Mono-N-benzyl cyclen: A highly selective, multi-functional “turn-on” fluorescence sensor for Pb2+, Hg2+ and Zn2+

Journal Pre-proofs Mono-N-benzyl cyclen: A highly selective, multi-functional “turn-on” fluorescence sensor for Pb2+, Hg2+ and Zn2+ Bruna B. Correia, Thomas R. Brown, Hee-Seung Lee, Joseph H. Reibenspies, Robert D. Hancock PII: DOI: Reference:

S0277-5387(20)30023-1 https://doi.org/10.1016/j.poly.2020.114366 POLY 114366

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Polyhedron

Received Date: Revised Date: Accepted Date:

17 September 2019 3 January 2020 13 January 2020

Please cite this article as: B.B. Correia, T.R. Brown, H-S. Lee, J.H. Reibenspies, R.D. Hancock, Mono-N-benzyl cyclen: A highly selective, multi-functional “turn-on” fluorescence sensor for Pb2+, Hg2+ and Zn2+, Polyhedron (2020), doi: https://doi.org/10.1016/j.poly.2020.114366

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Mono-N-benzyl cyclen: A highly selective, multi-functional “turn-on” fluorescence sensor for Pb2+, Hg2+ and Zn2+.

Bruna B. Correia,† Thomas R. Brown,† Hee-Seung Lee,†* Joseph H. Reibenspies,§ and Robert D. Hancock†*

Department of Chemistry and Biochemistry, University of North Carolina

†

Wilmington,Wilmington, North Carolina 28403,USA. §Department

of Chemistry,Texas A&M University, College Station, Texas 77843, USA.

*Corresponding authors: Robert D. Hancock: [email protected] Hee-Seung Lee: [email protected] Keywords: fluorescent sensors; aggregation induced emission; lead(II); mercury(II); DFT calculations

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ABSTRACT Metal complexing and fluorescence properties of MB-cyclen (1-phenylmethyl-1,4,7,10tetraazacyclododecane) and its complexes with metal ions in 50% CH3OH/H2O are described. The crystallographic structures of the complexes [(Zn(MB-cyclen))2OH](ClO4)3(2.13H2O), [Ag(MBcyclen)(ClO4)], [Pb(MB-cyclen)(ClO4)2], [Hg(MB-cyclen)(ClO4)2], [Cu(MB-cyclen)(ClO4)2], and [Ni(MB-cyclen)(ClO4)2] are reported. The normal fluorescence peak of the benzyl group was observed at 289 nm and only the ZnII ion showed a strong CHEF (chelation enhanced fluorescence) effect for the 289 nm peak, which is in line with its lower position in the well-known order of ability to form fluorescence quenching  contacts: ZnII << CdII < PbII < CuII ~ PdII ~ HgII < AgI. On the other hand, it is proposed that the apparent AIE (aggregation induced emission) nature of the 353 nm peak in the emission spectra of the HgII MB-cyclen complex and the neutral MBcyclen ligand in 50 % CH3OH/H2O arises from exciplex formation in nanoparticulate precipitates. The AIE nature of the 353 nm peak is supported by its appearance being accompanied by intense light scattering peaks in the absorbance spectra at about 200 nm. Finally, it was observed that the PbII complex exhibits an intense emission at 438 nm. The PbII MB-cyclen complex shows no Pb∙∙∙C π contacts in the crystal structure, which is consistent with its lower position in the order of ability to form such contacts. However, time-dependent DFT studies on the PbII complex suggest that a Pb∙∙∙C π contact is stabilized in the excited state, which gives rise to the 6s←6p transition of PbII coupled with transitions within the π-system of the benzyl fluorophore. Given that MB-cyclen responds to the bindings of PbII, HgII, and ZnII at very different wavelengths in their emission spectra, it can be used as a highly selective “turn-on’ sensor for the three metal ions.

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INTRODUCTION. The development of fluorescent sensors for heavy toxic metal ions such as CdII, HgII, and PbII is of continuing interest [1-6]. A difficulty in developing PET (photo-induced electron transfer) sensors for the very heavy HgII and PbII ions is their tendency to quench the fluorescence of simple small sensors. The currently accepted explanation for this [4] is that the large spin-orbit coupling constants (ξ) of these metal ions cause stabilization of the excited triplet state, which promotes radiationless return to the ground state. Recently, we have advanced an additional, or perhaps alternative, explanation for why heavy metal ions tend to quench the fluorecsence of simple PET sensors that have tethered fluorophores. Specifically, it has been shown that heavy metal ions tend to promote the formation of M∙∙∙C π contacts between the coordinated metal ion, and the fluorophore of the sensor [6-12], and the resulting mixing of the orbitals of the metal ion and of the fluorophore leads to quenching of fluorescence, as suggested by time-dependent DFT (TD-DFT) calculations [12]. Crystallographic and spectroscopic evidence suggest that the ability to form M∙∙∙C π contacts increases in the order of NiII ~ ZnII << CdII < PbII < CuII ~ PdII ~ HgII < AgI [9]. Note that the π contact based explanation of fluorescence quenching does not apply to sensors where the metal ion coordinates to donor atoms that are part of the fluorophore One finds that the several fluorescent sensors for HgII and PbII that act as the more desireable ‘turn-on’ CHEF (chelation enhanced fluorescence) sensors tend to involve features such as hydrolysis or irreversible structural change of the sensor so as to remove the PETinducing features of the sensor. It is also common to see that they are large complex sensors where the fluorophore is well removed from the metal binding site holding the quenching metal ion. On the other hand, our model where M∙∙∙C π contact formation with the fluorophore

