Oligo(p‐phenylene‐ethynylene)‐Derived Super‐π‐Gelators with Tunable Emission and Self‐Assembled Polymorphic Structures

Linear π‐conjugated oligomers are known to form organogels through noncovalent interactions. Herein, we report the effect of π‐repeat units on the gelation and morphological properties of three different oligo(p‐phenylene‐ethynylene)s: OPE3 , OPE5 , and OPE7 . All of these molecules form fluorescent gels in nonpolar solvents at low critical gel concentrations, thereby resulting in a blue gel for OPE3 , a green gel for OPE5 , and a greenish yellow gel for OPE7 . The molecule–molecule and molecule–substrate interactions in these OPEs are strongly influenced by the conjugation length of the molecules. Silicon wafer suppresses substrate–molecule interactions whereas a mica surface facilitates such interactions. At lower concentrations, OPE3 formed vesicular assemblies and OPE5 gave entangled fibers, whereas OPE7 resulted in spiral assemblies on a mica surface. At higher concentrations, OPE3 and OPE5 resulted in super‐bundles of fibers and flowerlike short‐fiber agglomerates when different conditions were applied. The number of polymorphic structures increases on increasing the conjugation length, as seen in the case of OPE7 with n=5, which resulted in a variety of exotic structures, the formation of which could be controlled by varying the substrate, concentration, and humidity.     

Oligo(p-phenyleneethynylene)-functionalized Perylenebisimidetriad: Synthesis, Photophysical Properties, and Self-assembly

The rod‐like oligo(p‐phenylene ethynylene)‐functionalized perylene bisimide triad was synthesized and characterized. Aggregation behavior in solvents of different polarity was investigated by absorption and fluorescent spectroscopy. The results showed that stronger aggregations took place in low‐polarity slovent. The experiments also indicated that the energy and electron transfer might takeplace between the two chromophores during the photoinduced excitation. Highly ordered two‐dimensional assemblies could be observed at solid/liquid interfaces.     

Carbo‐biphenyls and Carbo‐terphenyls: Oligo(phenylene ethynylene) Ring Carbo‐mers

Ring carbo‐mers of oligo(phenylene ethynylene)s (OPEn, n=0–2), made of C₂‐catenated C₁₈ carbo‐benzene rings, have been synthesized and characterized by NMR and UV‐vis spectroscopy, crystallography and voltammetry. Analyses of crystal and DFT‐optimized structures show that the C₁₈ rings preserve their individual aromatic character according to structural and magnetic criteria (NICS indices). Carbo‐terphenyls (n=2) are reversibly reduced at ca. ?0.42 V/SCE, i.e. 0.41 V more readily than the corresponding carbo‐benzene (?0.83 V/SCE), thus revealing efficient inter‐ring π‐conjugation. An accurate linear fit of E_1/2^red1 vs. the DFT LUMO energy suggests a notably higher value (?0.30 V/SCE) for a carbo‐quaterphenyl congener (n=3). Increase with n of the effective π‐conjugation is also evidenced by a red shift of two of the three main visible light absorption bands, all being assigned to TDDFT‐calculated excited states, one of them restricting to a HOMO→LUMO main one‐electron transition.     

Columnar Self‐Assembly in Electron‐Deficient Heterotriangulenes

A series of soluble carbonyl‐bridged heterotriangulenes, in which flexible n‐dodecyl chains are attached through different spacers to the planar nitrogen‐centered polycyclic core, have been synthesized. The introduction of triisopropylsilylethynyl moieties enabled, for the first time, the characterization of single‐crystal columnar packing of a substituted heterotriangulene by X‐ray crystallography. Electrochemical studies disclosed the carbonyl‐bridged heterotriangulene core as a reasonably strong acceptor for a reversible two‐electron transfer. The tendency of substituted heterotriangulenes to self‐assemble in solution, on surfaces, and in the bulk appeared to sensitively depend on the nature of the lateral substituents, their steric demand, and the applied solution processing conditions. It can be concluded that 1) additional phenylene moieties between the heterotriangulene core and the n‐dodecyl chains facilitate self‐assembly by extending the π‐conjugated polycyclic disc, 2) the rod‐like ethynylene spacers introduce some additional flexibility and hence lower the overall aggregation tendency, and 3) the combination of both features in the phenylene–ethynylene moieties induces thermotropic liquid crystallinity.     

