A donor-acceptor dyad with a highly lewis acidic boryl group

Article abstract of DOI:10.1021/om700520n

A donor - π - acceptor dyad with a highly electron-deficient B(C ₆F₅)₂ group as the acceptor has been prepared. 5-Diphenylamino-5′-bis(pentafluorophenyl)boryl-2,2′-bithiophene (3) was fully characterized by multinuclear NMR spectroscopy, mass spectrometry, and X-ray crystallography. The solid-state structure of 3 shows the presence of multiple π-stacking interactions rather than Lewis acid - base bonding between boron and nitrogen. Cocrystallized benzene molecules are incorporated into a channel-like framework of 3. The electronic structure and photophysical properties were investigated by UV-visible absorption and emission spectroscopy and DFT calculations. The absorption and emission maxima experience a strong bathochromic shift relative to the non-fluorinated analogues, the origin of which has been further examined by DFT calculations.

Full text of DOI:10.1021/om700520n

6126
Organometallics 2007, 26, 6126-6131
A Donor-Acceptor Dyad with a Highly Lewis Acidic Boryl Group
Anand Sundararaman,† Resmi Varughese,† Haiyan Li,† Lev N. Zakharov,‡,§
Arnold L. Rheingold,‡ and Frieder Ja¨kle*,†
Department of Chemistry, Rutgers UniVersity-Newark, 73 Warren Street, Newark, New Jersey 07102, and
Department of Chemistry and Biochemistry, UniVersity of California, San Diego,
La Jolla, California 92093
ReceiVed May 25, 2007
A donor-π-acceptor dyad with a highly electron-deficient B(C6F5)2 group as the acceptor has been
prepared. 5-Diphenylamino-5′-bis(pentafluorophenyl)boryl-2,2′-bithiophene (3) was fully characterized
by multinuclear NMR spectroscopy, mass spectrometry, and X-ray crystallography. The solid-state structure
of 3 shows the presence of multiple π-stacking interactions rather than Lewis acid-base bonding between
boron and nitrogen. Cocrystallized benzene molecules are incorporated into a channel-like framework of
3. The electronic structure and photophysical properties were investigated by UV-visible absorption
and emission spectroscopy and DFT calculations. The absorption and emission maxima experience a
strong bathochromic shift relative to the non-fluorinated analogues, the origin of which has been further
examined by DFT calculations.
withdrawing substituents on boron have not been examined in
detail. Indeed, one of the general features of donor-bridge-
acceptor compounds that have been previously studied is the
use of bulky mesityl groups to sterically stabilize the boron
center.
Introduction
The electron-deficient nature of triorganoboranes provides for
intriguing properties including their ability to act as efficient
optoelectronic device materials and sensors for nucleophiles.1
A key feature that enables these applications is the overlap
between the empty pπ orbital on boron and an organic π-system
of one (or more) of the substituents. Among these conjugated
organoboranes, donor-π-acceptor dyads (D-π-A) represent
a unique class of compounds. These push-pull systems, in
which typically a dimesitylboryl group acts as the acceptor that
is separated from a donor group such as an amine, a phosphine,
a ferrocene moiety, or other electron-rich groups through a
π-conjugated bridge, have been shown to display interesting
linear and nonlinear optical properties, including large hyper-
polarizabilities and two-photon excited fluorescence (TPEF).2,3
Changes in the optical properties upon binding of nucleophiles
to the empty pπ orbital on boron have recently been exploited
in fluoride anion recognition.4 Suitably substituted donor-π-
acceptor organoborane derivatives have also been studied as
bipolar materials for organic light emitting devices (OLEDs)
and photovoltaic applications.5,6
We have recently examined the effect of the electronic
structure of aryl groups attached to boron on the photophysical
and nucleophile binding properties of symmetric diboranes such
as compounds A (Ar ) 4-tBuC6H4, C6F5, Fc/C6F5), in which
two highly Lewis acidic boryl groups are connected via a
conjugated bithiophene bridge.7,8 We have also reported several
(3) Recent examples: (a) Yuan, Z.; Taylor, N. J.; Ramachandran, R.;
Marder, T. B. Appl. Organomet. Chem. 1996, 10, 305-316. (b) Branger,
C.; Lequan, M.; Lequan, R. M.; Barzoukas, M.; Fort, A. J. Mater. Chem.
1996, 6, 555-558. (c) Albrecht, K.; Kaiser, V.; Boese, R.; Adams, J.;
Kaufmann, D. E. J. Chem. Soc., Perkin Trans. 2 2000, 2153-2157. (d)
Yamaguchi, S.; Shirasaka, T.; Akiyama, S.; Tamao, K. J. Am. Chem. Soc.
2002, 124, 8816. (e) Liu, Z.-Q.; Fang, Q.; Wang, D.; Xue, G.; Yu, W.-T.;
Shao, Z.-S.; Jiang, M.-H. Chem. Commun. 2002, 2900-2901. (f) Liu, Z.-
Q.; Fang, Q.; Wang, D.; Cao, D.-X.; Xue, G.; Yu, W.-T.; Lei, H. Chem.-
Eur. J. 2003, 9, 5074-5084. (g) Kim, S.; Song, K.-H.; Kang, S. O.; Ko, J.
