Supramolecular Complexes:Chemosensors And Ruthenium-based Dye Sensitizers,Synthesis And Their Applications

The emergence of supramolecular chemistry has opened doors to many areas of scientific researches. The nature of this field is not only suggesting that chemical processes be spontaneous, but directly suggesting scientific researchers to review their methodology approaches to how they conduct syntheses and designs of reaction intermediate steps. The basic reaction mechanisms in supramolecular chemistry for complexes formation are mostly via self-assembly or self-association, the methods well-known in favored natural processes such as photosynthesis. The thesis presents seven complexes, chemosensors (1-5) in Part I and ruthenium bipyridyl-based dye sensitizers (6,7) in Part Ⅱ.The first part (Part Ⅰ), supramolecular chemistry is used as a guiding principle in chemosensing of ionic species in organic environments. The ever increasing of ionic species in biological and environmental systems, mostly caused by human actions due to raising in daily needs, has prompted researches towards new analytical methods, replacing old ones, to detect and monitor the concentration levels of ionic species in respective systems. The thresholds of ionic concentrations must always be maintained in any system for effective function. The concentration of anionic and cationic species particularly in biological systems must always be consistently monitored and maintained because any slight disturbance in metabolic homeostasis leads to abnormalities like conditions and ailments.The colorimetric anion sensor (complex1) based on phenylhydrazone was designed, synthesized, characterized and finally analyzed. The sensor can selectively discriminate biological important anionic species of acetate (AcO-), fluoride (F-) and partially dihydrogen phosphate (H2PO4-) ions among others. The discrimination was not only observed through UV-vis spectral shifts, but importantly by visible colour change detectable via naked eye observation without any instrumentation. While the limit of detection (LOD) of complex1was very excellent comparing to the current standard-set yardsticks or controls, the colorimetric activities detectable by naked eyes is just a much required value addition. The sensor (complex1) is well in accordance with the basic standard set of an efficient colorimetric sensor, that it must be easy to prepare, real time response and easy to apply. However, the uniqueness of complex1which differentiates it from other anion sensors is its logic functions. Upon investigating the logic operation functions, complex1displayed a remarkable reversible behaviors both in colour and spectral shifts, caused by the addition of F-and reversed by the molar additions of3d5-10, CdⅡ, HgⅡ, MgⅡ. The system recovery is in accordance with the rare logic operations based on two complementary inputs IMPLICATION/INHIBITION (IMP/INH) logic functions, modulated by F-/M (Ⅱ), M=3d5-10, CdⅡ, HgⅡ, MgⅡ. Logic operations were studied further, and it was concluded that complex1can be used as a molecular logic gate based on two input functions for ON/OFF system controls such as in keypad locks.Since it is highly likely to locate anions and cations within each other’s proximity, their activities are closely related, therefore must be treated as such. A fluorescent cation sensor (complex2) was designed, synthesized, characterized and analyzed. The sensor was based on mixed valence complex (complex2), of iron(Ⅱ), ferrocene based, and a d8based, platinum(Ⅱ) alkynyl complex. The sensors was selectively sensitive to the presence of silver ion (Ag(Ⅰ)) only, even when other closely related metal ions such as mercury ions were present. The chemosensing properties were propagated by the combinations of Fe(Ⅱ)/Fe(Ⅲ) redox behaviors and the optical/redox properties of platinum(Ⅱ) alkynyl complexes. The detection was observed through UV-vis spectra changes, but more pronounced by luminescent quenching.Apart from complex2, which can only discriminate Ag(Ⅰ), other fluorescent cation sensors (complex3,4and5) were synthesized and analyzed. The three sensors are all based on two ferrocene moieties conjoined together by a π-conjugated system of different benzene rings with/and disulfane bridges. The complexes were synthesized by a classic method of Schiff bases, through the condensation reaction of condensation of H2N(R)NH2amine,(R is bridging unit) with the carbonyl group of ferrocenyl aldehyde. All the sensors displayed sensitivity towards mercury (HgⅡ) and copper (CuⅡ) ions in organic media. The sensitivity and selectivity was through both colorimetric changes and spectrally (UV-vis and fluorescence). The addition of any of the metal ion saw the colour change from yellow to orange for each of the three sensors (complex3,4and5), this was concomitant with UV-vis spectral shifts. In addition to the two observations, both sensors experienced fluorescent quenching upon the molar additions of different quantities of HgⅡ or CuⅡ. Ferrocene based sensors have been explored in literature, but not by way of Schiff base synthetic methods, which seems to possess a specially affection towards HgⅡ or CuⅡ only. Electrochemistry studies were also studied through voltametric titrations of differential pulse voltametry (DPV), where upon the molar addition of cations displayed significant shifts respectively. This research project is still a work in progress, however, the major properties of colorimetric and fluorometric were all investigated and thereby presented.The second part (Part Ⅱ) of the thesis introduces the photocatalyst properties of two supramolecular systems containing ruthenium centers, and their applications in photocatalytic splitting of H2O for H2production using visible light irradiation. It is a widely known fact that the current sources of energy, the fossil fuels, are non-renewable, and predictions are rife that the current reserves are likely to run dry within a foreseeable future. In addition, the circumstances surrounding the energy resources like coal, natural gas and oil and their uneven distribution had significantly shaped the our history and the world. The problem is not only this, but with the issue of global warming being accelerated by the emission of greenhouse gases, for which carbon dioxide is the by-product of burning fossils, there is a great urgency for the world to start working towards the alternative solutions. Therefore it is imperative that people come up with alternative solutions, hopefully, sustainable, environmental friendly and renewable source of energy that can cater to the needs of the fast growing world and its subjects.Since the sunlight provides an abundant amount of energy to the earth’s surface, it is high time humans start coming up with ideas on how to make use of this free energy. Water is one of the abundant free natural resource available in plentiful, and at neutral pH and25℃, water can split into H2and O2via a multi-electron pathway that requires about1.23V. The sunlight contains more than the required energy to drive this thermodynamically uphill, multi-electron reaction, which even boiling water temperature is not enough. Notably, the sunlight reaching the earth surface directly absorbed by the water has no enough/required energy for such functions, therefore systems must be designed to efficiently absorb light and deliver the appropriate charges to H2O for H2evolution. Ruthenium bipyridyl complexes have been tested and used as dye sensitizer solar cells. The chemical properties of the ruthenium bipyridyl complexes are suitable for this function because unlike water, their photoexcitation (HOMO to LUMO) is achieved using sunlight or rather visible light radiation, high enough to oxidize water. Therefore, a ruthenium complex is used to initiate a process which will supply charges with required amount of energy enough to split the water. In this chapter, two supramolecular systems, complex6which contains a double core of ruthenium bipyridyl bridged by a π-electron conjugated system, and complex7which only contains one ruthenium bipyridyl core, have been synthesized, characterized and analyzed. Their applications towards water oxidation into H2and O2has been investigated and respectively compared with each other.