Feyza Kolcu*ab and
İsmet Kaya*a
aÇanakkale Onsekiz Mart University, Department of Chemistry, Polymer Synthesis and Analysis Laboratory, 17020, Çanakkale, Turkey. E-mail: feyzakolcu@comu.edu.tr; kayaismet@hotmail.com; Fax: +90 286 218 05 33
bÇanakkale Onsekiz Mart University, Lapseki Vocational School, Department of Chemistry and Chemical Processing Technologies, Çanakkale, Turkey
First published on 30th January 2017
The present work reports a simple and inexpensive way for the synthesis of a novel water-soluble conjugated oligo(acriflavine) (OAC). Oligo(acriflavine) was synthesized via chemical oxidative polycondensation by H2O2 (30%) as the oxidant at an optimum reaction temperature of 110 °C without use of an additional external template. The chemical structure of the synthesized compound was confirmed by FT-IR, UV-Vis, 1H-NMR and 13C-NMR techniques. Further characterization was employed using, photophysical, electrochemical, photovoltaic thermogravimetric-differential thermal analysis (TG-DTA), fluorescence (PL), cyclic voltammetry (CV) and differential scanning calorimetry (DSC) measurements. The number average molecular weight of the oligomer was found to be 4950 Da with a polydispersity index of 1.18. Photoluminescence (PL) properties of the synthesized materials were determined in solution form. The solvent effect on the optical properties of OAC was studied in selected solvents. The spectral analysis displayed a bicolor emission behavior when OAC solution was irradiated at different wavelengths. The HOMO–LUMO energy levels and electrochemical (E′g) band gap values of OAC were determined by CV measurement. Optical and electrochemical band gaps of OAC with more conjugated structure were lower than for AC. XRD analysis signified that the oligomer was in a semi-crystalline form. To further study the morphologic properties of the OAC, Scanning Electron Microscopy (SEM) images were illustrated at different amplifications. It was demonstrated that the structure of OAC was essentially a ladder oligomer with phenazine rings. The best performing photovoltaic cell, based on OAC reached a Jsc of 0.14 mA cm−2, a Voc of 0.37 V, and a FF of 0.50, giving a power conversion efficiency (% η) of 0.027% under AM 1.5 irradiation (100 mW cm−2).
Water–alcohol soluble conjugated polymers have attracted considerable attention owing to their important applications in polymer solar cells as cathode interlayers and in PLEDs, organic photovoltaics, polymer cells as well as in drug delivery and detection of biomacromolecules.21–25 Oxidative polymerization (OP) commonly used is one of the suitable techniques to prepare conjugated conductive polymers. Using OP, the solubility problem of a conductive polymer can be overcome using amino-functionalized conjugated polymers that are soluble in alcohol in the presence of trace acetic acid due to weak interaction between the nitrogen atom of amino group and acetic acid.26
On the other hand, highly crystalline conjugated polymers are very important for efficient charge carrier transport, and a crystalline structure of the polymers will effectively enhance performance of their electronic or optical devices.27 Several crystalline conjugated polymers have been synthesized using chemical routes.28–30 Photovoltaic (PV) devices emerge as an attractive way of converting energy directly from the sunlight. Among all types of photovoltaic devices, polymer-based organic solar cells (OSCs) are particularly intriguing since they have the advantages of low production cost, processing at room temperature, mechanical flexibility, and suitability for mass production in addition to being relatively quiet, not producing toxic substances or greenhouse gases, and requiring little maintenance during operations.31
Encouraged by these explanations in this article we report the synthesis and characterization of π-conjugated structure of acriflavine (AC) having phenazine units in its backbone. AC is a dye used as antiseptic agent, bactericidal inhibitory, genetically active agent and reagent for drug analyses.32–34
A novel water-soluble conjugated polymer is presented by containing same side-chains bearing primary amino (–NH2) groups responsible for dissolving in aqueous solution. Chemical oxidative polymerization of aromatic diamine-AC where the nitrogen atoms of diamine group were the most susceptible site for the polymerization was simple and environmentally friendly. Polymer consists of purely head-to-tail or ortho-coupled coupled acriflavine monomers by employing the direct chemical oxidation of acriflavine.
