Study on the Synthesis and Optoelectro Properties of Pyrimdine Containing Linear Molecules
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-p¹º½s¸¹:NSC 88-2113-M002-036 °õ¦æ´Á--: 1, August 1998- 31, July 1999
¥D«ù¤H: Ken-Tsung Wong (¨L®Úöæ) e-mail: [email protected]
°õ¦æ³æ¦ì: Department of Chemistry, National Taiwan University(¥xÆW¤j¾Ç¤Æ¾Ç¨t)
Abstract:
Take the advantage of different reactivity on the 2- and 5-position of 5-bromo-2-iodopyrimidine, an efficient one-pot synthetic protocol of sequential Sonogashira coupling reaction with different alkynes was established for synthesizing pyrimidine containing linear conjugated oligomers. A variety of linear oligomers with 2 pyrimidines ring were synthesized. Photophysical properties like UV-VIS and photoluminescence were measured in some of these oligomers.
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Research Background and Objectives:
The potential use of conjugated organic oligomers as active components in optoelectronic devices has attracted a great deal of attentions in recent years. Particularly, light emitting diodes (LEDs) have been extensively studied due to their promise for practical applications(1)
. However, it is well known that most organic materials suitable for LEDs have smaller hole injection barriers than electron injection barriers. In order to increase the efficiency of light emitting from these devices, an excellent electron transporting organic material is highly demanding. A more practical approach for searching such materials is to find a conjugated system with lower LUMO. Thus introduced CN groups on the vinylene bridges of poly(arylenevinylenes) has resulted in an increase in the electron affinity of polymeric materials for LEDs(2)
. In our last NSC proposal we subjected ourselves to synthesize a new type of oligomeric materials with high electron affinity. We chose pyrimidine according to its high ability to accept electrons(3)
as our candidate to incorporate into the oligomeric (1,4-phenylene)ethynylene backbone. The new-formed linear molecules then subject to evaluate its possibility of acting as an electron transporting and/or light emitting material by checking their photophysical and electrochemical properties.
N N N N N N
1. Synthesis of pyrimidine containing linear oligomeric molecules
First of all, we slightly modified the known procedure(4) for preparation of 5-bromo-2-iodopyrimidine 1. Well-grounded powder of 2-chloro-5-bromopyrimidine(5)
was added into a cold 55% HI solution, after stirring at 0-5 o
C for 12 hours, solid NaHCO3 was added slowly to quench reaction carefully and followed by decolorizing with saturated Na2SO3 aqueous solution, extract with CH2Cl2 and evaporate to afford pale-yellow solid. Pure white needle crystal of 5-bromo-2-iodopyrimidine was obtained by recrystallization from hexane in 64% yield.
At 50 o
C, Pd-catalyzed Sonogashira coupling reaction of 5-bromo-2-iodopyrimidine with excess alkynes afford bis-coupling product 2a-c in moderate to good yields. However, at room temperature, the coupling reaction of 5-bromo-2-iodopyrimidine 1 would selectively occur only on the 2-position in the presence of a catalytic amount of Pd(PPh3)4 and excess iPr2NH in dry THF solution, then trimethylsilylacetylene was added for the second coupling reaction on the 5-position without adding extra catalyst. The second coupling reaction was completed normally in 3 hours at 60 oC. The trimethylsilyl group was removed by adding either nBu4NF or 2N NaOH aq. solution in THF at room temperature to give 3a-c.
