Band gap control in conjugated oligomers via Lewis acids

Article abstract of DOI:10.1021/ja902789w

(Graph Presented) A simple and effective strategy for optical band gap control is demonstrated through the use of a novel small acceptor/donor/acceptor molecule, 1, and group-13 Lewis acids. Chromophore 1 contains a dithienolesilole donor unit end-capped

Full text of DOI:10.1021/ja902789w

Published on Web 07/08/2009
Band Gap Control in Conjugated Oligomers via Lewis Acids
Gregory C. Welch, Robert Coffin, Jeff Peet, and Guillermo C. Bazan*
Center for Polymers and Organic Solids, Department of Chemistry & Biochemistry and Department of Materials,
UniVersity of California, Santa Barbara, California 93106
Received April 7, 2009; E-mail: bazan@chem.ucsb.edu
New approaches to the design of organic chromophores are of
interest for a variety of optical and optoelectronic technologies. In
the area of solar energy conversion, it is essential that the absorption
of the light-harvesting material match the spectral characteristics
of the sun.1 This condition applies equally to dye-sensitized cells2
and devices based on organic semiconducting materials.3 Some of
the best-performing bulk heterojunction “plastic” solar cells are
fabricated with conjugated polymers that have backbones compris-
ing alternating donor/acceptor (D/A) comonomer units. This
structural motif leads to excited states with charge-transfer char-
acteristics and to energy transitions on the order of 1.4 eV.4 Several
of these conjugated copolymers contain benzo-2,1,3-thiadiazole
(BT) as the electron-acceptor unit and silicon- or carbon-fused
bithiophene as the donor fragment.5-7 Many oligomers containing
BT display absorption only within the blue region of the visible
spectrum.8
Figure 1. Absorption spectra of 1 plus varying amounts of B(C6F5)3 in
While the absorption of conjugated oligomers can be tuned via
structural modification, the required synthesis can be rigorous and
time-consuming. It occurred to us that it would be possible to
modulate the electronic properties of the BT fragment via interac-
tions with Lewis acids that bind nitrogen.9,10 Herein, we show that
this simple approach makes it possible to access a range of
chromophores starting with a single, well-defined oligomer. Band
gap control is readily achieved by varying the strength of the Lewis
acid.
o-DCB at 25 °C ([1] ) 3.78 × 10-4 M).
of B(C6F5)3 in increments of 0.1 mol revealed the existence of two
separate isosbestic points at ∼537 and ∼557 nm, indicating the
presence of more than two absorbing species in solution.12 These
data are consistent with the stepwise formation of mono- and bis-
Lewis acid adducts. On the basis of the propensity of boranes to
bind harder nitrogen bases over their softer sulfur counterparts, we
propose the structure of the adducts to be 2 and 3, respectively, as
shown in Scheme 1.14
Scheme 1. Synthetic Route to 1 and the Lewis Acid Adducts 2 and
3 (R ) C12H25
Deep-blue 3 was isolated as a solid in greater than 90% yield
after addition of 2 equiv of B(C6F5)3 to 1 followed by solvent
removal.
)
Its absorption characteristics are identical to those in Figure 1 for
the addition of 2 equiv of B(C6F5)3. The 1H NMR spectrum of 3 in
CD2Cl2 at 25 °C showed four CH resonances from 7.4 to 8.4 ppm;
the two B(C6F5)3 moieties are thus bound at opposite ends of the
molecule. The 11B and 19F NMR spectra showed no evidence of
free B(C6F5)3, and the 19F NMR spectrum exhibited 15 independent
signals upon cooling to -30 °C. There is thus restricted rotation
about the B-N and B-C6F5 bonds.12,15 Addition of excess PPh3
to 3 gave rise to an immediate color change from blue to the red
characteristic of 1 along with the formation of Ph3P-B(C6F5)3, as
determined by 31P NMR spectroscopy.16 These observations indicate
that displacement of the Lewis acid is possible by using a stronger
base. Significantly, there is no change in the conjugated framework
of 1.
Our studies focused on 1, which has an A/D/A structure and
was synthesized via the microwave-assisted Stille cross-coupling
reaction between BT-Br and DTSC12, as outlined in Scheme 1.11
Precipitation with methanol and purification via chromatography
gave the desired product as a red solid in 85% yield. Compound 1
was characterized by multinuclear NMR spectroscopy, high-
resolution mass spectrometry, and elemental analysis.12
1
Complex 2 could not be independently isolated. The H NMR
spectrum of a 1:1 ratio of B(C6F5)3/1 in CD2Cl2 exhibited broad
signals, indicating the exchange of borane between the nitrogen
atoms. When the solution was cooled to -30 °C, the exchange
was slowed, and a mixture of 1, 2, and 3 in a 1:2:1 ratio was
observed.12 Binding of B(C6F5)3 to the two BT units is therefore
statistical.
