The Theoretical Study On The Excited State Proton Transfer Between Hydrogen Bond For Two Molecular System

Due to the high quantum yield,large stokes displacement and strong charge recombination,the excited proton transfer molecules can be applied to the filds of various organic luminescent materials,fluorescent probes and photochemical reactions.The study of the mechanism of excited proton transfer can provide a theoretical support for the design of such molecules.In this thesis,the density functional theory(DFT)and time-dependent density functional theory(TDDFT)were used to study the mechanisms of excited state proton transfer processes for two molecular systems.The effect of electron-withdrawing group,cyano group,substituted at the different position with different number on the proton transfer processes of 10-hydroxybenzo[h]quinoline(HBQ)were studied in ethyl acetate.The optimized geometric structures and infrared spectra show that the intramolecular hydrogen bond of enol form structure weakens when the cyano group is on the pyridine ring while the intramolecular hydrogen bond strengthens when the cyano group is on the phenol ring in the ground(S₀)state.Electron spectra show that the substitution of cyano group also have a significant effect on electron spectra.The reason for the change in the electron spectra can be obtained from the analysis of frontier molecular orbitals(MOs)that is the substitution of the cyano group affects the energy gap of MOs.The calculated potential barriers for proton transfer of these four molecules are 6.83,7.83,4.87 and 2.79kcal/mol in the S₀state,respectively,which illustrated that the presence,position and number of cyano group can affect the proton transfer processes to some extent.After vertical excitation to the excited(S₁)state,the proton transfer processes of these four molecules are barrierless because of the promotion of the charge transfer.Through the analysis of Mulliken charge,it can be found that the cyano group,an electron-withdrawing group,changes the strength of hydrogen bond by sucking away the electronic charge from hydroxyl oxygen or neighboring nitrogen atom of its ring and then affects the barrier in the S₀state.In this work,we studied the effect of cyano group on the proton transfer processes for HBQ and give the reason for this effect in detail.The scientists found 6-azaindole(6AI)trimers by a scanning tunneling microscope,but its detailed mechanism of proton transfer is still unclear.We conducted a detailed theoretical study on the proton transfer mechanism of 6AI trimer in cycloethane solution and put forward our own views on the induction of experimental spectral values according to theoretical spectral values.Theoretical calculations show that the absorption and emission peaks of 6AI monomer are 262 nm and 355 nm respectively.After three 6AI monomers are connected to form trimer by hydrogen bonds,it can absorb energy at wavelengths of 276.7,276.2 and 274.8nm to S₁,S₂and S₃states respectively.The absorption peaks of 6AI monomer(about280nm)and 6AI trimer(310nm)obtained in the experiment are actually the absorption bands of 6AI monomer and trimer mixture solution and TPT tautomer(the configuration after three protons transfered)respectively.When 6AI Trimer is vertically excited to the S₂and S₃state,it will return to the S₁state soon through the internal conversion.Charge changes during excitation strengthen hydrogen bonds in the S₁state,which provide a driving force for proton transfer.Through the potential energy curves and the potential energy surface,it can be concluded that the 6AI trimer reaches the TPT tautomer in the S₁state through the concerted mechanism instead of the stepwise mechanism.After emitting fluorescence back to the ground state,TPT tautomer can return to the original normal structure through the reverse proton transfer process in two ways:one path is three protons transfer at the same time,the other path is one of the protons transfer to the DPT tautomer(the configuration after two protons transfered),and then the remaining two protons transfer at the same time.