Synthesis of Chiral MOF-74 Frameworks by Post-Synthetic Modification by Using an Amino Acid.

The synthesis of chiral metal-organic frameworks (MOFs) is highly relevant for asymmetric heterogenous catalysis, yet very challenging. Chiral MOFs with MOF-74 topology were synthesised by using post-synthetic modification with proline. Vibrational circular dichroism studies demonstrate that proline is the source of chirality. The solvents used in the synthesis play a key role in tuning the loading of proline and its interaction with the MOF-74 framework. In N,N'-dimethylformamide, proline coordinates monodentate to the Zn2+ ions within the MOF-74 framework, whereas it is only weakly bound to the framework when using methanol as solvent. Introducing chirality within the MOF-74 framework also leads to the formation of defects, with both the organic linker and metal ions missing from the framework. The formation of defects combined with the coordination of DMF and proline within the framework leads to a pore blocking effect. This is confirmed by adsorption studies and testing of the chiral MOFs in the asymmetric aldol reaction between acetone and para-nitrobenzaldehyde.

A Tunable, Fullerene-Based Molecular Amplifier for Vibrational Circular Dichroism.

Vibrational circular dichroism (VCD) studies are reported on a chiral compound in which a fullerene C60 moiety is used as an electron acceptor and local VCD amplifier for an alanine-based peptide chain. Four redox states are investigated in this study, of which three are reduced species that possess low-lying electronic states as confirmed by UV/Vis spectroelectrochemistry. VCD measurements in combination with (TD)DFT calculations are used to investigate (i) how the low-lying electronic states of the reduced species modulate the amplification of VCD signals, (ii) how this amplification depends on the distance between oscillator and amplifier, and (iii) how the spatial extent of the amplifier influences amplification. These results pave the way for further development of tailored molecular VCD amplifiers.

Opening 2,2-diphenyl-2H-chromene to infrared light.

Time-resolved vibrational spectroscopy studies are reported on the photoinduced structural dynamics of 2,2-diphenyl-2H-chromene, a prototypical photochromic compound that undergoes ring opening upon UV radiation. The transient IR absorption measurements in combination with (TD-)DFT calculations have been used to understand in detail the life cycle of such compounds. Excited-state decay and ring opening was found to occur on an ultrafast time scale. Three species have been identified in the time-resolved IR spectra with two short-lived species (on a picosecond timescale) and a final long-lived species that remains after the measurable ns delay range. These species have been assigned to various open isomers using quantum chemical calculations of equilibrium structures and force fields. From the experiments and calculations key conclusions can be drawn on previously suggested models for the photocycle of such compounds, as well as on possible ways to controllably influence the performance of these compounds.

Excited-State Electronic Asymmetry Prevents Photoswitching in Terthiophene Compounds.

The diarylethene moiety is one of the most extensively used switches in the field of molecular electronics. Here we report on spectroscopic and quantum chemical studies of two diarylethene-based compounds with a non- C3-symmetric triethynyl terthiophene core symmetrically substituted with RuCp*(dppe) or trimethylsilyl termini. The ethynyl linkers are strong IR markers that we use in time-resolved vibrational spectroscopic studies to get insight into the character and dynamics of the electronically excited states of these compounds on the picosecond to nanosecond time scale. In combination with electronic transient absorption studies and DFT calculations, our studies show that the conjugation of the non- C3-symmetric triethynyl terthiophene system in the excited state strongly affects one of the thiophene rings involved in the ring closure. As a result, cyclization of the otherwise photochromic 3,3″-dimethyl-2,2':3',2″-terthiophene core is inhibited. Instead, the photoexcited compounds undergo intersystem crossing to a long-lived triplet excited state from which they convert back to the ground state.

Interplay of Exciton Coupling and Large-Amplitude Motions in the Vibrational Circular Dichroism Spectrum of Dehydroquinidine.

A detailed analysis of the computed structure, energies, vibrational absorption (VA) and circular dichroism (VCD) spectra of 30 low-energy conformers of dehydroquinidine reveals the existence of families of pseudo-conformers, the structures of which differ mostly in the orientation of a single O-H bond. The pseudo-conformers in a family are separated by very small energy barriers (i.e., 1.0 kcal mol(-1) or smaller) and have very different VCD spectra. First, we demonstrate the unreliable character of the Boltzmann factors predicted with DFT. Then, we show that the large differences observed between the VCD spectra of the pseudo-conformers in a family are caused by large-amplitude motions involving the O-H bond, which trigger the appearance/disappearance of strong VCD exciton-coupling bands in the fingerprint region. This interplay between exciton coupling and large-amplitude-motion phenomena demonstrates that when dealing with flexible molecules with polar bonds, vibrational averaging of VCD spectra should not be neglected. In this regard, the dehydroquinidine molecule considered here is expected to be a typical example and not the exception to the rule.