Synthesis, Structures and Photophysical Properties of Tetra-and Hexanuclear Zinc Complexes Supported by Tridentate Schiff Base Ligands

: The synthesis, structure and photophysical properties of two polynuclear zinc complexes, namely [Zn 6 L 2 ( µ 3 -OH) 2 (OAc) 8 ] ( 1 ) and [Zn 4 L 4 ( µ 2 -OH) 2 ](ClO 4 ) 2 ( 2 ), supported by tridentate Schiff base ligand 2,6-bis(( N -benzyl)iminomethyl)-4- tert -butylphenol ( HL ) are presented. The synthesized compounds were investigated using ESI-MS, IR, NMR, UV-vis absorption spectroscopy, photoluminescence spectroscopy and single-crystal X-ray crystallography. The hexanuclear neutral complex 1 comprises six, ﬁve-and four-coordinated Zn 2+ ions coordinated by O and N atoms from the supporting ligand and OH-and acetate ligands. The Zn 2+ ions in complex cation [Zn 4 L 4 ( µ 2 -OH) 2 ] 2+ of 2 are all ﬁve-coordinated. The complexation of ligand HL by Zn 2+ ions leads to a six-fold increase in the intensity and a large blue shift of the ligand-based 1 ( π - π )* emission. Other biologically relevant ions, i.e., Na + , K + , Mg 2+ , Ca 2+ , Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ and Cu 2+ , did not give rise to a ﬂuorescence enhancement.


Introduction
Zinc as an essential trace metal for all living organisms is omnipresent in biogenic systems, such as in the cells of the human body. Meanwhile, it is well-known that Zn 2+ greatly contributes to biological processes, for example, as part of the immune system, as an active site or co-factor in enzymes or as a regulator for proteins, and it can also provide a stabilizing function to their structure [1][2][3][4][5][6][7][8][9][10]. The Zn 2+ ions can, however, also have toxic effects when toothier concentrations are too high, and it is presumably involved in the biochemical mechanisms of neuronal diseases [4,11]. Proper detection of free zinc, also in living cells, is highly desirable, but it still remains a challenge for common spectroscopic methods, with which free zinc is not obtainable. As a d 10 metal, Zn 2+ cannot be observed using UV-vis spectroscopy, but it is particularly suitable for fluorescence spectroscopy. For this technique, the CHEF effect (chelation-enhanced fluorescence effect) is exploited for a strong augmentation in the fluorescence intensity of the sensor molecule. Usually, the PET (photoinduced electron transfer) quenches the luminescence of some chemosensors, and it is turned off by the complexation of the closed-shell d 10 metal ion [12][13][14][15][16][17][18][19][20][21][22][23]. This combines two important factors: the high sensitivity of fluorescence spectroscopy and the excellent selectivity for Zn 2+ . A set of molecules, which provide these abilities, were already designed designed and explored: for example, 8-aminoquinoline-based sensor molecules such as 6methoxy-(8-p-toluenesulfonamide)quinoline (TSQ), Zinquin (ZQ) and 2-(hydroxmethyl)-4-methyl-6-((quinolinyl-8-imino)methyl)phenol (HMQP) [24][25][26][27][28][29][30]. However, what is often left out from consideration is the influence of different anions that may disturb the mechanism of complexation with zinc and the sensitivity of the method.
Recently, we have reported the synthesis, structures and properties of some discrete four-and five-coordinated Zn 2+ complexes supported by salicylaldiminato ligands [31][32][33][34][35]. As an extension, here, we report the synthesis of two zinc complexes derived from new Schiff base ligand HL (Scheme 1) bearing two imine functions and a phenolic oxygen as a N,O,N-donor set. The effect of the metal-ligand ratio variations on the coordination and luminescence properties is reported. The results are supported by accompanying DFT calculations of UV/vis spectra.

