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ISSN 1749-3889 (print), 1749-3897 (online) International Journal of Nonlinear Science Vol.7(2009) No.3,pp.290-300

Influence of Solvents Polarity on NLO Properties of Fluorone Dye

Ahmad Y. Nooraldeen1 , M. Palanichant2 , P. K. Palanisamy1

1 Center 2 Department

for Laser Technology, Department of Physics, Anna University of Chemistry, Anna University, Chennai 600025, India

(Received 17 May 2008, accepted 2 November 2008)

Abstract:We have investigated third order nonlinear optical properties of Rose Bengal dye in different organic solvents and measured both real and imaginary parts of [3 ]. The measurements were performed using SHG of diode pumped Nd-YAG laser CW at 532 nm wavelength by employing Z-scan technique. The nonlinear refractive index is found to be of the order of 10-9 cm2 /W.The magnitude of third order susceptibility is of the order of 10-6 esu. Their nonlinear absorption coefficient and absorption cross section have also been determined. The optical power limiting behavior of Rose Bengal was also investigated in four solvents (water, DMSO, methanol and ethanol). The results suggest that among the four organic solvents, methanol is a promising class of organic solvent for such nonlinear optical studies due to its relatively higher polarity when compared with other solvents. It also shows a very good optical limiting behavior with a limiting threshold of 1.138 mW at 0.1 mM concentration in Methanol solution. Hence this material may be tailored suitably for applications such as optical power limiters, optical switching and modulators. Key words:Nonlinear refractive index, optical limiting, solvent effect, Z-scan technique

1

Introduction

Nonlinear optical materials are required in a wide range of important applications, such as optical limiting, optical computing, and optical communication. Efforts are in progress in many laboratories for the realization of all optical devices. Such devices can do away with electrical/optical/ electrical conversion existing in the present communication system, which really limits the optimum use of available speed [1].Organic materials exhibiting nonlinear absorption are currently of interest because of their large third-order nonlinearities, instantaneous response time, high damage thresholds, ease of processing, structural modifications and their applicability over a wide range of wavelengths. By using the Z-scan technique the optical nonlinearity of many organic materials has been explored. The Z-scan technique is a simple and sensitive method introduced in 1990 to measure the nonlinear refractive index of optical materials [2, 3]. In this technique the sample is moved in the intensity pattern generated along the z-axis by a focused laser beam, and the on-axis transmitted signal is measured in the far field by a detector placed behind a small aperture (closed aperture Z-scan). Change of the refractive index due to optical nonlinearities in the sample is detected as characteristic intensity variation of the transmitted signal as a function of the sample position. The nonlinear refractive index is then calculated from the normalized peak­valley signal difference (Tp-v ). Furthermore, the z-positions at which the maximum and minimum occur are related to the order of the nonlinear process. Saturable absorption can be measured in the slightly different open aperture configuration, where the whole transmitted beam is collected and detected. Nonlinear absorption causes a characteristic peak in the transmitted signal when the sample is in the focus [4]. However, the closed-aperture Z-scan is also sensitive to refractive index changes due to density variations induced by thermal lens effects. For this reason the Z-scan setup has been used to detect small linear absorption using low power continuous wave (CW) lasers [5, 6].

Corresponding author.

E-mail address: [email protected] Copyright c World Academic Press, World Academic Union IJNS.2009.06.15/232

Ahmad Y. Nooraldeen, M. Palanichant, P. K. Palanisamy: Influence of Solvents Polarity on · · ·

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Optical limiting with organic materials or devices with transmission that decreases with light level is called optical limiters and has applications in eye and sensor protection. Ideally, an optical limiter should exhibit linear transmittance at low incident light fluences, but become opaque at high incident light fluence. To enhance the application viability of the conjugated organic NLO materials in optical limiter devices, a collective effort from physicists, chemists and material scientists is currently in progress to understand the fundamental relationship between optical limiting property with many of influencing parameters such as molecular structure etc. Most of the studies are focused on explaining that the large atomic number and the small atomic size may enhance the optical limiting properties of organic compounds. Some are focused on the effect of the conjugation length and the donor acceptor strength on the optical limiting properties of organic NLO materials, However little attention has been paid to the effect of the solvents [7, 8]. The Fluorone dyes (to which Rose Bengal dye belongs) has good photo-thermal stability, dissolvability and easy preparation virtue. Most importantly, its structure can be modified to change its absorption band according to needs. Its potential application is to work as novel optical limiter. By light illumination, Fluorone dye can generate isomerization reaction between trans and cis, which induces photochromic effect. The transition between trans and cis, which is dependent on the wavelength of irradiation light, is reversible and results in the change of the absorption coefficient [9-12].From this viewpoint, this work reports the nonlinear optical properties of Rose Bengal (Acid red 94 dye) in organic solvents of different polarity. The nonlinear absorption coefficient and nonlinear refractive index were obtained by using Z-scan technique. The third-order nonlinear susceptibility was also calculated. And also the optical limiting behavior of the Rose Bengal dye was studied in organic solvents of different polarities and different concentrations.

