Synthesis, Structure, and Luminescence of a Molecular Europium Tetracyanoplatinate Incorporating 4,5-Diazaﬂuoren-9-One

: Eu 2 (Pt(CN) 4 ) 3 (H 2 O) 12 ] · 4C 11 H 6 N 2 O · 6H 2 O represents a new member to the lanthanide tetra-cyanoplatinate family of materials. The synthesis, single-crystal X-ray structure, and photoluminescence properties of this compound are reported herein. The compound contains dimeric [Eu 2 (Pt(CN) 4 ) 3 (H 2 O) 12 ] molecules in which the Eu 3+ sites are connected with trans-bridging [Pt(CN)4] 2 − anions and co-crystallized along with 4,5-diazaﬂuoren-9-one (dafone) and lattice water. Prominent noncovalent interactions that stabilize the structure include H-bonding as well as short stacking interactions involving the planar tetracyanoplatinate anions in conjunction with the planar dafone molecules. Photoluminescence measurements illustrate that Eu 2 (Pt(CN) 4 ) 3 (H 2 O) 12 ] · 4C 11 H 6 N 2 O · 6H 2 O contains multiple emitting states and also displays enhanced Eu 3+ -based emission due to energy transfer within the compound.


Introduction
Cyanide-bridged, bimetallic compounds exhibit interesting structural and material properties, which have provided the basis for exploration of this research area [1][2][3][4].Magnetic applications have dominated the work centering around transition-metal hexacyanides or Prussian blue type compounds [5][6][7], while optical applications have been the main research focus regarding the dicyanometallates of Au and Ag and the tetracyanometallates of Pd and Pt [8][9][10].However, a multitude of research reports on the rare-earth cyanometallates focusing on the synthetic, structural, spectroscopic, and catalytic aspects of these materials are also available in the literature [11][12][13][14][15].In many such compounds, metallophilic interactions are recognized as a major force determining structural dimensionality and properties [16,17].For example, the square-planar [Pt(CN) 4 ] 2− anions readily associate into 1-D chains [18], or smaller geometric units [19], containing short Pt-Pt interactions and are prominent in the families of tetracyanoplatinate compounds.
Lanthanide ion luminescence is ideal for a number of applications, e.g., fluoroimmunoassays, [20,21] cellular imaging [22,23], and optical communications [24,25], due to its sharp f -f transitions and high color purity.However, because direct absorption of the f -f excited states is very inefficient, optimizing chromophoric ligands to enhance lanthanide-ion luminescence has been and continues to be an active area for researchers.Many organic [26,27] and inorganic [28,29] donors have been investigated over the years to efficiently transfer energy to acceptor lanthanide ions, including the aforementioned cyanometallate systems.

Single-Crystal X-ray Diffraction Studies
Single crystals of the title compound were selected, mounted on quartz fibers, and aligned with a digital camera on an Oxford Xcalibur E single-crystal X-ray diffractometer.Intensity measurements were performed using Mo Kα radiation from a sealed-tube Enhance X-ray source and an Eos area detector.CrysAlisPro [38] was used for the preliminary determination of the cell constants, for the data collection strategy, and for the data collection control.Following data collection, CrysAlisPro was also used to integrate the reflection intensities, apply an absorption correction to the data, and perform a global cell refinement.
For all structure analyses, the program suite SHELX (v.2014/3) was used for the structure solution (XS) and least-squares refinement (XL) [39].The initial structure solutions were carried out using direct methods, and the remaining atomic positions were located in difference maps.The final refinements included anisotropic displacement parameters for all non-hydrogen atoms.Some crystallographic details are listed in Table 1, and additional details are available as Supplementary Materials.CCDC 2169999 contains the supplementary crystallographic data for this paper.These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (accessed on 29 August 2022, or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336033; E-mail: deposit@ccdc.cam.ac.uk)

Photoluminescence Studies
The luminescence measurements were conducted utilizing a Photon Technology International (PTI) spectrometer system.This instrument utilizes a 75 W Xe source and PMT detectors (Hamamatsu model 928 tube) operating in digital (photon counting) mode to measure steady-state emission and excitation spectra.Time-dependent measurements were performed by utilizing a variable, pulsed Xe source and gated photomultiplier tubes (Hamamatsu model 928 tube).The selection of excitation and emission wavelengths were conducted by means of computer-controlled, autocalibrated "QuadraScopic" monochromators, which are equipped with aberration-corrected emission and excitation optics.The emission monochromators were calibrated, using a NIST traceable tungsten light, to compensate for the wavelength-dependent variation in the system on the emission channel.The instrument operation, data collection, and handling were all controlled using the FeliX32 fluorescence spectroscopic package.All of the spectroscopic experiments are conducted on neat crystalline samples, individually screened for phase purity using single-crystal X-ray diffraction and held in sealed quartz capillary tubes.

