Fluorenes, alternating copolymers

SCHEME 2.30 Synthesis of the Frechet-type dendrimer-substituted PFs and fluorene random and alternating copolymers. (From Marsitzky, D., Vestberg, R., Blainey, P., Tang, B.T., Hawker, C.J., and Carter, K.R., J. Am. Chem. Soc., 123, 6965, 2001.)... 

Another example of efficient Forster energy transfer in Eu3+ complexes of fluorene copolymers (similar to the alternating copolymers described in Scheme 2.49) was demonstrated by Huang and coworkers [414] for random copolymers. They synthesized copolymers 336 with a different ratio between the fluorene and the benzene units in the backbone and converted them into europium complexes 337 (Scheme 2.50) [414]. The complexes 337 were capable of both blue and red emission under UV excitation. In solution, blue emission was the dominant mode. However, the blue emission was significantly reduced or completely suppressed in the solid state and nearly monochromatic (fwhm 4 nm) red emission at 613 nm was observed. 

J. Lu, Y. Tao, M. D iorio, Y. Li, J. Ding, and M. Day, Pure deep blue light-emitting diodes from alternating fluorene/carbazole copolymers by using suitable hole-blocking materials, Macromolecules, 37 2442-2449, 2004. 

Q. Fang and T. Yamamoto, New alternative copolymer constituted of fluorene and triphenyla-mine units with a tunable -CHO group in the side chain. Quantitative transformation of the -CHO group to -CH=CHAr groups and optical and electrochemical properties of the polymers, Macromolecules, 37 5894-5899, 2004. 

W. Wang, J. Xu, and Y.-H. Lai, Alternating conjugated and transannular chromophores tunable property of fluorene-paracyclophane copolymers via transannular tt-tt interaction, Org. Lett., 5 2765-2768, 2003. 

S.-H. Jin, M.-Y. Kim, D.-S. Koo, and Y.-I. Kim, Synthesis and properties of poly(fluorene-a/t-cyanophenylene vinylenej-based alternating copolymers for light-emitting diodes, Chem. Mater., 16 3299-3307, 2004. 

Highly efficient green photoluminescence has also been realized from SCPs. Copolymers 11 (Fig. 5) derived from 2,7-fluorene and 2,3,4,5-tetraphenylsilole show absolute PL quantum yields up to 84%.28 A well-defined alternating copolymer 12 with a repeating unit made up of ter-(2,7-fluorene) and 2,5-silole possesses an absolute PL quantum yield >80%.29 SCPs 13 with a main chain structure of 3,6-carbazole-2,7-fluorene-2,5-silole also show absolute PL quantum yields up to 86%.30 An energy transfer copolymer 14 of 2,7-dibenzosilole and... 

Green electroluminescence is also achieved from the well-defined alternating copolymer 12 with a repeating unit made up of ter-(2,7-fluorene) and 2, 5-silole.29 With its neat film as the emissive layer, the EL device shows a maximum //Kr of 0.47%, but the device performance can be largely improved to a maximum //Ki. of 1.99% when using a copolymer/PF8 blend film as the emissive layer. Copolymer 14 derived from 2,7-dibenzosilole and 2,1,3-benzothiadiazole is also an excellent green EL polymer.26 A maximum 7el of 3.81% can be realized in EL devices. 

In a bulk-heterojunction photovoltaic cell with methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as an electron acceptor, alternating copolymer 19 (Fig. 9), derived from 2,7-fluorene and 2,5-dithienylsilole, can show impressive performance as the electron donor.31 In a device configuration of ITO/PEDOT/active layer/Ba/Al, the dark current density—bias curve shows a small leakage current, suggesting a continuous, pinhole-free active layer in the device. Under illumination of an AM 1.5 solar simulator at 100 mW/cm2, a high short-circuit current of 5.4 mA/cm2, an open-circuit voltage of 0.7 V, and a fill factor of 31.5% are achieved. The calculated energy conversion efficiency is 2.01%. 

FIGURE 10.23. Molecular subunits used with fluorene in alternating copolymers. (From Ref. 6.)... 

A route has been developed by Shu, Jen, and co-workers for the synthesis of a fluorene monomer 63 bearing two aryloxadiazole groups at the bridgehead by nucleophilic substitution of 4-fluorobenzonitrile with the fluorenyl anion, followed by conversion of the nitriles to oxadiazoles via tetrazole intermediates (Scheme 29) [98]. The EL efficiency of the alternating copolymer 64 in a single-layer device is considerably higher than for the PDAF homopolymer... 

