Article Cite This: J. Am. Chem. Soc. 2018, 140, 4588−4595
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Understanding and Controlling the Emission Brightness and Color of Molecular Cerium Luminophores Yusen Qiao,†,§ Dumitru-Claudiu Sergentu,‡,§ Haolin Yin,† Alexander V. Zabula,† Thibault Cheisson,† Alex McSkimming,† Brian C. Manor,† Patrick J. Carroll,† Jessica M. Anna,*,† Jochen Autschbach,*,‡ and Eric J. Schelter*,† †
P. Roy and Diana T. Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104, United States ‡ Department of Chemistry, University at Buffalo, State University of New York, Buffalo, New York 14260, United States S Supporting Information *
ABSTRACT: Molecular cerium complexes are a new class of tunable and energy-efficient visible- and UV-luminophores. Understanding and controlling the emission brightness and color are important for tailoring them for new and specialized applications. Herein, we describe the experimental and computational analyses for series of tris(guanidinate) (1−8, Ce{(R2N)C(NiPr)2}3, R = alkyl, silyl, or phenyl groups), guanidinate-amide [GA, A = N(SiMe3)2, G = (Me3Si)2NC(NiPr)2], and guanidinatearyloxide (GOAr, OAr = 2,6-di-tert-butylphenoxide) cerium(III) complexes to understand and develop predictive capabilities for their optical properties. Structural studies performed on complexes 1−8 revealed marked differences in the steric encumbrance around the cerium center induced by various guanidinate ligand backbone substituents, a property that was correlated to photoluminescent quantum yield. Computational studies revealed that consecutive replacements of the amide and aryloxide ligands by guanidinate ligand led to less nonradiative relaxation of bright excited states and smaller Stokes shifts. The results establish a comprehensive structure−luminescence model for molecular cerium(III) luminophores in terms of both quantum yields and colors. The results provide a clear basis for the design of tunable, molecular, cerium-based, luminescent materials.
I. INTRODUCTION Rare-earth-based luminophores have broad utility due to their favorable photostabilities and monochromatic purities. Solidstate optical materials doped with rare-earth elements have found a wide range of applications in lighting, displays, biological sensors, and lasers. Compared to the other rareearth cations, such as Eu3+ and Tb3+, in which luminescence results from parity-forbidden 4f → 4f transitions, the cerium(III) cation is characterized by its electric dipole allowed interconfigurational 4f → 5d transitions, giving rise to relatively broad absorption and emission bands, short lifetimes (ns), and high emission intensities.1,2 These features enable the application of cerium(III)-doped materials in light-emitting diodes (LEDs), inorganic scintillators, and photocatalysis.3−7 For example, the combination of Ce 3+ -doped YAG (Y3Al5O12:Ce3+) phosphors and blue-emitting LEDs generates white light and the phosphors feature broad emission bands (420−700 nm), high quantum yields (>90%), and fast decay times (∼63 ns).8 The Ce3+-activated Lu2SiO5 and LaBr3 scintillators show high quantum efficiencies (>30 000 photons/MeV) and short decay times (10−40 ns) and are good candidates for detector materials in high-energy physics.9 Despite these achievements in Ce3+-doped solid-state optical materials, challenges remain, including precise control of © 2018 American Chemical Society
emission colors and systematic tuning of photoluminescence quantum yields and lifetimes. For the prediction and control of quantum yields, recent results have suggested that rigid host lattices could result in high photoluminescence quantum yields (φ).3,10 The Debye temperature (ΘD), used as an indicator of structural rigidity,11 was indicated to have a positive correlation with quantum yields in Ce3+-activated phosphors. 12−14 However, other reports contradict the ΘD−φ correlation.15,16 Compared to bulk solid-state phosphors, molecular luminescent metal complexes offer attractive properties, such as their potential for sublimation in fabrication of organic LED (OLED) devices. More importantly, the photophysical properties of metal complexes can be sensitive to their local environment, i.e., ligand coordination sphere.17−20 In contrast to extensive studies reported for luminescent lanthanide(III) complexes deriving from 4f → 4f transitions,21−23 reported investigations of luminescent molecular cerium(III) complexes are still scarce, and the relationships between structure and quantum yield or emission color are unknown.2,5−7,24−34 Whereas molecular Eu3+ and Tb3+ show only small changes in emission wavelengths due to the 4f → 4f transition within Received: December 17, 2017 Published: January 23, 2018 4588
DOI: 10.1021/jacs.7b13339 J. Am. Chem. Soc. 2018, 140, 4588−4595
Article
Journal of the American Chemical Society
Figure 1. (a) GA cerium(III) and GOAr cerium(III) complexes.6 The following shorthand notation is used: A = N(SiMe3)2, G = (Me3Si)2NC(NiPr)2, and OAr = 2,6-di-tert-butylphenoxide. (b) Synthesis of cerium(III) tris(guanidinate) complexes. Apart from complexes 3 and 6, these complexes (including CeG3) were prepared by the salt metathesis reaction between CeI3 and M{(R2N)C(NiPr)2} (M = Li, Na or K) in toluene at 25 °C. (c) All cerium(III) complexes were isolated in moderate to good yields (36−71%) as pale yellow crystalline solids.
