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Mar 21, 2017 - Los Alamos National Laboratory, Post Office Box 1663, Mail Stop J514, Los Alamos, New Mexico 87545, United States. ‡. Department of ...
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Photophysical Dynamics and Relaxation Pathways of Ligand-toMetal Charge-Transfer States in the 5f1 [Np(VI)O2Cl4]2− Anion Beau J. Barker,† John M. Berg,† Stosh A. Kozimor,† Nicholas R. Wozniak,†,‡ and Marianne P. Wilkerson*,† †

Los Alamos National Laboratory, Post Office Box 1663, Mail Stop J514, Los Alamos, New Mexico 87545, United States Department of Chemistry, Radiochemistry Program, University of Nevada-Las Vegas, Las Vegas, Nevada 89154, United States

‡

ABSTRACT: Although several publications report on the electronic structure of the neptunyl ion, experimental measurements to detail the photophysical dynamics of this open-shell actinyl system are limited in number. Time-resolved photoluminescence has been a useful experimental approach for understanding photophysical dynamics and relaxation pathways of a variety of other molecular and ionic systems, including gaseous plutonium hexafluoride and solid-state uranyl compounds. Here, we investigate timeresolved photoluminescence emission of the 5f1 neptunyl tetrachloride ([Np(VI)O2Cl4]2−) dianion following visible excitation. Photoemission of the lowest energy neptunyl ligand-tometal charge-transfer (LMCT) transitions to both the ground and first electronically excited states is observed. Analyses of the decay lifetimes of the excited states suggest different relaxation pathways as a function of excitation energy. Vibronic progressions associated with the Np-oxo symmetric stretching mode are measured in emission spectra, and the energies from these progressions are compared with energies of vibronic progressions associated with the excitation spectra of [Np(VI)O2Cl4]2−. This study expands our understanding of this open-shell actinyl system beyond identification of excited states, allowing characterization of photophysical properties and evidence for the electronic character of the ground state, and suggests that this approach may be applicable to more complex actinide systems.

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[NpO2Cl4]2− are electric dipole forbidden, but may be allowed through magnetic dipole and electric quadrupole mechanisms, or electric dipole induced by coupling to infrared-active ungerade molecular vibrations.30 Energies of the vibrational modes of the ground electronic state of Cs2NpO2Cl4 are used to assign vibronic structure measured from the [NpO2Cl4]2− anion. In general, these assignments are identified based upon the energy difference between the origin and the vibronic transitions. The ground state vibrational modes of Cs2NpO2Cl4 are summarized in Table 1.8 Initial experimental work to measure optical spectra from the most stable 5f1 actinyl system, the neptunyl ion, has been published, but the number of reports is limited in comparison with the large body of work to characterize the closed-shell [U(VI)O2]2+ ion.5−29,31−45 Absorption spectra of transitions in Cs2Np(VI)O2Cl4 were observed early in reports by Stafsudd et al. and Gorshkov et al.5,6 Denning et al. greatly expanded on this work by recording spectra from single crystals of Cs2NpO2Cl4 and CsNpO2(NO3)3 doped into isotructural uranyl matrices.8,9 Combining results from multiple spectroscopic techniques (polarized absorption, magnetic circular dichroism, the Zeeman effect) with parametrized ligand field theory, they characterized 11 low-lying electronic states between 600 and 22 000 cm−1. The energies of the electronic

INTRODUCTION Employment of time-resolved photoluminescence is advantageous for characterization of the photophysical dynamics of complex electronic structures, including actinide systems.1−4 Detection of the optical transitions of 5f x (x ≥ 1) actinide ions and molecules, however, is challenged by electronic energy spacings in the near-infrared regime and a high density of electronic states. These characteristics increase the probability of nonradiative quenching through interactions of an emitted photon with either phonons (in the solid state) or solvent molecules (in solution), and contribute to relatively shorter lifetimes and typically lower quantum yields. Thus, reports of spectroscopic measurements to detail the photophysical dynamics of open-shell actinide molecules and ions are quite limited in number. The neptunyl (Np(VI)O22+) ion is useful for experimental and theoretical study of open-shell actinide systems due to the presence of a single electron in its electronic configuration, [Rn]5f1. Electronic transitions in NpO22+ ions can be broadly characterized as either LMCT or 5f−5f transitions.5−29 The LMCT transitions correspond to transfer of an electron from a formally metal-oxo bonding orbital to a nonbonding metalcentered 5f-type orbital. Conversely, 5f−5f transitions involve excitation of an electron from a nonbonding orbital of primarily 5f metal character to another orbital of predominantly 5f character. For centrosymmetric symmetries such as those of the pseudo-D4h structure of the [MO2Cl4]2− (M = U, Np, Pu) anions, these ungerade → ungerade transitions of NpO22+ or © 2017 American Chemical Society

