Photophysical Properties And Sensing Applications Of Aromatic End-capped Oligothiophenes

a-Oligothiophene and its derivatives have attracted much attention in the field of organic functional material and material science because of their well-defined, easily adjustable and controllable molecular structures, and remarkable electronic properties. Additionally, it also has advantages as strong absorption, high quantum yield and large Stokes'shift characterized by a good sensing fluorophore. Extensive studies found that this kind of fluorophores is photochemically unstable in sensing, particularly when they are immobilized onto substrate surfaces, which greatly limited its applications. Obviously, the first challenge we encountered in the work is to improve the photochemical stability of the oligothiophenes. The functionalization in its α-or β-position with a variety of aromatic moieties have been reported to adjust the energy gap of the compounds and increase their photochemical stabilities. This method has been widely applied in fabricating oligothiophene functionalized devices in the area of organic light-emitting diodes (OLEDs) and field-effect transistors (FETs).The development of fluorescent chemsensors has been an active research topic for many research fields. A fluorescent chemsensor realizes the sensing functions based on the change of sensor-relative fluorescence intensity and emission wavelength of them as signals. A typical fluorescent chemsensor includes three building units, namely, a receptor responsible for the molecular recognition of the analyte, a fluorophore responsible of signaling the recognition event and a linker between them. Compared with physical method, the self-assembled monolayers functionalized fluorescent film sensor, where the sensing element is immobilized on substrate surface via chemical bonds, can avoid the leaking of the chemicals and the contamination to the analytical system. In addition, the films prepared by chemical ways have shown several advantages of highly stable, re-useable, non-consumptive, and easy to be made into devices, etc. Design, synthesis and development of novel fluorophores for sensor construction are especially important to the sensitivity and selectivity of the fluorescent film sensors. On the basis of the considerations mentioned above, combining the reviews on chemically modified self-assembled fluorescent films and the current status of oligothiophenes, several film sensors were designed and prepared by immobilizing a-oligothiophene and its derivatives on glass slide surfaces via different flexible spacers. The obtained sensing films show high sensitivity and selectivity to formaldehyde vapor, nitro-containing explosives in the vapor phase and aqueous phase. Furthermore, their remarkable sensing ability, high sensitivity, ideal reversibility and long lifetime make them worthwhile to be exploited further. We also designed and synthesized several symmetrical aromatic end-capped oligothiophene derivatives and2,2':6',2"-terpyridine functionalized oligothiophene, which can be used to detect nitro-containing explosives and metal ions in aqueous solution, respectively.There are four parts in this thesis. The first part is about the reviews on chemically modified self-assembled fluorescent films and photophysical properties of oligothiophenes. The other three parts are about the experimental researches based on oligothiophene and its derivatives. More concretely, we get the following main results.In the second part of this thesis, novel fluorescent sensing films were fabricated by monomolecular layer assembly of oligothiophene onto an amine-terminated glass wafer surface. Contact angle, X-ray photoelectron spectroscopy (XPS) and steady-state fluorescence measurements demonstrated that the fluorophore was successfully immobilized on the substrate surface. The fluorescence emission of the film, however, is not stable in air, and it decreased along with increasing scanning number. Continuous irradiation of the film with UV-light decreased, but stabilized the fluorescence emission of the film. Exposure of the specially treated film into formaldehyde (HCHO) vapor generated a new fluorescence emission, which appeared in a shorter wavelength (458nm) in comparison with that of the original one. The intensity of the emission increased along with increasing the exposure time, as an example for2,2':5',2"-terthiophene (3T) functionalized sensing film, the intensity can be increased more than18.6times in30min. Furthermore, interference experiments revealed that the sensitization process is selective, and solvents including common acids, bases and alcohols have little effect upon the process. More interestingly, the process is reversible. We also proposed the possible sensing mechanism based on abundant blank tests and control tests. Accordingly, it is believed that the oligothiophene functionalized film should be a strong candidate for developing a novel and sensitive HCHO fluorescent film sensor.In the third part of this thesis, aromatic end-capped oligothiophene derivatives were synthesized and chemically immobilized onto a glass wafer surface via flexible spacers by employing a single-layer chemistry