Theoretical Study On The Optical Properties Of A Few Of Organic Functional Materials

The material is the cornerstone of the development of human society. Recently, organic functional materials, one kind of new materials that possess excellent photoelec-tric properties, attract a lot of attention, and become a hot research field. The disser-tation is devoted to study the optical properties and aggregation mechanism of organic functional materials with respect to the first-principles methods, molecular dynamics (MD) simulation, and combined quantum mechanical and molecular mechanical meth-ods (QM/MM) calculations. The impacts of aggregation structures, dynamics process and complex enviroment on the optical properties of the organic functional materials have been studied. The main contents of the dissertation are organized as follows.Chapter one gives a brief introduction on the research backgound, the specific objects and the existed problems, and the focus of this dissertation.Chapter two introduces the fundamentals of theoretical methods adopted in our studies, which include the wavefunction-based ab initio methods and charge density-based density functional theory (DFT), the molecular mechanism (MM) and the MD, the combined QM/MM method, etc.Chapter three presents the aggregation-induced emission (AIE) mechanism of dimethoxy-tetraphenylethylene (DMO-TPE) in water solution. We combine MD simulation and QM/MM calculations to investigate the aggregation process of DMO-TPE molecules in water solution, the detailed aggregation structures and mechanism of AIE. The MD simulations show that the aggregates start to appear in nano-second time scale, smaller than the time scale of common biological processes. The small molecular aggregates appear at the low concentration whereas the large aggregates with a chain-type struc-ture appear at the high concentration. The average radical distribution demonstrates that the waters join the aggregation process and that two types of hydrogen bonds between DMO-TPE and water molecules are built with the peaks at about 0.5 nm and 0.7 nm, re-spectively. The spectral features further reveal that the aggregates dominantly present J-type aggregation although they fluctuate between J-type and H-type. The molecu-lar environment play an significant role on the molecular geometries. For the isolated molecule in vacuo and solution, electronic excitation induces evident intramolecular rotation. In the aggregated phase, the molecular symmetry is fully broken down. The molecular condensed phase environment largely restricts the intramolecular rotation, in-creases the transition dipoles of the corresponding emission process, and subsequently enhance the emission efficiency. The statistical absorption and emission spectra agree well with the experimental measurements.Chapter four presents the studies on the mechanisms of unusual large Stokes shift and aggregation enhanced emission (AEE) of osmapentalyne cation. Osmapentalyne cations synthesized recently show remarkable optical properties, such as near-infrared emission, unusual large Stokes shift and aggregation-enhanced emission. Here, the mechanisms behind those novel optical behaviors are revealed from the combined MD simulations and QM/MM calculations. The results demonstrate that the large Stokes shift in gas phase comes from an photoexcitation-induced deformation of the osmium ring plane, whereas in solution it corresponds to the variation of osmium ring sym-metry. Although the central chromophore ring dominates the absorption and emission processes, the protecting groups PPh3 joins the emission. As osmapentalyne cations are aggregated together in solution, the radical distribution function of their mass-central distances displays several peaks immersed in a broad envelope due to different aggrega-tion pathways. However, the chromophore centers are protected by the PPh3 groups, the aggregation shapes do not affect the Stokes shift too much, and the calculated aggregate-enhanced emission is consistent with experimental measurements.Chapter five presents the studies on the optivcal properties of matel-free phthalo-cyanine (H2Pc) in the gas phase, solution and film. The studies revealed that in the gas phase, the cis-trans isomerization reaction can easily take place, and both the trans-H2Pc and cis-H2Pc contribute to the experimentally-measured optical properties. While in the tetrahydrofuran (THF) solution, only the trans-H2Pc has the dominant contribution to the optical properties. In the film, the aggregation effect and intermolecular charge transfer play an important role on the optical properties. At the optimized excited-state geometry of H2Pc dimer, the charge transfered between two monomers reaches to one unit of electron’s charge. The H2Pc trimer affects the absorption and emission energies a little compared with the dimer. The aggregation effect plays an important role in the spectra, and the position and lineshape of peaks are both affected obviously. The statis-tical absorption and emission spectra agree well with the experimental measurements.