Thiophene and furan containing pyrazoline luminescent materials for optoelectronics

Author’s Accepted Manuscript Thiophene and furan containing pyrazoline luminescent materials for optoelectronics V. Ramkumar, P. Kannan

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S0022-2313(15)00525-6 http://dx.doi.org/10.1016/j.jlumin.2015.09.020 LUMIN13605

To appear in: Journal of Luminescence Received date: 28 January 2015 Revised date: 5 September 2015 Accepted date: 8 September 2015 Cite this article as: V. Ramkumar and P. Kannan, Thiophene and furan containing pyrazoline luminescent materials for optoelectronics, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2015.09.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Thiophene and furan containing pyrazoline luminescent materials for optoelectronics V. Ramkumar and P. Kannan.* Department of Chemistry, Anna University, Chennai 600 025, India. Abstract Two pyrazoline core fluorescent materials were designed, synthesized and characterized by various approaches. The synthesized materials were confirmed by standard 1H-NMR and FTIR techniques. Porous like morphologies were found from SEM analysis and the thermal stability of the materials analyzed with TGA-DSC analysis. Emission properties, quantum yield, lifetime and bandgap energy of the materials have also been studied. Solvent effect of absorption and emission spectral results indicated that the wavelength was red-shifted to increase the solvent polarity. The semiconducting nature was studied by UV-visible spectra, cyclic voltammetry analysis and theoretical calculation. I-V characteristic analysis was used to determine turn-on voltage of the materials for optoelectronics. Keywords:

Pyrazoline materials, SEM analysis, thermal behavior, optical properties, bandgap energy, comparative statement.

*

Corresponding author details:

E-mail address: [email protected] (P. Kannan).

Telephone: +91-44 2235 8666, Fax: +91-44-2220 0889.

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1.

Introduction In recent years, organic light emitting materials have a significant consideration in

stimulating scientific research field, owing to incredible growth in the organic light emitting diodes (OLED’s) and enormous growth in organic optoelectronics field. One of the key steps towards development of opto-electronic materials lies in the choice of a suitable organic emitter. Organic π-conjugated heterocyclic compounds containing a sulfur atom (thiophene) and an oxygen atom (furan) exhibit an extensive variety of optical, electrical and photoelectric applications [1, 2]. Pyrazoline derivatives are valuable material in the organic emitter research domain, due to their high fluorescence quantum yield and reasonable fluorescence lifetime, thus wide-range of solicitation in industrial research. The pyrazoline derivatives with good thermal stability and high glass-transition temperature (Tg) have also been recognized as an excellent hole transport and emissive layer materials in organic electroluminescence devices [3]. The fluorescence efficiency and device stability of OLED’s have been improved by the incorporation of highly fluorescent molecules and some of them exist in nature. Blue light emitting OLED’s has been systematically studied for several years and a lot of new high performance molecules were proposed based on different approaches. Their performance features are still lower than for green and red light emitting OLED’s, and search for new, efficient blue light emitting materials is still in urge [4,5]. Some pyrazoline compounds are used in textile industry as an optical brightener because of their high fluorescence yield and blue-light emission. 1,3,5-Triaryl-2pyrazoline based fluorescent dyes are commonly used as an optical brighteners in some optoelectronic industries. Pyrazoline based materials are excellent hole-transport and light emitting materials, which has blue as well as green emission possessions. Recently, few

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pyrazoline derivatives were used in OLED both as carrier-transporting and emitting materials. Different properties of these pyrazoline derivatives were reported by studying the effect of substituents on the absorption and fluorescence properties of this class of compounds [6, 7]. Optical effect in pyrazoline materials is useful for controlling the wavelength of electroluminescent material, several pyrazoline derivatives have also been studied in molecularly doped polymer LED’s [8]. Solvent effect is a significant tool to explain internal properties of the desired materials with respect to polarity group present in a molecule, it is mainly because of stacking alignment and intracharge transfer of respective molecule. Theoretical calculations and experimental results indicates that the effect of substituent on electronic structure will provide insights that would be valuable to ongoing research on OLED’s and guide molecular design of pyrazoline-based electroluminescent materials. In the present work, an attempt has been made to synthesize and characterize two novel thiophene and furan substituted pyrazoline materials. The substituent effect of the materials was studied and discussed. The characterization part involves physical and optical behavior of these materials. The physical properties such as thermal and morphological analyses were subjected to TGA and SEM analysis. The optical properties such as absorption and emission behavior were subjected to UV-visible and fluorescence spectrophotometers. Bandgap energies were calculated from both theoretical and experimental analysis using UV-visible spectrophotometer, CV and DFT calculation. I-V characteristic analysis was used to find threshold voltage of the materials. The comparative studies of the materials were performed and discussed. 2.

Experiments and characterization

2.1.

