Tunable Multicolor Emission in Oligo(4-hydroxyquinoline) - The

Article pubs.acs.org/JPCC

Tunable Multicolor Emission in Oligo(4-hydroxyquinoline) Ali Bilici,*,† Fatih Doğan,*,‡ Mehmet Yıldırım,§ and Iṡ met Kaya§ †

Control Laboratory of Agricultural Ministry, 34153 Istanbul, Turkey Faculty of Education, Secondary Science and Mathematics Education, Canakkale Onsekiz Mart University, Canakkale 17100, Turkey § Ç anakkale Onsekiz Mart University, Department of Chemistry, 17020 Ç anakkale, Turkey ‡

S Supporting Information *

ABSTRACT: The results on the chemical oxidative polymerization of 4hydroxyquinoline (HQ) are reported. The treatment of HQ with ammonium peroxydisulphate in an acidic aqueous medium afforded a conjugated quinoline oligomer (OHQ). The spectral analysis results suggested that the polymerization of HQ occurred mainly at C3 and C8 positions. The obtained oligomer exhibited an uncommon multicolor emission behavior. Although a wide range of emission colors by using various conjugated oligomer/polymers has been reported in literature many times, quinoline oligomer reported here emitted multicolor when irradiated at different wavelengths. Moreover, a linear relationship was observed between the excitation energy and emission maxima obtained (λEm = −108.82EEx = +819.07, R2 = 0.986). This allows us to effectively tune the photoluminescence colors between blue and orange-red.

1. INTRODUCTION

the other words, blue, green, and red emissions are achieved by exciting the same sample at suitable wavelengths.

Polymer light-emitting devices (PLEDs) have gained much more interest in both academia and industrial field since their discovery in 1990.1 The conjugated polymers have been widely exploited for tuning of the emission colors in the PLEDs.2 Photoluminescence (PL) color tuning, in essence, is known as the alteration of transition-energy level of the studied molecule3 and can be achieved by different physical and chemical strategies. These include introducing various substituents into the polymer backbone4 or controlling polymer molecular weight5 or inter/intrachain interactions,6 or altering polymer concentration,7 particle size,8 or applied excitation wavelength.9,10 However, design of polymers with tunable optical properties often requires more complex device architectures and production process.9 Discovery of simpler techniques amenable to low-cost and large-area fabrication of devices for tunable emission is still one of the most challenging issues in polymer science.11 As a conventional synthetic method, oxidative polymerization (OP) has played an important role in polymer science. It has been shown to be an effective method for the facile preparation of high-quality conjugated polymers.12,13 This method allows high-scale production as well as simple isolation and purification procedures.14 In a previous work, we reported the facile and regioselective synthesis of conjugated poly(5-hydroxyquinoline).15 The obtained polymer exhibited fluorescence solvatochromism. In the present study, we report chemical OP of 4-hydroxyquinoline. The resulting product is constituted by mainly phenylene type linkages. It also displays a multicolor emission property. In © 2012 American Chemical Society

2. MATERIALS AND METHODS 2.1. Materials. HQ, ammonium peroxydisulphate, and all solvents were commercially obtained from Fluka and Merck Chemical and used as received. 2.2. Synthesis of Oligo (4-hydroxyquinoline) (OHQ). OHQ was prepared by chemically OP in an aqueous HCl medium. A typical procedure for the preparation of OHQ is as follows:16 HCl solution (1.0 M, 25 mL) and HQ (0.01 mol, 1.45 g) were added to a 100 mL glass flask in a water bath and stirred vigorously for 0.5 h. In a separate container, ammonium peroxydisulfate, (NH4)2S2O8, (0.01 mmol, 2.28 g), was dissolved in HCl (1.0 M, 25 mL) to prepare an oxidant solution. The oxidant solution was then added dropwise to the monomer solution at a rate of one drop every 3 s at 25 °C over a period of 30 min. The reaction mixture was then continuously stirred by using a magnetic stirrer for 48 h in a water bath at 25 °C. After the reaction, the salts formed were isolated from the reaction mixture by filtration or centrifugation and washed with excess of distilled water to remove the oxidant. 2.3. Characterization Techniques. The solubility tests were done in an ultrasonic bath using 1 mg sample and 1 mL of solvent at room temperature. The infrared and ultraviolet− visible spectra were measured by Perkin-Elmer FT-IR Spectrum One and Perkin-Elmer Lambda 25, respectively. The FT-IR spectra were recorded using universal ATR sampling accessory Received: May 7, 2012 Revised: July 21, 2012 Published: August 28, 2012 19934

