Phenylene bridged boron-nitrogen containing dendrimers

Article abstract of DOI:10.1021/ol101327h

Figure Presented. The synthesis and characterization of novel phenylene bridged boron-nitrogen containing φ-conjugated dendrimers N3B6 and N3B3, with peripheral boron atoms and 1,3,5-triaminobenzene moiety as a core, are presented. UV-vis absorption and e

Full text of DOI:10.1021/ol101327h

ORGANIC
LETTERS
2010
Vol. 12, No. 19
4236-4239
Phenylene Bridged Boron-Nitrogen
Containing Dendrimers
Agnieszka Pron´, 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 9, 2010
ABSTRACT
The synthesis and characterization of novel phenylene bridged boron-nitrogen containing π-conjugated dendrimers N3B6 and N3B3, with
peripheral boron atoms and 1,3,5-triaminobenzene moiety as a core, are presented. UV-vis absorption and emission measurements reveal
that the optical properties of the resulting compounds can be controlled by changing the donor/acceptor ratio: a 1:1 ratio results in a more
efficient charge transfer than the 1:2 ratio. This was proven by the red shift of the emission maxima and the stronger solvatochromic effect
in N3B3 compared to N3B6.
The synthesis of π-conjugated molecules with well-organized
chromophores1 has received attention due to their applica-
tions in organic electronics such as photovoltaics or organic
light-emitting diodes (OLED). Among them, π-conjugated
dendrimers2 are of interest in view of (i) the extension of
the conjugation into more than one dimension, (ii) the rigid
structure, (iii) the high chromophore to surface ratio, as well
as (iv) the monodispersity and purity. π-Conjugated den-
drimers containing electron-donating groups, for example,
amine3 or thiophene4 units, are widely studied in electronic
devices. In contrast, dendritic structures with electron-
acceptor units5 or both donor and acceptor moieties6 are
uncommon. Incorporation of boron moieties into conjugated
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10.1021/ol101327h  2010 American Chemical Society
Published on Web 09/07/2010

Scheme 1. Synthetic Pathway for Compounds N3B6 and N3B3
systems7 is one approach toward improving and tuning
as donors and boryl groups as acceptors, bridged by
phenylenes. This allowed us to study the effect of the donor/
acceptor (D-A) ratio on the optical properties of the resulting
compounds.
properties for use in organic electronics through lowering
the LUMO levels. A boron center possesses a vacant p-orbital
and can thus serve as an acceptor unit. Efficient overlap
between its empty orbital and the π-conjugated framework
then gives rise to a bathochromic shift of the UV-vis
absorption and emission signals. Boron acceptor units in
conjugation with appropriate electron donors show intense
intramolecular charge transfer.8
Although there are many papers on donor-acceptor
systems with boron units,9,10 these are mostly restricted to
systems with just one or two boron centers. Examples of
dendritic structures bearing boron moieties are very rare.5b,11
Kawashima et al.11 recently reported dendrimers based on
dibenzoazaborines in the periphery and a benzothiadiazole
(BTZ) unit as the core. Optical investigation of these
dendrimers indicated intramolecular charge transfer in the
excited state from the azaborine units to the BTZ unit. To
the best of our knowledge, there is no report concerning
π-conjugated dendrimers with boron atoms in the periphery
and nitrogen atoms as a core (and Vice Versa). Therefore,
we became interested in dendrimers bearing amine moieties
1,3,5-Tris(di(4-dimesitylboryl-phenyl)amino)benzene (N3B6)
and 1,3,5-tris((4-dimesitylboryl-phenyl)phenylamino)benzene
(N3B3) were synthesized in three steps as shown in Scheme 1.
The first step involved acid-catalyzed condensation of com-
mercially available phloroglucinol with 4-bromoaniline or
aniline to achieve 1,3,5-tris(4-bromophenylamino)benzene (1)
and 1,3,5-tris(phenylamino)benzene (2),12 respectively, in
75-80% yield. Compounds 1 and 2 were subjected to
Buchwald-Hartwig amination13 with 1,4-dibromobenzene in
the presence of palladium(II) acetate and bis(diphenylphosphi-
no)ferrocene (dppf) as a catalytic system to furnish compounds
3 and 4 in 50% yield. The final products N3B6 and N3B3 were
obtained in about 30% yield by treatment of 3 and 4,
respectively, with t-BuLi followed by addition of dimesitylboron
fluoride. Model compounds N1B114 and N1B2 were synthe-
sized via halogen-metal exchange from commercially available
bromo- or dibromo-triphenylamine, respectively (Supporting
Information). Pure samples were obtained after purification
by size exclusion chromatography. The synthesized com-
pounds are yellowish solids soluble in common organic
solvents. The bulky mesityl groups stabilize them against
air and moisture.
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The optical properties of N3B6 and N3B3 were studied
by UV-vis absorption and emission spectroscopy (Figure
1 and Table 1). Both of them exhibit two absorption bands
in the range from 300 to 400 nm. The weaker one (∼310
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Org. Lett., Vol. 12, No. 19, 2010
4237