2

2.1

Experiment

Materials

Fluorone dyes are sub group from Xanthene's dyes. The molecular structure of the Rose Bengal dye (Acid red 94) is shown in the Fig.1a. In general the Fluorone dyes are water- soluble organic compounds that contain colored cation; the intense color of this ion is caused by the extended conjugated system. The UV-VIS absorption spectra of the dye solution in different organic solvents (Distilled Water, Methanol, Ethanol, DMSO)(at 0.01 mM concentration) were recorded using the UV-VIS spectro-photometer (Perkin Elmer-Lambda 35) and are shown in Fig.1c. All the solvents were spectroscopic grade and were used without further purification. All the solutions used for the study were freshly prepared and kept in dark before the measurements. From Fig.1c we can see that the absorption spectra of Rose Bengal dye solutions in all the four species of solvent have a broad absorption band of (490 -560) nm. Also the UV-VIS spectra show that the absorption peaks of the dye solution shift towards the longer wavelength region proportional to the polarity of the solvents (Bathochromic shift).

Figure 1: (a) Fluorone general structure.( b) Molecular structure and the chemical formula of the Rose Bengal Dye. (c) UV-VIS spectra of the Rose Bengal (Acid red 94) Dye in different organic solvents at 0.01 mM concentration (W-Distilled water, M-Methanol, E-Ethanol, D-DMSO) IJNS homepage:http://www.nonlinearscience.org.uk/

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2.2

Z- scan experiment

The principle of the Z-scan technique is based on using a Gaussian laser beam in a tight focusing geometry, and moving the sample under investigation along the beam (along the z-axis) through the focal point. The transmittance in the far field is measured and normalized to 1 for linear absorption, and plotted as a function of sample position along the z-axis, with the focus of the laser beam chosen to be at z = 0. The Z-scan experiments were performed using a 532 nm Nd-YAG (SHG) CW laser beam (COHERENT ­Compass 215M diode-pumped laser) focused by a lens of 3.5 cm focal length. The laser beam waist wo at the focus was measured to be 17.30 µm and the Rayleigh length ZR was 1.76 mm (where ZR is the diffraction length of the laser beam, (ZR =Kw2 /2), and K=2/ is the wave number. o The schematic of the experimental setup used is shown in Fig.2. A 1 mm width quartz cell containing the aqueous solution of Rose Bengal was translated across the focal region along the axial direction, which was the direction of the propagation of the laser beam. The transmission of the beam through an aperture placed in the far field was measured using photodetector fed to the digital power meter (Field master Gs-coherent). For an open aperture Z-scan, a lens replaced the aperture to collect the entire laser beam transmitted through the sample. When the sample was moved from negative Z into focus, initially the beam irradiance was low and negligible nonlinear refraction occurred. Hence the transmittance remained relatively constant. As the sample was brought closer to focus the, beam irradiance increased leading to self-lensing in the sample. A negative self lensing prior to focus collimated the beam and caused beam narrowing at the aperture, which resulted in an increase in transmittance. As the sample was moved away from the focus i.e., towards positive Z the beam divergence caused a decrease in transmittance at the aperture. A pre-focal transmittance maximum (peak) followed by a post-focal transmittance minimum (valley) is the z scan nature of negative nonlinearity. The opposite effect is the nature of positive nonlinearity. The sensitivity to nonlinear refraction is entirely due to the aperture, and the removal of aperture completely eliminates the effect. The third order nonlinear refraction index of the sample (Dye solution) was evaluated from the Z-scan data .The Z-scan was performed for different polarity of organic solvents at different concentrations of the dye solution.

