Effect of Chromophore-Charge Distance on the Energy Transfer Properties of Water-Soluble Conjugated Oligomers

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Abstract
The synthesis of 1,4-bis(9,9‘-bis(3‘ ‘-(N,N,N-trimethylammonium)-propyl)-2‘-fluorenyl)benzene tetrabromide (C 3), 1,4-bis(9,9‘-bis(4‘ ‘-(N,N,N-trimethylammonium)-butyl)-2‘-fluorenyl)benzene tetrabromide (C 4), 1,4-bis(9,9‘-bis(6‘ ‘-(N,N,N-trimethylammonium)-hexyl)-2‘-fluorenyl)benzene tetrabromide (C 6), and 1,4-bis(9,9‘-bis(8‘ ‘-(N,N,N-trimethylammonium)-octyl)-2‘-fluorenyl)benzene tetrabromide (C 8) is reported. Fluorescence energy transfer experiments between C 3−C 8 and the acceptors pentasodium 1,4-bis(4‘(2‘ ‘,4‘ ‘-bis(butoxysulfonate)-styryl)styryl)-2-(butoxysulfonate)-5-methoxybenzene (3), fluorescein labeled single-stranded DNA and fluorescein labeled double-stranded DNA in water, buffer, and methanol reveal the importance of hydrophobic and electrostatic forces in determining chromophore−chromophore close proximity. In water, the oligomers with longer side chain length show better energy transfer, as well as higher Stern−Volmer quenching constants (Ksv), largely due to a stronger hydrophobic attraction between the optically active components. In methanol, the differences in energy transfer are leveled, and the oligomers with shorter side chain lengths show higher Ksv values. Compounds C 3, C 4, C 6, and C 8 were also used to dissect the different contributors to DNA hybridization assays based on cationic conjugated polymers.