10a-Aza-10b-borapyrenes: Heterocyclic analogues of pyrene with internalized BN moieties

Article abstract of DOI:10.1002/anie.200700591

(Chemical Equation Presented) Tres BN: The introduction of a B-N dipole into the internal positions of pyrene (see scheme) provides a novel platform for heterocyclic fluorescent species. Comparisons of the photophysical and redox properties between the CC

Full text of DOI:10.1002/anie.200700591

Communications
DOI: 10.1002/anie.200700591
Heterocyclic Compounds
10a-Aza-10b-borapyrenes: Heterocyclic Analogues of Pyrene with
Internalized BN Moieties**
Michael J. D. Bosdet, Warren E. Piers,* Ted S. Sorensen, and Masood Parvez
Polycyclic aromatic hydrocarbons (PAHs) have been at the
forefront of materials research owing to their exceptional
photophysical and electrochemical properties.[1] In the cur-
rent drive towards improved molecular devices such as
organic light-emitting devices and field-effect transistors, p-
conjugated materials containing boron have been intensely
investigated, as the incorporation of this heteroatom is known
to decrease the HOMO–LUMO gap while simultaneously
providing a versatile means of honing physical properties by
interaction with various Lewis bases.[2,3] Examples of poly-
cyclic aromatic heterocycles incorporating boron are scarce,
though isoelectronic analogues of the PAHs pyrene,[4] phen-
anthrene,[5] and naphthalene[6] have been synthesized by BN
substitution at the periphery. Herein we report the synthesis
and photophysical properties of an internally BN-substituted
isoelectronic analogue of the ubiquitous fluorophore
pyrene.[7,8]
Scheme 1. Synthesis of 10a-aza-10b-borapyrenes 5.
The BN analogues of pyrene (5, Scheme 1) were prepared
in two steps starting from boracyclohexadienes 1[9] and the
bis(ortho-alkynyl)-substituted pyridines 2. The borabenzene–
pyridine intermediates 3[9,10] rapidly converted into the BN
phenanthrene analogues 4, which in most instances were
isolable as pure compounds.[11] Aza-boraphenanthrenes 4
arise from a cycloisomerization reaction of one of the alkyne
moieties in 3; this reaction requires catalysis in the all-carbon
systems.[12] Here, the facile nature of the uncatalyzed reaction
under ambient conditions may be attributed to the nucleo-
philic character[5b,13] of the carbon atom in the position a to
the boron atom in the transient borabenzene 3. Although this
nucleophilic character is retained in derivatives 4, computed
structures of these derivatives suggest that the distance
between the nucleophilic a’ carbon atom and the terminal
carbon atom of the remaining alkyne group is about 3.5 Š in
this more rigid structure versus about 2.9 Š in the adducts 3.
Therefore, treatment of compounds 4 with 5 mol% of PtCl2 at
90–1008C is required to induce cyclization of the second
alkyne moiety to afford the pyrene analogues 5 as orange-
brown powders in moderate to high yields.
While the BN phenanthrenes 4 are moderately sensitive
to ambient atmosphere, the BN pyrenes 5 are air- and
moisture-stable, which allowed them to be purified by column
chromatography. Indeed, the pyrenes 5 were found to be
thermally and chemically quite robust, undergoing no reac-
À
tion at the B N linkage with protic acids like MeOH or even
HCl. Thermogravimetric analysis on 5b-H and 5c-H revealed
T5% values above 2108C, whereas 5a-H displayed a T5% value
of 1538C.
The structure of unsubstituted 5a-H has been confirmed
by X-ray crystallography (Figure 1a).[14] The B N bond is
À
short
(1.456(4) Š;
compare
borabenzene–pyridine:
1.558(3) Š),[15] which suggests significant B N double-bond
character. This value is in close agreement with comparable
BN PAH analogues with internalized BN moieties, such as
BN naphthalene (1.461(1) Š)[16] and B2N2 triphenylene
(1.464(4) Š).[17] The borabenzene and pyridine rings within
the pyrene framework retain significant p-electron delocal-
À
[*] M. J. D. Bosdet, W. E. Piers, T. S. Sorensen, M. Parvez
Department of Chemistry
À
ization, whereas the C4BN rings contain localized C C double
University of Calgary
2500 University Drive N.W., Calgary, Alberta, T2N 1N4 (Canada)
Fax: (+1)-403-289-9488
E-mail: wpiers@ucalgary.ca
Homepage: http://www.chem.ucalgary.ca/groups/wpiers
bonds (ca. 1.36 Š) between the peripheral carbon atoms. The
localization of these double bonds together with the very
slight deviation from planarity (Figure 1b) indicate structural
features analogous to those of pyrene.[18] NICS(1) calcula-
tions[19] support this view, revealing values of À13.7 and À15.1
for the pyridine and borabenzene rings, respectively, and a
value of À9.5 for the C4BN ring, suggesting that the pyridine
and borabenzene rings have significantly greater aromaticity
than the C4NB rings (Table S1 in the Supporting Informa-
tion). Comparative values for the all-carbon pyrene molecule
[**] M.J.D.B. acknowledges NSERC and AIF (Canada) for financial
support, Prof. D. Cramb, Prof. R. Turner, and Dr. C. A. Jaska of this
institution for invaluable discussions and/or access to instrumen-
tation, and H. Tuonnen for help with SOMO computations.
