Rare Earth, Rare Earth Luminescence, Luminescent Rare Earth Compounds, and Photofunctional Rare Earth Hybrid Materials

Abstract

This chapter mainly focuses on the fundamental principles of rare earth luminescence, which are important for the investigation of the photofunctional rare earth hybrid materials. It introduces the rare earth elements and their physiochemical properties, the atomic spectral term and energy level transition of rare earth ions associated with their electronic configuration, and the luminescence and spectroscopy of rare earth ions, rare earth inorganic compounds for phosphors, rare earth coordination compounds for molecular luminescence, and photofunctional rare earth hybrid materials combining the characteristics of both phosphor and molecular luminescence. This chapter provides a basis to understand the contents of the subsequent chapters.

1 Introduction

Rare earths (REs) are a special group of elements in the periodic table, consisting of sixth-period lanthanides , fourth-period scandium (Sc ), and fifth-period yttrium (Y). The lanthanides are a series of elements with atomic numbers from 57 to 71, including lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd ), samarium (Sm ), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). Sc and Y possess the nearly identical chemical properties as lanthanides , although lacking in 4f electrons, so they occur in the same ore bodies and have the same separation difficulties. Therefore, they are usually included in the rare earth category.

The rare earths can be generally divided into two classes, the “light rare earth elements (LREE)” (Ce fraction) and “heavy rare earth elements (HREE)” (Y fraction), i.e., <4f⁷and >4f⁷, respectively. Historically, the two classes were from different ores and studied independently. Y is a heavy rare earth element for its physicochemical property similar to Ho and Er. There are slight differences in physicochemical properties from each other which follow this rule of light and heavy rare earth elements as well. The term “rare earths” has been historically applied to the lanthanides because it was originally believed that these elements were sporadically distributed in nature. But in fact, some elements among them, such as La, Ce , and Y, are relatively abundant (Fig. 1.1) [1].

Fig. 1.1

The rare earth elements are often subdivided into light rare earth elements (LREE) and heavy rare earth elements (HREE)

One important rule correlating structure and physicochemical property of rare earth elements is so-called lanthanide contraction , a term used in chemistry to describe different but closely related concepts associated with smaller than expected atomic radii of the elements in the lanthanide series [2]. In multi-electron atoms, the decrease in radius brought about by increase in nuclear charge is partially offset by electrostatic repulsion among electrons. Particularly, a “shielding effect” operates, i.e., as electrons are added in outer shells, electrons already present shield the outer electrons from nuclear charge, making them a lower effective charge on the nucleus. The shielding effect exerted by the inner electrons decreases in order s > p > d > f. Usually when a particular subshell is filled up in a period, the atomic radii decrease. This effect is particularly pronounced in the case of lanthanides as their 4f subshells are being filled across the period, and they are least able to shield two outer (fifth and sixth) shell electrons. Thus, the shielding effect is least able to counter the decrease in radii caused by the increasing nuclear charge, which leads to “lanthanide contraction .” Radius drops from 1.020 Å in the case of Ce (III) to 0.861 Å in the case of Lu (III) [2]. Since the outer shells of the lanthanides do not change within the group, their chemical behavior is very similar, but the different atomic and ionic radii do affect their chemistry. Without the lanthanide contraction , the chemical separation of lanthanides will be extremely difficult. However, this contraction makes the chemical separation of period 5 and period 6 transition metals of the same group rather difficult. All elements following the lanthanides in the periodic table are influenced by the lanthanide contraction . The period 6 elements have very similar radii compared with the period 5 elements in the same group [2].

Rare earth elements are commonly associated to accessory minerals presenting structures that are large enough. They present high ionic charge and associated large ionic radius. They correspond to transition metals ranging from 4f ⁰to 4f ¹⁴, since they progressively fill the 4f electronic shell. Chemical properties slowly vary from La to Lu , according to their decreasing ionic radius. The trend usually decreases from LREE to HREE. However, within the 15 elements of the group, the respective ionization potentials present large variations. Values increase first, correlating with the increase in nuclear charge, which in turns increases the repulsion between electrons. However, a break is observed between f ⁶and f ⁷, i.e., Eu and Gd, caused by the exchange of energy resulting from coupling between paired electrons. It represents the half shell effect. Similar effects, but with reduced amplitude, occur at quarter and three-quarter filling of the electronic shell. REE are subdivided to four groups (or tetrads ) containing four elements each. The four groups include La to Nd , Pm to Gd, Gd to Ho , and Er to Lu . The four tetrads present specific variations superimposed to the linear trend from LREE to HREE. In each group, the tetrad effect is superimposed on the linear trend and manifested by a curved shape that develops on the four elements. But, depending on the chemical reactions that involve RE elements, those can be more favorably incorporated or not to the mineral assemblage. Two mutually opposite effects are observed, noted W and M types [3].

