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Rational Design of Highly Photoresponsive Surface-Confined SelfAssembly of Diarylethenes: Reversible Three-State Photoswitching at the Liquid/Solid Interface Naoki Maeda, Takashi Hirose, Soichi Yokoyama, and Kenji Matsuda* Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan S Supporting Information *

ABSTRACT: Photoresponsive behavior of systems in equilibrium at the liquid/solid interface can be modulated through the design of photochromic core structures and the process of self-assembly. In this study, we investigated the effects of altering the substitution position of a thienyl group on twodimensional (2-D) molecular ordering composed of photochromic diarylethenes (DAEs) at a liquid/highly oriented pyrolytic graphite (HOPG) interface using scanning tunneling microscopy (STM). We found that both the open- and the closed-ring isomers of a 3-thienyl-type DAE formed stripepatterned orderings on a HOPG substrate at similar concentrations and that the process of self-assembly on a 2D surface was highly cooperative for both isomers. In the case of 2-thienyl-type DAE, similar stripe-patterned ordering was observed only for the open-ring isomer, but no ordering was observed for the closed-ring isomer. Upon irradiation with UV and visible light, reversible three-state photoswitching over the formation/disappearance of 2-D ordering was observed using the 3thienyl-type DAE. The instability of the ordering of the 2-thienyl-type closed-ring isomer was rationalized by the bend angle of the DAE core framework. Fundamental understanding of the relationship between the photoresponsive behavior of 2-D molecular ordering and molecular structures has been deepened through quantitative analyses of the concentration dependence of surface coverage and computational studies that included molecular mechanics/molecular dynamics (MM/MD) calculations.

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DAE 2 (Figure 1).14 In this system, photocontrol over 2-D ordering was made possible by using materials with different adsorbabilities on highly oriented pyrolytic graphite (HOPG) between two photochromic isomers; the open-ring isomer 2o formed a stripe-patterned 2-D ordering at the octanoic acid/ HOPG interface, while the closed-ring isomer 2c did not. The photoresponses of 2-D molecular ordering upon photoisomerization can be designed by changing the chemical structures of photochromic cores. However, the relationship between molecular structures and the photoresponsive behavior of 2-D ordering remains unclear. Here, we report that by using a 3-thienyl-type DAE, both the open- and closed-ring isomers form their 2-D orderings at similar concentrations. Because the process of ordering formation for the both isomers is highly concentration dependent, a three-state photoswitchingwhich is highly photoresponsivewas suggested by a two-component model simulation which was developed for the first time in this study. The three-state photoswitching behavior of surfaceconfined self-assembly was successfully demonstrated by

INTRODUCTION Stimuli-responsive 2-D supramolecular networks have attracted attention because their dynamic nature gives rise to attractive functions using surface assemblies.1,2 For the exploration of stimuli-responsive 2-D supramolecular networks, the liquid/ solid interface is a feasible location at which adsorbate molecules are constantly exchanged between the substrate interface and the solution phase through adsorption/desorption dynamics.3−9 Among the stimuli-responsive 2-D systems, photoresponsive surfaces are a promising candidate for optoelectronic nanodevice applications that work at a molecular level. Upon light irradiation, substantial changes to their surface states can be induced based on the photochemical isomerization of adsorbates at the molecular scale. Indeed, photocontrol over 2-D self-assembly using photochromic compounds, such as azobenzenes 2,10,11 and diarylethenes (DAEs),12−18 has recently been reported by several groups, including our group. Among many photochromic compounds, DAEs have several advantages including resistance against fatigue resulting from repeated photoswitching cycles and the thermal stability of two photochemical isomers.19−21 Recently, we reported the phototriggered formation and disappearance of 2-D ordering composed of a 2-thienyl-type © 2016 American Chemical Society

Received: February 29, 2016 Revised: April 5, 2016 Published: April 6, 2016 9317

DOI: 10.1021/acs.jpcc.6b02115 J. Phys. Chem. C 2016, 120, 9317−9325

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The Journal of Physical Chemistry C

S3). To investigate each molecular ordering composed of 1o and 1c at the liquid/solid interface, the two photochromic isomers were isolated by HPLC (Supporting Information Figure S4) and then used for STM measurements. STM Observations of Molecular Orderings at the Octanoic Acid/HOPG Interface. When a solution of the open-ring isomer 1o in octanoic acid (500 μM, 10 μL) was dropped on a freshly cleaved HOPG surface, STM measurements revealed the formation of a stripe-patterned molecular ordering (Figure 2a). The observed 2-D ordering pattern of 3-

Figure 1. (a) Chemical structures of the photochromic cores of 3thienyl- and 2-thienyl-type DAEs in the closed form, whose bend angles were intrinsically different. (b) Photochromic reactions of the 3thienyl- (left, 1) and 2-thienyl-type DAEs (right, 2) bearing 2-(Nhexadecylcarbamoyl)ethyl groups.

experiment using scanning tunneling microscopy (STM) upon in situ photoirradiation at a liquid/solid interface.