Supporting information for this article, including full experimental
details, is available on the WWW under http://www.angewandte.org
or from the author.
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Chemie
Figure 1. a) Molecular structure of 5a-H (thermal ellipsoids shown at
50% probability). H atoms have been omitted for clarity. b) Side view
of 5a-H. c) Crystal packing of 5a-H as viewed down the crystallo-
graphic a axis. Select bond lengths [Š] and angles [8] (one of two
independent molecules): B1-N1 1.456(4), B1-C8 1.502(4), B1-C12
1.492(4), C1-C14 1.423(4), C13-C14 1.357(4), C12-C13 1.423(4), N1-C1
1.407(3), N1-C5 1.398(3); C1-N1-B1 119.8(2), N1-B1-C8 119.2(3), C9-
C8-B1 116.1(3), C10-C11-C12 121.8(3), C13-C12-B1 117.2(3), C8-B1-
C12 121.1(3), C11-C12-C13 126.5(3); C1-N1-B1-C8 179.1(2).
are À14.0 and À7.9 for the apical and flanking rings,
respectively.
Figure 2. a) Molecular structure of 5c-iPr (thermal ellipsoids shown at
Introduction of a dipolar BN unit affects the packing of
5a-H, which shows dimeric head-to-tail p stacking (Fig-
ure 1c), in which the BN dipoles oppose each other within
each dimer. The dimers exhibit an average separation of
3.50 Š between the molecular planes, and an intermolecular
B···N separation of 4.067 Š. The packing behavior of these
dimers differs from the herringbone motif observed in
crystalline pyrene.[18]
50% probability). H atoms have been omitted for clarity. b) Crystal
packing of 5c-iPr showing molecules from three parallel sheets (see
also the Supporting Information). Interplanar distance: 3.6 Š; closest
p–p overlap: 7.2 Š. Select bond lengths [Š] and angles [8] of the
monomer: B1-N1 1.454(2), B1-C1 1.498(2), B1-C11 1.507(2) C8-C9
1.425(2), C9-C10 1.378(2), C10-C11 1.435(2), N1-C8 1.399(2); N1-B1-
C1 119.47(13), N1-B1-C11 119.41(13); C8-N1-B1-C1 180.0.
The substituted BN pyrene 5c-iPr[20] shows analogous
metrical parameters to 5a-H (Figure 2a), but significantly
different solid-state packing (Figure 2b). The molecules are
arranged in parallel sheets, such that the dipoles of one sheet
are situated opposite to those of neighboring sheets. Notably,
there are no p-stacking interactions (closest p–p overlap:
7.24 Š) and consequently the closest intermolecular B···N
separation is very large (7.378 Š).
Electrochemically, pyrene is subject to multiple reductive
processes, with radical anionic, dianionic,[21] trianionic,[22] and
even tetraanionic[23] species having been reported. Cyclic
voltammetry on compounds 5 indicates they exhibit a
reversible one-electron reduction at potentials anodic to
that of pyrene. For example, 5a-H had a reversible one-
electron reduction at À1.98 V (Figure S1 in the Supporting
Information), whereas pyrene exhibits a first reduction at
À2.11 V.[24] Stirring a THF solution of 5a-H over a lithium
mirror for 24 h at 258C afforded the radical anion Li+ [5a-H]CÀ
as a deep purple solution, which was indefinitely stable under
an atmosphere of argon at À358C but immediately oxidized
back to neutral 5a-H upon exposure to oxygen. Radical
formation was further confirmed by EPR, with a singlet at
from B or N is anticipated; indeed a node exists along the
C2 axis.[25]
Pyrene is a ubiquitous fluorophore that has found
application in various sensors[26] and as a probe in biochemical
labeling studies.[7,27] As such, the photophysical properties of
BN pyrenes 5 were of interest. The absorption spectra of
derivatives 5 show a strong band in the UV region (320–
332 nm), with relatively weak visible bands (439–483 nm;
Table 1). Excitation of 5a-c in cyclohexane was found to
result in green fluorescence (lmax = 488–498 nm), with low
Table 1: UV/Vis and fluorescence data for BN pyrenes 5.
