Porphyrin lanthanide complexes for NIR emission

Coordination Chemistry Reviews 256 (2012) 1468–1478

Contents lists available at SciVerse ScienceDirect

Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr

Review

Porphyrin lanthanide complexes for NIR emission Véronique Bulach ∗ , Fabien Sguerra, Mir Wais Hosseini ∗ Laboratoire de Chimie de Coordination Organique, UMR CNRS 7140, Université de Strasbourg, F-67000 Strasbourg, France

Contents 1. 2.

3. 4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1468 Lanthanides “within” the porphyrin cavity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1469 2.1. Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1469 2.1.1. Acetylacetonate ligand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1469 2.1.2. Acetate ligand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1470 2.1.3. Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1471 2.1.4. Chloride or iodide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1471 2.1.5. Anionic tripodal ligand: TPB and LR Co . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1471 2.2. Photophysical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1472 Lanthanide coordinated by an external coordination site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1473 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1476 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1477 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1477

a r t i c l e

i n f o

Article history: Received 11 January 2012 Accepted 27 February 2012 Available online 14 March 2012 Keywords: Lanthanides Porphyrins NIR emission Energy transfer Coordination complexes

a b s t r a c t This review deals with the NIR emission of lanthanide cations sensitized by porphyrin derivatives acting as antenna. The critical literature survey revealed mainly two design strategies based on either the direct complexation of the Ln(III) cations by the tetraazacore of the porphyrin or by peripheral ligands covalently connected to the porphyrin backbone. For the reported examples, the design principles, structural analysis and photophysical studies are presented and discussed. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Near infrared luminescence (NIR) is of interest because of its potential application in biological imaging and in telecommunications [1–11]. Owing to their unique luminescent properties, in particular as NIR emitters, lanthanide cations such as Er, Nd and Yb

Abbreviations: TPP, meso-tetraphenylporphyrin; p-CH3 -TPP, p-OCH3 -TPP, meso-tetra(4meso-tetra(4-methylphenyl)porphyrin; p-F-TPP, meso-tetra(4-fluorophenyl)porphyrin; methoxyphenyl)porphyrin; meso-tetra(4-cyanophenyl)porphyrin; OEP, octaethylporp-CN-TPP, Por, porphyrin; TPB, hydrotris(pyrazole-1-yl)borate; LOMe Co, phyrin; cyclopentadienyltris(dimethylphosphito)cobaltate(I); LOEt Co, cyclopentadienyltris(diethylphosphito)cobaltate(I); DME, bis(methoxyethyl)ether. ∗ Corresponding authors. Tel.: +33 3 68 85 13 27; fax: +33 3 68 85 14 21. E-mail address: [email protected] (V. Bulach). 0010-8545/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.ccr.2012.02.027

complexes have been the subject of intense investigations over the last two decades [12–23]. All the Ln (III) ions, except La and Lu, are luminescent and cover a wide spectroscopic range. Indeed, Eu(III) (red), Tb(III) (green), Sm (orange), Dy (yellow) and Tm (blue) emit in the visible region. However, other spectroscopic regions are accessible by the emission of Ce and Gd in the near – UV and by Yb, Er, Ho and Nd in the near infrared [2]. Furthermore, lanthanide cations in the oxidation state 3 display sharp emission bands resulting from the shielding of the 4f orbitals by the filled 5s and 5p subshells and are thus not influenced by their coordination sphere. Finally, their luminescence lifetimes are within the micro to millisecond range [24]. However, owing to the f–f forbidden transitions, Ln(III) cations need to be sensitized by a chromophore acting as an antenna for light absorption. Thus, a bright emission of the Lanthanide cations requires an efficient energy transfer between the antenna and the emitter. This might be achieved by a good match between the donating level of the

V. Bulach et al. / Coordination Chemistry Reviews 256 (2012) 1468–1478

Fig. 1. Schematic representation of the commonly accepted mechanism for the sensitization of NIR Ln emitters by a porphyrin chromophore.

antenna and the accepting level of the Lanthanide, ideally with an energy gap in the 1000–2000 cm−1 range [16,25,26]. The energy migration process to Ln(III) may take place through various pathways such as from excited states (singlet or triplet) of the antenna or through charge transfer states such as metal to ligand (MLCT), ligand to metal (LMCT) or intra-ligand (ILCT) [27–30]. Moreover, the f–f transitions possess weak oscillator strengths and may thus be easily quenched by high energy vibrators such as O H or N H containing molecules present in the close proximity of the emitter. In order to avoid this deactivation pathway, it is essential to shield completely the Ln(III) cation and to avoid direct coordination of solvent molecules. The coordination number of Lanthanides usually varies between 8 and 12 and the complete shielding of the cation requires the coordination of at least four bulky chelating ligands. Moreover, the efficiency of the shielding depends strongly on the nature of the coordinating sites and their localization and preorganization. The porphyrin derivatives are perfect candidates to address the above mentioned requirements. Indeed, (i) the porphyrin is a strong chromophore absorbing both in the UV and visible regions; (ii) Its tetraaza macrocyclic core binds almost any Ln(III) cations; (iii) the backbone may be readily functionalized at meso and ␤-pyrrolic positions thus offering a large variety of possibilities and variations; (iv) most importantly, its excited state levels are perfectly adapted for the sensitization of Ln(III) NIR emitters (Fig. 1). In this contribution, we present a literature survey of the design, synthesis, structural analysis and photophysical studies dealing with the NIR emission of Ln(III) cations sensitized by a variety of porphyrin based antenna. The so-called double-decker and tripledecker sandwich type species composed of Ln cations and either two or three porphyrin units are not included in this report. In the past 15 years, numerous NIR emitters based on combinations of Ln(III) cations and porphyrin derivatives have been reported. The species described may be divided into the following two categories: (i) the Ln(III) cation is directly coordinated by the tetraaza core of the porphyrin (Fig. 2a and b); (ii) the Ln(III) cation is coordinated by ligands covalently linked to the porphyrin backbone (Fig. 2c and d). Whereas the first part of the review covers the synthesis and photophysical properties of NIR lanthanide emitters belonging to the first category, the second section deals with systems belonging to the second category. 2. Lanthanides “within” the porphyrin cavity The coordination of the Ln(III) cations within the macrocyclic core of the porphyrin behaving as an antenna is particularly

