Multiresponsive Photo-, Solvato-, Acido-, and Ionochromic Schiff Base Probe

Abstract

Nonclassical protomeric tautomerism in Schiff bases have the advantage of controlling and differentiating specific interactions in the −CN− linkage since its interactions are not governed by keto−enol tautomerism.Here, we report about the optical properties of a Schiff base probe (P1). X-ray structure analysis evidenced the existence of an intramolecular hydrogen bond which is responsible of a photochromic-fluorescent behavior. The properties of P1 were investigated by UV−vis and fluorescence spectroscopy in solution and solid state. A positive solvatochromism resulting from specific interactions taking place in P1 was studied by three different solvent scales, namely Lippert−Mataga, Kamlet−Taft, and Catalań, finding consistent results. Moreover, a strong acidochromic behavior was detected and the pK a and pK a * values were determined, finding a photobasic character. Further, an ionochromic behavior was stablished. P1 exhibits a λ-ratiometric fluorescence response toward Sn(IV) giving a luminescence color change from blue to green, displaying also a chromogenic response. Finally, theoretical calculations were conducted to analyze the probe mechanism in terms of natural transition orbitals (NTOs) and spatial extent of charge transfer excitations. The present contribution focused on the factors determining the ability of a single Schiff base probe to present photo-, solvato-, acido-, and ionochromism. ■ INTRODUCTIONOrganic multiresponsive molecules whose optical properties can be regulated by various chemical stimuli have attracted much attention due to their ability to signal different interactions. 1,2 On the other hand, Schiff bases are common organic structures which can be easily synthesized through a one-step synthetic procedure. 3,4 As a result, a vast library of Schiff bases has been developed for several applications, including optical data storage, 5−7 molecular electronics and computing, 8,9 molecular switches 10,11 and sensors. 12,13 However, most of these applications are based on the tautomeric preferences of the Schiff bases, 14 and very few reports regarding specific interactions in nonclassical protomeric tautomerism 15−17 which do not display the typical enolimine−ketoenamine tautomers have been reported. For example, specific interactions in the −CN− imine bond are highly valuable in the field of molecular assembly for micro-and nanostructure fabrication 18 and multiresponsive assemblies. 19−21 In this regard, single small molecules which present multiresponsive properties are highly desirable due to their simplicity to interact with the chemical environment. 22 Further, compared to other covalent bonds such as the click chemistry linkers, the Schiff base structure provides extraordinary reversibility with changing pH values and the stability of these bonds varies with pH, 23−25 or even when changing the polarity and polarizability of the media. 26−28 In addition, Schiff bases having a push−pull character bearing electron acceptor and donor groups connected by a conjugated bridge (D−π−A) ca...

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“…Schiff bases are common organic structures which can be easily synthesized through a one-step synthetic procedure [43]. The ionochromic behavior of Schiff bases and the interaction with a wide range of metal ions have been well documented [44]. Schiff bases containing salicyaldimine based ligands can be used in the development of cost-effective chemosensors because they show significant color changes of the solution and maxima of the absorption bond when they interact with anions and cations [45][46].…”

A photochromic–acidochromic HCl fluorescent probe. An unexpected chloride-directed recognition

“…The new red-shifted band was attributed to a π → π* electronic transition of the conjugated π-system with a protonated imine owing to intermolecular hydrogen interaction between the hydroxyl group and the imino nitrogen atom, similar to previously reported cinnamaldehyde-derived Schiff bases. 3,37 The solid-state fluorescence spectrum of S1 displays an emission band at around 520 nm. The fluorescence characteristics of the film were investigated during the irradiation experiment.…”

“…The field of multiresponsive molecules, of which the optical properties can be regulated by various chemical stimuli, has attracted much attention owing to their ability to signal different interactions. [1][2][3] Schiff bases represent common molecules that are easily synthesized by a one-step synthetic procedure. 4,5 Several applications using Schiff bases have been developed, including optical data storage, [6][7][8] molecular electronics and computing, 9,10 molecular switches 11,12 and sensors.…”

Two novel colorimetric probes (5-HMBA-FH and 3-HMBA-FH) based on fluorescein for copper(II) ion detection

“…The fluorescein and rhodanmine families, as excellent antenna chromophores, own outstanding optical properties, for instance, long emission and absorption wavelength, relatively large fluorescence quantum yield, large extinction co-efficient and high stability to light [21][22][23] . They are frequently used as raw materials synthesizing fluorescent probes and colorimetric probes, such as pyridoxal fluorescein hydrazone, 1-phenyl-3-methyl-5-hydroxypyrazole-4-benzoyl (fluorescein) hydrazone and 2,7-di(2-benzothiazolyl)-fluorescein 24,25 .…”