Synthesis and Characterization of Fluorene-Based Polymers Having Azine Unit for Blue Light Emission

. Two types of donor-acceptor copolymers were designed and synthesized by combination of an electron donor unit of fluorene sequences and an electron acceptor azine unit such as 1,2,4,5-tetrazine and 1,3,5-triazine. They were well soluble in common organic solvents with the number average molecular weight ( M n ) of 7.0 and 14.5 kg mol -1 , respectively, and have good thermal stability showing about at 360 °C with 5 wt% loss in TGA. Two copolymers exhibited intense blue photoluminescence with emission peak maxima at 437 and 421 nm in CHCl 3 , and 451 and 422 nm in the film state, respectively. These polymers exhibited good fluorescence quantum efficiencies in CHCl 3 (  ﬂ = 0.63, 0.97). Energy levels of the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels estimated by cyclic voltammetry were to be –5.83, –6.0 eV and –2.85, –2.88 eV,


Introduction
Conjugated polymers have gained large interest through various organic electronic applications such as organic light emitting diodes (OLEDs) [1][2][3], light emitting electrochemical cells (LECs) [4][5][6], organic solar cells [7,8] and organic thin-film transistors [9]. Blue light-emitting materials are of great significance for their unique applications in flat-panel displays and high-density information storage. Especially in full-color displays, the short-wavelength emission can serve as an excitation source for emission over the whole visible range [10]. Therefore, development of high-efficiency blue emitting materials is important. However, to develop stable pure blue polymeric emitters with their color coordinates in the Commission Internationale de L'Eclairage (CIE) chromaticity diagram within the standard blue (CIE: x< 0.15; y< 0.1) are still rare since the large energy bandgap (E g ) of these materials makes the achievement of high efficiency and a good charge balance difficult [11]. In this framework, the rational design of p-type conjugated polymers by incorporating electron-withdrawing groups, such as azine unit, seems to be a straightforward strategy to tune carrier injection and transporting properties of known materials [12]. A new donor-acceptor (D-A) system has been designed, which is expected to lower both the energy level of the highest occupied molecular orbital (E HOMO ) and energy level of the lowest unoccupied molecular orbital (E LUMO ), giving rise to a series of intriguing properties such as improved resistance to oxidation, facilitated electron injection, and ambipolar characteristics, which could simplify device fabrication and operation.

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Volume 62 placed in an ice bath for further solidification. The orange precipitate was collected by filtration and washed with cold ethanol (2 x 10 mL) to give a crude dihydrotetrazine, which was used in the next step without further purification.

General Method and Instrumentation
All synthetic manipulations were performed by a standard technique using a Schlenk tube under an argon atmosphere. Column chromatography was performed using a silica gel (Kanto Chem., 60 N, 63-120 mm). Nuclear magnetic resonance (NMR) spectra were recorded on a JEOL JNM-ECS 400 spectrometer. 1 H and 13 C chemical shifts are given in units of δ (ppm) relative to δ (TMS) = 0.00 and δ(CDCl 3 ) = 77.0 ppm, respectively. Photoabsorption in the range of ultraviolet-visible (UV-vis) and photoluminescence (PL) measurements of the polymer samples in CHCl 3 and in a form of a thin film coating on a quartz glass were performed using a Shimadzu UV-1800 spectrophotometer and an F-4500 fluorescence spectrophotometer (Hitachi). The fluorescence quantum yield in CHCl 3 were relative to 9,10-diphenylanthracene in cyclohexane ( fl = 0.90) as a standard. The number-average molecular weight (M n ) and the weight-average molecular weight (M w ) of the polymers were estimated by gel permeation chromatography (GPC) system (Shimadzu, LCsolution) using polystyrene standards with CHCl 3 as an eluent. Cyclic voltammetry (CV) of polymers in thin film on a Pt disk was performed at a scan rate of 50 mV/s in acetonitrile containing 0.1 M Et 4 NBF 4 at room temperature under Ar using a saturated calomel electrode (SCE) as the reference and a platinum wire as the counter electrode. The electrochemical data (vs SCE) obtained by cyclic voltammetry were made a correction with the redox potential (4.8 eV) of ferrocene/ferricinium [18,19]. Elemental analyses were carried out with a Perkin-Elmer type 2400 apparatus. Thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out by an Extra 6000 TG/DTA (Seiko) analyzer at a heating rate of 10 °C min -1 in an argon atmosphere.

