International Journal of Scientific & Engineering Research, Volume 4, Issue 12, December-2013 1593

ISSN 2229-5518

Correlating Spectral Variations of Malachite Green with Organic Co-Precursors in TEOS Silica Hybrid Glasses

D. Bora, S. Hazarika

AbstractMalachite Green (MG) doped inorganic-organic hybrid glasses are synthesized using monomers methyl methacrylate (MMA), 2-hydroxyethyl methacrylate (HEMA) and inorganic precursor tetraethylorthosilicate (TEOS) by sol-gel technique. Various spectral properties of MG in these hybrid matrices are evaluated and compared to those in silica glass to determine their utility as optical material. Inclusion of HEMA in matrix reduced fluorescence quantum yield and lifetime; presence of collisional quenching as dominant non-radiative process considered responsible for it, whereas presence of MMA in matrix improved fluorescence quantum yield, lifetime and non-radiative transition probability, which is attributed to reduction in free space and immobilization of dye mole- cules within rigid pores of silica matrix. Luminescent properties in the hybrid matrix with MMA and TEOS are comparable to those in silica matrix.

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Index Terms— Malachite Green, hybrid matrices, quantum yield, lifetime, Sol-gel glass, Luminescence Properties,TEM

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1 INTRODUCTION

HARACTERISTICS of dye molecules embedded in solid ma- trices are governed by the nature of matrices. Changes in nature
of matrix can shift peak absorption and luminescence wave-
lengths, induce variations in fluorescence lifetime and modify photo- stability of the embedded dye.[1]. Therefore, comparison of photo- physical processes of a dye embedded in matrices of varied constitu- ent and concentration can provide valuable information on the influ- ence of host matrix on dye properties including host’s feasibility with respect to application as luminescence conversion system[2], solid state dye laser material[3,4], nonlinear optics etc.. To obtain optimal optical, thermal and mechanodynamical properties of matrices for aforementioned applications the photo physical and photochemical property of the trapped dye requires proper adjustment of matrix structure and composition [5].

In context to the above discussion this paper reports a quantita- tive study of spectroscopic properties of Malachite Green (MG) (oxalate) doped in inorganic – organic hybrid glasses in an effort to develop a dye-host combination suitable for optical applica- tion. The spectroscopic properties in hybrid glasses are further compared to those in in-organic silica glass (reported in this work). Based on absorption recovery time study of the So → S1 transition in MG doped xerogel around 620 nm, Canva et.al[6] predicted potential optical applications for MG doped xerogel. However, reported investigations on absorbance and lumines- cence of MG doped in different inorganic matrices [7-9] do not present detailed quantitative analysis of spectroscopic properties required to characterize optically MG doped materials. Such characterization of MG doped in inorganic-organic hybrid matri- ces are not known to be reported.

D.Bora, is currently working in Assam University: Diphu, India, PH-

+919435084599. E-mail: dulen_bora@yahoo.com

S.Hazarika, is currently working in Assam University: Diphu, India,, PH-

+919435155925. E-mail: hazarikasubrata@rediffmail.com
In synthesis of organic compound based optical materials usually
(polymethyl methacrylate) (PMMA) is preferred because of its high transparency in visible region of the spectrum, high resistance to radiation, and compatibility to many organic compounds. But in preparation of inorganic -organic glasses for the present study organ- ic monomers methyl methacrylate (MMA) and 2-hydroxyethyl methacrylate (HEMA) are taken because copolymerization of methyl methacrylate (MMA) with other monomers like 2-hydroxyethyl methacrylate (HEMA) can lead to synthesis of materials with differ- ent properties. Besides, in hybrid matrices organic poly- mers/monomers combine with inorganic glasses and have contribu- tion from both components in their properties viz. flexibility, high optical uniformity, and low shrinkage of an organic component and rigidity, hardness, and thermal stability of the inorganic glasses [10].

2 MATERIALS AND METHODS:

2.1 Synthesis of glass:

Synthesis of inorganic and inorganic-organic hybrid glasses doped with 1× 10-5M of Malachite Green (oxalate) were carried out via
‘wet’ sol-gel process using acid (dilute HNO3) catalysts. Concentra- tion of MG was considered with respect to a total mixture of solvents consisting of Methanol, dilute nitric acid (HNO3), distilled water and
‘precursor’. Tetraethylorthosilicate (TEOS) was taken as the precur-
sor in the inorganic M1 glass, whereas in the inorganic-organic hy- brid glasses, TEOS mixed with adequate quantity of organic mono- mers viz. Methyl methacrylate (MMA) and 2-hydroxyethyl methac- rylate (HEMA) were used. The organic monomers and their quanti-

