Influence of supramolecular organization on energy transfer properties in chiral oligo(p-phenylene vinylene) porphyrin assemblies

A comparative study on oligo(p-phenylene vinylene) (OPV)-appended porphyrins containing all trans-vinylene (either hydrophilic or lipophilic) or amide linkages (lipophilic) is presented. The type of supramolecular arrangement obtained in organic solvents proves to be strongly dependent on the nature of the covalent connection. In the case of all trans-vinylene linkages, a J-type intermolecular packing is obtained and the assemblies are only of moderate stability. Conversely, the supramolecular structures obtained from the amide-linked system display an H-type stacking arrangement of enhanced stability and chirality as a consequence of intermolecular hydrogen bonding along the stack direction, favorably interlocking the stacked building blocks. Interestingly, the observed differences in stability and organization are qualitatively illustrated by monitoring the sequential energy transfer process in both types of assemblies. Efficient intramolecular energy transfer from the OPVs (donors) to the respective porphyrin cores is followed by energy transfer from Zn-porphyrin (donor) to free-base porphyrin (acceptor) in both systems. However, the improved intermolecular organization for the amide-linked system increases the energy transfer efficiency along the stack direction. In addition, the water-soluble (OPV)-appended porphyrin system forms highly stable assemblies in an aqueous environment. Nevertheless, the poor energy transfer efficiency along the stack direction reveals a relative lack of organization in these assemblies.     

Self-assembly and luminescence of oligo(p-phenylene vinylene) amphiphiles

We have synthesized a series of amphiphilic molecules consisting of oligo(phenylene vinylene) (OPV) asymmetrically end-substituted with a hydrophilic poly(ethylene glycol) (PEG) segment and a hydrophobic alkyl chain. This amphiphilic structure induces self-assembly into both thermotropic and lyotropic lamellar liquid crystalline (LC) phases. The molecules form strongly fluorescent, self-supporting gels in both water and polar organic solvents, even at high concentrations on the order of 30 wt %. These self-assembled structures have been characterized by small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), and polarized optical microscopy (POM). Photoluminescence (PL) is influenced by the structure of the material, with enhanced emission in the LC state due to assembly of the chromophore in confined two-dimensional layers. Self-assembly controlling molecular aggregation at the nanoscale could significantly improve the performance of OPV-based materials in optoelectronic devices.     

Highly emissive supramolecular oligo(p-phenylene vinylene) dendrimers

pi-Conjugated oligo(p-phenylene vinylene) (OPV) guest molecules for interaction with dendritic hosts were synthesized and fully characterized by NMR spectroscopy, MALDI-TOF-MS, elemental analysis and optical measurements. The binding properties of the five different OPV guests to a N,N-bis[(3-adamantyl ureido) propyl] methylamine host have been investigated. The guests that contained an aryl urea glycine spacer were bound with the highest association constant. Subsequently, an adamantyl urea modified fifth generation poly(propylene imine) dendrimer was synthesized as a multivalent host which contains 32 N,N-bis[(3-adamantyl ureido) propyl] amine binding sites. Size exclusion chromatography showed that 32 of the OPV guests strongly bind to the fifth generation adamantyl functionalized dendritic host. In the case of the supramolecular dendritic host/guest system smooth homogeneous thin films could be obtained by spin coating. The dendritic guest-host complexes showed a significantly higher emission upon binding then that of the individual molecules due to the three-dimensional orientation of the OPV guest molecules. In the solid state, this enhancement in luminescence was a factor of 10. The pi-conjugated oligomers are less aggregated in the supramolecular assemblies presumably because of a shielding effect of the bulky adamantyl units present in the hosts.     

Photoinduced electron transfer in hydrogen-bonded oligo(p-phenylene vinylene)-perylene bisimide chiral assemblies

Well-defined chiral fibers incorporating hydrogen-bonded oligo(p-phenylene vinylene) donor and perylene bisimide acceptor chromophores have been realized by self-assembly. Upon photoillumination of these fibers electron-transfer takes place, leading to charge separation within the aggregated dyes.     

Sequential energy and electron transfer in aggregates of tetrakis[oligo(p-phenylene vinylene)] porphyrins and C60 in water

Chiral aggregation of oligo(p-phenylene vinylene)-functionalized Zn and free-base porphyrins is observed in water. The formation of mixed assemblies containing both porphyrins results in sequential energy transfer from OPV via zinc porphyrin to free-base porphyrin. Furthermore, the incorporation of C60 as electron acceptor yields a charge separated state by ultimate electron transfer.     

Energy transfer in chiral co-assemblies of triple hydrogen-bonded oligo(p-phenylene vinylene)s and porphyrin

Pi-pi stacking of hydrogen-bonded porphyrin and oligo(p-phenylene vinylene)s (OPVs) yields helical co-assemblies which exhibit energy transfer from OPV to porphyrin.     

