Ultrafast Spectroscopy Study of the Photo-induced Electron Transfer and Coherent Population Transfer Based on Rhodamine Dye Molecules

Author:Zhang Wei

Supervisor:yang yan qiang


Degree Year:2019





Electron transfer and coherent population transfer are the most basic processes in the primary physical and chemical reactions.Electron transfer plays vital roles in redox reactions and even in photoelectric conversion devices.Coherent population transfer is also very important in researching molecular devices as well as quantum communication.In this dissertation,typical Rhodamine dyes(Rh101and Rh6G)are selected as the target molecules and some relevant ultrafast photo-induced electron transfer and coherent population transfer processes are investigated by ultrafast time-resolved spectroscopy.Firstly,photo-induced intra-molecular electron and vibrational energy redistribution is studied.After stimulated by a beam of resonant laser,the molecule would go through transition from electronic ground state to the electronic excited state,and the distribution of the electron cloud would be changed accordingly.Some electrons may transfer from one radical group to the other one,so the density increase of electron would cause vibtional enhancement that corresponds to typical electron acceptor chromophore.Based on the aforementioned principle,typical vibrational enhancement can be a probe to detect intra-molecular electron transfer process.In the experiments,we used Rh101 as the project.Quantum chemical calculations are performed to characterize the electron transfer in the electronic transition process.The calculated results show that a bit of electron transfer from xanthene to the phenyl ring.Ultrafast time-resolved transient grating(TG)spectroscopy is then performed to seek the experimental evidence.Through Fourier analysis at different wavelengths of TG signal and their specific value spectra,vibrational enhancement corresponding to electron acceptor chromophore(phenyl ring)is observed,containing the“wagging”and“rotation”of phenyl ring.So the ultrafast vibrational energy redistribution process is detected experimentally and the direction of electron flow is confirmed.Secondly,photo-induced inter-molecular electron transfer(PIET)versus molecular structure of acceptor molecules is investigated using ultrafast time-resolved TG spectroscopy.Typical laser dyes Rh101 and Rhodamine6G(Rh6G)in electron donor solvent-aniline(AN)are adopted as the objects.The forward electron transfer(FET)time constant from AN to the excited singlet state of two Rhodamine dyes and subsequent back electron transfer(BET)from two dyes to AN are measured.Through the data analysis,it is found that Rh6G presents faster ET rates with AN in both FET and BET processes.With chemical calculation and contrastive analysis,it can be concluded that the flexible molecular geometry of Rh6G leads to stronger electron coupling with donor solvent and give rise to larger ET rates.Lastly,the photo-induced coherent population transfer(CPT)by ultrafast spectroscopy is investigated.The Rh101 and methanol solution is selected as the target.In previous study on the CPT,gas molecules and mental ions are always selected as the research objective,researcher utilized typical fluorescence to detect population transfer process which happens on the order of nanoseconds.In this part,femtosecond time-resolved broad-band TG is proposed to study the CPT process in condensed materials with Rh101 in methanol as the sample.The vibration modes with frequencies of 1500 and 2900 cm-1 were found to be coherent populated through the TG approach.The subsequent dynamics is also monitored by BB-TG,which coincided with the referenced TA experimental results.We extend the CPT research field to the liquid phase and the CPT process which happens on the order of femtoseconds is tracked.The transient grating technique is proved to be a feasible approach to achieve and detect CPT process in liquid phase.