the core electrons, the 5d → 4f emission color of the Ce3+ cation should be tunable and respond to molecular design principles. There is a clear motivation to establish relationships between ligand effects and photophysical properties for molecular cerium(III) complexes, for design and optimization of luminescence properties. We recently reported that incorporation of guanidinate ligands into the coordination sphere of cerium(III) complexes resulted in increased quantum yields compared to that of amide ligands.5,6 The increase in quantum yield was postulated to occur through shielding of the cerium centers, to reduce nonradiative decay pathways such as quenching by inter- or intramolecular vibrational C−H oscillators.5,6 Herein, we describe the structural and photophysical properties of a new series of cerium(III) tris(guanidinate) complexes (Figure 1b) with tunable quantum yields and a clear steric basis. We also perform a computational analysis of the absorption and emission properties of a series of guanidinate-amide (GA) and guanidinate-aryloxide (GOAr) complexes (Figure 1a). From the combination of experimental and computational results, we have elucidated quantitative relationships between steric encumbrance about a cerium center and quantum yields and also ligand type and emission color. These findings demonstrate a comprehensive model for tuning the brightness and colors of molecular cerium(III) luminophores.
and the smallest quantum yield of 0.03, while CeG 3 demonstrated blue emission color and a much larger quantum yield of 0.81.5,6 The key spectroscopic feature of these compounds was their doublet to doublet transition, which afforded good conservation of absorption and emission energies without losses through spin-state changes. This feature allowed applications of the compounds, for example, as sensitizers for thermodynamically challenging substrates in photoredox catalysis. For the current work, it was also of interest to consider the role of complex structure in achieving variable brightnesses of the cerium(III) complexes, without changing their emission colors. For the current study, we focused on the complex CeG3, since the tris(guanidinate) framework only has one type of ligand and provides the smallest Stokes shift. We hypothesized that these features would allow us to investigate structure−photophysics relationships across the tris(guanidinate) series by changing the ligand backbone substituents. Syntheses and Structures. The cerium(III) tris(guanidinate) complexes described in this study are presented in Figure 1b. Except for complexes 3 and 6, the compounds were prepared by salt metathesis between CeI3 and M{(R2N)C(NiPr)2} (M = Li, Na, or K) in toluene at room temperature. The reaction between Ce(BH4)3(THF)3 and 3 equiv of Li{[(C10H15)2N]C(NiPr)2} afforded complex 3. Complex 6 was readily obtained through an insertion reaction of Ce2(NPh2)6 with excess N,N′-diisopropylcarbodiimide. Complexes 1−8 were characterized by X-ray diffraction studies [Figures 2a and S17−S24 (Supporting Information, SI)]. In all cases, the six coordinating nitrogen atoms formed a distorted trigonal prism with Ce−N bond lengths ranging from 2.48 to 2.54 Å, consistent with other reported cerium(III) tris(guanidinate/amidinate) complexes.35,36 By visual inspection of their space-filling models, there were differences in steric shielding of
II. RESULTS AND DISCUSSION Experimental Characterization of Cerium(III) Tris(guanidinate) Complexes. We recently demonstrated a unique series of luminescent molecular cerium complexes with a range of emission intensities (quantum yield 0.03−0.81) and colors (yellow to blue). Among these GA and GOAr cerium(III) complexes (Figure 1a), CeA3 featured yellow emission color 4589
DOI: 10.1021/jacs.7b13339 J. Am. Chem. Soc. 2018, 140, 4588−4595
Article
Journal of the American Chemical Society
Figure 2. (a) Thermal ellipsoid plots of 3 and 6 at the 30% probability level. Hydrogen atoms are removed for clarity. Atom colors: cerium, purple; nitrogen, blue; and carbon, gray. (b) Space-filling models of 3 and 6, representing different steric encumbrances at the cerium centers. As shown in panel b, the cerium cation in 3 is more shielded than that in 6. (c) Absorption (solid lines) and emission spectra (dashed lines) of 1 (blue), 2 (red), 3 (green), and 4 (pink) plotted in both energy (eV) and wavelength (nm). One electronvolt corresponds to a photon wavenumber of 8065.5 cm−1. Inset: images of toluene solutions of 1−4 in 1 cm path length quartz cuvettes (1.0 mM) under 365 nm UV irradiation. The spectra were collected in toluene. Complexes 1−4 have similar colors (yellow) and emission colors (blue or cyan). (d) Variable quantum yields (red bars) for 1−4, with standard deviations. Quantum yield is used to compare the brightnesses of these cerium luminophores; therefore, complex 1 is the brightest complex in this series.