Chem. Commun. 2004, 1, 68-69. (h) Charlot, M.; Porres, L.; Entwistle, C.
D.; Beeby, A.; Marder, T. B.; Blanchard-Desce, M. Phys. Chem. Chem.
Phys. 2005, 7, 600-606. (i) Thanthiriwatte, K. S.; Gwaltney, S. R. J. Phys.
Chem. A 2006, 110, 2434-2439. (j) Stahl, R.; Lambert, C.; Kaiser, C.;
Wortmann, R.; Jakober, R. Chem.-Eur. J. 2006, 12, 2358-2370. (k) Yuan,
Z.; Entwistle, C. D.; Collings, J. C.; Albesa-Jove´, D.; Batsanov, A. S.;
Howard, J. A. K.; Taylor, N. J.; Kaiser, H. M.; Kaufmann, D. E.; Poon,
S.-Y.; Wong, W.-Y.; Jardin, C.; Fathallah, S.; Boucekkine, A.; Halet, J.-
F.; Marder, T. B. Chem.-Eur. J. 2006, 12, 2758-2771.
In all of these intriguing applications, the nature of the donor
and the effect of different conjugated bridging groups have been
studied thoroughly.2 However, the nature of the boryl group
and, in particular, the effect of electron-donating or electron-
* To whom correspondence should be addressed. E-mail: fjaekle@
rutgers.edu.
(4) (a) Liu, Z.-Q.; Shi, M.; Li, F.-Y.; Fang, Q.; Chen, Z.-H.; Yi, T.;
Huang, C.-H. Org. Lett. 2005, 7, 5481-5484. (b) Liu, X. Y.; Bai, D. R.;
Wang, S. Angew. Chem., Int. Ed. 2006, 45, 5475-5478. (c) Bai, D. R.;
Liu, X. Y.; Wang, S. Chem.-Eur. J. 2007, 13, 5713-5723.
(5) Shirota, Y.; Kinoshita, M.; Noda, T.; Okumoto, K.; Ohara, T. J. Am.
Chem. Soc. 2000, 122, 11021-11022.
† Rutgers University-Newark.
‡ University of California, San Diego.
§ Current address: University of Oregon, Department of Chemistry,
Eugene, OR 97403-1253.
(1) Recent reviews: (a) Entwistle, C. D.; Marder, T. B. Angew. Chem.,
Int. Ed. 2002, 41, 2927-2931. (b) Entwistle, C. D.; Marder, T. B. Chem.
Mater. 2004, 16, 4574-4585. (c) Yamaguchi, S.; Tamao, K. Chem. Lett.
2005, 34, 2-7. (d) Ja¨kle, F. Coord. Chem. ReV. 2006, 250, 1107-1121.
(e) Ja¨kle, F. Boron: Organoboranes In Encyclopedia of Inorganic
Chemistry, 2nd ed.; King, R. B., Ed.; Wiley: Chichester, 2005; pp 560-
598. (f) Yamaguchi, S.; Wakamiya, A. Pure Appl. Chem. 2006, 78, 1413-
1424.
(6) (a) Okumoto, K.; Ohara, T.; Noda, T.; Shirota, Y. Synth. Met. 2001,
121, 1655-1656. (b) Jia, W.-L.; Song, D.; Wang, S. J. Org. Chem. 2003,
68, 701-705. (c) Jia, W.-L.; Bai, D.-R.; McCormick, T.; Liu, Q.-D.; Motala,
M.; Wang, R.-Y.; Seward, C.; Tao, Y.; Wang, S. Chem.-Eur. J. 2004, 10,
994-1006. (d) Jia, W. L.; Feng, X. D.; Bai, D. R.; Lu, Z. H.; Wang, S.;
Vamvounis, G. Chem. Mater. 2005, 17, 164-170. (e) Jia, W. L.; Moran,
M. J.; Yuan, Y.-Y.; Lu, Z. H.; Wang, S. J. Mater. Chem. 2005, 15, 3326-
3333.
(2) Yuan, Z.; Collings, J. C.; Taylor, N. J.; Marder, T. B.; Jardin, C.;
Halet, J.-F. J. Solid State Chem. 2000, 154, 5-12.
(7) Sundararaman, A.; Venkatasubbaiah, K.; Victor, M.; Zakharov, L.
N.; Rheingold, A. L.; Ja¨kle, F. J. Am. Chem. Soc. 2006, 128, 16554-16565.