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RESULTS AND DISCUSSION Molecular Design and Syntheses. As shown in Figure 1a, the bend angle of the core moiety of the closed-ring isomer of DAE is intrinsically different between the two derivatives, i.e., 2thienyl-type and 3-thienyl-type DAEs, because thiophene rings are not regular pentagons. Based on a density functional theory (DFT) calculation at the B3LYP/6-31g(d) level, 2-thienyl-type DAE was shown to have a smaller bend angle (ϕ ∼ 110°) by ca. 30° than the 3-thienyl-type DAE (ϕ ∼ 137°) (Figure S1). Because 3-thienyl and 2-thienyl-type DAEs, 1 and 2, have photochromic cores that are similar in size, the effects of modifying the geometry of a photochromic core on the photoresponsive behavior of 2-D ordering can be clearly discussed by comparing compounds 1 and 2. The synthesis of 2-thienyl-type DAE 2o was reported previously,14 and 3thienyl-type DAE 1o was synthesized according to a previously reported method (see the Supporting Information for synthetic details and characterization). Photochromism in Solution and Isolation of Photochromic Isomers. Upon irradiating an ethyl acetate solution of the open-ring isomer 1o with UV light (313 nm), a characteristic absorption band appeared at 517 nm with an isosbestic point at 315 nm, which corresponded to the generation of the closed-ring isomer 1c. Upon subsequent irradiation with visible light (546 nm), the initial spectrum of 1o was restored (Supporting Information Figure S2). On the other hand, when a solution was irradiated with UV light for a long time, the absorption spectrum irreversibly changed without passing through the isosbestic point at 315 nm, which suggests the generation of photochemical byproduct named as the annulated isomer.13,17 In contrast to the previous reports using a strong light intensity (ca. 300 mW/cm2),13 all experiments in this study was conducted using a weak light intensity (ca. 35 mW/cm2) to focus on the ordering transformation between two orderings composed of the open- and the closed-ring isomers, but not of the byproduct. Photoisomerization in octanoic acid, which was used as a solvent for STM measurements, was also characterized by UV− vis absorption spectroscopy (Supporting Information Figure

Figure 2. STM images of (a) the open-ring isomer 1o and (b) the closed-ring isomer 1c of 3-thienyl-type DAE at the octanoic acid/ HOPG interface (conditions: 1o, c = 500 μM, Iset = 7 pA, Vbias = −700 mV; 1c, c = 500 μM, Iset = 5 pA, Vbias = −680 mV). The orientations of the main symmetry axes of the underlying graphite (i.e., the ⟨112̅0⟩ zigzag directions) are indicated with white arrows. The direction of the alkyl chains was almost parallel to the main axis of graphite. The green line denotes the rotation angle (ψ) between the main axis of graphite and one of the unit cell vectors (along the b-axis). Molecular models of the ordering consisted of (c) 1o and (d) 1c, which were simulated by MM/MD calculations. Enlarged molecular models consisting of 10 molecules of (e) 1o and (f) 1c. One molecule is highlighted in red.

thienyl-type 1o was quite similar to a previously reported pattern of 2-thienyl-type 2o.14 High-resolution STM images demonstrated that the lattice parameters of the ordering of 1o were a = 5.9 ± 0.3 nm, b = 0.98 ± 0.01 nm, and α = 90 ± 1° (Figure 2a). The obtained ordering patterns and lattice parameters were adequately reproducible using molecular mechanics/molecular dynamics (MM/MD) calculations (Fig9318

