- Email: [email protected]
Synthesis and spectroscopic characterization of Nile Blue doped silica gel rods
Optik 124 (2013) 4287–4291
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Synthesis and spectroscopic characterization of Nile Blue doped silica gel rods Fozia Z. Haque a,∗ , Vazid Ali b , M. Husain b a b
Nanomaterials Lab, Department of Physics, Maulana Azad National Institute of Technology, Bhopal, MP, India Materials Science Laboratory, Department of Physics, Jamia Millia Islamia (Central University), Jamia Nagar, New Delhi 110025, India
a r t i c l e
i n f o
Article history: Received 12 August 2012 Accepted 2 January 2013
Keywords: Silica Sol–gel process Tetraethyl orthosilicate (TEOS) Organic dyes Nile-blue
a b s t r a c t We have studied silica-gel by sol–gel technique for the preparation of new dye-laser materials. Silica gel rods with dimension 50 mm × 10 mm have been prepared successfully without breaking. It shows high transparency and good mechanical strength. Tetraethylorthosilicate (TEOS), formamide in molar ratio (0.25:0.70), 80 ml ethanol, 20 ml dimethylformamide (DMF), 10 ml water, hydrochloric acid as a catalyst (at pH 6) and 0.5 ml silicone defoaming agent/surfactant have been used. The synthesis has been carried out in a beaker and the reaction mixture is caste in to the flat bottom glass tubes at 40 ◦ C after thoroughly mixing of all the ingredients. These complex reactions, that carried out by hydrolysis and condensation in the silica gel formation show less gel time ∼8–10 h at 40 ◦ C. The doping of dye (Nile Blue 690) has been taken during the preparation of all the ingredients solution mixture. It has been observed that the compatibility of Nile Blue dye with silica-gel promise good homogeneity with transparency. In the present study, synthesis, dye doping, FTIR, UV/vis–NIR and fluorescence spectroscopy have been studied. We have observed that Nile Blue doped silica gel rods material give good fluorescence showing sharp peaks in the visible range. UV–vis spectra of prepared silica gel material also indicate the absorption in visible range. Thermal stability of rods was 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. © 2013 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 techniques are low temperature materials processing, high homogeneity of final product and its capacity to generate materials with controlled surface property and shape and pore structures. The process includes the following steps: preparation of the solution, gelation, aging and finally drying of synthesized materials. Various works has been reported in literature on sol–gel processes for manufacturing glasses, ceramics and inorganic materials [1,2] but still there are some major drawbacks such as poor mechanical strength of silica gel and cracks formation during gelling. Therefore, modification of the characteristics of these materials will be very useful in the developments of new nano materials. Several research groups are involved in the development of smart materials [3,4] and their characterization by sol–gel process. Lenza and Vasconcelos [5] have reported the preparation of silica gel by sol–gel technique and its characterization by FTIR, which enabled formation of crack-free silica gel.
∗ Corresponding author. E-mail address: [email protected] (F.Z. Haque). 0030-4026/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2013.01.043
Recently, Stiegman et al. [6] have incorporated transition vanadium oxide into the silica matrix and the resultant material showed remarkable, hardness and optical transparency. Resfield et al. [7,8] have synthesized smart optical materials by sol–gel method and also studied their spectroscopic properties. The research work was focused on nanometer-sized particle of CdS, CdSe, CdTe and PbS, which have been formed by chemical method in silica glass films. These films were prepared either of 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 basis for their spectroscopic studies. They utilized a renovative method for the preparation of new family of materials with rare earths complexes by sol–gel technique. Further research work has also been carried out by Saradaov et al. [10] for the luminescent properties of silica and Zirconia xerogel doped with Eu(III) cryptate 3,3 -biisoquinoline-22-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, pyrene red, etc. 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
4288
F.Z. Haque et al. / Optik 124 (2013) 4287–4291
Fig. 1. Structure of Nile Blue 690.
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 (SSDL) 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] and Farooqui 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 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 Kiton-red organic dye for solid state dye laser is reported in our previous work [29]. Therefore several applications of silica gel materials as a smart optical device and many other opto-electronic/organic compounds such as dye laser materials and high temperature resistant spin coatings suggested their future possible engineering materials. In the present study, we have prepared silica-gel rods at room temperature. Here our basic objective is the formation and characterization of crack free dye doped silica-gel materials with enhanced mechanical properties. 2. Experimental procedure 2.1. Chemicals The following chemicals have been used in the experimental work for the preparation of silica gel rods of the monoliths; tetraethyl orthosilicate (TEOS) (Acros Organics, USA, 98% pure), 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 The first reported synthesis of Nile Blue dates back to 1896 by Mohlau and Uhlmann [30]. They condensed 1-naphthylamine with 4-nitroso-N,N-diethyl3-aminophenol to obtain a blue dye, which they later named as Nile Blue. Its derivative Nile Blue 690 is 5-amino-9(diethylamino)benzo(a)phenoxazin-7-ium perchlorate in the form of dark green crystals. Its chemical structure is shown in Fig. 1. 2.3. Methodology for silica gel rod formation Crack free silica gel rod has been prepared by using the following methods. Tetraethyl orthosilicate (TEOS) and formamide were taken in the molar ratio of 0.072:0.45, water 5 ml (v/v), ethanol
20 ml (v/v) and silicon defoaming agent/surfactant 0.04 ml (v/v) were mixed thoroughly and stirred at 25 ◦ C for 15 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 24 h. The samples casted in 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 wet gel rod was dried with N,N -dimethylformamide and methanol mixture at 60 ◦ C for about two weeks. These monolith rods were finally heated to 160 ◦ C for two h. and kept in airtight desiccators for final aging to achieve mechanical strength. The aging and final drying depend on the size and thickness of silica gel rod samples. The basic chemical reactions for silica gel formation are complicated but they can be summarized in the following simplified scheme: Si(OCH2 CH3 )4 + 2H2 O → SiO2 + 4HOCH2 CH3 However, the silica gel formation depends on the complex reaction mechanisms, i.e. hydrolysis, esterification, aldol condensation and finally water condensation as the mechanism elaborated below. The reactions of silica gel formation can be accelerated or slowed down by employing appropriate acidic and basic catalysts. Although silica gel formation seems simple but its synthesis is rather complex. A number of parameters such as pH, solvents, water content, temperature and drying conditions can change the properties of the final product.