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quenches fluorescence can explain why simple PET sensors having fluorophores with low electron density in their π systems often show a CHEF effect toward heavy metal ions. An example of such a sensor that we investigated recently is cdpa (see Figure 1 for key to ligand or sensor abbreviations). Crystal structures of cdpa complexes [11] show that PbII and HgII do not form π contacts with the low-electron density dimethoxycoumarin fluorophore, and so the CHEF effect is not strongly suppressed. In contrast, for adpa complexes, where the anthracenyl fluorophore has a high electron density in its π system, crystallography reveals that M∙∙∙C π contacts exist in the HgII and PbII complexes, and accordingly, these metal ions strongly quench the fluorescence of their adpa complexes [6-10]. Our present research has had a focus on finding other mechanisms of causing a CHEF effect, that are not controlled by ξ, or the ability to form M∙∙∙C π contacts with the fluorophore. Of recent interest in this regard is a study of the fluorescent properties of Di-N-benzyl cyclen (DB-cyclen) [13]. It was found that the PbII complex of DB-cyclen produced a unique emission at 447 nm, which was suggested to be due to a 6p→6s emission within the lone pair of PbII. On the other hand, a peak at 336 nm in the free neutral ligand appeared to be an aggregation induced emission (AIE). This was indicated by the peak appearing at the same pH in the absorbance spectra, which showed intense light scattering peaks. The emission peaks at 336 nm in neutral DB-cyclen and at 353 nm in its complex with ZnII were close to the emission peaks reported for dibenzylamine and other dibenzyl organic molecules, which have been assigned as intramolecular exciplex emissions [14]. However, models of DB-cyclen and its complexes with metal ions show that intramolecular exciplexes would be sterically very difficult to form. Thus, we suggested that if the emissions near 350 nm were indeed due to exciplexes, they must be intermolecular exciplexes occuring in the aggregates formed in an apparent AIE mechanism.

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In this paper we report the metal ion complexing and fluorescent properties of mono-Nbenzyl cyclen (MB-cyclen), which has a single N-benzyl group. Since there is only one benzyl group, MB-cyclen cannot form intramolecular exciplexes. To further investigate possible exciplex formation due to aggregation and the potential role of M∙∙∙C π contact formation in Nbenzyl cyclen complexes in general, we carried out crystallographic and spectroscopic studies of the ZnII, AgI, PbII, HgII, CuII and NiII complexes of MB-cyclen. We found that MB-cyclen shows a similar fluorescence response to the binding of PbII as seen in the DB-cyclen complex, but, interestingly, it shows a very different fluorescence response to HgII and ZnII. More importantly, MB-cyclen was found to show strong CHEF effects with PbII, HgII and ZnII at very different wavelengths, which renders MB-cyclen as a multi-functional “turn-on” fluorescent sensor.

EXPERIMENTAL and COMPUTATIONAL METHODS. Materials: The ligand MB-cyclen (1-benzyl-1,4,7,10-tetraazacyclododecane) was obtained from Alfa Aesar in 99.6% purity (lot analysis provided by manufacturer) and used as received. The metal perchlorates were obtained from VWR or Strem in ≥ 99% purity and used as received. All solutions were made up in deionized water (Milli-Q, Waters Corp.) of > 18 MΩ.cm-1 resistivity, plus HPLC grade methanol from Merck. Synthesis of [(Zn(MB-cyclen))2OH](ClO4)3 (1): One equivalent of MB-cyclen (0.07 mmol, 18.37 mg) was dissolved in 2 mL of methanol in a sample vial. Then 1 equivalent of Zn(ClO4)2·6H2O (0.07 mmol, 26.07 mg) was dissolved in 2 mL of Milli-Q water and added to the ligand solution. The vial was covered with parafilm punctured with a few small holes, and the solvent allowed to slowly evaporate. Colorless crystals formed after 4 days, and the crystals were filtered off and

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air-dried. Elemental anal. calc. for C30H55.25Zn2Cl3O14.13N8: C, 36.35; N, 11.30, H, 5.62. Found C, 36.65; N, 11.59, H, 5.56%. Synthesis of [(Ag(MB-cyclen))2](ClO4)2 (2): One equivalent of MB-cyclen (0.07 mmol, 18.37 mg) was dissolved in 2 mL of methanol in a sample vial. Then one equivalent of AgClO4·H2O (0.07 mmol, 15.77 mg) was dissolved in 1 mL of methanol and added to the ligand solution. Crystals were then obtained by the ether diffusion method used previously [9]: the vial containing the above Ag/MB-cyclen solution was placed uncapped and standing upright in a larger vial containing 5 mL of diethyl ether, which was sealed. Upon diffusion of ether into the methanol solution in the small vial, beige crystals formed rapidly and more were formed on standing for two days. The crystals were filtered off and air dried. Elemental anal. calc. for C15H26AgClO4N4: C, 38.27; H, 5.63%; N, 11.83. Found C, 38.45; H, 5.68%; N, 12.12%. Synthesis of [Pb(MB-cyclen)H2O(ClO4)2] (3): One equivalent of MB-cyclen (0.07 mmol, 18.37 mg) was dissolved in 2 mL of methanol in a sample vial. Then one equivalent of Pb(ClO4)2·3H2O (0.07 mmol, 32.21 mg) was dissolved in 2 mL of Milli-Q water and added to the ligand solution. The solution began to precipitate, so 3 mL of methanol and 2 mL of Milli-Q water was added to the vial and heated to re-dissolve. The vial was covered with parafilm and the solvent slowly evaporated through small holes in the parafilm. Colorless crystals formed after two weeks, which were filtered off and air-dried. Elemental anal. calc. for C15H23PbCl2O8N4: C, 27.07; H, 3.97; N, 8.42. Found C, 26.72; H, 4.06; N, 8.12%. Synthesis of [Hg(MB-cyclen)(ClO4)2] (4): This was synthesized using the ether diffusion method described for compound 2, based on Hg(ClO4)2·3H2O (0.06 mmol, 30.46 mg) plus 0.06 mmol MB-cyclen dissolved in 10 mL of methanol. After diffusion of ether into the methanol solution,

