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. 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 (103 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 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 cm1 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 Cl to I. 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 X are so stable and contracted that their over- bands which are split by ca. 2000 cm1 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 Cl, in the blue spectral region but can be shifted to much longer Br and I, 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 Cl to Br to I. In the case of Cl, 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 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

Qualitative MO scheme for CeIII -halide complexes 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 III for CeI3 a considerable 4f(Ce) ! 5d(I ) MLCT contribu- explored this possibility and selected the compound Ce 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-COO 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. MLCT states occur at low absorption in the near UV region [9,10]. The free acid energies if a ligand with empty low-energy orbitals is coor- pyz-COOH or its deprotonated anion pyz-COO is also dinated to an electron-rich metal center. The overwhelming colourless, because its absorption appears below 400 nm number of observations have been made on d ! p* MLCT [31]. Upon coordination these intraligand bands remain states of polypyridyl (or 1,2-diimine) complexes with elec- in the UV region, as indicated by the observation that II tron-donating transition metals such as Ru(II) [18,19], Zn (pyz-COO)2 is a white compound which does not show Re(I) [19–22] and Cu(I) [19,23,24]. However, the occurence any absorption in the visible region [33]. In distinction to III of MLCT states is not restricted to transition metals. this zinc complex, Ce (pyz-COO)3 is a yellow substance. MLCT bands have also been observed in the electronic This colour is caused by the longest-wavelength absorption spectra of complexes that contain reducing main group of the complex at kmax = 388 nm, which extends into the metals, including Sn(II), Sb(III) and Bi(III) [25]. In con- visible spectral region (Fig. 2). We suggest that this band trast to these transition and main group metal compounds, belongs to a MLCT transition from the Ce(III) 4f orbitals MLCT states of f-group metal complexes have apparently to the p* orbitals of the pyz-COO ligand. This assignment not yet been identified. The reason for this lack is not quite is consistent with the reducing character of Ce(III) and clear, but may be related to the fact that complexes of lan- electron-accepting nature of the pyz-COO ligand. CeIII thanides or actinides are only of limited stability. This (pyz-COO)3 shows a weak blue-green luminescence applies in particular to complexes with neutral ligands such (Fig. 2)atkmax = 470 nm [31]. It is assumed to originate as 2,20-bipyridine (bipy) or 9,10-phenanthroline. While from its MLCT state. Owing to the f1 electron configura- such lanthanide complexes are known [26,27], the affinity tion, absorption and emission are both spin-allowed pro- of Ln3+ for these ligands seems to be rather small. The elec- cesses. The small Stokes shift of DE = 4497 cm1 reflects tronic spectra provide evidence for this notion. Generally, the fact that the f-electron that takes part in the MLCT the longest-wavelength band of the bipy ligand undergoes transition is hardly involved in any bonding interaction. a distinct red shift upon complex formation [28,29]. How- ever, in the case of Ce(III) bipy complexes such a shift has not been observed [30], indicating a rather weak elec-

Ce III

O O Fig. 2. Electronic absorption (a) and emission (e) spectrum of 1.31 · 103 III 3 Ce (pyz-COO)3 Æ 1.5H2OinCH3CN/DMF = 100/1 at room temperature. Absorption: 1 cm cell (---) and 0.01 cm cell (––). Emission:

Structure 1. kexc = 360 nm, intensity in arbitrary units. A. Vogler, H. Kunkely / Inorganica Chimica Acta 359 (2006) 4130–4138 4133

