3.1. Structure Description
Structure description of 1 and 2: The PXRD patterns of
1 (
Eu) and
2 (
Tb), along with the simulated profile of
2, show peaks in identical 2θ positions for both (
Electronic Supplementary Information (ESI) Figure S1). The MOFs also show identical PXRD profiles as the simulated pattern of
1, which confirms that each sample crystallizes as a pure phase. Crystal structure data for single SCXA are presented in
Table 1. Both PXRD and SCXA analyses show that
1 and
2 are isostructural, so detailed analysis is given for
1 only. The crystal structure of
1 refines in a triclinic P-1 space group as a 3-D coordination polymeric network, with minimum voids and no measurable porosity.
Metal coordination: Structure
1 consists of two crystallographically identical Eu atoms per unit cell. There are five ADC units and two DMF molecules surrounding each Eu1 atom (
Figure 1a). The Eu1 atom has nine coordinates, all with Eu1–O bonds. Seven Eu–O bonds are with the oxygen atoms from the five ADC ligands and two Eu1–O bonds are with oxygen atoms of two DMF molecules, generating an irregular EuO
9 coordination polyhedron. A list of select bond lengths and angles for
1 and
2 is presented in
Table S1. The Eu–O bond lengths range from 2.363(1) Å for Eu–O3 to 2.572(2) Å for Eu–O1 and are similar to those observed in related coordination polymers [
33,
34]. The two Eu atoms are bridged by the oxygen atoms of two ADC ligands, with O1 atoms coordinating to Eu1 and creating an intradinuclear Eu···Eu distance of 4.036 Å.
Ligand coordination: The ADC unit coordinates in three modes, namely, μ
2:η
3 (
bis-bridging-chelating), μ
2:η
2 (
bis-bridging), and η
2 (
bis-chelating). The bridging ligands link four metal atoms, whereas the
bis-chelating ligands involve the linking of two. Various interconnected polymeric chains can be identified in the structure. Along the (100) direction, -(ADC-Eu
2)
n- chains are present, with the ADC units coordinating Eu atoms in
bis-bidentate chelating mode, and with two adjacent Eu1 atoms bridged by oxygen atoms of the ligands carboxylate functional group (
Figure 1b). Along the (010) direction, -(ADC-Eu
2)
n- chains are also present, with the ADC units coordinating Eu atoms in
bis-bidentate bridging/chelating mode through the ligands’ carboxylate groups. Each Eu atom is coordinated by three carboxylate oxygen atoms, and each carboxylate oxygen atom coordinates two Eu atoms. The -(ADC-Eu
2)
n- chains are also present along the
c direction with the ADC units in
bis-bidentate bridging mode; each carboxylate group coordinates two Eu atoms (
Figure 1c). The nearest Eu···Eu distance along the length of an ADC unit is 11.4 Å. Interestingly, the carboxylate groups are significantly twisted outside the plane of the anthracene moiety and measure 63.6° (C6-C2-C1-O2) for the
bis-chelating, 85.8° (C11-C10-C9-O3) for the
bis-bridging, and 68.7° for the
bis-bridging/chelating ligands. The twisting of the carboxylate groups, as compared to a coplanar configuration, is consistent with reported DFT calculations on the parent H
2ADC molecule, which showed its potential energy to be at a minimum with a 60° rotation of the COOH group [
35]. The interconnectivity of the chains through the Eu1 atoms creates a 3-D arrangement, with channels along the
b axis and the coordinated DMF solvent inside. Though non-continuous solvent accessible voids constitute 11% of the structure, nitrogen adsorption analysis at 77 K on the activated sample of
1 shows no appreciable porosity.
