Strong Host-Guest Dependence on the Emissive Properties of MOF-5 and [Zn₂(BTTB)(DMF)₂•(H₂O)₃]

3D-[Zn₄O(1,4-BDC)₃•x(solvent)] (MOF-5; BDC = 1,4-benzodicarboxylate) and 3D-[Zn₂(BTTB)(DMF)₂•(H₂O)₃] (MOF-D; BTTB = 4,4',4″,4‴-benzene-1,2,4,5-tetrayltetrabenzoate) have been investigated by means of steady-state UV-visible and fluorescence and time-resolved emission spectroscopy, as a function of solvent and power of the excitation irradiation. The low-temperature X-ray structures (173 K) were permitted to locate solvent molecules (here H₂O) in the lattice. They were found distributed in the middle in the voids with no evidence of specific interactions (H-bond, coulombic, and dipole-dipole) with the framework. The fluorescence decays of the ligands (ππ* excited state), τ_F, for the host-guest composites MOF-5@solvent and MOF-D@solvent (solvent = air, MeCN, EtCN, MeOH, EtOH, and DMF) were found bi-exponential (short τ_F1 (ps), and long τ_F2 (ns)) with one important feature: upon cooling from 298 to 77 K, MOF-5's τ_F1 decreases and τ_F2 increases, while the opposite trend is generally observed in MOF-D. The low values for τ_F1 (ps) in MOF-5 are associated with the augmented probability of solvent-ligand collisions leading to nonradiative deactivation, which upon cooling to 77 K increases further as the scaffolding contracts. The augmentation in τ_F2 is readily associated with the increased rigidity of the ligands that are not submitted to this effect (at the surface of the MOF and as pendent groups). For the low emitter MOF-D, the reversed situation is noted but not as clearly due to the uncertainties in the data. Upon increasing the excitation flux, the fluorescence intensity increases linearly with the laser power indicating the absence of singlet-singlet annihilation, inferring the absence of efficient exciton migration. This observation is explained by the small absorptivity coefficients, which leads to a small spectral overlap between absorption and fluorescence according to the Forster and Dexter theories, and consequently, a small rate for energy migration. This conclusion drastically changes the perception of the photocatalytic mechanism of MOF-5 and other MOFs exhibiting similar absorption features (i.e., no antenna effect).