dx.doi.org/10.1021/jp304432k | J. Phys. Chem. C 2012, 116, 19934−19940

The Journal of Physical Chemistry C

Article

(4000−550 cm−1). 1H and 13C NMR spectra (Bruker AC FTNMR spectrometer operating at 400 and 100.6 MHz, respectively) were also recorded by using deuterated DMSOd6 as a solvent at 25 °C. Thermal data were obtained by using a Perkin-Elmer Diamond Thermal Analysis system. TG-DTA measurements were made between 20 and 1000 °C (in N2, rate 10 °C/min). The number-average molecular weight (Mn) was determined by gel permeation chromatography (GPC) technique using Shimadzu VP-10A. For GPC investigations, an SGX (100 Å and 7 nm diameter loading material) 3.3 mm i.d. × 300 mm column was used; eluent: DMF (0.4 mL/min), polystyrene standards. Bruker microflex LT model matrixassisted laser desorption ionization time-of-flight mass spectroscopy (MALDI-TOF MS) was also used for molecularweight measurement. α-Cyano-4-hydroxycinnamicacid was used as matrix. A Shimadzu RF-5301PC spectrofluorophotometer was used in fluorescence measurements. The solution fluorescence quantum yields (Φf) of OHQ were determined using comparative method described by Williams et al.17 Fluorescein solution in 0.1 M NaOH was used as a wellcharacterized standard sample.18 Leica TCS SPE model confocal microscopy was used for multicolor fluorescence analysis and 405, 408, and 532 nm excitation wavelengths were selected to observe blue, green, and red emissions, respectively. Electrical conductivitiy of OHQ was measured on a Keithley 2400 electrometer using four-point probe technique. The oligomer was pressed on hydraulic press developing up to 1700 kg/cm2, and the conductivity measurements were conducted after the compressed pellet was exposed to saturated iodine vapor for 24 h.

MALDI-TOF-MS spectrum of OHQ shows the presence of approximately eight repeating monomeric units with the 143 m/z intervals. (See Figure S2 of the Supporting Information.) Molecular weight of OHQ estimated by GPC is much higher than that of the obtained by MALDI-TOF-MS. Similar differences have been also reported in the literature for other phenol-based oligomers and polymers.20 The molecular association of phenolic polymer is thought to be one reason for the difference in the apparent molecular weight of the polymer.20 The absorption spectra of HQ and OHQ in DMSO solutions are given in Figure S3a (Supporting Information). Compared with the monomer, OHQ shows a much broader absorption band that is attributed to the coexistence of both long and short effective conjugation length.21 Optical band gap (Eg) values of OHQ are determined by Touch-equation approach22 and found to be 3.1 eV. A typical Touch plot for OHQ is given in Figure S3b (Supporting Information). The structure of OHQ is confirmed by FTIR and NMR analysis. HQ is a phenolic compound with multireaction sites. Therefore, to gain an insight into the structure of OHQ, possible polymerization sites of HQ given in Figure 2 should be taken into account. It is widely acknowledged that the OP of aromatic monomers with ammonium persulphate proceeds through a cation-radical mechanism (Figure 2) and involves polyrecombination of cation-radical intermediates arising in the course of oxidation,23,24 and then radical cation is stabilized in several resonance forms. As shown in Figure 2, there are six main coupling sites (R1− R6), and the formation of the oxyphenylene units in the oligomer chains (C−O−C) is a probability. However, such etheric linkages should produce a characteristic band around 1280 cm−1, which is not appeared.25 The probability is the formation of C−N−C linkages (single bonds) in the chains (R3). However, these linkages should be ignored because the NMR analysis clearly indicates the presence of conjugated oligomer structure as discussed below. As a result, only phenylene type linkages (R2, R4, and R5) in the chains are expected. In the other words, 3, 6, and 8 positions of quinoline ring are preferred as the polymerization sites. Unlike monomer, a broad absorption is observed at 1720 cm−1 at the FTIR spectrum of the oligomer (Figure 3). This absorption peak arises over oxidation of hydroxyl groups in oligomer backbone to quinoid segments (−CO).25 However, the broad band from 3234 to 3600 cm−1 as well as the band at 1210 cm−1 indicates the presence of phenolic hydroxyl groups in the oligomer structure.25,26 The representative 1H NMR spectra of HQ and OHQ are given in Figure 4. The 1H NMR spectrum of the OHQ shows overlaps to some extent, which makes the analysis a little difficult. For comparison, 1H NMR spectrum of the monomer is also recorded. The proton signals of monomer observed at 7.50 (H−E) and 6.00 ppm (H−G) almost disappear after the polymerization. This indicates the elimination of H−E and H− G protons via couplings at C3 and C8 carbons. The broad proton signals from 10.88 to 12.69 ppm in the spectrum of the oligomer should be attributed to NH or OH protons with different chemical surroundings.27,28 The integration ratio of aromatic protons to total NH and OH protons (from 11 to 12.6 ppm) is found to be ∼4. This finding also indicates that nitrogen and oxygen species are not involved in bond formation

