Maarten J. Pouderoijen

White-Light Emitting Hydrogen-Bonded Supramolecular Copolymers Based on π-Conjugated Oligomers

Three different pi-conjugated oligomers (a blue-emitting oligofluorene, a green-emitting oligo(phenylene vinylene), and a red-emitting perylene bisimide) have been functionalized with self-complementary quadruple hydrogen bonding ureidopyrimidinone (UPy) units at both ends. The molecules self-assemble in solution and in the bulk, forming supramolecular polymers. When mixed together in solution, random noncovalent copolymers are formed that contain all three types of chromophores, resulting in energy transfer upon excitation of the oligofluorene energy donor. At a certain mixing ratio, a white emissive supramolecular polymer can be created in solution. In contrast to their unfunctionalized counterparts, bis-UPy-chromophores can easily be deposited as smooth thin films on surfaces by spin coating. No phase separation is observed in these films, and energy transfer is much more efficient than in solution, giving rise to white fluorescence at much lower ratios of energy acceptor to donor. Light emitting diodes based on these supramolecular polymers have been prepared from all three types of pure materials, yielding blue, green, and red devices, respectively. At appropriate mixing ratios of these three compounds, white electroluminescence is observed. This approach yields a toolbox of molecules that can be easily used to construct pi-conjugated supramolecular polymers with a variety of compositions, high solution viscosities, and tuneable emission colors.

Visualization of various supramolecular assemblies of oligo(para- phenylenevinylene)-melamine and perylene bisimide

A melamine derivative has been covalently equipped with two oligo(para-phenylenevinylene) (OPV) chromophores. This procedure yields a bifunctional molecule with two hydrogen-bonding arrays available for complementary binding to perylene bisimide derivatives. Depending on the solvent, hydrogen-bonded trimers, tetramers, and dimers on a graphite surface are observed for pure OPV-melamine by using scanning tunneling microscopy (STM). Upon the addition of perylene bisimide, linear tapes of perylene bisimide, 12-membered rosettes that consist of alternating hydrogen-bonded OPV-melamine and perylene bisimide moieties are visualized. These results provide direct evidence for the possible modes of hydrogen bonding within a supramolecular co-assembly in solution. Subsequently, the optical properties of pure OPV-melamine and co-assemblies with a perylene bisimide derivative were characterized in solution. In an apolar solvent, OPV-melamine self-assembles into chiral superstructures. Disassembly into molecularly dissolved species is reversibly controlled by concentration and temperature. Complementary hydrogen bonding to a perylene bisimide derivative in an apolar solvent yields multicomponent, pi-stacked dye assemblies of enhanced stability that are characterized by fluorescence quenching of the constituent chromophores. Titration experiments reveal that a mixture of hydrogen-bonded oligomers is present in solution, rather than a single discrete assembly. The solution experiments are consistent with the STM results, which revealed various supramolecular assemblies. Our system is likely not to be optimally programmed to obtain a discrete co-assembled structure in quantitative yield.

Identification of oligo(p-phenylene vinylene)–naphthalene diimide heterocomplexes by scanning tunneling microscopy and spectroscopy at the liquid–solid interface

Co-assembly of a melamine derivative covalently equipped with two oligo(p-phenylene vinylene) chromophores and a naphthalene diimide dye results in the formation of heterocomplexes at the liquid-solid interface which was shown using bias dependent imaging and scanning tunneling spectroscopy, despite the disordered nature of the assemblies.

Liquid crystalline hydrogen bonded oligo(p-phenylenevinylene)s

The thermotropic behaviour of a series of hydrogen bonded chiral and achiral oligo(p-phenylenevinylenes) (OPVs) has been investigated. The liquid crystalline properties are studied as a function of their molecular design that is based on hydrogen bonding units, the lengths of the π-conjugated and flexible segments and the branching of the side chains. Two-dimensional wide-angle X-ray scattering experiments indicate self-assembly of all OPVs into columnar superstructures which is the result of hydrogen bonds formed by the diaminotriazine or ureidotriazine units and π-stacking interactions accompanied by a local phase separation between the alkoxy side chains and the aromatic rods. The phase behaviour of the compounds bearing the diaminotriazine functionality depends on the length of the OPV segment. OPV trimers are already liquid crystalline at ambient conditions, while longer OPV tetramers are in a well-organized crystalline state at that temperature. In the liquid crystalline phase the intracolumnar order decreases due to molecular motion as confirmed by solid-state NMR studies. Interestingly, the introduction of branched (S)-2-methylbutoxy side chains increases significantly the π-stacking distance while the dimensions of the hexagonal unit cell decrease. The hydrogen bonding ureidotriazine groups decrease the thermal stability and lower the degree of order in comparison with the diaminotriazine derivatives. This low degree of order is probably the result of the ureidotriazine units that form hydrogen bonded dimers while the diaminotriazine arrays form hydrogen bonded hexameric rosettes. In the latter case a more densely packed columnar structure is formed.