Materials and method

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2,5-Thiophenedialdehyde, 2-acetylthiophene, 2-acetylfuran, sodium hydroxide, ethanol and phenyl hydrazine hydrochloride were purchased from Sigma Aldrich (USA). ClaisenSchmidt condensation reaction was used to synthesize intermediate chalcone compounds from respective aldehydes and ketones [9]. The condensation reaction was used between chalcone and phenylhydrazinehydrochloride to synthesize target pyrazoline compounds [10]. 2.1.1 Synthesis of 3,3'-(thiophene-2,5-diyl)bis(1-(thiophen-2-yl)prop-2-en-1-one) (Chalcone I) 2,5-Thiophenedialdehyde (0.009 mmol) and 1-acetylthiophene (0.018 mmol) were dissolved in absolute ethanol (60 mL) and the mixture was stirred at 10 ˚C for 30 min. An aqueous solution of sodium hydroxide (5 mL, 10%) was added slowly into the reaction mixture. At the end of the reaction (6 h), the mixture was poured into ice cold water (500 mL) and set aside for 10 h. The precipitated crude solid was collected by filtration, dried, and purified by repeated crystallization [9]. 2.1.2 Synthesis of 3,3'-(thiophene-2,5-diyl)bis(1-(furan-2-yl)prop-2-en-1-one) (Chalcone II) The aforementioned method was adopted for synthesis of 3,3'-(thiophene-2,5-diyl)bis(1(furan-2-yl)prop-2-en-1-one) (Chalcone II) from 2,5-thiophene dialdehyde (0.009 mol) and 1acetylfuran (0.018 mol) as starting materials to obtain a yellow color solid [9]. 2.2

Synthesis of pyrazoline material

2.2.1 Synthesis of 2,5-bis(1-phenyl-3-(thiophen-2-yl)-4,5-dihydro-1H-pyrazol-5-yl)thiophene (Material I)

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The synthesized chalcone I (3,3'-(thiophene-2,5-diyl)bis(1-(thiophen-2-yl)prop-2-en-1one) (0.009 mol) was dissolved with absolute ethanol, followed by the addition of phenylhydrazine- hydrochloride (0.018 mol) to it and the mixture was stirred under nitrogen atmosphere for 18 h at reflux condition. Then, the reaction mixture was quenched with ice water and allowed to stand for 6h, where it gets precipitated. The precipitated product was filtered and dried for 8 h in vacuum and the resultant pyrazoline material was further purified by column chromatography [10]. 2.2.2

Synthesis

of

2,5-bis(3-(furan-2-yl)-1-phenyl-4,5-dihydro-1H-pyrazol-5-yl)thiophene

(Material II) The above disclosed procedure was adopted for synthesis of 2,5-bis(3-(furan-2-yl)-1phenyl-4,5-dihydro-1H-pyrazol-5-yl)thiophene (Material II) from chalcone II (3,3'-(thiophene2,5-diyl)bis(1-(furan-2-yl)prop-2-en-1-one) [10]. Material I: Yield: 84 %: m.p. 215 ºC: FT-IR (Fig. S1 (ESI)) (KBr) (cm-1): 3060 aromatic (–CH), 2962 aliphatic (C–H), 1563 (C=N), 1064 (C–N). 1H-NMR (500 MHz, CDCl3) (δ / ppm): 3.25 (d, 4H, –CH2); 3.75 (d, 2H, –CH); 6.75 (d, 2H, thio–CH); 7.14 (d, 4H, thio–CH); 7.39 (m, 4H, Ph– H); 7.75 (m, 4H, Ph–H); 7.97 (d, 2H, thio–H). Elemental Analysis: Anal calculated: C = 67.13; H = 4.51; N = 10.44; S = 17.92, Found: C = 67.17; H = 4.46; N = 10.47; S = 17.39. Material II: Yield: 82%; m.p. 195 ºC. FT-IR (Fig. S1 (ESI)) (KBr) (cm-1): 3040 aromatic (C–H), 2962 aliphatic (C–H), 1527 (C=N), 1045 (C–N). 1H-NMR (500 MHz, CDCl3) (δ / ppm): 2.98 (d, 4H, –CH2); 3.25 (s, 2H, –CH); 4.80 (d, 2H, thio–CH); 6.4 (d, 4H, fur–CH); 6.9 (m, 4H, Ph–H); 7.19 (m, 4H, Ph–H); 7.37 (d, 2H, fur–H). Elemental Analysis: Anal calculated; C = 71.41; H = 4.79; N = 11.10; O = 6.34; S = 6.35, Found: C = 71.43; H = 4.76; N = 11.07; O = 6.38; S = 6.36.

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Scheme I Synthesis of material I and II. 2.3.

Measurements The Fourier transform infrared (FT-IR) spectra of materials I and II were recorded in the

range of 600 to 4000 cm-1 using a Bruker IFS 66V Fourier transform spectrometer as depicted in Fig. S1. 1H-NMR spectra were recorded on a 500 Hz AVANCE III spectrometer in CDCl 3 with TMS as an internal standard and the spectra displayed in Fig. S2 and S3. Thermogravimetric analysis (TGA) was performed on a Mettler TA 3000 thermal analyzer under nitrogen atmosphere at a heating rate of 5 ºC min-1 with sample weight of 3-5 mg. The absorption spectra were recorded between 190 to 800 nm on Shimadzu (2450) UV-Vis spectrophotometer using

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chloroform as solvent. Fluorescence spectra were obtained from Perkin-Elmer-II fluorescence spectrometer. Fluorescence lifetime of pyrazoline I and II were measured by time correlated single photon counting (TCSPC) method. The fluorescence decay curves were measured by exciting molecules at 375 nm, 200 ps light using Nano LED. Cyclic voltammetry measurements were performed on a CHI 600D electrochemical analyzer at room temperature with three electrode cell in a solution of Bu4NPF6 (0.1 M) in chloroform at a scanning rate of 100 mVs-1. GC electrode acts as working electrode; a platinum foil used as a counter electrode and an Ag/AgCl electrode as a reference electrode. Reference electrode was calibrated after each measurement with ferrocene (Fc). I-V device was made-up with an arrangement of ITO/TPD/TPBI:Pyrazoline material/TPBI/ Mg:Ag. ITO (indium tin oxide) glass plate as anode and pure TPD (N,N-diphenyl-N,N-bis(3-methylphenyl)-[1-1-biphenyl]-4-4-diamine) (60 nm) was used as a hole-transporting layer of HTL. TPBI (1,3,5-Tris(1-phenyl-1H-benzimidazol-2yl)benzene) doped with various pyrazoline derivatives was used as an emission layer (EML) (2 wt %, 30 nm) while pure TPBI used as an electron-transporting layer and Mg:Ag deposited as a cathode. 3.

