Photomodulated Self-Assembly of Hydrophobic Thiol Monolayer

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Photo-Modulated Self-Assembly of Hydrophobic Thiol Monolayer-Protected Gold Nanorods and Their Alignment in Thermotropic Liquid Crystal Chenming Xue, Karla Gutierrez-Cuevas, Min Gao, Augustine Urbas, and Quan Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp408081q • Publication Date (Web): 17 Sep 2013 Downloaded from http://pubs.acs.org on September 23, 2013

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The Journal of Physical Chemistry

Photo-Modulated Self-Assembly of Hydrophobic Thiol MonolayerProtected Gold Nanorods and Their Alignment in Thermotropic Liquid Crystal Chenming Xue†, Karla Gutierrez-Cuevas†, Min Gao†, Augustine Urbas§, and Quan Li*,† †

Liquid Crystal Institute and Chemical Physics Interdisciplinary Program, Kent State University, Kent, Ohio 44242, United States § Materials and Manufacturing Directorate, Air Force Research laboratory WPAFB, Ohio 45433, United States Supporting Information ABSTRACT: Three terminal thiols possessing azobenzene and perylene diimide (PDI) segments covalently linked by alkylene spacers of different length (P n SH, n = 4, 6, and 8) were synthesized to stabilize and functionalize gold nanorods (GNRs) via strong covalent Au-S bonds onto the gold surface. The resulting hydrophobic thiol monolayer-protected GNRs (P n GNRs) were stable in both organic solvent and solid state, and exhibited fascinating photoresponsive self-assembly behavior. The PDI moieties provided π-π interactions to promote GNR self-assemblies while the photoresponsive azobenzene moieties offered a way to phototune the assemblies in a reversible manner. Interestingly, when P n GNRs were mixed with a structurally similar room-temperature thermotropic liquid crystal perylene diimide (LCP), the UV-irradiated P n GNRs showed more compatibility with the LCP host than their corresponding un-irradiated ones. Furthermore, the P n GNRs with varied alkylene chain lengths showed different dispersion abilities in LCP. The UV-irradiated P 4 GNRs did not disperse well in LCP whereas the UV-irradiated P 6 GNRs and P 8 GNRs dispersed well in LCP and were further aligned upon mechanical shearing. In addition, preliminary molecular simulation was performed to explain this interesting photo-modulated self-assembly of the GNRs. Keywords: thiol monolayer protected; gold nanorod; photoresponsive; self-assembly; alignment; liquid crystal

1. INTRODUCTION Anisotropic gold nanorods (GNRs) are capturing intensive attention due to their applications in sensing, non-linear optical devices, imaging and diagnostics, etc.1-3 The surface plasmon resonance (SPR) of GNRs allows them to concentrate and manipulate light depending on their size, shape and proximity. If GNRs are self-assembled, they can couple and enhance local electric fields even by a factor as high as 105.4 The resultant near-field enhancements can be utilized in surfaceenhanced Raman spectroscopy (SERS).5 Undoubtedly, controlling and tuning GNR self-assembly are highly interesting and vital because they can further contribute to the preparation of metamaterials with unusual electromagnetic properties.6-7 ππ Intermolecular interactions were reported to create GNR self-assemblies.8-9 However, such GNR self-assembly might be further modulated if some tunable structural elements are introduced into the thiol molecules on GNR surface.10-11 It is also established that inducing GNR alignment in thin film is particularly important for their practical applications.6,12-15 Liquid crystal (LC) as a host material would be a viable candidate for such purpose, since not only LC can provide a long range orientational order to GNRs but also its orientation capable of responding to external filed such mechanical, electric and magnetic field can provide realignment and rearrangement of GNRs. However, aligning GNRs in LC host has not been explored much. So far one representative attempt has been to use water-containing lyotropic LC as host to align GNRs.12 But unstable lyotropic LC aqueous system would be an issue. Dispersing GNRs in organic media is more appealing