3. RESULT AND DISCUSSION Chemical OP of HQ with ammonium peroxydisulphate in acidic medium yielded to corresponding quinoline oligomer (OHQ) as brownish powders (78%). The general synthetic route is outlined in Figure 1.

Figure 1. Synthetic method for preparation of OHQ.

The isolated oligomer is well-soluble in polar solvents such as dimethylformamide (DMF), dimethylacetamide (DMA), dimethysulfoxide (DMSO), and partially soluble in tetrahydrofuran (THF), chloroform, methanol, and ethanol. The molecular weight of OHQ is determined by GPC; see Figure S1 (Supporting Information). GPC analysis indicates the presence of a polymeric product with the high molecular weight (Mn = 17 000 g/mol). However, as given below, relatively low char yield at 800 °C and sharp proton resonance signals suggest that OHQ has low molecular mass. The absence of new absorbance peaks in the wavelength range from 400 to 800 nm also supports this suggestion.19 Therefore, we used MALDITOF-MS to confirm the molecular weight of the product. The 19935



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Figure 2. Possible resonance forms of OHQ.

Figure 3. FTIR spectra of HQ (a) and OHQ (b).

Figure 4. Selected region of the 1HNMR spectra of HQ (a) and OHQ (b) in d6-DMSO. Note: Keto form of HQ is dominant in polar organic solvents.34

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Figure 5. Selected region of the 13CNMR spectra of HQ (a) and OHQ (b) in d6-DMSO.

and the polymerization selectively proceeds via phenylene units. The representative 13CNMR spectra of HQ and OHQ are given in Figure 5. Almost all carbon signals of HQ are shifted to lower fields in the oligomer spectrum due to increasing conjugation length.15 The monomer shows nine distinct resonances in the aromatic region. 13C NMR spectrum of OHQ exhibits new additional carbon signals at 162.56 and 153.25 ppm due to presence of azomethine and hydroxyl function in the OHQ chains, respectively (unit A in the oligomer chains). The IR vibration band at 1626 cm−1 also supports the presence of azomethine group. The coexistence of both carbonyl (180.12 ppm) and azomethine (162.56 ppm) functions in the 13C NMR spectra is assigned to the formation of keto-amine/phenol-imine tautomeric structures in the chains, as shown in Figure 1. The presence of new aromatic carbon signals (Figure 5b between 110 and 140 ppm) can be explained by the insertion of quinoidal character in oligomer chains. Similar spectral findings are also reported in our previous work.27 It is important to note that C3 and C8 carbon signals of OHQ are observed in significantly lower fields than those of the monomer. As a result, 3,8-carbons are the primarily linkage positions. It is acknowledged that the thermogravimetry is a useful tool for determining a polyphenol composition (the ratio of phenylene/oxyphenylene units). Therefore, TG-DTG analyses are performed. It is clear from DTG curve that OHQ has a three-step decomposition path (Figure 6). The first decomposition step occurs in the range of 50−120 C with a mass loss of 7%, resulting from the evaporation of trapped water in OHQ.29 The following decomposition step occurs in the range of 148−655 °C with a mass loss of 55%, which arises from the decomposition of phenylene units in the oligomer chains, and the last decomposition step occurs in range of 655−800 °C with a mass loss of 6%, resulting from the decomposition of oxyphenylene units in the oligomer chains.30 The TG-DTG

