**Rosita Diana and Barbara Panunzi \***

Department of Agriculture, University of Napoli Federico II, via Università 100, 80055 Portici NA, Italy; rosita.diana@unina.it

**\*** Correspondence: barbara.panunzi@unina.it

Academic Editors: Jorge Bañuelos Prieto and Ugo Caruso Received: 6 October 2020; Accepted: 25 October 2020; Published: 28 October 2020

**Abstract:** Tridentate ligands are simple low-cost pincers, easy to synthetize, and able to guarantee stability to the derived complexes. On the other hand, due to its unique mix of structural and optical properties, zinc(II) ion is an excellent candidate to modulate the emission pattern as desired. The present work is an overview of selected articles about zinc(II) complexes showing a tuned fluorescence response with respect to their tridentate ligands. A classification of the tridentate pincers was carried out according to the binding donor atom groups, specifically nitrogen, oxygen, and sulfur donor atoms, and depending on the structure obtained upon coordination. Fluorescence properties of the ligands and the related complexes were compared and discussed both in solution and in the solid state, keeping an eye on possible applications.

**Keywords:** zinc ion; fluorescence; tridentate ligand

#### **1. Introduction**

Over the past 20 years, fluorescence-responsive compounds are increasingly required for many technological applications, from lighting and switch devices to bio-imaging and analytical probes. Materials based on transition metal complexes were advantageously utilized. In this area, interest is growing in the abundant, less expensive, and environmentally "green" zinc(II) metal cation. Today, science is in great demand to address the challenge of sustainability. Scientific innovations and advances must play a major role in technological breakthroughs thanks to the choice of sustainable green matter as a substitute for highly toxic, expensive, and difficult to dispose products. So, green chemistry as the design of less hazardous chemical products and processes is a hot topic today. The replacement of heavy metal atoms in aromatic macrostructures with the small eco-friendly zinc cation, able to modulate the properties of coordination environments, easier to synthetize, and low cost, can be a way to meet the challenge.

From a research point of view, to the advantage of a large variety of coordination geometries and elaborate molecular architectures, zinc(II) complexes add the versatility of the luminescent levels, both in solution and in the solid state. Real breakthroughs for the novel luminescent technologies were obtained by employing highly efficient, stable, and cheap emitters. Among them, several zinc(II) complexes have to be included [1–6].

In the coordination complexes, electronic charge can be transferred between different molecular entities in the complex, from the electron donor zone to the receiving electron acceptor zone. In most complexes, charge-transfer electronic bands involve electron transfer between metal atoms and ligands. The charge-transfer bands in transition metal complexes are due to the shift of charge density between orbitals predominantly metal in character and those predominantly ligand in character. Most transitions are ligand-to-metal charge-transfer (LMCT) if the transfer occurs from the ligand orbitals to the metal, or metal-to-ligand charge-transfer (MLCT) in the reverse case. Due to its d<sup>10</sup> closed

shell configuration, zinc(II) ion has no optical signature. The d–d electronic transitions are not expected in zinc emissive complexes and the lowest energy excited states are mainly of a ligand-centered charge transfer (LCT) nature (as intramolecular charge transfer, ICT, and intraligand charge transfer, ILCT) and/or ligand-to-ligand charge transfer (LLCT) nature, rarely due to ligand-to-metal charge transfer (LMCT) states involving s or p empty orbitals of the metal [7]. The absence of ligand field stabilization energy leads to the formation of complexes with various coordination numbers, such as 4, 5, or 6, with tetrahedral, square pyramidal/trigonal bipyramidal, and octahedral geometries.

In dependence of the type of ligands and the coordination pattern imposed by the ligands, zinc(II) complexes exhibit fluorescence tuning both in intensity and/or emission maximum. Specifically, fluorescence enhancement (chelation enhanced fluorescence, CHEF mechanism [8–11]) or fluorescence reduction (metal-binding-induced fluorescence quenching [12]) can occur upon coordination. In addition, due to the lowering of the excited state of the bonded ligand upon coordination, a qualitative fluorescence tuning (blue or red shift of the emission maximum from ligand to metal) can be observed. The two different mechanisms can activate in solution and/or in the solid state, with drastic variations in fluorescence with respect to the ligand. Recently [13], the ability of the fluorophore ligand to form a π-contact with the metal cation was correlated with the fluorescence quenching or enhancement ability. Therefore, the information about the excited state geometry and the frontier orbital arrangement of the excited states is essential to understanding and foresight of the phenomenon.

