Aggregation Induced Enhanced Emission (AIEE) Based Colorimetric and Selective Turn-on Fluorescent Sensor for Cyanide

We have prepared and reporting a new cyanide colorimetric and turn-on uorescent chemo sensor L. Naked-eye colorimetric studies shows the prepared receptor is selectively sensing cyanide ion in acetonitrile medium. The sensor L shows very weak uorescence in acetonitrile medium. The emission intensity of L gets reduced (quenching of uorescence) upon addition of one equivalent of cyanide. Meanwhile further gradual addition of more than one equivalents of cyanide onto 1:1 cyanide complex of L shows remarkable uorescence enhancement. The weakly emissive receptor L show drastic enhancement in uorescence intensity upon addition of excessive cyanide ion which is mainly attributed due to the restriction of intramolecular rotation of 1:1 L•CN - complex and intermolecular aggregation of host: guest complex. This enhanced uorescence emission is attributed due to the aggregation induced enhanced emission (AIEE) is subsequently supported with UV-Vis, emission and 1 H-NMR spectroscopic studies. The preferential binding of cyanide by electron decient receptor L through hydrogen bonding and anion···π interaction over benzene analogue of L is also analyzed with help of MEP surface analyses. To the best of our knowledge, this is the rst example for selective colorimetric and turn- on cyanide sensor through AIEE mechanism. This type of uorine rich organic material having such an aggregation enhanced emission properties in response to cyanide addition will be useful to construct the new type of material for various sensory applications.

studies. The preferential binding of cyanide by electron de cient receptor L through hydrogen bonding and anion···π interaction over benzene analogue of L is also analyzed with help of MEP surface analyses.
To the best of our knowledge, this is the rst example for selective colorimetric and turn-on cyanide sensor through AIEE mechanism. This type of uorine rich organic material having such an aggregation enhanced emission properties in response to cyanide addition will be useful to construct the new type of material for various sensory applications.