DOI: 10.1021/acs.jpcc.6b02115 J. Phys. Chem. C 2016, 120, 9317−9325

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The Journal of Physical Chemistry C

stepsdescribed by the elongation equilibrium constant, Ke. The above-mentioned surface-diffusion model describes the intermolecular interaction by considering that the probability of random walk on surface depends on the number of adjacent adsorbates. On the other hand, in our model, the intermolecular interactions in 2-D molecular assembly are successfully taken into consideration by using the two different equilibrium constant Kn and Ke. According to the model, the fractional surface coverage (θ) can be described as follows:

ure 2c). The model simulation revealed that each DAE molecule extended its two alkyl chains in the same direction and was arranged in a head-to-head fashion to form a stripepatterned ordering with alternating intra- and intermolecular hydrogen bonds. The ordering was likely stabilized by multiple interactions: (i) each molecule of the open-ring isomer was oriented in a folded conformation through intramolecular hydrogen bonds of the amide groups, (ii) the fluorous head groups (i.e., hexafluorocyclopentene moieties) gathered at the center of each stripe, and (iii) intermolecular hydrogen-bond rows were formed via the amide groups along the direction of the bright line of the stripe. Unlike the closed-ring isomer of 2-thienyl-type DAE 2c that did not form any ordering at the octanoic acid/HOPG interface,14 another stripe-patterned ordering was observed for the closed-ring isomer of 3-thienyl-type 1c (Figure 2b). Although bright spots corresponding to the DAE core moiety appeared to be vague in the STM images, the alkyl chain moiety was clearly observed with enough resolution to determine the lattice parameters (a = 5.65 ± 0.03 nm, b = 0.52 ± 0.03 nm, and α = 89 ± 1°) (see Supporting Information Figure S5). The length of the b-axis for the closed-ring isomer 1c (0.52 nm) was significantly shorter than that of the openring isomer 1o (0.98 nm), which is consistent with observations from the MM model simulation of the ordering of 1c. The DAE core of the closed-ring isomer was arranged in an edge-on lamella structure with a twofold row of intermolecular hydrogen bonds, which were formed on both sides of the core moiety (Figure 2d). Although the two different stripe-patterned orderings composed of 1o and 1c were similar in appearance, they could be clearly distinguished by the rotation angle of the bright line of the stripe with respect to one of the main symmetry axes of the underlying graphite (i.e., the ⟨112̅0⟩ zigzag directions). The rotation angle (ψ) of the orderings of 1o and 1c was ψ1o = 10 ± 1° and ψ1c = 7 ± 1°, respectively (Figure 2a, b). The rotation angle can be a decisive factor to distinguish the ordering, especially in low-resolution STM images, as described later. Concentration Dependence of the Formation of 2-D Ordering. The ordering formation at a liquid/solid interface can be recognized as 2-D crystal growth.22−24 The process of crystal growth on 2-D surface, especially for epitaxial metal growth under ultrahigh vacuum (UHV) conditions, have been extensively investigated by means of experimental approach using STM25−30 and field ion microscopy (FIM),31−33 and theoretical approach using Monte Carlo-based simulations,34−37 suggesting that (1) random adsorption on surface followed by (2) surface diffusion and then (3) nucleation are the key steps in crystal growth on 2-D surface. On the other hand, in the formation of molecular ordering at the liquid/solid interface, randomly adsorbed molecules are usually not observed experimentally by STM. For this reason, thermodynamic models that are not based on the concept of surface diffusion have been recently developed by many groups to simulate concentration dependence of surface-confined selfassemblies at the liquid/solid interface.38,39 We recently developed a nucleation−elongation model for 2D self-assembly to simulate concentration dependence of ordering formation.14 In our model, we assumed that the first step of the adsorption/desorption equilibrium is a nucleation stepdescribed by the nucleation equilibrium constant, Kn and that the other subsequent equilibriums are elongation

θ = (1 − θ )

σKe(ct − αθ ) {1 − Ke(ct − αθ )}2

(1)

with α=

A sub LNAS

(2)

where σ is the degree of cooperativity and is defined as the ratio of two different equilibrium constants (Kn/Ke) and ct is the total concentration of the adsorbate molecule. α is a constant that can be determined from experimental conditions (Asub is the area of substrate, L is the volume of supernatant solution, NA is Avogadro’s constant, and S is the unit area occupied by one molecule on the substrate). Moreover, the Gibbs free energy for elongation of the 2-D ordering can be estimated with following equation,16,40 ΔGe = −RT ln Ke

(3)

where R is the gas constant and T represents temperature. The concentration dependence of surface coverage was experimentally investigated for the two isomers 1o and 1c using STM measurements (Figure 3). Both the open- and the closed-

