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A: Molecular Structure, Quantum Chemistry, and General Theory 2
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Electronic Spectra of CsNaYb(NO): Is There Quantum Cutting? Yuxia Luo, Zhenyu Liu, Sam C. K. Hau, Yau-yuen Yeung, Ka Leung Wong, KwokKeung Shiu, Xueyuan Chen, Haomiao Zhu, Guochen Bao, and Peter A. Tanner J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b01915 • Publication Date (Web): 11 Apr 2018 Downloaded from http://pubs.acs.org on April 12, 2018
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The Journal of Physical Chemistry
Electronic Spectra of Cs2NaYb(NO2)6: Is There Quantum Cutting? Yuxia Luo,a Zhenyu Liu,b,c Sam Chun-Kit Hau,a Yau Yuen Yeung,d Ka-Leung Wong,*a Kwok Keung Shiu,a Xueyuan Chen,e Haomiao Zhu,e Guochen Bao,a Peter A. Tanner*a a Department of Chemistry, The Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Hong Kong S.A.R., P. R. China b Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Hong Kong S.A.R., P. R. China c Department of Applied Biological and Chemical Technology, The Hong Kong Polytechnic University Shenzhen Research Institute, Shenzhen, P. R. China d Department of Science and Environmental Studies, The Education University of Hong Kong, 10 Lo Ping Road Tai Po, New Territories, Hong Kong, P. R. China e CAS Key Laboratory of Design and Assembly of Functional Nanostructures, and Fujian Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China
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Abstract ： The
crystal
structure
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electronic
spectra
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of
the
Th
symmetry
hexanitritoytterbate(III) anion have been studied in Cs2NaY0.96Yb0.04(NO2)6, which crystallizes in the cubic space group 3. The emission from Yb3+ can be excited via the NO2- antenna. The latter electronic transition is situated at more than twice the energy of the former, but at room temperature one photon absorbed at 470 nm in the triplet state produces no more than one photon emitted. Some degree of quantum cutting is observed at 298 K under 420 nm excitation into the singlet state, and at 25 K using excitation into either state. The quantum efficiency is ~10% at 25 K. The energy level scheme of Yb3+ has been deduced from excitation and emission spectra and calculated by crystal field theory. New improved energy level calculations are also reported for the Cs2NaLn(NO2)6 (Ln = Pr, Eu, Tb) series using the f-Spectra package. The neat crystal Cs2NaYb(NO2)6 has also been studied but results were unsatisfactory due to sample decomposition and this chemical instability makes it unsuitable for applications.
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1. Introduction The hexanitritolanthanate(III) ions1-4 differ from the corresponding transition metal ion systems in that the oxygen atoms of nitrate(III) groups are coordinated to the metal ion, rather than the metal-nitrogen coordination for the transition metals.5-9 In fact, at first it was envisaged that the Ln3+ ions occupied octahedral site symmetry with 6-nitrogen coordination.3 The lanthanate(III) ions possess the unusual Th site symmetry with Ln3+ coordinated to 12 oxygen atoms at ~2.6-2.8 Å, with N-O bond distances given as 1.07-1.29 Å.1 The calculated N-O bond distance for the free NO2- anion is 1.226 Å, with the O-N-O angle being 117.42o.10 The electronic spectra of Ln(NO2)63- comprise transitions of the NO2- group in addition to those of Ln3+. Early investigations11-13 of the electronic spectra of NO2- in NaNO2 showed that the lowest energy transition corresponds to a singlet-triplet absorption, 1A1 → 3B2, with the zero phonon line at 18959 cm-1. The spectrum is mainly vibronic, with dominant vibrational progression frequencies of 644 and 1121 cm-1, corresponding to totally symmetric bending and stretching modes, respectively. The frequencies in the corresponding phosphorescence spectrum are 829 and 1325 cm-1.11 Kirschner et al.14 demonstrated that the zero phonon line energy moves progressively to higher energy across the lanthanide series. The energies were plotted against Ln3+(VI) ionic radii (assumed to vary monotonically with Ln(XII) radii, in the absence of the data for 12-coordination) and gave a linear fit.15 The extrapolated triplet state energy for Ln = Yb is 21517 cm-1. This energy is slightly more than double the 2F5/2 excited J-multiplet energy of Yb3+. Hence, if energy transfer can occur between the NO2- ligand and Yb3+, it could possibly occur by quantum cutting so that one incoming photon could excite two Yb3+ ions. We aimed to excite the NO2- ion in a singlet state, followed by intersystem crossing to the lowest triplet state (enhanced by the heavy metal ion), and then achieve energy transfer to Yb3+.
