| With the progress of ultrafast laser technology, femtosecond time-resolvedspectroscopy has been developed unprecedentedly. Due to its high temporalresolution, this technology is being more and more applied to the study of physicaland chemical reaction process in femtosecond picosecond time scale, such aselectronic excited state radiative relaxation, electron-hole separation, as well ascovalentbondcleavage.ProfessorAhmedH.Zewailwasawardedthe1999NobelPrize,in recognition of his femtosecond time-resolved spectroscopy study on theintermediate state in achemical reaction.Thethesisisdividedintothefollowingthreeparts:The first part introduces the set-up of the femtosecond time-resolved transientabsorption spectroscopy system and fluorescence upconversion system. The ultrafastlaseramplifierSolsticegenerates100fs,250Hz,2mJ/pulse800nmpulsesasthemainlight source. The excitation light (wavelength300nm-650nm) is achieved by TOPASor a piece of BBO crystal by frequency mixing. The detection light is achieved byfocusing the800nm pulses on the sapphire crystal, which generates the Super-continuum white light (400nm-700nm). We also generated ultraviolet detectionlight(330nm-380nm)byfocusing400nmpulsesonthe same sapphirecrystal.Finally，we built up a set of upconversion system to detect decay kinetics of fluorescence.The second part introduces the study of the excited state charge transferproperties of the heterogeneous chlorophyll dimmer in the photosynthetic reactioncenter by time-resolved spectroscopy. The pigments that the electron undergoes inthe photosynthetic reaction center are arrayed into a symmetric structure, but thecharge separation of photosynthesis only takes place on one side. Homogeneouschlorophyll dimer is where the charge separation begins. We mutate thehomogeneous dimer to heterogeneous one, which has a stronger charge transferproperty. But this mutation does not affect the charge transfer channel. It only resultsin the reduced yield of the charge separation. At the same time we confirm that there is a strong coupling between the local excited state and the intramolecular chargetransfer state of the heterdimer, the weight of which is independent of the excitationwavelength.The third part proposed the two-step ring closure process of the spiropyran-derived merocyanine. Spiropyran is the model molecule in studying the photoisomerization. In recent years, the characteristics of the optical response have drawnmore and more attention due to its potential of molecular devices (photosensitivemolecularswitch,e.g.).Thelatestresearchdevelopedthree-dimensionaldatastorageand novel drug release methods based on spiropyran molecules. Merocyaninecommonly has two isomers, TTC and TTT, while the former is the stable form whenresolvedinsolution.Weinvestigatethe6-NO28-Br-BIPSbytime-resolvedspectroscopyexperiments. The results show that after1.4ns of light irradiation, a new absorptionpeak appears, which is60nm blue shifted from the original absorption peak. This newabsorption band has a lifetime less than10ns, which excludes the possibility of thetriplet state. According to the published literature, we determined that the latelyformed band results from the formation of the TTT isomer, which means that afterirradiation, the TTC isomer is first reorganized to TTT isomer, and then ring-closedspiropyran. Meanwhile, we performed a solvent dependent experiment onspiropyran-derived merocyanine. The result shows that the quantum yield of the ring-closure is related to the viscosity of the solvents rather than the polarity. The greaterthe viscosity of the solvent, the higher efficiency of the ring closure process.Understanding the basic ring-closure process of merocyanine will benefit us fromdeveloping the quick response molecular switch. The solvent viscosity dependence ofthering-closurereactionwillhelpustomakebetteruseofthesolventcontrollingdrugrelease.In short,the studyofthesebasicphysical processeshasapositivemeaningonthe spiropyran-based device in the future. |
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