Side-Chain Modifications in Polyfluorenes

Many studies on side-chain modifications in PF were initially based on the idea of excimer formation, resulting in the green emission during LED operation or in solid-state PL on annealing PF films. This resulted in several proposed strategies for the design of fluorene side-chain homopolymers, where bulky substituents at position 9 of the fluorene moiety should sterically prevent (hinder) interchain interaction and thus improve the stability of blue emission. 

In addition to 195-200, many other alkyl substituents and their derivatives have been introduced at position 9 of the fluorene nucleus in order to create a processible stable blue-emitting PF material, e.g., 203a-h [273-275,305], Chiral-substituted PFs 200 and 203g,h have been synthesized to study their chiroptical properties [306], particularly interesting due to polarized emission in such materials (see Chapter 5 in this book) (Chart 2.47). 

Patents of Dow Chemicals first described 9,9-diaryl-substituted PF homopolymers 204 and 205 by Yamamoto polymerization of the corresponding 2,7-dibromo monomers [272], although the methods for monomer preparation were not described. For unsubstituted fluorenone, a convenient method for its conversion into 9,9-(4-hydroxyphenyl)-[307-309] and 9,9-(4-alkoxyphenyl)fluorenes [310] was reported previously, which included condensation of fluorenone with phenol or its ethers in acidic conditions (dry HC1 [308,309] or H2SO4 [307,311]) in the presence of (3-mcrcaptopropionic or mercaptoacetic acids. Both polymers 204 and 205 showed similar Mn 21,000 with PDI of 1.48 and 1.75, respectively, and spectral data typical for PF (205 Aabs = 389 (r 50,000 //(mol cm) APL = 417, 439, and 473 mn (THF)) (Chart 2.48). 

SCHEME 2.28 Synthesis of poly[9,9-di(4-octyloxyphenyl)fluorenes). (From Lee, J.-H. and Hwang, D.-H., Chem. Commun., 28, 2836, 2003.) 

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