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Studies of Green Emission in Polyfluorenes Using a Model Polymer

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Studies of Green Emission in Polyfluorenes Using a Model Polymer Polymer Journal, Vol. 39, No. 12, pp. 1345–1350 (2007) #2007 The Society of Polymer Science, Japan Studies of Green Emission in Polyfluorenes Using a Model Polymer y Benqiao HE,1;2 Jing LI,2 Zhishan BO,2 and Yong HUANG2; 1Tianjin Key Laboratory of Fiber Modification and Functional Fiber, School of Materials and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300160, China 2State Key Laboratory of Polymer Physics & Chemistry and Joint Laboratory of Polymer Science and Material, Beijing National Laboratory of Molecular Science, Institute of Chemistry, CAS, Beijing 100080, China (Received May 14, 2007; Accepted September 5, 2007; Published October 23, 2007) ABSTRACT: The green emission (g-band) in polyfluorene-based conjugated materials is studied by various spec- 0 troscopic methods on defined poly(9,9 -dioctylfluorene) with one foluorenone unit (P20), which can be seen as a model compound for polyfluorene emitting g-band. The absorption and emission properties of P20 in the film and solution (room temperature) reveal the optical properties of the green emission emerging in polyfluorene-type polymer. All the experimental evidence obtained demonstrates that g-band in polyfluorene is attributed to the mono-chain fluo- renone; and the aggregation of the chains further suppresses the blue emission and enhances g-band. [doi:10.1295/polymj.PJ2007041] KEY WORDS Polyfluorene / Conjugated Polymer / Membrane / Green Emission / Conjugated polymers serving as emitters in light- not observed in PF/polystyrene blend film after UV- emitting diodes (LEDs) are attractive to the display in- exposure, which was believed to be due to polystyrene dustry because of their potential ability for easy and blocking the formation of fluorenone-based excimer. cost-effective processing by solution casting, excel- But the inter-chain distance in PF film (about 1.28 lent mechanical properties and structure controllabili- nm17) is beyond the distance for excimer formation ty to tune electro- and photophysical properties.1–6 (about 0.3 nm or less18). It is difficult to form the fluo- Among the conjugated polymers considered for LED renone-based excimer in such a situation. And the the- application, polyfluorenes (PFs) have been viewed as oretical calculation is also against the origin of the the most promising blue-light materials due to their fluorenone-based excimer.19 List et al. presented a extremely high solution and solid-state quantum effi- new origin of fluorenone defect.15 and other experi- ciency, good charge transport, excellent chemical mental evidence supported the opinion.20,21 But some and thermal stabilities and tunability of physical pa- experimental phenomena cannot be well explained by rameters through chemical modification and copoly- the opinion of fluorenone-defect yet.4,22–27 merization. However, a long-wave emission in the re- Though the true origin of g-band (from fluorenone gion of 490–550 nm,7–10 i.e. green emission (‘‘g-band’’ defect or fluorenone-based excimer) in the PF is for short), appears in the PFs spectra after ultraviolet not still clear, it is documented that the origin of g- light exposure, heating in the air or passage of current. band is related to the occurrence of the fluore- The appearance of g-band is undesirable for two rea- none.8,15,17,28–30 Therefore, a model molecule that is sons: One is the reduction of the overall quantum ef- a copolymer of fluorene and fluorenone had been used ficiency, and the other is the impurity of the blue to clarify the green emission in polyfluorene and the emission. Therefore, many research groups are study- mechanism of fluorenone defect was proposed.8 But ing the origin of g-band in order to suppress g-band. the occurrence of fluorenone segment (without alkyl G-band had been early believed to originate from substituent groups) in copolymer may allow the the aggregate of polyfluorene or polyfluorene-based approach of aromatic groups to distance of 0.3 nm excimer.11–13 But the PFs studied in the literatures for excimer formation. Therefore, the validity of the are often synthesized via the Yamamoto route whose model molecule was doubted. conditions are harsher than Suzuki coupling method In this work, a new model molecule that contains and often result in production of fluorenone during only one fluorenone unit in the midst of the PF chain 14–16 and after the polymerization. Therefore, the origin (denoted as ‘‘P20’’) was designed to clarify the exact of aggregate of polyfluorene cannot be accepted. Af- nature of g-band emission in the PF. The spectral terward, Sims et al. proposed a origin of fluorenone- properties of the model molecule in solution and film based excimer.17 In their experiment, g-band was are investigated in detail. All the experimental evi- yTo whom correspondence should be addressed (Tel: +86-10-68597350, Fax: +86-10-68597356, E-mail: [email protected]). 