Phosphaalkenes in π-conjugation with acetylenic arenes

Article abstract of DOI:10.1021/ol902688n

(Chemical Equation Presented) Phosphaalkene Inclusion at the periphery of acetylenlc arenes results In decreased band gaps of the title compounds as verified by spectroscopic and electrochemical techniques. The electronic coupling between two 1-phosphahex

Full text of DOI:10.1021/ol902688n

ORGANIC
LETTERS
2010
Vol. 12, No. 4
692-695
Phosphaalkenes in π-Conjugation with
Acetylenic Arenes
Xue-Li Geng, Qi Hu, Bernhard Scha¨fer, and Sascha Ott*
Department of Photochemistry and Molecular Science, Ångstro¨m Laboratories,
Uppsala UniVersity, Box 523, 75120 Uppsala, Sweden
sascha.ott@fotomol.uu.se
Received November 21, 2009
ABSTRACT
Phosphaalkene inclusion at the periphery of acetylenic arenes results in decreased band gaps of the title compounds as verified by spectroscopic
and electrochemical techniques. The electronic coupling between two 1-phosphahex-1-ene-3,5-diyne units is mediated by all para-substituted
arenes and increases from 4b to 4d.
Polyarylvinylenes and -acetylenes are organic semiconduc-
tors and as such interesting materials for a broad variety of
applications in electronic and photonic devices.1-4 While
the inclusion of heteroatoms in the form of heteroaromatics
is a common way to alter electronic properties of the
structures, much less research has been conducted on
replacing sp and sp2 hybridized carbon centers of the
acetylene and vinylene portion by other main group elements.
This is remarkable since such an arrangement would promote
the mixing of heteroatom-based orbitals with those of the
conjugated π-system, giving rise to new materials with
potentially interesting (opto)electronic properties. Only over
the last five years, the first polymers that resemble polyvi-
nylphenylenes but contain low-valent phosphorus centers in
the form of phosphaalkenes5 or diphosphenes6 instead of
vinylenes have emerged in the literature. Structurally defined
monodisperse segments of these intriguing polymers and
detailed studies of the interaction between the phosphorus-
containing parts of the molecules are however still scarce.
Bis(diphosphenes) that are separated by ferrocene7 or phe-
nylene8 linkers have been found to show substantial elec-
tronic coupling with a separation of the electrochemically
determined reduction potentials of the radical anion and the
dianion by 350 and 340 mV, respectively. We have recently
determined a coupling of similar magnitude between two
phosphaalkenes, mediated by a linear octatetrayne.9,10
A
shorter butadiyne linker in a cross-conjugated, expanded
dendralene segment in which the exotopic methylene groups
were exchanged by λ3-σ2 phosphorus leads to an increased
coupling of 530 mV.11
Herein, we present a series of PdC terminated acetylenic
arenes 4a-d that were prepared for two purposes: first, to
study the extent to which the different bridges would mediate
π-conjugation between the PdC units and second, to
investigate the effect that π-conjugated phosphorus hetero-
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(5) Wright, V. A.; Patrick, B. O.; Schneider, C.; Gates, D. P. J. Am.
Chem. Soc. 2006, 128, 8836–8844.
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M.; Rasmussen, T.; Ott, S. J. Org. Chem. 2009, 74, 9265–9273
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10.1021/ol902688n  2010 American Chemical Society
Published on Web 01/14/2010

