Experimental And Theoretical Study Of The Coherent Charge Transfer Process In The Excited-state Of Molecules

The excited-state coherent charge transfer process plays an important role in solar cells,photovoltaic devices,photoelectric sensors,photosynthesis and energy storages.Exploring the physical mechanism of the coherent charge transfer process from the perspective of ultrafast dynamics is helpful to the design of efficient organic photoelectric conversion devices.However,the microscopic mechanism of the coherent charge transfer process remains to be further studied.In this paper,we focused on the coherent charge transfer of molecules in the excited-state.The evolution process of the coherent charge transfer was observed in real-time.The physical image of the coherent charge transfer was illustrated.The microscopic mechanism of hydrogen bonds and the influence of the external environment on coherent charge transfer were revealed.The following four sections provide a summary of the specific research contents:(1)Applying density functional theory(DFT)with real-time time-dependent density functional theory(RT-TDDFT),the influence of intermolecular hydrogen bonds in guanine-cytosine(G-C)on coherent charge transfer in the excited-state was investigated.We found that the G-C base pair has three intermolecular hydrogen bonds and obvious charge transfer characteristics.The energies of molecular orbitals,hydrogen bond lengths and non-covalent interaction results showed that the intermolecular hydrogen bond interaction of the G-C base pair has a regular oscillation of 10 fs.The charge distribution on the G-C base pair was simulated by the charge density difference.The results illustrated that the distribution of charge on the donor and acceptor has a periodic oscillation of 10 fs.It proved that the charge transfer process of the G-C base pair is coherent,and the intermolecular hydrogen bond is conducive to the occurrence of the coherent charge transfer process.(2)The short-range coherent electron transfer of the thymine-adenine-thymine(TAT)base triplet was studied using DFT/RT-TDDFT theory.The molecular geometry,hydrogen bond lengths and frontier molecular orbitals of the TAT base triplet proved that the short-range electron transfer of the TAT base triplet is coherent.The charge density difference and transition density matrix visualized the electron-hole distribution,showing that there is a strong correlation between electron and hole.By analyzing the separation degree and the overlap degree of the electron-hole in the TAT base triplet,it is manifested that the charge transfer state promotes the short-range coherent electron transfer process of the TAT base triplet.The calculation of reorganization energy and free energy based on the time-dependent Marcus-Levich-Jortner theory revealed that the nucleus-electron vibration coupling is the root cause of the short-range coherent electron transfer process.(3)The influence of temperature(from 215 K to 355 K)on the coherent proton-coupled electron transfer of azobenzene derivatives(2,2’-DHAB)was investigated by femtosecond transient absorption spectroscopy.We measured the steady-state fluorescence spectra of 2,2’-DHAB at different temperatures,and found that 2,2’-DHAB has three fluorescence peaks.The calculation results explained that the fluorescence peaks are emitted by charge transfer and proton transfer states,respectively.The increase in temperature leads to the intensification of molecular vibration,which is not conducive to the deactivation of energy in the form of released photons,resulting in the weakening of fluorescence intensity.The measured transient absorption spectra indicated that the vibration of 2,2’-DHAB is more intense at higher temperatures.According to the phase information of the vibration signals,the vibration is classified as a Helzberg-Teller type oscillation.The kinetic fitting results explained that the coherent proton transfer can be accelerated by increasing temperature but has no effect on the coherent charge transfer process.Our study revealed the microscopic mechanism of the temperature effect on the coherent charge transfer process of 2,2’-DHAB.(4)The proton-coupled electron transfer process of triphenylamine derivatives(TPA-SD)was successfully transformed from incoherent to coherent by high pressure.The frontier molecular orbitals demonstrated that TPA-SD has proton-coupled electron transfer properties.The steady-state fluorescence spectra indicated that the fluorescence intensity increases obviously with the increase in pressure.The results of high pressure Raman spectra showed that the pressure can inhibit intramolecular rotation and reduce the distance between molecules,which is conducive to the occurrence of coherent proton-coupled electron transfer.We found that the transient absorption signals of TPA-SD do not oscillate periodically at atmospheric pressure,but the transient absorption signals at high pressure oscillate with a period of 250 fs.Our study successfully realized the transformation of the proton-coupled electron transfer process of TPA-SD from incoherent to coherent under high pressure.