Materials and Methods
All reagents were purchased and used without any further purification. 4-tert-butyl-2,6-diformylphenol was purchased from Sigma Aldrich. Solvents were of HPLC grade and not additionally purified. UV/vis spectra were recorded with a V-670 spectrophotometer from JASCO and analyzed with SPECTRA MANAGER v2.05.03 software. HELLMA 110-QS quartz cells with 10 mm path length were used as cuvettes. Measurements were made within the range of 190-650 nm. Fluorescence spectra were recorded on a Perkin Elmer FL6500 spectrometer using a constant slit width. Data acquisition was performed using the FL WINLAB V3.00 program from PERKINELMER [36]. Measurements were performed using precision SUPRASIL type 111-10-40-QS fused silica cells (10 mm cell diameter, 3500 μL volume) from HELLMA ANALYTICS. The infrared spectra were recorded with a Bruker Vertex 80V FTIR spectrometer utilizing KBr pellets. The ORIGINPRO 8G program from ORIGINLAB CORPORATION was used for data analysis and a graphical display of the spectra [37]. 1 H and 13 C NMR spectra measurements were recorded on BRUKER model DPX-400 ( 1 H: 400 MHz; 13 C: 100 MHz) or VARIAN Gemini 300 instruments ( 1 H: 300 MHz; 13 C: 75 MHz). The solvent signal served as the internal standard. The MestReNova v11.0 program from MESTRELAB RESEARCH S. L. was utilized for data analyses and graphical displays [38]. ESI mass spectra were recorded using MICROTOF or IMPACT II mass spectrometers from BRUKER DALTONIK GMBH. Elemental analyses were measured using an ELEMENTAR VARIO EL instrument from ELEMENTAR ANALYSESYSTEME GMBH.
X-ray crystallography. Single-crystal X-ray diffraction experiments were carried out on a STOE STADIVARI, equipped with an X-ray micro-source (Cu-Kα, λ = 1.54186 Å) and a DECTRIS Pilatus 300K detector at 180(2) K. The diffraction frames were processed with the STOE X-RED software package [39]. The structures were solved by direct methods [40] and refined by full-matrix least-squares techniques on the basis of all data against F 2 using Scheme 1. Synthesis of ligand HL and zinc complexes 1 and 2.

Materials and Methods
All reagents were purchased and used without any further purification. 4-tert-butyl-2,6-diformylphenol was purchased from Sigma Aldrich. Solvents were of HPLC grade and not additionally purified. UV/vis spectra were recorded with a V-670 spectrophotometer from JASCO and analyzed with SPECTRA MANAGER v2.05.03 software. HELLMA 110-QS quartz cells with 10 mm path length were used as cuvettes. Measurements were made within the range of 190-650 nm. Fluorescence spectra were recorded on a Perkin Elmer FL6500 spectrometer using a constant slit width. Data acquisition was performed using the FL WINLAB V3.00 program from PERKINELMER [36]. Measurements were performed using precision SUPRASIL type 111-10-40-QS fused silica cells (10 mm cell diameter, 3500 µL volume) from HELLMA ANALYTICS. The infrared spectra were recorded with a Bruker Vertex 80V FTIR spectrometer utilizing KBr pellets. The ORIGINPRO 8G program from ORIGINLAB CORPORATION was used for data analysis and a graphical display of the spectra [37]. 1 H and 13 C NMR spectra measurements were recorded on BRUKER model DPX-400 ( 1 H: 400 MHz; 13 C: 100 MHz) or VARIAN Gemini 300 instruments ( 1 H: 300 MHz; 13 C: 75 MHz). The solvent signal served as the internal standard. The MestReNova v11.0 program from MESTRELAB RESEARCH S. L. was utilized for data analyses and graphical displays [38]. ESI mass spectra were recorded using MICROTOF or IMPACT II mass spectrometers from BRUKER DALTONIK GMBH. Elemental analyses were measured using an ELEMENTAR VARIO EL instrument from ELEMENTAR ANALYSESYSTEME GMBH.
X-ray crystallography. Single-crystal X-ray diffraction experiments were carried out on a STOE STADIVARI, equipped with an X-ray micro-source (Cu-Kα, λ = 1.54186 Å) and a DECTRIS Pilatus 300K detector at 180(2) K. The diffraction frames were processed with the STOE X-RED software package [39]. The structures were solved by direct methods [40] and refined by full-matrix least-squares techniques on the basis of all data against F 2 using SHELXL-2018/3 [41]. PLATON was used to search for higher symmetry [42]. All non-hydrogen atoms were refined anisotropically. H atoms were placed in calculated positions and allowed to ride on their respective C atoms and treated isotropically using the 1.2-or 1.5-fold U iso value of the parent C atoms. All non-hydrogen atoms were refined anisotropically. All calculations were performed using the Olex2 crystallographic platform [43]. ORTEP-3 and POV-RAY were used for the artwork of the structures [44]. CCDC 2248758-2248760 contains the supplementary crystallographic data for this paper.

