Optik 123 (2012) 1010–1014
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Synthesis and spectroscopic characterization of Kiton Red-620 dye doped silica gel rods Fozia Z. Haque a,∗ , Vazid Ali b , M. Husain b a b
Department of Physics, Maulana Azad National Institute of Technology, Bhopal, MP, India Department of Physics, Materials Science Laboratory, Jamia Millia Islamia (Central University), New Delhi, India
a r t i c l e
i n f o
Article history: Received 22 January 2011 Accepted 10 June 2011
Keywords: Silica Sol–gel process Kiton Red-620 Organic dyes Spectroscopy
a b s t r a c t The work was carried out to achieve two different but interdependent objectives; one to synthesis a transparent silica matrix with enough strength and thermal stability, by sol–gel technique, to host an organic dye molecule without quenching its fluorescence and two to find the probability if the said material is suitable for the construction of solid state dye lasers. Crack-free transparent silica gel rods with good mechanical strength, thermal stability and dimensions were successfully synthesized by sol–gel techniques. The rods were doped with Kiton Red-620 dye in different concentration separately. Effect of various synthesis parameters like time, temperature and aging condition was extensively studied to obtain crack-free silica rods doped with dye. Optical properties of prepared rods were studied by FTIR, UV/VIS–NIR and fluorescence spectroscopy. It is observed that Kiton Red-620 dye doped silica gel rods show good fluorescence with sharp peaks in the visible range. Their UV–VIS spectrum indicates the absorption in visible range. Thermal stability of rods were studied by DSC/TGA methods. Eventually it is found that these dye doped silica gel materials explore the possibility for new solid-state dye laser materials. © 2011 Elsevier GmbH. All rights reserved.
1. Introduction The sol–gel technology has received enormous attention in the area of materials research. The unique advantages of the technique are low temperature materials processing, high homogeneity of final product and its capacity to generate materials with controlled surface property, shape and pore structures. Various works have been reported in the literature based on sol–gel process for manufacturing glasses, ceramics and inorganic materials [1,2]. Therefore, modification in the characteristics of these materials can be very useful in the developments of new nano materials via doping. Several research groups are involved in the development and characterization of sol–gel derived smart materials [3–5]. Recently, Stiegman et al. [6] have incorporated vanadium oxide into the silica matrix and the resultant material showed remarkable hardness and optical transparency. Reisfeld et al. [7,8] have synthesized smart optical materials by sol–gel method and also studied their spectroscopic properties. Nanometer-sized particles of CdS, CdSe, CdTe and PbS, have been doped silica glass films prepared by chemical using either pure silica or silica zirconia or combined zirconia with ormosils. Strek et al. [9] studied Eu(III) complex in silica gel and zirconia glasses, and have reported the theoretical
∗ Corresponding author. E-mail address:
[email protected] (F.Z. Haque). 0030-4026/$ – see front matter © 2011 Elsevier GmbH. All rights reserved. doi:10.1016/j.ijleo.2011.07.019
basis for their spectroscopic studies. Saraidarov et al. [10] have also carried out some research work to study the luminescent properties of silica and zirconia xerogel doped with Eu(III) cryptate 3,3 -biisoquinoline-2,2-dioxide. Dye laser utilizing a solid host is very attractive for wide range of applications been adequately discussed by a number of authors [11–14]. Various dyes like pyrromethanes, Rhodamines and pyrene red doped in modified PMMA (M-PMMA) silica-gels, or ormosils, and composite glasses have been quite successful in yielding efficient, longlived performance [12,15,16]. The primary life-limiting issue is the photochemical stability of the dyes, although recent work on identifying the degradation pathways of impregnated dyes has led to a better understanding of this phenomenon [12,17–21]. Good beam quality from polymer host dye laser has been addressed through appropriate resonator design [22–24] and short-pulse (ps) output from a solid-state dye laser has been demonstrated. The expectation is that the overall capabilities of solid-state dye lasers (SSDLs) will continue to expand with the search of new host matrix which is suitably having an optical window – a region of zero absorption, extending over the absorption and emission bands of the chromophoric dyes [25]. Various types of organic and inorganic materials and crystals are used for this purpose [26]. Zaidi and Farooqui [27] have studied several dyes and come across a finding that the silica glass prepared by sol–gel technique can be a good active host material for dye molecules [28]. But the poor mechanical strength of this matrix had been a long time quandary in the
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for final aging to achieve mechanical strength. The aging and final drying depend on the size and thickness of silica gel rod samples. 2.4. Trapping of dye molecules
Fig. 1. Structure of Kiton Red-620 or Sulforhodamine B.
