J Fluoresc DOI 10.1007/s10895-015-1730-y
ORIGINAL ARTICLE
Studies of Luminescence Performance on Carbazole Donor and Quinoline Acceptor Based Conjugated Polymer Anjali Upadhyay 1 & Karpagam S 1
Received: 22 September 2015 / Accepted: 26 November 2015 # Springer Science+Business Media New York 2015
Abstract We report on the synthesis of conjugated polymer (CV-QP) containing carbazole (donor) and quinoline (acceptor) using Wittig methodology. The structural, optical and thermal properties of the polymer were investigated by FT-IR, NMR, GPC, UV, PL, cyclic voltammetry, atomic force microscopy (AFM) and thermogravimetric analysis (TGA). The polymer exhibits thermal stability upto 200 °C and shows good solubility in common organic solvents. The polymer has optical absorption band in a thin film at 360 nm and emission band formed at 473 nm. The optical energy band gap was found to be 2.69 eV as calculated from the onset absorption edge. Fluorescence quenching of the polymer CV-QP was found by using DMA (electron donor) and DMTP (electron acceptor). AFM image indicated that triangular shaped particles were observed and the particle size was found as 1.1 μm. The electrochemical studies of CV-QP reveal that, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the CV-QP are 6.35 and 3.70 eV, which indicated that the polymers are expected to provide charge transporting properties for the development of polymer light-emitting diodes (PLEDs).
Keywords Carbazole . Quinoline . Wittig reaction . Cyclic voltammetry . Photophysical properties
* Karpagam S
[email protected] 1
Organic Chemistry Division, School of Advanced Science, VIT University, Vellore -14, Tamil Nadu, India
Introduction Conjugated polymers are very important class of materials in electro active and photoactive application. In particular, carbazole and its derivatives have attracted much attention because of owing high thermal stability, good solubility, extended glassy state. The nitrogen atom present in the carbazole can be easily functionalized with a large variety of substituent to modulate the properties without increasing the steric interactions and aromatic configuration provides good chemical and environment stabilities. Direct oxidative polymerization or electropolymerization of carbazole results in polymeric materials linked at 3, 6-positions [1]. In 1977, the first evidence of high electrical conductivity carbazole doped polyacetylene [2] generate a second life for polycarbazole derivatives as were many other aromatic polymers (polyaniline, poly(2,7fluorene), poly(p-phenylene), polythiophene, polypyrrole, etc. [3] Poly(3,6-carbazole)s were intensively investigated and widely used as organic photo-electronic materials, good photoconductors [4] and hole transporting active ingredients in electronic devices such as organic light emitting diodes (OLED)s [5] and DSSCs [6] due to their unique charge transporting properties and pronounced thermal stability. Carbazole is an attractive building block as it offers many nuclear sites for functional group incorporation [7]. Another π conjugated polymeric systems contain polyquinolines, which has excellent thermal stability, high mechanical strength and excellent electrical insulators. This was reported to exhibit electrical conductivity as high as 10 S/cm when doped with electron donors [8]. Quinoline containing conjugated polymer are useful in fabrication of light emitting devices employing them as electron –transporting layer [9], sensitizers in dye –sensitized solar cells [10], application in chemical sensors [11] and compatibilizer [12]. Functional quinoline have been mainly developed due to their
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broad optical absorption, bright photoluminescence and bipolar transport characteristics. There is a growing interest in the design and synthesis of quinoline enriched with functional chromophore to enhance their excellent emission and thermal properties [13]. Optical and electronic properties of polyquinolines have been studied extensively and their use in light emitting diodes which has shown excellent n-type (electron transport) properties [14]. Recently, the optoelectronic properties such as photoconductivity, photoluminescence, and nonlinearity of polyquinolines have been investigated by Jenekhe and co-workers [15]. The fluorescence method is useful for investigating the interaction between small probe molecules with human serum protein [16–19] amino acids or nucleotide analogues [20]. The degree of fluorescence quenching suggests changes in the protein or polymer structure with the microenvironment. The main target of research in conjugated polymer is to enhance electronic, optoelectronic or nonlinear optical properties with excellent mechanical properties, high thermal stability and good processibility [21]. In this regard, a wide range of conjugated polymers such as poly(p-phenylene vinylene) (PPV), poly(thiophene) (PT), poly(pyrrole) (PP), poly (p-phenylene) (PPP), poly(fluorine) (PF) and their derivatives have been investigated [22]. Alternating electron rich and electron deficient substituent in the macromolecular backbone are well known approach to obtain efficient devices. The work concerns design and synthesis of donor-acceptor (D-A) conjugated polymer by introducing highly electron donating carbazole moiety and electron accepting quinoline moiety into the polymer network. Vinylene groups were chosen as the side chains in carbazole unit to enhance the pi conjugation by delocalizing electrons along the backbone, furthermore, vinylene groups offer a more flexible polymer chain, which could improve solubility compared to directly coupled aryl-aryl conjugated polymers. The structure of the polymer has been established by spectral techniques. Optical and electrochemical properties of the polymer were studied and quenching performance was also studied thoroughly.
