to adopt of the photoactive antiparallel conformation. Unexpectedly, a significant shift of the isotherm to larger mean molecular areas was observed when films prepared from visible-irradiated samples of 3 were compressed. This apparent irreversibility of 3 was unanticipated, as it is clearly reversible in solution (inset of Figure 1), albeit with a significantly lower quantum yield than 1 and 2. To better understand the reason behind the observed Langmuir film expansion for 3 following visible irradiation, AFM and TEM were used to examine LS films of 3 under different irradiation conditions. LS Film Absorption Studies. Multilayer thin films of 1−3 were prepared using the LS method, whereby Langmuir films were transferred horizontally to a silicon substrate. This procedure was sequentially repeated to produce LS films composed of a specific number of depositions. The photochromism of the resulting LS films was then examined using absorption spectroscopy. LS films originally prepared from nonirradiated DTE stock solutions had absorption spectra similar to those observed in solution. However, only 3 exhibited reversible photoisomerization as a LS film. Again, this highlights the large change in molecular volume accompanying the photoisomerizations of 1 and 2, which must be conformationally restricted in a LS film. Similar to our solution studies, UV irradiation of 3o as a LS film prepared from 50 consecutive depositions shows an increase in absorbance in the visible region with a concomitant decrease in the UV region (Figure 3). These changes clearly represent the formation of the closed-ring isomer 3c in a multilayer thin film. As expected, the magnitude of the change in absorbance

Figure 2. Surface pressure−area isotherms of (a) 1, (b) 2, and (c) 3 on an ultrapure water subphase surface. Each plot contains a set of isotherms that represent Langmuir films prepared from the same DTE stock solution under three different irradiation conditions: no irradiation (black), irradiation with UV light (blue; 6 W UVA lamp; λc = 365 nm; 6 min), and irradiation with visible light (red; 53 W halogen lamp; 2 min). For each isotherm, the deposited volume was 120 μL and the DTE concentration was 0.15 mg mL−1.

Langmuir film of DTE isomers (or mixture of isomers) occupies, as the total number of molecules deposited was constant. For 1 and 2, a shift of the isotherms to larger mean molecular areas was observed for Langmuir films prepared from UV light-irradiated solutions (at their photostationary state) compared with films prepared from nonirradiated solutions. This suggests that films of 1 and 2 expand with UV irradiation. Correspondingly, this indicates that Langmuir films comprised of the closed-ring isomers of 1 and 2 occupy a larger area than films of their respective open-ring isomers. This correlates well with a previous study, which shows that the molecular volume of the closed-ring isomer of DTEs with phenylethynyl groups at the reactive carbons is larger than the D


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Figure 4. Normalized absorption spectra of 3c as LS films prepared from 10 (red), 30 (green), and 40 (blue) consecutive depositions. The inset shows a linear relationship between the normalized absorbance at 342 nm and the number of depositions.

Figure 3. (a) Normalized absorption spectra of 3, as a LS film prepared from 50 consecutive depositions, prior to irradiation (black) and after irradiation with UV light (6 W UVA lamp; λc = 365 nm) or visible light (53 W halogen lamp) in the following sequence: 1 min of UV (azure), 2 min of UV (blue), 3 min of UV (purple; photostationary state), 30 s of visible (green), 1 min of visible (olive), 2 min of visible (yellow), 3 min of visible (orange), 4 min of visible (red), and 6 min of visible (maroon). (b) An expansion of the 475−800 nm region.

