Three novel imidazole-based low bandgap polymers (P1-P3) were synthesized. Due to their donor-acceptor conjugations, these polymers showed excellent photo-physical and electrochemical properties (such as brilliant fluorescences, low bandgaps, large strokes shifts and low HOMO levels), which led them to be structurally stable semi-conducting molecular wires to act as promising transduction materials for chemosensory applications. Herein, the imidazole-based polymers showed remarkable sensing capabilities towards H+ and Fe2+ in semi-aqueous solutions. However, polymer P3 showed an unique sensitivity response compared with polymers P1 and P2. Upon titration with H+, polymer P3 showed reduced abosorption as well as fluorescence intensities due to the static quenchning. Whereas, polymers P1 and P2 showed hypsochromic shifts of absorption and PL maxima with enhanced fluorescence intensities under similar conditions. Compared with P1 and P2, the anomalous behaviour of P3 was proven via the computational analysis representing the electron densities over HOMO and LUMO of P1-P3 before and after the complexation with H+. Recoveries of their original fluorescence andabsorption were achieved by adding TEA.
Furthermore, P3 showed the best sensing ability to Fe2+ among P1-P3 due to the larger molecular wire effect. Correspondingly, in the presence of Fe2+, the fluorescence lifetime of P3 was extensively decreased (almost 11 times) compared with those of polymers P1 (4.6 times) and P2 (6.2 times). 1H NMR titrations with Fe2+ revealed the exceptional behaviour of P3 compared with P1 and P2. Recoveries of quenched fluorescences were achieved by adding Na2-EDTA/phenanthroline. Thus, imidazole-based polymers can act as efficient chemosensing materials in terms of selectivity, sensitivity and reversibility via the on-off-on fluorescence protocol for future environmental and biological applications.
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Chapter 5
5.1 Introduction:
The design of supra molecular assemblies in which the functional properties have the consequence of the tailored structure of the molecules are of growing interest in current research. Especially oligopyridyl ligands and their transition metal complexes have found ample applications as active materials in selfassembled molecular devices, electroluminescent materials and for storage applications in molecular electronics and photonics, luminescent sensors in molecular biology and medical diagnostics, in photocatalysis etc.117 Directional and effective electron and energy transfer could be achieved by the design of suitable multiple ligands and its complexation with transition metal ions. Among the N-heterocyclic ligands, the remarkably higher affinity of 2,2’:6’,2”-terpyridine towards transition-metal ions due to dπ-pπ* back-bonding of the metal ions to pyridine rings and a chelation effect make them useful for supramolecular construction. Compared with other transition metal ion complexes, due to the high binding strength of RuII to the terpyridine moiety, these complexes show a remarkable stability and can only be cleaved under extreme conditions like low pH value, high temperature, addition of strong competitive ligands.118 Suitable π-Conjugated substituents at the 4’-position of the 2,2’:6’,2”-terpyridine unit exhibit intriguing spectroscopic and redox properties which cause effective electronic communication between the metal and π ligand. Furthermore, their photophysical, electrochemical, and magnetic properties are strongly influenced by the nature of the π-Conjugated moiety attached to terpyridine.119 Furthermore, the electronic communication between the metal-complexed terpyridines with the attached π-Conjugated moiety is another fascinating feature directing their potentials for the design of new supramolecular architecture. Recently, terpyridyl Ru(II) complexes have got interest for the researchers to investigate their photovoltaic cells (PVC) application.120 Again, dyadic molecules comprised of a 4’-substituted terpyridine complex with ruthenium(II) and terpridine has been studied as specifically active candidates in organic
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photovoltaic cells.121 These systems have got extensive attention due to their very long-lived metal to ligand charge transfer (MLCT) excited states and high molar extinction coefficients in the visible range. In these dyads, the excited state that is generated upon the absorption of light leads to a charge separated state with high efficiency. During the synthesis of dyadic polymer, the polydispersity arising due to chains of various sizes and different molecular weights can extensively amend the carrier mobility when used in OPV devices.122a,b Therefore, monodispersed materials are desired for an efficient charge transport and device efficiency in organic photovoltaics applications because of their aptitude to control the morphology of blends.122c In this esteem, conjugated dendrimers offer an alternative to the conjugated polymers to be used in organic photovoltaics122d because of their following advantages. Dendrimers (1) possess well-defined molecular weight with monodispersity; (2) have shape persistency, which allows them to maintain structure in a solution-processable form; (3) can be synthesized with high purity compared to polymers. (4) an internal local electric field may be created during the charge transfer from the perifery to the core of the dendrimer. In addition, conjugated dendrimers are expected to show a high degree of ordering in OPV devices due to their small size and monodisperse nature. To the best of our knowledge, however, there are only a few reports on the applications of dendrimers in organic solar cells.123 Due to the three dimensional hyper-branched structure, the branches become denser with increasing distance from the core, which produces shell effect in denrimers.124 Thus, a high-generation dendrimer has highly dense branches towards the outer surface, which acts as a barrier to restrict the charge transfer between the inner and outer part of the dendrimer. Thiophene dendrimers has recently got promising attention for the application in photovoltaics. Terpyridine cored dendrimers contain metal binding terpyridine site to coordinate metal ions into it.
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