A new modular concept for the self-assembly of electron donor-acceptor complexes is presented that ensures (i) fine-tuning the strength of the complexation, (ii) controlling the electronic coupling to impact electron and energy transfer processes, and (iii) high solubility of the corresponding hybrid architectures. This task has been realized through developing a series of porphyrin-fullerene donor-acceptor systems held together by a Hamilton-receptor-based hydrogen-bonding motif. In this context, novel libraries of C60 monoadducts (1) containing cyanuric acid side chains and of tetraphenylporphyrin derivatives (2) involving the complementary Hamilton-receptor unit were synthesized. The association constants of the corresponding 1:1 complexes (1.2) connected by six hydrogen bonds were determined complementary by NMR and fluorescence assays. Their strength, which was found to be in the range between 3.7 x 10(3) and 7.9 x 10(5) M-1, depends on the nature of the spacers, namely, hexylene versus propylene chains. Finally, transient absorption studies revealed photoinduced electron transfer from ZnP to C60 in the corresponding 1.2 complexes, which generate radical ion pair states that are persistent well beyond the ns time scale. In the case of the analogous SnP complexes, energy instead of electron transfer was observed. This is due to the shift of oxidation potential caused by presence of Sn in the oxidation state of +4.
A concept is elaborated of pairing electron donors and electron acceptors that share a common trait, wire-like features, as a powerful means to realize a new and versatile class of electron donor-acceptor nanohybrids. Important variables are fine-tuning (i) the complexation strength, (ii) the electron/energy transfer behavior, and (iii) the solubilities of the resulting architectures. In particular, a series of supramolecular porphyrin/fullerene hybrids assembled by the hydrogen bonding of Hamilton receptor/cyanuric acid motif has been realized. Putting the aforementioned variables into action, the association constants (K(ass)), as they were determined from (1)H NMR and steady-state fluorescence assays, were successfully tweaked with values in the range of 10(4)-10(5) M(-1). In fact, our detailed studies corroborate that the latter reveal a dependence on the nature of the spacer, that is, p-phenylene-ethynylene, p-phenylene-vinylene, p-ethynylene, and fluorene, as well as on the length of the spacer. Complementary performed transient absorption studies confirm that electron transfer is indeed the modus operandi in our novel class of electron donor-acceptor nanohybrids, while energy transfer plays, if any, only a minor role. The accordingly formed electron transfer products, that is, one-electron oxidized porphyrins and one-electron reduced fullerenes, are long-lived with lifetimes that reach well into the time domain of tens of nanoseconds. Finally, we have used the distance dependence on electron transfer, charge separation and charge recombination, to determine for the first time a beta value (0.11 A(-1)) for hydrogen-bonding-mediated electron transfer.
In the current work, we have documented the use of two complementary supramolecular motifs, namely multipoint hydrogen bonding and metal complexation, as a means to control the stepby-step assembly of a panchromatically absorbing and highly versatile solar energy conversion system. On one hand, two different perylenediimides (1a/1b) have been integrated together with a metalloporphyrin (2) by means of the Hamilton receptor/cyanuric acid hydrogen bonding motif into energy transduction systems 1a•2 or 1b•2. Steady-state and time-resolved measurements corroborated that upon selective photoexcitation of the perylenediimides (1a/1b), an energy transfer evolved from the singlet excited state of the perylenediimides (1a/1b) to that of the metalloporphyrin (2). On the other hand, fullerene (3) and metalloporphyrin (2) form the electron donor-acceptor system 2•3 via axial complexation. Photophysical measurements confirm that an electron transfer prevails from the singlet excited state of 2 to the electron-accepting 3. The correspondingly formed radical ion pair state decays with a lifetime of 1.0 AE 0.1 ns. As a complement to the aforementioned, the energy transduction features of 1a•2 were combined with the electron donor-acceptor characteristics of 2•3 to afford 1a•2•3. To this end, time-resolved measurements reveal that the initially occurring energy-transfer interaction (53 AE 3 ps) between 1a/1b and 2 is followed by an electron transfer (12 AE 1 ps) from 2 to 3. From multiwavelength analyses, the lifetime of the radical ion pair state in 1a•2•3-as a product of a cascade of light-induced energy and electron transfer-was derived as 3.8 AE 0.2 ns. I n recent years, the conversion of solar energy into electrical energy has been one of the most important areas of scientific research due to the constantly growing human energy demands (1, 2). In order to design efficient, low-cost, environmentally responsible artificial photosynthetic systems, research follows the concepts of nature (3, 4). Similar to natural photosynthetic organisms (5-8), artificial systems undergo cascades of light-induced energy and electron transfer reactions. The absorbed energy is converted into chemical potential energy in the form of a radical ion pair state (9). These events take place in organized arrays of photofunctional chromophores within specifically tailored environments that allow an efficient light harvesting. As an important prerequisite, the lifetime of the excited states should be long enough to power efficient electron transfer in order to obtain radical ion pair states. Therefore, the development of panchromatically absorbing molecules with appropriate absorptive properties for light harvesting is desirable.Owing to their biological relevance and favorable properties as natural chromophores, porphyrinoids are widely used as molecular components in artificial photosynthetic systems. Their functions include panchromatic light harvesting through most of the visible part of the solar spectrum and electron transport (10-26). As another m...
Two orthogonal non-covalent binding sites, namely metal-ligand complexation of Ru and bipyridine and intermolecular hydrogen bonding, facilitate the self-assembly of a new type of supramolecular dendrimers.
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