Based on the foregoing information, we aimed to develop the potential applications of this new polymer in various disciplines, due to the fluorescent probe properties and high emission quantum yields. UV-Vis, CV, TGA, DSC, and PL techniques were carried out to characterize the photo physical, electrochemical and thermal properties of the new system. The electronic properties were analyzed using UV-Vis absorption spectrophotometer, fluorescence emission spectroscopy and cyclic voltammetry.
Solid state electrical conductivity of OAC in the form of a pellet was measured by four-point probe technique at room temperature and atmospheric pressure using Keithley 2400 Electrometer instrument. The oligomer was pressed in a hydraulic pressure developing up to 1687.2 kg cm−2 to prepare a pellet form.
Eight main coupling modes for the polymerization of acriflavine (AC) were identified and given in Scheme 2. The most probable coupling sites for the polymerization were through the radical formed as R1 and R2. AC was unit structure linked to each other via the C–NC or C–NH–C bonds. In such a way that two acridine units could be regarded as forming one pyrazine ring as result of an intramolecular cyclization reaction, leading to the ladder-like structure.
Therefore, R1 and R2 radicals could favor the formation of pyrazine ring during the polymerization via C–N coupled dimer (Scheme 1). The formation of a cycle reduces the energy of the molecule owing to the generation of an aromatic structure.39 The ortho position was subsequently oxidized to form phenazine-like moieties along the backbone of the polymer. Phenazine cycles could further participate in the subsequent chain growth. In this way, a phenazine-containing oligomer is able to initiate the chain growth. The computational results suggest that intramolecular cyclization is thermodynamically favorable, leading to the formation of ladder segments during the course of oxidative polymerization of aromatic diamine polymer growth mechanism.40
FT-IR spectroscopy was used to confirm the chemical structure of AC and OAC. The peaks at 3300 and 3169 cm−1 attributed to the N–H stretching vibration of the –NH2 groups were replaced by one broader band at 3154 cm−1 for OAC which signified the transformation of the primary amine to a secondary amine through polymerization that took place on nitrogen atoms of primary amine groups and resulted in forming the C–N coupled structure. The absorption peak at 3028 cm−1 ascribable to the stretching modes of aromatic C–H bond was observed at 3034 cm−1 in the oligomer structure of OAC. The peak bands at 2844 cm−1 and 2905 cm−1 were corresponded to the aliphatic methylene (–CH3) of AC and OAC, respectively. FT-IR spectrum of AC indicated two peaks at 1480 and 1377 cm−1 for the C–H scissoring and methyl rock, respectively, became broader in that of OAC. Two sharp peaks at 1622 and 1594 cm−1 attributed to the CN and CC stretching vibration modes for AC, became wider due to an increase in conjugation after polymerization and observed at 1660 and 1596 cm−1, respectively. The peaks at 1449 and 1510 cm−1 of OAC pointed to the ring stretching vibrations of newly formed pyrazine ring between two acridine units. The bending vibration of C–N bond for AC was observed at 1326 cm−1. In other words, phenazine ring could be considered as a part of the oligomer chain.41,42
1H NMR of AC was also recorded for a comparison to OAC for further enlightment of the synthesized structure and shown in Fig. 1 and 2. In the spectrum, the peaks in the aromatic region observed at 7.81 (doublet), 7.76 (doublet), 7.45 (singlet), 6.97 (doublet), 6.91 (doublet) and 6.85 (singlet) ppm were attributed to the Hb, Ha, Hc, He, Hf and Hg protons of AC, respectively. Because of 1–4 interactions with Hb and He protons, the dublet–dublet peak seen more downfield at 8.69 ppm was related to the Hd proton. The peaks for Ha protons were observed as terminal protons in the spectrum of OAC. The peak for the methylene (–CH3) protons was observed as a singlet at 3.90 ppm. The signals