2.4. Mechanisms ≡ Si − OR + H2 O
Hydrolysis
Si − OH + ROH
Esterification
≡ Si − OR + HO − Si ≡
Aldol condensation
Si − O − Si ≡ +ROH
Water condensation ≡ Si − OH + HO − Si ≡
Hydrolysis
≡ Si − O − Si ≡ +H2 O
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 (silicone defoaming agent) to reduce the stress in gelled sample. It has been observed that formamide and silicone defoaming agent reduce cracking that help to convert the wet silica gel into a monolithic gel within a reasonable time.
2.5. Trapping organic molecules in silica gel glasses 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. These involved combination of molecular precursors and polymerizing conditions. In this research work, we have emphasized here only the preparation of crack free silica gel rods doped with Nile Blue 690 (NB 690) transparent in desired shape and length, in the visible range. And these dye doped silica gel rods will be taken further for lasing study in our laboratory.
F.Z. Haque et al. / Optik 124 (2013) 4287–4291
4289
Fig. 2. FTIR spectra of synthesized silica gel rod without dye doping.
2.6. 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 rods preparation, diethylene glycol has been used to reduce the stress in gelled sample. Silica aero gels 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
Fig. 3. Absorption spectra at different concentration of Nile Blue 690 doped silica gel.
shrinkage of gel. In the present study, it has been found that formamide based silica gel rods formation 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. 1 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.
3.1. FTIR studies 3.2. UV–vis–NIR spectra Spectroscopic characterization is an important tool to understand the optical properties and the attached (interacting) groups after doping of the newly synthesized materials. Formed silica gel rods on the basis of above reactions mechanisms have been characterized by IR spectroscopy. IR spectra of pure silica gel rods, which exhibits several peaks (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 region of 2800–3000 cm−1 corresponds to symmetric and asymmetric stretching vibration of CH2 and CH3 groups of alkoxide and solvent residue. The main bands 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 stretching is typical of the gel structure that decreased the intensity and becomes insignificant when the material undergoes poly condensation 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
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 Nile Blue 690 is shown in Fig. 3. It has also been observed that during the processing/curing of doped silica gel rods there is some loss of dye concentration at high temperature that affects the UV/vis spectra sharpness. It has been shown that there is the same behavior of absorption peaks at 375, 380 and ∼400 nm. It is shown slightly red shifting of peak absorption, at different concentration of Nile Blue, which is also occurred. Thus, it concludes that silica gel material is ideally suitable for doping with various laser dyes or luminescent materials, which typically 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 Spectrophotometer, Hitachi). Fig. 4a shows fluorescence spectra for various concentrations of Nile Blue 690 (NB 690) doped silica gel rods. The dye (NB-690) doping was varied from 8.36 × 10−6 to 25.08 × 10−6 M/L in silica gel rods formation. The fluorescence intensity reaches its maximum at dopant concentration from
4290
F.Z. Haque et al. / Optik 124 (2013) 4287–4291 Table 1 Photophysical properties of Nile Blue 690 in various media. Medium
max (nm) abs
max em (nm)
Stokes shift (nm)
Ethanol Methanol N,N -dimethylformamide Silica gel
628 627 635 640
664 664 666 557
36 37 31 17
thermal stability from 100 to 150 ◦ C without significant change in the silica gel materials as well as dye doped silica gel rods matrix. 3.5. Spectroscopic effect of solvents on Nile Blue 690
Fig. 4. (a) Fluorescence spectra with different concentration of Nile Blue 690 doped silica gel. (b) Photoluminescence spectra with different concentration of Nile Blue 690 doped Silica gel.
8.36 × 10−6 to 25.08 × 10−6 M/L. It seems that the peaks of fluorescence emission wavelengths remain at position 220, 310 nm and slightly shifted between 650 and 700 nm but these peaks occurred with high fluorescence intensity. Fig. 4b also shows the luminescence spectra for different concentration doped (NB-690) silica gel rods. Similar trend to the case of the fluorescence spectra has been observed. But in luminescence spectra it has been seen interestingly that sharp peaks obtained in UV/vis range in silica gel host materials with shifting wavelength position (Red shift) Fig. 4b, when the concentration of Nile Blue 690 increased. The photo luminescence spectra change its shape with the various dye concentrations. The observed reduction in the luminance emission intensity is due to the NB-690 dye atoms/molecules forming aggregations in the silica gel matrix. In addition to this evidence it is clear from Fig. 4(a) (n3 spectra), which reduce the luminance intensity at high concentration of NB-690 as compared to n1 and n2 spectra. 3.4. Differential scanning calorimetric (DSC) study DSC of pure silica gel rods and Nile Blue doped silica gel rods have been studied at heat flow rate 10 ◦ C/min (Fig. 5a and b). It has been observed that the prepared cracks free silica gel rods show good
Fig. 5. DSC curves of (a) pure silica gel rods and (b) Nile Blue 690 doped silica gel rods.