2d). The quantum yields were found to increase significantly from 4 (0.17) to 3 (0.29), 2 (0.47), and 1 (0.81). The lifetime for the 2D excited state5,6 of 4 was determined to be shorter (37 ns) compared to that of 3 (95 ns), 2 (64 ns), and 1 (89 ns), and these values were consistent with reported lifetimes for molecular CeIII emitters.25 Radiative (krtol) and nonradiative (knrtol) decay rates were calculated (Tables S8 and S9, SI). The magnitudes of krtol values were consistent with predictions made from the Strickler−Berg equation (Table S11, SI).39 As expected from the decreasing trend in quantum yields for complexes 1−4, an increasing trend in krtol from (3.0 to 9.1) × 106 s−1 and a decreasing trend in knrtol from (22 to 2.2) × 106 s−1 was observed. Relationship between Steric Profiles and Photophysical Properties of Cerium(III) Tris(guanidinate) Complexes. The luminescence exhibited by complexes 1−4, absence of luminescence by complexes 5−8, differences in quantum yields and radiative/nonradiative decay rates among complexes 1−4, and inspection of the solid-state structures led us to postulate that the quantum yields of these complexes were highly sensitive to the degree of steric shielding of the cerium center. In this context, we analyzed the ligand shielding effects from the guanidinate ligands in two parts. First, we examined ligand shielding from the carbodiimide moieties. Percent buried volume (%Vbur) calculations have been used previously to quantify the degree of steric shielding provided by Nheterocyclic carbenes and guanidinate ligands.40,41 %Vbur was
the cerium center (Figure 2b). The photophysics of the tris(guanidinate) complexes was evaluated to detect and determine structure−property relationships. Photophysical Properties of Cerium(III) Tris(guanidinate) Complexes. Electronic absorption spectra of complexes 1−8 collected in toluene at room temperature displayed a band at ∼2.88 eV (430 nm) with ε ∼ 102 M−1 cm−1 [Figures 2c and S25−S33 (SI)]. Another band of roughly half intensity was observed at >3.10 eV ( 101°, whereas complexes 5−9 exhibited θ < 101° (Figure S68, SI), and (b) as θ increased, so did quantum yield with a corresponding decrease in knrtol. We hypothesized that the larger θ angle resulted in less energy dissipation of the CeIII excited states, particularly, by limiting nonradiative deactivation by exogenous solvent C−H vibrational modes. To further quantify the structure−quantum yield relationship and predict the quantum yield, an angle (C1−Ce−C2), α, was extracted from the solid-state structures for complexes 1−8 (Figure 3a). The α angle was defined as the apex angle that originates at the cerium center and contains two carbon atoms (through the center of the carbon atoms from the −NR2 moiety, C1 and C2). The C1 and C2 atoms were from the alkyl, silyl, or phenyl groups (R groups) of the ligand backbone substituents, which directly participated in shielding CeIII cation. A positive correlation between the α angles and the 4591
DOI: 10.1021/jacs.7b13339 J. Am. Chem. Soc. 2018, 140, 4588−4595
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Journal of the American Chemical Society
Figure 4. (a) Overlays of computed ground-state and emitting excited-state equilibrium geometries (B3LYP). Hydrogen atoms are omitted for clarity. The decrease in average Ce−N distance between ground states and excited states is 0.11 and 0.06 Å for CeA3 and CeG3, respectively. (b) Experimental absorption spectra (black solid line), emission spectra (black dashed line), calculated vertical absorption (λabs) and emission (λem) oscillator strengths, and Stokes shifts (eV). The nature of each vertical excitation is indicated in the legends. The Stokes shifts are given as the energy difference between λabs1 and λem. Calculated values are given in parentheses. (c) Schematic representation showing the evolution of the ground-state (gs, in blue) and emitting excited-state (es, in red) energies upon photoexcitation of the GA complexes. Stabilization/destabilization energies are in eV. Since the bond lengths in the excited state are contracted compared to those in the ground state, the minimum in the ground-state surface is drawn to the right of that in the excited-state surface.
