M -1 ). This difference in DNA binding activity between two photoisomers of the same compound is, to the best of our knowledge, significantly higher than those previously reported to date. Moreover, (E)-1 showed hypochromic and bathochromic shifts upon ligand binding, suggesting end-stacking ligand binding modes, where the energy of the π-π* transition responsible for the Soret band is lowered by the interactions of the ligand chromophores with the G-tetrad. Figure . a) UV/visible titration studies of (Z)-1 (right) and (E)-1 (left) with LTR-III (K + ) and b) binding isotherms.