Vibrational Paddlewheel Cu-Cu Node in Metal-Organic Frameworks: Probe of Nonradiative Relaxation

Nonradiative relaxation, a ubiquitous phenomenon in natural and artificial molecules and materials, has been extensively studied in contemporary chemistry. In this report, we show the nonradiative relaxation of Cu(II)-based paddlewheel metal-organic frameworks (MOFs), HKUST-1 and Cu-MOF-2, with Raman measurements. Irradiation of the Cu-based MOF crystals by a 532 nm laser with the minimum power of 1.5-8.0 mW results in the dissociation of the axially ligated solvent molecules at the paddlewheel Cu(II) sites. Dissociation arises by the accumulated thermal energy formed by nonradiative relaxation, and the minimum power necessary is dependent on both the type of MOF and the Lewis basic solvent molecule that is coordinated to the metal node. We demonstrate that the minimum power is associated with an equilibrium between the accumulation and dissipation of thermal energy and also that thermal dissipation is dependent on the coordination strength, molecular interaction energy, and kinetic energy of the solvent molecules residing in the pores. Finally, we show the nonradiative relaxation behavior of nonluminescent MOFs based on the comparison between the Cu-based MOFs and Zn-MOF-2, a structurally analogous MOF that does not exhibit nonradiative relaxation.