Lanthanide-Doped Layered Double Hydroxides Intercalated with Sensitizing Anions: Efficient Energy Transfer between Host and Guest Layers Poernomo Gunawan and Rong Xu* School of Chemical and Biomedical Engineering, Nanyang Technological UniVersity, 62 Nanyang DriVe, Singapore 637459 ReceiVed: June 23, 2009; ReVised Manuscript ReceiVed: July 25, 2009 Layered double hydroxides (LDHs) doped with Tb 3+ ions in the brucite-like layers were prepared by a simple one-step coprecipitation method at ambient conditions. When 4-biphenylacetate (BPA) anions were intercalated in the interlayer space, a high concentration of Tb 3+ up to around 19 wt % can be homogeneously incorporated in the octahedral lattice of LDHs. The luminescence study indicated that efficient energy transfer from the excited state of the intercalated BPA guest molecules to Tb 3+ centers in the host layers takes place. Compared with LDHs without a photosensitizer, Tb 3+ -doped LDHs intercalated with BPA display much enhanced green luminescence from Tb 3+ ions. Long lifetimes of around 1.3 ms and high quantum yields of 14-22% were obtained. In addition, the emission color of this type of hybrid materials can be tuned from blue to green by varying the amount of Tb 3+ ions. As the compositions of metal cations and interlayer anions in the LDH structure can be easily varied, the inorganic-organic hybrid system reported here opens great opportunity for developing efficient and functional luminescent materials by simple wet chemical methods. Introduction Light-emitting materials based on lanthanide ions have been widely investigated in view of their potential applications in optoelectronic devices, optical communications, light conversion molecular devices, biological labeling, etc. 1-3 Upon excitation, lanthanide ions emit sharp and intense luminescence based on their f-f electronic transitions in contrast to those from organic phosphors and quantum dots. 4,5 Long luminescence lifetime in a realm of microsecond to millisecond can be frequently obtained with Tb 3+ and Eu 3+ containing materials, thus offering the possibility for advanced applications, e.g., time-resolved fluoroimmunoassays. 6 However, the extinction coefficients of lanthanides are intrinsically low (less than 10 dm 3 /mol/cm) due to Laporte forbidden f-f transitions. 7 As a result, direct excitation of the metal ion center is often cumbersome and inefficient. This situation can be circumvented by using a light conversion process involving energy transfer from excited states of suitable chromophores to lanthanides. The chromophore acts as a sensitizer (or antenna) and the relaxed excited state transfers its energy to the lanthanide ion, which is referred to as an acceptor. 8 Energy transfer can only occur if the energy differ- ences between the ground and excited states of the sensitizer and acceptor are nearly equal (resonance condition) and if a suitable interaction between the two exists. 9 Considerable effort has been made in designing various kinds of sensitizing ligands, suchasmacrocyclicmolecules, 10-13 polymers, 14-16 anddendrimers, 17,18 with their triplet state energies comparable to those of the lanthanide acceptor states for enhanced luminescence efficiency. On the other hand, the incorporation of lanthanide ions and lanthanide complexes in solid materials has been a subject of increasing attention for practical device applications. The frequently used inorganic matrices are lanthanide oxides (e.g.,Y 2 O 3 19-23 ) and metal fluorides (e.g., NaYF 4 24-26 ) which also act as sensitizers. Some common metal oxides, such as SiO 2 , 27,28 TiO 2 , 28,29 and Fe 2 O 3 , 30 have also been used to host a small percentage of the lanthanide ions in their crystal lattices. Inorganic materials possessing a lamellar structure with guest inclusion capability are of great scientific interest. The properties of such materials can be tuned flexibly by varying the host and/ or guest composition and the interactions between the two. 3,31,32 To date, several types of layered compounds containing lan- thanides have been reported. Gandara et al. prepared layered lanthanide hydroxides of pure cationic rare earth (Yb 3+ , Dy 3+ , Ho 3+ ,Y 3+ ) intercalated with polyaromatic disulfonates via a hydrothermal route. 33 A series of layered hydroxynitrates and hydroxyhalides of various lanthanides were synthesized hydro- thermally. 34-37 The nitrate and halide anions in these compounds can be exchanged by organic carboxylate and sulfonate anions. Karmaoui et al. produced lanthanide oxide based lamellar hybrid materials intercalated with organic molecules (benzoate, biphe- nolate). The oxide layers were doped with optically active lanthanide ions, such as Tb 3+ , Eu 3+ , or Nd 3+ , to achieve green, red, or infrared luminescence, respectively. 38,39 Although in- corporation of Y 3+ in layered double hydroxides (LDHs) was reported a long time ago, 40 the use of LDHs as host materials for optically active lanthanides is still a relatively new topic. LDHs, as an important class of host-guest materials, have received great attention due to their versatile applications in catalysis, separation, electrochemistry, and the biomedical field. 41-47 LDHs consist of positively charged brucite-like layers and interlayer anions and their compositions can be represented by the general formula [M II 1-x M III x (OH) 2 ] x+ A n- x/n · mH 2 O. 48 One of the most striking features of LDHs lies in their compositional flexibility in accommodating numerous types of metal cations in the host layer and guest anions in the interlayer space. It has been reported that lanthanide complex anions of Eu 3+ , Gd 3+ , and Ce 3+ can be intercalated in LDHs. 49-52 The complex anions of Eu 3+ in the interlayer space displayed similar luminescence features as those of the free complexes. 49 A more recent study involved doping of Tb 3+ cations in the brucite-like layer. The * To whom correspondence should be addressed. Phone: +65 67906713. Fax: +65 67947553. E-mail: rxu@ntu.edu.sg. J. Phys. Chem. C 2009, 113, 17206–17214 17206 10.1021/jp905884n CCC: $40.75  2009 American Chemical Society Published on Web 09/10/2009