10.1021/om700520n CCC: $37.00 © 2007 American Chemical Society
Publication on Web 11/01/2007

D-A Dyad with a Highly Lewis Acidic Boryl Group
Organometallics, Vol. 26, No. 25, 2007 6127
Scheme 1. Synthesis of the Donor-π-Acceptor Dyad 3
related main chain organoboron polymers B (Ar ) 4-iPrC6H4,
C6F5, Fc) and studied their behavior as polymeric Lewis acids
in the binding of amines.9 In this Article, we explore how the
relative Lewis acidity of boron affects the properties of a
D-π-A dyad. Specifically, we discuss the synthesis and
properties of a new D-π-A compound that features a bis-
(pentafluorophenyl)boryl group as the acceptor moiety and a
diphenylamino group as the donor that is separated by a
bithiophene bridge. In comparison to previously studied com-
pounds that feature mesityl (Mes) groups on boron, such as the
bipolar emitting OLED material C (Ar ) mesityl, Ar′ ) p-tolyl)5
and the bithiophene-bridged compound D (Ar ) mesityl/4-t-
BuC6H4, Ar′ ) Ph),10 the strongly electron-withdrawing nature
of the pentafluorophenyl groups was expected to enhance the
Lewis acidity of boron, thereby possibly leading to greater
electronic interaction through the bithiophene linker.11
instantaneous color change from dark red to colorless upon
exposure to air and therefore has to be handled and stored under
nitrogen. This new D-π-A system was fully characterized by
1H, 13C, and 11B NMR spectroscopy and high-resolution
MALDI-TOF-TOF mass spectrometry, and elemental analysis.
The 11B NMR spectrum of 3 displays a resonance in the
expected region for triarylboranes at δ 50.5, which indicates
that there is no significant intermolecular interaction between
1
nitrogen and the Lewis acidic boryl group. In the H NMR
spectra, the presence of four doublets for the bithiophene linker
confirms the unsymmetric substitution pattern at the 5,5′-
positions of bithiophene. For 3, these signals appear in the region
from 7.45 to 6.20 ppm in C6D6, and some of the resonances
are partially overlapping with the phenyl resonances. The
attachment of the bis(pentafluorophenyl)boryl group in 3 is
reflected in the 19F NMR spectrum, which shows a set of three
resonances at δ -131.1 (Fo), -151.0 (Fp), -161.9 (Fm), where
the separation of the meta- and para-fluorine atoms of ∆δm,p
Results and Discussion
Synthesis and Characterization. Lithiation of the free 5′-
position of 5-diphenylamino-2,2′-bithiophene and subsequent
treatment with Me3SiCl gave 5-trimethylsilyl-5′-diphenylamino-
2,2′-bithiophene (1) as a pale yellow solid in 82% yield. The
5′-borylated diphenylaminobithiophene 2 was obtained readily
via silicon-boron exchange with BBr3. The progress of the
reaction can be conveniently monitored by a change of the color
from pale yellow to dark red. The intermediate 2 was isolated
in 66% yield upon repeated recrystallization from toluene, and
its structure was established by 1H, 13C, and 11B NMR
spectroscopy. Treatment of 2 with 2 equiv of pentafluorophenyl
copper12-14 at -35 °C resulted in selective transfer of two
pentafluorophenyl groups to boron (Scheme 1). Repeated
recrystallization of the product from small amounts of toluene/
hexanes gave 3 as dark red crystals in 75% yield. Compound 3
is highly sensitive to air and moisture as evident from an
) 10.9 is considerably smaller than that in B(C6F5)3 (∆δm,p
)
18.4)14 as may be expected due to more effective π-overlap of
the pπ orbital on boron with the bithiophene π-system. The
chemical shift difference is similar to that of PhB(C6F5)2 with
∆δm,p ) 12.6.14
Dark red rodlike single crystals of 3 suitable for X-ray
analysis were obtained by slow solvent evaporation from
deuterated benzene, which is also incorporated into the solid.
The X-ray structure of 3 confirms the tri-coordinate environment
of boron and nitrogen and reveals a nearly coplanar arrangement
of the central thiophene rings with a small angle of 11.7°
between the S1,C1-C4 and S2,C5-C8 average planes (Figure
1). Selected bond distances and angles are included in Table 1.
The sum of the C-B-C angles of 360.0° and that of the
C-N-C angles of 359.8° demonstrate that the environments
at boron and nitrogen are trigonal planar. The bond distances
and bond angles of 3 are similar to those of the symmetric
bifunctional Lewis acid (C6F5)2B-Th-Th-B(C6F5)2 (A).7 For
(8) Related diborylated bithiophene and terthiophene derivatives that
feature mesityl groups on boron (A, Ar ) 2,4,6-trimethylphenyl) have been
studied by Shirota and co-workers as organic light emitting device (OLED)
materials. See: (a) Noda, T.; Shirota, Y. J. Am. Chem. Soc. 1998, 120,
9714-9715. (b) Noda, T.; Ogawa, H.; Shirota, Y. AdV. Mater. 1999, 11,
283-285.
(9) Sundararaman, A.; Victor, M.; Varughese, R.; Ja¨kle, F. J. Am. Chem.
Soc. 2005, 127, 13748-13749.
(10) Parab, K.; Venkatasubbaiah, K.; Ja¨kle, F. J. Am. Chem. Soc. 2006,
128, 12879-12885.
(11) It is interesting to note that a D-π-A system with a highly Lewis
acidic B(C6F5)2 moiety that is linked to a PMes2 group through a short
C6F4 bridge has recently been reported by Stephan to reversibly bind
dihydrogen: (a) Welch, G. C.; San Juan, R. R.; Masuda, J. D.; Stephan, D.
