Photophysical Properties and Applications of Lanthanoid Helicates - Metallofoldamers: Supramolecular Architectures from Helicates to Biomimetics - page 194

lanthanoid helicates with this class of ligands is dealt with in Chapter 3 of this topic.
Moreover, programmed subtle differences between the two coordination sites of ditopic
ligands L3 result in the recognition of two unlike lanthanoid ions [7], while reducing the
denticity of one coordination site to two leads to the assembly of heterometallic d-f
helicates [8]. Finally the ligand backbone can be expanded to accommodate three or even
four coordination units, allowing the isolation of homometallic [9] or heterometallic [10]
4f-4f-4f helicates, or 4f-3d-4f [11] or 3d-4f-4f [12] edifices, or even a rare homometallic
tetranuclear helicate [13].
With respect to photophysical properties, the introduction of lanthanoid ions into heli-
cates has two major advantages. The first one is the highly protective coordination envi-
ronment generated by the wrapping of the ligand strands while the second lies in the
intermetallic communication that can be programmed along the helical axis. Energy
transfer between metal ions can be rationally programmed and leads to the control of
photophysical properties of one ion by the other. For instance, d-transition metal ions
with long excited-state lifetimes such as Cr III can populate the emissive states of near
infrared (NIR) emitting lanthanoid ions resulting in a shift of their apparent lifetimes
from the micro- to the millisecond range [14]. Another example stems from a pair of Cr III
ions sequentially transferring energy onto a single Er III

ion to generate molecular upcon-

version of NIR to visible light [15].
In this chapter, we describe basic photophysical properties of lanthanoid helicates,
along with photonic applications in biosciences and luminescent materials. Literature is
covered until August 2011 but only compounds for which quantitative luminescence data
(quantum yield, sensitization efficiency, lifetime) are available are taken into considera-
tion. A summary of lanthanoid photophysics can be found in reference [16]. The reader
interested in full details is referred to descriptions in earlier works by G. S. Ofelt [17], B.
R. Judd [18], G. H. Dieke [19], W. T. Carnall [20], C. A. Morrison [21] or S. H ufner [22].

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In addition, the reviews by C. G orller-Walrand and K. Binnemans on the rationalization of

ligand-field parameterization [23] and spectral intensities of f-f transitions [24] are very
useful and easy to read, with careful and precise definitions as well as very helpful dimen-
sional analysis. More recently, the topic edited by G. K. Liu and B. Jacquier sheds light on
lanthanoid-containing optical materials [25] while Judd-Ofelt theory has been summa-
rized in an elegant way by B. M. Walsh [26]. Applications of lanthanoid luminescence in
biosciences are summarized in a multiauthor topic edited by P. H

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published in 2011 [27].
Other relevant references deal with spin-orbit constants [28], energy level diagrams
[29], symmetry-related selection rules derived from group-theoretical considerations
[30], listings of f-f spectra [20,31], determination of radiative lifetimes [32], Judd-Ofelt
parameterization of f-f transition intensities [31,33], determination of radiative lifetimes
[34], mechanisms of energy transfer [35], influence of high-energy vibrations [36] or
interconfigurational 4f n $4f n 1 5d 1 transitions [37].

In view of the small absorption coefficients of f-f transitions, a sensitization process has
to be used, which is sketched in Figure 6.2. Q Ln is the overall quantum yield, obtained
upon indirect excitation into the ligand levels, while Q L Ln is the intrinsic quantum yield
measured upon indirect excitation into the excited f levels. They are related by the sensi-
tization efficiency h sens .When Q L Ln cannot be determined, it may be estimated with the
observed and radiative lifetimes (see Equation 6.1); in turn, the radiative lifetime can be