Received: February 1, 2017 Revised: March 3, 2017 Published: March 21, 2017 2353

DOI: 10.1021/acs.jpca.7b01029 J. Phys. Chem. A 2017, 121, 2353−2360

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The Journal of Physical Chemistry A Table 1. Vibrational Modes, Symmetries in D4h, Description of the Vibration Motion, and Ground State Vibrational Frequencies of [NpO2Cl4]2− 8 modea

symmetry D4h

ν1

A1g

ν2

A2u

ν3 ν4

Eu A1g

ν5 ν6 ν7 ν8 ν9

B2g Eu B1g Eu A2u

ν10

B1u

ν11

Eg

a

Other authors have attempted to detect neptunyl ions in solution using photoluminescence.20 While emission of the neptunyl ion is highly quenched via O−H oscillators, TalbotEeckelaers et al. reported time-resolved photoluminescence using the bare NpO22+ ion dissolved into D2O. They measured transitions in the near-infrared at 1490 and 1580 nm, and determined that the excited state lifetime is less than 10 ns. The authors attempted to exclude the solvent from the inner coordination sphere using polyoxometallates, which essentially encapsulated the NpO22+ ion within the [GeW9O34]10− and [PW9O34]9− clusters. In this system, the polyoxometallates replaced the solvent molecules in the actinide inner coordination sphere, and allowed the emission lifetime to increase to ∼62 ns. Subsequently, Woodall et al. observed visible emission from NpO22+ coordinated by chelating tetraphenylimidodiphosphinate (TPIP) ligand, which generated a [Np(VI)O2(TPIP)2(Ph3PO)] complex that was stable to redox behavior.27 Using Density Functional Theory calculations and the observation of a P−N stretching mode in the emission spectrum, they concluded that the emission was a combination of TPIP → NpO2 LMCT and oxo → Np LMCT character. The symmetries of the low-lying electronic states in Cs2NpO2Cl4 have been considered in detail by several groups. Denning et al. used ligand field theory to model the low-lying electronic states.8,9 After including the effects of spin−orbit coupling and the equatorial field, they found the ground state and the first excited state to both be of mixed 2Φu + 2Δu character. Pitzer et al. used a layered-cluster method and calculated the ground electronic state to be ~86% 2 δu character.14 The first electronically excited state was found to be 40% fδu and 49% fφu in character. More recently, restrictiveactive-space second-order perturbation theory (RASPT2) calculations by Su et al. have also found the ground and first electronic states to be mixed 2Φu + 2Δu, with the added detail that 2Φu makes the dominant contribution to the ground state and 2Δu makes the dominant contribution to the first excited state.24 In addition, they calculated the energies of the symmetric stretching mode and found it to be lower in the ground state (786 cm−1) than in the first excited state (812 cm−1). Herein, we use time-resolved photoluminescence spectroscopy to investigate emission in the near-infrared and visible regions, and to explore the photodynamics of the [NpO2Cl4]2− dianion doped into a Cs2UO2Cl4 crystal matrix, referred to hereafter as Cs2U(Np)O2Cl4. Emission is measured from the lowest-energy LMCT states to the 5f ground and first excited states. We detect strong progressions in the symmetric stretching mode for both ground and first excited states. Time-resolved waveforms show that the lifetime of the LMCT states are much shorter than the lifetime of previously reported intra-5f emission in Cs2U(Np)O2Cl4. Analyses of these data suggest relaxation pathways and symmetry of the ground electronic state.

vibrational energy (cm‑1)

motion Np-oxo symmetric stretch Np-oxo asymmetric stretch oxo-Np-oxo bend Np−Cl symmetric stretch Np−Cl stretch Np−Cl stretch Np−Cl in-plane bend Np−Cl in-plane bend Np−Cl out-of-plane bend Np−Cl out-of-plane bend oxo-Np-oxo rock

802 919 267 257 230 244 133 117 117 Raman and infrared inactive 185

The subscript refers to the vibrational mode.

Table 2. Excited State, Excited State Energy, Excited State Type, and Energy of the Symmetric Vibronic Stretch, ν1, Associated with the First 11 Electronic States of [NpO2Cl4]2− 8,9,11,29 state 0 I II III IV V VI VII VIII IX X a

energy (cm‑1) 0 ∼1000/915 6880.4/6880 ∼7990/7875 13 264.9/13 259 15 406.4/15 402 ∼15 683 16 799.8 17 241.4 19 375.2 20 080.8

type 5f 5f 5f 5f LMCT LMCT LMCT LMCT 5f LMCT 5f

ν1 (cm‑1) 8

802 830a 82629 80829 7229/71611/72029 7159/71611/71429 7119/71011 7139,11 7658/76311 6869/72611 7678/76611

Values from this study.

