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1.4 CHEMICAL SCIENCES
Ligands with a macrocyclic host unit attached to a photoactive metal core are of interest across a range of applications from luminescent or electrochemical sensors, light harvesting and energy-collection purposes. Their versatile properties and possibility of associating cyclodextrins with other molecules both covalently and non-covalently mean that these macrocyclic ligands can be employed in the design and the formation of supramolecular complexes. A particularly attractive proposition is to combine the selective inclusion capabilities of CD with luminophores which may act as reporters of binding or other interactions at the cyclodextrin cavity. Ruthenium, osmium, and iridium polypyridyl complexes are particularly attractive reporters in this regard because of their visible emission and their useful redox properties. Their long lived excited states are particularly attractive providing significant scope for interaction with an included species compared with organic fluorphores, which can be very insensitive to their environments due to their fast radiative decays. In this thesis, we exploited the properties of CD and ruthenium polypyridyl complexes in designing photoactive metallocyclodextrin where 5-amino-1, 10-phenanthroline has been attached to the primary side of β-cyclodextrin. This ligand. β -CD-aphen has been used to design [Ru(bpy)2(aphen-CD). [Ru(bpy)2(aphen-CD) ][PF6]2 exhibits an intense luminescence ascribed to a 3MLCT state at room temperature in DMF and water with , λmax at 618 nm, a lifetime of approximately 1 μs and quantum yield of φ= 0.013 in deaerated solution. The secondary side of cyclodextrin is still available. We exploited the idea that metals like Copper and Zinc can be coordinated to the secondary side in constructing of molecular assembly where the donor and acceptor are held together via covalent link through a bridge. We have designed a trinuclear metallocyclodextrin-based donor-acceptor complex where the dicopper (II) sites are hydroxo bridged to the secondary side of β-CD in [Ru(bpy)2(aphen-CD)]. The donor and acceptor in our system are linked via a β-Cyclodextrin On the account of their relatively hydrophobic interiors; CDs have the ability to form inclusion complexes with wide range of Substrates. This idea has been exploited to bring the donor and the acceptor in a close proximity and study the energy and/or electron transfer in these novel supramoluclar assemblies. The effect of anthraquinone and anthraquinonecarboxcyclic on the luminescence of ruthenium was investigated in chapter three. In addition we demonstrate, for the first time, that the luminescence quantum yield and lifetime of Ru (II) polypyridyl center shows remarkable sensitivity to pH and we present detailed pH titrations for luminescence intensity and lifetime. The range of pH response is extremely broad, from pH 1 to pH 13 and alterations to luminescence quantum yield of greater than 60% are observed. In chapter 6 we have examined photoinduced transfer process between Ru and Ir metal centers in water mediated by non-covalent interactions through a cyclodextrin cavity. On the basis of the inclusion ability of β-CD and the redox properties of pyrene we have studied Ru-Ir interaction in trimer [Ir-(RuCD)2] in aqueous solution. Formation of the Ru-Py-Ir trimer through cyclodextrins induces significant changes in the photophysical behavior of both pyrene and appended Ruthenium. We couldn’t detect a significant change in the photophysical of Iridium metal centre. This means that communication between the two metal centers is very weak. This behavior may be attributed to the presence of pyrene moiety which blocks the interaction between the two metal centers. In [Ir-(RuCD)2] pyrene acts as a store for the energy instead of mediating the transfer process. Future work on these complexes will include further studies on the nature of the electron transfer process. This will include, for example, more detailed transient absorption spectroscopy to discern the direction of transfer either Ir→ Ru or Py→Ru or both. We then extended our work to make homonuclear photoactive cyclodextrins dimers. These dimers comprised of two photoactive metal centers covalently attached to the primary side of γ-cyclodextrin. In chapter 5, we exploit the inclusion ability to design a tetrameric metallocyclodextrins containing photoactive Ru(II) polypyridyl units covalently bonded to γ-CD. Then the cyclodextrin complexes were self-assembled with a fullerene moiety in 2:1 ratio to produce the tetramer. The synthesis and characterization of the complexes and in particular their spectroscopic electrochemical and photophysical properties described. We provide evidence for photoinduced processes and discuss the possibility of electron and energy transfers in these novel supramolecular assemblies.
Atmeh, M.: Photoactive Supramolecular Cyclodextrin Assemblies. Doctoral Thesis. Dublin Institute of Technology, 2008.