Crystal structure and triboluminescence of europium(III) tetrakis-thenoyl trifluoroacetonate with outer-sphere organic cation

A novel complex of Eu(III) tetrakis-thenoyltrifluoroacetonate with the outer-sphere cation 2-(E)-1-(2-thenoyl)ethylidene)-1-hydrazonium carboxyimidoamide (Q) of the composition Q[Eu(TTA)4]·H2O (TTA, thenoyltrifluoroacetonate-ion; Q, outer-sphere cation) characterized with intense luminescence and triboluminescence has been synthesized. The structure of the centrosymmetric hydrazonium crystal is composed of complex anions of the composition [Eu(TTA)4]−, which are stacked in layers parallel to the plane (100), whereas interlayers of organic cations are located between the layers. A structural model for the formation of triboluminescent properties has been suggested.


Introduction
The study of triboluminescence (TBL) -the emission that occurs during the friction or destruction of crystals -is urgent from both fundamental (search for ways to convert mechanical energy into light) and practical (in relation to the development of highly sensitive optical sensors for monitoring the size and location of damages in critical objects) points of view [1][2][3][4][5][6]. Lanthanide-coordination compounds are promising candidates for the development of triboluminescent materials that can show intense luminescence when a mechanical impact is applied [2,5].
Several mechanisms of the TBL excitation were suggested: electrification under friction [7], electric discharge between oppositely charged fault planes of a crystal [8], and piezoelectricity [9,10]. Some authors, after analyzing a large number of crystal structures, claimed that only non-centrosymmetric crystals can exhibit TBL [10,11]. However, other authors [12,13] fabricated a number of centrosymmetric crystals of lanthanide complexes with TBL. Intense luminescent lanthanide complexes represent one of the promising classes of triboluminophores [14][15][16][17][18][19][20][21]. At present, there is no available unambiguous interpretation of the mechanism of Ln 3+ excitation in the triboluminescence TBL, so the study of the relation between the structure and triboluminescent properties of lanthanide complex compounds is of great urgency.
A logical question arises: is there a relationship between molecular design and TBL properties? To reveal the structure-TBL property correlation, it is necessary to develop an appropriate molecular strategy [22,23]. In this regard, an interesting example is the crystals of muscovite (mica) characterized with TBL and a unique cleavage type [24,25]. Based on the muscovite structure composed of infinite aluminosilicate layers linked by layers of potassium ions [25], it seems natural to assume the formation of an uncompensated charge on the crystal cleavage planes.
Mica is not a pyroelectric, because its crystals have a center of symmetry. However, mica crystals glow when cleaved in the dark. According to the data of [22,23], the separation of aluminosilicate packs under mechanical impact results is a significant strength of the field of the group charge on the mica cleavage plane. Here, when the mica crystals are mechanically cleaved, a glow emerges, which is associated with the fact that the mica sheets acquire a charge during the cleavage process, and an electric discharge in the gas occurs between the charged surfaces.
Earlier, we proposed a structural model of the formation of TBL in lanthanide complexes [18][19][20][21]. Important factors in the formation of mechanoluminescence are the layered type of crystal structure and the presence of charge-bearing 1 3 ligands in the destruction zone. An additional factor is the presence of crystallographic strict boundaries of destruction zones. Only a limited number of tetrakis [Ln(TTA) 4 ] complexes (TTA, thenoyltrifluoroacetonate-ion) with outersphere organic cations have been probed with respect to their TBL properties [4,18,26]. In continuation of our works on revealing structural aspects of the formation of triboluminescent properties [18][19][20][21], the present work is devoted to study of the crystal structure and photoluminescent (PL) and triboluminescent properties of the novel complex Q[Eu(TTA) 4 ]·H 2 O (I), where TTA is the thenoyltrifluoroacetonate ion; Q, the cation 2-((E) -1 (2-thenoyl)ethylidene) -1-hydrazonium carboxymidoamide (I) (Scheme 1).