The fluorescence enhancement effect is often the result of a stabilization of the excited state in poorly emissive ligands upon coordination [14]. Zinc(II) cation often causes a CHEF effect. Typically, in emissive zinc(II) complexes, the fluorescence emission results from a π-π\* LCT and the role of the zinc ion is to freeze the favorable re-emissive conformation. In this way, it is possible to obtain strongly emissive materials for lighting devices by increasing the emission of organic molecules upon zinc coordination. Ligands with flexible spacers and appropriate aromatic moieties able to fold over the metal in a locked conformation show good potential in analytical and bio-chemistry. Many sensing systems for zinc cation detection exploit the CHEF effect [15–17]. At the opposite, fluorescence quenching is quite unusual in zinc complexes. Nevertheless, ICT or ILCT transitions can be responsible of zinc binding-induced fluorescence quenching. Fluorescence quenching is observed in rigid pyridine-based ligands upon coordination, due to a decreased HOMO–LUMO energy gap, or in zinc-induced quenching of the protein intrinsic fluorescence due to conformational perturbations [18–21].

Finally, the aggregate nature of materials consisting of fluorophores frozen into polymeric chains or networks give rise to noticeable changes in the energetic levels of the ligands. The assembly of emissive pincers by zinc coordination produces the most varied polymeric structures: coordination polymers (CPs) obtained by zinc bridges [22–25], metallated polymers obtained by coordination with pre-formed chains [26,27], and polymeric networks obtained by interlacing of flexible zinc-crossed fluorophores [28–30]. Owing to both the restrictions imposed to fluorophore and the efficient electron hopping in the tight structure, relevant emission tuning with respect to the free ligands and to mononuclear structures is envisaged.

As modifications of the ligand are expected to change energy levels and structural features, the number of binding sites in the pincer ligand plays a decisive role on the spectroscopic properties. Tridentate ligands are quite simple pincers, often low cost and easy to synthetize. At the same time, tridentate ligands guarantee good stability to the complex thanks to a relevant chelate effect. Ligands that can bind zinc(II) through three donor atom groups, such as *O*, *N*, and *S* donor atom groups, represent an interesting class due to the variety of behavior and applications. Tridentate zinc complexes are known to produce simple and mononuclear or intricate even polymeric structures, by themselves or with auxiliary ligands. By doing so, they can cause relevant fluorescence tuning with respect to the free ligand, in solution and/or in the solid state. In addition, depending on the charge of the pincer

ligand, the zinc-binding reaction leaves unoccupied coordination sites for additional ligands, which in turn can modulate the structure and properties of the derived complexes.

Some representative functional groups involved in the build of fluorescence-responsive tridentate pincers can be identified. The Schiff base moiety, obtained by reaction of amines and carbonyl-containing compounds, is a good candidate for the synthesis of *N*, *O*, *S* donors containing ligands. In particular, in half-*salen*-type ligands, one nitrogen atom and one oxygen atom group chelate the metal, while the C=N functional group constitutes a versatile bridge between the *N*,*O* pincer and the branch of the ligand containing the third binding site. This role could be played by a third nitrogen, oxygen, or sulfur atom, as an example the carbonyl oxygen, the thione sulfur, or the donor atom of an heteroaromatic ring. The formation of a coordination core consisting of five- and/or six-membered rings between the pincer and zinc cation produces stable and sometimes highly emissive coordination complexes. Finally, the coordination core can be all made up of aromatic rings with *N*, *O*, *S* donor heteroatoms fixed in a rigid pincer or able to fold up as a flexible pincer.

