Identification Of Photochemical Sensors Based On Ion And Amino Acids

Fluorescent and colorimetric chemosensors in selectively recognizing ions and molecules are of current interest in supramolecular chemistry. The advantages of molecular fluorescence for sensing are high selectivity, sensitivity and simplicity. Colorimetric sensors are popular due to their capability to detect analyte by the naked eye without resorting to any expensive instruments. In this thesis, a series of new fluorescent and colorimetric chemosensors were synthesized and investigated for their applications in the detection of magnesium cation (Mg²⁺), fluoride anion (F^-), cysteine (Cys) and homocysteine (Hcy). The details are as follows:Three fluorescent compounds (C1, C2 and C3) based on 4-hydroxy coumarin were designed and synthesized to recognize Mg²⁺. The compiexation behavior of these compounds with magnesium ion was investigated by means of absorption and fluorescent spectra. All of theses fluorescent probes can selectively recognize magnesium ion. Especially, the solubility of C3 in water was increased by introducing ammonium. Under physiological pH conditions, the emission wavelength maximum of C3 was blue-shifted from 582 nm to 522 nm and the fluorescent quantum yield of C3 was increased from 0．09 to 0．32, when Mg²⁺ was added into the solution of C3．Moreover, no obvious interference in its fluorescence emission was observed when performing the titrations with Mg²⁺ in the mixture of other biologically important cations such as Na⁺, K⁺ and Ca²⁺. These facts indicated a high selectivity of C3 in sensing Mg²⁺. The apparent dissociation constant (K_d) of C3 is in theμM range, indicating the high sensitivity of C3 in sensing Mg²⁺.The terbium complex Tb(PMIP)₃(PhCN) was designed and synthesized as a reagent for anions. The triplet energy level of PhCN is a little higher than that of ⁵D₄ of Tb³⁺ and lower than that of PMIP, which induced a back-energy transfer from Tb³⁺ to PhCN. When three equivalents of fluoride anions were added into the acetonitrile solution of Tb(PMIP)₃(PhCN), PhCN could be replaced by fluoride anions and the above back-energy transfer was prohibited, which resulted in a fluorescence enhancement of the complex. After nine equivalents of fluoride anions were added, the replacement of PMIP with fluoride anions prohibited the ligand PMIP sensitized energy transfer of the terbium complex, resulting in the fluorescence quenching of the system. Similar phenomena were observed upon addition of acetate into the solution of Tb(PMIP)₃(PhCN). However, in the aqueous solution (CH₃CN/H₂O, 9:1, v/v), the terbium complex shows a remarkable selectivity of fluoride anions over the other anions (Cl^-, Br^-, I^-, ClO₄^-, NO₃^-, NO₂^-). As a reagent, the sensitivity of Tb(PMIP)₃(PhCN) is about 10^-8 mol·L^-1 for the fluoride anions.Two azo dyes DMPA and DHPA were synthesized and characterized as colorimetric probes for cysteine (Cys) and homocysteine (Hcy). Under neutral pH conditions, the recognition of DHPA for Cys and Hcy gave obvious color changes from pink to yellow, which was clearly visible to the naked eyes. Moreover, the competition experiments showed the interference from other amino acids was minimal. The NMR spectra indicated that the reaction of the azo dyes containing an aldehyde group with cysteine or homocysteine afforded very stable derivatives thiazolidines or thiazinanes. The method is selective and sensitive for cysteine and homocysteine detection without the interference of other amino acids. Importantly, the recognition of cysteine and homocysteine could be observed by naked eyes.Two diarylethenes derivatives 1,2-bis[2-methyl-5-aldehyde-3-thienyl]cyclo-pentene (1) and 1,2-bis[2-methyl-5-aldehyde-3-thienyl]perfluorocyclopentene (2) were successfully synthesized and exhibited photochromic property. Upon addition of cysteine or homocysteine, no obvious phenomenon was observed in open or closed isomer of 1, indicating inactivity of aldehyde in 1．However, obvious new absorption peaks appeared for open or closed isomer of 2 with six fluorine groups when cysteine or homocysteine were added. Especially, the absorption maximum of 2 in the photostationary state was blue-shifted from 617 to 461 nm upon addition of cysteine or homocysteine. The reaction mechanism is attributed to the special reaction between aldehyde and cysteine or homocysteine. The competition experiments showed the interference from other amino acids was minimal, indicated a high selectivity of 2 in sensing Cys and Hvy.