Vibronic Coupling Analysis of the Ligand-Centered Phosphorescence

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Vibronic Coupling Analysis of the Ligand-centered Phosphorescence of Gas-phase Gd(III) and Lu(III) 9-oxo-phenalen-1-one Complexes Ji#í Chmela, Jean-Francois Greisch, Michael E. Harding, Wim Klopper, Manfred M. Kappes, and Detlef Schooss J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b00006 • Publication Date (Web): 16 Feb 2018 Downloaded from http://pubs.acs.org on February 18, 2018

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The Journal of Physical Chemistry

Vibronic Coupling Analysis of the Ligand-Centered Phosphorescence of Gas-Phase Gd(III) and Lu(III) 9-Oxo-Phenalen-1-One Complexes Jiří Chmela1‡, Jean-François Greisch2‡*, Michael E. Harding2‡*, Wim Klopper1,2, Manfred M. Kappes1,2 and Detlef Schooss1,2 1

Institute of Physical Chemistry, Karlsruhe Institute of Technology, Fritz-Haber-Weg 2, 76131 Karlsruhe, Deutschland. 2

Institute of Nanotechnology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Deutschland. Supporting Information Placeholder

ABSTRACT: The gas-phase laser-induced photoluminescence of cationic mononuclear gadolinium and lutetium complexes involving two 9-oxo-phenalen-1-one ligands is reported. Performing measurements at a temperature of 83 K enables to resolve vibronic transitions. Via comparison to Franck-Condon computations, the main vibrational contributions to the ligand-centered phosphorescence are determined to involve rocking, wagging, and stretching of the 9oxo-phenalen-1-one–lanthanoid coordination in the low energy range, intraligand bending and stretching in the medium to high energy range, rocking of the carbonyl and methine groups, as well as C-H stretching beyond. While Franck-Condon calculations based on density-functional harmonic frequency computations reproduce the main features of the vibrationally resolved emission spectra, the absolute transition energies as determined by density functional theory are off by several thousand wavenumbers. This discrepancy is found to remain at higher computational levels. The relative energy of the Gd(III) and Lu(III) emission bands is only reproduced at the coupled-cluster singles and doubles level and beyond.

INTRODUCTION The magnetic-dipole transitions of lanthanoids and their complexes being often weak, their luminescence properties typically derive from the parity-forbidden and sometimes spin-forbidden character of their intra-4f electric dipole transitions. These transitions are partially allowed due to spinorbit mixing with spin-allowed transitions, resulting in molar absorption coefficients of about 0.01 − 0.1 m mol (0.1 − 1 cm M ) as well as relatively long excited state lifetimes in the ms range. The shielding of the 4 orbitals from the environment by the outer shell of 5s and 5p orbitals 1-6 yields particularly narrow transition lines. As a result, direct photoexcitation of lanthanoid luminescence is inefficient. Efficiency can be greatly improved by collection of electromagnetic excitation using -conjugated antenna ligands coordinated to the emitting centers.

Among -conjugated ligands, -diketonates have been found to be particularly well-suited for use with europium and terbium with applications ranging from optoelectronic 1, 4-5, 7 Rationalization of effidevices, sensors, to bioassays. cient energy transfers between ligand-centered donor and lanthanoid acceptor levels requires the knowledge of the ligand-centered excited state manifolds. It is commonly accepted that the leading channel for Eu(III) and Tb(III) optimal sensitization using -diketonates involves ← excitation of the ligand followed by ligand-centered → intersystem crossing. The lanthanoid centered excitation results from a resonant energy transfer between the ligandcentered donor vibronic levels and the lanthanoid-centered 4, 8-14 acceptor levels. This so-called lanthanoid sensitization leads, in absence of significant non-radiative decay channels, to high quantum yields for the lanthanoid-centered emis15 sion. The characterization of the ligand-centered triplet manifold of the complex typically involves phosphorescence measurements of synthetic analogues where europium and terbium have been replaced by gadolinium and lutetium 8, 16-18 ions. In the present work, small under-coordinated complexes of lanthanoids with the 9-oxo-phenalen-1-one (PLN) ligand (Figure 1) are studied. While PLN can be used 19-20 to sensitize Eu(III), Er(III), Nd(III), and Yb(III), the ab-1 sence of acceptor levels below 32000 cm prevents the sensitization of both Gd(III), with its half-full 4f-shell, and Lu(III), with its full 4f-shell leading to ligand-centered phosphorescence. This makes Gd(III) and Lu(III) containing complexes suitable for photophysical studies of the PLN ligand. Our long term goal is to better understand the sensitization of complexes of lanthanoids such as Eu(III) and Tb(III) characterized by a significant emission in the visible. For Eu(III), it is acknowledged that 1) effective sensitization involves energy transfer from ligand-centered vibronic levels mainly to the Eu(III) level in accordance with the selection rules for energy transfer via the Dexter mechanism, and 2) there is an optimum triplet energy enabling effective energy transfer to the lanthanoid while limiting thermally-driven 8, 21 energy back-transfer to the ligand. At room temperature,