changes indicated more rigid structures (Table S15, SI). This analysis allows us to understand how to control the emission color of these cerium luminophores. We discuss here in detail the GA series. The data of the GOAr series are shown in section 11 of the SI, as the two series of complexes have similar properties. Calculated Ground-State Equilibrium Geometries and Absorption Spectra of Cerium(III) Mixed-Ligand Complexes. To analyze the electronic structures of the GA and GOAr series, the structures were optimized with density functional theory (DFT), using the BP86 generalized gradient approximation (GGA) functional. For the GA series, the structures were also optimized using the B3LYP hybrid functional. Calculated equilibrium distances between the Ce center and the coordinating nitrogen atoms of the amide (Nam) and guanidinate (Ngu) ligands in the optimized ground state (gs) geometries are provided in Table S15 (SI). The maximum deviations in the computed structures from the experimental data did not exceed 0.05 Å. As discussed, the computed cerium(III) complexes exhibited generally two absorption bands, λabs1 (2.89−2.99 eV) and λabs2 (3.38−3.52 eV).5,6 Time-dependent DFT (TD-DFT) excitation energies, oscillator strengths, band assignments, and Stokes shifts, are provided in Figure 4b and Tables S17 and S18 (SI). In line with experimental deductions and with previous TDDFT computations,5,6 we find that the λabs1 and λabs2 absorption bands of the GA series correspond to transitions between the 4f
oscillators, as observed by comparative experiments using deuterated solvents. These analyses allow us to understand how to control the brightness of cerium luminophores using a structure−property relationship for the first time. In order to investigate the structure−photophysical property correlations further, especially the impact of ligand type on emission energies, quantum mechanical calculations were carried out. Computational Analysis of GA Cerium(III) and GOAr Cerium(III) Complexes. The experimental characterization indicated that the ligand shielding from the backbone substituents of the tris(guanidinate) framework were correlated with the brightnesses of the cerium(III) complexes. We were also interested in testing our hypothesis on the role of ligand identity in determining the emission color of the cerium(III) complexes. Among the series of GA and GOAr cerium(III) complexes, the complex CeA3 featured the largest Stokes shift of 138 nm (0.75 eV), while CeG 3 demonstrated a comparatively smaller Stokes shift of 35 nm (0.22 eV) (Figure 4b). Moreover, increasing the number of guanidinate ligands induced a systematic decrease of Stokes shifts. On the basis of these observations, we performed a computational analysis to determine the cause of the Stokes shifts, depending on the type of ligand and the rigidity of the structures of the cerium(III) complexes. The “rigidity” was defined as the rigidity of the coordination sphere of the Ce cations and was quantified as the differences of the metal−ligand bond distances between the ground-state and the excited-state structures, where small 4592
DOI: 10.1021/jacs.7b13339 J. Am. Chem. Soc. 2018, 140, 4588−4595
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Journal of the American Chemical Society
the Stokes shifts are reproduced, indicating the reliability of the employed computational scheme. When the complex relaxes to its equilibrium structure in the excited state, after photoexcitation, the electronic ground-state energy necessarily rises during this structural change (Figure 4c). Depending on the potential energy surfaces, the Stokes shifts may be dominated by changes in the excited-state or/and the ground-state energy. For the GA complexes, the excited-state relaxation is the largest for the more flexible CeA3 complex (0.5 eV) and drops toward much smaller values (0.1 eV) for the more rigid complex CeG3. This trend is mirrored in the ground state, albeit slightly less pronounced. The quantum mechanical calculations therefore confirm that the rigidity of the coordination sphere around the CeIII metal center strongly influences the photophysical complexes in the expected way, i.e., cerium(III) complexes with more rigid structures afford smaller Stokes shifts. Bonding interactions of the cerium cations with the ligands also impact the photophysical properties, as described in the following section. Electronic Structures of Cerium(III) Mixed-Ligand Complexes from Natural Bond Orbital (NBO) Analyses. Atomic charges and atomic orbital populations of the Ce center, in the absorbing ground-state and emitting excited-state optimized geometries of the GA series, are shown in Table 1.