Synthesis and Characterization of Compounds
The new di-imine ligand HL was prepared according to the route provided in Scheme 1. A mixture of 5-tert-butyl-2-hydroxybenzene-1,3-dialdehyde and two equivalents of benzylamine were allowed to react in a 1:2 molar ratio at room temperature for 2 h in EtOH in order to provide an intense yellow solution, from which the pure product could be isolated as a yellow microcrystalline solid in an almost quantitative yield.
Two new zinc(II) complexes, 1 and 2, were prepared according to the reactions in Scheme 1. The reaction of the ligand HL with Zn(OAc) 2 . 2H 2 O in a 3:1 ratio in acetonitrile at 80 • C for 30 min produced a pale yellow solution, from which a pale yellow, microcrystalline powder of composition [(Zn 6 (L) 2 (OH) 2 (OAc) 8 ]·4H 2 O (1) was reproducibly obtained in ca 87% yield. The treatment of HL with Zn(ClO 4 ) 2 . 6H 2 O in a 1:1 ratio in a CH 2 Cl 2 /MeOH/MeCN mixed solvent system furnished a yellow solution, from which a pale yellow solid of composition [Zn 4 L 4 (OH) 2 ](ClO 4 ) 2 · H 2 O (2) precipitated in an 86% yield. Both complexes exhibit good solubility in acetonitrile, DMSO and dichloromethane, but they are only sparingly soluble in alcohols and various hydrocarbons and virtually insoluble in water. The synthesized compounds resulted in a satisfactory elemental analyses, and their formulation was confirmed by electrospray ionization mass spectrometry (ESI-MS), FT-IR-spectroscopy, UV-vis and NMR spectroscopy as well as single-crystal X-ray diffractometric analyses.
The infrared spectrum of the free ligand shows a prominent band at 1634 cm −1 , which can be attributed to the C=N stretching frequencies of the imine groups ( Figure S1). The O-H stretching appears as a very broad band with a maximum at around 2600 cm −1 , indicating the presence of intramolecular hydrogen bonding interactions between the OH and imine groups. The IR spectrum of zinc complex 1 reveals two bands at 1580 and 1415 cm −1 attributed to the antisymmetric and symmetric stretching vibrations of the acetato ligand ( Figure S2). The imine group gives rise to two weak features at 1650 and 1630 cm −1 . The IR spectrum of perchlorate salt 2 ( Figure S2) reveals two strong absorption bands at 1090 and 625 cm −1 , characteristic values for the vibrations of a ClO 4 − counter anion. Again, as in 1, there are two bands at 1653 and 1631 cm −1 attributable to the stretching vibrations of a coordinated imine group.

Crystallographic Characterization
Single crystals of HL obtained from EtOH are triclinic and fall into space group P1. The structure shown in Figure 1 reveals that both imino groups are (E)-configured and coplanar with the phenol ring. The phenolic OH group forms an intramolecular hydrogen bond (O1-H1-N1) with the imine group as observed in other salicylaldimines. Figure S5 displays the packing diagram of HL. As observed, the phenol rings of two adjacent molecules are engaged in face-to-face π-π stacking interactions, as manifested by an interplanar distance of 3.4286(11) Å. The DFT-optimized geometry of the HL ligand at the PBE/TZ2P-D3(BJ) level of theory is in agreement with the experimental data ( Figure S16). Scheme 1. The reaction of the ligand HL with Zn(OAc)2 . 2H2O in a 3:1 ratio in aceto at 80 °C for 30 min produced a pale yellow solution, from which a pale yellow, mic talline powder of composition [(Zn6(L)2(OH)2(OAc)8]·4H2O (1) was reproducibly ob in ca 87 % yield. The treatment of HL with Zn(ClO4)2 . 6H2O in a 1:1 rati CH2Cl2/MeOH/MeCN mixed solvent system furnished a yellow solution, from w pale yellow solid of composition [Zn4L4(OH)2](ClO4)2 · H2O (2) precipitated in an 86 % Both complexes exhibit good solubility in acetonitrile, DMSO and dichlorometha they are only sparingly soluble in alcohols and various hydrocarbons and virtually uble in water. The synthesized compounds resulted in a satisfactory elemental an and their formulation was confirmed by electrospray ionization mass spectrometr MS), FT-IR-spectroscopy, UV-vis and NMR spectroscopy as well as single-crysta diffractometric analyses.