Synthesis of Symmetrical Multichromophoric Bodipy Dyes and Their Facile Transformation into Energy Transfer Cassettes

Multichromophoric boron‐dipyrromethene (Bodipy) dyes synthesized on phenylene‐ethynylene platforms have been be converted to energy transfer cassettes in a one‐step chemical transformation. Excitation energy transfer processes in these highly symmetrical derivatives were studied in detail, including time‐resolved fluorescence spectroscopy techniques. Excitation spectra and the emission lifetimes suggest efficient energy transfer between the donor and acceptor chromophore. These novel energy transfer cassettes, while highlighting a short‐cut approach to similar energy transfer systems, could be useful as large pseudo‐Stokes shift multichromophoric dyes with potential applications in diverse applications.     

Cooperative and FRET‐Assisted Brightness Enhancement in Oligo(phenylene ethynylene): Quantum Dot Organic–Inorganic Nanohybrids

Herein, we combine the ideas of concerted emission from fluorophore ensembles and its further amplification through FRET in an organic–inorganic hybrid approach. Spherical and highly fluorescent organic nanoparticles (FONPs, Φ_f=0.38), prepared by the self‐assembly of oligo(phenylene ethynylene) (OPE) molecules, were selected as a potential donor material. This organic core was then decorated with a shell of fluorescent CdSe/ZnS core–shell quantum dots (QDs; <d>?5.5 nm, Φ_f=0.27) with the aid of a bifunctional ligand, mercaptopropionic acid. Its high extinction coefficient (?≈4.1×10⁵ m ^?1 cm^?1) and good spectral match with the emission of the FONPs (J(λ)≈4.08×10¹⁶ m ^?1 cm^?1 nm⁴) made them a better acceptor candidate to constitute an efficient FRET pair (Φ_FRET=0.8). As a result, the QD fluorescence intensity was enhanced by more than twofold. The fundamental calculations carried out indicated an improvement in all the FRET parameters as the number of QDs around the FONPs was increased. This, together with the localization of multiple QDs in a nanometric dimension (volume≈1.8×10⁶ nm³), gave highly bright reddish luminescent hybrid particles as visualized under a fluorescence microscope.     

Synthesis,structure, and optical properties of an alternating calix[4]arene‐based meta‐linked phenylene ethynylene copolymer

Novel alternating copolymers comprising bis‐ calix[4]arene‐p‐phenylene ethynylene and m‐phenylene ethynylene units ( CALIX‐m‐PPE ) were synthesized using the Sonogashira‐Hagihara cross‐coupling polymerization. Good isolated yields (60–80%) were achieved for the polymers that show M_n ranging from 1.4 × 10⁴ to 5.1 × 10⁴ gmol^?1 (gel permeation chromatography analysis), depending on specific polymerization conditions. The structural analysis of CALIX‐m‐PPE was performed by ¹H, ¹³C, ¹³C–¹H heteronuclear single quantum correlation (HSQC), ¹³C–¹H heteronuclear multiple bond correlation (HMBC), correlation spectroscopy (COSY), and nuclear overhauser effect spectroscopy (NOESY) in addition to Fourier transform‐Infrared spectroscopy and microanalysis allowing its full characterization. Depending on the reaction setup, variable amounts (16–45%) of diyne units were found in polymers although their photophysical properties are essentially the same. It is demonstrated that CALIX‐m‐PPE does not form ground‐ or excited‐state interchain interactions owing to the highly crowded environment of the main‐chain imparted by both calix[4]arene side units which behave as insulators inhibiting main‐chain π–π staking. It was also found that the luminescent properties of CALIX‐m‐PPE are markedly different from those of an all‐p‐linked phenylene ethynylene copolymer ( CALIX‐p‐PPE ) previously reported. The unexpected appearance of a low‐energy emission band at 426 nm, in addition to the locally excited‐state emission (365 nm), together with a quite low fluorescence quantum yield (? = 0.02) and a double‐exponential decay dynamics led to the formulation of an intramolecular exciplex as the new emissive species. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Helical Folding Competing with Unfolded Aggregation in Phenylene Ethynylene Foldamers