The fluorescence and absorbance spectra have been performed to see the effect of solvent in Nile Blue 690 laser dye. Nile Blue 690 dyes belong to the class of oxazine dyes with a structure modified by an additional benzoic group. These have been reported to be efficient laser dyes too. If the central CH group of a pyronine dye is formally replaced by N , a compound is obtained whose absorption is shifted by about 100 nm to longer wavelength. Such an oxazine or phenooxazine dye is planar and rigid like its xanthene relative. The position of the absorption maximum depends on the end group of the chromophore. All oxazine dyes are photochemically more stable than the pyronines and rhodamine. However, there is no triplet problem in these days; it is comparatively difficult to suppress the process of internal conversion due to the smaller energy difference between S1 and S0. A continuous red shifting was observed in absorption spectrum on increasing polarity of the matrix. In emission spectrum the maximum emission band is slightly red shifted on increasing matrix polarity. It was seen that the emission band is highly blue shifted in sol–gel due to rigidochromism. Electronic transitions occur at rates rapid relative to the rates of inter nuclear motion in molecules. Hence, during an electronic transition the nuclei remain essentially stationary (the Franck–Condon principle). When a molecule in its ground state absorbs photon and reach a metastable excited state in which the molecular geometry and solvent configuration are those characteristic of the ground state. Solvent reorientation then occur approximately 10−1 to 10−12 s after excitation, producing an equilibrium excited state, in which the solvent configuration is optimal for the geometry and electron distribution of the molecule. It is obvious that due to drastic changes in molecular geometry and electronic charge distribution on excitation, the energy difference between equilibrium and Franck–Condon excited state can be quite large in some cases. There is no reason to expect precise correspondence between absorption and fluorescence solvent effect because the relevant ground and excited state involved in absorption and emission are different. In polar molecules, it was found that the excited state is more polar than the ground state. So, the excited state becomes more stabilized than the ground state on increasing the polarity of the solvent. A red-shift in absorption and emission spectra is observed and this red-shifting depends on the nature of the solute–solvent interaction. It is usually observed that on increasing the rigidity of the matrix, the fluorescence spectrum of some dye molecules is blue shifted compared to the spectrum in liquid solution because after excitation the solvent reorientation is less facile in a rigid medium than it is in solution. Nile Blue shows progressively longer absorption and emission maxima as the solvent polarity is increased. The Stokes’ shifts observed for the Blue compound is exceptionally high (almost 100 nm), but in polar solvents it is reduced to around 40 nm (Table 1). This also reveals that the spectroscopic characteristics of Nile Blue are pH dependent. This is because under basic conditions the minimum group will be deprotonated, whereas under
F.Z. Haque et al. / Optik 124 (2013) 4287–4291
strongly acidic conditions the 5-amino might even become protonated. Curiously, the spectroscopic properties of Nile Blue are somewhat dependent on the counter ions. We speculate that this could even be a reflection on intimate ion pairing influencing the solvent sphere of the dye. 4. Conclusions 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 silicon defoaming agent (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 important information on the compactness of silica gel. At high frequency, absorption bands of the Si O Si stretching indicate a strongly cross-linked structure. The UV–vis–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 peaks appearance in UV/vis range with dye NB-690 doped silica gel. Acknowledgment Thanks are due to DRDO, New Delhi, India for providing financial support in the form of a major research project to Prof. M. Husain (Principal Investigator). References [1] Z.S. Hu, J.X. Dong, G.X. Chen, Replacing solvent drying technique for nanometer particle preparation, J. Colloid Interface Sci. 208 (2) (1998) 367–372. [2] A.B. Mrowiec-Bialon, J. Jarzebski, A.I. Lachowski, J.J. Malinowski, Twocomponent aerogel adsorbents of water vapour, J. Non-Cryst. Solids 225 (1998) 184–189. [3] Z. Zhu, Y. Tsung, M. Tomkiewicz, Morphology of TiO2 aerogels. Electron microscopy, J. Phys. Chem. 99 (1995) 15945–15949. [4] C.A. Muller, M. Schneider, I. Maullat, A. Baker, Titania–silica epoxidation catalysts modified by polar organic functional groups, J. Catal. 189 (2000) 221–232. [5] R.F.S. Lenza, W.L. Vasconcelos, Structural evolution of silica sols modified with formamide, Mater. Res. 4 (3) (2001) 175–179. [6] A.E. Stiegman, H. Eckert, G. Plett, S.S. Kim, M. Anderson, A. Yavourian, Vanadia/silica xerogels and nanocomposites, Chem. Mater. 5 (1993) 1591– 1595. [7] R. Reisfeld, in: C.K. Jorgensen (Ed.), Optical Properties of Colorants or Luminescents Species in Sol–Gel Glasses; Structure and Bonding, vol. 77, Springer Verlag, 1992, pp. 207–256.
