Controlling the charge transfer in phenylene-bridged borylene-amine π-conjugated systems

Article abstract of DOI:10.1021/ol9012487

Image Presented Novel boron-nitrogen-containing π-conjugated compounds 3,3′- and 4,4′-((2,4,6-triisopropylphenyl)borylene)bis(N, N-diarylbenzenamine) (1-2), m- and p-phenylene bridged to the boron center, respectively, have been synthesized and characterized. Optical studies by means of UV-vis absorption and emission measurements as well as DFT calculations reveal a different charge transfer behavior between the para series and the meta series at ground and excited states.

Full text of DOI:10.1021/ol9012487

ORGANIC
LETTERS
2009
Vol. 11, No. 16
3550-3553
Controlling the Charge Transfer in
Phenylene-Bridged Borylene-Amine
π-Conjugated Systems
Agnieszka Pron´, Gang Zhou, Hassan Norouzi-Arasi, Martin Baumgarten, and
Klaus Mu¨llen*
Max Planck Institute for Polymer Research, Ackermannweg 10,
D-55128 Mainz, Germany
muellen@mpip-mainz.mpg.de
Received June 4, 2009
ABSTRACT
Novel boron-nitrogen-containing π-conjugated compounds 3,3′- and 4,4′-((2,4,6-triisopropylphenyl)borylene)bis(N,N-diarylbenzenamine) (1-2),
m- and p-phenylene bridged to the boron center, respectively, have been synthesized and characterized. Optical studies by means of UV-vis
absorption and emission measurements as well as DFT calculations reveal a different charge transfer behavior between the para series and
the meta series at ground and excited states.
Boron-containing π-conjugated systems have recently at-
tracted attention1 due to their extensive applications in
material science, e.g., as emission and electron conduction
layers in organic light-emitting diodes (OLEDs),2 fluoride
sensors,3 and in components for nonlinear optics.4 The
intriguing properties of boron-containing compounds, which
include low-lying LUMO orbitals, ease of reduction, and a
bathochromic shift of the absorption and emission spectra,
result from the efficient overlap between an empty p orbital
on the boron center and the π-conjugated framework. When
an appropriate electron donor is in conjugation with a boron
acceptor unit, an intense intramolecular charge transfer from
the electron-rich moiety to the boron center occurs (Figure
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10.1021/ol9012487 CCC: $40.75
Published on Web 07/22/2009
 2009 American Chemical Society

S1, Supporting Information).2c The electronic properties and
the geometry of the linker have a significant impact on the
optical characteristics of organoboron compounds. Yamagu-
chi et al.5 showed that incorporation of a strong electron-
donating group into boron-containing systems results in
efficient charge transfer, and therefore a bathochromic shift
in both absorption and emission spectra is observed. Re-
cently, Wang et al.6 investigated the absorption and emission
spectra of linear boron- and nitrogen-containing molecules
with different shape and conjugation type. This study
revealed that the most efficient charge transfer was present
in the linear system due to the conjugative interaction.
Scheme 1. Syntetic Route for Compounds 1p, 1m, and 2m
During our research into π-conjugated donor-acceptor
triarylboron compounds,7 we noticed a significant lack of
systematic studies in terms of the effect of the position of
the donor substituents with respect to the boron center. The
change in the location of the donor moiety influences the
properties of the resulting compounds, especially their charge
transfer behavior. Such investigations offer a promising
route to understanding the effects of conjugation in these
systems, and thereby controlling the absorption and emission
characteristics as well as the HOMO-LUMO gap of
different oligomeric or polymeric donor-acceptor systems.
To investigate the impact of the electronic properties of linkers
and donors on the intramolecular charge transfer interaction in
organoboron compounds, we studied model compounds
3,3′- and 4,4′-((2,4,6-triisopropylphenyl)borylene)bis(N,N-
diarylbenzenamine) (1-2), which are m- (1m, 2m) and p-
(1p, 2p) phenylene bridged to the boron center, respectively.
Different amino groups were incorporated to probe the
influence of the strength of the donor moiety on the optical
properties of the resulting compounds. The charge transfer
interaction between the amino center and the boron center
was studied by means of UV-vis absorption and emission
spectroscopy and density functional theory (DFT) calcula-
tions. Additionally, fluoride sensing properties were inves-
tigated.
As shown in Scheme 1, compounds 1p, 1m, and 2m were
synthesized via Hartwig-Buchwald amination9 of 3p and
3m with either di-p-tolylamine or di-p-anisylamine. More-
over, in this amination step, varying conditions were required.
The meta-substituted compounds 1m and 2m were achieved
with palladium(II) acetate and 1,1′-bis(diphenylphosphino)-
ferrocene (dppf) as the catalytic system.10 Synthesis of
compound 1p was performed by using tris(dibenzylidene-
acetone)dipalladium (Pd2dba3) and tri-tert-butylphosphine
(t-Bu3P) as catalyst. It was not possible to synthesize 2p via
amination. To synthesize compound 2p, an alternative
approach was introduced as demonstrated in Scheme 2. The
Bis(4-bromophenyl)-2,4,6-triisopropylphenylborane (3p)
and bis(3-bromophenyl)-2,4,6-triisopropylphenylborane (3m)
were obtained by treating commercially available 1,4- or 1,3-
dibromobenzene with n-butyllithium (n-BuLi) followed by
addition of dimethyl 2,4,6-triisopropylphenylboronate (TripB-
(OMe)2),8 respectively (Scheme 1). Surprisingly, different
conditions had to be used to synthesize these two compounds.
In the case of the para-substituted species (3p), halogen-metal
exchange was completed after stirring at -78 °C in tetrahy-
drofuran (THF) for 1 h and the product was acquired as
colorless crystals which were stable against air and moisture.
For the meta-isomer 3m, reaction at room temperature in
diethyl ether was required and the product was obtained as
a pale viscous oil after column chromatography.
Scheme 2. Synthetic Route for Compound 2p
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130, 12477.
(8) Pelter, A.; Smith, K.; Buss, D.; Jin, Z. Heteroat. Chem. 1992, 3,
275.
Org. Lett., Vol. 11, No. 16, 2009
3551

