Chemistry Reference
In-Depth Information
6.3.3.3 Molecular Upconversion in a CrErCr Helicate
Upconversion is a phenomenon in which absorption of two (or more) photons of same
energy by a chromophore results in the emission of light with a shorter wavelength. The
phenomenon was evidenced by F. Auzel in 1966 and several lanthanoid ions are amenable
to this phenomenon, particularly Er
III
which is extensively provided in telecommunica-
tions [99], upconverting nanoparticles for luminescent immunoassays [56,75], and which
is being tested in energy conversion of the NIR solar spectrum [100]. Several mechanisms
lead to upconversion and two of them, excited state absorption (ESA) and energy transfer
upconversion (ETU) are sketched in Figure 6.18.
The tritopic ligand L29 (Scheme 6.6) self-assembles with triads of ions to generate
molecular upconversion in the CrErCr helicate, both in solid state and in acetonitrile solu-
tion [15]. NIR excitation of this helicate in the Cr
2
T
1
;
2
E
4
A
2
Þ
absorption band
around 750 nm with a Ti-sapphire laser indeed results in the green emission of Er
III
at
540 nm. The mechanism involved is described on the upper right part of Figure 6.18: there
is sequential energy transfer, first from the
2
E level of an excited Cr
III
ð
ion, leading to
4
I
9=2
9=2
Þ
,folowedbyan r romthe condC
III
Er
ð
ion,
inducing the
Er
III
4
I
11=2
11=2
transition);
fast nonradiative deactivation then populates the emissive
4
S
3=2
3=2
level. At 77 K, the yield
of the Cr to Er energy transfer is 50%. Logarithmic plots of the intensity of the upcon-
verted green emission versus the laser power yield straight lines with slopes 1.8 (solid
state) and 2.1 (acetonitrile solution), diagnostic of upconversion. The implied mechanism
is most probably ETU. This phenomenon is made possible for the following reasons: (i)
the high local concentration of the Cr
III
2
G
4
F
4
I
9=2
9=2
transition (or, alternatively, the
4
F
5=2
5=2
ð
;
Þ
9=2
sensitizers (equivalent to 0.55M), (ii) the
Figure 6.18 Energy scheme for excited state absorption (ESA) and energy transfer (ETU)
mechanisms and molecular up-conversion in [CrErCr(L29)
3
] (CF
3
SO
3
)
9
.
Redrawn with per-
mission from [15]. Copyright 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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