Absorption and Fluorescence Spectra of the 1a in different Solvents
To investigate the effects of solvents, UV and fluorescence spectra of sensor 1a were obtained. As shown in Fig. 1a, the maximum absorption wavelengths of the compound 1a were at about 350 nm in common organic solvent, such as acetone, dimethylsulfoxide, acetonitrile, tetrahydrofuran, dichloromethane, ethyl acetate, methanol, ethanol, glycol, isopropanol. But no absorption could be found in water. The same phenomenon could be seen from Fig. 1b, no fluorescence could be found in water. But maximum emission wavelength around 455 nm could be found in common organic solvent. All these suggested that 1a could well dissolvent in most solvents except water.
Fluorescence determination of water content in ethanol
Water exists in all kinds of organic solvents and is the most common impurity in organic solvents. As the most common contaminant in organic solvents, water is often adverse in many organic synthetic reactions and industrial production processes, which negatively affected not only the yield of chemicals but also their activities. Therefore, the determination of the water content in the organic solvent is one of the most important and most commonly encountered analytical problems.
In order to explore the possibility for application of the synthesized sensor in the detection of water content in organic solvents, the fluorescence emission spectra excited by the light of 350 nm of 1a dispersed in ethanol were further examined. As shown in Fig. 2 (a), when the ethanol solution of 1a changed to 10% water in ethanol, the fluorescent intensity increased significantly. This indicates that 1a can be used as fluorescent probes to quickly detect water content in organic solvents. Figure 2(a and b) shows the fluorescence intensity of 1a changed in ethanol with different concentration of H2O (0, 10, 20, 30, 50, 60, 70, 80, 90 v/v, %). It can be seen that with the increase of water content, the fluorescence intensity decreases gradually and the emission peak get a red shift. In order to get more details, Fig. 2(c and d) shows the fluorescence intensity of 1a changed in ethanol with different concentration of H2O (0, 1, 2, 3, 4, 5, 6, 7, 8, 9 v/v, %). It can be seen that with the increase of water content, the fluorescence intensity increased firstly then decreased. The maximum fluorescence intensity could be detected in 6% water in ethanol. The limit of detection was as low as 0.22% based on five times the standard deviation rule.
Absorption and Fluorescence Spectra of the Sensor 1a with Metal Ion
The absorption and fluorescence spectra of sensor 1a were examined following the treatment with different metal ions (5.0 eq.) in 6% water in ethanol solutions. As shown in Fig. 3, after adding Na+, K+, Mg2+, Ca2+, Cr3+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, Cd2+, Hg2+, Pb2+, Al3+, the absorption intensity (I350 nm) and fluorescence intensity (I455 nm) of 1a basically no changed. All these suggested that 1a couldn’t recognize any metal ion, and the metal in solution couldn’t affect the interaction of 1a with water.
Absorption and Fluorescence Spectra of 1a in acidic and basic solutions
Since the compound 1a comprise both acidic (-COOH) and basic (Ar-N) groups, their properties can be very different under acidic or basic conditions, and this was next evaluated. Aqueous/ethanol (1:1 v/v) solutions were used, ensuring complete dissolution of the fluorophores, and the observed pH of the solutions adjusted by aqueous/ethanol (1:1 v/v) NaOH/HCl solutions. As shown in Fig. 4a, fluorophore 1a showed highly varied absorption peaks in different acidic and basic solutions. Absorption peak of 1a displayed a red-shift with increasing pH from 2.31 to 10.72. Then fluorescence spectra of 1a in acidic and basic solutions could be observed. As shown in Fig. 4b, fluorophore 1a showed highly varied fluorescence in different acidic and basic solutions. The fluorophore 1a showed strong emission under basic conditions while being weakly emissive in acidic solutions, similar to azo chromophores. [28] In general, azo chromophores can take H-aggregation as the typical aggregation manner under different conditions. [29–32] And H-aggregation denotes the aggregation showing a blue-shifted band to the molecular absorption band. All these suggested that compound 1a could take H-aggregation with pH decreasing and disaggregate with pH increasing.
The possible mechanism for trace water detection in organic solvents
In order to explore the mechanism for the detection of water content in organic solvents, compound 1b was synthesized. The absorption and fluorescence spectra of 1b dispersed in different organic solvents (dichloromethane, H2O, acetonitrile, tetrahydrofuran, ethanol) were further examined. As shown in Fig. S5 and S6, the maximum emission wavelength of 1b dispersed in dichloromethane, tetrahydrofuran and acetonitrile was around 455 nm. But nearly no fluorescence could be found in water and ethanol. All these suggested that the long carboxyl chain in 1a could increase the solubility in ethanol and denote to increase the fluorescent intensity with the addition of water.
Figure 5a shows the absorption spectra of 1a changed in ethanol with different concentration of H2O (0, 10, 20, 30, 40, 50, 60, 70 v/v, %). It can be seen that with the increase of water content, the absorption intensity decreases gradually and the absorption peak get a red shift. Figure 5b shows the absorption spectra of 1a at 350 nm in ethanol with different concentration of H2O (0, 1, 2, 3, 4, 5, 6, 7, 8, 9 v/v, %). It can be seen that with the increase of water content, the absorption intensity first increase and then decrease.
All these indicate that the presence of hydroxyl in water will lead to the red-shift effect for the absorption peak and change of absorption intensity. Combined with the results of Fig. 4a, H-aggregation of 1a could disaggregate with addition of water, and 1a could precipitate with the increase of water. Because the electron density around N is higher, this atom serves as a stronger acceptor of hydrogen bonds, which decreases the possibility of molecular aggregation caused by π-π interaction in the presence of water. In a word, the change of absorption and fluorescence of compound 1a is attributed to the aggregation and disaggregate caused by addition of water and partially to the increase in polarity of the solvent caused by the addition of water.