1469

interesting for both synthetic reasons and photophysical requirements. Indeed, owing to the propensity of the tetraaza core to bind Ln(III) cations, the introduction of the latter within the cavity of the porphyrin is usually straightforward allowing thus to prepare a series of complexes by combining different porphyrin derivatives with a variety of lanthanide cations. The localization of the Ln(III) cations within the porphyrin cavity increases the efficiency of the energy transfer owing to the proximity of the emitting center and the antenna. As already mentioned above, interactions between ligands and Ln3+ cations are largely electrostatic in nature and the geometry of the complex is imposed by steric effects rather than by electronic factors. For Ln–porphyrin complexes, the coordination sphere of the lanthanide is composed of four nitrogen atoms of the porphyrin and at least 3 external monodentate ligands. The external ligands are usually quite labile leading to the decomposition of the complex. Taking advantage of the oxophilicity of Ln3+ ions, oxygen containing chelate type ligands are used in order to prevent decomposition. 2.1. Synthesis Using octaethylporphyrin (OEP), the first [Ln(III)Por]+ complexes were reported in 1969 [31,32]. A few years later, the synthesis was extended to the tetraphenyl porphyrin (TPP) series [33]. All Ln(III) cations, excepted the radioactive Promethium, have been introduced within the porphyrin cavity. Due to the size of the Ln(III) cations and their coordination number varying between 8 and 9, the metal ion is usually located above the porphyrin main plane with several solvent molecules bound to the metal centre completing its coordination sphere. These complexes are usually unstable in solution, especially in the case of Ln(III) possessing large ionic radii. They decompose rapidly to generate either the free base porphyrin and/or, under specific conditions, the double-decker or triple decker complexes such as Ln(Por)2 or Ln2 (Por)3 compounds. In order to stabilize [Ln(III)Por(X)], solvent molecules are replaced by chelate type ligands and monodentate anionic species X by multidentate anions. The replacement of solvent molecules containing OH groups by chelates avoids non radiative quenching of the emission process and thus enhances the NIR emission of the Ln(III) ions. In the following section, [Ln(III)Por(X)] complexes are classified according to the nature of the ligand X coordinated to the metal ion. 2.1.1. Acetylacetonate ligand The synthesis of Eu(p-CH3 -TPP)Acac complex was first described in 1974 by Horrocks et al. [33]. The complex was obtained upon refluxing a mixture of H2 TPP and Eu(acac)3 ·3H2 O in 1,2,4trichlorobenzene. The synthetic procedure was extended to most of the Ln(III) cations [34] and to porphyrins bearing substituents at the meso and/or at the ␤-pyrrolic positions [35–38]. Crystal structures of Yb(Por)Acac (Por = p-F-TPP or p-CN-TPP) complexes were reported in 2008 [39] and revealed the presence of only one solvent molecule (either a water or a MeOH molecule) coordinated to the metal ion which is surrounded by the N4 core of the porphyrin and the bidentate Acac ligand (Fig. 3). The metal ion is thus 7 coordinate and lies ca 1.1 A˚ above the N4 mean plane. Structural information on Gd(TPP)Acac complex were obtained by EXAFS spectroscopy and are consistent with an octacoordinated Gd(III) with the presence of two water molecules bound to the metal ion [40]. The insertion of the lanthanide could also be achieved in melted imidazole at 210 ◦ C [41]. This procedure was used for the metallation of a unsymmetrical porphyrin bearing one meso4-myristyloxyphenyl substituent. Owing to the presence of 4-myristyloxyphenyl group, the resulting [Ln(Por)Acac]

1470

V. Bulach et al. / Coordination Chemistry Reviews 256 (2012) 1468–1478

Fig. 2. Schematic representations of the two strategies used for the sensitization of Ln(III) ions: coordination of the lanthanide ion by the porphyrin tetraaza core (a and b) or by ligating fragments covalently attached to the porphyrin backbone (c and d).

complexes (Ln = Gd, Tb, Dy, Ho and Er) display liquid crystalline behavior. In 2003, Tamiakia et al. [42] described the preparation of series of gadolinium meso-tetraarylporphyrin complexes bearing both achiral and chiral ␤-diketonates ligands. The interaction of chiral ␣-aminoacids with the [Gd(Por)X] complexes (bearing chiral or achiral X ligands) displays intense CD peaks in the Soret region implying the formation of highly coordinated 1:1 supercomplexes. The Gd(III)porphyrin complexes act as efficient CD receptors in chirality sensing of aminoacids. Furthermore, these chiral complexes allow chiral differentiation of dipeptides. Wong and co-workers [43] reported in 2003 the synthesis of a diethyl malonate appended porphyrin. The combination of the latter with Ln(III) cations such as Yb, Er, and Nd, leads to neutral complexes implying that the porphyrin acts as a hexadentate ligand with the appended malonate group coordinated to the metal ion (Fig. 4).

Fig. 3. X-ray crystal structure of [Yb(p-CN-TPP)Acac(MeOH)]. H atoms are omitted for clarity.

The absorption spectra of [Ln(Por)Acac] complexes exhibit the usual features of metalloporphyrins with a Soret band within the 420–440 nm range corresponding to meso-aryl porphyrins and two weaker Q bands (500–650 nm). These bands are typical of intra-ligand ␲ → ␲* transitions of the porphyrinate ligand and correspond to the transition from the ground state to the second singlet state (S2 ) and to the first excited singlet state (S1 ) respectively. Compared with the free porphyrin, the presence of Ln(III) induces a moderate red shift (ca 500 cm−1 ) of the Soret band and, due to an increase of the porphyrin symmetry, an almost complete disappearance of two out of the four Q bands of the free base [44]. These features are similar for all the Ln(III) cations and not influenced by the presence of substituents in the periphery of the macrocycle [41,43,45–47]. 2.1.2. Acetate ligand The first X-ray structure of a [Ln(Por)Acetate] complex was reported at the end of the 90 s for the terbium complex of the meso-tetraphenylporphyrin bearing chlorine atoms at the ␤pyrrolic positions: [Tb(␤-Cl8 TPP)(Acetate)(DMSO)2 ] (Fig. 5) [48]. Surprisingly, this complex was obtained by refluxing the porphyrin derivative with an excess of Tb(Acac)3 (15 eq.) in TCB. The presence of acetate anion bound to Tb(III) cation was assumed to result from the decomposition of Acac chelate at high temperature. The Tb(III) centre, coordinated by the 4 N-pyrrolic atoms, the acetate anion acting as a bidentate ligand and two DMSO molecules, is located 1.28 A˚ out of the porphyrin N4 mean plane and adopts a square antiprismatic geometry. He et al. reported the synthesis of [Yb(TPP)(O2 CMe)(CH3 OH)2 ] in 65% yield by refluxing Yb(O2 CMe)3 ·3H2 O with the porphyrin in TCB during 48 h followed by stirring with methanol under reflux during 2 h [49]. Its crystal structure revealed that the metal ion is

Adapted from Ref. [39].

Fig. 4. Schematic representation of the Ln complexes with the diethyl malonate appended porphyrin.

Fig. 5. X-ray structure of [Tb(␤-Cl8 TPP)(O2 CCH3 )(DMSO)2 ]. H atoms are omitted for clarity.

Adapted from Ref. [43].

Adapted from Ref. [48].

V. Bulach et al. / Coordination Chemistry Reviews 256 (2012) 1468–1478

1471

Fig. 6. Schematic representation of [Yb(TPP)(O2 CMe) L1]. The acetate ligand being either monodentate (a) or bidentate (b). Adapted from Ref. [50].

Fig. 7. X-ray structure of [Yb(TPP)(H2 O)(8-hydroxyquinolinate)]. H atoms are omitted for clarity.

seven-coordinated, with the acetate anion acting as a monodentate ligand. The metal ion is located 1.078 A˚ above the porphyrin mean plane (N4). [Yb(TPP)(O2 CMe)(CH3 OH)2 ] complex contains two labile MeOH ligands and reacts readily with neutral bidentate ligand L1 such as substituted phenanthroline or 2,2 -dipyridylamine [50] leading to the formation of [Yb(TPP)(O2 CMe)L1] complexes (Fig. 6). Interestingly, in the solid state, for L1 = 2,2 -dipyridylamine, the acetate is acting as a monodentate ligand and the Yb(III) is thus 7-coordinate (Fig. 6a), while with substituted phenanthroline, the acetate ligand is bidentate leading to an octacoordinate Yb(III) cation (Fig. 6b).

Adapted from Ref. [54].