4291
[8] R. Reisfeld, Sol gel processed lasers, in: S. Sakka (Ed.), Sol–Gel Technology (Handbook), vol. 3, 2004, pp. 239–261 (Chapter 12). [9] W. Strek, J. Sokolnichi, J. Legendziewics, K. Maruszewski, B. Nissen, R. Reisfeld, T. Pivich, Optical properties of Eu(III) chelates trapped in silica gel glasses, Opt. Mater. 13 (1999) 41–48. [10] T. Saraidarov, R. Reisfeld, M. Pietraszkiewicz, Luminescent properties of silica and zirconia xerogels doped with europium(III) salts and europium(III) cryptate 3,3 -biisoquinoline-22 -dioxide, Chem. Phys. Lett. 330 (2000) 515–520. [11] C. Stewen, M. Larionov, A. Giesen, Yb:YAG thin disk laser with 1 kW output power, in: H. Injeyan, U. Keller, C. Marshall (Eds.), OSA Trends in Optics and Photonics, Advanced Solid-State Lasers, Optical Society of America, Washington, DC, 2000, pp. 35–41. [12] E. Yariv, S. Schultheiss, T. Saraidarov, R. Reisfeld, Efficiency and photostability of dye-doped solid-state lasers in different hosts, Opt. Mater. 16 (1–2) (2001) 29–38. [13] A. Costela, I. Garcia-Moreno, H. Tian, J. Su, K. Chen, F. Amat-Guerri, M. Carrascoso, J. Barroso, R. Sastre, Internal photostabilization of polymeric solid-state dye lasers based on trichromophoric rhodamine 6G molecules, Chem. Phys. Lett. 277 (1997) 392–396. [14] K.M. Dyumaev, A.A. Manenkov, A.P. Maslyukov, G.A. Matyushin, V.S. Nechitailo, A.M. Prokhorov, Dyes in modified polymers: problems of photostability and conversion efficiency at high intensities, J. Opt. Soc. Am. B9 (1992) 143–151. [15] Ya.V. Kravchenko, A.A. Manenkov, G.A. Matyushin, V.M. Mizin, D.P. Pacheco, H.R. Aldag, New high-efficiency pyrromethene-580-doped modified PMMA solid state dye laser, Proc. SPIE 2986 (1997) 124–131. [16] D.P. Pacheco, H.R. Aldag, Solid state lasers VII, comparison of the laser performance of solid state dye media under long- and short-pulse excitation, Proc. SPIE 3265 (1998) 2–12. [17] M.D. Rahn, T.A. King, A.A. Gorman, I. Hamblett, Photostability enhancement of Pyrromethene 567 and Perylene Orange in oxygen-free liquid and solid dye lasers, Appl. Opt. 36 (1997) 5862–5871. [18] H. Tran Nhung, M. Canva, T.A. Truong Dao, F. Chaput, A. Brun, N.D. Hung, J.P. Boilot, Stable doped hybrid sol–gel materials for solid-state dye laser, Appl. Opt. 4 (12) (2003) 2213–2218. [19] T. Suratwala, K. Davidson, Z. garlund, D.R. Uhlmann, Reversing gels and water soluble and photostability of laser dyes within sol–gel hosts. Solid state lasers VI, SPIE Proc. Series 2986 (1997) 141–152. [20] M. Ahmad, M.D. Rahn, T.A. King, Singlet oxygen and dye-triplet-state quenching in solid-state dye lasers consisting of Pyrromethene 567-doped poly(methyl methacrylate), Appl. Opt. 38 (30) (1999) 6337–6342. [21] G. Jones II, O. Klueva, S. Kumar, D. Pacheco, Photochemical and lasing properties of pyrromethane dyes, Proc. SPIE 4267 (2001) 24–35. [22] S. Chandra, H. Toomas, J. Allik, A. Hutchinson, J. Fox, C. Swim, Tunable ultraviolet laser source based on solid-state dye laser technology and CsLiB6O10 harmonic generation, Opt. Lett. 22 (4) (1997) 209–211. [23] A. Mandl, A. Zavriyev, D.E. Klimek, Energy scaling and beam quality studies of a zigzag solid-state plastic dye laser, IEEE J. Quantum Electron 32 (1996) 1723–1726. [24] J.A. Russell, D.P. Pacheco, H.R. Aldag, S.M. Dolotov, M.F. Koldunov, V.Ya. Kravchenko, A.A. Manenkov, A.V. Reznichenko, G.P. Roskova, Beam-quality measurements on solid-state dye lasers using nonconfocal unstable resonators, Proc. SPIE 4267 (2001) 36–45. [25] Q. Zhang, Z. Jiang, Pyrromethene doped silica laser glasses by sol–gel processes, Mater. Chem. Phys. 69 (1–3) (2001) 95–98. [26] B. Dunn, J.I. Zink, Molecules in glass: probes, ordered assemblies, and functional materials, Acc. Chem. Res. 40 (2007) 747–755. [27] Z.H. Zaidi, A.S. Farooqui, Spectroscopic characteristics of coumarin 540 A in sol–gel, Spectrosc. Lett. 34 (2) (2001) 301–309. [28] A.S. Farooqui, Spectroscopic investigation and prediction of laser action properties of 4-hydroxycoumarin dye in sol–gel. Ph.D. Thesis, Department of Physics, Jamia Millia Islamia, New Delhi-25, India (2001). [29] Z. Fozia Haque, V. Ali, M. Husain, Synthesis and spectroscopic characterization of Kiton Red-620 dye doped silica gel rods, Optik 123 (2012) 1010–1014. [30] J. Jose, Y. Ueno, K. Burgess, Water-soluble Nile Blue derivatives: syntheses and photophysical properties, Chem. Eur. J. 15 (2009) 418–423.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Synthesis and spectroscopic characterization of Nile Blue doped silica gel rods Fozia Z. Haque a,∗ , Vazid Ali b , M. Husain b a b
Nanomaterials Lab, Department of Physics, Maulana Azad National Institute of Technology, Bhopal, MP, India Materials Science Laboratory, Department of Physics, Jamia Millia Islamia (Central University), Jamia Nagar, New Delhi 110025, India
a r t i c l e
i n f o
Article history: Received 12 August 2012 Accepted 2 January 2013
Keywords: Silica Sol–gel process Tetraethyl orthosilicate (TEOS) Organic dyes Nile-blue
a b s t r a c t We have studied silica-gel by sol–gel technique for the preparation of new dye-laser materials. Silica gel rods with dimension 50 mm × 10 mm have been prepared successfully without breaking. It shows high transparency and good mechanical strength. Tetraethylorthosilicate (TEOS), formamide in molar ratio (0.25:0.70), 80 ml ethanol, 20 ml dimethylformamide (DMF), 10 ml water, hydrochloric acid as a catalyst (at pH 6) and 0.5 ml silicone defoaming agent/surfactant have been used. The synthesis has been carried out in a beaker and the reaction mixture is caste in to the flat bottom glass tubes at 40 ◦ C after thoroughly mixing of all the ingredients. These complex reactions, that carried out by hydrolysis and condensation in the silica gel formation show less gel time ∼8–10 h at 40 ◦ C. The doping of dye (Nile Blue 690) has been taken during the preparation of all the ingredients