of hydrogens in the phenylene unit shifted to lower fields after polymerization, which was significant for the π-conjugated polymers.43 The broad proton signals with respect to AC unit indicated that oligomer OAC consisted of repeating units with different chemical surroundings. The proton signals of NH2 group at 6.76 and 7.27 ppm almost disappeared and observed at 11.20 ppm as a secondary amine after polymerization due to C–N couplings of monomer units during the polycondensation reaction. The doublet signal for He proton of AC turned into a singlet peak at 7.36 ppm indicating that a ring closure between two radicals and ladder structure formation took place in the polymerization pathway of AC. The results elucidated that OAC was formed primarily through the formation of R1 and R2 radicals and the polymerization occurs via the formation of C–NC and C–NH–C.44
The polymerization proceeded by C–N coupling mechanism. The formation of C–N bond through cyclization resulted in shifting peaks for C1, C2, C8 and C9 downfield between 163.18 and 169.12 ppm. The peaks at 117.42, 120.49, 158.55 and 159.17 ppm indicated the presence of terminal C1, C9, C2 and C8, respectively. C10 and C13 peaks were observed at 116.92 and 116.89 ppm for AC appeared at 124.71 ppm in the spectrum of OAC the proposed structure of oligomer is confirmed according to the 1H NMR and 13C NMR spectral results in Fig. 2 and 3. The proposed structure for the oligomer was supported by UV-Vis and FT-IR analyses. Our claim for the polymerization of AC was that polymerization primarily took place via C1 and C2 phenylene units form both sides of acriflavine.
The molecular weight of OAC was determined using GPC chromatogram indicating that Mn and Mw weight were calculated as 4200 and 4950 Da, respectively, with a mono modal particle size distribution by RID detector, resulting in that OAC was oligomeric structured. OAC had a polydispersity index (PDI) value of 1.18. According to the average molecular weight value, the average constitutional units in OAC were nearly 19–22.
To research the crystal structures of AC and OAC, the powder XRD analysis was conducted. The crystallite sizes of the monomer and the oligomer are given in Table 1. The calculated (D) values gave information about the types of crystal structure and the interlayer spacing (d). The analyzed samples of AC and OAC were finely grounded and homogenized. Using the diffraction data, the mean crystallite sizes of the monomer and the oligomer, D, were determined according to the Debye–Scherer equation D = (0.9λ)/(βcosθ), where λ was X-ray wavelength (1.5406 Å), θ was Bragg diffraction angle, and β was the full width at half maximum of the diffraction peak.45,46 In view of the importance of knowing crystal sizes and interlayer spacings (d) obtained from the angle at which the corresponding peak is diffracted in AC and OAC, it can be proposed the percent of the crystallinity of AC and OAC. The peak at 2θ = 26.8° for AC disappeared, substituted by a new weakened broad peak at around 2θ = 27° for OAC. A decrease in the percentage crystalline was observed from 33 to 19% for AC and OAC, respectively, due to the loss of peaks at 2θ value of 18.4° and the decrease in peak intensities. Highly intense four diffraction peaks were observed at 2θ = 32.8°, 44.7°, 65.1° and 78.25° and four less intense peaks observed at 2θ = 18.4°, 26.8° and 47.6° for the monomer AC. The four intense peaks were observed at 2θ = 32.8°, 44.7°, 65.03° and 78.24° for the oligomer OAC. The peak corresponding to AC seen at 2θ = 26.17° became lower and broader in OAC, which revealed that the sample was partially crystallized.47 The low crystallinity could result from the long range ordering of the oligomer chains.48 Less intense peaks of OAC were attributed to the periodicity parallel to oligomer chains. A decrease in the crystallite size caused an increase in the width of the diffraction as peak broadening. On either side of the Bragg angle, the diffracted beam destructively interfered and resulted in a sharp peak.