2.1.3. Water The hydrated [Ln(Por)(H2 O)3 ]Cl (Ln = Yb(IIII) or Er(III)) complexes may be obtained in high yield by condensation of the porphyrin with Ln[N(SiMe2 )3 ]×[LiCl(THF)3 ] in DME [51]. The procedure was unsuccessful for other lanthanide cations with greater ionic radii such as Nd(III) owing to the instability of [Nd(Por)(H2 O)3 ]Cl precursor which rapidly decomposes through demetallation. The X-ray structures of [Ln(Por)(H2 O)3 ]Cl (Por = p-OCH3 TPP, Ln = Y(III), Yb(III) and Er(III)) were reported by Wong et al. in 1999 [51]. The metal ion is coordinated to 3 water molecules and 4 porphyrin nitrogen atoms and lie ca 1.1 A˚ above the N4 mean plane of ˚ 1.11 A˚ and 1.14 A˚ for Yb, Er and Y respecthe porphyrin (1.08 A, tively). Complexes incorporating one or two coordinated water, [Yb(Por)(H2 O)2 THF]Cl [51] and [Yb(Por)(H2 O)(THF)Cl] [52] were also reported. The water molecules coordinated to Yb are labile and can readily be exchanged with solvent molecules. This behavior was illustrated through the formation of single crystals of the DMF solvate [Yb(TPP)(DMF)2 Cl] upon slow diffusion of diethylether into a solution of [Yb(TPP)(H2 O)3 ]Cl in DMF [53]. Water molecules can also be replaced by ancillary anionic ligands L2 such as 8-hydroxyquinoline, 1-hydroxy2-benzimidazolylguanidine, and 7-azabenzotriazole, hexafluoroacetylacetonate leading to neutral [Yb(TPP)(H2 O)L2] complexes [54]. The structure of the complex containing the 8-hydroxyquinoline was determined by X-ray diffraction methods (Fig. 7). The Yb3+ is 7-coordinated and located 1.09 A˚ above the porphyrin mean plane (N4). The absorption spectra of the aqua complexes with mesoarylporphyrins are all similar to those obtained with the other Ln monoporphyrin complexes with a Soret band at ca 425 nm together with two Q bands at ca 550 and 590 nm [55]. 2.1.4. Chloride or iodide Anhydrous lanthanide chloride complexes [Ln(TPP)Cl(DME)] (Ln = Yb, Tm, Er and Ho) were obtained in high yield (up to 85%)

upon reaction of Li2 TPP(DME)2 with LnCl3 ·3THF in toluene [56]. The Ho and Yb complexes have been characterized by single crystal X-ray diffraction. These complexes are similar. Indeed, in both cases, the metal ion is 7-coordinated and surrounded by 4 N, 1 Cl and 2 O atoms and located 1.105 A˚ and 1.154 A˚ above the porphyrin mean plane (N4) for the Yb and Ho, respectively. The iodide complexes [Nd(TPP)I(THF)2 ] and [Pr(TPP)I(THF)2 ] were obtained starting from the iodide salts of Nd(III) and Pr(III) respectively and Li2 TPP(DME)2 in THF [57]. [Nd(TPP)I(THF)2 ] [57], [Yb(TPP)Cl(DME)] [56] and [Ho(TPP)Cl(DME)] [56] complexes have been characterized on single crystals by X-ray diffraction methods. All three complexes are similar in the crystalline phase. The lanthanide ion is 7-coordinated and its coordination sphere is composed of 4 N, 1 Cl or I and 2 O atoms belonging to the solvent molecule (Fig. 8). In agreement with the size of lanthanide, the latter is located 1.286, 1.154 and 1.105 A˚ above the porphyrin mean plane (N4) for the Nd, Ho and Yb, respectively. 2.1.5. Anionic tripodal ligand: TPB and LR Co In order to minimize the interactions between the lanthanide emitter and quenching agents such as water, considerable efforts have been devoted to the introduction of multidentate monoanionic ligands such as hydrotris(pyrazole-1-yl)borate (TPB) and cyclopentadienyltris(dialkoxyphosphito)cobaltate(I) (LR Co). Indeed, such ligands introduce a congested environment around the Ln(III) ion and prevent direct interactions with solvent molecules. The introduction of the tridentate ligand could not be achieved starting from the [Ln(Por)(Acac)]+ or [Ln(Por)(OAc)]+ precursors because of the difficulty to remove Acac or acetate ligands.

Fig. 8. X-ray structure of [Ho(TPP)(Cl)(DME). H atoms are omitted for clarity. Adapted from Ref. [56].

1472

V. Bulach et al. / Coordination Chemistry Reviews 256 (2012) 1468–1478

Fig. 9. X-ray structure of [Yb(p-OMe-TPP)(LOEt Co)]. H atoms are omitted for clarity. Adapted from Ref. [60].

The exchange processes are usually achieved starting with the aqua or the halogenated complexes such as [Ln(Por)(H2 O)3 ]Cl or [Ln(TPP)X(solvent)]. 2.1.5.1. Trispyrazolylborate: TPB. The formation of [LnTPP(TPB)] complexes (Ln = Yb, Tm, Er, Ho, Nd, Pr) was described by Boncella et al. in 2003. These complexes were prepared under anhydrous conditions and under nitrogen atmosphere by reacting [Ln(TPP)X(solvent)] complexes with 1 eq of NaTPB [57]. In the solid state, these complexes are stable in air. However, they decompose over an extended period of time due to slow hydrolysis of Ln NTPB bonds. The synthesis of [Ln(Por)(TPB)] complexes (Ln = Yb(III), Er and Nd(III)) with meso-tetraarylporphyrin derivatives bearing various substituents in the para position of the aryl groups was also reported. These complexes have been obtained upon addition of the KTPB to a solution containing the aqua complexes [Ln(Por)(H2 O)3 ]Cl [58,59]. Crystal structures of [Ln(Por)(TPB)] (Ln = Nd, Er, Tm, Yb) are isomorphous. The metal ion is 7-coordinated and located above the porphyrin mean plane with the distance ranging from 1.30 A˚ for the Nd(III) to 1.16 A˚ for the Yb(III). No solvent molecule is coordinated to the metal centre. Groups present in the meso position of the porphyrin have no effect to the structure. The main difference between different complexes is due to the radii of the metal ions which lead to a decrease of the M–Nporphyrin bond distances from ca 2.43 A˚ for Nd(III) to 2.34 A˚ for Yb(III). 2.1.5.2. Cyclopentadienyltris(dialkoxyphosphito)cobaltate(I): LR Co. The coordination of the anionic ligands such as LOEt Co or LOMe Co in the apical position of a Lanthanide located within the cavity of a porphyrin backbone has been described either for [Ln(Por)Cl(DME)] [57] or [Ln(Por)H2 O)3 ]+ [60]. Several X-ray structures for the Yb, Er, and Nd complexes have been reported (Fig. 9) [57,60–65]. All complexes of the type [Ln(Por)(LR Co)] are stable in air and display the same structural features: the Ln(III) ion is 7-coordinate and located above the N4 porphyrin mean plane (with a distance in the 1.18–1.30 A˚ range). Within the complex, the average Ln–N ˚ when compared with the averbond distances are longer (>0.1 A) age Ln–O distances indicating that Ln3+ ion have higher affinity for O than for N. 2.2. Photophysical properties All the monoporphyrinate [Ln(Por)]+ complexes exhibit typical absorption spectra. The Soret band, due to the allowed S0 → S2 transition, appears close to 425 nm, while the two Q bands, due to the forbidden S0 → S1 transitions, are in the 550–600 nm region.

