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Study of Photo-Induced Dichroism in Sudan III Doped in Poly(Methyl Methacrylate) Thin Films B

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Study of Photo-Induced Dichroism in Sudan III Doped in Poly(Methyl Methacrylate) Thin Films B Vol. 127 (2015) ACTA PHYSICA POLONICA A No. 3 Study of Photo-Induced Dichroism in Sudan III Doped in Poly(Methyl Methacrylate) Thin Films B. Abbasa;* and Y.T. Salmanb aAtomic Energy Commission, P.O. Box 6091, Damascus, Syria bPhysics Department, Faculty of Science, Damascus University, Damascus, Syria (Received December 4, 2013; in nal form December 10, 2014) Linear dichroism, dichroic ratio, contrast ratio and order parameters of Sudan III/PMMA guesthost thin lms have been investigated with visible polarized laser light. Dichroism increased in an exponential fashion with increase of the pump intensity. Light-induced dichroism and polar order of the dye molecules within the polymer network were reversible. These parameters showed fast increase in their values in an exponential fashion as the pumping process takes place. Also, they decay rapidly in an exponential fashion when the pump light is cut o. Photoisomerisation and polar orientation are dependent on both the molecular structure of the dye and polymer. Two forms of Sudan III molecules (trans and cis) are responsible for the interaction with laser light and forming an anisotropic structure inside the PMMA/Sudan III lms. However, there is another set of forms of the dye (keto and enol) has to be considered, which may contribute to dichroism. DOI: 10.12693/APhysPolA.127.780 PACS: 78.20.Ek, 33.55.+b, 78.66.Qn 1. Introduction Sudan III in solvents and doped in polymeric lms using the Z-scan technique. Multiple diraction rings were ob- Photoinduced optical anisotropy in polymer/organic served when the dye doped in both the liquid and solid dyes systems is the subject of intensive care and inves- media was exposed to a diode-pumped Nd:YAG laser tigations [1]. The importance of these nonlinear opti- at 532 nm. On the other hand, De Boni and his col- cal (NLO) materials for applications are quite obvious leagues [7] studied two-photon absorption (2PA) spec- in many elds of modern optics [2], among which, op- tra of Sudan III, Sudan IV and Sudan Red diazoaro- toelectronics, photonics, and optical signal processing. matic dyes dissolved in dimethyl sulfoxide (DMSO) us- Such materials have been used for polarization hologra- ing Z-scan technique with fs pulses. They showed that phy, optical data storage, integrated optics, all-optical the absence of 2PA to the ππ∗ band is related to the modulation, second-order nonlinear optical (NLO) ef- weaker donor/acceptor groups, and lower symmetry of fects [3], optical recording, optically-controlled optical diazoaromatic compounds. Also, the decrease of 2PA elements, optical switches [2] and are promising can- cross-section with temperature is attributed to thermally didates in optical shielding. Azobenzene-based dye induced torsions, which decreases the eective conjuga- molecules form an important set of molecules which tion of diazoaromatic molecules. This kind of optical have vast interest in research and applications alike. investigations indicated the crucial role of the photoin- For example, Schab-Balcerzak et al. [4] described the duced anisotropy in the dye doped systems. formation of supramolecular azopolymers based on hy- drogen bonds as perspective materials for laser in- Anisotropy may be induced either by photoinduced duced surface relief gratings (SRGs) and for polar- isomerization and ordering of the absorbing molecules' ization gratings. They realized supramolecular lms dipoles in directions in which light aects them less [2], on the basis of hydrogen bonds between the func- or by tautomerization where a compound can be rep- tional groups of 4-[4-(3-hydroxypropyloxy)phenylazo]- resented by two structures that are related by an in- pyridine polymer and azobenzene derivative 4-[4-(6- tramolecular movement of hydrogen from one atom to hydroxyhexyloxy)phenylazo]pyridine. Zidan et al. [5] in- another [8]. The induced anisotropy manifests itself in vestigated the optical limiting performance of Sudan III dichroism of long-term stability but can be eliminated dye doped into ethylene propylene dienepolymethylene by either optical or thermal means [9]. Dichroism is a polymer (EPDM) using 532 nm pulses from a frequency- measure of the orientation of the nonlinear optical dye doubled Nd:YAG laser. They found that this dye man- molecules in the host polymer [2]. Dye molecules are well ifests optical limiting eects, with a concentration de- known for their properties of photochromism [10], which pendent eciency. Gayathri and Ramalingam [6] also is a reversible change between two species having dier- investigated the third order nonlinear optical property of ent absorption spectra, which can be induced by photoir- radiation. Because photochromism of organic molecules is sensitive to the micro-environment factors such as po- larity and free volume, the perturbation of reaction me- *corresponding author; e-mail: [email protected] dia on photochromic behaviors oers novel information concerning the microstructures of matrices around the (780) Study of Photo-Induced Dichroism in Sudan III Doped. 