Volume 2 Issue 4 - November 23, 2007
Novel Hyperbranched Polyfluorenes Containing Electron-Transporting Aromatic Triazole as Branch Unit
Yun Chen*, Lin-Ren Tsai

Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan

Macromolecules, 40, 2984-2992 (2007)

Recently, electroluminescent (EL) polymers have been extensively investigated owing to their good film-forming properties, easy fabrication via spin-coating, and their tunable luminescence properties, which make them excellent candidates in both single- and multilayer polymer light-emitting diodes (PLEDs). The most extensively studied EL polymers are the linear conjugated ones, such as poly(p-phenylenevinylene) (PPV), polyfluorene (PF), and their derivatives. Polyfluorenes (PFs) are promising materials for blue light-emitting diodes because of their high photoluminescence (PL) and electroluminescence (EL) efficiencies. However, there are some drawbacks that hamper their potential applicability, such as the undesired green emission that appears upon thermal annealing or device operation. This low-energy emission band has been attributed to the formation of interchain interaction (excimer or aggregate). Hyperbranched polymers have drawn a lot of attention and consideration to diminish interchain interaction due to their highly branched and globular molecular structure. Therefore, light-emitting hyperbranched polymers are of current interest for developing efficient EL devices and other photonic devices. Compared to highly symmetric dendrimers, hyperbranched polymers not only are easier to synthesize but also possess comparable properties. Moreover, hyperbranched structure is advantageous over its linear counterpart in good processability and reduced unfavorable intermolecular interactions and crystallization.
Scheme 1. Synthesis of linear and hyperbranched copolyfluorenes.

In order to avoid the excimer/aggregate formation and to improve the electron affinity and transport ability of PFs, we introduced a 3,4,5-triphenyl-1,2,4-triazole branching unit to prepare hyperbranched copolyfluorenes. A novel A2 + A2' + B3 approach based on Suzuki condensation coupling was employed for the synthesis of hyperbranched the copolyfluorenes PF1~PF5, in which A2, A2and B3 are 9,9-bis[4-(9-carbazolyl)butoxyphenyl]-2,7- dibromofluorene (M1), 9,9-dihexylfluorene-2,7-bis(trimethylene boronate) (M3) and 3,4,5-tris(4-bromophenyl)-4H-1,2,4-triazole (M2), respectively (Scheme 1). The reaction time was controlled to avoid gelation of the hyperbranched polymers. The resulting hyperbranched triazole-containing copolyfluorenes showed stable blue light emission even in the air at elevated temperatures.

These polymers show absorption maxima around 349~378 nm, both in solution and in film state, which are attributed to π-π* transition of the conjugated oligofluorene segments. The absorption maximum of PF1 (378 nm) shows a blue-shift of 29 nm relative to PF5 (349 nm). Obviously, the conjugation length decreases gradually with increasing branching triazole content (from PF1 to PF5). The PL peaks of P1 and PF1~PF5 in CHCl3 locate around 415~421 nm, which red-shift slightly (ca. 7 nm) relative to those in film state. Previous literature pointed out that although the absorption spectral maxima of oligofluorenes continue to shift to the longer wavelength through n = 10 (n: number of fluorene unit), their emission maxima remain virtually unchanged beyond n = 6. In our system, the conjugation length should extend with decreasing molar percent of triazole branch unit in the hyperbranched polymers. For instance, absorption maximum of PF5 (349 nm) shifts bathochromically to 378 nm of PF1 and P1. The emission spectral maxima, however, remains virtually unchanged, i.e. around 415-421 nm and 423-427 nm in CHCl3 and film state, respectively. This should be owing to efficient excitation energy transfer from short oligofluorenes to longer ones whose fluorene unit number (n) is over 6. Interestingly, a linear relationship between 1/λmax and 1/(1 - ntriazole) has been correlated (Fig. 1), indicating that the effective conjugation length of the oligofluorenes decreases smoothly with increasing triazole contents. Accordingly, the average conjugation length of a hyperbranched oligofluorene can be estimated from its absorption maximum using the linear plot of 1/λmax versus 1/(1 - ntriazole).
Figure 1. Plot of 1/λmax versus 1/(1 - ntriazole), where λmax: absorption maximum in film state; ntriazole: molar fraction of branching triazole in the hyperbranched polymers.
Figure 2. PL spectra of P1 and PF1~PF5 films after annealing at 200°C in air for 1 h.

