The effects of the rigidity of molecular recognition sites in fluorene-based conjugated polymers P1 and P2 on metal ion sensing have been investigated. The structures of polymers P1 and P2 have twisted 2,2'-bipyridine and planar 1,10-phenanthroline units, respectively, which alternate with one fluorene monomer unit. It is found that the absorption and emission bands of 1,10-phenanthroline-based polymer P2 exposed to metal ions can be red-shifted up to 30 nm, and emission intensity can be quenched up to 100%, depending on metal ions present, which is very similar to that of the 2,2'-bipyridine-based analogue PI. However, polymer P2 shows much higher sensitivity to metal ions than P1. The origins of ionochromic effects of the 2,2'-bipyridine-based conjugated polymer due to the metal ion chelation have been attributed to both conformational changes and electron density variations on the polymer chains caused by introducing positively charged metal ions (Chen et al. J. Phys. Chem., B 2000, 104, 1950-1960). On the basis of the comparison of P2 with P1, conformational changes are not required in the ion responsive process of the phen ion-recognition unit. We demonstrate that the electron density variations play more important roles in metal ion-induced red-shifts in absorption and fluorescence quenching in photoluminescence.