The ultrafast proton transfer and the following dynamics for aromatic Schiff bases N , N ′-bis(salicylidene)ethylenediamine (salen) and N , N ′-bis(salicylidene)-1,4-butylenediamine (salbn) were investigated with experimental and theoretical methods. A dual emission property with a large Stokes shift in salen and salbn indicates that excited state intramolecular proton transfer occurs with photoexcitation. An efficient single proton transfer was confirmed within 200 fs for both molecules. Subsequently, a fast twisted motion of the keto moiety carries cis -keto to a relaxed stable geometry in the S 1 state. Following the twisted motion, the phenol ring at keto moiety further rotates to a conical intersection with the ground state and a cis – trans isomerization occurs. The isomerization rate is high, which dominates the competition with the radiative transition, resulting in weak emission intensity. It is confirmed that the length of alkyl chain affects the direction of phenol ring twisting and rotation during the whole subsequent relaxation of excited cis -keto tautomer. Compared with polar solvent acetonitrile, the barrier of isomerization is higher and the hydrogen bond on keto moiety is stronger in nonpolar solvent toluene. It makes fluorescence radiation channels competing with isomerism more likely to occur, contributing to the observed difference of enol/keto emission ratios of salen and salbn in toluene and acetonitrile.
Huie Jiang,Lijuan Chen,Zhijian Li,Junwei Li,Huaiqin Ma,Lulu Ning,Nihao Li,Xinhua Liu
A facile AIE fluorescent probe which exhibits a large stokes shift of 202 nm has been successfully developed. The probe can recognize Cd 2+ with high selectivity by inhibiting ESIPT process, leading to observably blue-shift and significantly enhanced fluorescence with the colour changed from green to cyan. Excitedly, the recognition of Cd 2+ is completely free from the interference of Zn 2+ and Cd 2+ could be quantitatively detected in aqueous solution with the detection limit of 1.5 × 10 −7 M. Moreover, the probe has been successfully applied to detect Cd 2+ in real water samples and image Cd 2+ in living cells.
The investigation on the construction strategy, relationship between the structure and photophysical properties of fluorescein analogs plays an important role in the rational design of specific fluorescent dyes. In this work, a fluorescein analog Fc-1 , containing a reactive aldehyde group was prepared using 2′, 7′-dichlorofluorescein (DCF) as the raw material and Vilsmeier–Haack reaction mechanism. Meanwhile, two more fluorescein analogs Fc-2 and Fc-3 were synthesized based on Fc-1 and the Knoevenagel reaction mechanism. The results of Fc-1 indicated that the yield of Fc-1 was higher than 95% after a step of the low-temperature hydrolysis (40 °C) from an intermediate imine salt DCF-3 . The fluorescence quantum yield ( Φ ) of Fc-1 was relatively high in polar protic solvents, even in H 2 O ( Φ = 0.25). Fc-1 had large Stokes shifts in different solvents (≧ 123 nm), which was indentified by the Gaussian calculation of Fc-1 . Meanwhile, Fc-1 was also used as a ratio and colorimetric fluorescent probe for H 2 S in vitro , and the dianion ( [Fc-1-2H] 2- ) mechanism for detecting H 2 S was proposed by nuclear magnetic resonance (NMR), pH titration, and Gaussian calculation. Additionally, Fc-1 can be used in leukocyte classification because of its relatively high Φ in water, cell permeability, and cytotoxicity. All the data of dyes Fc-2 and Fc-3 indicated that the derivatization of the carboxyl group for Fc-2 did not affect the photophysical properties of Fc-2 . Both of them had relatively large Stokes shifts and long emission wavelengths . The large Stokes shift of Fc-2 and Fc-3 was due to the distribution of electron clouds in the excited states of Fc-2 and Fc-3 . The results of pH titration proved that dyes Fc-2 and Fc-3 were sensitive to the changes in pH and exhibited their potentials as pH probes. Thus, the strategy of constructing fluorescent dyes based on Vilsmeier–Haack and Knoevenagel reactions and the relationship between structure and properties will be beneficial to developing more dyes with large Stokes shift.
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