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colorless crystals formed after three weeks. Elemental anal. calc. for C15H26Cl2N4O8Hg: C, 27.22; H, 3.96; N, 8.46. Found C, 27.20; H, 4.32; N, 8.42. Synthesis of [Cu(MB-cyclen)(ClO4)2] (5): This was synthesized using the ether diffusion method described for compound 2, based on Cu(ClO4)2·6H2O (0.07 mmol, 25.94 mg) in 1 mL of methanol added to 0.07 mmol MB-cyclen (18.37 mg) dissolved in 2 mL of methanol. After diffusion of ether into the methanol solution, dark blue crystals formed after two days. Elemental anal. calc. for C15H26Cl2N4O8Cu: C, 34.20; H, 4.97; N, 10.63. Found C, 34.51; H, 4.97; N, 10.57%. Synthesis of [Ni(MB-cyclen)(CH3CN)2 ](ClO4)2 (6): This was synthesized by mixing Ni(ClO4)2·6H2O (0.07 mmol, 25.60 mg) in 1 mL of acetonitrile with 0.07 mmol MB-cyclen (18.37 mg) dissolved in 2 mL of methanol. Dark blue crystals formed after two days. Elemental anal. calc. for C19H32NiCl2N6O8: C, 37.90; H, 5.36; N, 13.96. Found C, 37.58; H, 5.47; N, 13.77%. Molecular Structure Determination: Details of structure determination are given in Table 1. Single crystals of compounds 1-6 were selected and diffraction data collected on a 'Bruker APEXII CCD' diffractometer. The crystals were kept at 100-110 K during data collection, as indicated in Table 1. Using Olex2 [15], the structure was solved with the ShelXT [16] structure solution program using Direct Methods and refined with the ShelXL refinement package using Least Squares minimization. The structures were deposited with the Cambridge Structural Database, and are available under deposition numbers 1862045, 1862051, 1862081, 1862082, 1862083 and 1862084. Fluorescence Measurements. Excitation-emission matrix (EEM) fluorescence properties were determined on a Jobin Yvon SPEX Fluoromax-3 scanning fluorometer equipped with a 150 W Xe arc lamp and a R928P detector. The instrument was configured to collect the signal in ratio mode with dark offset using 5 nm band passes on both the excitation and emission

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monochromators. The fluorescence of the DB-cyclen free ligand and metal ion complex solutions was recorded in 50% MeOH/water. DFT calculations: All DFT calculations reported in this work were carried out with the ab initio quantum chemistry package GAMESS.[17] Excited state (S1) geometry optimization of the PbIIMB-cyclen complexe was performed within the framework of TD-DFT with CAM-B3LYP [18] and B3LYP [19,20] exchange-correlation functionals. The SV(P) basis set [21] was used for the main group elements, whereas the Lanl2DZ [22] effective core potential was employed for the Pb. To mimic solution environment, we used polarizable continuum model (PCM) as implemented in GAMESS. The vertical excitation frequencies and the corresponding oscillator strengths were also computed via TD-DFT with the optimized excited state geometry.

RESULTS AND DISCUSSION. Structure of [(Zn(MB-cyclen))2OH](ClO4)3(2.13H2O) (1): The structure of 1 is seen in Figure 2, where one of a pair of disordered [(Zn(MB-cyclen))2OH]3+ individuals is shown. The disorder involves some of the perchlorates, and the lattice water molecules having only partial occupations. There are two distinct [(Zn(MB-cyclen))2OH]3+ dimers in the unit cell, one of which has the pair of N-benzyl groups in an eclipsed arrangement when viewed from above, which dimer is shown in Figure 2. The second [(Zn(MB-cyclen))2OH]3+ dimer in the unit cell has the two N-benzyl groups near each other, and they are nearly exactly staggered, being rotated by some 48º away from being eclipsed, as judged from the orientation of the Zn-N bonds. Each of the ZnII ions forming the [(Zn(MB-cyclen))2OH]3+ dimers has approximately square pyramidal coordination geometry, and is bridged by the single hydroxides occupying the axial sites. What is of particular interest in structure 1 is the position of the eclipsed N-benzyl groups of the dimer in

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Figure 2. In our previous paper on DB-cyclen complexes [13], it was proposed based on the results from DFT calculations that the emissions around 353 nm that appeared in the ZnII complex were due to an AIE effect caused by intermolecular exciplex formation in the nanoparticular precipitates, involving a ZnII DB-cyclen dimer. The N-benzyl groups of a ZnII DB-cyclen nitrate-bridged dimer were proposed to be well positioned to form an exciplex. It is thus encouraging to see that the [(Zn(MB-cyclen))2OH]3+ dimer conforms with this idea, and that a pair of N-benzyl groups are in reasonably close proximity, and potentially able to form an exciplex. The closest approach of aromatic carbon atoms of the N-benzyl groups in Figure 2 is 4.809 Å. It might not be clear whether the distance between two N-benzyl groups in Figure 2 is close enough for forming a π stacked arrangement present in an exciplex. Exciplexes involving two aromatic groups are thought to resemble π stacked aromatics, which for the pyrene excimer are indicated by DFT calculation to have interplanar separations of 3.25 Å [24]. Mikata et al. [25] have proposed that emissions that occur at about 475 nm in ZnII complexes of TPEN analogues such as N,N-1-isoBQBPEN (Figure 1) are due to exciplex formation involving the isoquinoline groups coordinated to the ZnII. Their study shows that in the solid state, the proposed exciplex forming isoquinoline groups of [Zn(N,N-1-isoBQBPEN)]2+ are not ideally oriented for forming the π stacked type of structure required, being at angles of about 90º to each other as expected from their coordination to the ZnII. It is revealing that there is an energy drop of 48 kJ/mol derived from TD-DFT calculations, from the ‘Frank-Condon’ excited state form of [Zn(N,N-1-isoBQBPEN)]2+, in which the atoms have remained in the same position as that in the ground state, and the exciplex form, where the complex has rearranged to give the π stacked isoquinoline groups. This shows that, in spite of a measured lifetime of nanoseconds [25] for the

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exciplex peak at 475 nm, the [Zn(N,N-1-isoBQBPEN)]2+ was able to rearrange extensively in the short-lived excited state to form the exciplex from the Frank-Condon structure. One is thus given some confidence that the closer approach of the aromatic rings of the N-benzyl groups of the [(Zn(MB-cyclen))2OH]3+ dimer in Figure 2 could be accomplished in the excited state, which would involve simple tilting of the Zn-O-Zn bridge to produce the π stacked arrangement for forming an exciplex. Structure of [Ag(MB-cyclen)(ClO4)] (2): The structure of 2 is seen in Figure 3. The AgI ions in 2 have a square pyramidal coordination geometry, with the axial site occupied by a η2 πcontacted benzyl group from a neighboring AgI MB-cyclen individual, regarded as occupying a single coordination site. As was observed for the AgI DB-cyclen complex [13], the AgI MBcyclen individuals are bound together by extremely short intermolecular η2 Ag∙∙∙C π-contacts, which for 2 are 2.481 Å and 2.536 Å. Adjacent AgI MB-cyclen individuals are oriented in an alternating up-down fashion to generate a continuous chain held together by the Ag∙∙∙C πcontacts, which is illustrated in Figure S1 in the supplementary material. A very short π-contact distance seen in structure 2 is not much longer than what would be regarded as fully covalent bonds. One must presume that these very short Ag∙∙∙C contacts are indeed highy covalent bonds similar to those found in, for example, [Cr(benzene)2], with Cr-C = 2.141 Å [26]. The latter shorter Cr-C bonds than the Ag-C bonds in 2 reflect the smaller size of the Cr than Ag: i.e. the covalent single bond radii are Cr = 1.27 Å and Ag = 1.53 Å [27]. In Figure 4 is shown a plot of frequency of occurrence of η2 Ag∙∙∙C π-contacts in the CSD [28], as a function of ranges of Ag∙∙∙C π-contact lengths. Use of the CSD to draw up such a diagram nust be carried out with great care. If one simply searches for non-bonded Ag∙∙∙C distances, one misses out many very short Ag∙∙∙C distances because these are registered by the