was less when compared with that observed in solution over a similar irradiation period. Nonetheless, the photoisomerization of 3c was completely reversible as LS films upon irradiation with visible light. In addition, it is important to note that the relative shape of the absorption spectra appears to be independent of the number of depositions. This suggests that the layers within a multilayer film retain their twodimensional structure (Figure 4). Further, a strong linear correlation was observed between the intensity of the absorption bands and the number of depositions (i.e., R2 = 0.9963; inset of Figure 4). As a result, the consecutive deposition of DTE Langmuir films appears to produce homogeneous multilayer LS films with relatively high precision over the film thickness. LS Film Imaging Studies. To better understand the unexpected expansion observed for a Langmuir film of 3 following visible irradiation, AFM was used to qualitatively examine the structure of this molecular assembly. Langmuir films of 3 were transferred, as a single deposition, to silicon substrates using the LS method. As shown in Figure 5, AFM images of these LS films under different irradiation conditions display significantly different structural features. A LS film prepared from a solution of 3o shows aggregated structures with slightly rounded shapes with an average diameter of ca. 70

Figure 5. AFM images of LS films of 3 prior to irradiation with UV light (a) and following irradiation with UV light (b; 6 W UVA lamp; λc = 365 nm; 6 min) and visible light (c; 53 W halogen lamp; 2 min). E


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Langmuir nm (Figures 5a and S4). This suggests that 3o forms multimolecular aggregates either during the formation of a Langmuir film or during the preparation of a LS film. There is a precedent for the former as the aggregations of amphiphilic azobenzenes5,6 and spiropyrans7−12 have been observed in Langmuir films at low surface pressures. Although amphiphilic DTEs have never been examined as a single component in Langmuir films, the aggregation of amphiphilic DTEs in aqueous solution and photoinduced changes in the morphology of these aggregates have been reported.20,21,28−32 Interestingly, a LS film prepared from a UV-irradiated solution of 3o (i.e., 3c at the photostationary state) shows large continuous structures with irregular shapes, which suggests a coalescence of the smaller circular structures of 3o (Figure 5b). Further, this photoinduced change in morphology is most likely due to differences in intermolecular packing between photoisomers within these aggregated structures. The rigid closed-ring isomers appear to form more planar aggregates relative to the conformationally flexible open-ring isomers. A similar observation was recently reported for an amphiphilic DTE with oligo(ethylene glycol) side chains in water.29 This DTE was shown to undergo a reversible photoinduced morphological change in water between colorless microspheres and colored fibers. In contrast, AFM images of 1o and 2o show predominantly continuous films with relatively small void spaces throughout (Figures S2 and S3). However, upon UV irradiation, these films of 1c and 2c become more uniform with fewer, albeit larger, void spaces. For all DTEs, the average thickness of the LS films was ca. 4 nm (Figures S5−S7). This suggests that the aggregated structures of 3 and the relatively continuous films of 1 and 2 are not solely composed of a single monolayer. For the latter, we hypothesize that monolayers are partially disrupted by bilayer or multilayer constructions, which most likely form during deposition. To further support the photoinduced morphological changes observed for 3, TEM was also used to analyze these films, as a single deposition on silicon nitride substrates. Similarly, a LS film of 3o clearly shows well-defined circular structures, whereas following irradiation with UV light these structures coalesce into irregular wormlike structures (Figure 6). Unlike the photochromism observed in LS films via absorption spectroscopy, this photoinduced change in morphology is not readily reversible. As shown in Figure 5c, an AFM image of a LS film prepared from a solution of 3c irradiated with visible light displays further coalescence, producing larger continuous aggregates. We hypothesize that these large aggregates and the large void spaces between them are most likely responsible for the significant increase in surface pressure observed for the compression isotherm of 3c following visible irradiation. Although stock solutions of 3c in chloroform were irradiated with visible light for appropriate periods of time, it appears that even a small amount of 3c hinders the formation of smaller circular structures observed for 3o and favors the formation of large irregular aggregated structures. In fact, a TEM image of this same stock solution clearly shows the reappearance of well-defined circular structures that are not visible by AFM (Figure 6c). Although photoinduced morphological changes are not completely reversible for these aggregated structures, the photochromism of 3 is completely conserved in multilayer LS films. We are currently examining these photoinduced morphological changes in aqueous solution using dynamic light scattering and in-liquid TEM to better understand the structure and dynamics of this self-assembly.

