Blue fluorescent 4a-aza-4b-boraphenanthrenes

Article abstract of DOI:10.1021/ol070328y

Figure presented Phenanthrene analogues with internalized B-N moieties were found to afford blue light emission with good quantum efficiencies, whereas the isomeric species with peripheral B-N moieties displayed only UV emission behavior, like the all-carbon framework.

Full text of DOI:10.1021/ol070328y

ORGANIC
LETTERS
2
007
Vol. 9, No. 7
395-1398
Blue Fluorescent
a-Aza-4b-boraphenanthrenes
1
4
Michael J. D. Bosdet, Cory A. Jaska, Warren E. Piers,* Ted S. Sorensen, and
Masood Parvez
Department of Chemistry, UniVersity of Calgary, 2500 UniVersity DriVe, N.W.
Calgary, Alberta, Canada, T2N 1N4
wpiers@ucalgary.ca
Received February 8, 2007
ABSTRACT
Phenanthrene analogues with internalized B−N moieties were found to afford blue light emission with good quantum efficiencies, whereas the
isomeric species with peripheral B N moieties displayed only UV emission behavior, like the all-carbon framework.
−
The development of luminescent materials based on highly
conjugated species such as polycyclic aromatic hydrocarbons
from those of their all-carbon counterparts. Herein, we report
the synthesis of two different isomers of BN-phenanthrene,
which display dramatically different fluorescence emission
behavior that is dependent upon the location of the B-N
moiety within the aromatic framework.
(PAHs) has led to applications in electronic devices such as
organic light emitting diodes and thin film transistors.
Although luminescent materials that provide primary color
(
red, green, and blue) emission are highly desirable for
The reaction of 1-chloro-2-trimethylsilyl-boracyclohexa-
2,5-diene with a variety of 2-ethynyl-pyridines was found
to cleanly afford the desired phenanthrene analogues 1-35
(Scheme 1) with BN-substitution at the internal 4a,4b
positions. The cycloisomerization of the pendent ethynyl
group was found to be unusually facile, as similar transfor-
display applications, those that possess pure blue emission
coupled with high stability and high quantum efficiency are
the most difficult to synthesize due to the large HOMO-
LUMO energy gap that is required. The incorporation of
fluorescent phenanthrene moieties into such materials has
1
2
6
7
only recently attracted significant attention. Our group has
mations in all-carbon or other heteroaromatic systems are
found that the incorporation of B-N moieties into PAHs
3
4
such as triphenylene or pyrene can afford conjugated
materials with photophysical properties distinctly different
Scheme 1. Synthesis of 1-5
(
1) Kim, D. Y.; Cho, H. N.; Kim, C. Y. Prog. Polym. Sci. 2000, 25,
089.
2) (a) Tian, H.; Wang, J.; Shi, J.; Yan, D.; Wang, L.; Geng, Y.; Wang,
1
(
F. J. Mater. Chem. 2005, 15, 3026. (b) Suh, H.; Jin, Y.; Park, S. H.; Kim,
D.; Kim, J.; Kim, C.; Kim, J. Y.; Lee, K. Macromolecules 2005, 38, 6285.
(c) Tian, H.; Shi, J.; Dong, S.; Yan, D.; Wang, L.; Geng, Y.; Wang, F.
Chem. Commun. 2006, 3498. (d) Boden, B. N.; Jardine, K. J.; Leung, A.
C. W.; MacLachlan, M. J. Org. Lett. 2006, 8, 1855. (e) Yang, C.; Scheiber,
H.; List, E. J. W.; Jacob, J.; M u¨ llen, K. Macromolecules 2006, 39, 5213.
(3) (a) Jaska, C. A.; Emslie, D. J. H.; Bosdet, M. J. D.; Piers, W. E.;
Sorensen, T. S.; Parvez, M. J. Am. Chem. Soc. 2006, 128, 10885. (b) Emslie,
D. J. H.; Piers, W. E.; Parvez, M. Angew. Chem., Int. Ed. 2003, 42, 1252.
(4) Bosdet, M. J. D.; Piers, W. E.; Sorensen, T. S.; Parvez, M. Angew.
Chem., Int. Ed., in press.
1
0.1021/ol070328y CCC: $37.00
© 2007 American Chemical Society
Published on Web 03/06/2007

