Theoretical Study On The Proton Transfer For 2′-substituted Phenyl(Pyridyl) Benzazole Compounds And Its Chemical Sensor For Fluoride Anion

Proton transfer process is very important in many chemical and biological reactions, which is always a topic of much interest in chemical research. In this thesis, the ground state and excited state proton transfer properties for a number of 2′-substituted phenyl (pyridyl)benzazole compounds have been studied by quantum chemical calculations. In addition, the chemical sensor for fluoride ion based on the study to these chemical substances of excited state proton transfer for the fluorescence signal group has been study. The main results obtained are listed as follows:1. The properties of intramolecular proton transfer reaction of 2-(3′-amino-2′-pyridyl) benzimidazole(3′-APyBI) in gas were studied at the B3LYP/6-31+G(d) and TD B3LYP/6-311++G(d,p)//CIS/6-31+G(d) level. The emission spectrum of 3′-APyBI in solvent (water, acetonitrile, ethanol, and THF) was calculated. The ground-state potential energy surface has been studied; two reaction paths of the stepwise mechanism of double proton transfer have been suggested and the Path 1 (a1 i1 i2) is the favorable in reaction kinetics. The study of excited states indicates that there mainly existed intramoleclar single proton transfer for 3′-APyBI. The calculation of emission spectrum of 3′-APyBI shows dual fluorescence characteristic, the short wavelength fluorescence emission is about 390nm and Stokes fluorescence emission is about 490nm.2. The excited state intramolecular proton transfer (ESIPT) and the photophysical mechanism of UV absorbers of 2-phenylbenzotriazole derivatives [2-(2′-hydroxyphenyl)benzotriazole (H-TIN), 2-(2′-aminophenyl)benzotriazole (APyBT) and 2-(2′-mercaptophenyl)benzotriazole (MPyBT)] with different proton donor that have been performed by density functional theory (DFT) and second order M?ller-Plesset perturbation theory (MP2). The results show that the most stable tautomer is normal configuration N that has intramolecular hydrogen bond for different tautomers of three compounds in ground state, while the tautomer T and twisted configuration Ttwisted are unstable in ground state. The potential energy curves for ESIPT of three compounds show that H-TIN and APyBT could occur ESIPT overcame the energy barrier of ca. 7.06kJ/mol and 20.7kJ/mol, respectively. However, the ESIPT of MPyBT can occur without a barrier; meanwhile combined the results of molecular orbital and charge difference density three-dimensional cube for the three compounds, it indicates that H-TIN, APyBT and MPyBT can occur the fast ESIPT and twisted intramolecular charge transfer, and these reasons show that they all have good UV stabilization.3. The light-induced intramolecular proton and charge transfer properties of 2-(3′-hydroxy-2′-pyridyl)benzoxazole(HPyBO) in the gas phase, water, chloroform, acetonitrile and cyclohexane have been studied by theoretical method. The results show that the enol configuration E1 of HPyBO is the most stable configuration, in gas phase, cis-keto configuration K1 is unstable i while it can be stable in the solvent environment. With the solvent polarity increasing, E1 can more easily change to the E2, E3 and E4 configurations. The results of ground and excited potential energy curves show that HPyBO can occur ESIPT overcame a little energy barrier in the solvent and the gas, and ESIPT is a fast process. The K1 to the K2 rotation isomerization process can occur after consider the solvent effects; meanwhile isomers of HPyBO have intramolecular charge transfer. Light-induced charge transfer accompanies ESIPT of E1 to K1. The fluorescence emission of HPyBO shows dual fluorescence emission character in solvent conditions which is conform to the experimental study results, the solvent polarity influence significantly to the fluorescence emission of HPyBO, with the polarity increases, the opposite of its dual-fluorescence emission displacement that the short-wave peak red shifted, while the Stokes peak is blue-shifted.4. The effects of chemical substitution on the photophysical and photochemical properties of 2-(2′-aminophenyl)benzimidazole(APBI) have been theoretically studied which replacing the hydrogen atom of the amino group in APBI by -CH₃ (E-C), -SiH₃ (E-OSi), -NH2 (E-N), -COH (E-CO), -NO₂ (E-NO₂), -CF₃ (E-F), -CN (E-CN₃), -OMe (E-OMe), -COCH₃ (E-CC), Ts (E-S), p-CH₃-C₆H₄-CO- (E-C=O), p-CH₃-C₆H₄-NHCO- (E-CN). The results show that the introduction of substituents does not change the slight deviation from the plane configuration for APBI, in ground state enol E configuration for all derivatives is the most stable structures; sub-stable structure is the R configuration that is rotamer to the E around the C2-C7 single bond rotation. The keto K form becomes stable in the ground state when the E-CN3, E-F, E-NO₂, E-N or E-OMe substituents are present in the molecule. The results of NICS ground state of the ring show that substituents affect the APBI ring electron delocalization. Excited state proton transfer potential energy surface studies have shown that all of the derivatives could occur ESIPT, when the introducing of substituents for the E-CN3, E-N, or E-OMe, the ESIPT of the APBI derivatives in S1 state is barrierless process. Whereas E-C, E-C=O or E-OSi substituents are present in the molecule the ESIPT potential energy surface of APBI remains almost unaltered by the effect of the substituent in S1, the K*and E* conformation are almost at the same energy level, the energy barrier for ESIPT converts the enol to the keto tautomer is about 20kJ/mol. When substitutents is E-CC, E-CN, E-CO, E-F, E-NO₂, or E-S which make K* configuration more stable than E* in S1 states, the occurrence of this type of ESIPT of the difficulty for overcoming the energy barrier at the level of order is E-S (0.2kJ/mol) < E-F(3.0kJ/mol) < E-NO₂(7.2kJ/mol) < E-CN(12.6kJ/mol) < E-CC(15.8kJ/mol) < E-CO(17.0kJ/mol). In summary, the introduction substituent to the amino group of APBI still has the ESIPT property of the parent structure.5. The chemical sensor for fluoride anion of twelve 2-substituted amino phenyl (or pyridyl) benzoxazole compounds, which a hydrogen atom of the amino group replaced by hydrogen bond identification group for F - (p-methyl phenylsulfonyl or p-methyl carbaniloyl), have been studied by theoretical method. The results show that the compound e(2-(2′-tosylaminophenyl)benzimidazole) which has the strongest interaction with the F- and better detectable fluorescent signal in the all compounds. The mechanism of these compounds of chemical sensor for fluoride anions is that first through intermolecular hydrogen bonding to form complex, and then lead to the binding site of proton from the compound to block the ESIPT of probe in excited state, at last the fluorescence emission properties of complexes will be different from probe molecules. Interaction energy, ground state and excited state structure and spectra calculations show that a(2-(2′-tosylaminophenyl)benzothiazole), c(2-(2′-tosylaminophenyl)benzoxazole), e(2-(2′-tosylaminophenyl)benzimidazole), g(2-(2′- p-methylphenylureaphenyl)benzothiazole), j(2-(2′-p-methylphenylureapyridyl)benzothiazole) and l(2-(2′-p-methylphenylureapyriyl)benzoxazole) have a great potential for such compounds applied to ratiometric fluorescent chemosensor for F-ion .