W. Science 2006, 314, 1124-1126. A related ortho-substituted system
1-(NPh2)-2-[B(C6F5)2]C6H4 has been reported by Roesler and Piers: (b)
Roesler, R.; Piers, W. E.; Parvez, M. J. Organomet. Chem. 2003, 680, 218-
222.
(12) Cairncross, A.; Sheppard, W. A. J. Am. Chem. Soc. 1968, 90, 2186.
(13) (a) Cairncross, A.; Sheppard, W. A.; Wonchoba, E.; Guildford, W.
J.; House, C. B.; Coates, R. M. Org. Synth. 1980, 59, 122-131. (b)
Sundararaman, A.; Lalancette, R. A.; Zakharov, L. N.; Rheingold, A. L.;
Ja¨kle, F. Organometallics 2003, 22, 3526-3532.
(14) (a) Sundararaman, A.; Ja¨kle, F. J. Organomet. Chem. 2003, 681,
134-142. (b) Ja¨kle, F. Dalton Trans. 2007, 2851-2858.
Figure 1. X-ray crystal structure of 3 with thermal ellipsoids at
the 50% probability level; hydrogen atoms and a benzene solvent
molecule are omitted for clarity.

6128 Organometallics, Vol. 26, No. 25, 2007
Sundararaman et al.
Figure 2. Plot illustrating the extended structure of 3 (view along the crystallographic b-axis); color code: blue (N), yellow (S), dark green
(B), light green (F).
Figure 3. Plot illustrating the incorporation of C6D6 in channels of 3 (view along the crystallographic c-axis); color code: blue (N), yellow
(S), dark green (B), light green (F).
instance, the distance between the two adjacent thiophene rings
amounts to C(4)-C(5) ) 1.438(3) Å for 3, which is comparable
to 1.445(7) Å for A. The B-C(thiophene) distance of 1.502(3)
Å is also essentially identical to that of A (1.507(3) Å).
significant angle of 29.4° with respect to each other. π-Stacking
between C6F5 moieties of adjacent molecules is also evident
from the extended structure plot in Figure 2.
Intriguingly, the inclusion of one molecule of benzene for
every two main molecules in the unit cell results in a channel-
like extended structure. The diphenylaminobithiophene moieties
form a highly symmetric framework that allows for inclusion
of the benzene molecules (Figure 3). The shortest H‚‚‚H
distances between thiophene hydrogens across the channels are
ca. 6.0 Å, and the F‚‚‚F distances across the channels are ca.
11.1 Å. However, the phenyl rings of the diphenylamino groups
extend far into the channels, thus limiting the effective channel
diameters significantly. The exterior channel walls are decorated
Extended Structure. As shown in Figure 2, adjacent
molecules in the crystal lattice are oriented in a parallel and
head-to-tail manner, such that the amine nitrogen is in
proximity to the boron-bound thiophene ring (shortest distances
C4(x,y,z)‚‚‚N1(2 - x,y,0.5 - z) ) 4.212 Å). Moreover, the
nitrogen-attached thiophene rings are at a distance that suggests
the presence of π-stacking interactions with the shortest
intermolecular contact being C6(x,y,z)‚‚‚C6(2 - x,y,0.5 - z) )
3.357 Å; however, the thiophene rings are oriented at a

D-A Dyad with a Highly Lewis Acidic Boryl Group
Organometallics, Vol. 26, No. 25, 2007 6129
Table 1. Comparison of Selected Bond Lengths [Å] from
X-ray Analysis of 3 with Data from DFT Calculations on 3
and Ph2B-Th-Th-NPh2 (4)
3 (X-ray)
3 (DFT)
4 (DFT)
B(1)-C(1)
B(1)-C(9)
B(1)-C(15)
S(1)-C(1)
S(1)-C(4)
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
N(1)-C(8)
N(1)-C(21)
N(1)-C(27)
S(2)-C(5)
S(2)-C(8)
C(5)-C(6)
C(6)-C(7)
C(7)-C(8)
C(4)-C(5)
1.502(3)
1.583(3)
1.572(3)
1.734(2)
1.710(2)
1.387(3)
1.392(3)
1.385(3)
1.392(3)
1.425(3)
1.427(3)
1.734(2)
1.735(2)
1.360(3)
1.400(3)
1.360(3)
1.438(3)
1.5116
1.5824
1.5837
1.7643
1.7418
1.3959
1.3998
1.3961
1.3874
1.4296
1.4302
1.7633
1.7627
1.3821
1.4121
1.3822
1.4378
1.5442
1.5716
1.5707
1.7640
1.7455
1.3900
1.4094
1.3878
1.3947
1.4261
1.4273
1.7601
1.7659
1.3799
1.4168
1.3767
1.4448
Figure 4. Comparison of the absorption and emission spectra of
3 with those of the non-fluorinated analogue (tBuPh)MesB-ThTh-
NPh2 (D) in CH2Cl2.
with C6F5 moieties that show multiple short intermolecular
F‚‚‚F contacts ranging from 2.718 to 2.754 Å (∑vdW ) 2.94
Å).
