Photoluminescence properties of copolymers derived ...

Photoluminescence properties of copolymers derived of 3-alkylthiophenes and thiophenes containing NLO chromophores J. Castrellon-Uribe*, M. Güizado-Rodríguez*, C. M. Rueda-Anaya Centro de Investigación en Ingeniería y Ciencias Aplicadas (CIICAp). Universidad Autónoma del Estado de Morelos (UAEM). Av. Universidad No. 1001, Col. Chamilpa, C.P. 62210, Cuernavaca, Morelos, México. ABSTRACT In this works, synthesis and evaluation of the luminescence properties of novel polythiophenes derivatives derived of 3alkylthiophenes (alkyl=hexyl, octyl) and thiophenes functionalized with a NLO chromophore: 2-[Ethyl[4-[2-(4nitrophenyl)ethenyl]phenyl]amino]ethanol are reported. The optical response of copolymers films obtained by spin coating technique when they were excited optically was analyzed. The fluorescence signals measured to about 650 nm when the copolymers were excited at 488 nm (blue light) were studied to different excitation powers. The optical response of the copolymers films shows a change ratio of around 6%/mW. The evaluations of photoluminescence properties of conjugated polythiophenes are important due to their potential applications in light-emitting diodes (LEDs), solar cells and chemical sensors. Keywords: Conducting Polythiophenes; NLO choromophores, Photoluminescence; Fluorescence; laser

1. INTRODUCTION Since conjugated polymers with delocalized π-electrons offer unique physicochemical properties, unobtainable for conventional polymers, significant research efforts directed to better understanding of their chemistry, physics and engineering have been undertaken in the past two and half decades. Conducting polymers (CPs) have been found to have applications as optical, electronic, drug-delivery, memory and biosensing devices such as polymer light-emitting diodes, polymer lasers photovoltaic cells, field-effect transistors, etc.1-3 The greater part of the research study on heterocyclic conjugated polymers has centered on poly(thiophene)s because of their good thermal and environmental stability and easy synthesis and structural versatility. They have certain order that is known as regioregularity: conformational ordering along the backbone, π-stacking of flat polymer chains, and lamellar stacking between chains.4-6 One member of the polythiophene family is the poly(3-alkylthiophene)s. The introduction of alkyl groups longer than butyl to the 3position of the thiophene unit yields moderate to high molecular weight materials soluble in common organic solvents. The length of the alkyl side group affects melting point, conductivity etc. Moreover, the presence of substituents in 3and/or 4-position can produce new materials which combine physical properties characteristic of the polyconjugated backbone with specific properties of a given substituent. For example, the introduction of photochromic chromophore as a side group to the conjugated polythiophene backbone significantly alters electrochemical, spectroelectrochemical, photochromism behavior as well as conductivity and NLO properties of these new polymers as compared to the corresponding parent forms.7-24 Specifically, the functionalization with an NLO chromophore, which contains electrondonor (push) and acceptor (pull) groups bonded to azobenzenic structures, allows one to combine second- and thirdorder NLO properties in one material. Non-linear optical (NLO) materials have been widely investigated because of their potential applications such as in frequency doublers, electro-optical switches and ultrafast devices for information processing, storage and ultrafast devices for information processing, storage and computing.7-24

* [email protected], [email protected]. Phone: +521 7773297084. www.uaem.mx.

22nd Congress of the International Commission for Optics: Light for the Development of the World, edited by Ramón Rodríguez-Vera, Rufino Díaz-Uribe, Proc. of SPIE Vol. 8011, 80113B 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.901968 Proc. of SPIE Vol. 8011 80113B-1 Downloaded from SPIE Digital Library on 11 Nov 2011 to 138.253.100.121. Terms of Use: http://spiedl.org/terms

In this manuscript, we report experimental results of the synthesis and evaluation of the luminescence properties of poly(3-HT-co-3-TPhNO2) and poly(3-OT-co-3-TPhNO2) films which were deposited on substrate glass by spin coating technique. The luminescence response of the copolymers films was studied when they were excited at 488 nm to different optical powers. Besides, atomic force microscopy (AFM) images of the surface morphology of the copolymers films are showed.

