Fluorophores were successfully used in several areas of chemistry and biochemistry. For many purposes, however, it is necessary that the fluorescence compound features a high fluorescence quantum yield as well as a large Stokes shift. The latter is, for example, achieved by the use of a twisted intramolecular charge-transfer (TICT) compound, which shows a twisted geometry in the excited state. However, the higher the twisting is, the lower becomes in general the fluorescence quantum yield as the resulting emission from the twisted state is forbidden. In order to escape this dilemma, we propose the model of planarized intramolecular charge-transfer (PLICT) states. These compounds are completely twisted in the ground states and planar in the excited states. By means of quantum chemical calculations (time-dependent (TD)-B3LYP and CC2) and experimental studies, we could demonstrate that 1-aminoindole and its derivatives form photoinduced PLICT states. They show both very large Stokes shifts ( =9000–13 500 cm⁻¹, i.e., λ=100–150 nm) and high fluorescence quantum yields. These characteristics and their easy availability starting from the corresponding indoles, make them very attractive for the use as optical switches in various fields of chemistry as well as biological probes.

The double “pancake” bonding in the dimers of the six-membered heterocycles 1,3-dithia-2,4,6-triazine (4) and 1,3-dithia-2,4-diazine (16) were investigated by means of high-level quantum chemical calculations (B3LYP and CCSD(T)). It was found that the S–S dimers, 20 a and 27, are not the most stable isomers, but the dimers showing short S−N (21 a) and S−C (25, 28) bonds. An investigation of the 5-phenyl-1,3-dithia-2,4,6-triazine (4 b) yields that the syn dimer with two S−S bonds (2.57 Å) is the most stable one. In this dimer, the phenyl groups are placed on top of each other. The additional dispersion energy of the phenyl rings causes a stabilization of the syn-S–S (C_2v-like) isomer. As a result, two weak albeit relevant single S−S bonds (2.57 Å) are predicted. These findings contradict the recently published concept of double “pancake” bonding in the dimer 4 b₂.

Oligomers of azole peptides have been isolated from a multitude of marine organisms. Up to now, these azole-containing dipeptide-analogue oligomers have only been found as cyclic n-mers (mostly tri- and tetramers) in nature. Herein, we show that imidazole-containing pseudopeptides form helixlike secondary structures in different solvents. The screw sense of the helix can be determined by attaching a single chiral imidazole unit to the N terminus of the oligomer. Investigation by means of circular dichroism (CD) spectroscopy showed that the folding process of the helix depends on the water content of the solvent in a parabolic way. In a pure organic medium, the helix is stabilized by hydrogen bonds between the hydrogen atoms of the amide groups and the nitrogen atoms of the azole ring. In aqueous solution, the formation of the helix is driven by dispersion interactions. The formation of the helix is more pronounced in aqueous solution than in organic solvents.

A simple way of rationalizing the structures of cyclic, bicyclic, and tricyclic sulfur–nitrogen species and their congeners is presented. Starting from a planar tetrasulfur tetranitride with 12π electrons, we formally derived on paper a number of heterocyclic eight-membered 10π electron species by reacting the 3p orbitals of two opposite sulfur centers with one radical each, or by replacing these centers by other atoms with five (P) or four (Si, C) valence electrons. This led to planar aromatic 10π electron systems, nonplanar bicyclic structures with a transannular SS bond, and tricyclic structures by bridging the planar rings with an acceptor or donor unit. The final structures depend on the number of π electrons in the bridges. Intermediate biradicals are stabilized by Jahn–Teller distortion, giving transannular SS bonds between the NSN units. This procedure may be summarized by two rules, which provide a rationale for the structures of a large number of sulfur–nitrogen-based molecules. The long bonds between the NSN units show a p character of >95 %. The qualitative results have been compared with known molecular structures and the results of B3LYP/cc-pVTZ calculations as well as CASSCF and CASVB calculations. B3LYP/cc-pVTZ calculations have also provided the UV/Vis spectra and the NICS values of the planar 10π systems.

A chirality switch in which the intrinsic chirality of a 4,4′-bipyridine is combined with a metal-ion-induced switching principle is described. In the uncomplexed state the 4,4′-bipyridine unit, which is linked to an S,S,S,S-configured cyclic imidazole peptide, is P-configured. The addition of zinc ions leads to a rotation around the CC bond axis of the 4,4′-bipyridine and the M isomer of the metal complex is formed. By addition of a stronger complexing agent the metal ions are removed and the switch returns to its initial position. The combination of the chirality switch with a second switching unit allows the construction of a molecular pushing motor, which is driven chemically and by light.

Biphenyl derivatives with small substituents in the ortho and ortho′ positions are called tropos. Due to the low rotation barrier around the C–C bond connecting the two phenyl units, the isolation of only one conformer is not possible; thus they are conformationally unstable. Using DFT calculations, we were able to show that using a suitable peptidic bridging unit, biphenyl systems can become conformationally stable. This stabilization should be independent of the type of substituent in the ortho and ortho′ positions. Some of the proposed biphenyl derivatives were successfully synthesized and studied in solution and solid state. The recorded VT-NMR, 2D-NMR and CD spectra show that all biphenyl derivatives exhibit the P conformation. The preference for the P conformation is confirmed by the structure of a biphenyl derivative in solid state.

A remarkable challenge for the design of molecular machines is the realization of a synchronized and unidirectional movement caused by an external stimulus. Such a movement can be achieved by a unidirectionally controlled change of the conformation or the configuration. Biphenol derivatives are one possibility to realize a redox-driven unidirectional molecular switch. For this reason, a 4,4′-biphenol derivative was fixed to a chiral cyclopeptidic scaffold and stimulated by chemical oxidants and reduction agents. The conformation of the switch was determined by DFT calculations by using B3LYP and the 6-31G* basis set. The switching process was observed by UV and circular dichroism (CD) spectroscopic measurements. Several oxidation agents and various conditions were tested, among which (diacetoxy)iodobenzene (DAIB) in methanol proved to be the best. In this way it was possible to synthesize a redox-stimulated molecular switch with a movement that is part of a rotation around a biaryl binding axis.