solution mixture. It has been observed that the compatibility of Nile Blue dye with silica-gel promise good homogeneity with transparency. In the present study, synthesis, dye doping, FTIR, UV/vis–NIR and fluorescence spectroscopy have been studied. We have observed that Nile Blue doped silica gel rods material give good fluorescence showing sharp peaks in the visible range. UV–vis spectra of prepared silica gel material also indicate the absorption in visible range. Thermal stability of rods was 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. © 2013 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 techniques are low temperature materials processing, high homogeneity of final product and its capacity to generate materials with controlled surface property and shape and pore structures. The process includes the following steps: preparation of the solution, gelation, aging and finally drying of synthesized materials. Various works has been reported in literature on sol–gel processes for manufacturing glasses, ceramics and inorganic materials [1,2] but still there are some major drawbacks such as poor mechanical strength of silica gel and cracks formation during gelling. Therefore, modification of the characteristics of these materials will be very useful in the developments of new nano materials. Several research groups are involved in the development of smart materials [3,4] and their characterization by sol–gel process. Lenza and Vasconcelos [5] have reported the preparation of silica gel by sol–gel technique and its characterization by FTIR, which enabled formation of crack-free silica gel.
∗ Corresponding author. E-mail address: [email protected] (F.Z. Haque). 0030-4026/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2013.01.043
Recently, Stiegman et al. [6] have incorporated transition vanadium oxide into the silica matrix and the resultant material showed remarkable, hardness and optical transparency. Resfield et al. [7,8] have synthesized smart optical materials by sol–gel method and also studied their spectroscopic properties. The research work was focused on nanometer-sized particle of CdS, CdSe, CdTe and PbS, which have been formed by chemical method in silica glass films. These films were prepared either of 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 basis for their spectroscopic studies. They utilized a renovative method for the preparation of new family of materials with rare earths complexes by sol–gel technique. Further research work has also been carried out by Saradaov et al. [10] for the luminescent properties of silica and Zirconia xerogel doped with Eu(III) cryptate 3,3 -biisoquinoline-22-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, pyrene red, etc. 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
4288
F.Z. Haque et al. / Optik 124 (2013) 4287–4291
Fig. 1. Structure of Nile Blue 690.
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 (SSDL) 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] and Farooqui 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 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 Kiton-red organic dye for solid state dye laser is reported in our previous work [29]. Therefore several applications of silica gel materials as a smart optical device and many other opto-electronic/organic compounds such as dye laser materials and high temperature resistant spin coatings suggested their future possible engineering materials. In the present study, we have prepared silica-gel rods at room temperature. Here our basic objective is the formation and characterization of crack free dye doped silica-gel materials with enhanced mechanical properties. 2. Experimental procedure 2.1. Chemicals The following chemicals have been used in the experimental work for the preparation of silica gel rods of the monoliths; tetraethyl orthosilicate (TEOS) (Acros Organics, USA, 98% pure), 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 The first reported synthesis of Nile Blue dates back to 1896 by Mohlau and Uhlmann [30]. They condensed 1-naphthylamine with 4-nitroso-N,N-diethyl3-aminophenol to obtain a blue dye, which they later named as Nile Blue. Its derivative Nile Blue 690 is 5-amino-9(diethylamino)benzo(a)phenoxazin-7-ium perchlorate in the form of dark green crystals. Its chemical structure is shown in Fig. 1. 2.3. Methodology for silica gel rod formation Crack free silica gel rod has been prepared by using the following methods. Tetraethyl orthosilicate (TEOS) and formamide were taken in the molar ratio of 0.072:0.45, water 5 ml (v/v), ethanol
20 ml (v/v) and silicon defoaming agent/surfactant 0.04 ml (v/v) were mixed thoroughly and stirred at 25 ◦ C for 15 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 24 h. The samples casted in 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 wet gel rod was dried with N,N -dimethylformamide and methanol mixture at 60 ◦ C for about two weeks. These monolith rods were finally heated to 160 ◦ C for two h. and kept in airtight desiccators for final aging to achieve mechanical strength. The aging and final drying depend on the size and thickness of silica gel rod samples. The basic chemical reactions for silica gel formation are complicated but they can be summarized in the following simplified scheme: Si(OCH2 CH3 )4 + 2H2 O → SiO2 + 4HOCH2 CH3 However, the silica gel formation depends on the complex reaction mechanisms, i.e. hydrolysis, esterification, aldol condensation and finally water condensation as the mechanism elaborated below. The reactions of silica gel formation can be accelerated or slowed down by employing appropriate acidic and basic catalysts. Although silica gel formation seems simple but its synthesis is rather complex. A number of parameters such as pH, solvents, water content, temperature and drying conditions can change the properties of the final product.