Compd
UV/Vis[a]
Fluorescence[b]
ff[c]
tf[b,d]
lmax [nm]
lmax [nm]
[ns]
5a-H
5b-H
5b-iPr
5c-H
5c-iPr
321 (5.8), 443 (0.29)
328 (9.7), 446 (0.10)
480, 493, 498,
506, 514
489, 522
0.15
0.11
0.11
0.16
0.19
70 (7)[e]
76
332 (5.6), 453 (0.19),
483 (0.06)
320 (8.4), 341 (1.8),
439 (0.12)
323 (8.9), 446 (0.10),
477 (0.02)
498, 532
478, 511
488, 521
53
79
giso = 2.003 (Figure S2 in the Supporting Information). The
59
EPR spectra were observed to be featureless with no hyper-
fine coupling, even when the radical anion was generated
in situelectrochemically with a [ nBu4N]+ counterion. The
SOMO of 5a-H (Figure S3 in the Supporting Information)
shows that the unpaired electron is located predominantly on
the carbon atoms, and thus only minimal hyperfine coupling
[a] Recorded in CH2Cl2. Number in parentheses is the molar extinction
coefficient, e, in 104 LmolÀ1 cmÀ1. [b] Recorded in cyclohexane with
excitation at 330 nm. [c] Quantum yield reported relative to 9,10-
diphenylanthracene (ff =0.90). [d] Fluorescence lifetime. [e] Excimer
fluorescence lifetime determined at greater than 10À3 m (lex =430 nm).
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quantum yields (ff = 0.11–0.19) and moderate lifetimes (tf =
53–79 ns; Table 1). Conversely, pyrene fluoresces in the UV
region (lmax = 383 nm) with both a higher quantum efficiency
ground-state dimer absorption/fluorescence. While there is
likely some contribution of inner filter effects to the overall
shape of the optically dense solutions,[7] the existence of a
second lifetime component in these solutions together with
the single-crystal emission maxima is suggestive of an
excimeric species. Analogous features exist in the excitation
spectrum from a single crystal of 5c-iPr (Figure S5 in the
Supporting Information), in which no p stacking is observed
(see above). Clearly, this packing excludes fluorescence from
a dimer, and indeed the emission spectrum closely resembles
that of the monomer in dilute cyclohexane solution. This
result is notable, as BN pyrene 5c-iPr also displays excimer
emission in concentrated solution.
In summary, we report synthetic routes to a new family of
heterocycles based on the pyrene framework, with an
internalized BN fragment in place of CC. While retaining
some of the photophysical properties of pyrenes (excimer
formation), the 10a-aza-10b-bora derivatives 5 have higher
electron affinities, feature a lower band gap, and incorporate
an internal dipole which influences crystal packing. Together
these features suggest these compounds may form the basis
for n-channel organic semiconductors.[32] The versatility of the
synthetic route presented, along with ongoing functionaliza-
tion studies of 5a-H, augurs well for the fine-tuning of
photophysical and redox properties for this promising family
of compounds.
(ff = 0.60 in cyclohexane) and
a longer lifetime (tf =
450 ns).[28] The polar nature of 5a-c might be expected to
give rise to solvatochromic behavior in the emission spectra,
but this has not been investigated in detail as of yet.
In all BN pyrenes the red edge of the emission band was
seen to increase in intensity with increasing sample concen-
tration (Figure S4 in the Supporting Information), suggestive
of excimer formation at higher concentrations. Pyrene is the
textbook example of a species which forms an excimer in
concentrated solution, the fluorescence from which is a broad,
bathochromically shifted peak at 470 nm.[29] In contrast, the
fluorescence from the 5a-H excimer is only modestly bath-
ochromically shifted relative to the monomer (broadening
centered around 510–550 nm), such that no clear maxima
could be resolved,[30] though a second lifetime component was
identified in highly concentrated solutions of 5a-H (see
Table 1), which may be attributed to an excimer. Further
insight into the fluorescence behavior of this emissive species
was gained from a study of the emission and excitation of
single crystals of 5a-H and 5c-iPr.[31] Thus, the p-stacked
motif of 5a-H gave rise to a broad emission with lmax
=
520 nm (Figure 3a), similar to the emission position for
concentrated solutions of this species. The excitation spec-
trum (Figure 3b) shows predominantly bands which corre-
spond to monomer excitation, arguing against the notion of
Experimental Section
5a-H: Under argon, a solution of 1-H (90 mg, 0.49 mmol) in toluene
(1 mL) was added to a solution of 2a (62 mg, 0.49 mmol) in toluene
(6 mL) and stirred for 3 days. Volatile components were removed
in vacuo and the brown solid was subsequently slurried in n-hexane
(10 mL), sonicated for 30 minutes, and filtered through a swivel frit.