than dispersing them in aqueous media because their low interfacial energies allow for a high degree of control during solution and surface processing. Thus, to enhance GNR’s stability and compatibility with organic LC media, hydrophobic thiol monolayer-protected GNRs in thermotropic LCs would be desirable. In this study, a series of new thiols (P n SH) comprising a perylene diimide unit (PDI) and a photoresponsive azobenzene moiety linked by flexible alkylene chains of different length were synthesized to stabilize and functionalize GNRs (Figure 1). The resultant hydrophobic thiol monolayerprotected GNRs were stable in both organic solvent and solid state and exhibited interesting photoresponsive self-assembly behavior. When P n GNRs were mixed with a structurally similar room-temperature thermotropic LC perylene diimide (LCP) host (see Figure 1),16 the UV-irradiated P n GNRs were more compatible with the LCP host than their corresponding unirradiated ones, which showed a better alignment upon mechanical shearing. To the best of our knowledge, this is the first report on photo-modulated self-assembly of GNRs.

2. EXPERIMENTAL SECTION All chemicals and solvents were purchased from commercial supplies and used without further purification. HAuCl4 is 30 wt% in diluted HCl solution. 1H NMR spectra were recorded on a Bruker 400 MHz NMR spectrometer, with deuterated chloroform (CDCl3) as solvent at 25 °C. The chemical shifts were reported using residue CHCl3 (δ = 7.26 ppm) as the internal standard. 13C NMR spectra were recorded on a Varian 200 MHz NMR spectrometer. The chemical shifts were reported using CDCl3 (δ = 77.16 ppm) as the internal standard.

ACS Paragon Plus Environment

The Journal of Physical Chemistry High resolution mass spectrometry (HRMS) was performed by Mass Spectrometry & Proteomics Facility of Ohio State University. UV-visible spectra were collected on a PerkinElmer Lambda 25 UV-Vis spectrometer at the resolution of 1 nm. Fluorescence spectra were recorded on a FluoroMax-3 spectrafluorometer of Horiba scientific. For transmission electron microscopy (TEM) observation, solution samples were first dispersed on TEM Cu grids pre-coated with thin carbon film (Cu-400 CN) purchased from Pacific Grid Tech. After completely dried, they were studied using a FEI Tecnai TF20 FEG TEM equipped with a 4k UltraScan CCD camera for digital. A C6H12O

O

O

N

N O O

O O

N OCnH2nO

nescence of PDI. P 4 SH displayed red color while P 6 SH and P 8 SH were yellow (insets in Figure 2, top). Upon UV irradiation, all of them presented bright fluorescence, of which P 6 SH and P 8 SH were brighter than P 4 SH as shown in Figure S3. P4SH

P8SH

P6SH

400 P4GNR

400 P6GNR

500

500

400 P8GNR

500

OCnH2nSH

N

PnSH n = 4, 6 and 8

C12H25O

O

O

N

N

O O

O O

400 OC12H25

530 nm

2.3 nm

365 nm

600

800

400

600 800 Wavelength (nm)

400

600

800

LCP

B

4.5 nm

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GNR

GNR

Figure 1. (A) Molecular structures of the thiols P n SH (n = 4, 6, and 8) with varied alkyl chain length and liquid crystalline host LCP. (B) Reversible photoisomerization of P 8 SH upon light irradiation. Note: The thiol structures were based on molecular simulation by Gaussian software. Hydrogen atoms were omitted for clarity.