Figure 6. TEM images of LS films of 3 prior to irradiation with UV light (a) and following irradiation with UV light (b; 6 W UVA lamp; λc = 365 nm; 6 min) and visible light (c; 53 W halogen lamp; 2 min).

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CONCLUSIONS In summary, single-component Langmuir films of three DTEs 1−3 were prepared and pressure−area isotherms were recorded under different irradiation conditions. Langmuir films of 1 and 2 showed an expansion and contraction following UV and visible light irradiation, respectively. In contrast, cycloreversion of 3c only led to a further expansion of the Langmuir film. Upon further study, both AFM and TEM images reveal that 3o forms circular aggregates. In addition, 3o F


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(4) Shembekar, V. R.; Contractor, A. Q.; Major, S. S.; Talwar, S. S. Photoisomerization of Amphiphilic Azobenzene Derivatives in Langmuir−Blodgett Films Prepared as Polyion Complexes, Using Ionic Polymers. Thin Solid Films 2006, 510, 297−304. (5) Song, X.; Perlstein, J.; Whitten, D. G. Supramolecular Aggregates of Azobenzene Phospholipids and Related Compounds in Bilayer Assemblies and Other Microheterogeneous Media: Structure, Properties, and Photoreactivity. J. Am. Chem. Soc. 1997, 119, 9144−9159. (6) Chyla, A.; Bieńkowski, M.; Sworakowski, J.; Koźlecki, T.; Wilk, K. A. Photochromic Properties of Anionic Azobenzene Amphiphiles in Solution and Langmuir−Blodgett Films. In Trends in Colloid and Interface Science XI; Rosenholm, J. B., Lindman, B., Stenius, P., Eds.; Steinkopff-Verlag Heidelberg: Darmstadt, 1997; pp 153−159. (7) Hobley, J.; Oori, T.; Kajimoto, S.; Gorelik, S.; Hönig, D.; Hatanaka, K.; Fukumura, H. Laser-Induced Phase Change in Langmuir Films Observed Using Nanosecond Pump-Probe Brewster Angle Microscopy. Appl. Phys. A 2008, 93, 947−954. (8) Nakazawa, T.; Azumi, R.; Sakai, H.; Abe, M.; Matsumoto, M. Brewster Angle Microscopic Observations of the Langmuir Films of Amphiphilic Spiropyran During Compression and Under UV Illumination. Langmuir 2004, 20, 5439−5444. (9) Taguchi, M.; Li, G.; Gu, Z.; Sato, O.; Einaga, Y. Magnetic Vesicles of Amphiphilic Spiropyran Containing Iron Oxide Particles on a Solid State Substrate. Chem. Mater. 2003, 15, 4756−4760. (10) Tachibana, H.; Yamanaka, Y.; Matsumoto, M. Surface and Photochemical Properties of Langmuir Monolayer and Langmuir− Blodgett Films of a Spiropyran Derivative. J. Mater. Chem. 2002, 12, 938−942. (11) Matsumoto, M.; Nakazawa, T.; Azumi, R.; Tachibana, H.; Yamanaka, Y.; Sakai, H.; Abe, M. Light-Induced J-Aggregation of Merocyanine in Langmuir and Langmuir−Blodgett Films. J. Phys. Chem. B 2002, 106, 11487−11491. (12) Gong, H.; Wang, C.; Liu, M.; Fan, M. Acidichromism in the Langmuir−Blodgett Films of Novel Photochromic Spiropyran and Spirooxazine Derivatives. J. Mater. Chem. 2001, 11, 3049−3052. (13) Holden, D. A.; Ringsdorf, H.; Deblauwe, V.; Smets, G. Photosensitive Monolayers. Studies of Surface-Active Spiropyrans at the Air-Water Interface. J. Phys. Chem. 1984, 88, 716−720. (14) Fujise, T.; Takeshita, M.; Sakaguchi, K.; Era, M. Preparation of Dithienylethene with Amino Groups, and Photochromic Behavior in Langmuir−Blodgett Film of the Amphiphilic Dithienylethene. Int. J. Org. Chem. 2013, 3, 158−161. (15) Abe, S.; Uchida, K.; Yamazaki, I.; Irie, M. Fatigue-Resistance Property of Diarylethene LB Films in Repeating Photochromic Reaction. Langmuir 1997, 13, 5504−5506. (16) Abe, S.; Sugai, A.; Yamazaki, I.; Irie, M. Photochromic Reaction of a Diarylethene in Langmuir−Blodgett Films. Chem. Lett. 1995, 24, 69−70. (17) Kandasamy, Y. S.; Cai, J.; Beler, A.; Sang, M.-S. J.; Andrews, P. D.; Murphy, R. S. Photocontrol of Ion Permeation in Lipid Vesicles with Amphiphilic Dithienylethenes. Org. Bimol. Chem. 2015, 13, 2652−2663. (18) Rossos, A. K.; Gengler, R. Y. N.; Badali, D. S.; Miller, R. J. D. Synthesis of Bidimensional Prussian Blue Analogue Using an Inverted Langmuir−Schaefer Method. Langmuir 2016, 32, 9706−9713. (19) Gilat, S. L.; Kawai, S. H.; Lehn, J.-M. Light-Triggered Molecular Devices: Photochemical Switching of Optical and Electrochemical Properties in Molecular Wire Type Diarylethene Species. Chem. - Eur. J. 1995, 1, 275−284. (20) Zou, Y.; Yi, T.; Xiao, S.; Li, F.; Li, C.; Gao, X.; Wu, J.; Yu, M.; Huang, C. Amphiphilic Diarylethene as a Photoswitchable Probe for Imaging Living Cells. J. Am. Chem. Soc. 2008, 130, 15750−15751. (21) van Herpt, J. T.; Areephong, J.; Stuart, M. C. A.; Browne, W. R.; Feringa, B. L. Light-Controlled Formation of Vesicles and Supramolecular Organogels by a Cholesterol-Bearing Amphiphilic Molecular Switch. Chem. - Eur. J. 2014, 20, 1737−1742. (22) Hu, F.; Jiang, L.; Cao, M.; Xu, Z.; Huang, J.; Wu, D.; Yang, W.; Liu, S. H.; Yin, J. Cyanine-Based Dithienylethenes: Synthesis,