2.4. Mechanisms ≡ Si − OR + H2 O
Hydrolysis
Si − OH + ROH
Esterification
≡ Si − OR + HO − Si ≡
Aldol condensation
Si − O − Si ≡ +ROH
Water condensation ≡ Si − OH + HO − Si ≡
Hydrolysis
≡ Si − O − Si ≡ +H2 O
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 (silicone defoaming agent) to reduce the stress in gelled sample. It has been observed that formamide and silicone defoaming agent reduce cracking that help to convert the wet silica gel into a monolithic gel within a reasonable time.
2.5. Trapping organic molecules in silica gel glasses 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. These involved combination of molecular precursors and polymerizing conditions. In this research work, we have emphasized here only the preparation of crack free silica gel rods doped with Nile Blue 690 (NB 690) transparent in desired shape and length, in the visible range. And these dye doped silica gel rods will be taken further for lasing study in our laboratory.
F.Z. Haque et al. / Optik 124 (2013) 4287–4291
4289
Fig. 2. FTIR spectra of synthesized silica gel rod without dye doping.
2.6. 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 rods preparation, diethylene glycol has been used to reduce the stress in gelled sample. Silica aero gels 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
Fig. 3. Absorption spectra at different concentration of Nile Blue 690 doped silica gel.
shrinkage of gel. In the present study, it has been found that formamide based silica gel rods formation 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. 1 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.
3.1. FTIR studies 3.2. UV–vis–NIR spectra Spectroscopic characterization is an important tool to understand the optical properties and the attached (interacting) groups after doping of the newly synthesized materials. Formed silica gel rods on the basis of above reactions mechanisms have been characterized by IR spectroscopy. IR spectra of pure silica gel rods, which exhibits several peaks (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 region of 2800–3000 cm−1 corresponds to symmetric and asymmetric stretching vibration of CH2 and CH3 groups of alkoxide and solvent residue. The main bands 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 stretching is typical of the gel structure that decreased the intensity and becomes insignificant when the material undergoes poly condensation 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
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 Nile Blue 690 is shown in Fig. 3. It has also been observed that during the processing/curing of doped silica gel rods there is some loss of dye concentration at high temperature that affects the UV/vis spectra sharpness. It has been shown that there is the same behavior of absorption peaks at 375, 380 and ∼400 nm. It is shown slightly red shifting of peak absorption, at different concentration of Nile Blue, which is also occurred. Thus, it concludes that silica gel material is ideally suitable for doping with various laser dyes or luminescent materials, which typically 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 Spectrophotometer, Hitachi). Fig. 4a shows fluorescence spectra for various concentrations of Nile Blue 690 (NB 690) doped silica gel rods. The dye (NB-690) doping was varied from 8.36 × 10−6 to 25.08 × 10−6 M/L in silica gel rods formation. The fluorescence intensity reaches its maximum at dopant concentration from
4290
F.Z. Haque et al. / Optik 124 (2013) 4287–4291 Table 1 Photophysical properties of Nile Blue 690 in various media. Medium
max (nm) abs
max em (nm)
Stokes shift (nm)
Ethanol Methanol N,N -dimethylformamide Silica gel
628 627 635 640
664 664 666 557
36 37 31 17
thermal stability from 100 to 150 ◦ C without significant change in the silica gel materials as well as dye doped silica gel rods matrix. 3.5. Spectroscopic effect of solvents on Nile Blue 690
Fig. 4. (a) Fluorescence spectra with different concentration of Nile Blue 690 doped silica gel. (b) Photoluminescence spectra with different concentration of Nile Blue 690 doped Silica gel.
8.36 × 10−6 to 25.08 × 10−6 M/L. It seems that the peaks of fluorescence emission wavelengths remain at position 220, 310 nm and slightly shifted between 650 and 700 nm but these peaks occurred with high fluorescence intensity. Fig. 4b also shows the luminescence spectra for different concentration doped (NB-690) silica gel rods. Similar trend to the case of the fluorescence spectra has been observed. But in luminescence spectra it has been seen interestingly that sharp peaks obtained in UV/vis range in silica gel host materials with shifting wavelength position (Red shift) Fig. 4b, when the concentration of Nile Blue 690 increased. The photo luminescence spectra change its shape with the various dye concentrations. The observed reduction in the luminance emission intensity is due to the NB-690 dye atoms/molecules forming aggregations in the silica gel matrix. In addition to this evidence it is clear from Fig. 4(a) (n3 spectra), which reduce the luminance intensity at high concentration of NB-690 as compared to n1 and n2 spectra. 3.4. Differential scanning calorimetric (DSC) study DSC of pure silica gel rods and Nile Blue doped silica gel rods have been studied at heat flow rate 10 ◦ C/min (Fig. 5a and b). It has been observed that the prepared cracks free silica gel rods show good
Fig. 5. DSC curves of (a) pure silica gel rods and (b) Nile Blue 690 doped silica gel rods.