It is well-known that thermal annealing of polyfluorene usually leads to formation of interchain interaction (excimers). Fig. 2 shows the normalized PL emission spectra of P1 and PF1~PF5 films after annealing at 200°C in the air for 1 h. It is noteworthy that no additional band emerges in the PL spectra of P1 and PF1 whose triazole molar contents are 0% and 4.3%, respectively. The appearance of additional band is usually undesirable since it changes the pure blue emission to blue-green color. To our best of knowledge no similar stable polyfluorene derivatives annealed at elevated temperatures in air have been reported. Apparently, with the incorporation of carbarzole pendant groups (P1) and/or triazole branching unit (PF1), the red-shift and excimer phenomena of polyfluorene can be reduced significantly. However, PL emission spectra of PF2-PF5 show additional broad band at about 520 nm after the annealing, and the band intensity enhances gradually with increasing triazole content. In order to elucidate the origin of this abnormal green emission induced by thermal annealing, we measured the absorption spectra of monomer mixtures (M1 + M2) in CHCl3 (Fig. 3-a). Interestingly, the mixtures with M1:M2 =1:1, 2:1, 3:1 and 5:1 all exhibited broad absorption band at about 440-450 nm. Moreover, the PL spectra (excitation at 440 nm) of the mixtures with M1:M2 =1:0, 1:1, 2:1, 3:1 and 5:1 all exhibited green emission as shown in Fig. 3-b. However, no green emission was observed for M1:M2 = 10:1 and 1:0, which is similar to the result obtained in annealing for PF1 and P1. Therefore, the green emission possibly originates from the complex of triazole and carbazole units. Accordingly, the green emission becomes more obvious when molar ratio of M2 and M1 is approaching to one. The energy levels of M1 and M2 were also estimated from their onset reduction and oxidation potentials measured in acetonitrile. The HOMO and LUMO energy levels of M1 (M2) are -5.41 eV (-6.44 eV) and -1.90 eV (-2.54 eV), respectively. The band gaps between HOMO level of M1 and LUMO level of M2 is 2.87 eV, which is corresponding well to the excitation wavelength (440 nm), suggesting that the two monomers form as complex readily.
Figure 3. (a) Absorption spectra of M1, M2, and their mixtures (molar ratio of M1:M2 = 1:0, 10:1, 5:1, 3:1, 2:1 and 1:1) in CHCl3. (b) PL spectra of mixtures of M1 and M2 in CHCl3 (excitation: 440 nm).
Figure 4. Optimized geometries and molecular orbital of linked M1 and M2 residues using semi-empirical MNDO calculation.

The HOMO and LUMO energy levels of PF1~PF5, estimated from electrochemical data, are in the range of -5.41~-5.36 eV and -2.44~-2.42 eV, respectively, whereas those of P1 are -5.34 eV and -2.19 eV. The optimized geometries and molecular orbital for linked M1 and M2 residues are depicted in Fig. 4, in which the HOMO and LUMO situate at hole- (carbazole) and electron-transporting (triazole) segments, respectively. Accordingly, the oxidation and reduction under a bias will start from carbazole and triazole, respectively. This redox behavior is also expected for PF1~PF5 since they also comprise hole-transporting carbazole pendant groups and electron-transporting triazole segments.

Two-layer EL devices (ITO/PEDOT:PSS/ PF1 or PF2/Al) were fabricated to investigate their EL spectral characteristics. The EL spectrum of PF1 is similar to its PL spectra and PF2 reveals a much broader emission in longer wavelength region, which might be due to partial complex formation during device operation. However, no green-blue band emission at 550 nm was observed in EL spectra of PF1 and PF2. This result indicates the hyperbranched copolymers prevent the aggregation effectively. The maximum brightness (current efficiency) of PF1 and PF2 devices were 161 cd/m2 (0.056 cd/A) and 212 cd/m2 (0.059 cd/A), respectively, which are much higher than that of P1 device (48 cd/m2, 0.014 cd/A). This result suggests that incorporation of branching electron-transporting triazole units is effective in improving EL performance of the devices.

In summary, linear (P1) and hyperbranched copolyfluorenes (PF1~PF5) containing carbazole as pendant and aromatic 1,2,4-triazole as branching units were synthesized and characterized. These hyperbranched copolymers was soluble in common organic solvents and exhibited good thermal stability (Td > 420°C). Their absorption maxima (λmax) locate at 349~378 nm and, furthermore, linear relationship between 1/λmax and 1/(1-ntriazole) has been correlated. New green emission (~520 nm) appeared in the PL spectra of PF2-PF5 after thermal annealing at 200°C, which have been attributed to complexes formed from carbazole and aromatic 1,2,4-triazole chromophores. The oxidation and reduction start from the carbazole and triazole groups, respectively. Two-layer PLED devices (ITO/PEDOT/ PF1 or PF2/Al) were fabricated and their optical properties investigated, the maximum brightness of PF1 and PF2 were 161~212 cd/m2 at about 19 V.
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