Results and discussion

3.1.

Physical properties

3.1.1. Thermal analysis TGA and DSC curves of the synthesized material I and II are presented in Figs. 1 and 2; the evaluation and their comparison of the results are given in Table 1. They revealed that the material I and II are thermally stable (Tdi) up to 295 ºC and 198 ºC respectively. Both I and II have two well-defined decomposition profiles. First decomposition of I (Fig. 1) start with 22 % weight loss at 295 ºC, and the second one starts weight loss of 54 % at 668 ºC followed by 3 %

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char yield at 700 ºC. Whereas, II starts decomposing at 198 ºC with 23 % weight loss, second decomposition at 553 ºC with 45 % weight loss and has a char yield of 2% at 700 ºC (Fig. 2). Differential scanning calorimetry (DSC) analysis of I and II display exothermic peak at 223 ºC and 198 ºC, respectively, as evidenced in Fig. 1 and 2. This different thermal behavior is attributed to the substituent effect of thiophene and furan in the pyrazoline materials. The results revealed that thiophene is a stiffer moiety than that of furan in the pyrazoline material, and both the materials (I & II) possesses good thermal stability which is useful for high temperature organic electronics.

3.1.2. SEM analysis The SEM micro images (Fig. 3) depicts different morphologies of synthesized products I and II. Porous surface morphology was obtained for I, while II exhibits semi porous morphology with layers, ascribed to different molecular arrangement of respective pyrazoline materials. Porous like morphology with various size structures was observed on the surface of I, formation of pores is attributed to self -assembly of free conjugated thiophene group present at the end of the molecule [11, 12]. These materials possess smooth surface and increase the efficiency of the OLED device. Porous structure materials enhance out-coupling efficiency of organic light emitting device [13]. SEM analysis revealed that material I may contribute to more out-coupling efficiency than material II.

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3.2.

Optical properties

3.2.1. Emission behavior of materials The emission spectra of the materials I and II in chloroform were depicted in Figs. 4 & 5. Material I displayed green emission at 455 nm while II exhibited blue emission at 405 nm as similar to pyrazoline family [14, 15]. This indicates that the pyrazoline derivatives emit different color fluorescence by changing the substitution in pyrazoline structure. Fluorescence spectra of the two compounds are similar as compared with various pyrazoline derivatives reported in the literature [16, 17]. The concentration effect of compound I and II on the fluorescence emission spectral intensity was studied. From the fluorescence spectra, it could be observed that the peak intensities increase in concentration as depicted in Figs. 4 and 5. Since the stacking nature is high in both the compounds, the emission spectra intensity increases with respect to concentration. The emission peak between these two materials are red shifted by 50 nm themselves, ascribed to the effect of thiophene and furan present in the respective material and different ICT and electronic effect on these compounds. Thiophene moiety enhances ICT and electronic effect in I and thus shifts fluorescence to longer wavelengths. The less electronic effect and ICT in II contribute to lower absorption and emission than I. As discussed in SEM analysis, material I has a more porous like morphology, which leads to stacking of molecules with each other. Whereas, in material II, the molecular stereo structure is complicated and interaction between each other becomes weak, that leads to semi porous morphology with less stacking nature.

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The fluorescence quantum yield (Ф) of materials I and II was measured using quinine sulfate (Ф = 0.55) as a standard (ESI) [5, 15]. The different Ф value was obtained on different solvents and is listed in Table 2. Material I possess high Ф value than material II. This difference of quantum yield (Ф) is ascribed to change of electronic push-pull substitution and stacking (planarity) of conjugated part of the molecule towards solvents [5, 18, 19].

The fluorescence lifetime of I and II was measured by TCSPC analysis using different solvents such as hexane, dioxane, chloroform, acetone and DMSO. The both mono and non monoexponential has observed for these materials I and II in various solvents as indicated in Fig. 6. The reason for non-monoexponential attributed to intra charge transfer (ICT) of the molecule. The ICT of the molecules were differing with respect to the solvent interactions. Where the high polar solvent such as acetone, DMSO may increase the ICT of the molecules, thus the exponential decay differs. On the other hand, the less polar solvent such as hexane, dioxane, chloroform has not much to play in the ICT of the molecule. The polarity of the solvent may interact with ICT of the molecules, thus changes are the reasons for non-monoexponential decay. Thus obtained a monoexponentials, the data are listed in Table 2. The obtained fluorescence decay time of materials I and II were increased with respect to polarity of solvents. It is realized that the lifetime of material II is considerably faster than the material I as material I has more planarity and stacking than material II. 3.2.3. Optical spectral shifts A small red shift (5 nm) in absorption spectra and significant red shift (50 nm) in fluorescence between the material I and II were observed and depicted in Fig. 7 and the data in

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Table 2. The presence of different terminal substituent (such as thiophene and furan) in the materials results in a small red shift (5nm) in absorption spectra and drastic red shift (50 nm) in the fluorescence spectra. This is ascribed to the material I is more planar in nature than II and enhance more flow of electrons. Further, material I possess more ICT than II and makes better absorption and emission. This is ascribed to conjugation delocalization of the electron cloud is more pronounced in thiophene substitution than furan based material. Hence, thiophene acts as a better chromophore than furan.

