Experimental And Theoretical Investigation On The Influence Of External Environmental On Excited-state Proton Transfer Process

The excited-state proton transfer process has always been a hot topic in the field of atomic and molecular physics.The molecules with excited-state proton transfer characteristic have important application values and development prospects in many fields such as organic optical materials,probe molecules and chemical sensors.By changing the external environmental factors and adjusting the photophysical properties of molecules with excited-state proton transfer,the excited-state proton transfer process mechanism can be controlled,which is expected to provide theoretical basis for optimizing the performance of light emitting devices,designing molecular switches and sensors,etc.In our thesis,we use femtosecond transient absorption spectroscopy,high-pressure anvil cell technology and time-dependent density functional theory to study the effects of solvent and pressure on the excited-state proton transfer process of organic molecules.The specific research contents are summarized as follows:?1?Using femtosecond transient absorption spectroscopy and time-dependent density functional theory?DFT/TDDFT?,the effect of solvent polarity on the luminescence characteristics and excited-state ultrafast dynamics of coumarin E-8-(?4-?dimethylamino?phenyllimino?methyl-7-hydroxy-4-methyl-2H-chromen-2-one?CDPA?molecule was studied.With the increase of solvent polarity,the fluorescence intensity of coumarin CDPA molecule gradually weakened,and the fluorescence peak position was red shifted.Ultrafast kinetic fitting results show that the increase of solvent polarity is helpful to accelerate the excited state intramolecular proton transfer?ESIPT?process,torsional intramolecular charge transfer?TICT?process of coumarin CDPA.DFT/TDDFT theoretical calculation results show that the first electronically excited state configuration of coumarin CDPA in n-Hexane has a structural twist of nearly 70degrees compared with the ground state configuration,while only 8 degrees in acetonitrile?ACN?solvent.Frontier molecular orbital analysis showed that the structural twist of coumarin CDPA greatly changed intramolecular charge distribution.In addition,structural torsion in n-Hexane solvent causes ESIPT process of coumarin CDPA to overcome the energy barrier of 8.95 kcal/mol,while small torsion angle in ACN solvent causes ESIPT process of coumarin CDPA to overcome the energy barrier of 1.73 kcal/mol,thus the TICT process in coumarin CDPA system can inhibit the occurrence of ESIPT process.?2?The high-pressure anvil cell technology is adopted to realize white-light luminescence and significantly enhance luminescence of 1-Hydroxyanthraquinone?1-HAQ?,which cannot emit white-light under atmospheric pressure.The physical mechanism of fluorescence enhancement under pressure is explored by means of high-pressure transient absorption technology.The steady-state absorption spectrum results show that the 1-HAQ molecule transitions to the second electronic excited state?S2?after excitation,and the absorption peak position redshift with the increase of pressure,which effectively reduces the energy level position of 1-HAQ.1-HAQ molecule observed a weak fluorescence band from enol?Enol*?structure and a strong fluorescence band from keto?Keto*?structure under atmospheric pressure,covering a spectral range of 450 nm to 700 nm.By introducing high-pressure technology,the fluorescence spectrum of 1-HAQ is extended to 350 nm to 800 nm under high-pressure.When the pressure is increased to 2.2 GPa,the fluorescence intensity of Enol*structure is increased by 13 times and that of Keto*structure is increased by3 times.In addition,it is worth noting that 1-HAQ achieve white-light luminescence at 0.8 GPa,corresponding to the Commission Internationale de L'Eclairage?CIE?chromaticity coordinates?0.31,0.32?.It is found that the decrease of band gap between S2 state and Enol*state and Keto*state under high-pressure is the cause of fluorescence enhancement.The high-pressure excited-state ultrafast dynamics results show that the increase of excited-state proton transfer energy barrier is the main reason for the different enhancement amplitudes of fluorescence intensity of Enol*and Keto*structures.?3?The effect of ethanol?EtOH?solvent on Dipyrido[2,3-a:3',2'-i]carbazole?DPC?and the mechanism of excited state intermolecular double proton transfer?ESDPT?of DPC-EtOH complexes were studied by time-dependent density functional theory?DFT/TDDFT?.The addition of EtOH solvent causes the DPC monomer to generate two intermolecular hydrogen bonds N1-H1···O1 and O1-H2···N2,and forms the DPC-EtOH complex.Frontier molecular orbital analysis shows that the addition of EtOH solvent breaks the symmetrical charge distribution of DPC monomer,reducing the charge distribution at N1-H1 site and increasing the charge distribution at O1-H2 site in DPC-EtOH complex,which is helpful for the occurrence of double proton transfer process of DPC-EtOH complex.The results of molecular structure optimization and noncovalent interactions analysis show that the intermolecular hydrogen bond interaction of DPC-EtOH complex in the first electronic excited state is enhanced compared with that in the ground state.Infrared vibration spectrum analysis shows that N1-H1 and O1-H2 stretching vibration peaks of DPC-EtOH complex have obvious redshift,further confirming the enhancement of intermolecular hydrogen bond interaction in DPC-EtOH complex.Potential energy surface analysis results show that the double proton transfer process of DPC-EtOH complex in the ground state cannot be carried out due to high energy barrier,while the intermolecular double proton transfer process of DPC-EtOH complex in the first electronic excited state is realized through a synergistic mechanism.Our study realized the ESDPT process by introducing ethanol solvent into DPC monomer that cannot undergo proton transfer,and revealed that ESDPT mechanism of DPC-EtOH complex is cooperative mechanism.?4?Time-dependent density functional theory?DFT/TDDFT?is used to study the excited state multi-proton transfer?ESMPT?process of 1-H-pyrrolo[3,2-h]quinoline?PQ?connecting different numbers of methanol?MeOH?molecules and the physical mechanism behind the significant fluorescence difference.The results of potential energy curves show that both PQ-MeOH and PQ-?MeOH?₂ complexes can realize multi-proton transfer in the first electronic excited state.The energy barrier of multi-proton transfer can be effectively adjusted by changing the number of MeOH molecules.In addition,it is revealed that the non-fluorescence property of PQ-MeOH complex is due to the cooperation of fast ESMPT process and intersystem crossing?ISC?process,indicating that ESMPT process and ISC process in PQ-MeOH complex are cooperative mechanisms.The fluorescence observed in PQ-?MeOH?₂ complex is due to the increased contribution of the ESMPT energy barrier to fluorescence than the consumption of fluorescence by ISC process,indicating that ESMPT process and ISC process are competitive mechanisms in PQ-?MeOH?₂ complex.