Long-Lived Charge Carrier Photogeneration in a Cooperative Supramolecular Double-Cable Polymer.

A newly designed C3-symmetric disc-shaped chromophore, BTT(NDI)3, features electron accepting naphthalene diimides linked to an electron donor BTT core. BTT(NDI)3 self-assembles in apolar solvents into highly ordered, chiral supramolecular fibers through π-π and 3-fold hydrogen-bonding interactions. This leads to a cooperative formation of plane-to-plane stacking of BTTs and J-aggregation of the outer NDIs. Such a structure ensures high charge mobility. Only photoexcitation of BTT in the BTT(NDI)3 polymers triggers a unidirectional electron transfer from BTT to NDI and results in (BTT•+-NDI•-) lifetimes that are by up to 3 orders of magnitude longer compared to (NDI•+-NDI•-) that is formed upon NDI photoexcitation. A multiphasic decay implies ambipolar pathways for charge carriers, that is, electron and hole delocalization along the respective BTT and NDI stacks. Our supramolecular approach offers potential for developing functional supramolecular polymers with continuous pathways for electrons and holes and, in turn, minimizing charge recombination losses in organic photovoltaic devices.

Ferro- and ferrielectricity and negative piezoelectricity in thioamide-based supramolecular organic discotics.

Amide-based discotic supramolecular organic materials are of interest for fundamental understanding of cooperative self-assembly and collective dipole switching mechanisms as well as for practically relevant ferroelectric and piezoelectric properties. Here, we show how replacing amides (dipole moment of ∼3.5 D) with thioamides (∼5.1 D) as dipolar moieties in the archetypal C3-symmetric discotic molecule BTA leads to ferroelectric materials with a higher remnant polarization and lower coercive field. The thioamide-based materials also demonstrate a rare negative piezoelectricity and a previously predicted, yet never experimentally observed, polarization reversal via asymmetric intermediate states, that is, ferrielectric switching.

Unveiling Polymerization Mechanism in pH-regulated Supramolecular Fibers in Aqueous Media.

An amine functionalized C3 -symmetric benzotrithiophene (BTT) monomer has been designed and synthetized in order to form pH responsive one-dimensional supramolecular polymers in aqueous media. While most of the reported studies looked at the effect of pH on the size of the aggregates, herein, a detailed mechanistic study is reported, carried out upon modifying the pH to trigger the formation of positively charged ammonium groups. A dramatic and reversible change in the polymerization mechanism and size of the supramolecular fibers is observed and ascribed to the combination of Coulombic repulsive forces and higher monomer solubility. Furthermore, the induced frustrated growth of the fibers is further employed to finely control the one-dimensional supramolecular polymerisation and copolymerization processes.

Supramolecular Copolymers: Structure and Composition Revealed by Theoretical Modeling.

Supramolecular copolymers, non-covalent analogues of synthetic copolymers, constitute a new and promising class of polymers. In contrast to their covalent counterparts, the details of their mechanism of formation, as well as the factors determining their composition and length, are still poorly understood. Here, the supramolecular copolymerization between two slightly structurally different benzene-1,3,5-tricarboxamide (BTA) monomers functionalized with either oligodimethylsiloxane (oDMSi) or alkyl side chains is unraveled by combining experimental and theoretical approaches. By applying the "sergeant-and-soldiers" approach using circular dichroism (CD) experiments, we are able to obtain detailed insights into the structure and composition of these supramolecular copolymers. Moreover, we observe an unexpected chiral induction upon mixing two independently CD-silent solutions of the achiral (soldier) and chiral (sergeant) monomers. We find that the subtle differences in the chemical structure of the two monomers impact their homopolymerization mechanism: whereas alkyl-BTAs cooperatively self-assemble, oDMSi-BTAs self-assemble in an isodesmic manner. The effect of these mechanistic differences in the supramolecular copolymerization process is investigated as a function of the composition of the two monomers and explicitly rationalized by mathematical modeling. The results show that, at low fractions of oDMSi-BTA sergeants (<10 mol%), the polymerization process is cooperative and the supramolecular helicity is biased toward the helical preference of the sergeant. However, at higher fractions of oDMSi-BTA sergeant (>25 mol%), the isodesmic assembly of the increasing amounts of sergeant becomes more dominant, and different species start to coexist in the copolymerization process. The analysis of the experimental data with a newly developed theoretical model allows us to quantify the thermodynamic parameters, the distribution of different species, and the compositions and stack lengths of the formed supramolecular copolymers existing at various feed ratios of the two monomers.

