Zn(II) Complexes Based on a Schiff Base: Mechanochromism- and Solvent Molecule-Dependent Acidochromism

Abstract

A new Schiff base molecule (HL) with a triphenylamine group and its three zinc(II) complexes (1, 1-THF, and 1-DMF) were synthesized and characterized. As expected, three kinds of loosely packed structures that are susceptible to mechanical stimulation were adjusted through twisted conformations and weak intermolecular interactions (C−H•••π, π•••π, and hydrogen bonds). They all exhibit remarkable mechanochromic luminescence (MCL) behavior with high contrast upon mechanical grinding due to the phase transition between a crystalline and an amorphous state, as confirmed by powder X-ray diffraction (PXRD) studies. Especially, grinding of 1-THF was accompanied by the departure of lattice tetrahydrofuran (THF) molecules. Further investigation revealed that 1-THF and 1-DMF showed outstanding acidochromic behavior. The PXRD and thermogravimetric analysis (TGA) results proved that fumigation with HCl could destroy the interaction between the solvent and ligand molecules, resulting in the phase transition from the crystalline state to the amorphous state, which is essential for their acidochromic properties. The acidochromic mechanism is different from the reported protonation and deprotonation process. Both HL and complex 1 exhibit no acidochromism. This work will shed light on the mechanism of acidochromic properties of Schiff base zinc(II) complexes and expand the potential solid-state stimuli-responsive luminescence materials.

Stimuli-Responsive Properties of a Zinc(II) Salen-Type Schiff-Base Complex and Vapochromic Detection of Volatile Organic Compounds

“…In summary, the variegated vapochromic response of the cast films of 1 (Scheme ) represents a unique feature in the panorama of stimuli-responsive Zn­(II) molecular materials. − ,− In fact, the possibility to detect various VOCs having Lewis basicity, and in particular, the discrimination of primary aliphatic amines and pyridine, represents a clear advance compared to our previous work , and to other vapochromic investigations, the latter mostly related to acidochromism. − …”

“…To further explore the responsive behavior of Zn(salen)type molecular materials toward volatile Lewis bases, we chose the Zn(salmal) derivative, 1, having the 5,5′-tert-butyl-bulky substituent on the salicylidene rings, allowing comparison with previous investigated complexes 27 −29,31 having different substituents and substitution patterns. This complex was found to possess unique thermochromic and vapochromic characteristics, compared to those of our previous investigations 29,31 and literature studies on stimuli-responsive molecular materials of Zn(II) coordination complexes, [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28]30 allowing the detection of various VOCs having Lewis basicity and, among them, the discrimination of primary aliphatic amines and pyridine.…”

“…In this regard, Zn­(II) metal complexes and Zn­(II) coordination polymers represent an interesting family of stimuli-responsive materials, − although most of these studies refer to their mechanochromic and acidochromic properties, the latter mainly related to the chromic response when exposed to hydrochloric acid or ammonia vapors. Analogously, Zn­(salen)-type complexes, in the absence of coordinating species, form molecular aggregates via intermolecular Zn···O interactions − and are characterized by multiresponsive behavior. − Moreover, thanks to their Lewis acidic characteristics, they are potentially suitable materials for sensing volatile Lewis bases. − However, their application as chemosensors of volatile Lewis bases is rather limited. ,, …”

Multi-stimuli responsive behavior of two Zn(ii) complexes based on a Schiff base with high contrast

“…7,9,22–43 In addition, vapochromic luminescence is also an important type of stimulus response to certain vapors. 44–51 Among vapochromic properties, the responses to HCl vapor and NH 3 vapor are of particular importance because they are generated in electroplating, textile, fertilizer production, and rubber industries. Inhalation of HCl vapor can lead to respiratory ulcers and pulmonary edema, while excessive exposure to NH 3 vapor can cause corrosion of the skin, eyes, and respiratory system.…”

Efficient emission of Zn(ii) and Cd(ii) complexes with nopinane-annelated 4,5-diazafluorene and 4,5-diazafluoren-9-one ligands: how slight structural modification alters fluorescence mechanism

“…Zinc( ii ) and cadmium( ii ) complexes establish one of the most important classes of emissive compounds. 71–121 Earlier, we synthesized a series of mononuclear zinc( ii ) and cadmium( ii ) complexes with hybrid ligands containing a 2,2′-bipyridine moiety and natural terpene (+)-limonene or (+)-3-carene moieties and demonstrated that they exhibit luminescence with the quantum yield increasing in the following order: free ligand < cadmium( ii ) complex < zinc( ii ) complex. 122 The complexes with the limonene derivative demonstrate excitation wavelength–dependent emission with nanosecond and microsecond lifetimes of the excited states.…”