This is an overview of selected cutting-edge examples of tridentate zinc(II) complexes causing fluorescence tuning with respect to the ligand. In this research area, many articles have been produced in the last 15 years. Starting from the synthetic and purely phenomenological approach, to the complete and detailed analysis of the energy levels, until the synergistic approach to the structure–property relationship, there is much to tell on this subject. –

In the following sections, ligands will be classified according to the binding donor atom groups, and fluorescence properties of ligands and related complexes will be comparatively discussed. By use of an easy "cartoon" representation (as in Figure 1), we will propose a quick intuitive overview of groups of structures, examined according to the photoluminescence (PL) response and the application area. use of an easy "cartoon" repr

**Figure 1.** Schematic representation of most of the coordination cores achievable from tridentate pincers binding zinc(II) cation (in green). X, X ′ , and X" can be *N*, *O*, and *S* atom groups.

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#### **2. Nitrogen Binding Sites**

– The study of polydentate ligands with available *N* donor sites is a prolific research area in coordination chemistry [31–43]. Typically, nitrogen binding sites are neutral sites where the lone pair of nitrogen atom is available for donation. Nitrogen aromatic heterocycles are excellent building blocks for the synthesis of *N*-donor polydentate ligands. Single or fused five- and/or six-term rings produce stable and soluble pincers able to direct the chemical and chemo-physical properties. A substantial amount of literature articles on pyridine-containing tridentate ligands has emerged. Related to the enhanced π-electron delocalization upon zinc coordination, in some cases, the zinc-binding-induced fluorescence quenching phenomenon was detected. At the opposite, pyridine and other nitrogen heterocycles assembled in flexible architectures often produce a CHEF effect.

#### *2.1. Terpyridine-Type Ligands*

Rigid pyridine-based tridentate ligands as terpyridine (Tpy) are well-known chelate ligands for transition metals [44–46]. Many Tpy ligands were found to be emissive in the solid state, with photoluminescence quantum yields (PLQYs) strongly depending on their own molecular structure and on substituents, with a relevant emission tuning due to metal coordination. Zinc-binding-induced fluorescence reduction/quenching was often detected, as well strong color emission tuning. In 2009, a series of zinc(II) bis(4 ′ - phenyl-terpyridine) complexes with substituted Tpy were studied by J. Popp and coworkers [47] (Figure 2). The zinc ion is coordinated with the three nitrogen atoms from each of two terpyridine ligands and with two PF<sup>6</sup> <sup>−</sup> as auxiliary ligands. Tuning of the color emission from violet to cyan (425–487 nm) in dependence of the extension of the π-conjugated system of the ligand was observed, and from low to high PLQYs (from 6 to 64%) were recorded. Thin solid films obtained by low-concentration dye-doped poly(methylmethacrylate) (PMMA) matrix show bright emission, with PLQYs up to 0.30. By DFT study, the HOMO energy level was also significantly influenced by the ligand structure, whereas the LUMO energy appeared to be independent of the electronic pattern of the Tpy ligand, with the enhanced π-electron delocalization leading to a decreased HOMO–LUMO energy gap.

**Figure 2.** Tpy-type tridentate pincers causing zinc-binding fluorescence reduction/quenching.

' The crystalline structure of a Tpy-type ligand upon coordination with different transition metals was examined in 2015 by K. Rissanen and coworkers [48] (Figure 2). The ligand displayed bright blue emission and high PLQYs dissolved in several organic solvents. A distorted octahedral arrangement with two tridentate terpyridine ligands was detected in the zinc complex and a significant

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greenish-yellow tuned emission attributed to ILCT states, still appreciable but lowered in intensity with respect to the ligand. DFT analysis rationalized the ICT-type electronic transitions involved from the diphenylacetylene moiety to the terpyridine group. Except for the other d<sup>10</sup> closed shell cadmium (II) ion, a complete metal-binding-induced fluorescence quenching was observed in the presence of the other divalent metal ions.