first step involved the synthesis of bromo-substituted amine
4.11 This was treated with tert-butyllithium (t-BuLi), followed
by addition of TripB(OMe)2 to furnish the target compound
2p.
Compounds 1-2 are yellow-greenish oils and well soluble
in common organic solvents. Figure 1 shows UV-vis
to the boron center. This was further confirmed by an
hypsochromic shift of 60 nm in the corresponding emission
maximum after binding to F-. The sensing behavior of the
meta-substituted compound 1m is almost the same (Sup-
porting Information) as described for 1p, which suggests that
the B-N containing compounds 1-2 can be used as fluoride
sensors.4b,12
Interestingly, the relative intensity of the absorption bands
around 295 and 400 nm changes dramatically upon changing
the position of the donor substituents on the phenylene linkers
at the boron moiety (Figure 1).
The much smaller intensity of the charge transfer band in
the case of meta-substituted compounds 1m and 2m as
compared to para-substituted compounds 1p and 2p can be
attributed to the weaker donor-acceptor interactions in the
ground states. This is a result of the position of substitution,
since m-phenylene positioning of N relative to B interrupts
the conjugation. Increasing the strength of the donor sub-
stituents in the meta-series, by introducing methoxy groups
on the amine moieties (compound 2m), resulted in only a
small bathochromic shift in the absorption spectra (from 398
to 409 nm).
Figure 1. UV-vis absorption (dash lines) and emission (solid lines)
spectra of compounds 1-2 in DCM.
Much lower fluorescence quantum yields (Table 1) were
observed for m-phenylene bridged compounds 1m and 2m
absorption and emission spectra of 1-2. The optical band
gap was calculated by using the onset of the absorption
spectra. All four compounds exhibit two absorption bands
around 295 and 400 nm, respectively. The former one can
be identified as a band originating from the aromatic
backbone, while the latter one is attributed to a charge
transfer transition from the amino group to the boron center.
To confirm this charge transfer interaction, titration experi-
ments with fluoride ions were investigated, as F- can convert
a boron center from an acceptor to a donor species.7 Upon
addition of tetrabutylammonium fluoride (TBAF) to the
dichloromethane (DCM) solution of compound 1p, the band
at 397 nm disappeared and the intensity of the band at 295
nm significantly increased (Figure 2). This result indicates
Table 1. Photophysical Data for Compounds 1-2
abs λmax
(nm) (log ε)
optical band
gap (eV)
emission
λmax(nm)
compd
QYb
0.40
1p
295 (4.46)
397 (4.85)
292 (-)a
2.86
2.84
2.76
2.67
492
542
537
568
2p
376 (-)a
1m
2m
292 (4.88)
398 (3.51)
295 (4.75)
409 (3.45)
0.09
0.06
a Pure sample obtained by analytical HPLC. b In cyclohexane.
compared to para-linked compound 1p. This also can be
attributed to the less efficient conjugation pathway.6 Time-
resolved fluorescence measurements revealed that compound
1p has a lifetime of approximately 2 ns in hexane and a
considerably longer lifetime in DCM due to the charge
transfer character of the excited state. Compound 2m in both
hexane and DCM has a fluorescence lifetime that is on the
order of tenths of nanoseconds indicating the much slower
relaxation of the excited state due to the meta substitution.
In contrast to the very weak solvatochromism in the
absorption spectra, the difference in emission spectra is
Figure 2. UV-vis absorption (dash lines) and emission (solid lines)
spectra of compound 1p (red) upon treatment with TBAF (blue) in
DCM (1.3 × 10-5 mol/L). Inset photograph: Change of color of
the DCM solution of 1p upon treatment with TBAF. The flasks
were illuminated with a hand-held UV lamp.
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123, 11372.
occupation of the empty orbital of the boron atom by F-
and inhibition of the charge transfer from the nitrogen center
3552
Org. Lett., Vol. 11, No. 16, 2009

