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Electron transfer over long distances is important for many future applications in molecular electronics and solar energy harvesting. In these contexts, it is of great interest to find molecular systems that are able to efficiently mediate electrons in a controlled manner over nanometer distances, that is, structures that function as molecular wires. Here we investigate a series of butadiyne-linked porphyrin oligomers with ferrocene and fullerene (C-60) terminals separated by one, two, or four porphyrin units (P-n, n = 1, 2, or 4). When the porphyrin oligomer bridges are photoexcited, long-range charge separated states are formed through a series of electron-transfer steps and the rates of photoinduced charge separation and charge recombination in these systems were elucidated using time-resolved absorption and emission measurements. The rates of long-range charge recombination, through these conjugated porphyrin oligomers, are remarkably fast (k(CR2) = 15 - 1.3 x 10(8) s(-1)) and exhibit very weak distance dependence, particularly comparing the systems with n = 2 and n = 4. The observation that the porphyrin tetramer mediates fast long-range charge transfer, over 65 angstrom, is significant for the application of these structures as molecular wires.

A systematic investigation is reported into the influence of the counter cations on the optical, electrical and electroluminescent properties of polyelectrolytic conjugated polymers and of their cyclodextrin-threaded rotaxanes. We compare conjugated polyelectrolytes with sulfonated side groups where the anionic charge is balanced by Li+, K+, Cs +, tetramethylammonium (Me4N+) and cryptate-encapsulated potassium (K+ at [2.2.2]). Narrowing (for the unthreaded analogues) and a slight red-shift of the absorption spectra (for the rotaxanes) are found upon exchange of Li+ for larger cations, together with a blue-shift and an efficiency enhancement of the luminescence. These effects are similar in nature to those induced by rotaxination, and are therefore assigned to a marked reduction of intermolecular interactions between the conjugated cores. Exchange of Li+ for K+, Cs +, or Me4N+ results in a higher electroluminescence external quantum efficiency (EQE) for both polyrotaxanes and unthreaded polymers. For polyrotaxane-based devices the EQE increased approximately 7 times upon substitution of Li+ with Cs+ or Me4N+, thereby demonstrating the importance of the selection of the counter-cations for optimizing the performance of polyelectrolytic conjugated polymers in light-emitting devices. © 2008 WILEY-VCH Verlag GmbH and Co. KGaA.

Four generations of conjugated dendrimers that contain 1,3,5-tris(distyrylbenzenyl)benzene cores, stilbene dendrons, and t-butyl surface groups have been synthesized. The dendrimers were synthesized by coupling benzylphosphonate-focused dendrons with 1,3,5-tris(4-formylstilbenyl)benzene to give the dendrimers in yields in the range 60-82 %. We have probed the optoelectronic properties of the dendrimers by electrochemistry, photoluminescence, and in light-emitting device structures. we have found that the degree of aggregation is strongly generation dependent. We compared the properties of these benzene-centered dendrimers with an equivalent family of dendrimers that differs only in having a nitrogen atom as the central unit. We found that the aggregation of dendrimers was strongly dependent on the degree of delocalization across the central unit. the dendrimers with the benzene central unit, which have three localized distyrylbenzene chromophores, were found to aggregate more strongly in the solid state that those with nitrogen as the central unit. In the latter case the electroactive component is comprised of all three distyrylbenzene units and the nitrogen atom.

We have synthesised a new family of dendrimers with stilbene dendrons attached to a porphyrin core via a stilbene unit and compared their properties with a family of dendrimers with the same core and dendrons but with the dendrons attached via a phenyl unit. The oxidation and reduction half potentials of the two dendrimer families were found to be the same and independent of generation indicating that the dendrons were not creating a micro-environment for the core. However, the rate of heterogeneous electron transfer was found to be strongly dependent on link type and generation. The photoluminescence quantum yield (PLQY) of the dendrimers was also found to be strongly dependent on the method of attachment of the core. In solution the dendrimers with the stilbene link between core and dendrons had PLQYs 1.5 times higher than their phenyl counterparts but in the solid state the trend was reversed with the phenyl linked dendrimers generally having a higher PLQY. The difference in properties has been assigned to the comparative openness of the dendrimer architectures and the effect of the dendrons on the shape of the porphyrin core.

An efficient strategy has been developed for the preparation of four generations of electroluminescent dendrimers that contain tris(distyrylbenzenyl)amine cores, stilbene dendrons, and tert-butyl surface groups. The synthesis involved coupling of benzylphosphonate focused dendrons with tris(4'-formylstilbenyl)amine to give the dendrimers in yields ranging from 63 to 86%. The dendrimers were found to be monodisperse by gel-permeation chromatography. The zeroeth generation dendrimer underwent two chemically reversible oxidations while for the higher generations only one chemically reversible oxidation was observed. On reduction, the dendrimers were found to aggregate with the level of aggregation dependent on the switching potential. The four dendrimer generations were investigated by means of optical spectroscopy. Time-resolved luminescence of the dendrimers in solution showed that the excited state of each of the generations had a monoexponential decay with a lifetime of 1.8 ns. The photoluminescence quantum yield (PLQY) of the dendrimers in solution was independent of generation and was found to be in the region of 0.62. This suggests that the origin of the luminescence is the same for all dendrimer generations. In thin films, time-resolved luminescence of the zeroeth dendrimer generation revealed a long-lived luminescence component in the red part of the spectrum with a lifetime of 7.5 ns. This emission component could not be found in the first, second, and third generation dendrimers, where the long-lived luminescence had a lifetime of 1.5-3 ns at all detection wavelengths. Furthermore, the PLQY of the dendrimer films was found to be dependent on generation and significantly lower than the solution PLQYs. The dendrimer film PLQY increased with generation from 5% for the zereoth generation to 12% for the third generation. The differences observed in the time-resolved luminescence and PLQY of the dendrimers in the solid state arise from the fact that intermolecular interactions between the emissive cores of the dendrimers are considerably stronger in the zeroeth generation than in higher generations. The intermolecular interactions result in an aggregate, which we ascribe to an excited-state species, such as an excimer.

