Light-tunable DNA interactions enable spatiotemporal assembly and relaxation-driven crystallization of colloids.

Self-assembly of colloidal particles enables the formation of complex superstructures, yet precise control over assembled structures remains challenging and assembly pathways primarily rely on temperature, a global trigger. Here, we introduce a strategy to dynamically modulate DNA-mediated colloidal interactions using azobenzene-functionalized DNA strands (azoDNA) grafted on colloidal particles, programming the interaction landscapes in space and time. Photo-isomerization of the azobenzene moiety allows reversible and continuous tuning of the stability of DNA duplexes, enabling light-controlled regulation of interparticle binding under isothermal conditions. By varying illumination conditions, the effective melting temperature of the colloids can be adjusted over a wide range, allowing reversible assembly, spatially patterned aggregation, and dynamic reconfiguration of colloidal structures at the scale of a few particles. Beyond reversible switching, we show that the slow thermal relaxation of azobenzene provides a new route to relaxation-mediated colloidal crystallization, in which the gradual recovery of DNA stickiness promotes ordered crystal growth. These results demonstrate how light can be used to program both the strength and the temporal evolution of DNA-mediated interactions, offering a versatile platform for spatiotemporally controlled self-assembly and adaptive colloidal materials.