ethyl acetate range from 0.26 to 0.52 (Table 1). The
photoluminescence (PL) features reveal typical characteristics
of conjugated fluorene derivatives.13 More importantly, the
emission maxima (Figure 2) of FTn can be controlled by
Figure 3. Comparison of the cyclic voltammogram of FT2 and
FT4.
shows a smaller potential difference between two redox
processes. However, the pronounced potential differences
between the two oxidations (410-280 mV) and two reduc-
tions (240-140 mV) indicate that the monocationic and
monoanionic species could efficiently delocalize the charge
over the entire conjugated backbone. These results are
consistent with the significant red shifts in the absorption
bands. The increasing conjugation length is due to an
increasing delocalization of the π-electron system along the
backbone.
Figure 2. Photoluminescent spectra of FTn (n ) 1-4). Inset shows
the color of emission in CHCl3, from top to bottom, FT1, FT2,
FT3, FT4.
varying the number n (n ) 1-4) of the oligothiophene
moieties of the conjugated backbone. The color of the
fluorescence can be achieved from light blue to bright yellow
(Figure 2, inset).
Figure 4 shows the spectral response of FT2 at various
applied voltages. Between 0 and 1.00 V, the intensity of
absorption at 405 nm of the neutral FT2 decreased, while
new peaks grew at 609, 680, 1022, and 1202 nm. Upon
increasing the applied voltage, the intensity of these new
peaks was continuously enhanced with an isosbestic point
at 460 nm (Figure 4a). These new forming visible bands (609
and 680 nm) and strong near-IR absorption bands (1022 and
1202 nm) were ascribed to the absorption of the first
oxidation state. When the applied voltages were above 1.00
mV, the intensity of the long wavelength peaks started to
diminish in intensity, while a new peak appeared at 351 nm
with increasing intensity until the applied voltage reached
1.40 V. These spectral changes could be fully restored when
the voltage was cycled between 0.0 and 1.40 V. The spectral
changes at various applied voltages were in agreement with
the CV experiments, in which the first and second oxidation
potentials of FT2 were observed at 1.01 and 1.36 V,
respectively. The spectroelectrochemistry indicated that the
oxidation only involves a one-step one-electron redox
process. In contrast, the spectral response of FT2 in the
reduction region by using platinum gauze or reticulated
vitreous carbon as a working electrode did not exhibit any
isosbestic point(s) (Supporting Information). For the latter
case, the intensity of a new broad absorption (580 nm)
increased upon increasing the negative voltage from -0.55
to -2.12 V. This result suggests that the radical anion is
relatively more reactive during the electrolysis as compared
to its radical cation.
Cyclic voltammetry (CV) experiments were conducted on
fluorene-capped oligothiophenes FTn at room temperature
to probe their electrochemical properties. For FTn, two
discrete reversible oxidation potentials were observed in CH2-
Cl2 (0.1 M nBu4NPF6 as a supporting electrolyte). Unexpect-
edly, two reversible reduction potentials were also detected
in THF (glassy carbon electrode, 0.1 M nBu4NClO4 as a
supporting electrolyte) (Table 1). This finding indicates the
possibility that an n-doping property is induced in the
oligomers when 9,9-diphenylfluorenes are incorporated into
the oligothiophenes as the terminal groups. Figure 3 shows
a comparison of the cyclic voltammogram of FT2 and FT4.
The oxidation potentials tend to decrease as the chain of the
oliogothiophene core is lengthened, while FTn with shorter
oligothiophene cores exhibit more negative reduction po-
tentials. The oligomer with the longer conjugation length
(12) Representative Procedure for Synthesis of FT2. 9,9-Diphenylfluo-
rene-2-pinacol boronate (2) (930 mg, 2.1 mmol), 5,5′-dibromo-2,2′-
bithiophene (3, n ) 2) (324 mg, 1.0 mmol), Pd(PPh3)4 (23 mg, 0.02 mmol),
2 M K3PO4 (2 mL), 1,4-dioxane (20 mL), and tri-tert-butylphosphine (0.05
M in toluene, 1.2 mL, 0.06 mmol) were stirred and refluxed under argon
for 2-3 days. The cooled reaction mixture was quenched with saturated
sodium bicarbonate and extracted with chloroform. The combined organic
extracts were dried over MgSO4 and concentrated by rotary evaporation.
The crude product after washing with hot hexane was recrystallized from
CHCl3/hexanes to afford FT2 (567 mg, 71%) as a yellow solid (see the
Supporting Information for spectroscopic characterization).
(13) (a) Katsis, D.; Geng, Y. H.; Ou, J. J.; Culligan, S. W.; Trajkovska,
A.; Chen, S. H.; Rothberg, L. J. Chem. Mater. 2002, 14, 1332. (b) Belleteˆte,
M.; Ranger, M.; Beaupre´, S.; Leclerc, M.; Durocher, G. Chem. Phys. Lett.
2000, 316, 101.(c) Weinfurtner, K.-H.; Weissortel, F.; Harmgarth, G.;
Salbeck, J. Proc. SPIE-Int. Soc. Opt. Eng. 1998, 3476, 40.
Org. Lett., Vol. 4, No. 25, 2002
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