Thiophene-fused borepins as directly functionalizable boron-containing π-electron systems

Article abstract of DOI:10.1021/ja502644e

Synthetic protocols were developed for the gram-scale preparation of two isomeric dithienoborepins (DTBs), boron-containing polycyclic aromatics featuring the fusion of borepin and thiophene rings. DTBs exhibit reversible cathodic electrochemistry and boron-centered Lewis acidity in addition to enhanced electronic delocalization relative to benzo-fused analogues. Borons precise position within the conjugation pathway of DTBs significantly affected electronic structure, most clearly demonstrated by the variation in spectroscopic responses of each isomer to fluoride ion binding. In addition to excellent stability in the presence of air and moisture, DTBs could also be subjected to electrophilic aromatic substitution and metalation chemistry, the latter enabling the direct, regiospecific functionalization of the unsubstituted thiophene rings. Subsequent tuning of molecular properties was achieved through installation of donor and acceptor π-substituents, leading to compounds featuring multistep electrochemical reductions and polarizable electronic structures. As rare examples of directly functionalizable, π-conjugated, boron-containing polycyclic aromatics, DTBs are promising building blocks for the next generation of organoboron π-electron materials whose development will demand broad scope for molecular diversification in addition to chemical robustness.

Full text of DOI:10.1021/ja502644e

Article
pubs.acs.org/JACS
Thiophene-Fused Borepins As Directly Functionalizable Boron-
Containing π‑Electron Systems
David R. Levine,† Maxime A. Siegler,† and John D. Tovar*,†,‡
‡
†Department of Chemistry and Department of Materials Science and Engineering, Johns Hopkins University, 3400 North Charles
Street, Baltimore, Maryland 21218, United States
S
* Supporting Information
ABSTRACT: Synthetic protocols were developed for the
gram-scale preparation of two isomeric dithienoborepins
(DTBs), boron-containing polycyclic aromatics featuring the
fusion of borepin and thiophene rings. DTBs exhibit reversible
cathodic electrochemistry and boron-centered Lewis acidity in
addition to enhanced electronic delocalization relative to
benzo-fused analogues. Boron’s precise position within the
conjugation pathway of DTBs significantly affected electronic
structure, most clearly demonstrated by the variation in
spectroscopic responses of each isomer to fluoride ion binding. In addition to excellent stability in the presence of air and
moisture, DTBs could also be subjected to electrophilic aromatic substitution and metalation chemistry, the latter enabling the
direct, regiospecific functionalization of the unsubstituted thiophene rings. Subsequent tuning of molecular properties was
achieved through installation of donor and acceptor π-substituents, leading to compounds featuring multistep electrochemical
reductions and polarizable electronic structures. As rare examples of directly functionalizable, π-conjugated, boron-containing
polycyclic aromatics, DTBs are promising building blocks for the next generation of organoboron π-electron materials whose
development will demand broad scope for molecular diversification in addition to chemical robustness.
To fully realize the potential of organo-main group
heterocycles as candidates for functional materials, it is critical
to develop streamlined procedures for their synthesis as well as
their incorporation into more complex, extended π-conjugated
systems. This presents a particular challenge in the context of
compounds containing tricoordinate boron centers, which are
inherently sensitive to oxygen, moisture, and many typical
reaction conditions.16 Therefore, despite elegant synthetic
preparation and intriguing molecular properties, many unique,
cutting-edge π-conjugated organoboron architectures are
reported as final targets without mention of continued chemical
tailoring.
