1,721,074 research outputs found
THE CONTROL OF THE CHOLESTERIC PITCH BY SOME AZO PHOTOCHEMICAL CHIRAL SWITCHES
No full textA few chiral azo compounds, which undergo reversible photochemical switching, are presented. Of these, the most interesting contain the binaphthyl moiety and belong to the C-2 (derivatives 1 and 2) or C-1 symmetry group (derivatives 3 and 4). These binaphthyl compounds display intense CD and high values. Photochemical switching has profound effects on both the CD and values of these compounds; in the case of compound 3, the sign of beta changes upon isomerisation. Compound 2 has, to our knowledge, the highest P of the switches reported in the literature and also seems the most interesting owing to its fast response to photochemical stimuli. Nematic phases can be transformed into cholesteric phases with reflection bands in the visible region by doping with reasonable amounts of 1 and 2. The reflection colours can be changed reversibly by photoisomerisation of the switches. Thermal reversion of the colourless UV photostationary state to the green isomeric EE state or to intermediate coloured states is temperature dependent. This can allow the thermal history of a sample to be traced
REVERSIBLE INTERCONVERSION BETWEEN A SUPRAMOLECULAR POLYMER AND A DISCRETE OCTAMERIC SPECIES FROM A GUANOSINE DERIVATIVE BY DYNAMIC CATION BINDING AND RELEASE
No full textThe tunable interconversion between two highly ordered supramolecular motifs (G-quartet K+-templated column and G-ribbon) of a lipophilic guanosine derivative fueled by cation complexation and release in a cryptand [2.2.2] containing guanosine solution is reported. The process is controlled by the sequential addition of acid and base
Architetture supramolecolari di guanosine semisintetiche per lo "scaffolding" di gruppi funzionali
No full textSintesi e architetture supramolecolari di derivati lipofili della guanina
No full textSelf-assembly relies on the association of pre-programmed building blocks through non-covalent interactions to give complex supramolecular architectures. Previous studies provided evidence for the unique self-assembly properties of semi-synthetic lipophilic guanosine derivatives which can sequestrate ions from an aqueous phase, carry them into an organic phase where they promote the generation of well-defined supramolecular assemblies. In the presence of cations lipophilic guanosines form columnar aggregates while in their absence they generate supramolecular ribbons. The aim of this thesis has been the synthesis of guanine derivatives, in particular N9-alkylated guanines and a guanosine functionalized as a perchlorotriphenylmetil moiety (Gace-a-HPTM) in order to observe their supramolecular behaviour in the absence of sugar (ribose or deoxyribose) and in the presence of a bulky and chiral substituent respectively. By using guanine instead of guanosine, while maintaining all the hydrogen bond acceptor and donor groups required for supramolecular aggregation, the steric hindrance to supramolecular aggregation is notably reduced because (i.e. guanines with groups in N9 different from sugar are expected to have a greatest conformational freedom even in presence of bulky groups in C8). Supramolecular self-assembly of these derivatives has been accomplished in solutions by NMR and CD spectroscopy and on surface by STM technique. In analogy with other guanosine derivatives, also N9-substituted guanines and GAceHPTM form either ribbon-like aggregates or cation-templated G-quartet based columnar structures.L’auto-assemblaggio è descritto come il processo mediante il quale un sistema disordinato di componenti pre-esistenti forma una struttura supramolecolare, in conseguenza di specifiche interazioni deboli tra i componenti del sistema stesso. Studi precedenti, hanno evidenziato la capacità di auto-assemblaggio di derivati lipofili delle guanosine; tali composti sono capaci di estrarre degli ioni dalla fase acquosa e trasportarli in fase organica mediante la formazione di specifiche strutture supramolecolari. In presenza di cationi le guanosine lipofile formano aggregati colonnari mentre in loro assenza generano delle strutture nastriformi. Lo scopo principale della seguente tesi è stato la sintesi di derivati lipofili della guanine, in particolare di derivati della guanina alchilati in posizione N9 e di un derivato della guanosina funzionalizzato con un sostituente perclorotrifenilmetilico(PTM). Lo scopo principale era osservare in che modo l’assenza dello zucchero in un caso e la presenza di un sostituente chirale con un elevato ingombro sterico (GaceHPTM) nell’altro potessero influenzare il comportamento supramolecolare delle molecole sintetizzate. Utilizzando le guanine invece delle guanosine, non solo vengono conservati tutti i gruppi donatori e accettori di legami a idrogeno richiesti per l’aggregazione supramolecolare, ma viene ridotto l’ingombro sterico che in alcuni casi ostacola la formazione di aggregati (guanine con sostituenti in N9 diversi dallo zucchero possiediono una maggiore libertà conformazionale anche in presenza di un gruppo ingombrato in posizione C8 della guanosina). L’auto-assemblaggio di derivati lipofili della guanina è stato studiato in soluzione mediante spettroscopia NMR e dicroismo circolare mentre su superficie lo studio è stato effettuato mediante STM (scanning tunneling microscopy). Allo stesso modo dei derivati lipofili della guanosina, i seguenti derivati formano strutture nastriformi e colonnari variando le condizioni sperimentali
