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Two pyrazoline (PYR) materials with phenyl and thiophene terminal group substituents (PPYR and TPYR, respectively) were synthesized, and their optical properties were examined. Variation of the terminal group allowed manipulation of the physicochemical properties of PYR based on differences in the intracharge transfer of the molecules. The peak maxima of the absorption/emission spectra PPYR and TPYR differed, occurring at 348/439 and 366/446 nm, respectively. The difference in the optical properties was due to greater electron transfer of the thiophene group compared with the phenyl group, which led to an increase in the intracharge transfer of the thiophene-containing molecule. These results demonstrate that the physicochemical properties of PYR materials can be tuned by modification with suitable substituents.
© 2016 Optical Society of America
1. Introduction
Organic light-emitting materials have received considerable attention in photonics because of their potential utility in lights, back-lights, and organic light-emitting diode (OLED) displays [1,2]. Many recent studies have focused on improving the efficiency of electroluminescent devices using multilayered structures, doped emitting layers, and efficient injection contacts [3–5]. One of the key issues is to increase the current efficiency of OLEDs by controlling the charge carrier transport by adjusting the hole and electron injection. A normal OLED device has a sandwich-type multi-layer structure consisting of an anode, a hole-transporting layer (HTL), an emission layer (EML), an electron-transport layer, and a cathode. The development of EML materials is critical for achieving high color purity, current efficiency, and luminance. In addition, judicious combination of the highest occupied molecular orbital (HOMO) and lowest occupied molecular orbital (LUMO) levels is also important for achieving high luminance and efficiency. Among the known EML materials, small organic molecules are excellent candidates for OLEDs because of their advantages, such as high purity, easy processing, and high efficiency. Thus, the development of novel fluorescent molecules is necessary to enhance the efficiency and stability of OLED devices [6–8].
Recently, pyrazoline (PYR)-based materials have attracted increasing attention for optoelectronic applications, such as non-linear optical devices, fluorescence brighteners, OLEDs, and solar cells [9–11]. PYR-based materials exhibit excellent luminescent properties because they act as both the HTL and EML and provide optimum HOMO and LUMO levels [11–13]. The nitrogen groups in PYR facilitate electron transfer, and the extended conjugation in PYR results in bright luminescence of the molecule [4–6]. Intramolecular charge transfer (ICT) is a beneficial feature of many organic materials. The optical and physicochemical properties of the material depend on the overall charge-transfer of the molecule. For instance, reducing doping defects can suppress the recombination of photogenerated charge carriers, resulting in improved photocatalytic activity [14]. ICT arises from donor/acceptor interactions in such charge-transfer materials, and these interactions in turn affect the ferromagnetic, nonlinear optical, and conducting properties of the respective materials [15].
The tuning colors are important in OLED displays; thus, extensive effort to develop organic light-emitting device (OLED)-based technology for commercially viable displays and lighting devices is underway. Blue, green, and red OLEDs are required to develop full color displays. It is reported that the performance of blue emitting materials in blue OLEDs is not as efficient as that of green or red emitting materials, where the main problems are the color purity, stability, and lifetime of blue OLEDs [16]. To achieve a full-color display, efficient tunability of the emission spectrum to a desired color is an important consideration when designing materials. Modification of the core and peripheral units of organic molecules and substitution of various groups are potential approaches for tuning the emission of these materials [17,18]. Through variation of the substituent pattern and the concomitant change of the backbone distortion, the emission color of organic materials can be tuned from blue to deep red. The emission color of the materials is governed by the substituent groups due to overlap of the intermolecular orbital; thus, variation of the substituents is considered a much easier, more efficient, and competitive approach for manufacturing different types of displays compared to other approaches.
In this study, we investigate the synthesis and characterization of two PYR materials with phenyl and thiophene terminal-group substituents (PPYR and TPYR, respectively). The structural and optical properties of PPYR and TPYR are analyzed using field-emission scanning electron microscopy (FE-SEM), Fourier-transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), ultraviolet-visible (UV-Vis) spectrophotometry, and electroluminescence (EL) spectroscopy. The bandgap energy (Eg) of these species is calculated based on both experimental and theoretical analysis.
2. Experimental
Synthesis of (3E,3′E,5E,5′E)-3,3′,5,5′-tetrabenzylidene-[1,1'-bi(cyclohexane)]-4,4'-dione (chalcone I)
1,1'-Bi(cyclohexane)-4,4'-dione (0.009 mmol) and benzaldehyde (0.036 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 slowly added to the reaction mixture. At the end of the reaction (6 h), the mixture was poured into ice water (500 mL) and set aside for 10 h. The precipitated crude yellow solid was collected by filtration and dried and purified using the process described above [9–12].