Interest has recently re-emerged in polycyclic π-electron
compounds featuring borepin rings,17 charge-neutral, boron-
containing heteroaromatic analogues of the tropylium cation.18
In contrast to prevailing synthetic precedent at the time, a key
component of our contribution to borepin research was to
achieve molecular and polymeric functionalization of benzo-
fused borepins17e−h via transition-metal-catalyzed cross-cou-
pling chemistry, a general approach which has drawn increasing
attention as a route to a variety of functionalized organoboron
heterocycles.19 However, this strategy has typically relied upon
the preinstallation of transformable functional groups (generally
a halogen or its equivalent) onto molecular precursors in order
I. INTRODUCTION
Embedding main group elements within rigidly planar
polycyclic aromatic molecules has emerged as a promising
strategy to construct organic materials with unique and
desirable optoelectronic properties.1 For example, in addition
to modulating frontier molecular orbital levels, exotic main
group elements (such as B,2 Si,3 P,4 Se,5 and Te6) provide
opportunities for reversible heteroatom coordination number
change7 and irreversible covalent modification (e.g., via
heteroatom oxidation8 or functionalization9) as gateways to
dynamic or permanent property tuning. This has allowed for
the continued evolution of more traditional materials derived
from polycyclic aromatics based on the elements C, N, O, and
S.10 A remarkably broad scope of function has been
demonstrated by the main group π-electron family of
compounds: planar, boron-doped nanographenes11 exhibit
high electron-accepting ability and emission extending into
the near-IR; stimulus-responsive “phosphole-lipids”12 self-
assemble into organogels demonstrating efficient solid state
and solution FRET; and compounds containing azaborine
motifs take advantage of C−C/B−N isosterism to act broadly
as either enzyme-inhibitory benzene mimics13 or as hole
carriers in p-type OFET devices.14 Replacement of carbon
centers with main group atoms in π-conjugated systems also
provides many opportunities to glean new fundamental
chemical insight. For example, this approach has deepened
our understanding of aromaticity and antiaromaticity in
modified benzenoid and nonbenzenoid systems.15
Received: March 15, 2014
Published: April 16, 2014
© 2014 American Chemical Society
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to later carry out the desired reactions on the fully constructed
boron-containing species.
We now report the synthesis, characterization, and chemical
reactivity of two structurally isomeric B-mesityl (Mes; 2,4,6-
trimethylphenyl) dithienoborepins (DTBs; Chart 1, “this
chemistry24 to functionalize the unsubstituted DTB cores
(thieno “R” sites, Chart 1).
Annelation of borepin and thiophene presents a choice with
respect to the orientation of the fused rings. Altering the fusion
motif leads to different structural isomers with unique
conjugation pathways. Based on our previous studies of meta-
and para- substituted DBBs,17e−h we speculated this could
provide a basis for molecular property tuning. Therefore, we
targeted DTBs possessing clear differences in the electronic
relationship between boron and the extended π-system (Chart
1, “this work”). Because the dominant pathway for electron
delocalization involving thiophene rings typically extends
outward from the α-positions (Chart 1, blue and red
conjugation pathways), these model compounds can be
thought of as possessing thiophene R-substituents with weak
(left) or strong (right) conjugation to the boron center. At the
same time, R−R electronic communication should be more
effective when the thiophene rings are α-conjugated to one
another via the olefinic borepin bridge (Chart 1, R−R
conjugation “strong”) than when cross-conjugated to one
another via the boron center (Chart 1, R−R conjugation
“weak”).
Chart 1. Structures of Dibenzo- and Dithieno-Fused
Borepins
Synthesis of DTBs. We prepared the parent B-Mes DTBs 1
and 2 by employing a synthetic strategy based on the
condensation of dimetalated (Z)-dithienylethenes25 with a
mesityl boronic ester (Scheme 1). Lithium−bromine exchange
Scheme 1. Synthesis of Parent B-Mes DTBs
a
b
c
d
Reference 17a. Reference 17d. Reference 17e, f. Reference 21.
work”). These molecules were prepared on gram scales in a
straightforward manner and exhibited excellent tolerance for
moisture and oxygen while preserving the important Lewis
acidic character of boron. The precise position of boron within
isomeric DTB scaffolds leads to key differences in electronic
structure, strongly influencing molecular properties. The
chemically robust nature of DTBs allows the use of standard
thiophene synthetic chemistry, thus enabling direct, late-stage
molecular diversification of the parent systems for rapid tuning of
electronic and redox properties.
of 326 and 421a with tert-butyllithium (t-BuLi), followed by
treatment with MesB(OMe)2, provided DTBs 1 and 2 in 43%
and 70% yields, respectively. This straightforward preparative
method is more step-economic than the stannocyclization/tin−
boron exchange routes commonly used to construct doubly
fused borepin rings17d−h and avoids the toxic organotin
reagents and boron halides associated with the latter protocol.
