Colorimetric quantification of linking in thermoreversible nanocrystal gel assemblies

Nanocrystal gel networks can be responsive, tunable materials, but deliberately designing their structure and controlling their properties have been challenging. By employing reversibly bonded molecular linkers, gelation can be realized under conditions predicted by thermodynamics. But, simulations have offered the only microscopic insights, with no experimental means to monitor linking leading to gelation. Here, we introduce a metal coordination linkage with a distinct optical signature allowing us to quantify linking in situ and establish the structural and thermodynamic basis for assembly. Due to coupling between linked indium tin oxide nanocrystals, their infrared absorption shifts abruptly at a chemically tunable gelation temperature. We quantify bonding spectroscopically and use molecular dynamics simulations to understand bonding motifs as a function of temperature, revealing that gel formation is governed by reaching a critical number of effective links that extend the nanocrystal network. Microscopic insights from our colorimetric linking chemistry enable switchable gels based on equilibrium thermodynamic principles, opening the door to rational design of programmable nanocrystal net-work assemblies.