Luminescent supramolecular heterometallic macrocycles and their encapsulation on cholate gels

The metal complex formed by coordination of Zn(II) to 1,7-bis(4-methylpyridine)-4-(2naphthylmethyl)-1,4,7-triazaheptane (ZnL) was reacted in aqueous solution with [Pd(NO3)2(en)] and [Pt(NO3)2(en)] salts to form the self-assembled heterometallic macrocycles [Zn2L2Pd2(en)2] and [Zn2L2Pt2(en)2], respectively. Pd(II) and Pt(II)coordination modulates the original emission of ZnL arising from the presence of the naphthalene chromophore and the formation of the macrocycles can be monitored due to the PET process occurring with coordination of Pd(II) and Pt(II) to the pyridine units of ZnL. Additionally, several studies reveal that these heteromacrocycles can be encapsulated in Zn(II)-cholate hydrogels giving rise to soft materials with tunable emissive properties. Preliminary analysis show also that the addition of the metallic species in micromole concentration to the cholate hydrogels resulted in an improved mechanical strength of the final materials.


Introduction
Supramolecular metallomacrocycles [1] and related supramolecular architectures [2][3][4] have attracted great attention on the last decades and 2D metallocycles with different geometries (such as triangles, squares, pentagons and hexagons) have been synthesized. [5,6] Several potential applications have been reported for these metallomacrocycles such as reactor cavities, catalysts, sensors, photonic devices and platforms for the design of materials with tailored properties, among others. In concrete, heterometallic supramolecular macrocycles where the second metal can introduce additional functionality and complexity to the final architectures are of special interest.
In that sense, the use of Pt(II) and Pd(II) as second metal centers to be coordinated to pyridine moieties and related ligands has been very profitable [6][7][8] . This is mainly due to the structural diversity of pyridyl-based ligands, the geometry of the metal centers and the facility for tuning the lability of the coordination bond.
The use of this kind of supramolecular macrocycles in applications for daily life requires their supporting on a solid-like phase. A versatile approach involves the encapsulation in supramolecular gels. The latter are well known soft materials that are formed via non-covalent interactions of small molecules. [9] These interactions enable the reversible formation of large self-assemblies or supramolecular polymers, forming a solid-like network (minor component) which is able to entrap either an organic solvent or water (major component) through surface tension or capillary forces. [10] Due to their particular properties, in the last decades the use of supramolecular gels has been studied in a wide range of applications such as drug delivery and tissue engineering, [11,12] crystallization of pharmaceuticals, [13] luminescent materials, [14] reaction vessels and reusable catalysts, [15] and pollutant removal [16] , among others.
Within this context, it is known that the interaction between the cholate anions and divalent or trivalent metal cations is able to form a 3D-netwotk which entraps water forming supramolecular hydrogels. [17,18] It is believed that the mechanism of gelation is related to metal-carboxyl cholate coordination, hydrogen bonding between cholate molecules and a delicate hydrophilic-hydrophobic balance. [17] The employment of trivalent lanthanides as metal cations has allowed the development of emissive soft materials with tunable luminescence. [18][19][20][21] Other applications for divalent-cholate 3 hydrogels have been reported such as their use as precursors for the development of lithium-based batteries. [21] In this paper, we report the formation of heterometallic macrocycles through the interaction of a Zn(II)-polyamine complex with Pd(II) and Pt(II) species. The encapsulation of these macrocycles in Zn(II)-cholate hydrogels yields soft materials with tunable emission and improved mechanical properties.

Formation of heterometallic macrocycles with ZnL 2+
Polyamine ligand 1,7-bis(4-methylpyridine)-4-(2-naphthylmethyl)-1,4,7triazaheptane (L) that contains two N-donor groups at terminal positions ( Figure 1) was reacted with a Zn(II) salt and subsequently with square planar [M(en)](NO3)2 (M = Pd 2+ , Pt 2+ ) complexes in order to synthesize luminescent water soluble heterometallic macrocycles, taking advantage of the affinity of the pyridine moieties towards Pt(II) and Pd(II) cations. The intrinsic luminescent properties of L were very useful to follow the self-assembly processes (see below). Compound L was previously described [22] and consists of a triamine chain functionalized at both terminal positions with 4-picolyl units and at the central nitrogen with a methylnaphthyl fragment. It was observed that zinc(II) coordinates to the polyamine chain giving rise to the formation of ZnL 2+ (Figure 2) which promotes the presence of a luminescence pH window by an "off -on -off" activity. The highest emission of ZnL 2+ is observed at around pH 8. This is due to the fact that at higher pH values (pH > 7.9) a photoinduced electron transfer (PET) phenomenon occurs from the amine lone pair to the ππ* excited state of the naphthalene, quenching the emission of the signaling unit. On the other hand, at lower pH values (pH < 7.9) PET process also occurs, but in this case from the π-π* excited state of the naphthalene to the protonated pyridinium unit. [23,24] It is necessary to adjust the pH at around 7.9 in order to succeed with the synthesis of the corresponding macrocycles. In this way, amine groups are coordinated to the Zn 2+ cation being a very useful situation to avoid competition with N- pyridyl atoms in the coordination with Pt(II) and Pd(II) in the self-assembly process ( Figure 2). [22,25] 1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64  65 ZnL 2+ (5x10 -5 M) and changes on the spectroscopical properties were followed at each point. Variations on the absorption spectra are mainly focused on the pyridyl unit with an increase of intensity at ca. 260 nm ( Figure S1 in SUPPORTING INFORMATION).
Moreover, the addition of the increasing amounts of Pd(II) or Pt(II) complex causes a decrease on fluorescence intensity of the naphthalene at ca. 335 nm ( Figure 3) and reaches a plateau after the addition of ca. 2 equivalents Pd(II) or Pt(II) salt, being indicative of the formation of the expected supramolecular macrocycle. The decrease on the fluorophore's emission implies the activation of a quenching process. These changes in emission and absorption are analogous to those produced by protonation of pyridyl units [22] and suggest that the nitrogen atom of the aromatic amine could be coordinated to the square planar complex giving rise to the formation of heterometallic macrocycle (Scheme 1). Association constant values (Table 1) were calculated using the HypSpec 1.1.33 software for Windows [26] and in both cases, the system fits well to the formation of the 2+2 complex taking place in two steps: (eq. 1) where A refers to the ZnL 2+ complex and B to the heavy metal species, Pd(II) or Pt (II) respectively. It can be seen in Table 1, that whereas the formation of the A2B complex is equally favoured for both heavy metal species, the formation of the 2:2 complex is easier in the case of Pd(II), as expected for the kinetically easier formation of Pd(II)pyridyl bond. [27] 1 2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63 Table 1. (m/z = 984.5) were detected as a confirmation of the successful self-assembled macrocycle formation ( Figure S2 in SUPPORTING INFORMATION) .