Understanding and Controlling the Emission Brightness and Color of

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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 J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b13339 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 23, 2018

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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, PA 19104, United States ‡ Department of Chemistry, University at Buffalo, State University of New York, Buffalo, NY 14260, United States 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 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 guanidinate-aryloxide (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 steric encumbrance around the cerium center induced by various guanidinate ligand backbones 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 structureluminescence 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. Solid-state 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 rare earth 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 Ce3+-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 (>30000 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 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 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 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 ligand sphere of cerium(III) complexes resulted in increased quantum yields compared to amide ligands.5,6 The increase in quantum yield was postulated to occur through shielding of the cerium centers, to reduce non-radiative decay pathways such as quenching by inter- or intra-molecular vibrational C‒H oscillators.5,6 Herein, we describe the structural

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and photophysical properties of a new series of cerium(III)

tris(guanidinate)

Page 2 of 9 complexes

(Figure

1b)

with

tuna-

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 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.

ble quantum yields and a clear steric basis. We also perform a computational analysis of the absorption and emission properties of the series of guanidinate-amide (GA) and guanidinatearyloxide (GOAr) complexes (Figure 1a). From the combination of experimental and computational results, we have elucidated quantitative relationships between steric encumbrance about cerium center and quantum yields, and ligand type and emission color. These findings demonstrate a comprehensive model for tuning the brightness and colors of molecular cerium(III) luminophores.

II. RESULTS AND DISCUSSIONS 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‒blue). Among these guanidinate-amide (GA) and guanidinate-aryloxide (GOAr) cerium(III) complexes (Figure 1a), CeA3 featured yellow emission color and the smallest quantum yield of 0.03, while CeG3 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 emission, 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 these features would allow us to investigate structurephotophysics 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} = Li, Na or K) in toluene at room temperature. 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 Figures S17-S24). In all cases, the six coordinating nitrogen atoms formed a distorted trigonal prism with Ce−N bond lengths ranging from 2.48−2.54 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 the cerium center (Figure 2b). The photophysics of the tris(guanidinate) complexes were evaluated to detect and determine structure-property relationships. Photophysical Properties of Cerium(III) Tris(guanidinate) Complexes. Electronic absorption spectra of complexes 1−8

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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 (grey). b, Space-filling models of 3 and 6, representing different steric encumbrances at the cerium centers. As shown in 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). 1 eV corresponds to a photon wave–1 number of 8065.5 cm . 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.

collected in toluene at room temperature displayed a band at ~2.88 eV (430 nm) with ɛ ~ 102 M–1 cm–1 (Figure 2c and Figures S25-S33). Another band of roughly half intensity was observed at > 3.10 eV (< 400 nm) for all the complexes. These bands corresponded to 4f → 5d transitions and are consistent with our reported spectra.5,6 The assignment of the observed transitions was further supported by time dependent density functional theory (TD-DFT) calculations and subsequent natural transition orbitals (NTOs) analyses (see the Supporting Information Section 6).37,38 Despite the similarity in colors and absorption spectra for complexes 1‒8, only complexes 1‒4 were luminescent. Excitation and emission spectra of complexes 1‒4 were recorded in toluene solutions (Figure 2c and Figures S34-S37). All complexes showed emission maxima between 2.60 and 2.70 eV. Complexes 1‒3 were blue emitting whereas 4 was cyan emitting (Figure 2c, inset). The Stokes shifts for complexes 1‒4 spanned from 0.22‒0.26 eV (35‒43 nm) (see the Supporting Information Sections 5 and 8), a remarkably smaller range than that for our previously reported GA complexes (0.22‒0.75 eV, 35‒138 nm).5,6 Photoluminescence quantum yields (φPLtol) and lifetimes (τ ) were obtained in toluene at room temperature (Figure 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 tol

2

D excited state5,6 of 4 was determined to be shorter (37 ns) compared to 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 decay rates (knrtol) were calculated (Tables S8 and S9). The magnitudes of krtol values were consistent with predictions made from the Strickler−Berg equation (Table S11).39 As expected from the decreasing trend in quantum yields for complexes 1‒4, an increasing trend in krtol from 3.0−9.1×106 s‒1 and a decreasing trend in knrtol from 22−2.2×106 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/non-radiative decay rates between 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



calculated on complexes 1‒8 and the radius of the sphere around the cerium center was set to R = 4.5 Å to include all the iPr groups of the guanidinate ligand but exclude the ‒NR2