Fractal-like self-assembly of oligo(p-phenylene vinylene) capped gold nanoparticles

Reversible interparticle self-organization is reported for gold nanoparticles functionalized with an oligo(p-phenylene vinylene) moiety in butanol. The aggregates show a clear melting temperature at 80 degrees C, and atomic force microscopy and transmission electron microscopy indicate a fractal-like organization of the particles.     

Supramolecular polymerization and polymorphs of oligo(p-phenylene vinylene)-functionalized bis- and monoureas

Bis- and monoureas hybridized with the oligo(p-phenylene vinylene) (OPV) pi-electronic segment and 3,4,5-tridodecyloxyphenyl wedge were synthesized and their supramolecular polymerization in diluted solution, gel formation in concentrated solution, and liquid crystallinity in bulk state were investigated. Bisurea 1a featuring a hexamethylene linker showed the highest supramolecular polymerization ability and formed tapelike nanofibers that can gelate various organic solvents. On the other hand, bisurea 1b featuring a dodecamethylene linker and monourea 2 showed a lower degree of supramolecular polymerization, resulting in gel formations in a smaller variety of solvents. These results clearly reflect a high level of cooperativity between the two urea sites and the two OPV segments of 1a upon hydrogen-bonding and pi-pi stacking interactions, respectively. When the gels of 1a, 1b, and 2 were dried, all the compounds self-organized into multilamellar superstructures. Thermal treatment of these lamellae at high temperatures induces columnar liquid-crystalline mesophases as a result of microsegregation between the rigid OPV parts and the molten aliphatic wedges. These results demonstrate that the present molecular constituent is very useful for fabricating dye-based functional assemblies providing nanoscale pi-electronic fibers, and solvent-incorporated and bulk soft materials.     

Supramolecular p-n-heterojunctions by co-self-organization of oligo(p-phenylene vinylene) and perylene bisimide dyes

Comparative studies on hydrogen-bonded versus covalently linked donor-acceptor-donor dye arrays obtained from oligo(p-phenylene vinylene)s (OPVs) as donor and bay-substituted perylene bisimides (PERYs) as acceptor dyes are presented. Both systems form well-ordered J-type aggregates in methylcyclohexane, but only hydrogen-bonded arrays afford hierarchically assembled chiral OPV-PERY dye superstructures consisting of left-handed helical pi-pi co-aggregates (CD spectroscopy) of the two dyes that further assemble into right-handed nanometer-scale supercoils in the solid state (AFM study). In the case of hydrogen-bonded arrays, the stability of the aggregates in solution increases with increasing conjugation length of the OPV unit. The well-defined co-aggregated dyes presented here exhibit photoinduced electron transfer on subpicosecond time scale, and thus, these supramolecular entities might serve as valuable nanoscopic functional units.     

Donor-acceptor polymers: a conjugated oligo(p-phenylene vinylene) main chain with dangling perylene bisimides

Two new donor-acceptor copolymers that consist of an enantiomerically pure oligo(p-phenylene vinylene) main chain with dangling perylene bisimides have been synthesized by using a Suzuki cross-coupling polymerization. Absorption and circular dichroism spectroscopy revealed that the transition dipole moments of the donor in the main chain and the dangling acceptor moieties of the copolymers are coupled and in a helical orientation in solution, even at elevated temperatures. A strong fluorescence quenching of both chromophores indicates an efficient photoinduced charge transfer after photoexcitation of either donor or acceptor. The formation and recombination kinetics of the charge-separated state were investigated in detail with femtosecond and near-steady-state photoinduced absorption spectroscopy. The charge-separated state forms within 1 ps after excitation, and recombination occurs with a time constant of 45-60 ps, both in solution and in the solid state. These optical characteristics indicate a short distance and appreciable interaction between the electron-rich donor chain and the dangling electron-poor acceptor chromophores.     

Energy-transfer efficiency in stacked oligo(p-phenylene vinylene)s: pronounced effects of order.

The supramolecular structure of two types of oligo(p-phenylene vinylene) (OPV) building blocks in dodecane solution is studied. Monofunctional chromophores (MOPV) form well-defined helical assemblies, whereas bifunctional molecules (BOPV) aggregate into so-called frustrated stacks, lacking any higher helical order. This difference in organization has a major influence on the transfer of excitation energy through the assemblies. Energy transfer to supramolecularly incorporated guests (MOPV with lower bandgap) is used to probe the intrinsic differences in exciton mobility in these two types of mixed aggregates. From the observed donor fluorescence quenching, it can be concluded that the helically ordered nature of the MOPV stacks facilitates the transfer of excitation energy, yielding evidence for higher exciton mobility in the well-ordered assemblies than in the frustrated stacks. Finally, the concept of energy transfer in supramolecular assemblies is extended to the solid state by the successful implementation in a light-emitting diode (LED).     