1345 B. HE et al. then heated in oil bath at 50 C with stirring for 24 h P2 2n = 2 to fully dissolve. P6 2n = 6 P20 2n = 20 Spectral Characterization The UV-vis absorption spectra were recorded by n n a spectrophotometer of SHIMADZU, UV-1601PC. Steady state photo-luminescent (PL) spectra were re- O corded by a fluorescence photometer (VARIAN, Cary Figure 1. The structure formula of P2n. Eclipse, FLR025). The photo-luminescent lifetime measurements were performed with an Edinburgh dence obtained demonstrates g-band in polyfluorene is Analytical Instruments (FLS-920). All spectral char- attributed to the mono-chain fluorenone; more impor- acterizations were carried out at room temperature. tant, the aggregation of the chains suppresses the blue emission and enhances g-band. RESULTS AND DISCUSSION EXPERIMENTAL Spectral Characterization of P20 Film and PF Film After UV-light Exposure for 24 h Materials There are two reasons to choose P20 molecule as the The model polymer (denoted as ‘‘P20’’) was poly- model molecule. Firstly, the structure of P20 is nearly fluorene containing one fluorenone unit in the chain, identical to that of the PF except one fluorenone unit, which was synthesized and characterized in our labo- which makes the P20 well miscible with the PF. Sec- ratory. The detailedly synthetical procedure was intro- ondly, the interaction between fluorenone units will duced in our previous literature.31 The ratio of fluore- be restricted because the fluorenone unit is packed none unit to fluorene unit in P20 was around 1:20 by fluorene units, which is different the random co- according to the fed ratio and there is only one fluore- polymers of fluorene and fluorenone reported in liter- 8 none unit in each P20 chain. Other two oligofluorenes atures. containing one fluorenone unit (denoted as ‘‘P2’’ and In order to ensure that P20 is a valid model polymer ‘‘ P 6’’) were also synthesized and studied in order to for the PF emitting green band, the spectral studies compare with P20. The P20,P6 and P2 (the figure is of the pure PF exposed to UV-light have been firstly the number of repeat fluorene units) were called P2n carried out. It is found that a new emission peak at by a joint name. The structure formula of P2n is 530 nm appears when PF film is exposed to UV-light shown in Figure 1. The poly(9,90-dioctylfluorene) for 1 h. There is no emission peak at 530 nm in PF film (PF) was synthesized according to Suzuki polycon- before exposing (in the insert of Figure 2a). And the 32 densation method. The molecular weight (Mn ¼ strength of g-band increases with increasing UV-light 9000, Mw=Mn ¼ 2:6) of PF was determined by gel exposure time, which is identical to the phenomena permeation chromatography (GPC) calibrated with reported by List et al.15 Figure 2 shows the PL and polystyrene standard. absorption spectra of PF film after UV-light exposure for 24 h (denoted as ‘‘PF-UV-24h’’) and the blend Solution Preparation film containing 75% P20 and 25% PF (denoted as All solutions were prepared by first dissolving the ‘‘ P 20PF25’’). The PL spectra of the two films have appropriated amount of polymer into a solvent and weak peaks at 424 nm and strong peaks at 532 nm. 100 a 0.16 b P PF25 20 80 0.12 PF-UV-24h 60 0.08 350 400 450 500 550 40 Wavelength (nm) 0.04 Intensity(a.u.) Intensity(a.u.) P PF25 20 20 PF-UV-24h 0.00 0 400 500 600 700 800 200 300 400 500 600 Wavelength(nm) Wavelength(nm) Figure 2. The emission (a) and absorbance (b) of PF-UV-24 h (dash) and P20PF25 film (solid). The insert in (a) is the emission of pure PF before UV-exposure. 1346 Polym. J., Vol. 39, No. 12, 2007 Studies of Green Emission in Polyfluorenes Using a Model Polymer The PL peaks at 424 nm come from PF or PF segment Spectra of P2n Solutions and the peaks at 532 nm are g-band mentioned above. Figure 4 shows the absorption spectra of P2n and PF The absorption spectra of the two films have -Ã in tetrahydrofuran (THF). The maximum absorptions transitions at 400 nm with shoulder peaks at 433 nm. at 360, 376, 387 nm are assigned to -Ã transition It is suggested that the PL and absorption spectra of of main chain of P2,P6,P20, respectively, and a the PF-UV-24h film are the same as those of the long-wave absorption peak at 453 nm is all clearly P20PF25 film. observed in P2,P6,P20 spectra and no absorption at Figure 3 shows the PL decays for g-band emissions 453 nm is observed in PF.
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