Structure description of 3 and 4: The PXRD patterns of
3 (
Er) and
4 (
Tm), along with the simulated profile of
4, show peaks in identical 2θ positions (
ESI Figure S2). The structures show PXRD profiles that are identical to the simulated pattern of
4, which confirms that they both crystallize as pure phases. Both PXRD and SCXA (
Table 1) show that
3 and
4 are isostructural, so detailed analysis is given for
4 only. The structure is a 3-D network consisting of two crystallographically inequivalent Tm atoms, six ADC ligands, two coordinated, and two lattice, DMF molecules, and two coordinated, and one lattice, water molecules.
Metal Coordination: The coordination environment for each of the two crystallographically inequivalent Tm atoms is shown in
Figure 2. The Tm1 atom has eight coordinates. Six are Tm1-O
CARB bonds with oxygen atoms from four ADC units, while the seventh and eight are Tm1-O
H2O and Tm1-O
DMF bonds, respectively (
Figure 2a). The distorted Tm1O
8 coordination polyhedron has Tm1-O bond lengths ranging from 2.228(2) Å for Tm1-O10 to 2.512(3) Å for Tm1-O3, and O-Tm1-O bond angles ranging from 53.8(8)° to 89.1(4)°. The Tm2 atom coordinates seven oxygen atoms to form an irregular Tm2O
7 coordination polyhedron (
Figure 2a). There are five Tm2-O
CARB bonds from four ADC ligands, one Tm2-O
DMF bond, and one Tm2-O
H2O. A list of selected bond lengths and angles are presented in
Table S1. The Tm2-O bond lengths range between 2.219(3) Å for Tm2-O13 and 2.393(3) Å for Tm2-O5. The O-Tm2-O bond angles range from 55.7(3)° to 86.3(2)°. The Tm-O bond lengths are similar to those observed in related MOF structures [
12]. The intradinuclear Tm1···Tm2 distance measures 4.687 Å.
Ligand coordination: Three coordination modes can be identified for the ADC linker, namely, bridging (μ
2:η
2),
bis-chelating (η
2), and
bis-monodentate (η
1). The
bis-chelating ligands coordinate Tm1 and Tm2 atoms along both the
b and
c axes to create a 2-D “ladder-like” conformation, with the “ladder rungs” along the
b axis (
Figure 2b). The “ladder” structure is like that reported by Wang et al. in a related but different structure [
13]. Along the
b axis, the
bis-chelating and
bis-monodentate coordinating ADC units are coordinated to Tm1···Tm2 centers in an alternating arrangement. The
bis-bridging ADC units, which are aligned along the
a axis, intersect the 2-D ladder arrangement at the bimetallic Tm1···Tm2 centers to complete the 3-D network (
Figure 2b). As in the case of
1 and
2, the carboxylate groups are significantly twisted outside the plane of the anthracene moiety, with a twist angle of 62.2° (C30-C27-C26-O7) for the
bis-bridging ligand and 77.6° (C3-C2-C1-O2) for the
bis-chelating ligands. As discussed earlier, the twisting is dictated by the minimum energy conformation from the 60° rotation of the carboxylate, as shown by DFT calculations [
34]. The uncoordinated oxygen atoms of the carboxylate groups on the
bis-monodentate coordinating ADC units form strong hydrogen bonds with nearby coordinated water molecules, thus further stabilizing the ligand and providing restriction to its rotation (
Figure 2c). The closest interchromophore distance is 12.5 Å. Narrow channels are present along the
b direction. However, like
1, nitrogen adsorption analysis at 77 K on an activated sample of
4 shows no appreciable porosity.