Influence of the Solvent and the Enantiomeric Purity on the Transition between Different Supramolecular Polymers

The self-assembly of two enantiomerically pure hexa(oligo(p-phenylene vinylene))-substituted benzenes having 24 stereocenters was studied in pure methylcyclohexane (MCH) and in a mixture of MCH/toluene (4:1). Irrespective of the solvent a cooperative supramolecular polymerization mechanism was determined for these star-shaped molecules by using temperature-dependent CD and UV/Vis spectroscopy. Quite remarkably, a transition from one helical supramolecular state (A) to a second more thermodynamically stable supramolecular helical assembly (B) was observed. The rate of the A→B transition was strongly dependent on the nature of the solvent; being faster in the solvent mixture than in pure MCH. By using size exclusion chromatography we could relate the increased rate to a decreased stability of the supramolecular A state in the solvent mixture. Next, we mixed the two enantiomerically pure hexa-substituted benzene derivatives in a so-called majority-rules experiment, which lead to the anitcipated chiral amplification in the A state. More importantly it appeared that the A→B transition was significantly hampered in these mixed systems. Furthermore, the absence of chiral amplification in the B state revealed the formation of separated enantiomerically pure assemblies. Therefore, by using a wide variety of spectroscopic and chromatographic techniques we determined the influence of solvent and enantiomeric purity on the transition between different supramolecular states.

Synthesis and properties of ?,?-phenyl-capped bithiophene derivatives

Nine different α,ω-phenyl-endcapped bithiophenes were synthesised, and the effect of the different side chains on the liquid crystalline properties, alignment ability and charge carrier mobility have been studied. An increase in chain length leads to a decrease in the liquid crystalline–isotropic phase (clearing) transition temperature. Remarkably, introduction of an asymmetric carbon centre close to the π-conjugated segment resulted in the loss of the liquid crystalline phase. Alignment on rubbed polyimide was obtained for the liquid crystalline thiophene derivatives lacking heteroatoms in the side chain and for the chiral α,ω-phenyl-endcapped bithiophene. Some bithiophenes showed bipolar charge transport in time-of-flight (TOF) measurements, with mobilities up to 3 × 10−3 cm2 V−1 s−1 in the liquid crystalline state. Field effect transistors revealed mobility for holes up to 1 × 10−3 cm2 V−1 s−1 (crystalline state). From the data set obtained, it can be concluded that the use of linear hydrocarbon chains as solubilising tails in these types of π-conjugated building blocks gives the best overall electronic performance.

Synthesis and self-assembly of bay-substituted perylene diimide gemini-type surfactants as off-on fluorescent probes for lipid bilayers

Abstract Interest in bay‐substituted perylene‐3,4:9,10‐tetracarboxylic diimides (PDIs) for solution‐based applications is growing due to their improved solubility and altered optical and electronic properties compared to unsubstituted PDIs. Synthetic routes to 1,12‐bay‐substituted PDIs have been very demanding due to issues with steric hindrance and poor regioselectivity. Here we report a simple one‐step regioselective and high yielding synthesis of a 1,12‐dihydroxylated PDI derivative that can subsequently be alkylated in a straightforward fashion to produce nonplanar 1,12‐dialkoxy PDIs. These PDIs show a large Stokes shift, which is specifically useful for bioimaging applications. A particular cationic PDI gemini‐type surfactant has been developed that forms nonfluorescent self‐assembled particles in water (“off state”), which exerts a high fluorescence upon incorporation into lipophilic bilayers (“on state”). Therefore, this probe is appealing as a highly sensitive fluorescent labelling marker with a low background signal for imaging artificial and cellular membranes.