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program as actual sigma bonds: one should select the distance type as ‘any’ when carrying out the search, rather than ‘intramolecular distance’ or ‘contact’. Other search considerations were discussed previously [9]. Figure 4 shows a bimodal distribution for AgI, and also for HgII, which is included in the diagram. This accords with the idea that Ag∙∙∙C and Hg∙∙∙C π-contacts shorter than about 2.8 Å form a separate group to the longer contacts, which are typically longer than 3.0 Å. The short contacts are possibly better regarded as being highly covalent bonds like those observed in organometallic comounds such as [Cr(benzene)2]. The great ability of AgI, and also HgII as discussed for structure 4 below, to form strong M∙∙∙C π-contacts possibly accounts for their pronounced ability to quench fluorescence. Structure of [Pb(MB-cyclen)H2O(ClO4)2] (3): The structure of 3 is seen in Figure 5(a). The PbII in 3 appears to have a coordination number of eight, with a close to square antiprismatic coordination geometry as seen in Figure 5(b). The view of the complex in Figure 5(b) shows Pb-N bond lengths ranging from 2.463 to 2.681 Å, whereas the Pb-O bond lengths range from 2.754 Å (to a coordinated water molcule) to 2.916, 3.071 and 3.072 Å for oxygen donors of three unuidentate perchlorates. The longer Pb-O as compared to Pb-N bonds, unexpected in that M-O bonds are normally shorter than M-N bonds, is thought to arise from the presence of a stereochemically active lone pair on the PbII [29-31], The lone pair causes progressively longer Pb-L bonds as one approaches the proposed site of the lone pair, with the shortest Pb-L bonds occuring opposite the site of the lone pair, the antipodal site [30,31]. Very long Pb-L distances occur in the vicinity of the lone pair, as seen with one of the perchlorates in 3 with the Pb-O(1) distance of 3.072 Å. It is often difficult to decide whether such a long Pb-O distance corresponds to a bond or not, since this is considerably longer than, for example, the sum of the ionic radii of Pb2+ and O2-, of 2.59 Å [32]. The idea of PbII as a spherical ion with a single ionic radius does

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not apply when it has a sterochemically active lone pair, and very long bonds in the vicinity of the lone pair are to be expected [33]. Our experience with PbII suggests that Pb-O distances at least as long as 3.50 Å are to be regarded as bonds, so that the coordination number of the PbII in 3 is to be regarded as eight. This is comprised as discussed above of the four Pb-N bonds involving the cyclen moiety of 2.681 to 2.497 Å, and the four Pb-O bonds, to one perchlorate at 3.072(4) Å, bridging perchlorates at 3.071(4) and 2.916 Å, and to a coordinated water molecule at 2.753(4) Å. The Pb-N bonds being much shorter than the Pb-O bonds indicates [29-31] that the lone pair lies between the four oxygen donors, on the side of the PbII facing the viewer in Figure 5. In contrast to AgI in structure 2, and HgII in 4, the PbII here does not form any π contacts with the benzyl fluorophore of MB-cyclen. This follows an interesting pattern found with cyclens having different numbers of N-benzyl groups. Habata et al. [34] have found that in [Ag(TB-cyclen)]+, all four N-benzyl groups form long intramolecular π-contacts, but in [Ag(DBcyclen)]+ [13] and [Ag(MB-cyclen)]+ here (2), only very short single intermolecular π-contacts are formed. Similarly, an as yet unpublished structure of [Pb(TB-cyclen)]2+ we have just determined shows four intramolecular π-contacts, while [Pb(DB-cyclen)]2+ has only single intermolecular π-contacts [13]. It may be that intermolecular π-contacts are disfavored with four N-benzyl groups present, due to steric crowding, and that in the solid state (but not necessarily in solution) with fewer N-benzyl groups present, short and strong intermolecular π-contacts are favored. The lack of Pb∙∙∙C π-contacts in 3 is in accord with the lower tendency of PbII to form π contacts in the proposed order NiII ~ ZnII << CdII < PbII < CuII ~ PdII ~ HgII < AgI [9]. Structure of [Hg(MB-cyclen)](ClO4)2 (4): The structure of 4 is seen in Figure 6. The HgII in 4 has quite similar coordination geometry to the AgI in 2, with very short intermolecular η2 Hg∙∙∙C

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π-contacts of Hg-C(13) = 2.484(3) and Hg-C(12) = 2.576(3) Å with a benzyl group from a neighboring complex occupying the axial coordination site. The HgII is coordinated to the cyclen moiety with Hg-N(1) = 2.474(2) Å, Hg-N(2) = 2.3555(2) Å, Hg-N(3) = 2.373(2) Å, Hg-N(4) = 2.313(2) Å. Again, the coordination geometry of the HgII is square pyramidal if the coordination of the η2 benzyl group on the axial site is regarded as occupying a single coordination site. As with the AgI complex in 2, the bridging N-benzyl groups in 4 lead to a chain structure: a difference is that the η2 π contacts in 2 invole the ortho and meta carbon atoms of the N-benzyl group, which leads to a linear chain, whereas the η2 π contacts in 4 involve the meta and para carbon atoms, which leads to a zig-zag structure. The distribution of η2 Hg∙∙∙C π-contact lengths in Figure 4 shows, as with AgI, a bimodal distribution, suggesting that two distinct types of interaction occur between HgII and aromatic rings. As with AgI these would be contacts of less than about 2.8 Å, which would best be regarded as resembling the bonding in complexes such as [Cr(benzene)2], and longer contacts that would be due to simple attraction between the metal ion and the electrons of the aromatic π-system. One notes from Figure 4 that AgI appears to be more prolific at forming short M∙∙∙C π-contacts than is HgII. This may be an artifact of the HgII being more prone to mercuration, displacing the hydrogen atoms where these are available on aromatic rings, and being largely limited to aromatic rings that are protected from such substitution by having carbon substituents at appropriate positions on the aromatic ring. Structure of [Cu(MB-cyclen)(ClO4)2] (5): The structure of 5 is seen in Figure 7, where the CuII has square pyramidal coordination geometry: the cyclen ligand provides the base of the structure, with two distinct complex individuals giving Cu-N(1A) = 2.0280(15) Å, Cu-N(2A) = 2.0158(16) Å, Cu-N(3A) = 1.9978(15) Å, Cu-N(4A) = 2.0210(16) Å, and Cu-N(1B) = 2.0288(14) Å, CuN(2B) = 2.0109(15) Å, Cu-N(3B) = 2.0105(15) Å, Cu-N(4B) = 2.0212(15) Å. The axial sites are

