Convergent synthesis and photoinduced processes in multi-chromophoric rotaxanes

Design, synthesis and photoinduced processes in molecular interlocked photosynthetic [60]fullerene systems

The mechanical bond, an entanglement in space between component parts that cannot be separated without breaking or distorting chemical bonds between atoms, can be used as a versatile organizing principle in the design of artificial photosynthetic systems.

Conjugated Porphyrin Dimers: Cooperative Effects and Electronic Communication in Supramolecular Ensembles with C60

Two new conjugated porphyrin-based systems (dimers 3 and 4) endowed with suitable crown ethers have been synthesized as receptors for a fullerene-ammonium salt derivative (1). Association constants in solution have been determined by UV–vis titration experiments in CH₂Cl₂ at room temperature. The designed hosts are able to associate up to two fullerene-based guest molecules and present association constants as high as ∼5 × 10⁸ M^–1. Calculation of the allosteric cooperative factor α for supramolecular complexes [3·1₂] and [4·1₂] showed a negative cooperative effect in both cases. The interactions accounting for the formation of the associates are based, first, on the complementary ammonium-crown ether interaction and, second, on the π–π interactions between the porphyrin rings and the C₆₀ moieties. Theoretical calculations have evidenced a significant decrease of the electron density in the porphyrin dimers 3 and 4 upon complexation of the first C₆₀ molecule, in good agreement with the negative cooperativity found in these systems. This negative effect is partially compensated by the stabilizing C₆₀–C₆₀ interactions that take place in the more stable syn-disposition of [4·1₂].

A multicomponent molecular approach to artificial photosynthesis – the role of fullerenes and endohedral metallofullerenes

In this review article, we highlight recent advances in the field of solar energy conversion at a molecular level.

Multistep energy and electron transfer processes in novel rotaxane donor-acceptor hybrids generating microsecond-lived charge separated states

A new set of [Cu(phen)2]+ based rotaxanes, featuring [60]-fullerene as an electron acceptor and a variety of electron donating moieties, namely zinc porphyrin (ZnP), zinc phthalocyanine (ZnPc) and ferrocene (Fc), has been synthesized and fully characterized with respect to electrochemical and photophysical properties. The assembly of the rotaxanes has been achieved using a slight variation of our previously reported synthetic strategy that combines the Cu(i)-catalyzed azide-alkyne cycloaddition reaction (the "click" or CuAAC reaction) with Sauvage's metal-template protocol. To underline our results, complementary model rotaxanes and catenanes have been prepared using the same strategy and their electrochemistry and photo-induced processes have been investigated. Insights into excited state interactions have been afforded from steady state and time resolved emission spectroscopy as well as transient absorption spectroscopy. It has been found that photo-excitation of the present rotaxanes triggers a cascade of multi-step energy and electron transfer events that ultimately leads to remarkably long-lived charge separated states featuring one-electron reduced C60 radical anion (C60˙-) and either one-electron oxidized porphyrin (ZnP˙+) or one-electron oxidized ferrocene (Fc˙+) with lifetimes up to 61 microseconds. In addition, shorter-lived charge separated states involving one-electron oxidized copper complexes ([Cu(phen)2]2+ (τ < 100 ns)), one-electron oxidized zinc phthalocyanine (ZnPc˙+; τ = 380-560 ns), or ZnP˙+ (τ = 2.3-8.4 μs), and C60˙- have been identified as intermediates during the sequence. Detailed energy diagrams illustrate the sequence and rate constants of the photophysical events occurring with the mechanically-linked chromophores. This work pioneers the exploration of mechanically-linked systems as platforms to position three distinct chromophores, which are able to absorb light over a very wide range of the visible region, triggering a cascade of short-range energy and electron transfer processes to afford long-lived charge separated states.

