Design, Synthesis, and Density Functional Theory Studies of Indole Hydrazones as Colorimetric “Naked Eye” Sensors for F Ions

Abstract

A new series of sensors SM-1 to SM-3 was designed and synthesized using indole carboxaldehydes (2a-2c) and 2,4-dinitrophenyl hydrazine. Accompanied by the synthesis, density functional theory investigation was also accomplished at the M06-2X/6-311G+(d,p) functional. A reduction in band gap (ΔE = 4.702-4.230 eV) along with a bathochromic shift (λmax = 433.223-471.584 nm) was seen in deprotonated chromophores than their neutral sensors. Further, significant charge transference from indole toward dinitrophenyl hydrazine was also examined. Global reactivity parameters also expressed the greater stability of sensors than that of their deprotonated form. SM-3 displayed high selectivity toward F ions as compared to SM-1 and SM-2, which respond to both F- and CN- ions. The electronic absorption spectrum was recorded in CH3CN. The sensor SM-3 showed high selectivity toward F- ions with a low detection limit (8.69 × 10-8), and the binding constant for SM-3 was determined as 7.7 × 105. The sensor displayed naked eye views as the color of solution changed from mustard to purple with a red shift of 96 nm. The mechanism suggests deprotonation from the NH group, which was confirmed by 1H NMR. The sensor is found to be useful for detection of F- ions in the real sample and for analytical application (test strip).