Fluorescence Reporting Based on FRET Between Conjugated Polyelectrolyte and Organic Dye for Biosensor Applications - Advanced Fluorescence Reporters in Chemistry and Biology - page 431

levels are ( 5.6 eV, 2.8 eV) for 1,( 5.4 eV, 2.6 eV) for 9,( 5.8 eV, 2.7 eV)

for 10, and ( 5.4 eV, 2.6 eV) for 11, respectively. Fluorine substitution raises the

energy levels by 0.2 eV, while the methoxy groups lower the levels by 0.2 eV,

relative to those of 1Br. The LUMO energy of Fl ( 3.6 eV) is well located within

the band gap of these polymers. However, the HOMO energy of Fl ( 5.9 eV) is

lower than that of 1Br ( 5.4 eV). As a result, PET

process could occur between 1Br, 9 or 11, and Fl, and to less extent between 10 and
Fl. The HOMO energy of TR (

5.6 eV), 9 ( 5.4 eV) and 11 (

5.4 eV) is higher than that of 1Br and 10, and is
close to that of 9. From the HOMO-LUMO energy level point of view, TR matches
these polymers better than F1. FRET experiments between 1Br, 9 and 10 and
ssDNA 8 -F1 or ssDNA 8 -TR were performed by monitoring dye emission upon
excitation of 1Br (385 nm), 9 (365 nm), and 10 (365 nm). The polymer concentra-
tions were based on polymer repeat unit (RU). As shown in Fig. 3a , the most intense
Fl emission was observed for 10/ssDNA 8 -Fl, which is approximately twofold more
intense than that for 1Br /ssDNA 8 -Fl and is over an order of magnitude higher than
that for 9/ssDNA 8 -Fl. For 10/ssDNA 8 -Fl, the integrated Fl emission is approxi-
mately fivefold greater than that obtained by direct excitation of Fl at its absorption
maximum (495 nm) in the absence of CCPs. In contrast to the low F1 emission for
9/ssDNA 8 -F1, TR emission from 9/ssDNA 8 -TR is much higher (Fig. 3b ). This is
due to better matched energy levels between 9 and TR than that for 9/F1. This study
indicates the importance of energy level alignment between the donor and acceptor
in determining the CCP-sensitized dye emission.
On the other hand, it is known that PET is essentially a contact process described
by an exponential distance dependence, which functions effectively at a donor-
acceptor distance considerably shorter than that for FRET processes [ 72 ]. Thus, it is
also feasible to increase donor-acceptor distance to reduce PET. A poly(fluorene-
alt -1,4-phenylene) derivative with spiroanthracenyl groups (12) was synthesized

a b

510 5

510 5

410 5 410 5

310 5 310 5

210 5

210 5

110 5

110 5

0

0

500 520 540 560

Wavelength (nm)

580 600 620

580 600 620 640 660 680

Wavelength (nm)

Fig. 3 PL spectra of ssDNA 1 -Fl (a) and ssDNA 1 -TR (b) in the presence of 1Br ( black ), 9 ( blue ),
and 10 ( red ) in 25 mM phosphate buffer at (ssDNA 1 -Fl or ssDNA 1 -TR) ¼ 2 10 8 M and
[RU] ¼ 4 10 7 M. The excitation wavelengths are 385 nm for 1Br and 365 nm for 9 and 10.
Direct excitation of ssDNA 1 -Fl and ssDNA 1 -TR prior to polymer addition is also shown in green
and orange , respectively [ 71 ]