Green to Red Luminescence Switchable by Excitation Light in

Jérôme Long , Daisuke Asakura , Masashi Okubo , Atsuo Yamada , Yannick .... Augustin M. Madalan , Céline Pichon , Carine Duhayon , Jean-Pascal Sutter ...
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Green to Red Luminescence Switchable by Excitation Light in Cyanido-Bridged TbIII−WV Ferromagnet Szymon Chorazy,†,‡ Koji Nakabayashi,‡ Shin-ichi Ohkoshi,*,‡,§ and Barbara Sieklucka*,† †

Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krakow, Poland Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan § CREST, Japan Science and Technology Agency, K’s Gobancho, 7 Gobancho, Chiyoda-ku, Tokyo 102-0076, Japan ‡

S Supporting Information *

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two luminophors, it is expected to observe an energy transfer or a selective excitation, both potentially leading to switchable luminescence,22−24 which was not yet implemented into molecule-based magnets. We focused on bimetallic [WV(CN)8]-based materials which were shown to be a versatile platform for various functionalities,25−28 including visible or near-infrared emission in LnIII−WV (Ln = Nd, Eu, Gd) helices revealing magnetic coupling.22,28 We report the synthesis, crystal structure, and properties of {[TbIII(Box)2(dmf)2][WV(CN)8]}·H2O (1) (Box = bis(oxazoline), dmf = N,N′dimethylformamide) cyanido-bridged system exhibiting ferromagnetism and visible green-red luminescence, switchable by the selective excitation of organic ligand or metal excited states. Yellow block crystals of 1 were prepared by layering of diethyl ether onto acetonitrile solution of Tb(NO3)3·6H2O, Box, and TBA3[WV(CN)8], with a small addition of dmf. Single-crystal X-ray diffraction indicated that 1 are cyanidobridged layers of a mixed 4- and 8-membered metal rings topology (Figure 1; also see Tables S1−S6 and Figures S1−S3 in the Supporting Information). The layers consist of octametallic Tb4W4(CN)8 units with dimensions of 14.8 × 14.8 Å, built of four [WV(CN)8]3− ions alternately connected with four [TbIII(Box)2(dmf)2]3+ complexes. The same building blocks create also smaller tetrametallic Tb2W2(CN)4 squares with dimensions of 5.8 × 5.8 Å which link four different Tb4W4(CN)8 units giving 2-D coordination skeleton (Figure 1c). The asymmetric unit contains one [WV(CN)8]3−, one TbIII, and one H2O (Figure 1b). The [WV(CN)8]3− ion forms three cyanide bridges and reveals a geometry of a bicapped trigonal prism with a small contribution of a square antiprism (Supporting Information Tables S3 and S4). TbIII coordinates three nitrogen atoms of cyanides, two oxygen atoms of dmf, and four nitrogen atoms of two Box ligands, which results in the coordination number, C.N. = 9, and the geometry of a capped square antiprism (CSAPR-9, Supporting Information Tables S5 and S6). All layers of 1 are situated within the [1̅01] plane, but they are located with the significant shift between neighboring layers as the smaller Tb2W2 squares of one layer is above the larger Tb4W4 ring of the second (Figure 1c). This hampers the size of channels which are the broadest, 8.4 × 10.6 Å, in the [100] and [001] crystallographic directions