Compounds | Peak pos. [°2θ] | β (B obs. [°2θ]) | cosθ | Crystallite size [nm] | d-Spacing [Å] |
---|---|---|---|---|---|
Major peaks of AC | 32.84 | 0.307 | 0.95 | 31.08 | 0.44 |
44.71 | 0.307 | 0.92 | 32.24 | 0.31 | |
65.11 | 0.307 | 0.92 | 35.48 | 0.20 | |
78.25 | 0.409 | 0.78 | 28.63 | 0.21 | |
Major peaks of OAC | 32.84 | 0.205 | 0.96 | 47.98 | 0.28 |
44.72 | 0.307 | 0.92 | 32.24 | 0.31 | |
65.03 | 0.205 | 0.96 | 54.87 | 0.13 | |
78.24 | 0.409 | 0.78 | 28.32 | 0.21 |
Fig. 4 Absorption spectra of (a) (AC) and (OAC) in DMSO, (b) the colors of AC and OAC under room light and UV light, (c) absorption spectra of OAC in different solvents. |
From an analysis of the absorption spectra, it is possible to evaluate the optical band gap energy. The optical band gap values (Eg) are obtained by using the following equation as in the literature and were given in Table 2.
Eg = 1242/λonset | (1) |
The effect of solvent polarity on UV-vis spectra was also examined and shown in Fig. 4c. The effect of solvent polarity was clearly seen on the positions, intensities and shapes of the absorption bands of OAC. The spectral shifts in DMSO, DMF, methanol and water with different polarities mainly depends on the strength of the intermolecular hydrogen bonds between the –NH group of OAC and the –OH, carbonyl (CO) or sulfonyl (SO) functional groups of the solvent molecules.51,52 The maxima of the absorption curves appear to undergo blue shifts with increasing solvent polarity. As seen in Fig. 4c, λmax values were found to be 490, 459, 458 and 414 in DMF, DMSO, methanol and water, respectively. The free pair of electrons on nitrogen atom of OAC were lost via H-bonding leading to the hyposochromically (blue shift) shift of π–π* bands due to a decrease in conjugation and delocalization of the π-electrons when the solvent polarity increased. The shift to shorter wavelength was more enhanced in polar protic solvents like methanol and water than that in apolar protic solvents. The clarification could be based on that a blue shift induced by increasing the polarity of the solvent signified a decrease in the polar quinoid resonance forms. The benzene substituents in OAC are electron donors with unshared electron pairs that may partake in hydrogen bonds with the solvent. Blue shifts would be accounted for that the interaction with polar solvents contributes to the stability of the ground state more than it does to the excited state.53
Therefore, OAC was expected to show a semiconductor behavior as result of oxidative polymerization. Additionally, the lowest optical band gap was expected to observe in DMF or DMSO due to their aprotic polar solvent property to prevent the formation of a hydrogen bond between OAC and the solvent, concurrent with an increase in conjugation and delocalization of the π-electrons.
The electrochemical properties and redox behavior of AC and its oligomer were studied using cyclic voltammograms (CVs) carried out in acetonitrile as the supporting electrolyte at room temperature under argon atmosphere are given in Fig. 5. Based on the correlations between the HOMO and LUMO energy levels and redox potentials, the value of the energy levels can be calculated using the onset values of the oxidation and reduction peaks. Making the correlation between redox process and the energy levels, it was possible to determine the electrochemical band gap energy (E′g) of the oligomer. The HOMO–LUMO energy levels and electrochemical band gaps (E′g) were calculated using the oxidation (Eox) and reduction peak potentials (Ered) as shown in Table 2. The determination of HOMO–LUMO energy levels were possible by using the empirical formulas:54
EHOMO = −(4.39 + Eox) | (2) |
ELUMO = −(4.39 + Ered) | (3) |
E′g = ELUMO − EHOMO | (4) |
As displayed in Fig. 5, the oxidation of acriflavine initially produced the radical cation that reacted with another monomer molecule to produce oligo(acriflavine). Two oxidation peaks observed at 1.34 and 1.30 V can be attributed to the oxidation of free NH2 group of AC to form polaron structure (˙+NH), and the oxidation of –NH2 group to form cation radical for OAC in the anodic region, respectively, which are not stable and undergo chemical reactions with parent molecule or dimerization reactions. The acriflavine unit is first oxidized by the formation of a radical cation which then, combines to form dimer and higher oligomers.55 The reduction peak potentials were concerned with the neutralization of the oxidized states of the oligomer.