Synthesis of complex I
The complex I was synthesized as follows: 4.0 mmol of TTA (thenoyltrifluoroacetone) was dissolved at heating in 15 ml ethanol, and then, 4.0 ml of 1 N NaOH (Roskhimreaktiv) solution was added. Aminoguanidine hydrochloride (1.5 mmol) was dissolved at heating in 10 ml a mixture of ethanol and water (5:1) and, then, mixed with a solution of TTA. A total of 1 mmol of EuCl 3 6H 2 O (Roskhimreaktiv) dissolved in a small amount of water (5 ml) was added drop-by-drop to the hot solution. The mixture was heated to boil at stirring and, then, left to cool down. Within a week, a red oil was formed, from which crystals precipitated 2 weeks later. The resulting crystals were washed with acetone and dried in air. The obtained compound comprised yellow-brown crystals soluble in ethanol, acetone, and benzene. The product yields 61%. The element analysis was performed using a EuroEA 3000 analyzer.

Measurements
A complete X-ray diffraction study was performed using a KAPPA APEXII CCD system (MoKα-radiation, graphite monochromator). Data collection and editing, refinement of unit cell parameters, and conversion of integral intensities into modules of structural amplitudes were carried out via the software [27]. The structure was determined by the direct method with subsequent refinement of the positional and thermal parameters in the anisotropic approximation for all non-hydrogen atoms via the software [28]. The positions of hydrogen atoms of the ligands of the complex, in addition to the position of a hydrogen atom of a water molecule, albeit being revealed in the electron density syntheses, however, did not determine the fundamental novelty and, in further work, those calculated and refined according to the "rider" model were used. The position of the basis hydrogen atom of a water molecule was found from the electron density synthesis. In the process of refining the structure, it was found that the thenoyl groups of TTA ligands are disordered by A and B positions (Fig. 3). The main crystallographic parameters of the studied sample, the characteristics of the X-ray diffraction experiment, and the details of the refinement of the structure model by the least squares method are provided in Table S1, the main interatomic distances and valence angles are summarized in Table S2 (Supplementary information). The CIF file containing full information on the studied structure was deposited in the CCDC under the number 2050196, where it can be obtained on request at www. ccdc. cam. ac. uk/ data_ reque st/ cif.
The luminescence and excitation spectra of the crystal samples were recorded using an RF-5301 spectrofluorometer (Shimadzu).
To measure the triboluminescence spectra, we used an experimental complex based on the Flame Vision PRO System multichannel optical analyzer (OMA) consisting of a SPECTRA-PRO monochromator from Acton Research Corporation (USA) and a DiCAM-PRO module (optical brightness amplifier, 1024*768 CCD camera, 12-bit ADC). The crystals were destructed by the impact of a steel ball with a weight of 110 g falling from a height of 1m.

Luminescent properties of I
When the complex I is irradiated with ultraviolet light, an intense red PL is observed. Figure 1 shows the excitation and PL spectra of the complex I.
A broad band in the excitation spectrum of I is determined by the intramolecular π * ←π transition of TTA.
The luminescence spectrum I does not differ significantly from the luminescence spectra of known Eu(III) complex compounds and is determined by the f-f transitions of the lanthanide ion. The maximum luminescence intensity was observed for the 611 nm band corresponding to the electric dipole transition 5 D 0 -7 F 2 , in which the main part of the emission energy is concentrated. The Stark structure of the spectrum I indicates a low symmetry of the crystal field of the nearest surrounding of Eu 3+ ion.
The complex I exhibits triboluminescent properties. The intensity of the TBL of complex I was compared with the TBL of the well-known triboluminophore Et 3 NH[Eu(DBM) 4 ] (II) [2] (DBM, dibenzoylmethanate anion; Et 3 NH + , triethylammonium cation) (Fig. 2). The analysis showed that the integral intensity of the TBL band of complex I is 6.5 times less than for complex II.