Figure 3. Concentration dependence of surface coverage of 3-thienyltype (1o, open circle; 1c, filled circle) and 2-thienyl-type (2o, open diamond; 2c, filled diamond) DAEs at the octanoic acid/HOPG interface. Solid and dashed lines denote the curves that best fit the nucleation−elongation model for 2-D self-assemblies. Data points for 2o and 2c were taken from ref 28.

ring isomers showed steep changes in surface coverage within a narrow range of concentrations (270−300 μM for 1o and 240− 280 μM for 1c). The sensitive ordering formation in response to the concentration changes has been previously observed for 2-thienyl-type DAE 2o (at 190−230 μM).14 We note that compounds 1o and 2o did not form any aggregates in solution phase at concentrations less than 400 μM, as evidenced by the 9319

DOI: 10.1021/acs.jpcc.6b02115 J. Phys. Chem. C 2016, 120, 9317−9325

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The Journal of Physical Chemistry C Table 1. Lattice Parameters and Adsorption Parameters of the 2-D Orderings at the Octanoic Acid/HOPG Interface compd 1o 1c 2od 2ce 3of 4of

a/nm 5.9 ± 0.3 5.65 ± 0.03 6.3 ± 0.2 − 6.8 ± 0.1 7.2 ± 0.1

b/nm 0.98 ± 0.01 0.52 ± 0.03 1.04 ± 0.04 − 0.98 ± 0.02 0.98 ± 0.03

α/deg 90 ± 1 89 ± 1 89 ± 1 − 88 ± 1 87 ± 2

Za 2 1 2 − 2 2

S/nm2b 2.89 2.94 3.28 − 3.33 3.52

Kn/M−1 −3

≤4 × 10 0.4 ± 0.3 1.3 ± 0.5 − (1.2 ± 0.8) × 103 ≤0.6

Ke/103 M−1

σ

ΔGe/kJ·mol−1c

3.62 ± 0.04 3.78 ± 0.03 4.60 ± 0.02 − 36 ± 2 570 ± 170

−6

−20.3 −20.4 −20.9 − −26.0 −32.9

≤10 1.0 × 10−4 4.5 × 10−4 − 3.2 × 10−2 ≤10−6

a The number of molecules in a unit cell. bThe unit area occupied by one molecule on the substrate (S), which was calculated as follows: S = (ab sin α)/Z. cThe Gibbs free energy for elongation of molecular ordering, which was calculated as follows: ΔGe = −RT ln Ke (where T = 298 K). d See ref 14. eNo 2-D ordering was observed for compound 2c. f2-Thienyl-type DAE analogues bearing an N-octadesyl group (−C18H37) (3o) and an N-eicosyl group (−C20H41) (4o), instead of an N-hexadecyl group (−C16H33) of 2o. See ref 18.

group (−C16H33) (2o) was −26.0 kJ·mol−1, which decreased further to −32.9 kJ·mol−1 for the analogue bearing an N-eicosyl group (−C20H41) (4o). The critical concentration (cc), at which surface coverage saturates, of 4o (cc ∼ 10 μM) was 25 times smaller than that of 2o (cc ∼ 250 μM).32 This effect was rationally explained by the increase in van der Waals interactions between the alkyl side chains and HOPG, as well as the interactions between adjacent alkyl chains that resulted from an increase in alkyl chain lengths. Because 1o and 1c have the same alkyl chain lengths and similar sized cores, these compounds showed similar ΔGe’s; these values were smaller in absolute value than those of 3o and 4o, which had longer alkyl chain lengths. Molecular Mechanics/Molecular Dynamics Calculations for 2-D Orderings on an HOPG Substrate. Despite the similar chemical structures between the closed-ring isomers 1c and 2c, critical differences were observed in their adsorbabilities on an HOPG surface. To explore these differences in adsorbability between 3-thienyl-type 1c and 2thienyl-type 2c, molecular dynamics (MD) calculations were conducted where each ordering model comprised 10 molecules on an HOPG substrate (Figure 4). The initial structure of 1c was inspired by high-resolution STM images, and that of 2c was constructed based on the structure of 1c. Note that the initial structure of 2c was obtained as a local minimum structure after geometrical optimization using MM calculations. Interestingly, the edge-on lamella structure of 1c was retained during an MD simulation time of 20 ps, whereas regularity of the structure was significantly lost for the 2-thienyl-type 2c. For the latter case, the orientation of the DAE core moiety of 2c tended to change from edge-on to face-on contact during the MD calculation. The results obtained for the open-ring isomers suggested that each 2-D ordering composed of 1o and 2o was stable during the MD calculations under the same conditions (Supporting Information Figures S9 and S10). In relation to this result, we noticed that the core part of 2thienyl-type DAE 2c had less contact with the HOPG surface compared to 3-thienyl-type compound 1c (Figure 4c, d). The inadequate contact of the core moiety could be reasonably explained by the bend angle of the DAE framework (as illustrated in Figure 1a). 2-Thienyl-type DAE has a small bend angle (ϕ ∼ 110°); therefore, the edge-on lamella structure is not a preferable arrangement for 2-thienyl-type compound 2c. Indeed, MM calculations suggested that the molecule− substrate interaction energy (Emol−sub), which is mainly a van der Waals interaction, for 2c was weaker than that of 3-thienyltype 1c by 20.1 kJ·mol−1 (Table 2, Supporting Information Figure S13). Moreover, the strain energy of 2c for the extended flat conformation adopted on the HOPG surface (Estrain) was