2. EXPERIMENTAL PROCEDURE All preparative work was executed in an atmosphere of dry oxygen free nitrogen, using conventional Schlenk techniques. Solvent were carefully dried; Compounds, 3-hexylthiophene and 3-octylthiophene monomers were purchased from Sigma-Aldrich, Biochemika and J.T. Baker and used without further purification. The average molecular number (Mn), weight (Mw), Z average (Mz) and polydispersity (PD) were determined by a size exclusion chromatograph (HPLC 600 Controller Waters 996 photodiode array detector and a PL Gel column (300 mm x 7.5 mm, pore size of 10,000 Å (fractionation range 4000 – 40,000 molecular weight) and a GPC 5UM precolumn). The column was eluted with THF at 1.0 mL/min at 40 oC. The samples concentration was around 2 g/L and the injection volume was 20 μL. Polymer regioregularity was analyzed by 1H NMR Varian Inova 400 MHz. Colymers were dissolved in CDCl3 (ca. 5 mg of polymer in 0.5 mL) in 5 mm (o.d.) tubes and measured at 25 °C with TMS as internal standard. Atomic force microscopy (AFM, nano scope IV multimode scanning probe microscope) was used to analyze the surface morphology of the polymer deposited on glass substrates. 2.1 Synthesis of copolymers Synthesis of copolymer of 2-(ethyl(4-(4-nitrostyryl)phenyl)amino)ethyl 2-(thiophen-3-yl)acetate with 3-hexylthiophene, poly(3-HT-co-3-TPhNO2). The polymerization was carried out in a three-neck flask under constant flow of dry nitrogen. In a typical preparation a solution of 5.7 g (34.3 mmol) of FeCl3 in 80 mL of chloroform and 20 mL of nitromethane was placed into the reactor cooled with a bath of ice with salt. Then a solution of 150 mg (0.3 mmol) of monomer 2-(ethyl (4-(4-nitrostyryl) phenyl) amino) ethyl 2-(thiophen-3-yl) acetate (synthesis of monomer will be described soon in another publication) in 20 mL of chloroform was added in one portion. In the next step a solution of 1.7 mL (9.5 mmol) of 3-hexylthiophene in 50 mL chloroform was added dropwise. The polymerization was let at room temperature with constant stirring and was terminated by addition of 400 mL of methanol. The precipitated polymer was then separated and washed repeatedly with methanol. Since the crude polymer is partially doped, it was treated with a mixture of NH3(aq) /MeOH, 500 mL for 1 h order to dedoped it. The neutral polymer was solved with chloroform in a soxhlet apparatus, after washed with methanol, evaporated the solvent and recovered as a plastic with a small quantity of dichloromethane and finally dried to constant mass, obtaining 1.06 g of pure copolymer. Relation between monomers is 95 % of 3-hexylthiophene and 5 % of functionalized thiophene. The purity of the copolymer is 99.4 %, Mn (g/mol) = 69,544 ± 318, Mw = 86,556 ± 174, Mz = 92,549 ± 82, PD = 1.24 ± 0.004 (confidence interval of 90 %). Configuration of diads (%) = 33 (HH), 67 (HT), of triads (%) = 43 (HT-HT), 18 (TT-HT), 20 (HTHH), 19 (TT-HH).

Synthesis of copolymer of 2-(ethyl(4-(4-nitrostyryl)phenyl)amino)ethyl 2-(thiophen-3-yl)acetate with 3-octylthiophene, poly(3-OT-co-3-TPhNO2). This copolymer was synthesized using the same synthetic procedure as for poly(3-HT-co-3-TPhNO2). Relation between monomers is 98.6 % of 3-hexylthiophene and 1.4 % of functionalized thiophene. The purity of the copolymer is 99.8 %, Mn (g/mol) = 59,394 ± 2467, Mw = 65,728 ± 853, Mz = 69,061 ± 396, PD = 1.11 ± 0.03 (confidence interval of 90 %). Configuration of diads (%) = 24 (HH), 76 (HT), of triads (%) = 58 (HT-HT), 14 (TT-HT), 21 (HT-HH), 8 (TT-HH). The structure of the polymers is showed in Fig. 1. Finally, the polymeric films were deposited on corning glass substrates by spin-coating technique with a spin frequency of 4000 rpm. The solution concentration of copolymers in toluene was 50 mg/mL.

Proc. of SPIE Vol. 8011 80113B-2 Downloaded from SPIE Digital Library on 11 Nov 2011 to 138.253.100.121. Terms of Use: http://spiedl.org/terms

Figure 1. Structure of the copolymers.