dvanced materials combining ferromagnetism with luminescence attract a special interest in materials chemistry, as they can find vital applications in optoelectronics and multimodal sensing, including biological imaging.1−8 The great effort is focused on the preparation of luminescent magnetic nanocomposites constructed of fluorescent quantum dots (QDs), made from CdSe, CdS, or ZnS, bounded with magnetic nanoparticles (MNPs), mainly built of iron oxides.1−4 Other luminophors, organic dyes, lanthanides(III) complexes, and carbon nanoparticles were also applied with MNPs in the construction of magneto-luminescent nanocomposites.5−8 In these materials, emission and ferromagnetism come from two independent ingredients, blended into a heteromeric particle or encapsulated in a polymer or silica matrix. An essentially different approach is realized for functional molecular materials, giving the chance to introduce both functionalities into a single coordination framework, as was shown for chirality, proton conductivity, or microporosity, which were fruitfully combined with magnetic ordering.9−15 The luminescent molecular magnet has to be constructed of building blocks ensuring both magnetic coupling and an effective luminescence. However, the most of d-metal ions, widely used in molecular magnetism, are not emissive due to the nonradiative relaxation through interactions with low-lying excited states.16 It makes design and synthesis of emissive magnets extremely difficult, and very few trials within this area were reported.17−21 They were realized by three synthetic strategies. The first, (i), applies luminophors only coexisting in a supramolecular network with a magnetic system. This approach succeeded in an oxamato-bridged Co II −Cu II ferrimagnet (Tc = 19 K) with interlayer emissive [RuII(2,2′bpy)3]2+ counterions,17 and MnII−CuII ferromagnet (Tc = 19 K) with luminescent methylviologen counterions.18 In the second approach, (ii), diamagnetic luminophors are used in the construction of magnetic system. They can mediate in magnetic coupling as presented for a carboxylate-bridged NiII network showing ferromagnetism (Tc = 9.5 K) and emission from bridging ligand.19 The third strategy, (iii), employs metal centers which are both paramagnetic and luminescent. This approach was shown only for TbIII−WV (Tc = 2.8 K) or TbIII− EuIII−WV (Tc = 2.5 K) ferromagnets built of emissive paramagnetic TbIII and paramagnetic WV.20,21 In the present work, to construct a switchable luminescent magnet, we combined two strategies, (ii) and (iii), by the exploration of luminescent bis(oxazoline), and TbIII, exhibiting the paramagnetic 7F6 ground state. For such an approach with © 2014 American Chemical Society

Received: May 28, 2014 Revised: June 24, 2014 Published: June 30, 2014 4072

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Figure 1. Crystal structure of 1: (a) the single coordination layer, (b) the asymmetric unit, and (c) the spatial arrangement of layers and water molecules between them. Figure 2. dc magnetism of 1: T dependence of χMT (H = 2 kOe) (top), H dependence of M at T = 1.8 K (top, the inset), FCM curves at various H with its first derivative in the inset (middle), and M−H hysteresis loop at T = 1.8 K (bottom).

(Supporting Information Figure S1). The interlayer space is filled by water molecules, connected with terminal cyanides of [WV(CN)8]3− units through hydrogen bonds. The supramolecular network is also controlled by weak π−π stacking between oxazoline rings of Box ligands (Supporting Information Figure S2). The powder X-ray diffraction pattern of 1 confronted with the calculated one indicates that the structural model from single-crystal XRD experiment is valid for the bulk sample used in other physical measurements (Supporting Information Figure S3). The direct-current (dc) magnetism of 1 is presented in Figure 2 and Supporting Information Figure S4. The room temperature magnetic susceptibility−temperature (χMT) product for the TbIII−WV unit is 12.2 K cm3 mol−1 which is in a good agreement with 12.2 K cm3 mol−1 calculated for uncoupled TbIII (J = 6, gJ = 3/2) − WV (S = 1/2, g = 2) units. Upon cooling, χMT is stable down to 100 K and decreases slowly below this point. The minimum of 11.0 K cm3 mol−1 at 12 K is followed by the abrupt increase up to 19.3 K cm3 mol−1 at 2.5 K. At the lowest T, χMT decreases again to 16.7 K cm3 mol−1 at 1.8 K (Supporting Information Figure S4). The

minimum on the χMT−T plot can be ascribed to the depopulation of the Stark levels arising from the splitting of the 7F6 ground state of TbIII, while the abrupt increase of χMT below 10 K is ascribed to magnetic coupling between TbIII and WV. The nature of CN−-mediated interactions is suggested by the H dependence of the magnetization (M) at 1.8 K. The magnetization reveals a fast increase with the increasing field, reaching 4 Nβ at 3.5 kOe, which is followed by the slower increase to 5.9 Nβ at 70 kOe. The observed M at 70 kOe is very close to 6.0 Nβ expected for ferromagnetic interactions between TbIII and WV, assuming a spin of S = 1/2 with g = 2 for WV and an effective spin of S = 1/2 with the strong uniaxial Ising-spin anisotropy of the g-tensor (g∥ = 10, g⊥ = 0) for TbIII. At the same time, the experimental M at 70 kOe is much higher than 4 Nβ related to antiferromagnetically coupled TbIII−WV units, proving the ferromagnetic coupling in 1.20,21 4073