linear relationship between absorbance and sample concentration (Figures S6 and S7 in the Supporting Information). The proposed model described by eq 1 is applicable only in the case that the ordering formation process is in thermodynamic equilibrium and the surface coverage is path independent. This point should be carefully checked because whether an ordering formation process at the liquid/solid interface is under kinetic or thermodynamic control is strongly depends on the structure of adsorbates.39 In the case of compounds 1 and 2, (1) molecular orderings were clearly observed by STM just after the drop of sample solution onto freshly cleaved HOPG substrate, and the surface coverage was stable at least during several hours after the sample deposition; (2) ordering formation and disappearance was reversibly induced by photoirradiation and the photoinduced ordering transformation can be reasonably explained by the concentration change concomitant with photoisomerization; and (3) the molecular ordering was instantly disappeared within the time scale of STM measurement (i.e., a few minutes or less that is needed for dropping a solvent followed by engaging the tip into the interface and then scanning STM images) when the solution over an established molecular ordering was diluted by the addition of the solvent (Supporting Information Figure S8). From these results, we concluded that the ordering formation process of compounds 1 and 2 is in thermodynamic equilibrium. The experimentally obtained concentration dependence of the surface coverage of 1o and 1c was analyzed by the nucleation−elongation model. The experimental results regarding surface coverage depending on concentrations was well reproduced, as shown in Figure 3 (solid line for the closed-ring isomer 1c and dashed lines for open-ring isomers 1o and 2o). The resulting adsorption parameters (i.e., Kn, Ke, σ, and ΔGe) are summarized in Table 1. We note that the concentration dependence of surface coverage can be expressed using only the two parametersthe elongation equilibrium constant Ke and the degree of cooperativity σand is independent of the rate of surface diffusion. Despite the differences in the chemical structures of the core moieties (i.e., open- or closed-ring isomers, and 2-thienyl- or 3-thienyl-type core structures), the ΔGe values at 298 K for compounds 1o, 1c, and 2o (−20.3 to −20.9 kJ·mol−1) were comparable to one another. The differences in ΔGe values (ΔΔGe < 1 kJ·mol−1) were smaller than the average thermal energy at room temperature (i.e., RT = 2.5 kJ·mol−1 at 25 °C). Another important insight is that the length of the alkyl side chain had a significant impact on the ΔGe value; for example, the ΔGe values of the 2-thienyl-type DAE analogue bearing an N-octadecyl group (−C18H37) (3o) instead of an N-hexadecyl 9320

DOI: 10.1021/acs.jpcc.6b02115 J. Phys. Chem. C 2016, 120, 9317−9325

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adsorption model, we assumed a stepwise adsorption/ desorption equilibrium for each component, which is described as follows: substrate + Asol ⇌ A1sub A1sub + Asol ⇌ A sub 2 sol sub A sub 2 + A ⇌ A3

