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Chemical Sensors Based on Fluorescence Turn-On Mechanism by Using Excited State Intramolecular Proton Transfer

Chen, Weihua
http://rave.ohiolink.edu/etdc/view?acc_num=akron1334772708

Abstract Details

2012, Doctor of Philosophy, University of Akron, Chemistry.
Exited state intramolecular proton transfer (ESIPT) occurs in aromatic molecules that contain hydrogen bond donor and acceptor groups located at a close distance. In the event of proton transfer, a proton migrates from one group to the other, thereby producing a tautomeric form which emits fluorescence with a large Stokes shift. My research focuses on the development of novel fluorescent sensors with an ESIPT turn-on mechanism, which intends to address some of the problems that we human being are facing today. As a starting point, we choose 2-(2-hydroxyphenyl)benzoxazole (HBO) as a model compound which absorbs at 320 nm and emits at 500 nm. Our strategy is to design a sensor molecule in which either the H-donor or acceptor is masked to block the ESIPT pathway. Upon specific binding with the target analytes, the masked H-donor (or acceptor) can be released, thereby enabling the ESIPT. Such kind of ESIPT turn-on approaches as a sensing event trigger in chemical sensors were rarely reported in literature when we started our research in 2009. My dissertation is devoted to seek fundamental understanding on the ESIPT mechanism, and to utilize the ESIPT in chemical sensor design. First, an efficient method for constructing HBO derivatives has been developed by using palladium (II) mediated oxidative cyclization in Chapter 2. The reaction was carried out in mild conditions with easily-synthesized Schiff base as starting material and was compatible to a wide range of function groups. The Pd (II) involved in the reaction was consumed in the β-hydride elimination step and regenerated in the presence of oxygen. This method opens a path for efficient construction of HBO molecules with various functional groups, which has been used throughout my research to synthesize the HBO derivatives of interest. HBO compounds often exhibit dual fluorescence in polar solvents due to rotamerism around the single bond connecting the benzoxazole to the 2-hydroxyphenyl ring. Although the two rotamers of HBO are assumed to convert to each other through a bond rotation, the magnitude of the rotational barrier is not known in literature. By designing 2-(2,6-dihydroxyphenyl) benzoxazole (DHBO), we incorporated two hydroxyl groups in the molecule, which allows to monitor two related hydrogen bondings, i.e. N...H-O and O-H...O, simultaneously. NMR study of DHBO over variant temperature detected two sets of OH peaks that coalescence at -20 degree. This observation led to the first estimation of rotational energy barrier for HBO derivatives (rotational barrier energy = 10.5 kcal/mol), which is described in Chapter 3. In addition, 2-D NMR and fluorescence study of compound 4-bromo-2- (4-methyl-2-benzoxazolyl)phenol revealed that the syn-rotamer is predominant in CDCl3. Pyrophosphate (PPi) is a biologically important target which is involved in many cellular processes. To selectively and sensitively detect this species over other anion, for example phosphate and ATP, is a challenging task. In chapter 4, we demonstrated that significantly improved PPi detection can be achieved by using the binuclear compound 3•2Zn, in which the proton donor is masked to disable ESIPT. Unique geometry of the designed binuclear compound allows 3•2Zn to selectively accommodate a PPi molecule into the two zinc centers, thereby initiating the thermodynamically favored Zn-O bond cleavage to enable ESIPT upon PPi binding. Because of its high selectivity over other anions, it can detect PPi released from a PCR experiment and has the potential to be further developed for DNA pyrosequencing. In addition we also have synthesized the mono nuclear compounds 8•Zn in order to understand the mechanism of this sensing event. Palladium catalyzed reactions are widely used in organic synthesis such as Suzuki-Miyaura coupling, Heck coupling and Buchwald-Hartwig reaction. To analyze the residual palladium (often in the form of Pd(II)) in the final product is important because it may be a health hazard. The government threshold for palladium in drugs is 5¿¿¿¿¿¿¿¿¿¿¿¿“10 ppm. In Chapter 5, we examined how the substituents at the 5 position of the phenol ring affect the Pd(II) promoted oxidative cyclization and we found electron withdrawing group can enhance the reaction rate following a linear Hammett plot pattern. Based on these findings, a strong electron withdrawing group dicyanomethyl was introduced at the 5 position to obtain a new sensor for Pd(II). The sensor itself was non-fluorescent, however, upon addition of Pd(II) in acetonitrile/water at room temperature, it underwent oxidative cyclization to form the benzoxazole ring, therefore turning on the ESIPT fluorescence. The sensor showed high selectivity and was able to detect Pd(II) in the presence of other competing metal ions. Serendipitously, the sensor gave an emission band in the near-infrared (NIR) region with the peak maximum at 780 nm, thus providing the first NIR sensor for palladium detection.
Yi Pang, Dr. (Advisor)
Christopher Ziegler, Dr. (Committee Member)
Michael Taschner, Dr. (Committee Member)
Jun Hu, Dr. (Committee Member)
Newby Bi-min Zhang, Dr. (Committee Member)
196 p.
Chemistry
fluorescent sensor; benzoxazole; intromolecular hydrogen bonding; ESIPT; pyrophosphate sensor; palladium sensor,

Recommended Citations

Citations

  • Chen, W. (2012). Chemical Sensors Based on Fluorescence Turn-On Mechanism by Using Excited State Intramolecular Proton Transfer [Doctoral dissertation, University of Akron]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=akron1334772708

    APA Style (7th edition)

  • Chen, Weihua. Chemical Sensors Based on Fluorescence Turn-On Mechanism by Using Excited State Intramolecular Proton Transfer. 2012. University of Akron, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=akron1334772708.

    MLA Style (8th edition)

  • Chen, Weihua. "Chemical Sensors Based on Fluorescence Turn-On Mechanism by Using Excited State Intramolecular Proton Transfer." Doctoral dissertation, University of Akron, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=akron1334772708

    Chicago Manual of Style (17th edition)

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This open access ETD is published by University of Akron and OhioLINK.