Structural Studies
The structure of the europium complex in [Eu 2 (Pt(CN is molecular, and contains a dimeric core with the two Eu(III) sites joined by a trans-bridging Pt(CN) 4 2− anion.A thermal ellipsoid plot of the asymmetric unit with the included atomic labeling scheme is illustrated in Figure 1, and the dimer is illustrated in Figure 2.This dimeric core is reminiscent of that contained in [{Eu(DMSO) 2 (C 12 H 8 N 2 )(H 2 O) 3 } 2 Pt(CN) 4 ] 2+ complex cations [40].The slightly distorted, square-antiprismatic coordination geometry around each Eu(III) center results from one monodentate N-bound Pt(CN) 4 2− anion, one trans-bridging N-bound Pt(CN) 4 2− anion, and six water molecules.Pt1, occupying the trans-bridging Pt(CN) 4 2− , resides on an inversion center, which relates the two halves of the dimeric complex cation by symmetry.
trans-bridging Pt(CN)4 2− , resides on an inversion center, which relates the two halves of the dimeric complex cation by symmetry.In addition to the metal-centered complexes, dafone molecules and waters of hydration are co-crystallized within the lattice and add to its structural stability with many and varied H-bonding and pi-stacking interactions.The number of H-bonds for the complex waters is maximized via the formation of two H-bond donor interactions, as described in detail in Table 2.All of the following acceptors are involved in at least one H-bonding interaction in the structure: uncoordinated N-terminated cyanide, lattice water, dafone-N, or dafone-O.Furthermore, every non-coordinated cyanide from each tetracyanoplatinate anion is involved in an H-bonding acceptor interaction with an O−H donor from either a coordinated or uncoordinated water molecule.In addition to these common types of van der Waals interactions mentioned above, [Eu 2 (Pt(CN) 4 ) 3 (H 2 O) 12 ]•4C 11 H N 2 O•6H 2 O contains additional intermolecular interactions as shown in Figure 2. The structure contains stacks of alternating pairs of dafone molecules and tetracyanoplatinate anions.The pistacking distances between dafone molecules are ~3.42Å, and the plane-centroid distances between dafone and tetracyanoplatinate are ~3.55Å within these 1-D stacks.Previous structural studies containing square planar Pt(II) and organic aromatics have described similar interactions [41] [31,32,35].3, and band locations and assignments are given in Table 4.These data illustrate that [Eu 2 (Pt(CN) 4 ) 3 (H 2 O) 12 ]•4C 11 H 6 N 2 O•6H 2 O displays a number of sharp emission bands, some strong, stretching across the visible region, in addition to a weak, broad emission band centered at approximately 510 nm.The assignment of the sharp emission bands in the spectrum is straightforward as these originate from f −f emissions of two different Eu(III) excited states.Expectedly, direct Eu(III) excitation, e.g., the 5 D 2 ← 7 F 0 transition at 467 nm, provides relatively weak Eu-based emission due to low absorption of the f -element ion, however, excitation with higher energy UV radiation in the range of 270 to 450 nm leads to similar emission profiles with much greater intensities.