Figure 6. TG-DTG curve of OHQ.

analysis indicates that the phenylene content of OHQ is ∼90 wt %, and this value is in harmony with the spectral analysis results, as discussed above. The large endothermic peaks observed in DTG curve are considered to originate from a different arrangement of Units A and B in the repeating units. The virgin conductivity value of OHQ determined by fourpoint probe is 3 × 10−8 S/cm. After doping with iodine vapor for 24 h, the conductivity is reached to 7 × 10−4 S/cm. The key issue of the study is fluorescence behaviors of the oligomer, which are investigated by means of PL and confocal microscope analysis. Although the difference between the absorption spectra of HQ and OHQ is relatively small (Figure S3a of the Supporting Information), there is a substantial difference in the PL spectra due to a difference in the energy levels of the two materials.31 The emission spectra of OHQ are different from those of HQ in respect to their shape, position, and the intensity. The emission spectrum of the monomer in DMSO solution is characterized by an emission peak observed at 393 nm with a shoulder at 450 nm as excited at 350 nm (Figure 7A). However, under the same experimental conditions, OHQ exhibits an emission maximum at 462 nm. This 69 nm red19937



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Figure 7. (A) PL spectra of HQ in DMSO excited by different wavelengths. λEx: 350 (a), 360 (b), 370 (c), 380 (d), 405 (e), 480 (f), and 532 (g). (B) Emission spectra of OHQ solution (in DMSO) with progressively longer excitation wavelengths from 420 to 540 nm: (a) 420, (b) 440, (c) 460, (d) 480, (e) 500, (f) 510, (g) 520, and (h) 540 nm. (C) Normalized emission spectra for three excitation values; 405, 480, and 532 nm, respectively. Slit width: 5 nm and conc.: 10 mg L−1 in all measurements. (D) 3D view of the PL spectra of OHQ in DMF with excited at various wavelengths.

shifted emission maximum of OHQ is attributed to its polyconjugated backbone. As is well-known, the polymerization of quinoline fluorescent chromophores results in decreased fluorescence due to increased local concentration quenching effects,32,33 but to our surprise, the emission intensity of OHQ in DMSO is about two times stronger than that of HQ. DMSO solution of OHQ has a broad fluorescence spectrum like its absorption spectrum (Figure 7B). As seen in this Figure, emission maxima of OHQ solution are red-shifted from 480 to 565 nm when excited with progressively longer wavelengths in 20 increments from 420 to 540 nm. To monitorize the bathochromic shifts, we also recorded the normalized emission spectra (Figure 7C). The spectral changes related to applied excitation wavelengths are more visible in 3D view (Figure 7D). However, as seen in Figure 7A, no red-shifting in emission maxima is observed for the monomer, as the excitation wavelength is increased gradually from 350 to 532 nm. In addition, its emission is quenched with increasing excitation wavelengths. When excited with 405, 480, and 532 nm, OHQ emits blue, green, and red light, respectively. These are clearly seen by the naked eye (Figure 8), and the emission quantum yields are found to be 1.7, 1.2, and 0.7% for blue, green, and red PL, respectively. Stoke’s losses are 0.496, 0.313, and 0.129 eV, and the full width at half-maximum (fwhm) values are 0.536, 0.317, and 0.244 eV for blue, green, and red emissions, respectively. fwhm value of blue emission is larger than that of similar values for green and red emissions. This means that in OHQ a set of blue-emitting centers exists with a ground-state energy level distribution wider than that of the green and particularly the red ones.10

Figure 8. Photographs of OHQ solution excited at 405 (a), 480 (b), and 532 (c) nm and corresponding emission spectra OHQ in DMSO. Photographs were recorded in PL analysis cell.

