A biphenyl containing two electron-donating and two electron-accepting moieties: A rigid and small donor-acceptor-donor ladder system

Article abstract of DOI:10.1039/c5cc03063c

Ladder π-conjugated materials and also push-pull systems belong to important classes of compounds for the development of organic electronic devices. In this communication, a novel π-conjugated material that unifies the properties of both of these classes is presented. The material comprises a rigid biphenyl framework, which bears two bridging electron-accepting phosphine oxide moieties as well as two electron-donating amino groups. The structure and photophysical properties of this compound are discussed and compared with those of a related system lacking the second P-moiety.

Full text of DOI:10.1039/c5cc03063c

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DOI: 10.1039/C5CC03063C
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A biphenyl containing two electron-donating
and two accepting moieties: a rigid and small
donor-acceptor-donor ladder system
Cite this: DOI: 10.1039/x0xx00000x
Tobias W. Greulich,a Naoya Suzuki,b Constantin G. Daniliuc,a Aiko Fukazawa,b
Eriko Yamaguchi,b Armido Studer*a and Shigehiro Yamaguchi*b
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
smallest known ladder-type donor-acceptor-donor π-systems (Figure
1c).
Radical phosphanylation has recently been shown to be a highly
useful and practical method for the synthesis of arylphosphanes.8 For
example, we successfully applied radical phosphanylation to the
Ladder π-conjugated materials and also push-pull systems
belong to important classes of compounds for the
development of organic electronic devices. In this
communication, a novel π-conjugated material that unifies
properties of both of these classes is presented. The material
comprises a rigid biphenyl framework which bears two
bridging electron-accepting phosphine oxide moieties as well
as two electron-donating amino groups. Structure and
photophysical properties of this compound are discussed and
compared with those of a related system lacking the second P-
moiety.
a) classic ladder π-conjugated materials
E
E
π
π
π
π
E
E
E
E = B, Si, N, P etc.
b) donor-acceptor chromophore
The development of functional materials has received great
attention in the past decades. The constantly growing requirements
of electronic devices such as organic light-emitting diodes
(OLEDs),1 field-effect transistors (FETs)2 and photovoltaic cells3
concerning performance, size and weight ask for novel types of
organic π-systems. Along these lines, various classes of compounds
have been developed, for example π-conjugated oligomers or
polymers,4 polycyclic aromatic hydrocarbons5 or ladder-type π-
electron systems.6 Incorporation of main group elements into the π-
skeleton of these materials as a bridging unit has been shown to be a
valuable strategy for the fine-tuning of functions and properties
(Figure 1a).6b A key aspect is that effective π-conjugation comes
along with the rigidity and flatness of the given structure. In addition,
a “dopant” element, in particular phosphorus, allows for variation of
the physical properties by simple chemical modification, such as
complexation with Lewis acids or metals, and alkylation towards
phosphonium salts.7 Oxidation to phosphine oxides or sulfides also
offers ready access to electron-accepting π-conjugated materials.
Beyond incorporating main group elements into an organic π-
conjugated framework, the combination of strong electron-donating
groups with highly electron-accepting moieties thereby forming so
called push-pull chromophores leads to promising π-materials
(Figure 1b).3c Herein, we present the synthesis and photophysical
properties of a biphenyl system featuring electron-accepting bridging
units in a ladder-type system that further contains electron-donating
substituents. This compound can be considered as one of the
D
A
π-Spacer
D = electron-donating group
A = electron-accepting group
c) ladder-type push-pull system
A
D
π
π
D
A
Fig 1 a) ladder-type organic π-conjugated materials with main group element
bridges and b) a ladder-type donor-acceptor-donor system studied in this work.
preparation of bis(phosphoryl)-bridged biphenyls.9 Similarly, Liu
and coworkers used this method to access bisphosphole-bridged
ladder oligophenylenes.10,11 Encouraged by these results, we decided
to use this reaction as a key step for the synthesis of conjugated
polycyclic π-systems bearing phospholes as rigidifying electron-
accepting entities along with nitrogen substituents as electron-
donating moieties,12 representing small and rigid ladder-type donor-
acceptor-donor biphenyls.