8
9
typically incurred via a soft metal catalyst, UV irradiation,
or flash vacuum pyrolysis.10 The observed increase in
reactivity in this case may be due to the enhanced nucleo-
philic nature of the carbon atom R to boron1 in the
borabenzene-pyridine adduct intermediates 1′-5′. However,
the intermediate borabenzene-pyridine adduct was isolated
1
3
in the case of R ) SiMe (4′), due to slow cycloisomerization
reactivity at 25 °C. This decrease in activity is likely a result
of the reduced electrophilicity of the terminal carbon atom
of the alkyne moiety due to the electropositive nature of the
silicon atom. The cycloisomerization of 4′ was induced by
heating at 80 °C for 24 h to afford 4, which was readily
monitored by B NMR spectroscopy (4′: Bδ: 33.6; 4: Bδ:
11
2
7.1). Finally, the reaction of 1-chloro-2-trimethylsilyl-4-
iso-propyl-boracyclohexa-2,5-diene with 2-ethynyl-pyridine
was found to afford the iPr analogue 5, with alkyl substitution
located at the 7- rather than at the 9-position. Compounds
1-5 were found to be completely moisture stable and
moderately air stable and were conveniently purified via
column chromatography using dry, neutral alumina under
Ar.
A single-crystal X-ray diffraction study of 1 (Figure 1a)
indicated the presence of three independent molecules in the
unit cell, two of which were arranged in a π-stacked, head-
to-tail dimer with close B‚‚‚N dipole interactions (3.74 and
3.72 Å), and the third was situated orthogonal to the dimeric
unit with one relatively close edge carbon-to-π-face interac-
tion of ca. 3.5 Å (Figure 1b). There were no significant
differences found in the bond lengths and angles for the three
molecules, all of which possessed planar BN-phenanthrene
rings (Figure 1c). For one particular molecule of 1, the B-N
bond length was found to be 1.491(2) Å, which is suggestive
of some partial BdN double bond character as it is
intermediate in length between a single bond (e.g., BN-
Figure 1. (a) Molecular structure of 1. All hydrogen atoms have
been omitted for clarity. Selected bond lengths (Å) and angles (deg)
for one particular molecule: B(1)-N(1) 1.491(2), N(1)-C(5)
1.390(2), C(5)-C(6) 1.406(2), C(6)-C(7) 1.376(2), C(7)-C(8)
1
.407(2), B(1)-C(8) 1.499(2); N(1)-B(1)-C(8) 116.5(1), N(1)-
biphenyl, 1.558(3) Å)12 and a double bond (e.g., B
B(1)-C(12) 124.9(1), C(8)-B(1)-C(12) 118.6(2), C(1)-N(1)-
2
2
N -
C(5) 118.5(1), C(1)-N(1)-B(1) 121.1(1), C(5)-N(1)-B(1) 120.4(1).
3
triphenylene, 1.464(4) Å). Similar to B
2 2
N -triphenylene,
(b) Arrangement of the three independent molecules in the unit
partially localized CdC double bonds were suggested by the
presence of alternating long (av 1.41 Å) and short (av 1.36
Å) C-C bond lengths around the carbon periphery. The
molecular structure of 2 (Figure S1, Supporting Information)
was found to possess metrical parameters similar to that of
cell (B‚‚‚N: 3.74 and 3.72 Å). (c) Side view of 1.
1, along with a large twist angle of ca. 60° between the Ph
group and the BNC ring.
4
Because compounds 1-5 all contained a B-N fragment
at the annulation points of the aromatic rings, it was of
significant interest to explore the effects of shifting this polar
moiety to the molecular periphery. Thus, the isomeric BN-
phenanthrene analogues 7 and 8, in which the B-N fragment
occupies the 9,10-positions, were synthesized via a modified
(
5) In the case of 2, a small amount (ca. 10 %) of the 9,10-diphenyl
substituted species (2′′) was consistently observed, the structure of which
was confirmed by NMR, mass spectrometry, and X-ray crystallography (see
Figure S3, Supporting Information). At present, we do not know how this
compound is formed; ongoing mechanistic experiments suggest that solvent
does not play a significant role, that the process is not influenced by
adventitious HCl, and that residual palladium in the pyridine compound is
likely not involved.
1
3
procedure originally reported by Dewar et al. Microwave-
assisted dehydrohalogenation of 2-biphenylamino-dichloro-
(
(
(
6) Mamane, V.; Hannen, P.; F u¨ rstner, A. Chem.-Eur. J. 2004, 10, 4556.
7) Zimmermann, G. Eur. J. Org. Chem. 2001, 457.
8) Mamane, V.; Gress, T.; Krause, H.; F u¨ rstner, A. J. Am. Chem. Soc.
3
borane in the presence of AlCl was found to afford the
2
004, 126, 8654.
9) Lewis, F. D.; Karagiannis, P. C.; Sajimon, M. C.; Lovejoy, K. S.;
Zuo, X.; Rubin, M.; Gevorgyan, V. Photochem. Photobiol. Sci. 2006, 5,
chlorinated precursor 6, which upon reduction with LiAlH
4
(
gave 7 as a white crystalline solid (Scheme 2). The Ph
derivative 8 was prepared from the reaction of 6 with
3
69.
(10) (a) Dix, I.; Doll, C.; Hopf, H.; Jones, P. G. Eur. J. Org. Chem.
11
PhMgBr. The B NMR of 7 displayed a broad doublet at
2
002, 2547. (b) Imamura, K.; Hirayama, D.; Yoshimura, H.; Takimiya, K.;
Aso, Y.; Otsubo, T. Tetrahedron Lett. 1999, 40, 2789.
11) Ghesner, I.; Piers, W. E.; Parvez, M.; McDonald, R. Chem. Commun.
005, 2480.
12) Boese, R.; Finke, N.; Henkelmann, J.; Maier, G.; Paetzold, P.;
Reisenauer, H. P.; Schmid, G. Chem. Ber. 1985, 118, 1644.
34.5 ppm (JHB ) 115 Hz), which was found to agree with
the calculated chemical shift of BN-substituted benzene (34.4
(
2
(
(13) Dewar, M. J. S.; Kubba, V. P.; Pettit, R. J. Chem. Soc. 1958, 3073.
Org. Lett., Vol. 9, No. 7, 2007
1
396

