Phosphorus-based heteropentacenes: Efficiently tunable materials for organic n-type semiconductors

Article abstract of DOI:10.1002/chem.200801549

Benzo-condensed dithieno[3,2-b:2',3'-d]phospholes have been synthesized that allow convenient tuning of properties that are essential for application as semiconductor materials in organic field-effect transistor (OFET) devices. The versatile reactivity of the trivalent phosphorus atom in these heteropentacenes provides access to a series of materials that show different photophysical properties, significantly different organization in the solid state, and distinctly different electrochemical properties that can be achieved by simple chemical modifications. The materials show strong photoluminescence in solution and in the solid state that depends on the electronic nature of the phosphorus center. Electrochemical studies revealed that the phosphorus atom intrinsically furnishes materials with n-channel or ambipolar behavior, also depending on its electronic nature. The experimental data were verified by DFT quantum chemical calculations and suggest that the phosphorus-based heteropentacenes could be excellent candidates for n-channel OFET semiconductor materials.

Full text of DOI:10.1002/chem.200801549

DOI: 10.1002/chem.200801549
Phosphorus-Based Heteropentacenes: Efficiently Tunable Materials for
Organic n-Type Semiconductors
Yvonne Dienes,[a] Matthias Eggenstein,[a] Tamµs Kµrpµti,[b] Todd C. Sutherland,[c]
Lµszló Nyulµszi,*[b] and Thomas Baumgartner*[a, c]
9878
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 9878 – 9889

FULL PAPER
Abstract: Benzo-condensed dithie-
no[3,2-b:2’,3’-d]phospholes have been
synthesized that allow convenient
tuning of properties that are essential
for application as semiconductor mate-
rials in organicfield-effetc transistor
(OFET) devices. The versatile reactivi-
ty of the trivalent phosphorus atom in
these heteropentacenes provides access
to a series of materials that show differ-
ent photophysical properties, signifi-
cantly different organization in the
solid state, and distinctly different elec-
trochemical properties that can be ach-
ieved by simple chemical modifications.
The materials show strong photolumi-
nescence in solution and in the solid
state that depends on the electronic
nature of the phosphorus center. Elec-
trochemical studies revealed that the
phosphorus atom intrinsically furnishes
materials with n-channel or ambipolar
behavior, also depending on its elec-
tronicnature. The experimental data
were verified by DFT quantum chemi-
cal calculations and suggest that the
Keywords: density functional calcu-
lations · luminescence · phosphorus
heterocycles · semiconductors · sul-
fur heterocycles
phosphorus-based
heteropentacenes
could be excellent candidates for n-
channel OFET semiconductor materi-
als.
Introduction
state packing of the conjugated system. Greater intermolec-
ular overlap leads to increased bandwidth, which is directly
related to carrier mobility.[10] Pentacene was one of the first
organic materials successfully employed in OTFTs and
shows high field effect mobility and properties comparable
to those of classical transistors that rely on amorphous sili-
con as active material.[2] However, due the low solubility
and poor stability of pentacene towards oxidation, investiga-
tions focusing on improved materials that circumvent these
problems have been performed.[1,2] Heteropentacenes con-
taining annelated thiophene moieties such as pentathieno-
Organic p-conjugated oligomers and polymers exhibit semi-
conducting properties and hence are used increasingly as
active materials in organic field-effect/thin-film transistors
(OFET/OTFTs),[1,2]
(OLEDs),[2–4] photovoltaiceclls,
organiclight-emitting
diodes
[5]
and sensor materials.[6]
The organicnature of these materials, when suitably modi-
fied, allows for low-cost deposition processes such as print-
ing, stamping, spin coating, and evaporation, and expensive
[7]
lithographicor vacuum-deposition steps are not required.
Low-temperature solution processing, in particular, expands
the repertoire of tolerant substrates and allows fabrication
of flexible, lightweight, and inexpensive devices.[3,6a] A great
deal of attention has been focused on modification of the
electronic nature of the materials to obtain properties suita-
ble for the desired function (high charge-carrier mobility,
electroluminescence, etc.). The band gap of organic materi-
als and the degree of p-conjugation can generally be adjust-
ed by chemical transformations to tailor the optoelectronic
properties of these materials; these modifications can also
significantly influence their organization in the solid
state.[2,8] The solid-state morphology of conjugated materials,
in particular, plays an important role in the performance
characteristics of electronic devices.[2,6a,9] Both exciton mi-
gration and carrier mobility strongly depend on the solid-
U
G
hibit higher stability than the parent pentacene and also ex-
hibit good FET performance. However, most of these mate-
rials require tedious syntheses and purification processes.
[a] Dr. Y. Dienes, M. Eggenstein, Prof. Dr. T. Baumgartner
Institute of InorganicChemistry, RWTH-Aachen University
Landoltweg 1, 52074 Aachen (Germany)
Recently,
dibenzo[d,d’]thieno[3,2-b;4,5-b’]dithiophene
A
E-mail: thomas.baumgartner@ucalgary.ca
(DBTDT) was investigated as a new material for organic
electronics and showed not only promising semiconducting
properties, but also high thermal and photostability, as well
as easy accessibility.[14]
In the context of organic electronics, incorporation of
phosphorus centers into oligomeric or macromolecular ma-
terials has also recently drawn significant attention.[15]
Phosphole-containing materials are of particular interest. In
contrast to planar pyrroles, due to the pyramidal environ-
ment of the tricoordinate phosphorus center, efficient orbi-
tal interaction of the phosphorus lone pair with the conju-
[b] Dr. T. Kµrpµti, Prof. Dr. L. Nyulµszi
Department of Inorganicand Analytical Chemistry
Budapest University of Technology and Economics
1521 Budapest GellØrt tØr 4 (Hungary)
Fax : (+36)1463-3363
E-mail: nyulaszi@chem.bmu.hu
[c] Prof. Dr. T. C. Sutherland, Prof. Dr. T. Baumgartner
Department of Chemistry, University of Calgary
2500 University Drive NW, Calgary, ABT2N1N4 (Canada)
Fax : (+1)403-289-9488
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801549.
Chem. Eur. J. 2008, 14, 9878 – 9889
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9879

