Size-Dependent Optical Band Gaps in Metal-Organic Framework Nanoparticles

Decades of research into size-dependent semiconductor optical gaps have focused on quantum confinement as the dominant mechanism. Emerging reports indicate that lattice strain─intentional or incidental─can impart optical shifts similar or greater in magnitude. Here, we report evidence of optical absorption and photoluminescence spectra of M(1,2,3-triazolate)₂ (M = Mg, Cr, Mn, Fe, Co, Cu, Zn, or Cd) nanoparticles that blueshift from bulk values with decreasing particle sizes in a manner that defies explanation by conventional quantum confinement. The phenomenon persists for particle sizes as large as 200 nm, whereas quantum confinement generally ceases beyond 20-30 nm diameters and follows a weaker dependence on the particle radius. Computational simulations and crystallographic analysis suggest that this behavior arises from size-dependent changes to metal-linker bonding that manifest in strain values comparable to literature reports of strain-induced optical shifts in other classes of materials. This behavior appears beyond this family of materials in other notable examples of metal-organic frameworks (MOFs), including the well-studied Cu₃(trimesate)₂ (CuBTC), where smaller sizes correlate with blueshifted optical gaps. Taken together, these results represent one of the few examples of size-dependent strain in crystalline materials and reinforce the emerging view that MOFs become softer materials when isolated as nanoparticles.