way of making solid state dye lasers from such materials. Recently we have been able to overcome the problem and the synthesis of crack free tough silica rods doped with Nile-Blue 690 organic dye for solid state dye laser is reported in our previous work [25]. This paper reports an extensive study of the effect of synthesis parameters on the formation of crack free silica rods. It also gives detail of the process of doping laser dye in silica gel matrix. It is found that the size of silica rods depends on the aging and drying process of silica gel. The optical and mechanical properties of these rods explore their utility as a laser material. The rods were characterized by FTIR, UV–VIS and fluorescence spectroscopy techniques. This research work not only reports the successful formation silica gel rods with desirable shape and strength but also explores the information about its utility as dye laser materials. 2. Experimental 2.1. Chemicals The following chemicals were used for the preparation of silica gel rods or the monoliths. Tetraethyl orthosilicate (TEOS) (Acros Organics USA, 98% pure), Kiton Red-620 (Exciton USA), silicone defoaming agent (Metro Arc Co., Calcutta, India), tetrahydrofuran (LR Grade stabilized for synthesis, S.D. Fine Chem. Ltd., Mumbai, India), formamide (LR Grade, Central Drug House (P) Ltd., New Delhi, India, 98.5%), concentrate nitric acid (LR Grade, Qualigen Fine Chemicals, Mumbai, India), ethanol (Merck, Germany) and dimethyl formamide (LR Grade, S.D. Fine Chem. Ltd., Mumbai, India, 99% pure). 2.2. Chemical formula and structure of Kiton Red-620 Kiton Red-620 dye is N-[6-(diethylamino)-9-(2,4disulfophenyl)-3H-xanthen-3-ylidene]-N-ethyl-ethanaminium hydroxide) and in some literature described as Sulforhodamine B [C27 H29 N2 O7 S2 Na]. Its chemical structure is shown in Fig. 1.
Doping of organic molecules such as laser dyes in sol–gel derived silica gel matrix will open new opportunities for optical and electrooptical applications. The methods used for achieving this goal are related to the control and processing of sol–gel glasses which in turn, demands for the perfect combination of molecular precursors and polymerizing conditions. In this work, emphasis is given to the preparation of crack free silica gel rods doped with the nanoparticles of dye Kiton Red-620 (KR-620). KR-620 dye solution of different concentration, from 11.6 × 10−5 , 23.2 × 10−5 and 34.8 × 10−5 M/L is used for doping in silica gel matrix. Dilute concentration of dye molecules is taken to avoid segregation of dye molecules in silica gel matrix. These dye doped silica gel rods will be taken for further lasing study in our laboratory with desired shape, strength, length and preferably transparent in the visible range. 2.5. Prevention of cracks A major concern in the preparation of dried gels is to prevent the cracks during aging and drying. As the solvent escapes from within the gel or the changes occur in pore size during these processes, stresses are produced in the silica network that may cause cracking. Here in this work we have tried to overcome this problem by the use of chemical additive. In the present case of silica gel rod preparation, diethylene glycol has been used to reduce the stress in gelled sample. Silica aerogels contain primary particle of 2–5 nm diameters. Such small sizes of silica particles have an extraordinary high specific surface area (∼900 m2 /g). It is, not surprisingly, therefore that the chemistry of the interior surface of an aerogel plays a dominant role in its chemical and physical behavior. The nature of surface groups of a silica aerogel is strongly dependent on the conditions used in its preparation. It is observed that formamide and diethylene glycol reduce cracks that help to convert the wet silica gel into a monolithic gel within a reasonable time. It has been observed that formamide improve bonding strength with silica gel network. We assume that the –OH groups of diethylene glycol shows strong interaction with silica gel network and thus change the interior surface chemistry of silica aerogel. Conclusively we observed that diethylene glycol and formamide play significant role to increase the strength of silica gel materials. 3. Characterization 3.1. FTIR study
2.3. Methodology for fabrication doped silica gel rods Crack free silica gel rod has been prepared in a very sophisticated manner using the following method. Tetraethyl orthosilicate (TEOS) and formamide were taken in the molar ratio of 0.072:0.45 (w/w), water 5 ml (v/v), diethylene glycol 4 ml and ethanol 20 ml (v/v) and then mixed thoroughly and stirred at 25 ◦ C for 30 min. The reaction mixture is made and adjusted to a pH 3 by adding catalyst nitric acid. The reaction mixture (pH 3) is then casted into a flat bottom glass tubes and kept at 40 ◦ C for 48 h. Moulded samples in the tubes start to gel. During this processes sol–gel transition took place and phase transformation is occurred with the growth of silica gel rod. After one week the prepared silica gel rod was washed with distilled water and then wet gel rod was dried with N,N -dimethylformamide and methanol mixture and put at 60 ◦ C for about two weeks. These monolith rods were finally heated to 140 ◦ C for desired period of time and kept in airtight desiccators
Spectroscopic characterization is an important tool to provide the information for understanding the optical properties and the attached (interacting) groups after doping of the newly synthesized material. Silica gel rods formed on the basis of above reaction mechanisms have been characterized by FTIR spectroscopy. The FTIR-spectra of pure silica gel rod show several peaks in Fig. 2. The absorption band in the region 4000–3000 cm−1 is mainly due to combination of vibrations of Si–OH or water. The broad absorption band is generally composed of the stretching modes. Region 3740–3660 cm−1 showing free Si–OH bonds and the region of 2800–3000 cm−1 correspond to symmetric and asymmetric stretching vibration of CH2 and CH3 groups of alkoxide and solvent residue. The main band in the region 1300–400 cm−1 , is associated with combination of vibration of silica network. Region 1200–1000 cm−1 corresponds to stretching vibrations of –Si–O–Si– bonding. The band region around 960 cm−1 associated with Si–OH
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Fig. 2. FTIR spectra of synthesized silica gel rod without dye doping.
