Benzo- and naphthoborepins: Blue-emitting boron analogues of higher acenes

Full text of DOI:10.1002/anie.200902803

Communications
DOI: 10.1002/anie.200902803
Boron Heterocycles
Benzo- and Naphthoborepins: Blue-Emitting Boron Analogues of
Higher Acenes**
Lauren G. Mercier, Warren E. Piers,* and Masood Parvez
Unsaturated rings containing boron have been of fundamen-
tal and practical interest for many years.[1] Five-membered
borole rings I are isoelectronic to the antiaromatic cyclo-
pentadienyl cation and have been important in addressing
concepts of aromaticity.[2] The six-membered borabenzene
and boratabenzene compounds II (Nu = nucleophile),[3] how-
ever, are aromatic and have found application as cyclo-
pentadienide-like ligands with substantial tunability at the
boron substituent.[4] More recently, these cores have been
tested as organic materials, finding application as fluoride
sensors.[5]
The seven-membered borepin framework III is by com-
parison less explored, even though it is an aromatic six-p-
electron system, isoelectronic to the tropylium ion.[6] This
paucity of information is in part due to the lack of general
synthetic routes to this functionality, but the tendency of the
Scheme 1. Synthesis of B-mesityl annulated borepins 3a–c.
unsupported borepin ring to undergo thermal transforma-
tions is also problematic.[7] For the latter reason, those
borepins that are known tend to be annulated by other
rings,[8] which provides a stabilizing influence on the borepin
core structure.[9]
We have been interested in boron-containing analogues of
higher acenes,[10] given the utility of the all-carbon species in
organic semiconductor applications.[11] Accordingly, we have
prepared dibenzoborepin, benzonaphthoborepin, and
the first step, a Wittig reaction is utilized to prepare the cis-
dibromo diaryl ethylenes 1a–c from the appropriate ortho-
bromo aryl aldehyde and the ortho-bromo benzylbromide
partners in excellent yield and high cis:trans (c:t) selectivity.[12]
Closure to compounds 2a–c, featuring the seven-membered
stannacycle, was accomplished in variable yield by lithiating
at low temperature and quenching the resulting dianionic
species with Me2SnCl2; attempts at direct quenching with
dinaphthoborepin derivatives incorporating
a B-mesityl
[13]
group as analogues of anthracene, tetracene, and pentacene,
respectively.
The route developed is shown in Scheme 1 and relies on a
tin–boron exchange reaction that has been applied with wide
success in other synthetic protocols for boron heterocycles. In
boron reagents such as MesB(OMe)2 were not successful.
Nevertheless, stannepins 2 were readily converted to the
targeted borepins 3a–c using an excess of BCl3 and subse-
quent treatment with MesLi without isolation of the highly
moisture-sensitive borepin chloride.[5a] The mesityl borepins
were stable enough towards air and adventitious moisture to
be handled in the ambient atmosphere and purified by
column chromatography, but over the course of several hours,
they did react with water to form B-O-B borinic esters.
NMR spectroscopic data for compounds 3 are consistent
with their formulation as shown in Scheme 1. The olefinic
protons in the isolated double bond of the borepin ring appear
as a singlet for 3a and 3c at 6.80 and 6.99 ppm, respectively, in
the 1H NMR spectra; these protons resonate as two doublets
(7.41 and 7.20, 3JHH = 12.8 Hz) for the asymmetrical com-
pound 3b. The 11B NMR spectra feature signals characteristic
[*] L. G. Mercier, W. E. Piers, M. Parvez
Department of Chemistry, University of Calgary
2500 University Drive N.W., Calgary, AB, T2N1N4 (Canada)
Fax: (+1)403-289-9488
http://www.chem.ucalgary.ca/research/groups/wpiers/
E-mail: wpiers@ucalgary.ca
[**] Funding for this work was provided by NSERC of Canada and the
Xerox Research Foundation.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200902803.
