A soluble PPV derivative (C-PPV) with high molecular weight, substituted with a crown ether in direct π-conjugation at the 2,3-position, has been synthesized via a modified Gilch route in good yield. The double layer LED based on C-PPV (ITO/C-PPV/Alq3/Al) exhibits a high luminance of 730 cd m−2, and emits yellow–green light with an external EL quantum efficiency of 0.04%. The fluorescence attenuation of the polymer solution is found upon K+, Na+, or Li+ binding. The AFM provides high-resolution images of C-PPV in thin films constructed from a very dilute solution in the absence and presence of metal ions. Worm-like images with well-defined width and height are observed only in the presence of K+, which is in striking contrast to the individual ball shape of C-PPV molecules in the absence of K+, or in the presence of Na+, or Li+. It was also observed that the worm-like images grew longer and longer as the solution of C-PPV in the presence of K+ stood longer. These unique self-assembling features of the C-PPV molecules may be attributed to the complexation of K+.