In this report, we synthesized a series of TO conjugates containing different amino side chains and investigated their binding to telomeric G-quadruplex DNA (G4) using several biophysical methods including fluorometric titration and thermal denaturation monitored by fluorescence and circular dichroism. The composition of side chains strongly affects the binding of these molecules to G-quadruplex DNA. Incorporation of amino side chains increases the binding affinity of TO toward G4 but has a minimal effect on its selectivity for G4 over duplex DNA. The plausible binding modes are a synergistic effect of end-stacking and groove interactions as indicated by docking studies. Inhibition of human telomerase activity by TO derivatives was determined in vitro by the TRAP assay. Several derivatives can selectively inhibit the activity of telomerase over DNA polymerase at low concentrations. More significantly, TO-spermine conjugate (16) exhibits a remarkable effect on telomerase inhibition in the submicromolar range, which is comparable to the inhibition effect of a well-known G4 ligand, BRACO-19. Our results here provide guidance of utilizing TO derivatives as a viable scaffold to design novel G4 ligands, G4 probes, and potent telomerase inhibitors.