states of [NpO2Cl4]2− identified by Denning et al. are summarized in Table 2 along with the energy for the symmetric vibrational mode, ν1, associated with each state.8,9,11,29 In the current work, we retain the labeling scheme used by Denning to identify the electronic states, such that the states are labeled with 0 for the ground state, I for the first excited state, II for the second excited state, and so on.8,9 Wilkerson, Berg and co-workers were the first to report lowlying intra-5f photoluminescence from [NpO2Cl4]2−.16,17,19,22,23,29 They detected strong emission from a near-infrared state to the ground state from Cs2U(Np)O2Cl4 cooled to 75 K and at room temperature. The temporal decay at 75 K is single exponential with a lifetime of 71 μs, and a lifetime of 20 μs is measured at room temperature.17 In a subsequent paper, they reported excitation spectra from the near-infrared through the visible region while monitoring this low energy state.29 Photoluminescence from NpO22+ in the visible portion of the spectrum was first observed by Stone (while working as a Ph.D. student in Robert Denning’s laboratory), but this work is only documented in his thesis.11 These combined studies suggest that further investigation of LMCT emission is warranted.11,16,17,19,22,23,29

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EXPERIMENT The preparation and structural characterization of Cs2U(Np)O2Cl4, crystals are described elsewhere.5,7,8,11,16,17,19,22,29 The ratio of Np:U is ∼3%, based upon the relative concentrations of Np and U dissolved in the parent solution. Polycrystalline needles are mounted inside a flame-sealed borosilicate capillary, and the sealed capillary is placed in an NMR tube, sealed, and then cooled to liquid nitrogen temperature. 2354

DOI: 10.1021/acs.jpca.7b01029 J. Phys. Chem. A 2017, 121, 2353−2360

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The Journal of Physical Chemistry A The apparatus used in this study is described elsewhere.29,46 The excitation source is a tunable optical parametric oscillator (Continuum Panther) pumped by a pulsed Nd:YAG laser (Continuum Surelight II-10) operating at 10 Hz. The OPO has a spectral bandwidth of ∼5 cm−1 and a temporal width of 10 ns. Photoluminescence from the sample was collected at 90° from the excitation laser and dispersed using a 300 mm focal length spectrograph (Acton Research Corporation SpectraPro-300i). Emission spectra were collected using a 512 element InGaAs photodiode array (50 × 500 μm). The emission spectra were calibrated using atomic lines from either a krypton or neon lamp.47 The uncertainty in the reported peak positions from the emission spectrum is ±5 cm−1. Spectral intensities are corrected for detector and spectrograph responses. To record time-resolved decay curves, photoluminescence was imaged onto a photomultiplier tube (PMT) (Hamamatsu R5509-72). The signal from the PMT was amplified (Stanford Research SR445) and sent to a multichannel scaler/averager (SR430), each having a response time of ∼5 ns. The bin size was set to 5 ns, and waveforms from 3000 to 10 000 laser pulses were coadded. An instrumental spike likely due to radiofrequency noise was observed from 0 to ∼100 ns, and therefore, this portion of each temporal waveform was not fit.

Figure 2. Photoluminescence emission spectrum (solid line) and excitation spectrum (dotted line) of the V → 0 vibronic structure of Cs2U(Np)O2Cl4 at 75 K following pulsed excitation of the neptunyl state IX + νIX3/6 at ∼19 600 cm−1.29 Peaks labeled with an asterisk (*) are associated with a different origin.

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RESULTS AND DISCUSSION Transition Assignments. Photoluminescence of Cs2U(Np)O2Cl4 crystals cooled to 75 K was measured between 6100 and 18 300 cm−1 using pulsed excitation of the strong absorption transition at ∼19 600 cm−1 (510 nm) and is shown in Figure 1. The absorption transition at ∼19 600 cm−1 was first

Figure 3. Photoluminescence emission spectrum (solid line) and excitation spectrum (dotted line) of the IV → 0 vibronic structure of Cs2U(Np)O2Cl4 at 75 K following pulsed excitation of the neptunyl state IX + νIX3/6 at ∼19 600 cm−1.29 Peaks labeled with an asterisk (*) are associated with a different origin.

Figure 1. Photoluminescence emission spectrum of 3% Cs2U(Np)O2Cl4 at 75 K following pulsed excitation of the neptunyl state IX + νIX3/6 at ∼19 600 cm−1.

assigned to 0 → IX + νIX3/6 by Denning and co-workers.9 The most intense features in Figure 1 are in the near-infrared region between 6100 and 7100 cm−1. These transitions have been studied in detail using low-temperature absorption and photoluminescence spectroscopy.5,6,8−11,16,17,19,22,23,29 The transition at 6880 cm−1 was assigned to the II → 0 origin. For the following discussion, portions of the photoluminescence emission spectrum are divided between Figures 2 through 5 in order to facilitate discussion of vibronic structure associated with electronic transitions built on V → 0 (Figure 2), IV → 0 (Figure 3), V → I (Figure 4), and IV → I (Figure 5). Figure 2 shows the vibronic transitions associated with V → 0 following excitation of the neptunyl state 0 → IX + νIX3/6.

Figure 4. Photoluminescence emission spectrum of the V → I vibronic structure of Cs2U(Np)O2Cl4 at 75 K following pulsed excitation of the neptunyl state IX + νIX3/6 at ∼19 600 cm−1.29 Peaks labeled with an asterisk (*) are associated with a different origin.

