Study On The Geometrical Structures, Interactions And Photophysical Properties Of Four Hydrogen Bond Clusters

The geometrical structures, UV-Vis absorption spectra, fluorescence spectra and the infrared spectra ofthe hydrogen-bonded clusters formed by esculetin, etc and dimethylsulfoxide, etc have been studied withthe time-dependent density functional theory method in this paper. By analysing the changes of thehydrogen bonding interactions within the various hydrogen-bonded clusters and the photophysicalprocesses such as spectral shifts, intermolecular electron transfer and intramolecualr charge transfer, wehave explained the influences of the intermolecular hydrogen bonding interactions on the geometricalstructures and photophysical properties of the hydrogen-bonded clusters.In Chapter1, the research status of atomic and molecular physics have been introduced, thephotoexcitation and decay processes as well as the influences of the intermolecular hydrogen bondinginteractions on the photophysical processes of molecule systems such as fluorescence are discussed and themain research contents of this paper are introduced. In Chapter2, the frequently-used quantum mechanicsmethods to study the electronic structures of multi-electron systems, such as configuration interaction withsingle excitations method, density functional theory method and time-dependent density functional theorymethod are introduced.In Chapter3, for the different absorption and fluorescence wavelengths of esculetin in the three aproticsolvents such as dimethylsulfoxide, we have calculated the absorption and fluorescence spectra of thehydrogen-bonded clusters formed by esculetin with dimethylsulfoxide, tetrahydrofuran and acetonitrile. Wefind that the different spectral redshifts happened to esculetin molecule in the three different aproticsolvents should be due to the different hydrogen-bond accepting abilities of the three solvents which resultin the different strength of the intermolecular hydrogen bonds. The hydrogen bond interaction betweenesculetin and dimethylsulfoxide is the strongest which imposes the most significant stabilization effect onthe ground-state esculetin molecule and induces the largest spectral redshift13nm. The hydrogen bondinteraction between esculetin and tetrahydrofuran is a little weaker which leads to a small spectral redshift11nm. The hydrogen bond interaction between esculetin and acetonitrile is the weakest which results in thesmallest spectral redshift6nm. After the photoexcitation, the hydrogen binding energies of the twointermolecular hydrogen bonds in the three esculetin clusters are increased from85.85,70.62and51.72 kJ/mol in the ground state S0to104.23,82.18and61.44kJ/mol in the first singlet excited state S1, whichare increased by18.38,11.56and9.72kJ/mol, respectively. The different increases of the intermolecularhydrogen bond strength lead to different changes of the S0-S1energy gaps of esculetin molecule in the threeaprotic solvents. The largest increase of hydrogen binding energy in dimethylsulfoxide leads to the largestfluorescence spectrum peak448nm while those in solvents tetrahydrofuran and acetonitrile are at442and443nm, respectively. Moreover, we have calculated the absorption and fluorescence spectra of the threedeprotonated forms of the esculetin molecule, and find that the minor absorption and fluorescence spectraobserved in the experiments should be mainly due to the first deprotonated form, namely the de-H1form.In Chapter4, we have optimized the ground-state geometrical structures of the varioushydrogen-bonded clusters formed by resorufin anion with different numbers of water molecules. Thecalculated results indicate that hydrogen bonds HB-II and HB-III formed at oxygen atoms O2and O3arestronger than hydrogen bonds HB-I and HB-IV formed at heteroatoms O1and N1, which should be due tothe different electronegativity of the four atoms. Furthermore, as the number of the water molecules in thehydrogen-bonded cluster increases, each of the hydrogen bonds becomes weak, which should be ascribedto the competitions between the hydrogen bonds. Moreover, based on the comparisons between theexcitation energies of the resorufin anion monomer and the various hydrogen-bonded clusters andaccording to the relationship between electronic spectral shifts and the electronic excited-state hydrogenbonding changes first clarified by Zhao, we predict that the intermolecular hydrogen bonding interactionswill be weakened in states S2, S3and strengthened in states S10, S11and S12. In Section2, in order tounderstand the changes of the four intermolecular hydrogen bonds upon photoexcitation to the first singletexcited state S1, we have calculated the S1-state geometrical structures, infrared spectra and the hydrogenbinding energies of clusters Res--Water and Res--4Water. Our calculated results indicate that, after thephotoexcitation, hydrogen bond HB-I is weakened, hydrogen bonds HB-II and HB-III are strengthenedslightly while hydrogen bond HB-IV is significantly strengthened, which should be due to the remarkableintramolecular charge transfer to the heteroatom N1from the rest part of the resorufin anion.In Chapter5, we have investigated the photoinduced electron transfer between coumarin C337andaniline molecules. We have demonstrated that the ground-state hydrogen-bonded clusters C337-AN/MANwill be initially excited to the second singlet excited state S2upon photoexcitation at400nm. Then the S2-state hydrogen-bonded clusters will undergo vibrational relaxation and internal conversion to get to thefirst singlet excited state S1. The bond length of the intermolecular hydrogen bond is significantly shortenedand the directions of the groups involved in the formation of the hydrogen bonds tend to become the samein the internal conversion process, which significantly enhance the electronic coupling between the electrondonator and the electron acceptor and efficiently facilitate the intermolecular electron transfer from anilineor methylaniline to the excited-state C337molecule.In Chapter6, for the gradual redshifts of the absorption and fluorescence spectra of formylperylene(FPe) in the MeOH/ACN mixture solvents with the increase of the MeOH concentration, we havecalculated the geometrical structures, electronic spectra, infrared spectra and the dipole moments of the FPemonomer as well as its hydrogen-bonded clusters formed with1-3MeOH molecules. We find that, in theground state, from the singly hydrogen-bonded clusters FPe-MeOH to the quadruply hydrogen-bondedcluster FPe-3MeOH, the intermolecular hydrogen bonding interactions between the FPe molecule and theMeOH molecules are gradually strengthened, which should efficiently facilitate the intramolecular chargetransfer from the perylene moiety to the carbonyl group and be responsible for the observable intensityincrease in the red tail of the absorption spectra. Upon photoexcitation, the intermolecular hydrogenbonding interactions between the FPe molecule and the MeOH molecules as well as those among theMeOH molecules are further strengthened, which results in more intramolecular charge transfer andstronger electronegativity of the carbonyl group oxygen atom. Eventually, the strong negative charge on theC=O group will be delocalized along the strengthened hydrogen bond networks to the MeOH moleculesand the charge transfer state S1can be consequently further stabilized which results in the obviousfluorescence spectra redshifts.