Cationic, water-soluble, fluorene-containing poly(arylene ethynylene)s: Effects of water solubility on aggregation, photoluminescence efficiency, and amplified fluorescence quenching in aqueous solutions

Three novel fluorene-containing poly(arylene ethynylene)s with amino-functionalized side groups were synthesized through the Sonogashira reaction. They were poly {9,9-bis[6'-(NN-diethylamino)hexyl]-2,7-fluorenylene ethynylene}-alt-co-{2,5-bis[3'-(N,N-diethylamino)-1'-oxapropyl]-1,4-phenylene} (P1), poly{9,9-bis[6'-(NN-diethylamino)hexyl]-2,7-fluorenylene ethynylene} (P2), and poly({9,9-bis[6'-(N,N-diethylamino) hexyl]-2,7-fluorenylene ethynylene}-alt-co-(1,4-phenylene)) (P3). Through the post-quaternization treatment of P1-P3 with methyl iodide, we obtained their cationic water-soluble conjugated polyelectrolytes (WSCPs): P1'-P3'. The water solubility was gradually improved from P3' to P1' with increasing contents of hydrophilic side chains. After examining the ultraviolet-visible absorption and photoluminescence (PL) spectra, fluorescence lifetimes, and dynamic light scattering data, we propose that with the reduction of the water solubility from P1' to P3', they exhibited a gradually increased degree of aggregation in H2O. The PL quantum yields of P1'-P3' in H2O displayed a decreasing tendency consistent with the increased degree of aggregation, suggesting that the pronounced degree of aggregation was an important reason for the low PL quantum yields of WSCPs in H2O. Two structurally analogous water-soluble trimers of P2' and P3', model compounds 2,7-bis(9,9-bis{6'-[(N,N-diethyl)-N-methylammonium] hexyl}-2-fluorenylethynyl)-9,9-bis{(6'-[(N,N-diethyl)-N-methylammonium]hexyl}fluorene hexaiodide and 1,4-bis(9',9'-bis{(6-[(NN-diethyl)-N-methylammonium]hexyl}-2'-fluorenylethynyl)benzene tetraiodide, were synthesized. The amplified fluorescence quenching of these WSCPs by Fe(CN)(6)(4-) in H2O was studied by comparison with a corresponding analogous trimer. The effects of aggregation on the fluorescence quenching may be two-edged in these cases. (c) 2006 Wiley Periodicals, Inc.