and 5d Ce orbitals. The 5d orbitals are predominantly nonbonding, but they exhibit some mixing with ligand orbitals. Since the complexes CeA3 and CeG3 are approximately C3vsymmetric, and CeA2G and CeAG2 are approximately C2vsymmetric, the transitions can occur among linear combinations of 4f and 5d orbitals similar to those belonging to selected irreducible representations of these point groups. The λabs1 band is unambiguously assigned to a 4f → 5dz2 transition for all the GA complexes. The orbitals involved for λabs2 would formally belong to the e irreducible representation of C3v for complexes CeA3 and CeG3 or to any of the irreducible representations of C2v for complexes CeA2G and CeAG2, but excluding the a1 symmetric 5dz2 orbital implicated in λabs1. Additional absorption bands are evident around 320 nm or below in the measured spectra of complexes CeA2G to CeG3, but not CeA3.5,6 The TD-DFT calculations assign these bands as ligand to metal charge transfer (LMCT) from Ngu-centered molecular orbitals (MOs) to 4f Ce acceptor orbitals. The MOs implicated in these LMCT bands (λabs3) are shown in Figures S70 and S71 (SI). LMCT bands in the absorption spectrum of the CeA3 complex appear at much higher energies. Excited-State Geometries and Emission Spectra of Cerium(III) Mixed-Ligand Complexes. Emission is expected to occur from the ground vibrational level of the electronic excited state for the cerium luminophore. The structures of the λabs1 electronic excited states were optimized by TD-DFT. Upon optimization, the excited state therefore becomes the structurally relaxed emissive state, λem. The corresponding equilibrium structural parameters and the photophysical data for the λem states are given in the section 11 of the SI. The excited-state structures of complexes CeA3 and Ce(OAr)3 deviate the most from the ground-state structures, as they afford noticeably shorter (by ∼0.11 Å) bond lengths between the Ce center and the coordinating Nam or O atoms. Figure 4a shows overlays of the ground- and excited-state structures: The Ce−Ngu distances are, to a lesser extent, contracted (by ∼0.06 Å) in the excited states, which agree with the expectation of comparatively larger rigidity of the tris(guanidinate) framework. Therefore, the consecutive replacement of amide and aryloxide ligand by guanidinate ligands leads to increasingly rigid structures that have smaller geometric relaxation after the excitation (Table S15, SI). Interestingly, negative correlations were found between coordination number (CN) and contraction of the Ce−N bonds (ΔdCe−N) and CN and Stokes shifts (Figures S73 and S74, SI). Moreover, molecular Ce(III) luminophores with high coordination number tend to have small Stokes shifts, such as [G2CeI]2 (CN = 6, Stokes shift = 0.25 eV),6 [NEt4]3[CeCl6] (CN = 6, Stokes shift = 0.28 eV),7 Ce(III) cryptates (CN = 7, Stokes shift = 0.26 eV),49 Ce(III) complexes with N-substituted tris(N-alkylbenzimidazol-2ylmethyl)amine (NTB) ligands (CN = 8, Stokes shift = 0.31 eV),2 and [K(THF)2][(C5Me5)2CeI2] (CN = 12, Stokes shift = 0.37 eV).29 Similarly, from our previous work, coordination of Cl− to CeA3 increases CN from 3 to 4 and decreases Stokes shift from 0.75 to 0.66 eV.6 On the basis of these observations, we postulate that the coordination number might be a component of the rigidity of the coordination sphere and might affect the Stokes shifts of molecular Ce(III) luminophores. The computational data for the emission energies are in excellent agreement with the experimental band maxima, with average deviations of only 0.11 eV or less for the GA series and 0.13 eV for the GOAr series. Also, the experimental trends in
Table 1. NBO Population and Charge Analysis for the Electronic Ground State (gs) and the Emitting Excited State (es) in the GA Cerium(III) Complexes at Their Corresponding Equilibrium Geometriesa complex CeA3 CeA2G CeAG2 CeG3
El. state gs es gs es gs es gs es
Ce charge 1.99 1.86 2.07 1.92 2.15 1.98 2.23 2.04
Ce populations 1.14
0.68
0.07
0.13
4f 5d 6s 6d 4f0.355d1.266s0.396d0.16 4f1.125d0.616s0.086d0.12 4f0.305d1.356s0.296d0.14 4f1.115d0.556s0.086d0.11 4f0.295d1.376s0.226d0.12 4f1.115d0.486s0.076d0.08 4f0.275d1.416s0.166d0.09
Nam/Ngu chargeb −1.84/− −1.81/− −1.85/−0.83 −1.81/−0.81 −1.85/−0.82 −1.81/−0.81 −/−0.82 −/−0.80
a
(TD-)B3LYP//(TD-)B3LYP calculations. Populations in units of electrons. bNatural charge averaged over all Nam/Ngu centers of the Ce first coordination sphere.
