Structure-Driven Photoluminescence Enhancement in a Zn-Based Metal–Organic Framework

We investigate the much debated mechanism that leads to an enhancement of the photoluminescence upon small aromatic molecule adsorption in the Zn-based metal–organic framework (MOF) material Zn2(bdc)2(dpNDI) (PCP-Zn). We present evidence based on ab initio simulations, coupled with in situ IR spectroscopy and photoluminescence measurements, that (i) the PCP-Zn "turn on" photoluminescence enhancement effect upon guest molecule adsorption mainly occurs as the result of an underlying guest-induced strucural change and (ii) a further, but smaller, enhancement occurs along with a change in emission color due to interactions of the guest molecules with the framework, altering the MOF's electronic structure. We provide a detailed analysis of both mechanisms. The clear separation of a dominating structural effect and minor electronic effect is important, for example, for sensing applications, where they can help to design improved MOF-based sensors with increased efficiency and selectivity.