Page 4 of 9

backbone substituents (Figure 3a, also see the Supporting Information Section 10). The values of %Vbur for complexes 1‒8 were similar (84-89 %). Therefore, we concluded the dif

Figure 3. a, Determination of percent buried volume (%Vbur), cone angle (θ), and the α angle of complex 1. The solid-state structure of complex 1 is shown in a mixture of space-filling model and ball and stick model. Some carbodiimide and amide groups are depicted in wireframe model for clarity. The green transparent sphere in the space-filling model is at a radius of 4.5 Å from the metal center. The cone angle (θ) is the apex angle of the cone shown in blue. The α angle is shown in red. Details of determination of these metrics are shown in the Supporting Information Section 10. b, Correlation plot of the quantum yields versus α angles for complexes 1‒8. The points for the luminescent complexes (1‒4) are shown in blue circles, whereas non-luminescent ones (5‒8) are shown in red. The black dotted line is derived from complexes 1‒5.

ferences in shielding of the CeIII cations through the first coordination sphere were not profound among the complexes, due to the conserved structural profiles of the carbodiimide moieties. We postulated the extended coordination sphere, namely, the ‒NR2 moieties, might play a more important role in effecting the photophysical properties. Second, we investigated ligand shielding effects from the ‒NR2 moieties. Structure-property relationships for metal complexes have been rationalized using cone angles for phosphine ligands and amine ligands.42,43 We defined cones for the ‒NR2 moieties and calculated the cone angles (θ angle) of complexes 1‒8 (Figure 3a, also see the Supporting Information Section 10).44 Notably, a) all the luminescent complexes 1‒4 had θ > 101°, whereas complexes 5‒9 exhibited θ < 101° (Figure S68) 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). 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 quantum yields was evident (Figure 3b); a linear relationship existed for 1‒5 (R2 = 0.997). Moreover, the trend line indicated that any complexes with α angles below 56.8° were not luminescent, consistent with the experimental observations. The electronic structures of these complexes exhibit an excited state where the unpaired electron resides in a 5dz2 orbital. We reasoned that nonradiative decay rates would be reduced by sterically restricting the 5dz2 corresponding excited state relaxation. To rationalize the observed correlation, photoluminescence quantum yields (φPLtol-d8) and lifetimes (τtol-d8) were also collected in toluene-d8 at room temperature (see the Supporting Information Sections 7 and 8). Deuterated solvents have been widely adopted to probe the effects of solvent X‒H/D quenching, X = C, N, and O, on lanthanide(III) luminescence.45-48 Nonradiative decay rates (knr tol-d8 ) were determined from the quantum yields and excited state lifetimes. A negative correlation between ∆knr (∆knr = knr tol ‒ knr tol-d8) and α angles was observed in complexes 1‒4 (Figure S65). The quenching ability of a C‒H/D oscillator is inversely proportional to r6 (∆knr ∝ r‒6), where r equals the distance between the C‒H/D oscillators and the metal center.45 We expected the larger α angle represented more steric encumbrance for the populated 5dz2 excited state, hence shifting the C‒H/D oscillators further away from the cerium center. As a result, complexes that have larger steric profiles along their primary axes of symmetry (aligned with the 5dz2 orbital) are expected to exhibit smaller ∆knr values and thereby higher quantum yields.


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Journal of the American Chemical Society The experimental data demonstrate the first quantitative structure-photophysical property correlation for molecular cerium(III) luminophores. We discovered that the quantum

yields of tris(guanidinate) complexes correlated with the bulkiness of the group appended to the guanidinate ligand. The physical basis for this correlation is the extent of vibrational

Figure 4. a, Overlays of computed ground- 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 Å 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) oscillators, and Stokes shifts. The nature of each vertical excitation is indicated in the legends. The Stokes shifts are given as 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 parabola is to the right of that in the excited state parabola.