Scheme 2. Synthesis of 6-8
Table 1. UV-vis, Fluorescence, Quantum Yield, and Lifetime
Data for 1-8a
UV-vis
nm)
fluorescenceb
(nm)
τFd
(ns)
(
ΦFc
1
2
3
4
5
6
7
8
446
448
445
457
452
326
326
329
293
450
459
451
464
459
330
327
336
347
0.58
0.49
0.56
0.38
0.52
0.43
0.61
0.31
0.09
5.3
4.6
5.6
6.2
5.4
3.5
5.9
4.7
52.4
ppm).14 Compounds 7 and 8 were found to be completely
air and moisture stable, likely due in part to the absence of
reactive BdC double bonds that are present when the B-N
moiety occupies the ring annulation points. Although we
were unable to obtain X-ray quality crystals of either 7 or 8,
the molecular structure of the 10-hydroxy species has been
previously reported.15
e
C14H10
a
b
All experiments were performed in cyclohexane solution. 1-5: λex
c
) 330 nm. 6-8, phenanthrene: λex ) 260 nm. ΦF ) Quantum yield.
d
Reported relative to 9,10-diphenylanthracene (ΦF ) 0.90).
τF )
Although the aromatic nature of Dewar’s BN-phenanthrene
Fluorescence lifetime. e C14H10 ) phenanthrene.
16
7
has been contested in the literature, our group has recently
employed nucleus independent shift calculations (NICS) as
an effective means of evaluating the aromaticity of various
heterocyclic rings.3 Thus, the resulting NICS(1) values
determined for 1 suggested reasonably high aromatic char-
a
acter for both the C
ppm), whereas the outer C
to possess slightly reduced aromaticity. Similar trends were
observed for 7 (C , -10.0 and -10.4 ppm; C BN, -4.6 ppm)
as well as for the all-carbon phenanthrene (inner, -10.7;
5
N (-9.5 ppm) and C
5
B rings (-8.7
4
BN ring (-6.3 ppm) was observed
6
4
17
outer, -8.2 ppm), which suggests similar aromatic behavior
for all three phenanthrene species regardless of heteroatom
substitution.
Phenanthrene is a well-known fluorophore with emission
in the UV (λem ) 347 nm) and a relatively low quantum
Figure 3. (a) Cyclohexane solutions of 1 and 7. (b) Fluorescence
of 1 and 7 upon irradiation at 365 nm.
yield (Φ ) 0.09); despite this, it has recently found attention
F
2
as a conjugated component in luminescent organic materials.
In contrast, the emission spectrum of 1 was observed to
undergo a large bathochromic shift (103 nm) to give a
maximum at 450 nm (Figure 2) along with a substantial
3
). A small Stokes shift value was observed for 1 (4 nm)
compared to that of phenanthrene (54 nm), which suggests
that geometric distortions in the excited-state are minimal
in 1. The alkyl- and aryl-substituted species 2-5 also
displayed blue fluorescence with slightly red-shifted emission
maxima (λem ) 451-464 nm) along with moderately
increase in the quantum efficiency to Φ ) 0.58 (Table 1),
F
which as a result afforded bright blue light emission (Figure
decreased quantum efficiencies (Φ ) 0.56-0.38). The
F
fluorescent lifetimes for 1-5 were found to be in the range
of 4.6-6.2 ns, significantly shorter than for phenanthrene
(
(
τ
τ
F
) 52.4 ns) but comparable to that of B
) 5.0 ns). On the other hand, the emission spectrum of
2
N
2
-triphenylene
3
a
F
7
displayed a maximum in the UV at 327 nm (Figure 2)
with a reasonably high quantum efficiency of Φ ) 0.61.
F
Because this is a 123 nm hypsochromic shift relative to the
isomeric 1, the emission bands in 7 were found to be more
similar to that of phenanthrene or even phenanthridine (λem
)
350 nm), which also possesses N-heteroatom substitution
(
(
14) Ashe, A. J., III; Fang, X. Org. Lett. 2000, 2, 2089.
15) Harris, K. D. M.; Kariuki, B. M.; Lambropoulos, C.; Philp, D.;
Robinson, J. M. A. Tetrahedron 1997, 53, 8599.
16) Mikhailov, B. M.; Kuimova, M. E. J. Organomet. Chem. 1976, 116,
23.
(
1
(
17) NICS(1) values of -10.2 (inner) and -6.5 ppm (outer) have been
Figure 2. Fluorescence emission spectra of phenanthrene (black
line), 1 (red line), and 7 (blue line) in cyclohexane.
previously reported for phenanthrene: Schleyer, P. v. R.; Maerker, C.;
Dransfeld, A.; Jiao, H.; van Eikema Hommes, N. J. R. J. Am. Chem. Soc.
996, 118, 6317.
1
Org. Lett., Vol. 9, No. 7, 2007
1397