3þ The excited state behavior of lanthanide(III) compounds The system Eu =N3 in water has been selected for var- is not only determined by photophysical processes. Some ious reasons. Since N3 has a rather low optical electroneg- 3+ 4+ 3+ Ln ions which can be oxidized to Ln have been shown ativity (2.8) [32],N3 to Eu LMCT absorptions should to undergo this oxidation also by photolysis [34]. For occur at relatively long wavelengths. Secondary photooxi- example, Ce3+ in an acidic aqueous solution is photooxidation of Eu2+ which requires short-wavelength irradiation dized according to the equation [35] could thus be diminished or avoided. Even more impor- 3þ IV tant, azide is well known to undergo a rapid irreversible Ce þ Hþ hm ! Ce þ 1H ð1Þ aq 2 2 oxidation to nitrogen. Recombination of the radical pair 2+ The nature of the reactive excited state is not quite clear. Eu /N3, which is generated by LMCT excitation of euro- Generally, photooxidations of metal complexes which pro- pium(III) azide complexes, is then less efficient. Finally, the ceed with a concomitant reduction of the solvent are initi- choice of water as solvent offers several advantages. In par- ated by CTTS (charge transfer to solvent) excitation but a ticular, any interference by organic radicals which are 3+ CTTS absorption of Ce in H2O has apparently not been formed in organic solvents is excluded. identified. However, as mentioned above the MC f ! d The LMCT band of Eu(III) bromide complexes appears 3+ transition of Ce can also be viewed as a special type of at 320 nm [36]. Since Br and N3 have the same optical III 3+ (Ce ! H2O) MLCT transition, since the 5d orbital of electronegativity (2.8) [32], the N3 to Eu LMCT absorp- Ce(III) is very large and diffuse and extends thus to the tion is expected to appear at nearly the same wavelength. water molecules in the first and possibly second coordina- Indeed, upon addition of azide to an aqueous solution of tion sphere of the cerium ion [16]. Nevertheless, the molec- EuCl3 a new band shows up (Fig. 3)atkmax = 324 nm 3+ ular process of the photolysis of Ce in water remains (e = 43.5) [39]. Upon irradiation (kirr > 300 nm) of an aque- obscure. ous solution containing 0.01 M EuCl3 Æ 6H2O and 0.05 M 3+ The photooxidation of Ce also takes place in the pres- NaN3, photolysis takes place as indicated by an increase 2+ 2 2+ ence of oxidants such as Cu ,S2O8 and Eu [34].In of the absorption above 260 nm. Simultaneously, N2 bub- this case, an excited state electron transfer involving the bles evolve. The photolysis of the Eu3+ azide complex pro- fd state of Ce(III) seems to be in operation. ceeds according to the simple equation Photooxidations of Ln(III) to Ln(IV) have also been Eu3þN ! Eu2þ þ 1:5N ð2Þ observed for Tb3+ and Pr3+ [34]. In contrast to Ce(III), 3 2 the lowest excited states of Tb(III) and Pr(III) are ff states The photochemical formation of Eu2+ was confirmed by which are apparently also able to participate in electron luminescence spectroscopy. While aqueous Eu2+ is not transfer processes. emissive, it forms a fairly stable complex with the crown ether 15-crown-5 which shows an intense luminescence at 2.2. LMCT states kmax = 432 nm [40]. Indeed, when 15-crown-5 in methanol is added to the photolyzed solution, this emission is nicely 3+ Complexes of the cations Ln , which can be reduced reproduced (Fig. 3). At kirr = 333 nm, the quantum yield of to Ln2+, are expected to display LMCT absorptions at the Eu2+ formation is / =7· 104. When the photolysis of 7 14 3+ relatively low energies. Owing to their f and f configu- the Eu azide complex is performed in the presence of O2, ration, Eu(II) and Yb(II) compounds, respectively, are spectral changes are not observed since the Eu3+ azide quite stable. Moreover, the oxidation state II is also complexes are completely regenerated. accessible for Nd, Tm, and in particular for Sm. Indeed, LMCT absorptions have been detected for complexes of the corresponding Ln3+ ions [36–38]. However, the LMCT states and MC ff states have comparable energies. With less reducing ligands, the ff states may occur below the LMCT states. The LMCT states of Ln(III) compounds are apparently not luminescent, but are frequently reactive [34]. Generally, LMCT excitation of metal complexes leads to the reduction of the metal ions and the concomitant oxidation of ligands. In agreement with this notion, the photoreduction of Eu(III) and Sm(III) to Eu(II) and Sm(II) has been reported but a clear relationship to LMCT excitation has been rarely established [34]. Moreover, oxidation products are often unknown. In addition, an accumulation of Ln(II) does usually not take place since it undergoes an efficient Fig. 3. Absorption spectrum (a) of an aqueous solution of 0.02 M EuCl3 Æ 6H2O and 0.1 M NaN3. Emission spectrum (e) of this solution reoxidation. With these shortcomings in mind, we have after irradiation and addition of a solution of 15-crown-5 in methanol 3+ recently studied Eu in the presence of azide in aqueous (kexc = 320 nm, 1-cm cell, emission intensity in arbitrary units). All solution [39]. solutions were saturated with argon. 4134 A. Vogler, H. Kunkely / Inorganica Chimica Acta 359 (2006) 4130–4138