Importantly, this procedure enabled the synthesis of 1 and 2 on
gram scales.
DTBs exhibited excellent tolerance for oxygen, moisture, and
silica gel, allowing for aqueous reaction workup and purification
by chromatography under ambient laboratory conditions.27
Additionally, both compounds could be stored under air for
several months without showing any signs of decomposition as
indicated by NMR and UV−vis spectroscopy. This is quite
notable given that the B-Mes bond of DBB-Mes is hydrolyzed
by atmospheric water over the course of several hours to form
the B−O−B anhydride,17d requiring the use of the bulkier B-
Mes* protective group to impart prolonged stability.17e Thus,
the high degree of robustness exhibited by DTBs suggests a
special stability conferred by thiophene-borepin ring fusion.
II. RESULTS AND DISCUSSION
Design Considerations. The key structural feature of these
molecules is the fusion of two thiophene rings to the b- and f-
positions of a central borepin core. This pattern of ring fusion
has been employed in the context of dibenzo-fused borepins
[such as DBB-OH,17a DBB-Mes,17d and DBB-Mes*17e,f (Mes*
= 2,4,6-tri-tert-butylphenyl); Chart 1] to obtain compounds
which possess greater degrees of stability than unfused borepin
systems and, with appropriate steric shielding for boron, can be
rendered highly robust toward air and moisture. Inspired by
analogous dithienotropylium cations,20 Gronowitz and co-
workers demonstrated over 40 years ago that replacement of
the benzo rings in dibenzoborepins with thieno rings21,22 (as in
DTB-OR, DTB-Me, and DTB-OB; Chart 1) lent additional
stability to the borepin substructure. However, it was reported
soon thereafter that DTB-OB decomposed upon further
attempted synthetic manipulation.23 We envisioned that
installation of an appropriate boron-protecting group (such as
Mes) might provide compounds with sufficiently robust
character to allow the use of well-established thiophene
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Characterization of DTBs. The identity and purity of the
parent DTBs were confirmed with NMR spectroscopy (1H, 11B,
13C) and high-resolution mass spectrometry in conjunction
with unambiguous structural determination via X-ray crystallog-
raphy. UV−vis and photoluminescence (PL) spectroscopy were
utilized to elucidate electronic properties, and electrochemical
properties were examined by cyclic voltammetry (CV).
Theoretical calculations were carried out to obtain optimized
geometries, molecular orbital surfaces/energies, optical tran-
sitions, and nucleus-independent chemical shift (NICS)28
values. For comparison, we reference data for DBB-Mes as
reported by Piers17d and bis(2-thienyl)mesitylborane (BTMB)
as reported by Kawashima.29
featuring thieno fusions (∼ −4.9 ppm) are consistent with
weak aromaticity (as has been reported for other hetero-
annelated borepins),30 the values for the dibenzo-fused borepin
model (−2.90 ppm) indicated even less aromatic character.
These results correlate well with the stronger deshielding of
borepin rings protons (Hc) in DTBs compared to DBB-Mes, as
well as crystallographic data (vide infra) showing considerably
reduced borepin C−C bond length alternation in DTBs (1:
0.078 Å; 2: 0.088 Å) compared to DBB-Mes (0.111 Å). The
enhancement in aromaticity of borepin rings in DTBs
compared to DBB can be rationalized in terms of the weaker
localization of aromatic π-electron sextets within fused thieno
rings relative to benzo rings, allowing greater diatropic ring
currents to circulate within the central borepin substructures of
the DTB systems.
