Synthesis and Application of Chiral Conjugated Polymers and Dendrimers
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ABSTRACT 1,1'-Binaphthyl-based chiral polymers and dendrimers have been synthesized and their potential applications have been explored. These materials have shown a variety of interesting properties such as electroluminescence, optical nonlinearity, enantioselective catalysis and chiral sensing.
1,1'-Binaphthyl molecules represent a very important class of atropisomers .1 Particularly , when the 2,2'-positions of these molecules are substituted with oxygen, nitrogen or phosphorus atoms , molecules with a very stable chiral configuration have been obtained. They have been extensively used in processes such as asymmetric catalysis , molecular recognition and chiral separation and have demonstrated outstanding chiral induction in many cases .2 Starting from 1996, we have reported the u s e of 2,2'substituted 1,1'-binaphthyl molecules to construct a new class of chiral conjugated materials .2a Both linear polymers and dendritic macromolecules have been prepared based on the 1,1'-binaphthyl motif. These materials have shown potentially very useful properties . Recently , we have found that the binaphthylbased macromolecules have exhibited interesting properties such as electroluminescence, optical nonlinearity, enantioselective catalysis and chiral sensing. In this article, these studies are high-lighted. Polymer 2 represents the simplest binaphthyl polymer we have prepared. It was synthesized from the polymerization of 6,6'-dibromo -2,2'-dihexyloxy -1,1'-binaphthyl (1) in the presence of a catalytic amount of NiCl2 and excess zinc (Scheme 1).3 This polymer was also prepared from the Suzuki coupling of 1 with a 6,6'diboronic acid binaphthyl monomer.4 Both the optically active and optically inactive forms of the polymer were obtained. The molecular weights of these polymers are in the range of 11,300 to 21,500 (M w) as measured by gel permeation chromatography (GPC) relative to polystyrene standards. W e also prepared another polymer 3 that contains bulky side chain substituents by polymerization of the corresponding racemic 6,6'-dibromo binaphthyl monomer in the presence of NiCl2 and Zn.3 This polymer has a much higher glass transition temperature and higher thermal stability than the hexyl substituted polymer 2. The UV absorption spectrum of polymer 2 in methylene chloride
RO
1
5
6
RO OR
RO OR
RO OR 4
3 2
Br NiCl2(cat) / Zn(excess)
7 8
RO Br 1, R = n-C 6H 13
PPh3, bipyridine DMF ∆∆ RO
OR
RO
RO
OR
2, R = n-C6H 13
O O
O
O
O
O
3
O O
Scheme 1. Synthesis of Polybinaphthyl 2.
O O
O O
OR
solution displayed λmax at 262, 284 and 316 nm. When excited at 316 nm, its fluorescence spectrum showed two maximums at 386 and 406 nm. The π−π* band gap energy of 2 was estimated to be 3.33 eV by studying the cyclic voltammetry and UV absorption of the thin films of this polymer.5 The thin film of 2 spin -coated on a quartz plat displayed emission maximums at 390, 408, 450 (sh) nm. A broad secondary peak between 500 - 560 nm was also observed which may be due to the formation of excimers in the solid state vi
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