Experimental Section Materials Carbazole, 2, 6-dimethylquinoline, phosphorus oxy chloride, Allyl bromide, Triphenylphosphine and N-bromo succinimide (NBS) were purchased from Sigma Aldrich, Mumbai, India. Benzoyl peroxide, chloroform, acetonitrile, methanol, acetone, benzene, carbon tetrachloride, N, N-dimethyl formamide were purchased from SD fine chemicals, Chennai, India.
Characterization 1
H NMR spectra were recorded on Bruker 400 MHz spectrometer using CDCL3 as the solvent. FT-IR spectra were measured as KBr pellets with a midrange (500–4000 cm−1) using a JASCO-V-670 spectrometer. UV-Vis spectra were carried out on a Hitachi U2910 spectrophotometer and the photoluminescence spectra were taken on a Hitachi F7000 spectrofluorometer. The exact molecular weight of the intermediates was measured from Perkin Elmer 680 mass spectrometer with high resolution data system. A cyclic voltammogram waves obtained using a CH instrument, with a 0.1 M Tetrabutylammonium hexafluorophosphate (BU4NPF6) as the supporting electrolyte at a scan rate of 50 m V/s. Indium tin oxide (ITO), Pt wire and Ag/AgCl were utilized as the working electrode, counter electrode and reference electrode respectively. The HOMO and LUMO levels of the polymer were determined using the oxidation and reduction onset value. Molecular weight of the final polymer was obtained by using GPC Waters cooperation RI Detector 2414. TGA measurements were performed on a SEIKO TG DTA 7000 instrument under nitrogen flow condition at a rate of 30 cm3 min−1 and a heating rate of 10 °C min−1. Topographic images of the active layers were obtained through atomic force microscopy (AFM) in a tapping mode under ambient conditions using Nano Surf Easy scan II instrument. AFM measurements were performed on thin polymer films on glass slides (10 × 20 × 1 mm) This was prepared by using programmable spin coating system (Spin-NXG –P1). A solution of 1 mg polymer in 1 ml chloroform is used with the spin- rate of 2000 rpm. The film was left to evaporate solvent for up to1hr [23].