2.2 Optical measurements In order to evaluate the optical properties of the copolymers films deposited on a glass substrate (corning glass) by spin coating technique, the procedure was as follows. First, the optical absorbance of the copolymers was measured using a tungsten halogen light source. Then, the absorbance spectrum of the copolymers films was measured at room temperature in the interval from 300 nm to 650 nm of wavelength. Afterwards, in a second optical experiment the luminescence of the copolymers films was investigated. The fluorescence signal of the poly(3-HT-co-3-TPhNO2) film was observed to about 655 nm (red light), whereas to poly(3OT-co-3-TPhNO2) film was at 640 nm, when the copolymers were excited whit a laser at 488 nm (blue light). Next, the fluorescence response of the copolymers films was studied when they were excited to different optical powers. The fluorescence signal of the copolymers films was analyzed at room temperature. The fluorescence spectrums were recorded using a spectrometer (Ocean Optics, Dunedin, Florida, USA).

3. EXPERIMENTAL RESULTS AND DISCUSSION 3.1 UV-Vis Absorbance of polymers films Figure 2 shows the absorbance of the copolymers films. The absorbance spectrum of the copolymers films exhibited a broad absorption band from 350 nm to 650 nm with a maximum peak centered to about 500 nm with a green-shifted absorption. The poly(3-HT-co-3-TPhNO2) film had a maximum optical absorbance of 2.75 at 510 nm; whereas for poly(3-OT-co-3-TPhNO2) film it was about 2 at 503 nm of wavelength. Therefore, according to the results obtained the fluorescence response of the copolymers films, they were excited with blue radiation corresponding to 488 nm. 3.2 Fluorescence spectroscopy of the copolymer films Conductive polymers as the polythiophene (PTh) and their derivatives combines interesting optical properties as non linear optical (NLO) and photoluminescence (PL). Typically, when the polythiophenes are excited with photons of a particular energy, the luminophores are promoted toward a higher energy level. The excited electron returns to the unexcited ground state by a combination of radiative and radiationless processes which arise through several different mechanisms. Emission occurs through the radiative processes called luminescence. The transition from excited level to


the state electronic fundamental occurs through of the photons emission with a longer wavelength (lower energy) than that of excitation. The radiation transition from the lowest excited singlet state to the ground state is called fluorescence. The photoluminescence properties of conjugated polythiophenes as fluorescence efficiency can be controlled by changing the substitution patterns and the steric interactions of the side-chain substituents on the polythiophene backbone. Generally, fluorescence efficiency is determined by the concentration of the polymer solution and the side chain of polythiophene plays an important role in fluorescence quenching which is utilized in chemical sensing application.25-27 Figure 3 shows the fluorescence spectrums of the copolymers films. The fluorescence signal of the poly(3-HT-co-3TPhNO2) was measured at 655 nm, whereas for poly(3-OT-co-3-TPhNO2) film at 640 nm (red light) when the polymers were irradiated whit a laser at 488 nm. A large Stokes shift of about 145 nm is observed for poly(3-HT-co-3-TPhNO2); whereas for poly(3-OT-co-3-TPhNO2) film is of 137 nm with a smaller intensity of fluorescence. The Stokes shift of these copolymers films it exhibit a red-shift.

3,5

Absorbance [u.a.]

3,0

λ1 = 510 nm

2,5 λ2 = 503 nm

2,0 1,5 poly(3-HT-co-3-TPhNO

)

poly(3-OT-co-3-TPhNO

)

2

1,0

2

0,5 0,0 400

500

600

700

800

900

Wavelength [nm]

Figure 2. Measured absorbance spectrums of copolymers thin films.

180 poly(3-HT-co-3-TPhNO ) 2

Fluorescence [u.a.]

160 140

poly(3-OT-co-3-TPhNO ) 2

Excitation λ= 488 nm

λfluor = 655 nm

120 100

λ

80

fluor

= 640 nm

60 40 20 0 400

500

600

700

800

900

Wavelength [nm]

Figure 3. Fluorescence spectrums of copolymers thin films excited at 488 nm.