Information, bathochromic shifts of 27 and 61 nm for
N3B6 and N3B3, respectively, were observed when the
polarity of the solvent increased from hexane to acetone.
This can be attributed to the charge transfer transition from
the nitrogen donor to the boron acceptor.15 As proven by
solvent-dependent emission measurements, the strength
of these interactions is influenced by the D-A ratio. In
the case of N3B3, in which each amine moiety interacts
with only one boron center, more efficient charge transfer
occurs and the red shift of the emission band is stronger
than that of N3B6.
In general, three-coordinated organoboron compounds
can be used as fluoride sensors.5b,16 A fluoride anion
occupies the empty orbital of the boron atom and thus inhibits
the charge transfer from the nitrogen center. The character
of the boron center is thereby changed from an acceptor to
a donor. As shown in Figure 2, upon stepwise addition of
TBAF to the solution of N3B6 the emission band was red-
shifted17 by 100 nm. When more than 3 equiv of F- was
added, the band at 540 nm disappeared and a new band at
410 nm could be seen. This phenomenon can be explained
by switching the character of one of the boron atoms attached
to an amine moiety from an acceptor to a donor. This gives
rise to new D-D-A chromophore that shows stronger D-A
interactions in the excited state than the parent N3B6
molecule. Further addition of F- switches off all acceptor
centers, inhibiting any charge transfer and making the entire
molecule a donor with a new band at 410 nm. Treatment of
compound N3B3 (Figure 2) with TBAF results in a blue
shift of the emission spectra. This can be attributed to the
fact that addition of each eq of F- turns off D-A properties
of one B-N charge transfer moiety. In contrast to N3B6, no
new D-A interactions appear. A similar tendency was
observed for the model compounds N1B2 and N1B1 (Figure
S14 in Supporting Information). The charge transfer band
in the UV-vis absorption spectra gradually decreased upon
addition of F- anions (Figures S9-S12 in Supporting
Information). No stepwise changes were observed, and thus
it was impossible to determine binding constants for the
intermediates.
Figure 1. UV-vis absorption (solid lines) and emission spectra
(dashed lines) of N3B6, N3B3, N1B2, and N1B1 in DCM (1.3 ×
10-5 M). Excited at 375 nm.
nm) can be identified as originating from the aromatic
backbone and the stronger one (∼385 nm) as a charge
transfer transition from the amino group to the boron center.
This was further confirmed by fluoride titration experiments
as discussed below. The charge transfer band of N3B6
consists of a shoulder at 385 nm and a maximum at 395
nm, while N3B3 shows a band at 385 nm. As documented
in Figures S1 and S2 in Supporting Information, absorption
spectra do not change upon increasing the solvent polarity.
Interestingly, the difference in the emission spectra of
both compounds is significant. In the case of N3B6, with
a D-A ratio 1:2, blue emission at 445 nm is observed.
Changing the D-A ratio to 1:1 (N3B3) causes a batho-
chromic shift of 26 nm. Obviously, in the case of N3B3
every amine moiety interacts with just one boron atom,
whereas in N3B6 the donating strength of amine group is
“distributed” over two boron atoms thus weakening D-A
interactions. Comparing the emission maxima of den-
drimers with those of model compounds N1B1 and N1B2
(Figure 1), no red shift was observed, indicating lack of
conjugation between dendrons due to the dihedral angle
between these moieties and their 1,3,5-benzene core.
Because of the strong blue emission (quantum yield )
0.6) compound N3B6 is of potential interest as an emitter
in OLEDs.
The electrochemical properties of N3B6 and N3B3 were
studied by cyclic voltammetry (CV) as shown in Table 1,
which revealed similar values of HOMO and LUMO levels
for N3B6 and N3B3. The energy gaps of these compounds
were calculated to be 2.64 and 3.63, respectively, which is
in good agreement with the optical band gap.
In view of the lack of solvatochromism in the absorption
spectra, the solvent dependence in the emission spectra is
remarkable. As shown in Figures S5 and S6 in Supporting
Table 1. UV-vis Absorption, Emission, and CV Data of Compounds N3B6 and N3B3
a
a
λmax [nm],
Emax
optical band
gapc [eV]
HOMOd
[eV]
LUMOe
[eV]
∆E
[eV]
HOMOf
[eV]
LUMOf
[eV]
compound
log εa
[nm]
QYa,b
N3B6
N3B3
395, 5.00
385, 4.92
445
471
0.6
0.5
2.89
2.95
-5.51
-5.53
-2.87
-2.90
2.64
2.63
-5.19
-5.10
-1.82
-1.63
a Measured in DCM. b Calculated using DPA as standard. c Calculated using onset of the absorption spectra; HOMO and LUMO levels calculated from
cyclic voltametry measurements vs Ag/AgCl using ferrocene as standard. d Measured in DCM. e Measured in THF. f Calculated using B3LYP/6-31G*.
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Org. Lett., Vol. 12, No. 19, 2010