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Chemie
of three-coordinate, neutral boron nuclei[14] at 64.3, 67.7, and
66.7 ppm for 3a–c, respectively.[15]
While the spectroscopic data are consistent with planar
structures, X-ray crystallographic analysis[16] of each com-
pound reveals bowed geometries; this deformation is likely
due to packing effects (see below) rather than a ground-state
preference for this geometry. Figure 1 shows the molecular
structures of 3b and 3c, while Figure 2 illustrates key
intermolecular packing interactions; 3a[15] is similar to 3c
and will not be discussed in detail.
Figure 2. Thermal ellipsoid (50%) diagrams showing packing motifs in
3b (top) and 3c (bottom). In 3b, the faces of the naphthyl moieties
are approximately 3.5 ꢀ apart in the Ba and Bb dimers; these units are
approximately orthogonal and connected through edge-to-face interac-
tions. In 3c (similar to 3a, see the Supporting Information), molecules
pack in an edge-to-face herringbone motif.
Figure 1. Thermal ellipsoid (50%) diagrams of the molecular struc-
tures of 3b (top) and 3c (bottom). Selected bond lengths [ꢀ], angles,
and dihedral angle [8] for 3b: C1–B1 1.568(3), C1–C10 1.449(3), C10–
C11 1.449(3), C11–C12 1.346(3), C12–C13 1.455(3), C13–C18 1.426(3),
C18–B1 1.561(3), B1–C19 1.587(3); C1-B1-C18 126.33(19), C18-B1-C19
116.49(18), C1-B1-C19 117.10(19); C1-B1-C19-C24 88.1(3). Selected
bond lengths [ꢀ], angles, and dihedral angle [8] for 3c: C1–B1
1.559(3), C1–C10 1.440(3), C10–C11 1.457(3), C11–C12 1.338(3), C12–
C13 1.451(3), C13–C22 1.446(3), C22–B1 1.566(3), B1–C23 1.587(3),
C1-B1-C22 125.98(18), C22-B1-C23 117.23(19), C1-B1-C23 116.79(18);
C1-B1-C23-C24 91.4(2).
compounds 3a and 3c, the curvature of the carbon spine
engendered by the seven-membered central ring causes a
bowing of the rings from planarity as a consequence of edge-
to-face interactions in the herringbone packing pattern.[11] In
3c, close interactions between C11 and C12 of one molecule
with C1 and C18 of an adjacent molecule (3.6–3.8 ꢀ) anchor
this motif along the entire length of the ring system. As
computations indicate that 3a–c have planar ground-state
structures (see below), these packing effects are enough to
compensate for the energy required to bend the molecule
1
The borepin rings in each compound exhibit similar
from planarity. Notably, H NMR spectra for 3a are temper-
À
metrical properties. Intraring B C bond lengths are approx-
ature-invariant to 223 K.
À
imately 0.02 ꢀ shorter than the values for the B CMes bonds,
In the asymmetric heterotetracene 3b, the primary
packing interaction is a face-to-face motif[11] between the
two naphthyl groups of adjacent molecules (separation
ca. 3.5 ꢀ). These face-to-face dimers pack orthogonally such
that an interaction between the edge of a naphthyl group and
the face of a phenyl group in an adjacent dimer can be
attained. The asymmetry of this molecule may be an
exploitable molecular design principle to favor face-to-face
over edge-to-face interactions in heteroacenes.
indicating a degree of conjugation through the boron center in
the borepin core. The C11 C12 bond gets progressively
shorter for 3a (1.386(3) ꢀ), 3b (1.346(3) ꢀ), and 3c
(1.338(3) ꢀ) and indicates a strongly localized C C bond.
The sum of the angles around the boron center is close 3608
for all three compounds, with the internal angle of the seven-
membered ring 7–98 larger than the others; the mesityl group
is oriented essentially perpendicular to the boron trigonal
plane.
À
=
Annulated borepins 3 are weaker Lewis acids than related
acyclic boranes not constrained in the aromatic borepin ring.