atoms have on the electronic properties of the acetylenic
arenes. Considering that the frontier molecular orbitals of
phosphaalkenes are closer in energy than those of
ethenes,12,13 we were interested to see whether phospha-
alkene incorporation could be used to decrease the HOMO-
LUMO gap of the arenes.
exception of a phenyl group at the butadiyne terminus (δ )
311 ppm). It thus seems that the P-centers are largely
unaffected by the nature of the arene linker. This behavior
is in contrast to previous findings that phenyl groups with
electron-withdrawing nitro (δ ) 319 ppm) and electron-
donating amino substituents (δ ) 304 ppm) at the butadiyne
terminus have a noticeable effect.
Single crystals of 4d were grown from a mixture of
dichloromethane and methanol, and the structure of 4d was
determined by X-ray diffraction analysis (Figure 1). Reflect-
TMS-protected, butadiyne-substituted phosphaalkene 3 can
be prepared from the diacetylenic chloride 2, which in turn
is accessible from alcohol 1 (Scheme 1).10 Because of the
Scheme 1
.
Synthesis of Bis(1-phosphahex-1-ene-3,5-diyn-6-yl)
Arenes 4a-d (Mes* ) 2,4,6-tBu3Ph)
Figure 1. Molecular structure of 4d (35% probability ellipsoids).
All hydrogen atoms are omitted for clarity. Selected bond lengths
[Å] and angles [deg]: P1-C6, 1.697; P1-C13, 1.842; C1-C2,
1.427; C2-C3, 1.207; C3-C4, 1.366; C4-C5, 1.206; C5-C6,
1.419; C13-P1-C6, 101.4; P1-C6-C5, 122.0; P1-C6-C7,
121.5; C5-C6-C7, 116.5. Dihedral angle between phenyl and the
plane of P1-C6-C5 [deg]: 18.1. Dihedral angle between an-
thracene and the plane of P1-C6-C5 [deg]: 52.9. Dihedral angle
between Mes* and the plane of P1-C6-C5 [deg]: 85.9.
limited stability of terminally unsubstituted 1-phosphahex-
1-ene-3,5-diyne, we employed a one-pot deprotection-
Sonogashira coupling protocol14 to prepare 4a-d. It is vital
for the success of the reaction to use diiodoarenes as coupling
partners as the lower reactivity of the corresponding bromides
results in decomposition of deprotected 3. Commercially
available 1,4-dibromonaphthalene and 9,10-dibromoan-
thracene were thus converted to the diiodo derivatives prior
to the coupling experiments.15
ing the dimeric character of 4d, its solid state structure
exhibits a crystallographically imposed inversion center of
symmetry with a P···P distance of 18.0 Å. The butadiyne is cis
to the Mes* group and features bond distances in the typical
range for CtC triple and Csp-Csp single bonds. The crystal
structure of 4d gives valuable insights into the degree of
communication between the phosphaalkenes and the remain-
ing π-conjugated units of the molecule. As expected, the
Mes* group is nearly orthogonal to the plane defined by
1-phosphahex-1-ene-3,5-diyne (PC5) with an angle of 85.9°.
The phenyl group at the PdC carbon is twisted out of the
PC5 plane by only 18.1°, allowing sizable conjugation
between the two units. In contrast, the interplanar angle
between PC5 and the anthracene unit is much greater at 52.9°,
giving a nonoptimal geometry for π-orbital overlap. It
appears that this unfavorable orientation is enforced by steric
constraints that would emerge between the anthracene core
and the p-tert-butyl substituent of the Mes* group if the
anthracene were more coplanar with PC5. In other words,
since the Mes* group resides under/above the anthracene,
π-conjugation between the latter and PC5 is impeded.
The different bridging units in 4a-d have a marked effect
on the color of the compounds, which progressively changes
from light yellow to yellow and red (Scheme 1). Electronic
In a typical experiment, compound 3 was treated with the
diiodoarene under customary Sonogashira reaction condition
(CuI and PdCl2(PPh3)2) in the presence of excess K2CO3.
Whereas 4b and 4d could be purified by column chroma-
tography on silica, 4a and 4c were isolated by preparative
HPLC on a C18 chromatographic column using a mixture
of THF and methanol as eluent, and 4a-d were afforded in
acceptable isolated yields. The 31P NMR spectra of 4a-d
show almost identical chemical shifts of δ ) 313.7, 313.8,
314.1, and 314.2 ppm, respectively, which are also very
similar to that of a monomeric reference compound 5
(Mes*PdC(Ph)-C4-Ph)9 that is identical to 3 with the
(12) Rozhenko, A. B.; Schoeller, W. W.; Povolotskii, M. I. Magn. Reson.
Chem. 1999, 37, 551–563
(13) Wannere, C. S.; Bansal, R. K.; Schleyer, P. v. R. J. Org. Chem.
2002, 67, 9162–9174
.
.
(14) Pak, J. J.; Weakley, T. J. R.; Haley, M. M. J. Am. Chem. Soc.
1999, 121, 8182–8192.
(15) Khan, M. S.; Al-Mandhary, M. R. A.; Al-Suti, M. K.; Al-Battashi,
F. R.; Al-Saadi, S.; Ahrens, B.; Bjernemose, J. K.; Mahon, M. F.; Raithby,
P. R.; Younus, M.; Chawdhury, N.; Ko¨hler, A.; Marseglia, E. A.; Tedesco,
E.; Feeder, N.; Teat, S. J. Dalton Trans. 2004, 2377–2385.
Org. Lett., Vol. 12, No. 4, 2010
693