Here we only give a special example to demonstrate the lanthanide contraction and tetrad effect , which is shown in the cyano-bridged aqua (N,N-dimethylacetamide ) (cyanometal) rare earth (Sm , Gd, Ho , Er) nitrate and potassium hexacyanometallate. There are four typical different coordination geometries and crystal structures in these series of complexes with the same terminal ligand N,N-dimethylacetamide (DMA ): one-dimensional chain for Sm {[Sm (DMA)₂(H₂O)₄Fe(CN)₆⋅5H₂O]_n, [SmFe]_n} with approximately parallel transpositioned bridging CN ligands between the Sm and Fe atoms, trinuclear for Gd ([(Gd(DMA)₃(H₂O)₄)₂Fe(CN)₆][Fe(CN)₆]⋅3H₂O, Gd₂Fe) with two approximately perpendicular cis-positioned bridging CN ligands between the two Gd and Fe atoms, homonuclear for Ho ([Ho (DMA)₃(H₂O)₃Fe(CN)₆]⋅3H₂O, HoFe) with only one bridging CN between the Ho and Fe atom, and ion pair for Er ([Er(DMA)₃(H₂O)₄][Cr(CN)₆], Er-Cr) without any CN bridging between Er and Cr atoms [4, 5].

Besides, there also exists some special phenomenon, which depends on the physiochemical property of rare earth compounds. For example, in the synthesis of NaRE(MoO₄)₂ (RE = Y, La, Nd , Eu, Gd, Tb, Er, Yb) and Na₅Lu(MoO₄)₄ compounds by a room-temperature solid-state reaction and hydrothermal crystallization process, it indicates that the higher temperature and moisture can speed up the reaction process, and especially the existence of crystalline water molecules in the precursor is necessary for the solid-state reaction at room temperature. It is found that different rare earth nitrate precursors present the different reactivity to sodium molybdate at room temperature. The crystallization degree of the products after room-temperature solid-state reactio n depends on the melting point of rare earth nitrate precursors [6].

For rare earth atoms, their electronic configurations are Sc 1s²2s²2p⁶3s²3p⁶3d¹4s², Y 1s²2s²2p⁶3s²3p⁶3d¹⁰4s²4p⁶4d¹5s², and Ln 1s²2s²2p⁶3s²3p⁶3d¹⁰4s²4p⁶4d¹⁰4fⁿ5s²5p⁶5d^m6s²(m = 0, 1; n = 0–14). For trivalent rare earth ions, their electronic configurations are Sc (III) 1s²2s²2p⁶3s²3p⁶, Y(III) 1s²2s²2p⁶3s²3p⁶3d¹⁰4s²4p⁶, and Ln(III) 1s²2s²2p⁶3s²3p⁶3d¹⁰4s²4p⁶4d¹⁰4fⁿ5s²5p⁶(n = 0–14; among Ln = La, Ce , Gd, and Lu : 5d¹). Besides the common trivalent state, some rare earth elements can form the divalent (Eu(II), Sm (II), Tm(II), Yb(II)) and tetravalent (Ce (IV) (CeO₂), Pr(IV) (Pr₆O₁₁), Tb(IV) (Tb₄O₇)) under normal conditions. In aqueous solution, only Ce (IV), Eu(II), and Yb(II) can exist stably.

2 Atomic Spectral Term and Energy Level Transition of Rare Earth Ions

The incompletely filled 4f shell of rare earth ions possesses single electrons and produces the non-zero spin quantum number (S). So the spin angular momentum splits and disturbs the orbital angular momentum (L) in the same space region, resulting in the fine splitting of orbital energy levels, so-called the spin–orbit coupling e ffect. Rare earth ions possess many single electrons and show large quantum number, whose spin–orbit coupling effec t is strong. Therefore, the common spectral term^2S+1L-based S and L cannot show the fine energy levels of rare earth ions; we have to consider the strong spin–orbit coupling effec t and introduce the angular quantum number (J) originated from L and S. Subsequently, the electron configurations and energy levels of rare earth ions are presented as a spectral sub-term ^2S+1L_J.

For all the rare earth ions, the number of single electrons in the 4f orbital shows the symmetrical distribution centered with Gd³⁺ion, so there is a rule for their quantum number (L and S). For L and J, a so-called gadolinium breaking effect shows the two different formulas: in front of Gd³⁺, J = L-S, L = −1/2(n(n−7)) and behind Gd³⁺, J = L + S, L = −1/2((n−7)(n−14)), where L can be represented as S, P, D, F, G, H, and I, respectively. So being centered with Gd³⁺, the light rare earth ion and the heavy one show the same L and S symmetrically except for the different J.

Figure 1.2 presents a substantial part of the energy levels originating from the 4fⁿconfiguration as a function of n for the trivalent rare earth ions; the width of the bars in the figure gives the order of magnitude of the crystal field splitting . So the number of possible energy level transitions for these rare earth ions is great, revealing that rare earth ions and their compounds are a huge treasure of luminescent materials [7].

Fig. 1.2

A summary of electronic excited state energy levels for RE³⁺ions. Reprinted with permission from Ref. [7]. Copyright 2009 the American Chemical Society

3 Luminescence and Spectroscopy of Rare Earth Ions

Trivalent rare earth ions such as Sc³⁺, Y³⁺, La³⁺, and Lu³⁺possess the stable, empty, or filled state, so they are optically inert ions and cannot show the photoactive behavior. Gd³⁺has half-empty 4f shell and produces a very stable⁸S_7/2 ground state with excitation energy higher than 32,000 cm⁻¹, so the emission of Gd³⁺is situated in the ultraviolet region (<318 nm).⁸S_7/2 level belongs to non-degenerate orbital and cannot be split by the crystal field , and so its low-temperature emission spectrum only shows a line peak originated from the transition (⁶P_7/2 → ⁸S_7/2). Although Gd³⁺has the same electron configuration as Eu²⁺, its 4f⁶5d state is situated at a relatively high energy state, and it is impossible to observe the band emission from 4f⁶5d → 4f⁷transition. The luminescence of Gd³⁺mainly comes from⁶P → ⁸S transition, a line emission at 313 nm. At its high-energy position, the transition emission can be observed in the host lattice with high-energy optical absorption . Besides, the energy transfer is easy from the⁶P energy level of Gd³⁺to other rare earth ions or host lattice groups . Therefore, Gd³⁺generally is considered as an inert ion just like Sc³⁺, Y³⁺, La³⁺, and Lu³⁺[8, 9].