The folding and aggregation behavior of a pair of oligo(phenylene ethynylene) (OPE) foldamers are investigated by means of UV/Vis absorption and circular dichroism spectroscopy. With identical OPE backbones, two foldamers, 1 with alkyl side groups and 2 with triethylene glycol side chains, manifest similar helical conformations in solutions in n‐hexane and methanol, respectively. However, disparate and competing folding and aggregation processes are observed in alternative solvents. In cyclohexane, oligomer 1 initially adopts the helical conformation, but the self‐aggregation of unfolded chains, as a minor component, gradually drives the folding–unfolding transition eventually to the unfolded aggregate state completely. In contrast, in aqueous solution (CH₃OH/H₂O) both folded and unfolded oligomer 2 appear to undergo self‐association; aggregates of the folded chains are thermodynamically more stable. In solutions with a high H₂O content, self‐aggregation among unfolded oligomers is kinetically favored; these oligomers very slowly transform into aggregates of helical structures with greater thermodynamic stability. The folded–unfolded conformational switch thus takes place with the free (nonaggregated) molecules, and the very slow folding transition is due to the low concentration of molecularly dispersed oligomers.     

Photophysical and Laser Properties of Cassettes based on a BODIPY and Rhodamine Pair

This work deals with the synthesis and the photophysical and laser properties of new BODIPY‐rhodamine cassettes. These dyads differ in their rigid and conjugated spacer group (phenyl or acetylenephenyl) and in their linking positions (meta or para). The photophysical properties of these cassettes are controlled by the formation/opening of the spirolactone ring, which, in turn, switches off/on an energy‐transfer process between the chromophores. Herein, we thoroughly describe the influence of the attached spacer group, as well as the distance and orientation between the donor–acceptor pair, on the excitation energy transfer. The observed fast dynamics and efficiency suggest that the process mainly takes place “through‐bond”, although the “through‐space” mechanism also contributes to the whole process. As a result, efficient laser emission from the rhodamine is achieved upon excitation of the BODIPY, in particular for the cassette that contains an acetylenephenyl spacer group in a para disposition.     

Electrical conductance of conjugated oligomers at the single molecule level

We determine and compare, at the single molecule level and under identical environmental conditions, the electrical conductance of four conjugated phenylene oligomers comprising terminal sulfur anchor groups with simple structural and conjugation variations. The comparison shows that the conductance of oligo(phenylene vinylene) (OPV) is slightly higher than that of oligo(phenylene ethynylene) (OPE). We find that solubilizing side groups do neither prevent the molecules from being anchored within a break junction nor noticeably influence the conductance value.     

Molecular junctions based on SAMs of cruciform oligo(phenylene ethynylene)s

Cruciform oligo(phenylene ethynylene)s (OPEs) with an extended tetrathiafulvalene (TTF) donor moiety (OPE5-TTF and OPE3-TTF) and their simple analogues (OPE5-S and OPE3) without conjugated substituents were used to form high-quality self-assembled monolayers (SAMs) on ultraflat gold substrates. Molecular junctions based on these SAMs were investigated using conducting-probe atomic force microscopy (CP-AFM). The TTF substituent changes the molecular orbital energy levels and decreases the HOMO-LUMO energy gap, resulting in a 9-fold increase in conductance for both TTF cruciform OPEs compared to the unsubstituted analogues. The difference in electrical transport properties of the SAMs was reproduced by the theoretical transport calculations for the single molecules.     