The infrared spectrum of the free ligand shows a prominent band at 1634 cm −1 , can be attributed to the C=N stretching frequencies of the imine groups ( Figure

Crystallographic Characterization
Single crystals of HL obtained from EtOH are triclinic and fall into space gro The structure shown in Figure 1 reveals that both imino groups are (E)-configur coplanar with the phenol ring. The phenolic OH group forms an intramolecular hyd bond (O1-H1-N1) with the imine group as observed in other salicylaldimines.   Crystals of [Zn 6 L 2 (OH) 2 (OAc) 8 ]·3MeCN·0.5H 2 O were grown by the slow evaporation of an acetonitrile solution. The crystal structure comprises a discrete, hexanuclear, mixed-ligand complex, [Zn 6 L 2 (OH) 2 (OAc) 8 ] (1), and several co-crystallized MeCN and H 2 O molecules. Figure 2 shows the structure of the neutral zinc complex. The zinc complex exhibits idealized C 2 symmetry (neglecting the four benzyl groups of the supporting ligand). Two nearly isostructural [Zn 3 L(OH)(OAc) 2 ] 2+ moieties are connected by four µ 1,3 -briding acetate groups.  (1), and several co-crystallized MeCN and H2O ecules. Figure 2 shows the structure of the neutral zinc complex. The zinc complex exh idealized C2 symmetry (neglecting the four benzyl groups of the supporting ligand). nearly isostructural [Zn3L(OH)(OAc)2] 2+ moieties are connected by four µ1,3-briding tate groups. The three Zn 2+ ions within the trinuclear subunits exhibit different coordination vironments: Zn1 is six-coordinated by the imine N and phenolate O donors from the porting ligand, three O atoms from three µ1,3-bridging acetato ligands and one t bridging OH group in a distorted octahedral fashion. The Zn2 atom, on the other han five-coordinated, being surrounded by imine N and phenolate O donors, two O a from two µ1,3-bridging acetates and the OH bridge. The coordination geometry of Z severely distorted and lies between the ideal trigonal bipyramidal or square pyram environments, as suggested by the τ value of 0.49 [58], or the SHAPE symmetry fa (Table S1) [59]. Finally, one Zn atom is coordinated in a distorted tetrahedral geometr three O atoms from three µ1,3-bridging acetate groups and the triply bridging OH gr A selection of bond lengths and angles is given in Table 1. The bond lengths and an show no unusual features and compare well with those in other six-, five-and fourdinated Zn complexes. The corresponding bond lengths and angles within the two t clear subunits do not differ much. The angles around Zn1, for example, range The three Zn 2+ ions within the trinuclear subunits exhibit different coordination environments: Zn1 is six-coordinated by the imine N and phenolate O donors from the supporting ligand, three O atoms from three µ 1,3 -bridging acetato ligands and one triply bridging OH group in a distorted octahedral fashion. The Zn2 atom, on the other hand, is five-coordinated, being surrounded by imine N and phenolate O donors, two O atoms from two µ 1,3 -bridging acetates and the OH bridge. The coordination geometry of Zn2 is severely distorted and lies between the ideal trigonal bipyramidal or square pyramidal environments, as suggested by the τ value of 0.49 [58], or the SHAPE symmetry factors (Table S1) [59]. Finally, one Zn atom is coordinated in a distorted tetrahedral geometry by three O atoms from three µ 1,3 -bridging acetate groups and the triply bridging OH group. A selection of bond lengths and angles is given in  8 ] supported by the ligand 2,6-bis((N-benzyl)iminomethyl)-4-methylphenol. This complex is also composed of trinuclear [Zn 3 L(OH)(OAc) 2 ] 2+ moieties [11]. Other polynuclear zinc carboxylato complexes are known; basic zinc acetate, Zn 4 O(CH 3 COO) 6 , is a prominent example [60].

