Journal of the American Chemical Society
Communication
acidity of Cl-SiNCs. The present PDI is typical for oxidative
polymerization methods.
obtained from Cl-SiWFs were 4.0k, 9.5k, and 2.4, respectively.
14
Polymerization of 1 induced by SiCl provided P3HT with H/
4
Evaluation of the optical properties of the present P3HT is
H-terminated P3HT as a major product, as evidenced by
MALDI-TOF analysis (Figure S4b). GPC evaluation provided
M , M , and PDI of 3.9k, 8.4k, and 2.1, respectively. Similar to
crucial if these materials are to find future applications in
1
4,29,30
optoelectronic devices.
UV−vis spectroscopy was
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performed on chloroform solutions of P3HT and thin solid
films drop-cast from this solution onto quartz wafers. The
solution-phase spectrum (Figure 4a, dotted trace) shows a
the observations made for polymers prepared using Cl-SiNCs,
1
H NMR analysis (Figure S5) revealed the polymers obtained
from the reactions of 1 with Cl-SiWFs and SiCl to be 64 and
4
6
2% regioregular (HT), respectively. The optical properties of
the polymers obtained from these reactions are equivalent to
those of the P3HT obtained from Cl-SiNCs (Figure S6).
To obtain a more direct comparison of the catalytic
efficiencies of Si−Cl-containing reagents, we performed
reactions using each catalyst under identical conditions. The
present comparison of catalytic performance cannot be viewed
as strictly quantitative because knowing the exact number of
catalytic sites for Cl-SiNCs and Cl-SiWFs is impossible. Still, we
believe that this comparison provides valuable insight into the
origin of our observations. The results are summarized in Table
S1. Reactions with all three catalysts produced polymer, and the
M , M , and yield were highest for the product of the reaction
Figure 4. (a) UV−vis absorption spectra and (b) PL emission spectra
upon excitation at 350 nm for P3HT obtained from a 6 h reaction with
Cl-SiNCs, recorded on a 0.05 mg/mL solution in CHCl (dotted
3
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trace) and a solid film (solid trace). Insets show the P3HT solution in
involving Cl-SiNCs. These data are consistent with the
qualitatively slower reactions noted for the Cl-SiWF and
CHCl and the solid film under ambient light and UV excitation,
3
respectively.
SiCl catalysts and indicate that the high surface area and
4
surface Lewis acidity of the Cl-SiNCs play important roles in
the observed reactivity.
characteristic broad absorption at λmax = 431 nm; a slight red
shift to λmax = 451 nm is noted for the cast films (Figure 4a,
solid trace). This red shift has been previously attributed to the
increased conjugation resulting from interchain interactions of
In conclusion, three silicon-based metal-free catalysts were
employed to prepare P3HT. These catalysts have various
attractive features, including reasonable reaction times and
straightforward separation from the target products. The
presented syntheses of the workhorse conjugated polymer
P3HT indicate that abundant silicon-based catalysts offer
alternative approaches for the preparation of functional
materials free from costly and potentially harmful transition
metals.
1
4,30,31
P3HT in solid films due to π−π stacking.
In addition, the
absorption bandwidth is broader for the films, tailing out to ca.
6
50 nm.
Another characteristic P3HT property is its photolumines-
cence. PL spectra of the present P3HT are shown in Figure 4b.
The spectrum obtained from a CHCl solution affords a
3
featureless emission with a maximum at 570 nm tailing out to
ASSOCIATED CONTENT
Supporting Information
Experimental procedures, polymer characterization, and addi-
8
00 nm (dotted trace). The PL maximum shifts to longer
■
*
S
wavelengths for the solid film deposited from CHCl solution,
3
showing a peak emission at 647 nm and two shoulder features
at 725 and 823 nm (solid trace). The appearance of the low-
energy shoulder features has previously been attributed to
32
polaron formation in solid films.
AUTHOR INFORMATION
Notes
To support our hypothesis that the polymerization reaction
arises as a consequence of the Lewis acidity of the Cl-SiNCs
and to eliminate possible contributions from monomer
photooxidation, we performed identical reactions in the
presence and absence of light (Table S1 in the SI). The two
reactions afforded equivalent products. Furthermore, an
additional control reaction performed under ambient light in
the absence of Cl-SiNCs yielded no polymer (Figure S3).
Having established that the reaction of 1 with Cl-SiNCs
provides P3HT under the presented conditions, we endeavored
to evaluate other Si−Cl-containing species for similar reactivity.
In this context, we performed equivalent reactions with Cl-
SiWFs [p-type Si(111) wafers; double side polished; 1 × 2
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors recognize NSERC for continued generous support.
Dr. Eric Rivard and William Torres Delgado are thanked for the
assistance with GPC analysis. Thanks are conveyed to all
Veinot Team members, including Dr. Muhammad Iqbal, Dr.
Mita Dasog, and Leah Coumont for their thoughtful
discussions and experimental assistance.
2
cm ] and silicon tetrachloride. Qualitative inspection of the
color of the reaction mixture suggested negligible polymer
formation after 6 h. Extending the reaction time to 24 h
afforded a red solution consistent with polymer formation.
MALDI-TOF analysis of the isolated products revealed that the
reaction catalyzed by Cl-SiWFs afforded H/Br- and Br/Br-
terminated P3HT with the number of repeat units ranging from
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(
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(
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to 22 (Figure S4a). The M , M , and PDI of the polymer
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C
dx.doi.org/10.1021/ja5075739 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX