| Organic photoelectric materials with excellently optical and electrical properties, thermodynamic stability, are widely used in the field of advanced functional materials. Due to low-cost, light-weight and easy-to-carry advantages of new organic functional materials, organic solar cell (OSC) and organic light emitting diode (OLED) are new stars in the field of research and development. Therefore, the development of organic photoelectric functional materials, are always the focus of physical, chemical, and materials researchers. Recently, through the incorporations of donor-acceptor architectures, a series of materials with favorable properties were synthetized.Narrow band gapπ-conjugated polymers have been applied to p-type materials, since the donor-acceptor approach has proved remarkably effective in the synthetic design of narrow band gap polymers absorbing at long wavelengths by Havinga et al in 1993. It is particularly interesting that donor-acceptor alternating copolymers since their electronic and optoelectronic properties can be tuned efficiently by intramolecular charge transfer (ICT). The interaction between the electron donor and acceptor moieties in such an alternating donor-acceptor copolymer can result in the hybridization of the high-lying HOMO energy level of the donor and the low-lying energy levels of the acceptor, leading to a relatively small band gap. Moreover, the donor-acceptor architectures enhance the level of intramolecular charge transfer. Small molecules, polymers and organic-inorganic complexes with donor-acceptor architectures show unusual optical and electrical properties. There are seven chapters in this paper. In Chapter 1, we made a summary of organic solar cell and organic emitting diode, the development of the chemical enhanced mechanism of surface-enhanced Raman scattering and theoretical and practical meaning of this study. In Chapter 2, an overview of theoretical methods including molecular orbital theory, configuration interaction theory, perturbation theory, density functional theory, electronic excited state theory, the choice of basis set, electronic spectrum and surface-enhanced Raman scattering theory. We carried on the theoretically investigations on the electronic structures and properties of different donor-acceptor organic molecular systems.1.The effect of the substituent groups (alkyl or aryl) on the structure, electronic, optical properties, ionization potentials (IPs), electron affinities (EAs), and reorganization energy of the donor-acceptor monomers 5,8-di-2-thienyl-quinoxaline (T[Q]T), 4,9-di-2-thienylpyrazine[2,3-g]quinoxaline (T[PQ]T) were studied theoretically. The lowest-lying absorption in assigned toπ→π* transition, and the fluorescence can be described as originating from the 1[ππ*] excited state. The lowest-lying absorption and emission spectrum of T[Q]T and T[PQ]T with alkyl groups exhibit blue-shifted, while T[Q]T and T[PQ]T with aryl groups exhibit the opposite result. The extra absorption bond at 400 nm of T[Q]T-Bph is contributed by theπ-π* transitions between the biphenyl and acceptor fragment. Orbital compositions transfer coefficient (Χ) of the donor in LUMOs is reduced with the aryl groups on the acceptor, which illuminates that the aryl contributes to intramolecular charger transfer, and the result is in accord with the analysis of reorganization energy. IPs is brought down by both of the alkyl and aryl groups, but EAs is raised only by aryl, therefore, aryl is conductive to forming excitons for D-A-D molecules. Consequently, T[Q]T and T[PQ]T with aryl groups are more reasonable monomers of donor-acceptor copolymers as a solar cell materials comparing with the alkyl-introduced ones.2. The novel linear and star molecules based on benzothiadiazole as a central core, TPA as end groups and different functional groups asπ-spacers were theoretically and exploringly investigated here. The geometry structures and electronic structures are studied by advance quantum chemical methods. The ground and the excited state geometries were optimized by B3LYP and CIS methods with 6-31G* basis sets, respectively. Ionization potentials (IPs), electron affinities (EAs), reorganization energy (λhole andλelectron) of linear and star molecules were studied by DFT methods. The absorption and the emission spectra were calculated by TD-BHandHLYP method. The results of IPs, EAs,λhole andλelectron show star molecules with ethynyl groups asπ-spacers are more suitable for generating hole, and exhibit excellent charge transfer character. Moreover, the diethynyl groups improve the charge transfer character and extend the absorption wavelength towards the range with the maximum photon flux.3. A series of DA alternating oligomers (BTBTB, BCBCB, BSiBSiB, BTTTB, BCCCB, BSiSiSiB, BCTCB and BSiTSiB) taken from the corresponding DA alternating copolymers, were studied theoretically. With the gain of the D/A ratio, charge transfer and hole-reorganization energy are both increased, but both of IPs and EAs are reduced, illuminating the gain of the D/A ratio is beneficial for the copolymers used as the p-type materials in the BHJ solar cells. While increasing the D/A ratio, the high energy transition mainly attributed toπ(C-C band)→π* (N-S band) transition in the acceptor has a redshift. And the low energy transition attributed toπ→π* transition from donor to acceptor, has a blueshift. The trend in the nearness of two transitions results in broadening the absorption band in the visible range.4. The effect of the incorporation of 4-N,N-diphenylaminostilbene(DPS) and oxadiazole (OXD) dopants on the structural and electronic properties of a stable UV-blue-light emission poly[2,7-(9,9-dioctylfluorene)-alt-1,3-(5-carbazolphenylene)] (PFCz) host copolymer were investigated theoretically by means of quantum mechanical calculations using density functional theory (DFT) and time-dependent DFT (TD-DFT). It was found the backbones of these host/dopant copolymers are nonplanar in its ground state and excited state. The absorption bands around 340 nm observed experimentally were originated from the intrachain charge transfer from PFCz groups to backbone and the charge recombination along the backbone, whereas the fluorescence bands around 422 nm observed experimentally were originated from the intrachain charge transfer from DPS groups to backbone. The calculated results of fluorescence and radiative lifetime implied the intensity of emissions and radiative lifetime were concerned with composition of DPS group. Moreover, electron mobility can be increased through improving the composition of OXD group.5. The chemical enhanced mechanism of surface-enhanced Raman scattering (SERS) spectrum was investigated by using the density functional theory (DFT). We studied the Raman spectra of Ag13/MPH and Ag13/MPH/TiO2 systems under the 514.5 nm excitation process. We found that the intensities of non-totally symmetric vibration modes were selective enhanced after the TiO2 was introduced into Ag13/MPH system. Through analyzing the ground state and excited states of charge transfer (CT) complex, it was found that the system result in a photo-induced CT state from Ag to MPH-TiO2 Complex when the excitation wavelength exceeds the optical absorption threshold (635 nm) of MPH-TiO2 complex. The selective enhancement of the b2 modes in SERS spectra is suggested originating from the Herzberg-Teller mechanism via the coupling of the corresponding modes with the CT transition. Our theoretical results not only supported the experimental results, but also defined the adsorption threshold of the CT complex clearly and provided an intelligible physical elaboration for the laser wavelength-dependent SERS phenomenon.
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