Preparation of 9-allyl-9 H-carbazole-3, 6-dicarbaldehyde (Monomer 1) Monomer 1 was prepared by two step procedure. Firstly, 9allyl carbazole was prepared from carbazole (3.0 g, 18 mmol) and allyl bromide (3.2 g, 27 mmol). This was mixed with 20 ml DMF containing sodium hydride (0.86 g, 36 mmol) and heated at 80 °C under stirring. The solution was allowed for 14 h and then washed with 150 ml water after cooling to ambient temperature. The compound was evaporated and purified by column chromatography (Silica gel; hexane/ethyl acetate 20/1) gave a brown powder (Yield: 74 %). Secondly, prepared 9-allyl carbazole (2.0 g, 10 mmol) was dissolved with phosphorus oxy chloride (1.85 ml, 20 mmol) and 3 ml of dimethylformamide. This solution was refluxed for an 18 h. After completion of the reaction, it was poured into a large amount of water and the product was extracted with chloroform. The organic layer was dried over magnesium sulfate and evaporated in vaccum. The final residue (monomer-1) was separated by silica-gel column
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chromatography using hexane–ethyl acetate (10:1) as eluting solvent. Yield: 55.2 %; m.p. 140 °C, Light brown crystals grew over a period of one week. FTIR (KBr, cm−1), 1680 (C = O), 2840 (N-CH2); 1 H-NMR (CDCl3, ppm): δ 10.02(s, 2 H, CHO), 4.28 (d, 2 H, N-CH 2 ); MS calculated for C21H23NO2 (m/z) = 263.1, found 263.2. Preparation of 2, 6-bis (bromo methyl quinoline) 2, 6-dimethylquinoline (1.57 g, 10 mmol) and N-bromo succinimide (NBS) (3.64 g, 20 mmol) were dissolved in the solvent mixture of carbon tetrachloride (CCl4) and benzene (2:1). A small amount of benzoyl peroxide was added as an initiator. The solution was refluxed for 18 h at 70 °C under nitrogen atmosphere. Upon cooling, a part of the product precipitated along with succinimide. The precipitate was filtered off and washed with CCl4 to redissolve the product that had precipitated out. Two filtrates were combined and solvents were evaporated to obtain yellow solid wetted by red oil. The solid was washed with cold methanol to remove red oil and a light yellow solid was obtained. It was further washed with water and recrystallized with methanol to obtain pure product. Yield: 68 %; m.p. 120 °C. FTIR (KBr, cm−1): 3082 (aromatic-CH), 1641 (aromatic C = N), 1400 (aromatic C = C), 559 (CH2Br); 1H NMR (CDCl3, ppm): δ 7.71 (t, J1 = 1.8 Hz, J2 = 4.8 Hz, 4 H), 7.4–7.6 (d, J = 8.4 Hz, 4 H), 4.54 (s, 4 H). MS calculated for C11H9Br2N (m/z) = 315, found, 315.90. Preparation of 2, 6-dimethyl (triphenyl phosphonium dibromo-methyl) Quinoline (Monomer 2) In a 250 ml three necked round bottom flask, 2, 6-bis (bromomethylquinoline) (0.31 g, 1 mmol) and triphenylphosphine (0.52 g, 2 mmol) were dissolved with 15 ml of acetonitrile. The solution was stirred overnight at 40 °C under nitrogen atmosphere. The resulting brown precipitate was recrystallized from toluene-methanol mixture (2:1 ratio). Yield: 66 %; m.p. 110–113 °C. FTIR (KBr, cm−1): 3045 (aromatic-CH), 1641 (aromatic C = N), 1471 (aromatic C = C), 758(C-P), 694 (C-Br), 511(P-Br); 1H NMR (CDCl3, ppm): 7.3, (3 H aromatic), 4.6 (4 H, CH2Br). MS calculated for C47H39Br2NP2 (m/z) = 837.1, found, 837.23. Preparation of Quinoline and Carbazole Substituted Vinylene Polymer (CV-QP) Monomer-1 (0.052 g, 0.2 mmol) and monomer-2 (0.167 g, 0.2 mmol) was dissolved with 15 ml of methanol and 5 ml of chloroform. This was mixed with calculated quantity of sodium methoxide in 10 ml of methanol under a nitrogen atmosphere [24]. The resulting solution was stirred at 50 °C
overnight. Precipitation occurred by adding little amount of methanol, which was reprecipitated from dichloromethanemethanol mixture. After filteration and removal of solvent, the crude product was purified via silica gel column chromatography (eluent: ethyl acetate/n-hexane = 1/4) gave 1.2 g (68 %) as green solid compound. FTIR (KBr, cm−1), 3049 (aromatic CH), 1469 (aromatic C = C), 1624 (phenyl), 995 (trans-vinylene). 1H NMR (CDCl3, ppm):1.10 (m, aliphatic protons), 7.35 (m, phenylene), 6.95 (trans vinylene) 7.18 (m, vinylene), 8.01 (quinoline). 13C NMR (CDCl3, ppm): 131.88 (Phenyl carbons), 129.89 (vinylene carbons), 135.93, 156.17 (quinoline).