In order to investigate the optical response of the copolymers films, fluorescence measurements were carried out to different optical powers of excitation. Figure 4 a) depicts the normalized fluorescence spectrum of the poly(3-HT-co-3TPhNO2) film measured at 655 nm when it was excited at 488 nm to different optical powers in the interval of [6 mw – 20 mW]. The fluorescence maximum peak measured in the copolymers films shown here is comparable with previous works. 28 To evaluate the behavior of the fluorescence response of the poly(3-HT-co-3-TPhNO2) film, measurement for ascending and descending optical powers were carried out. Then, the optical response of this film measured at 655 nm to ascending and descending optical powers of excitation is showed in Fig. 4 b). The fluorescence intensity varies roughly linearly with optical power of excitation in the interval from 6 mW to 20 mW and a nearly linear increase in the y-intercepts to a ratio of about 6%/mW. We observed an error of hysteresis, particularly when the measurement is acquired at the same time as optical power of excitation is descending (see Fig. 4 b)). This error could be associated to the inhomogeneous distribution and thickness of the copolymer film on the surface of the corning glass when it was deposited by spin coating technique and also due to degradation of the copolymer thin film. Likewise, the fluorescence response of the poly(3-OT-co-3-TPhNO2) film when it was excited at 488 nm of wavelength was investigated. Figure 5 depicts the fluorescence signal of the copolymer film measured at 640 nm and also their behavior under ascending and descending excitation in the interval of [6 mw – 20 mW]. The results obtained are comparables with the other copolymer film but a minimum difference in the Stokes shift of about 10 nm is observed between them. Besides, the fluorescence quantum efficiency of poly(3-OT-co-3-TPhNO2) film is 50% smaller and with a change ratio of about 5%/mW compared to its analogue. Typically, the fluorescence intensity is greater in liquids than in the films. In solution, the distance between neighboring molecules is too large to allow intermolecular interaction, whereas in films the distance is small enough that excitation energy transfer (EET) can occur. It is important to notice that one must distinguish that the EET occurs before the emission of the photon whereas that the self-absorption after photon-emission and is therefore depending on the optical density and geometry of the sample. In solutions, quenching effects are depending of the concentration which causes a decrease in the fluorescence intensity. On the other hand, two effects can be considered to be responsible for luminescence quenching in films: static quenching by formation of aggregates in the unexcited ground state and collisional quenching due to interaction in the excited state. Nevertheless, in our experiments the fluorescence response of both copolymers films is gradually increased without showing a significant fluorescence quenching when the films were irradiated at 488 nm for different optical powers of excitation.

120

120

a)

100

5.88 6.88 9.12 9.95 10.88 12.02 12.84 14.02 14.88 15.84 17.01 18.18 19.08 20.10

Excitación λ= 488 nm

60

40

20

Fluorescence [%]

Fluorescence [%]

100

80

b)

Excitation power [mW]

λ= 655 nm

Ascending optical power Descending optical power

80

λ = 655 nm

60

40

20

poly(3-HT-co-3-TPhNO ) 2

0

0

400

500

600

700

800

900

1000

0

2

4

6

Wavelength [nm]

8

10

12

14

16

18

20

22

24


Figure 4. Normalized florescence signal of the poly(3-HT-co-3-TPhNO2) film. (a) Fluorescence spectrum measured at 655 nm under excitation at 488 nm, (b) fluorescence response for ascending and descending optical power of excitation.


120

120 a)


λ= 640 nm

5.88 6.88 9.12 9.95 10.88 12.02 12.84 14.02 14.88 15.84 17.01 18.18 19.08 20.10

80

Excitation λ= 488 nm

60

40

b)

100

Fluorescence [%]

Fluorescence [%]

100

Ascending optical power Descending optical power

80

λ = 640 nm

60

40

20

20

0

0

poly(3-OT-co-3-TPhNO ) 2

400

500

600

700

800

900

1000

0

2

4

6

Wavelength [nm]

8

10

12

14

16

18

20

22

24


Figure 5. Normalized florescence signal of the poly(3-OT-co-3-TPhNO2) film. a) Fluorescence spectrum measured at 640 nm under excitation at 488 nm, b) fluorescence response for ascending and descending optical power of excitation.

3.3 Analysis of the morphology of copolymer films In the poly(3-HT-co-3-TPhNO2) films, we observed spherical particles over a flat surface, regularly distributed. Those particles have an average height of 338 ± 41 nm and a width of 4.45 ± 0.40 μm (confidence interval of 99 %), with a roughness of 70.2 nm. This morphology is characteristic of block copolymers. In the case of poly(3-OT-co-3-TPhNO2), we founded smaller particles, and a flat surface which is proven by the roughness of 12.9 nm (see Fig. 6). The thickness of the films is about 120 nm. (a)

(b)

40

Figure 6. Atomic force microscopy images of the surface morphology of the copolymers films chemically deposited on glass substrate by spin coating technique. (a) poly(3-HT-co-3-TPhNO2), and (b) poly(3-OT-co-3-TPhNO2).