The population analysis reveals important deviations from the idealized 4f15d06s0 and 4f05d16s0 ground- and excited-state configurations of the cerium cation. There is clear evidence of ligand to metal electron donation, as indicated by excess metal electron populations. The sum of the Ce 6s, 4f, and 5d populations amounts to 1.89 electrons in the ground state of complex CeA3 and systematically decreases to 1.66 for complex CeG3, which concurs with the increasing bond distances (by ∼0.23 Å) between the Ce center and the coordinating nitrogen atoms. The population sums for the excited states follow a similar trend and so do the population sums for the GOAr complexes (Table S20, SI). The data in Table 1 show that, for the structurally more flexible ligands, the coordinating atoms can engage the Ce center better and donate more strongly to the metal cation [see Figures 4a and section 11 (SI)]. A stronger donation engenders a stronger electronic coupling between the metal and the ligands, which results in increased nonradiative relaxation to the ground state. It is also interesting to note that the excited states of the complexes afford a sizable 4f population between 0.27 4593
DOI: 10.1021/jacs.7b13339 J. Am. Chem. Soc. 2018, 140, 4588−4595
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Journal of the American Chemical Society and 0.35 for the GA series (Table 1) and between 0.34 and 0.45 for the GOAr series. In the ground states, the excess 4f populations are smaller (0.11−0.18) and most of the ligand donation is into the Ce 5d shell. It is worth noting that similar trends were identified for CeIV versus CeIII ground-state bonding, namely, that in CeIV complexes much more of the electron donation by the ligands ends up in the 4f shell than in CeIIIcomplexes.50 Due to the distribution of the 5dz2 orbital relative to the ligands, the excited Ce(III) center appears to the coordinating atoms somewhat similar to a Ce(IV) ion. Together with the increased ligand to metal donation in the excited states, as evidenced by the NBO analysis, this explains the structural contraction of the complexes around the metal center upon photoexcitation. Although the scalar relativistic (SR) TD-DFT results rationalize much of the experimental findings, further corroboration can be obtained from robust multiconfigurational wave function theory calculations, including a spin−orbit coupling (SOC) treatment. The electronic states of lanthanide complexes with open valence f and d shells are likely multiconfigurational and strongly influenced by SOC. Furthermore, the measured emission spectra for the cerium(III) complexes investigated herein were previously deconvoluted into two overlapping bands, which were postulated to arise from de-excitations from a doublet excited state of 2D ion parentage to the two SOC components of the 2F ground state, namely, the 2F5/2 (ground) and 2F7/2 (excited) ion parentage levels.5,6 The measured splitting of the two emission bands, for each GA complex, was reported to be ∼0.25 eV. To confirm this hypothesis, multiconfigurational complete-active space selfconsistent field (CASSCF) calculations with SOC were conducted. Absorption and Emission Spectra of Cerium(III) Mixed-Ligand Complexes According to Multiconfigurational Wave-Function-Based Methods. Scalar relativistic (SR) and SOC (including SR effects) photophysical data were obtained from complete active space wave function calculations without (“CAS-SR”) and with (“CAS-SO”) SOC, respectively (see section 11, SI). The CAS-SR state energy differences were in excellent agreement with TD-DFT and the experimental λabs1 and λem band maxima and Stokes shifts, showing that the TDDFT calculations were sufficiently accurate, despite the pronounced multiconfigurational character of the electronic states evidenced by the CAS calculations. The final electronic features of interest are therefore the split emission bands. For the cerium(III) cation in low-symmetry crystal fields, the 2J + 1 degeneracy of the 2F5/2 and 2F7/2 ion manifolds is lifted into Kramers doublets (where J is the total angular momentum). For each GA complex, the averaged energy difference of the states deriving from the 2F5/2 and 2F7/2 ion manifolds was ∼0.25 eV, in agreement with the experimental data (Figure 5). Moreover, the calculated SOC Stokes shifts are in excellent agreement with the experimental data. The CASSO results confirm that the emission spectra of the GA complexes indeed feature two distinct bands, associated with emission from states of 2D parentage and 5dz2 character, to states derived from the 2F5/2 and 2F7/2 components of the ion ground state. This first comprehensive computational analysis of molecular cerium(III) luminophores reproduced experimentally measured absorption and emission spectra in addition to Stokes shifts of GA and GOAr cerium(III) complexes. Moreover, the results strongly indicated that applying rigid coordination spheres to
