Fluorescence Behavior Of Aromatic Compounds Immobilized On A Substrate Surface: A Monte Carlo Simulation

Assembly of molecules on thin films to form highly ordered architectures is one of the important approaches to prepare functional films. In addition to other advantages, molecules in assembled state are much easier to be made into devices. Thus, a series of sensing films based upon the supra-molecular behavior of aromatic compounds immobilized on substrate surfaces and its dependence to the composition of the medium have been successfully prepared recently. It was found that the properties and performances of the films depend upon various factors, including the nature of the sensing element (the molecule assembled), the way of immobilization, the nature of the substrate, and the density of the sensing element immobilized etc. To accelerate and deepen the studies, it was decided to carry out theoretical simulation of the sensing films based upon the proposed principle. All the work included in this dissertation is based upon the idea mentioned above.In the first section, the fluorescence behavior of a functional plate, of which the surface is functionalized by single layer immobilization of aromatic compounds (sensing elements), is simulated by Monte Carlo simulation method. The effects of the immobilization density of a sensing element, the excitation efficiency, and the association efficiency, P, which is a characteristic parameter of an aromatic compound, between the molecules of the sensing element on the ratio of the excimer emission to the monomer emission of the plate have been systematically examined. But the effects of the length and flexibility of a spacer are neglected, because in this simulation aromatic compounds are immobilized on a plate by short stiff spacer. It was demonstrated that, for a functionalized plate of a short stiff spacer in a polar solvent (in this case, the association tendency is strong, and P is close to 1), aromatic compounds of middle excitation efficiency might be suitable as sensing elements. In addition, the immobilization density of the sensing element should be controlled. A recommended value is about 50%.In the second section, the fluorescence behavior of a function plate, of which thesurface is functionalized by single layer immobilization of aromatic compounds (sensing elements), is simulated by Monte Carlo simulation method. The effects of the immobilization density of a sensing element, the excitation efficiency, and the association efficiency, P, which is a characteristic parameter of an aromatic compound, between the molecules of the sensing element on the ratio of the excimer emission to the monomer emission of the plate have been systematically examined. Different to the first section, the effects of the length and flexibility of a spacer are considered, because in this simulation aromatic compounds are immobilized on a plate by a flexible spacer and the length of the spacer are adjustable. It was demonstrated that the characteristic parameter P of a given sensing element has limited effect upon the film's IE/IM ratios, for a sensing film of spacer length of 3-12, a IE/IM value between 0.5-2 may be obtained only when the density of the sensing element immobilized on the inert substrate is controlled between 30-70%. It is to be noted that lower photon excitation efficiency corresponds to a greater IE/IM values. But, too low excitation efficiency means weaker fluorescence, resulting in difficult in measurement, thereby, excitation efficiency should not be too low.