11B NMR spectra of DTBs show broad peaks near 50 ppm,
which are shielded by 12−15 ppm relative to DBB-Mes (Table
1). This suggests that thieno-ring fusion substantially enhances
Single crystals suitable for X-ray structure determination
were grown by slow diffusion of MeOH into concentrated
solutions of 1/CHCl3 and 2/CH2Cl2 (Figure 1). All C−B−C
Table 1. Selected 1H NMR, 11B NMR, and NICS(1) Data for
1, 2, DBB-Mes, and BTMB
1
2
DBB-Mes
BTMB
a
c
d
11B (δ, ppm)
51.9
48.7
64.3
54.9
a
a
a
d
1H (δ, ppm)
1H (δ, ppm)
1H (δ, ppm)
7.47
7.95
−
−
7.37
7.81
b
d
7.38
7.69
7.70
7.74
7.29
c
a
c
−
−
Figure 1. Displacement ellipsoid plots for (a) 1 and (b) 2 at 110(2) K
(50% probability level) with views perpendicular (top) and parallel to
(bottom) the polycyclic planes. Selected bond lengths (Å), 1: B1−C1
= 1.533(2), B1−C10 = 1.538(2), B1−C11 = 1.581(2), C1−C4 =
1.409(2), C4−C5 = 1.426(2), C5−C6 = 1.348(2), C6−C7 =
1.426(2), C7−C10 = 1.404(2); 2: B1−C1 = 1.522(2), B1−C10 =
1.521(2), B1−C11 = 1.583(2), C1−C4 = 1.406(2), C4−C5 =
1.440(2), C5−C6 = 1.353(2), C6−C7 = 1.441(2), C7−C10 =
1.403(2).
b
NICS(1)borepin
−4.93
−4.90
−2.90
a
b
R = Mes; spectra collected in CDCl3 solution. R = H; GIAO
method, DFT, B3LYP/6-31+G*. Reference 17d. Reference 29.
c
d
the π-electron density at boron relative to benzo-fusion. The
rigidly planar geometry provided by the centrally fused borepin
rings also appears to promote effective π-electron delocalization
to boron; relative to the unconstrained (yet similarly
thiophene-conjugated) congener BTMB, the boron center of
2 is shielded by 6.2 ppm. The two DTB isomers also show
substantial differences in the magnetic environments of their
polycyclic core protons. Thieno protons (Ha, Hb; Table 1) in 2
are noticeably deshielded relative to 1, presumably due to the
stronger electron-withdrawing effect of the α-conjugated boron
center. On the other hand, chemical shifts for the “olefinic”
borepin ring protons (Hc, Table 1) in both DTBs are quite
close in value, yet are deshielded by over 0.3 ppm compared to
DBB-Mes.
bond angles fall within the range 118.5°−123.0°, confirming sp2
hybridization of the boron centers. The DTB backbones
possess highly planar geometries [angle between thiophene
(Th) mean planes: 1Th−Th = 2.4°, 2Th−Th = 3.0°] with the
pendant Mes groups oriented in near-perpendicular fashion to
the polycyclic core [borepin-Mes dihedral angles: 1 = 76.6(2)°,
2 = 82.5(2)°]. These solid-state molecular geometries were
well-reproduced by DFT calculations (B3LYP/6-31G*, Sup-
porting Information). The planarity of the DTBs differentiates
them from DBB-Mes, which shows a slightly bowed borepin
ring in the solid state.17d Despite the planar X-ray structures of
1 and 2, we did not observe extensive intermolecular π-stacking
in the crystal lattices (Supporting Information), likely due to
the steric hindrance of the Mes groups.
1
The 11B and Hc chemical shift values suggested a possible
difference in aromatic character between the dibenzo-fused and
dithieno-fused borepins. To gain further insight regarding the
contribution of borepin-centered ring currents to the observed
NMR shifts, we carried out NICS calculations on simplified
models of 1, 2, and DBB-Mes, replacing the Mes groups with H
(Table 1). While NICS(1) values for the borepin rings
B−C bond lengths (Figure 1) can be used to gauge boron’s
participation within the delocalized DTB π-electron networks.
