| Photochromic materials have received a great attention because of their potentials in various applications including optoelectronics such as optical memory, switching, and display devices. Diarylethene is one of the most well-knows and extensively investigated classes of photochromic molecules because of its excellent thermally stable and fatigue resistant properties required for optoelectronic applications. The photochromic reaction of diarylethenes is based on the reversible ring-closing and ring-opening reactions. Upon irradiation with UV light, the opening isomer is converted into the closed-ring isomer, which has a new absorption band in the visible wavelength region. The closed-ring isomer reverts to the open-ring isomer upon irradiation with visible light. During the photochromic reactions, various physical properties such as refractive index, optical rotation, magnetic interaction, IR peak intensity, redox potential, and fluorescence as well as the absorption spectrum are changed. From the viewpoint of the optical memory and swirching applications, the change in the physical properties can be utilized to achieve a non-destructive readout. To optimize those physical properties for the purpose of a practical device application, many researches have been used to achieve this goal.More than a decade ago, Irie and Lehn were the first authors to investigate the photochromic characteristics of diarylethene (DA) derivatives. These molecules are indeed excellent candidates for molecule-switch applications, as they can be found in closing-ring and opened-ring forms presenting very different electronic properties, and the conversion between these states is allowed by irradiation at different wavelengths. DA derivatives are unconjugated and colorless in opened-ring form but conjugated and colored when in the closed-ring state. As several DA compounds are thermally stable, able to undergo many photochromic cycles, and present large quantum yields of conversion as well, they have been the interest of extensive research program, that is still vivid today.Using diarylethenes molecules in these different devices requires a fine control of both the photochromic and spectroscopic properties. To determine which diarylethene derivative may be the optimal candidate for use as an optical molecule switch, it is necessary to understand the influence of the molecule structure ,energy gap ,the absorption maximum and the solvent effect on the diarylethene molecules properties. For this purpose, we have systemically investigated the photochromic properties of different diarylethene molecules and the solvent effects on the absorption wavelength and other physical properties, and the reaction mechanism of BTF6→BTFO4.In chapter 3 , we have systemically investigated the sulfur-oxidized reaction on the benzothiophenes of diarylethene . After the full geometry optimizations of the diarylethene, vibrational analysis for the set of sulfur-oxidized diarylethenes, the calculated results indicate that①i n the reaction , R O1-O2(the band lengths between O2 and O4) is longer than before , and the bond is ruptured. R S4-O1 is shorter than before, hydroxyl links to the S atom; H atom departs from the O1, and joins on the O2, R O1-H is longer and R O2-H is shorter.②The electron densities of the atom O1 and O2 decreased, and the electron densities of the atom H decreased and then increased. Among the reaction of BTF6→BTFO2 (BTFOO), the electron density of the atom S increased, but among the reaction of BTFO2 (BTFOO)→BTFO4, the electron density of the atom S increased and then decreased.③the reaction of BTF6→BTFO1,and BTFO1→BTFOO, BTFO2or BTFOO→BTFO3 are endothermic reaction, and the reaction of BTFO1→BTFO2, BTFO3→BTFO4 is exothermic reaction. The energy gap of the reaction of BTFO1→BTFO2 is lower than the reaction of BTFO1→BTFOO, so BTFO2 is the main production. It agrees well with the experimental conclusion. And the reaction mechanism is hydroxyl of 3-chloroper benzoic acid is gone to the S atom on the benzothiophene, the electron density of the S atom increases, and then the oxygen hydrogen bond of hydroxyl rupture, hydrogen attacks COO - to form 3 - chlorine acid. It makes the S atom on the benzothiophene ring into sulfoxide structure, and has been BTFO1. After such a reaction, then the sulfoxide structure of the benzothiophene ring becomes sulfoxyl, and has been BTFO2. Another benzothiophene of the diarylethenes occures the above reactions, and have been BTFO3 and BTFO4.The various properties presented by a diarylethene are strongly related with the oxidation state of the sulfur atoms in the diarylethene. In chapter 4, on the basis of the geometric structure of the molecules BTF6 and BTFOn (n=1,2,3,4,O), the calculations clearly demonstrate that the present of oxygen atoms in the benzothiophene unit of the diarylethenes causes a blue shift of the maxima absorption lengths. The number of the oxygen on the S atom of the benzothiophene increases, the distance between two reactive carbons is shorter, the value of BLA is linear with the oxidation of the S atom on the benzothiophene ring, the conjugated of the molecules increased and the molecules are more stable, which energy can be decreased. The bond lengths tend to be average. The ground-state energy of the closed-ring is higher than the corresponding opened-ring. So the closed-ring is more stable. The ground energy of BTF6 is the smallest for no oxidation. With the increase of the oxidation on the S atom, the ground-state energy gaps between the opened-ring and the closed-ring become bigger. For the ground-state energy gap which BFTOO conquers is low, therefore BTFOO closed-ring converts to opened form easily. As the number of oxygen atom of the diarylethene increased, the maximum absorption wavelength of the closed-ring isomers are blueshifted from that of unoxidized BTF6. The energy gap between HOMO and LUMO is also decreased.In the experiment, the solvent effect can change the photochomic behavior of diarylethene. In chapter 5, theoretical studies on diarylethenes with benzothiophenes an aryl groups and dithiole-2-one and dithiole-2-thione at the ethenic bond have been carried out using B3LYP/6-311G method in different solvents which are evaluated by means of the Polarizable Continuum Model(PCM). On the basis of geometric structure in the ground state, we calculated the electronic absorption spectra using TD/B3LYP/6-311G approach. The results agreed well with experimental data which measured in experiment and show that:①the benzothiophenes units of BTDT and BTDO can increase the symmetrical molecular structure. So their cyclization is easier. With increasing the solvent polarity, the distance between two reactive carbons are longer and the dihedral angels are bigger. The quantum yield decreases, and the cyclization is prohibited.②With increasing the solvent polarity, the potential energy that the cycloreversion conqued is lower. The closed isomer is less stable. The dipole moment of the molecules increased, and the charge of the molecule separated obvioursly.③The first triplets excited state results from the electron transition of diarylethenes with benzothiophenes an aryl groups and dithiole-2-one are from HOMO to LUMO(π→π*), but the first triplets excited state results from the electron transition of diarylethenes with benzothiophenes an aryl groups and dithiole-2-thione are from HOMO to LUMO(n→π*). The different solvent effects on the absorption spectra have been reasonably explained through analyzing the frontier molecular orbitals. With increasing the solvent polarity, the maximum absorption length is red shift. The energy gap between HOMO and LUMO is smaller. |
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