Journal of the Korean Physical Society, Vol. 64, No. 3, February 2014, pp. 451∼454

The Cause of Absorption and Luminescence Band Shift of Disperse Red-13 inSilica Spheres

Byoung-Ju Kim and Kwang-Sun Kang∗

Department of New and Renewable Energy, Kyungil University, Gyeongsan 712-701, Korea

(Received 26 September 2013, in final form 6 November 2013)

Optical absorption and luminescence band shifts have been observed for a disperse red-13 (DR-13)attached on silica spheres. A covalent bond is formed between an isocyanatopropyl triethoxysilane(ICPTES) and a DR-13 prior to DR-13 being incorporated inside the spheres. The Stober synthe-sis process was performed with the synthesized ICPTES-DR-13 (ICPDR), tetraethoxy orthosilane(TEOS), 2-propanol and NH4OH solution. The final silica spheres with ICPDR (ICPDRSS) wereanalyzed with Fourier transform infrared, UV-visible and fluorescence spectrometers. Characteris-tic absorption peaks at 1700 and 1704 cm−1 representing –C=O stretching vibrations for ICPDRand ICPDRSS indicate the formation of urethane linkages. Although the absorption band at 502nm did not shift with increasing concentration of DR-13 in methanol, the absorption band for theICPDRSS shifted 28 nm toward shorter wavelength due to the antiparallel geometry of the azo-chromophores. A broad luminescence was observed for ICPDRSS and shifted toward the blue withincreasing excitation wavelength, which was due to a transformation of the DR-13 in silica spheresfrom a trans-form to a cis-form.

PACS numbers: 78.40.MeKeywords: peak shift, Luminescence peak shift, Disperse red (DR), Silica spheresDOI: 10.3938/jkps.64.451

I. INTRODUCTION

Extensive efforts have been devoted to fabricating effi-cient second-order nonlinear optical (NLO) devices. Be-cause the intermolecular interactions and the interac-tions with the environment significantly affect the macro-scopic NLO performances, relative arrangements and ag-gregations of molecules are detrimental to the NLO re-sponse. The self-assembly of monodisperse silica sphereshas attracted enormous attention within the scientificcommunities due to its wide range of applications, in-cluding optical data storage using surfaces modified withfluorescing dyes [1], photonics [2,3], catalysis [4], opticalmodulation [5] and optical switching. Core-shell struc-tures comprised of a fluorescence dye and an optically-inert soft shell have been developed for optical data stor-age [6]. The density of the optical data storage has beenincreased by more than a factor of two with this core-shell structure compared with a homogeneous storagemedium. Next-generation optical data recording mate-rials to protect secure document have been developedwith microspheres comprised of UV, visible and near-infrared fluorescence dyes [7]. Therefore, the develop-ment of submicrometer silica spheres with fluorescent

∗E-mail: kkang@kiu.ac.kr; Tel: +82-53-850-7189; Fax: +82-53-850-7190

dyes is a promising field for many optical device appli-cations. Furthermore, the uniform addition of fluoresc-ing dyes to submicrometer silica spheres with large-areadefect-free structures, instead of a core-shell structure,is more attractive due to its being an easy, simple andlow-cost process.

In this paper, we report two-step synthesis routesto attach disperse red-13 (DR-13) to silica spheres.This report also includes the Fourier-transform in-frared (FTIR) spectra for the 3-isocyanatopropyl tri-ethoxysilane (ICPTES)-DR-13 (ICPDR) and ICPDR-silica spheres (ICPDRSS) after synthesis, field-emissionscanning electron microscopy (FESEM) image of ICP-DRSS, UV-visible absorption/transmission spectra, andphotoluminescence (PL) characteristics of ICPDRSS.

II. EXPERIMENTAL

Tetraethylorthosilicate (TEOS, 98%), methanol(HPLC grade), isopropanol (99%), NH4OH solution(28%), ICPTES (95%) and DR-13 (95%) were purchasedfrom Sigma Aldrich Co, LTD. and used without furtherpurification. The DR-13 (60 mg) was dissolved inpyridine, and dry nitrogen was purged in reaction flask.The ICPTES (280 mg) was added to the reaction flaskwith stirring and was maintained at a temperature of

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Fig. 1. Schematic representation of (a) urethane bond for-mation between an ICPTES and a DR-13 and (b) covalentbond formation between ICPDR and silica spheres.

60 ◦C. The reaction products (ICPDR) were pouredinto 100 ml of an aqueous NaCO3 solution and werefiltered through nylon filter paper. The synthesizedICPDR was purified with an excess amount of deionizedwater. Into the silica spheres in a 100 ml round-bottomflask which had been synthesized using the Stobersynthetic method, ICPDR (20 mg), 2-propanol (50 ml)and TEOS (750 mg) were charged. NH4OH solution (50ml) was quickly added to the round- bottom flask andwas stirred vigorously for 6 h. The resulting spheres(ICPDRSS) were centrifuged at 3000 rpm for 30 minand washed with methanol several times. The ICPDRSSwas re-dispersed in methanol, and the solution was droponto the glass substrate and dried to make an ICPDRSSfilm for use in obtaining the UV-visible spectrum.The FTIR, UV-visible and PL spectra were obtainedfor the synthesized materials by using a Nicolet iS5FTIR spectrometer, Thermo-Scientific Genesys 10S UV-visible spectrometer, and Hitachi F-4500 fluorescencespectrometer, respectively. The FESEM image of theICPDRSS was obtained by using a JEOL ISM-7402Ffield-emission scanning electron microscope.

