Determination of the Attenuation Factor () in Hybrid Covalent/Non-Covalent Molecular Wires

We have established for the first time the molecular wire behaviour in a new set of hybrid covalent/supramolecular porphyrinfullerene structures, in which hydrogen-bond interactions and pphenylene oligomers of different length act as highly efficient molecular wires exhibiting a remarkably low attenuation factor ( = 0.07 ± 0.01 Å1 )

Charge-Gating Dibenzothiophene-S,S-dioxide Bridges in Electron Donor–Bridge–Acceptor Conjugates

The synthesis of a series of new electron donor–bridge–acceptor (D–B–A) conjugates (18–20) comprising electron-donating zinc(II) porphyrins (ZnPs) and electron-accepting fullerenes (C60s) connected through various co-oligomer bridges containing both dibenzothiophene-S,S-dioxide and fluorene units is reported. Detailed investigations using cyclic voltammetry, absorption, fluorescence, and femto/nanosecond transient absorption spectroscopy in combination with quantum chemical calculations have enabled us to develop a detailed mechanistic view of the charge-transfer processes that follow photoexcitation of ZnP, the bridge, or C60. Variations in the dynamics of charge separation, charge recombination, and charge-transfer gating are primarily consequences of the electronic properties of the co-oligomer bridges, including their electron affinity and the energy levels of the excited states. In particular, placing one dibenzothiophene-S,S-dioxide building block at the center of the molecular bridge flanked by two fluorene building blocks, as in 20, favors hole rather than electron transfer between the remote electron donors and acceptors, as demonstrated by exciting C60 rather than ZnP. In 18 and 19, in which one dibenzothiophene-S,S-dioxide and one fluorene building block constitute the molecular bridge, photoexcitation of either ZnP or C60 results in both hole and electron transfer. Dibenzothiophene-S,S-dioxide is thus shown to be an excellent building block for probing how subtle structural and electronic variations in the bridge affect unidirectional charge transport in D–B–A conjugates. The experimental results are supported by computational calculations

Understanding and controlling short- and long-range electron/charge transfer processes in electron donor-acceptor conjugates

We have probed a series of multicomponent electron donor2-donor1-acceptor1 conjugates, both experimentally and computationally. The conjugates are based on the light harvester and primary electron-donor zinc-porphyrin (ZnP, donor1), to whose β-positions a secondary electron-donor ferrocene (Fc, donor2) and the primary electron-acceptor C60-fullerene (C60, acceptor1) are linked via p-phenylene-acetylene bridges of different lengths. This modular approach makes full control over shuttling electrons and holes between C60, ZnP, and Fc possible. Different charge-separation, charge-transfer, and charge-recombination routes have been demonstrated, both by transient absorption spectroscopy measurements on the femto, pico-, nano-, and microsecond time scales and by multi-wavelength and target analyses. The molecular wire-like nature of the p-phenylene-acetylene bridges as a function of C60-ZnP and ZnP-Fc distances is decisive in the context of generating distant and long-lived C60•‒ ZnP Fc•+ charge-separated states. For the first time, we confirm the presence of two adjacent charge-transfer states, a C60 ZnP•‒ Fc•+ intermediate in addition to C60•‒ ZnP•+ Fc, en route to the distant C60•‒ ZnP Fc•+ charge-separated state. Our studies demonstrate how the interplay of changes in the reorganization energy and the damping factor of the molecular bridges, in addition to variation in the solvent polarity, affect the outcome of charge-transfer and the corresponding rate constants. The different regions of the Marcus parabola are highly relevant: The charge-recombination of, for example, the adjacent C60•‒ ZnP•+ Fc charge-separated state is located in the inverted region, while that of the distant C60•‒ ZnP Fc•+ charge-separated state lies in the normal region. Here, the larger reorganization energy of Fc relative to ZnP makes the difference

Charge-Gating Dibenzothiophene‑<i>S</i>,<i>S</i>‑dioxide Bridges in Electron Donor<b>–</b>Bridge<b>–</b>Acceptor Conjugates