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the solvent-dependent optimal excess energy for the ligandcentered triplet compared to Eu(III) level is reportedly -1 between 700 and 1400 cm . Similarly, Tb(III) effective sensitization is best achieved when the ligand-centered triplet -1 states have an electronic energy in excess of 1500-3000 cm compared to the Tb(III) emitting state, reportedly in order to avoid back transfer of population from the Tb(III) level to the ligand-centered excited states via thermally 8, 18 populated vibronic levels. Although these conclusions are commonly accepted, they neither explicitly consider the Stark level splitting of the lanthanoid acceptor levels nor the shift of the triplet level in the antenna induced by lanthanoid complexation (differences in triplet shifts as induced by Eu(III), Gd(III), and Tb(III) respectively are typically presumed to be negligible). They also do not take into account the spin-orbit mixing of the ligand-centered singlet and triplet states nor the zero field splitting of the ligand-centered triplet. All of these effects are expected to depend on the environment to differing degrees. Further insight into this can be obtained by performing experiments (and calculations) on the corresponding lanthanoid complexes without solvent environment – as realized here by isolating them in an ion trap for spectroscopic study. The species of interest in the present work are monocations whose luminescence properties are rationalized with the help of density functional calculations and FranckCondon simulations. The deconvolution of the vibrationally resolved gas-phase phosphorescence spectra of the + + [Gd(PLN)2] and [Lu(PLN)2] is used to infer information on the vibronic levels involved in the sensitization of lanthanoids such as Eu(III) and thereby also contribute to a better understanding of associated temperature effects. The paper is structured as follows. After the introduction of the experimental and theoretical techniques, the observed gas-phase ligand-centered phosphorescence is presented. This is followed by the comparison of emission spectra of the Gd(III) and Lu(III) complexes. Finally, the active modes of the phosphorescence spectra are analyzed employing the FranckCondon model.

Figure 1. Structure of the 9-oxo-phenalen-1-one monoanion ligand (PLN ).

MATERIALS AND METHODS +

3+

3+

[Ln(PLN)2] cations with Ln = Gd and Lu result from the decomposition of complexes with higher coordination numbers such as [Ln(PLN)4Na] or [Ln9(PLN)16(OH)10]Cl in coordinating solvents such as DMSO and alcohols. Their 22-25 synthesis was reported in previous publications. Isolated

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+

[Ln(PLN)2] species are obtained by electrospray ionization from solution.

Experimental setup The experimental setup has been detailed in Refs. 23 and 26. Briefly, it consists of an ion trap cooled down using liquid nitrogen in which the ions are mass-selected, stored, and thermalized by collision with a helium buffer gas at a pressure of ~0.2 mbar. Upon continuous-wave photoexcitation the emitted light is collected by a microscope objective fitted into one of the ion trap endcaps and resolved in energy using a spectrograph/CCD camera assembly.

Computational methods The geometry optimizations were performed with 27 28-33 TURBOMOLE at the B3LYP/def2-TZVPP level of theory. The geometries are provided in the xyz format as Supporting Information. The large core relativistic effective core potentials (RECPs) ECP53MWB and ECP60MWB of Refs. 34 and 35 were used for Gd and Lu, respectively. From the harmonic frequencies and the corresponding normal modes computed at the optimized geometries, Franck-Condon factors and relative intensities of the vibronic bands were calculated (see next section). Finally, upon comparison with experimental spectra, the modes contributing the most to given transitions have been identified. To further investigate the position of the 0-0 transition, single point computations on the B3LYP/def2-TZVPP geometries were carried out at the BP, 28, 36-38 MP2, CCSD, and CCSD(T) levels of theory. Unless stated otherwise the convergence thresholds in all computations -8 -5 were 10 and 10 / for the self-constistent field calculations and the Cartesian gradient, respectively. The frozencore approximation was used for all MP2, CCSD, and CCSD(T) computations. The resolution-of-the-identity (density fitting) approximation was used in all computations with 33, 39-40 the corresponding auxiliary basis sets.