relaxations of bright cerium(III) excited states by C‒H/D 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 CeG3 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. Based on these observa-

tions, we performed a computational analysis to determine the cause of the Stokes shifts, depending on the type of the 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 changes indicated more rigid structures (Table S15). 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 the Supporting Information, Section 11, 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. 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-S18. 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 and 5d Ce orbitals. The 5d orbitals are predominantly nonbonding, but exhibit some mixing with ligand orbitals. Since the complexes CeA3 and CeG3 are approximately C3vsymmetric, and CeA2G and CeAG2 are approximately C2v symmetric, 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 Ngucentered molecular orbitals (MOs) to 4f Ce acceptor orbitals. The MOs implicated in these LMCT bands (λabs3) are shown in Figures S68-S69. 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 Supporting Information Section 11. 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 vertical excitation (Table S15). Interestingly, negative correlations were found between coordination number (CN) and contraction of the Ce–N bonds (∆dCe–N) and CN and Stokes shifts (Figures S73-S74). Moreover, molecular Ce(III) luminophores with high coordination number tend to have

Page 6 of 9

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, Stoke shift = 0.26 eV),49 Ce(III) complexes with N-substituted tris(N-alkylbenzimidazol-2ylmethyl)amine (NTB) ligands (CN = 8, Stoke shift = 0.31 eV),2 and [K(THF)2][(C5Me5)2CeI2] (CN = 12, Stoke 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 eV to 0.66 eV.6 Based on these observations, we postulate that 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 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 photo-excitation, 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 influence 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 Analyses. Atomic charges and atomic orbital populations of the Ce center, in the absorbing ground- and emitting excited-state optimized geometries of the GA series, are shown in Table 1. 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). 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 geometries.a Complex

El. state

Ce charge

Ce populations

Nam/Ngu chargeb


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Journal of the American Chemical Society CeA3 CeA2G CeAG2 CeG3

gs

1.99

4f1.145d0.686s0.076d0.13

−1.84/-

es

1.86

4f0.355d1.266s0.396d0.16

−1.81/-

gs

2.07

4f1.125d0.616s0.086d0.12

−1.85/−0.83

0.30

1.35

0.29

0.14

−1.81/−0.81 −1.85/−0.82

es

1.92

4f

gs

2.15

4f1.115d0.556s0.086d0.11 0.29

5d

1.37

6s

6d

0.22

0.12

es

1.98

4f

gs

2.23

4f1.115d0.486s0.076d0.08

es

2.04

0.27

4f

5d 5d

1.41

6s

6d

0.16

6s

6d

0.09

−1.81/−0.81

Supporting Information Section 11). 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 TD-DFT calculations were sufficiently accurate, despite a pronounced multi-configurational character of the electronic states evidenced by the CAS calculations. The final electronic features of interest are therefore the split emission bands.

-/−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 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 Figure 4a and Figure S70). A stronger donation engenders a stronger electronic coupling between the metal and the ligands, which results in increased non-radiative 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 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 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 photo-excitation. 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 deexcitations from a doublet excited state of 2D ion parentage to the two SOC components of the 2F ground state, namely, the 2 F5/2 (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) MixedLigand Complexes according to Multiconfigurational Wave-Function-Based Methods. Scalar relativistic (SR) and SOC (including SR effects) photophysical data were obtained from complete active space wavefunction calculations without (‘CAS-SR’) and with SOC (‘CAS-SO’), respectively (see the

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). The λabs1 SOC excitation energies were calculated relative to the average energy of the six Kramers doublets of 2 F5/2 parentage for each GA complex. Like the CAS-SR counterparts, these energies were in good agreement with the experimental data. Moreover, the calculated SOC Stokes shifts are in excellent agreement with the experimental data. The CAS-SO 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.

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, 1 2 which are further split into Kramers doublets. The λem and λem SOC emission energies are relative to the average energies of the eight/six 2F7/2 and 2F5/2 state components, respectively.

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 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 colors and brightness 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 metalligand electronic coupling in those compounds with the largest Stokes shift. Finally, multi-configurational 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 the results of this work can be applied to tailor optical properties to achieve new and specialized applications.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Synthetic details and characterizations, electronic absorption, excitation, and emission data, computational details, and determination of structural metrics (PDF) Crystallographic data (CIF).

AUTHOR INFORMATION Corresponding Authors *[email protected] *[email protected] *[email protected] §

These authors contributed equally.