Results and Discussion Synthesis and Characterization of Monomers and Polymers The monomer 1 was synthesized from 9-allyl-9 H-carbazole3, 6-dicarbaldehyde in the presence of phosphorus oxy chloride by the well-known Vilsmeier reaction in a way similar to that reported literature [25]. The monomer 2 was prepared from bis (2, 6-bromo methyl) quinoline with triphenylphosphine. The final polymer (CV-QP) was synthesized by the Wittig polymerization with carbazole dicarbaldehyde (monomer 1) and quinoline diphosphonium salt (monomer 2) which is shown in Scheme 1. The structures of the intermediates and the target polymer confirmed by 1H NMR and FTIR spectroscopic measurements. The structure of monomer 1 was further confirmed by FTIR and NMR analysis. The chemical shift of aldehyde protons formed at 10.02 ppm that respond to aldehydic proton and its FTIR spectrum showed a strong peak at 1680 cm−1, which confirms the presence of aldehyde proton. Firstly, bromomethyl quinoline was prepared by using NBS reagent and benzoyl peroxide. Presence of bands at around 559 cm−1, 3082 cm−1 in the FT-IR spectrum clearly indicated that the functional groups of C-Br and phenyl nucleus. This fact was further confirmed by appearance of signals at 4.5 ppm (CH2 protons attached in heterocycles and bromo group) in 1H NMR spectra. The exact mass of the compound from MS spectrum was found as 315.90. The monomer 2 (phosphonium salt) was obtained from bromomethyl quinoline and triphenylphosphine in an overall yield of 82 %. The structure of the monomer 2 was confirmed by appearance of strong bands at 758 and 694 cm−1 (C-P and P-Br stretching frequency) in IR spectra and multiplet at 7.5– 7.8 ppm (aromatic hydrogens of phosphonium salt) in 1H NMR spectra. Methylene signal of 4.6–4.8 ppm was shifted to 2.5–2.7 ppm (1 H NMR) and 28.3–33.6 ppm was shifted to 49.6–52.2 ppm (13C NMR) which indicates –CH2 group attached with phosphonium salt was confirmed.
J Fluoresc Scheme 1 Synthetic routes of carbazole and quinoline functionalized conjugated Polymer (CV-QP)
Characterization of CV-QP The synthetic procedure of the polymer (CV-QP) was sketched in Scheme 1. The present polymer (CV-QP) was prepared from the carbazole dicarbaldehyde monomer (Monomer-1) and quinoline phosphonium salt (Monomer-2) in a mixed solvent (ethanol/chloroform) using the well-known Wittig reaction. Sodium methoxide was selected as a base and used to deprotonate the phosphonium salt. The chemical structure of CV-QP was verified by FT-IR, NMR spectroscopy and GPC chromatography. The FT-IR spectra revealed (Fig. 1) that CV-QP gave a sharp peak at around 995 cm−1 assigned to the trans-vinylene out-of-plane stretch which suggested that the configuration of trans-vinylene was dominant among the newly formed vinylene double bonds. The 1H NMR spectrum of CV-QP in CDCl3 solvent is shown in Fig. 2. In the 1H NMR spectrum, proton signals formed at 6.755–8.019 ppm may be assigned to each of the corresponding hydrogen present in quinoline and carbazole ring in the polymer. The signal formed at 6.95 is due to the presence of trans vinylene present in the polymer (CV-QP). These aromatic carbon regions were also found in 13C NMR spectroscopy (Fig. 3). Carbazole, quinoline and vinylene
carbon regions were noticed in the range of 110–156 ppm. From the GPC analysis, the polymer exhibited the average molecular weight was 3455 which fall within a reasonable range expected from Wittig condensation reaction. This was more compared with the literature related to quinoline containing OPV [26]. The polymer was soluble in common organic solvents, such as tetrahydrofuran (THF), dichloromethane (DCM), chloroform, toluene, methanol and ethyl acetate at room temperature. These factors are employed to improve the interaction between polymer and solvent molecules. So, optically clear films were prepared easily by spin coating technique. Optical Characterization Absorption Spectra The UV-vis absorption spectra of CV-QP in different solvents such as, THF, chloroform, ethyl acetate, methanol and DMF and thin film are shown in Fig. 4a respectively. Study of the effect of solvents on the electronic absorption play an important role in the photophysics of the excited state. The magnitude of the spectral shift in various solvents (with different
J Fluoresc Fig. 1 FT-IR spectra of CV-QP
polarities) mainly depends on the strength of the intermolecular hydrogen bond (s) or spectral shift may be ascribed, due to the specific solute-solute and solute-solvent interaction in the form of hydrogen bonding [27]. As the solvent polarity increases from moderately polar to very polar, lowest energy absorption maxima was obtained undergo a significant red shift accompanied by spectral shape changes. Positive solvatochromism i.e. a bathochromic shift of the absorption band with increasing solvent polarity was observed indicating