D-H•••A D-H H•••A D•••A D-H•••A
illustrate that [Eu2(Pt(CN)4)3(H2O)12}]•4C11H6N2O•6H2O displays a number of sharp emission bands, some strong, stretching across the visible region, in addition to a weak, broad emission band centered at approximately 510 nm.The assignment of the sharp emission bands in the spectrum is straightforward as these originate from f−f emissions of two different Eu(III) excited states.Expectedly, direct Eu(III) excitation, e.g., the 5 D2← 7 F0 transition at 467 nm, provides relatively weak Eu-based emission due to low absorption of the f-element ion, however, excitation with higher energy UV radiation in the range of 270 to 450 nm leads to similar emission profiles with much greater intensities.The 5 D0→ 7 F0 transition, oftentimes just referred to as the 0−0 transition, is observed at 579 nm and signifies a strong electric dipole contribution.The observance of this 0−0 transition with only a single band is in line with the crystallographic structure analysis  The 5 D 0 → 7 F 0 transition, oftentimes just referred to as the 0−0 transition, is observed at 579 nm and signifies a strong electric dipole contribution.The observance of this 0−0 transition with only a single band is in line with the crystallographic structure analysis that shows a C 1 site symmetry for the single Eu(III) site in the compound.The breaking of the degeneracy of the 5 D 0 → 7 F 1 transition resulting in the observance of multiple Stark components is also consistent with a low site symmetry for Eu(III) (C 1 symmetry) [42].The 5 D 0 → 7 F 2 transition shows splitting into five Stark components even at room temperature.Although the two outer bands are not completely resolved, this splitting is estimated at 100 cm −1 .The Eu 3+ ion emission has a lifetime of 120 µs, measured for the 5 D 0 → 7 F 1 transition, in the title compound.While this value is in line with the reported Eu 3+ lifetimes Crystals 2023, 13, 317 7 of 10 in other systems, the differences can be attributed to the number of H 2 O molecules directly coordinated to the Eu(III) ion in the different compounds [43] or to the relative amount of hydrogen bonding involved in systems containing charge transfer interactions with Eu(III) acceptors [44].
The assignment of the broad emission band centered at approximately 510 nm is not as straightforward.Performing time-delay emission experiments, with pulsed excitation, reveals that the lifetime of this emission is much shorter than the Eu 3+ f-f emission bands.Figure 4 shows the decoupled emission from the title compound at different time delays.This indicates that the emission originates from different emitting species within the compound.Since dafone and tetracyanoplatinate can both independently present broad emission bands within this spectral region [30,31], the emission spectra alone is inconclusive.However, the excitation spectra shown in Figure 5 are not reminiscent of the absorption features of either dafone or tetracyanoplatinate alone.Further, the similarity of the excitation bands upon monitoring the Eu(III) emission or the ligand-based emission band is direct evidence for the existence of energy transfer in the system.All of this together indicates the existence of exciplex formation within this compound.
that shows a C1 site symmetry for the single Eu(III) site in the compound.The breaking the degeneracy of the 5 D0→ 7 F1 transition resulting in the observance of multiple Sta components is also consistent with a low site symmetry for Eu(III) (C1 symmetry) [42].T 5 D0→ 7 F2 transition shows splitting into five Stark components even at room temperatu Although the two outer bands are not completely resolved, this splitting is estimated 100 cm −1 .The Eu 3+ ion emission has a lifetime of 120 μs, measured for the 5 D0→ 7 F1 tran tion, in the title compound.While this value is in line with the reported Eu 3+ lifetimes other systems, the differences can be attributed to the number of H2O molecules direc coordinated to the Eu(III) ion in the different compounds [43] or to the relative amount hydrogen bonding involved in systems containing charge transfer interactions w Eu(III) acceptors [44].
The assignment of the broad emission band centered at approximately 510 nm is n as straightforward.Performing time-delay emission experiments, with pulsed excitatio reveals that the lifetime of this emission is much shorter than the Eu 3+ f-f emission ban Figure 4 shows the decoupled emission from the title compound at different time dela This indicates that the emission originates from different emitting species within the co pound.Since dafone and tetracyanoplatinate can both independently present broad em sion bands within this spectral region [30,31], the emission spectra alone is inconclusi However, the excitation spectra shown in Figure 5 are not reminiscent of the absorpti features of either dafone or tetracyanoplatinate alone.Further, the similarity of the exci tion bands upon monitoring the Eu(III) emission or the ligand-based emission band direct evidence for the existence of energy transfer in the system.All of this together in cates the existence of exciplex formation within this compound.

Figure 1 .
Figure 1.A thermal ellipsoid plot (50%) shows the asymmetric unit of the title compound.The Eu and Pt sites contain square-antiprismatic and square-planar coordination environments, respectively.Pt1 is located on an inversion center.The atoms in the dafone molecules are not labeled for clarity.

Figure 2 .
Figure 2. A ball and stick plot illustrating the molecular dimer with the central trans-bridging tetracyanoplatinate anion.The stacking of the dafone and tetracyanoplatinate anions is also shown.

Figure 1 .
Figure 1.A thermal ellipsoid plot (50%) shows the asymmetric unit of the title compound.The Eu and Pt sites contain square-antiprismatic and square-planar coordination environments, respectively.Pt1 is located on an inversion center.The atoms in the dafone molecules are not labeled for clarity.

Figure 1 .
Figure 1.A thermal ellipsoid plot (50%) shows the asymmetric unit of the title compound.The Eu and Pt sites contain square-antiprismatic and square-planar coordination environments, respectively.Pt1 is located on an inversion center.The atoms in the dafone molecules are not labeled for clarity.

Figure 2 .
Figure 2. A ball and stick plot illustrating the molecular dimer with the central trans-bridging tetracyanoplatinate anion.The stacking of the dafone and tetracyanoplatinate anions is also shown.

Figure 2 .
Figure 2. A ball and stick plot illustrating the molecular dimer with the central trans-bridging tetracyanoplatinate anion.The stacking of the dafone and tetracyanoplatinate anions is also shown.

Figure 4 .
Figure 4. Room temperature, gated emission spectra collected with 370 nm excitation and 10 or 1 μs time delays.

Figure 4 .
Figure 4. Room temperature, gated emission spectra collected with 370 nm excitation and 10 or 130 µs time delays.