FTIR Analysis: The nature of the ADC units in the structures was further investigated by analysis of their Fourier transformed infrared spectra (FTIR) and a comparison with that of H
2ADC (
ESI Figure S3). The band at 3448 cm
−1 in all samples indicates the presence of O–H from adsorbed water on the MOFs and on H
2ADC. The 2925 cm
−1 and 2967 cm
−1 bands observed in H
2ADC were assigned to weak intramolecular O⋅⋅⋅H bonds between non-planar C=O and H on the aromatic ring at the 1, 4, 5, and 8 carbon positions. These bands were not observed in
1–
4, and their absence is attributed to C=O coordination to the metal atom, thus limiting their interactions with aromatic H. The band observed at 1687 cm
−1 in the spectrum of H
2ADC is attributed to the HO–C=O, with localized charges on the ligand’s carboxylic acid functional groups. This band was not observed in
1–
4; instead, two individual bands were observed at 1601 cm
−1 and 1551 cm
−1, which are attributed to variation in stretching vibrations of the C–O bonds in the three different ligand conformations. These observations indicate that the ligand is deprotonated (as ADC) within the MOF structures. The band at 1562 cm
−1 in the MOF spectra is attributed to of metal-oxygen bonds [
36].
Thermal Analysis: The thermal behavior of the structures was investigated by thermogravimetric analysis. The TGA curves of 1 and 2 show weight loss between 100 and 450 °C representing the loss of coordinated and uncoordinated DMF molecules (~30 wt %). Weight loss commencing around 440 °C (~40 wt %) is attributed to the loss of ADC units, and residue (~30 wt %) is attributed to lanthanide oxides. The TGA curves of 3 and 4 show small weight loss events up to 100 °C, attributed to the loss of H2O molecules (~10 wt %). Weight loss up to 400 °C (~15 wt %) is attributed to loss of DMF. Weight loss event commencing around 400 °C (~45 wt %) is attributed to the loss of ADC units, and residue (~30 %) is attributed to lanthanide oxides.
3.2. Photoluminescence
The photoluminescence behavior of each compound was investigated. The room temperature solid-state photoluminescence emission spectra of the structures along with that of solid H
2ADC are presented in
Figure 3. The spectrum of Na
2ADC in dilute aqueous solution was also recorded for comparison (
ESI Figure S5). The spectrum of Na
2ADC in aqueous solution shows two defined vibronic peaks: one with λ
max at 425 nm and a shoulder at 450 nm. This is similar to that reported for pure anthracene [
37], except that a smaller left shoulder peak expected at ~400 nm was not defined. The MOF structures (except
3) show emission spectra with distinct vibronic peaks that are similar in profile to those observed for Na
2ADC in aqueous solution, thus suggesting that the emission is linker-based.
The emission peaks from the structures are within the 400–600 nm region, with their wavelength maxima (λ
max) observed between that of the Na
2ADC in dilute aqueous solution at 425 nm and that of the H
2ADC powder at 500 nm. The emission maxima of the structures are therefore red-shifted compared to the ADC sodium salt solution and blue-shifted compared to the H
2ADC powder. Further, a Stokes shift was observed among the structures as follows: 60 nm for
1 (380 nm
ex-max to 440 nm
em-max), 65 nm for
2 (380 nm
ex-max to 435 nm
em-max), 50 nm for
3 (389 nm
ex-max to 430 nm
em-max) and 30 nm for
4 (400 nm
ex-max to 430 nm
em-max), all of which are smaller than the 87 nm observed for H
2ADC. Except for
4, the Stokes shift values are larger than the 41 nm observed in Zn-PCN-14, which contains the larger and rotatable anthracene liker, 5,5′-(anthracene-9,10-diyl)diisophthalinic acid (DPATC) [
2]. With the exception of Structure
3, the peaks are more defined than those observed in Zn-PCN-14. By comparison, the emission spectrum of H
2ADC is broad with less defined peaks and a larger Stokes shift (
Figure 4). The broad spectral features of H
2ADC are likely ascribed to changes in the excited state geometry of the molecule due to the rotation of its –COOH groups to near coplanar conformation with the anthracene moiety. This can result in a lower energy excited state complex due to increased resonance, π overlap, and charge transfer interactions between the functional group and the ring system [
34]. However, the ADC units in Structures
1–
4 are deprotonated and are rigidified by coordination to the metal atoms. Rotation of the carboxylate groups is expected to be restricted as a result. This ridification, coupled with the separation of individual ADC units in the structures, will therefore reduce the level of interligand interactions and, by extension, reduce the extent of non-radiative relaxation pathways that would otherwise exist in solid forms of both anthracene and H
2ADC.
