Design of Luminescent Polymers for Leds
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DESIGN OF LUMINESCENT POLYMERS FOR LEDS A. B. Holmes*, A. D. Bond*, J. E. Davies*, C. Fischmeister*, J. Frey*, U. Hennecke*, J. Li*, Y. Ma*, R. E. Martin*, I. D. Rees*, K. Robinson*, T. Sano*, F. Cacialli**, S. F. Lim**, R. H. Friend** *Melville Laboratory for Polymer Synthesis, Department of Chemistry, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, U.K. **Cavendish Laboratory, Department of Physics, University of Cambridge, Madingley Road, Cambridge CB3 0HE, U.K.
ABSTRACT A 2,3-dibutoxy-1,4-phenylenevinylene comonomer was incorporated into a distyrylbenzene derivative 11. Novel 1,2-disubstituted-3,6-dibromobenzene comonomers 15 and 18 were prepared by directed metallation. Copolymerization of 11 with a 9,9-dioctylfluorene2,7-diboronate ester 1 yielded a green fluorescent polymer while copolymerization of 15 and 18 with the 9,9-dihexylfluorene-2,7-diboronate 22 afforded promising blue fluorescent polymers 23 and 24 respectively.
INTRODUCTION There is much interest in the possible use of conjugated fluorescent polymers as the active layer in organic light emitting devices [1-7]. Early progress was described with materials based on poly(2,5-disubstituted-1,4-phenylenevinylene) derivatives where the important principle of color tunability through the use of copolymers was first exemplified [8]. More recently polymers based on poly(9,9-disubstituted-fluorene-2,7-diyl) repeat units (e.g. 3) have enjoyed considerable success, owing to the efficiency of their synthesis, the realization of high molecular weights, and the ability to tune the color of emission by use of copolymers [9]. The method of choice for the synthesis of these materials is the step growth polycondensation involving the Suzuki cross coupling of the diboronate 1 with the dibromide 2 [9]. This process has been enhanced by the recent report from the Cambridge Display Technology group that tetra-alkyl ammonium hydroxide is a preferred base catalyst for the Suzuki polymerization (Scheme 1) [10]. O B O
C8H 17 1
C8H17
B O O
+
Br
Br
C8H 17
C8H17
Pd(PPh3 )4, Et4NOH Toluene, reflux
2
C8H 17
C8H17
n
3
Scheme 1. Suzuki cross coupling route to polyfluorene (PF8) 3. One of the major complications of polyfluorenes is their propensity towards excimer formation which results in an undesired long wavelength emission during device operation [11]. This is obviously undesirable. Various approaches have been developed to minimize the BB5.2.1
aggregation phenomena in polyfluorenes. These include the synthesis of various copolymers [11, 12], the planarization of the fluorene structure by the development of ladder-like polymers [13] and polyindenofluorenes [14], the attachment of dendrons to increase the steric bulk around the electroactive segments [15] and the use of hole traps [16]. In this work we report the copolymer approach [12] based on building blocks which were expected to have high photoluminescence efficiencies as a result of steric inhibition of coplanarity through the bulk of side chain substituents.
RESULTS The Gilch dehydrohaloge
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