The presence of the Ln(III), possessing low-lying f states (Yb, Er, Nd, Ho, Pr), leads to a quenching of the fluorescence and/or phosphorescence of the porphyrin in the visible region and to the appearance of Ln emission (Fig. 1). The excitation spectrum of this emission corresponds to the absorption spectrum of the porphyrin indicating that the sensitization proceeds through the excited state of the porphyrin. The usually accepted mechanism for this sensitization is depicted in Fig. 1: the porphyrin is excited to its singlet state S1, the energy transfer from the chromophore to the lanthanide ions is generally considered to take place through the triplet state of the porphyrin (T1) via an intersystem crossing (ISC) which is followed in part by the energy transfer to the excited state of the Lanthanide. Both the singlet and the triplet states of the ligand may be involved in the energy transfer onto the metal, but, since the singlet state is short lived, this process is often not considered [66]. Assuming a Porphyrin-S1 → Porphyrin-T1 → Ln* energy flow, the efficiency of the overall process is quantified by the overall quantum yield ˚tot which is the ratio between the number of photons emitted by the Ln cation divided by the number of photons absorbed by the porphyrin. The emission process is regulated by the ISC efficiency (S1 → T1), the Energy Transfer efficiency (LigandT1 → Ln*) and the intrinsic Ln quantum yield (Ln* → Ln), ˚Ln . The overall quantum yield ˚tot thus corresponds to the product of these three steps: ˚tot = ˚ISC ˚ET ˚Ln . It is important to point out that the values of the overall quantum yield reported in the literature should be compared with caution. Indeed, due to instrumental limitations, often the measurement of the intrinsic luminescence quantum yield ˚Ln is not carried out. Thus, most of the reported quantum yield were obtained using estimated values for the intrinsic ˚Ln deduced from the equation ˚Ln =  obs / O with  obs the observed life time and  O the natural lifetime [67–69]. The energy transfer efficiency is related, among others factors, to the energy gap between T1 and Ln*, which should be at least equal to 1850 cm−1 to avoid back energy transfer and O2 quenching. [25]. In order to achieve an efficient and fast energy transfer, a short distance between the porphyrin and the antenna is obviously an advantage. This transfer can proceed through a Förster (dipole–dipole) or a Dexter (exchange) mechanism. The exchange mechanism requires a good overlap between the ligand and the metal orbitals while the Förster mechanism is associated with the coupling between the dipole moment associated with T1 and the dipole moment of 4f orbitals. The energy transfer to Ln(III) ions usually proceeds through the Förster [66,70]. The intrinsic quantum yield reflects the extent of non-radiative deactivation of the excited state of the lanthanide Ln(III)* that occurs through the interactions with its surrounding [24]. The best way to minimize this deactivation process is to design a rigid environment free of high energy vibrators (such as OH, NH or to a lower extends C–H) and to protect the Ln(III) cations from solvent interactions. Nevertheless, the intrinsic quantum yield depends mainly on the energy gap between the Ln lowest lying excited state and the highest sublevel of the Ln ground state. The smaller is this gap, more efficient is the deactivation through vibrations of bound ligands with high energy vibrators. It is thus obvious why Ln(III) ion, except Eu, Gd and Tb, display all very low quantum yield in aqueous solution. Concerning the NIR emitters, eliminating high energy vibrators from the coordination sphere (inner or outer sphere) of the Ln(III) is usually not sufficient. Indeed, the Ln* → Ln energy gap for Nd(III), Er(III) is equal to ca 5500 cm−1 , 6500 cm−1 and the observed ˚Ln are usually very low (ranging from 0.02% to 0.5%). For Yb(III), the gap is around 10,000 cm−1 and the ˚Ln value is higher but still low (up to 4 or 5%) [2,25]. This explains why most of the photophysical studies on NIR emitters have been performed on Yb complexes.

V. Bulach et al. / Coordination Chemistry Reviews 256 (2012) 1468–1478

As mentioned above, the quantum yield of Ln monoporphyrinate complexes is strongly influenced by the presence of solvent molecules (mostly OH-containing) in the coordination sphere of the emitter. Water molecules coordinated to the metal ion lead to a quenching of the NIR emission due to vibronic coupling. This has been illustrated by the significant increase of the NIR emission intensity at 980 nm observed for [Yb(TPP)(DMF)2 Cl] complex when compared with [Yb (TPP)(H2 O)3 ]Cl [53]. Indeed, the replacement of the three water molecules by DMF and chloride anion leads to an increase of the Ln emission lifetime from 1.24 to 1.84 ␮s and thus to an increase of the intensity of the NIR emission by a factor of ca 1.5. The Yb emission lifetimes obtained for [Yb(TPP)(H2 O)L2 ] complexes (L2 = ancillary anionic ligands such as 8-hydroxyquinoline, 1-hydroxy-7-azabenzotriazole, 2benzimidazolylguanidine, and hexafluoroacetylacetonate) [54] are within the 10–20 ␮s range and the quantum yields ˚Ln in the 9.1 × 10−3 to 16.7 × 10−3 range (assuming that  o = 1.2 ms [68]). A NIR emission lifetime of ca 2.5 ␮s was reported for [Yb(Por)(acac) X] (X = H2 O or CH3 OH) complex showing almost identical quenching ability of coordinated CH3 OH and H2 O molecules [39]. Moreover, the lifetime of the NIR emission reported for the diethyl-malonate appended porphyrin Yb complex was found to be 2.5 ␮s [43] indicating that the covalent linkage of the Acac moiety to the antenna, although leading to significant increase of stability of the complex, has no significant influence on its photophysical features. However, it has been reported that the presence of substituents in the ␤-pyrrolic positions such as a propylbenzene leads to an increase of the luminescence [71]. The values obtained for Yb complexes bearing a coordinated acetate anion follow similar trends. The observed half-life  obs for the emission at 978 nm upon excitation at 415 nm is close to 1.5 ␮s for [Yb(TPP)(acetate)(CH3 OH)2 ] while it is 12-fold higher for [Yb(TPP)(acetate)(phenanthroline)]. This observation demonstrates the significant quenching effect of the coordinated methanol molecules on the NIR emission. The corresponding reported ˚Ln values are 0.12% and 0.86% respectively (with  o = 1.2 ms [68]). A comparative study on the photophysics of Yb monoporphyrinate complexes [Yb(TPP)(O2 CMe) L1] (L1 = substituted phenanthroline) [50] shows that the overall NIR emission efficiency is of the same order of magnitude as the one reported for [Yb(TPP)(H2 O)L2 ] complexes bearing ancillary bidentate ligands. The comparison of the intensity of the NIR emission of [Ln(TPP)H2 O)3 ]Cl (Ln = Yb, and Er) clearly shows that the intensity is an order of magnitude higher for Yb3+ than for Er3+ [55]. Moreover in both cases, the NIR emission is enhanced in the presence of methoxy at the para position of the meso-phenyl group [55,61], while the presence of 4-hydroxyphenyl at the meso position leads to a decrease of the emission. The effect of the number of halogen atoms located at the ␤pyrrolic positions on the intensity and the lifetime of the Ln emission for Er-porphyrin complexes has been reported recently by Pizzoferrato et al. [47]. The overall quantum yield ˚tot , estimated for Er(TPP)acac to be 4 × 10−4 , increases by a factor two when eight bromides were introduced in the ␤-pyrrolic positions. As expected, the quantum yield ˚tot decreases by a factor of two in the presence of EtOH [70]. The replacement of emission quenchers such as water molecules by the tripodal TPB and LR Co ligands lead to an increase of the intensity of the emission in the NIR together with an increase in the stability of the complexes. This is due to the shielding of the metal ion by both the porphyrin moiety and the capping tripodal ligand. When compared with [Yb(TPP)(H2 O)]+ , the intensity of the emission at 980 nm is increase by a factor of 4.5 for [Yb(TPP)(LOEt Co)] [55]. In 2003, Schanze and co-workers studied [57] the efficiency of the energy transfer and the NIR quantum yields of [Ln(TPP)(TBP)]

1473

Fig. 10. Schematic representation of the conjugated polymer based on [Er(Por)]+ complexes. Adapted from Ref. [76].