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Borepin B−C bonds (1.52−1.53 Å) are much shorter than B−
C bonds to the exocyclic Mes groups (∼1.58 Å), revealing
substantial boron−carbon π-bonding within the fused,
planarized portions of the molecules. The enhancement of
B−C π-bonding due to ring fusion is highlighted by the fact
that 2 has 0.02−0.03 Å shorter B−Cthieno bonds than unfused
BTMB [1.544(4), 1.549(6) Å],29 despite the otherwise similar
connectivities between boron and the thiophene rings. We also
observe considerable shortening of the borepin B−C bonds
(0.03−0.04 Å) in DTBs relative to DBB-Mes [1.563(3),
1.564(3) Å],17d demonstrating that more efficient boron-π-
overlap is achieved through the polarizable electronic structures
of the thiophene-fused arrays relative to the benzo-fused
analogue.
accepting character of these systems. Despite the presence of
terminal thieno moieties in DTBs, we were unable to observe
anodic CV waves or other follow-up electrochemical processes
(such as electropolymerization) within the THF solvent
window (<0.90 V).
DTB Reactivity. Noting the high stability of the DTBs
toward air and water exposure, we were curious to see if they
could withstand reaction conditions required for continued
synthetic modification. Thus, we subjected 1 and 2 to typical
reactions used to functionalize unsubstituted thiophene rings:
electrophilic aromatic substitution (SEAr) and metalation/
electrophile quench (Scheme 2).
Scheme 2. Reactivity of Parent DTBs: SEAr and Direct
a
UV−vis spectra indicate a slight bathochromic shift in
absorption onset for 1 (390 nm) relative to 2 (378 nm) (Figure
2a, c). This is consistent with a smaller calculated HOMO−
Metalation Chemistry
a
Reaction conditions: (a) NBS (2.0 equiv), DMF, rt, dark. (b) (i) t-
BuLi (2.2 equiv), Et2O, −78 °C; (ii) D2O.
SEAr reactions demonstrated that bromine substituents could
be installed electrophilically onto DTBs without causing
decomposition of the boron-containing cores (Scheme 2,
condition a).32 Unexpectedly, we found the site selectivity of
bromination to be isomer-dependent. Upon treatment of 1
with 2 equiv of N-bromosuccinimide (NBS), bromine
incorporation was observed at the typical thiophene α-positions
to give dibrominated 1a in 52% yield and a lesser amount
(∼30%) of a singly thieno-brominated product. However,
subjecting 2 to identical reaction conditions led instead to
bromination of the pendant mesityl ring, giving 2-Mes-Br in
51% yield in addition to recovered starting material. It appears
that strong conjugation between boron and the thiophene rings
in 2 deactivates the latter toward SEAr chemistry, leading to a
preference for mesityl bromination. The protection afforded to
boron by the B-Mes group appears to be critical for the stability
Figure 2. Photophysical and electrochemical data for 1 (a, b) and 2
(c,d). UV−vis/PL spectra obtained in CHCl3 solution. CV scans
obtained in 0.1 M n-Bu4NPF6/THF.
LUMO gap for the former (Supporting Information) and
reflects the notion that 1 possesses greater electron
delocalization owing to a longer effective conjugation pathway
(via the olefinic bridge of the borepin, Chart 1). In the low-
energy spectral region (310−380 nm), 1 shows much greater
absorption intensity than 2, in agreement with TD-DFT
calculated transitions revealing oscillator strengths an order of
magnitude larger for the former (Supporting Information).
DTBs were luminescent under 365 nm light both as solids and
in solution; however, solution state PL quantum yields for 1
(ΦPL = 0.06) and 2 (ΦPL = 0.05) were low in comparison to
DBB-Mes (ΦPL = 0.70).
CV showed single, reversible electrochemical reductions for
both DTBs in THF (E1/2 red 1 = −2.26 V, E1/2 red 2 = −2.42 V
vs Ag/Ag+; Figure 2b, 2d). The slightly more difficult reduction
of isomer 2 is in qualitative agreement with a higher calculated
LUMO level (Supporting Information) and can be ascribed to
mitigation of boron’s electron-deficient character by strong
conjugation with the thiophene rings. Interestingly, the
reversible reduction of DBB-Mes (E1/2 red = −2.36 vs Ag/
Ag+)31 is observed at a similar potential to DTBs, indicating
that replacement of benzo rings with more electron-rich
thiophene rings does not significantly diminish the electron-
́ ́
of the DTB core during bromination, since Parkanyi reported
DTB-OB to undergo B−C bond cleavage upon treatment with
a brominating agent.23
Despite the fact that SEAr preferentially occurred at the
mesityl group of 2, we found direct metalation with
organolithium reagents to be a very effective method for
exclusive α-thieno-functionalization of both DTB isomers.