Synthesis and photophysical properties of new catenated electron donor-acceptor materials with magnesium and free base porphyrins as donors and C60 as the acceptor

A new series of nanoscale electron donor-acceptor systems with [2]catenane architectures has been synthesized, incorporating magnesium porphyrin (MgP) or free base porphyrin (H2P) as electron donor and C60 as electron acceptor, surrounding a central tetrahedral Cu(I)-1,10-phenanthroline (phen) complex. Model catenated compounds incorporating only one or none of these photoactive moieties were also prepared. The synthesis involved the use of Sauvage's metal template protocol in combination with the 1,3-dipolar cycloaddition of azides and alkynes ("click chemistry"), as in other recent reports from our laboratories. Ground state electron interactions between the individual constituents was probed using electrochemistry and UV-vis absorption spectroscopy, while events occurring following photoexcitation in tetrahydrofuran (under both aerobic and anaerobic conditions) at various wavelengths were followed by means of time-resolved transient absorption and emission spectroscopies on the femtosecond and nanosecond time scales, respectively, complemented by measurements of quantum yields for generation of singlet oxygen. From similar studies with model catenates containing one or neither of the chromophores, the events following photoexcitation could be elucidated. The results were compared with those previously reported for analogous catenates based on zinc porphyrin (ZnP). It was determined that a series of energy transfer (EnT) and electron transfer (ET) processes take place in the present catenates, ultimately generating long-distance charge separated (CS) states involving oxidized porphyrin and reduced C60 moieties, with lifetimes ranging from 400 to 1060 nanoseconds. Shorter lived short-distance CS states possessing oxidized copper complexes and reduced C60, with lifetimes ranging from 15 to 60 ns, were formed en route to the long-distance CS states. The dynamics of the ET processes were analyzed in terms of their thermodynamic driving forces. It was clear that intramolecular back ET was occurring in the inverted region of the Marcus parabola correlating rates and driving forces for electron transfer processes. In addition, evidence for triplet excited states as a product of either incomplete ET or back ET was found. The differences in behavior of the three catenates upon photoexcitation are analyzed in terms of the energy levels of the various intermediate states and the driving forces for EnT and ET processes.

Diarylferrocene tweezers for cation binding

Diarylferrocenes can act as molecular tweezers of cations. Their unique molecular shape and low torsional potentials allow for strong binding of small cations in the gas phase.

New polydentate trimethylsilyl chalcogenide reagents for the assembly of polyferrocenyl architectures

A series of polychalcogenotrimethylsilane complexes Ar(CH2ESiMe3)n, (Ar = aryl; E = S, Se; n = 2, 3, and 4) can be prepared from the corresponding polyorganobromide and M[ESiMe3] (M = Na, Li). These represent the first examples of the incorporation of such a large number of reactive -ESiMe3 moieties onto an organic molecular framework. They are shown to be convenient reagents for the preparation of the polyferrocenylseleno- and thioesters from ferrocenoyl chloride. The synthesis, structures, and spectroscopic properties of the new silyl chalcogen complexes 1,4-(Me3SiECH2)2(C6Me4) (E = S, 1; E = Se, 2), 1,3,5-(Me3SiECH2)3(C6Me3) (E = S, 3; E = Se, 4) and 1,2,4,5-(Me3SiECH2)4(C6H2) (E = S, 5; E = Se, 6) and the polyferrocenyl chalcogenoesters [1,4-{FcC(O)ECH2}2(C6Me4)] (E = S, 7; E = Se, 8), [1,3,5-{FcC(O)ECH2}3(C6Me3)] (E = S, 9; E = Se, 10) and [1,2,4,5-{FcC(O)ECH2}4(C6H2)] (E = S, 11 illustrated; E = Se, 12) are reported. The new polysilylated reagents and polyferrocenyl chalcogenoesters have been characterized by multinuclear NMR spectroscopy ((1)H, (13)C, (77)Se), electrospray ionization mass spectrometry and, for complexes 1, 2, 3, 4, 7, 8, and 11, single-crystal X-ray diffraction. The cyclic voltammograms of complexes 7-11 are presented.