4. CONCLUSIONS We have presented experimental results of the synthesis and evaluation of the luminescence properties of poly(3-HT-co3-TPhNO2) and poly(3-OT-co-3-TPhNO2) films which were deposited on substrate glass by spin coating technique. We have determined that the fluorescence signal of the first copolymer film is found at 655 nm which exhibit a Stokes shift of 145 nm, whereas for the second copolymer film at 640 nm (red light) with 137 nm and with a smaller intensity signal when they were irradiated whit a laser at 488 nm (blue light). The behavior of the fluorescence response of the copolymers films was studied under excitation at 488 nm to different optical powers in the interval of [6-20 mW]. The fluorescence response of the copolymers varies roughly linearly with optical excitation to a ratio of about 6 and 5%/mW


respectively for ascending and descending optical power and a minimum error of hysteresis is observed which could be associated with the distribution of the copolymer on surface of the glass, thickness and degradation of the copolymers thin films. The behavior of the photoluminescence signal is associated with the percentage of functionalized thiophenes in the copolymers (5 % vs 1.4 %), which indicates a larger proportion of active chromophores in the derivative of 3hexylthiophene. Also, the polydispersity (1.24 vs 1.11) is greater in this copolymer which is related to the proportion of monomers mentioned above. Thus, we have evaluated the photoluminescence properties of novel NLO conjugated polythiophenes which show potential applications in optoelectronic devices.

ACKNOWLEDGMENTS The authors thank Daniel Bahena Uribe for his help with AFM images. This project was funded by REDES PROMEP second year project, PROMEP-UAEM-PTC92, and CONACYT CB2007-81383-Q.

REFERENCES [1] Pron, A. and Rannou, P., “Processible conjugated polymers: from organic semiconductors to organic metals and superconductors,” Prog. Polym. Sci. 27(1), 135-190 (2002). [2] Saxena, V. and Malhotra, B. D. “Prospects of confucting polymers in molecular electronics,” Current. Appl. Phys. 3(2-3), 293-305 (2003). [3] Cheng, Y.-J., Yang, Sh.-H. and Hsu, Ch.-Sh. “Synthesis of Conjugated Polymers for Organic Solar Cell Applications,” Chem. Rev. 109, 5868-5923 (2009). [4] Woo, C. H., Thompon, B. C., Kim, B. J., Toney, M. F. and Fréchet, J. M., “The Influence of Poly(3hexylthiophene) Regioregularity on Fullerene-Composite Solar Cell Performance,” J. Am. Chem. Soc. 130 (48), 16324-16329 (2008). [5] Osaka, I. and McCullough, D., “Advances in Molecular Design and Synthesis of Regioregulr Polythiophenes,” Acc. Chem. Res. 41 (9), 1202-1214, 2008. [6] Zhai, L., Pilston, R. L., Zaiger, K. L. Stokes, K. K. and McCullough, R. D., “A Simple Method to Generate Side-Chain Derivatives of Regioregular Polythiophene via the GRIM Metathesis and Post-polymerization Functionalization,” Macromolecules 36, 61-64, 2003. [7] Lévesque, I. and Leclerc, M., “Novel Dual Photochromism in Polythiophene Derivatives,” Macromolecules 30, 4347-4352 (1997). [8] Zagórska, M., Kulszewicz-Bajer, I., Proń, A. and Sukiennik, J., “Preparation and Spectroscopic and Spectroelectrochemical Characterization of Copolymers of 3-Alkylthiophenes and Thiophene Functionalized with an Azo Compound,” Macromolecules, 31, 9146-9153 (1998). [9] Zagórska, M., Kulszewicz-Bajer, I., Proń, A., Raimond, P., Kajzar, F. and Attias, A.-J., “Polythiophenes functionalized with Disperse Red 1 chromophore,” Synth. Met. 102, 1141-1142 (1999). [10] Samyn, C.; Verbiest, T. and Persoons, A., “Second-order non-linear optical polymers,” Macromol. Rapid. Comun. 21, 1-15 (2000). [11] Della-Casa, C., Fraleoni, A., Costa-Bizzarri, P. and Lanzi, M., “New 3-alkylthiophene copolymers functionalized with a NLO chromophore,” Synth. Met. 124, 467-470 (2001). [12] Abd-EI-Aziz, A. S., Afifi, T. H., Budakowski, W. R., Friesen, K. J. and Todd, E. K. “The First Example of Cationic Iron-Coordinated Polyaromatic Ethers and Thioethers with Azo Dye-Functionalized Side Chains,” Macromolecules 35 (24), 8929-8932 (2002). [13] Lanzi, M., Paganin, L., Costa-Bizarri, P., Della-Casa, C. and Fraleoni, A., “Facile Synthesis of Soluble Multifunctional Polyalkylthiophenes,” Macromol. Rapid Commun. 23, 630-633 (2002). [14] Della-Casa, C., Costa-Bizzarri, P., Lanzi, M., Paganin, L., Bertinelli, F., Pizzoferrato, R., Sarcinelli, F. and Casalboni, M., “Monomers of 3-alkyl-substituted thiophene: synthetic routtes for the functionalization with non-linear optical chromophores,” Synth. Met. 138, 409-417 (2003). [15] Lanzi, M., Paganin, L. and Costa-Bizzarri, P., “Synthesis and polymerization of a new thiophene functionalized with both NLO-active chromophore and an alkylic self-plastifying chain,” Eur. Polym. J. 40, 2117-2127 (2004).