Treatment of 1 or 2 with t-BuLi followed by addition of
D2O provided deuterated DTBs 1-d2 and 2-d2 in 93% and 79%
yields, respectively (Scheme 2). 1H NOE spectra of the
deuterated products confirmed the regiospecific nature of the
metalation process (Supporting Information). The high yields
obtained are evidence of the impressive stability of DTB cores
toward strongly basic reagents without the requirement for very
severe steric shielding of boron (such as found in B-
33
Mes*19b,17e−h or BMes2 functional groups). Importantly, in
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a
Scheme 3. Molecular Diversification of DTBs via Direct Functionalization
a
Reaction conditions: (a) t-BuLi (2.5 equiv), Et2O, −78 °C. (b) For 1a/2a: Br2 (2.4 equiv), hexanes. (c) For 1b/2b: n-Bu3SnCl (3.0 equiv). (d) For
1c/2c: DMF (3.2 equiv); (e) For 1a → 1d/2a → 2d: 4-ethynyl-N,N-dimethylaniline (2.4 equiv), Pd[P(t-Bu)3]2 (0.1 equiv), CuI (0.08 equiv), i-
Pr2NH (2.4 equiv), toluene, rt; (f) For 1b → 1e/2b → 2e: 4-bromobenzonitrile (2.2 equiv), Pd(PPh3)4 (0.05 equiv), 1,4-dioxane, reflux. (g) For 1c
b
c
→ 1f/2c → 2f: 4-tert-butylaniline (2.1 equiv), TFA (cat.), CHCl3, 60 °C. Used without purification. Yield given over two steps.
addition to D2O, dilithio-DTBs were found to react with other
synthetically relevant electrophiles (vide infra).
isomeric variation has been shown to lead to changes in anion-
binding-induced electronic responses in vinyl side-chain
borylated materials,35 to our knowledge DTBs represent the
only examples of such phenomena being caused by structural
isomerism within a boron-conjugated main chain.
Stability in the presence of t-BuLi provoked another
question: could the tricoordinate boron centers of DTBs still
kinetically function as Lewis acids? Indeed, the ability of
tricoordinate boron to bind with anions is the key functional
property upon which organoboron-based sensing and catalysis
schemes rely.34 Previously, we were unable to observe such
interactions in DBB-Mes*, presumably due to the extreme
steric bulk presented by the B-Mes* groups. However, the less
demanding steric environment in DTBs suggested that they
might be able to show anion binding behavior. In fact, addition
of increasing amounts of tetrabutylammonium fluoride (TBAF)
to NMR samples of 1 and 2 in CDCl3 led to the gradual
disappearance of the native 11B NMR signals near 50 ppm,
accompanied by the appearance of new peaks at 2.8 and 2.1
ppm, respectively (Supporting Information). These spectral
shifts are characteristic of coordinative saturation of triarylbor-
anes to form tetracoordinate fluoroborates, confirming that the
Lewis acidity of boron in DTBs remains intact.
Notably, fluoride binding had quite different effects on the
electronics of the two DTB isomers (Figure 3). While TBAF
addition to 1 led only to very subtle changes in the UV−vis
profile (Figure 3a), 2 was dramatically impacted as key bands
were strongly hypsochromically shifted or suppressed entirely
(Figure 3b). This further supports the notion that the degree of
boron−π interaction varies according to the specific nature of
the available conjugation pathways in each DTB; though
Molecular Diversification. Regiospecific lithiation of
DTBs provided a robust method to synthesize functionalized
analogues directly from the unsubstituted parent compounds
(Scheme 3).36 Thus, treatment of dilithio-DTBs with molecular
bromine, tri-n-butylstannyl chloride, or DMF furnished
dibromide (1a, 2a), distannane (1b, 2b), and dialdehyde (1c,
2c) derivatives, respectively. The bromides and stannanes were
employed in subsequent Pd-catalyzed cross-couplings: Sonoga-
shira37 cross-couplings between 1a/2a and 4-ethynyl-N,N-
dimethylaniline and Stille38 cross-couplings between 1b/2b and
4-bromobenzonitrile were used to prepare DTBs with donor-
(1d, 2d) and acceptor-type (1e, 2e) π-conjugated substituents,
respectively. The flexibility to employ DTBs as either
electrophilic (1a, 2a) or nucleophilic (1b, 2b) cross-coupling
partners suggests that a wide scope of functionalized DTBs
should be accessible via Pd-catalyzed methods.