The emission spectral lines of Eu³⁺are situated in a red color region, which correspond to the transitions (4f⁶) from excited state⁵D₀ to⁷F_J (J = 0, 1, 2, 3, 4, 5, 6). Because the⁵D₀ level cannot be split by the crystal field (J = 0), the crystal field splitting of⁷F_J levels is produced by the splitting of the emission transitions. Besides, the emission from the higher⁵D energy level (⁵D₁,⁵D₂,⁵D₃) can also be observed.⁵D₀ → ⁷F_J emission is suitable to determine the spectral features of Eu³⁺. If Eu³⁺is located in the lattice with inversion symmetry , electric-dipole transitions between levels of 4fⁿconfiguration are strictly forbidden by parity selection rule , and only the weak magnetic-dipole transitions or vibronic electric-dipole transitions occur. If Eu³⁺is situated in the lattice without inversion symmetry , electric-dipole transition is not strictly forbidden and shows weak spectral lines, so-called forced electric-dipole transitio n. These transition emissions need two conditions: one is the crystallographic lattice of Eu³⁺does not have an inversion symmetry center, and the other is the charge transition state which is situated at the low energy level [8, 10].

According to the split number of⁷F_J energy level and the transition number of⁵D₀ → ⁷F_J transition, it is easy to predict the point group symmetry of the environment where Eu³⁺is situated: (a) When Eu³⁺is situated in the lattice with a strict inversion symmetry center, the allowed magnetic-dipole transition ⁵D₀ → ⁷F₁ with orange light (about 590 nm) is dominant. (b) When Eu³⁺is situated in the point group symmetry C_i, C_2h and D_2h, its⁵D₀ → ⁷F₁ transition shows three spectral lines for the splitting of three states from the completely removed degeneration of⁷F₁ level in symmetric crystal field . (c) When Eu³⁺is situated in the point group symmetry C_4h, D_4h, D_3d, S₆, C_6h, and D_6h, its⁷F₁ level splits into two states to show two⁵D₀ → ⁷F₁ transition spectral lines. (d) When Eu³⁺is situated in the point group symmetry T_h and O_h with a high cubic symmetry,⁷F₁ level only has one split and shows one⁵D₀ → ⁷F₁ transition spectral line.

On the other hand, when Eu³⁺is situated in the lattice with deviated inversion symmetry center, the forbidden electric-dipole transition ⁵D₀ → ⁷F₀ with red light (about 615 nm) is dominant: (a) When Eu³⁺is situated in the point group symmetry C_s, C_n, and C_nv, its⁵D₀ → ⁷F₀ transition emission appears at 580 nm because the expanding crystal field potential energy surface transition needs to involve odd-order crystal field term. (b)⁵D₀ → ⁷F₀ transition of Eu³⁺cannot be split by the crystal field and shows only one emission peak, and then each peak corresponds to one crystal lattice. Therefore, the number of⁵D₀ → ⁷F₁ emission peaks can be utilized to predict the lattice number of point group symmetry where Eu³⁺is located (C_s, C_n, and C_nv). (c) When Eu³⁺is situated in point groups with low symmetry, such as triclinic crystal system C₁ and monoclinic crystal system C_s, C₂,⁷F₁, and⁷F₂ levels completely remove the degeneration state and are split into three and five sublevels, respectively. Subsequently, there are one⁵D₀ → ⁷F₀ peak, three⁵D₀ → ⁷F₁ peaks, and five⁵D₀ → ⁷F₂ peaks, respectively; among them⁵D₀ → ⁷F₂ transition shows the dominant red emission .

Tb³⁺ emission has originated from its⁵D₄ → ⁷F_J transition, mainly in the green color region. Besides, the⁵D₃ → ⁷F_J transition from the high energy level also shows an emission mainly in blue region, which is easily quenched by the crossing relaxation energy transfer process Tb(⁵D₃) + Tb(⁷F₆) → Tb(⁵D₄) + Tb(⁷F₆). Sm³⁺ emission is situated in the orange–red color region, corresponding to⁴G_5/2 → ⁶H_J (J = 5/2, 7/2, 9/2, 11/2, 13/2) [11]. Dy³⁺shows two main emissions in visible light region, corresponding to⁴F_9/2 → ⁶H_15/2 (470–500 nm, blue region) and⁴F_9/2 → ⁶H_13/2 (570–600 nm, yellow region) [12]. Among them the yellow emission ⁴F_9/2 → ⁶H_13/2 (570–600 nm) belongs to hypersensitive transition. The luminescence color of Dy³⁺often is close to white. When the hypersensitive transition of host lattice is dominant, it shows yellow color emission . The yellow/blue (Y/B) ratio is the important luminescence parameters. The direct excitation of Dy³⁺in ultraviolet region is not successful because its charge transfer state and 5d energy level are both over 50,000 cm⁻¹. During the energy transfer from host lattice to Dy³⁺, only YVO₄: Dy³⁺shows highly effective luminescence.