Figure 2: Experimental set-up for the z-scan technique

2.3

Optical limiting experiment:

Limiting with organic materials or devices with transmission that decreases with increasing light level is called optical limiters and has applications in eye and sensor protection. Rose Bengal dye solution in specific solvents exhibits optical limiting due to nonlinear absorption coefficient. The sample was located at the valley point of z-scan curve and since the sample is a negative nonlinear material the valley point was closely behind the focus. The laser of 532 nm irradiated the sample and the power of the transmitted beam corresponding to that of illumination beam was measured and the relation curves between them in different concentrations and solvents are shown in Fig.5 .The input power of the laser beam and the corresponding output power through the aperture were detected by the power meter .The laser output was varied by using variable beam splitter which was mounted in the laser path. When the illumination intensity was small, the output power increased with the increase of input power. When the illumination intensity was greater than the threshold value the output power decreased as the input power increased. IJNS email for contribution: [email protected]

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3

3.1

Results and Discussions

Z- scans results:

Experiment was performed for different solvents and concentrations of the dye solution. A typical closedaperture Z-scan curve of the dye solution (at 0.05 and 0.06 mM concentrations) exhibiting the normalized transmittance is shown in the Fig.3 at incident intensity Io = 7.27 kW/cm2 .The curve is characterized by a pre-focal peak followed by a post-focal valley, which implies that the nonlinear refractive index is negative (n2 < 0).To avoid any discrepancy caused by deviations from an ideal Gaussian profile, all the measurements were taken with the experimental configuration kept identical for all the concentrations of the dye in different organic solvents. The closed aperture Z-scan results reveal that the change in peak to valley normalized transmittance increases with increase of concentration and decrease of solvent polarity and the corresponding third-order nonlinear refractive index also increases (as seen from fig.4). Also the defocusing nonlinearity (negative nonlinearity) is mainly due to the thermally induced changes in the refractive index. The nonlinear refractive index (n2 ) and nonlinear absorption coefficient () values of the dye solution are given in Table 4 and the corresponding 3 values calculated are tabulated in Table 5.The recorded Z-scan curves exhibit a peak-valley distance Zp-v =1.7ZR which confirms the presence of the third order optical nonlinearity .The NLO efficiency at the specific wavelength is of primary importance for accessing the suitability of a material for all-optical integrated devices. This parameter is usually evaluated through the figure of merit given by T=/n2 . It is necessary to achieve T < 1. The dye samples satisfy this condition and hence may have potential application in optical devices. We have shown (Fig.3 a, b) that the nonlinear absorption can be attributed to a saturation absorption process, while the nonlinear refraction leading to self-defocusing in this dye may be due to thermal lensing effect. Measurements were performed on the pure solvents (Distilled Water, Methanol, Ethanol and DMSO) taken in the quartz cell under the same measuring conditions. The experimental results show that pure solvents do not exhibit any nonlinearity contribution. This verifies that the valley and peak in the Z-scan curves originate from the dye molecules and not from the solvent or the quartz cell. All the experiments were done at room temperature.

3.2

Optical limiting results

To characterize the optical limiting performance of Rose Bengal dye solution, we have directly measured the transmitted power as a function of fluence at 532 nm CW laser. The results of Rose Bengal solution dye in four species of solvents are shown in fig.5.The experimentally determined optical limiting threshold values are given in Table 1. Table 1: The optical limiting threshold value of Rose Bengal dye solution at different solvents and concentrations Optical Limiting Threshold Values in mW Concentration 0.05 mM 0.06 mM 0.08 mM 0.1 mM Dye Solution Water 8.10 7.95 7.35 5.44 DMSO 5.75 4.05 3.24 1.72 Ethanol 5.60 3.91 3.12 1.420 Methanol 4.79 3.38 3.02 1.138

3.3

Solvent effect

The physical and chemical properties of organic molecules utilized in different scientific and technological applications can strongly depend on the properties of surrounding media [13,14]. For liquid solutions, the solvent plays a fundamental role in photo physical processes, leading to the modification of the ground IJNS homepage:http://www.nonlinearscience.org.uk/

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Figure 3: Z scan data of the 0.05 mM and 0.06 mm aqueous solution of Rose Bengal at I0 = 7.824kW/cm2 (a) (b) Open aperture scan (S=1) , (c) (d) closed aperture scan (S=0.4), (e) and (f) the division of closed by open Z - scan data.

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Ahmad Y. Nooraldeen, M. Palanichant, P. K. Palanisamy: Influence of Solvents Polarity on · · ·

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Figure 4: Z scan data of the 0.05mM and 0.06 mm solution of rose Bengal at I0 = 7.824kW/cm2 (a) (b) the division closed by open aperture Z- scan data in Methanol solvent (c) (d) the division closed by open aperture Z- Scan data in Ethanol solvent (e) (f) the division closed by open aperture Z-scan data in DMSO solvent.