Readily accessible N,N-dibenzyl-3,5-dibromoaniline (1) was first
homocoupled with FeCl3 to benzidine derivative 2 (Scheme 1).13
Biphenyl 2 was characterized by X-ray analysis revealing, as
expected, that the phenyl planes of the biphenyl core show a torsion
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angle of around 90° (see Supporting Information). Radical
phosphanylation of with PhP(SnMe3)2 initiated by 1,1´-
R
O
Br
2
P
i
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azobis(cyclohexanecarbonitrile) (V-40) and subsequent oxidation
with H2O2 afforded the desired trans-bis(phenylphosphoryl)-bridged
benzidine 3a as a single diastereoisomer. As this P-phenylated
derivative has only limited solubility to CH2Cl2, we also tried to
synthesize a P-alkylated derivative. Using n-OctylP(SnMe3)2 as a
new phosphorus source, we obtained an octyl-substituted derivative
3b, which has improved solubility to various solvents even including
toluene.
Bn2N
NBn2
Bn2N
NBn2
DOI: 10.1039/C5CC03063C
Br
5a (R = Ph) 34%
5b (R = n-C8H17) 41%
4
Scheme 2 Phosphorylation to benzidine 5; i: a) PhP(SnMe3)2 or n-OctylP(SnMe3)2,
V-40, PhCF3, 125 °C, 48 h; b) H2O2, CH2Cl2, rt, 3 h.
In order to elucidate the influence of the two phosphole moieties
on the structure and properties of 3, we also prepared the
corresponding mono(phosphoryl)-bridged benzidines 5a and 5b,
which represent less strained congeners of 3 (Scheme 2). These
compounds were obtained by using the same chemistry starting with
dibromide 4.14 The structure of 5a was confirmed by X-ray structural
analysis, which revealed that the endocyclic C–P bonds in 5a are, as
compared to those of benzidine 3a, much shorter (C2–P1 =
1.792(12) Å; C8–P1 = 1.803(11) Å, see Supporting Information).
Again the two N-lone pairs are in conjugation with the biphenyl π-
system. However, due to the low quality of the single crystals, the
exact bond lengths are afflicted with a marginal error in the case of
phosphoryl-bridged benzidine 5a (see the ESI).
Br Br
Br
i
Bn2N
NBn2
Bn2N
Br Br
Br
1
2 44%
R
O
R
P
P
ii
Bn2N
NBn2
O
Next, we studied the effect of the substitution pattern of the
biphenyl core on the molecular orbital energies and performed
calculations at the B3LYP/6-31G(d) level of theory.15 For better
comparison we also included compound 6 which was previously
prepared in our laboratories9 into these studies. It was known that the
introduction of electron-withdrawing phosphoryl groups
3a (R = Ph) 24%
3b (R = n-C8H17) 18%
Scheme 1 Synthesis of donor-acceptor-donor system 3; i: FeCl3·6 H2O, PhCH3,
85 °C, 24 h, ii: a) PhP(SnMe3)2 or n-OctylP(SnMe3)2, V-40, PhCF3, 125 °C, 72 h; b)
H2O2, CH2Cl2, rt, 3 h.