Pr³⁺ emission color strongly depends on the host lattice of doped Pr³⁺. Red emission derives from its¹D₂ level, and green color emission comes from its³P₀ level. Besides, it can also show blue emission from¹S₀ level and visible emission from⁴f⁵d state [12]. Nd³⁺(4f³), Er³⁺(4f¹¹), and Yb³⁺(4f¹³) often show the emission in near-infrared (NIR ) region. For Nd³⁺, its emission is ascribed to three transitions of⁴F_3/2 → ⁴I_J (J = 9/2, 11/2, and 13/2) [12]. For Yb³⁺, it shows the typical emission coming from its²F_5/2 → ²F_7/2 transition. For Er³⁺, it exhibits emission in both NIR regions (⁴I_13/2 → ⁴I_15/2) and visible region (²H_11/2 → ⁴I_15/2 and⁴S_3/2 → ⁴I_15/2 transition) [12]. Tm³⁺shows a weak blue emission originating from¹G₄ → ³H₆ transition [12].

Besides the line emission , some rare earth ions also show the wideband emission , such as trivalent ions (Ce³⁺, Pr³⁺, and Nd³⁺) and divalent ions (Eu²⁺, Sm²⁺, and Yb²⁺). Ce³⁺has only one 4f electron with excited state electron configuration 5d¹, whose ground state 4f¹contains two energy levels (²F_5/2 and²F_7/2). So Ce³⁺exhibits two 5d → 4f transition emissions. Ce³⁺ emission generally is situated at ultraviolet or blue region. In few systems, the long wavelength emission in green or red region also can be found, in which the 5d¹ energy level is relatively low and the crystal field strength is high [13]. The most popular divalent ion is Eu²⁺(4f⁷), whose 5d → 4f emission changes in the rage from long wavelength ultraviolet to yellow regions. Eu²⁺mainly shows the wideband emission due to 4f⁶5d → 4f⁷transition , which also shows line emission under low-temperature condition [14].

4 Rare Earth Phosphors

Rare earth ions and their compounds are a huge treasure of luminescent materials, which can behave as both photoactive ions and host elements. Almost all kinds of rare earth ions can be utilized in phosphors. Rare earth phosphors become the most useful species for commercial applications, due to their typical luminescence processes and mechanisms.

Trivalent rare earth ions have excellent photoactive properties such as sharp emission spectra for high color purity, broad emission bands covering the ultraviolet (UV)-visible –near-infrared (NIR ) region, a wide range of lifetimes from the microsecond to the second level, high luminescence quantum efficiencies , etc. These properties have attracted much attention for a wide variety of applications in the fields of lighting devices (television and computer displays, optical fibers , optical amplifiers , lasers ) and biomedical analysis (medical diagnosis and cell imaging ).

However, it is difficult for rare earth ions to directly behave as luminescent materials because of their poor light absorption abilities. As it is known, the f–f transitions of rare earth ions are spin forbidden, so it is hard to generate efficient emission by direct excitation . The doping of rare earth ions into special matrices such as metallic oxides and oxysalts is traditionally an efficient method to obtain excellent luminescent material. In these kinds of phosphors, the host absorption is the main energy origin for the energy transfer and sensitization of the luminescence of active rare earth ions. Besides, the charge transfer states between rare earth ions and their coordinated oxygen atoms (CTS) also exist in some special phosphor systems. The two main wide energy absorptions can be further excited to activate rare earth ions, resulting in the effective luminescence. Certainly, the f–f transition absorption of rare earth ions themselves also appears (Fig. 1.3).

Fig. 1.3

The scheme for the luminescence and energy transfer process in rare earth solid-state phosphors

The binary rare earth compounds used as hosts for the luminescence of rare earth ions mainly include rare earth oxides and fluorides . Among them Y₂O₃:Eu³⁺is the most famous one because it is used as the red color phosphor for trichromatic lamp. Besides, La₂O₂S:Eu³⁺is an important cathode ray-luminescent phosphor applied in color television. The binary rare earth fluoride can be used as the host to activate the luminescence of rare earth ions with visible or NIR emission . But their luminescent performance is not satisfied. Recently, the ternary or quaternary fluorides become the important hosts for special luminescence of rare earth ions, including MREF₄ (M = Na, Li), BaREF₅, BaMgF₄, K₃MF₇ (M = Zr, Hf) Na₃ScF₆, M₂NaScF₆ (M = K, Rb, Cs), and Na₃Li₃Sc₂F₁₂ , While the most popular one is the NaREF₄ series, especially NaYF₄, which is a good host for the upconversion luminescence.