Within each structure, the cofacial alignment of ADC units in the (100) direction is interrupted by ADC units in the (010) direction. The closest cofacial distances are 14.5 Å in
1 and 12.5 Å in
4, which are beyond the distance within which significant interchromophore coupling interactions among the phenyl rings of ADC units would be present [
38]. The nearest distances between the planar face of one anthracene moiety and the hydrogen atoms on the edges of another, range between 3.689 (H12···C3) and 5.242 Å (H12···C7) for
1, and between 3.397 (H4···C24) and 5.542 Å (H4···C21) for
4. These edge-to-face distances are in the range within which C–H···π interactions are possible between the-orbitals of the hydrogens and the π system of the anthracene moieties. Such interactions could contribute to the non-radiative decay pathways and to the observed Stokes shifts. The broad featureless emission spectrum of
3 could be the result of more severe structural changes that brought ADC units closer on exposure to UV excitation. The possibility of the inductive effects of the Er atoms on the linker that can cause perturbation of the electronic transitions occurring in the ligand to result in more diffuse spectrum is also worthy of consideration and warrants further investigation.
Of note is that no luminescence spectral features from the lanthanide ions were detected in the visible region (for Eu
3+ and Tb
3+) and were not measured in the near-infrared region (for Tm
3+ and Er
3+) which is beyond the range of our standard fluorimeter. It is speculated, however, that following direct excitation the photoemissions of the latter two metals ions would be weak or non-existent due to their low molar absorption coefficient (typically lower that 10 L mol
−1 cm
−1) [
1,
2,
3,
4].
Time-resolved photoluminescence decay: Time-resolved photoluminescence measurements were also acquired to further investigate the local environment of the anthracene units in the 3-D networks. Structures
1 Eu (not shown),
3 (
Er), and
4 (
Tm) yielded photoluminescence decay curves of low intensity that almost overlap with the instrument response function (IRF) (
ESI Figure S6), while the decay curve for
2 (
Tb), like that of H
2ADC, was quite distinct from the IRF (
Figure 4). The decay curves were fitted with the biexponentional function,
, which corresponds to two different photo emissive rates, where I is the intensity, τ is the time, τ
1 and
τ2 are their corresponding excited state decay lifetimes, and α is the pre-exponential factor. The faster a radiative lifetime a major component has, the more τ
1 is attributed to emission from monomeric-like ADC units, and the more τ
2 is attributed to ADC units involved in coupling interactions, as observed for anthracene dimers. For Structure
2 (
Tb), lifetimes τ
1 = 0.2 ns and τ
2 = 0.5 ns and weighted average lifetime τ
o = 0.3 ns are much shorter than the 4.9 ns, 16 ns, and 9.5 ns, respectively, we determined for H
2ADC (
ESI Table S2). These lifetimes are also shorter than the average lifetime value of τ
o = 2.0 ns reported elsewhere for anthracene in monomeric isolated arrangements [
39] and shorter than τ
o = 5 ns reported for Zn-PCN-14 [
2].
As discussed earlier, changes in the excited state geometry of bulk ligand molecules due to the rotation of its –COOH groups to near coplanar conformation with the anthracene moiety and reorganization towards end-to-face herringbone arrangement, which can facilitate excimer formation and strong interchromophore interactions, are quite likely. This could have contributed to the much longer lifetime (τ
2) compared to the MOF structures [
8,
34].
The short lifetimes observed for Structures
1–
4 in comparison to those of H
2ADC and Zn-PCN-14 suggest that there is significant fluorescence quenching in the structures. Similar quenching of ligand fluorescence in complexes of lanthanides ions, including Tb
3+ and Eu
3+ have been previously observed [
40] and is postulated to occur by the energy transfer between the ligand and the paramagnetic lanthanide ions via a cross-relaxation mechanism.