Ratiometric Naphthalimide Fluorescent Probes For Cations Based On ICT Mechanism

Fluorescent probes can transfer molecular recognition events into fluorescence signals and then make a bridge between man and molecule. The advantages of molecular fluorescence for sensing can be summarized: high sensitivity of detection down to the single molecule, "on-off switchability, subnanometer spatial resolution and submillisecond temporal resolution, observation in situ, remote sensing by using optical fibres, et al. Ratiometric fluorescent signals, in contrast with intentisy-responsive ones, have higher sensitivity and can be used in quantitative detection. Ratiometric measurements involve the observation of changes in the ratio of the intensity of the absorption or the emission at two wavelengths. They permit signal rationing and thus increase the dynamic range and provide built-in correction for environmental effects.4-amino-l,8-naphthalimide fluorophore contains an electron-donating group (often an amino group) conjugated to an electron-withdrawing group, it undergoes ICT, intramolecular charge transfer, from the donor to the receptor upon excitation by light. When the electron-donating character is changed, a shift in fluorescence emission spectra would be expected. In this thesis, a series of ratiometric 1,8-naphthalimide fluorescent probes for cations based on the ICT mechanism were designed and synthesized. On the basis of the high reactivity of the precursor N-butyl-4-bromo-5-nitro- 1,8-naphthalimide, we introduced different cation receptors into the 4 and 5 positions, and then obtained ratiometric fluorescent probes for Cu²⁺, Zn²⁺, Cd²⁺, and pH, respectively.Two types of Cu²⁺ ratiometric fluorescent probes, totally including fifteen coupounds, have been designed and synthesized. The binding of Cu²⁺ by CuBF compounds reduces the electron-donating ability of the electron donor. Owing to the reduction of conjugating, a blue shift in emission spectra is observed. Probes CuBF1 and CuBF7-ll exhibit Cu²⁺-only sensitive among metal ions such as Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Fe³⁺, Mn²⁺, Al³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Hg²⁺, Ag⁺, Pb²⁺. CuRF coupounds are based on the combination of deprotonation and ICT mechanisms. The secondary amines conjugated to 1,8-naphthalimide could be deprotonated by Cu²⁺, and as a result, large red shifts in both absorption and fluorescence spectra were obtained, from which one could sense Cu²⁺ colorimetrically and ratiometrically. CuRF are also sense only Cu²⁺ among metal ions. Two methods about how to ensure fluorescence not to be quenched by Cu²⁺ have been summarized, and a third one ispresented, that is to compensate the electron-deficiency of Cu2+ with a proper donor. Thus the ET (electron transfer or energy transfer) would be suppressed from fluorophore to copper, and as a result fluorescence is maintained. The donors in CuBF and CuRF are aromatic n electrons and deprotonated nitrogen atoms, respectively.Two ratiometric and colorimetric fluorescent probes ZnRFl-2 for Zn2+ have been designed and synthesized. The secondary amine conjugated to 1,8-naphthalimide could be deprotonated by Zn2+, and as a result, large red shifts in both absorption and fluorescence spectra were obtained, from which one could sense Zn2+ colorimetrically and ratiometrically. These visible emissions, the solutions color changing from piromrose yellow to red and fluorescence from green to red, allow Zn2+ to be readily recognized by the naked eye. Probe ZnRFl-2 for Zn2+ was developed from our former Cu2+ sensors on the base of the same deprotonation mechanism. With the improvement of selectivity for metal ion receptors, we believe that this design strategy would help to extend the development of ratiometric fluorescent probes for other metal ions.Two ratiometric fluorescent probes for Cd2+ CdBFl-2 have been designed and synthesized. The binding of Cd2+ by CdBFl reduced the electron-donating ability of the conjugated nitrogen atom, and blue-shifted the fluorescence emission from 531 nm to 487 nm. However, in the case of Zn2+, the secondary amine conjugated to 1,8-naphthalimide could be deprotonated, and fluorescence emission red-shifted from 531 nm to 558 nm. Thus, CdBFl is a multi-analytes sensing probe.Two fluorescent probes HF1-2 for ratiometric low pH measurements have been designed and synthesized. The two probes have narrow pH-sensitive windows. For HF1 based on ICT mechanism, the fluorescence emission blue-shifted from 566 nm to 522 nm when the solution became more acidic from pH 2.57 to 1.70. In addition, HF1 is non-metal ion sensitive. For HF2 based on intramolecular hydrogen binding, the fluorescence emission red-shifted from 497 nm to 527 nm when the solution became more acidic from pH 2.23 to 1.30.