The most extensively used host species are the rare earth oxysalts (complex oxides), which are very abundant and versatile, including silicate , vanadate , niobate , tantalate , titanate , borate , aluminate , tungstate , molybdate , and so on. These oxysalts with oxide polyhedron frameworks can produce the effective host photo-absorption to further sensitize the luminescence of photoactive rare earth ions. In some rare earth ion (such as Eu³⁺)-doped systems, the charge transfer process can occur between Eu³⁺ion and the oxygen center of the oxide framework, resulting in a wide charge transfer band (CTB ). This is beneficial for the energy transfer and luminescence sensitization of rare earth ions. Besides, the characteristic f–f transition of photoactive rare earth ion can also be observed. Some host systems belong to self-activated phosphors, such as tungstate . The doping of activator ions may exhibit the emissions of both host and activator ; different luminescence color can be integrated to realize white light output.

Rare earth silicate phosphors include many different types, such as oxygen-rich silicon oxides RE₂SiO₅ (especially Y₂SiO₅), oxyapatites M₂RE₈(SiO₄)₆O₂ (M = Mg, Ca, Sr), Ca₄RE₆(SiO₄)₆O (M = Ca, Sr), Zn₂SiO₄, Zr₂SiO₄, CaSiO₃, Sr₃La₆(SiO₄)₆, Na₃RESi₃O₉, CaAl₂SiO₇, etc. And also they can form M₅RE₅(SiO₄)₃(PO₄)₃O₂ (M = alkali earth) with other oxysalts such as PO₄ ³⁻. Rare earth aluminates include REAlO₃: Eu³⁺, RE₃Al₅O₁₂, MREAlO₄, MAl₂O₄, BaMgAl₁₁O₁₇, Sr₄Al₁₄O₂₅, MAl₂O₄, MREAlO₄, etc. Among them the most important one is yttrium aluminate garnet Y₃Al₅O₁₂ (YAG). Besides, the famous green and blue color phosphors are in trichromatic systems, MAl₁₁O₁₉:Ce³⁺, Tb³⁺(CAT), and BaMg₂Al₁₆O₂₇:Eu²⁺(Mn²⁺) (BAM). Rare earth borates include REBO₃, RE₃BO₆, Zn₄B₆O₁₃, MB₆O₁₀ (M = alkali earth), REAl₃(BO₃)₄, etc. Certainly , both aluminate and borate can be introduced into the same phosphor system, such as REAl₃(BO₃)₄, due to their similar physical properties.

The main rare earth vanadates are REVO₄ and M₃RE(VO₄)₃, respectively. Among them YVO₄: Eu³⁺is an important phosphor . Certainly, rare earth ions and vanadate ions can be replaced by other similar valence anions or cations, resulting in new phosphors such as REP_xV_1-xO₄ or Bi_xRE_1-xVO₄. Rare earth niobate and tantalate phosphors include many systems such as RENbO₄, RETaO₄, Zn₃Nb₂O₈, and REVTa₂O₉, respectively. Rare earth phosphates include REPO₄, Zn₃(PO₄)₂, La₃PO₇, M₃Ln(PO₄)₃, NH₄ZnPO₄, etc.

Rare earth tungstates belong to self-activated phosphors, which show strong blue luminescence. The rare earth-doped tungstate phosphors include M’WO₄: Eu/Tb (M’ = Ca, Sr, Ba, Pb, Zn, Mn, Fe, Cd), RE₂WO₆, Bi₂WO₆, Ag₂WO₄, and Ag₂W₂O₇. The most common rare earth molybdate is NaRE(MoO₄)₂ (and Na₅Lu(MoO₄)₄). Besides, there are other systems, such as MMoO₄ (M = Ca, Sr, Ba, Pb, Zn, Mn, Fe, Cd) and RE₂(MoO₄)₃. For rare earth titanates, the main host species include MTiO₃ (M = Ca, Sr), RE₂Ti₂O₇, RE₂Ti₂O₇, NaRETi₂O₆, and Na₂RE₂Ti₃O₁₀, whose main activator ions are Pr³⁺or Eu³⁺.

5 Luminescent Rare Earth Coordination Compounds

Rare earth coordination compounds or complexes cover different coordination numbers from 3 to 12 and structures from zero-dimensional to three-dimensional for rare earth ions with large ion radius. A majority of them are the complexes with oxygen as the coordinated atom, whose ligands involve carboxylic acids , β-diketones , macrocyclic compounds , etc. Most of these ligands are photoactive due to their localized conjugated structure which can produce the electron transition. β-Diketones possess the double ketone groups linked with methylene group to produce the chelation with rare earth ions. The well-known β-diketones contain acetylacetone (AcAc ), trifluoroacetone (TFA ), hexafluoroacetone (HFA ), 2-thenoyltrifluoroacetone (TTA ), dibenzoylmethane (DBM ), etc. The introduction of unsymmetrical substituted groups can enhance the photoactive absorption; in particular, TTA is the best ligand to sensitize the luminescence of Eu³⁺. The coordination interaction is simple between rare earth ions and β-diketones, mainly the chelated coordination. Generally, the β-diketone ligands transform to their enol structure (so-called keto–enol tautomerism) and lose one proton under alkali environment to form β-diketonate, which is coordinated to rare earth ions to form rare earth tris- or tetrais-β-diketonates. The macrocyclic compounds , such as crown ether , calixarene , Schiff base, etc., also can be used as ligands of rare earth ions, whose luminescent performance is not eminent but they can be used for special purposes. The most abundant ligand for rare earth ions is carboxylic acid , especially the aromatic carboxylic acid for the photofunctional application. These ligands contain one or multi-carboxylic groups or other substituted groups, which provide the abundant coordination positions for rare earth ions. Particularly, the complicated coordination modes of carboxylic groups to RE³⁺endow their complexes with the diverse structures, always resulting in polymeric structures (1D chain, 2D layer, or 3D network). Especially the porous coordination polymers (PCPs ) or metal–organic frameworks (MOFs ) attract wide interest.