absorption spectra of 4a-d in CH2Cl2 are shown in Figure
2. Whereas the m-phenyl-bridged 4a exhibits a longest
compounds feature an irreversible oxidation at Ep,a )
1.03-1.08 V, which is thus largely invariant to the nature
of the bridging unit and similar to that of monomeric 5 (Table
1). The presence of only one oxidation wave shows that these
Table 1. Electrochemical Data for 1 mM Solutions in CH2Cl2
(0.1 M NBu4PF6), Glassy C-Electrode, ν ) 100 mV/sa
compound
reduction E1/2 [V]
oxidation Ep,a [V]
5b
-1.98
-2.03
-1.87, -1.97
-1.80, -1.92
-1.58, -1.77
1.05
1.08
1.03
1.06
4a
4b
4c
4d
0.92c,d
1.04d
a All potentials are given versus Fc+/0. E1/2 ) (Epa + Epc)/2. b 5 ) Mes*
P ) C(Ph)-C4-Ph.9 c Anthracene-based oxidation. d Resolved by differential
pulse voltammetry.
Figure 2. UV-vis Absorption spectra of compounds 4a (- · · -),
4b (· · ·), 4c (- - -), and 4d (s) in CH2Cl2 at 25 °C.
processes are isolated to each monomeric subunit, which is
in accordance with a recent study of related PC5-based
compounds.9 A completely different picture emerges from
the cathodic scans (Figure 3). Consistent with the optical
wavelength absorption maximum almost identical to that of
monomeric 5 (λ ) 379 nm, ε ) 17000 M-1cm-1),10 the
lowest energy transitions of 4b-d with the para-substituted
bridges are bathochromically shifted, pointing toward a
sizable electronic coupling between the two PC5 portions.
Owing to the most quinoid character of the anthracene bridge,
the lowest energy transition of 4d is red-shifted by 67 and
89 nm compared to that of 4c and 4b, respectively.
The influence of the phosphorus centers in 4a-d on the
electronic properties of the appended π-systems was evalu-
ated in comparison with structurally related reference
compounds that are exclusively based on carbon. The
literature compounds that match 4a-d most closely are based
on trans-1,2-diethynylethenes (DEE), two of which have
been interconnected by the same bridging aromatic units as
in 4b,d.16 p-Phenyl-bridged bis-DEE has its lowest energy
absorption at 374 nm and thus at higher energy by almost
50 nm compared to 4b (λ ) 423 nm, ε ) 36500 M-1 cm-1),
further supporting the notion that the inclusion of phosphorus
heteroatoms leads to decreased HOMO-LUMO gaps. A less
dramatic effect is observed for 4d where the phosphorus
centers only lead to a shift of the longest wavelength
absorption maximum by 17 nm compared to that of the
anthracene-bridged bis-DEE. The partial failure of the
P-centers in 4d to impose a larger impact is presumably due
to the poor π-conjugation between PdC and anthracene that
is caused by steric repulsions between the latter and the Mes*
group (see above). As a further consequence, the lowest
energy absorptions of 4b and 4d differ by only 89 nm,
compared to 121 nm in the DEE series. Compounds 4a-d
are not emissive at ambient temperature presumably because
of rapid quenching processes that involve the phosphorus
centers.6,17-19
Figure 3. Cyclic voltammograms of 4a (- · · -), 4b (· · ·), 4c
(- - -), and 4d (s) (1 mM solutions of compounds in CH2Cl2, 0.1
M NBu4PF6, ν ) 100 mV/s).
absorption data, anthracene containing 4d features the lowest
HOMO-LUMO gap and is reversibly reduced at mildest
potential of E1/2 ) -1.58 V. This first electron uptake seems
to involve the entire π-conjugated system and renders the
second reduction less facile by 190 mV. The extent of
electronic coupling is often described by the comproportion-
ation constant Kc for the radical anion that can be calculated
20
2-/-
from Kc ) exp(F∆E1/2/RT) with ∆E1/2 ) E1/2-/0 - E1/2
.
In case of 4d, Kc for the radical anion is 1630. Less
(17) Partyka, D. V.; Washington, M. P.; Gray, T. G.; Updegraff, J. B.;
Turner, J. F.; Protasiewicz, J. D. J. Am. Chem. Soc. 2009, 131, 10041–
10048.
Further insights into the communication between the two
PC5 units in 4a-d was sought from cyclic voltammetry. All
(18) Peng, H.-L.; Payton, J. L.; Protasiewicz, J. D.; Simpson, M. C. J.
Phys. Chem. A 2009, 113, 7054–7063.
(16) Martin, R. E.; Wytko, J. A.; Diederich, F.; Boudon, C.; Gisselbrecht,
J.-P.; Gross, M. HelV. Chim. Acta 1999, 82, 1470–1485.
(19) Amatatsu, Y. J. Phys. Chem. A 2008, 112, 8824–8828.
694
Org. Lett., Vol. 12, No. 4, 2010

