Nanoparticles of Low Optical Band Gap Conjugated Polymers

Sonogashira coupling of 9,9-bis(2-ethylhexyl)-2,7-diethynyl-9 H -fluorene ( 1 ) with 2,7-dibromo-9,9-bis(2-ethylhexyl)-9 H -fluorene ( 2 ) and dibromo-substituted electron accepting arylenes 5,7-bis(5-bromothiophen-2-yl)-2,3-dihexylthieno[3,4- b ]pyrazine ( 3 ), 4,7-bis(5-bromo-4-(2-ethylhexyl)thiophene-2-yl)bis(benzothiadiazole) ( 4 ) or 4,9-bis(5-bromo-4-(2-ethylhexyl)thiophen-2-yl)-6,7-dihexylthiadiazolo[3,4- g ]quinoxaline ( 5 ), respectively, in a miniemulsion polymerization process afforded colloidally stable dispersions of poly(arylene ethynylene) nanoparticles with an average size in the range of 50 to 120 nm. For these poly[( 1 - alt -( 2 - co - X )] ( X = 3, 4, or 5 ), poly( 1 - alt - 3 ), poly( 1 - alt - 4 ), and poly( 1 - alt - 5 ) aqueous nanoparticle dispersions absorption occurs increasingly in the red and NIR regime with increasing incorporation of acceptor monomer, with a strong absorption up to λ abs = 1 μm for poly( 1 - alt - 4 ) dispersions. For poly[ 1 - alt -( 2 - co - 3 )] nanoparticles fluorescence is also observed at λ em = ca. 700 nm, which occurs at this long wavelength exclusively even at an incorporation of only 1 mol % of 3 due to energy transfer to the low-energy chromophores. An alternative postpolymerization approach by change of solvent quality yielded dilute aqueous dispersions of self-stabilized nanoparticles from an amphiphilic block copolymer, poly(ethylene glycol)- block -poly( 1 - alt- 6 )- block -poly(ethylene glycol) generated from alkyne-terminated poly( 1 - alt- 6 ) via azide–alkyne coupling ( 6 = 4,7-bis(5-bromo-4-(2-ethylhexyl)thiophene-2-yl)benzothiadiazole), which fluoresce at λ em max = 672 nm.