While good-quality single crystals of 2 and 3 could not be
obtained, it was possible to do so for the adduct containing B(C6F5)3
and BT-Br, i.e., species 4. The solid-state structure of 4 determined
by X-ray diffraction studies is summarized in Figure 2.12 Consistent
with our expectations, B(C6F5)3 binds to nitrogen instead of sulfur.
Figure 1 shows that the absorbance spectrum of 1 in o-
dichlorobenzene (o-DCB) exhibits an absorption maximum (λmax
)
at 503 nm with an onset (λonset) at 577 nm. A color change from
red to green to blue was observed by visual inspection upon addition
of 2 equiv of the Lewis acid B(C6F5)3.13 Analysis by UV-vis
spectroscopy revealed the disappearance of the primary band of 1
and the appearance of a new transition with λmax ) 647 nm.
Addition of more than 2 equiv of B(C6F5)3 resulted in minimal
intensity change at 647 nm and the emergence of a band at λmax
)
305 nm due to unbound B(C6F5)3. Stepwise addition of 0-2 equiv
9
10802 J. AM. CHEM. SOC. 2009, 131, 10802–10803
10.1021/ja902789w CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
Furthermore, it chooses to do so at the 1-position of the BT unit,
opposite the Br atom, presumably because of steric constraints. A
similar regiochemistry is expected in 2 and 3 (Scheme 1). Addition
of excess borane to 4 or 3 did not result in formation of a second
or third B-N bond, respectively, from 25 to -50 °C, as determined
by 1H and 19F NMR spectroscopy.12 This lack of subsequent
reactivity is likely due to the combination of steric constraints and
the depletion of electronic density within the BT fragment upon
binding the first equivalent of B(C6F5)3.
Additionally, it is noteworthy that there is a synergistic lowering
of the absolute energies of the two frontier molecular orbitals. The
LUMO, however, is lowered by an additional 0.43 eV relative to
the HOMO, thereby giving rise to the narrower band gap.
In conclusion, the synthesis, structural characterization, and
optical properties of 1, 2, and 3 demonstrate a simple strategy for
tuning the optical properties of an A/D/A chromophore with charge-
transfer excited-state characteristics. The basic strategy involves
Lewis acid complexation to a basic site within the π-delocalized
framework. Our current thinking is that this complexation increases
the electron affinity of the BT acceptor group, thereby stabilizing
the charge-transfer characteristics of the excited state. While the
red-shifting of nitrogen-containing conjugated organic materials can
be achieved by protonation19 or metal-ion complexation,20 the
present method offers the ability to progressivelly shift to lower-
energy transitions by increasing the strength of the Lewis acid. We
anticipate that the approach will be general for small molecules,
oligomers, and even D/A-conjugated copolymers in which the
acceptor fragments provide sterically unencumbered lone pairs of
electrons.
Figure 2. POV-Ray depiction of 4. Hydrogen atoms have been omitted
for clarity.
Acknowledgment. Financial support from the National Science
Foundation (DMR 0606414), the Institute for Collaborative Bio-
technologies, and the Office of Naval Research is gratefully
acknowledged. G.C.W. is the recipient of a Canadian NSERC PDF
Scholarship.
The absorption spectra of 1 and 3 in the solid state were
compared to those in solution. The spectrum of solid 1 showed
minimal change in λmax relative to that for 1 in solution, although
the appearance of a shoulder at 544 nm is indicative of multichro-
mophore interactions.12 No such differences were observed for 3,
consistent with the fact that the bound B(C6F5)3 increases the
distance between optically active fragments to the point where
significant through-space interactions do not take place.17
Note Added after ASAP Publication. The Supporting Information
PDF file published ASAP July 8, 2009, was an incomplete early version.
The final version Supporting Information PDF file was published July
16, 2009.
Supporting Information Available: General experimental and
characterization procedures and a CIF file for 4. This material is
available free of charge via the Internet at http://pubs.acs.org.
References
(1) Roncali, J. Macromol. Rapid Commun. 2007, 28, 1761.
(2) Gratzel, M. J. Photochem. Photobiol., C 2003, 4, 145.
(3) Scharber, M. C.; Wuhlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.;
Heeger, A. J.; Brabec, C. L. AdV. Mater. 2006, 18, 789.
(4) Dennler, G.; Scharber, M. C.; Brabec, C. J. AdV. Mater. 2009, 21, 1323.
(5) Hou, J.; Chen, H. Y.; Zhang, S.; Li, G.; Yang, Y. J. Am. Chem. Soc. 2008,
130, 16144.
