Theoretical Study On The Excited State Hydrogen Bond Dynamics Of Several Molecular Systems

The excited-state proton transfer process is one of the most fundamental and important processes in biology,physics and chemistry.The dye molecules that can produce excited-state proton transfer generally have high fluorescence quantum yield and large stokes shift,which can be applied to organic luminescent materials.Through in-depth study the excited-state proton transfer properties of the molecular,we can provide theoretical support for the development of some efficient organic luminescent materials.In this paper,the excited-state intra-or intermolecular proton transfer?ESIPT?processes in several molecules systems were investigated by theoretical method.The main research contents are listed as follows:?1?The effect of solvent polarity on the ESIPT of 6-amino-2-?2'-hydroxyphenyl?benzoxazole?6A-HBO?were studied by theoretical method.We selected nonpolar solvent of dichloromethane?DCM?and the polar solvent of the methanol?MeOH?as the solvent.By scanning the potential energy curves,we found that the polarity of the solution have an effect on the ESIPT process of 6A-HBO.In the polar MeOH solvent,an intermolecular hydrogen bond O···H between 6A-HBO and MeOH can form.The potential barrier of the 6A-HBO in MeOH solvent is 5.16 kcal/mol,which is 3.57 kcal/mol lower than the barrier in DCM solvent.It can be concluded that the intermolecular hydrogen bond does not block the ESIPT process of 6A-HBO.On the contrary,the ESIPT procoess is more likely to occur due to the presence of the intermolecular hydrogen bond.This work will provide some guidance for us to discuss the excited-state properties of some molecules in different solvents.?2?The effect of different types of substituents on the ESIPT processes of2-?2'-hydroxyphenyl?benzimidazole?HBI?molecular were studied.We have selected two different types of substituents,one is electron donor group?-NO₂?and the other is electron acceptor group?-NH₂?.The geometric parameters,infrared?IR?vibrational spectra,frontier molecular orbitals?MOs?,charge distribution analysis,and potential energy curves were calculated to investigate the effect of electron acceptor and donor groups on the ESIPT reactions of the HBI molecules.By comparing the parameters,we found that the influence of the electron donor and the electron acceptor on the ESIPT processes of HBI molecular is very different.The presence of the electron acceptor-NO₂ group can facilitate the ESIPT process,while the presence of electron donor-NH₂ group can hinder the ESIPT process.This work will provide some guidance for the synthesis of molecules with ESIPT properties.?3?The ESIPT processes of non-fused five-membered ring structure of 2-?1H-pyrrol-2-yl?pyridine?5-HB?and the fused six-membered ring structure of 10,11,12,13-tetrahydro-9H-quinolino[8,7-a]carbazole?6-HB?were investigated by theoretical method.5-HB is more flexible than 6-HB to form the intramolecular hydrogen bond between pyrrole and pyridine,while 6-HB possesses more favorable distance and orientation between pyrrole and pyridine.The geometric parameters,IR vibrational spectra,bond critical point?BCP?parameters and reduced density gradient?RDG?,potential energy curves and time evolution of selected bond distances were obtained to reveal the properties of these two structures.We found that although the non-fused structure is more flexible than fused structure,the intramolecular hydrogen bond of fused six-membered ring pyrrole-pyridine system?6-HB?is much stronger than that of the non-fused five-membered ring system?5-HB?due to its favorable distance and orientation and the ESIPT process of 6-HB is easier and faster than that of 5-HB.?4?The excited-state double proton transfer?ESDPT?mechanism in model DNA base pairs,7-azaindole?7AI?dimer,was investigated.In this work,the potential energy surfaces of 7AI dimer in the ground state?S₀?and the lowest energy excited singlet state?S₁?were constructed.The proton transfer process of the 7AI dimer in S₀ state is prohibited due to the high potential barrier.After vertical excitation to S₁ state,the single proton transfer can occur.The second proton transfer process in the stepwise mechanism is blocked by a high potential barrier?36.4 kcal/mol?.However,the single proton transfer process is compatible with the concerted mechanism.We show that the single proton transfer process rather than the concerted mechanism dominates the dynamics.The concerted process is unfavorable in the S₁ state compared with the barrierless single proton transfer process.In addition,the proton transfer process in S₀state is revealed.The single proton transfer tautomer in the S₁ state returns to the S₀ state and transfers the second proton via a barrierless process.Finally,the double proton transfer tautomer at S₀ state can recover to the normal dimer through the reverse proton transfer reaction.?5?The effect of fluorine substitution on the strength of C-H···O hydrogen bond in the double hydrogen bonding cyclic structure was examined through theoretical method.Firstly,we have studied the effect of different bonding motif of the 2-FP···HOAc dimer on the strength of C-H···O hydrogen bond.It has been found that the C-H···O hydrogen bond of structure c is stronger than that of structure b.The fluorine favors the C-H···O hydrogen bonding interaction through the cooperating of O-H···F and C-H···O hydrogen bonds.Then we have investigated the effect of different degree of fluorination on the strength of C-H···O hydrogen bond.With increasing in fluorination,the C-H···O bond strength increases and the O-H···N and O-H···F bonds strength decrease.For the same degree of fluorination complexes,the C-H···O hydrogen bond of c type bonding motif is stronger than that of b type bonding motif.Through the natural population analysis?NPA?,we found that the hydrogen bond strength is correlation with the charge redistribution of the proton donor and acceptor atoms.The molecular electrostatic potentials?ESP?allow us to realized that the substituent F atom act as a role of electron-withdrawing.The pyridine ring N atom and2-position F atom become less negative and the remaining H atom?s?become more electropositive with increasing degree of fluorination.In general,the charge distribution of the molecular can be changed by the fluorination,and then effect the strength of the hydrogen bonds.