Electronic Absorption and Emission Spectroscopy
The synthesized compounds were further characterized using UV-vis absorption and photoluminescence spectroscopy. The electronic absorption spectra for the free ligand HL

Electronic Absorption and Emission Spectroscopy
The synthesized compounds were further characterized using UV-vis absorption and photoluminescence spectroscopy. The electronic absorption spectra for the free ligand HL and its deprotonated phenolate form, L − , were measured in a mixture of DCM/MeCN (3:2) at room temperature. The spectrum of the free ligand displays three main absorption maxima ( Figure 4). The absorption maxima and corresponding extinction coefficients are listed in Table 3. The broad band with an absorption maximum at 248 nm is attributed to the π-π* transitions localized on the phenol and phenyl rings of HL, according to time-dependent density functional theory (TDDFT) calculations (see details below). The lower-energy bands with absorption maxima at 348 nm and 451 nm, on the other hand, are assigned to electronic transitions within a more extended π system involving the phenolate ring and the two imine groups. The UV-vis spectrum of the deprotonated ligand differs significantly from that of the protonated ligand. It reveals two well-resolved absorption maxima at 234 and 349 nm. Instead of the band at 450 nm observed in HL, there is a tail on the 349 nm band that extends into the visible range but with no well-developed maximum. Thus, deprotonation has a strong effect on the spectroscopic properties of HL. Table 3. UV-vis spectroscopic data for HL and the deprotonated ligand; luminescence spectroscopic data for HL, L − and 1. π-π* (ArOH) π-π*(ArOH) π-π*(ArOH+Ph)   π-π* (ArOH) π-π*(ArOH) π-π*(ArOH+Ph) The broad band with an absorption maximum at 248 nm is attributed to the π-π* transitions localized on the phenol and phenyl rings of HL, according to time-dependent density functional theory (TDDFT) calculations (see details below). The lower-energy bands with absorption maxima at 348 nm and 451 nm, on the other hand, are assigned to electronic transitions within a more extended π system involving the phenolate ring and the two imine groups. The UV-vis spectrum of the deprotonated ligand differs significantly from that of the protonated ligand. It reveals two well-resolved absorption maxima at 234 and 349 nm. Instead of the band at 450 nm observed in HL, there is a tail on the 349 nm band that extends into the visible range but with no well-developed maximum. Thus, deprotonation has a strong effect on the spectroscopic properties of HL.
To study the complexation reactions of HL with Zn 2+ ions in the solution, UV-vis spectrophotometric batch titrations were carried out. The method of YOE and JONES was used here, in which the ligand concentration is kept constant over the entire range and the amount of Zn 2+ is raised stepwise until a stable complex species is formed [61][62][63][64]. Tetra-n-butylammonium hexafluorophosphate (10 mM) was added to the used solvent mixture (3:2-DCM/MeOH) to ensure a constant ionic strength during the complexation reactions.
The addition of aliquots of Zn(OAc) 2 (from 0.1 to 10 equiv.) leads to clear changes in the UV-vis spectrum of the ligand ( Figure 5). Thus, the bands at 350 and 450 nm for HL vanish with increasing Zn 2+ concentrations, and a new band with a maximum at 387 nm emerges (at a Zn/L ratio of 1.0 equiv.), providing clear support that Zn 2+ binding to the bis(iminophenolate)ligand L − occurs. Adding the further equivalents of Zn(OAc) 2 leads to a weak but significant hyperchromic effect manifested by the slight increase in the intensity of the 387 nm band. The YOE and JONES method was applied in order to determine the stoichiometric ratio of the complex. As observed from the inset of Figure 5, the plot of the absorbance value versus stoichiometric ratio [Zn 2+ ]/[HL] increases very steeply up to 1:1, and then there is a further, but much less pronounced, increase in intensity, suggesting that additional Zn 2+ ions will not directly interact with the donor atoms of HL. The spectroscopic titration of HL with Zn(ClO 4 ) 2 . 6H 2 O behaved in a very similar fashion (SI). Overall, these findings suggest that the ligand examined here has a strong propensity to form Zn 2+ complexes with a 1:1 metal/ligand ratio. This ratio would be consistent with the 2:2 stoichiometries found in the solid state and implies that such 2:2 complexes also exist in the solution state. In the presence of additional co-ligands, this ratio may be increased, but it is accompanied by much less pronounced changes in the spectroscopic features of supporting ligand HL. These findings are also consistent with those made by fluorescence spectroscopy described below.