The Role of the Axial Substituent in Subphthalocyanine Acceptors for Bulk-Heterojunction Solar Cells.

Four hexachlorosubphthalocyanines SubPcCl6 -X bearing different axial substituents (X) have been synthesized for use as novel electron acceptors in solution-processed bulk-heterojunction organic solar cells. Subphthalocyanines are aromatic chromophoric molecules with cone-shaped structure, good solution processability, intense optical absorption in the visible spectral region, appropriate electron mobilities, and tunable energy levels. Solar cells with subphthalocyanines as the electron acceptor and PTB7-Th as the electron donor exhibit a power conversion efficiency up to 4 % and an external quantum efficiency approaching 60 % due to significant contributions from both the electron donor and the electron acceptor to the photocurrent, indicating a promising prospect of non-fullerene acceptors based on subphthalocyanines and structurally related systems.

A Versatile Method for the Preparation of Ferroelectric Supramolecular Materials via Radical End-Functionalization of Vinylidene Fluoride Oligomers.

A synthetic method for the end-functionalization of vinylidene fluoride oligomers (OVDF) via a radical reaction between terminal olefins and I-OVDF is described. The method shows a wide substrate scope and excellent conversions, and permits the preparation of different disc-shaped cores such as benzene-1,3,5-tricarboxamides (BTAs), perylenes bisimide (PBI), and phthalocyanines (Pc) bearing three to eight ferroelectric oligomers at their periphery. The formation, purity, OVDF conformation, and morphology of the final adducts has been assessed by a combination of techniques, such as NMR, size exclusion chromatography, differential scanning calorimetry, polarized optical microscopy, and atomic force microscopy. Finally, PBI-OVDF and Pc-OVDF materials show ferroelectric hysteresis behavior together with high remnant polarizations, with values as high as Pr ≈ 37 mC/m(2) for Pc-OVDF. This work demonstrates the potential of preparing a new set of ferroelectric materials simply by attaching OVDF oligomers to different small molecules. The use of carefully chosen small molecules paves the way to new functional materials in which ferroelectricity and electrical conductivity or light-harvesting properties coexist in a single compound.

Making the invisible visible: improved electrospray ion formation of metalloporphyrins/-phthalocyanines by attachment of the formate anion (HCOO(-)).

A protocol is developed for the coordination of the formate anion (HCOO(-)) to neutral metalloporphyrins (Pors) and -phthalocyanines (Pcs) containing divalent metals as a means to improve their ion formation in electrospray ionization (ESI). This method is particularly useful when the oxidation of the neutral metallomacrocycle fails. While focusing on Zn(II)Pors and Zn(II)Pcs, we show that formate is also readily attached to Mn(II), Mg(II) and Co(II)Pcs. However, for the Co(II)Pc secondary reactions can be observed. Upon collision-induced dissociation (CID), Zn(II)Por/Pc·formate supramolecular complexes can undergo the loss of CO2 in combination with transfer of a hydride anion (H(-)) to the zinc metal center. Further dissociation leads to electron transfer and hydrogen atom loss, generating a route to the radical anion of the Zn(II)Por/Pc without the need for electrochemical reduction, although the Zn(II)Por/Pc may have a too low electron affinity to allow electron transfer directly from the formate anion. In addition to single Por molecules, multi Por arrays were successfully analyzed by this method. In this case, multiple addition of formate occurs, giving rise to multiply charged species. In these multi Por arrays, complexation of the formate anion occurs by two surrounding Por units (sandwich). Therefore, the maximum attainment of formate anions in these arrays corresponds to the number of such sandwich complexes rather than the number of porphyrin moieties. The same bonding motif leads to dimers of the composition [(Zn(II)Por/Pc)2·HCOO](-). In these, the formate anion can act as a structural probe, allowing the distinction of isomeric ions with the formate bridging two macrocycles or being attached to a dimer of directly connected macrocycles.

