2,4-Pentanedione (HL1) (also known as acetylacetone), which was prepared by Claisen more than 100 years ago, is the simplest aliphatic (mono)β-diketone ligand. Its development and important properties and applications have recently been elegantly reviewed by Reedijk [8].
A vast number of HL1 derivatives have been synthesized so far. Among them, fluorinated β-diketones have received special attention [22–35], and are summarized in Figure 2.3. This type of ligands were synthesized with the purposes of enhancing the extraction power, by reducing the acidity of theβ-diketones [22], and of improving photoluminescence and electro- luminescence properties, by reducing the level of the higher energy frequency oscillator C–H vibrational quenching [23]. Additional advantages of using fluorinated β-diketone ligands
CF3 F3C CF3 F3C CF3
F
CF3 S
CF3
O O
CF3
O O
CF2CF3
O O
CF2CF3
O O
CF3
F3CF2C CH2CF3
O O CF3
O O
CF2CF3
O O
N
N HL2
CF3
O O
Fe
HL3 HL4 HL5
HL6 HL7 HL8
CF2CF3
O O
HL9 HL10
CF2CF2
O O
S
HL12 HL13
F3CF2C CF2CF3
O O
HL11
HL14
HL15
HL16
CF2CF3
O O
HL17 Figure 2.3 Some representative fluorinatedβ-diketones.
include enhanced thermal stability and volatility of their lanthanide complexes, which are of significance in practical applications.
Although lanthanide emitters have the potential advantages of sharp emission bands, with high purities of colors ranging from blue to red, compared with fluorescent organic
emitters in photonic electroluminescent (EL) applications, the emission intensity of lan- thanide ions is usually very weak due to the poor charge transportation capabilities of the lanthanide complexes, hindering their applications in EL displays. In order to improve the performance, many lanthanide β-diketonates, grafted with hole-transporting carbazole and triphenylamine, or electron-transporting 1,3,4-oxadiazole groups, have been reported. Some representative nonfluorinatedβ-diketones [36–45] based on the parent 1,3-propanedione are shown in Figure 2.4. If theβ-diketones were grafted with H-bond forming groups such as hydroxyl, for example H2L27 as shown in Figure 2.4, a high dimensional supramolecular network could be formed [40]. If the β-diketones have additional coordination groups like pyridine, such as HL31,32,35[43, 45], they could often function as useful building blocks for the construction of supramolecular assemblies.
The next category of β-diketones are 4-acyl-1-phenyl-3-methyl-5-pyrazolones (see Figure 2.5) and their analogs of 3-phenyl-4-acyl-5-isoxazolones (see Figure 2.6). The lat- ter type ofβ-diketones have stronger acidities (lower pKa values) than the former, and have recently been studied as promising light conversion molecular devices [46–51].
Figure 2.7 shows a family of inorganic analogs ofβ-diketonates, aryl-functionalized imi- dodiphosphinate ligands (HL46−48) [52], which are bulky ligands around the lanthanide ion, providing shell-type protection of the ion from coordinated solvent molecules. HL48consists of fully fluorinatedN-{P,P-di(pentafluorophinoyl)}-P,P-dipentafluorophenylphosphinimidic acid, and can form ideal fluorinated shells around all visible and NIR (near-infrared) emitting lanthanides [52c].
2.2.2 Bis( β -Diketones) Ligands
Molecular structures of some representative bis(β-diketones) ligands are shown in Figure 2.8.
Bis(β-diketones) ligands, were proved to be efficient motifs or structural elements for self- assembling highly luminescent metallo-supramolecular lanthanide complexes [53–59] and representative examples (H2L49−61) are shown in Figure 2.8. Special attention has also been paid to the use of enantiomerically pure bis-β-diketones of H2L54−56[58]. H2L50in Figure 2.8 was shown to have the ability to form d–f–d molecular magnetic materials [60].
2.2.3 Dendritic β -Diketones Ligands
Dendrimers are tree-like branched macromolecules that consist of a core, one or more den- drons, and surface groups [61]. Several recently reported dendrimerβ-diketones are shown in Figure 2.9. They have attracted special interest due to their unique structures and properties.
The tree-like shape of dendrimers provides a large surface area that can be grafted with chro- mophores such as carbazole, resulting in a large absorption cross-section and, accordingly, the efficient capture of photons, as well as a tuning of the carrier-transporting capability and site-isolation effect [62].
The other interesting properties of dendritic molecules are the site-isolation effect of den- drons, creating a micro-environment to prevent the intermolecular interaction and avoiding a self-quenching effect. Indeed, it was found that the dendritic shell could achieve site iso- lation of the Ln3+cation and maximize the luminescent characteristics. The introduction of such functional groups as carrier-transporting carbazole groups into the dendrimer diketone
O O
O O
N
HL21
HL19
HL18 HL20
HL22
O O
O O
O N N
HL23
O O
O(CH2)4 N
HL24
(HC)6O N
O O
N
HL25
O O
N
HL26
O O O O
N
O O
3 O
N N
CF
O O
O O
OH
H2L27
HL28
HL29
Figure 2.4 Some representative nonfluorinatedβ-diketones based on parent 1,3-propanedione.
O O OH
N O2N
N NH
N O
COOH O
COOH O
O
HL30
HL34
HL31 HL32
N N
O O
N N
O O
H3L35 Figure 2.4 (Continued)
N N
O O
HL38
N HN
O O
N
HL40 N
HN
O O
N
HL39 N
N
O O
HL36
N N
O O
HL37
N N
O O
HL41 Figure 2.5 Molecular structures of 1-phenyl-3-methyl-4-acyl-5-pyrazolones.
N O
O O
N O
O O
N O
O O
N O
O O
HL42 HL43 HL44 HL45
Figure 2.6 Molecular structures of 3-phenyl-4-acyl-5-isoxazolones.
P HN
P O O
HL46 HL47 HL48
P HN
P O O
P HN O
P O F5 F
F5 F5
F
F
Figure 2.7 Aryl-functionalized imidodiphosphinate ligands.
lanthanide complexes would be an attractive aspect of dendrimer chemistry. The dendritic β-diketonate ligands consisting of dibenzoylmethane cores, Fréchet-type poly(aryl ether) den- drons, and the carrier-transporting group–grafted peripheral functional groups may not only tune the triplet energy level, but also exhibit a strong light-harvesting potential, resulting in an intense emission from the central lanthanide(III) ion via sensitization. Therefore, some den- driticβ-diketonates (see Figure 2.9) and their corresponding lanthanide complexes have been synthesized [62–65]. Among such wide ranging applications as drug delivery, light harvest- ing, solar cells, organic light-emitting diode (OLED)s, and sensors [66–68], the application ofβ-diketone lanthanide dendrimer as (OLED) materials have attracted particular attention, due tothe presence of controllable key features such as intermolecular interactions and charge transport, which are important for all OLED.