The corresponding, previously reported excitation spectrum for 0 → V, collected by monitoring the intensity of II → 0 at 6880 cm−1, is included for comparison.29 The V → 0 origin may be evident in the weak feature at ∼15 402 cm−1. The weak intensity is consistent with that previously reported for the V → 0 origin in excitation (see Table 3). Vibronic structure in the 2355

DOI: 10.1021/acs.jpca.7b01029 J. Phys. Chem. A 2017, 121, 2353−2360

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The Journal of Physical Chemistry A

Table 4. Assignments of Vibronic Structure and Transition Energies for the Transition IV → 0 transition IV IV IV IV IV IV IV

Table 3. Assignments of Vibronic Structure and Transition Energies for the Transition V → 0 V V V V V V V V V

→ → → → → → → → →

0 0 0 0 0 0 0 0 0

(Origin) + lattice + ν8/9 + ν3/6 + ν3 + ν11 + ν2 + ν2 + ν8/9 + ν3/6 + ν1 + ν2 + ν1

energy (cm‑1)

energy difference from origin (cm‑1)

15 402 15 342 15 273 15 125 14 954 14 479 14 363 14 327 13 688

0 60 129 277 448 923 1039 1075 1714

0 0 0 0 0 0 0

(Origin) + ν8/9 + ν3/6 + ν2 + ν2 + ν1 + ν2 + 2ν1 + ν2 + 3ν1

energy difference from origin (cm‑1)

13 265 13 133 12 974 12 335 11 541 10 751 9956

0 132 291 930 1724 2514 3309

respectively. Corresponding transitions involving the same modes in excited state structure are evident with similar relative intensities in the excitation spectrum. An emission peak at 12 335 cm−1, 930 cm−1 lower in energy than the origin, is assigned IV → 0 + ν2, and a progression in ν1 was identified as IV → 0 + ν2 + nv1, where n = 0−3 is observed with an average vibrational spacing of 793 cm−1. The low intensity of the excitation spectrum reported in ref 29 likely precludes the ability to identify a corresponding progression in the photoluminescence excitation spectrum. Transitions mirroring the 0 → IV + νIV1 and 0 → IV + νV8/9 + νV1 excitation transitions are not apparent in the emission spectrum. A number of emission features between 10 000 and 14 500 cm−1 could be assigned to structure built on the V → I origin (Figure 4) and IV → I origin (Figure 5). As described in prior work by Stone, transitions ending on I are challenging to assign in the luminescence spectra because the energy of I is approximately coincident with the energy of 0 + ν2.11 Transitions ending on I are likely masked by transitions to 0 + ν2. To identify the energy of I, Stone collected luminescence spectra from 18O-labeled Cs2U(Np)18O2Cl4. The presence of the 18O isotope shifted Np-oxo vibronic modes to lower energy, while leaving the energy of electronic origins essentially unchanged. This isotopic substitution provided a means to discriminate between vibronic structure belonging to neptunium-oxo modes and electronic origins. The isotopic shift of 16 cm−1 of V → 0 + ν2 in Stone’s photoluminescence spectrum revealed the origin band belonging to V → I transition, and a more accurate energy for I was determined (915 cm−1). Stone assigned an emission peak at 14 491 cm−1 to the V → I origin, which is consistent with the calculated difference in energy between V (15 406.4/15 402 cm−1) and I (915 cm−1).11 In our spectrum, the V → I origin is likely obscured by the V → 0 + ν2 emission peak. The energies of the vibronic peaks associated with this origin and the difference in energies between assignments and origin are given in Table 5. The energy spacing between the peaks at 14 202, 13 369, 12 544,

Figure 5. Photoluminescence emission spectrum of the IV → I vibronic structure of Cs2U(Np)O2Cl4 at 75 K following pulsed excitation of the neptunyl state IX + νIX3/6 at ∼19 600 cm−1.29 Peaks labeled with an asterisk (*) are associated with a different origin.

transition

→ → → → → → →

energy (cm‑1)