Our target linear oligomers 4a-c with 5 aromatic rings were synthesized in excellent yields by treating compound 3a-c with diiodoaromatic compound using our standard Sonogashira coupling reaction condition. In order to diverse our targets, different diiodoaromatic compounds for example 1,1’-diiodobipehnyl has been used as connector for 3a to afford 4f. Instead of 4d and 4e are very soluble in polar organic solvents for example CH2Cl2, CHCl3, 1,1’-dichloroethane, 1,1’,2,2’-tetrachloroethane, 4a-c and 4f are slightly soluble in chloroform but more soluble in 1,1’,2,2’-tetrachloroethane. We are now trying to use the same synthetic strategy for synthesizing a more soluble fully alkyl-substituted linear
N N Cl Br N N I Br 55 % HI 0 oC 1 64 % G G N N G N N I Br 1 + Pd(PPh3)4 CuI, iPr2NH THF, 60 oC 2a: G= t-Bu 2b: G= OC4H9 2c: G= OC8H17 34% 70% 75% excess G N N I Br 1 + Pd(PPh3)4 CuI, iPr2NH THF, r.t. excess Me3SiCCH G Me3Si N N G Me3Si N N TBAF or 2N NaOH THF, r.t. G H N N 3a: G= t-Bu 3b: G= OC4H9 3c: G= OC8H17 50% 64% 60%
2. Photophysical Properties of Linear Molecules containing Pyrimidines
UV-VIS absorption spectra of 2a, 2b, 4a, 4c in CHCl3 were shown in Figure 1. As the length of conjugation increases, the absorption maximum shows slightly red shift. Compound
2b and 4c with octoxyl group as substituent absorb light at longer wavelength compared with
their t-butyl-substituted analogs 2a and 4a. Upon UV excitation, the photoluminescence spectra of 2a, 2b, 4a, 4c, are shown in Figure 2. The maximum wavelength of emissions was observed with slightly red shift as the length of conjugation increased. The same substituent effect as observed in UV-VIS spectra was also obtained in emission spectra. Compound 2b and 4c with octoxyl group as substituent emit blue light with longer wavelength compared to their t-butyl-substituted analogs 2a and 2b. Compound 4a with blue fluorescent emission maxima at 397 nm and 418 nm in different concentrations is shown in Figure 3. Both emissive intensities are concentration-dependent. The emissive intensity at 397 nm decreased as the concentration increased. Whereas, the intensity of emission at 418nm increased as the concentration increased. The changes of the relative intensities of these two peaks could be attributed to the formation of excimer. Thus, the self-quenching effect of 4a was observed at relatively high concentration (7.5 x 10-5
M). The photoluminescent spectrum of 4c with different concentration is also shown in Figure 4.
G N N I I R R R R G N N G H N N Pd(PPh3)4 CuI, iPr2NH THF, 60 oC 4a: G= t-Bu, R=H 86% 4b: G=OC4H9, R=H 74% 4c: G=OC8H17, R=H 44% 4d: G=OC4H9, R=OC8H17 76% 4e: G=OC8H17, R=OC8H17 68% 2 N N N N N N C8H17 H17C8 N N 4f 4g
300 350 400 450 500 550 600 0.00 0.05 0.10 0.15 0.20 0.25 A b s . Wavelengt h(nm)
F igure 1. UV-VIS Spectra
N N N N N N OC8 C8O 1.72 x 10-6 M 350 400 450 500 550 600 0 1 2 3 4 A rb it ra ry S c a le Em issio n(n m ) N N N N OC8 C8O 1.72 x 10-6 M F igure 2. Photoluminescence 350 400 450 500 550 600 0 1 2 3 4 5 6 7 In te n s it y Emiss ion (nm) N N N N 1.50 x 10-7 M 1.50 x 10-6 M 1.50 x 10-5 M 7.50 x 10-5 M
F igure 3. Concentration Dependent Photoluminescence
400 450 500 550 600 0 1 2 3 4 5 6 7 In t e n s it y Emis sion (nm) N N N N OC8 C8O 1.72 x 10-6 M 1.72 x 10-5 M
F igure 4. Concentration Dependent Photoluminescence
Self-Evaluation
In our last NSC proposal, we set our first goal to develop an efficient synthetic strategy for synthesizing pyrimidine containing linear conjugated oligomers. According to this progress report, take the advantage of different reactivity on the 2- and 5-position of 5-bromo-2-iodopyrimidine, we have already established an one-pot synthetic protocol of sequential Sonogashira coupling reaction with different alkynes. A variety of linear oligomers with 2 pyrimidines ring were synthesized very efficiently. We set our second goal to study the photophysical and electrochemical properties of these oligomers. All measurements now are undergoing in order to evaluate the possibility for applications as an electron transporting and/or light emitting materials in organic LEDs.
References
[1] Miyata, S.; Nalwa, H. S. ed. Gordon and Breach Publishers, 1997 Amsterdam “ Organic Electroluminescent Materials and Devices” and references therein.
[2] (a) Greenham, N. C.; Moratti, S. C.; Bradley, D. D. C.; Friend, R. H.; Holmes, A. B.
Nature, 1993, 365, 628. (b) Moratti, S. C.; Cervini, R.; Holmes, A. B.; Baigent, D. R.;
Friend, R. H.; Greenham, N. C.;Gruner, J.; Hamer, P. J. Synth. Met. 1995, 71, 2117. (c) Peng, Z.; Galvain, M. E. Chem Mater. 1998, 1785.
[3] Gommper, R.; Mair, H. –J.; Polborn, K. Synthesis, 1997, 696.
[4] Goodly, J. W.; Hird, M.; Lewis R. A.; Toyne, K. J. Chem. Commun. 1996, 2719. [5] Which is synthesized from bromination of 2-hydroxypyrimidine hydrochloride with Br2 followed by chlorination on the 2-position with POCl3
