Two fluorene-based alternating copolymers P1 and P2 with the N,N-diethylaniline pendent group in different contents on the polymer backbone were synthesized through Suzuki Coupling Reaction, and were fully characterized by H-1 NMR, elemental analysis, and gel-permeation chromatography (GPC). Photophysical studies show that changes of the contents of the N,N-diethylaniline groups have little effect on the ground state electronic structure, but take a significant effect on the excited state. The intensities of green emission band in these polymers changes with the different content of diethylaniline groups, and are supposed to be originated from an intramolecular charge transfer process. Both of the P1 and P2 show remarkable fluorescence emission "turn-on" responses exclusively to Hg2+. Moreover, fluorescent titration experiments indicate that the two copolymers have high sensitivity for Hg2+. The coordination between metal ions and N atoms could greatly weaken the electron-donating ability of N atoms, consequently inhibit the intramolecular charge transfer process, leading to fluorescence emission enhancement. As far as we know, this is the first report for detection of Hg2+ with "turn-on" output signals based on polyfluorenes. (C) 2009 Elsevier Ltd. All rights reserved.