Hydrocarbon links in an octet truss

We use the octet truss of R. Buckminster Fuller to develop a geometric placement method for synthesizing braid representations of knots and links of oligo (phenylene ethynylene)s using the 60° ortho, 120° meta or 180° para phenyl ring substitution angles and respecting the van der Waals repulsion constraints. We show that any knot or link can be realized by a phenylene ethynylene oligomer modeled on the octet truss. Use of this lattice is motivated by the structural constraints of these phenylene ethynylene units. Where in bio-organic chemistry, questions often involve identifying existing knots, for example in DNA strands, organic synthesis is concerned with assembling molecular structures that can be verified to exist in a desired knot topology. This physical realization of a knot as a construction of common organic molecular subunits then facilitates further study of the properties of knotted molecules in general.     

Synthesis of Polyisoprene Terpenoid Dendrons and their Applications in Oligo(phenylene ethynylene)s as “Shells”

The novel double‐stage convergent synthesis of a new class of polyisoprene terpenoid (PIPTP) dendrons is described. PIPTP dendrons bear a highly branched aliphatic hydrocarbon skeleton and a hydrophilic hydroxy focal point functionality. These dendrons have the specific formula C_(5×2^G+1_‐5)H_(5×2^G+2_‐8)O, and each dendritic layer is constructed from an isoprene unit. The key branching steps involve a double alkyl‐metal addition to an ester functionality, followed by deoxygenation of the resulting tertiary alcohol by triethylsilane and trifluoroacetic acid, then hydrogenation or hydrogenolysis. The dendrons were also attached to oligo(phenylene ethynylene)s (OPEs) so as to function as protective shells to allow fine tuning of the nanoscopic environment around the OPE moiety, and to exert precise control of the packing density and intermolecular interaction between the OPE cores. Fluorescence quantum yield data reveal that the OPE core is better encapsulated by the PIPTP dendrons than by Fréchet dendrons.     

Synthesis,Structure, and Optical Studies of Donor–Acceptor‐Type Near‐Infrared (NIR) Aza–Boron‐Dipyrromethene (BODIPY) Dyes

Six donor–acceptor‐type near‐infrared (NIR) aza–boron‐dipyrromethene (BODIPY) dyes and their corresponding aza–dipyrrins were designed and synthesized. The donor moieties at the 1,7‐positions of the aza–BODIPY core were varied from naphthyl to N‐phenylcarbazole to N‐butylcarbazole. The 3,5‐positions were also substituted with phenyl or thienyl groups in the aza–BODIPYs. Photophysical, electrochemical, and computational studies were carried out. The absorption and emission spectra of aza–BODIPYs were significantly redshifted (≈100 nm) relative to the parent tetraphenylaza–BODIPY. Fluorescence studies suggested effective energy transfer (up to 93 %) from donor groups to the aza–BODIPY core in all of the compounds under study. Time‐dependent (TD)‐DFT studies indicated effective electronic interactions between energy donor groups and aza–dipyrrin unit in all the aza–BODIPYs studied. The HOMO–LUMO gap (ΔE) calculated from cyclic voltammetry data was found to be lower for six aza–BODIPYs relative to their corresponding aza–dipyrrins.     

