The anionic diamido N‐heterocyclic carbene 1 is used to prepare a series of linear as well as trigonal, heteroleptic CuI complexes with small molecular ligands such as pyridine derivatives or triphenylphosphine. A key role lies in the versatile precursor for these complexes, a moisture‐ and air‐stable 1D coordination polymer [1 ⋅ Cu]n composed of only the NHC ligand and CuI, such that the copper is linearly coordinated by the carbene carbon atom and one oxygen atom of the backbone of the carbene. This polymer can easily be cleaved into monomeric complexes by addition of the desired ligand to dispersions of the polymer in dichloromethane. In solution, the complexes are in equilibrium with this highly insoluble polymer and free ligand. Thus, analytical and spectroscopical experiments with the compounds are limited to their crystalline state, characterized by single crystal X‐ray diffraction experiments. Some of the complexes exhibit visible luminescence in the solid state upon irradiation with ultraviolet light. The spectral features (emission wavelength, Stokes shift, width of the emission band, vibrational fine structure) significantly differ among the complexes. Quantum mechanical computations reveal a subtle interplay of several factors such as coordination number and charge transfer character of the emissive state.
The anionic diamido N-heterocyclic carbene 1 is used to prepare a series of linear as well as trigonal, heteroleptic CuI complexes with small molecular ligands such as pyridine derivatives or triphenylphosphine. A key role lies in the versatile precursor for these complexes, a moisture- and air-stable 1D coordination polymer [1·Cu]n composed of only the NHC ligand and CuI, such that the copper is linearly coordinated by the carbene carbon atom and one oxygen atom of the backbone of the carbene. This polymer can easily be cleaved into monomeric complexes by addition of the desired ligand to dispersions of the polymer in dichloromethane. In solution, the complexes are in equilibrium with this highly insoluble polymer and free ligand. Thus, analytical and spectroscopical experiments with the compounds are limited to their crystalline state, characterized by single crystal X-ray diffraction experiments. Some of the complexes exhibit visible luminescence in the solid state upon irradiation with ultraviolet light. The spectral features (emission wavelength, Stokes shift, width of the emission band, vibrational fine structure) significantly differ among the complexes. Quantum mechanical computations reveal a subtle interplay of several factors such as coordination number and charge transfer character of the emissive state.
A previous quantum chemical study (M. Bracker et al., Phys. Chem. Chem. Phys. 2019, 21, 9912–9923) on the excited state properties of fluorinated derivatives of the flavin chromophore promised an increased fluorescence performance of the derivative 7,8‐difluoro‐10‐methyl‐isoalloxazine (7,8‐dF‐MIA). Here, we describe the synthesis of 7,8‐dF‐MIA, its ribityl derivative, and for reason of comparison 9‐F‐MIA. The compounds dissolved in water (H2O and D2O) were characterized by steady state, time resolved, and fluorescence correlation spectroscopy. The experiments confirm the increase of the fluorescence quantum yield of 7,8‐dF‐MIA (0.42 in H2O) compared to MIA (0.22) predicted by quantum chemistry. The anticipated reduction of the fluorescence quantum yield for 9‐F‐MIA is also confirmed experimentally. The quantum chemical computations as well as the spectroscopic observations attribute the increased fluorescence quantum yield of 7,8‐dF‐MIA predominantly to a decrease of the rate constant of intersystem crossing. Switching from H2O to D2O as a solvent is shown to increase fluorescence quantum yields (0.53 for 7,8‐dF‐MIA) and lifetimes of all fluorinated MIA derivatives. This can be attributed to a Förster type energy transfer from the excited chromophore to vibrational overtones of water and further water‐mediated deactivation processes.
The Front Cover illustrates a full strike in fluorescence! Double fluorination of isoalloxazines at positions 7 and 8, to which quantum chemistry has pointed at, gives compounds with the highest fluorescence quantum yields reported so far for its class. The mallet shows the power of fluorination for the modulation of intersystem crossing when aimed delicately. The corresponding difluorinated flavins show even higher quantum yields when using deuterated solvents. Cover design by Marie Glasewald. More information can be found in the Research Article by Claus A. M. Seidel, Peter Gilch, Constantin Czekelius and co‐workers.
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