Successful preparation of 3a and 3b demonstrates the great
potential of this multiple phosphanylation process, which occurs via
sequential fourfold homolytic substitution at phosphorus by aryl
radicals. Notably, the produced four ring-fused P-heterocyclic
compounds build up high ring strain within the π-system, according
to the X-ray crystal structural analysis of 3a (Figure 2). A
characteristic feature of this remarkable class of triarylphosphine
oxides is the elongated endocyclic C–P bond (1.840(3)–1.841(3) Å)
LUMO+2 –0.14 eV
LUMO+1 –0.17 eV
LUMO+1 –0.50 eV
LUMO+1 –0.60 eV
LUMO+1 –1.02 eV
LUMO+1 –1.16 eV
LUMO+1 –1.48 eV
LUMO+1 –1.65 eV
which belongs to the longest among the reported examples.9
A
LUMO+1 –1.75 eV
further indication of a strained π-system is the length of the central
C–C bond (1.437(6) Å). Compared to precursor 2 (1.482(5) Å), this
C–C bond is significantly shortened in order to overcome the
induced ring strain that is caused by the two annulated phosphine
oxide moieties. The arene entities in the biphenyl core are coplanar
4.01 eV (309 nm)
f = 0.9363
3.35 eV (370 nm)
f = 0.1498
3.06 eV (405 nm)
f = 0.0024
(torsion angle (C1C4C4*C1*)
= 0°), suggesting effective π-
3.87 eV (320 nm)
HOMO –4.87 eV
conjugation. Importantly, both of the terminal amino groups adopt a
planar geometry, in which the lone pairs of the nitrogen atoms are in
perfect conjugation with the π-system. Similar characteristic features
are also observed for the crystal structure of the octyl-substituted 3b
(Figure S4 in the ESI).
HOMO –4.94 eV
f = 0.0000
Bn2N
NBn2
HOMO –5.17 eV
Ph
O
P
7
Bn2N
NBn2
Ph
O
P
P
5a
HOMO –6.41 eV
Bn2N
NBn2
Ph
O
P
O
Ph
3a
P
Ph
O
6
Fig 3 Relative molecular orbital energies of bisphosphole 6 and donor-acceptor-
donor systems 3 and 5 (B3LYP/6-31G(d) level) and the S0–S1 transition energies
estimated by the TD DFT calculations at the same level of theory. The results of
bis(dibenzylamino)biphenyl 7 fixed in a coplanar geometry are also shown for
comparison. The vertical solid and dotted arrows represent the major and minor
components for each transition.
Fig 2 X-ray crystal structure of 3a. Top view (left) and side view (right). Thermal
ellipsoids are shown with 30% (left) and 15% (right) probability.
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considerably stabilizes the energy of the LUMO of bisphosphole 6
To gain insights into the effect of the donor-acceptor-donor
compared to the parent biphenyl.9 On the other hand, two electron- skeleton in 3 on the properties, the solvent effects were investigated
View Article Online
DOI: 10.1039/C5CC03063C
donating amino groups increase the electron density of the π-system by employing more soluble octyl-substituted derivative 3b, which
and thus the orbital energies of bis(phosphoryl)-bridged benzidine 3a has slightly shorter absorption and emission maximum wavelengths
(Figure 3). The fact that the LUMO energy is less affected than the compared to 3a. Interestingly, unlike the ordinary donor-acceptor-
HOMO by these substituents results in a significantly narrower donor systems, 3b did not show any noticeable solvatochromism
HOMO–LUMO gap of ΔE = 3.69 eV for 3a as compared to 4.84 eV
for 6. The second phosphine oxide moiety is essential to gain the
narrow HOMO–LUMO gap, as one can see by comparison of the
calculated energies for the mono-phosphorylated congener 5a, which
shows a HOMO–LUMO gap of ΔE = 3.92 eV.
The photophysical properties of both mono- and bis-phospholes
3 and 5 were evaluated next and compared with the data reported for
6. The absorption as well as emission spectra are depicted in Figure
4 and the data are summarized in Table 1. The UV/Vis absorption
spectra of bis(phenylphosphoryl)-bridged benzidine 3a in CH2Cl2
both in the absorption and fluorescence spectra (λabs and λem: 446
and 556 nm in toluene; 453 and 572 nm in CHCl3; 452 and 569 nm
in CH2Cl2; 442 and 565 nm in acetone, respectively). This result
implies no significant intramolecular charge transfer (ICT) character
in the excited state for this compound. Time dependent (TD) DFT
calculation at the B3LYP/6-31G(d) level indeed showed that the S0–
S1 transition in 3 is attributable to not the ICT transition, but the π–
π* transition from HOMO to LUMO, both of which delocalize over
the biphenyl skeleton. This transition is, however, essentially
symmetrically forbidden, in sharp contrast to the symmetrically
allowed transition for the parent benzidine derivative 7 without the
phosphoryl bridges (Figure 3). This difference results from the
switching of the orbital symmetry of the LUMO and LUMO+1
between 7 and the phosphoryl bridged derivatives 3 and 5. This is
one of notable impacts of the incorporation of the electron-
withdrawing phosphoryl groups. It is also noted that, despite the no
ICT character in the excited state, the bisphospholes 3 have
significantly large Stokes shift around 4600 cm–1, which are much
larger than those of 5 (ca. 3600 cm–1). This difference is responsible
for the significantly red-shifted fluorescence for 3 and represents
unique character of the present small donor-acceptor-donor system.