Self-assembled nanotapes of oligo(p-phenylene vinylene)s: sol-gel-controlled optical properties in fluorescent pi-electronic gels

A rational approach to the design of supramolecular organogels of all-trans oligo(p-phenylene vinylene) (OPV) derivatives, a class of well-known organic semiconductor precursors, is reported. Self-assembly of these molecules induced gelation of hydrocarbon solvents at low concentrations (<1 mM), resulting in high aspect ratio nanostructures. Electron microscopy and atomic force microscopy (AFM) studies revealed twisted and entangled supramolecular tapes of an average of 50-200 nm in width, 12-20 nm in thickness, and several micrometers in length. The hierarchical growth of the entangled tapes and the consequent gelation is attributed to the lamellar-type packing of the molecules, facilitated by cooperative hydrogen bonding, pi stacking, and van der Waals interactions between the OPV units. Gelation of OPVs induced remarkable changes in the absorption and emission properties, which indicated strong electronic interaction in the aggregated chromophores. Comparison of the absorption and emission spectra in the gel form and in the solid film indicated a similar chromophore organization in both phases. The presence of self-assembled aggregates of OPVs was confirmed by solvent- and temperature-dependent changes in the absorption and emission properties, and by selective excitation experiments. This is the first detailed report of the gelation-induced formation of OPV nanotapes, assisted by weak, nondirectional hydrogen-bonding motifs and pi-pi stacking. These findings may provide opportunities for the design of a new class of functional soft materials and nanoarchitectures, based on pi-conjugated organic semiconductor-type molecules, thereby enabling the manipulation of their optical properties.     

Synthesis of oligo(p-phenylene vinylene)-porphyrin-oligo(p-phenylene vinylene) triads as antenna molecules for energy transfer

The oligo(p-phenylene vinylene)-porphyrin-oligo(p-phenylene vinylene) (P-OPVn, n=2, 4, where n is the number of phenyl rings) and the complex with Zn²⁺ based on P-OPVn were synthesized for investigating their photophysical properties via UV-vis, voltammetry, steady-state and time-resolved fluorescence spectra. In these molecules two OPV moieties as energy donors were linked to porphyrin center by virtue of Wittig reaction. The detailed studies of photophysical properties indicate that OPV group can act as an antenna unit for effective intramolecular energy transfer.     

Synthesis of n-type perylene bisimide derivatives and their orthogonal self-assembly with p-type oligo(p-phenylene vinylene)s

Four different (chiral) electron-deficient (n-type) perylene bisimides containing aliphatic, aromatic, or ethyleneoxide side chaines have been synthesized and fully characterized. All of them form supramolecular stacks in apolar methylcyclohexane (MCH) solution as demonstrated by concentration- and temperature-dependent absorption, circular dichroism, and fluorescence studies. One derivative was investigated in more detail in the solid state and proven to be liquid crystalline and capable of forming nanometer-sized fiberlike networks when drop-cast from MCH. Optical spectroscopy techniques show that perylene bisimide and an oligo(p-phenylene vinylene) (p-type) derivative orthogonally self-assemble into separate nanosized p-and n-type stacks in MCH. In contrast in toluene only molecularly dissolved species are present. In films deposited from MCH as well as from toluene photoinduced electron transfer takes place from the p-type material to the n-type material. As a result of the orthogonal self-assembly process, in films from MCH an ordered network of fibers was formed, whereas in films from toluene no ordering was observed. However, probably due to the lateral orientation on the surface and the presence of long aliphatic chains pointing toward the electrodes, efficient bulk heterojunction solar cells could not be constructed.     

Fabrication of poly(p-phenylene vinylene) (PPV) nanoheterocomposite films via layer-by-layer adsorption

The fabrication of polyelectrolyte multilayer film architectures composed of a polycation precursor (Pre-PPV) of the electroluminescent poly(p-phenylene vinylene) (PPV) and two different counter-polyanions, cellulosesulfate and poly(vinylsulfate), are reported. All multilayers were characterized by UV/VIS-spectroscopy and X-ray reflectometry. Due to the differences in spatial arrangement of charged groups, rigidity, and conformation of the polyanions, the corresponding multilayer films differ in properties such as average thickness increments and surface roughness. The adsorbed amounts per layer can be adjusted by addition of inorganic salts. Thermal conversion of Pre-PPV to PPV is achieved already slightly above 100 °C, yielding identical absorption spectra for after either 3 h at 160 °C or 20 h at 120 °C. The heat treatment causes the film thickness to be reduced by 24–40% due to elimination of dimethylsulfide and HCl and also the loss of water, but the films stay optically transparent.     

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.