Novel pyrazoline and pyrazole “turn on” fluorescent sensors selective for Zn²⁺/Cd²⁺ at λ_em 480 nm and Fe³⁺/Fe²⁺ at λ_em 465 nm in MeCN

Abstract

A small series of simple pyrazoline and pyrazole based sensors, all derived from the same chalcone precursors, were synthesised, characterised and screened for their fluorescence “turn on” properties in the presence of multiple metals.

One-pot preparation of pyrazole “turn on” and “turn off” fluorescent sensors for Zn2+ and Cd2+ directly from chalcones via in situ aromatisation

“…Previous work 10 was used as a baseline (entry 1, Table 1) and 1 H NMR was used to determine the percentage (%) conversion of C1 to P1 using the ratio of the chemically distinct methyl (CH 3 ) groups in the pyrazoline P1* at approx. 2.90 ppm and the pyrazole methyl (CH 3 ) at approx.…”

“…1). 9 “Turn on” fluorescent sensors are characterised by increased λ em with analyte, 3 demonstrates Fe 3+ /Fe 2+ selectivity 10 and P1 displays a “turn on” response with Zn 2+ and Cd 2+ in MeCN (Fig. 1).…”

“…15 3 and P1 demonstrate structural complexity is not a prerequisite for complex functionality and that simple molecular structures can detect biologically important analytes. 10,11…”

“…To date, there are few examples of direct chalcone to pyrazole syntheses, often requiring vigorous heating under acidic conditions 18 or use of a catalyst. 19 The traditional two step synthesis involves 1,2 addition of hydrazine to the enone 10,11,20 forming a pyrazoline 21 which is isolated, purified, and undergoes subsequent aromatisation to a pyrazole (blue and red respectively (Scheme 1).…”

“…A range of chemical oxidants have been reported to perform this pyrazoline to pyrazole aromatisation including MnO 2 , 22 FeCl 3 , 23 CoCl 2 24 and CuCl 2 . 25 Previous work 10 demonstrated the addition of 8.0 equivalents (eq.) methylhydrazine at room temperature resulted in minor formation of pyrazole (yield 8–18%).…”