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ties used in the synthesis of three hybrid glasses are presented in Table-1. In synthesis of the glasses 2ml of ‘precursor’ with requisite proportion of the monomers was mixed with a previously prepared solution of Malachite Green in 10.5 ml mixture of methanol(8.75 ml), distilled water (1.25 ml) and dilute nitric acid (0.5 ml), taken in proportion of 70(Methanol): 10 (H2O): 4 (HNO3) parts and stirred in a magnetic stirrer with Teflon coated stirrer bar for 1h. The 2ml pre- cursor constituted 16 parts in a total solvent mixture of 12.5 ml used in each of these preparations. Hydrolysis and polymerization of the
‘precursor’ solution in methanol under the catalyst action of doubly
distilled water and dilute nitric acid results in formation of gel after about 1h of stirring. The gel was then poured into plastic moulds and left to dry and solidify at room temperature (25-270C). With further progress of hydrolysis the gel solidified to form a deep orange col- oured stiff hard mass (Xerogel / sol-gel glass) in 48-72 hrs.
The radiative (K r ) and non-radiative (K nr ) transition probabilities are calculated using the following relations [11]:

2.2 Instruments and Methods:

A commercial UV-VIS-NIR spectrophotometer (CARY-5E) was

K = φ

r τ

(2)

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used to record the VIS absorption spectra of Malachite Green doped
in the inorganic and hybrid matrices in the range 400-700 nm and their corresponding fluorescence spectra were taken in a JY Fluorolog-3-11 spectrofluorometer, excited by 445 nm wavelength of
and

K nr

= (1 − φ )

τ

(3)
a 450W Xenon lamp, keeping slit widths at 5nm, integration time of
0.1s and step size of 1nm. Fluorescence life time measurements of the same samples at their peak emission wavelength were performed in FLUOROCUBE - a life time measurement system from JOBIN- YVON. The Instrument Response Function (IRF) of the instrument
Where φ and τ are measured fluorescence quantum yield (QY) and
life time (non-intrinsic), respectively, of MG in the matrices.
Fluorescence Quantum Yield (φ) is measured by comparison to a reference fluorophore of known QY using the single point method from the equation [12].
was 1.263ns obtained using LUDOX SM-30 colloidal suspension in water. The excitation source for decay time measurement is a 460nm LED. Transmission Electron Micrographs (TEM) and diffraction pattern were recorded in a JEOL JEM-2100 ultra high resolution Transmission Electron Microscope. The microscope was operated at an accelerating voltage of 200 kV. All measurements and recordings were done in room temperature (25-270C).

3 THEORETICAL CONSIDERATIONS:

Several spectroscopic properties of MG are evaluated from recorded spectra of different glasses synthesized for a comparative analysis described in this paper. Oscillator strength of bands at absorbance maxima are derived from integrated absorption coefficients using the formula

I

φ = φr × ×

I r

ODr

OD

(4)

f = 4.32 ×10−9 ε (υ )dυ ,where ε (υ )dυ ε (υ ) × ∆υ


…..(1)

In Eqn. (1), ε (υ )

is the molar absorption co-efficient at frequency
υ (cm-1) and υ is the band width at

12 ε (υ ) (FWHM) measured

directly from absorption spectra. Molar absorption co-efficient of
MG is calculated directly from the absorption profile presented in Fig .1 using the relation for Absorbance, A = ε (υ )lc (where l=path length, c=Molar concentration). The absorbance values in Fig. 1 are for identical sample path length (l) of 0.1 cm and MG concentration
(c) of 1x 10-5M. Beer –Lambert law is obeyed for the given concen-
tration.
In Eqn.(4), φr
is the quantum yield of the reference fluorophore.

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Riboflavin with φr = 0.3 is taken dissolved in ethanol as the refer- ence fluorophore. I is the integrated fluorescence intensity, OD is the optical density. Subscript r refers to the reference fluoro- phore.Fluorescence life time (τ) of MG in these glasses is estimated by double exponent iteration of the recorded curves given in Fig. 2 above.

4 RESULTS

4.1 Absorption Spectra and Properties:



Absorption spectra of the four glasses are presented in Fig.1. Each spectra has two peaks - one in the blue spectral region and the other in the yellow-red region. The peaks in hybrid matrices are red shifted with respect to peaks recorded in inorganic M1 glass. As absorption maxima in the four matrices lie in the blue spectral region, quantita- tive analysis of the bands in the blue region is only carried out. The peak absorption wavelength (λabs), oscillator strength (f) , FWHM (Δυ ) of these bands and the molar absorption co-efficient of MG (ε(υ) ) at absorption maxima derived for the four glasses as discussed in Sec.2 are compiled in Table- 2 for comparison. The oscillator strength and molar absorption co-efficient of MG in the hybrid ma- trices are enhanced with respect to values in inorganic M1 glass, but the corresponding FWHM is increased only in THM1 glass, i.e. the hybrid matrix with 1:1 ml of HEMA: TOES. In presence MMA i.e. in THMM1 and TMM1 glasses the oscillator strength, molar absorp- tion co-efficient and FWHM of the bands are almost equal.