spectrum is assigned based upon previous ground state vibrational mode analysis (Table 3). The first features lower in energy than the origin are assigned to V → 0 + lattice modes (15 342 cm−1), V → 0 + ν8/9 (15 273 cm−1), V → 0 + ν3/6 (15 125 cm−1), and V → 0 + ν3 + ν11 (14 954 cm−1), which mirror peaks that were assigned as 0 → V + lattice modes, 0 → V + νV8/9, 0 → V + νV3, 0 → V + νV6̧ and 0 → V + νV3 + νV11 in the excitation spectrum. Transitions mirroring the 0 → V + νV1, 0 → V + νV1 + νV8/9, or 0 → V + νV3 + νV11 + νV1 excitation transitions are not apparent in the emission spectrum. Using the frequency of the ground vibrational mode ν2, a transition band at 14 479 cm−1 is assigned to V → 0 + ν2, which is supported by the corresponding assignment in the excitation spectrum. A peak approximately 116 cm−1 lower in energy is assigned to V → 0 + ν2 + ν8/9, although a corresponding assignment is not noted in the excitation spectrum The remaining peaks at 14 327 and 13 688 cm−1 are assigned to V → 0 + ν3/6+ ν1 and V → 0 + ν2+ ν1, respectively, which also mirror assignments in the excitation spectrum. Vibronic features assigned to the IV → 0 transition are labeled in Figure 3, and their energies are listed in Table 4. The corresponding 0 → IV excitation spectrum is shown in the figure for comparison.29 Assignment of the IV → 0 origin is supported by the appearance of a peak at 13 265 cm−1 in both the emission and the excitation spectra. The emission peaks at 13 133 and 12 974 cm−1 are 132 and 291 cm−1 lower in energy than the origin, intervals matching the energies of vibrational modes ν8/9, ν3, and ν6. Therefore, these are assigned to the vibronic transitions IV → 0 + ν8/9 and IV→ 0 + ν3/6,

Table 5. Assignments of Vibronic Structure and Transition Energies for the Transition V → I transition V V V V V V V a

2356

→ → → → → → →

I I I I I I I

(Origin) + ν3 + ν3 + ν1 + ν2 + ν1 + ν3 + 2ν1 + ν3 + 3ν1 + ν3 + 4ν1

energy (cm‑1) a

14 491 14 202 13 369 12 733 12 544 11 724 10 909

energy difference from origin (cm‑1) 0 289 1122 1758 1947 2767 3582

Energy from ref 11. DOI: 10.1021/acs.jpca.7b01029 J. Phys. Chem. A 2017, 121, 2353−2360

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The Journal of Physical Chemistry A 11 724, and 10 909 cm−1 is ∼823 cm−1. This separation, along with the difference in energy between V → I and the intense peak at 14 202 (Δ = 289 cm−1), suggests assignment of these peaks as a progression on ν1 (V → I + ν3 + nν1; n = 0−4). An additional peak at 12 733 cm−1 is lower in energy than V → I by 1758 cm−1, and is assigned tentatively to V → I + ν1 + ν2. The difference in energy between I (915 cm−1) and IV (13 264.9 cm−1) suggests that the IV → I origin should be found at ∼12 350 cm−1, but this region is masked by a large and intense peak, assigned IV → 0 + ν2. The energies of the vibronic peaks associated with this origin and the difference in energies between assignments and origin are given in Table 6. Table 6. Assignments of Vibronic Structure and Transition Energies for the Transition IV → I transition IV IV IV IV IV IV IV IV IV IV a

→ → → → → → → → → →

I I I I I I I I I I

(Origin) + ν8/9 + ν6 + ν3 + ν8/9 + ν1 + ν6 + ν1 + ν3 + ν1 + ν8/9 + 2ν1 + ν6 + 2ν1 + ν3 + 2ν1

energy (cm‑1) a

12 344 12 223 12 086 12 050 11 390 11 257 11 217 10 563 10 429 10 393

Figure 6. Symbolic representation of the excitation and decay paths of the various time-resolved photoluminescence waveforms recorded. A is excitation of V + νV3 (15 654 cm−1) while monitoring V + ν3/6 (15 125 cm−1); B is excitation of IX + νIX3/6 (∼19 600 cm−1) while monitoring V + ν3/6 (15 125 cm−1); C is excitation of IV + νIV8/9 (13 352 cm−1) while monitoring IV → 0 + ν2 + ν1 (11 541 cm−1); D is excitation of V + νV3 (15 654 cm−1) while monitoring IV → 0 + ν2 + ν1 (11 541 cm−1); E is excitation of IX + νIX3/6 (∼19 600 cm−1) while monitoring IV → 0 + ν2 + ν1 (11 541 cm−1).

energy difference from origin (cm‑1) 0 121 258 294 954 1087 1127 1781 1915 1951

Table 7. Results from Fit of Eq 5 to Luminescence Decay Traces Observed While Monitoring Emission from States IV and V Following Excitation of Various States

Energy from ref 11.