In the absence of oxygen, Eu2+ ions can be photochem- but is of considerable interest. It has been known for a long ically oxidized by water [34,41,42]. This process prevents time that lanthanides can mediate their heavy-atom effect the efficient accumulation of Eu2+ as a final product of to ligands by inducing increased spin–orbit coupling. As the Eu(III) photoreduction. However, in the region of the a consequence, the fluorescence of the ligand is quenched LMCT band of Eu(III) azide complexes the extinction since intersystem crossing becomes faster. For the same coefficient of Eu2+ is sufficiently small (e < 500) to restrict reason, the radiative lifetime of the IL triplet decreases this interference. In contrast, previous studies have been and the phosphorescence quantum yield grows. However, carried out with EuCl3 or Eu(ClO4)3 in aqueous or alco- previous studies were essentially limited to measurements holic solution [34,43–45]. In these cases, LMCT excitation at low temperatures, but we have recently shown that this requires much shorter-wavelength irradiation. Since in this IL phosphorescence appears also at r.t. [52,53]. This obser- spectral region Eu2+ is strongly absorbing and efficiently vation is important for potential applications. In order to photooxidized (e.g. at 250 nm: e = 1778 and / = 0.2) [41], observe an IL emission, other excited states of different ori- the secondary photolysis severely interferes with the pri- gin (e.g. MC, CT) must be absent or at least located at mary photoreduction of Eu3+. Moreover, while the azide energies well above the emitting IL state. An excellent can- radicals as primary photooxidation products undergo a didate for this purpose is Gd3+. Owing to the very high sta- rapid irreversible decay, chloride atoms which are gener- bility of its half-filled f shell (f7), ff transitions occur at very 2+ 4+ ated by LMCT excitation of EuCl3 are much more stable high energies. Since neither Gd nor Gd is accessible, and accordingly favor a recombination (EuCl2 +Cl! MLCT or LMCT excited states are also not available. 3+ EuCl3). Even if a cage escape of chlorine atoms should Finally, the paramagnetism of Gd with 7 unpaired elec- be successful and the formation of Cl2 takes place, a subse- trons also facilitates intersystem crossing in ligands. 2+ quent reoxidation of Eu by Cl2 would certainly occur. In this context, we have recently examined the emission All these unfavorable conditions prevent the accumulation behavior of the gadolinium(III) chelates Gd(dtpaH2), 2+ of Eu as a permanent photoproduct of EuCl3. These Gd(hfac)3, Gd(tta)3 and Gd(qu)3 with dtpa = 1,1,4,7,7- observations may also be of importance for the photore- diethylene-triaminepentaacetate, hfac = hexafluoroacetyl- duction of other Ln3+ ions such as Sm3+ [46]. acetonate, tta = thenoyltrifluoroacetonate and qu = 8-quinolinolate (or oxinate) (Structure 2) under ambient 2.3. MMCT states conditions [52]. The dtpaH2 ligand does not provide IL excited states at Since Ln(III) can serve as electron donor (see MLCT) or low energies owing to the absence of a conjugated p-elec- electron acceptor (see LMCT), it can be anticipated that tron system. Accordingly, the IL absorption of MMCT transitions in suitable mixed metal systems will Gd(dtpaH2) appears only at rather short-wavelength occur. Indeed, Ce3+ and Tb3+ in combination with oxidiz- (k < 265 nm). The metal-centered ff absorptions occur at ing d0 metals such as Ti(IV), V(V) and Ta(V) show longer wavelength. In particular, the structured absorption 3+ 0 (Ln ! d ) MMCT absorptions [6,47,48]. On the other at kmax = 273 nm is rather characteristic. It is thus not sur- III 3+ II 4 hand, the ion pair [Eu (2.2.1.cryptand)] [M (CN)6] prising that Gd(dtpaH2) does not exhibit an IL lumines- with M = Fe, Ru and Os is characterized by MMCT tran- cence, but rather exhibits the typical narrow UV emission sitions from the reducing cyanide complexes to the oxidiz- at kmax = 312 nm which belongs to the spin-forbidden 3+ 6 8 3+ ing Ln cation [49,50]. P7/2 ! S7/2 ff transition of the Gd ion. Since it is asso- Very little is known about the excited state processes ciated with a multiplicity change of two, it is equivalent from such MMCT states. They are apparently not emissive with a phosphorescence (or triplet emission) of diamagnetic [6,47]. On the contrary, they quench the emission of other compounds. states which are located at higher energies. At this point, it should be mentioned that lanthanides also form deeply coloured mixed-valence compounds O 3+ 4+ which contain the combination of either Ln /Ln or O H2C O 2+ 3+ Ln /Ln [47]. However, the extent of mixed-valence H2 H2 H2 O C N C C N interaction is largely unknown in these cases. H2C OH dtpaH2 2.4. IL states O 2