at a peripheral position.18 As such, the B-N moiety in 7
acts more as a bridge to planarize the true fluorophore (e.g.,
biphenyl), whereas in 1, the internalized B-N fragment is
more intimately involved in the fluorogenic unit.
In conclusion, we have observed that the location of polar
B-N moieties within the aromatic framework of PAHs can
dramatically alter the photophysical properties of the resulting
materials. This is dependent upon whether the B-N species
is intimately involved in the fluorogenic unit (e.g., 1) or
simply acts as a bridging group for the true fluorophore (e.g.,
Acknowledgment. This work was funded by the Natural
Science and Engineering Research Council (NSERC) of
Canada in the form of a Discovery Grant to W.E.P. Alberta
Ingenuity and NSERC are thanked for providing support to
M.J.D.B. (2003-2008) and C.A.J. (2005-2007). The authors
would also like to thank Prof. Ray Turner (Calgary) for the
use of his fluorescence spectrometer.
Supporting Information Available: Full experimental
details and compound characterization; crystallographic
information in CIF format; molecular structures of 2
and 2′′; NICS values for 1, 7, and phenanthrene; and
7). This strategy of isoelectronic BN incorporation might
provide an alternative method for tuning the emissive
properties of phenanthrene-based materials2 for use in
optoelectronic devices.
1
13
1
emission, H and C{ H} NMR spectra for 1-8. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(18) (a) Kropp, J. L.; Lou, J. J. J. Phys. Chem. 1971, 75, 2690. (b) Van
Duuren, B. L. Anal. Chem. 1960, 32, 1436.
OL070328Y
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Org. Lett., Vol. 9, No. 7, 2007