Chemistry 2023, 5, FOR PEER REVIEW 10 used here, in which the ligand concentration is kept constant over the entire range and the amount of Zn 2+ is raised stepwise until a stable complex species is formed [61][62][63][64]. Tetran-butylammonium hexafluorophosphate (10 mM) was added to the used solvent mixture (3:2-DCM/MeOH) to ensure a constant ionic strength during the complexation reactions.
The addition of aliquots of Zn(OAc)2 (from 0.1 to 10 equiv.) leads to clear changes in the UV-vis spectrum of the ligand ( Figure 5). Thus, the bands at 350 and 450 nm for HL vanish with increasing Zn 2+ concentrations, and a new band with a maximum at 387 nm emerges (at a Zn/L ratio of 1.0 equiv.), providing clear support that Zn 2+ binding to the bis(iminophenolate)ligand L − occurs. Adding the further equivalents of Zn(OAc)2 leads to a weak but significant hyperchromic effect manifested by the slight increase in the intensity of the 387 nm band. The YOE and JONES method was applied in order to determine the stoichiometric ratio of the complex. As observed from the inset of Figure 5, the plot of the absorbance value versus stoichiometric ratio [Zn 2+ ]/[HL] increases very steeply up to 1:1, and then there is a further, but much less pronounced, increase in intensity, suggesting that additional Zn 2+ ions will not directly interact with the donor atoms of HL. The spectroscopic titration of HL with Zn(ClO4)2 . 6H2O behaved in a very similar fashion (SI). Overall, these findings suggest that the ligand examined here has a strong propensity to form Zn 2+ complexes with a 1:1 metal/ligand ratio. This ratio would be consistent with the 2:2 stoichiometries found in the solid state and implies that such 2:2 complexes also exist in the solution state. In the presence of additional co-ligands, this ratio may be increased, but it is accompanied by much less pronounced changes in the spectroscopic features of supporting ligand HL. These findings are also consistent with those made by fluorescence spectroscopy described below. The complexation reactions were further examined using fluorescence spectroscopy. The emission spectra of compounds HL, L − , and of a 1:3 solution of HL and Zn(OAc)2 are displayed in Figure 6 along with the corresponding excitation spectra. The emission spectra were measured in acetonitrile solution at the 10 −5 M concentration. Table 3 lists the data. The free ligand fluorescence is blue-green, reflected in a single emission band with a maximum at 497 nm when excited at 447 nm. The excitation spectrum of HL monitored The complexation reactions were further examined using fluorescence spectroscopy. The emission spectra of compounds HL, L − , and of a 1:3 solution of HL and Zn(OAc) 2 are displayed in Figure 6 along with the corresponding excitation spectra. The emission spectra were measured in acetonitrile solution at the 10 −5 M concentration. Table 3 lists the data. The free ligand fluorescence is blue-green, reflected in a single emission band with a maximum at 497 nm when excited at 447 nm. The excitation spectrum of HL monitored at 497 nm corresponds to the absorption spectrum and reveals an absorption band at 447 nm that is assigned to a transition into the 1 (π-π*) state of the bis(iminomethyl)phenol system. The deprotonation of the ligand has a strong effect on the luminescence properties. This can be detected by the naked eye. Thus, in contrast to HL, L − exhibits a very weak, intraligand 1 (π-π)* fluorescence emission band at 563 nm. The luminescence intensity decreases by a factor of 15, and the Stokes shift increases from 2250 cm −1 for HL to 11,139 cm −1 in L − . Remarkably, the complexation of the ligand by Zn 2+ leads to a circa six-fold enhancement of the luminescence intensity, which is also accompanied by a hypsochromic shift in the emission band to 452 nm. This behavior has been observed for other Zn 2+ complexes supported by o-hydroxy-aryl SCHIFF base ligands [65][66][67][68]. The enhancement of the emission intensity can be explained by the CHEF effect (chelation-enhanced fluorescence effect) due to the inhibition of photo-induced electron transfer processes [69,70].