Table 2. Calculated Electronic Transitions from TD-DFT
Calculations
Photophysical Properties. Compound 3 is dark red colored
due to a strong absorption at λmax ) 520 nm (lg ꢀ ) 4.0) in
CH2Cl2 solution (Figure 4). This band is red-shifted by ca. 80
nm relative to the lowest energy absorption of the symmetric
diboron analog A with λmax ) 413, 437(sh) nm. The band is
attributed to charge transfer from the amino group to the highly
Lewis acidic boron center. Upon excitation at 520 nm in CH2-
Cl2 solution, 3 emits red light with a fluorescence maximum at
λem,max ) 620 nm. In comparison, the bithiophene-bridged
species (tBuPh)MesB-Th-Th-NPh2 (D)10 has a λmax,abs ) 436
nm (lg ꢀ ) 4.6) and emits green light at λmax,em ) 540 nm in
CH2Cl2 solution. Thus, the absorption and emission bands for
3 are clearly red-shifted as a result of substitution of boron with
electron-withdrawing C6F5 groups.15 As expected for charge-
transfer bands in donor-π-acceptor systems, a pronounced
solvatochromic effect is observed. The absorption and to an even
larger extent the emission maximum are significantly blue-
shifted in hexanes (λmax,abs ) 513 nm and λmax,em ) 558 nm).
More polar, coordinating solvents such as pyridine, MeCN,
DMF, or DMSO bind to boron as evident from the spontaneous
discoloration of the red solution. For instance, addition of excess
pyridine to 3 in CH2Cl2 leads to a colorless solution with λmax,abs
) 369 nm that shows, similar to the non-borylated dipheny-
laminobithiophene, an intense blue luminescence. The quantum
efficiency of 2% for compound 3 in CH2Cl2 is low in
comparison to related mesityl-substituted donor-acceptor dyads
(e.g., 74% for (tBuPh)MesB-Th-Th-NPh2 (D)10), but not
unlike the 3% found for the symmetric C6F5-substituted diboron
analogue (B). The solid-state emission of 3 (635 nm) is further
red-shifted by about 15 nm relative to that in CH2Cl2 solution,
likely as a result of favorable π-stacking interactions in the solid
state as evident from the crystal structure analysis.
excitation energy oscillator
compound transition MO contributions
eV (nm)
strength/f
3
4
S0fS1 HOMOfLUMO
S0fS2 HOMO-1fLUMO
HOMOfLUMO+1
S0fS3 HOMO-2fLUMO
S0fS1 HOMOfLUMO
S0fS2 HOMOfLUMO+1
S0fS3 HOMO-1fLUMO
2.44 (508)
3.52 (352)
0.8857
0.3450
3.63 (341)
2.65 (468)
3.59 (345)
3.66 (339)
0.0011
0.7679
0.1369
0.2540
the comparatively short B-C distance to the bithiophene bridge
of 1.502(3) Å is well reproduced in the calculations (1.5116
Å). Similarly, an unusually small degree of bond alternation in
the thiophene ring that is attached to boron is found from both
the X-ray and the computed structure (X-ray, C-C distances
from 1.385(3) to 1.392(3) Å; DFT, C-C distances from 1.396
to 1.400 Å). Most revealing is a comparison with the data
computed for the non-fluorinated derivative 4. The B-C
distance for 4 is considerably longer with 1.5442 Å and the
bond alternation more pronounced (C-C distances of 1.388-
1.409 Å). These observations suggest enhanced electronic
delocalization between boron and the thiophene ring with
increasing Lewis acidity at boron. In this context, it is also
interesting to note that a higher Mulliken charge of +0.220 is
calculated for B in 3 than in 4 (+0.092).
Orbital plots for compounds 3 and 4 are displayed in Figure
5. The most obvious difference in the frontier orbitals is found
for the HOMO level, where a significantly stronger contribution
of the boron empty p-orbital leads to enhanced overlap with
the organic π-system in 3. The orbital energies for both the
HOMO and the LUMO are lower in 3 in comparison to 4 as a
(16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K.
N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,
A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,
revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
DFT Calculations. To gain further insight into the origin of
the observed bathochromic shift of the absorption and emission
of fluorinated 3 in comparison to donor-acceptor dyads with
mesityl or phenyl substituents on boron, we optimized the
structure of 3 and of the non-fluorinated analogue 4 (Figure 5)
using DFT calculations and determined for both compounds the
vertical transition energies using TD-DFT calculations (Gaussian
2003, B3LYP).16
The calculated bond lengths for 3 correlate well with those
obtained from X-ray diffraction analysis (Table 1). Importantly,
(15) The absorption maximum for 1-(NPh2)-2-[B(C6F5)2]C6H4 (λmax
517 nm in CH2Cl2) is observed at similar energy. See ref 11b.
)

6130 Organometallics, Vol. 26, No. 25, 2007
Sundararaman et al.
Figure 5. Computed contour plots and orbital energies (eV) of the frontier orbitals for 3 and 4 (isovalue ) 0.02).