3.2.4. Solvent effect Solvent polarity plays a fundamental role in the photo-physical properties of these compounds. The solvent effect of material I and II were studied by UV-visible and fluorescence spectrophotometers. Fig. 8 demonstrated the images under visible, long UV and short UV light illumination of I and II in low polar to highly polar solvents. The images of sample bottles demonstrate various emission colors of I and II depending upon the polarity of solvents. The color of materials in solution was increased from blue to green with respect to the polarity of solvent. The results indicate that there is a difference in dipole moment of I and II in the excited state and ground state. The magnitude of this shift indicated that ground state of the molecule was polar [20]. The dipole moment of solution increases during electronic transition, as ground state dipole moment is smaller than excited-state dipole moment. In addition, molecules stacking will be liberal in less polar solvents whereas molecular separation and stacking will be spreading in high polar solvents and thus shifts towards blue to green emission.

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A small red shift (5nm) was observed in absorption spectra and proximal red shift obtained in emission spectra. Furthermore, the solvent effect on the fluorescence indicated that the emission wavelength of the compound was red-shifted due to increase in solvent polarity (Fig. 9 and 10) [21,22]. The absorption peak was hypsochromic with increasing the polarity of solvents, as the ground state of the compound was more stabilized in a solvent cage of already partly oriented solvent molecules with stronger polarity [23]. Stoke’s shifts value increase with increasing the solvent polarity and is consistent with stabilization of polar excited states by polar solvents. Optical property of the materials I and II on various solvents are listed in Tables. 2 and S.1. Charge transfer molecules are sensitive to changes in the external environment and produces dramatic fluorescence spectral changes. In highly polar solvents such as DMSO, the emissive S1 state of intramolecular charge transfer (ICT) character is strongly solvated and it’s energy is dramatically lowered. The charge transfer mechanism of pyrazoline occurs between N1→N2→C3, where N1 atom of pyrazoline ring is the donating-electron source of charge transfer, and replace moiety of linking C3 acts as an electron acceptor. As already observed for aromatic π-conjugated pyrazoline materials, the solvatochromism and Stoke’s shift can be useful to describe the intracharge transfer character of a set of compounds [24]. Consequently, the energy gap (Eg) is enlarged so that the coupling of S1 state directly to ground state stays open and inter system crossing from singlet to triplet state is enhanced. In less polar solvents such as toluene, all the compounds emit blue light with moderate intensity. On increasing solvent polarity the emission band also shifts bathochromically and in DMSO it emits green light as depicted in Figs. 9 and 10. The relaxed, excited state would be energetically stabilized relative to the ground state with increasing polarity of solvents and a significant red shift to fluorescence

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band was observed. Furthermore, as listed in Table 2, the increase of solvent polarity led to decrease of fluorescence the quantum yield. Fluorescence quantum yield decreased from hexane to DMSO and then increased with decrease of solvent polarity, because the solvents with high polarity (such as DMF, DMSO and ethanol) can form hydrogen bonds [24]. For example, quantum yield (Ф) of compound I in hexane and DMSO was found to be 0.72 and 0.69 respectively, due to the non-radioactive relaxation process which facilitated an increase in solvent polarity and yield lowest emission energy [23].

3.2.4. The Lippert-Mataga relationship The Lippert-Mataga correlations of the materials I and II are plotted against Stoke’s shift (∆ν cm-1) versus orientation polarizability of the solvents having various polarity parameters as shown in Figure 11. The Stoke’s shift (∆ν cm-1), dielectric constant (ε) (electric permittivity at low frequencies), refractive index of a solvent (n) and ∆f (ε, n) have been calculated from equation 1 and are listed in Table 3. Fig. 5 demonstrate that the plot deviates from linearity, similar to the previously reported literature [25,26]. In general Lippert-Mataga plot is linear, as the fluorophore energy depends on change in dipole moment when excited. Thus, when they come in contact with solvent molecules possessing various particles, they do not undergo any chemical interaction between solute and solvent, and hence a linear plot is obtained. It may be concluded that from nonlinearity of Lippert-Mataga plot, the probe molecule either forms hydrogen bonding with protic solvents or take part in a chemical reaction [26]. Accordingly, the

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non-linearity of correlations is due to the specific solvent effect, most probably the formation of intermolecular hydrogen bonding between the solute and solvent molecules [25, 26]. It is observed from the slope of these plots that the change in dipole moment for Material I is higher than that of materials II. In protic solvents specific solvent-fluorophore interaction such as hydrogen bonding is possible and the extent of this interaction would depend upon the functional groups present in the fluorophore. The protic and aprotic solvents are clearly divided, i.e. two isolated domains appear in the plot. Besides, the exclusive regression of the two isolated domains gives good linearity. Thus, the correlation of Stokes shift helps in identifying specific solventfluorophore interactions.

∆𝑓 (𝜀, 𝑛) = (

(𝜀 − 1) (n − 1) − ) (2𝜀 + 1) (2n + 1)

……….(1)

Where, Δf is the orientation polarizability, ε is the dielectric constant (electric permittivity at low frequencies), ‘n’ is the refractive index of the solvent and Δν stands for the Stokes’ shift.

3.3.