4.2 Luminescence Spectra and properties:

Luminescence properties calculated using Eqn. 2-4 together with peak emission wavelength (λp), effective band width (Δλeff ) of tran- sitions measured from the luminescence spectra presented in Fig. 3 and measured fluorescence lifetime of MG in the inorganic and inor- ganic-organic hybrid glasses are complied in Table -3. Large red shifts up to ~ 100 nm in peak emission wavelength and narrower band width of MG transitions compared to those in the inorganic SiO2 glass are observed in hybrid glasses. Further, its observed that inclusion of monomer HEMA in the SiO2 in matrix (inTHM1glass) drastically reduces fluorescence quantum yield, fluorescence life time and increases the non-radiative transition probability compared to SiO2 matrix. The situation improves with addition of MMA (in THMM1 glass) and in the matrix with MMA + TEOS (in TMM1 glass) fluorescence intensity and quantum yield, lifetime are en- hanced and non-radiative transition probability is reduced (compared to SiO2 matrix).

4.3 TEM micrographs and diffraction pattern of matri- ces:

Transmission Electron Micrographs (TEM) and their diffraction pat- terns of the matrices presented in Fig. 4(a) and 4(b) clearly shows variation in structure with matrix composition. From the radiative properties compiled in Table-3 its evident that the matrix with coarse texture and diffraction pattern of spotless rings (TMM1 glass) is the most rigid among the hybrid matrices. Appearance of TEM diffrac- tion pattern reflects on the phase nature of the specimens. Patterns of micro –crystalline or amorphous matrices like polymers and metallic glasses that lack long – range order in atomic lattice consists of con- centric rings [13]. Diffraction ring patterns marked by reflection spots, as in THM1 and THMM1 glasses, often refers to matrices formed by large collection of crystallites with different orientations. The individual reflections from such crystals appear as spots on the rings. Luminescence parameters derived for these two hybrid glasses are not encouraging.

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and thereby limits non-radiative relaxation channels of excited trapped molecules. Impregnation of PMMA in such SiO2 matrix further fills up the pores and increases rigidity of matrices by reduc- ing free space within the pores, and eventually the quantum yield [15].
The simultaneous decrease observed in fluorescence lifetime and
intensity (quantum yield) of MG in THM1 glass is indicative of the presence of collisional quenching as a dominant non-radiative pro- cess [12], where fluorophore de-excites non-radiatively to its sur- rounding environment via collision with fluorescence quenchers like O2(in presence of HEMA in matrix). The collisional quenching rate is defined as Kc = ko [C], where ko is related to the diffusivity and the hydrodynamics radii of the reactants and [C] is the quencher concentration [12]. As such, reduction in the quantity of HEMA i.e.[C] (adding equal quantity of MMA ) in SiO2 matrix( as in THMM1 glass) decreases non-radiative transition probability rate and causes improvement in fluorescence quantum yield and lifetime, but the values still remain much smaller than that in SiO2 matrix.
The slight enhancement in fluorescence intensity and quantum yield in TMM1 glass with respect to SiO2 matrix is a result of reasons cited in case of ‘impregnation of PMMA’ in SiO2 matrix. Observa- tion of a simultaneous decrease in non-radiative transition probabil- ity in the glass is indicative of restrictions imposed on non-radiative channels of de-excitation of excited trapped molecules. But, a fluo- rescence lifetime greater than in M1 glass slightly reduces radiative transition probability in TMM1 glass.
The red shift observed in the peak wavelengths of transition bands in the hybrid matrices (compared to the SiO2 matrix) may be attributed to the ligand field of the matrices acting on the vibrational levels of the trapped molecules [16]. The different TEM diffraction pattern of the hybrid matrices also infers to a variation of field in the matrices that resulted in red shifts of different magnitude.

6 CONCLUSIONS:

Several absorption and luminescence properties of MG were evalu- ated in inorganic-organic hybrid glasses synthesized with organic
monomers MMA, HEMA and inorganic precursor TEOS by the sol- gel technique and studied in comparison to SiO 2 matrix derived from TEOS. The investigation revealed how presence of different mono- mers in SiO2 matrices controlled the luminescence properties and hence the efficiency of the matrices either by increasing rigidity of matrix and /or promoting collisional quenching. The quantum yield , fluorescence lifetime of MG evaluated for the MMA-TEOS hybrid matrix ( TMM1 glass) were slightly improved compared to TEOS inorganic matrix ( M1 glass) and when considered together with their radiative and non-radiative transition probabilities, TMM1 glass emerged as the most efficient amongst the three hybrid matrices con- sidered in the present study. However, conclusive evidence on its utility as optical material can be had only after study of its photosta- bility.

5 DISCUSSIONS:

Malachite Green (oxalate) dye -a member of the Triarylmethane
group-is considered a non-fluorescent dye which in non-viscous sol- vents has a low quantum yield of as low as ~10-4[14]. But when it is confined within rigid pores of SiO2 solid matrix the quantum yield is enhanced ~103 times (Ref. Table-2). This is caused by confinement of the dye molecules within the rigid pores of matrix, which restricts non-radiative routes for deactivation of excited confined molecules and enhances photoluminescence intensity and quantum yield. Rigid- ity of the pores reduces space for rotational motion of the molecules

ACKNOWLEDGMENT

The authors acknowledge the support of SAIF-IIT (Madras) and SAIF-NEHU for providing instrumentation facilities used in sample analysis.

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