A competition experiment between 3a and Ph2BMes reveals
Packing motifs in this series of acene analogues vary with
the symmetry of the wingspan (Figure 2). In symmetrical
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that dimethylamino pyridine preferentially binds to
increases. Comparing 3a to an all-carbon analogue, suberene,
it is evident that incorporation of boron influences the
photophysics of the molecule by decreasing the gap between
the highest occupied and lowest unoccupied molecular
orbitals (HOMOs and LUMOs) as well as by lowering the
energy of the LUMO. This change results in absorption at
longer wavelengths and more intense blue fluorescence in 3a
than in suberene (Table 1). The charge-transfer character of
the lowest-energy transitions is indicated by significant
solvatochromism effects on the emission wavelength;[15] as a
result, quantum yields and fluorescence lifetimes are signifi-
cantly increased in the boron compounds relative to suberene.
DFT calculations[15] (RB3LYP 6-31 + + G (d,p)) on each
compound yield planar structures as energy minima when
molecular coordinates for the bent solid-state structures are
used as input data. Compound 3a shows HOMOs and
LUMOs based on the entire boron–carbon framework
(Figure S3 in the Supporting Information), whereas com-
pounds 3b and 3c have HOMOs based on the carbon
framework and LUMOs with contributions from boron-based
orbitals (Figures S4 and S5 in the Supporting Information).
However, time-dependent DFT computations show that the
major transitions in all of these molecules come from low-
lying orbitals based on the annulated carbon system and go to
unoccupied orbitals with contributions from boron, in keep-
ing with the observed low-intensity charge-transfer transi-
tions.
Ph2BMes.[15] This preference is likely due to a combination
of lower availability of the empty p orbital on boron by virtue
of its conjugation with the extended p system and the steric
constraints inherent to the boron center rigidly held in the
borepin ring. Accordingly, compounds 3 do not strongly bind
donor solvents such as THF or diethylether. Electrochemical
measurements[15] reveal reduction potentials of À2.56, À2.25,
and À2.20 V for 3a–c, relative to Fc/Fc+ at scan rates of
50 mVsÀ1 in THF. The reductions are quasi-reversible and
occur at less negative potentials than observed for BMes3.[17]
In solution, the compounds are pale yellow with strong
absorptions in the 260–314 nm range and weaker absorptions
(e < 10000) at higher wavelength assignable to intramolecular
charge-transfer transitions; a bathochromic shift is observed
as the p system is extended. Compounds 3a and 3b emit at
400 and 445 nm and display relatively high quantum yields
(Figure 3, Table 1); the quantum yield for 3c emission at
477 nm drops precipitously. This finding is in keeping with the
trend for all-carbon acenes[18] and reflects the greater
propensity for nonradiative decay paths in more conjugated
systems. The optical band gaps, taken from the longest
absorption wavelength, also decrease as conjugation
In summary, air- and moisture-tolerant analogues of
polycyclic aromatic acene hydrocarbons containing a borepin
core have been synthesized and characterized. In the solid
state they display edge-to-face and face-to-face interactions,
and in solution they exhibit blue fluorescence, displaying in
some cases high quantum yields.
Experimental Section[15]
Synthesis of 3a: BCl3 gas (510 mg, 4.35 mmol) was condensed into a
solution of 2a (114 mg, 0.337 mmol) in toluene (90 mL) at À788C.
The reaction mixture turned from clear and colorless to light brown.