⋮ Asub N−1

sol

+ A ⇌ Asub N

(4)

substrate + Bsol ⇌ B1sub B1sub + Bsol ⇌ Bsub 2 sol Bsub ⇌ B3sub 2 + B

⋮ sol Bsub ⇌ Bsub N−1 + B N

Figure 4. Molecular dynamics (MD) simulations of the orderings composed of 10 molecules of (a) 1c and (b) 2c on an HOPG substrate (force field, Dreiding). During the simulation time of 20 ps, the edge-on lamella structure was retained for the 3-thienyl-type DAE 1c, whereas the regularity of the ordering was significantly lost for the 2-thienyl-type DAE 2c. (c, d) Side view of an adsorbed molecule before MD simulation for (c) 1c and (d) 2c. Dashed circles denote the contact region of the DAE core moieties and the HOPG surface.

where X is a molecule in the solution phase, and is an N-mer domain on the substrate. Interactions between two different components for the formation of each 2-D ordering were not taken into account in this model. In a similar manner to the monocomponent model, we assumed that only the first step of the equilibrium was a nucleation step described by KnX and the other subsequent equilibriums were elongation steps described by KeX. Then, surface coverage of each component could be determined as follows:42

Table 2. Adsorption Energies on an HOPG Substratea compd

Emol−sub/kJ·mol−1b

Estrain/kJ·mol−1c

Eads/kJ·mol−1d

1o 1c 2o 2c

−474.3 −474.1 −470.4 −454.0

5.2 15.9 11.9 18.3

−469.1 −458.2 −458.5 −435.7

(5)

XNsub

sol

θ A = (1 − θ A − θ B) θ B = (1 − θ A − θ B)

a

Calculated based on a simple model composed of one molecule on a graphite substrate using MM calculations in the gas phase (see Supporting Information for details). bNonbonding energy between the molecule and the substrate. cStrain energy of the flat conformation on a HOPG surface with respect to the relaxed conformation in the gas phase. dAdsorption energy, which is defined as follows: Eads = Emol−sub + Estrain. Other contributions such as (1) molecule−molecule interactions in 2-D ordering (Emol−mol), (2) effects of entropy, and (3) solvent interactions were not taken into account in this calculation.

σ A K e A (c t A − α A θ A ) {1 − KeA(ct A − α Aθ A )}2

(6)

σ BKe B(ct B − α Bθ B) {1 − Ke B(ct B − α Bθ B)}2

(7)

with αA =

αB =

A sub LNAS A

(8)

A sub LNAS B

(9)

where superscripts A and B denote parameters of each component and the parameters are identical to those used in eqs 1 and 2. By simultaneously solving eqs 6 and 7, a set of θA and θB could be uniquely determined when the total concentration (ctX) and adsorption parameters (i.e., KeX and σX) of each component were provided (see the Supporting Information for details of the multicomponent model). The simulation of surface coverages for 1o and 1c using the experimentally determined adsorption parameters are shown in Figure 5a. Because of the highly cooperative nature of the 2-D ordering formation of 1o and 1c (i.e., σ ≪ 1), clear rectangular regions of concentrations at which 2-D ordering composed of either 1o or 1c fully covered substrate were simulated. This sharp concentration dependence of ordering formation suggested that the molecular ordering composed of 1o and 1c was highly sensitive to photoisomerization. When the ordering formation was not cooperative (i.e., σ ≅ 1), surface coverage gradually changed against concentration of each isomer, as shown in Figure 5b.

2.4 kJ·mol−1 higher than that of 1c (Supporting Information Figure S14). Consequently, the energy gain upon adsorption (Eads = Emol−sub + Estrain) of 2-thienyl-type 2c was smaller than that of 3-thienyl-type 1c by 22.5 kJ·mol−1, which was comparable to the experimentally determined ΔGe value for 1c (−20.4 kJ·mol−1).41 Thus, the bend angle differencee.g., ϕ = 137° in 1c and ϕ = 110° in 2ccould substantially contribute to the difference in stability of 2-D ordering composed of rigid aromatic molecules with edge-on lamella structures on a HOPG substrate. Two-Component Model Simulation for the Formation of 2-D Ordering. Upon irradiation of a photochromic compound at a liquid/solid interface, photoisomerization occurs, and the generated photochemical isomers competitively adsorb on substrates. In order to simulate the competitive adsorption occurring upon in situ photoirradiation on a 2-D substrate, a two-component model was developed for the first time in this study. Considering two adsorbates (A and B) in the 9321