The diagrams revealed the tendency of the HOMO elevation and on the contrary, the LUMO reduction through the oligomer formation (Table 2). Since the electrochemical band gap of OAC was lower than that of AC, the electrochemical band gap (E′g) values confirmed the optical band gap values (Eg).
Fig. 6 Emission spectra of AC (a) and OAC (b) solutions in selected solvents. Slit width: 3 nm and conc.: 10 mg L−1 in all measurements. |
Photoluminescence (PL) spectra of monomer (AC) could be recorded in only DMF and DMSO with a slit width of 3 nm, because the emission exceeded 1000 a.u of PL intensity in water and methanol. The monomer was a green light emitter with an emission maximum at 517 and 497 nm in DMF and DMSO, respectively. The intensities in DMF and DMSO were observed as 659 and 151 a.u, respectively (Fig. 6a). On the other hand, the oligomer had a different property from the monomer with an extra emission of blue color when excited by UV radiation. Fig. 6b gives information about the emission spectra of OAC in various solvents. As seen in Fig. 5b, the fluorescence intensity of OAC decreases with increasing the polarity of solvent. Maximum emission wavelength (λmax) of OAC in DMSO, DMF, H2O and MeOH appeared at 437, 440, 441 and 441 nm with the appearance of a blue color emission and 521, 518, 510 and 521 nm with the emission of yellow color when excited by UV and visible light, respectively. The obtained results revealed that the spectrum was affected by polarity depending on protic–aprotic nature of solvent.
In MeOH and H2O protic solvents, OAC emitted bicolored fluorescent light with blue and green colors when excited at 380 and 460 nm, respectively as seen in Fig. 7. In DMF and DMSO aprotic solvents, the excitation of OAC with 380 and 460 nm resulted in the blue and green fluorescence light at around 440 and 520 nm, respectively, as seen in Fig. 8. The intensities of bicolored fluorescent light emission were 300 and 800 with the excitation wavelengths at 380 and 460 nm, respectively. The behavior of bicolor emission when excited at different wavelengths can be explained in a way that polymers have a broad chain length distribution, and the longer conjugated chains have lower energy gaps with an emission light at longer wavelengths. Therefore, the wavelength of fluorescent emission increased as the wavelength of the excitation source increased. Quantum yield (QY) is another important contribution to PL characteristics. The oligomer OAC has the emission QY of 6% in DMSO and 3% in DMF for green PL emission.
Fig. 7 Photographs and the corresponding of maximum emission spectra of OAC solution in CH3OH and H2O. Slit width: 3 nm and conc.: 10 mg L−1 in all measurements. |
Fig. 8 Photographs and the corresponding of maximum emission spectra of OAC solution in DMF and DMSO. Slit width: 3 nm and conc.: 10 mg L−1 in all measurements. |
To observe the multicolor emission with change of excitation wavelength, confocal microscope analysis was also performed in Fig. 9. A certain amount of OAC solution was dropped on a glass sheet and dried. The emission colors of blue and green colors were separately obtained by using confocal microscope with the excitation wavelengths of 360 and 470 nm agreed with the PL ones very well. OAC displayed bicolor emission behavior in all selected polar solvents. OAC showed nearly similar emission characteristics under the same excitation wavelength in selected polar solvents such as DMF, DMSO, MeOH and water.