Nanopatterned superlattices in self-assembled C2 -symmetric oligodimethylsiloxane-based benzene-1,3,5-tricarboxamides.

The synthesis of C3 - and C2 -symmetric benzene-1,3,5-tricarboxamides (BTAs) containing well-defined oligodimethylsiloxane (oDMS) and/or alkyl side chains has been carried out. The influence of the bulkiness of the oDMS chains in the aggregation behavior of dilute solutions of the oDMS-BTAs in methylcyclohexane was studied by temperature-dependent UV spectroscopy. The formation of hierarchically self-assembled aggregates was observed at different BTA concentrations, the tendency of aggregation increases by shortening or removing oDMS chains. Chiral BTAs were investigated with circular dichroism (CD) spectroscopy, showing a stronger tendency to aggregate than the achiral ones. Majority rules experiments show a linear behavior consistent with the existence of a high mismatch penalty energy. The most efficient oDMS-BTAs organogelators have the ability to form stable organogels at 5 mg mL(-1) (0.75 wt %) in hexane. Solid-state characterization techniques indicate the formation of an intermolecular threefold hydrogen bonding between adjacent molecules forming thermotropic liquid crystals, exhibiting a hexagonal columnar organization from room temperature to above 150 °C. A decrease of the clearing temperatures was observed when increasing the number and length of the oligodimethylsiloxane chains. In addition to the three-fold hydrogen bonding that leads to columnar liquid crystalline phase, segregation between the oDMS and aliphatic chains takes place in the BTA functionalized with two alkyl and one oDMS chain leading to a superlattice within the hexagonal structure with potential applications in lithography.

Supramolecular assembly of multicomponent photoactive systems via cooperatively coupled equilibria.

Here, we show that the synergistic interplay between two binding equilibria, acting at different sites of a (Zn)phthalocyanine-amidine molecule (Pc1), enables the dissociation of the photoinactive phthalocyanine dimer (Pc1)2 into a three-component system, in which a sequence of light harvesting, charge separation, and charge shift is successfully proven. The aforementioned dimer is assembled by dual amidine-Zn(II) coordination between neighboring Pc1 molecules and gives rise to high association constants (KD ≈ 10(11) M(-1)). Such extraordinary stability hampers the individual binding of either carboxylic acid ligands through the amidine group or pyridine-type ligands through the Zn(II) metal atom to (Pc1)2. However, the combined addition of both ligands, which cooperatively bind to different sites of Pc1 through distinct noncovalent interactions, efficiently shifts the overall equilibrium toward a photoactive tricomponent species. In particular, when a fullerene-carboxylic acid (C60A) and either a dimethylamino-pyridine (DMAP) or a phenothiazine-pyridine ligand (PTZP) are simultaneously present, the photoactivity is turned on and evidence is given for an electron transfer from photoexcited Pc1 to the electron-accepting C60A that affords the DMAP-Pc1(•+)-C60A(•-) or PTZP-Pc1(•+)-C60A(•-) radical ion pair states. Only in the latter case does a cascade of photoinduced electron transfer processes afford the PTZP(•+)-Pc1-C60A(•-) radical ion pair state. The latter is formed via a thermodynamically driven charge shift evolving from PTZP-Pc1(•+)-C60A(•-) and exhibits lifetimes that are notably longer than those of DMAP-Pc1(•+)-C60A(•-).

Tuning the stability of graphene layers by phthalocyanine-based oPPV oligomers towards photo- and redoxactive materials.