and [Ln(TPP)(LOEt Co)] complexes. Although higher than other [Ln(Por)]+ complexes, the overall quantum yield values are still rather low and roughly the same for both tripodal ligand. Quantum yields ˚tot of ca 0.1%, 0.2% and 3% were obtained for Er, Nd, and Yb complexes respectively. Wong and co-workers studied the solvent and the substituent effects on the NIR photoluminescence of such complexes. The presence of electron donating substituents on the meso-phenyl decreases the NIR emission [58] of Yb and Er whereas it increases the emission in the case of Nd [62]. Recently, a water soluble Yb porphyrin complex bearing a pentafluorophenyl groups in the meso position together with a rhodamine moiety covalently connected to the porphyrin was reported [72]. Upon excitation at 430 nm, the system displays NIR emission with an impressive 2.5% quantum yield in aqueous solution. Ytterbium porphyrins capped with acetylacetonate [73], cobaltocene (LOEt Co) [74] or tris-pyrazolyborate (TBP)[75] ligands have been incorporated in a blend of substituted polymer in order to generate NIR electroluminescent devices. The efficiency of the resulting system was measured and was ten times higher than for Ytterbium complexes. This probably results from a better spectroscopic overlap between the polymer and the porphyrin together with a good solubility of the porphyrin in the polymeric blend. Pizzoferrato et al. [76] have observed similar efficiency for a system based on a Erbium porphyrin directly incorporated by covalent linkage into a polymeric chain (Fig. 10).

3. Lanthanide coordinated by an external coordination site The coordination of Lanthanide cations by peripheral coordinating sites covalently linked to the porphyrin core is synthetically challenging. Consequently, when compared with the previous section describing numerous examples of lanthanide complexes, only very few cases dealing with porphyrin derivatives covalently connected to lanthanide complexes have been reported. An advantage associated with such systems is the possibility of fine tuning the energy level of the excited state of the chromophore through the binding of metal cations within the tetraaza core of the porphyrin. Furthermore, through the nature of the spacer connecting the peripheral ligand to the porphyrin backbone, it is possible to control the distance between the antenna, i.e. the porphyrin and the NIR lanthanide emitter. Beeby et al. were the first to explore this strategy. In 2000, they reported [77] the synthesis of a palladium porphyrin bearing a chiral cyclen moiety covalently linked to the porphyrin backbone using a ␤-pyrrolic position (Fig. 11). The ligand was synthesized in six steps starting from the 2-aminoporphyrin. The procedure used involved the synthesis of the iodoacetamide precursor and

1474

V. Bulach et al. / Coordination Chemistry Reviews 256 (2012) 1468–1478

Fig. 11. Schematic representation of the Pd(Por) covalently linked to a chiral cyclen group. Adapted from Ref. [77].

its coupling with cyclen followed by the introduction of the chiral aminoacid arms in the last step. The excitation of the porphyrin at 529 nm leads to the emission of the Ln at 980 nm for the Yb complex and 870 nm and 1064 for the Nd complex. For both cases, the phosphorescence of the Pd porphyrin is observed. In the presence of O2 , the NIR emission is quenched by a factor 7 and of 2 for Yb and Nd respectively. Moreover, the emission is enhanced in deuterated CD3 OD. These features are in agreement with the conventional energy transfer mechanism from the triplet state of the porphyrin to the excited state of the Lanthanide. The synthesis of multidentate ligands based on a porphyrin core covalently linked to a penta- or hepta-dentate ligand (Fig. 12) has been reported recently [78]. These ligands were obtained upon coupling the mono-4-aminotetraphenylporphyrin with the anhydride of the ethylenediaminetetraacetic acid (EDTA) or the diethylenetriaminepentaacetic acid (DTPA). The photophysical properties of the corresponding Yb complexes together with the heterometallic complexes (Zn and Cu) were studied. The investigation revealed that an energy transfer from the porphyrin antenna to the Yb ion takes place. Owing to a better shielding of the emitter, the NIR emission is slightly enhanced for the heptadentate ligand. The presence of Cu or Zn cations in the cavity of the porphyrin leads to a quenching of the NIR emission. Kim and coworkers [79] studied the NIR emission properties of the Pt-Erbium heterometallic complexes depicted in Fig. 13. Concerning the synthesis, the formation of Er(Pt-Por)3 (H2 O)3 complex is rather straightforward when compared with the previous examples mentioned above. The complex is obtained upon reaction of Pt(II) meso-tetraphenylporphyrin bearing a carboxylate moiety in the para position of one of the meso-phenyl group (PtPor), with ErCl3 . Upon excitation of Er(Pt-Por)3 (H2 O)3 complex at 430 nm, NIR emission is observed at 1525 nm corresponding to the 4I 4 13/2 → I15/2 transition of Er(III). The exchange of all three water molecules by a terpyridine ligand (Fig. 13) leads to an increase of the NIR emission by a factor of 6. The Zn(II) complex, Er(Zn-Por)3 (Terpyridine), was also reported [80]. It shows no NIR emission upon excitation at 514 indicating that the Zn-porphyrin is not an appropriate sensitizer for Er(III). This observation is in agreement with a higher Inter System Crossing efficiency for the conversion of S1 to T1 for the platinum complex when compared with Zn-porphyrin. Furthermore, it indicates that the triplet state is involved in the energy transfer mechanism from Pt-Porphyrin to Er(III) in Er(Pt-Por)3 (Terpyridine). For the latter, the absence of NIR emission in the presence of O2 , as well as the quenching of the

Fig. 12. Chemical structure of (MPor)-LnDTPA and (MPor)-LnEDTA. Adapted from Ref. [78].

phosphorescence of the Pt-Po chromophore at 660 nm are further arguments in favor of the mechanism involving the T1 state of the porphyrin. For the Er(Pt-Por)3 (Terpyridine) complex, a decay time of the NIR emission of 1.2 ␮s [81] and a quantum yield ˚Ln of 0.015% (deduced from the equation ˚Ln =  obs / O , with  O = 8.00 ms [82]) were determined. A distance between the antenna and the NIR emitter of 11.4 A˚ was calculated by molecular mechanics. Taking the Gd complex as a reference compound, a Förster type mechanism through the triplet state of the platinum porphyrin was proposed for the energy transfer. The latter involves three steps: the S1 state of the Pt complex transfers its energy to the T1 state thought an Inter System Crossing followed by an energy transfer to the receiving levels 4 F9/2 and the 4 I9/2 of the Er(III) emitter with 83% and 17% contributions respectively. Interestingly, the incorporation of a Gn -aryl-ether functionalized dendron in the Er(Pt-Por)3 (Terpyridine) complex [80,83] leads to the dendritic type complex Er(Pt-PorGn )3 (Terpyridine) displaying a rather good stability (Fig. 13). The aryl-ethertypes dendrons are not involved in the sensitization process but exert an efficient shielding effect leading to an increase in the NIR emission efficiency. In thin films, upon excitation of Er(Pt-PorGn )3 (Terpyridine) complex at 410 nm, the NIR emission intensity is increased by a factor of ca 2, 3.5 and 20 for n = 1, 2 and 3 respectively when compared with the parent Er(PtPor)3 (Terpyridine) complex. In a parallel fashion, the energy transfer efficiency between the Pt-Antenna and the Er(III) increases with n from 12 to 43%. The sensitization of Yb with a Pd, Pt or Zn porphyrin was also reported by Liu and coworkers [64]. The system, of the bisporphyrin

V. Bulach et al. / Coordination Chemistry Reviews 256 (2012) 1468–1478

1475

Fig. 13. Chemical structures of of Er(Pt-Por)3 (H2 O)3 , Er(Pt-Por)3 (Terpyridine) and Er(Pt-PorGn )3 (Terpyridine) complexes. Adapted from Ref. [79].

type (Fig. 14), is based on a metalloporphyrin (M = Pd, Pt or Zn) covalently linked to an Yb-porphyrin through a flexible linker. The bisporphyrin is generated upon coupling the stable [Yb(pOH-TPP)(LCoOMe)] complex with a meso-tetraphenylporphyrin bearing a 3-bromopropoxy group at the para position of one of the meso-phenyl groups. The introduction of Zn(II), Pd(II) and Pt(II) is achieved in the last step in 83%, 88% and 60% yield respectively.

