Cis-to-trans photoconversion of azobenzene self-assembled monolayers on gold nanoparticle surfaces investigated by Raman spectroscopy

Abstract

Cis-trans photoconversion behaviors of 4-dimethylaminoazobenzenecarboxylate (4DAzC) on gold nanoparticle surfaces have been investigated by means of Raman spectroscopy. The vibrational features indicated that both the cis- and trans-conformation should coexist after the adsorption on gold nanoparticle surfaces. After the irradiation of the blue light, the intensities of the cis-form peaks were found to decrease on gold. Our work can demonstrate that Raman spectroscopy can be utilized in studying the photoconversion of azobenzene-containing self-assembled monolayers fabricated on metal surfaces.

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Introduction

Although the first finding of photochromic molecules can be traced back to the nineteenth century,¹ their systematic investigation was not conducted until recent years. In the past several decades, photochromic molecules have attracted renewed interest due to the applicability to various photonic devices, such as erasable optical memory media and optical switch components.^2–6 Since photochromic molecules have two isomers with different physical properties in the absorption spectrum, refractive index, oxidation/reduction potential, and vibrational (infrared or Raman) spectrum, they can be an excellent candidate materials of erasable media for the next generation.^7–11

During the past decade, an increasing interest in azobenzene dyes has emerged, accompanied by research activities in various research fields.^12,13Azobenzene-containing molecules have two isomers, which are determined by dihedral angles between two phenyl rings and the CN=NC plane.¹⁴Trans-azobenzene can be transformed to cis-azobenzene by rotating or inverting a phenyl ring through the excitation with ultraviolet light; the reverse transformation from cis-azobenzene to trans-azobenzene can be obtained through visible light or thermal isomerization.¹⁵ In general, irradiation at the π→π* state lies close to the cis-isomer, whereas irradiation at the n→π* band shifts to the trans-isomer.

One of the major goals in molecular electronics is to fabricate molecules that function as switches on metal surfaces.¹⁶ Considering that photochromic molecules can reversibly switch between the two isomeric states in response to an external trigger of light, a better understanding of the molecular structures of photochromic molecules on metal surfaces should be important for practical reasons.¹⁷

Surface-enhanced Raman scattering (SERS) has proved one of the most sensitive techniques to monitor the adsorbates on metal substrates at the submonolayer coverage limit despite the equivocal selection rule and limited applicability to a few metals.¹⁸ The interfacial structures on nanostructures could be explained in terms of vibrational features by means of SERS.¹⁹ Since the report made by Faraday,²⁰ the physicochemical characterizations of colloidal dispersions of gold as one of the SERS substrates have not been performed systematically.^21–23

Although a number of spectroscopic studies were previously reported, any detailed SERS study on the cis-transisomerization of the azobenzene compounds on metal surfaces has not yet been reported to the best of our knowledge. To better understand the isomerization behaviors of self-assembled monolayers based on azobenzene, we performed a SERS study of 4DAzC on gold nanoparticle surfaces.

Experimental

Raman spectra were obtained using a Renishaw Raman confocal system model 1000 spectrometer equipped with an integral microscope (Leica DM LM). The 633 nm irradiation from a 25 mW air-cooled HeNe laser (Melles Griot Model 25 LHP 928) with the plasma line rejection filter was used as the excitation source for the Au SERS experiments. The blue LED at 460 nm with a maximum power of 11 W was purchased from Luxlam in Korea. UV-Vis absorbance spectra were checked by a Shimadzu UV-3101 PC spectrophotometer.²⁴

The citrate-stabilized gold nanoparticles were synthesized by adopting the recipes in the literature.²⁵ A 133.5 mg portion of KAuCl₄ (from Aldrich) was initially dissolved in 250 ml of water, and the solution was brought to boiling. A solution of 1% sodium citrate (25 ml) was then added to the KAuCl₄ solution under vigorous stirring, and boiling was continued for ca. 20 min. The statistical analysis revealed a size distribution in diameter for 15 nm particles with a relative standard deviation of ∼10%. 4DAzC (>98.0%) were purchased from Tokyo Kasei. The concentration of 4DAzC in Au nanoparticle solution was estimated to be ∼2 × 10⁻⁵ M. All the chemicals otherwise specified were reagent-grade and triply distilled water, of resistivity greater than 18.0 MΩ cm, was used in making aqueous solutions. The sample was prepared as the self-assembled layers in the colloidal solution phase. It is also possible that the temperature may be heated up after the UV light irradiation, although we cannot directly measure the deviation. Since the SERS experiments are performed in the aqueous solutions phase, it is only expected that the temperature should not rise up so quickly due to high heat capacity of water. All ab initio molecular orbital calculations were carried out using the Gaussian ′03 suite of programs.²⁶

Results and discussion

Fig. 1 compares relative SERS spectral intensities of 4DAzC on Au nanoparticle surfaces upon irradiation with blue light as a function of time. The relative bands at 1138 and 1442 cm⁻¹ appeared to increase whereas those at 1168, 1268, and 1502 cm⁻¹ decreased with irradiation time. The bands at 1442 and 1502 cm⁻¹ can be ascribed to the N=N stretching bands for the trans and cis isomer, respectively.¹⁴ To support our experimental data, we performed a density functional theory (DFT) calculation for 4DAzC. The dihedral angles of C–N–N–C and N–N–C–C for the cis isomer are calculated to be 11.2 and 50.0°, respectively, in compliance with the recent calculation results.¹⁴ The energy difference between the cis and trans isomers of 4DAzC was calculated to be 16 kcal mol⁻¹, as in line with the previous report.²⁷ The spectral vibrational assignments and DFT results are summarized in the electronic supplementary information, ESI.†

Fig. 1 (a) SERS spectra on Au nanoparticle surfaces after irradiation at 460 nm in the wavenumber region between 1000 and 1700 cm⁻¹ for 0–60 min. (b) The magnified view of the SERS spectra on Au nanoparticle surfaces after the irradiation at 460 nm in the wavenumber region between 1400 and 1550 cm⁻¹.