(6) Peet, J.; Kim, Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A. J.;
Bazan, G. C. Nat. Mater. 2007, 6, 497.
(7) Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T. Q.; Dante, M.;
Heeger, A. J. Science 2007, 317, 222.
(8) Anant, P.; Lucas, N. T.; Jacob, J. Org. Lett. 2008, 10, 5533. (b) Kato, S.;
Matsumoto, T.; Shigeiwa, M.; Gorohmaru, H.; Maeda, S.; Ishi-i, T.; Mataka,
S. Chem.sEur. J. 2006, 12, 2303. (c) van Mullekon, H. A. M.; Vekemans,
J. A. J. M.; Meijer, E. W. Chem.sEur. J. 1998, 4, 1235.
(9) Focante, F.; Mercandelli, P.; Sironi, A.; Resconi, L. Coord. Chem. ReV.
2006, 250, 170.
Figure 3. Normalized absorption spectra of 1 in o-DCB in the presence
of 2 equiv of various Lewis acids.
(10) (a) Fraleoni-Morgera, A.; Giorgini, L.; Zanirato, P. Dyes Pigments 2008,
76, 394. (b) Cazenobe, I.; Ledoux, I.; Zyss, J.; Thornton, A.; Bruce, D. W.;
Kakkar, A. K.; Lesley, G. M. J.; Woodward, A.; Taylor, N. J.; Marder,
T. B. Chem. Mater. 1998, 10, 1355. (c) Lopez-Garriga, J. J.; Babcock,
G. T.; Harrison, J. F. J. Am. Chem. Soc. 1986, 108, 7131. (d) Apblett, A.;
Chivers, T.; Richardson, J. F. Can. J. Chem. 1986, 64, 849.
(11) Son, H. J.; Han, W. S.; Chun, J. Y.; Kwon, S. N.; Ko, J.; Kang, S. O.
Organometallics 2008, 27, 2464.
To explore the generality of the band gap modification, a range
of Lewis acids [AlMe3, AlEtCl2, Al(C6F5)3, BCl3, and BBr3] were
added to 1 in o-DCB and the UV-vis spectra of the resulting
mixtures recorded. As can be seen in Figure 3, a series of red-
shifted complexes that cover most of the visible region were
obtained. Addition of the relatively weak acid AlMe3 resulted in
the formation of a violet solution, a 70 nm red shift in λmax, and a
109 nm red shift in λonset. The effect of the strongest Lewis acid,
BBr3, was more pronounced, yielding a blue-green-colored complex.
In this case, λmax and λonset were remarkably red-shifted by 178 and
257 nm, respectively, relative to 1.12 Indeed, the optical absorption
of the BBr3 analogue of 3 approaches that of known polymers
containing DTS and BT repeat units.5 The general trend shows that
the larger red shifts are obtained with stronger Lewis acids.
The nature of the bathochromic shifts was investigated compu-
tationally.12 Comparison of the DFT-optimized structures of 1 and
318 shows that the HOMO and LUMO become less delocalized
across the π-conjugated backbone upon Lewis acid attachment. In
other words, the Lewis acid withdraws electron density away from
the BT unit, altering the electron density of the chromophore.
(12) See the Supporting Information.
(13) Piers, W. E. AdV. Organomet. Chem. 2005, 52, 1.
(14) Pearson, R. G. J. Am. Chem. Soc. 1963, 85, 3533.
(15) Focante, F.; Camurati, I.; Resconi, L.; Guidotti, S.; Beringhelli, T.;
D’Alfonso, G.; Donghi, D.; Maggioni, D.; Mercandelli, P.; Sironi, A. Inorg.
Chem. 2006, 45, 1683.
(16) Jacobsen, H.; Berke, H.; Do¨ring, S.; Kehr, G.; Erker, G.; Fro¨hlich, R.;
Meyer, O. Organometallics 1999, 18, 1724.
(17) Bazan, G. C. J. Org. Chem. 2007, 72, 8651.
(18) Methyl and chlorine substituents were used in place of the dodecyl side
chains and C6F5 aryl rings, respectively.
(19) (a) Hancock, J. M.; Jenekhe, S. A. Macromolecules 2008, 41, 6864. (b)
Fukumoto, H.; Yamamoto, T. J. Polym. Sci., Part A: Polym. Chem. 2008,
46, 2975. (c) Monkman, A. P.; Pa˚lsson, L. O.; Higgins, R. W. T.; Wang,
C.; Bryce, M. R.; Batsanov, A. S.; Howard, J. A. K. J. Am. Chem. Soc.
2002, 124, 6049.
(20) Tolosa, J.; Zucchero, A. J.; Bunz, U. H. F. J. Am. Chem. Soc. 2008, 130,
6498.
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