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Excited State Properties of Lanthanide Complexes: Beyond Ff States

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Excited State Properties of Lanthanide Complexes: Beyond Ff States Inorganica Chimica Acta 359 (2006) 4130–4138 www.elsevier.com/locate/ica Excited state properties of lanthanide complexes: Beyond ff states Arnd Vogler *, Horst Kunkely Institut fu¨r Anorganische Chemie, Universita¨t Regensburg, D-93040 Regensburg, Germany Received 3 May 2006; accepted 27 May 2006 Available online 7 June 2006 Abstract Generally, metal-centered ff states dominate the discussion of the excited state properties of lanthanide complexes. In particular, the luminescence properties of Eu(III) and Tb(III) compounds have been studied in great detail for many decades. However, other types of excited states such as MC fd, MLCT, LMCT, MMCT and IL are also of interest. In this context, we have recently examined the excited state behavior of selected Ce(III), Ce(IV), Eu(II) and Gd(III) complexes which are luminescent and/or photoreactive. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Electronic spectra; Luminescence; Photochemistry; Lanthanides; Cerium; Europium; Gadolinium 1. Introduction coefficients and the radiative lifetimes of ff states are rather large (10À3 s). Owing to the small absorption coefficients Lanthanide (Ln) compounds play an important role in of Ln3+, the excitation can be facilitated by suitable ligands the field of luminescence spectroscopy. In the ground which absorb the light and subsequently transfer the exci- states, the electron configuration of lanthanide cations tation energy to the emissive Ln3+ ion. In addition, appro- extends from f0 to f14. All lanthanides form stable com- priate ligands may prevent radiationless deactivations. This pounds in the oxidation state III, representing the ground behavior is illustrated by various Eu3+ and Tb3+ com- state configuration f1 (Ce3+)tof14 (Lu3+). Moreover, the plexes, which emit an intense red and green luminescence, empty (f0:Ce4+), half-filled (f7:Eu2+,Gd3+,Tb4+) and respectively [7,8], e.g. the completely filled f shell (f14:Yb2+,Lu3+) are also stable EuIII(TTA) k = 612 nm, and are of special importance. The majority of spectro- 3 max acetone, r.t. scopic studies deals with Ln(III) compounds, which are TTA = thenoyl-trifluoro-acetonate / = 0.56, s = 565 ls characterized by electronic transitions within the 4f shell III [1–6]. Since the f electrons are largely shielded from the Tb (acac)3 kmax = 543 nm, environment, they behave as inner and not valence elec- ethanol, r.t. trons. Accordingly, the absorption and emission spectra acac = acetylacetonate / = 0.19, s = 820 ls consist very narrow bands. Transitions between f orbitals of Ln3+ are strictly parity forbidden. Moreover, many ff transitions are also spin- In the following sections, any further discussion of the ff forbidden although spin–orbit coupling attenuates the states is omitted. For more details, the reader is referred to forbiddenness. Nevertheless, both restrictions have impor- an extensive body of literature which is summarized in var- tant consequences. The bands have very low absorption ious books and reviews [1–6]. In our short report, we emphasize some other types of excited states: MC (metal- centered) fd, MLCT (metal-to-ligand charge transfer), * Corresponding author. Tel.: +49 941 943 4716. LMCT (ligand-to-metal charge transfer), MMCT (metal- E-mail address: [email protected] (A. Vogler). to-metal charge transfer) and IL (intraligand) states. How- 0020-1693/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2006.05.025 A. Vogler, H. Kunkely / Inorganica Chimica Acta 359 (2006) 4130–4138 4131 ever, these topics are not comprehensively covered. Our 390 nm for CeBr3, 464 and 514 nm for CeI3. For all three account is essentially restricted to recent observations in compounds, the separation of both emission maxima our laboratory. amounts to approximately 2000 cmÀ1 corresponding to 2 2 the energy difference of both f states ( F7/2 and F5/2). In 2. Ln(III) the excitation spectra, these transitions appear as longest wavelength bands at kmax = 294 and 312 nm for CeCl3, 2.1. MC fd and MLCT states 295 and 325 nm for CeBr3 (Fig. 1), 384 and 417 nm for CeI3. In addition to ff transitions, MC fd transitions are prin- What is the reason for the red shift of the excitation and cipally accessible for lanthanide ions. Generally, they occur emission in the series CeCl3, CeBr3 and CeI3? The cerium at energies which are much higher than those of ff transi- 5d orbitals must be modified by overlap with the valence tions. However, the spectroscopy of Ce(III) [6,9–15] is quite orbitals of the halide ligands. Generally, these are the nsnp different from that of other Ln(III) compounds. The lowest- orbitals which are filled. They are certainly located at much energy transition of