Aldrich. BBr3 was distilled under high vacuum prior to use.
result of fluorination of the phenyl rings (Figure 5). However,
the difference is considerably less pronounced for the HOMO
than the LUMO level, which ultimately results in a smaller
HOMO-LUMO gap of 2.694 eV for 3 in comparison to 2.993
eV for 4. We next calculated the vertical electronic transitions
for 3 and 4, which are summarized in Table 2. For both
compounds, the lowest energy transition corresponds to a high
probability excitation from the HOMO to the LUMO level. The
calculated excitation energy of 508 nm for 3 is in excellent
agreement with the experimentally determined absorption
maximum of 520 nm. Moreover, the lowest energy absorption
for non-fluorinated 4 is much higher in energy (468 nm) and in
a range similar to that of experimental data reported for (tBuPh)-
MesB-Th-Th-NPh2 (D).10
13,14
2-Diphenylamino-5,5′-bithiophene17 and (C6F5Cu)4
were pre-
pared according to literature procedures, and 2-diphenylamino-2′-
trimethylsilyl-5,5′-bithiophene was synthesized in analogy to the
preparation of 2-diphenylamino-2′-tributylstannyl-5,5′-bithiophene.18
All reactions and manipulations were carried out under an
atmosphere of prepurified nitrogen using either Schlenk techniques
or an inert-atmosphere glovebox (Innovative Technologies). Ether
solvents were distilled from Na/benzophenone prior to use.
Hydrocarbon and chlorinated solvents were purified using a solvent
purification system (Innovative Technologies; alumina/copper
columns for hydrocarbon solvents), and the chlorinated solvents
were subsequently distilled from CaH2 and degassed via several
freeze pump thaw cycles.
Elemental analyses were performed by Quantitative Technologies
Inc., Whitehouse, NJ.
Conclusions
All 499.9 MHz 1H, 125.7 MHz 13C, 160.4 MHz 11B, 470.2 MHz
19F, and 79.45 MHz 29Si NMR spectra were recorded at room
temperature on a Varian INOVA spectrometer equipped with a
boron-free 5 mm dual broadband gradient probe (Nalorac, Varian
Inc., Martinez, CA). Solution 1H and 13C NMR spectra were
referenced internally to solvent signals. 11B NMR spectra were
referenced externally to BF3‚Et2O (δ ) 0), 19F NMR spectra to
R,R′,R′′-trifluorotoluene (0.05% in C6D6; δ ) -63.73), and 29Si
NMR spectra to SiMe4 (δ ) 0). 11B NMR spectra were acquired
with boron-free quartz NMR tubes. The abbreviations Th (thiophene),
Pf (2,3,4,5,6-pentafluorophenyl), and pst (pseudotriplet) were used
for peak assignments.
The donor-π-acceptor dyad (3) with a highly electron-
deficient bis(pentafluorophenyl)boryl group as the acceptor is
readily prepared through successive treatment of 5-trimethyl-
silyl-5′-diphenylamino-2,2′-bithiophene with BBr3 and the mild
aryl transfer reagent (CuC6F5)4. Because of the front strain
exerted by the diphenylamino group, Lewis acid-base com-
plexation is observed neither in solution nor in the solid state.
However, multiple π-stacking interactions give rise to a channel-
like structure that leads to inclusion of benzene solvent
molecules. Most intriguing is the distinct bathochromic shift of
both the absorption and the emission maxima for 3 in com-
parison to related non-fluorinated donor-acceptor species with
diarylborane moieties as the acceptor functionality. The origin
of this shift can be traced back to a considerably smaller
HOMO-LUMO gap in 3, which is a result of the different
impact of fluorination on the energy of these frontier orbitals.
UV-visible absorption data were acquired on a Varian Cary 500
UV-vis/NIR spectrophotometer. The fluorescence data and quan-
tum yields were measured on a Varian Cary Eclipse fluorescence
spectrophotometer. Anthracene was used as the standard for
determination of the quantum yields. The quantum yield of
anthracene was adopted from the Handbook of Photochemistry.19
Experimental Section
(17) Hooper, M. W.; Utsunomiya, M.; Hartwig, J. F. J. Org. Chem. 2003,
68, 2861-2873.
(18) Tabet, A.; Schro¨der, A.; Hartmann, H.; Rhode, D.; Dunsch, L. Org.
Lett. 2003, 5, 1817-1820.
n
Materials and General Methods. Me3SiCl and BuLi (1.6 M
in hexanes) were purchased from Acros, and BBr3 was from

D-A Dyad with a Highly Lewis Acidic Boryl Group
Organometallics, Vol. 26, No. 25, 2007 6131
Sample solutions were prepared using a microbalance ((0.1 mg)
and volumetric glassware.
129.4 (Ph-Cm), 124.3 (Th-C), 123.4 (Ph-Cp), 122.9 (Ph-Co),
122.5 (Th-C), 121.4 (Th-C), 0.1 (SiMe3). 29Si NMR (CDCl3, 99.3
MHz): δ ) -6.3. Anal. Calcd for C23H23NS2Si (405.65): C, 68.10;
H, 5.71; N, 3.45. Found: C, 68.02; H, 5.57; N, 3.41.
