Investigation On The Ultrafast Dynamics Of Electron Transfer Process In CdSe/ZnS Quantum Dot-anthraquinone Derivatives Complexes

Electron transfer(ET)process between quantum dots(QDs)and acceptor molecules as a basic and crucial photophysical process has been widely used in photodetectors,photocatalysis,photovoltaic devices,light emitting diodes and solar cells.The effective regulation of the ET process,especially the acceleration of it,plays a significant role in improving the efficiency of photoelectric conversion.Therefore,an in-depth understanding of the internal mechanism and influencing factors of the ET process are helpful for us to better realize the regulation of the ET process.In our thesis,we chose CdSe/ZnS core/shell QDs as electron donor and anthraquinone(AQ)derivatives as electron acceptors.Based on the Marcus ET theory,the femtosecond transient absorption(TA)spectra technology was carried out to study the influence of different acceptor molecules,solvents and pressure on the ET process between of them.The specific research contents are summarized as follows:(1)Femtosecond TA spectra and cyclic voltammetry were employed to investigate the effects of different number of chlorine substituted AQ molecules on the ET process between CdSe/ZnS core/shell QDs and AQ derivatives molecules.The fluorescence quenching in CdSe/ZnS and AQ derivatives complexes indicated that the ET occurs from QDs to AQ derivatives through comparing with the fluorescence intensity with pure QDs.The TA spectra in three systems indicated that the 1S bleach signal amplitude showed an obvious decrease with the acceptor changing from 1,4,5,8-TTAQ,1,8-DCAQ to 1-CAQ.The ultrafast kinetic fitting results in three systems indicated that the ET rate increased gradually with the acceptor changing from 1,4,5,8-TTAQ,1,8-DCAQ to 1-CAQ.Then,we measured the LUMO orbital energy level of 1,4,5,8-TTAQ,1,8-DCAQ and 1-CAQ through cyclic voltammetry.It was found that the energy value of the LUMO orbital energy level increases gradually with the decrease of the number of chlorine substituent in the AQ molecule,which leads to a gradual increase in the driving force of ET process from CdSe/ZnS-1,4,5,8-TTAQ,CdSe/ZnS-1,8-DCAQ to CdSe/ZnS-1-CAQ,thus promoting the ET rate.Our results indicated that the regulation of ET rate can be achieved by changing the number of substituent of acceptor molecule.(2)The effect of different solvents(carbon disulfide,dichloromethane,trichloromethane,n-hexane,tetrachloromethane,and n-dodecane)on ET process between CdSe/ZnS core/shell QDs and 1-chloroanthraquinone(1-CAQ)was studied by the femtosecond TA spectra and density functional theory(DFT)methods.The results of steady-state fluorescence spectra showed that obvious fluorescence quenching exists in CdSe/ZnS-1-CAQ system in six different solvents,which proved that the ET processes from CdSe/ZnS to 1-CAQ molecule have occurred in different solvents.The TA spectra of CdSe/ZnS-1-CAQ complex in six solvents indicated that the 1S bleaching peak had a red shift with changing delay time.The ultrafast kinetic fitting results of CdSe/ZnS-1-CAQ complex in six solvents showed that the ET rate was strongly dependent on the change of the solvent,especially the ET rate in carbon disulfide was about 300 times that in n-dodecane.Then,DFT theory was applied to calculate the recombination energy of CdSe/ZnS-1-CAQ complex in six solvents.The calculation results showed that the recombination energy value of the acceptor molecule 1-CAQ was very large,and it also accounted for a large proportion in the recombination energy of the system.What's more,different solvents had great influence on the recombination energy of acceptor molecule.Our results show that the effects of recombinant energy of acceptor molecule in different solvents on ET rate can not be neglected.(3)The effect of pressure on the ET process between CdSe/ZnS core/shell QDs to AQ was studied by the means of femtosecond TA spectra and in situ high-pressure technology.By comparing the steady-state fluorescence spectra of CdSe/ZnS and CdSe/ZnS-AQ complex at atmospheric pressure,it was found that the steady-state fluorescence had a significant quenching phenomenon with the addition of AQ molecule,which proved that the ET process from CdSe/ZnS to AQ molecule has occurred.The TA spectra of CdSe/ZnS-AQ complex under high pressure showed that the 1S bleaching peak of CdSe/ZnS had a blue shift as the pressure increases.The ultrafast kinetic fitting results of CdSe/ZnS-AQ complex under high pressure showed that the ET rate presented a step-wise change as the pressure increases.The pressure can cause the phase transformation and volume reduction of the solvent cyclohexane,which resulted in the decrease of the distance between CdSe/ZnS and AQ,thus promoting the ET rate.Our results suggest that pressure can be used as a novel means to regulate the ET rate.