Configurationally stable triaryl phosphane oxides are important for reactions with transfer of chiral information. Apart from introducing bulky substituents to suppress fast inversion of helicity at room temperature, the use of a second chiral element which induces chirality in the triaryl phosphane oxide, so that it adopts only one configuration, is suitable. With regard to chirality transfer, C₂-symmetric imidazole cyclopeptides were tested for obtaining a configurationally stable phosphane oxide. Density functional calculations showed almost equal energies of the three possible triaryl phosphane oxides (MMM)-1, (PPP)-1, and (MP)-1. Surprisingly, after synthesis only the MMM conformer is present in solution, and its configurational stability was proved by variable-temperature and 2D NMR experiments as well as CD measurements. In view of the results of the DFT calculations, formation of stable (MMM)-1 cannot be explained thermodynamically but by kinetic reaction control. This concept of freezing the conformation of a triaryl phosphane oxide can in future be used to easily prepare configurationally stable stereoisomeric propellerlike compounds.

A straightforward synthesis of C₃-symmetric, imidazole-containing, macrocyclic peptides with different binding arms is presented. The chirality of the backbone and the selection of adequate receptor arms make these systems highly selective receptors for α-chiral primary organoammonium ions. Furthermore, the receptors have the ability to discriminate between enantiomeric guests with selectivity ratios of up to 87:13. The binding constants and the selectivity ratios were estimated by standard ¹H NMR titration techniques in CDCl₃. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

A straightforward synthesis of a C₃-symmetric imidazole-containing macrocyclic peptide with three hydroxyquinoline side arms is presented. Complex formation with various metal ions (Al³⁺, Ga³⁺, Fe³⁺, La³⁺ and Y³⁺) was investigated by spectrophotometric methods. CD spectroscopy revealed a highly diastereoselective binding of these ions at room temperature. In the case of Al³⁺, Ga³⁺ and La³⁺ complexes, Job plot analyses gave evidence of pure 1:1 stoichiometry. Ab initio calculations for the Al³⁺ and Ga³⁺ complexes showed that the Λ isomers are considerably stabilized relative to the Δ isomers. Furthermore, the calculated CD spectra for the Al³⁺ complexes confirm the formation of the Λ isomers in solution. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

A straightforward synthesis of C₂-symmetric azole-containing macrocyclic peptides is presented. This type of macrocycle possesses four amide groups directed into the interior of the scaffold that act as hydrogen-bond donors and two nitrogen atoms from the azole unit that act as hydrogen-bond acceptors. This arrangement makes them sensitive receptors for Y-shaped anions like AcO^– and H₂PO₄^– with a selectivity for dihydrogen phosphate versus acetate, as was shown with ¹H NMR titration techniques in [D₆]DMSO/5 % CDCl₃.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

A basic requirement for each molecular system that is supposed to perform work is a synchronized and unidirectional movement. Unidirectionality can be achieved by a change of configuration or conformation that is controllable by external stimulation. Molecular hinges based on a bipyridine unit work unidirectionally and are able to reach an amplitude of motion that amounts to about 180°. To analyze if it is possible to adjust the height of the unidirectional amplitude of motion, three planar chiral molecular hinge systems with a 2,2′-bipyridine unit as functional element were designed and stimulated with various divalent metal ions in different solvents. The configurations of the hinges were determined by DFT calculations using B3LYP and the 6-31G* basis set and experimentally verified by 2D NMR NOESY spectra. Circular dichroism (CD) and UV spectroscopy were used to study the properties of the hinges by the addition of metal ions (primarily Zn²⁺ and Hg²⁺) in dichloromethane and methanol. The choice of metal ions and solvents determines whether or not and how far the hinges are closed. Furthermore, a drastic change in the height of the amplitude of motion can be reached by modifying the position of the bipyridine unit in the hinge. Amplitude values from 45 up to 190° were obtained from quantum mechanical calculations. This control of the amplitude of motion can in the future be used for more complex switching processes of molecular machines.

A straightforward synthesis of C₃-symmetric oxazole-containing macrocyclic peptide scaffolds is presented. This type of macrocycles bears three functional groups on the oxazole rings, which allows fixing of various receptor arms on them in an easy manner. The chiral backbone of the macrocycle proved to be a powerful tool for chirality induction, thus predetermining the configuration of helically coordinated metal centres. The diastereoselective formation of Co^II, Ni^II, Cu^II and Zn^II complexes with tripodal bipyridyl ligand 4 was proved by UV- and CD-absorption spectrophotometric titration experiments.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

A chiral C₃-symmetric enterobactin analogue (1) has been synthesized by attachment of three 2,3-dihydroxybenzoyl units to a chiral oxazole-containing macrocyclic peptide scaffold. Complex formation kinetics and stoichiometry with various metal ions were investigated by spectrophotometric methods. In the cases of Al^III, In^III and Fe^III complexes, UV absorption and CD kinetics showed nonlinearity, which results from slow conformational changes of the octahedral complexes. Virtual binding constants were determined from UV absorption data and showed selective binding of Ga^III in preference to Fe^III, by two orders of magnitude. CD spectroscopy revealed highly diastereoselective binding of Al^III, Ga^III, In^III, Fe^III and Ge^IV ions at room temperature, corresponding to the helical chirality opposite to that of the analogous enterobactin complexes. Ab initio calculations confirmed the energetic stabilization of the Λ isomers relative to the Δ isomers.