Theoretical Studies On Optical Excitation And Intramolecular Charge Transfer Phenomena In Several Complex Systems

Small organic molecules or polymers with excellent optical properties behave actively among the most important kinds of functional materials. Not only that they can be used as a component in solar cell’s architecture to transfer light into current, or can be used in OLED to transfer voltage power into light, but also that they have been widely used as molecule labels in biomedical studies or for environmental pollution detection. It is people’s tremendous attention and continuous passion on designing and synthesizing unique materials for different usages makes the knowledge about the relationship between structures and properties an important requirment. Density functional theory (DFT) method is believed to be a useful tool in doing such kind of researches as it always provide trustable results. Moreover, since the development of B3LYP functional, together with some other newly established functionals, DFT method have been able to handle the problems in a much more broad fields. The work of this thesis mainly rely on DFT method, doing quantum mechanism (QM) or quantum mechanism combined with molecular mechanism (QM/MM) to study a few interesting systems, trying to reveal the relationship among optical properties, molecular structures and electronic structures. The main context of this thesis can be summarized as follows:1. Geometries, electron structures and optical characters of substituted polycarbazole derivatives Carbazole, like phenylethylene and thiophene, is one kind of basic units in designing photovoltaic or OLED materials. It can be polymerized on2,7-carbons or on3,6-carbons, then the rest positions also can be variously modified. Experimentally, quite different optical properties has been found in triarylborane substituted2,7-polycarbazole and3,6-polycarbazole. After studying the ground state properties and the excitation properties by DFT and TDDFT methods, we found that the bonding or anti-bonding characters of frontier orbitals differ on different polymerizations. Although the orbitals of triarylborane have little effect on the orbitals from polycarbazole mainchain, they might be involved in electron excitation as an electron acceptor, as long as the suitable repetitve number was achevied, resulting in a charge transfer property on excitation. Additionally, the three different functionals we used in calculations from monomer to pentamer, B3LYP, CAM-B3LYP and ωB97X, tells us that the long-range corrected functionals can predict the backbone excitation in2,7-polycarbazole properly, in consistent with experimental observations.2. Study of firefly luciferase-luciferin bioluminescence system Bioluminescence is a quite common natural phenomenon. Among all the species the firefly bioluminescence is the most efficient one. The experimental technique to utilize it is relatively mature and has been broadly used in biological and biomedical researches. However, for better usage people would prefer to tune the fluorescence accordingly, and among which the red fluorescnece is always welcomed, therefore we should be clear about the factors that can influence the fluorescence. With this purpose, we applied QM method with impilicit solvent model (PCM) and QM/MM method in succession to study the fluorescence difference induced by different environment. Our results suggest the complicated luciferase protein system would play a key role in tuning the fluorescnent color and the simplified solvent model can not properly represent such kind of effect, because although there is a slight change in optimized structures from gas phase and protein system, such tiny difference will not be able to change the fluorescence according to TDDFT results. With the assumption of linear contribution in emission spectrum from amino acide residues, we carefully study the residue effect by taking them into consideration both from point charges viewpoint and QM atoms viewpoint, then we found the Coulomb interaction is still the important one. From theoretical results, we could point out some residues with great effect on fluorescence, and we expect they will be validated by experimental studies.3. Electron transfer and energy transfer effect on fluorescence process(a) Dye molecules can be designed as chemsensors to detect specific molecule/ion in solvent or in gas. The basic concept is that the fluorescence of chemsensor can be modified before and after binding to the target molecule. The design stratege of introducing photo induced charge transfer (ICT) process is very useful way in tuning the fluorescence. Based on the experimentally synthesized BODIPY chemsensor for Hg2+ion detection, we analysed the frontier orbital from DFT calculations together with the BODIPY excitation modes from TDDFT, and qualitatively explained the reason for the high selection of Hg2+ion while almost no responses on other metal ions.(b) During recent years, metal nanoparticles, especially the gold nanoparticles has been widely used in biochemistry and biological research, since they are a very effective fluorescent quencher. In the presence of gold nanoparticles, the fluorescence of dye molecule will be tuned according to the distance between them, accompanying with the dynamic motions of biomolecule itself or biochemical reactions. where the excitation energy transfer happens when the fluorescence is quenched. Practically there is no such QM method can handle the size of nanoparticles. Instead, we started from the Au20and Au34clusters to study their excitation modes, as well as the model dye molecule PDI, and FRET theory was used to investigate the energy transfer process dependent on the distance.