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In this study we synthesized two acid-/base-controllable [2]rotaxanes featuring aminodiazobenzene and aminocoumarin units, respectively, as chromophores and dibenzo[24]crown-8 and dibenzo[25]crown-8 units, respectively, as their macrocyclic components. Each [2]rotaxane contained N-alkylarylamine (ammonium) and N, N-dialkylamine (ammonium) centers as binding sites for their crown ether components. The absorption patterns of the chromophores were dependent on the position of the encircling macrocyclic component and the degree of protonation, with three distinct states (under acidic, neutral, and basic conditions) evident for each [2]rotaxane. The mixed [2]rotaxane system displayed stepwise and independent molecular shuttling behavior based on the degree of protonation of the amino groups in response to both the amount and strength of added acids or bases; as such, the system provided five different absorption signals as outputs that could be read using UV/Vis spectroscopy. [ABSTRACT FROM AUTHOR]

Two functional [2]rotaxanes, with a 4-morpholin-naphthalimide fluorophore as one of the two stoppers and a difluoroboradiaza- s -indacene functionalized dibenzo[24]crown-8 as macrocycle, were designed, synthesized and well characterized. [2]Rotaxane 1 and 2 had almost the same structural skeleton, and a ferrocene unit was introduced in [2]rotaxane 1 as the other stopper, while a 3,5-dimethoxybenzene stopper in [2]rotaxane 2 . It has shown that efficient energy transfer process in both two [2]rotaxanes occurred from the 4-morpholin-naphthalimide donor to the difluoroboradiaza-s-indacene acceptor. Moreover, [2]rotaxane 1 exhibited a remarkable fluorescence increase in response to the addition of base that can drive the shuttling motion because of the introduction of ferrocene electron donor. This work can pave the way for the design and construction of complex and functional molecular systems based on the mechanically interlocked molecules. [ABSTRACT FROM AUTHOR]

We developed the synthesis of a multicomponent molecular machine by employing the inclusion complex of an azobenzene-containing pyridinium salt (G1) with α-cyclodextrin (α-CD) and tetrasulfonated 1,5-dinaphtho-32-crown-8 (DNC) in water. The interconversion of the heterowheel trans-G1·DNC·α-CD [3]pseudorotaxane into cis-G1·DNC [2]pseudorotaxane was modulated by the photoinduced isomerization of the azobenzene moiety accompanied by the release of α-CD from the molecular axle. In control experiments, the azobenzene center of the bis(pyridinium)-modified azobenzene (G2) could not be isomerized in the presence of DNC and α-CD. Only polymeric aggregates were obtained with the G2·DNC·α-CD system, which indicates that the inclusion complex with α-CD and the electrostatic attraction with DNC are crucial to govern the reversible photoswitchable process. This work may provide a method to induce conformational changes of supramolecular assemblies that have multiple macrocyclic receptors. [ABSTRACT FROM AUTHOR]

A bistable bis-branched [3]rotaxane with a perylene bisimide (PBI) chromophore that separates two mechanically interlocked [2]rotaxane arms including a diferrocene- functionalized dibenzo-24-crown-8 ring, was designed, prepared, and characterized by 1H NMR and 13C NMR spectroscopy and HR-ESI mass spectrometry. The uniform relative shuttling movement of target [3]rotaxane in response to external acid-base stimuli was confirmed by 1H NMR spectroscopy. Furthermore, the shuttling motion of the macrocycles in the system was accompanied by a remarkably visual fluorescence change due to the distance-dependent photoinduced electron transfer process from the ferrocene units to the PBI fluorophore. This kind of rotaxane has important potential for designing more sophisticated mechanically interlocked molecules with controllable functions. [ABSTRACT FROM AUTHOR]

The photothermal effect is the generation of heat by molecules or particles upon high-energy laser irradiation, and near-infrared absorbers such as gold nanoparticles and organic dyes have a range of potential photothermal applications. The favourable photothermal properties of thiophene-functionalised croconaine dyes were recently discovered. The synthesis and properties of novel croconaine rotaxane and pseudorotaxane architectures capable of efficient photothermal performance in both organic and aqueous environments are reported. The versatility of this dye-encapsulation strategy was demonstrated by the preparation of two organic croconaine rotaxanes using different synthetic methods: the formation of an aqueous pseudorotaxane association complex, and the synthesis of water-soluble, croconaine-doped silicated micelle nanoparticles. All of these near-infrared-absorbing systems exhibit excellent photothermal behaviour, with pseudorotaxane and rotaxane formation vital for effective aqueous heat generation. Dye encapsulation provides steric protection to enhance the stability of a water-sensitive croconaine dye, while rotaxane-doped nanoparticles avoid detrimental band broadening caused by chromophore coupling. [ABSTRACT FROM AUTHOR]

The development of multicomponent (supramolecular) systems that can perform predetermined functions under external control -- i.e., molecular devices -- is a challenging task in chemistry and a fascinating objective in the frame of a bottom-up approach to nanostructures. In this context light signals can be conveniently used both for supplying energy to the system and for probing its states and transformations. The aim of this tutorial review is to recall a few basic aspects of light-induced processes that can be used to "write" and "read" onto molecular and supramolecular systems. These principles are illustrated through some examples of artificial molecular devices and machines taken from our work, which provide a flavour of current research. They are molecular and supramolecular systems that operate and/or perform valuable functions by exploiting photoinduced energy- or electron-transfer processes, photoisomerization reactions, or photoinduced proton transfer. The choice of these examples was based on both their intrinsic importance for the referred topic and their educational value. In the last section of the review potential applications, limitations and future directions of the research in the field of artificial molecular devices and machines are also discussed. [ABSTRACT FROM AUTHOR]

Abstract: A main-chain hyperbranched polyrotaxane (HBP) assembled by the interaction between dibenzo[24]crown-8 (DB24C8) and dibenzyl ammonium (DBA) was synthesized through Cu(I)-catalyzed azide-alkyne cycloaddition. Energy donors and acceptors were attached to DB24C8 and DBA units, respectively, to investigate the effect of such polyrotaxane structure on the intramolecular and intermolecular energy transfer channel. Our investigations indicated that HBP with higher molecular weight was obtained in relatively lower concentration. AFM, DLS, and SLS analyses revealed that our HBP molecule was described as a ball-shaped nanoparticle with the radius of about 4–8 nm. Benefitting from such unique topology, the aggregation of the chromophores through π-π stacking was effectively suppressed both in solution and in solid state. Meanwhile, the steady-state and time-resolved fluorescence characterization demonstrated that HBP exhibited the excellent energy funneling ability from A3 to B2 unit with high energy transfer efficiency and fast rate, which was attributed to the large spectral overlap between the emission features of the donors and the absorption of the acceptors, and the unique highly branched mechanically interlocked architecture. The energy transfer quantum yield of HBP was calculated to be as high as 99%, and this energy transfer process was estimated to occur with a rate of 7.58 × 109 s−1 with single exponential. [Copyright &y& Elsevier]

A straightforward non-covalent synthetic strategy for producing donor–acceptor rotaxanes is reported. Femtosecond absorption spectroscopy illustrates that the use of this strategy gives rise to supramolecular chromophores with different electron transfer behavior. [ABSTRACT FROM AUTHOR]