There is an increasing demand for monitoring environmental pollutants and the control requires new sensing materials with better sensitivity, selectivity and reliability. In this study, a series of Co7 clusters incorporating various flexible polyhydroxyamine ligands are explored, with the first report of thiocyanate recognition triggered by crystal formation using a Co7 crystal (1). For this, we have fortunately synthesized three new mixed metal Co7 clusters with fascinating structural features. The clusters were characterized by spectroscopic and single crystal X-ray diffraction methods and later by DFT calculations. Due to its better emission spectrum, 1 was further utilized for evaluating its sensing ability towards various anions in water. Surprisingly, 1 shows better quenching ability towards the recognition of SCN- with a better binding constant. The luminescence quenching towards SCN- detection was further verified by the single crystal method, HSAB principle (symbiosis) and theoretical calculations such as DFT studies. The SCXRD data clearly suggest that the Co7 (1) can be converted into Co14 (1a) by direct reaction with NaSCN under ambient conditions. Besides the soft/hard acid-base concept (symbiosis), the energies of formation, and Co-NCS and Co-OH2 bond energies (as unravelled by DFT) are responsible for this transformation. Therefore, 1 can be used as a selective and sensitive sensor for the detection of thiocyanate anions based on the fluorescence amplification and quenching method. Further, the designed cluster has also been utilized to detect anions in human blood samples to differentiate a smoker and a non-smoker. It has been concluded that the samples of smokers have a high degree of thiocyanate (∼12 or 9.5 mg L-1) in comparison to those of non-smokers (2-3 mg L-1). Thus, this kind of cluster material has high potentiality in the field of bio-medical science in future endeavours for identification of the extent of thiocyanate content in smokers.
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