Effect of n-alkyl substitution on Cu(II)-selective chemosensing of Rhodamine-B derivatives

Abstract

A series of rhodamine-B hydrazide based molecular probes (1-10) were synthesized by their derivatization with n-alkyl chains of different lengths at hydrazide amino- end. These probes exhibited absorption (A_~557) and fluorescence (I_~580) ‘off-on’ signal transduction along with colourless→magenta colour transition in the selective presence of Cu(II) ion among all the competitive metal ions investigated. The effective coordination of these probes to Cu(II) ion under the investigated environment forming [Cu.L]2+,(L=1-5) and [Cu.L₂]²⁺,(L=6-10) complexes led to their spiro-ring opening, which in turn, expressed through signatory spectral peaks of ring-opened rhodamine. All these probes exhibited Cu(II) selectivity in signaling despite structural modifications to the core receptor unit through variation in nature of the 1-alkyl substituents. However, the sensitivity of signaling and kinetics of spiro-ring opening varied and could be correlated with the number of carbon atoms present in 1-alkyl substituents. The structural elucidation with X-ray diffraction and X-ray photoemission spectroscopic analyses rendered further insight on structure-function correlation in their Cu(II)-complexes. These probes with Cu(II)-coordination induced selectivity in signaling, high complexation affinity (log Ka =4.8-8.8), high sensitivity(LOD = 80nM-4.1 nM), faster response time (rate =0.0017-0.0159 s^-1) and reversibility with counter anions ascertained their potential utility as chemosensors for Cu(II) ion detection.