Truxene-to-Fluorenone Energy Transfer in a Robust Mesoporous Zn-MOF.

A new metal-organic framework (MOF; [Zn 4 O( hett ) 4/3 ( fluo ) 1/2 ( bdc ) 1/2 ] n ; TFT-MOF ) constructed on chromophoric ligands 5,5',10,10',15,15'-hexaethyltruxene-2,7,12-triacetate ( hett ), 9-fluorenone-2,7-dicarboxylate ( fluo ), terephthalate ( bdc ), and the Zn 4 O node has been prepared and identified by powder X-ray diffraction. This luminescent MOF exhibits large mesoporous pores of 2.7 nm based on computer modeling using density functional theory (DFT) calculations. The steady-state and time-resolved fluorescence spectra and photophysical parameters of TFT-MOF have been investigated and compared with those of the free ligands and their basic chromophores. All in all, TFT-MOF exhibits particularly efficient singlet-singlet energy-transfer processes described as 1 ( hett )* → ( fluo ) and 1 ( bdc )* → ( fluo ), leading to fluorescence arising for the fluo lumophore operating only through Förster resonance energy transfer (FRET) with an efficiency of transfer of up to >95%. This experimental conclusion was corroborated by DFT and time-dependent DFT (TDDFT). For the 1 ( hett )* → ( fluo ) process, the approximated overall rate constant of energy transfer was evaluated to be at most 2.04 × 10 10 s -1 (using a Stern-Volmer approach of solution data and the relationship between distance and concentration). This process was analyzed using the Förster theory, where two intrapore energy transfer paths of center-to-center distances of 13 and 25 Å have been identified. TFT-MOF photosensitizes the formation of singlet oxygen ( 1 O 2 ( 1 Σ g )) as detected by its phosphorescence signal at 1275 nm.