| Molecular fluorescence has many remarkable advantages, such as high sensitivityof detection down to the single molecule, rapid "on-off" switchability, feasibility ofhuman-molecule communication, subnanometer spatial resolution with submicronvisualization and submillisecond temporal resolution. Based on the simple review of itsevolution from molecular sensors to molecular switches and then to molecular logiccircuits, this paper emphasized the unique advantages of molecular fluorescence invarious fields, such as analytical chemistry, biochemistry, semiochemistry, medicinalchemistry and environmental sciences. Anthracence, a typical fluorescent molecule,was exemplified to indicate the scientific significance of developing a new fluorophorewith high quantum yield.Highly developed organic synthesis, deep understanding of molecular recognitionand increasing skills on molecular design provided the solid basis for our researchworks herein. Except following the general rules of designing a new fluorescentmolecule, we also emphasized the volatilities of preparation and derivation, especially,the diversity in various coming applications. Therefore,5-methoxy-2-(2-pyridyl)thiazole (2-MPT) emerged as the target molecule in ourapplication-directed design. Through such deliberate introduction of the basic group,the metal ion chelating fragment and the thiazole as the good candidate of thethermally-stable photochromic diarylethelene derivatives in one molecular skeleton,we hope that the designed thiazole could satisfy the demands of both fluorescentmolecular sensors (pH sensor and the metal ion sensor) and the photochromic systems.Then, we hope to construct some molecular switches and the logic gates because theresponses to the various stimuli provided the wide opportunities to molecular devices.Considering the integrality of the research, 5-methoxy-2-(3, or 4-pyridyl)thiazoles (3-MPT, 4-MPT) were also reported in our works. Studies of the opticalproperties of the three compounds indicated that all of the three 5-methoxy-2-pyridylthiazoles (MPT) are the new fluorescent molecules. Especially, the high fluorescentquantum yields of 2-MPT in polar solvents (φFL≈1 in acetonitrile or methanol) arereally the surprised gift to us. And their preparation is also very simple and easy.According to both the experimental studies of the solvent effects and the lifetime of the excited states and the theoretical calculations on the basis of density functionalfunction (DFT), we ascribed the fluorescence of MPT to the internal charge transfer(ICT) excited states.MPT shown a unique pH dependent fluorescence, and a series of pH fluorescentsensors was proposed, which could measure the pH value based On the ratio of the dualemissions upon one excited lights. Further, 2-MPT was also a selective CuⅡionfluorescent sensor through the formation of non-fluorescent 2-MPT chelated CuⅡcomplex. The stoichiometry between 2-MPT and CuⅡion was confirmed to be 1:1through the Job-method. The static quenching mechanism was also supported by theStern-Volmer curves. The complexation constants and the detection limits were alsofitted according to the results of the spectrometric titrations.Detection of FeⅢions based on the dual emissions of MPT upon two excitedlights was an astounding discovery in this work. Based on the experimental results ofthe spectrometric and electrochemical cyclovoltmetric titrations and the studies of theStern-Volmer curves, the sensing mechanism of FeⅢwas believed to be a redoxreaction between FeⅢand MPT. We thus proposed 2-MPT and 4-MPT as fluorescentFeⅢsensors, which could take the ratiometric measurement of FeⅢion due to the dualemissions under two excited lights.Studies of MPT as the molecular fluorescent sensors disclosed that they had thedifferent responses to the different chemical stimuli (H+, CuⅡand FeⅢ), whichprovided the possibilities to construct some molecular logic circuits. Therefore, weproposed the two combined logic gates, NOR-INH for trifluoroacetic acid (I1) and CuⅡ(I2) and NOR-OR for trifluoroacetic acid (I1) and FeⅢ(I2), respectively.Further derivations of MPT to the photochromic diarylethelenes were proved tobe successful because of the thermal irreversibility and photo-reversibility. They werethe photo-optical molecular switches just as we expected. To our surprise, effects of theprotonation of these prepared photochromic compounds not only resulted inacidichromism, which provided multiple colors for display, but also introduced theproton-transfer equilibrium at the excited states, which resulted in the proton-lockedread-out form for data memories. Accordingly, we constructed a digital molecular logiccircuits with four-states; three inputs(UV lights, visible lights and trifluoroacetic acid)and three outputs, which could execute AND, INH and NOR logic operations.
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