Solvent-induced switching between two supramolecular assemblies of a guanosine–terthiophene conjugate
No full textWe report here our findings on a lipophilic guanosine derivative armed with a terthiophene unit that undergoes a pronounced variation of its supramolecular organisation by changing the polarity of the solvent. In chloroform the guanosine derivative, templated by alkali metal ions, assembles via H-bonding in G-quartet based D-4-symmetric octamers; the polar guanine bases are located into the inner part of the assembly and act as a scaffold for the terthienyl pendants. On the other hand, in the more polar (and H-bond competing) acetonitrile, different aggregates are observed in which the terthiophene chains are pi-pi stacked in a helicoidal (left-handed) arrangement in the central core, and the guanine bases (free from hydrogen bonding) are located at the periphery and exposed to the solvent. The system can be switched back and forth by subsequent addition of chloroform and acetonitrile. The solvent-induced switching can be easily followed by circular dichroism spectroscopy: the CD exciton-couplet in the guanine chromophore absorption region observed in chloroform disappears after addition of acetonitrile, indicating the disassembly of the G-quartet based octameric structure, while an intense quasi-conservative exciton splitting in the 300-450 nm spectral region becomes predominant in the CD spectrum. This latter strong bisignate optical activity can be ascribed to the helical packing of conjugated terthiophene moieties stabilised by pi-pi interactions. NMR spectra and photophysical investigations confirm the structures of the guanine-directed and thiophene-directed assemblies in chloroform and acetonitrile, respectively
Supramolecular hybrid organic-inorganic multicomponent architectures in solution and on surface
No full textSupramolecular architectures can be built-up from a single molecular component (building
block) to obtain a complex of organic or inorganic interactions creating a new emergent
condensed phase of matter, such as gels, liquid crystals and solid crystal. Further the
generation of multicomponent supramolecular hybrid architecture, a mix of organic and
inorganic components, increases the complexity of the condensed aggregate with functional
properties useful for important areas of research, like material science, medicine and
nanotechnology.
One may design a molecule storing a recognition pattern and programming a informed
self-organization process enables to grow-up into a hierarchical architecture. From a
molecular level to a supramolecular level, in a bottom-up fashion, it is possible to create a
new emergent structure-function, where the system, as a whole, is open to its own
environment to exchange energy, matter and information. “The emergent property of the
whole assembly is superior to the sum of a singles parts”.
In this thesis I present new architectures and functional materials built through the selfassembly
of guanosine, in the absence or in the presence of a cation, in solution and on the
surface.
By appropriate manipulation of intermolecular non-covalent interactions the spatial
(structural) and temporal (dynamic) features of these supramolecular architectures are
controlled.
Guanosine G7 (5',3'-di-decanoil-deoxi-guanosine) is able to interconvert reversibly
between a supramolecular polymer and a discrete octameric species by dynamic cation
binding and release.
Guanosine G16 (2',3'-O-Isopropylidene-5'-O-decylguanosine) shows selectivity binding
from a mix of different cation's nature.
Remarkably, reversibility, selectivity, adaptability and serendipity are mutual features to
appreciate the creativity of a molecular self-organization complex system into a multilevelscale
hierarchical growth.
The creativity - in general sense, the creation of a new thing, a new thinking, a new
functionality or a new structure - emerges from a contamination process of different
disciplines such as biology, chemistry, physics, architecture, design, philosophy and science
of complexity
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