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Figure 5: Optical limiting response of Rose Bengal in different organic solvents and concentrations (a) In aqueous solution at different concentrations. (b) In ethanol solution at different concentrations. (c) In methanol solution at different concentrations. (d) In DMSO solution at different concentrations. (e) In different solvents at 0.08 mM concentration. (f) The sample in different solvents at 0.10 mM concentration. and excited-state energies of the molecules. The interaction of solute with surrounding solvent leads to solvation effect, which refers to the reorientation of the solvent molecules around a solute molecule. The result of interaction of solute with solvent molecules depends on the nature of arising forces (specific, such as hydrogen bonding, and/or nonspecific, universal interactions) [14], which are determined by charge distribution and polarizability of the solvent and solute molecules. Considerable changes in the energy of solvated solute molecules may occur with changes in the solvent, especially in polar media [15-19]. The max values of Rose Bengal dye derived from the absorption bands in water, DMSO, ethanol and methanol are presented in Table 3. In addition, the absorbance at 532nm in different solvent is also presented in the same table. The max increases from water to methanol and the increase is explained as follows: The absorbance of Rose Bengal dye is due to its quinanoid structure shown in Fig.1b, The quinanoid structure is in conjugation with the adjacent aromatic rings. There is equal probability for both the rings to have the quinanoid structure as shown below:

Figure 6: The probability of changing position of ONA group This conjugation is slightly suppressed in water, due to association of it with ONa group, which is actually extending the conjugation between its own aromatic ring with the quinanoid structure. Hence the electronic excitation due to - * transition occurs at shorter wavelength .In all other solvents such an association with ONa group might not be as pronounced as in water. Actually the conjugation increases with the decrease of the polarity of the solvents. Hence the max of - * transition is shifted to longer wavelengths in DMSO, ethanol and methanol .The absorbance increases from water to methanol and the increase is explained as follows: Since in water the aromatic ring carrying the ONa group is not much conjugated with the quinanoid structure there is less probability to interact with electromagnetic radiation. Conversely in DMSO, ethanol and methanol, the aromatic ring carrying the ONa group is more conjugated with the quinanoid structure and as a result the absorbance gradually increases from water to methanol. Decrease in the polarity of the solvent from water to methanol as indicated in Table 2 is also in support of our suggestions. IJNS email for contribution: [email protected]

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The increase of absorbance at 532 nm from water to ethanol can also be similarly explained .The FWHM increases with the increase of the polarity of the medium. This observation suggests that the difference in the energy spacing between and * for all the dye molecules may not be large in water, i.e. the Rose Bengal dye has nearly similar level of association with water thus providing an absorbance band with less width. In all the other solvents because of less polarity, the association may not be as perfect as that in water. Hence there might be enhanced distribution of energy spacing which results in enhanced broadening of the absorbing band .The NLO property of Rose Bengal dye increases from water to methanol which is a compelling support for the increasing level of conjugative interaction between the rings due to decrease in the polarity of the solvents . According to the results obtained from the Z-scan experiment of the Rose Bengal dye solution in four species of solvent, we found that n2 and of the dye solution are dependent on the surrounding medium parameter, especially on solvent polarity. Fig.7a illustrates the relation between n2 and the polarity of the solvents which has been used in this study at different concentrations, and Fig.8b illustrates the relation between optical limiting threshold value and the solvent polarity at different concentrations. Comparing the optical limiting performance with their corresponding linear absorption at 532 nm, it is observed that the higher the linear absorption, better the limiting performance. Changing the solvent not only affects the linear absorption spectra, but also has a dramatic influence on the nonlinearity and optical limiting characteristics of the organic compound. Fig.7b shows the relation between the values of optical limiting threshold and solvents polarity. Also according to the Z-scan experiment result we find that nonlinear refractive index and Tp-v (peak to valley distance) values of the dye solutions are depending on dye solutions concentrations. Fig.8a, b illustrate the relation between n2 and Tp-v with dye solution concentration in different solvents. Table 2: Some physical properties of the solvents which used in the study Polarity Dipole Linear Dielectric Density Acidity Viscosity index moment ref. index constant gm/lit 9 1.80 D 1.333 80 1.000 7 1.05 7.2 3.96 D 1.479 47 1.092 9 1.99 5.2 1.69 D 1.361 24 0.789 6.91 0.949 5.0 1.69 D 1.328 33 0.791 6.42 0.593

Solvent Water DMSO Ethanol Methanol

Table 3: The spectral parameters of Rose Bengal dye solution in different solvents at 0.01mM concentration Solvent max(nm) Abso. at max Abso. at 532nm FWHM nm Water 540 0.549 0.309 31 DMSO 549 0.620 0.377 34 Ethanol 551 0.656 0.473 37 Methanol 557 0.824 0.776 39

where a represent the Real part of Third order non-linear susceptibility Re 3 (X10-6 esu), b are the imaginary part of the Third order non-linear susceptibility Im 3 (X10-6 esu) and c are the absolute value of the Third order non-linear susceptibility 3 (X10-6 esu).