Ce(III) involves the promotion of an lower energies than the empty Ce 5d orbital. It follows that electron from the 4f to the 5d orbitals. Since the ground this interaction should shift the fd transitions to higher 1 state and the excited states of this f ion are spin doublets, energies from CeCl3 to CeI3 since the s and p orbital ener- all transitions are spin-allowed. The corresponding absorp- gies increase from ClÀ to IÀ. On the contrary, the emission tions appear in the UV spectral region [9,10]. The emission shows the opposite behavior. We suggest that the valence from this metal-centered fd state consists of essentially two orbitals of XÀ are so stable and contracted that their over- bands which are split by ca. 2000 cmÀ1 owing to spin–orbit lap with the diffuse, high energy 5d orbitals of Ce3+ is neg- coupling. Generally, this emission occurs in the UV and/or ligible. However, the empty 3d, 4d and 5d orbitals of ClÀ, in the blue spectral region but can be shifted to much longer BrÀ and IÀ, respectively, are also located at quite high ener- wavelength depending on the environment of the Ce3+ ion gies and are well-suited for the overlap with the 5d metal [11–15]. Any reliable explanation for this shift is not avail- orbitals (Scheme 1). able, but it has been emphasized that it is a consequence The energy of the empty halide d-orbital should increase of the interaction with cerium 5d orbitals since the 4f orbi- from ClÀ to BrÀ to IÀ. In the case of ClÀ, the 3d-orbital tals are hardly affected by the environment. The d-orbital energy is apparently much higher than that of the Ce3+ splitting was attributed to crystal field effects. In addition, 5d orbital. Accordingly, the overlap is also moderate. For À covalency has been mentioned as a further influence. Unfor- CeI3, the 5d orbital energy of I may come close to that tunately, these notions have never been related to simple MO models. However, it has been pointed out that the metal-centered fd transition can be viewed also as a MLCT − Ce 3+ CeIII−X transition since the 5d orbitals are rather diffuse and extend X to the ligands of Ce(III) [16]. In this context, we have recently studied the electronic spectra of cerium(III) halides [17]. The emission spectra nd of solid anhydrous CeCl3, CeBr3 (Fig. 1) and CeI3 display a rather simple pattern. The emission is relatively intense also at r.t., but the bands are better resolved at 77 K. They 5d appear at kmax = 340 and 362 nm for CeCl3, 362 and A E 4f Qualitative MO scheme for CeIII -halide com- plexes including the lowest-energy transition in absorption (A) and emission (E) Fig. 1. Electronic excitation (kem = 350 nm) and emission (kexc = 290 nm) spectrum of solid CeBr3 under argon at 77 K, intensity in arbitrary units. Scheme 1. 4132 A. Vogler, H. Kunkely / Inorganica Chimica Acta 359 (2006) 4130–4138 of the 5d Ce orbital. The overlap now becomes much lar- tronic interaction between cerium and bipy. On the other ger. As a result, the lowest-energy empty MO of CeX3 must hand, anionic ligands form relatively stable complexes with shift to lower energies from X = Cl to Br and to I in agree- Ln3+, owing to the electrostatic attraction between metal ment with our observation. Simultaneously, this MO con- cations and ligand anions. Accordingly, a complex consist- tains an increasing nd halide contribution in this order. It ing of a reducing f-group metal cation and an electron- follows that in the case of CeCl3 the lowest-energy transi- accepting anionic ligand should be a promising candidate tion may still be considered to be largely metal-based, while for the observation of an optical MLCT transition. We À III for CeI3 a considerable 4f(Ce) ! 5d(I ) MLCT contribu- explored this possibility and selected the compound Ce À tion must be taken into account. On the basis of this model, (pyz-COO)3 (Structure 1) with pyz-COO = pyrazine-2- it is also concluded that in the ground state metal–halide carboxylate for a recent study [31]. interaction is ionic while in the fd/MLCT excited state This choice was based on the following considerations. metal–ligand bonding exists, but it should be rather weak Ce(III) is a one-electron donor of moderate reducing since it is caused by just one single electron. In this context, strength. Pyrazine has been shown to be a rather strong it is of interest to if low-energy MLCT states of Ce(III) acceptor for MLCT transitions [32]. As an electron-with- complexes with conventional CT accepting ligands can also drawing substituent, the carboxylate group of pyz-COOÀ be observed. should even enhance the acceptor strength of pyrazine. Metal-to-ligand charge transfer (MLCT) excited states Generally, simple Ce(III) compounds are colourless, play a very important role in the photophysics and photo- since the metal-centered f ! d transition gives rise to an chemistry of metal complexes.
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