Franck-Condon computations 41

In the present work, the HOTFCHT program, which takes into account vibrational mode mixing effects 42 (Duschinsky) in the calculation of the vibronic (Franck43-45 Condon) spectra of polyatomic molecules at non-zero temperatures, is used. In this program, the number of integrals calculated is reduced by the implementation of the ap46 proach developed by Berger and coworkers. This involves a prescreening of Franck-Condon profiles enabling the assessment of the discrepancies induced by the neglect of overtones beyond a given quantum number and of integrals with a high (given) number of simultaneously excited vibrational modes. While transitions originating from vibrationally excited levels in the initial state (hot bands) are taken into account, anharmonicities are ignored which may explain, in part, deviations from the experimental spectra (vide infra). The calculation of electronic spectra may be achieved either by time-dependent approaches or in terms of FranckCondon overlaps of the initial nuclear wave functions with time-independent vibrational eigenfunctions of the final electronic state. For low resolution and very high density of states, that is, for large molecules at high temperatures, the use of time-dependent wave packets for computing spectra is particularly advantageous, whereas for high resolution and low temperatures time-independent approaches might be 47-48 preferable. In our computations, the time-independent


approach is used for the spectra at cryogenic temperatures while the time-dependent approach is used for spectra at both temperatures (cryogenic and room temperature).

RESULTS AND DISCUSSION Gas-phase luminescence In order to determine the energy of the PLN triplet relative to its ground state and the manifold of vibronic states when coordinated to a lanthanoid ion, gas-phase photoluminescence measurements have been performed on the + + [Gd(PLN)2] and [Lu(PLN)2] complexes. Although the PLN ligand is rigid, vibrational features can only be resolved at cryogenic ion trap temperatures for which thermalization via collisional cooling is sufficient to remove the excess energy – energy deposited upon photoexcitation and left after photo+ emission – as displayed in Figure 2 for the [Gd(PLN)2] complex. The temperatures mentioned in Figure 2 correspond to the temperature of the walls of the ion trap. Contrarily to 49 measurements involving matrix isolation, the effective temperature of the trapped ions is unknown and depends on the He buffer gas pressure, the trapped species as well as the irradiance used for photoexcitation. Emission wavelength / (nm) 750 700 650 600 550 500 450 1

-1

to 18221 cm for Lu(III). This is consistent with the con9 densed-phase results of Crosby et al. which also display a -1 systematic ~100 cm blue shift of the phosphorescence of the Lu(III) complexes compared to Gd(III) ones (using other chelating ligands). It underscores that replacing atomic emitters such as Eu(III) and Tb(III) by Gd(III) and Lu(III) to gain access to the vibronic manifolds induces a shift that needs to be accounted for in quantitative resonant energy transfer studies. Emission wavelength / (nm) 640 620 600 580 560 540 1

Gd (83K, 400 W cm-2) Lu (83K, 400 W cm-2)

0 16000

-2

Gd (~293K, 600 W cm ) Gd (83K, 800 W cm-2) Gd (83K, 400 W cm-2)

Normalized intensity

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

The Journal of Physical Chemistry

Normalized intensity

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0 14000

16000

18000

20000

22000

24000

-1

Emission wavenumber / (cm )

Figure 2. Temperature and irradiance dependence of the gasphase ligand-centered phosphorescence spectrum of the Gd(PLN)2+ complex (458 nm excitation, 458 nm longpass emission filter, ~0.2 mbar He buffer gas). The vibronic transitions of the phosphorescence spectra at cryogenic temperatures are well-defined, as expected when the ground and excited states are structurally similar and vibrational enhancement of transition probabilities is limited. Also taking into account additional measurements at low laser power in order to avoid hot bands, this leads to the highest-energy most-intense transition being assigned as → 0-0. Interestingly, while the energy of the emission -1 -1 maximum is 18102 cm at 83 K, it shifts to 18001 cm at room temperature due to the combined effect of band broadening and hot bands. This is expected to directly impact the sensitization efficiency of lanthanoid ions with temperature. Some -1 high energy (> 19000 cm ) luminescence attributed to ligand 17, 50-51 fluorescence is also observed at room temperature. Re-1 placing Gd(III) by Lu(III) (Figure 3) induces a ~120 cm shift -1 of the PLN → 0-0 transition from 18102 cm for Gd(III)