After several washes with n-hexane, the solvent was removed
in vacuo, and the brown solid was slurried in toluene and filtered
through celite to afford an orange solution. PtCl2 (3 mg, 11 mmol) was
added, and the mixture stirred for 16 h at 958C. After cooling and
removing the toluene in vacuo, the brown solid was dissolved in Et2O
(10 mL) and filtered through neutral alumina. Yellow plates suitable
for X-ray analysis were obtained by slow evaporation from this
1
solution at 258C. Yield: 43 mg (43%). H NMR (C6D6): d = 8.84 (d,
3
3
2H, JH,H 7.5 Hz, CH7), 8.78 (d, 2H, JH,H 8.5 Hz, CH5), 8.25 (t, 1H,
3JH,H 7.5 Hz, CH8), 7.62 (d, 4H, 3JH,H 8.3 Hz, CH2 + CH4), 7.45 ppm (t,
3
1H, JH,H 7.5 Hz, CH1); 13C{1H} NMR (C6D6): d = 139.42 (s, C3),
138.92 (s, CH5), 133.04 (s, C6), 130.44 (s, CH7), 127.27 (s, CH1), 123.69
(s, C8), 120.42 (s, CH2), 117.35 ppm (s, CH4); 11B{1H} NMR (C6D6):
d = 22.3 (brs); UV/Vis (e given in 104 LmolÀ1 cmÀ1): lmax(CH2Cl2) =
443 (0.3), 321 (5.8), 285 (2.2) nm; lmax(n-hexane) = 479 (0.1), 447
(0.3), 322 (5.6), 286 (2.3) nm; fluorescence (cyclohexane): lmax = 480,
493, 498, 506, 514 nm; ff = 0.15, tf = 70 ns (c2 = 0.30); cyclic voltam-
metry (in THF, vs. SCE): E1/2 = À1.98 V. HRMS calcd for C14H10N11B
([M+]): 203.09063; found: 203.09112. TGA: Tonset = 1018C, T5%
1538C. M.p. 1388C (differential scanning calorimetry).
=
Received: February 8, 2007
Published online: April 5, 2007
Figure 3. a) Emission spectra (lex =330 nm) of 5a-H single crystal and
cyclohexane solutions normalized at 510 nm. Solid line: single crystal;
dashed line: 4.92 mm solution; dotted line: 5.91 mm solution. b) Exci-
tation spectra (lem =520 nm) of 5a-H single crystal and dilute
cyclohexane solution. Solid line: single crystal; dotted line: 5.91 mm
solution. For single-crystal spectra, excitation and emission band-
widths were set to 2.5 and 10 nm, respectively.
Keywords: aromaticity · boron · cycloisomerization ·
.
fluorescence · heterocycles
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Chemie
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[20] Crystal Data for 5c-iPr: C25H32BN, Mr = 357.33, monoclinic,
space group P21/m, a = 12.295(6), b = 7.242(3), c = 12.806(7) Š,
a = 90, b = 114.731(19), g = 908, V= 1035.7(9) Š3, Z = 2, 1calcd
=
1.146 gcmÀ3, MoKa radiation, l = 0.71073 Š, T= 173(2) K, 5716
measured reflections, 3205 unique (Rint = 0.0356), 2191 reflec-
tions with Inet > 2s(Inet), m = 0.064 mmÀ1, min/max transmission
0.9872 and 0.9923, R1(I>2s) = 0.0538, wR2 = 0.1397, GoF =
1.037, 160 parameters, final difference map within + 0.342 and
À0.253 eŠ3. CCDC-636013 contains the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
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[14] Crystal data for 5a-H: C14H10BN, Mr = 203.04, monoclinic, space
group P21/c, a = 8.471(6), b = 17.106(13), c = 14.372(6) Š, a = 90,
b = 102.14(4),
g = 908,
V= 2036(2) Š3,
Z = 8,
1calcd =
1.325 gcmÀ3, MoKa radiation, l = 0.71073 Š, T= 173(2) K, 8518
measured reflections, 4639 unique (Rint = 0.0843), 1661 reflec-
tions with Inet > 2s(Inet), m = 0.076 mmÀ1, min/max transmission
0.9894 and 0.9955, R1(I>2s) = 0.0637, wR2 = 0.1251, GoF =
0.974, 289 parameters, final difference map within + 0.169 and
À0.175 eŠ3. CCDC-636012 contains the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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[19] The NICS method is a measure of the abnormal proton chemical
shift of aromatic molecules by the induced ring current that is
due to cyclic s- or p-electron delocalization, and whose values
can either be negative (suggestive of diatropic ring currents and
[31] Single-crystal emission and excitation spectra of pyrene afforded
some of the early confirmation of excimer formation in solution;
see: B. Stevens, Spectrochim. Acta 1962, 18, 439.
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