As we all know, all rare earth ions only possess weak light absorption because the molar absorption coefficients of most transitions in the absorption spectra of RE³⁺are smaller than 10 Lmol⁻¹ cm⁻¹; only a very limited amount of radiation is absorbed by direct excitation in the 4f levels to obtain weak luminescence. To overcome this disadvantage, Weissman has discovered “antenna effect” within the rare earth complexes of organic ligands, which is based upon the excitation in an absorption band of the organic ligand and the subsequent intramolecular energy transfer between the organic ligands and rare earth ions [15]. Afterward, the mechanism of the energy transfer is studied in detail, putting emphasis on the rare earth β-diketonate complexes , because they are potentially active species in liquid lasers . Combining with luminescence process of organic ligands and the results in rare earth organic complexes , the commonly accepted mechanism by Crosby and Whan is shown in the inset of Fig. 1.4 [16, 17]. Upon irradiation with ultraviolet radiation, organic molecules are excited to a vibrational level of the excited singlet state (S_i) and then may undergo fast internal conversion process (IC) to relax to the lowest vibrational level of the S₁ state, which is a very quick process and easily occurs in solution for the interaction with solvent molecules. Commonly, the excited singlet state of organic molecule can be deactivated radiatively to the ground state, resulting in molecular fluorescence with the spin which allows transition between two electronic states with the same spin multiplicity from S₁ to S₀. On the other hand, the excited singlet state may occur as conversion with different spin multiplicity from singlet S_i to triplet T_i by the microscopic quantum mechanics perturbation, called intersystem crossing (ISC ). The excited triplet state of organic molecule can be deactivated radiatively to the singlet ground state, resulting in molecular phosphorescence with the spin-forbidden transition from T₁ to S₀. When organic molecule behaves like a ligand in rare earth complexes through coordination interaction, the complex may undergo a non-radiative transition from the triplet state to an excited state of the rare earth ion through a so-called intramolecular energy transfer process . The sensitized rare earth ion may undergo a radiative transition to a lower 4f state by characteristic line-like photoluminescence or may be deactivated by non-radiative processes to produce heat. From the emission transition position of common rare earth ions, the certain resonance emission levels can be determined, which are⁴G_5/2 for Sm³⁺(17,800 cm⁻¹),⁵D₀ for Eu³⁺(17,250 cm⁻¹),⁵D₄ for Tb³⁺(20,500 cm⁻¹), and⁴F_9/2 for Dy³⁺(20,960 cm⁻¹), respectively. If the rare earth ion is excited to a non-emitting level, the excitation energy is dissipated via non-radiative processes until a resonance level is reached either directly by excitation in the 4f levels or indirectly by energy transfer . In this case, radiative transitions will compete with the non-radiative processes to display RE³⁺-centered emission . So the final line emission of rare earth ion depends on the minimization of non-radiative deactivation or other emissions of molecular fluorescence and phosphorescence . To populate a resonance level of a RE³⁺ion, the necessary prerequisite is the lowest triplet state of the ligand, or the complex is higher than the resonant energy level of the rare earth ion. If not, no effective emission of RE³⁺is found, and molecular luminescence (fluorescence or phosphorescence ) may be observed. So the intramolecular energy transfer within the rare earth complexes may depend on the match between the energy of the lowest triplet level of the ligands or complexes and the resonance level of RE³⁺. Because the position of the triplet level depends on the type of ligand, it is therefore possible to control the luminescence intensity observed for a given rare earth ion by variation of the ligand.

Fig. 1.4

The scheme for the luminescence and energy transfer process in rare earth coordination compounds

Sato and Wada have investigated the relationship between the intramolecular energy transfer efficiencies and triplet state energies in the rare earth β-diketones chelates [18]. Gadolinium complexes are selected as model complexes for the determination of the lowest triplet state energies of organic ligands due to their enhanced phosphorescence /fluorescence ratios (Φ_ph/Φ_fl > 100) compared to those of other rare earth complexes , and the emitting level energy of Gd³⁺is so much higher than the triplet state energies of organic ligands; therefore, it cannot be sensitized by ligands. Moreover, the presence of a heavy paramagnetic Gd³⁺ion enhances the ISC from the singlet to the triplet state because of the mixing of the triplet and singlet states, so-called paramagnetic effect. The spin–orbit coupling interaction endows the triplet state a partially singlet character to relax the spectral selection rules. The efficiency of the energy transfer is proportional to the overlap between the phosphorescence spectrum of the ligand and the absorption spectrum of the rare earth ion. The overlap decreases as the triplet state energy increases. Therefore, the intramolecular energy migration efficiency from the organic ligands to the central RE³⁺is the most important factor influencing the luminescence properties of rare earth complexes , which depends mainly on the two energy transfer processes. One is from the lowest triplet state energy of organic ligand to the resonant energy level by Dexter’s resonant exchange interaction theory (Eq. 1.1) [19]:

$$ {k}_{\mathrm{ET}}=2\pi {Z}^2{\displaystyle \int {F}_{\mathrm{d}}(E){F}_{\mathrm{a}}(E)\mathrm{d}\mathrm{E}\ \exp \left(-2{R}_{\mathrm{d}\mathrm{a}}/L\right)} $$