Chemistry 2023, 5, FOR PEER REVIEW nm that is assigned to a transition into the 1 (π-π*) state of the bis(iminomethy system. The deprotonation of the ligand has a strong effect on the luminescence ties. This can be detected by the naked eye. Thus, in contrast to HL, L − exhibit weak, intraligand 1 (π-π)* fluorescence emission band at 563 nm. The luminescen sity decreases by a factor of 15, and the Stokes shift increases from 2250 cm −1 fo 11,139 cm −1 in L − . Remarkably, the complexation of the ligand by Zn 2+ leads to a c fold enhancement of the luminescence intensity, which is also accompanied by a chromic shift in the emission band to 452 nm. This behavior has been observed f Zn 2+ complexes supported by o-hydroxy-aryl SCHIFF base ligands [65][66][67][68]. The e ment of the emission intensity can be explained by the CHEF effect (chelationfluorescence effect) due to the inhibition of photo-induced electron transfer p [69,70].     The luminescence properties of the ligand HL were investigated in the presence of other metal ions in view of reports from the literature that o-hydroxy-aryl compounds allow for the sensitive optical detection of Zn 2+ ions among other metal cations [11]. Thus, methanolic solutions containing the equimolar quantities of the ligand and a series of abundant and biologically relevant p-and d-bock metals were prepared, and their emission properties were determined. Figure 8 shows that none of the investigated metal ions reveals a fluorescence enhancement. This is either attributed to the poor binding affinity of some metal ions to the SCHIFF-base ligands (in the case of Na + , K + , Mg 2+ and Ca 2+ ) or the quenching of the excited singlet states by electron transfer reactions involving the d-states of open-shell transition metal ions (in case of Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ and Cu 2+ ).
Several other fluorescent OFF _ ON sensors for Zn 2+ ions based on salicylidene aldimine binding sites have been reported [11,20,65]. Zn 2+ binding enhances the rather weak intraligand 1 (π-π) fluorescence emission of the free ligands significantly due to the inhibition of photo-induced electron transfer processes [65]. Zn 2+ ion binding also rigidifies the ligand architecture, thereby decreasing the probability of the vibrational deactivation of the excited singlet state [34,35].
The fluorescence intensity of HL as a function of the Zn 2+ concentration was investigated in order to determine the LOD value (limit of detection value), which provides the lowest concentration that can be measured. Fluorescence intensity was found to be linear between 1 × 10 −6 and 1 × 10 −7 M Zn 2+ and produced an LOD value of 0.24 (1) ppm. This value is comparable to those of other Zn 2+ fluorescence sensor systems featuring iminophenolate units [11,34,35]. The luminescence properties of the ligand HL were investigated in the presence of other metal ions in view of reports from the literature that o-hydroxy-aryl compounds allow for the sensitive optical detection of Zn 2+ ions among other metal cations [11]. Thus, methanolic solutions containing the equimolar quantities of the ligand and a series of abundant and biologically relevant pand d-bock metals were prepared, and their emission properties were determined. Figure 8 shows that none of the investigated metal ions reveals a fluorescence enhancement. This is either attributed to the poor binding affinity of some metal ions to the SCHIFF-base ligands (in the case of Na + , K + , Mg 2+ and Ca 2+ ) or the quenching of the excited singlet states by electron transfer reactions involving the d-states of open-shell transition metal ions (in case of Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ and Cu 2+ ). In order to support experimental findings, we have carried out TD-DFT calculations on ligand HL, differently (de-)protonated species and a [Zn2L2] 2+ complex to simulate the UV-vis absorption spectra (for details, see the Experimental Section). By considering various forms of HL, dynamic and solvent effects could be accounted for, allowing the full Several other fluorescent OFF _ ON sensors for Zn 2+ ions based on salicylidene aldimine binding sites have been reported [11,20,65]. Zn 2+ binding enhances the rather weak intraligand 1 (π-π) fluorescence emission of the free ligands significantly due to the inhibition of photo-induced electron transfer processes [65]. Zn 2+ ion binding also rigidifies the ligand architecture, thereby decreasing the probability of the vibrational deactivation of the excited singlet state [34,35].
The fluorescence intensity of HL as a function of the Zn 2+ concentration was investigated in order to determine the LOD value (limit of detection value), which provides the lowest concentration that can be measured. Fluorescence intensity was found to be linear between 1 × 10 −6 and 1 × 10 −7 M Zn 2+ and produced an LOD value of 0.24(1) ppm. This value is comparable to those of other Zn 2+ fluorescence sensor systems featuring imino-phenolate units [11,34,35].