[16] Della-Casa, C., Fraleoni-Morgera, A., Lanzi, M., Costa-Bizzarri, P., Paganin, L., Bertinelli, F., Schenetti, L., Mucci, A., Casalboni, M., Sarcinelli, F. and Quatela, A. “Preparation and characterization of thiophene copolymers with second order non-linear optical properties,” Eur. Polym. J. 41, 2360-2369 (2005). [17] Gon alves, V. C. and Balogh, D. T. “Synthesis and characterization of a dye-functionalized polythiophene with different chromic properties,” Eur. Polym. J. 42, 3303-3310 (2006). [18] Lanzi, M., Bertinelli, F., Paganin, L., Costa-Bizzarri, P. and Cesari, G. “Electronic Transitions of Polyalkylthiophenes Partially Derivatized with NLO Chromophores: A Theoretical and Experimental Study,” Macromol. Chem. Phys. 207, 1253-1261 (2006). [19] Caruso, U., Diana, R., Fort, A., Panunzi, B. and Roviello, A. “Synthesis of Polymers Containing Second Order NLO Active Thiophene and Thiazole Based Chromophores,” Macromol. Symp. 234, 87-93, 2006. [20] Batista, R. M. F., Costa, S. P. G., Belsley, M. and Raposo, M. M. M. “Synthesis and second-order nonlinear optical properties of new chromophores containing benzimidazole, thiophene, and pyrrole heterocycles,” Tetrahedron 63, 9842-9849 (2007). [21] Kim, D. W., Choi, J. J., Lim, J.-S., Lee, Ch. “Push-Pull Chromophore with Phenylene and Thiophene as Conjugation Bridge for Electro-Optic Applications,” Mol. Cryst. Liq. Cryst. 463, 43/[325]-53/[335] (2007). [22] Ma, X., Liang, R., Yang, F., Zhao, Z., Zhang, A., Song, N., Zhou, Q. and Zhang, J. “Synthesis and properties of novel second-order NLO chromophores containing pyrrole as an auxiliary electron donor,” J. Mater. Chem. 18, 1756-1764 (2008). [23] Shi, Z., Zhang, X. and Cui, Z. “Synthesis and characterization of thiophene-containing chromophores for nonlinear optical (NLO) materials,” 17(3), 243-254, 2008. [24] Zhang, B.-z. and Zhao, X.-y. “Synthesis, characterization, and photochromic behaviors of polythiophene derivarites in the solid state,” J. Mater. Sci. 44, 2765-2773 (2009). [25] Maiti, J. and Dolui, S. K., “Photoluminescence properties of poly(thiophene-3yl-acetic acid 8-quinolinyl ester) in solution and in acid medium,” J. Lumin. 129, 611-614, 2009. [26] Lodi, A., Caselli, M., Zanfrognini, B., Cagnoli, R., Mucci, A. and Parenti, F. “Strategies to reduce inter-chain aggregation and fluorescence quenching in alternated multilayers of a polythiophene” Thin Solid Films 516, 8731-8735 (2008). [27] Lu, P. and Xia, G. M., “Linear and nonlinear luminescence properties of thiophene based materials,” 70 (2), 201-208 (2005). [28] Zhao, X. Hu, X. and Huat Gan L. “Photoluminescent behavior of poly(3-hexylthiophene) derivatives with a high azobenzene content in the side chains”, Polym. Adv. Technol. 16, 370–377, 2005.
