Strong Host-Guest Dependence on the Emissive Properties of MOF-5 and [Zn-2(BTTB)(DMF)(2)& BULL;(H2O)(3)]( n )

Two metal-organic frameworks (MOFs)have been characterizedin the presence of different guests (empty pores, DMF, MeOH, EtOH,MeCN, and EtCN) to ascertain the influence of these guests on theexcited state dynamic processes taking place in the MOF structures.We find that the solvent molecules have a significant impact on theluminescence intensity and excited state lifetimes when compared toempty MOFs and that no exciton migration occurs on either MOF structure. 3D-[Zn4O(1,4-BDC)(3)& BULL;x(solvent)]( n ) (MOF-5; BDC= 1,4-benzodicarboxylate) and 3D-[Zn-2(BTTB)(DMF)(2)& BULL;(H2O)(3)]( n ) (MOF-D; BTTB =4,4 & PRIME;,4 & DPRIME;,4'-benzene-1,2,4,5-tetrayltetrabenzoate)have been investigated by means of steady-state UV-visibleand fluorescence and time-resolved emission spectroscopy, as a functionof solvent and power of the excitation irradiation. The low-temperatureX-ray structures (173 K) were permitted to locate solvent molecules(here H2O) in the lattice. They were found distributedin 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 (& pi;& pi;* excited state),& tau;(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 & tau;(F1) (ps), and long & tau;(F2) (ns)) with one importantfeature: upon cooling from 298 to 77 K, MOF-5's & tau;(F1) decreases and & tau;(F2) increases,while the opposite trend is generally observed in MOF-D. The low values for & tau;(F1) (ps) in MOF-5 are associated with the augmented probability of solvent-ligandcollisions leading to nonradiative deactivation, which upon coolingto 77 K increases further as the scaffolding contracts. The augmentationin & tau;(F2) is readily associated with the increased rigidityof the ligands that are not submitted to this effect (at the surfaceof the MOF and as pendent groups). For the low emitter MOF-D, the reversed situation is noted but not as clearly due to the uncertaintiesin the data. Upon increasing the excitation flux, the fluorescenceintensity increases linearly with the laser power indicating the absenceof singlet-singlet annihilation, inferring the absence of efficientexciton migration. This observation is explained by the small absorptivitycoefficients, which leads to a small J spectral overlap between absorption and fluorescence according tothe Forster and Dexter theories, and consequently, a small rate forenergy migration. This conclusion drastically changes the perceptionof the photocatalytic mechanism of MOF-5 and other MOFsexhibiting similar absorption features (i.e., no antenna effect).

Automated summary

Two metal-organic frameworks (MOFs) were characterized in the presence of different guests to investigate the influence of these guests on the excited state dynamic processes in the MOFs. Solvent molecules have a significant impact on the luminescence intensity and excited state lifetimes, and no exciton migration occurs in either MOF structure.