Chemistry Reference
In-Depth Information
N
N
2P
S
1
ET
FRET
O
O
a
S
1
c
S
1
O
NH
O
HN
FRET
O
isc
O
O
O
HN
NH
O
O
c
T
1
N
N
H
N
N
N
H
N
H
2PA
O
O
ET
N
Pt
O
O
O
O
N
N
phos
O
O
N
O
O
N
O
N
O
O
O
H
H
[PtP
-
-C343
+
]
N
N
HN
O
NH
O
a
S
1
c
S
0
O
O
NH
HN
O
O
O
O
φ
φ
FRET
= 0.9
phos
= 0.0065
N
N
FIGURE 14.18
Pt porphyrin octa-coumarin-343 construct and associated energy
pathways [97].
that in the simpler compound in Figure 14.16, suggesting that other pathways of
energy dissipation exist and may be important for the design of 2P-enhanced
phosphorescent sensors. The measurements performed on the compound with
the number of antenna fragments doubled (Figure 14.18) fully agreed with this
assumption. While the quantum yield of FRET in this Pt porphyrin octa-
coumarin-343 construct was still high (
f
FRET
¼
0.9), the yield of phosphores-
cence was extremely low (
0.0065), most likely due to the competing
electron transfer involving the long-lived triplet state of the Pt porphyrin and
C343-antenna.
In this molecule, C343 fragments are attached to the meta-positions in the
meso-aryl rings on the PtP core via flexible linkers, while the reduction potential
of the octacarboxy core is lower than that of the tetracarboxy core due to the
electron-withdrawing effect of carboxyl groups. The combined effect is that
the ET process in this case is significantly more effective than in the case of the
tetra-C343 adduct. In order to avoid this undesirable ET, the redox potentials of
the core and the antenna need to be adjusted; and the antenna fragments need to be
placed far enough from the core, but still within the Forster radius r
0
,sothatthe
probability of the ET is minimized, while the efficiency of FRET remains high. It
is, therefore, reasonable to assign an additional role to the dendrimer in two-
photon oxygen probes. Namely, the dendrimer should provide a supporting
distance-tuning matrix, in which the active elements of the device (antenna and
the core) would be positioned at optimal distances from one another to ensure the
desired energy distribution.
The interplay between 2PA, FRET, and ET processes and phosphorescence was
studied using model bichromophoric systems based on PtTBPs and rhodamine B
(RhB) derivatives [100]. RhB is a significantly stronger 2P absorber than C343.
f
phos
¼