The feature at 12 223 cm−1 is assigned to IV → I + ν8/9. Likewise, two features at 12 086 cm−1 and 12 050 cm−1 are assigned to the IV → I + ν6 and IV → I + ν3 vibronic transitions, respectively, based on the difference between these frequencies and that of the IV → I origin. There are three transitions built on a progression in ν1: IV → I + ν8/9 + nν1; n = 0−2; IV → I + ν6 + nν1; n = 0−2; IV → I + ν3 + nν1; n = 0− 2. These progressions exhibit average values of 830, 829, and 829 cm−1, respectively, for ν1. The highest energy features in Figure 1 include two peaks at 16 695 and 17 520 cm−1. These energies closely coincide in energy with the I → 0 + ν2 + nν1 vibronic bands (n = 2−3) of Cs2UO2Cl4.32,35 An attempt was made to confirm this assignment using time-resolved luminescence, but we did not have sufficient signal-to-noise to record a temporal waveform. These transitions are mostly likely due to photoluminescence emission from the uranyl tetrachloride host following twophoton absorption. Dynamics. The time evolution of state-selective excitation is controlled by decay processes of the emitting state and, in the case of indirect population via pumping of higher energy states, by the decay processes from those higher lying states to the emitting state. An overview of the excitation and emission schemes used to analyze the photophysical dynamics of the LMCT states is given in Figure 6. Fitting results (vide inf ra) are provided in Table 7. Time-resolved V → 0 + ν3/6 photoluminescence (15 125 cm−1) was measured using two different excitation schemes (represented by A and B in Figure 6). Transients collected following direct excitation of V via 0 → V + νv3 (15 654 cm−1) showed single exponential decay with a fitted time constant of τ = 0.28 μs. We take this result to be the characteristic decay lifetime of V. The same V → 0 + ν3/6 photoluminescence populated indirectly via pumping of 0 → IX + νIX3/6 (∼19 600

a

pumped state/monitored state

τl (μs)

τu (μs)

V/V IX/V IV/IV V/IV IX/IV

0.28 0.29 0.38 0.36 0.41

-N/Aa -0.28 0.25

Upper state lifetime is too short to be resolved in our experiment.

cm−1), which is then followed by relaxation to V, also showed single exponential decay with a time constant of τ = 0.29 μs. Figure 7 shows both of these transients. Essentially we resolve

Figure 7. Time-resolved photoluminescence waveforms of V → 0 + ν3/6 (15 125 cm−1) following direct excitation of state V via V + νV3 at 15 654 cm−1 (black) and following excitation of the neptunyl state IX + νIX3/6 at ∼19 600 cm−1 (red).

no difference between the emission dynamics of V either from indirect population of V via decay from IX or from direct pumping of V. As noted in the Experimental Section, we observe an instrumental spike in our laser system between 0 and ∼100 ns. Thus, any potential decay from IX to V and/or intermediate states shorter than ∼100 ns would not be detected. 2357

DOI: 10.1021/acs.jpca.7b01029 J. Phys. Chem. A 2017, 121, 2353−2360

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The Journal of Physical Chemistry A Time-resolved photoluminescence transients were recorded from IV → 0 + ν2 + ν1 (11 541 cm−1) emission using three different pulsed excitation schemes: (1) direct excitation of IV via pumping 0 → IV + νIV8/9 (13 352 cm−1) (Figure 6c); (2) indirect population of IV via pumping 0 → V + νv3 (15 654 cm−1) (Figure 6d); and (3) indirect population of IV via pumping of 0 → IX + νIX3/6 (19 600 cm−1) (Figure 6e). The resulting transients are compared in Figure 8. The population

n u = n u0e(−t / τu) nl =

(3)

n u0Al τuτl (−t / τl) [e − e(−t / τu)] τl − τu

(4)

where nu0 is the initial population of the upper state (u) immediately after the pump pulse, Al = bulAu + wu and 1/τu = (Au + wu), 1/τl = (Al + wl). Equation 4 was further simplified into eq 5: nl = D[e(−t / τl) − e(−t / τu)]

where D = nu0Alτuτl/(τl − τu). In the case of direct pumping, a single exponential term was used. The results of fitting eq 5 to the waveforms are given in Table 7. The IV → 0 + ν2 + ν1 transient measured following excitation of V + νV3 yielded a fit to eq 5 having τu = 0.28 μs, and τl = 0.36 μs. This result is consistent with the separately observed direct excitation lifetimes of states V and IV, which were about 0.28 and 0.38 μs, respectively. Fit of the emission transient from IV following excitation of IX + νIX3/6 gave τu = 0.25 μs, and τl = 0.41 μs, lifetimes that are not significantly different from those measured following indirect population via pumping V. These results suggest that the decay from IX to IV proceeds through the same long-lived intermediate state as the decay from V to IV. The similarity of the fitted τu values with the lifetime of the direct excitation and emission from V implies that V itself could be this intermediate state. However, one cannot rule out the involvement of one or more additional intermediate states from these data alone. Bonding and Excited State Energies of ν1. It is useful to point out that, with measurement of emission to I, the energy of the ground state metal-oxo symmetric stretch, ν1, (802 cm−1) can now be compared with the energies of ν1 in the 5f excited states I (830 cm−1), II (826 cm−1), and III (808 cm−1) that have now all been observed in the excitation and photoluminescence spectra.11,29 The energy of ν1 in I and II is measurably greater (≥15 cm−1) than the energy of ν1 in the ground electronic state or in III. This result is notable because in all of these states, the unpaired electron resides in a 5fδ or 5fϕ orbital, illustrated in Figure 9, both of which are nonbonding with respect to the Np-oxo bond. Electronic transitions involving only these orbitals would not be expected to directly alter the Np-oxo bonding and therefore the vibrational frequencies of ν1. However, the 5fφu orbitals are formally antibonding with respect to the metal-chloride