Bandgap energy determination Bandgap energy (Eg) is an essential tool for constructing optical devices. Determination

of energy gap of material is necessary to bring up the display manufacturing. Chemists usually describe the energy associated with light in terms of its wavelength in nanometers (nm), while physicists tend to refer to its energy in electron volts (eV). The color of the emission obviously depends upon the size of HOMO and LUMO energy gap, for which visible light (380-780 nm)

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corresponds to 1.5 - 3.2 eV is used. The efficiency of an OLED depends greatly upon the choice of electrode materials, as the efficiency of charge injection depends upon the size of energy barrier between the work function of electrode and energy level of HOMO (anode) or LUMO (cathode). Since balanced charge injection is required (emission requires a hole and an electron to combine), energy barriers should also be of comparable sizes. As a result, in designing new materials for use in an OLED, care must be taken to match HOMO and LUMO energies as closely as possible to decide electrode materials. An attempt has been made to produce pure green emitting pyrazoline materials. Pyrazoline are interesting candidates for green light emission, as they exhibit good stability toward light, heat and oxygen, despite their high tendency toward π-π* aggregate formation with concomitant quenching of fluorescence [27]. Various methods are in use to predict the bandgap energy (Eg), in which we used some theoretical as well as experimental calculations. Almost similar band gap energies were obtained by UV-visible spectra, cyclic voltammetry analysis and theoretical (DFT) calculations. The results revealed that the materials belong to semiconductor category. 3.3.1. UV-Vis Spectra (Kubelka-Munk function) The UV-Vis absorption spectra of material I and II were shown in Figs. 12 and 13 and their data are presented in Table 2. The spectra were recorded in chloroform, that evidenced absorption peaks at 358 nm and at 356 nm for I and II, respectively, which are assigned to allowed π-π* transition of conjugated backbone localized on the pyrazoline ring system. The absorption peaks ranged at 358 and 356 nm, as both compounds possessed similar structure and the only difference is its substituent (thiophene and furan) at the end of pyrazoline. Material I is more electron-withdrawing nature (due to the presence of thiophene) than material II (due to furan, which is less electron donating group) and this makes the charge transfer (from π-π*

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transition)more easier for Material I [28]. The absorption of the materials is red shifted from 356 nm to 358 nm, due to lack of planarity in the molecule and an increase in electronic transition energy.

The calculated band gap energy from the plot of Kubelka-Munk function (αhν)2 versus photon energy for direct band gap semiconductors is exhibited in Figs. 12 and 13 [29]. The obtained optical bandgap (Eg opt), which was determined from the absorption edge of solution spectra is given in Table 4. The optical band gap varies from 2.32 eV and 2.48 eV for I and II, which is slightly greater than 4,8-diphenyl-1,5-anthrazolines, that indicated lower band gap and largest

λmax with high thermal stability (Ti 295 ºC, Ti 205 ºC) and can be applied as an

electroluminescent material.

3.3.2. Cyclic voltammetry The electrochemical properties of both materials I and II were analyzed by cyclic voltammetry in chloroform in the presence of tetrabutylammonium hexafluorophosphate (0.10 mol L-1) as supporting electrolyte and their results are depicted in Fig. 14, and their values are given in Table 4. Electrochemical band gaps were calculated from onset potentials of anodic and cathodic waves. The cyclic voltammetry (CV) data of materials exhibited a reversible process. HOMO, LUMO and bandgap energies (Eg) were calculated using formal empirical formula (HOMO = - ([EAonset]ox + 4.8) eV, LUMO = - ([EAonset]red + 4.8) eV and Eg = LUMO - HOMO)

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[30,31]. The oxidation onsets of I and II were estimated to be 0.6 and 1.8 V, which correspond to HOMO energy levels of -5.6 and -6.6 eV, respectively [30, 32, 33]. The reduction potential onsets of I and II are -1.4 and -1.2 V, corresponds to LUMO energy levels of -3.4 and -3.6 eV, respectively, the energy gap (Eg) of the materials was found to be 2.2 eV and 2.35 eV, whose values

are

near

to

the

most

widely

used

hole-transport

material

[4,4-

bis(1naphthylphenylamino)biphenyl (NPB) (HOMO = -5.5 eV, LUMO = -2.4 eV)]. Further, this low energy gap indicates good semiconducting behavior of the material. The electron-donating strength of thiophene substituted pyrazoline unit shifts the formal reduction potential to less positive values than furan substitution. Moreover, intra-molecular charge transfer may take place between the pyrazoline and the heterocyclic rings due to the high electron cloud density of N atoms in phenyl ring. Further, this leads to a decrease of the electron cloud density in the pyrazoline ring. The band gap energy details in the pyrazoline derivatives are listed in Table 4. It may be concluded from the results that the influence of heterocyclic substitution on molecular structure leads to changes in electronic structure of these materials [34]. It has been suggested that thiophene and furan containing pyrazoline is responsible for low bandgap energy of molecule. Moreover, HOMO levels were in the range of -5.6 and -6.6 eV, which is near to the most widely used hole-transport materials. These small molecules are expected to be applicable as an electron-transporting electroluminescent material for OLED’s [7].

3.3.3. DFT calculation The ground-state geometry of compounds was optimized by hybrid density functional theory (B3LYP) with 6-31G* basis set in the Gaussian 03 program package [9]. Fig. 15

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illustrates the calculated spatial distributions of HOMO and LUMO levels of materials I and II. In ground state, material I possesses more electron density and distributions over the molecule, thus leads to form a more planar structure than material II. The theoretically predicted energy (HOMO/LUMO) energy levels and bandgap energy of compounds are listed in Table 4. [7].