Fig. 2 1H NMR spectra of CV-QP
the charge transfer nature of the absorption band and that excited state of each oligomer is more stabilized than the ground state [28]. The absorption spectra of CV-QP in the different solvents are characterized by two bands, one near 250–290 nm (broad band) and other at around 310–340 nm (Sharp band). The former band can be assigned to π- π* electronic transition occurred in the conjugated system whereas the lowestenergy band, which is less intense, is largely of charge transfer
J Fluoresc Fig. 3 QP
13
C NMR spectra of CV-
character. It is this lowest-energy band that is most sensitive to the solvent polarity and to the donor moiety (carbazole). This confirms intra molecular charge transfer (ICT) effects strongly occurred in carbazole containing polymer. From the Fig. 4a, it is observed that the polymer solution has apparently shifted the wavelength more too blue than thin film (360 nm) which may be attributed to the expansion of polymer chain in the solvent. It was explained that the electronic character of a substituent influences the optical properties of the polymers. The introduction of electron donating and
accepting groups directly affects the conjugation length and HOMO and LUMO levels of the polymers. It is clear that carbazole and quinoline unit modulated the absorption curves of the oligomer. The absorption onset wavelength of this polymer in film is 460 nm, which corresponds to a band gap 2.69 eV. The optical band gap (E opg Þ of the present polymer is lower as compared to the carbazole containing polymer [29] this is may be due to a possible photo-induced electron transfer from the carbazole donor to quinoline –acceptor in the excited state.
Fig. 4 a UV-Vis absorption b PL spectra of CV-QP with various solvents and Film
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Photoluminescence Spectra Emission behaviour of the electronically excited compounds was also examined in a series of solvents with varying polarity to identify the impact of solvent polarity on the excited state. Fluorescence band maxima are more red shifted when the solvent polarity increases compared to absorption spectra under the same condition. This fact indicates an increase in dipole moment of excited state compared to ground state. Fig. 4b represents the photoluminescence (PL) spectra of the polymers with various solvents. The fluorescence emission spectra were obtained by irradiative excitation at the wavelength of the absorption maximum. The polymers exhibited a strong solvatochromic effect [30]. The emitting wavelengths of the PL polymers depend on the structure of the polymers. The emission band was formed in the range of 385–450 nm with the different solvents. Fluorescence emission maximum shifted to longer wavelength when compared with the literature based on carbazole linked conjugated polymer [31]. This is mainly due to electronic character of substituent present in the carbazole moieties which is responsible for the higher electron-donating character [32]. Transparent and uniform films of the polymers were prepared on quartz plates by spin –coating at room temperature. A thin film PL spectrum of the polymer was shown in Fig. 4b. The maximum emission wavelength for thin film was observed at 473 nm. This was red shifted (bathochromic shift) compared with the solution spectrum by 32 nm.This may be due to the π- π* interchain interaction or aggregation of πconjugated polymer chains in solid state results in a higher conjugation chains in the solid polymers, leading to a red shift in the emission spectrum relative to the solution state. Stokes shift is the difference between PL and UV-Vis absorption peaks. If the stokes shift is too small, the emission and absorption spectra will overlap more. Then the emitting light will be self-absorbed and the luminescent efficiency will decrease in the devices. In our study, stokes shift of the present polymer was found to have 104 nm. The larger stokes shift and less selfabsorption of emitting light were observed. This was suggested as best material for organic light emitting diodes [29, 33]. Electrochemical Studies Cyclic voltammetry was used to estimate the HOMO and LUMO energy of the polymers. The cyclic voltammogram of the polymer coated on a glassy electrode by evaporating the chloroform solution of the polymer, using a Pt counter electrode and a Ag/AgCl reference electrode, immersed in the electrolyte with 0.1 M tetra butyl ammonium-hexa fluorophosphates in acetonitrile at a scan rate 25 mV/S. Cyclic voltammograms cycles remain unchanged under multiple successive potential scans, indicating its excellent stability against electrochemical oxidation as shown in Fig. 5, while
Fig. 5 Cyclic voltammogram of CV-QP