Figure 8. Time-resolved photoluminescence waveforms of IV → 0 + ν2 + ν1 (11 541 cm−1) emission following direct excitation of IV via IV + νIV8/9 at 13 352 cm−1 (black), V + νV3 at 15 654 cm−1 (blue), and IX + νIX3/6 at ∼19 600 cm−1 (red).

dynamics of IV are clearly quite different when direct excitation is used compared with either type of indirect population. Direct excitation of IV yielded a decay dominated by a single exponential term with a fitted time constant of τ = 0.38 μs. Both indirect population schemes showed delayed IV → 0 + ν2 + ν1 emission onsets, with emission rising to a maximum rate ∼0.3 μs after the excitation pulses. This result indicates that one or more steps in the energy decay paths from IX or V to IV are slow enough to observably affect the emission dynamics. To model the more complex population dynamics of IV → 0 following indirect population, we make several simplifying assumptions: (1) that the population growth of the emitting lower level (IV) is controlled by the decay rate of a single longlived upper state in the decay chain from the pumped state to IV; (2) that the initial excitation process is effectively instantaneous; (3) that subsequent energy transfer is only intramolecular such that there are no cross-relaxation processes occurring between [NpO2Cl4]2− sites; and (4) that there is no energy transfer to or from the Cs2UO2Cl4 host. The populations of the excited states decay by radiative emission or by a nonradiative mechanism, such as multiphonon or internal conversion processes. The rate equations for such a system are as follows:48,49 dnu = −(A u + wu)n u(t ) dt

(1)

dnl = −(Al + wl)nl(t ) + (bulA u + wu)n u(t ) dt

(2)

(5)

The symbols nu,l, wu,l, and Au,l are the populations, nonradiative decay rate, and radiative decay rate for the upper and lower states respectively; bul is the branching ratio from the upper state to the lower state, and t is the time after the excitation pulse. The above differential equations can be solved to give the populations of nu and nl at time t:

Figure 9. Orbital structures of the NpO22+ ion. 2358

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The Journal of Physical Chemistry A bonds.21,26 Excitation from a 5fδu to a 5fφu orbital should remove electron density from the metal-oxo axis and equatorial plane, and into the equatorial plane itself. This response would have the effect of elongating the metal-chloride bonds and decreasing the electrostatic interaction between the metal and the chloride ligands, moving them further away and allowing the metal-oxo bond to become shorter. The result would be a slightly stronger metal-oxo bond and higher energy metal-oxo symmetric stretch as evidenced by the larger vibrational frequency of ν1 in states I and II. Because the unpaired electron resides in a formally nonbonding orbital with respect to the Np-oxo bond, this analysis suggests that changes in the Np−Cl bond length may influence the Np-oxo bond strength and may provide evidence that excitations to I or II involve a transition to a 5fϕ orbital, while in the ground electronic state and III, the electron likely resides in a 5fδ orbital. We refer the reader to Su et al. for a more detailed discussion of these effects in the context of theoretical models of electronic structure.24