Figure 2
.
Changes in the emission spectra of compounds N3B6 (1.3 × 10-5 M) and N3B3 (1.3 × 10-5 M) in THF upon stepwise addition
of TBAF solution. Excited at 370 nm.
Theoretical calculations on compounds N3B6 and N3B3
using density functional theory (B3LYP/6-31G*)18 indicated
similar HOMO and LUMO orbital energies for both mol-
ecules and thus similar energy gaps. Although the calculated
LUMO levels (-1.82 and -1.63 eV, respectively) are higher
than the experimental values, there is good agreement
between calculated HOMO levels (-5.19 and -5.10 eV,
respectively) and experimental data (see Table 1).
In conclusion, we have synthesized and characterized the
first multichromophore π-conjugated dendrimers bearing
phenylene bridged boron and nitrogen moieties with different
D-A ratio. Optical investigations revealed that changing the
D-A ratio did not influence significantly the ground state
properties of the B-N containing dendrimers. In contrast,
the excited state characteristics, such as emission wavelength
can be tuned. In the case of a 1:1 ratio of D-A a more
efficient charge transfer occurred as compared to a 1:2 ratio.
As a result, a red shift of the emission bands and a stronger
solvent dependence were observed for compound N3B3.
Synthesized dendritic molecules will be further tested as
materials for blue OLED and NLO. Synthesis of higher
generations of dendritic structures is in progress.
Acknowledgment. This work was financially supported
by the Marie Curie SolarNType project (MRTN-CT-2006-
035533) and German Science Foundation (SFB 625, Korean-
German graduate school IRTG).
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Supporting Information Available: Experimental pro-
cedures, characterization, and spectra. This material is
available free of charge via the Internet at http://pubs.acs.org.
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(18) See Supporting Information for reference.
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