stretching is typical of the gel structure that decreased the intensity and becomes insignificant when the material undergoes polycondensation mechanism during drying. It has also been observed when the gel dried with increasing temperature. The physical and mechanical properties improved and thus led to increase in the Si–O–Si bonds on heating. Moreover, as the temperature increases the surface area and pore volume decreases, thus resulting the shrinkage of gel. In the present study, it has been found that formamide based silica gel rod formations show stronger networks, smooth finish, low shrinkage and high monolithicity. The objective of the IR study was to observe the structural bonding formation for better mechanical and optical properties. Fig. 2 shows that the –OH and alkoxy groups (–OR), which appeared in the IR spectra around 3500–2900 cm−1 respectively. These alkoxy groups are not only residuals due to hydrolysis and poly-condensation reaction but also these are the products of esterification reaction. In IR spectra scattering becomes less important and standard molecular vibrations result in the spectral structure. A strong broad IR signal absorption band is generally observed at 3500 cm−1 due to –OH stretching vibrations. Similarly, weak –OH bending band is seen at 1600 cm−1 . Both absorption bands are due to adsorbed water and surface OH groups as exhibited in these bands. The –Si–O–Si– fundamental vibrations give the strong band at ≈1100 cm−1 . The region of high infrared transparency is between 3300 and 2000 cm−1 . Further increase in the addition of the additives result in the absorption of radiation in this region or scatter in infrared radiation. One of the most important additives is elemental carbon that absorbs the infrared radiation and in some cases, actually increases the mechanical strength of gel.
Fig. 3. Absorption spectra at different concentration of Kiton Red doped silica gel (k1 = 11.6 × 10−5 , k2 = 23.2 × 10−5 and k3 = 34.8 × 10−5 M/L).
tometer, Hitachi). Fig. 4(a) shows fluorescence spectra for various concentrations of Kiton Red-620 in silica gel rods. The dye (Kiton Red-620) doping was varied from 11.6 × 10−5 , 23.2 × 10−5 and 34.8 × 10−5 M/L in silica gel rod formation. The fluorescence intensity appears at different dopant concentrations from 11.6 × 10−5 to 34.8 × 10−5 M/L. It seems that the peaks of fluorescence emission wavelengths slightly shifted between 650 and 700 nm with broadened shape, but these peaks occurred with high fluorescence intensity. Fig. 4(b) also shows the luminescence spectra for different concentration doped (KR-620) silica gel rods. Here, luminescence spectra of silica gel rods doped with varied concentration of KR620; exhibited similar trend as in the case of the fluorescence spectra. But luminescence spectra broaden the peak, while it has been seen interestingly, that sharp peaks obtained in UV/VIS range in silica gel host materials shifted wavelength position (red line shift, Fig. 4(b)) when the concentration of KR-620 increased. The photoluminescence spectra change its shape with the various dye concentrations. The observed reduction in the luminance emission intensity is due to the KR-620 dye atoms/molecules forming segregations in the silica gel matrix.