Tpy-type ligands with different substituents were examined for their binding ability specifically toward zinc(II) cation in 2016 [20] and in 2017 [21] (Figure 2). In [21], Zhen Ma and coworkers examined the compound obtained by reacting 4′ -phenyl-terpyridine and ZnSO4·7H2O by X-ray crystallographic analysis. A neutral 1:1 (Zn:Tpy) complex with a coordinated sulfate group, qualitatively emissive in the solid state, was obtained. The emission spectrum displayed two bands at ca. 377 and 499 nm red-shifted with respect to 361 and 373 nm for the ligand in the solid state. The former higher energy band of the complex was assigned to LLCT whereas the latter band to LMCT [49,50]. In 2016, the influence of the coordination stoichiometry was explored by Yi Pang and coworkers [20], by reacting substituted Tpy-type ligands (R in Figure 2 is a *p*-substituted phenyl ring bearing a donor group and R'=H or R'=CH<sup>3</sup> on the lateral pyridine groups) with ZnCl2. The authors pointed out that the zinc complex forms in a 2:1 (ligand: metal) ratio with a low zinc(II) concentration and in a 1:1 ratio with a high concentration of metal. In ethanol, fluorescence quenching occurs in 2:1 complexes, whereas, turning into 1:1 complexes, fluorescence increases in the opposite direction, marking the role of the coordination pattern in the emission intensity. In 1:1 complexes, temperature-dependent fluorescence spectroscopy elucidated the role of the ICT mechanism between donor and acceptor groups. Due to the occurrence of a relevant ICT process, the strong donor substituents induce zinc-binding fluorescence quenching and red-shift of the emission maximum. Conversely, the weak donor substituent *p*-CH3-phenyl group causes an increase in the fluorescence emission of the 1:1 complex.

Very recently [51], B.N. Gosh and coworkers examined the X-ray crystal structure of 4 ′ -functionalized terpyridine complexes (Figure 2). Zinc cation did show a trans-arrangement of the terminal pyridine nitrogen atoms with respect to the central pyridine ring in the 2:1 ligand zinc tridentate complex. The terpyridine ligand exhibits a bright blue emission in dichloromethane with 68% PLQY and produces a different coordination pattern in dependence of the coordinated metal. Upon zinc complexation, ligand emission shows a significant reduction while other transition metal ions completely quench the fluorescence of the ligand. The weak fluorescence of the zinc complex can be imputed as an ILCT transition from the amine moiety to the metal coordinated terpyridine fragment [48,52,53]. The non-fluorescence nature of the complexes with other metal cations can be imputed as MLCT-type transitions.

Recently, fluorogens exhibiting aggregation-induced emissive properties (AIEgens) have grabbed scholars' attention in various scientific areas. In contrast to conventional aggregation-caused quenching (ACQ) molecules, AIEgens are weakly fluorescent/non-emissive in diluted solution and emit intensely in their aggregate form (concentrated solution and solid state), owing to the restriction of intramolecular motions [54]. Tpy-based zinc complexes AIEgens have been proved to be attractive and versatile tools for biological imaging and chemical sensing. Under physiological conditions, Tpy-type ligands are employed in bio-medical applications, such as for living cell imaging. As an example, in 2005 [55], Valery N. Kozhevnikov and Burkhard König presented 2,2′ -bi- and 2,2′ :6′ ,2"-terpyridines with aminomethyl and aryl substituents employable as luminescent probes coordinating in a pentadentate coordination core zinc cation in physiological media.

In 2019, a thiophene bridged Tpy tridentate Zn(II) complex (Figure 3) was designed for RNA-specific targeting thanks to its AIE bright yellow-green fluorescence emission under physiological conditions by Ju Yupeng Tian, Dandan Li, and coworkers [56]. On the other hand, in 2016, a new fluorescence sensor for citrate (acting itself as a tridentate ligand) detection was developed by integrating an AIE Tpy ligand with zinc(II) by Ju Mei, Jianli Hua, and coworkers [57]. The enhanced electron-withdrawing ability of the complex gave rise to a red-shifted fluorescence compared with the organic ligand. The zinc-based probe was not an AIEgen but showed significant fluorescence

enhancement (fluorescence turn-on mechanism) under substitution of the auxiliary ligands with citrate. The bathochromic shift of the absorption maximum and the decrease of fluorescence intensity may be ascribed to the stronger ICT in the complex as compared to the ligand (Figure 3).

