Communications to the Editor Bull. Korean Chem. Soc. 2001, Vol. 22, No. 11 1181
Synthesis and Characterization of New P
이y(terphenylene vinylene) Derivative as Blue Emitting Material
Yun-HiKim,Jun-HwanAhn,Dong-Che이 Shin, Jin-Hak Kim, Young-Wook Park, Don-Soo Choi： Young-Kwan Kim,f andSoon-KiKwon*
Department of Polymer Science & Engineeringand ResearchInstitute of Industrial Technology, GyeongsanNation시 University, Chinju 660-701, Korea
‘Department of Chemical Engineering, Hongik University, Seoul121-791, Korea Received August27,2001
Keywords: Poly(terphenylene vinylene),BlueLED, Suzuki coupling,Energy bandgap.
Efficientthermally stableblue light emittingmaterials are neededboth to complete the color spectrum and serve as energy transfer media for incorporated fluorophores. The extended delocalization lengths ofmost fully conjugated polymers, however, resultin smallelectronic bandgaps and red-shiftedemissions.
Thepoly(p-phenylenevinylene) (PPV),1-6 which is thefirst reported electroluminescent polymer, and PPV derivatives have several advantages as anemitting material; high thermal stability, good film quality, suitable color tunability, etc.
However, it has low oxidativestability and somedifficulties in processability and emitting blue color.
Poly(p-phenylene) (PPP), andits derivatives7-12have also been extensively investigated for light emitting materials because theyarethermally and oxidatively stablepolymers.
PPP derivatives show large band gaps since the aromatic rings are twisted torelive unfavorable steric interactions in the backbone, which limitsthe effective conjugation length.
Unsubstituted PPPis highly insoluble,limitingthemolecular weightsandprocessability.Large, solubilizing substituents may also be incorperated,resulting in improved processability, but thisusuallyexacerbatesthestericinteractionsin thepolymer main chain. Thus, PPP derivatives are intrinsically violet blueemitter tohavesomedifficulties in color tunability.13
Solubility, oxidativestability,lowturn-on voltage, and color tunability (especiallyblue emission) are desirableproperties for conjugated polymers used in LEDs.14 Moreover, good thermal stability is neededbecause heat is generated when currentpassesthrough thedevice.
Thus, wetried to synthesize a blue-light-emitting polymer, which has the advantages of both PPP and PPV Recently, we reported synthesis and properties of poly(biphenylene- vinylene) (PBPV)derivatives witha controlled conjugation length of biphenylene vinyleneunit, which is composed of alternating PPP and PPVunits.
Inthiscommunication,we describe thesynthesisand charac terization ofblue-light-emitting, poly(a-phenylene terphenylene vinylene)derivative.And,on the basisofpoly(terphenylene- vinylene), our research alsoinvolvedintroducing substituent into the vinyl bridge. Introducing phenyl pendant group to a vinyl bridge may expect to lead to enhanced solubility, oxidative and thermalstability, andreduced theformation of eximers owing tointerchaininteractions.
Scheme 1
The synthetic route ofpolymer was shownin Scheme 1.
Benzene diboronic acid was obtained by Grignard reaction of4,4-dibromobenzene with triethylborate. [1,2-(4',4''-Di- bromophenyl) -1 -phenyl ethylene] were synthesizedby Wittig reaction of (4-bromobezyl)triphenylphosponium bromide with4-bromobenzophenone. The polymerizationwas carried out using a typicalSuzukicoupling reaction with good yield.
After the polymerization, the end-capped reaction of the bromine and boronic acid end group, which hamper thermal stability and efficiencyofPL, was accomplished by bromo
benzene and benzene boronic acid. The polymer structure shown was consistent with the elemental analysis and the spectroscopic data from 1H-NMRand FT-IR. The obtained polymer was readily soluble in common organic solvents such as chloroform,dichloromethaneandtolueneeventhough thepolymerwas composed rigidaromatic and vinyl carbons.
The weightaverage molecular weight (Mw) of polymer, as determined by gel permeation chromatography usingpoly
styrene standard,wasMw= 13500 (PDI= 1.6). Thethermal characterizationof the polymer was accomplished by DSC and TGA. The DSCthermogram obtained fromthe second heating of thepolymer. The polymer had glass transition at around 146 oC. TGA curve showed that the weight loss of the polymer is lessthan 5% onheatingto 430 oC. From the above results,theobtainedpolymer has goodthermal stability.
1182 Bull. Korean Chem. Soc. 2001, Vol. 22, No. 11 Communications to the Editor
300 400 500 600
Wavelength (nm)
Figure 1. The optical absorption and photoluminescence spectra of the polymer in chloroform.
Figure 1 shows the optical absorption and photolumine scencespectra of a dilute solution ofthe polymer inchloro form. Themaximumabsorptionof polymershowed 350 nm.
The maximumabsorption peak of thepolymerare70-90 nm blue shifted as comparedwiththatof PPV, 10-20 nm blue shifted as compared with that of PBPV and 10-20 nm red shifted as compared with that of PPP. These shifts may originate from theterphenylene vinylne units composed of PPPand PPV units. The band-gap energy ofthe new poly
merestimated from extrapolationof thelowenergy absorp tion spectrawasabout 2.98eV.
The PL spectrum ofthe dilute solution of the polymer pumped by UV light(350nm) hasa maximumpeak at450nm. Al
thoughthePL spectrum ofa solidfilm of polymer shows a slight bathochromic shift from that of solution, it is blue shiftedby 10-20 nmfromthoseof poly(biphenylenevinylene) derivatives14,15 whichwere composedalternatingPPP and PPV.
It may beresulted from increased phenylunit inthemain chain.
Fromthe cyclic voltammogramof a polymer,thepolymer is reductive as well as oxidative electrochemicallyshowing EA and IP vales 2.96 and 5.73 eV, respectively. The n-n* band gap of 2.98 eV is almostthe same as that determined
«
§
=
-J LLJ
250 300 400 500 600
Wavelength (nm)
Figure 2. The electroluminescence spectrum of the ITO/polymer/
Al device.
2.85 eV
ITO 4.6 eV '
Al 4.3 eV
Figure 3. The position (with respect to the vacuum) of the HOMO and of the LUMO for the polymet with no field. The work functions of the indium tin oxide and Al are also reported.
fromtheonsetofopticalabsorption(420nm).
Single-layer light-emitting diode was fabricated by using ITO as the anode and an Al as the cathode. The electro luminescence spectrumof the polymeris shown in Figure2.
Blue lightemission was observedfrom the EL device ofthe polymer.The EL spectrum ofthe polymerdepicted a close correspondence to the PL spectrum, indicating that the recombination processes were the same forboth cases, as oftenobserved forpolymer-basedLEDs. It was also ofnote thattailing in thelong wavelengthregion observed for the electroluminescence spectrum suggested that emissionfrom some excimer states should be taken into consideration.
Figure 3 showstheelectronic energy bandgapschemeof the new polymer as evaluated from cyclic voltammogram and UV-vis absorption spectrum. The efficient device for the obtainedpolymerasemittinglayerissuggested that thehole injectionand lowwork functionmetal are needed. Nowwe are pursuing the study of the optimized devices and poly(terphenylenevinylene)derivatives.
Acknowledgment. ThisFinancial supportof this work by KRF (Y00275) andKOSEF (R05-2000-00016) is gratefully acknowledged.
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