was shown to undergo a photoinduced change in morphology, as these circular structures coalesced into irregular wormlike structures. Interestingly, AFM images show that visible irradiation of 3c primarily leads to the formation of larger aggregated structures; however, evidence of a reversible change in morphology was apparent in TEM images. We suggest that these larger aggregates are responsible for the significant increase in surface pressure observed for the compression isotherm of 3. Further, we postulate that incomplete cycloreversion impedes the reformation of smaller circular structures observed for 3o. Irrespective of these photoinduced morphological changes, the photoisomerization of 3 was completely reversible as single-component multilayer thin films upon direct UV or visible light irradiation. Consequently, amphiphilic DTEs do have potential application in the development of high-capacity data-storage devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b02484.

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Instrumentation; materials; synthetic procedures for 3, including NMR and high-resolution mass spectra; procedures for quantum yield, photoconversion and photostability studies; compression−decompression cycles of Langmuir films of 1−3; and AFM images and analysis of LS films of 1−3 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

R. J. Dwayne Miller: 0000-0003-0884-0541 R. Scott Murphy: 0000-0002-8346-2846 Author Contributions ⊥

A.K.R., M.K., and J.C. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support for this work has been provided by the Natural Sciences and Engineering Research Council (NSERC) Canada, University of Regina, Max Planck Society, and the excellence cluster “The Hamburg Centre for Ultrafast ImagingStructure, Dynamics and Control of Matter at the Atomic Scale” of the Deutsche Forschungsgemeinschaft.

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REFERENCES

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Photochromism of Amphiphilic Dithienylethenes as Langmuir

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