After 80 min, the reaction was warmed to room temperature and
concentrated to a total volume of 15 mL. In argon atmosphere, a
solution of mesityl lithium (238 mg, 1.89 mmol) in toluene (3 mL) was
added dropwise, and the reaction mixture turned yellow. After 16 h,
the solvent was removed in vacuo and the product was purified by
column chromatography (SiO2, hexanes—25% EtOAc/hexanes—
EtOAc). This solid was then recrystallized by slow evaporation of
toluene and washed with hexanes to yield 3a (60 mg, 58%) as large
yellow-brownish plates. 1H NMR (CDCl3): d = 8.06 (dd, JHH = 7.2,
1.2 Hz, 2H), 7.83 (dd, JHH = 7.2, 0.8 Hz, 2H), 7.73 (dd dd, JHH = 7.6,
1.2 Hz, 2H), 7.43 (dd dd, JHH = 7.2, 1.2 Hz, 2H), 7.37 (s, 2H), 6.96 (s,
2H), 2.44 (s, 3H), 1.53 ppm (s, 6H); 13C{1H} NMR (CDCl3): d = 144.1,
141.4, 137.8, 136.2, 134.0, 132.9, 132.7, 127.2, 127.0, 22.9, 21.3 ppm
(two peaks for carbon atoms bonded to the boron atom were not
observed owing to quadrupolar relaxation). 11B NMR (CDCl3): d =
64.3 ppm. HRMS calcd for C23H21B: 308.1736; found: 308.1733.
Elemental analysis (%) calcd for C23H21B: C 89.63, H 6.87; found:
C 89.26, H 7.25.
Figure 3. Normalized UV/Vis absorbance spectra (solid shapes) and
emission spectra (open shapes) for 3a (squares), 3b (circles) and 3c
(triangles); dichloromethane, 2.25ꢁ10À5 m.
Table 1: Summary of optical data for 3a–c and hydrocarbon analogue
suberene.[a]
Compound Abs lmax [nm] Em lmax [nm] F,[b] t [ns] Optical BG [eV][c]
3a
260
280
314
285
400
445
477
384
0.70, 11
0.39, 13
0.01, 24
3.11
2.81
2.61
3b
3c[d]
Suberene
0.02, 3.6 3.67
[a] see Table S1 in the Supporting Information for a full listing of
absorbance bands and extinction coefficients. [b] Absolute quantum
yields measured with an integrating sphere. [c] Optical band gap
estimated from the onset of absorption in CH2Cl2 (2.25ꢁ10À5 m).
[d] Measured in CH2Cl2 (5.0ꢁ10À5 m).
Received: May 25, 2009
Revised: June 15, 2009
Published online: July 13, 2009
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Keywords: borepins · boron · fluorescence · heteroacenes ·
heterocycles
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[16] Crystal data for 3a: C23H21B, Mr = 308.21, monoclinic, P21/n, a =
9.408(6), b = 8.469(5), c = 22.011(12) ꢀ, b = 93.56(2)8, V=
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, MoKa radiation, l =
0.71073 ꢀ, T= 173(2) K, 7463 measured reflections, 3940
unique, min/max transmission = 0.9820 and 0.9846, R1 (I >
2s) = 0.0554, wR2 = 0.1308, GoF = 1.041, No. of parameters =
217, final difference map within + 0.272 and À0.201 eꢀ3. Crystal
data for 3b: C27H23B, Mr = 358.26, monoclinic, C21/c, a =
34.623(7), b = 7.992(3), c = 15.450(5) ꢀ, b = 113.71(3)8, V=
3914(2) ꢀ3, Z = 8, 1 = 1.216 gcmÀ3
,
MoKa radiation, l =
0.71073 ꢀ, T= 173(2) K, 6081 measured reflection, 3567
unique, min/max transmission = 0.9953 and 0.9886, R1 (I >
2s) = 0.0510, wR2 = 0.1117, GoF = 1.040, No. of parameters =
254, final difference map within + 0.214 and À0.227 eꢀ3. Crystal
data for 3c: C31H25B, Mr = 408.32, monoclinic, P21/n, a =
11.642(3), b = 8.660(4), c = 22.879(7) ꢀ, b = 103.90(2)8, V=
2239.1(14) ꢀ3, Z = 4, 1 = 1.211 gcmÀ3
, MoKa radiation, l =
0.71073 ꢀ, T= 173(2) K, 8599 measured reflection, 5039
unique, min/max transmission = 0. 9893 and 0.9906 R1 (I >
2s) = 0.0671, wR2 = 0.1393, GoF = 1.081, No. of parameters =
289, final difference map within + 0.325 and À0.250 eꢀ3.
CCDC 730729 (3a), 730730 (3b), and 730731 (3c) contain the
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