Rigid small-molecule DNA hybrids (rSMDHs) have been synthesized with three DNA strands attached to a rigid tris (phenyl acetylene) core. When combined under dilute conditions, complementary rSMDHs form cage dimers that melt at >10 degrees C higher and much sharper than either unmodified DNA duplexes or rSMDH aggregates formed at higher concentrations. With a 2.97 average number of cooperative duplexes, these caged dimers constitute the first example of cooperative melting in well-defined DNA-small-molecule structures, demonstrating the important roles that local geometry and ion concentration play in the hybridization/dehybridization of DNA-based materials.