sweeping catholically, the polymer showed a reduction peak at around −0.70 V. These reduction potentials are lower than those of thiophene containing oxadiazole [34]. In the anodic sweep, polymer showed an oxidation peak at around +1.95 V. The onset potential processes can be used to estimate the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of a conjugated polymer. According to the equations [35] HOMO ðeVÞ ¼ e Eonset ox þ 4:4 ðeVÞ LUMO ðeVÞ ¼ −e Eonset red þ 4:4 ðeVÞ The HOMO energy level can be calculated with the empir+ 4.4) (eV), where ical equation HOMO ðeVÞ ¼ eðEonset ox Eox is the onset oxidation potential. The value of EHOMO and ELUMO was calculated as −6.35 and 3.70 eV respectively. The HOMO energy level of the present polymer is almost the same as that of carbazole containing 8-hydroxyquinoline [36], The electrochemical energy band gap (E ec g )has been estimated as 2.65 eV, which is high as compared with literature on N-aryl carbazole derivatives as donor and cyanoacrylic acid as acceptor bridged by benzothiadiazole unit [37].
Morphological Properties The morphology of the thin film layer deposits are expected to be explained by the self-assembling behaviour of the materials. Thin solid–state deposits from conjugated compound molecularly dispersed in solution, upon slow evaporation, mainly show three types of interactions (molecule-molecule, molecule –solvent, and molecule –surface). The surface morphology has been measured for the polymer and shown in the Fig. 6. AFM is becoming important in polymer characterization which provides information on surface morphology of the material and a powerful surface technique that can be used to obtain high-resolution images of the surface of organic film.
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Fig. 6 AFM images of CV-QP
The carbazole linked quinoline polymer (CV-QP) showed triangular shaped particles and these have aggregated as cluster and the particle size was measured as 80 nm. Polymer seems to be having smooth surface. AFM topography was also used to find out the average surface roughness (Sa), root mean square (RMS) (Sq) and maximum peak valley (Sq- Sv) [38]. From the results, Average (Sa) and RMS surface roughness (Sa) of CV-QP were noticed as 3.4 and 4.8 nm respectively, which was higher when compared to carbazole containing polymer [39]. Increased surface roughness may be due to the strong interaction between carbazole moieties with quinoline. But the roughness of the present polymer was lower than that of cyano substituted oligo phenylene vinylene [40]. In summary, the morphology changes can be assigned to the crystallisation of quinoline on the surface of polymer films. The tendency of crystallization is higher for the polymer CV-QP due to the possibility of better π stacking. Thermal Analysis Thermogravimetric analysis of polymer was carried out under nitrogen atmosphere at a heating rate of 5 °C/min. Thermal property of CV-QP was examined by thermogravimetric analysis (TGA) as shown in Fig. 7a. The polymer showed moderate thermal stability. The polymer showed weight loss below 200 °C. The significant weight loss of 10 % was observed at around 200 °C. The char residue at 400 °C was calculated to be 95 %. CV-QP had higher thermal stability than imidazole containing carbazole [41].
Fluorescence Quenching by Using DMA (Donor) and DMTP (Acceptor) The fluorescence quenching technique is an important tool for the study of the mechanism of molecular interaction, energy transfer or charge transfer. Quenching of fluorescence is a process of decreasing the fluorescence intensity of the fluorophores. The collisional interaction (appropriate electronic orbital overlapping) between a fluorophore and the quencher molecules is a basic requirement for effective quenching. Fluorescence quenching processes in solution fall under two categories. The first one is static quenching through the formation of a ground state complex and the second one is dynamic quenching due to diffusive collisions between the photoluminescence (PL) emitter and the quencher [42]. The fluorescence of CVQP can be quenched efficiently by both dimethyl aniline [DMA] as a donor quencher and dimethyl terephthalate [DMTP] as acceptor quencher by static quenching process. As shown in Fig. 8, quenching performance of CV-QP was analysed by using DMA and DMTP. The quenching effects were examined for each concentration of CV-QP, so that the differences in Stern – Volmer activity could be obtained. We monitored the fluorescence emission spectral changes of polymer upon adding different concentration of DMA and DMTP in chloroform. We found that the emission intensities of polymer decreased gradually with increasing concentration of DMA, because DMA has high potential, which decreases the densities of electron on CVQP and weakens the conjugation extent of CV-QP (Fig. 8a).