The calculated band gap of materials I, II are 2.32 and 2.87 eV respectively. The HOMO level energy of I and II are -5.38 and -6.32 eV. The LUMO energy level of I and II are -3.06 and -3.45 eV, respectively. The molecule takes a non-planar configuration, which helps to impede pep stacking interaction with solid state to some extent. Fig. 15 illustrates the calculated spatial distributions of HOMO, LUMO levels of I and II. As noticed clearly, HOMO is a π orbital concentrated on central pyrazoline moiety; LUMO is of π* character distributed on pyrazoline ring and benzene ring for pyrazoline ring of I and II energy diagram. The main orbitals (Fig. 15) visualize rather typical changes in the electron density distributions. The experimental values of cyclic voltammetry and UV-Vis spectra and theoretically calculated values are listed in Table 4. 3.4.

I-V characterization A typical device with configuration of ITO/TPD/TPBI:Pyrazoline material/TPBI/Mg:Ag

was used to find threshold (turn on) voltage of materials I and II. The current density (I) - bias voltage (V) curve of device points a turn on voltage (Von) of 1.8 V for I and 2.7 V for II and are shown in Fig. 16. The threshold voltage to operate these devices, defined here as the voltage at which 1.8 and 2.7 V. The reason for material I to has less turn-on voltage is the presence of sulphur (thiophene) which has more conducting or semiconducting nature than oxygen (contains furan), and pyrazoline containing thiophene moiety acts as a good semiconductor. Since this

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device has excellent hole-injection and transport layers, the low turn on voltage of this device suggests that I and II must have high electron injection ability. In comparison, material I has good semiconducting nature and possess more efficiency than material II.

3.5.

Comparison and structural relationship The comparative statement explains about similarities and dissimilarity of the synthesized

materials I & II. The difference between these two materials is that one has thiophene as and the other has furan terminal group. These differences results in conjugation degree and electronic effect of the compounds [35]. Fig. 17 demonstrate the electronic structure of the materials I and II, which describes that I has more electronic cloud while II has less electronic cloud at HOMO level. Thiophene is more flexible and possesses semiconducting property, extended conjugation, electron cloud, better stacking between each other and enhanced ICT when compared with furan, as interpreted from characterization studies. Thiophene containing pyrazoline acts as a good opto-electronic material than furan based derivative. Thus by varying the substituent in pyrazoline, their derivatives exit different fluorescence. The results also evidence to pyrazoline derivatives emitting different color emissions [15]. In general, the absorption and fluorescence spectra of these materials are similar to pyrazoline derivatives reported in the literature [16,17]. Though both materials I and II are good in optoelectronic applications, I is more potential than II.

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4.

Conclusions In summary, two novel thiophene and furan substituted pyrazoline compounds have been

synthesized and characterized by various techniques. Their optical, electrochemical properties and theoretical (DFT) studied were explained. The emission spectra of these two compounds exhibit different emission behavior from green to blue. Moreover, both compounds exhibit high quantum yields (Ф) (0.72, 0.69) and fluorescence lifetime (3.8, 2.6 ns). The band gap energy of these two materials was examined by UV-visible spectrophotometer, Cyclic voltammetry and DFT calculations establishing ideal semiconducting nature. I-V characteristic results demonstrate the turn-on voltage of materials I and II to be 1.8 and 2.3 V. The change of molecule in the end position had a distinct effect while side-group substitution of different electronic cloud materials. The results predict that the synthesized pyrazoline compounds are potential materials for optoelectronics application. Acknowledgements The instrumentation facility provided under FIST-DST and DRS-UGC to Department of Chemistry, Anna University is sincerely acknowledged. VR gratefully acknowledges the award of Meritorious UGC-BSR fellowship, New Delhi. Help rendered by Dr. Hubert, Joe towards the theoretical calculation is greatly acknowledged. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/xxx/xxx. References

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24

List of Tables. 1). Table 1 Thermal behavior of material I and II. 2). Table 2 Optical properties of material I and II on selective low to high polar solvents. 3). Table 3 Lippert-Mataga relationship parameters of material I and II. 4). Table 4 Bandgap energy comparisons of material I and II.

25

Table 1 Thermal behavior of material I and II.

Material

Tdi

Tdf

TRmax

Tm

( ºC)

( ºC)

( ºC)

( ºC)

I

295

700

556

223

II

205

681

502

198

Initial decomposition temperatures (Tdi), final decomposition temperatures (Tdf), maximum degradation rate temperature (TRmax) and melting point (Tm) of material I & II establish from TGA-DSC curves.

26

Table 2 Optical properties of material I and II on selected low to high polar solvents.

Material I

Material II

Abs

Ems

∆

QY

LT(τ)

Abs

Ems

∆

QY

LT(τ)

(nm)

(nm)

(nm)

(Ф)

ns

(nm)

(nm)

(nm)

(Ф)

ns

Hexane

355

400

45

0.72

1.9

355

390

35

0.69

1.4

Dioxane

360

423

63

0.70

2.3

355

399

44

0.67

1.9

CHCl3

358

440

82

0.72

3.8

356

405

49

0.62

2.6

Acetone

360

450

90

0.68

4.2

358

420

62

0.60

3.4

DMSO

365

470

105

0.63

4.9

360

445

85

0.61

4.1

Solvent

27

Table 3

Lippert-Mataga relationship parameters of material I and II.