It should be mentioned that the intensities of the cis isomer are predicted to be smaller by several times than those of the trans isomer from our DFT calculation. Although not shown here, the ordinary Raman spectra of 4DAzc in its solid state and solutions in the absence of Au nanoparticles mostly revealed the peaks for the trans conformers with quite weak features for the cis isomer. Considering that the intensities of the cis isomer are predicted to be smaller by several times than those of the trans isomer, our SERS data in Fig. 1(a) indicate that the cis isomer should be quite dominant on Au nanoparticle surfaces. As indicated in Fig. 1, the intensities of the trans-isomer were increased, whereas those of the cis isomer decreased upon 460 nm irradiation. As magnified in Fig. 1(b), the N=N stretching band intensities for the cis isomer at 1502 cm⁻¹ appeared to decrease relative to those for the trans isomer at 1442 cm⁻¹ upon irradiation of the blue light at 460 nm. The decrease in intensity can be plotted as shown in Fig. 2, suggesting an exponential decay with respect to time. It should be mentioned that the reverse trans-to-cisisomerization is not easy to observe under our current experimental condition. Hence our results should be ascribed to an irreversible photoreaction phenomenon on metal surfaces.

Fig. 2 Relative decrease in intensity of the cis-N=N stretching band with that of the trans-N=N stretching bands in increasing the irradiation time with 10% error bars.

Fig. 3 shows the UV-Vis absorption spectra of 4DAzC before (solid line) and after (dashed line) the irradiation of the blue light at 460 nm in ethyl acetate. Before irradiation, it shows a one band pattern at 430–460 nm, including the π–π* and n–π* transition, owing to the reduction of the band gaps of the excited states presumably caused by the paraamino group.^28,29 Note that the absorption maxima of 4-aminoazobenzene and 4-dimethylaminoazobenzene (methyl yellow) were found at 383 and 410 nm, respectively, as consistent with the previous report.³⁰ Considering that the absorption band intensity at 430–460 nm increased via a thermal relaxation process in the absence of any light, this band may be assumed to be ascribed to the trans-conformer. Upon 460 nm irradiation, the absorption band at 430–460 nm decreased in intensity, while a new band at 370 nm appeared in ethyl acetate solution. This conversion process was not observed, however, in an aqueous solution. Moreover, the conversion process in ethyl acetate was found to be reversible, different from the case of our SERS data on Au nanoparticles. The difference may be due to a less flexibility of cis-transisomerization for 4DAzC after anchoring on Au surfaces, although not completely certain. A further examination is in progress using temperature-dependent UV-Vis absorption spectroscopy and DFT calculations of the HOMO and LUMO energy levels of 4DAzC. According to the present UV-Vis absorption spectroscopy data, the cis-trans conversion of 4DAzC in solutions occurred differently from the case of surfaces; it thus suggests that our SERS results should be due to a surface photoreaction phenomenon. It is not absolutely certain why the cis-isomer is found to be dominant in Au nanoparticle solution, different from the case of the solution. It is expected that the adsorption characteristics of the cis isomer should be different from those of the trans isomer due to dissimilar orientations on surfaces. Under our experimental conditions, the cis isomer was assumed to adsorb faster or more strongly on Au nanoparticles surfaces, although not definite. The results of this study thus demonstrate that Raman spectroscopy can be utilized in studying the photo-conversion of azobenzene-containing self-assembled monolayers fabricated on metal surfaces.

Fig. 3 UV-Vis absorption spectra of 4DAzC before (solid line) and after (dashed line) irradiation at 460 nm for 30 min in ethyl acetate solution.

Conclusions

The cis-transisomerization behaviors of 4-dimethylaminoazobenzenecarboxylate (4DAzC) on gold nanoparticle surfaces have been investigated by means of Raman spectroscopy. The vibrational features also supported that the cis- and trans-conformations should coexist after the adsorption on gold nanoparticle surfaces. After the irradiation of the blue-light, the intensities of the cis-form peaks were found to decrease on gold nanoparticle surfaces.

Acknowledgements

Sang-Woo Joo would like to thank Mr Hyoung-Woo Choi, Mr Jin Seol Park, and Ms Yu Jin Lee for their help with the experiments. This research was supported by the ‘‘GRRC’’ Project of Gyeonggi Provincial Government, Republic of Korea (S.I.Y.). This work was supported by the Korea Science and Engineering Foundation (R01-2006-000-10017-0), the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2007-314-C00146), the Seoul R & BD Program, the Soongsil University Research Fund.

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Footnote

† Electronic supplementary information (ESI) available: Additional tables. See DOI: 10.1039/b808903e

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