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each occupied by an oxygen from a perchlorate anion with Cu-O(1A) = 2.2051(13) Å and CuO(1B) = 2.1680(13) Å. The N-benzyl groups do not form π-contacts with the CuII, but of interest is the fact that, as seen in Figure 7, they are situated near each other in the solid state with separations of the closest aromatic carbons being 3.956 Å for C(10A)∙∙∙C(13A) and 3.892 Å for C(10B)∙∙∙C(13B). these separations are perhaps close enough for exciplex formation in the solid state, as discussed for the ZnII MB-cyclen with the hydroxy-bridged dimer in structure 1. Structure of [Ni(MB-cyclen)(CH3CN)2](ClO4)2 (6): The structure of 6 is seen in Figure 8, where the NiII has distorted octahedral coordination geometry, The four nitrogen donors of the cyclen moiety give bond lengths of Ni-N(1) = 2.156(3) Å, Ni-N(2) = 2.074(3) Å, Ni-N(3) = 2.122(3) Å, Ni-N(4) = 2.070(3) Å, while the acetonitriles give Ni-N(5) = 2.099(3) Å, Ni-N(6) = 2.100(3) Å. The cyclen ligand is folded with what might be referred to as a cis-I ++++ conformation [35], which means that the two donor groups apart from the macrocycle are cis to each other, the I and the ++++ indicate that all four substitituents on the nitrogen donors of the cyclen moiety are on the same side of the macrocyle. A search of the CSD [28] shows that of 20 structures of NiII with cyclen with two more donor groups completing an approximate octahedral coordination grometry, the cis-I ++++ is mostly the observed conformation of the complex, apart from seven examples that have the +++- arrangement. There is no evidence of intermolecular close contact of the N-benzyl groups in 6 that might lead to exciplex formation. The Coordination geometry of MB-cyclen complexes. A reviewer has made the useful suggestion that a more detailed analysis of the coordination geometry of MB-cyclen complexes is of interest. This is particularly true because of the participation of π-contacted aromatic groups in the coordination sphere of the complexed metal ions. Potentially useful approaches to analysis in this regard are provided by the calculation of the τ parameter [36] that provides a measure of

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how closely the metal ion approaches, for example, either square pyramidal or disphenoidal geometry for five-coordination, or use of the SHAPE program [37] to analyze, for example, the coordination geometry of the Pb(II) complex. The τ parameter has a value of 0.00 for exactly square pyramidal complexes, and 1.00 for exactly trigonal bipyramidal complexes [36]. The τ parameters are for the five-coordinate complexes reported here are: 0.05 (Zn(II), structure 1) and 0.25 (Cu(II), structure 5). Both the Ag(I) and Hg(II) are also five-coordinate if the η2 coordinated benzyl groups are regarded as having unidentate coordination, with a ‘donor atom’ at the midpoint of the η2 π contacted C-C bond, in which case τ parameters of 0.04 (Ag(I), structure 2) and 0.02 (Hg(II) , structure 4) are calculated. The small τ parameters for these five-coordinate complexes suggest that they are all very close to having square pyramidal coordination geometry, which is to be expected in that the square base for all of them is formed by the rigid macrocyclic cyclen moiety. The SHAPE program has been used to indicate that in many cases a coordinated olefin or aromatic groups is best described as being a unidentate ligand [37], which it would be reasonable to assume is the case for the structures here with η2 coordinated benzyl groups, with the η2 benzyl group occupying a single coordination site. For the case of the Pb(II) complex (structure 3), as has already been discussed, and is apparent in Figure 5, the geometry is quite well described as being square anti-prismatic. A problem with a SHAPE analysis as applied to structure 3 is that such an analysis is best applied to a homoleptic complex, which is further complicated by the presence of a rigid macrocylic cyclen moeity controlling the geometry of the complex. Fluorescence of the MB-cyclen free ligand. In Figure 9 are shown the fluorescence spectra of MB-cyclen in 50% CH3OH/H2O over a wide range of pH. The peak at 289 nm must be the usual fluorescence peak associated with the benzyl group [14], where, as expected, it decreases in

15

intensity with increasing pH. This is expected from the presence of a PET effect in the deprotonated ligand. The peak at 353 nm is at a similar wavelength to intramolecular exciplex peaks observed with molecules such as dibenzylamine [14], but cannot be an intramolecular exciplex peak since MB-cyclen has only a single benzyl group. The 353 nm peak is absent or very weak up until pH 7.86, and then increases in intensity up until pH 11.84. The appearance of the 353 nm peak occurs over a pH range corresponding to expected deprotonation of the MB-cyclen ligand: the protonation constants of cyclen itself are 10.53 and 9.60 [38], which would lead to deprotonation over a similar pH range. An indication of the nature of the 353 nm peak comes from the fact that in acetonitrile, in which the MB-cyclen is very soluble, no 353 nm peak is observed, suggesting an aggregation induced emission (AIE) peak. The absorbance spectra in Figure 10 observed for a repeat of the titration seen in Figure 9 also support the idea of an AIE peak. Above pH 7 a large broad light scattering peak at about 200 nm appears in the absorbance spectrum that grows to high intensity as the pH approaches 12. Clearly a precipitate of deprotonated MB-cyclen is forming, possibly on the order of a nanoparticle size range, since it is not visible to the naked eye. Thus, it seems reasonable to propose that the 353 nm peak in Figure 9 is due to AIE. Usual AIE often also involves benzene type rings [2]. It has been proposed that the fluorescence that appears only in the solid state [39,40] for a molecule such as 1-methyl-1,2,3,4,5-pentaphenyl-1Hsilole (Silole in Figure 1), or AgI and HgII complexes of ligands with phenyl substituted ethylene fluorophores [41], is due to restriction of rotation of the phenyl groups in the solid state, which rotation is proposed to quench fluorescence in solution [2,40,41]. The latter mechanism, if it is applied to MB-cyclen, might be expected to increase the intensity of the 289 nm band, rather than leading to a new band at 353 nm. It may be that in a nanocluster of MB-cyclen, the N-benzyl groups of adjacent MB-cyclen molecules are close enough together to overlap and form an