DOI: 10.1021/acs.jpcc.6b02115 J. Phys. Chem. C 2016, 120, 9317−9325

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other hand, when the ordering formation process was highly cooperative, complete switching could be induced by changing the conversion ratio by less than 10% (solid lines in Figure 5c). Thus, highly sensitive surface-confined self-assembly could be rationally designed considering the cooperative process of ordering formation. Investigation of Surface Coverage using ConversionRatio-Specified Solution Samples. The two-component model simulation of surface coverage shown in Figure 5 was experimentally verified using STM. A solution of 1o in octanoic acid ([1o]t = 400 μM) was prepared and irradiated with UV light (313 nm) to isomerize 1o to 1c to obtain a sample solution with a desired conversion ratio. The conversion ratio of the sample solution was precisely determined by UV−vis absorption spectroscopy (examined conversion ratios were 0, 23, 36, 49, 65, and 75%, see Supporting Information for details). The solution (∼10 μL) was then deposited on a HOPG substrate. Resulting STM images of the conversionratio-specified samples are shown in Figure 6.43 Stripe-patterned ordering was clearly observed with conversion ratios of 0, 23, 65, and 75%, whereas no ordering was obtained for samples at intermediate conversion ratios (36 and 49%). Judging from the rotation angle (ψ) between the bright line of the stripe and a main symmetry axis of the underlying graphite, the orderings observed at low conversion ratios (0 and 23%) correspond to the open-ring isomer 1o (ψ = 10 ± 1°). The other orderings observed at high conversion ratios (65 and 75%) correspond to the closed-ring isomer 1c (ψ = 7 ± 1°), as expected (see Figure 2). Experimentally determined surface coverages of 1o and 1c are plotted in Figure 5c. The experimental data were almost completely consistent with the curve simulated using the cooperative parameters (solid lines in Figure 5c). On the other hand, they largely deviated from the simulated curves when the isodesmic parameters were used (dashed lines in Figure 5c). This result suggests that (1) the highly cooperative nature of ordering formation for isolated compounds 1o and 1c is maintained even when the two isomers coexist, and (2) the adsorption parameter of each isomer determined by a monocomponent system is independent of the presence of the photochromic counterpart. Reversible Three-State Switching over the Formation/ Disappearance of 2-D Orderings upon in Situ Photoirradiation. The two photochemical isomers of 3-thienyl-type DAE 1o and 1c formed their 2-D ordering at similar concentrations (cc = 280−300 μM), and the ordering formation process was highly cooperative for both isomers (σ ≤ 10−4). We anticipated that the condition of 3-thienyl-type DAE 1 at a suitable concentration ([1o]t + [1c]t = 400 μM) would be appropriate for demonstrating the three-state switching of 2-D ordering induced by in situ photoirradiation. Indeed, the 2-D ordering composed of 1o completely disappeared after in situ irradiation with UV light for 5 min to a solution of 1o ([1o]t = 400 μM) at the octanoic acid/HOPG interface (Figure 7a). Successive irradiation with UV light for a long time (10 min) resulted in the formation of the other orderings composed of 1c, as expected. Three-state switching in the opposite direction (i.e., in situ visible light irradiation starting from a solution of 1c ([1c]t = 400 μM)) was also confirmed to proceed as expected. Thus, reversible control over the formation/disappearance of 2D ordering between the three-states, (i.e., (1) the stripepatterned ordering composed of 1o, (2) no ordering, and (3) the ordering composed of 1c) was realized using 3-thienyl-type DAE 1.44 This multistep photoinduced ordering transformation

Figure 5. Contour plot of simulated surface coverages of the 2-D orderings composed of 1o and 1c as a function of their concentrations, obtained from (a) a cooperative model using experimentally determined adsorption parameters (1o: Kn = 4 × 10−3 M−1, Ke = 3.62 × 103 M−1; 1c: Kn = 4 × 10−1 M−1, Ke = 3.78 × 103 M−1) and from (b) an isodesmic model (1o: Kn = Ke = 3.62 × 103 M−1; 1c: Kn = Ke = 3.78 × 103 M−1). The diagonal-patterned region at the upper right corner denotes the concentration region at which surface coverage can not be evaluated from the model simulation (i.e., both parameters Ke1o[1o]sol and Ke1c[1c]sol exceed unity, see the Supporting Information for details). The black dashed line denotes the trajectory of the concentration pairs along which photoisomerization between 1o and 1c proceeded when the total concentration ([1o]t + [1c]t) was set to 400 μM. (c) Simulated curves of surface coverage of 1o (red line) and 1c (blue line) as a function of conversion ratios (i.e., [1c]t/([1o]t + [1c]t)). Solid and dashed lines in part (c) correspond to the cross section along the black dashed line in parts (a) and (b). Red square and blue circle denote the surface coverages of 1o and 1c, respectively, determined from STM measurements.