significant (Figures S6-S9, Supporting Information).13 Ba-
thochromic shifts of 97, 86, 99, and 100 nm were observed
for compounds 1p, 2p, 1m, and 2m, respectively, when the
polarity of the solvent changed from hexane to acetonitrile.
Such a remarkable bathochromic shift is characteristic for
an efficient charge transfer from the nitrogen donor to the
boron acceptor.14
One can notice a larger effect of solvatochromism for
methoxy series (compounds 2p and 2m) (14 nm) than for
the methyl one (1p and 1m) (2 nm) probobly due to the
strong donating character of methoxy groups. Comparison
of the emission maxima of compound 1p (492 nm) and
compound 1m (537 nm) in DCM (Figure 1) indicated a
stronger charge transfer in the excited state in the case of
meta substitution. This can be explained by the more
electron-deficient character of the meta-substituted boron as
compared to the para-substituted example. When a more
electron-rich amine is introduced as in compound 2m, a
further bathochromic shift of 40 nm is observed compared
with compound 1m.
For a better understanding of the optical properties
displayed by 1-2, we have investigated the geometry and
electronic structure of these molecules using density func-
tional theory calculation (B3LYP/6-31G*).15 Calculated
HOMO and LUMO orbitals of compounds 1-2 are shown
in Figure S11 in the Supporting Information. These calcula-
tions reveal that in all compounds the LUMO orbitals are
localized on the boron subunit and the HOMO orbitals reside
on the amine moieties. In contrast, the HOMO orbitals in
compounds 1m and 2m are located on one and the HOMO-1
orbitals on the other donor subunit.16 In all compounds the
dihedral angles of the aryl rings attached to the boron are
about 23°, which is in agreement with the results of Marder
et al.17 According to our calculations shown in Figure 3,
energy gap, can be tuned simply by changing the position
of the donor substituent in the boron-nitrogen system.
Compound 2m has the smallest calculated gap, although
optical gaps are smaller than those calculated, in agreement
with the experimentally observed trend for compounds 1-2.
In the case of meta-substituted compounds 1m and 2m,
significant decreases of LUMO levels were observed com-
pared to the corresponding para-substituted compounds 1p
and 2p. This phenomenon is consistent with the stronger
acceptor properties of boron in 1m and 2m, due to the weak
electronic communication between the amine and boron
centers.
In conclusion, novel boron-containing donor-acceptor
compounds 1-2 have been synthesized and characterized.
UV-vis absorption and emission spectroscopic measure-
ments as well as performed calculations confirm that (i)
compounds 1p and 2p, with p-phenylene bridges, show much
stronger charge transfer abilities at the ground state, due to
a favorable position of substitution compared to the meta-
series 1m and 2m, and (ii) compounds 1m and 2m, with
m-phenylene bridges, possess stronger electron acceptor
abilities in the excited states compared to 1p and 2p because
the donor moiety cannot compensate for the electron-
accepting character of the boron center. Additionally, fluoride
sensing behavior of these compounds was proven by a strong
hypsochromic shift of the emission spectra and disappearance
of the charge transfer band in the absorption spectra upon
treatment with F-. Our studies offer an effective protocol to
understand and control the conjugation effect, and therefore
photophysical properties of different D-A systems.
Acknowledgment. This work was financially supported
by the German Science Foundation (SFB 625, Korean-
German graduate school IRTG) and the Marie Curie So-
larNType project (MRTN-CT-2006-035533). G.Z. gratefully
acknowledges the Alexander von Humboldt Stiftung for a
research fellowship. We thank Dr. F. Laquai and Mr. W.
Kamm (MPIP) for lifetime measurements and fruitful discus-
sion.
Supporting Information Available: Experimental pro-
cedures, characterization, and spectra. This material is
available free of charge via the Internet at http://pubs.acs.org.
Figure 3
1-2.
. Calculated (B3LYP/6-31G*) energy levels for compounds
OL9012487
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