To demonstrate an alternative approach toward DTB
functionalization, we carried out trifluoroacetic acid (TFA)-
catalyzed condensations between 4-tert-butylaniline and DTB
dialdehydes 1c and 2c to give conjugated imines 1f and 2f. In
addition to demonstrating the compatibility of DTBs with a
strong Brønsted acid, these reactions suggest that reversible,
self-correcting imine bond formation39 could offer oppor-
tunities to access structurally and functionally complex π-
conjugated DTB-based organoboron architectures via dynamic
covalent chemistry.40
Properties of π-Functionalized DTBs. Characterization
of π-donor and π-acceptor substituted DTBs confirmed that
functionalization could be used to tune electrochemical and
photophysical properties (Figure 4). CV revealed that
extending π-conjugation with either donor or acceptor
substituents led to earlier electrochemical reduction onsets
(Figure 4a, 4b), in agreement with lower calculated LUMO
levels for substituted DTBs relative to the unsubstituted parent
systems (Supporting Information). Donor-substituted DTBs
1d and 2d showed strong second irreversible reductions
following the first reversible processes (Figure 4a). However,
Figure 3. UV/vis spectra of (a) 1 and (b) 2 before and after addition
of excess fluoride (TBAF).
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The donor−acceptor character of 1d and 2d suggested that
fluoride binding (to give putative DTB fluoroborates 1d-F− and
2d-F−, Figure 5a, d) could provoke even more dramatic
Figure 4. Electrochemical and photophysical data for donor- (1d, 2d)
and acceptor-functionalized (1e, 2e) DTBs: CV scans of (a) 1d and
2d and (b) 1e and 2e in 0.1 M n-Bu4NPF6/THF. Normalized UV−vis
and PL spectra of (c) 1d and 2d and (d) 1e and 2e in CHCl3 solution.
Figure 5. Photophysical responses upon fluoride addition to 1d and
2d in THF. PL spectra are normalized for clarity and do not reflect
emission intensity quenching after F− addition. (a) Representation of
fluoride bound 1d-F−, with unbroken major conjugation pathway in
red. (b) UV−vis and (c) normalized PL spectra before and after TBAF
addition to 1d. (d) Representation of fluoride-bound 2d-F− with
broken major conjugation pathway in red. (e) UV−vis spectra and (f)
normalized PL spectra before and after TBAF addition. Photo insets
illustrate sample appearance before (left cuvette) and after (right
cuvette) addition of fluoride under ambient light (b, e) and 365 nm
irradiation (c, f).
acceptor-functionalized DTBs 1e and 2e exhibited up to four
separate reversible reduction waves within a comparable
electrochemical window (Figure 4b), a phenomenon we
attribute to the high electron-accepting capacity of the boryl
and PhCN groups. The ability shown by acceptor-function-
alized DTBs to reversibly attain stable tetraanionic states in a
stepwise fashion is quite rare in the realm of small organic
molecules and suggests that such compounds could be of
interest as capacitive elements for multistate switching41 in n-
type logic or memory devices.42
Electronic spectra of π-functionalized DTBs in CHCl3
showed clear bathochromic shifts in absorption and emission
onsets relative to the parent DTBs (Figure 4c, 4d), consistent
with DFT calculations indicating narrowed HOMO−LUMO
gaps for the functionalized DTB analogues (Supporting
Information). This demonstrates that DTB effective con-
jugation lengths are efficiently extended by installation of π-
substituents. As found for the parent systems, the absorption
onsets for π-extended derivatives of 1 were bathochromically
shifted from the corresponding derivatives of 2. Structured,
bimodal emission spectra with low-energy shoulders were
observed for compounds 1d, 1e, and 2e; however, donor-
functionalized DTB 2d exhibited a broad, featureless emission
spectrum with a much larger Stokes shift than the isomeric
donor-substituted 1d. These features are consistent with
optically induced intramolecular charge transfer between
boryl moieties and the strongly conjugated donor groups.43
To further investigate the polarizable character of donor-
functionalized DTBs, we collected two-photon absorption
(TPA) data for 1d and 2d, obtaining TPA cross section values
of 63 and 75 GM in THF, respectively (excitation at 780 nm,
Supporting Information). While the values are small compared
to performance-oriented nonlinear optical chromophores,44 the
flexibility of DTB functionalization chemistry should offer many
options to optimize these properties through molecular
tailoring.