The fluorescence and absorbance spectra have been performed to see the effect of solvent in Nile Blue 690 laser dye. Nile Blue 690 dyes belong to the class of oxazine dyes with a structure modified by an additional benzoic group. These have been reported to be efficient laser dyes too. If the central CH group of a pyronine dye is formally replaced by N , a compound is obtained whose absorption is shifted by about 100 nm to longer wavelength. Such an oxazine or phenooxazine dye is planar and rigid like its xanthene relative. The position of the absorption maximum depends on the end group of the chromophore. All oxazine dyes are photochemically more stable than the pyronines and rhodamine. However, there is no triplet problem in these days; it is comparatively difficult to suppress the process of internal conversion due to the smaller energy difference between S1 and S0. A continuous red shifting was observed in absorption spectrum on increasing polarity of the matrix. In emission spectrum the maximum emission band is slightly red shifted on increasing matrix polarity. It was seen that the emission band is highly blue shifted in sol–gel due to rigidochromism. Electronic transitions occur at rates rapid relative to the rates of inter nuclear motion in molecules. Hence, during an electronic transition the nuclei remain essentially stationary (the Franck–Condon principle). When a molecule in its ground state absorbs photon and reach a metastable excited state in which the molecular geometry and solvent configuration are those characteristic of the ground state. Solvent reorientation then occur approximately 10−1 to 10−12 s after excitation, producing an equilibrium excited state, in which the solvent configuration is optimal for the geometry and electron distribution of the molecule. It is obvious that due to drastic changes in molecular geometry and electronic charge distribution on excitation, the energy difference between equilibrium and Franck–Condon excited state can be quite large in some cases. There is no reason to expect precise correspondence between absorption and fluorescence solvent effect because the relevant ground and excited state involved in absorption and emission are different. In polar molecules, it was found that the excited state is more polar than the ground state. So, the excited state becomes more stabilized than the ground state on increasing the polarity of the solvent. A red-shift in absorption and emission spectra is observed and this red-shifting depends on the nature of the solute–solvent interaction. It is usually observed that on increasing the rigidity of the matrix, the fluorescence spectrum of some dye molecules is blue shifted compared to the spectrum in liquid solution because after excitation the solvent reorientation is less facile in a rigid medium than it is in solution. Nile Blue shows progressively longer absorption and emission maxima as the solvent polarity is increased. The Stokes’ shifts observed for the Blue compound is exceptionally high (almost 100 nm), but in polar solvents it is reduced to around 40 nm (Table 1). This also reveals that the spectroscopic characteristics of Nile Blue are pH dependent. This is because under basic conditions the minimum group will be deprotonated, whereas under
F.Z. Haque et al. / Optik 124 (2013) 4287–4291
strongly acidic conditions the 5-amino might even become protonated. Curiously, the spectroscopic properties of Nile Blue are somewhat dependent on the counter ions. We speculate that this could even be a reflection on intimate ion pairing influencing the solvent sphere of the dye. 4. Conclusions 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 silicon defoaming agent (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 important information on the compactness of silica gel. At high frequency, absorption bands of the Si O Si stretching indicate a strongly cross-linked structure. The UV–vis–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 peaks appearance in UV/vis range with dye NB-690 doped silica gel. Acknowledgment Thanks are due to DRDO, New Delhi, India for providing financial support in the form of a major research project to Prof. M. Husain (Principal Investigator). References [1] Z.S. Hu, J.X. Dong, G.X. Chen, Replacing solvent drying technique for nanometer particle preparation, J. Colloid Interface Sci. 208 (2) (1998) 367–372. [2] A.B. Mrowiec-Bialon, J. Jarzebski, A.I. Lachowski, J.J. Malinowski, Twocomponent aerogel adsorbents of water vapour, J. Non-Cryst. Solids 225 (1998) 184–189. [3] Z. Zhu, Y. Tsung, M. Tomkiewicz, Morphology of TiO2 aerogels. Electron microscopy, J. Phys. Chem. 99 (1995) 15945–15949. [4] C.A. Muller, M. Schneider, I. Maullat, A. Baker, Titania–silica epoxidation catalysts modified by polar organic functional groups, J. Catal. 189 (2000) 221–232. [5] R.F.S. Lenza, W.L. Vasconcelos, Structural evolution of silica sols modified with formamide, Mater. Res. 4 (3) (2001) 175–179. [6] A.E. Stiegman, H. Eckert, G. Plett, S.S. Kim, M. Anderson, A. Yavourian, Vanadia/silica xerogels and nanocomposites, Chem. Mater. 5 (1993) 1591– 1595. [7] R. Reisfeld, in: C.K. Jorgensen (Ed.), Optical Properties of Colorants or Luminescents Species in Sol–Gel Glasses; Structure and Bonding, vol. 77, Springer Verlag, 1992, pp. 207–256.