ACKNOWLEDGMENT This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences Separations and Analysis program under Award Number DE-SC0017259. J.A. thanks the National Science Foundation (CHE-1560881) for financial support. 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. 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, Dr. Shuguang Zhang (all UPenn) are acknowledged for helpful discussions.

REFERENCES (1) Cotton, S. Lanthanide and Actinide Chemistry; John Wiley and Sons: West Sussex, U. K., 2006; pp 61-77. (2) Zheng, X.-L.; Liu, Y.; Pan, M.; Lü, X.-Q.; Zhang, J.-Y.; Zhao, C.-Y.; Tong, Y.-X.; Su, C.-Y. Angew. Chem. Int. Ed. 2007, 46, 7399. (3) Qin, X.; Liu, X.; Huang, W.; Bettinelli, M.; Liu, X. Chem. Rev. 2017, 117, 4488. (4) Suzuki, K.; Tang, F.; Kikukawa, Y.; Yamaguchi, K.; Mizuno, N. Angew. Chem. Int. Ed. 2014, 53, 5356.

Page 8 of 9

(5) Yin, H.; Carroll, P. J.; Anna, J. M.; Schelter, E. J. J. Am. Chem. Soc. 2015, 137, 9234. (6) Yin, H.; Carroll, P. J.; Manor, B. C.; Anna, J. M.; Schelter, E. J. J. Am. Chem. Soc. 2016, 138, 5984. (7) Yin, H.; Jin, Y.; Hertzog, J. E.; Mullane, K. C.; Carroll, P. J.; Manor, B. C.; Anna, J. M.; Schelter, E. J. J. Am. Chem. Soc. 2016, 138, 16266. (8) Smet, P. F.; Parmentier, A. B.; Poelman, D. J. Electrochem. Soc. 2011, 158, R37. (9) Melcher, C. L.; Schweitzer, J. S. IEEE Trans. Nucl. Sci. 1992, 39, 502. (10) Piao, X.; Machida, K.-i.; Horikawa, T.; Hanzawa, H.; Shimomura, Y.; Kijima, N. Chem. Mater. 2007, 19, 4592. (11) Ashcroft, N. W. M., N. D. Solid State Physics; Holt, Rinehart and Winston: New York, 1976. (12) Bachmann, V.; Ronda, C.; Meijerink, A. Chem. Mater. 2009, 21, 2077. (13) George, N. C.; Birkel, A.; Brgoch, J.; Hong, B.C.; Mikhailovsky, A. A.; Page, K.; Llobet, A.; Seshadri, R. Inorg. Chem. 2013, 52, 13730. (14) Brgoch, J.; DenBaars, S. P.; Seshadri, R. J. Phys. Chem. C 2013, 117, 17955. (15) Ha, J.; Wang, Z.; Novitskaya, E.; Hirata, G. A.; Graeve, O. A.; Ong, S. P.; McKittrick, J. J. Lumin. 2016, 179, 297. (16) Han, J. K.; Hannah, M. E.; Piquette, A.; Talbot, J. B.; Mishra, K. C.; McKittrick, J. ECS J. Solid State Sci. Technol. 2012, 1, R98. (17) Che, C.-M.; Lai, S.-W. Coord. Chem. Rev. 2005, 249, 1296. (18) Haas, K. L.; Franz, K. J. Chem. Rev. 2009, 109, 4921. (19) Visbal, R.; Gimeno, M. C. Chem. Soc. Rev. 2014, 43, 3551. (20) Williams, J. A. G. Chem. Soc. Rev. 2009, 38, 1783. (21) Armelao, L.; Quici, S.; Barigelletti, F.; Accorsi, G.; Bottaro, G.; Cavazzini, M.; Tondello, E. Coord. Chem. Rev. 2010, 254, 487. (22) Petoud, S.; Cohen, S. M.; Bünzli, J.-C. G.; Raymond, K. N. J. Am. Chem. Soc. 2003, 125, 13324. (23) Sabbatini, N.; Guardigli, M.; Lehn, J.-M. Coord. Chem. Rev. 1993, 123, 201. (24) Baschieri, A.; Mazzanti, A.; Stagni, S.; Sambri, L. Eur. J. Inorg. Chem. 2013, 2013, 2432. (25) Yu, T.; Su, W.; Li, W.; Hua, R.; Chu, B.; Li, B. Solid·State Electron. 2007, 51, 894. (26) Zhao, L.; Liu, Y.; He, C.; Wang, J.; Duan, C. Dalton Trans. 2014, 43, 335. (27) Jiao, Y.; Wang, J.; Wu, P.; Zhao, L.; He, C.; Zhang, J.; Duan, C. Chem. Eur. J. 2014, 20, 2224. (28) Kunkely, H.; Vogler, A. J. Photochem. Photobiol. A: Chem. 2002, 151, 45. (29) Hazin, P. N.; Lakshminarayan, C.; Brinen, L. S.; Knee, J. L.; Bruno, J. W.; Streib, W. E.; Folting, K. Inorg. Chem. 1988, 27, 1393. (30) Rausch, M. D.; Moriarty, K. J.; Atwood, J. L.; Weeks, J. A.; Hunter, W. E.; Brittain, H. G. Organometallics 1986, 5, 1281. (31) Hazin, P. N.; Bruno, J. W.; Brittain, H. G. Organometallics 1987, 6, 913. (32) Kuzyaev, D. M.; Balashova, T. V.; Burin, M. E.; Fukin, G. K.; Rumyantcev, R. V.; Pushkarev, A. P.; Ilichev, V. A.; Grishin, I. D.; Vorozhtsov, D. L.; Bochkarev, M. N. Dalton Trans. 2016, 45, 3464. (33) Matthes, P. R.; Müller-Buschbaum, K. Z. Anorg. Allg. Chem. 2014, 640, 2847. (34) Azenha, M. E.; Burrows, H. D.; Fonseca, S. M.; Ramos, M. L.; Rovisco, J.; de Melo, J. S.; Sobral, A. J. F. N.; Kogej, K. New J. Chem. 2008, 32, 1531.