Crystal structure of I
The molecular structure of the compound I is shown in  formation of hydrogen bonds of a water molecule with oxygen atoms of two neighboring thenoyltrifluoroacetonate ions (Fig. 3).
The . This arrangement of the structural elements, multiplied by translations in the direction of the crystallographic axis "c," builds a somewhat corrugated unit layer of repeatability in the structure of the compound (Fig. 4).

Structural model of formation of TBL
The unit layers of repeatability in the structure of the compound I multiplied in the direction [110] form a somewhat enlarged and relatively "free" space between themselves (destruction zone) [18][19][20][21], which can be conditionally considered a cleavage area in the crystal, along which cracking will proceed during its destruction under mechanical impact. The unit layers are arranged in a three-dimensional framework by the impact of van der Waals interactions, as well as by hydrogen bonds N(1)-H(1)...O(1). An important feature of this structure consists in the fact that the thenoyl ring S(21)C(25)C(26) C(27)C(28) (Fig. 3) from the second thenoyltrifluoroacetonate ion, which is also located at the top of the corrugation of the unit layer, is extended far (by 3.233 Å beyond the middle of the destruction zone towards the neighboring layer (Fig. 5).
Here, the thenoyl ring is statistically distributed over A and B positions with the occupations of the atomic positions of 0.751(1) and 0.248(2), respectively.
The locations of the thenoyl rings of the second thenoyltrifluoroacetonate ion in the destruction zone are highlighted in pink in Fig. 5. Such a feature of the structure formation could promote a weakening of the relative strength of the bond between the elementary layers of the structure and, with shear deformation during the destruction of the crystal, result in ionization of the second thenoyltrifluoroacetonate ion. In such a way, the cleavage plane is determined, as well as the emergence of uncompensated charge on the cleavage surfaces and, as a result, the development of the TBL property. One should mention that the cations as charge carriers do not noticeably go out to the surfaces of the cleavage planes but are "hidden" among the complex ions [Eu(TTA) 4 ] − in the cavities formed by "voids'" between (TTA)-ions and, therefore, cannot participate in the formation of a charge on the cleavage surface during the destruction.
Therefore, under mechanical impact, the crystals will crack, first of all, along the cleavage planes between the layers parallel to the plane (−110) (Fig. 5).
The authors of [18][19][20][21] noted that the TBL properties of crystals were determined by the following structural features of the compound: (1) the presence of cleavage planes, along which a crystal cracks, (2) the exit of the charge-carrying structural elements of the compound to the crystal destruction zone.
The boundaries of the destruction zone and its width are determined by the surfaces passing through the central Eu atoms of the molecular complexes, on which the chargecarrying thenoyltrifluoroacetonate ion is destructed under mechanical impact. In the case of the studied compound Q[Eu(TTA) 4 ]·H 2 O, the width of such a destruction zone is 6.109 Å. (Fig. 5). Here, the thenoyl ring of the second (TTA)-ion goes far beyond the middle of the destruction zone towards the neighboring layer by the C(27) atom by 3.233 Å, while this atom goes even beyond the boundary of the destruction zone of the neighboring layer by 0.179 Å. The plane of the middle of the destruction zone intersects the bond line C(24)-C(25) near the atom C(24) (Fig. 3).

Conclusions
In the present paper, we have suggested the structural model and possible mechanism for the emergence of an uncompensated charge on the cleavage surfaces of the compound I crystals under mechanical impact. Here, the charge-carrying ligands that contribute to the emergence of an uncompensated charge on the cleavage surfaces are thenoyltrifluoroacetonate anions. The electric field created between the oppositely charged cleavage surfaces in the crystal generates electrons that excite the triplet levels of the ligand. As a result of the intramolecular energy transfer from the excited levels of ligands to the resonant 5 D 0 level of Eu (III), TBL is excited.