Synthesis of (3E,3′E,5E,5′E)-3,3′,5,5′-tetrakis(thiophen-2-ylmethylene)-[1,1'-bi(cyclohexane)]-4,4'-dione (chalcone II)
The process described above was also used to synthesize (3E,3′E,5E,5′E)-3,3′,5,5′-tetrakis(thiophen-2-ylmethylene)-[1,1'-bi(cyclohexane)]-4,4'-dione (chalcone II) from 1,1'-bi(cyclohexane)-4,4'-dione (0.009 mmol) and 2-thiophenecarboxaldehyde (0.036 mmol) as starting materials to obtain a yellow solid.
Synthesis of (7E,7'E)-7,7'-dibenzylidene-2,2',3,3′-tetraphenyl-3,3a,3′,3′a,4,4',5,5′,6,6',7,7'-dodecahydro-2H,2'H-5,5′-biindazole (PPYR)
The synthesized chalcone I (0.009 mmol) was dissolved in absolute ethanol and mixed with phenylhydrazine hydrochloride (0.018 mmol). The mixture was stirred under nitrogen for 18 h under reflux. The reaction mixture was quenched with ice water and allowed to stand for 6 h. The precipitated material was filtered and dried for 8 h under vacuum and the resultant PYR material was further purified by column chromatography [9–11]. The yield of PPYR was ~84% with Fourier-transform (FT-IR; KBr, cm−1) peaks at 2998 (aromatic; Ar–CH), 2880 (aliphatic; A–CH), 1596 (C = N), and 1182 cm−1 (C = N) and 1H-NMR (500 MHz, CDCl3) (δ/ppm) peaks at 1.37 (m, 2H, BiCy–CH), 1.51 (m, 4H, BiCy–CH), 1.81 (t, 2H, BiCy–CH), 2.18 (t, 2H, BiCy–CH), 3.21 (m, 2H, Pyr–CH), 4.43 (d, 2H, Pyr–CH), 5.95 (d, 2H, Bz–H), 7.13 (m, 2H, Ph–H), 7.23 (m, 10H, Ph–H), 7.31 (m, 4H, Ph–H), 7.36 (m, 10H, Ph–H), and 7.49 (m, 6H, Ph–H) ppm.
Synthesis of (7E,7'E)-2,2'-diphenyl-3,3′-di(thiophen-2-yl)-7,7'-bis(thiophen-2-ylmethylene)-3,3a,3′,3′a,4,4',5,5′,6,6',7,7'-dodecahydro-2H,2'H-5,5′-biindazole (TPYR)
The procedure described above was adopted for synthesis of TPYR from chalcone II. The yield of TPYR was ~81% with FT-IR (KBr, cm−1) peaks at 2995 (Ar–CH), 2816 (A–CH), 1568 (C = N), and 1164 (C = N) cm−1 and 1H-NMR (500 MHz, CDCl3) (δ/ppm) peaks at 1.31 (m, 2H, BiCy–CH), 1.67 (m, 4H, BiCy–CH), 2.01 (m, 2H, BiCy–CH), 2.46 (m, 2H, BiCy–CH), 3.25 (m, 2H, Pyr–CH), 5.01 (d, 2H, Pyr–CH), 6.17 (d, 2H, Bz–H), 6.97 (m, 4H, Ph–H), 7.16 (m, 4H, Ph–H), 7.24 (m, 2H, Ph–H), 7.31 (m, 4H, Ph–H), and 7.48 (m, 8H, Thio & Ph–H) ppm.
Characterization
The FT-IR spectra of PPYR and TPYR were obtained using an FT-IR spectrometer (Bruker IFS 66 V). High-resolution 1H-NMR spectra were obtained using a 500 Hz AVANCE III spectrometer using CDCl3 with tetramethylsilane as an internal standard. The absorption spectra of the synthesized materials in ethanol were obtained using a Shimadzu (2450) UV-Vis spectrophotometer, and the emission spectra were obtained using a fluorescence spectrophotometer (Perkin-Elmer II). The surface morphologies of the materials were examined using FE-SEM (Hitachi S 3000H).