F3C Intraligand transitions play an important role in photo- O chemistry and photophysics of Ln(III) complexes. In par- F3C N O ticular, the antenna effect has been studied in great detail O S O [38,51]. This effect refers to the ability of a ligand to absorb O light and to transfer subsequently the excitation energy to F3C qu the Ln3+ ion, which finally emits from ff states. We will dis- hfac tta cuss here another aspect of IL states which is less known Structure 2. A. Vogler, H. Kunkely / Inorganica Chimica Acta 359 (2006) 4130–4138 4135

The other chelates used in this work have their IL states GdCp3 shows an emission from this type of interligand 6 available at energies well below the P7/2 ff state. The lon- excited state (Fig. 4) [53], which is bonding with respect 3 gest-wavelength absorptions of Gd(hfac)3, Gd(tta)3 and to the interaction within the Cp3 moiety. However, Gd(qu)3 at kmax = 302, 337 and 368 nm, respectively, are fluorescence does not occur while a strong green phospho- assigned to the lowest-energy spin-allowed IL transition rescence (kmax 500 nm) appears even in solution at r.t. of the hfac, tta and qu ligands. Spin forbidden IL absorp- (/ = 0.2 in ether). tions are not observed since they are apparently too weak. The interligand triplet of solid GdCp3 decays with As previously shown, Gd(hfac)3 [54], Gd(tta)3 [54] and s = 2.3 ls. Of course, other LnCp3 complexes with lantha- Gd(qu)3 [55] show an IL phosphorescence at low tempera- nides which have available low-energy ff states (e.g. TbCp3 tures. Gd(qu)3 displays an additional fluorescence at and YbCp3) do not show interligand emissions. In this shorter wavelength. However, singlet/triplet mixing in case, the luminescence originates from MC ff states [61–63]. Gd(III) chelates is so strong that the IL phosphorescence Recently, we have observed another type of heavy-atom of these chelates does not only appear at low temperatures effect. In this case, it is not transmitted to an organic ligand but also appears under ambient conditions [52] in analogy but to another metal complex as a whole [64]. With regard to the phosphorescence of many other complexes of heavy to transition metals, the heavy-atom effect is largely metals [56]. restricted to the second and third transition series [56]. The greenish blue phosphorescence of Gd(hfac)3 at Accordingly, the phosphorescence of first-row transition kmax = 470 nm and the green phosphorescence of Gd(tta)3 metal complexes has frequently rather long lifetimes. The at kmax = 510 nm in fluid acetonitrile solution are very emission may be then quite strong at low temperatures weak. In contrast, these emissions are quite strong if the but is absent or only very weak at r.t. However, it is con- compounds