781 photoactive molecules [11]. One of the most powerful methods for studying the structure and physical behavior of nonlinear optical guesthost polymeric systems is the measurement of the absorption of linearly polarized light by an ensemble of oriented molecules [12]. To the best of our knowledge, linear dichroism of PMMA/Sudan III thin lms has never been studied before. In this work we report results of photoinduced linear dichroism proper- ties of Sudan III molecules that have been incorporated (dissolved) in PMMA polymeric network. 2. Experimental 2.1. Sample preparation The starting materials for preparing the guest host polymeric samples were poly(methymethacrylate) (PMMA, MW: 36000) from Acros Organics, and Su- dan III (95% dye content), from Aldrich. 2 g of PMMA was dissolved in 20 ml of dichloromethane (C2H2Cl2). Sudan III was added to the clear dissolved solution of PMMA (equivalent to 5% of PMMA by weight). Fig. 1. The experimental setup: P polarizer, ND The mixture was stirred for 3 h until the clear solution neutral density lter, B/S beam splitter, λ/2 half- was obtained with dye molecules fully dissolved. Thin wave plate, and PD photodiode. lms were dip-coated on transparent glass substrates (lm thicknesses of the order 1 µm were measured by to the electric vector of the pump beam become dierent. prism coupling technique and the absorption maximum The absorbances were then calculated from [13]: was at 508.4 nm). Samples were baked in an oven and Ik ◦ held at 70 C for 1 h in order to eliminate the residual sol- Ak = − log (1) vent. Finally, samples were kept in a desiccator at 22 ◦C I0 and in dark environment. The samples' best absorption wavelength was found I? A? = − log ; (2) using UVvisible spectrophotometer (Photodiode Ar- I0 ray Photospectrometer (PDA) Specord S100, from An- where I0 is the intensity of the probe beam when there alytik Jena) to record the absorption spectrum of is no sample, and Ik and I? are the intensities of the PMMA/Sudan III samples. probe beam transmitted through the sample when it is polarized parallel and perpendicular to the pump beam 2.2. Experimental setup polarization, respectively. The linear dichroism is com- puted from [2]: Guesthost polymeric thin lms become dichroic when pumped with linearly polarized beams from an Ar+ laser Ak − A? D = : (3) at λ = 514 nm. The probe signal of the beam falling on Ak + 2A? the sample and subsequently on a photosensor is fed to a personal computer through a low noise current pream- 3. Results and discussion plier (SR570, from Stanford Research Systems), and a Figure 2 shows the chemical structure of PMMA poly- DSP lock-in amplier (SR850, from Stanford Research mer (a), and the chemical structure of Sudan III molecule Systems). An IEEE 488.2 GPIB (from National Instru- and its tautomeric forms (b). Figure 3 shows that the ments) card was used to control and record the experi- maximum absorption wavelength for PMMA/Sudan III mental data along with a special program written in Bor- prepared samples is at λ = 508:4 nm. Since it is most land C++. Figure 1 shows the experimental arrangement suitable for linear dichroism investigations to pump and of the setup. probe the samples with laser wavelengths identical or Dichroism is measured by a probe beam linearly po- close to the maximum absorption of the samples, a wave- larized passing through the sample while it is being sub- length of λ = 514 nm from a multi-line Ar-ion laser ject to the pump beam. The probe beam intensity was (543-MAP-A02, from MellesGriot) was used. A series kept about 0.03% of the pump beam intensity. This is of experiments was carried out in order to elucidate an important procedure to use a very weak probe beam the inuence of linearly polarized light. Samples were with a wavelength identical or very close to the absorp- pumped (one sample at a time) with several intensity tion maximum of the dye molecules where any changes in levels, at which the transmitted probe beam intensities the absorption will be greatest, without inuencing the were recorded simultaneously. Photoinduced changes pumping eect of the sample. In this manner, the probe in the transmitted probe beam intensities are plotted beam absorbance parallel (Ajj) and perpendicular (A?) in Fig. 4. This gure shows the transmission spectra of 782 B. Abbas, Y.T. Salman PMMA/Sudan III thin lms at dierent parallel polar- ized laser intensities of the probe, and depicts the increase in transmitted light intensities with respect to increase in the laser pump intensities. Fig. 4. Transmitted spectra of PMMA/Sudan III at dierent parallel polarized laser intensities. Fig. 2. The chemical structure of PMMA polymer (a), Sudan III molecule and its tautomeric forms (b). Fig. 5. Transmitted spectra of PMMA/Sudan III at dierent perpendicular polarized laser intensities.
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