Influencing Factors Of Excited State Intramolecular Proton Transfer And Mechanisms In Different Reactions

With the development of experimental technology and computational level,the research on the photophysical and photochemical properties of excited-state molecules had become possible.Among the organic chromophores with intramolecular hydrogen bonding structures,an important excited state hydrogen bonding dynamic behavior,that was,the excited state intramolecular proton transfer?ESIPT?had attracted widespread attentions of researchers.The unique photophysical properties of the dual fluorescence emission of the ESIPT reaction played a key role in many fields.For example,the synthesis of polymer UV stabilizers,high-efficiency organic light-emitting diodes?OLEDs?,laser dyes and fluorescent probes,etc.^[1-3]It was well known that the ESIPT reaction was the most important photochemical conversion process in the excited-state hydrogen bonding dynamics^[4].The properties of the hydrogen bond determined the feasibility of proton transfer reaction and significant impact on the dynamic behaviours of internal conversion,photo-induced electron transfer,intramolecular charge transfer,fluorescence quenching or enhancement^[5].Therefore,in the first chapter of this text,we would detailedly introduce the history of hydrogen bonding researches and developments,the definitions of hydrogen bond,the quantum chemistry studies of hydrogen bond,the characteristics and forming criteria of hydrogen bond,the hydrogen bonding strength,categories,importance as well as the proton transfer reactions along hydrogen bond structures^[6-9].The investigation of hydrogen bonding properties helped us to correctly explain the ESIPT mechanism and to essentially illustrate the fundamental reasons that determine the ESIPT mechanisms.Because the hydrogen bonds widely exist in various substances in nature,the researches of the hydrogen bonding properties and dynamics had paid the attention of researchers in various fields.Since the photophysical phenomena of ESIPT were experimentally observed for the first time^[10],the ESIPT reactions had become an indispensable research hotspot in photochemical reactions.A mountain of related research work on ESIPT mechanisms,conditions,products and so forth had been reported.Some unknown ESIPT mechanisms were constantly emerging under different influencing factors,which makes the application prospects of ESIPT reactions more extensive^[11,12].At the end of the first chapter,we would introduce the current research status of ESIPT reactions at home and abroad,for example,the effects of intermolecular hydrogen bonding sites on ESIPT reactions^[13];the effects of excited-state intermolecular proton transfer on ESIPT reactions^[14,15];the effects of different temperatures on ESIPT reaction^[16,17].ESIPT reaction,which acted as the most important hydrogen bonding dynamic behaviour,was widely applied in industrial production and human daily life.Many researchers had observed the optical phenomena and measured the related properties of the ESIPT chromophore under different influencing factors,furthermore,they had predicted the potential properties and functions of these molecules.However,the experimental work lacked the fundamental explanations and researches on the ESIPT mechanisms.This text mainly used quantum chemistry calculational methods,such as the density functional theory?DFT?and time-dependent density functional theory?TDDFT?to detailedly study the ESIPT mechanisms under different influencing factors and in different reactions.Therefore,in Chapter 2,we would mainly introduce the establishment and development of density functional theory as well as some basic concepts^[18].The research reports of the ESIPT mechanisms under different influencing factors and in different reactions had been consulted and referenced as much as possible.It was found that the research on the mechanisms and application of the ESIPT reactions was still incomplete,and the reaction mechanisms were still lack of the proper understanding and interpretation.The main research work was as follows:In Chapter 3,we investigated the effect of solvent polarity on the ESIPT reaction by using theoretical calculation methods.First,we calculated the bonding parameters of hydrogen bonds and the infrared vibrational frequencies under different solvent polarities,which showed that the increase of the solvent polarity suppressed the strength of the excited-state intramolecular hydrogen bonds.The effect of solvent polarities on hydrogen bonding was given quantum-level explanations via analysing the properties of electron structures.Then,through constructing the potential energy surfaces and comparing the activation energy barriers of the ESIPT reactions,the influence of the solvent polarities on the ESIPT reactions was quantitatively demonstrated,that was,the increase of the solvent polarity prevented the occurrence of the ESIPT reaction.In Chapter 4,we used theoretical calculations to study the effect of different substituent positions on the ESIPT reaction.Firstly,the excited-state intramolecular hydrogen-bond strengthening mechanism was confirmed by analysing the bonding parameters of hydrogen bonds and the infrared vibrational frequencies.Then we calculated the electron-hole distribution and the frontier molecular orbital isosurfaces,found that when the substitution positions of the substituents were different,the molecules had different charge transfer degrees and affected the molecular photophysical properties.In order to reveal the effect of different substituent positions on ESIPT mechanisms,we established the minimum energy paths for proton transfer in different electronic states.The results showed that the ESIPT reaction was only feasible in the excited state.By calculating the activation energy barrier of the ESIPT reaction,the effect of the substitution effect on the mechanism of the ESIPT reaction was quantitatively demonstrated.In Chapter 5,we conducted a quantum-level study on the ESIPT mechanism of photodeamination reaction for anthrol molecule.First,the analyses of molecular hydrogen-bond bonding parameters and infrared vibrational frequencies showed that the strength of hydrogen bond O1-H2···N3 was enhanced during photoexcitation process,the strength of chemical bond N3-C4 was weakened during isomerization process.Then we calculated the frontier molecular orbitals and Mayer bond order of the anthrol molecule,which essentially explained the intensity changes of the hydrogen bonds O1-H2···N3 and the chemical bonds N3-C4.In the end,we used a more accurate theoretical calculation method to construct the molecular three-dimensional reaction potential energy surfaces,found that the photodeamination reaction of the anthrol molecule could only occur under the induction of ESIPT reaction.In Chapter 6,we used DFT and TDDFT quantum chemistry calculational methods to study the ESIPT mechanism in the reaction that used fluorescent probes to detect Hg²⁺and OCl^-ions.?i?The fluorescence phenomena observed in the experiment were accurately reproduced in theory,and the fluorescence signal of detecting Hg²⁺ions was reassigned,it is believed that this signal originated from the keto structure S-P generated by ESIPT reaction instead of the enol structure S-N.?ii?By comparing the bonding parameters of hydrogen bonds in different electronic states and isosurfaces of reduced density gradients,it was confirmed that enhanced hydrogen bonding provided a driving force for the ESIPT reaction.?iii?The feasibility of the ESIPT reaction was confirmed by calculating the Gibbs free energy of the photo-induced isomerization.Through describing the frontier molecular orbitals and hole-electron overlap integral isosurfaces of different structures,the excited-state intramolecular charge transfer?ESICT?characteristics of the molecules were visually manifested.Finally,based on the investigation of ESIPT and ESICT mechanisms,the fluorescence detection mechanism for detecting Hg²⁺and OCl^-ions was explained in detail.