Spirobifluorenes have been investigated by Salbeck [100] and found to be promising materials for use in blue LEDs, and so some effort has been made to incorporate these units into polymers (Scheme 30). As poly(diarylfluorene)s have proven to be stable blue emitters it comes as no surprise to find that polymers containing spirobifluorenes such as 66 [101] and 67 [102] also produce stable blue emission. A fluorene-spirobifluorene alternating copolymer 68 has been made and was found to give stabler emission than the fluorene homopolymer 16, but green emission was still observed upon heating in air at 150 °C [103]. 

For PFs with HTM grafting as side chain, the alternating copolymer 18 with electron-deficient moiety (4-ferf-butylphcnyl-l,3,4-oxadiazole) functionalized fluorene and monomer of PFO was synthesized by Shu and coworkers [31]. The device with the configuration ITO/PEDOT PSS/18/Ca/Ag showed improved performance turn-on voltage of 5.3 V (defined as voltage needed for brightness of 1 cdm-2), maximum brightness 2770 cdm-2 at 10.8 V, and maximum external quantum efficiency 0.52% at 537 cdm-2 rela-... 

Andersson LM, Inganas O (2006) Acceptor influence on hole mobility in fullerene blends with alternating copolymers of fluorene. Appl Phys Lett 88 082103... 

The formation of alternating copolymers through the polymerization of pairs of monomers, one of which is the donor and the other the acceptor of an electron, is well known. We shall mention only a few studies out of a great number of those recently published. First, those dealing with the nature of active centers in such systems will be examined. When radical initiators are used, e.g., benzoyl peroxide as in17), and the reaction is inhibited with different radical polymerization inhibitors, such as stable radicals like 2,2,6,6-tetramethylpiperidine 1-oxide, quinones, fluorene etc., questions concerning the nature of active centers can be regarded as solved. 

The Yamamoto method can be used to synthesize fluorene homopolymers (Scheme 7.3A) as well as random alternating copolymers [52], Alternating copolymers can be synthesized through Suzuki coupling (Scheme 7.3B), which tolerates a wide variety of different functional groups [53],... 

Clearly, if metal-binding sites are required, one strategy is to prepare alternating copolymers where one monomer is based on a well-known ligand (e.g., bipyridine, phenanthroline, and acetylacetonate), which is capable of coordinating to a metal ion. One such example was a study carried out by Liu et al. In this case, three similar conjugated polymers 17 (R = Oct), 18, and 19 based on fluorene and bipyridine were prepared and tested for their metal ion sensing capabilities [55],... 

Zhang et al. describe the properties of two alternating copolymers based on fluorene and a bipyridyl unit 17 (R = Hexyl) or a phenanthroline unit 20 to coordinate... 

Recently, n-channel transistors of poly(9,9-di-n-octylfluorene-alt-benzothiadia-zole) (51d) have been reported using thin films fabricated by spin-coating.[298] Electron mobilities ranging from 6 x lO. S x 10 cm V s were observed with the use of a 50 nm BCB polymer layer on top of the Si02 dielectric and Ca electrodes. Alternating copolymers containing 1,3,4-heterodiazoles and fluorenes have also been synthesized and show very low n-channel mobilities (2.2 x 10 cm V s for compound 51e).[299]... 

Copolymers with fluorene and 1,3,4-oxadiazole show highly efficient photoluminescence. A double layer device consisting of PVK and an alternating copolymer of 9,9 -didodecylfluorene-2,7-diyl and (l,4-bis-( 1,3,4-oxadiazole)-2,5-di(2-ethylhexyloxy)phenylene)-5,5 -diyl exhibits a narrow blue electroluminescence with a maximum at 430 nm. Electrochemical analysis of the pol5miers using cycUc voltametry suggests that they can be used both as electron transport materials and as blue emission materials for LEDs. 

Conjugated alternating copolymers based on the BODIPY core and various donors (Figure 5.20) have also been synthesized [160] via the Sonogashira polymerization reaction, to place BODIPY units in alternation with comonomers such as 9,9-bis(2-ethylhexyl)-9H-fluorene (FL),... 

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