Material I

Material II

Abs

Ems

ν cm-1

ν cm-1

∆ν cm-1 ∆f (ε, n)

(nm)

(nm)

(Abs)

(Ems)

Hexane

355

400

28169

25000

3169

Benzene

358

401

27932

24937

Ether

357

402

28011

Dioxane

360

423

CHCl3

358

Acetone

∆ν cm-1 ∆f (ε, n)

Abs

Ems

ν cm-1

ν cm-1

(nm)

(nm)

(Abs)

(Ems)

0.2953

355

390

28169

25641

2527

0.2953

2995

0.1485

356

392

28089

25510

2579

0.1485

24875

3135

0.0095

356

391

28089

25575

2514

0.0095

27777

23640

4137

0.1991

355

399

28169

25062

3106

0.1991

455

27932

21978

5954

0.1661

356

405

28089

24691

3398

0.1661

360

450

27777

22222

5555

0.0013

358

420

27932

23809

4123

0.0013

EtOAc

361

455

27700

21978

5722

0.0048

354

433

28248

23094

5153

0.0048

THF

360

459

27777

21786

5991

0.3088

356

431

28089

23201

4888

0.3088

EtOH

360

456

27777

21929

5847

0.2889

356

435

28089

22988

5101

0.2889

MeOH

362

458

27624

21834

5790

0.2087

357

437

28011

22883

5127

0.2087

DMAF

361

465

27700

21505

6195

0.2629

359

441

27855

22675

5179

0.2629

DMF

363

466

27548

21459

6088

0.2756

359

442

27855

22624

5230

0.2756

DMSO

365

470

27397

21276

6120

0.2826

360

445

27777

22471

5305

0.2826

Solvent

28

Table 4

Bandgap energy comparisons of material I and II.

DFT Material

CV

UV

I-V

HOMO

LUMO

Eg

HOMO

LUMO

Eg

Eg

(eV)

(eV)

(eV)

(eV)

(eV)

(eV)

(eV)

voltage(V)

I

-5.38

-3.06

2.32

-5.6

-3.4

2.2

2.32

1.8

II

-6.32

-3.45

2.87

-6.6

-3.6

2.35

2.48

2.7

Turn

on

29

Figure Legends 1). Fig. 1 TGA-DSC thermogram of material I. 2). Fig. 2 TGA-DSC thermogram of material II. 3). Fig. 3 SEM morphology images of material I and II. 4). Fig. 4 Emission spectra of material I (λex = 372 nm) at 0.006 - 0.001 % concentrations in chloroform solvent. 5). Fig. 5 Emission spectra of material II (λex = 357 nm) at 0.006 - 0.001 % concentrations in chloroform solvent. 6). Fig. 6 Fluorescence decay curves of material I and II. 7). Fig. 7 Fluorescence spectra shift between material I and II. 8). Fig. 8 Solvent effect of material I and II on various solvent under short UV (254 nm), long UV (365 nm) and visible (420 nm) light illumination. a). Acetone b). chloroform c). Dioxane d). Ethyl acetate e). Ether f). Hexane g). Benzene h). MeOH i). EtOH j). THF k). DMSO l). DMF m). DMAF. 9).

Fig.

9

Emission

spectra

(Excited

wavelength

at,

Hexane

=

355

nm,

Dioxane = 360 nm, chloroform = 358 nm, Acetone = 360 nm, DMSO = 365 nm) of material I in different solvents (0.006 %). 10). Fig. 10 Emission spectra (Excited wavelength, Hexane = 355 nm,

30

Dioxane = 355 nm, CHCl3 = 356 nm, Acetone = 358 nm, DMSO = 360 nm) of material II in different solvents (0.006 %). 11). Fig. 11 Lippert-Mataga plot of material I and II, constructed by plotting the Stokes’ shift of I and II in the different solvents (1. Hexane, 2. Benzene, 3. Ether, 4. Dioxane, 5. Chloroform, 6. Acetone, 7. Ethylacetate, 8. Tetrahydrofuran, 9. Ethanol, 10. Methanol, 11. DMAF, 12. DMF, 13. DMSO) reported with their corresponding Lippert solvent parameters. 12). Fig. 12 Absorption spectrum (a) and bandgap plot (b) of material I. 13). Fig. 13 Absorption spectrum (a) and bandgap plot (b) of material II. 14). Fig. 14 Cyclic voltammogram of material I and II. 15). Fig. 15 HOMO-LUMO (DFT) energy diagrams of material I and II. 16). Fig. 16 Current density (I) Vs voltage (V) characteristics of material I and II. 17). Fig. 17 HOMO energy differentiation of material I and II.

31

Heat flow (mW/mg)

Fig. 1 TGA-DSC thermogram of material I.

32

Heat flow (mW/mg)

Fig. 2 TGA-DSC thermogram of material II.

33

Material I

Material II

Fig. 3 SEM morphology images of material I and II.

34

350

(a.u) Fluorescence Indensity (a.u) (a.u) Intensity Fluorescence (a.u) Intensity Fluorescence Indensity Fluorescence

1000 300

0.006 % CHCl3

250 800 200

600 150

100 400 50

200 0 350

400

0.001 % CHCl3 450

500

550

600

Wavelength (nm)

0 400

450

500

550

600

650

Wavelength (nm) Fig. 4 Emission spectra of material I (λex = 372 nm) at 0.006 - 0.001 % concentrations in chloroform solvent.

35

(a.u) Intensity Fluorescence (a.u) Indensity Fluorescence (a.u) Intensity Fluorescence (a.u) Indensity Fluorescence

350 350

0.006 % CHCl3

300 300 250 250 200 200 150 150 100 100 50 50 0 0

350 350

0.001 % CHCl3

400 400

450 500 450 500 Wavelength (nm)

550 550

600 600

Wavelength (nm)

Fig. 5 Emission spectra of material II (λex = 357 nm) at 0.006 - 0.001 % concentrations in chloroform solvent.

