The role of borole in a fully conjugated electron-rich system

Article abstract of DOI:10.1039/b312028g

The reaction of the 3,3′-dilithiobithieno complex with Tip-B(OMe)₂ affords a borole with an extended conjugated electron-rich π-electron system; the electronic perturbation by the introduction of push-pull substituents is described.

Full text of DOI:10.1039/b312028g

The role of borole in a fully conjugated electron-rich system
Sanghoon Kim,a Kee-hyung Song,b Sang Ook Kang*a and Jaejung Ko*a
a Department of Chemistry, Korea University, Chochiwon, Chungnam 339-700, Korea.
E-mail: jko@korea.ac.kr; Fax: (+)82-41-867-5396; Tel: (+)82-41-860-1337
b Department of Chemical Education, Korea National University of Education, Chungbuk 363-791, Korea
Received (in Cambridge, UK) 29th September 2003, Accepted 23rd October 2003
First published as an Advance Article on the web 3rd November 2003
The reaction of the 3,3A-dilithiobithieno complex with Tip-
B(OMe)2 affords a borole with an extended conjugated electron-
rich p-electron system; the electronic perturbation by the
introduction of push–pull substituents is described.
light. Compound 2 exhibits an absorption band at 369 nm attributed
to the p–p* transition of the p-electron unit and an intense band at
438 nm. Compound 2 displays a green fluorescence at 534 nm in
THF. The positions of the absorption and fluorescence bands in 2
are independent of the solvents (Table 1). The somewhat large
Stokes shift (100 nm) suggests a significant contribution of the
boron vacant p-orbital to the photophysical properties. In contrast
to the fluorescence band of 2, that of 4 is solvent dependent. When
the solvent is changed from hexane or THF to DMF, the emission
maximum shifts to a longer wavelength (Fig. 1). The shift is about
30 nm for THF and ~ 50 nm for hexane. The solvatochromism
exhibited in 4 is attributable to the relatively easy removal of the
Interest in the chemistry of borirenes,1 borepines,2 and boroles3
stems from the concept of p-electron aromatic and antiaromatic
character, and conjugation across the sp2-hybridized boron.
Boroles, the most representative example of this class of com-
pounds, have recently attracted increasing attention as potent Lewis
acids for ethylene polymerization4 and a new type of fluorescent
sensor for a specific anion.5 Several borole catalysts have been
designed that are based on the substituent effects on the aryl rings.
Less attention has been focused on how the perturbation of the p-
electron systems influences the photophysical properties6 and
Lewis acidity. One promising way is the control of the pp–p*
conjugation by removal of the boron p-orbital from conjugation in
the borole ring. Another potential way may be to take advantage of
the bipolar character by the incorporation of the triphenylamine and
boryl moieties, which are p-conjugated through the electron-rich