16

exciplex in the excited state. This suggestion would account for the 353 nm band being at a wavelength observed for an intramolecular exciplex peak in dibenzylamine [14]. A major difference between the fluorescence behavior of MB-cyclen and DB cyclen free ligands on titration with base between pH 2-12, is that a peak at 353 nm in the MB-cyclen titration in Figure 9 is formed rapidly, whereas with DB-cyclen the peak appears over a matter of hours, and in the case of TB-cyclen, over a period of days. It seems possible that the peak is formed by the conformer adopted by the non-protonated ligand, which is very different from that of the protonated form of the ligand. With TB-cyclen, the diprotonated form of the free ligand has a ++++ type of conformation as required for complexing metal ions, but the non-protonated form of the ligand has a ++-- conformation with the nitrogen donors buried within surrounding benzyl groups [42]. Thus, we suggest that the bulky N-benzyl groups would slow down conformational change as the number of benzyl groups increased in number along the series MB-cyclen < DB-cyclen < TB-cyclen. Fluorescence of MB-cyclen complexes with metal ions: The fluorescence spectra of metal ions 1:1 with MB-cyclen in 50% CH3OH/H2O are seen in Figure 11. As is usually the case, the Zn(II) complex shows a very strong CHEF (chelation enhanced fluorescence) effect involving the 289 nm band. Such a CHEF effect is typically considered to be due to its relatively small spin-orbit coupling constant (ζ) that does not lead to quenching of fluorescence. Coordination of the ZnII to the MB-cyclen results in prevention of the PET (photo-induced electron transfer) effect due to lone pairs on the non-protonated nitrogen donors of the ligand. Small or no CHEF effects are observed for the 289 nm band of MB-cyclen with the other metal ions tested, which either have large ζ values, or are paramagnetic like CuII and NiII, which lead to quenching of fluorescence of PET sensors [4]. Alternatively, one could interpret the large CHEF effect of the ZnII complex as being

17

due to its inability to form fluorescence-quenching π contacts with the benzyl group of the ligand, whereas the other metal ions are far more efficient at forming such π contacts, as shown by a study of adpa complexes [7-10]. The Hg(II) MB-cyclen complex shows a band at 353 nm, as observed for the MB-cyclen ligand at pH values above 7.0. However, the 353 nm band appears at a low pH of 3.0 and above for the HgIIMB-cyclen complex, which might be the expected pH for formation of the HgII complex. The absorption spectra show light scattering peaks above this pH (Figure S2), indicating that the 353 nm peak of the HgIIMB-cyclen complex may be associated with an AIE effect via intermolecular exciplex formation: the N-benzyl groups of the ligand would be coming into close enough contact in the nanocluster to undergo exciplex formation. None of the other metal ions tested show a 353 nm AIE peak, which would suggest that if they precipitate at all, they do not have a structure that would promote exciplex formation. The PbII complex of MB-cyclen produces an intense fluorescence peak at 438 nm, as seen in Figure 11. The 438 nm peak starts appearing at low pH and quickly becomes near the maximum strength at pH = 4 (Figure 11). An absorbance titration of PbIIMB-cyclen analogous to that in Figure 9 for the fluorescence of the MB-cyclen system shows light scattering as the complex forms, suggesting that one is seeing an AIE effect (Figure S3). None of the other metal ions tested produces an emission at 438 nm. A similar band was observed at 447 nm for the PbIIDB-cyclen complex in our previous study [13], for which we speculated that it is due to a metal-centered transition. To elucidate the origin of the 438 nm emission peak observed in PbIIMB-cyclen, we performed TD-DFT studies (see below) and it suggest that the 438 nm peak is indeed mostly from metal-centered transition.

18

In short, given the distinct fluorescence responses of MB-cyclen toward metal ions, one has here a very interesting ligand MB-cyclen that can produce emissions that would allow for the selective detection of ZnII, HgII, and PbII with peaks at very different wavelengths. TD-DFT calculations on the 438 nm peak in the PbII MB-cyclen complex: If the 438 nm peak is due to intermolecular exciplex formation between two N-benzyl groups as in HgIIMB-cyclen, it should appear around 353 nm, which indicates that the mechanism associated with the emission of PbII MB-cyclen is different. A further unusual feature for the PbII MB-cyclen spectra is the greater quenching of the 289 nm peak (Figure 11) than that which occurs with any of the other metal ions tested, suggesting a rather different interaction of the PbII with the N-benzyl fluorophore of the MB-cyclen, We suspected that the 438 nm peak is related to the possible formation of a contact between the benzyl group and PbII, which is also the likely source of a significantly quenched normal fluorescence peak (289 nm). In order to confirm such a possibility and identify the source of the 438 nm peak, we performed TD-DFT calculations with the CAM-B3LYP [18] exchange-correlation functional. We also employed a polarizable continuum model (PCM) of water to mimic a solution environment (see computational section). In order to estimate the effect of a -contact on the emission frequency of the PbIIMB-cyclen complex, we optimized the excited state (S1) geometries of the PbII complex with and without a -contact. The resulting excited state geometries are shown in Figure 12. The vertical emission frequency corresponding to each optimized excited state geometry was calculated via the TD-DFT method with the same functional and the basis set used in the geometry optimizations. As reported in Table 2, the emission frequencies from the two structures are quite different. The emission from the structure without a  contact occurs at shorter wavelength with a small oscillator strength, which can be considered to be the normal fluorescence peak. As shown in Figure 12, the MOs 19