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Herein, we have investigated this scenario and found that quantum cutting does not occur at room temperature under triplet state excitation. The crystal structure, vibrational and electronic spectra of Cs2NaYb(NO2)6 enabled us to make this conclusion and they are presented in the following. The energy level structure and magnetic property of Yb3+ in this high symmetry system have also been investigated. In addition, the f-Spectra package has been utilized to reexamine the Ln3+ energy level dataset fittings of other hexanitritolanthanate(III) anions.
2. Experimental 2.1 Synthesis of Cs2NaY0.96Yb0.04(NO2)6 and Cs2NaYb(NO2)6 Cs2NaY0.96Yb0.04(NO2)6 and Cs2NaYb(NO2)6 were synthesized by dissolving Y2O3 (0.48 mmol for Cs2NaY0.96Yb0.04(NO2)6, Acros, 99.99%), Yb2O3 (0.02 mmol for Cs2NaY0.96Yb0.04(NO2)6, 1 mmol for Cs2NaYb(NO2)6, Sigma-Aldrich, 99.99%) in 37% HCl at 150 oC for ~15 min until all the lanthanide oxide had dissolved and a clear solution was obtained. Then, NaCl (1 mmol, Dieckman, AR) and CsCl (2 mmol, Strem, 99.999%) were added to the solution. Heat was applied at 150 °C for about 25 min until all the salts dissolved. The solution was heated continuously until all the acid evaporated out and a white solid powder was obtained. This powder was dissolved into 2 ml water and a solution of NaNO2 (AR >97%) (1 ml, 9 M for Cs2NaY0.96Yb0.04(NO2)6, 1ml, 10.8 M for Cs2NaYb(NO2)6) was added to the solution and a transparent solution was obtained. The solution was housed in a desiccator kept in a refrigerator at 4 °C. After two days, transparent crystals of Cs2NaY0.96Yb0.04(NO2)6 and white powder of Cs2NaYb(NO2)6 were obtained. Cs2NaY0.99Yb0.01(NO2)6 was prepared analogously.
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The Journal of Physical Chemistry
2.2 Characterization X-ray diffraction patterns of crystals were recorded with a Bruker AXS D8 Advance X-Ray Diffractometer equipped with non-monochromated Cu Kα X-Rays (λ = 1.5418 Å). Raman spectra were recorded at room temperature by a Perkin-Elmer Spectrum 2000 spectrometer using the resolution of 4 cm−1. FT-IR spectra were recorded at room temperature in the range from 400-4000 cm-1 using with a Perkin Elmer Paragon 1000 PC spectrometer with the resolution of 4 cm-1. X-ray photoelectron spectra (XPS) were recorded by a SKL-12 spectrometer modified with a VG CLAM 4 multichannel hemispherical analyser. Emission spectra were recorded using a Horiba 0.5 m monochromator (iHR550) equipped with a 600 groove mm-1 grating blazed at 800 nm and with a CCD detector (Syncerity, 300-1100 nm). The laser systems at City University (CityU) and Baptist University (HKBU) consisted of a Nd:YAG pumping laser, a third-order harmonic generator (THG at 355 nm, 120 mJ) and an optical parameter oscillator (OPO, SpectraPhysics versaScan and UVScan) with a pulse duration of 8 ns, repetition frequency of 10 Hz. The crystal or powders were housed in an Advanced Research Systems Instruments Optical Cryostat (10 - 800 K) at HKBU and an Oxford Instruments liquid helium cryostat at CityU. A tungsten lamp was employed as the source to record the Yb3+ absorption spectra at 20 K but the result was not conclusive. Luminescence lifetimes were measured by a Digital Phosphor Oscilloscope (1 GHz, 20 GS/s) with the Nd:YAG pulsed laser as excitation source. The emission and excitation spectra of Cs2NaYb(NO2)6 were recorded at Fujian Key Laboratory of Nanomaterials by an Edinburgh Instrument FLS 920 spectrometer using laser (a frequency doubled Ti:Sapphire laser, 700-1000 nm, pulse duration