The photophysical properties of the four dyads have been studied. For these complexes, the porphyrin bearing Yb cation acts as an antenna and the other porphyrin covalently linked to the antenna enhance the NIR emission by energy transfer to the emitting centre. Upon excitation at 420 nm, both Yb-2H and Yb-Zn complexes (Fig. 14) fluoresce at 652 nm ( = 12.96 ns) and at 611 nm ( = 3.88 ns) respectively, whereas for the Yb-Pd and Yb-Pt

Fig. 14. Chemical structure of bis-porphyrin dyads. Adapted from Ref. [64].

1476

V. Bulach et al. / Coordination Chemistry Reviews 256 (2012) 1468–1478

complexes, owing to the enhanced Inter System Crossing for Pd and Pt porphyrins, phosphorescence of the triplet level is observed at 693 nm ( = 950 ␮s) and at 660 nm ( = 100 ␮s) respectively. For all dyads, the emission in the visible is quenched when compared with the related MTPP complexes whereas the NIR emission is observed. This is in favor of an energy transfer from the porphyrin to the Yb cation. In the NIR region, all complexes exhibit emissions at 998 nm assigned to the 2 F5/2 → 2 F7/2 transition of Yb(III). When compared with [Yb(TPP)(LOMe Co)] complex, relative intensities of 1.8, 4 and 5 are obtained for Yb-Zn, Yb-Pd and Yb-Pt species respectively. For both Yb-H2 and Yb-Zn complexes, the NIR emission lifetimes of ca 30 ␮s is comparable to the one determined for [Yb(TPP)(LCoOMe)]. For the Yb-Pd and Yb-Pt complexes, the NIR emission lifetimes is increased to 40 ␮s and 70 ␮s respectively. These extended lifetimes correspond to the long-lived triplet state present in the palladium and platinum dyads ( = 950 ␮s and 100 ␮s respectively) and indicate that the efficiency of the energy transfer from the porphyrin triplet state is higher for Yb-Pt when compared with Yb-Pd. The rate constant for the energy transfer from the Pd and Pt porphyrin to Yb(III) are equal to 306 and 1667 s−1 respectively. Our group has proposed to use a ligand based on mesotetraphenylporphyrin as an antenna bearing four chelating units preorganised on the same face of the backbone for the binding of the lanthanide cation. The design principle was based on the generation of isolable atropoisomers resulting from the presence of bulky substituent at the ortho position of the meso-phenyl groups. Among the four possible isomers, the ␣4 atropoisomer was targeted. As a first attempt the porphyrin was equipped with four catecholate units [84,85]. Unfortunately, owing to the dianionic nature of the catecholate unit, the binding of lanthanide cations resulted in insoluble materials. In order to avoid that, the catechol moiety was replaced by 8-hydroxyquinolinyl group leading to ␣4 -H2 Por(QOH)4 ligand (Fig. 15) [86,87]. The connection of the four 8-hydroxyquinoline moieties to the porphyrin backbone is achieved by amide groups. The ligand was obtained in 80% yield at room temperature avoiding thus atropoisomerisation by condensation of 8-hydroxy7-quinolinecarboxylic acid with the ␣4 atropoisomer of the meso-tetrakis(o-aminophenyl)-porphyrin in the presence of HBTU as the coupling reagent. The introduction of the Nd(III) in the pre-organized cavity is achieved in 83% yield upon treatment of ␣4 -H2 Por(QOH)4 with Nd(Acac)3 in the presence of one equivalent of TBAOH. Owing to the presence of TBA as the counter ion, the [␣4 -H2 Por(QO)4 Nd]TBA complex is soluble in most organic solvents. Using CPK models and X-ray structures of analogous porphyrin based ligands, a distance of ca 5 A˚ between the Nd(III) and the centroid of the porphyrin is estimated. The synthesis of d–f complex [␣4 -PdPor(QO)4 Nd]TBA was achieved following a similar strategy using the palladium complex of the tetra-o-aminoporphyrin. Both [␣4 -H2 Por(QO)4 Nd]TBA complex and the heterobinuclear [␣4 -PdPor(QO)4 Nd]TBA species have been studied (Fig. 16) and their photophysical

Fig. 15. Schematic representation of ␣4 -H2 Por(QOH)4 . Adapted from Ref. [87].

characteristics determined. Upon excitation at 425 nm, both complexes exhibit up to 75% quenching of the visible emission of the porphyrin with concomitant appearance of two NIR emission bands at 1064 nm and 1340 nm corresponding to the 4 F3/2 → 4 I11/2 and 4 F3/2 → 4 I13/2 transitions respectively of the Nd(III) centre. The observed emission results from an energy transfer from the porphyrin to the Nd(III). An overall quantum yield ˚tot of 0.7 × 10−4 and 0.8 × 10−4 is determined for the [␣4 -H2 Por(QO)4 Nd]TBA and [␣4 -PdPor(QO)4 Nd]TBA complexes respectively. Interestingly, for both Nd complexes [␣4 -H2 Por(QO)4 Nd]− and [␣4 -PdPor(QO)4 Nd]− , the NIR emission intensity is only slightly decreased by the presence of O2 . Moreover, the determined lifetimes of 561 ␮s and 565 ␮s at 700 nm for the phosphorescence of the Palladium complexes [␣4 -PdPor(QOH)4 ] and [␣4 -PdPor(QO)4 Nd]TBA respectively are roughly the same [88]. These observations indicate that the triplet state is probably not the main pathway in the energy transfer mechanism and that the sensitization of the Nd(III) proceeds through the singlet state of the porphyrin. 4. Conclusion This review describes the synthesis and the photophysical properties of near infra-red emitting porphyrin lanthanide complexes. The reported examples may be described under two categories depending on the location of the Ln(III) ion with respect to the antenna. Indeed, two strategies based either on the binding of the lanthanide ion by the tetraaza core of the porphyrin or by peripheral ligands have been described. From a synthetic point of view, whereas the coordination of Ln(III) ion by a porphyrin backbone is rather straightforward, its localization in the proximity of the porphyrin unit using peripheral ligands is synthetically much more demanding. Concerning the photophysical properties, to reach acceptable NIR emission efficiencies, a close proximity between the Ln(III)

Fig. 16. Schematic representations of ␣4 -H2 Por(QOH)4 and the corresponding complexes. Adapted from Ref. [88].