are incorporated in a rigid matrix. Generally, ceivable that a heavy-atom effect can be induced in first- Gd(III) complexes have variable coordination numbers, row transition metal complexes by the introduction of a mainly six and nine. Accordingly, the chelate structures suitable second metal. We explored this possibility and III III are certainly rather flexible. This flexibility may provide a selected the compounds Gd [M (CN)6] with M = Cr channel for radiationless deactivation, which is restricted and Co for a recent study [64]. This choice was guided by in a rigid matrix. Besides, flexibility of the diketonate the following considerations. The salts K3Cr(CN)6 [65] ligand itself might play a role. and K3Co(CN)6 [66,67] show an intense long-lived LF In analogy to various other oxinates of heavy metals (ligand-field) phosphorescence but only at low tempera- [57], Gd(qu)3 shows fluorescence at kmax = 510 nm and tures. It follows that a strong LF phosphorescence may phosphorescence at r.t. The red phosphorescence which appear at r.t. if gadolinium transmits its heavy-atom effect appears as a shoulder at 650 nm is quenched by oxygen. to Cr(III) and Co(III). This expectation is based on the 3 The appearance of a r.t. phosphorescence in solution which observation that Gd(III) coordinates to [Co(CN)6] [68] 3 can be quenched by O2 is a typical feature of heavy metal and [Cr(CN)6] [69–72] via the nitrogen atoms. Moreover, oxinates [57]. it has been shown that the cyanide bridges mediate an elec- The organometallic compound GdCp3 (Structure 3) tronic interaction between both metals as indicated by the with Cp = cyclopentadienyl is another interesting example magnetic coupling in complexes which contain, for exam- of the heavy-atom effect [53]. Generally, the lanthanides ple, the CrIII–CN–GdIII moiety [69–72]. It follows that form LnCp3 complexes which in solution exist as discrete LnCp3 molecules or as solvates of the type LnCp3L with L = solvent [58]. The structure of LnCp3 consists of a reg- ular triangle with the centers of the g5-coordinated Cp planes at the corners and the metal in the middle (D3h sym- metry). The LnCp3L complexes have a pseudotetrahedral structure with a trigonal-pyramidal LnCp3 fragment (C3v). 3 Since the bonding in LnCp3 is largely ionic, the Cp3 ligand frame can be treated separately [59,60]. The interligand interaction in LnCp3 leads to a HOMO/LUMO separation, which is smaller than that of a single Cp ligand.

Gd Gd

Fig. 4. Electronic emission (a) and excitation (b) spectrum of GdCp3 in dry diethylether at room temperature, kexc = 250 nm and kem = 500 nm, Structure 3. intensity in arbitrary units. 4136 A. Vogler, H. Kunkely / Inorganica Chimica Acta 359 (2006) 4130–4138