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The maximum of χMT at 2.5 K, which is ascribed to the saturation of M, suggests the onset of magnetic ordering, which is visible in the field-cooled magnetization (FCM) plots for small magnetic fields (Figure 2, middle). FCM curves reveal the abrupt increase of the signal below 3 K, followed by the saturation of M which is characteristic of the transition to the ferromagnetically ordered state. The Curie temperature, TC = 2.4 K, was found from the maximum of the first derivative of FCM. 1 reveals the magnetic hysteresis in the M−H plot, with small coercive field of 15 Oe, indicating a soft character of this ferromagnet (Figure 2, bottom). The ac magnetic susceptibility, both the in-phase χ′M and out-of-phase χ″M show also peaks typical for the magnetic transition (Supporting Information Figure S5). However, they are weakly frequency-dependent which can be interpreted in terms of the contribution of spinglass behavior, coexisting with ferromagnetism in 1.29 The low temperature spin-glass-like state of 1, proved by the precise analysis of ac data (Supporting Information), can be attributed to the structural disorder of interlayer H2O, and terminal dmf, weakening the propagation of a 3-D magnetic ordering. The luminescence properties of 1 were investigated by solid state emission and excitation spectra at T = 77 K. Using different wavelengths of UV excitation light, 1 reveals various luminescent characteristics. When excited by λexc = 260 nm, the strong sharp lines at 490, 545, 585, and 622 nm, with weaker peaks at 647, 667, and 678 nm, leading to overall green luminescence, are observed (Figure 3, top). This emission is

(HAE) on TbIII coordinating Box, promoting the fast S1 → T1 intersystem crossing (ISC), and consequent T 1 → S 0 transition.30−32 In addition, the residual S1 → S0 fluorescence of organic ligand with the maximum at 435 nm, accompanied by weak peaks of TbIII-centered luminescence, were also detected (Figure 3, bottom). The drastic difference in emission depending on the excitation light of 1 comes from the alignment of TbIII and Box energy levels (Figure 3). Using UV light below 300 nm, only excited states of TbIII are populated, as Box does not possess so high energy levels. The excitation explores spinforbidden interconfigurational 4f8 → 4f75d1 transitions, which is proved by the characteristic broad bands at 275 and 295 nm in the excitation spectrum for the emission of 545 nm (Supporting Information Figure S7).33,34 The UV light above 300 nm modifies the emission, as it results in the population of both excited states of TbIII and Box. The energy is most efficiently transferred to the T1 state of Box, giving dominated red emission. This is connected with the fast S1 → T1 ISC due to HAE, and with the relative position of T1 state, much lower than emissive 5D0 level of TbIII, excluding the possible ligandto-metal energy transfer. This is also visible in the excitation spectrum for the emission of 655 nm, which shows only one broad UV band at 330 nm, similar to the spectrum of a pure ligand (Supporting Information Figure S7). There is no evidence on the contribution of [WV(CN)8]3− ions to the emission, which means the energy absorbed by their ligand-tometal charge transfer states is simply relaxed through nonradiative pathways,28 not influencing on the equilibrium between Tb and ligand luminescence. In summary, we have prepared 2-D Tb III (Box)−W V multifunctional coordination polymer, combining ferromagnetism with visible luminescence, the color of which, green or red, can be switched by excitation light. This molecular system opens a new perspective in the area of magneto-luminescent materials revealing tunable luminescence and ferromagnetism, as both results from the property of a single coordination network. For such a bifunctional system the synergetic effect between luminescence and magnetic ordering can be expected. This is, however, difficult to detect due to low TC of 2.4 K. In the future, we will try to prepare the appropriate measurement system to prove the interaction between ferromagnetism and multicolored emission. The alternative way is to construct emissive magnets with higher TC, by the increase of coordination connectivity through CN− bridges. Despite this, the presented material can be utilized in the preparation of luminescent ferromagnetic thin films or nanoparticles. It also provides the chance to introduce other functionalities, such as chirality, which can be implemented using chiral luminescent ligands. It will aid in realizing another idea, which is the observation of circularly polarized luminescence, and magnetochiral effects in the ferromagnetically ordered systems.9,35 Research along this line is in progress.

Figure 3. Emission spectra at T = 77 K of 1 excited by 260 nm (top) and 340 nm (bottom). Right: energy level diagrams with electronic transitions: A = absorption, P = phosphorescence, F = fluorescence, ISC = intersystem crossing.