Optical properties.
The photophysical properties of dilute solution and thin films of all the polymers were investigated with UV-vis and PL, and the results are shown in Fig. 3. The UV-vis absorption and the emission spectra data for polymer were summarized in Table 2. International Letters of Chemistry, Physics and Astronomy Vol. 62 Absorption and PL spectra of PF-tetrAz and PF-triAz in CHCl 3 are shown in Fig. 3a. The absorption maxima in wavelength (λ max ) at 390 nm for PF-tetrAz and 394 nm for PF-triAz are due to π-π* transition of the conjugated main chains. The absorption λ max of PF-triAz is in long wavelengths compared with that of PF-tetrAz, which indicates that π-conjugation of PF-triAz is longer than that of PF-tetrAz. The emission peaks in solution were observed at 437 nm for PF-tetrAz and 421 nm for PF-triAz. Stokes shifts of PF-triAz in the solution was 27 nm, which was much smaller than that of PF-tetrAz (47 nm). These small Stokes shifts also support PF-triAz having rigid and regular conformation, which enabled utilization of absorbed energy to PL efficiently.
The absorption and PL spectra of PF-tetrAz and PF-triAz in thin solid film state are shown in Fig. 3b. Compared with absorption spectra in solution, the absorption of PF-tetrAz and PF-triAz is slightly broaden and red shift compared with their absorption in film. The broadening and red shift of the film absorption spectra indicated that there were some aggregations or interactions of the polymer chains in the solid state. The optical band gap (E g opt ) estimated from the onset of the absorption spectrum in the film state was 2.8 eV for PF-tetrAz, which was smaller than that of PF-triAz (2.98 eV). Compared with PL spectra of PF-tetrAz and PF-triAz in solution, a red shift was observed in their film. The PL maxmum of PF-triAz in the film state was shorter in wavelength than that of PF-tetrAz. This can be attributed to the higher polarizability of the tetrAz group and restrained intermolecular aggregation [22]. PF-tetrAz and PF-triAz showed intense blue PL emission with CIE coordinates (x, y = 0.16, 0.12; 0.16, 0.07, respectively) in the film state and good relative  fl of 0.63, 0.97 in CHCl 3 , which suggest that PF-tetrAz and PF-triAz are considered to be good polymeric blue emitters superior to a series of polyfluorene derivatives [23].

Electrochemical Properties.
Electrochemical analysis of the thin-film sample of the polymers was employed to estimate E HOMO and E LUMO . The CV results of PF-tetrAz and PF-triAz are shown in Fig. 4 and the estimated values are summarized in Table 2. During the anodic scan, both of the compounds showed irreversible oxidation peaks, which are assigned to the oxidation of the fluorene units. PF-tetrAz had E HOMO of -5.83 eV, which was higher about 0.17 eV than that of PF-triAz (-6.0 eV). This could be caused by the delocalization of the HOMO because of good planarity between the fluorene and tetrAz segments. Upon the cathodic sweep, PF-triAz exhibited quasireversible reduction waves, whereas PF-tetrAz showed weak irreversible reduction waves. The E LUMO of PF-tetrAz (-2.85 eV) is slightly lower than that of PF-triAz (-2.74 eV), on account of higher electron-deficiency of tetrAz compared to triAz. The above results suggest that electronegativity of the azine unit is responsible for lowering of both E HOMO and E LUMO of the polymers.

Conclusion
In summary, two efficient blue luminescence polymers PF-tetrAz and PF-triAz, have been developed by combination of the fluorene sequenes as a electron-donor and the tetrAz or triAz core as an electron-acceptor. These polymers were obtained in high yields, and they had a good solubility in common organic solvents and a good thermal stability. In the CHCl 3 solution, they showed similar electronic properties of absorption and emission, the values of λ max (abs) and