Photophysical Properties and Applications of Lanthanoid Helicates - Metallofoldamers: Supramolecular Architectures from Helicates to Biomimetics - page 233

NEt 2

R N N

K

OH O

OH

7.8 Å

R=H H 2 L30

OCNEt 2 H 2 L31

Figure 6.19 Formula of ligands L30 and L31 and structure of [KYb 2 (L31) 3 ] þ . Adapted with
permission from [108]. Copyright 2007 American Chemical Society.

Er transfer efficiency, (iii) the large Cr( 4 T 2 )-Cr( 2 E) energy gap
(7600 cm 1 ) so that only the Cr

reasonably large Cr !

level can feed the Er III

2 E

ð Þ ion and (iv) the long lifetime

2 E

of Cr . This observation is remarkable in that many attempts of designing molecular

complexes with organic ligands for Er III upconversion failed, except in two cases, because
of the very easy deactivation of Er III by vibrations such as C

ð Þ

H oscillators.

6.3.4 Sensitizing NIR-Emitting Lanthanoid Ions

A strongly chelating group which adequately sensitizes the luminescence of NIR-emitting
lanthanoid ions is 8-hydroxyquinolinate [101]. This chromophore has been used either
directly [102], after suitable derivatization [103], or grafted onto tripodal [104] or tetrapo-
dal [105] ligands or onto benzoxazole [106] or BODIPY [107] frameworks. Finally, two
of these units have been fused together into ligands L30, L31 to yield heterometallic tri-
nuclear helicates (Figure 6.19) [KAlYb(L30) 3 ] þ ,[KYb 2 (L30) 3 ] þ [108], [KLn 2 (L31) 3 ] þ ,
[KAlLn(L31) 3 ] þ (Ln

Nd, Er, Yb) and [M I Yb 2 (L31) 3 ] þ (K

Rb, Cs) [109] which have
been fully characterized by NMR, ES-MS and X-ray crystallography. Relevant spectro-
scopic parameters are reported in Table 6.15. Although modest, quantum yields are size-
able for complexes with organic ligands in view of the problematic presence of C

¼ ¼

H
vibrators [36,101]. Comparing KAlNd with KLn 2 helicates, one notes that Al III has a
small beneficial influence on the quantum yields and lifetimes. In contrast, quantum

Table 6.15 Luminescence lifetimes and quantum yields for microcrystalline samples
of heterometallic trinuclear helicates with ligands L30 and L31 [108,109].

Q Ln (%) Q Ln (%)

Ligand Sample

t Ln ( m s)

Ligand Sample

t Ln ( m s)

L30 KAlYb 22.6(2) 1.17(3) L31 KAlNd 1.29(1) 0.29(1)

L31 KYb 2 18.8(1) 1.04(7) KAlEr 2.58(3) 0.028(4)

RbYb 2 18.3(2) 1.96(9) KNd 2 1.11(1) 0.24(1)

CsYb 2 17.3(1) 1.74(2) KEr 2 2.44(6) 0.026(4)