thiophene ring.7 It should be noted that such push–pull types of p-
electron systems have been well documented as potent nonlinear
optical materials8 or emitting compounds.7 In this communication,
we present the first example of controlling the photophysical
properties and Lewis acidity by the introduction of a fully
conjugated electron-rich unit into the borole.
To examine this perturbation, we employed a borole having
extended p-conjugated thienyl substituents. The introduction of
9,9-dimethylfluorenyl, amine, boryl, and p-electron bridge units
into the borole is intended to provide amorphous glass, electron-
donating, electron-accepting, control of the HOMO–LUMO gap
and charge-carrier transport, respectively. A bulky 2,4,6-triisopro-
pylphenyl (Tip) unit is introduced at a boron atom for kinetic
stabilization.
Our synthesis of bis{2-[4-(bis(9,9-dimethylfluorenyl)amino)-
phenyl]-5-thienyl}dithienoborole (2), starting from the 3,3A-di-
bromobithieno compound (1), was prepared by the metalation of 1
with TipB(OMe)2 in THF under a nitrogen atmosphere (Scheme
1).† The compound 2 is air- and moisture-stable and was identified
by various spectroscopic methods, mass spectrometry, and ele-
mental analysis. A key feature in the 11B NMR spectrum of 2 occurs
at dB 216.2, about 60 ppm and 10 ppm upfield of the resonance for
the fluorinated 9-borafluorene, and tris(9-anthryl) fluorobor-
ate·K+·[2.2.2] cryptand,9 respectively. This remarkable property
strongly suggests the existence of strong electronic interactions
between the boron and fully conjugated electron-rich groups along
the chain, resulting in charge donation from the p-electron systems
toward the boron atom. Further support for this interaction is
provided by the similar property of the conjugated borane 4
prepared by the reaction of lithiated 2-[N,N-bis(9,9-dimethyl-
Scheme 1 Reagents and conditions: (i) 4-bromoaniline, CuCl, 1,10-phenan-
throline, KOH in toluene at 125 °C for 12 h, (ii) 2-thiophene boronic acid,
Pd(PPh3)4, K2CO3 in THF at rt for 12–16 h, (iii) n-BuLi, Me3SnCl in THF
at 0 °C?rt, (iv) 3,3A-5,5A-tetrabromo-2,2A-bithiophene, PdCl2(PPh3)2, LiCl
in toluene at 125 °C, (v) TipB(OMe)2, n-BuLi, THF.
fluoren-2-yl)aminophenyl]-5,5A-bithienyl
with
TipB(OMe)2
(Scheme 2).‡ The borane 4 is a pale yellow solid exhibiting 11B
NMR resonance at dB 28.6. Such an abnormal upfield shift of the
resonance was observed in the unsaturated polymers containing the
boron and thiophenyl units.10
Compounds 2 and 4 are bright green and greenish-blue emitters
in THF, respectively and in the solid state when irradiated by UV
Scheme 2 Reagents and conditions: (i) 5-[2,2A-bithiophene]boronic acid,
Pd(PPh3)4, K2CO3 in THF at rt, (ii) TipB(OMe)2, n-BuLi, THF.
68
C h e m . C o m m u n . , 2 0 0 4 , 6 8 – 6 9
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

