A new water-soluble zwitterionic oligofluorene bearing carboxylic acid and quaternary ammonium as pendant groups (OF-1) is synthesized and characterized. It forms aggregates by intermolecular electrostatic interactions and exhibits similar light-harvesting ability as that of conjugated polymers. Efficient fluorescence resonance energy transfer (FRET) occurs from OF-1 to double-stranded DNA tagged with fluorescein (dsDNA-F1). A photoresponsive oligofluorene (OF-3) is also synthesized by protecting OF-1 with 1-(2-nitrophenyl)ethanol. Photolysis of OF-3 can produce OF-1 to result in a fluorescence "turn-on" response, thus the FRET from OF-3 to dsDNA-F1 can be turned on by light irradiation. OF-3 offers the potential for remote DNA sensing.