The MALDI TOF/TOF measurement was performed on an
Applied Biosystems 4700 Proteomics Analyzer in reflectron (+)-
mode with delayed extraction. Benzo[a]pyrene was used as the
matrix (10 mg/mL toluene). The sample was dissolved in toluene
(10 mg/mL), mixed with the matrix in a 1:10 ratio, and then spotted
on the wells of a sample plate inside a glovebox. Peptides were
used for calibration (Des-Arg-Bradykinin (904.4681), Angiotensin
I (1296.6853), Glu-Fibrinopeptide B (1570.6774), ACTH (clip
1-17) (2093.0867), ACTH (clip 18-39) (2465.1989), and ACTH
(clip 7-38) (3657.9294)) with R-hydroxy-4-cyanocinnamic acid
as the matrix.
X-ray diffraction intensities for 3 were collected on a Bruker
SMART APEX CCD diffractometer at T ) 100(2) K using Mo
KR (λ ) 0.71073 Å) radiation. Crystallographic data and details
of the X-ray diffraction experiment and crystal structure refinement
are given in Table S1. SADABS20 absorption correction was
applied, and the structure was solved using direct methods and
completed by subsequent difference Fourier syntheses and refined
by full matrix least-squares procedures on F2. All non-hydrogen
atoms were refined with anisotropic displacement coefficients. The
H atoms were placed at calculated positions and were refined as
riding atoms. All software and sources scattering factors are
contained in the SHELXTL (5.10) program package.21 Crystal-
lographic data for 3 have been deposited with the Cambridge
Crystallographic Data Center as supplementary publication no.
CCDC-657125. Copies of the data can be obtained free of charge
on application to CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK (fax, (+44) 1223-336-033; e-mail, deposit@ccdc.cam.ac.uk).
DFT calculations were performed with the Gaussian 03 pro-
gram.16,22 Geometries and orbital energies were calculated by means
of the hybrid density functional B3LYP with the basis set of 6-31G-
(d). The input files and orbital representations were generated with
Gaussview (isovalue of 0.02). Excitation data were determined using
TD-DFT (B3LYP) calculations.
Reaction of 1 with BBr3. Synthesis of 2. A solution of BBr3
(1.02 g, 4.08 mmol) in toluene (10 mL) was added at -78 °C to a
solution of 1 (1.65 g, 4.07 mmol) in toluene (40 mL). The reaction
mixture was allowed to slowly warm to room temperature and left
stirring overnight. All volatile components were removed under
high vacuum, and the crude red material was recrystallized twice
from toluene/hexanes mixture at -35 °C. Isolated yield: 1.35 g
1
3
(66%). H NMR (CDCl3, 500 MHz): δ ) 7.90 (d, J ) 4.0 Hz,
3
1H, Th-H8), 7.32 (pst, J ) 7.0 Hz, 4H, Ph-Hm), 7.22 (m, 5H,
3
Ph-Ho, Th-H4/7), 7.19 (d, J ) 4.0 Hz, 1H, Th-H4/7), 7.13 (t,
3J ) 8.0 Hz, 2H, Ph-Hp), 6.55 (d, 3J ) 4.0 Hz, 1H, Th-H3). 13
C
NMR (CDCl3, 125.7 MHz): δ ) 155.1 (Th-C), 154.9 (Th-C),
147.3 (Ph-Ci), 145.3 (Th-C), 129.7 (Ph-Cm), 127.5 (Th-C),
126.1 (Th-C), 125.1 (Th-C), 124.5 (Ph-Cp), 124.0 (Ph-Co),
118.7 (Th-C), Th-C9 not observed. 11B NMR (CDCl3, 160.7
MHz): δ ) 45.7 (w1/2 ) 600 Hz). Anal. Calcd for C20H14BBr2NS2
(503.08): C, 47.75; H, 2.80; N, 2.78. Found: C, 47.55; H, 2.59;
N, 2.72.
Reaction of 2 with 2 equiv of C6F5Cu: Synthesis of 3. A
solution of “C6F5Cu” (0.46 g, 1.99 mmol) in toluene (10 mL) was
cooled to -35 °C and then added dropwise to a solution of 2 (0.50
g, 0.99 mmol) in toluene (10 mL) at -35 °C. The reaction mixture
was allowed to slowly warm to room temperature and left stirring
for 1 h. The mixture was then filtered through a fritted glass funnel,
and all volatile components were removed under high vacuum. The
product was obtained upon recrystallization from a fairly concen-
trated solution in a toluene/hexanes mixture at -35 °C. Isolated
yield: 0.50 g (75%). 1H NMR (C6D6, 500 MHz): δ ) 7.45 (d, 3J
3
) 4.0 Hz, 1H, Th-H), 7.06 (d, J ) 7.5 Hz, 4H, Ph-Ho), 7.01
3
(pst, J ) 8.0 Hz, 4H, Ph-Hm), 6.88-6.85 (m, 4H, Ph-Hp and
Th-H), 6.20 (d, 3J ) 4.0 Hz, 1H, Th-H). 19F NMR (C6D6, 470.2
MHz): δ ) -131.1 (m, 4F, Pf-Fo), -151.0 (t, 3JFF ) 20 Hz, 2F,
Pf-Fp), -161.9 (m, 4F, Pf-Fm). 13C NMR (C6D6, 125.7 MHz): δ
) 159.1 (Th-C), 157.3 (Th-C), 149.1 (Ph-Ci), 147.6 (Th-C),
Caution! BBr3 is toxic and highly corrosive and should be
handled appropriately with great care. Fluorinated grease was used
for ground glass joints in reactions involving BBr3.