In order to support experimental findings, we have carried out TD-DFT calculations on ligand HL, differently (de-)protonated species and a [Zn 2 L 2 ] 2+ complex to simulate the UV-vis absorption spectra (for details, see the Experimental Section). By considering various forms of HL, dynamic and solvent effects could be accounted for, allowing the full assignment of absorption bands in the experimental spectra (see Figure 9). In order to support experimental findings, we have carried out TD-DFT cal on ligand HL, differently (de-)protonated species and a [Zn2L2] 2+ complex to sim UV-vis absorption spectra (for details, see the Experimental Section). By conside ious forms of HL, dynamic and solvent effects could be accounted for, allowing assignment of absorption bands in the experimental spectra (see Figure 9).  The spectra were obtained using AMS code [53].
The band with the largest intensity was assigned to various π-π* transitions, while the band at around 350 nm was assigned to a π-π* transition with a contribution from the n-orbitals of the OH group. The lowest excitations at 450 nm were assigned to a π-π* transition with contributions from the deprotonated phenol group, and it was observed that the deprotonation resulted in a large redshift into the visible light range. As it is shown in Figure 10 the experimental results are in good agreement with the calculated ones. the band at around 350 nm was assigned to a π-π* transition with a contributi n-orbitals of the OH group. The lowest excitations at 450 nm were assigned to sition with contributions from the deprotonated phenol group, and it was ob the deprotonation resulted in a large redshift into the visible light range. As it Figure 10 the experimental results are in good agreement with the calcu Figure 10. Summation of the calculated excitations for all species in Figure 9. The ex convolved with a 30 nm wide Gaussian to fit the experimental spectrum well.

Conclusions
The syntheses of the new tridentate Schiff-base ligand 2,6-bis((N-benzy thyl)-4-tert-butylphenol (HL) and the corresponding polynuclear zinc [Zn6L2(µ3-OH)2(OAc)8] (1) and [Zn4L4(µ2-OH)2](ClO4)2 (2) are described. The ex sition of the two complexes was confirmed via X-ray crystallography, which r influence of the coordinating acetate anion. The molecular structure of 1 co ions, which are six-, five-and four-coordinated by the O and the two N atoms o and OH − and acetate ligands. In the discrete complex cation [Zn4L4(µ2-OH)2] 2+ ions, there are five-coordinated with τ values ranging from 0.49 to 0.52. The properties of the ligand have been investigated with UV-vis spectroscopy. Th ergy band at 348 nm and the visible transition at 451 nm are designated as elec sitions within the extended π system of the iminophenolate. Via the deproton ligand, the band at 451 nm disappears, whereby the spectroscopic properties o change significantly. The UV-vis titrations with the acetate and the perchlo 2+ Figure 10. Summation of the calculated excitations for all species in Figure 9. The excitations are convolved with a 30 nm wide Gaussian to fit the experimental spectrum well.

Conclusions
The syntheses of the new tridentate Schiff-base ligand 2,6-bis((N-benzyl)iminomethyl)-4-tert-butylphenol (HL) and the corresponding polynuclear zinc complexes [Zn 6 L 2 (µ 3 -OH) 2 (OAc) 8 ] (1) and [Zn 4 L 4 (µ 2 -OH) 2 ](ClO 4 ) 2 (2) are described. The exact composition of the two complexes was confirmed via X-ray crystallography, which revealed the influence of the coordinating acetate anion. The molecular structure of 1 contains Zn 2+ ions, which are six-, five-and four-coordinated by the O and the two N atoms of the ligand and OH − and acetate ligands. In the discrete complex cation [Zn 4 L 4 (µ 2 -OH) 2 ] 2+ of 2 all Zn 2+ ions, there are five-coordinated with τ values ranging from 0.49 to 0.52. The absorption properties of the ligand have been investigated with UV-vis spectroscopy. The lower energy band at 348 nm and the visible transition at 451 nm are designated as electronic transitions within the extended π system of the iminophenolate. Via the deprotonation of the ligand, the band at 451 nm disappears, whereby the spectroscopic properties of the ligand change significantly. The UV-vis titrations with the acetate and the perchlorate salts of Zn 2+ exhibit the tendency of HL to form complexes with a stoichiometric ratio of 1:1. The results of luminescence spectroscopy studies support this observation. The complexation of Zn 2+ leads to a further enhancement of the fluorescence intensity by factor six (CHEF effect), which was not detected for Na + , K + , Mg 2+ , Ca 2+ , Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ or Cu 2+ .
Data Availability Statement: Deposition Numbers 2248758-2248760 contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service. TD-DFT data will be available soon and will be announced.