support from the LANL LDRD program office. LA-UR-1720444

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(1) Hollas, J. M. High Resolution Spectroscopy; John Wiley & Sons: New York, 1998. (2) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Kluwer Academic/Plenum Publishers: New York, 1999. (3) Valeur, B. Molecular Fluorescence Principles and Applications; Wiley-VCH: New York, 2002. (4) Liu, G.; Beitz, J. V. Optical Spectra and Electronic Structure. In The Chemistry of the Actinide and Transactinide Elements, 3rd ed.; Morss, L. R., Edelstein, N. M., Fuger, J., Katz, J. J., Eds.; Springer: Dordrecht, The Netherlands, 2006; Chapter 18, pp 2013−2111. (5) Stafsudd, O. M.; Leung, A. F.; Wong, E. Y. Absorption Spectrum of NpO22+ in Cs2UO2Cl4. Phys. Rev. 1969, 180, 339−343. (6) Gorshkov, N. G.; Ladygin, I. N.; Mashirov, L. G.; Suglobov, D. N. Electronic Absorption Spectra of Oxocations of Np(VI) and Pu(VI). Relationship Between the Electronic Configurations of these Oxocations in the Excited State and the Strength of the Metal-oxygen Bond. Radiokhimiya 1975, 17, 896−899. (7) Weigel, F.; Werner, G. D.; Kalus, Ch. The Crystal Structure of the Dicesium Neptunyl Chloride, Cs2NpO2Cl4. Physica B+C 1980, 102B, 308. (8) Denning, R. G.; Norris, J. O. W.; Brown, D. The Electronic Structure of Actinyl Ions V. f-f Transitions in [NpO2Cl4]= and [NpO2(NO3)3]−. Mol. Phys. 1982, 46, 287−323. (9) Denning, R. G.; Norris, J. O. W.; Brown, D. The Electronic Structure of Actinyl Ions VI. Charge Transfer Transitions in Cs2NpO2Cl4 and CsNpO2(NO3)3. Mol. Phys. 1982, 46, 325−364. (10) Gorshkov, N. G.; Mashirov, L. G. Vibronic Spectrum of Cs2NpO2Cl4. Radiokhimiya 1984, 26, 540−549. (11) Stone, P. J. The Electronic Structure of Actinyl Ions. Ph.D. Dissertation, University of Oxford, 1985. (12) Makhyoun, M. A. Electronic Structure and Ligand Field States of NpO2Cl42‑ and NpO2(NO3)3−: A Relativistic MS-Xα Study. Inorg. Chem. 1987, 26, 3592−3595. (13) Matsika, S.; Pitzer, R. M. Electronic Spectrum of the NpO22+ and NpO2+ Ions. J. Phys. Chem. A 2000, 104, 4064−4068. (14) Matsika, S.; Pitzer, R. M. Actinyl Ions in Cs2UO2Cl4. J. Phys. Chem. A 2001, 105, 637−645. (15) Matsika, S.; Zhang, Z.; Brozell, S. R.; Blaudeau, J.−P.; Wang, Q.; Pitzer, R. M. Electronic Structure and Spectra of Actinyl Ions. J. Phys. Chem. A 2001, 105, 3825−3828. (16) Wilkerson, M. P.; Barefield, J. E.; Berg, J. M.; Dewey, H. J.; Hopkins, T. A. A Spectroscopic Investigation of the Electronic Structure of Neptunyl Ions. J. Nucl. Sci. Technol. 2002, 39, 129−131. (17) Wilkerson, M. P.; Berg, J. M.; Hopkins, T. A.; Dewey, H. J. First Observation of Intra-5f Fluorescence from an Actinyl Center: Np(VI) Near-IR Emission in Cs2U(Np)O2Cl4. J. Solid State Chem. 2005, 178, 584−588. (18) Infante, I.; Gomes, A. S. P.; Visscher, L. On the Performance of the Intermediate Hamiltonian Fock-Space Coupled-Cluster Method on Linear Triatomic Molecules: The Electronic Spectra of NpO2+, NpO22+, and PuO22+. J. Chem. Phys. 2006, 125, 074301. (19) Wilkerson, M. P.; Arrington, C. A.; Berg, J. M.; Scott, B. L. Crystal Structure and Spectroscopic Measurements of Room Temperature Intra-5f Fluorescence of Cs2Np(VI)O2Cl4. J. Alloys Compd. 2007, 444−445, 634−639. (20) Talbot-Eeckelaers, C.; Pope, S. J. A.; Hynes, A. J.; Copping, R.; Jones, C. J.; Taylor, R. J.; Faulkner, S.; Sykes, D.; Livens, F. R.; May, I. Luminescence from Neptunyl(VI) Species In Solution. J. Am. Chem. Soc. 2007, 129, 2442−2443. (21) Denning, R. G. Electronic Structure and Bonding in Actinyl Ions and their Analogs. J. Phys. Chem. A 2007, 111, 4125−4143. (22) Wilkerson, M. P.; Berg, J. M. Excitation Spectra of Near-infrared Photoluminescence from Np(VI) in Cs2U(Np)O2Cl4. Radiochim. Acta 2009, 97, 223−226.

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CONCLUSIONS In this study, we report photoluminescence from the lowest energy LMCT states of the [NpO2Cl4]2− dianion to either the ground or first electronically excited states. We assign weakly emitting transitions at 13 265, and 15 402 cm−1 to the IV → 0 and V → 0 origins based upon comparison with excitation spectra, but are unable to detect the IV → I and V → I origins. Assignments of vibronic structure assignments are consistent with the energies of the ungerade vibrational modes ν2, ν3, ν6, ν8, ν9, and ν10, and the gerade mode ν1. We also investigated the population decay kinetics of the LMCT states. Time-resolved photoluminescence decay waveforms were recorded while monitoring emission from both states IV and V. Emission from V can be described by a single exponential decay after either direct excitation of this state or excitation of IX, in the visible region. However, the presence of a delayed emission onset in the temporal waveform measured while monitoring emission from state IV following excitation of states IX or V suggests that V decays to IV. A qualitative analysis of the energy of the vibrational frequencies of the symmetric stretch associated with the ground state and excited states I, II, and III suggests that in I and II, the unpaired electron resides in a 5fϕ type orbital, while the unpaired electron resides in a 5fδ orbital in 0 and III.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (505) 667-5922. Fax: (505) 665-4955. ORCID

Marianne P. Wilkerson: 0000-0001-8540-0465 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 Science. Los Alamos National Laboratory is managed and operated by Los Alamos National Security, LLC (LANS), under Contract Number DE-AC52-06NA25396 for the U.S. Department of Energy’s National Nuclear Security Administration (NNSA). B.J.B. gratefully acknowledges a LANL Seaborg Institute Fellowship. N.R.W. gratefully acknowledges 2359

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