technique. Unlike the film fabricated in the same way but with3T as the fluorophore, the film fabricated in the present study possesses unprecedented photochemical stability at ambient conditions. Fluorescence studies revealed that the emission of the film as fabricated is significantly and selectively quenched by the presence of nitro-containing explosives both in the vapor phase and in aqueous solution. Experimental and theoretical studies demonstrated that the quenching may be a result of electron transfer from the electron-rich aromatic end-capped oligothiophene derivatives to the electron-deficient nitro-containing explosives. It was found that the response time and the quenching efficiency of the systems are dominantly determined by the vapor pressures of the nitro-containing explosives tested for the vapor phase sensing. The sensing performances of the film to nitro-containing explosives in aqueous phase were also investigated. In this case, however, the specific binding of the film to picric acid (PA) makes the compound show a superior quenching efficiency than other nitro-containing explosives. Moreover, the response is fast and reaches equilibrium within90s. Furthermore, acids, bases, apple juice, perfume and some commonly found organic solvents show little effect upon the sensing process. Both the vapor phase sensing and the aqueous solution sensing are reversible. Furthermore, the film is stable for at least6months provided it is properly preserved. The basic contribution of the present work is not only creating a new fluorescent film of superior sensing properties to nitro-containing explosives in the vapor phase, in particular to PA in the aqueous phase, but also providing a series of novel photochemically stable fluorophores, which may combine the advantages of small molecular fluorophores and those of conjugated polymers/oligomers, for developing new fluorescent sensing films. At the same time, a series of novel π-electron rich fluorophores which differ by the terminal units of3T have been synthesized and found that all the fluorophores are sensitive to the presence of nitro-containing explosives, in particular, PA and3,5-dinitro-2,6-bispicrylamino pyridine (PYX). On the basis of their structure-property relationships, a mechanism was proposed to explain the possible interactions between the compounds as prepared and the nitro-containing explosives under study. Considering the optical and sensing properties of these novel π-conjugated aromatic end-capped oligothiophenes, it is anticipated that they can be developed into fluorescent sensory materials for real applications.In the fourth part of this thesis, it is reported that design and synthesis of an oligothiophene derivative modified by2,2':6',2"-terpyridine unit as the fluorescent binding group. The photochromism of the free ligand and the influence of metal ions on the photochemical properties have been thoroughly characterized by monitoring the changes in their fluorescence emission spectra. Furthermore, the fluorescence results show that this kind of material can be used for selective and sensitive detecting of metal ions. The chemical structure of the product was identified by fluorescence, UV-Vis, FT-IR, MS and NMR. The photochromic properties of the ligand have been found to be strongly influenced by the bridging transition metal ions, the results indicate that Cd2+and Zn2+can interact with the fluorophore, make the fluorescence emission red-shifted and the fluorescence intensity increased for Cd2+and decreased for Zn2+, respectively. Metal ions such as Co2+, Cu2+, Ni2+, Pb2+Pd2+and Hg2+can effectively quench the fluorescence intensity. Whereas, Na+, K+NH4+, H+, Mg2+, Ba2+, Fe3+, OH-and H2O do not produce obvious fluorescence changes of the ligand. It is worth noting that the optical properties of π-conjugated oligomer incorporating2,2':6',2"-terpyridine as recognition site can be greatly improved and adjusted by functionalized with terpyridine units and further coordinated with metal ions.The key contributions of this study can be generalized as follows:(1) We employ α-oligothiophenes as the sensing fluorophores, deeply study their fluorescence properties and broaden their applications for the fabraction of fluorescent self-assembled monolayers.(2) We firstly discover the special interactions between UV-light treated fluorescent sensing film based on oligothiophene and formaldehyde vapor, propose the possible sensing mechanism and realize a new sensing way for detecting formaldehyde vapor.(3) In order to enhance the photophysical stability of a-oligothiophenes, we design and synthesis several novel aromatic end-capped oligothiophene derivatives and obtain excellent fluorescent sensing materials for nitro-containing explosives in the aqueous and vapor phase.(4) A new fluorescent chemosensor, using oligothiophene derivative modified by2,2':6',2"-terpyridine unit as the binding group is fabricated. The fluorescence results show that this kind of materials can be used for selective and sensitive detecting for metal ions. The optical properties of oligothiophene derivatives can be greatly improved by functionalized by terpyridine unit and further coordinated with metal ions.