In contrast to pristine zinc phthalocyanine (1), zinc phthalocyanine based oPPV-oligomers (2-4) of different chain lengths interact tightly and reversibly with graphite, affording stable and finely dispersed suspensions of mono- to few-layer graphene-nanographene (NG)-that are photoactive. The p-type character of the oPPV backbones and the increasing length of the oPPV backbones facilitate the overall π-π interactions with the graphene layers. In NG/2, NG/3, and NG/4 hybrids, strong electronic coupling between the individual components gives rise to charge transfer from the photoexcited zinc phthalocyanines to NG to form hundreds of picoseconds lived charge transfer states. The resulting features, namely photo- and redoxactivity, serve as incentives to construct and to test novel solar cells. Solar cells made out of NG/4 feature stable and repeatable photocurrent generation during several 'on-off' cycles of illumination with monochromatic IPCE values of around 1%.

A self-assembled subphthalocyanine-based nanophotosensitiser for photodynamic therapy.

A subphthalocyanine substituted with nine tetra(ethylene glycol) chains on the periphery has been synthesised. This novel amphiphilic and cone-shaped compound can self-assemble in water into spherical nanoparticles with a hydrodynamic diameter of 154 nm. These nanoparticles can be taken up readily by cancer cells and localised predominately in lysosomes where they disassemble gradually, leading to activation in fluorescence emission and, photocytotoxicity, showing IC50 values of as low as 1.2 µM.

Porphyrin-based supramolecular nanofibres as a dynamic and activatable photosensitiser for photodynamic therapy.

Photodynamic therapy (PDT) represents a promising treatment modality for a range of cancers and other non-malignant diseases due to its non-invasive nature arising from the light-dependent activation. However, PDT has not been the first-line treatment of cancer thus far as a consequence of, among others, the lack of effective transport and activation strategies, and the undesired side effect caused by skin photosensitisation induced by the "always on" photosensitisers. To overcome this "Achilles' heel", we present herein a non-covalent approach to construct a one-component dynamic supramolecular nanophotosensitising system based on a carefully designed porphyrin. The control of the photoactivities of the resulting supramolecular fibres lies in the spatiotemporal control of the monomer-polymer equilibrium. Both the thermodynamics and kinetics of this nanosystem have been carefully studied by different techniques. Moreover, in vitro and in vivo studies have also been performed, showing that these supramolecular aggregates exhibit facile cell internalisation and progressive disassembly after being endocyted by targeted cells, leading to activation of the photosensitising units and eventually cell death and tumour eradication under photoirradiation.

Elucidating dynamic behavior of synthetic supramolecular polymers in water by hydrogen/deuterium exchange mass spectrometry.

A comprehensive understanding of the structure, self-assembly mechanism, and dynamics of one-dimensional supramolecular polymers in water is essential for their application as biomaterials. Although a plethora of techniques are available to study the first two properties, there is a paucity in possibilities to study dynamic exchange of monomers between supramolecular polymers in solution. We recently introduced hydrogen/deuterium exchange mass spectrometry (HDX-MS) to characterize the dynamic nature of synthetic supramolecular polymers with only a minimal perturbation of the chemical structure. To further expand the application of this powerful technique some essential experimental aspects have been reaffirmed and the technique has been applied to a diverse library of assemblies. HDX-MS is widely applicable if there are exchangeable hydrogen atoms protected from direct contact with the solvent and if the monomer concentration is sufficiently high to ensure the presence of supramolecular polymers during dilution. In addition, we demonstrate that the kinetic behavior as probed by HDX-MS is influenced by the internal order within the supramolecular polymers and by the self-assembly mechanism.

Structure-function relationships in unsymmetrical zinc phthalocyanines for dye-sensitized solar cells.

