Study On Synthesis And Properties Of α-Monosubstituted Pyridinium Salts

Pyridinium salts are a type of organic aromatic compounds with six-numbered N-heterocyclic cations and have wide applications in many fields covering chemistry, biology and materials science. Pyridinium salts can be used as non-linear optical materials, fluorescence materials, photosensitive materials, photoinitiators due to their excellent photophysical and photochemical properties. On the other hand, owing to their unique ionic structures, they can also be used as phase transfer catalysts, cationic surfactants, ionic liquids. In recent years, new applications expanded to gene transfer and supramolecular chemistry bring new energy to the pyridinium research. In this case, design and synthesis new pyridinum salts are of great significance.Among several synthetic approaches for pyridinium salts, the reaction of pyrylium salts with primary amines is a predominant method due to its mild reaction condition, easy separation of production and high yield. However, there is no report about the synthesis of a-monosubstituted pyridinium salts by using this method. The properties of pyridinium salts depend on the kind and the position of substituents on pyridinium rings. Due to the a-H of a-monosubstituted pyrydinium salts, it is possible to get a series of new pyridinium salts with novel functions. In our previous research, we found a new method to synthesize a-monosubstituted pyrylium salts from polysubstituted cyclopentadienes, such as2,4,5-triphenylpyrylium salt and2,3,4,5-tetraphenylpyrylium salt. We tried the reaction of them with various primary amines. It was found surprisingly that, in our case, a-monosubstituted pyridiniums were obtained directly from their a-monosubstituted pyrylium precursors. This result also proved the previous conclusion was wrong. This paper is aimed at the synthesis of new a-unsubstituted pyridinium salts, reaction mechanism discussion and applications in synthetic chemistry and supramolecular chemistry fields. The study consists of four parts as below:(1) Synthesis of a-unsubstituted pyridinium salts. Performed the reaction of2,4,5-triphenylpyrylium salt and2,3,4,5-tetraphenyl pyrylium salt with various primary amines. A series of a-monosubstituted pyridinium salts were synthesized and carefully characterized. It was found that a-monosubstituted pyridinium salts can be obtained directly from their a-monosubstituted pyrylium precursors, the number of the phenyl gourps is one of the key roles of this reaction;(2) The photocyclization of α-unsubstituted pyridinium salts. Phenanthridinium salts were synthesized by α-unsubstituted pyridinium salts photocyclization. It was found that the reaction did not occur when pyridinium salts with strong electron-donating substituents. And the spectra indicated that phenanthridinium salts have good fluorescence properties. Meanwhile, under different conditions phenanthridinium salts can form different crystal morphologies;(3) a-unsubstituted pyridinium salts as anions sensors. Pyridinium salts with amino substituent and dipodand bipyridinium salt were studied as anions sensors, the sensing and induce-fit process were examined by fluorescence spectra, UV-vis spectra, H NMR titration and single crystal. The results show that1-(2-aminophenyl)-2,4,5-triphenylpyridinium can be served as an efficient chemodosimeter for sensing fluoride ion, an intramolecular cyclization from pyridinium3q to pyrido[1,2-a]benzimidazole7a initiated by F-.1,1-(1,3-phenylene)bis-2,4,5-triphenylpyridinium could be served as an efficent NO3-sensor. Due to the match of moleculars’ structures, hydrogen bond and the charge induction enhanced planarity of molecular5e. And the planarity of molecular induced the increasing of the fluorescence intensity;(4) Intramolecular cyclization of a-unsubstituted pyridinium salts. The intramolecular cyclizations from2-aminophenyl-pyridinium salts to pyrido[1,2-a]benzimidazole in the presence of base were studied. By studying reaction conditions, such as solvents, base, temperature, atmosphere, reaction time, the best reaction condition was confirmed. A series of pyrido[1,2-α]benzimidazoles were synthesized and carefully characterized. And the fluorescence spectra indicated that this kind of compounds with high quantum yield could be served as good fluorescence materials. The nitro substituent can modulate the fluorescence.