3. Results and discussion
A schematic of the synthesis of the target materials, PPYR and TPYR, is presented in Fig. 1(a). The standard Claisen-Schmidt condensation reaction was used to synthesize intermediate chalcone compounds from the respective aldehydes and ketones [9]. The condensation reaction between chalcone and phenylhydrazine hydrochloride was used to obtain the PYR compounds [10–13]. The FE-SEM images indicate that PPYR and TPYR exhibited different morphologies with an uneven flake-like shape and spherical shape, respectively, because of the effect of the terminal group. The phenyl group induced layer-by-layer aggregation of the molecules and π–π stacking interaction between the molecules, resulting in an uneven flake-like structure. However, the five-membered thiophene group induced the push–pull effect in the molecules to form a spherical morphology in the case of TPYR. Figure 1(c) presents two different images of the blue emission of PPYR and TPYR under UV (254 and 365 nm) and visible (520 nm) illumination. The emission of PPYR and TPYR is related to the charge transfer mechanism of the molecule, where charge transfer occurs between the N1–N2–C3 regions of the PYR group. The substituent in the 3-position acts as an electron-transfer and linking group in relation to the 1-position of PYR. In the excited state, the molecule is polarized via intramolecular conjugated charge transfer. Delocalization of electrons occurs within the molecules, which results in strong fluorescence of the material [19,20].
Fig. 1 (a) Synthesis scheme, (b) FE-SEM images, and (c) fluorescence emission (blue) under exposure of PPYR and TPYR to UV light.
The synthesized materials were characterized using FT-IR and 1H-NMR analysis, and the results are presented in Fig. 2. The FT-IR spectra of PPYR and TPYR are presented in Fig. 2(a). The stretching vibration at 1483 cm−1 corresponds to aromatic C = C. The characteristic peak of the C–H group appears at 800–1200 cm−1. The characteristic N = N band was observed at 1557 cm−1 [12]. An intense sharp band at 1171 cm−1 due to the C–N stretching vibration was also observed, and a –CH2 ring stretching peak was observed at 2857 cm−1. The 1H-NMR spectra of the PYR compounds (Fig. 2(b)) show multiplets between 1.25 and 6.25 ppm, corresponding to the respective aliphatic –CH and –CH2 protons. The multiplet peaks at 7–9 ppm are due to the presence of aromatic protons in PPYR and TPYR [10].
Figure 3 presents the absorption and emission spectra of PPYR and TPYR. Figure 3(a) presents the UV-Vis absorption spectra of PPYR and TPYR acquired using ethanol as the solvent. The absorption peaks of PPYR at ~348 and ~366 nm respectively correspond to the vibronic bands of the first electronic transition (S0–S1) of the PYR ring [12,13]. The UV-Vis absorption peak at ~250 nm is due to an allowed π−π* transition of the material [21,22]. The longer-wavelength absorption in the region of 2500–3500 cm−1 of these materials can be assigned to an aromatic stretching band of C–H vibration and intracharge transfer (ICT) of the conjugated localized PYR ring system [10]. The shortwave shoulder in the region of 348–366 nm is attributed to vibronic transitions in PYR, and the long-wave maximum is associated with the LUMO–HOMO radiative transition in the PYR molecule [13]. The phenyl and thiophene substituents in PYR absorb in different ranges. Because TPYR has greater intracharge transfer than PPYR, the TPYR peak is red-shifted by 18 nm compared with the PPYR peak.
Fig. 3 (a) UV-Vis absorption spectra, (b) fluorescence emission spectra, and EL spectra of PPYR and TPYR.
The emission spectra of PPYR and TPYR in ethanol solvent are shown in Fig. 3(b). In general, PYR compounds are capable of very strong emission after excitation [11–13]. PPYR and TPYR exhibited blue emission with peak maxima at ~439 and ~446 nm, respectively. TPYR has a peak at longer wavelength because of its thiophene group. PPYR and TPYR both have a phenyl group at the 1-position of the PYR ring. The differences in the fluorescence properties of PPYR and TPYR are due to the presence of the phenyl and thiophene group at the end. The molecular structures of PPYR and TPYR have conjugated–N1–N2 = C3– groups. TPYR exhibits stronger fluorescence and longer emission wavelength than PPYR because of the stronger charge transfer of TPYR than that of PPYR. The carbon atom at the 5-position of PYR is sp3 hybridized. If PYR is substituted with a phenyl group, the group will be located at the corner of a pyramid. PYR derivatives can exhibit different emissions if suitable substituents are present in the material. The emission peak of TPYR was red-shifted by ~7 nm relative to the emission peak of PPYR because of the greater ICT and electronic effect in TPYR. The thiophene group in TPYR enhances the ICT and electronic delocalization. The electronic delocalization of PPYR is less pronounced than that of TPYR [23–25].