A series of unsymmetrical zinc phthalocyanines bearing an anchoring carboxylic function linked to the phthalocyanine ring through different spacers were designed for dye-sensitised solar cells (DSSC). The modification of the spacer group allows not only a variable distance between the dye and the nanocrystalline TiO(2), but also a distinct orientation of the phthalocyanine on the semiconductor surface. The photovoltaic data show that the nature of the spacer group plays a significant role in the electron injection from the photo-excited dye into the nanocrystalline TiO(2) semiconductor, the recombination rates and the efficiency of the cells. The incident monochromatic photon-to-current conversion efficiency (IPCE) for phthalocyanines bearing an insulating spacer is as low as 9%, whereas for those with a conducting spacer an outstanding IPCE 80% was obtained.

Self-Assembly of Diacetylene-Bridged Phenylenevinylene Oligomers in Water and Organic Solvents.

Rodlike π-conjugated molecules in which two OPV fragments are connected through a diacetylene bond self-assemble in aqueous and organic media. Optical spectroscopy and AFM measurements indicated that, in water, strong hydrophobic interactions between π-cores promote aggregation into robust, uniform micellar structures. In contrast, in apolar solvents, a fibrilar morphology is obtained by coiling of columnar stacks. These stacks are formed in a nucleation-elongation process with degrees of cooperativity of 0.006, that is influenced by the low rotation barriers around the σ-bonds in the diacetylene linker.

Resistive switching in an organic supramolecular semiconducting ferroelectric.

The combination of switchable dipolar side groups and the semiconducting core of the newly synthetized C3-symmetric benzotrithiophene molecule (BTTTA) leads to an ordered columnar material showing continuous tunability from injection- to bulk-limited conductivity modulation.

Selenoamides modulate dipole-dipole interactions in hydrogen bonded supramolecular polymers of 1,3,5-substituted benzenes.

We report the synthesis and self-assembly behavior of a chiral C3-symmetrical benzene-tricarboselenoamide. The introduction of the selenoamide moiety enhances the dipolar character of the supramolecular interaction and confers a remarkable thermal stability to the supramolecular polymers obtained.

From isodesmic to highly cooperative: reverting the supramolecular polymerization mechanism in water by fine monomer design.

Two structurally-similar discotic molecules able to self-assemble in water, forming supramolecular fibers, are reported. While both self-assembled polymers are indistinguishable from a morphological point-of-view, a dramatic change in their polymerization mechanism is observed (i.e., one self-assemble via an isodesmic mechanism, while the other shows one of the highest cooperativity values).

Photoinduced Energy Transfer in ZnCdSeS Quantum Dot-Phthalocyanines Hybrids.

In this article, interaction between ZnCdSeS quantum dot (QD) and phthalocyanines with variable linker has been reported. Steady-state and time-resolved spectroscopic investigation reveals that only photoinduced energy transfer occurs from QD to phthalocyanines. To evaluate quantitatively the energy transfer, the Poisson statistics of QD-dye complex formation was used in the analysis of steady-state and time-resolved emission quenching, which allows to estimate the energy transfer rate constant for an ideal one-to-one complex. The measured rate constants are compared to the rates evaluated based on the classic Förster theory, which shows roughly 1 nm discrepancy in the energy transfer distance estimation, or one order in magnitude discrepancy in the transfer rate constants.

Ferroelectric self-assembled molecular materials showing both rectifying and switchable conductivity.

Advanced molecular materials that combine two or more physical properties are typically constructed by combining different molecules, each being responsible for one of the properties required. Ideally, single molecules could take care of this combined functionality, provided they are self-assembled correctly and endowed with different functional subunits whose strong electronic coupling may lead to the emergence of unprecedented and exciting properties. We present a class of disc-like semiconducting organic molecules that are functionalized with strong dipolar side groups. Supramolecular organization of these materials provides long-range polar order that supports collective ferroelectric behavior of the side groups as well as charge transport through the stacked semiconducting cores. The ferroelectric polarization in these supramolecular polymers is found to couple to the charge transport and leads to a bulk conductivity that is both switchable and rectifying. An intuitive model is developed and found to quantitatively reproduce the experimental observations. In a larger perspective, these results highlight the possibility of modulating material properties using the large electric fields associated with ferroelectric polarization.