Page 9 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Journal of the American Chemical Society (35) Dröse, P.; Blaurock, S.; Hrib, Cristian G.; Hilfert, L.; Edelmann, Frank T. Z. Anorg. Allg. Chem. 2011, 637, 186. (36) Werner, D.; Deacon, G. B.; Junk, P. C.; Anwander, R. Chem. Eur. J. 2014, 20, 4426. (37) Tretiak, S.; Saxena, A.; Martin, R. L.; Bishop, A. R. Chem. Phys. Lett. 2000, 331, 561. (38) Martin, R. L. J. Chem. Phys. 2003, 118, 4775. (39) Strickler, S. J.; Berg, R. A. J. Chem. Phys. 1962, 37, 814. (40) Falivene, L.; Credendino, R.; Poater, A.; Petta, A.; Serra, L.; Oliva, R.; Scarano, V.; Cavallo, L. Organometallics 2016, 35, 2286. (41) Maity, A. K.; Fortier, S.; Griego, L.; MettaMagaña, A. J. Inorg. Chem. 2014, 53, 8155. (42) Brown, T. L.; Lee, K. J. Coord. Chem. Rev. 1993, 128, 89. (43) Tolman, C. A. Chem. Rev. 1977, 77, 313.

(44) Bilbrey, J. A.; Kazez, A. H.; Locklin, J.; Allen, W. D. J. Comput. Chem. 2013, 34, 1189. (45) Bischof, C.; Wahsner, J.; Scholten, J.; Trosien, S.; Seitz, M. J. Am. Chem. Soc. 2010, 132, 14334. (46) Bunzli, J.-C. G.; Piguet, C. Chem. Soc. Rev. 2005, 34, 1048. (47) Hasegawa, Y.; Ohkubo, T.; Sogabe, K.; Kawamura, Y.; Wada, Y.; Nakashima, N.; Yanagida, S. Angew. Chem. 2000, 112, 365. (48) Yanagida, S.; Hasegawa, Y.; Murakoshi, K.; Wada, Y.; Nakashima, N.; Yamanaka, T. Coord. Chem. Rev. 1998, 171, 461. (49) Blasse, G.; Dirksen, G. J.; Sabbatini, N.; Perathoner, S. Inorg. Chim. Acta 1987, 133, 167. (50) Duignan, T. J.; Autschbach, J. J. Chem. Theory Comput. 2016, 12, 3109.

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