(1.1)

Where k _ET is the rate constant of the intermolecular energy transfer and P _da is the transition probability of the resonant exchange interaction. 2πZ ²/R is the constant relating to the specific mutual distance between the central RE³⁺ion and its coordinated atoms. F _d(E) and E _a (E) are the experimental luminescent spectrum of energy donor (ligands) and the experimental absorption spectrum of energy acceptor (RE³⁺), respectively, R _da is the intermolecular distance between donor atoms and acceptor atoms, and L is the van der Waals radius. Both R _da and L may be considered to be constant in intramolecular transfer processes, so k _ET is proportional to the overlap of F _d(E) and E _a (E).

On the other hand, a close match between the energy of the triplet state and the energy of the receiving 4f level of the rare earth ion is not desirable either, because energy back transfer of the rare earth ion to the triplet state can occur. When the energy transfer is not very efficient, it is possible to observe some remaining ligand emission in combination with the rare earth-centered emission . The inverse energy transition by the thermal deactivation mechanism is shown as follows (Eq. 1.2) [20, 21]:

$$ k(T)=A \exp \left(-\varDelta E/RT\right) $$

(1.2)

Both of them represent the overlap spectrum of RE³⁺. With the decrease in the energy difference between the triplet state energy of conjugated carboxylic acid and RE³⁺, the overlap of F _d(E) and E _a (E) is increased. So from the above equation, it can be concluded that the larger the overlap between the luminescent spectrum of organic ligands and the emissive energy of RE³⁺, the larger is the intramolecular energy rate constant k_ET. On the other hand, the activation energy ΔE in Eq. (1.2) is equal to the energy difference ΔE(Tr-RE³⁺), while from the formula, the inverse energy transfer rate constant k(T) increased with decreasing ΔE(Tr-RE³⁺) [22,23,24,25]. As discussed above, there should exist an optimal energy difference between the triplet position of organic ligands and the emissive energy level RE³⁺; a larger or a smaller ΔE(Tr-RE³⁺) will decrease the luminescence of rare earth complexes .

Another possibility to sensitize rare earth luminescence is via charge transfer states [26,27,28]. This is especially the case for trivalent rare earth ions that can easily be reduced to the divalent state (redox-sensitive rare earth ions) like Sm³⁺, Eu³⁺, and Yb³⁺, where light can be absorbed by an intense ligand-to-metal charge transfer state (LMCT state) from which the excitation energy can be transferred to the 4f levels of the rare earth ion. This process only works well if the LMCT state lies at high-enough energy. For instance, sensitization of Eu³⁺through a LMCT state is efficient if the LMCT lies above 40,000 cm⁻¹. Low-lying LMCT states will partially or totally quench the luminescence. In the case of Eu³⁺, metal-centered luminescence is totally quenched if the LMCT energy is less than 25,000 cm⁻¹[29]. Also strongly absorbing chromophores containing d-block metals can be used for sensitizing rare earth luminescence [30]. Because these chromophores absorb in general at longer wavelengths than the most often used organic chromophores (typically in the visible spectral region), d-block chromophores are especially useful for sensitizing the near-infrared luminescence of rare earth ions like Nd³⁺, Er³⁺, and Yb³⁺.

Besides, Kleinerman proposes a mechanism of direct transfer of energy from the excited singlet state S₁ to the energy levels of the rare earth ion [31], which is now not considered to be of great importance because it is often not efficient due to the short lifetime of the excited singlet state. In this way, the energetic constraints from the T₁ state of the ligand can be avoided. Horrocks and co-workers later examine the energy transfe r processes in rare earth ion-binding proteins and assume that it is the excited singlet state of the chromophore that sensitizes the rare earth examine [6]. However, owing to the lack of information regarding the emission from the excited states of the coordinated ligand and the difficulties in the determination of the ligand-localized triplet –triplet absorption spectra for rare earth chelates, it has been very difficult to prove for certain which state is responsible for the energy transfer process [32]. All the experimental work conducted on the sensitization of rare earth chelate luminescence seems to support the triplet pathway, whereas the singlet pathway for the sensitization of the Eu complex has not been observed experimentally [32]. Yang et al. report the successful extension of the excitation window to the visible region for a Eu complex, which subsequently exhibits highly efficient EuIII-centered luminescence. The sensitization mechanism is shown to take place through the singlet pathway by means of time-resolved luminescence spectroscopic studies. They have demonstrated the first observable case of excitation energy transfer from the ligand to the luminescent states of EuIII ion through the singlet pathway in a visible light-sensitized europium complex. The excitation window for this complex has been extended up to 460 nm (Fig. 1.5).