3. RESULTS AND DISCUSSION The thiol molecules P n SH were prepared by a facile synthesis (see Supporting Information). Their optical properties in CH 2 Cl 2 were investigated by UV-vis spectra (Figure 2, top). All the three thiols exhibited typical PDI absorption peaks at 493 nm and 529 nm and azobenzene peak at 373 nm. After 365 nm UV irradiation for 5 minutes, the intensity of azobenzene peak significantly decreased while its intensity returned when irradiated by visible light at 530 nm for 5 minutes (Figure S1). Although azobenzene has a reversible cis-trans change resulting from its thermal relaxation, it was much slower in comparison to the reversal on visible light irradiation (Figure S2).17-20 Among the three thiols P n SHs, the intensity decrease of the azobenzene peak in P 4 SH with the shortest distance between PDI and azobenzene had the smallest change while the P 8 SH with the longest distance between PDI and azobenzene had the largest change. This could be ascribed to the interaction between PDI and azobenzene groups which lowers the degree of the trans-cis photoisomerization of azobenzene.21 This interaction could also reduce the photolumi-

Figure 2. UV-vis spectra of P n SH (10-6 M) (top) and P n GNR (bottom) in CH 2 Cl 2 . Black line: initial P n SH or P n GNR, Red line: after 5 min UV (365 nm) irradiation, Blue line: after 5 min visible light (530 nm) irradiation, and Dash line: CTAB-GNR. Inset: the pictures of corresponding solutions.

After successful thiol exchange with prepared CTABGNRs,22 the resulting organo-soluble P n GNR solutions showed clear light-blue color without precipitates (Figure 2, bottom). Same initial CTAB-GNR solutions were chosen for the thiol exchange reaction to yield P n GNRs. The GNRs had an average size of 36.2 nm × 13.0 nm and an average aspect ratio of 3.0 based on calculation of 500 GNRs’ size and aspect ratio. As shown in Figure 2 (bottom), the P n SHs caused significant GNR SPR absorption peak shift. Comparing to the initial CTAB-GNR, the transverse peak at 520 nm shifted to ca. 580 nm and the longitudinal peak at ca. 700 nm shifted to 800 nm. The transverse peak showed significantly enhanced intensity. The longitudinal peaks at ca. 800 nm became much broader, indicating agglomeration and the self-assembly of GNRs. Upon UV and following visible light exposure, the weak characteristic azobenzene peak at ca. 370 nm of the P n GNRs showed intensity changes, indicating the surface thiol-azo molecules also exhibit reversible trans-cis photoisomerization. The fluorescence spectra of P n GNRs are also shown in Figure S3. Compared to the free P n SHs, their fluorescence was significantly quenched due to the proximity of these PDI groups to the surface of GNRs.8-9 When the solutions were dropped on TEM grids and dried, the P n GNR self-assemblies were observed (Figure 3). For the samples from un-irradiated solutions, there were condensed GNR assemblies while there were less condensed assemblies from the UV-irradiated solutions. When the UV-irradiated solutions were further irradiated by visible light, the samples displayed condensed assemblies again, with an increase in side-by-side arrangements. This indicated that, during the UV irradiation, trans-cis azobenzene photoisomerization weakened the interactions among GNRs which reduced the GNR assemblies. Upon visible light irradiation, the azobenzene groups exhibited a cis-trans change which in turn strengthened the interactions, resulting in increased GNR assembly. In addition, this dynamic cis-trans change process provided an opportunity for PDI groups on adjacent GNRs to intercalate with each other in a more energetically favorable way, thus increasing the presence of side-by-side self-assemblies. This was also


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confirmed by the UV absorption spectra in Figure 2, where the absorption after visible light irradiation shows broader plasmon resonance features. P4GNR

P6GNR

Initial Sheared Before shear

UV Before shear Sheared

P8GNR

P4GNR+LCP Initial

P6GNR+LCP UV

P8GNR+LCP Figure 5. Images of 0.5 wt% P n GNR in LCP thin films under crossed polarized optical microscope. Scale bar: 100 µm. Shearing direction was about 45° to the polarizer.

Vis

Before shear Figure 3. TEM images of P n GNRs showing different selfassembly states dried from CH 2 Cl 2 solutions of initial, after UV irradiation, and after visible light irradiation. Scale bar: 50 nm.

