Nanoparticles of low optical band gap conjugated polymers

Abstract

Sonogashira coupling of 9,9-bis(2-ethylhexyl)-2,7-diethynyl-9H-fluorene (1) with 2,7-dibromo-9,9-bis(2-ethylhexyl)-9H-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 λabs = 1 μm for poly(1-alt-4) dispersions. For poly[1-alt-(2-co-3)] nanoparticles fluorescence is also observed at λem = 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 λem max = 672 nm.