shows an absorption band with the maximum wavelength of λabs
=
464 nm with rather small absorption coefficient of ε = 870. Notably,
compound 3a exhibits an orange emission with the maximum
wavelength of λem = 594 nm in the fluorescence spectrum in CH2Cl2.
Compared to the parent bis(phosphoryl)-bridged biphenyl 6 (λabs
=
347 nm, λem = 387 nm) or mono(phosphoryl)-bridged benzidine 5a
(λabs = 431 nm and λem = 505 nm),9 both the absorption and emission
wavelengths of 3a are significantly red-shifted. These differences
demonstrate the significant impacts of the combination of two
electron-donating amino groups and two electron-accepting
phosphoryl moieties on the photophysical properties.
Whereas the monophosphole derivative 5a have
a high
fluorescent quantum yield of ΦF = 0.67 in CH2Cl2, that of 3a is low
of ΦF = 0.03. This difference is largely due to the longer-wavelength
emission as well as the forbidden character of the S0–S1 transition
for 3, and thereby suppressed radiative decay process from the
excited state.
Table 1: Photophysical data for the phospholes.a
UV/Vis Absorption
Emission
Compound
λabs [nm]b
ε [×103]
λem [nm]c
In summary, we have presented the synthesis of phosphoryl-
bridged benzidines by using radical phosphanylation and also
evaluated their photophysical properties. Calculation of the
molecular orbital energies showed narrow HOMO–LUMO gaps of
3.69 eV and 3.92 eV for benzidines 3 and 5, respectively. Notably,
these π-systems can be formally regarded as one of the smallest
donor-acceptor-donor systems bearing two electron-donating and
two electron-withdrawing moieties within a biphenyl core. Unlike
the ordinary donor-acceptor-donor systems, however, these π-
systems have no significant intramolecular charge transfer character
in the excited state and thereby do not show any noticeable
fluorescent solvatochromism. Nevertheless, these compounds still
exhibited a large bathochromic shift of the emission wavelength and
showed orange fluorescence. This work highlights the value of
organic π-materials possessing a phosphoryl-bridged arene backbone
conjoined with electron-donating substituents and their great
potential to be applied as building blocks in functional materials.
3a
3b
5a
5b
6d
464
452
431
415
347
0.87
1.03
3.08
3.47
0.37
594
569
505
488
387
a In CH2Cl2. b Only the absorption maxima at the longest wavelength are
shown. c Excited at the absorption maximum wavelength. d Reference 9.
5.0
Absorption
Emission
3a
3b
5a
5b
4.0
3.0
This work was supported by the Deutsche Forschungsgemeinschaft
(DFG) within the framework of the IRTG Münster/Nagoya and JSPS
Core-to-Core Program, A. Advanced Research Networks, as well as
JST, CREST (S.Y.).
2.0
1.0
ε
Notes and references
a Westfälische Wilhelms-Universität Münster, Corrensstraße 40, 48149
Münster, Germany.
b Department of Chemistry, Graduate School of Science and Institute of
Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Furo,
Chikusa, Nagoya 464-8602, Japan.
300
400
500
600
700
Wavelength / nm
Email: yamaguchi@chem.nagoya-u.ac.jp; studer@uni-muenster.de
Fig 4 Absorption (dotted line) and emission (solid line) spectra of 3 and 5.
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Electronic Supplementary Information (ESI) available: Experimental
procedures, spectroscopic and crystallographic data as well as
computational details are included in the SI. See DOI: 10.1039/c000000x/
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DOI: 10.1039/C5CC03063C
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15 For detailed information see Supporting Information.
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