To observe the multicolor emission with change of excitation wavelength, confocal microscope analysis is also performed (Figure 9). For this purpose, a certain amount of OHQ solution is dropped on a glass sheet and dried. The obtained sample then is separately excited at 405, 480, and 532 nm, resulting in blue, green, and red light emission, respectively. This emission behavior is different from the organic fluorophores,5,34 and it is highly reproducible. It is also important to note that a linear relationship is observed between the excitation energies and the obtained emission maxima; λEm = −108.82EEx +819.07, R2 = 0.986, 19938



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such a linear relationship has not been reported up until now. This also allows tuning the PL color on the desired scale. This unusual emission behavior has been reported in the literature for several polymeric systems including oligopyrene nanofibers,9 polyvinylpyridine gels,10 polyamidoamine nanoparticles,5 hyperbranched polyamidoamines,35 and nitro-substituted pyrene oligomers36 The origin of this phenomenon is considered to originate from the broad chain dispersity and high structural heterogeneity of the polymeric system studied.9,10 In this content, the conjugated polymers are considered as multichromophoric systems,37 and each unit consisting of the different conjugation lengths behaves as a different chromophore group. As a result, the light is absorbed by a multitude of chromophores. In other words, the longer conjugated chains emit the light with longer wavelength (532 nm), whereas the shorter conjugated chains emit the light with shorter wavelength (405 nm), and thus the different fluorescence emission colors can be achieved by changing the applied excitation wavelength.9The relatively broad molecular-weight distribution curve (Figure S1 of the Supporting Information) and also broad and extended UV spectrum (Figure S3b of the Supporting Information) of OHQ assign the presence of such a possible multitude chromophores. However, a detailed investigation about this line will be given in a forthcoming publication.

Figure 9. Confocal microscope images of OHQ. The excitation wavelengths were 405 (a), 488 (b), and 532 nm (c), respectively. (d) Merged picture of a−c.

where Eex and λEm are excitation light source energy and obtained emission peak wavelengths, respectively. (See Figure 10 and Table 1.) There are only a few reports on the

4. CONCLUSIONS In this study, the facile and regio-controlled synthesis of a conjugated quinoline oligomer, OHQ, was shown. It was also shown that emission colors of the obtained product could be tuned from blue to red by adjusting of the excitation source energy. The origin of this phenomenon is believed to mainly result from different conjugation length distribution of the OHQ, which was suggested based on the UV−vis and GPC analysis. Linear change between the excitation energy and the peak wavelength of the emitted light was reported as an uncommon property, and this made the OHQ adjustable as PL color. It is expected that the obtained product has a great potential for applications in biology and displays and optical technologies like the other versatile conjugated-quinoline polymers.38

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Figure 10. Linear relationship between emission maxima observed versus applied excitation wavelength.

S Supporting Information *

Table 1. Fluorescence-Related Data of OHQ Solutions in DMSO λEx (nm)

EEx (eV)

λEm (nm)

380 400 420 440 460 480 500 520 540 560

3.27 3.11 2.96 2.82 2.70 2.59 2.48 2.39 2.30 2.22

470 480 488 512 526 541 548 558 565 583

ASSOCIATED CONTENT

GPC and MALDI-TOF-MS spectrum of OHQ, UV−vis spectra of HQ and OHQ, and touch plot for OHQ. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (A.B.); fatihdogan@comu. edu.tr (F.D.). Fax: +90 212 663 42 96, +90 286 218 05 33. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS ̇ AK Grants This work is financially supported by TUBIT Commission (project no: TBAG-109T914).

oligomers/polymers exhibiting multicolored emission, as previously mentioned. However, to best of our knowledge, 19939



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(34) Jaiswal, J. K.; Mattoussi, H.; Mauro, J. M.; Simon, S. M. Nat. Biotechnol. 2003, 21, 47−51. (35) Feist, F. A.; Basch, T. J. Phys. Chem. B 2008, 112, 9700−9708. (36) Lu, B.; Xu, J.; Fan, C.; Jiang, F.; Miao, H. Electrochim. Acta 2008, 54, 334−340. (37) Unver, H.; Yıldız, M.; Zengin, D. M.; Ozbey, S.; Kendi, E. J. Chem. Crystallogr. 2001, 31, 211−216. (38) Huang, M. R.; Huang, S. J.; Li, X. G. J. Phys. Chem. C 2011, 115, 5301−5315.

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Tunable Multicolor Emission in Oligo(4-hydroxyquinoline) - The

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