Relationship Between Molecular Structure And Two-Photon Fluorescence Properties Of Multi-Branched/Multi-Chain Molecules Studied By Ultrafast Spectroscopy

Due to their special electronic structure, the organicπ-conjugated materials have been widely used in optoelectronic applications such as electroluminescence, two-photon absorption(TPA) and so on. The organic materials have attracted more and more attentions due to their advantages compared with inorganic materials, such as low cost, conveniency of synthesis, good physical properties et al. However, the physical machinism corresponding to the optoelectronic properties is still unclear, and the relationship between the molecular structure and optoelectronic properties is not well understood. This deficiency makes the research and engineer of materials proceeding relatively slowly. Thus, the research of fundument processes after photoexcitation in organicπ-conjugated materials including the properties of excited state and energy relaxation process should be performed, in order to design more excellent optoelectronic materials. Ultrafast laser spectroscopy such as femtosecond time-resolved transient absorption spectroscopy, time-resolved fluorescence spectroscopy, is an important experimental method to investigate the excited state energy relaxation process in organic materials. Femtosecond time-resolved transient absorption spectroscopy and fluorescence lifetime measurement combined with solvent dependent absorption and fluorescence spectroscopy have been used to investigate the excited state energy relaxation process of an organicπ-conjugated molecule 1,4-di(4'-N,N-diphenyl- aminostyryl) benzene (DPA-DSB). The structure-properties relationship is analysed. Furthermore, the enhancement of two-photon absorption in the multi-branched derivations and double chain derivation of DPA-DSB have been studied.Organic molecules with electronic donor/acceptor usually have good TPA ability due to their intramolecular charge transfer(ICT). However, the molecular structure usually varies during ICT process, which decreases the fluorescence quantum yield. In this thesis, the energy relaxation of excited states has been investigated by femtosecond transient absorption spectroscopy. It is shown that the molecular structure did not have big change during the intramolecular charge transfer (ICT) thanks to their symmetric D-π-D structure. The formation and relaxation of the ICT state are the major process in the excited state energy relaxation process. The twist ICT (TICT) state with the donor twisted is absence in DPA-DSB. The experimental results are supported by the quantum chemical calculation performed by Gaussian 03. The optimized molecular structure in the ground state and excited state indicated that the ICT state has a planar structure which is similar to the ground state. The little structure change during the ICT process makes the ICT state have high fluorescence quantum yield, which makes the potential application of DPA-DSB to the two-photon fluorescence.Multi-branched molecules have been proved to have cooperative enhanced TPA compared to single branched molecule. However, the has not been an unified conclusion of the mechanism of the cooperative enhancement. We use solvent polarity dependent spectroscopy to investigate the excited state properties of a series of multi-branched molecule based on DPA-DSB and analysis the reason for the change of TPA cross section with the increasing of the branches. The results suggest that the determinant of TPA properties is the extent of ICT instead of the coupling interaction between branches.Double chain molecule has better performance in TPA enhancement compared to multi-branched molecules, as the TPA cross section of double chain molecule DPA-TSB is 3.5 times of the single chain molecule DPA-DSB. We used femtosecond time-resolved transient absorption spectroscopy to investigate the excited state energy relaxation process of DPA-TSB compared to DPA-DSB. The results suggest that the double chain molecule has an enhanced ICT compared to the single chain molecule, which makes a larger dipole moment in the excited state and an cooperative enhancement of TPA. Thus, in the case of DPA-DSB, both double chain molecule and multi-branched molecules have cooperative enhancement of TPA compared to DPA-DSB, the reason for which is the enhancement of the extent of ICT in excited state. So the extent of ICT in excited state of molecule should be considered in molecule engineer to enhance the TPA ability.In this work, we used the time-resolved spectroscopy combined with quantum chemical calculation to investigate the molecular structure and other properties of excited states in DPA-DSB and their multi-branched double chain derivations. The investigation of such molecules provides theoretical basis for the study and design of better organic optoelectronic informational materials.