To explore GNR self-assembly and alignment, P n GNRs were mixed with LCP. Sample preparation was depicted in Figure 4. The film samples were dried from the initial solutions or UV irradiated solutions followed by heating them to the LCP’s clearing point and cooling to room temperature, assuring LCP was in a uniform LC state. Their textures were examined by polarized optical microscope as showed in Figure 5. LCP with and without shearing were also presented for comparison (Figure 6). When sheared, thin films of the LCP/P n GNR mixtures showed uniform orientation. However, some black spots, i.e. GNR assemblies could be observed in the sheared P 4 GNR/LCP samples particularly the sample before UV irradiation. For P 6 GNRs/LCP and P 8 GNRs/LCP samples, there could not find such obvious black spots.

shearing a

b

c d

f

e

Figure 4. Preparation of LCP mixing with P n GNR samples for TEM. a: drop cast and dried on a glass slide. b: heat to 210 °C and cooled. c: mechanical shearing. d: soak in 5% HF aqueous solution. e: transfer the floating solid pieces to water and break to tiny pieces. f: transfer a tiny piece to a TEM grid.

Sheared

PDI

Figure 6. Images of LCP under crossed polarized optical microscope. Note: Shearing direction is about 45° with polarizer.

The sample films from the above treatments were transferred to TEM grids and examined by TEM (Figure 7 for P 4 GNR, Figure 8 for P 6 GNR, and Figure 9 for P 8 GNR). For the initial P n GNRs, they all exhibited self-assemblies in LCP (Figure 7A, 8A, and 9A). After UV irradiation, the P 4 GNRs still showed some self-assemblies in LCP (Figure 7B) and some dispersed ones(Figure 7C) while P 6 GNRs and P 8 GNRs showed much better dispersion in LCP (Figure 8B and 9B). The dispersion of P 8 GNR in LCP was better than P 6 GNR since P 6 GNR still showed some small agglomerates in LCP. For P 8 GNRs, they showed the best compatibility in LCP as no GNR self-assembly was observed in the TEM image (Figure 9B). After shearing, the aligned GNRs with longitudinal direction parallel to the shearing direction were obtained (Figure 8C and 9C). The red arrows indicated the shearing direction. The results indicated that UV irradiation reduced the selfassembly of GNR in LCP, which became decreased in strength with the increase of alkyl chain length from 4 to 8, resulting in well-dispersed GNRs in LCP. Therefore, with varied alkyl chain lengths, different self-assembly behaviors and alignment of GNRs were achieved in room-temperature LC film. It is worth noting that the P 6 GNRs and P 8 GNRs still dispersed well in LCP and they did not re-assemble even following visible light irradiation. This could be that in the LCP after UV irradiation the surface of P 6 GNRs and P 8 GNRs was covered with LCP molecules which kept them well dispersed. They could not reform strong inter-rod interactions with surface thiol molecules on adjacent GNRs.



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A

50 nm

B

Figure 9. TEM images of P 8 GNR in LCP. (A) Before UV irradiation. (B) After UV irradiation. (C) After shearing. C is from scanning TEM.

C

To explain the above photoresponsive self-assembly behaviors of P n GNRs and P n GNRs in LCP as schematically presented in Figure 10, molecular simulation based on Gaussian 09 was applied to investigate the influences of P n SH molecular structure changes on GNR self-assemblies (Figure 10), which were based on the most stable forms from energy minimization calculations (DFT, B3LYP).23 In the trans configuration, more π-π interaction pairs from P n SH molecules on the GNR surface existed and there were stronger π-π interactions than for the cis configuration (Figure 11A). With reversible configuration changes upon photo irradiations, the corresponding strong/weak π-π interactions resulted in tight/loose GNR assemblies, which were consistent with observations in Figure 2 and 3. Different from the reversible self-assembly behavior of P n GNRs themselves in organic solvent, P n GNRs in LCP disassembled upon UV irradiation but could not re-assemble after visible light irradiation (Figure 11B). For P 4 GNRs, assemblies did not disperse after UV irradiation (Figure 7B). Reduced trans-cis photoisomerization of azobenzene group in P 4 SH as observed in Figure 2 could account for this result: the strong interaction between PDI and azobenzene groups in P 4 SH hindered conformation change on the GNR surface, and could not reduce the inter-rod interactions sufficiently. Therefore P 4 GNR could not disperse well in LCP.