respectively. However, Ln4+ ions are oxidizing. Accord- ingly, complexes with those metals will display low-energy LMCT absorptions. Studies of excited state properties had been essentially restricted to Ce(IV) complexes [34]. Since the f shell is empty, any interference by MC transitions is excluded. Numerous Cer(IV) compounds are coloured because their LMCT bands appear in the visible spectral region. Such LMCT excited states are apparently not emissive but reactive [34]. Generally, Ce(IV) complexes undergo a photoreduction to Ce(III) and a concomitant oxidation of ligands or the solvent. Recently, we have studied the Fig. 5. Electronic excitation (kmax = 626 nm) and emission (kexc = excited state behavior of CeIV(tmhd) (Structure 4) with 300 nm) spectrum of solid Gd[Co(CN) ] at r.t., intensity in arbitrary units. 4 6 tmhd = 2,2,6,6-tetramethyl-3,5-heptanedionato and made some surprising observations [74]. the compounds Gd[M(CN)6] with M = Cr and Co are In agreement with a previous report, the broad band of quite promising candidates for the appearance of an Ce(tmhd)4 at kmax = 372 nm (Fig. 6) is assigned to a tmhd intense LF phosphorescence at r.t. to Ce(IV) LMCT transition. The shorter-wavelength 3 The electronic spectrum of [Co(CN)6] is characterized absorption at kmax = 276 nm is attributed to a tmhd IL by long-wavelength absorptions at kmax = 313 (e = transition. The photolysis of Ce(tmhd)4 proceeds according 243 M1 cm1) and 260 (180) nm [73], which belong to to the equation: 1 1 1 the spin-allowed LF transitions A1g ! T1g and T2g, IV hm=LMCT III respectively [32,73]. The lowest-energy spin-forbidden LF Ce ðtmhdÞ4 ! Ce ðtmhdÞ3 þ oxidized tmhd ð3Þ 1 3 transition A1g ! T1g is only observed in absorption as While the photoredox behavior of Ce(tmhd)4 is not unu- a shoulder at 385 nm for solid K3[Co(CN)6]at15K[67]. 3 sual, the luminescence of this complex (Fig. 6) is quite sur- However, the T1g state is emissive but also only at low prising. First of all, in addition to the UO 2þ ion [56], temperatures (T <77K) [66,67]. The phosphorescence of 2 K3[Co(CN)6] appears at kmax = 694 nm with a lifetime of s = 0.65 ms at 77 K. This lifetime is quite large, since H (H3C)3C C C(CH3)3 cobalt as a member of the first transition series does not C C exert a strong heavy-atom effect. In solution or low-tem- 3 (H3C)3C OO C(CH3)3 perature glasses, [Co(CN)6] is not luminescent owing to a facile photosubstitution which is induced by LF excita- C O O C tion. In contrast to the potassium salt, the solid compound HC CeIV CH Gd[Co(CN)6] exhibits a strong LF phosphorescence C O O C (k = 626 nm) at r.t. (Fig. 5) [64]. O O max (H3C)3C C(CH3)3 Compared to the potassium salt (kmax = 694 nm), it C C undergoes a considerable blue shift. This is a clear indica- (H3C)3C C C(CH3)3 H tion that Gd3+ does not only act as a counterion, but is also directly coordinated to cyanide via nitrogen. The forma- Structure 4. tion of a cyanide bridge between Gd3+ and Co3+ is certainly also required for an electronic interaction of both metals. As a consequence, Gd(III) exerts a strong heavy- atom effect at Co(III) which in turn leads to the appearance of an intense LF phosphorescence at r.t. In agreement with III this assumption, the LF triplet of Co in Gd[Co(CN)6] undergoes a relatively fast decay (s = 7.5 · 107 s). This type of heavy-atom effect applies also to Gd[Cr(CN)6]. 3 The complex [Cr(CN)6] shows a weak phosphorescece at kmax = 820 nm, which originates from the lowest-energy LF doublet. For Gd[Cr(CN)6], this LF phosphorescence is quite intense even at r.t. [64].

3. Ln(IV)

Fig. 6. Electronic absorption (a) and emission (e) spectrum of The oxidation state IV is relatively stable for cerium and 3.34 · 105 MCeIV(tmhd) in CH CN at r.t., 1-cm cell. Emission: 0 7 4 3 terbium owing to the electron conﬁguration f and f , kexc = 370 nm, intensity in arbitrary units. A. Vogler, H. Kunkely / Inorganica Chimica Acta 359 (2006) 4130–4138 4137