delocalization of the radical anion in 4c is expressed by a
smaller difference between the two reduction waves of only
120 mV (Kc ≈ 100). This trend continues via the p-phenyl-
linked system (100 mV, Kc ≈ 60) to the m-phenyl-linked
4a for which only one reduction can be resolved by cyclic
voltammetry. Another effect of the decreased coupling
between the monomeric units when going from 4d to 4a
is that the potential required for the first reduction becomes
increasingly negative until it is similar to that of mono-
meric 5.
entire π-conjugates in comparison with all-carbon-based
reference compounds. Considering the importance of higher
acenes with acetylene substituents in organic electronics,21
additional inclusion of phosphaalkene groups as in 4a-d
may become a new strategy for further band gap reduction.
Some of this capacity is however lost when steric constraints
force the PdC unit out of conjugation with the arene. These
factors need to be considered when designing future low band
gap oligomers and polymers that contain low valent λ3-σ2
phosphorus.
Comparing the electrochemical data for 4a-d with those
of the DEE analogues,16 some striking differences become
obvious. The latter are generally more difficult to reduce and
to oxidize than the former. For example in the phenyl-bridged
bis-DEE, the two processes are separated (Epa - Epc) by more
than 3.5 V (Epc ) -2.44, Epa ) 1.22 V), which is
considerably more than the difference found in 4b (∼ 2.9
V). A similar effect can be observed when comparing the
data of 4d with those of the anthracene-bridged bis-DEE
(E1/2,red ) -1.72 V, Ep,a anthracene ) 1.22 V).
Acknowledgment. This work was supported by the
Swedish Research Council, the Go¨ran Gustafsson Founda-
tion, the Carl Trygger Foundation and Uppsala University
through the U3MEC molecular electronics priority initiative.
We thank Jia Wang and Prof. Jin Qu (Nankai University,
China) for the X-ray structure determination of 4d.
Supporting Information Available: Detailed experimen-
tal procedures and characterization of all new compounds;
cif file for 4d. This material is available free of charge via
the Internet at http://pubs.acs.org.
In summary, it could be shown that mixing of phospha-
alkene-based fragment orbitals with those of acetylenic arene
frameworks leads to decreased HOMO-LUMO gaps of the
OL902688N
(20) Richardson, D. E.; Taube, H. Inorg. Chem. 1981, 20, 1278–1285.
(21) Anthony, J. E. Angew. Chem., Int. Ed. 2008, 47, 452–483.
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