Tailoring intra- and intermolecular properties : from cyclophanes to daisy chains

Todays high-tech society is striving for faster, cheaper and higher performing devices in all circumstances. Traditional materials have long since reached their limits making the search for new materials with altered properties inevitable. Easy processable, long-term stable polymeric materials with improved handling or switchable properties for use in plastics, conducting single molecular wires, switches, rectifiers in computer industry, nonlinear optic materials for data transmission at the speed of light, devices for flexible displays and luminescent materials operating with less energy are needed in the nearby future. Hence fundamental research in material science and nanotechnology is urgently necessary to provide the base for potential future applications.
Part A: The synthesis and aggregation studies of a hermaphroditic molecular rod comprising a terminal water soluble loop are presented. Functional group transformations on a mono-functionalized Diederich-type cylophane, acting as the loop subunit, containing a hydrophobic cavity and final coupling of an oligophenylene-ethynylene hydrophobic molecular rod provided the envisaged target compound in 21 or 17 synthetic steps. In polar solvents the hydrophobic rod threads into the cavity of the macrocycle driven by a strong hydrophobic effect. Aggregation studies by 1H-NMR titrations, fluorescence titrations and mass spectrometry confirmed the formation of dimers at low concentrations and longer oligomers at higher concentrations. Such molecular daisy chains are potential candidates towards new polymers with altered macroscopic properties
Part B: The functionalization of the molecular rod of the amphiphilic daisy chain monomer at the terminus with thiol anchoring groups for investigations in molecular electronics is shown. Aggregation studies revealed a similar self-complexation behavior as the unfunctionalized version. Below concentrations of 0.37 mM the formation of [c2]daisy chains was observed. Such thiol-functionalized pseudorotaxanes are potential candidates for investigations of bimolecular bridges in mechanically controlled break junctions (MCBJ) potentially resulting in new design possibilities in single molecular electronics. Furthermore the mechanically adjustable stacking surface of the molecular rods allows for mimicking a macroscopic potentiometer by mechanical minute opening and closing the gap between two electrodes.
Part C: Terminal piperidinyl and nitro functionalized biphenyls, bridged between 2 and 2’ position by a variable number of methylene groups, were synthesized and fully characterized. These push-pull systems with defined and restricted torsion angles between their phenyl rings are ideal model compounds to investigate the influence of the chromophore’s conjugation in nonlinear optic (NLO) responses. A general implementable synthetic route towards these model compounds is reported. NLO properties of the series of torsionally constrained push-pull biphenyls were collected by electric field induced second harmonic generation (EFISH) experiments. The results agree qualitatively with semi-empirical simulations based on the AM1 Hamiltonian. A linear dependence of the quadratic response on the cos2Φ of the inter-aryl dihedral angle is observed, which points to oscillator strength loss as the dominant effect of increasing backbone twist.
To get insight into the conformational stability, the thermodynamics of the atropisomerization of these torsion angle restricted, axial chiral biphenyl based push-pull cyclophanes were studied. Using 1H-NMR coalescence and dynamic HPLC measurements the rotation barrier around the central C-C bond was determined, indicating that the tendency of the push-pull system to planarize may be considered as a driving force for the atropisomerization.
Part D: The influence of 2,2’ propyl-bridged and 4,4’ electron donor or electron acceptor substituted axial chiral biphenyl cyclophanes on their atropisomerization process was studied. Estimated free energies of the rotation around the central biphenyl bond – obtained from 1H-NMR coalescence measurements – were correlated to the Hammett-parameters σp as a measure for electron donor and acceptor strength. It is demonstrated that the resulting linear correlation is mainly based on the influence of the different substituents on the π-system of the biphenyl cyclophanes. DFT calculations show a planar transition state of these isomerization processes and the calculated energy barriers based on these mechanistic studies are in good agreement with the experimentally obtained free energies. In addition the butyl-bridged derivatives were studied by dynamic HPLC. The same trends than for the propyl-bridged cyclophanes were observed.
Part E: A series of mono-thiolated torsion angle restricted biphenyl based cyclophanes were synthesized. These model compounds allowed for modulation of the field-effect mobility and threshold voltage of organic thin film transistors.