Synthesis,Electrochemical, and Optical Properties of a Novel PPV/PPE Block‐Copolymer

Summary: A novel poly(p‐phenylene vinylene) (PPV)/poly(p‐phenylene ethynylene) (PPE) block‐copolymer was synthesized by a cross‐coupling polycondensation with Pd(PPh₃)₂Cl₂ and a phase‐transfer catalyst, and was confirmed by ¹H NMR and IR spectroscopy and elemental analysis. The thermal, electrochemical, and photoluminescent properties of the new copolymer have been investigated. The incorporation of triple bonds into the cyano‐substituted PPV (CN‐PPV) backbone leads to higher oxidation and reduction potentials than poly(2‐methoxy‐5‐(2‐ethylhexyloxy)‐p‐phenylene vinylene) (MEH‐PPV) and CN‐PPV, potentially making the copolymer a good electron‐transporting material for use in a light‐emitting‐diode device.

The cyclic voltammogram of the novel poly(p‐phenylene vinylene) (PPV)/poly(p‐phenylene ethynylene) (PPE) block‐copolymer synthesized here.     

Polymers with alternating anthracene and phenylene building blocks linked by ethynylene and/or vinylene units: Studying structure‐properties‐relationships

Four conjugated polymers ( P1 – P4 ) consisting of alternating anthracene‐9,10‐diyl and 1,4‐phenylene building blocks connected via ethynylene as well as vinylene ( P1 and P2 ), ethynylene‐only ( P3 ), and vinylene‐only ( P4 ) moieties, respectively, were synthesized and studied. The phenylene units in all four polymers bear 2‐ethylhexyloxy side‐chains to promote good solubility. The three polymers with vinylene units ( P1 , P2 , and P4 ) were prepared using the Horner–Wadsworth–Emmons reaction. For the synthesis of the arylene‐ethynylene polymer P3, the palladium‐catalyzed Sonogashira cross‐coupling reaction was used. The polymers were characterized by NMR, Fourier transform infrared spectroscopy, and Raman spectroscopy. Photophysical, absorption and photoluminescence, and electrochemical properties were studied. Spectroscopic ellipsometry measurements were performed to gain more insight on the optical properties. In addition, the transport properties were investigated using admittance spectroscopy. The bulk hole mobility and its dependence on the electric field were evaluated for P1 and P2 . © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 129–143     

Supramolecular Polymerization of C3‐Symmetric Organogelators: Cooperativity,Solvent, and Gelation Relationship

A systematic study of the influence of solvent and the size of C₃‐symmetric discotics on their supramolecular polymerization mechanism is presented. The cooperativity of the self‐assembly of the reported compounds is directly related to their gelation ability. The two series of C₃‐symmetric discotics investigated herein are based on benzene‐1,3,5‐tricarboxamides (BTAs) and oligo(phenylene ethynylene)‐based tricarboxamides (OPE? TAs) that are peripherally decorated with achiral ( 1 a and 2 a ) or chiral N‐(2‐aminoethyl)‐3,4,5‐trialkoxybenzamide units ( 1 b and 2 b ). The supramolecular polymerization of compounds 1 a , b and 2 a , b has been exhaustively investigated in a number of solvents and by using various techniques: variable‐temperature circular dichroism (VT‐CD) spectroscopy, concentration‐dependent ¹H NMR spectroscopy, and isothermal titration calorimetry (ITC). The supramolecular polymerization mechanism of compounds 2 is highly cooperative in solvents such as methylcyclohexane and toluene and is isodesmic in CHCl₃. Unexpectedly, chiral compound 1 b is practically CD‐silent, in contrast with previously reported BTAs. ITC measurements in CHCl₃ demonstrated that the supramolecular polymerization of BTA 1 a is isodesmic. These results confirm the strong influence of the π‐surface of the central aromatic core of the studied discotic and the branched nature of the peripheral side chains on the supramolecular polymerization. The gelation ability of these organogelators is negated in CHCl₃, in which the supramolecular polymerization mechanism is isodesmic.     