Fig. 2. (Color online) (a) FTIR spectra of pure silicaspheres, ICPDR and ICPDRSS and (b) FESEM image ofthe ICPDRSS.

III. RESULTS AND DISCUSSION

A covalent bond between a DR-13 molecule and anICPTES molecule was formed by an –OH group of theDR-13 reacting with an –N=C=O group of the ICPTESin pyridine at 60 ◦C for 3 h. The synthesized ICPDRwas poured into an aqueous NaCO3 solution to removethe unreacted DR-13 and ICPTES. The mixture was fil-tered with a 200 nm porous nylon filter, and the reten-tate was washed with deionized water. The Stober syn-thesis method was used for ICPDR, TEOS, 2-propanoland NH4OH. An ICPDR molecule has three ethoxygroups, which hydrolyze to form silanol groups. Thesesilanol groups condense with the silanol groups of thehydrolyzed TEOS. Eventually, the hydrolyzed ICPDRmolecules incorporate mainly inside the silica spheres.The resulting color of the spheres was an intense red.Schematic views of the synthesis processes for ICPDRand ICPDRSS are shown in Figs. 1(a) and 1(b), respec-tively.

The isocynate group has a strong infrared absorptionpeak at 2270 cm−1 corresponding to an asymmetric-

The Cause of Absorption and Luminescence Band Shift· · · – Byoung-Ju Kim and Kwang-Sun Kang -453-

Fig. 3. (Color online) (a) Concentration-dependent UV-visible absorption spectra of DR-13 in methanol and (b) com-parison of the absorption spectra for DR-13 in methanol withthat for the ICPDRSS film.

stretching vibration. The FTIR spectrum of the ICP-DRSS shows no isocynate peak, as shown in Fig 2(a),which indicates complete reaction of the isocynate groupdue to the basic condition during the synthesis. TheFTIR spectrum of the ICPDRSS has additional newpeaks at 2954, 2852, 1704, 1371 and 839 cm−1 comparedwith the spectrum of the pure silica spheres. The ab-sorption peaks at 2954 and 2852 are assigned to asym-metric stretching and symmetric stretching of the –CHgroup, respectively. The absorption peak at 1704 cm−1

represents a –C=O stretching vibration for the urethanebond. The absorption peaks at 1371 and 839 cm−1 aredue to the symmetric deformation of the –CH bond andthe deformation of the aromatic –CH bond. These re-sults clearly show the existence of azo-chromophores inthe silica spheres. The FESEM image of the synthesizedICPDRSS is shown in Fig. 2(b). The ICPDR is not ob-servable in this image, which implies that the ICPDR ismainly in the spheres.

The molecular structure of the DR-13 has π-electronswith a donor (–N) and an acceptor group (–NO2); this

Fig. 4. (Color online) Excitation wavelength-dependentPL spectra for the ICPDRSS film with various excitationwavelengths.

structure forms intense intermolecular charge transferband in the range of 400-600 nm, depending on the po-larity [8] and can be expected to form various absorptionbands, depending on the intermolecular distance and thepolarity of the surrounding environment. The UV-visibleabsorption peak of the DR-13 in methanol is at 502 nm,as shown in Fig. 3(a). A comparison of the absorptionspectra of the DR-13 in methanol and the ICPDRSS filmis shown in Fig. 3(b). The absorption peak is shifted 28nm toward blue with respect to that of the DR-13 inmethanol, which may be due to the antiparallel geom-etry of the chromophores. When there are interactionsbetween chromophores, mixed excited states are gener-ated, which create two delocalized sub-units. Only thehighest exciton energy state is allowed for the antiparallelchromophores, resulting in a blue shift of the spectrum[9].

Figure 4 shows the fluorescence spectra for the ICP-DRSS film under 260, 270, 280, 290 and 300 nm excita-tion wavelengths. The overall luminescence-peak pro-files slightly shift toward blue with increasing excita-tion wavelength, which might be due to the increasedtrans-to-cis photoisomerization. The π-electron delocal-ization and conjugation are better in the trans-form ofthe azobenzene, which results in a red-shift. However,the cis-form can be obtained by transforming the trans-form and has a geometry with phenyl rings twisted per-pendicular to the plane determined by the C–N=N–Cbond, resulting in a blue shift of the luminescence [10].

IV. CONCLUSION

An ICPDR has been synthesized with an ICPTES anda DR-13 in pyridine prior to embodying the ICPDR insilica spheres. The Stober synthesis method was utilizedto fabricate the ICPDR-embodied silica spheres. The

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FTIR spectrum provides evidence of the existence of theDR-13 in the silica spheres. The UV-visible absorptionpeak of the ICPDRSS shifts toward blue compared withthat of DR-13 in methanol. The PL spectral profile shiftstoward blue due to the transformation from the trans-form to cis-form of the azobenzene chromophore.

ACKNOWLEDGMENTS

This work has been supported by Kyungil Universityin Korea.

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