Chemistry Reference
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for this purpose [
73
]. The 10
H
-spiroanthracenyl groups are orthogonal to the main
backbone vector but do not contribute to electron delocalization of the backbone,
which also serve as “molecular spacers” to effectively shield the backbone and
increase the donor-acceptor distance upon complexation with dye-labeled
nucleic acid. FRET between 12 and ssDNA
8
-Fl was studied and compared with
11 which have nearly the same energy levels as 12. These polymers have similar
polymer fluorescence quenching upon addition of ssDNA
8
, while the polymer-
sensitized Fl emission is totally different. As shown in Fig. 4, excitation of 11
results in negligible Fl emission, whereas strong Fl emission is obtained by using 12
as the energy donor. Likewise, Woo et al. have shown that changing the counterions
of 1Br from bromide into tetraphenylborate can also minimize PET process, which
was attributed to increased donor-acceptor distance owing to the increased size of
counterions [
74
].
To optimize the molecular orientation (
k
), nonlinear CCP and star-shaped CCP
oligomer were synthesized. The introduction of
m-
phenyl groups into 1Br leads to a
CCP (13, Scheme
8
) with nonlinear “kinks” along the backbone [
75
]. It was found
that although there is a progressive blueshift in absorption with increasing
m-
phenyl
content, FRET efficiency is increased, leading to twofold more intense polymer-
sensitized fluorescence of dsDNA
8
-Fl (dsDNA
8
-Fl was obtained by hybridization
of the ssDNA
8
-Fl with ssDNA
9
:5
0
-AGC ACC CAC ATA GTC AAG AT-3
0
)as
compared to that of 1Br. A water-soluble cationic tetrahedral molecule 14 was also
synthesized, and the energy transfer from 14 to dsDNA
8
-F1 was studied and
compared with its arm and 1Br [
76
]. The results show that 14 provides the highest
Fl emission intensity. Fig.
5
shows the comparison of FRET between 1Br and 14 to
Fig. 4 PL spectra of (a) 11/ssDNA
8
-Fl and (b) 12/ssDNA
8
-Fl in water at pH
¼
8: ([ssDNA
8
-
Fl]
10
7
M); excitation at 380 nm. The spectra are not
normalized according to the polymer emission [
73
]
10
8
M, [12]
¼
1.5
¼
[11]
¼
3
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