The synthesis of a series of new electron donor–bridge–acceptor (D–B–A) conjugates (<b>18</b>–<b>20</b>) comprising electron-donating zinc­(II) porphyrins (ZnPs) and electron-accepting fullerenes (C₆₀s) connected through various co-oligomer bridges containing both dibenzothiophene-<i>S</i>,<i>S</i>-dioxide and fluorene units is reported. Detailed investigations using cyclic voltammetry, absorption, fluorescence, and femto/nanosecond transient absorption spectroscopy in combination with quantum chemical calculations have enabled us to develop a detailed mechanistic view of the charge-transfer processes that follow photoexcitation of ZnP, the bridge, or C₆₀. Variations in the dynamics of charge separation, charge recombination, and charge-transfer gating are primarily consequences of the electronic properties of the co-oligomer bridges, including their electron affinity and the energy levels of the excited states. In particular, placing one dibenzothiophene-<i>S</i>,<i>S</i>-dioxide building block at the center of the molecular bridge flanked by two fluorene building blocks, as in <b>20</b>, favors hole rather than electron transfer between the remote electron donors and acceptors, as demonstrated by exciting C₆₀ rather than ZnP. In <b>18</b> and <b>19</b>, in which one dibenzothiophene-<i>S</i>,<i>S</i>-dioxide and one fluorene building block constitute the molecular bridge, photoexcitation of either ZnP or C₆₀ results in both hole and electron transfer. Dibenzothiophene-<i>S</i>,<i>S</i>-dioxide is thus shown to be an excellent building block for probing how subtle structural and electronic variations in the bridge affect unidirectional charge transport in D–B–A conjugates. The experimental results are supported by computational calculations

Tuning the Carbon Nanotube Selectivity: Optimizing Reduction Potentials and Distortion Angles in Perylenediimides

Different water-soluble perylenediimides (PDIs) have been used to individualize and stabilize single-walled carbon nanotubes (SWCNTs) in aqueous media. A key feature of the PDIs is that they can be substituted at the bay positions via the addition of two and/or four bromines. This enables control over structural and electronic PDI characteristics, which prompted us to conduct comparative assays with focus on SWCNTs' chirality and charge transfer. Electrochemical, microscopic, and spectroscopic experiments were used to investigate the SWCNT chiral selectivity of PDIs, on the one hand, and charge-transfer reactions between SWCNTs and PDIs, on the other hand

Synergie von elektrostatischen und π-π-Wechselwirkungen für die Verwirklichung von künstlichen photosynthetischen Modellsystemen auf Nano-Ebene

Peer reviewe

Synergy of Electrostatic and π–π Interactions in the Realization of Nanoscale Artificial Photosynthetic Model Systems

In the scientific race to build up photoactive electron donor-acceptor systems with increasing efficiencies, little is known about the interplay of their building blocks when integrated into supramolecular nanoscale arrays, particularly in aqueous environments. Here, we describe an aqueous donor-acceptor ensemble whose emergence as a nanoscale material renders it remarkably stable and efficient. We have focused on a tetracationic zinc phthalocyanine (ZnPc) featuring pyrenes, which shows an unprecedented mode of aggregation, driven by subtle cooperation between electrostatic and π–π interactions. Our studies demonstrate monocrystalline growth in solution and a symmetry-breaking intermolecular charge transfer between adjacent ZnPcs upon photoexcitation. Immobilizing a negatively charged fullerene (C60) as electron acceptor onto the monocrystalline ZnPc assemblies was found to enhance the overall stability, and to suppress the energy-wasting charge recombination found in the absence of C60. Overall, the resulting artificial photosynthetic model system exhibits a high degree of preorganization, which facilitates efficient charge separation and subsequent charge transport.Peer reviewe

Modified bibenzimidazole ligands as spectator ligands in photoactive molecular functional Ru-polypyridine units? Implications from spectroscopy

The photophysical properties of Ruthenium-bipyridine complexes bearing a bibenzimidazole ligand were investigated. The nitrogens on the bibenzimidazole-ligand were protected, by adding either a phenylene group or a 1,2-ethandiyl group, to remove the photophysical dependence of the complex on the protonation state of the bibenzimidazole ligand. This protection results in the bibenzimidazole ligand contributing to the MLCT transition, which is experimentally evidenced by (resonance) Raman scattering in concert with DFT calculations for a detailed mode assignment in the (resonance) Raman spectra