Study Of The Proton Transfer Of2-substituted Benzimidazole In The Cucurbit[]urils Nanocavity

Proton transfer exist in a variety of chemical and biochemical processes, as oneof the basic reactions in biochemical systems. Some of the reactions such as theproton transfer in the ion channel of creatures, the proton transfer in the tunel in theprocess of enzyme catalysis, the tansport of proton through membrance in solutionand the proton transfer in the process of catalysis of zeolite molecular sieve all takeplace in the millipore at nano-scale. The confined proton transfer process are studiedby using cucurbit[]urils for a namo-scarle cavum which simulate the environment ofthe namo-scarle cavum in the creature, and the method of quantum chemicalcalculation follows many other spectrascopy equipment are used for the studing, andthen, the proton transfer at different pHs are at the work as follows.1、The excited-state intramolecular proton transfer (ESIPT) reaction of theprotropic forms of2-(2-amino-3-pyridyl)-benzimidazole was studied at the TDB3LYP/6-31++G(d,p)//CIS/6-31G(d,p) level. The interaction of2-(2-amino-3-pyridyl)-benzimidazole (2-A3PyBI) and cucurbit[8]uril (CB8) and the effect to theproton transfer of2-A3PyBI were studied by using fluorescence, absorption spectrum and~1H NMR spectra. The study reveals that2-A3PyBI has a dual emission fluorescence which corresponding to the two proton transfer molecule structure, the effeciency of proton transfer in the strong acid and base solution is bigger than in nature solution. The cationic form of2-A3PyBI can binds with CB8and forms1:2inclusion complex, which determined by Benesi-Hildebrand equation. At pH=3.0, the long wavelength emission band and the short wavelength emission band decreased in the present of CB8, the decrease ratio of the long wavelength emission peak is less than the short wavelength emission peak.At pH=5.0, the short wavelength emission peak decreased in the present of CB8, but increased in the long wavelength emission peak. They both indicated that, the encapsulation of2-A3PyBI into CB8cavity enhanced the excited-state intramolecular proton transfer process. The~1H NMR results indicted that the benzene ring entrapped into the CB8cavity.2、The effect of the macrocyclic host, cucurbit[7]uril (CB7)，on the photophysicalproperties of the2-(2-amino-3-pyridyl)-benzimidazole (2-A3PyBI) dye have been studied in aqueous solution at selected pHs by using fluorescence and UV–Visspectroscopy. Different phototropic forms except the anionic form of the dye (bivalentcationic, univalent cationic, neutral forms) forms inclusion complexes with CB7macrocyclic cavity, with different binding constant each other. Dual fluorescencewere got at the selected pHs, and an obvious enhancement of the blue emission bandwhile a disappearance of the red emission band were observed in the presence of CB7,it indicates that the process of the intramolecular excited-state proton transfer (ESIPT)of2-A3PyBI is retarded by encapsulating into the CB7macrocyclic hydrophobiccavity, and then hence its subsequent conversion to the imino form. The~1H NMRspectra result shows that the inclusion complex forms with the benzene ring inside butthe pyridine ring outside the cavity.3、The interaction of2-(3-Acetamido-2-Pyridyl)-benzimidazole (2-3Am2PyBI)with cucurbit[7]uril (CB7) macrocycle and the effect of inclusion on the protontransfer of2-3Am2PyBI were studied by using emission fluorescence,~1H NMRspectra and quantum chemical calculation. By the way, the proton transfer of the dyewith and without Acetyl (2-3A2PyBI) were also studied. The results indictes that theAcetyl promoted the process of proton transfer of the benzimidazole and the cationicform binds with CB7easier than the other forms of the dye. The fluorescence studingat selected pHs suggest that the proton transfer process of the dye was retarded byinteracting with CB7nanocavity and the mole ratia is host:guest=1:1analyzied bythe Benesi-Hildebrand method. The dye entered into the CB7cavity with its benzenering was proved by the~1H NMR titration experiment and the protonated forms wereverified by the quantum calculation with fluorescence spectra.