Influence of Metal,Ligand and Solvent on Supramolecular Polymerizations with Transition‐Metal Compounds: A Theoretical Study

The nature of intermolecular interactions governing supramolecular polymerizations is very important for controlling their cooperativity. In order to address this problem, supramolecular columns made of Pt^II and Pd^II complexes of oligo(phenylene ethynylene)‐based pyridine (OPE) and tetrazolylpyridine ligands (TEP) were investigated through the dispersion‐corrected PM6 method. Aromatic, CH–π, M–Cl and metallophilic interactions helped stabilize the supramolecules studied, and their geometries and associated cooperativities were in excellent agreement with experimental data. The OPE ligand and/or the presence of Pt^II led to stronger metallophilic interactions and also to cooperative supramolecular polymerizations, which clearly suggests that metallophilic interactions are a key factor for controlling cooperativity. The results indicate that sequential monomer addition is in general less spontaneous than the combination of two larger preformed stacks. The present theoretical investigations contribute to the further understanding of the relation between the thermodynamics of supramolecular polymerizations and the nature of different synthons.     

Charge carrier mobility,photovoltaic, and electroluminescent properties of anthracene‐based conjugated polymers bearing randomly distributed side chains

This article reports on the synthesis, characterization, and properties of various anthracene‐containing poly (p‐phenylene‐ethynylene)‐alt‐poly(p‐phenylene‐vinylene) (PPE‐PPV) polymers (AnE‐PVs) bearing statistical distributions of various side chains. Primarily, the ratio of linear octyloxy and branched 2‐ethylhexyloxy side chains at the poly(p‐phenylene vinylene) (PPV) parts was varied, leading to the polymers stat, stat1, and stat2. Furthermore, polymers also containing asymmetric substituted PPV and poly(p‐phenylene ethynylene) units (bearing methoxy and 2‐ethylhexyloxy side chains) were prepared yielding stat3, stat4, and stat5. These materials exhibit a broad variation in their photovoltaic properties. It is once more shown that side chains and their distribution can crucially affect the photovoltaic device performance. The introduction of units with asymmetric substitution into these systems seems to be harmful for their utilization in photovoltaic applications. Organic field‐effect transistors were fabricated to investigate hole mobilities in these new materials. Large variance was observed, falling in the range of almost two orders of magnitude, indicating rather different π–π stacking behavior of the polymer backbones owing to side‐chain modifications. Moreover, a selection of the new polymeric systems was investigated regarding their potential for light‐emitting diode (LED) applications. Polymer LEDs using the polymers AnE‐PVstat, ‐stat3, ‐stat4, and ‐stat5, as the active layer showed turn‐on voltage of ～2 V and exhibited red light emission. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

A Bis‐Manganese(II)–DOTA Complex for Pulsed Dipolar Spectroscopy

High‐spin gadolinium(III) and manganese(II) complexes have emerged as alternatives to standard nitroxide radical spin labels for measuring nanometric distances by using pulsed electron–electron double resonance (PELDOR or DEER) at high fields/frequencies. For certain complexes, particularly those with relatively small zero‐field splitting (ZFS) and short distances between the two metal centers, the pseudosecular term of the dipolar coupling Hamiltonian is non‐negligible. However, in general, the contribution from this term during conventional data analysis is masked by the flexibility of the molecule of interest and/or the long tethers connecting them to the spin labels. The efficient synthesis of a model system consisting of two [Mn(dota)]^2? (MnDOTA; DOTA^4?=1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetate) directly connected to the ends of a central rodlike oligo(phenylene–ethynylene) (OPE) spacer is reported. The rigidity of the OPE is confirmed by Q‐band PELDOR measurements on a bis‐nitroxide analogue. The Mn^II?Mn^II distance distribution profile determined by W‐band PELDOR is in reasonable agreement with one simulated by using a simple rotamer analysis. The small degree of flexibility arising from the linking MnDOTA arm appears to outweigh the contribution from the pseudosecular term at this interspin distance. This study illustrates the potential of MnDOTA‐based spin labels for measuring fairly short nanometer distances, and also presents an interesting candidate for in‐depth studies of pulsed dipolar spectroscopy methods on Mn^II?Mn^II systems.