3.2. UV–Visible–NIR spectra of silica gel rods The silica gel rods have been studied for their optical characterization using (UV/VIS spectrophotometer). The absorption spectra of silica gel rods doped with dye Kiton Red-620 are shown in Fig. 3. It has also been observed that during the processing/curing of doped silica gel rods there was some loss in dye concentration at high temperature that affects the UV/VIS spectra sharpness. It has been shown that they exhibit the same behavior of absorption peaks at 375, 380 and 400 nm. It is shown slightly shifting of peaks absorption, at different concentrations of Kiton Red due to polarity of solvents used in the synthesis, which may have occurred due to organic dye atoms. Thus, it can be concluded that silica gel material is ideally suitable for doping with various laser dyes or luminescent materials, which may emit in tunable fashion in this region. 3.3. Fluorescence studies Fluorescence spectroscopy of prepared dye doped silica gel rods have been performed with (F-4500 Fluorescence Spectropho-
Fig. 4. (a) Fluorescence spectra with different concentration of Kiton Red doped silica gel (k1 = 11.6 × 10−5 , k2 = 23.2 × 10−5 and k3 = 34.8 × 10−5 M/L). (b) Photoluminescence spectra with different concentration of Kiton Red doped silica gel (k1 = 11.6 × 10−5 , k2 = 23.2 × 10−5 and k3 = 34.8 × 10−5 M/L).
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seem reasonable to expect similar explanation for the absorption and emission spectra of Kiton Red in different matrixes like organic solvents and sol–gel silica. Thus with low lying (, *) cited singlet the frequencies of both absorption and fluorescence are shifted to lower frequency site as the hydrogen bond donating ability of the matrix increase, and the magnitude of the fluorescence shifts is greater than those in absorption. 4. Conclusions
Fig. 5. (a) DSC curves of pure silica gel rods and (b) Kiton Red-620 doped silica gel rods.
3.4. Differential scanning calorimetry (DSC) study DSC of pure silica gel rods and Kiton Red-620 doped silica gel rods have been studied at heat flow rate 10 ◦ C/min as shown in Fig. 5(a) and (b). It has been observed that the prepared crack free silica gel rods showed the good thermal stability from 100 ◦ C to 150 ◦ C without significant change in the silica gel materials as well as dye doped silica gel rods matrix.
In the present work, the silica gel rods have been prepared by sol–gel process and characterized by FTIR, UV/VIS spectroscopy and fluorescence studies. FTIR spectroscopy is employed to understand structural changes that occur at surface and in the network of silica gel obtained. Formamide and diethylene glycol (additives) show significant role to reduce the differential stresses produced by capillary forces in the pores of the drying gel resulted to improve mechanical strength of silica gel rods. In the absence of formamide, the silica gel structure showed poor strength. Formamide based silica gel rods present small shrinkage, high monolithicity and a large amount of silanol groups on its surface, which produces a more active surface of silica gel. FTIR studies of prepared samples also provide the information about the compactness of silica gel. At high frequency, absorption bands of the Si–O–Si stretching indicate a strongly cross-linked structure. The UV–Visible–NIR Spectra studies of prepared silica gel rods show the sharp absorption band in UV/VIS region that further explore the possibility for developments in advance optical materials. Fluorescence studies also show the sharp peak appearance in UV/VIS range with dye Kiton Red-620 doped silica gel. Both samples, with and without dye, show good thermal stability for a wide temperature range.
3.5. Effect of solvents in Kiton Red-620 (spectroscopic studies) The emission and absorbance spectra have also been performed to see the affect of solvents on Kiton Red-620 laser dye. In the case of Kiton Red a red shift was observed in absorption spectra as well as in emission spectra on increasing polarity of the solvent. It was found that the absorption and emission spectra are highly red shifted in sol–gel sample as compared to spectra in solution. It was discussed earlier that the magnitude of red shifting depends on the specific nature of the solute–solvent interaction [25,29]. The absorption and emission spectra of Kiton Red were taken in ethanol, methanol, N,N -dimethylformamide and silica gel and the results are compiled in Table 1. Sol–gel samples are often filled with liquids (water and ethanol). Intra-molecular hydrogen bonding and hydrogen bonding interaction of molecules with the solvent or with other can greatly affect their absorption and fluorescence spectra. The influence of hydrogen bonding on the fluorescence of organic molecules is not limited to interaction with the solvent but the fluorescence yield can also be correlated with the ability of the solvent to act as a hydrogen-bond acceptor. The general effects of the excited-state hydrogen bonding on the luminescence of nitrogen heterocyclics are dependent on whether the lowest excited singlet is (n, *) or (, *). It is known that excited (, *) singlets of nitrogen heterocyclic are more strong basic than the ground state. We can thus conclude that the excited state is more favorable to hydrogen bond, stronger with protonic solvents than the ground state. It therefore might Table 1 Photophysical properties of Kiton Red-620 in various media. Medium
max (nm) abs
max em (nm)
Stokes shift (nm)
Ethanol Methanol N,N -dimethylformamide Silica gel
554 557 554 568
592 590 595 608
36 33 41 40
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