17000

18000

19000

-1

Emission wavenumber / (cm )

Figure 3. Comparison of the gas-phase ligand-centered phos+ + phorescence spectra of the [Gd(PLN)2] and [Lu(PLN)2] -1 complexes. The emission maxima are 18102 cm for Gd(III) -1 and 18221 cm for Lu(III). The excitation wavelength is 458 2 nm (~400 W/cm ). He buffer gas pressure of ~0.2 mbar. Compared to the phosphorescence spectrum of the matrix 52-53 isolated (neutral) HPLN ligand, the gas-phase spectra of + + -1 [Gd(PLN)2] and [Lu(PLN)2] are blue shifted: by ~877 cm -1 -1 for Gd(III) at 18102 cm and by ~996 cm for Lu(III) at 18221 -1 cm . It is worth noting that the gas-phase measurements do not suffer from inhomogeneous broadening in contrast to the matrix measurements. The phosphorescence origins of neutral HPLN molecules in a n-hexane Shpol'skii matrix are -1 -1 about 17225 cm and 17277 cm for HPLN, and about 60 -1 cm lower for DPLN – the ligand with a deuterium instead of -1 a hydrogen involved in the hydrogen-bond – at 17166 cm -1 and 17216 cm . Pairs of values are provided for matrixisolated measurements due to the coexistence of two mo54 lecular environments. +

Upon shifting the vibronic spectra of the [Gd(PLN)2] and + [Lu(PLN)2] complexes such that their likely 0-0 origins are superimposed (Figure S1), the following vibrational bands could be resolved for the Gd(III) complex: 209(w), 462(s), 542(s), 652(vw), 746(w), 975(w), 1125(w), 1249(w), 1365(w), -1 and 1585(w) cm , while those resolved for the Lu(III) complex are about 127(s), 203(w), 476(s), 556(s), 677(w), 756(vw), -1 1251(w), 1398(w), and 1598(vw) cm . The discrepancy in the number of bands observed is due to differences in the signal to noise ratio of Gd(III) and Lu(III) measurements and the associated difficulty to determine band positions. For comparable ion numbers, the Lu(III) complex luminescence is about five times weaker than for Gd(III) under the excitation + condition used. The vibrational frequencies of [Lu(PLN)2]


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are slightly blue shifted compared to [Gd(PLN)2] . The vibrational bands of both complexes are also found to overlap reasonably well with some of the vibrational lines observed in the phosphorescence spectrum of the deuterated free ligand (DPLN) measured in an n-hexane Shpol'skii matrix and -1 located at 422, 545, 659, 712, 969, 1240, and 1359 cm , respec52-56 tively. It is therefore suggested that the first major vi+ + bronic band in the [Gd(PLN)2] and [Lu(PLN)2] spectra, -1 shifted by approximately 600 cm compared to the 0–0 transition, corresponds primarily to intraligand vibrations (see the computational analysis below). On the other hand, the + + [Gd(PLN)2] and [Lu(PLN)2] complexes display three vibra-1 tional bands past 1000 cm while HPLN displays none and DPLN only two. Conceivably, these bands, particularly in the -1 1585-1598 cm range, could affect sensitization as they likely correspond to C-0 or C-C vibrations proximal to the coordi50, 57-58 nated lanthanoid.

Gas-phase structures +

Our calculations indicate that the [Ln(PLN)2] complexes with Ln = Gd (III) and Lu (III) have tetragonal disphenoid, + , (see Figure 4) symmetry. The [Ln(PLN)2] structure with coplanar PLN ligands has been found to correspond to a transition state for Gd(III) and Lu(III) ions.

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perimentally found to be ~120 cm . Table 1 shows the comparison between experimental and computed ΔΔELu-Gd. All methods with the exception of HF show qualitatively the + energy of 0-0 transition being higher in [Lu(PLN)2] complex + than in [Gd(PLN)2] complex. The MP2 method shows particularly poor agreement with experiment arguably due to poor description of the T1 state. The best agreement is obtained with the CCSD and CCSD(T) methods. The computed Franck-Condon phosphorescence spectra also agree remarkably well with experimental results, thereby enabling the deconvolution of the experimental spectrum. As can be seen from the time-independent computations (Figure 5) the main contributions to the spectra come from vibrations belonging to the " and irreducible representations. The character and nature of the most intense vibronic transitions displayed in the Franck-Condon spectra of Figure + 5 are analyzed in Table 2. The spectra of [Gd(PLN)2] and + [Lu(PLN)2] are very similar.