A donor-acceptor system is presented in which the electron-transfer rates can be sensitively controlled by means of excitation wavelength and temperature. The electron donor is a butadiyne-linked zinc porphyrin dimer that is connected to a C(60) electron acceptor. The broad distribution of conformations allowed by the butadiyne linker makes it possible to selectively excite perpendicular or planar donor conformers and thereby prepare separate initial states with driving forces for electron transfer that differ by almost 0.2 eV. This, as well as significant differences in electronic coupling, leads to distinctly different rate constants for electron transfer, which in consequence can be controlled by changing excitation wavelength. By extending the system with a secondary donor (ferrocene), a second, long-range charge-separated state can be formed. This system has been used to test the influence of conformational heterogeneity on electron transfer mediated by the porphyrin dimer in the ground state. It was found that if the dimer is forced to a planar conformation by means of a bidentate ligand, the charge recombination rate increased by an order of magnitude relative to the unconstrained system. This illustrates how control of conformation of a molecular wire can affect its behaviour.

beta,meso,beta-Fused porphyrin oligomers have many attractive photophysical features such as strong absorption in the near-IR at wavelengths greater than 1000 nm, and high two-photon cross sections. However their ultrafast S(1)-S(0) deactivation (k(d) > 10(11) s(-1)) limits potential applications. We have synthesised a deuterated fused porphyrin dimer to test whether deuteration influences the rate of non-radiative deactivation. An efficient synthetic strategy was developed, starting with deuteration of dipyrromethane. Deuteration of the zinc porphyrin dimer does not affect its fluorescence quantum yield in CD(2)Cl(2)(Phi(fD)/Phi(fH)= 1.00 +/- 0.05). This implies that the ultrafast non-radiative deactivation is not simply a consequence of the small S(1)-S(0) energy gap. Comparison with other conjugated porphyrin oligomers confirms that the deactivation rate in the edge-fused oligomers is faster than would be expected from the energy gap law. This result indicates that it should be possible to create near-IR dyes with similar S(1)-S(0) energy gaps to the beta,meso,beta-fused porphyrin oligomers but with slower rates of S(1)-S(0) decay.

[structure: see text] A vinylene-linked porphyrin dimer, with no substituents at the beta-positions, has been synthesized by CuI/CsF promoted Stille coupling. In the crystal structure of this dimer, the C(2)H(2) bridge is twisted by 45 degrees relative to the plane of the porphyrins. The absorption, emission spectra, and electrochemistry reveal substantial porphyrin-porphyrin pi-conjugation. The triplet excited-state absorption spectrum of this dimer makes it suitable for reverse saturable absorption at 710-900 nm.

Dendrimers can be used as the emissive layer in organic light-emitting diodes. We have synthesised conjugated dendrimers containing meta-linked stilbene dendrons and luminescent porphyrin or triarylamine cores. The HOMO energy levels of the amine-cored dendrimers were studied by cyclic voltammetry and modelling of device characteristics. Both techniques showed that charge was injected directly into the core and importantly that the energy levels did not change with dendrimer generation.

The crystal structure of a cyanine dye rotaxane shows that the cyclodextrin is tightly threaded round the polymethine bridge of the dye; encapsulation dramatically increases the kinetic chemical stability of the radicals formed on oxidation and reduction of the dye, making it possible to observe the rotaxane radical dication by ESR and UV-vis-NIR spectroscopy.

We report methodology for the preparation of symmetric and asymmetric solution processable phosphorescent dendrimers that are comprised of 2-ethylhexyloxy surface groups, biphenyl based dendrons, and iridium(III) complex cores. The symmetric dendrimer has three dendritic 2-benzo[b]thiophene-2′-ylpyridyl (BTP) ligands with the dendritic ligands responsible for red emission. The asymmetric dendrimer has two dendritic 2-phenylpyridyl ligands and one unsubstituted BTP ligand. Iridium(III) complexes comprised of 2-phenylpyridyl ligands are normally associated with green emission whereas those containing BTP ligands emit red light. Red emission is observed from the asymmetric dendrimer demonstrating that emission occurs primarily from the metal-to-ligand charge transfer state associated with the ligand with the lowest HOMO–LUMO energy gap. The photoluminescence quantum yields (PLQYs) of the symmetric and asymmetric dendrimers were strongly dependent on the dendrimer structure. In solution the PLQYs of the asymmetric and symmetric dendrimers were 47 ± 5% and 29 ± 3% respectively. The photoluminescence lifetime of the emissive state of both dendrimers in solution was 7.3 ± 0.1 µs. In the solid state the comparative PLQYs were reversed with the symmetric dendrimer having a PLQY of 10 ± 1% and the asymmetric dendrimer a PLQY of 7 ± 1%. The comparatively larger decrease in PLQY for the asymmetric dendrimer in the solid state is attributed to increased core–core interactions. The intermolecular interactions are greater in the asymmetric dendrimer because there is no dendron on the BTP ligand. Electrochemical analysis shows that charge is injected directly into the cores of the dendrimers.