Main Text
Development of simple and effective chemo sensors for selective detection of hazardous anions is really challenging area of research in the eld of synthetic anion receptor chemistry [1][2][3][4][5] . Further, designing a selective colorimetric sensor for a particular anion is even a complicated one since anions having different shapes and charges. 6 Among the hazardous anions, cyanide ion is one of the most poisonous anions because trace amounts of its presence lead to deactivates many biological processes such the activity of cyto-c and cellular respiration. 7 Cyanide is extensively used in industries such as in metal mining and electroplating process. The extensive usage of cyanide in industry also increases the risk of environmental pollution. 7 Giving the importance of design and sensing the cyanide ion, we have recently reported bipodal, tripodal based electron de cient selective colorimetric and turn-on uorescent sensors for cyanide ion. 8-10 Majority of these sensor systems are working through either charge transfer or electron transfer mechanisms. [11][12][13][14][15][16] Thus, there are several chemo sensor systems for cyanide anion so far developed based on change in uorescence emission intensity, naked-eye colorimetric detection of cyanide, displacement and chemodosimetric approaches. [17][18][19][20][21] It has been observed that several types of organic compounds were utilized as uorescent sensors for detecting anionic species through aggregation induced enhanced emission (AIEE) phenomenon; whereas upon binding of guest species, the emission of normal uorophore would be quenched, in particular at higher concentration level. [22][23][24][25][26] Unlike normal receptors, AIEE active receptors show minimal or zero relative emission in dilute condition. On the other hand it turned in to remarkable turn-on uorescent receptors in the higher concentration. This enhanced uorescent emission is mainly attributed due to the restriction of intramolecular rotation of 1:1 L•CNcomplex and intermolecular aggregation of host: guest complex.
At this juncture, developing AIEE based receptor molecules which will helps us to open a window for new opportunity to selectively detect a particular anion at their lower concentration level. With the long term interest to develop new receptor molecules for selective cyanide ion detection, here in we are reporting a per uro-hydrazone based organic synthetic receptor L, Scheme 1.
Instantly, we have tested the colorimetric responses of sensor L by adding tetrabutyl ammonium salts of various anions into the respective acetonitrile solution of L. Only the solution contains receptor L with cyanide and uoride ions show immediate color change from colorless to faint brown and yellow respectively, gure 1 inset picture. In the respective UV-Vis spectrum a new absorption band is appeared in the visible region. Appearance of new absorption band in the visible region after addition of cyanide and uoride into the acetonitrile solution of L which is support our naked-eye colorimetric nding of L with cyanide and uoride. As evidenced from the naked eye experiment and UV-Vis spectra ( gure 1 and inset picture), the receptor L is almost intact with other anions. There were no color changes or spectral changes observed for complex 1 upon addition of any other anions, which clearly suggesting that the complex 1 shows high selectivity towards azide anion, (see ESI).
The receptor L alone has no signi cance in its emission in the spectrum. The above synthesized receptor L shows relatively weak uorescent emission in acetonitrile solution. When we excite the molecule at 260 nm, the sensor molecule L shows a weak mono uorescence emission with two centered maxima at 309 nm and 417 nm in dry acetonitile. Further, upon 1:1 stocichiometric addition of cyanide and uoride anion into the respective acetonitrile solution of L, the respective emission intensity is further quenched, gure 2. There were no considerable changes were observed with addition of other anions as tetrabutyammonium salts. As depicted in the gure 2, it is clear that upon UV illumination, the intensities of uoride and cyanide complexes of L were quenched which will justify our obtained emission spectral data.
The binding e ciency of the receptor L towards uoride and cyanide ion is estimated through emission titration. The emission titration was performed with monitoring the emission intensity of L at 309 nm and 417 nm upon addition of calculated amount of uoride and cyanide ion into the acetonitrile solution of L.
The emission intensity of receptor L at 309nm and 417nm are started to decrease upon addition of increasing amount of uoride. Finally it has reached minimum, gure 3. Further we have calculated the stoichiometric ratio of this ligand-cyanide/ uoride host: guest system and it is found to be 1:1 by job's plot (ESI). The association constants and LoDs are calculated and tabulated in table 1. In order to check the trend of emission spectral changes upon addition of cyanide beyond the one equivalent, we have decided to continue the cyanide anion addition beyond 1 one equivalent i.e. up to 10 equivalents with gradual additions (in to 1:1 -L: cyanide complex), gure 4. Interestingly, it has been observed that the intensity of uorescent spectrum of L: cyanide complex is started to increase upon gradual addition of cyanide and reached the maximum when 10 equivalents of cyanide is added. In addition, the emission spectrum also gets blue shifted from 417 nm to 471 nm (~57 nm), gure 5. The observed strong binding towards cyanide/ uoride can be explained with the help of molecular electrostatic potential (MEP) maps of receptor L and their anionic complexes. The molecular electrostatic potential surface maps are generated by using MolView programmer, and the results are displayed in the gure 8. 27 The blue color dominated regions in the MEP surface of L are electron de cient areas in which anion can bind through anion···π interactions. From the gure 8a, it is very clear that the uoro-substituted substituted benzene rings are holding positive clouds inside the aromatic ring (blue in color) and hence the respective penta uoro substituted benzene rings became more positive which is a favorable condition for approaching anion towards electron de cient aromatic rings. Meanwhile on the other hand when we don't have electron withdrawing natured uoro substitution on the phenyl rims the electrons are poised middle of the aromatic ring and those became more positive (deep red in color). This could not be the favorable conditions to approach a negatively charged anion, gure 8b.
In conclusion, We have prepared and reporting a new cyanide colorimetric and turn-on uorescent chemo sensor L. Naked-eye colorimetric studies shows the prepared receptor is selectively sensing cyanide ion in acetonitrile medium. The sensor L shows very weak uorescence in acetonitrile medium. The emission intensity of L gets reduced (quenching of uorescence) upon addition of one equivalent of cyanide.
Meanwhile further gradual addition of more than one equivalents of cyanide onto 1:1 cyanide complex of L shows remarkable uorescence enhancement. The weakly emissive receptor L show drastic enhancement in uorescence intensity upon addition of excessive cyanide ion which is mainly attributed due to the restriction of intramolecular rotation of 1:1 L•CNcomplex and intermolecular aggregation of host: guest complex. This enhanced uorescence emission is attributed due to the aggregation induced enhanced emission (AIEE) is subsequently supported with UV-Vis, emission and 1 H-NMR spectroscopic studies. The preferential binding of cyanide by electron de cient receptor L through hydrogen bonding and anion···π interaction over benzene analogue of L is also analyzed with help of MEP surface analyses.
To the best of our knowledge, this is the rst example for selective colorimetric and turn-on cyanide sensor through AIEE mechanism. The overall concept and results are depicted in the gure 9. This type of uorine rich organic material having such an aggregation enhanced emission properties in response to cyanide addition will be useful to construct the new type of material for various sensory applications.