Ce(tmhd)4 seems to be only the second example of an emit- At this point, an analogy between the emission of 0 IV III 3+ ting f complex. Even more intriguing is the fact that the Ce (tmhd)4 and the chemiluminescence of [Ru (bipy)3] luminescence does apparently not originate from the low- should be emphasized. The luminescence of Ce(tmhd)4 can est-energy LMCT state since the emission and the lon- be viewed as an emission of Ce(III) from a MC fd excited gest-wavelength LMCT absorption appear at similar state, which is generated by electron transfer from a tmhd energies (Fig. 6). We suggest that this emission involves a ligand to Ce(IV). This behavior corresponds to that of 3+ radiative transition from a higher-energy LMCT state to [Ru(bipy)3] . Electron transfer from strong reductants 2+ the lowest-energy LMCT state. Ce(IV) has available two yields [Ru(bipy)3] not only in the ground state but also different acceptor orbitals: the 5d orbitals at higher energy in the emissive MLCT excited state [18,19]. and the 4f orbitals at lower energies. Accordingly, the longest-wavelength band of Ce(tmhd)4 at kmax = 372 nm is 4. Ln(II) attributed to the (ligand ! 4f CeIV) LMCT transition. The shorter-wavelength (ligand ! 5d CeIV) LMCT absorp- In agreement with the stability of the f7 and f14 configu- tion has not been identified but may be obscured by the in- ration, Eu(II) and Yb(II) compounds are accessible and tense IL band at kmax = 276 nm. However, irrespective of well characterized. In addition, Sm(II) plays an important the nature of the shorter wavelength bands, higher-energy role in the chemistry of lanthanides. All three M2+ ions excitation can lead to the population of the (ligand ! 5d are strongly reducing. Accordingly, the corresponding CeIV) LMCT state. Its radiative transition to the Ln(II) complexes are not expected to display LMCT (ligand ! CeIV 4f) LMCT state is then nothing else but absorptions. However, with suitable ligands MLCT transi- an emission from a fd excited state of Ce(III). Our sugges- tions should occur at low energies, but have not yet been III tion is supported by the observation that [Ce (tmhd)4] identified. The reason for this is not quite clear but may III IV and Ce (acac)3 emit at similar energies as Ce (tmhd)4 be related to the lack of stability of such Ln(II) complexes. [74]. Usually, the emission of Ce(III) compounds is rela- So we are left with MC transitions. Indeed, compounds of tively intense and short-lived (ns-range) since it is a spin-al- Sm2+,Eu2+ and Yb2+ are characterized by low-energy MC lowed process. According to these considerations, the fd and ff states which are of comparable energies. It 2+ excited state behavior of Ce(tmhd)4 can now be summa- depends on the environment of Ln if the fd or ff states rized by the following qualitative energy diagram (Scheme are lowest [6]. 2). Ln(II) compounds are frequently emissive and the nat- The population of the LMCT (CeIII d1) state may be ure of the emitting state can be recognized by the structure facilitated by its energetic proximity to the IL state and of the spectrum. In particular, the line-type pattern of ff by the larger extension or the cerium 5d orbitals, which cer- transitions is rather characteristic while the emission from tainly provide a better electronic coupling with the ligands fd states gives broad bands which, however, can show than the well-shielded 4f orbitals. Since the emission is a vibrational features. The majority of Eu(II) compounds rather rapid process, it apparently competes successfully [6] including EuI2 [75] emits from fd states. In addition, with radiationless deactivation including photoredox pro- SmI2 and YbI2 also show luminescence from fd states even cesses, which are assumed to start from the lowest-energy in solution at r.t. [75]. In contrast, Sm2+ photochemically LMCT state. This conclusion is supported by the observa- generated in a variety of glasses displays a ff emission tion that the photolysis of Ce(tmhd)4 is also initiated by [76–78]. long-wavelength irradiation [74]. Owing to their reducing character, it is not surprising that Ln2+ ions are easily photooxidized. In particular, the photoreactivity of Eu(II) [34] and Sm(II) [79–81] compounds has been studied in some detail. In analogy IL state to Ce(III) (see above), the photooxidations of Ln(II) seem to originate from fd states since the fd transitions III 1 LMCT(Ce d ) terminate at d orbitals which are exposed to the ligands or the solvent. Water (or H+) has been shown to be a 2+ photooxidant for Eu [41,42]. Recently, SmI2 has been Emission applied as a very useful photoreductant in organic syn- Absorption thesis [79]. III 1 LMCT (Ce f ) 5. Summary

Generally, the discussion of excited state properties of lanthanide complexes is restricted to MC ﬀ states. In our photoredox reaction short account, other types of excited states are emphasized. ground state It is shown that the luminescence and photochemistry of Scheme 2. various lanthanide complexes in the oxidation states II, 4138 A. Vogler, H. Kunkely / Inorganica Chimica Acta 359 (2006) 4130–4138

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