Upon photoisomerization, the concentration of one isomer decreases with the concomitant generation of the same amount of the other isomer, during which the sum of the amount of the two isomers remains constant. Therefore, concentration changes upon photoisomerization are displayed as downward sloping straight lines, as shown in Figure 5a, b (black dashed line). Simulated curves of surface coverage along the black dashed lines in Figure 5a, b are shown in Figure 5c. The x-axis of Figure 5c is the conversion ratio, which is defined as the ratio of the closed-ring isomer to the open-ring isomer (i.e., [1c]t /([1o]t + [1c]t)). When the total concentration was set to 400 μM, the open-ring isomer 1o formed its ordering at a conversion ratio from 0 to ∼0.3, whereas the closed-ring isomer 1c formed ordering at a range between ∼0.6 and 1. At the intermediate range (i.e., conversion ratio from ∼0.3 to ∼0.6), no ordering formation was simulated. In marked contrast, when isodesmic parameters were applied to the same model simulation (i.e., Kn = Ke, σ = 1), an intermediate region with no ordering was absent (dashed lines in Figure 5c). In this situation, a ca. 30% change in conversion ratio was needed to induce a complete switch between formation and disappearance of molecular ordering. On the 9322

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Figure 6. STM images of conversion-ratio-determined solutions of 1o and 1c at the octanoic acid/HOPG interface ([1o]t + [1c]t = 400 μM, Iset = 5−30 pA, Vbias = −800 to −650 mV). Conversion ratios of the sample were (a) 0, (b) 23, (c) 36, (d) 49, (e) 65, and (f) 75%. Red and blue dashed lines denote the surface areas that are covered with the ordering composed of either 1o or 1c, respectively. The conversion ratio of each sample was precisely determined by UV−vis absorption spectroscopy before deposition of the solution on the substrate (see the Supporting Information).

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CONCLUSIONS In summary, we demonstrated that the stability of 2-D crystals (i.e., the Gibbs free energy gained upon adsorption on a substrate) of each of the photochemical isomers and the process of ordering formation are essential aspects in the design of highly photoresponsive 2-D orderings using photochromic molecules. The bend angles of the photochromic core frameworks substantially influenced the adsorbability of rigid aromatic molecules bearing alkyl side chains at the liquid/ HOPG interface. The fundamental understanding of the relationship between the photoresponsive behavior of 2-D molecular ordering and molecular structures has been deepened by the approach combining the quantitative analyses of concentration dependence of surface coverage and computational studies using MM/MD calculations. By using a 3-thienyltype DAE 1, reversible three-state switching over the formation/disappearance of 2-D ordering was observed at the octanoic acid/HOPG interface upon in situ photoirradiation. The photoinduced ordering formation/disappearance was reasonably explained by the concentration change in the solution phase concomitant with photoisomerization reactions. The cooperative process of ordering formation enabled 2-D molecular ordering at the liquid/solid interface to be highly photosensitive. Further investigations using photochromic molecules that have a variety of photochromic and photophysical properties will open up more sophisticated systems of photoresponsive 2-D ordering.

Figure 7. Reversible control over the formation/disappearance of 2-D orderings composed of (a) 3-thienyl-type DAE 1 (c = 400 μM, Iset = 5−10 pA, Vbias = −700 to −660 mV) and (b) 2-thienyl-type DAE 2 (c = 500 μM, Iset = 30 pA, Vbias = −800 mV) upon in situ photoirradiation at the octanoic acid/HOPG interface. Insets in (a) show magnified stripe-patterned orderings and the rotation axis of the bright line of the stripe with respect to the main symmetry axis of the underlying graphite (green solid line).

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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.jpcc.6b02115. Experimental procedures, absorption spectra, HPLC chart for separation of isomers, details of the twocomponent model simulation, additional figures, and 1H and 13C NMR spectra (PDF)

was in marked contrast to the previously reported UV-lightinduced disappearance of 2-D ordering composed of 2-thienyltype DAE 2, in which switching between only two states was possible (Figure 7b). 9323

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-75-3832738. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas “Photosynergetics” (No. 26107008) from the MEXT, Japan and a Grant-in-Aid for Young Scientists (B) (No. 25810048) from the JSPS, Japan.

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