involved in this transition are mainly  and * type orbitals of the benzyl group with some contributions from 6s and 6p orbitals of PbII. On the other hand, the emission from the structure with a  contact occurs at much longer wavelength with significant oscillator strength. It is exclusively between HOMO and LUMO (Figure 12(c)), involving mostly a 6p  6s transition of PbII mixed with some contributions from a -* type transition. Thus, our TD-DFT results suggest that the origin of the 438 nm peak observed in the PbIIMB-cyclen fluorescence spectrum is a metalcentered transition, induced most likely by the intermolecular  contact in the excited state with the benzyl group caused by aggregation. Although the emission frequencies reported in Table 2 are significantly blue shifted compared to the experimental frequencies, it is expected that they will move toward longer wavelengths upon aggregation, especially the one due to a  contact. To further verify our calculations, we repeated the same calculations with the B3LYP [19,20] functional and obtained similar results (even better agreement in frequencies. see Table 2). SUMMARY and CONCLUSION The free ligand MB-cyclen shows a band at 289 nm that appears to be the normal emission associated with benzyl groups. Among the metal ions we tested, only the ZnII MB-cyclen complex shows a strong CHEF effect involving the 289 nm emission of the MB-cyclen ligand. Solid state structures of AgI and HgII complexes with MB-cyclen show very short intermolecular M∙∙∙C πcontacts with the N-benzyl group of the ligand, in line with the idea that these may be important in quenching of fluorescence. Note that ZnII is less likely to form π-contacts than any other metal ions studied here. At higher pH a new band at 353 nm is observed for the free ligand, which appears to be an aggregation induced emission (AIE). This is supported by the fact that this band is not formed in acetonitrile in which the neutral MB-cyclen is highly soluble, and that the absorption spectra of MB-cyclen show large light-scattering peaks at higher pH where the neutral 20

MB-cyclen ligand is expected to form. Among the metal ions tested, only the HgII complex also shows a 353 nm band, which also appears to be an AIE. The structures of the ZnII and CuII complexes of MB-cyclen (Figure 2 and Figure 7) show the N-benzyl groups to be well placed in the solid state for overlapping to form the π-stacked structures needed to form exciplexes. This supports the idea that bands observed at around 353 nm in the complexes of metal ions with MBcyclen and DB-cyclen [13] might involve AIE based on exciplex formation. The PbII complex of MB-cyclen shows a broad intense emission band in solution, which occurs at 438 nm, similar to that observed in the PbIIDB-cyclen complex [13]. TD-DFT calculations suggest that this peak arises mostly due to a 6p6s transition of PbII, with some contributions from a -* type transition in the benzyl fluorophore. No other metal ion tested produces such a peak, suggesting that the 438 peak in the emission of PbII complex could form the basis of a simple and highly selective sensor for PbII. In summary, we showed in this work that MB-cyclen, a simple ligand with a tethered benzene fluorophore can be used as a highly selective, novel “turn-on” sensor for PbII, HgII, and ZnII. It is important to point out that PbII and HgII typically quench the fluorescence of simple fluorescent sensors. A unique feature of MB-cyclen is that it responds to these three metal ions at very different wavelengths in the emission spectrum: at 289 nm for ZnII, 353 nm for HgII, and 438 nm for PbII, Therefore, there is no ambiguity in distinguishing these ions. No other metal ion tested showed an increase in emission intensity near these wavelengths upon binding the MB-cyclen. The origins of each of the three peaks were also investigated through crystallographic and TDDFT studies. Our study suggested that nanoparticular aggregation might play an important role in the fluorescence spectra of MB-cyclen and its metal ion complexes in general, although the detailed mechanism associated with each peak in the emission spectra is rather different.

21

Appendix A. Supplementary data: The CCDC contains the supplementary crystallographic data for

1862045, 1862051, 1862081, 1862082, 1862083 and 1862084. These data can be obtained free of charge via https://nam05.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.ccdc.cam.ac.uk%2Fconts% 2Fretrieving.html&data=01%7C01%7Chancockr%40uncw.edu%7Cb67ba0ee16b84cd152a508d767 81f0f9%7C2213678197534c75af2868a078871ebf%7C1&sdata=mH17BY5Y5GZZHBArXTWWmx yNp58tcl%2BMCfjm8ZBT3h0%3D&reserved=0, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected].

ACKNOWLEDGEMENTS The work done in the paper was supported by the National Science Foundation under CHE – 1565981. The authors also thank the University of North Carolina Wilmington for support for this work.

22

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Table 1. Crystal data and details of structure refinement for [(Zn(MBcyclen))2OH](ClO4)3(2.13H2O) (1); [Ag(MB-cyclen)(ClO4)] (2); [Pb(MB-cyclen)(ClO4)2] (3); [Hg(MB-cyclen)(ClO4)2] (4); [Cu(MB-cyclen)(ClO4)2] (5); [Ni(MB-cyclen)(ClO4)2] (6) _________________________________________________________________________________ 1

2

3

Empirical formula: Formula weight: Temperature (K) Wavelength (Å): Crystal system: Space group: Unit cell dimensions:

C30H55.25Cl3N8O14.13Zn2 991.16 100(2) 0.71073 triclinic P-1

C15H26AgClN4O4 469.72 110(2) 0.71073 monoclinic Pc

C15H26PbCl2N4O9 684.49 110(2) 0.71073 monoclinic P21/c

a (Å) b(Å)

11.2921(5) 18.7267(9)

8.2982(4) 16.6043(8)

15.5867(6) 10.8277(4)

c (Å) α (deg) β (deg) γ (deg) Volume (Å3): Z

20.2546(10) 87.326(2) 89.0720(10) 78.5270(10) 4192.9(3) 4

13.3373(8) 90 96.610(2) 90 1825.47(17) 4

13.9041(5) 90 111.522(2) 90 2182.96(14) 4

Final R indices [I > 2σ(I)]

R1 = 0.0686 wR2 = 0.1469 R1 = 0.1133 wR2 = 0.1670

R1 = 0.0598 wR2 = 0.0815 R1 = 0.1167 wR2 = 0.0921

R1 = 0.0331 wR2 = 0.0787 R1 = 0.0482 wR2 = 0.0862

R indices (all data)

______________________________________________________________________________

26

Table 1. (contd.) _________________________________________________________________________________ 4