V. Bulach et al. / Coordination Chemistry Reviews 256 (2012) 1468–1478

centre and the antenna as well as a proper shielding of the emitting centre are required. It is worth noting that even when both requirements are fulfilled, the highest reported quantum yield is ca 3% for Yb(III) porphyrin complexes of the type b presented in Fig. 2. The complex is based on a Yb(III) ion located above the porphyrin mean plane and further complexed by a tridentate ligands such TPB or LR Co. For the second strategy based on the binding of the Ln(III) cation by peripheral ligands covalently linked to the porphyrin backbone, since the distance between the emitting metal centre and the antenna is longer, a lower energy transfer efficiency is observed as expected. However, the major advantage of this design principle is related to the possibility of fine-tuning both the distance and the energy level of the excited state of the chromophore through either functionalization of the porphyrin or through the binding of metal cations in the porphyrin centre (see Fig. 2c and d). Acknowledgements This research is supported by the University of Strasbourg, the International Centre for Frontier Research in Chemistry (FRC) Strasbourg, the CNRS, the Institut Universitaire de France and the Ministère de l’Enseignement Supérieur et de la Recherche (Ph.D. Fellowships for F.S.) References [1] J.-C.G. Bünzli, Chem. Rev. 110 (2010) 2729. [2] J.-C.G. Bünzli, S.V. Eliseeva, J. Rare Earths 28 (2010) 824. [3] F.-F. Chen, Z.-Q. Chen, Z.-Q. Bian, C.-H. Huang, Coord. Chem. Rev. 254 (2010) 991. [4] S. Comby, J.-C.G. Bünzli, Lanthanide near-infrared luminescence in molecular probes and devices, in: J. Gschneidner, J.-C.G. Bünzli, V.K. Pecharsky (Eds.), Handbook on the Physics and Chemistry of Rare Earths, Elsevier Science, Amsterdam, 2007, p. 217. [5] M.D. Ward, Coord. Chem. Rev. 251 (2007) 1663. [6] C. Andraud, O. Maury, Eur. J. Inorg. Chem. 2009 (2009) 4357. [7] G.A. Hebbink, J.W. Stouwdam, D.N. Reinhoudt, F.C.J.M. van Veggel, Adv. Mater. 14 (2002) 1147. [8] A.M. Nonat, C. Allain, S. Faulkner, T. Gunnlaugsson, Inorg. Chem. 49 (2010) 8449. [9] M.P. Oude Wolbers, F.C.J.M. van Veggel, B.H.M. Snellink-Ruel, J.W. Hofstraat, F.A.J. Geurts, D.N. Reinhoudt, J. Chem. Soc., Perkin Trans. 2 (1998) 2141. [10] V. Vicinelli, P. Ceroni, M. Maestri, V. Balzani, M. Gorka, F. Vögtle, J. Am. Chem. Soc. 124 (2002) 6461. [11] J. Zhang, S. Petoud, Chem. Eur. J. 14 (2008) 1264. [12] X. Zhu, W.-K. Wong, W.-Y. Wong, X. Yang, Eur. J. Inorg. Chem. 2011 (2011) 4651. [13] A. Kornienko, B.F. Moore, G.A. Kumar, M.-C. Tan, R.E. Riman, M.G. Brik, T.J. Emge, J.G. Brennan, Inorg. Chem. 50 (2011) 9184. [14] M.E. Gallina, C. Giansante, P. Ceroni, M. Venturi, J. Sakamoto, A.D. Schlueter, Eur. J. Inorg. Chem. (2011) 1479. [15] B.F. Moore, G.A. Kumar, M.-C. Tan, J. Kohl, R.E. Riman, M.G. Brik, T.J. Emge, J.G. Brennan, J. Am. Chem. Soc. 133 (2011) 373. [16] H. Uh, S. Petoud, C.R. Chim. 13 (2010) 668. [17] X.-Y. Chen, X. Yang, B.J. Holliday, Inorg. Chem. 49 (2010) 2583. [18] J. Chakraborty, A. Ray, G. Pilet, G. Chastanet, D. Luneau, R.F. Ziessel, L.J. Charbonniere, L. Carrella, E. Rentschler, F.M.S. El, S. Mitra, Dalton Trans. (2009) 10263. [19] W. Huang, D. Wu, D. Guo, X. Zhu, C. He, Q. Meng, C. Duan, Dalton Trans. (2009) 2081. [20] M. Mato-Iglesias, T. Rodriguez-Blas, C. Platas-Iglesias, M. Starck, P. Kadjane, R. Ziessel, L. Charbonniere, Inorg. Chem. 48 (2009) 1507. [21] L.-N. Sun, Y. Zhang, J.-B. Yu, S.-Y. Yu, S. Dang, C.-Y. Peng, H.-J. Zhang, Micropor. Mesopor. Mater. 115 (2008) 535. [22] L.-N. Sun, Y. Zhang, J.-B. Yu, C.-Y. Peng, H.-J. Zhang, J. Photochem. Photobiol. A 199 (2008) 57. [23] G.-L. Law, K.-L. Wong, Y.-Y. Yang, H.-L. Yang, W.-T. Wong, M.H.-W. Lam, H.-L. Tam, K.-W. Cheah, J. Fluoresc. 18 (2008) 749. [24] J.-C.G. Bünzli, A.-S. Chauvin, H.K. Kim, E. Deiters, S.V. Eliseeva, Coord. Chem. Rev. 254 (2010) 2623. [25] L. Armelao, S. Quici, F. Barigelletti, G. Accorsi, G. Bottaro, M. Cavazzini, E. Tondello, Coord. Chem. Rev. 254 (2010) 487. [26] G.F. de Sá, O.L. Malta, C. de Mello Donegá, A.M. Simas, R.L. Longo, P.A. Santa-Cruz, E.F. da Silva Jr., Coord. Chem. Rev. 196 (2000) 165. [27] A. Vogler, H. Kunkely, Inorg. Chim. Acta 359 (2006) 4130. [28] M. Kleinerman, J. Chem. Phys. 51 (1969) 2370. [29] A. Aebischer, F. Gumy, J.-C.G. Bunzli, Phys. Chem. Chem. Phys. 11 (2009) 1346.