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attributed to intra-f8 5D4 → 7F0−6 transitions of TbIII.20 When 1 is excited by λexc = 340 nm, it reveals the complex emission pattern with dominated broad band at 650 nm, giving the red luminescence. This emission is related to the 3(π−π*)T1 → S0 phosphorescence of Box ligand. As such transition was not found for a pure Box, showing only blue 1(π−π*)S1 → S0 fluorescence (Supporting Information Figure S6), ligand phosphorescence in 1 originates from the heavy atom effect

ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data, experimental details, additional structural views, results of analyses of coordination polyhedrons, powder diffractograms, excitation spectra, and detailed analysis of ac magnetic data. This material is available free of charge via the Internet at http://pubs.acs.org. 4074

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(21) Chelebaeva, E.; Long, J.; Larionova, J.; Ferreira, R. A. S.; Carlos, L. D.; Almeida Paz, F. A.; Gomes, J. B. R.; Trifonov, A.; Guérin, C.; Guari, Y. Inorg. Chem. 2012, 51, 9005. (22) Chorazy, S.; Nakabayashi, K.; Ozaki, N.; Pełka, R.; Fic, T.; Mlynarski, J.; Sieklucka, B.; Ohkoshi, S. RSC Adv. 2013, 3, 1065. (23) Zhang, H.; Xu, X.; Ji, H.-F. Chem. Commun. 2010, 46, 1917. (24) Wu, W.; Xia, Z. RSC Adv. 2013, 3, 6051. (25) Nowicka, B.; Korzeniak, T.; Stefańczyk, O.; Pinkowicz, D.; Chorazy, S.; Podgajny, R.; Sieklucka, B. Coord. Chem. Rev. 2012, 256, 1946. (26) Chorazy, S.; Nakabayashi, K.; Imoto, K.; Mlynarski, J.; Sieklucka, B.; Ohkoshi, S. J. Am. Chem. Soc. 2012, 134, 16151. (27) Chorazy, S.; Podgajny, R.; Nitek, W.; Fic, T.; Görlich, E.; Rams, M.; Sieklucka, B. Chem. Commun. 2013, 49, 6731. (28) Chorazy, S.; Nakabayashi, K.; Arczynski, M.; Pełka, R.; Ohkoshi, S.; Sieklucka, B. Chem.Eur. J. 2014, 20, 7144. (29) Zhao, H.; Lopez, N.; Prosvirin, A.; Chifotides, H. T.; Dunbar, K. R. Dalton Trans. 2007, 878. (30) Tobita, S.; Arakawa, M.; Tanaka, I. J. Phys. Chem. 1985, 89, 5649. (31) Guldi, D. M.; Mody, T. D.; Gerasimchuk, N. N.; Magda, D.; Sessler, J. L. J. Am. Chem. Soc. 2000, 122, 8289. (32) Balamurugan, A.; Gupta, A. K.; Boomishankar, R.; Reddy, M. L.; Jayakannan, M. ChemPlusChem 2013, 78, 737. (33) van Pieterson, L.; Reid, M. F.; Burdick, G. W.; Meijerink, A. Phys. Rev. B 2002, 65, 045114. (34) Chang, Y.-S.; Lin, H.-J.; Li, Y.-C.; Chai, Y.-L.; Tsai, Y.-Y. J. Solid State Chem. 2007, 180, 3076. (35) Carr, R.; Evans, N. H.; Parker, D. Chem. Soc. Rev. 2012, 41, 7673.

AUTHOR INFORMATION

Corresponding Authors

*(B.S.) E-mail: [email protected]. *(S.O.) E-mail: ohkoshi.chem.s.u-tokyo.ac.jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the International PhD-studies Programme at the Faculty of Chemistry, Jagiellonian Univ., within the Foundation for Polish Science MPD Programme, cofinanced by the European Regional Development Fund, by Polish National Sci. Centre within Research Project DEC2011/01/B/ST5/00716, by the Core Research for Evolutional Science and Technology (CREST) project of the Japan Science and Technology Agency (JST), the Cryogenic Research Center, and the Center for Nano Lithography & Analysis, The University of Tokyo, which are supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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dx.doi.org/10.1021/cm501954e | Chem. Mater. 2014, 26, 4072−4075