36

B

B 10000

10000

10000

Material I in Hexane

Material I in Dioxane

Material II in Hexane

Material II in Dioxane

1000 1000

1000

100

Counts

Counts

Counts

Counts

1000

100

100

100

10 8

10

12

14

16

18

202

4

6

8

10

12

14

16

18

20

0

10

20

Time (ns)

Time (ns)

Material I

30

50

B

Material I in Acetone

Material II

in CHCl3

40

Time (ns)

5000

in CHCl34000 3000

Counts

Counts

6

0

10

20

30

B 40

Time (ns)

Material II in Acetone 1000

100

2000

1000

10 0 0

B

Material I in DMSO

10

20

30

40

B

Time (ns)

0 50

10

20

Time (ns)

Material II in DMSO

1000

Counts

4

Counts

2

1000

100

10

100 0

10

20

30

Time (ns)

40

50

0

5

10

15

20

25

30

35

40

Time (ns)

Fig. 6 Fluorescence decay curves of material I and II on different solvents.

30

40

37

350 350

Fluorescence (a.u) Intensity Fluorescence (a.u) Intensity(a.u) Fluorescence (a.u) Indensity FluorescenceIndensity

50 nm

300 300

250 250

200 200

(II)

150 150

(I)

100 100 50 50 00 350 350

400 400

450 450

500 500

550 550

Wavelength (nm) Wavelength (nm) Fig. 7 Fluorescence spectra shift between material I and II.

600 600

650

38

Visible light

Material I a

b

c

d

e

g

f

h

j

i

l

k

m Long UV

a

c

b

d

e

g

f

h

j

i

k

m

l

Short UV a

b

c

d

e

f

g

i

h

j

k

l

Material II a

b

m

Visible light c

d

e

f

g

h

i

j

k

l

m

Long UV a

a

b

b

c

c

d

d

e

e

f

f

g

g

h

h

i

i

j

j

k

k

l

m

Short UV m l

Fig. 8 Solvent effect of materials I and II in various solvent under short UV (254 nm), long UV (365 nm) and visible (420 nm) light illumination. a). Acetone b). Chloroform c). Dioxane d). Ethyl acetate e). Ether f). Hexane g). Benzene h). MeOH i). EtOH j). THF k). DMSO l). DMF m). DMAF.

39

1000 350

(a.u) Intensity (a.u) Fluorescence Fluorescence Indensity

(a.u) Indensity Fluorescence (a.u) Intensity Fluorescence



300

800 250

Hexane Hexane CHCl 3 Dioxane Dioxane CHCl3 Acetone Acetone DMSO DMSO

600 200 150

400

200

100

50

0 350

400

450

500

550

600

Wavelength (nm)

0 350

400

450

500

550

600

Wavelength (nm) Fig. 9 Emission spectra (Excited wavelength, Hexane = 355 nm, Dioxane = 360 nm, chloroform = 358 nm, Acetone = 360 nm, DMSO = 365 nm) of material I in different solvents (0.006 %).

40

350

(a.u) Fluorescence Intensity (a.u) Fluorescence Indensity (a.u) Intensity Fluorescence Fluorescence Indensity (a.u)

1000 300

Hexane hexane Dioxane dioxane CHCl3 chcl3 Acetone acetone DMSO dmso

800 250

200

600

400

200

150

100

50

0

0

350

350

400

400

450

500

Wavelength (nm) 450

500

550

550

600

600

Wavelength (nm) Fig. 10 Emission spectra (Excited wavelength, Hexane = 355 nm, Dioxane = 355 nm, chloroform = 356 nm, Acetone = 358 nm, DMSO = 360 nm) of material II in different solvents (0.006 %).

41

6500 6000

9 11

7

Material I 6

5500

5000

12

8

5

5500

13

10

4500

∆ν cm-1

∆ν cm-1

5000

3

4000

Material II

10

9

4000

6 5

2

4

2

8

4500

3000

1 3000

12 11

7

3500

3500

13

4

1

2500

3

2500 0.00

0.05

0.10

0.15

0.20

∆f (ε, n)

0.25

0.30

0.35

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

∆f (ε, n)

Fig. 11 Lippert-Mataga plot of material I and II, constructed by plotting the Stokes’ shift of I and II in the different solvents (1. Hexane, 2. Benzene, 3. Ether, 4. Dioxane, 5. Chloroform, 6. Acetone, 7. Ethylacetate, 8. Tetrahydrofuran, 9. Ethanol, 10. Methanol, 11. DMAF, 12. DMF, 13. DMSO) reported with their corresponding Lippert solvent parameters.

a) a)

Fig. 12 Absorption spectrum (a) and bandgap plot (b) of material I.

42

a) a)

b)

Fig. 13 Absorption spectrum (a) and bandgap plot (b) of material II.

43

44

Fig. 14 Cyclic voltammogram of material I and II.

45

Material I

LUMO = -3.06 eV

Eg = 2.32 eV

HOMO = -5.38 eV

Material II

LUMO = -3.45 eV Eg = 2.87 eV

HOMO = - 6.32 eV

Fig. 15 HOMO-LUMO (DFT) energy diagrams of materials I and II.

46

Fig. 16 Current density (I) vs. Voltage (V) characteristics of material I and II.

47

Fig. 17 HOMO energy differentiation of material I and II.

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