Figure 5. Schematic representation of the SOC emission energies. The scheme explicitly shows the SOC splitting of the 2F ground state, for each GA complex, into the 2F5/2 and 2F7/2 components, which are further split into Kramers doublets. The λem1 and λem2 SOC emission energies are relative to the average energies of the eight/six 2F7/2 and 2 F5/2 state components, respectively.
cerium centers results in small Stokes shifts. These analyses allow us to understand how to control and predict the emission colors of molecular cerium luminophores.
III. CONCLUSION As shown by our combination of experimental and computational studies, leveraging steric effects and ligand types can efficiently control the emission brightness and color of cerium luminophores. The results from the tris(guanidinate) series of complexes 1−8 demonstrate that complexes with large ligand steric demand in C3 symmetry show tunable quantum yields and, therefore, brightness. The key physical process at work here is the exclusion of exogenous solvent molecules from the coordination sphere, namely along the C3 axis, that provide quenching processes. Furthermore, our computational results include the first comprehensive computational analysis of molecular cerium(III) luminophores. We successfully reproduced the experimental absorption and emission spectra and rationalized the Stokes shifts for two series of compounds. We conclude that, by applying a rigid coordination sphere to the cerium center, the geometry relaxation is suppressed, yielding higher-energy emitted light. NBO calculations support these observations by identifying relatively strong metal−ligand electronic coupling in those compounds with the largest Stokes shift. Finally, multiconfigurational calculations further support the spectral assignments and optical band splitting due to SOC. Ce3+-phosphors have a range of promising optical properties, including in OLEDs and as photoredox mediators. We expect that the results of this work can be applied to tailor optical properties to achieve new and specialized applications.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b13339. Synthetic details and characterizations; electronic absorption, excitation, and emission data; computational details; and determination of structural metrics (PDF) Crystallographic data in CIF format for 2−4, 6−8, and 3I (CIF) 4594
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AUTHOR INFORMATION
Corresponding Authors
*
[email protected] *jochena@buffalo.edu *
[email protected] ORCID
Yusen Qiao: 0000-0001-7654-8636 Dumitru-Claudiu Sergentu: 0000-0001-6570-5245 Thibault Cheisson: 0000-0003-4359-5115 Jessica M. Anna: 0000-0001-5440-6987 Jochen Autschbach: 0000-0001-9392-877X Eric J. Schelter: 0000-0002-8143-6206 Author Contributions §
Y.Q. and D.-C.S. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Separation Science program under Award Number DE-SC0017259 (E.J.S.) and Heavy Element Chemistry Program under Award Number DE-SC0001136 (J.A.). T.C. acknowledges the Dreyfus Foundation Postdoctoral Program in Environmental Chemistry for fellowship support. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by U.S. National Science Foundation Grant Number OCI-1053575. We also thank the Center for Computational Chemistry (University at Buffalo) for providing computational resources. The Petersson, Chenoweth, and Park groups at the University of Pennsylvania are thanked for use of their fluorometers. Dr. George Furst, Dr. Gu Jun, Prof. Huayi Fang, Bren Cole, and Dr. Shuguang Zhang (all UPenn) are acknowledged for helpful discussions.
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DOI: 10.1021/jacs.7b13339 J. Am. Chem. Soc. 2018, 140, 4588−4595