Photoinduced multistep charge separation in a heteroleptic Cu(I) bis(phenanthroline)-based donor-chromophore-acceptor triad

A molecular triad assembly consisting of an electron donor, a bis(phenanthroline)copper(I) chromophore, and an electron acceptor has been prepared. Under visible-light excitation, this assembly undergoes efficient (ca. 50%) photoinduced, multistep formation of a diradical cation charge-separated state that has a lifetime of >100 ns and stores >1.0 eV of energy. This system constitutes an earth-abundant functional analogue of related Ru(bpy)(3) triad systems.

Topological and Conformational Effects on Electron Transfer Dynamics in Porphyrin-[60]Fullerene Interlocked Systems

The effect of molecular topology, and conformation on the dynamics of photoinduced electron transfer (ET) processes has been studied in interlocked electron donor-acceptor systems, specifically rotaxanes with zinc(II)-tetraphenylporphyrin (ZnP) electron donor and [60]fullerene (C(60)) as the electron acceptor. Formation or cleavage of coordinative bonds was used to induce major topological and conformational changes in the interlocked architecture. In the first approach, the tweezers-like structure created by the two ZnP stopper groups on the thread was used as a recognition site for complexation of 1,4-diazabicyclo[2.2.2]octane (DABCO), which creates a bridge between the two ZnP moieties on the rotaxane, generating a catenane structure. The photoinduced processes in the DABCO-complexed (ZnP)(2)-[2]catenate-C(60) system were compared with those of the (ZnP)(2)-rotaxane-C(60) precursor and the previously reported ZnP-[2]catenate-C(60). Steady-state emission and transient absorption studies showed that a similar multistep ET pathway emerged for rotaxanes and catenanes upon photoexcitation at various wavelengths, ultimately resulting in a long-lived ZnP(•+)/C(60) (•-) charge separated radical pair state. However, the decay kinetics of the latter states clearly reflect the topological differences between the rotaxane, the catenate, and DABCO-complexed-catenate architectures. The lifetime of the long-distance ZnP(•+)-[Cu(I)phen(2)](+)-C(60) (•-) charge separated state is more than four times longer in 3 (1.03 µs) than in 1 (0.24 µs) and approaches that in catenate 2 (1.1 µs). The results clearly showed that adoption of a catenane from a rotaxane topology inhibits the charge recombination process. In a second approach, the Cu(I) ion used as template to assemble the (ZnP)(2)-[Cu(I)phen(2)](+)-C(60) rotaxane was removed, and structural analysis suggested a major topographical change occurred, such that charge separation between the chromophores was no longer observed upon photoexcitation in nonpolar as well as polar solvents. Only ZnP and C(60) triplet excited states were observed upon laser excitation. These results highlighted the critical importance of the central Cu(I) ion for long range ET processes in these large interlocked electron donor-acceptor systems.

Strategies and tactics for the metal-directed synthesis of rotaxanes, knots, catenanes, and higher order links

More than a quarter of a century after the first metal template synthesis of a [2]catenane in Strasbourg, there now exists a plethora of strategies available for the construction of mechanically bonded and entwined molecular level structures. Catenanes, rotaxanes, knots and Borromean rings have all been successfully accessed by methods in which metal ions play a pivotal role. Originally metal ions were used solely for their coordination chemistry; acting either to gather and position the building blocks such that subsequent reactions generated the interlocked products or by being an integral part of the rings or "stoppers" of the interlocked assembly. Recently the role of the metal has evolved to encompass catalysis: the metal ions not only organize the building blocks in an entwined or threaded arrangement but also actively promote the reaction that covalently captures the interlocked structure. This Review outlines the diverse strategies that currently exist for forming mechanically bonded molecular structures with metal ions and details the tactics that the chemist can utilize for creating cross-over points, maximizing the yield of interlocked over non-interlocked products, and the reactions-of-choice for the covalent capture of threaded and entwined intermediates.

Alternative demetalation method for Cu(I)-phenanthroline-based catenanes and rotaxanes

A new and less hazardous procedure for demetalation of Cu(I)-phenanthroline-based interlocked molecules, using aqueous NH(4)OH rather than toxic KCN, has been developed. The conditions are compatible with materials containing nucleophile-sensitive appended groups such as C(60), and coordinating moieties such as zinc(II)-porphyrins.