electronic structure changes than those observed in 1 and 2
(vide supra). In fact, upon treatment with TBAF, samples of 1d
and 2d underwent significant electronic changes that were
observable both spectroscopically and with the naked eye
(Figure 5). As with the parent DTBs, the degree of change
observed here was isomer-dependent. In the UV−vis spectrum
of compound 1d (with the boron center weakly conjugated to
the peripheral donor groups), only small hypsochromic shifts of
the main spectral bands occurred upon TBAF addition (Figure
5b), along with a subtle color change of the sample (Figure 5b,
photo inset). In the emission spectra, a λPL max hypsochromic
shift of 66 nm was evident (Figure 5c), and a visually detectable
emission color change could be seen under 365 nm irradiation
(Figure 5c, photo inset).
The response of DTB 2d (in which boron is strongly
conjugated to the donor groups) to fluoride treatment was
much more dramatic. A large hypsochromic shift of 95 nm for
the λAbs max was accompanied by a visible color change of the
sample from yellow to completely colorless (Figure 5e). This
was accompanied by a hypsochromic shift of 118 nm for the
λPL max along with a strong quenching of emission intensity
(Figure 5f). These changes were visually apparent as “on−off”
switching of sample luminescence (Figure 5f, photo inset). The
distinct differences in electronic responses of 1d and 2d to
fluoride binding can be rationalized in terms of the impact on
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Journal of the American Chemical Society
Article
Notes
π-conjugation in the two isomers (Figure 5a, d, red pathways):
fluoride binding leaves the all-carbon major conjugation
pathway of 1d relatively unaffected (Figure 5a) while 2d’s
intramolecular charge-transfer pathway is instead shut down
due to occupation of the boron p-orbital (Figure 5d). These
observations should prove useful in the rational design of new
DTBs or other organoboron-based sensor materials which not
only consider the placement of boron within the “main chain”
or “side chain” but also take into account the strength of
conjugation between boron and the extended π-electron system
to provide additional opportunities for electronic property
tuning.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support for this work was provided by Johns Hopkins
University. We gratefully acknowledge Prof. William L. Wilson
(University of Illinois at Urbana−Champaign) for collection of
two-photon absorption data and Prof. Warren E. Piers
(University of Calgary) for helpful correspondence regarding
DBB-Mes data. We thank Dr. Cathy D. Moore (JHU) for
assistance with 11B NMR and H NOE experiments and Ms.
1
Tiffani C. Chance (JHU) for help with the collection of UV−
vis and PL data.
III. CONCLUSION
We have reported the synthesis of two new isomeric B-Mes
dithienoborepins (DTBs), air- and moisture-stable boron-
containing polycyclic aromatics featuring the fusion of
thiophene and borepin rings. These compounds could be
prepared on gram scales and exhibit the desirable properties
characteristic of typical boron-containing π-electron materials,
such as polarizable electronic structures, reversible electro-
chemical reductions, and fluoride-binding ability. We have
shown that DTB isomerism, by virtue of the innate differences
in electronic structure between 1 and 2, can be used as a design
motif to tune electronic properties, most clearly highlighted by
the strongly isomer-dependent electronic responses to fluoride
binding. This strategy should prove generally useful for
maximizing the tunability of other boron-embedded π-electron
systems.
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ASSOCIATED CONTENT
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