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Fig. 7 a TGA thermogram b DTA graph of CV-QP
This indicates carbazole unit remarkably enhance the electron transfer between the polymer and DMA due to its electron rich character. It was clear that the Stern –Volmer plots were obtained from the fluorescence quenching titration which means photo-induced electron transfer happened between the emitters and the quenchers. The quenching process can be analysed by Stern –Volmer relationship. F 0 = F ¼ 1 þ Ksv ½Q
From this equation, the quencher concentration is [Q], the Stern –Volmer constant is Ksv, F0 is the measured fluorescence intensity without quencher present, and F is the measured fluorescence intensity with [Q] present. After plotting (F0/F) against [Q], the slope can be determined to give the value of Ksv, the Stern –Volmer constant [43]. The linear Stern –Volmer constant (Ksv) values obtained from the slopes of the fitted lines are 5.6 × 105 M−1S−1 for [DMA], which is higher value compared
Fig. 8 a Fluorescence quenching spectra of CV-QP at different concentration of DMA. Concentration of CV-QP, 5 × 10−4 mo/L; concentration of DMA (mol/L), (1) 2 × 10−4; (2) 4 × 10−4; (3) 6 × 10−4; (4) 8 × 10−4; (5) 1 × 10−3; (6) 1.2 × 10−3; (7) 1.4 × 10−3; (8) 1.6 × 10−3; b
Fluorescence quenching spectra of CV-QP at different concentration of DMTP. Concentration of CV-QP, 5 × 10−4mo/L; concentration of DMA (mol/L), (1) 2 × 10−4; (2) 4 × 10−4; (3) 6 × 10−4; (4) 8 × 10−4; (5) 1 × 10−3; (6) 1.2 × 10−3; (7) 1.4 × 10−3; (8) 1.6 × 10−3;
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to fluorine containing conjugated polymer [44]. This is due to the electron donating carbazole unit present in the backbone which enhance the electron transfer between CV-QP and DMA due to its electron rich character. Interaction between the donor and acceptor in charge transfer complex (CTCs) could influence the conformation of the polymer chain and these results suggest that the intramolecular charge transfer (ICT) state of the conjugated polymer CV-QP is more stable. The fluorescence quenching process of CV-QP with DMTP was also examined and shown in Fig. 8b. From the figure, the emission intensity of CV-QP was initially decreased then increased and finally decreased. This may be due to strong interaction between carbazole and DMTP happens first and later with quinoline which would lead to changes in the fluorescence intensity of the polymer CV-QP.
Conclusion In summary, a novel quinoline and carbazole type donoracceptor conjugated polymer (CV-QP) has been successfully synthesized. The resulting polymer was characterized by FTIR, NMR, UVand photoluminescence. The absorption maxima of CV-QP in solution and film states were located at 250– 340 nm and 360 nm. The spectrum shows the emission maximum of CV-QP is found in the range of 390–441 nm in solution and 473 nm in thin film, which has a bathochromic shift by 32 nm. The present polymer has shown good solubility in common organic solvents and capable of exhibiting enhanced fluo rescent properties. The optical Eopg and electrochemical of the present polymer is 2.69 and 2.65 eV. band gap Eec g The polymeric complex has moderate thermal stability with initial decomposing temperature at above 200 °C. The fluorescence intensity of CV-QP can be quenched by both electron donor (N, N-dimethylaniline) and electron acceptor (dimethylterephalate). The morphology of the final polymer showed triangular shaped particles with particle size measured as 1.1 μm. Based on complete studies, our polymeric materials are well suitable for light emitting diode application.
2. 3. 4. 5.
6. 7. 8. 9.
10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.
24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34.
Acknowledgement The authors are grateful to the VIT University for providing the necessary facilities and DST/VIT-SIF for recording spectral data.
35. 36. 37.
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