Fig. 1.5

The scheme for the singlet direct energy transfer pathway diagram (bottom) in complex Eu(TTA )3 L (L = N,N-dialkyl aniline moiety-modified dipyrazolyltriazine derivative) (Reprinted with permission from Ref. [32]. Copyright 2004 John Wiley & Sons Ltd)

In addition, in ternary rare earth complexes , there also exists the energy transfer between different ligands. According to the phosphorescent spectra or lifetimes of ternary and binary gadolinium complexes , it can be found that in some ternary rare earth complexes , only one ligand may become the main energy donor for the energy transfer from another ligand to this ligand, whose energy transfer also depends on the triplet state energy level of the two ligands. This phenomenon often occurs in the ternary rare earth complexes with aromatic carboxylic acids and heterocyclic ligands [33]. On the other hand, some inert RE ions (La³⁺, Gd³⁺, Y³⁺, Lu³⁺) also enhance ultraviolet absorption intensity of ligands and then enhance phosphorescence intensity to improve intersystem crossing efficiency from S₁ to T₁ of ligands. Subsequently, the intramolecular energy transfer efficiency from ligands to RE³⁺will be increased to obtain the strong luminescence of RE³⁺[34].

6 Photofunctional Rare Earth Hybrid Materials

Photofunctional rare earth hybrid materials are more complicated than molecular systems. The initial aim is to prepare the multicomponent materials and modify their properties, such as thermal stability or photostability. But now the photofunctional hybrid materials are endowed with the abundant connotations. These hybrids display special characteristics of both solid-state luminescence and molecular luminescence. The hybrids not only involve the structural components as the building units but also contain the photofunctional units. The aggregate state and microstructure of the photofunctional hybrid materials have been studied, and the final aim is to realize the property tuning and function integration. In these photofunctional rare earth hybrid systems, all kinds of interaction forces are involved, including coordination bonds, covalent bonds, and other interactions. On the other hand, the composition of hybrid materials is multi-scale, from molecular scale to nanoscale and to bulk materials.

As the above discussion attests, the luminescent rare earth compounds mainly have two types: one is the atomic or ionic system, i.e., rare earth solid-state compounds as phosphors (continuous solids), and the other is the molecular system, i.e., rare earth coordination compounds or complexes (molecular solids). The main distinction between the luminescence of the two kinds of compounds is different energy donors , i.e., host absorption for the former and ligand absorption for the latter. But in fact, the nature of the interaction between photoactive rare earth ions and hosts or ligands is identical, which is the coordination and depends on the coordination environment surrounding rare earth ions. Certainly, the charge transfer state absorption also exists between certain rare earth ions and ligand or host, such as Eu³⁺-O CTS.

For rare earth photofunctional hybrid materials, the initial aim is only to put photoactive species (mainly rare earth complexes ) into some hosts to improve their thermal or photostability [35,36,37,38,39]. The combination of organic and inorganic components at a molecular or nanometer scale integrates certain strong points of organic compounds (easy processing with conventional techniques, elasticity, and organic functionalities) with the advantages of inorganic components (hardness, thermal and chemical stabilities). According to the different interaction forces between the two phases, these hybrid materials are divided into two major classes [40]. Physically doped hybrids have weak interactions (such as hydrogen bonding , van der Waals forces, or weak static effects) between the organic and inorganic phases, which cannot overcome many problems such as clustering of emitters, inhomogeneous dispersion of the components, or leaching of the dopants [41,42,43,44,45,46]. Chemically bonded hybrids have strong interactions such as covalent, ion–covalent, coordination, or Lewis acid/base bonds, which can graft the rare earth complexes onto hosts [47, 48]. The latter class of hybrids overcome the disadvantages of the former one, since they are homogeneous and can avoid the self-quenching of rare earth ions, which results in increased incorporation concentrations.

Further, it needs to mention the nature of the luminescence and its significance for photofunctional hybrid materials. The whole photofunctional hybrid material system belongs to the complicated frameworks or networks that contain various kinds of building blocks. The important component is the ligand or linker functionalized host. The scheme of the photofunctional hybrid systems is shown in Fig. 1.6. The linker is a special ligand which plays a key role in producing the antenna effect to transfer energy to sensitize the luminescence of coordinated photoactive RE³⁺. The linker functionalized host in the hybrid system shows the characteristics of both of the ligand and the host, which integrates the luminescence of solid-state phosphors and molecular complexes . The common hosts include sol–gel -derived silica, which is mainly from organically modified silane, mesoporous silica , microporous zeolite , metal–organic framework or porous coordination polymer, organic polymer or its composites, etc. Certainly, the photo-absorption is often affected by the metal ions coordinated to linker-modified hosts in some systems such as MOFs (or PCPs ). The final hybrid system can display multicolo r luminescence under energy transfer with antenna effect and also achieve the white color luminescence integration. Some hybrids can be further studied to explore their application in sensing .

Fig. 1.6

The scheme for the compositions and photophysical applications of photofunctional rare earth hybrid materials

In this book, seven topics are discussed in the following chapters. Chapter 2 focuses on photofunctional rare earth hybrid materials based on sol–ge l-derived silica, mainly with the organically modified silanes as the linkers. Chapter 3 focuses on photofunctional rare earth hybrid materials based on organically modified mesoporous silica . Chapter 4 focuses on photofunctional rare earth hybrid materials based on microporous zeolite . Chapter 5 focuses on photofunctional rare earth hybrid materials based on metal–organic frameworks. Chapter 6 focuses on photofunctional rare earth hybrid materials based on organic polymer or its composites. Chapter 7 focuses on photofunctional rare earth hybrid materials with other functional units achieved by multicomponent assembly . Chapter 8 focuses on the photophysical applications of photofunctional rare earth hybrid materials, especially on chemical sensing . The seven topics are chosen because there are extensive reports on them.