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 aminofunctionalized side groups were synthesized through the Sonogashira reaction. They were poly{9,9-bis[6′-(N,N-diethylamino)hexyl]-2,7- fluorenylene ethynylene(-alt-co-(2,5-bis[3′-(N,N-diethylamino)-1′- oxapropyl]-1,4-phenylenel (P1), poly{9,9-bis[6′-(N,N-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)-JV-methylammonium]hexyl)fluorene hexaiodide and 1,4-bis(9′,9′-bis{6″-[(N,N-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.