Table 1. Comparison of experimental and computed ligand-centered #$ → %& 0-0 transitions, ΔELu, ΔEGd, and their differences, ΔΔELu-Gd (see text). ΔELu -1

+

Figure 4. Gas-phase structures of the [Gd(PLN)2] complex computed at the B3LYP/def2-TZVPP level of theory illustrating its symmetry.

Analysis of electronic interactions The phenomenological differences between the phosphorescence of free and lanthanoid-complexing ligands, in particular the phosphorescence enhancement in the Lu(III) and Gd(III) complexes, as well as the difference between + + [Gd(PLN)2] and [Lu(PLN)2] result from a number of (related) effects. Chief among these are differences in the spinorbit interaction, differences in the zero-field splitting of the triplet state, configuration mixing (4 to 4 ! promotion energy), exchange interaction between the Gd(III) ion and the ligands (delocalization of the triplet state) as well as non-adiabatic effects related to the structure. Also contributing are the (vibronic-)spin-orbit interactions as well as the interactions with paramagnetic species – which together 16, 59 comprise the generic heavy atom effect. The investigation of these ligand-centered excited states involves using “large-core” relativistic effective core potentials (RECP) including the 4f electrons to describe the Ln(III) ions. The absolute position of the 0-0 transition for the respective complexes is difficult to compute as illustrated in Table 1. All methods either underestimate the transition en-1 ergy by ~2000 cm or overestimate it by a similar amount with the exception of MP2 which yields values too high by a factor 2. This discrepancy between theory and experiment remains unexplained at this point. Nevertheless, computations at the CCSD and CCSD(T) levels agree tentatively with the trend observed experimentally. As was already mentioned, the difference between the 0-0 + + transitions of [Gd(PLN)2] and [Lu(PLN)2] complexes is ex-

ΔEGd

ΔΔELu-Gd

-1

-1

[cm ]

[cm ]

[cm ]

Experiment

18221

18102

+120

BP

16116

15880

+236

B3LYP

20961

20657

+304

HF

15991

16053

–62

MP2

37543

36343

+1200

CCSD

20638

20532

+105

CCSD(T)

20851

20712

+138

The computations were performed at the B3LYP geometry, the def2-TZVPP basis set was used in all cases. Harmonic zero-point vibrational energy corrections at the B3LYP/def2-1 TZVPP level of theory are included and contribute –147 cm , -1 -1 –135 cm , and –12 cm for ΔELu, ΔEGd, and ΔΔELu-Gd, respectively. Note that for Gd the large-core RECP of Ref. 34 was used rather than the corrected RECP of Ref. 35.

Table 2. [Ln(PLN)2]+, with Ln = Gd and Lu, most intense vibronic transitions (0 K time-independent computation) and their irreducible representations (IR). cm

-1

IR

Type of vibration

Gd

Lu

18*

21**

e

Rocking and wagging of PLN-Ln bonds

36*

41**

e

Rocking and wagging of PLN-Ln bonds

128

128

a1

Symmetrical bonds

476

479

a1

Intraligand planar bending near C=O

568

568

a1

Intraligand planar bending of C-C bonds

775

778

a1

Intraligand stretching of C-C bonds

1294

1296

a1

In plane rocking of C=O and C-H bonds

1656

1656

a1

Intraligand collective C-C stretching


stretching

of

PLN-Ln

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The Journal of Physical Chemistry * Overtone of vibration at 9 cm

-1

** Overtone of vibration at 10 cm

-1

For the systems studied here, the ligand-centered triplet + + state of the [Gd(PLN)2] and [Lu(PLN)2] complexes is at -1 least 18102 cm above the ground state. For a suitable lanthanoid ion, efficient sensitization would therefore involve resonant energy transfer from associated vibronic states populated on the time scale of the transition. The analysis of the phosphorescence spectra by simulation of the FranckCondon envelopes and the identification of the vibrational modes involved helps to identify, for a given coordination complex geometry, the vibrational levels whose population is likely to contribute to the sensitization of complexed lanthanoid ions.

