Molecular Dynamics Study On The Structural Properties Of Monolayer Fluorescent Thin Films

In the trace detection of nitro compounds, the fluorescent self-assembled monolayer film sensor has a lot of advantages and its performane are mainly dependent on its fluorescent behavior. Experimental results have showed that the fluorescent behavior of monomolecular layer fluorescent film sensor is closely related to its microstructure. A fundamental understanding of the structure-related properties of the film requires the solid evidences to confirm the experimental indication. Unfortunately, the thickness of the monomolecular layer chemistry-based fluorescent film is less than a few hundred angstroms, and the structural information is very hard to "see" or obtain, which is always far beyond the chemical resolution methods or other physical anlysis. Hence, the fluorescent behavior of the monomolecular layer fluorescent film is alternatively studied by the method of molecular dynamics (MD) simulation, which has been quickly developed in the recent decades and has become a powerful tool for study the structural characteristics of the thin films of this kind. Thus, based on the previous experements and combining with all the experimental resulsts, we employ MD simulation method to reveal the physical mechanism of the excellent optical behavior of monolayer fluorescent films induced by the structural modification. The main results of this dissertation are listed as follows:1. The solvent effects on the structure of the conjugated polymers-functionalized fluorescent films with side-chain and without side-chain modifications are studied by MD simulations. The simulation results show that the fluorescent molecules with side-chain have more superior performance on fluorescent responses, which is due to the stronger reaction of the side chains to the solvents, leading to obvious structure modifaction compare to that of the fluorescent molecules without side chains. The fluorescent molecules of M-PPEs (2,5-dihexadecyloxy-phenyleneethynylene) on SiO2substrate prefer to aggregate in a dry state, but are more likely to stay in a monomolecular state when the good solvent THF is added. The simulation shows that there is no π-π stacking between benzene rings, but the intermolecular hydrogen bonds between H (N) and O (ether), H (C) and O (ether), as well as H (benzene) and O (ether) pairs are clearly shown. However, only the populations of the H(C)-O(ether) pairs and H(benzene)-O(ether) pairs change with the introduction of solvent, showing an observable decrease of the population of hydrogen bonds between them. In conclusion, the aggregation and the separation of the molecules are mainly controlled by the contraction and extension of the side chains connected to the PPEs backbones. The dipole-dipole attraction between the side chains is mainly responsible to those changes. The weak van de Waals force results in the out spreading of the side chains in the presence of tetrahydrofuran molecules(THFs). At some proper concentration of the solvent molecules, the side chains may stretch out, and at some particular concentration, the side chains are even perpendicular to the backbones of the molecules.2. The MD simulation of fluorescent properties on pyrene-dilL-pyrene (PDP) phosphor in water and methanol are carried out in chapter4. The simulation results show that, at equilibrium state,(1) the ring-to-ring distance (within6A) between pyrene rings of the PDP phosphor molecules in methanol is more disordered than in water;(2) the center-to-center distance of the neighboring pyrene ring pairs shifts from-4.1A to-6.0A. The numerical evidence clearly indicates a population decrease of pyrene excimers and thus reduces the excimer emission of the phosphor, which greatly supports experimental results. By introducing trinitrotoluene (TNT) molecules in water and methanol respectively, MD simulation reveals that (1) the concentration of TNT in water is higher than that in methanol due to its molecular polarization;(2) the inserting of TNT molecules between pyrene rings destroy the proper distance that is suitable for the formation of pyrene excimers, and tend to form a TNT-pyrene complex with pyrene molecules instead. The dynamical results suggest a significant decrease of excimer emission of phosphor in water than that in methanol. The MD simulation is consistent with our experimental observations, ie., after adding TNT into these two systems, the fluorescent quenching efficiency in water is much higher than that in the methanol. Therefore, the conformal variations of the PDP phosphors and the molecular distributions of TNT in water and in methanol may result in the changes of the photophysical property of such phosphors.3. Experimentally, the fluorescent intensity of the pyrene-functionalized fluorescent film is reduced by introducing nitrobenzene. In order to reveal the mechanism of this quenching effect, another molecular dynamics simulation is adopted in chapter5. The MD simulation clealy reveals that, in the vacuum environment, the center distance between a pair of pyrene rings distributes from about4A to10A, and the most probable distance populates at5.3A, which is a suitable distance for excimer emission. The adding of nitrobenzenes results in a decrease of the population of pyrene molecules within that distance, indicating a decrease of excimer emission of the film. In addition, the pyrene molecule rings in the film prone to adopt a quasi-coplanar structure in the vacuum, but less likely arrange into a coplanar structure when nitrobenzenes are added. Moreever, the structural changes are mainly induced by inserting the incoming nitrobenzene molecules into the previously coplanar pyrene rings. Finally, the analysis of the length and the orientation of molecular chains show that the flexibility and the spatial order of the side chains are declined when nitrobenzenes are introduced. Therefore, the structural modification of the pyrene molecules in the film is a main reason for the fluorescent quenching effect.4. The MD simulations on the oligo(diphenylsilane)-functionalized fluorescent film in different environment are carried out in chapter6. By simulating a relative large system for a longer sampling period, useful results have been obtained to give a quantitative analysis beyond the chemical method. Firstly, the conformation variations of the oligo(diphenylsilane) molecules in toluene and the oligo (diphenylsilane)-functionalized fluorescent film in dry states are directly "observed" in phase space. In order to simulate a real device in a more practical environment, two kinds of molecular dynamical systems are constructed. One is a pure system with an isolate monomolecular layer fluorescent film; the other monomolecular layer film is mounted on the SiC>2substrate. An interesting feature of the two systems is that the first peak of the radial distribution function (RDF) for the two systems appears at almost the same position, which confirms our previous prediction that chemical immobilization does not affect the fluorescence properties of the oligomer. Then, we analyze the RDF of the centriod-to-centriod of the oligo(diphenylsilane) molecules. The calculation of RDF in the absence and the presence of44TNT molecules (corresponds to a concentration of in the experimet) shows that the maximum peak shifts from a.c.11.4A to12.1A when TNT molecules are added on the film, suggesting that the oligo(diphenylsilane) molecules on the SiO2substrate are scattered by TNT molecules. Moreever, the incoming TNT molecules prefer to insert in rather than adsorb on the oligo(diphenylsilane) molecules and form a complex with them. This indicates that the immersing TNT molecules stretch the molecular chain and reduce the interchain interaction which decreasing the eximer fluorescent emission and quench the fluorescence of the oligo(diphenylsilane)-functionalized fluorescent film. All the MD simulation results are basically consistent with the experimental observations as reported earlier.