1
1
146.2 (dm, JCF ) 244 Hz, Pf-Co), 142.9 (dm, JCF ) 256 Hz,
Pf-Cp), 141.5 (Th-C9), 138.1 (dm, 1JCF ) 252 Hz, Pf-Cm), 130.3
(Ph-Cm), 126.5 (Th-C), 125.9 (Th-C), 125.4 (Ph-Cp), 124.8
(Ph-Co), 117.9 (Th-C), 114.3 (t, 2JCF ) 33 Hz, Pf-Ci). 11B NMR
(CDCl3, 160.7 MHz): δ ) 50.5 (w1/2 ) 1200 Hz). UV-vis (CH2-
Cl2, 4.7 × 10-5 M), λmax ) 520 nm (lg ꢀ ) 4.0) and 350 (lg ꢀ )
4.0); fluorescence (CH2Cl2, 4.7 × 10-5 M), λem,max ) 620 nm, Φ
Synthesis of 2-Diphenylamino-2′-trimethylsilyl-5,5′-bithiophene
n
(1). A solution of BuLi in hexanes (5.6 mL, 1.6 M in hexanes,
9.0 mmol) was added dropwise to a solution of diphenylamino-
bithiophene (2.50 g, 7.50 mmol) in THF (150 mL) at -78 °C. The
reaction mixture was allowed to warm to room temperature and
kept stirring for 3 h. The mixture was cooled back to -78 °C, Me3-
SiCl (1.20 mL, 9.30 mmol) was added dropwise, and the mixture
was then allowed to warm to room temperature overnight. The crude
product was loaded on an alumina column and eluted with 5%
diethyl ether/hexanes mixture. The product was obtained as a
colorless solid upon removal of the solvents. Yield: 2.50 g (82%).
1H NMR (CDCl3, 499.893 MHz): δ ) 7.33 (pst, 3J ) 7.5 Hz, 4H,
) 0.02 (λexc ) 520 nm); UV-vis/fluorescence in hexanes, λmax
)
513 nm, λem,max ) 558 nm, Φ ) 0.09 (λexc ) 513 nm); fluorescence
of solid, λem,max ) 635 nm (λexc ) 515 nm). High-resolution MALDI
TOF/TOF: m/z ) 677.0212 (calcd for
C
1H1411B19F1014N32S2
12
32
677.0501). Anal. Calcd for C32H14BF10NS2 (677.39): C, 56.74; H,
2.08; N, 2.07. Found: C, 56.80; H, 1.94; N, 2.11.
3
Ph-Hm), 7.24 (d, J ) 8.0 Hz, 4H, Ph-Ho), 7.16 (br, 2H, Th-
Acknowledgment. We thank the National Science Founda-
tion (CAREER award CHE-0346828 to F.J.) and the Alfred P.
Sloan Foundation (research fellowship to F.J.) for support of
this research. We are grateful to Reynaldo Rodriguez from
Irvington High School, New Jersey for his contributions to the
synthesis of compound 1.
H7,8), 7.10 (t, 3J ) 7.0 Hz, 2H, Ph-Hp), 7.02 (d, 3J ) 3.5 Hz, 1H,
Th-H4), 6.64 (d, 3J ) 3.5 Hz, 1H, Th-H3), 0.39 (s, 9H, SiMe3).
13C NMR (CDCl3, 125.7 MHz): δ ) 150.7 (Th-C), 147.8 (Ph-
Ci), 143.1 (Th-C), 139.2 (Th-C), 134.9 (Th-C), 131.6 (Th-C),
(19) Murov, S. L., Carmichael, I., Hug, G. L., Eds. Handbook of
Photochemistry, 2nd ed.; Marcel Dekker Inc.: New York, 1993.
(20) Sheldrick, G. M. SADABS (2.01), Bruker/Siemens Area Detector
Correction Program; Bruker AXS: Madison, WI, 1998.
(21) Sheldrick, G. SHELXTL (5.10); Bruker XRD: Madison, WI.
(22) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652. (b) Tozer,
D. J.; Handy, N. C. Phys. Chem. Chem. Phys. 2000, 2, 2117-2121. (c)
Becke, A. D. J. Chem. Phys. 1996, 104, 1040-1046. (d) Casida, M. E.;
Jamorski, C.; Casida, K. C.; Salahub, D. R. J. Chem. Phys. 1998, 108,
4439-4449.
Supporting Information Available: Crystallographic data for
compound 3 including tables of crystal data, atomic coordinates,
bond lengths and angles, and anisotropic thermal parameters.
Computed contour plots of selected molecular orbitals for 3 and 4
and calculated orbital energies (eV) from DFT calculations. This
materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
OM700520N