4

Conclusion

Z-scan and nonlinear transmittance measurements have been carried out on Rose Bengal dye solution in four species of polar solvents using (SHG) Nd-YAG CW laser at 532 nm. The nonlinear optical properties and nonlinear absorption spectra were influenced by solvent polarity. The observed nonlinearities were large and negative in nature .The experimental results show that n2 of Rose Bengal dye solution is strongly dependent on the polarity of the solvent. IJNS homepage:http://www.nonlinearscience.org.uk/

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Figure 7: (a) Relation between nonlinear refractive index Vs Solvents polarity at different concentrations. (b) Relation between the optical limiting threshold values at 0.1 mM concentration Vs solvent polarity

Figure 8: (a) nonlinear refractive index Vs concentration for solution of rose Bengal in different solvents (W: Distilled water, M: Methanol, E: Ethanol, D: DMSO), (b) concentration dependence of the normalized transmittance change Tp-v.

Table 4: Nonlinear ref. Water DMSO Concentration n2 n2 X X X -9 -5 10 10 10-9 2 /W cm/W cm cm2 /W 0.02 mM 5.49 6.50 12.29 0.03 mM 8.94 12.28 15.61 0.04 mM 12.02 13.37 19.22 0.05 mM 13.28 15.90 24.52 0.06 mM 14.83 21.32 25.82

index & nonlinear absorption coefficient Ethanol Methanol n2 n2 X X X X X -5 -9 -5 -9 10 10 10 10 10-5 2 /W cm/W 2 /W cm/W cm/W cm cm 16.62 13.43 27.83 15.64 22.77 21.32 16.58 34.33 17.56 45.18 26.38 20.06 56.38 20.26 67.59 34.69 26.47 63.61 30.28 72.29 41.92 30.02 69.39 33.21 86.02

Water Concentrations a b 0.05 mM 5.98 3.02 0.06 mM 6.68 4.05

Table 5: Third order non-linear susceptibility DMSO Ethanol c a b c a b c 3.07 11.05 6.59 6.68 11.93 12.08 1.213 4.1 11.63 7.96 8.04 13.53 13.18 1.32

Methanol a b c 13.64 13.73 1.37 14.96 16.34 1.64

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Nonlinear transmission studies on different solutions demonstrate that nonlinear absorption contributes to the optical limiting behavior, and the results indicate that it possesses high quality of optical limiting. The reported nonlinearities are primarily thermal in nature owing to the CW excitation based on nonlinear refraction. The nonlinear refractive index is found to be of the order of (10-9 cm2 /W ). The magnitude of third order susceptibility is of the order of (10-6 esu). Their nonlinear absorption coefficient and absorption cross section have also been determined. The dye solution (Rose Bengal) dissolved in methanol behaves as a good optical limiter.

References

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[16] Wouter Verbouwe, Lucien Viaene, Mark Van der Auweraer, Frans C. De Schryver, H. Masuhara, R. Pansu, J. Faure : Photoinduced Intramolecular Charge Transfer in Diphenylamino-Substituted Triphenylbenzene, Biphenyl, and Fluorene . J. Physical chemistry A. 101 (44):8157 - 8165(1997) [17] A.M.Hammam,N.M.Rageh,S.A.Ibrahim: Solvatochromic studies on 2-[(2-hydroxypheny)azo]-4-5diphenylimidazole.Dyes Pig.35(3) :289 - 296 (1997) [18] Lixin Tian ,Jinling Fan, Ruihua Tian: The Attractor on Viscosity Peakon b-Family of Equations. Int.J.Nonlinear Science.4 (3):163-170(2007) [19] Jianlong Wen , Jing Wang, Zhentao Wang, Xiaoying Wang: Electrostatic Charged Two-Phase Turbulent Flow Model. Int.J.Nonlinear Science.5 (1):65-70(2008)

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