4291
[8] R. Reisfeld, Sol gel processed lasers, in: S. Sakka (Ed.), Sol–Gel Technology (Handbook), vol. 3, 2004, pp. 239–261 (Chapter 12). [9] W. Strek, J. Sokolnichi, J. Legendziewics, K. Maruszewski, B. Nissen, R. Reisfeld, T. Pivich, Optical properties of Eu(III) chelates trapped in silica gel glasses, Opt. Mater. 13 (1999) 41–48. [10] T. Saraidarov, R. Reisfeld, M. Pietraszkiewicz, Luminescent properties of silica and zirconia xerogels doped with europium(III) salts and europium(III) cryptate 3,3 -biisoquinoline-22 -dioxide, Chem. Phys. Lett. 330 (2000) 515–520. [11] C. Stewen, M. Larionov, A. Giesen, Yb:YAG thin disk laser with 1 kW output power, in: H. Injeyan, U. Keller, C. Marshall (Eds.), OSA Trends in Optics and Photonics, Advanced Solid-State Lasers, Optical Society of America, Washington, DC, 2000, pp. 35–41. [12] E. Yariv, S. Schultheiss, T. Saraidarov, R. Reisfeld, Efficiency and photostability of dye-doped solid-state lasers in different hosts, Opt. Mater. 16 (1–2) (2001) 29–38. [13] A. Costela, I. Garcia-Moreno, H. Tian, J. Su, K. Chen, F. Amat-Guerri, M. Carrascoso, J. Barroso, R. Sastre, Internal photostabilization of polymeric solid-state dye lasers based on trichromophoric rhodamine 6G molecules, Chem. Phys. Lett. 277 (1997) 392–396. [14] K.M. Dyumaev, A.A. Manenkov, A.P. Maslyukov, G.A. Matyushin, V.S. Nechitailo, A.M. Prokhorov, Dyes in modified polymers: problems of photostability and conversion efficiency at high intensities, J. Opt. Soc. Am. B9 (1992) 143–151. [15] Ya.V. Kravchenko, A.A. Manenkov, G.A. Matyushin, V.M. Mizin, D.P. Pacheco, H.R. Aldag, New high-efficiency pyrromethene-580-doped modified PMMA solid state dye laser, Proc. SPIE 2986 (1997) 124–131. [16] D.P. Pacheco, H.R. Aldag, Solid state lasers VII, comparison of the laser performance of solid state dye media under long- and short-pulse excitation, Proc. SPIE 3265 (1998) 2–12. [17] M.D. Rahn, T.A. King, A.A. Gorman, I. Hamblett, Photostability enhancement of Pyrromethene 567 and Perylene Orange in oxygen-free liquid and solid dye lasers, Appl. Opt. 36 (1997) 5862–5871. [18] H. Tran Nhung, M. Canva, T.A. Truong Dao, F. Chaput, A. Brun, N.D. Hung, J.P. Boilot, Stable doped hybrid sol–gel materials for solid-state dye laser, Appl. Opt. 4 (12) (2003) 2213–2218. [19] T. Suratwala, K. Davidson, Z. garlund, D.R. Uhlmann, Reversing gels and water soluble and photostability of laser dyes within sol–gel hosts. Solid state lasers VI, SPIE Proc. Series 2986 (1997) 141–152. [20] M. Ahmad, M.D. Rahn, T.A. King, Singlet oxygen and dye-triplet-state quenching in solid-state dye lasers consisting of Pyrromethene 567-doped poly(methyl methacrylate), Appl. Opt. 38 (30) (1999) 6337–6342. [21] G. Jones II, O. Klueva, S. Kumar, D. Pacheco, Photochemical and lasing properties of pyrromethane dyes, Proc. SPIE 4267 (2001) 24–35. [22] S. Chandra, H. Toomas, J. Allik, A. Hutchinson, J. Fox, C. Swim, Tunable ultraviolet laser source based on solid-state dye laser technology and CsLiB6O10 harmonic generation, Opt. Lett. 22 (4) (1997) 209–211. [23] A. Mandl, A. Zavriyev, D.E. Klimek, Energy scaling and beam quality studies of a zigzag solid-state plastic dye laser, IEEE J. Quantum Electron 32 (1996) 1723–1726. [24] J.A. Russell, D.P. Pacheco, H.R. Aldag, S.M. Dolotov, M.F. Koldunov, V.Ya. Kravchenko, A.A. Manenkov, A.V. Reznichenko, G.P. Roskova, Beam-quality measurements on solid-state dye lasers using nonconfocal unstable resonators, Proc. SPIE 4267 (2001) 36–45. [25] Q. Zhang, Z. Jiang, Pyrromethene doped silica laser glasses by sol–gel processes, Mater. Chem. Phys. 69 (1–3) (2001) 95–98. [26] B. Dunn, J.I. Zink, Molecules in glass: probes, ordered assemblies, and functional materials, Acc. Chem. Res. 40 (2007) 747–755. [27] Z.H. Zaidi, A.S. Farooqui, Spectroscopic characteristics of coumarin 540 A in sol–gel, Spectrosc. Lett. 34 (2) (2001) 301–309. [28] A.S. Farooqui, Spectroscopic investigation and prediction of laser action properties of 4-hydroxycoumarin dye in sol–gel. Ph.D. Thesis, Department of Physics, Jamia Millia Islamia, New Delhi-25, India (2001). [29] Z. Fozia Haque, V. Ali, M. Husain, Synthesis and spectroscopic characterization of Kiton Red-620 dye doped silica gel rods, Optik 123 (2012) 1010–1014. [30] J. Jose, Y. Ueno, K. Burgess, Water-soluble Nile Blue derivatives: syntheses and photophysical properties, Chem. Eur. J. 15 (2009) 418–423.