Fig. 9 Confocal microscope images of OAC. The excitation wavelengths were 380 (a), and 470 (b), respectively. |
Based on foregoing results, OAC can be a very promising candidate for materials in light-emitting diodes due to their bicolor (blue and green) light-emitting property as fluorescent water soluble conjugated oligomer.
Compounds/thermal data | AC | OAC |
---|---|---|
a The onset temperature.b Maximum weight loss temperature.c Glass transition temperature.d Change of specific heat during glass transition. | ||
Tona | 290 | 160 |
Tmaxb | 333, 430 | 206 |
20% weight loss | 373 | 266 |
50% weight loss | 628 | 719 |
% char at 1000 °C | 49.7 | 37.7 |
DTA (exo/endo) | —/— | —/— |
DSC [Tgc (°C)/ΔCpd (J g−1 K−1)] | —/— | 139/0.821 |
(5) |
(6) |
The characteristic intersections with abscissa and ordinate are the open circuit voltage (Voc) and the short circuit current (Isc), respectively, as seen in Fig. 11. The largest power output (Pmax) is determined using the point at which the product of voltage and current has maximum value. Division of Pmax by the product of Isc and Voc yields the fill factor FF. We have achieved 0.027 as % η under AM 1.5 irradiation (100 mW cm−2) with a dye-sensitized solar cells based on OAC, exhibiting a short circuit photocurrent density Jsc of 0.14 mA cm−2, open circuit voltage Voc of 0.37 V, and the fill factor FF of 0.50. It can be concluded that OAC is moderately effective as an organic-dye photosensitizer for use in dye-sensitized solar cells. The reason regarding to performance of the organic photovoltaics depends on the following factors. (1) Broad and strong absorption in visible and near IR region to match with solar spectrum (2) suitable LUMO energy level of acceptor and HOMO energy level of donor to maintain Voc (3) optimum morphology donor–acceptor with nanoscale phase separation within active layer which influences Jsc, Voc and FF.60
Fig. 11 J–V curves under different light intensities (1:100 mW cm−2; 2:80 mW cm−2; 3:60 mW cm−2; 4:40 mW cm−2; 5:20 mW cm−2). |
By the exposure of the pellet to iodine vapor in a dessicator at room temperature, the conductivity of iodine doped oligomer depending on time could be followed.61 The virgin electrical conductivity of OAC was measured to be 2.35 × 10−9 S cm−1. Upon doping with iodine at room temperature in a vacuum desiccator, its conductivity increased to 5.90 × 10−9 S cm−1 in 6 h and increased by about two orders of magnitude reached to 1.86 × 10−7 S cm−1 in 24 hours. The values were 3.1 × 10−7, 3.96 × 10−7, 7 × 10−7, 9.94 × 10−7 and 1.08 × 10−6 S cm−1 by doping in 48, 72, 96 and 120 hours, respectively. Since nitrogen is an electronegative element, iodine doped oligomer generally occurs by coordination of iodine with nitrogen atoms in the oligomer structure. The results revealed that the electrical conductivity of iodine-doped pellet of OAC increased about three orders of magnitude and reached to about 10−6 S cm−1 value in 120 hours. The conductivity first increased with doping time, then tend to level-off. As emphasized, there is an opposite correlation between the values of HOMO–LUMO band gap and the conductivity. A lower band gap makes the electronic transition easier with obtaining higher conductivity values. Consequently, it can be said that OAC is semi-conducting structured.
Conjugated organic dyes, which have OAC moiety as the electron donor showed moderate performance as a photosensitizer in dye-sensitized solar cells. A maximum solar-energy-to-electricity conversion efficiency (η) of 0.027% under AM 1.5 irradiation (100 mW cm−2) with a dye-sensitized solar cell. The crystalline nature and bicolored emission property of oligo(acriflavine) provide potential applications in bicolored fluorescent labeling in bioscience and the fabrication of nano-devices. The study may be carried forward by designing a better structure of OAC with light absorbing low band gap donor–acceptor morphology in nanoscale for better photon harvesting and thereby may increase the photocurrent in solar cell.
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