yields a surprisingly good agreement. The most intense vibronic bands observed in the experiment are well reproduced by the time-dependent computations when assuming a 50 -1 cm linewidth. Correspondingly the contributing vibrational modes can be assigned. We find that the lowest vibrational modes are floppy in nature involving rocking and wagging of the ligands compared to one another. The population of the-1 se low energy modes, computed to be on the order of 10 cm in the harmonic approximation, is most likely responsible for the broadening of the Gd(III) complex spectrum compared to the Lu(III) one as overtones of these modes are populated in both cases but are slightly higher in energy for Lu(III). Most interesting are the modes leading to a significant distortion of the carbonyl groups. These affect the coordination geometry of the ligands with respect to the lanthanoids. We have identified four such modes directly related to major bands in the Franck-Condon spectra of both Gd(III) and Lu(III) and involved in a distortion of the coordination ge-1 ometry. The lowest (about 128 cm ) involves a significant distortion of the coordinating bonds while the distortion for -1 the three subsequent modes (between 128 and 1000 cm ) is -1 less pronounced. The modes above 1000 cm are almost strictly intraligand vibrations (without any distortion of the lanthanoid coordination sphere). +

Electronically, the [Lu(PLN)2] 0-0 transition is about 120 -1 cm higher in energy than the approximately five times stronger Gd(III) transition. Only the CCSD and CCSD(T) computational approaches reproduce this shift. Whether this is due to error cancellation or accounting for electron correlation effects is currently unknown. However, the magnitude of the shift indicates that computational studies of the dynamics of resonant energy transfer between the ligand centered vibronic states and lanthanoid-centered acceptor states should include an accurate description of the contributing vibronic manifolds. Relying solely on the phosphorescence spectra of species where the lanthanoid emitter (e.g. Eu(III)) has been substituted by Gd(III) or Lu(III) is not sufficient as -1 a difference of 100 cm can lead to dramatically different populations of states at cryogenic temperatures.

CONCLUSION

Figure 5. Comparison of experimental (83 K) and computed at 0 K (time-independent sticks plot) and at 83 K (time-1 dependent envelope spectra; 50 cm linewidth) phosphores+ + cence spectra for a) the [Gd(PLN)2] and b) the [Lu(PLN)2] complexes. The time-independent computations at 0 K suggest the 0-0 transition to belong to the " irreducible representation for both complexes. The equilibrium geometries and the corresponding vibrational frequencies were obtained at the B3LYP/def2-TZVPP level. The computed spectra are shifted to match the 0-0 transitions of the respective experiments. Comparison of the Franck-Condon computations performed for the only structural energy minimum found for + + [Gd(PLN)2] and [Lu(PLN)2] with the experimental spectra

In this work, we compare vibrationally resolved gas-phase phosphorescence spectra of mass-selected Gd(III) and Lu(III) coordination complexes. These emit from their ligand-based T1 states following UV excitation. We find that the + -1 [Lu(PLN)2] 0-0 transition is about 120 cm higher in energy than for Gd(III). We observe a significant temperature dependence in the emission spectra which, at liquid nitrogen temperature, are nicely reproduced by our Franck-Condon computations. An analysis of the character and nature of the vibrational modes involved in the most intense transitions is provided. The approach sheds light on the manifold of states populated upon gas-phase photoexcitation and emission in these systems. We also expected our results to contribute to the design of more efficient lanthanoid⋯antenna emitters via a better understanding of the role played by the environment or the absence thereof. While large RECP’s considerably speed up computations and reproduce the ligand-centered character of the “triplet” state reached upon photoexcitation, a marked discrepancy is observed for absolute transition energies. Higher CCSD and CCSD(T) levels of computation



reproduce the relative behavior, but predict T → S 0-0 transition energies ca. 0.25 eV larger than seen in experiment.

ASSOCIATED CONTENT Supporting Information Figure comparing the gas-phase ligand-centered phospho+ + rescence spectra of the [Gd(PLN)2] and [Lu(PLN)2] complexes. XYZ of the computed structures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]

Author Contributions ‡These authors contributed equally.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work has been supported by the Deutsche Forschungsgemeinschaft (DFG) through CRC/TRR 88 „Cooperative Effects in Homo- and Hetero-Metallic Complexes (3MET)“ (Projects C1 and C7). We also acknowledge the Bundesministerium für Bildung und Forschung (BMBF) through the Helmholtz Research Program POF Science and Technology of Nano-systems for support and for providing the necessary infrastructure.

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Vibronic Coupling Analysis of the Ligand-Centered Phosphorescence

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