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(a)
(b)
[Eu
2
(L11)
3
]
[E (L11) ]
[Eu
2
(L7)
3
]
[Eu(dpa) ]
3-
3
]
3
×10
575
625
675
725
λ
/ nm
Figure 6.9 (a) 2PA-([Eu
2
(L11)
3
]) and 3PA-([Eu
2
(L7)
3
], [Eu(dpa)
3
]
3
)] excited emission spectra
(l
exc
¼800 nm) of the helicates tested for cell staining and, as a comparison, the tris(dipicoli-
nate) mononuclear complex. (b) Superposition of bright field and 2PA luminescence confocal
microscopy images of HeLa cells incubated with [Eu
2
(L11)
3
] 200 mM and excited at 750 nm.
Redrawn with permission from [78]. Copyright 2010 American Chemical Society.
therefore be as long as 700-800 nm, that is in the transparency window of biological tis-
sues. Helicates [Ln
2
(L7)
3
]and[Ln
2
(L11)
3
](Ln
Eu, Tb) can indeed be excited in this
way, the former by a three-photon process (3PA) and the latter by two-photon absorption
(2PA) [78]. Luminescence spectra for the Eu
III
helicates obtained upon excitation at
800 nm are shown in Figure 6.9 (left). The multiphoton cross-sections remain modest
(Table 6.8), but they are much larger than for the reference mononuclear tris(dipicolinate)
complexes. Note that units of 2PA and 3PA cross-sections are not the same and therefore
the corresponding data cannot be directly compared, so that there is no contradiction in
the fact that the emission spectrum of [Eu
2
(L11)
3
] shown in Figure 6.9 has a much larger
intensity compared to emission from [Eu
2
(L7)
3
] despite a seemingly threefold increase
only in multiphoton cross-section! It is not known why [Eu
2
(L11)
3
] does not present 3PA
absorption, which would correspond to a spectral range (267 nm) where its molar absorp-
tion coefficient is larger than for [Eu
2
(L7)
3
]. Further, instrumentation limitations pre-
vented the determination of s
¼
(2)
at a wavelength of 760 nm corresponding to the
Table 6.8 Multiphoton absorption cross sections for mono- and di-nuclear complexes in
aqueous solutions at pH 7.4 (Tris-HCl 0.1M) determined upon irradiation at 800 nm[78].
Compound
Cs
3
[Eu(dpa)
3
] s
3
[Tb(dpa)
3
]
[Eu
2
(L7)
3
]
[Tb
2
(L7)
3
]
[Eu
2
(L11)
3
]
max
abs
l
(nm)
271
271
322
322
350
max
abs
(M
1
cm
1
)
13 200
13 200
77 300
77 300
80 300
e
Q
1PA
a
0.29
0.21
0.21
0.11
0.09
eff
abs
(nm)
b
l
267
267
267
267
400
eff
abs
(M
1
cm
1
)
9500
9500
42 700
42 700
920
e
Process
3PA
3PA
3PA
3PA
2PA
(
n
)
(GM)
s
0.011
0.014
0.26
0.28
0.75
a
Quantum yield upon one-photon excitation.
b
(
n¼
2 or 3).
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