| The photosynthesis is the most important and highest efficient photochemical process found in photosynthetic organisms, including plants, algae and a variety of types of bacteria in the nature. All these organisms utilize sunlight to power cellular processes and ultimately drive the chemical reaction after the ultrafast energy transfer and charge separation. The primary process of photosynthesis involves the capture of light energy, energy transfer and a series of electron transfer which all take place in the picosecond or subpicosecond time region. After the initial electron transfer event, a series of electron transfer reactions takes place that eventually stabilizes the stored energy in forms that can be used by the cell. The development of molecular biology, genetic engineering and the site mutation technique establish the fundamental molecular basis for us to study the mechanisms of photophysics and photochemistry in the primary processes involved. Meanwhile, the fast developments of ultrafast spectroscopic techniques in the past decades provided us a significant experimental approach to investigate the ultrafast processes in the primary reaction of photosynthesis. Therefore, further investigating energy transfer and charge separation on the process of photosynthesis are significant not only for thoroughly understanding the mechanisms of photophysics on the process of photosynthesis but also especially for building elentron apparatuses in the molecular size such as optical switches, transistors, selenium rectifier etc. Furthermore, it has important theoretical and applied values for the development of molectroics. At present, the hotspots of research are mainly focusing on the human-designed molecular systems and the energy transfer and charge separation of complex systems by introducing mediuming molecules.The materials with porphyrin groups distribute abroad in nature, their properties are stable and the photophysical characters of their monomers have been known to people, so porphyrin-based D-B-A complex has good theoretical and experimental foundation/groundwork, which attract many people's attention.The principal work in this paper is to investigate the process of energy tranfer in porphyrin-based D-B-A complex mainly by steady/transient state spectrum technique with 532 nm and 1064 nm excitation. The main results can be summarized as below.1. In the system of donor-bridge molecule-acceptor, porphyrin zinc (ZnP) is the energy donor, benzene bridge eliminated conjugation (OB), benzene bridge (BB), naphthalene bridge (NB) and anthracene bridge (AB) are the bridge molecules, and porphyrin iron (Fe(Cl)P) is the energy acceptor. The molecules of the three parts are relatively independent chromophores, in which the bridge molecule is the medium of energy tranfer between donor and acceptor. By contrast, the energy tranfer of single state and triple state in the system exists simultaneously while the bridge molecule is existent, and the energy tranfer speed of system is fifferent with the change of the electron conformation of the bridge molecule. The steady state fluorescence is also weakened gradually with the change of the electron conformation of the bridge molecule, which can be observed from fluorescence spectra. Take BB bridge for example, the energy tranfer between the triple state of the donor and the acceptor and its mechanism in the supermolecular system of assembled ZnP-BB-Fe(Cl)P are studied by steady state and transient state spectrum technique. The results showed that there existed ultrafast energy tranfer from the triple state of donor ZnP to the triple state of acceptor Fe(Cl)P in the system. By exciting the donor at both room and low temperature, the triple state of ZnP is populated by intersystem transition, which energy can be tranfered to the acceptor through the bridge molecule B while the Fe(Cl)P exists. The tranfer rate at room temperature is 7.2×105 s-1. Because of the spacial distance between donor and acceptor is about 2.5 nm, the tranfer mechanism of direct coupling between donor and acceptor can be excluded, so the ultraexchanged mechanism by bridge molecule mediuming is the main physical mechanism of the energy tranfer.The photophysical properties of porphyrin-based assembled molecular system was investigated using steady-state fluorescence and transient absorption. The obtained results indicate the existence of energy transfer process from the triplet state of donor ZnP to the accepter Fe(Cl)P, mediated by the bridge molecule. At room temperature, the fluorescence of ZnP was quenched dramatically with the presence of acceptor Fe(Cl)P, implying the energy transfer between donor and acceptor. By comparison of the triplet state lifetime of ZnP in reference and assembled system, it was observed the excitation energy of donor's triplet state is quenched by a factor of 5, and the rate constant for energy transfer is around 7.2×105 s-1. The temperature dependence of energy transfer rate constant indicates the influence of conformation on the electronic coupling between donor and acceptor mediating by the bridge molecule. Considering the spatial distance between donor and accepter being upto 2.5 nm, it could be concluded that superexchange mechanism via bridge dominates the deactivation of excited triplet state of ZnP in the assembled porphyrin dimer. 2. The properties of two materials with two-photon absorption, THHP2 and T(4-VP)P, have been studied by pico-second pulse laser. The results showed that they displayed obvious optical limiting effects. By curving, the two-photon absorption coefficients can be obtained, which values are 4.06×10-20 cm3/W2 and 5.02×10-20 cm3/W2, and their two-photon absorption sections also can be calculated, which values are 7.85×10-77 cm6s2 and 9.72×10-77cm6s2, respectively. The two-photon fluorescence and the optical limiting effects of samples are obvious, which are suitable to ultrafast process of optical limiting.3. The effect of substituents and metal ions on the linear and nonlinear optical properties of porphyrines have been investigated. The absorption and PL emission peaks of porphyrines substituted by donating electron group shifted towards long wavelength compared with TPPH2, and the former's third-order nonlinear effects enhanced a lot. The nonlinear absorption at 532 nm and at 1064 nm were observed. The former can be attributed to reverse saturated absorption and the latter can be attributed to two-photon absorption. Then the nonlinear absorption coefficientβ, the nonlinear refractive index n2 and the third-order nonlinear susceptibilityχ(3) have been calculated. When introducing Zn2+ in porphyrine ring to form metal compounds, porphyrin plane might stand out and the spin orbit coupling of Zn2+ enhanced, which reduced the relaxing time of single state and increased the relaxing time of triple state simultaneously, all of which made the metal porphyrin compounds have larger nonlinear refractive index than according free porphyrin compounds.In a word, the energy tranfer in porphyrin-based D-B-A complex and the third-order nonlinear properties of porphyrin compounds are researched by steady state and transient state spectrum technique and Z scan technique. The results will provide important experimental foundation for studying photosynthesis, solar battery and molecular photoelectron apparatus, and for the research and design of optical limiters, wave-guide electron-photon modulators, photo-information storage components etc.
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