The EL spectra of PPYR and TPYR are presented in Fig. 3(c). The electroluminescent devices were fabricated with the indium tin oxide (100 nm)/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (20 nm)/poly(N-vinylcarbazole) (20 nm)/PPYR (60 nm) or TPYR (60 nm)/ZnO (40 nm)/Al (100 nm) configuration. Sharp emission peaks at 440 and 450 nm with full-width-at-half-maximum (FWHM) values of 41 and 45 nm were observed (Fig. 3(c)) for PPYR and TPYR, respectively. A narrow FWHM was obtained because of the dominant single transition in PPYR and TPYR and exciplex formation between PYR and the ZnO layer [26,27]. An exciplex can be formed between the ground state of a donor molecule and the excited state of an acceptor molecule [28]. The exciplex band corresponds to the transition from the excited state of the acceptor to the ground state of the donor. The intrinsic emission band corresponds to the transition between the excited and ground states of the molecule [29]. The EL spectrum of the TPYR device showed an obvious shift toward the long-wavelength region, which may be attributed to ICT of TPYR. Different emission peaks were observed in the fluorescence and EL spectra of PPYR and TPYR because fluorescence occurs when the molecules participate in excitation–relaxation, whereas EL relaxation occurs in recombination zones, typically at the center of the material.
The Eg values of the materials were determined from the absorption edge of the solution spectra. The Eg values of the materials were obtained from a Kubelka–Munk function (αhʋ)2 vs. photon energy (hʋ) plot (Fig. 4). Using Eq. (1), the Eg values of PPYR and TPYR were calculated to be 2.78 and 3.16 eV, respectively.
Here, A is an absorbance parameter independent of hʋ and Eg, and α is the absorption coefficient [10,30].The HOMO–LUMO energy level and theoretical Eg values of PPYR and TPYR were calculated using hybrid density functional theory (B3LYP) with the 6-31G* basis set in the Gaussian 03 program package [9,10]. Figure 5 shows the spatial distributions of the HOMO and LUMO levels of PPYR and TPYR. The theoretically predicted energy levels were determined using ethanol as a solvent, and the HOMO–LUMO energy levels and Eg values of PPYR and TPYR are listed in Table 1. Electron transfer from the HOMO to the LUMO level caused a decrease in the electron density of the electron-donating group of the PYR-ring-based system [31]. The HOMO is characterized by a π-orbital electronic configuration and the LUMO has π* character that is delocalized over the PYR compound. The HOMO, LUMO, and Eg reflect the photo-physical activity of the molecule. A smaller Eg between the HOMO and LUMO facilitates excitation of the HOMO electrons in the molecule [32]. The HOMO–LUMO level energy of PPYR and TPYR were −4.73/-1.50 and −4.84/-1.64 eV, respectively. The calculated Eg values of PPYR and TPYR were 3.23 and 3.20 eV, respectively. PPYR has a phenyl unit as an end group, which leads to delocalization of the electron cloud of the material. Thus, PPYR molecules were well aligned in the aggregates, and the stacking of the molecules was well ordered. In TPYR, the terminal group is the electron donating thiophene group; this group contains sulfur and there is greater molecular interaction within the molecular system. The results indicated that the synthesized materials were semiconducting category.
Table 1. Optical properties and Eg values of PPYR and TPYR
4. Summary
In summary, two PYR materials with phenyl or thiophene terminal group substituents were fabricated. The phenyl substituent exhibits uneven flake-like morphology, whereas the thiophene substituent possesses sphere-like morphology. Analysis of the photo-physical properties of PPYR and TPYR showed absorption peaks at 348 and 368 nm, respectively, and emission peaks at 439 and 446 nm, respectively. The difference in the emission of the molecules was mainly due to the different terminal substituents and charge transporting ability of PPYR and TPYR. Thiophene substitution results in stronger emission at longer wavelength than phenyl substitution because of the excellent charge transfer and π-conjugated properties of the thiophene group. These results indicate that the emission peak can be manipulated using phenyl and thiophene as different terminal group substituents.
Funding
National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (2014R1A2A1A11049450, 2015R1A2A1A15053268, and 2016M3A7B4910458).
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