**Figure 3.** AIE behavior of zinc complexes from Tpy-type tridentate pincers.

## *2.2. N,N,N Schi*ff *Base-Type Ligands*

π π Schiff base ligands containing an additional nitrogen binding site, usually derived from a nitrogen heterocycle, have been widely reported. The versatile CH=N bridge allows the employment of a large variety of substituents and to build the most varied architectures. Unlike their complexes, this kind of ligand is often non-emissive, and a general behavior due to the CHEF effect was mostly found. In 2011, Kaushik Ghosh and coworkers [58] explored the crystal structure and photophysical properties of four zinc(II) complexes derived from the tridentate ligand Pyimpy (see Figure 4) with different auxiliary ligands. In the complexes, two pyridinic nitrogen groups are involved in the equatorial plane binding along with the iminic nitrogen group. The complexes show various fluorescence emission in toluene solution upon excitation of the charge transfer band near 350 nm (ascribable to π,π\* LC of the ligand [59]) while the free ligand Pyimpy displays no fluorescence emission in the same experimental conditions. The phenomenon was ascribed to the loss of vibrational energy decay due to ligand stiffening under coordination. In addition to the zinc-binding fluorescence intensity enhancement, a shift of the maximum of emission was detected, in dependence of the auxiliary ligands.

Other nitrogen aromatic heterocycles have recently emerged as electron donor-containing moieties. Among them, pyrimidine groups attract attention for their role in biological systems [60,61]. Susanta Kumar Kar and coworkers in 2012 [62] prepared two tridentate *N*,*N*,*N* donor Schiff base ligands [63] using pyrimidine- and pyridine-containing carbonyl compounds (see Figure 4). Whereas the ligand with a methyl substituent was fluorescent silent, its zinc complex showed a strong CHEF effect in methylcyclohexane. Probably due to photoinduced electron transfer processes in the presence of several nonbonding electron pairs on the nitrogen donor atom groups, the ligand π,π\* transitions are not allowed and the flexible bonds of the ligands cause the activation of the non-radiative channel. CHEF activates by the increase in conformational rigidity of the ligands upon strong zinc binding, which prevents non-radiative channels [64]. Pyrimidine-containing Schiff base ligands were studied in 2015 by Saugata Konar [65] for the zinc-binding ability into 1-D coordination polymers held together by µ1,5-bridged dicyanamide ions. The difference due to the activation of a *N*,*N*,*O* bonding site (ligand L1, achieved by the presence of a half-*salen* group) with respect to the *N*,*N*,*N* bonding site (ligand L2, see Figure 4) was pointed out. Both the ligands display low fluorescence intensity in methanol. Conversely, both their polymeric complexes show red-shifted enhanced fluorescence emission. The low emission intensity of the ligands was ascribed to photo-induced electron transfer processes, while the CHEF effect to an increase in conformational rigidity of the ligands upon complexation. The complex derived from the *N*,*N*,*O* ligand shows the highest CHEF effect compared to the complex derived from the *N*,*N*,*N* ligand. This has been attributed to the strong binding of the *salen*-type ligand L1 thanks to the *N*,*O* donor pincer compared to the *N*,*N* pincer of ligand L2. In the first case, the conformation of the coordination core is strongly trapped in a planar conjugated *habitus*.

**Figure 4.** *N*,*N*,*N* Schiff base-type ligands with different nitrogen aromatic rings and their CHEF active complexes.

π π Polydentate flexible ligands containing two or more benzimidazole donor units have long been used in coordination and supramolecular chemistry [66–68]. The fused benzimidazole ring contains the *N*-binding site of the imidazole moiety. Feng-Mei Nie and coworkers [69,70] synthetized solid-state emissive zinc(II) complexes derived from tridentate and polydentate benzimidazole (Figure 4), solid-state emissive by themselves. Different architectures were obtained in dependence from the number of chelating site and on the auxiliary ligands, as analyzed by the X-ray diffraction technique. In presence of oxalate, a dinuclear oxalate-bridges Zn2L<sup>2</sup> coordination core was obtained (in 2014, [69]), whereas the same ligand produced a five-coordinate ZnL coordination core in the presence of *N*,*O* chelating picolinate (in 2016, [70]). In both cases, the fluorescence emission was assigned to π-π\* ILCT bands. The red shift of the emission maxima from ligand to complexes is influenced by the coordination pattern. The remarkable fluorescence enhancement compared to the free ligand was ascribed to extensive π-conjugated structure formation.