boron p-orbital from conjugation relative to the borole compound 2
by coordinating the donor solvent to the boron atom.
To evaluate the electron-donating and -accepting properties,
cyclic voltammograms for compounds 1, 2, and 4 were recorded.
Compound 2 undergoes a reversible anodic oxidation. No oxidation
counterpart corresponding to the anodic peak was observed in spite
of the presence of the boryl unit. This observation is presumably
due to the fact that Lewis acidity in 2 no longer exists. Both
compounds 1 and 4 exhibited two sequential cathodic and the
corresponding anodic waves. The first half-wave oxidation poten-
tials (E1/2oxid) of 2 and 4 were almost the same irrespective of the
difference in the geometric configuration of the borole and borane,
being 0.44 V vs. Ag/Ag+. The oxidation potential is comparable to
or lower than that for the p-electron rich bithienyl complexes such
as bis(triarylamines) based on 3,3A-diphenyl-2,2A-bithiophene11 and
2,5-bis{4-[bis(4-methylphenyl)amino]phenyl}thiophene.12
In order to obtain the characteristic features of the electronic
structure, molecular orbital calculations of 2 were performed with
the B3LYP/3-21G*. The HOMO is delocalized over the p-
conjugated systems via the boron vacant p-orbital (Fig. 2). On the
other hand, the LUMO is localized on a boron with a dithienyl unit.
Examination of the HOMO and the LUMO of 2 indicates that
photoexcitation results in a net charge transfer from the conjugated
p-electron system to the borole ring.
Fig. 2 Representation of (a) HOMO and (b) LUMO of 2 based on the
B3LYP/3-21G* calculation.
Notes and references
† Experimental procedure for 2: To a stirred THF solution (30 mL) of 1
(0.95 g, 0.67 mmol) was added n-BuLi (1 ml of 1.6 M solution in hexane,
1.6 mmol) at 0 °C. The solution was warmed to room temperature and
continually stirred for 2 h at that temperature. The THF solution (5 mL) of
TipB(OMe)2 (0.22 g, 0.8 mmol) was slowly added to the solution at 0 °C
and then the mixture was warmed to room temperature. The solution was
stirred for 12 h. The product was extracted with methylene chloride and
washed with hexane (5 mL 3 3) to afford 2 in 71% yield. 1H NMR (CDCl3):
d 7.65 (d, 4H, J = 7.5 Hz), 7.61 (d, 4H, J = 8.1 Hz), 7.49 (d, 4H, J = 8.4
Hz), 7.39 (d, 4H, J = 7.5 Hz), 7.32–7.24 (m, 16H), 7.18 (d, 2H, J = 8.1 Hz),
7.15 (d, 2H, J = 4.8 Hz), 7.12 (d, 4H, J = 8.1 Hz), 7.08 (s, 2H), 6.91 (s, 2H),
2.87 (m, 2H), 2.65 (m, 1H), 1.42 (s, 24H), 1.24 (m, 18H). 13C{1H} NMR
(CDCl3): d 155.2, 153.6, 147.6, 147.1, 142.9, 130.9, 136.1, 134.6, 134.5,
128.2, 127.1, 126.7, 126.4, 126.2, 125.3, 124.5, 124.1, 123.7, 123.2, 122.8,
122.3, 122.0, 121.0, 120.5, 119.3, 118.4, 47.0, 35.8, 35.4, 34.2, 33.7, 27.3.
11B NMR: d 216.2. MS:m/z 1494 [M+].
‡ Experimental procedure for 4: Compound 4 was prepared using the same
procedure as that described for 2. 1H NMR (CDCl3): d 7.68 (d, 4H, J = 7.5
Hz), 7.64 (d, 4H, J = 8.1 Hz), 7.52 (d, 4H, J = 8.1 Hz), 7.42 (d, 4H, J =
7.2 Hz), 7.35–7.25 (m, 10H), 7.23–7.14 (m, 16H), 7.04 (d, 2H, J = 4.2 Hz),
6.95 (s, 2H), 2.91 (m, 2H), 2.70 (m, 1H), 1.45 (s, 24H), 1.30 (m, 18H).
13C{1H} NMR (CDCl3): d 155.2, 153.6, 147.6, 147.1, 143.1, 130.9, 137.7,
135.9, 134.5, 128.8, 128.2, 127.8, 127.4, 127.1, 126.7, 126.3, 125.3, 124.8,
124.1, 123.7, 123.2, 122.9, 122.3, 120.9, 120.6, 119.7, 119.4, 119.1, 46.9,
35.2, 34.8, 34.0, 33.6, 27.1. 11B NMR: d 28.6. MS: m/z 1496 [M+].
In summary, we have disclosed the synthesis of a fully
conjugated electron-rich borole. The perturbation of the p-electron
systems by introducing amine and boryl moieties influences the
photophysical properties and Lewis acidity. The extremely upfield
shift of the resonance in the 11B NMR spectrum of 2 may be
attributable to the charge donation from the p-electron systems to
the boron atom. This intrinsic property should be considered in the
design of compounds targeted for specific applications.
We are grateful to BK21(2003) for generous financial support.
Table 1 Photophysical and electrochemical data of 2 and 4
First oxidation
Com-
pound
Absorption
(lmax/nm)
Fluorescencea potential/V
(lmax/nm)
Solvent
vs. Ag/Ag+b
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2
4
DMF
THF
Hexane
DMF
THF
369, 438
370, 436
370, 433
376
541
534
438, 518
511
481
0.45
0.43
375
Hexane
373
440, 463
a Excited at the longest absorption maximum wavelengths. b Peak potential
measured on a 1 mM solution in dichloromethane containing 100 mM of
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Fig. 1 Photograph of 2 and 4 solutions under UV irradiation (365 nm). From
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C h e m . C o m m u n . , 2 0 0 4 , 6 8 – 6 9
69