5

6

Empirical formula: C15H26Cl2N4O8Hg C30H52Cl4Cu2N8O16 C19H32NiCl2N6O8 Formula weight: 661.89 1049.67 602.11 Temperature (K) 110(2) 99.99 110(2) Wavelength (Å): 0.71073 0.71073 0.71073 Crystal system: orthorhombic monoclinic monoclinic Space group: Pbca P21/n P21/c Unit cell dimensions: a (Å) 16.0330(8) 20.6880(5) 14.7208(10) b(Å) 13.6863(6) 8.7556(2) 13.1807(9) c (Å) 18.7410(10) 23.1313(5) 13.5351(9) α (deg) 90 90 90 β (deg) 90 93.2460(10) 90.254(3) γ (deg) 90 90 90 Volume (Å3): 4112.4(4) 4183.19(17) 2626.2(3) Z 8 4 4 Final R indices R1 = 0.0228 R1 = 0.0290 R1 = 0.0526 [I > 2σ(I)] wR2 = 0.0430 wR2 = 0.0661 wR2 = 0.1288 R indices (all data) R1 = 0.0427 R1 = 0.0341 R1 = 0.0680 wR2 = 0.0487 wR2 = 0.0685 wR2 = 0.1428 ____________________________________________________________________________________

___________________________________________________________________________

27

Table 2. Vertical excitation frequencies of PbIIMB-cyclen complex obtained from TD-DFT calculations using CAM-B3LYP [18] exchange-correlation functional with the optimized excited state geometries shown in Figure 12(a). The corresponding oscillator strengths and the major contributions to each transition are included as well. The transition wavelengths reported in paranthesis were obtained with the B3LYP [19,20] exchange-correlation functional.  TransitionEnergy  Oscillator MO Contributions [/eV] [/nm] Strength contact NO S0 S1 5.236 237 0.001 H-1  L (34%) (246) H  L+2 (33%) H-1  L+3 (17%) YES S0 S1 3.372 368 0.109 H  L (97%) (397)

28

H

N

N

N

N

H

H

H

MB-cyclen

N

N

N

N

H

DB-cyclen

N

N

N

TB-cyclen CH3 O

O O

O

N

N

CH3

Si

N

N

CH3

N

N

silole

N

adpa

cdpa

N

N

N

N N

N

N

N N

N

H

N

N

N

N

H

N N

TPEN

N,N-isoBQBPEN

DA-cyclen

Figure 1. Ligands and sensors discussed in this paper.

29

Figure 2. Structure of the hydroxide bridged dimer [(Zn(MB-cyclen))2OH]3+ present in 1. Hydrogens, except on the bridging hydroxide, omitted for clarity. Thermal ellipsoids drawn at the 50% probability level. Drawing made with ORTEP [23]

30

Figure 3. AgI complex of MB-cyclen present in structure 2, showing the very short intermolecular Ag∙∙∙C  contacts between the Ag and a benzyl group from an adjacent complex. The Ag∙∙∙C η2 -contacts hold together the alternating [Ag(MB-cyclen)] individuals to form a continuous chain. Hydrogens are omitted for clarity. Thermal ellipsoids are drawn at the 50% probability level. Drawing made with ORTEP [23].

31

Figure 4. Distribution of numbers of examples of η2 Ag∙∙∙C and Hg∙∙∙C π-contact distances in contacts with aromatic rings, as a function of the numbers of examples found in the indicated ranges of M∙∙∙C π-contact distances in the CSD [28]. The contacts include both intermolecular and intramolecular contacts with carbons in aromatic rings, where the contacted carbons have only hydrogen atoms bound to them in the case of AgI, but also alkyl substituents in the case of HgII.

32

Figure 5. PbII complex of MB-cyclen present in structure 3. Hydrogens on carbon atoms of the macrocyclic ring are omitted for clarity, and Pb-O bonds longer than the sum of ionic radii of Pb and O are drawn as dashed bonds, since these were not recognized as bonds by ORTEP [23]. The complex is viewed down what is roughly a four-fold rotational axis (considering only the positions of the donor atoms), to indicate what would best be described as its square antiprismatic coordination geometry. Thermal ellipsoids are drawn at the 50% probability level. Drawing made with ORTEP [23].

33

Figure 6. HgII complex of MB-cyclen present in structure 4, showing the very short intermolecular Hg∙∙∙C  contacts between the Hg and a benzyl group from an adjacent complex. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at the 50% probability level. Drawing made with ORTEP [23].

34

Figure 7. CuII complex of MB-cyclen present in structure 5 showing two complex individuals and the orientation of the N-benzyl groups in the solid state. Hydrogen atoms are omitted for clarity, and thermal ellipsoids are drawn at the 50% probability level. Drawing made with ORTEP [23].

35

Figure 8. NiII complex of MB-cyclen present in structure 6, showing the coordinated acetonitrile groups that complete an approximately octahedral coordination sphere around the nickel. Hydrogen atoms are omitted from the carbon atoms of the cyclen moiety for clarity. Thermal ellipsoids are drawn at the 50% probability level. Drawing made with ORTEP [23].

36

Figure 9. Fluorescence spectra of 10-4 M MB-cyclen from pH 2.36 to 11.84 in 50% CH3OH/H2O, Excitation wavelength = 265 nm.

37

Figure 10. Absorbance spectra of 10-4 M MB-cyclen from pH 2.36 to 11.84 in 50% CH3OH/H2O, The intense broad band that appears above pH 7.58 and grows greatly in intensity up towards pH 12.75 is a typical light scattering peak associated with precipitation of very small particles.

38

Figure 11. Fluorescence spectra of MB-cyclen and its complexes with some metal ions, all at 10-4 M, and close to pH 7.0, in 50% (v/v) CH3OH/H2O. Excitation wavelength = 265 nm.

39

Figure 12. (a) DFT optimized excited state geometries of the PbIIMB-cyclen complex with (right) and without (left) a -contact. (b) Plots of frontier orbitals associated with the emission (b) without  contact (normal fluorescence) and (c) with  contact. All DFT and TD-DFT calculations were performed by using the GAMESS package [17].

40

All authors have been credited

41

All authors have been credited

42

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

43

The metal ion complexing and fluorescent properties of N-monobenzyl cyclen are reported showing that it responds as a sensor for Zn(II), Hg(II) and Pb(II) at three different wavelengths. Crystallographic and DFT studies are used to support suggested an aggregation induced emission mechanism for the Hg(II) complex, and that the mechanism for the Pb(II) complexes involves transmissions within the inert pair on the Pb(II)

44