50 nm

50 nm

Figure 7. TEM images of 0.5 wt% P 4 GNR in LCP. (A) Before UV irradiation showing assembled GNRs. (B), (C) After UV irradiation showing assembled and some dispersed GNRs.

A

B

C

100 nm

100 nm

100 nm

Figure 8. TEM images of P 6 GNR in LCP. (A) Before UV irradiation. (B) After UV irradiation. (C) After shearing.

A

100 nm

B

C

500 nm

100 nm

Au

Au 530 nm

Au

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Au 365 nm

Au

Au

Figure 10. Molecular simulation of P n GNR with azobenzene in trans and cis configurations on GNR surface.


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The Journal of Physical Chemistry A

PnSH

UV Vis

LCP

B

UV

Vis

Figure 11. (A) Schematic depiction of photoresponsive P n GNRs showing different strengths of inter-rod interactions before and after UV irradiation. In this model, on one side of the GNR surface, along the longitudinal direction there exhibits 5 molecules as a representative example to offer π-π interactions to intercalate with surface molecules on adjacent GNRs (in the blue region). Before UV irradiation, there are 9 π-π interaction pairs; after UV irradiation, only 5 π-π interaction pairs exist. This assembly behavior is reversible upon UV/visible light irradiations. (B) P n GNRs (n = 6 and 8) mixing with LCP. This assembly behavior is irreversible upon UV/visible light irradiations. The red disks represent for PDIs.

4. CONCLUSIONS In conclusion, three new thiol molecules P n SHs containing azobenzene and PDI moieties linked together with alkylene chains of different length were synthesized and attached onto GNRs via strong covalent Au-S bonds. The resulting hydrophobic P n GNRs were stable and exhibited photoresponsive self-assembly behavior in organic solvent. The initial unirradiated P n GNRs in CH 2 Cl 2 showed intensive selfassemblies. Interestingly, the self-assemblies became looser after UV irradiation while intensive assemblies appeared again after re-exposure with visible irradiation. When the hydrophobic GNRs were mixed with a structurally similar roomtemperature thermotropic LCP which possessed the identical PDI moiety as the P n SH molecules, the UV-irradiated P nGNRs exhibited less self-assembly in the LCP host as compared with their corresponding un-irradiated ones. The P nGNRs with varied alkyl chain lengths exhibited different photoresponsive structural changes, resulting in different selfassembly and dispersion behaviors in LCP thin films. The UVirradiated P 4 GNRs did not disperse well in LCP host whereas P 6 GNRs and P 8 GNRs dispersed well in LCP and could be aligned upon mechanical shearing. Furthermore, a preliminary molecular simulation was performed to explain this interesting photo-modulated self-assembly behavior of P n GNRs. This work provides a method to tune GNR self-assemblies and further offers a way to disperse and align GNRs in a LC film. These studies not only could give insight into controlling the self-assembly of anisotropic nanoparticles which have altered SPR properties arising from the switchable assembled struc-

tures, but also suggested a method to align the anisotropic nanoparticles with desired orientations which could further guide to fabricate novel nanophotonic and optical metamaterials.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Tel: (330) 672 1537 (Q. L.).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Air Force Office of Scientific Research (AFOSR FA9550-09-1-0254). The TEM data were obtained at the (cryo) TEM facility at the Liquid Crystal Institute, Kent State University, supported by the Ohio Research Scholars Program Research Cluster on Surfaces in Advanced Materials. Supporting Information Available. Synthesis of thiols P n SH, CTAB-GNRs and P n GNRs, time-dependent UV-vis spectra of P 4 SH, This information is available free of charge via the Internet at http://pubs.acs.org



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Photomodulated Self-Assembly of Hydrophobic Thiol Monolayer

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