1477

[30] A.P. Souza, L.C.V. Rodrigues, H.F. Brito, S. Alves Jr., O.L. Malta, J. Lumin. 130 (2010) 181. [31] A. Treibs, Just. Liebigs Annalen 728 (1969) 115. [32] J.W. Buchler, G. Eikelmann, L. Puppe, K. Rohbock, H.H. Schneehage, D. Weck, Just. Liebigs Annalen 745 (1971) 135. [33] C.-P. Wong, R.F. Venteicher, W.D. Horrocks, J. Am. Chem. Soc. 96 (1974) 7149. [34] W.D. Horrocks, C.-P. Wong, J. Am. Chem. Soc. 98 (1976) 7157. [35] Z.Z. Xin, L. Wei, J. Ming, L.G. Fa, Synth. React. Inorg. Met. Org. Nano-Met. Chem. 30 (2000) 1747. [36] D.-M. Li, Z.-X. Zhao, S.-Q. Liu, G.-F. Liu, T.-S. Shi, X.-X. Liu, Synth. Commun. 30 (2000) 4017. [37] D.-M. Li, Z.-X. Zhao, H.-R. Sun, G.-F. Liu, T.-S. Shi, X.-X. Liu, S.-J. Dong, Synth. React. Inorg. Met. Org. Nano-Met. Chem. 30 (2000) 1899. [38] Z.X. Zhao, G.F. Liu, Synth. React. Inorg. Met. Org. Nano-Met. Chem. 32 (2002) 465. [39] H. He, A.G. Sykes, D. Galipeau, S.W. Ng, M. Ropp, Inorg. Chem. Commun. 11 (2008) 1051. [40] J.H. Agondanou, I. Nicolis, E. Curis, J. Purans, G.A. Spyroulias, A.G. Coutsolelos, S. Bénazeth, Inorg. Chem. 46 (2007) 6871. [41] X.L. Cui, G.F. Liu, M. Yu, J. Coord. Chem. 59 (2006) 1361. [42] H. Tamiaki, S. Unno, E. Takeuchi, N. Tameshige, S. Shinoda, H. Tsukube, Tetrahedron 59 (2003) 10477. [43] H.-S. He, Z.-X. Zhao, W.-K. Wong, K.-F. Li, J.-X. Meng, K.-W. Cheah, Dalton Trans. (2003) 980. [44] C.P. Wong, Inorg. Synth. 22 (1983) 156. [45] Z.X. Zhao, T.F. Xie, D.M. Li, D.J. Wang, G.F. Liu, Synth. Met. 123 (2001) 33. [46] Z.X. Zhao, Q.H. Xu, D.M. Li, G.F. Liu, L.S. Li, R.R. Xu, Solid State Sci. 3 (2001) 339. [47] R. Pizzoferrato, R. Francini, S. Pietrantoni, R. Paolesse, F. Mandoj, A. Monguzzi, F. Meinardi, J. Chem Phys. A 114 (2010) 4163. [48] G.A. Spyroulias, A. Despotopoulos, C.P. Raptopoulou, A. Terzis, A.G. Coutsolelos, Chem. Commun. (1997) 783. [49] H. He, A.G. Sykes, Inorg. Chem. Commun. 11 (2008) 1304. [50] H. He, A.G. Sykes, P.S. May, G. He, Dalton Trans. (2009) 7454. [51] W.-K. Wong, L. Zhang, W.-T. Wong, F. Xue, T.C.W. Mak, J. Chem. Soc. Dalton Trans. (1999) 615. [52] W.-K. Wong, L. Zhang, F. Xue, T.C.W. Mak, J. Chem. Soc. Dalton Trans. (1999) 3053. [53] H. He, X. Zhu, A. Hou, J. Guo, W.-K. Wong, W.-Y. Wong, K.-F. Li, K.-W. Cheah, Dalton Trans. (2004) 4064. [54] H. He, P.S. May, D. Galipeau, Dalton Trans. (2009) 4766. [55] H.S. He, W.-K. Wong, K.-F. Li, K.-W. Cheah, Synth. Met. 143 (2004) 81. [56] T.J. Foley, K.A. Abboud, J.M. Boncella, Inorg. Chem. 41 (2002) 1704. [57] T.J. Foley, B.S. Harrison, A.S. Knefely, K.A. Abboud, J.R. Reynolds, K.S. Schanze, J.M. Boncella, Inorg. Chem. 42 (2003) 5023. [58] H. He, J. Guo, Z. Zhao, W.-K. Wong, W.-Y. Wong, W.-K. Lo, K.-F. Li, L. Luo, K.-W. Cheah, Eur. J. Inorg. Chem. 2004 (2004) 837. [59] H. He, W.K. Wong, J. Guo, K.F. Li, W.Y. Wong, W.K. Lo, K.W. Cheah, Aust. J. Chem. 57 (2004) 803. [60] W.-K. Wong, A. Hou, J. Guo, H. He, L. Zhang, W.-Y. Wong, K.-F. Li, K.-W. Cheah, F. Xue, T.C.W. Mak, J. Chem. Soc. Dalton Trans. (2001) 3092. [61] J.X. Meng, K.F. Li, J. Yuan, L.L. Zhang, W.K. Wong, K.W. Cheah, Chem. Phys. Lett. 332 (2000) 313. [62] H. He, W.-K. Wong, J. Guo, K.-F. Li, W.-Y. Wong, W.-K. Lo, K.-W. Cheah, Inorg. Chim. Acta 357 (2004) 4379. [63] S. Fu, X. Zhu, G. Zhou, W.-Y. Wong, C. Ye, W.-K. Wong, Z. Li, Eur. J. Inorg. Chem. 2007 (2007) 2004. [64] F.-L. Jiang, W.-K. Wong, X.-J. Zhu, G.-J. Zhou, W.-Y. Wong, P.-L. Wu, H.-L. Tam, K.-W. Cheah, C. Ye, Y. Liu, Eur. J. Inorg. Chem. 2007 (2007) 3365. [65] X.-J. Zhu, T. Zhang, S. Zhao, W.-K. Wong, W.-Y. Wong, Eur. J. Inorg. Chem. 2011 (2011) 3314. [66] J.-C.G. Bünzli, C. Piguet, Chem. Soc. Rev. 34 (2005) 1048. [67] S. Klink, G. Hebbink, L. Grave, F. Van Veggel, D. Reinhoudt, L. Slooff, A. Polman, J. Hofstraat, J. Appl. Phys. 86 (1999) 1181. [68] M.H.V. Werts, R.T.F. Jukes, J.W. Verhoeven, Phys. Chem. Chem. Phys. 4 (2002) 1542. [69] F. Artizzu, M.L. Mercuri, A. Serpe, P. Deplano, Coord. Chem. Rev. 255 (2011) 2514. [70] R. Pizzoferrato, L. Lagonigro, T. Ziller, A. Di Carlo, R. Paolesse, F. Mandoj, A. Ricci, C. Lo Sterzo, Chem. Phys. 300 (2004) 217. [71] S. Zhuravlyov, N. Rusakova, Y. Korovin, J. Alloys Compd. 451 (2008) 334. [72] T. Zhang, X. Zhu, C.C.W. Cheng, W.-M. Kwok, H.-L. Tam, J. Hao, D.W.J. Kwong, W.-K. Wong, K.-L. Wong, J. Am. Chem. Soc. 133 (2011) 20120. [73] B. Harrison, Appl. Phys. Lett. 79 (2001) 3770. [74] T.S. Kang, B.S. Harrison, M. Bouguettaya, T.J. Foley, J.M. Boncella, K.S. Schanze, J.R. Reynolds, Adv. Funct. Mater. 13 (2003) 205. [75] K.S. Schanze, J.R. Reynolds, J.M. Boncella, B.S. Harrison, T.J. Foley, M. Bouguettaya, T.-S. Kang, Synth. Met. 137 (2003) 1013. [76] R. Pizzoferrato, T. Ziller, R. Paolesse, F. Mandoj, A. Micozzi, A. Ricci, C. Lo Sterzo, Chem. Phys. Lett. 426 (2006) 124. [77] A. Beeby, R.S. Dickins, S. FitzGerald, L.J. Govenlock, C.L. Maupin, D. Parker, J.P. Riehl, G. Siligardi, J.A.G. Williams, Chem. Commun. (2000) 1183. [78] N. Rusakova, N. Semenishyn, Y. Korovin, J. Porphyrins Phthalocyanines 14 (2010) 166. [79] J.B. Oh, K.L. Paik, J.-W. Ka, S.-G. Roh, M.-K. Nah, H.K. Kim, Mater. Sci. Eng. C 24 (2004) 257.

1478

V. Bulach et al. / Coordination Chemistry Reviews 256 (2012) 1468–1478

[80] J.B. Oh, Y.H. Kim, M.K. Nah, H.K. Kim, J. Lumin. 111 (2005) 255. [81] M.-K. Nah, J.B. Oh, H.K. Kim, K.-H. Choi, Y.-R. Kim, J.-G. Kang, J. Phys. Chem. A 111 (2007) 6157. [82] M.J. Weber, Phys. Rev. 171 (1968) 283. [83] J.B. Oh, M.K. Nah, Y.H. Kim, M.S. Kang, J.W. Ka, H.K. Kim, Adv. Funct. Mater. 17 (2007) 413. [84] C. Drexler, M. Wais Hosseini, J.-M. Planeix, G. Stupka, A. De Cian, J. Fischer, Chem. Commun. (1998) 689.

[85] B. Zimmer, V. Bulach, C. Drexler, S. Erhardt, M.W. Hosseini, A. De Cian, New J. Chem. 26 (2002) 43. [86] F. Eckes, V. Bulach, A. Guenet, C.A. Strassert, L. De Cola, M.W. Hosseini, Chem. Commun. 46 (2010) 619. [87] F. Eckes, E. Deiters, A. Métivet, V. Bulach, M.W. Hosseini, Eur. J. Org. Chem. 2011 (2011) 2531. [88] A. Guenet, F. Eckes, V. Bulach, C.A. Strassert, L. De Cola, M.W. Hosseini, unpublished results.