| Fluorescent probes have attracted much attention because of their various advantages,such as high sensitivity for single-molecules detection, switching operations, visualsub-nanometer spatial resolution capacities for submicron and the sub-millisecondtime resolution submicrons, in situ detection (fluorescence imaging techniques),long-range detection using optical fibers, and many other advantages. Meanwhile,owing to their rich and diverse nature, as well as their wide applicationcharacteristics, organic luminescent materials have become a research hotspot inrecent years. In the studies of both photoluminescence and electroluminescence, aswell as the growing field of dye-sensitized solar cell research, organic light-emittingmaterials play important roles in all of them. As the sensing signal output unit influorescent probes, the photophysical and photochemical properties of fluorescentdyes could directly affect the quality of the sensors.Over the past two decades, BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene)has attracted much interest among numerous fluorescent dyes. The BODIPY dyesshow excellent photophysical and photochemical properties:(1) narrow full widthhalf maxima of absorption and emission band that render them good candidates fluorescent labels;(2) relatively high molar absorption coefficients, implying highabsorbance efficiencies;(3) high fluorescence quantum yields, some of whichreaching100%;(4) robustness against photo bleaching and chemical bleaching.Owing to their good photophysical and photochemical properties, BODIPYdyeshave good potentials as chemosensors, biological labels and organic solar cells.In order to obtain highly sensitive and selective fluorescent chemosensors,receptor groups with different recognization abilities could be used. Therecognization ability of a fluorescent chemosensor can be estimated by investigatingthe stimulus-response relationship of the probe. According to the above strategy, aseries of fluorescent chemosensors based on BODIPY dye has been successesfullydesigned and synthesized, and their structures have also been rationalized andoptimized. The responses of these probes to solvent polarity, acidity and metal ionshave been investigated, and this has provided guiding principles for the design andsynthesis of highly sensitive and selectives fluorescent chemosensors.In the first part of the work, a series of novel fluorescent probes based onBODIPY dyes has been synthesized and characterized by decoratingN-phenylaza-15-crown-5as the host group at the3-position and4-hydroxyphenylgroup at meso-position of the BODIPY core. Both their basic photophysicalproperties and responsive properties to the metal ions and protons have been studiedby determining their UV-vis absorption spectra, emission spectra and mass spectra.From the single crystal structure of compound2.2, the dihedral angle between theindacene and the styryl group is18.70°, which suggests less effective conjugationwithin the entire chromophore. Although there are no π–π stacking interactions, ahead-to-tail dimeric arrangement is observed between two adjacent molecules withintermolecular hydrogen bonds. With the dimer, a short contact distance of1.97between the hydroxy group of one molecule and the azacrown unit of anothermolecule is observed. When2.2is dissolved in different solvents, it shows obvioussolvatochromic behaviors. Their emission energies in various solvents show a lineardependence on the Lippert solvent parameter, showing that it is a good probe for the detection of solvent polarity. Upon addition of HClO4into the acetonitrile solution of2.2, the ICT process from the azacrown moiety to the BODIPY is blocked. Thisreduces the intensity of the UV-vis absorption and fluorescent emission spectra, andgives rise to new absorption and emission bands of higher energies. It showsdifferent photophysical properties when binding to different metal ions with variouscharge and radii. When monovalent Li+and Na+were added, the UV-vis absorptionand emission spectra show negligible changes due to their smaller radii and chargesand lower valence. As the cavity size of the azacrown ether matches better with theionic radius and charge of Mg2+, significant emission enhancement has beenobserved. Accroding to the results of the non-linear curve-fitting method, compound2.2and Mg2+show a1:1stoichiometry. On the other hand, since compound2.3contains two azacrown moieties, it is capable of binding two metal ions at the sametime. Upon addition of Mg2+ions, cooperative binding behavior has been determined.A Hill coefficient of1.90with an overall binding constant (log K) of3.55have beenobserved. Furthermore, binding of Zn2+gives a Hill coefficient of1.05, suggestingthat the two Zn2+binding processes of each azacrown unit are completelyindependent of each other.In the second part of the project, two series of compounds based on BODIPY withflexible alkoxy and rigid acetylenes as spacers have been synthesized. From theUV-vis absorption spectra of3.4-3.6, linear combination of the absorbances of thedonor and the acceptor can be observed. Their emission spectra have been obtainedupon excitation at480nm. Apart from a stronger emission band at638nm region,compounds3.4-3.6show an additional weak emission band at528nm region, whichcan be assigned to the residual emission of the yellow chromophore, suggestinggood energy transfer between the donor and the acceptor. The ratios of the yellowemission intensity at ca.528nm to that of the red emission at ca.638nm, ID/IA, incompounds3.4-3.6are almost identical, suggesting that the length of alkoxy chainshas no significant influence the energy transfer efficiency. The UV-vis absorptionsepctra of compounds3.7and3.8can also be regard as the linear combination of the donor and the acceptor. Their emission spectra have also been obtained by excitationat480nm. In the emission spectrum of compound3.7, only the acceptor emissioncan be observed, while the emission of the donor moiety has almost disappeared.Similarly for compound3.8, strong emission originated from the acceptor with littleresidual emission from the donor. These observations reveal that efficient energytransfer has occurred in compounds3.7and3.8.In the third part of the project, a series of BODIPY compounds containingArBMes2groups at various positions has been designed and synthesized. Theirphotophysical and ion-binding properties have been studied by UV-vis absorption,emission and19F NMR spectra. Compounds4.1and4.2show strong fluorescencewhen fluoride ion was absent. Upon additon of fluoride ion, these anions interactswith the empty π orbital of the boron atoms to form a Bmes2-F-adduct. Therefore,the emission has been quenched as the PET process was triggered, leading to anemission decrease of intensity band. For compound4.3, the absorbance band at632nm decreased and a new band at603nm appeared when Mg2+was added. Inaddition, the anion-binding properties have been further investigated by19F NMRspectroscopy. Thes spectra only show fluoride signals originated from BODIPYwhen F-was absent. Interestingly, upon addition of excess fluoride ions, apart fromthe fluoride signals from the BODIPY compound itself, additional signals of theBmes2-F-adduct as well as free fluoride ions can be observed. For compounds4.4-4.8, increasing the conjugation would lead to a red shift in their UV-visabsorption band. As the ICT process is present, a weak emission intensity was alsoobserved. Compound4.7includes yellow and red emitting chromofores. The yellowemitting band shows weaker emission intensity when excited at480nm, suggestiveof high-efficiency energy transfer. Because of the relatively large pseudo Stokes shift,compounds4.7and4.8can be used as more efficient sensors for metal ions. |
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