T. Baumgartner, L. Nyulµszi et al.
gated system is inhibited, and a low degree of lone-pair de-
localization results (Figure 1).[16] In addition, the phosphole
system exhibits a peculiar electronic structure resulting from
properties with respect to emission wavelengths, intensity,
and tunability. Parent dithienophospholes are very strong
blue-light emitters with photoluminescence efficiencies of
up to 90%. Notably, their photophysical properties can effi-
ciently be tuned by simple modifications at either the phos-
phorus center (E=lone pair) by oxidation (E=O, S), com-
plexation with main group centers (E=BH3, CH3+) or tran-
sition metals (E=metal), or by variation of the substitution
pattern (R’ on the thieno units or R on the central phospho-
rus atom).[19e,22,24]
Figure 1. Pyrrole versus phosphole versus thiophene.
We now report the synthesis and properties of heteropen-
tacenes based on the dithieno[3,2-b:2’,3’-d]phosphole system
with potential application in OFET/OTFT devices in mind.
Expanding the p-conjugated framework over five rings, as
opposed to the three rings of the parent dithienophosphole
system, is expected to lead to improved intermolecular
packing, better p overlap, and higher thermal stability, all of
which are of central importance for the target application.
Not only we will show that the central trivalent phosphorus
atom plays an important part in tuning the electronic prop-
erties, but it also has a central role in the solid-state organi-
zation of the materials. Furthermore, interaction of the low-
some interaction between the endocyclic p system of the bu-
À
tadiene moiety and the s* orbital of the exocyclic P C bond
(s–p hyperconjugation).[17] This interaction leads to a low-
lying LUMO energy level that is particularly attractive for
organic electronics with respect to n-channel (or n-type)
semiconducting behavior. Furthermore, trivalent phosphorus
species can react with oxidizing agents or Lewis acids, and
they can also coordinate to transition metals.[18] This offers a
variety of synthetically facile possibilities that can be used
to efficiently modify the electronic properties of the product
materials.[19] RØau and co-workers were among the first to
incorporate the phosphole moiety into extended p-conjugat-
ed materials (A);[15,20] their systematicstudies demonstrated
the advantageous electronic features of the incorporated
phosphorus centers in a series of phosphole-based OLED
devices.[21]
À
lying s* P C orbital with the low-lying unoccupied orbitals
of the extended p* system is expected to facilitate n-channel
conductivity in these compounds. The phosphorus center is
thus a tool for efficiently and effectively tuning a range of
materials properties, commonly not possible in this simplici-
ty with genuine organic semiconductors.
Results and Discussion
Synthesis and photophysical properties: In a modified pro-
cedure reported by Dahlmann and Neidlein,[25] oxidative
coupling of lithiated 2,3-dibromobenzo[b]thiophene (1) with
copper(II) chloride in diethyl ether at À788C gave 3,3’-di-
bromo-2,2’-di
A
(2).
Benzannelated
A few years ago, we introduced the dithieno[3,2-b:2’,3’-
d]phosphole system B into the field of organic electronics.[22]
This building block conjoins two thiophene subunits with a
central phosphole moiety by annelation and allows selective
tuning of the electronic properties of the materials by func-
tionalization of the phosphorus atom.[19e] Fused bithiophene
materials in general are promising candidates for application
in organic electronics, as they provide a high degree of p
conjugation due to their rigidified, planar structure, which
intrinsically affords smaller HOMO–LUMO gaps.[8,23] Our
extensive, systematicinvestigations to date have revealed
that this is also true of the dithienophosphole system B, and
phosphole 3 was obtained in good yield by our general
route to dithienophospholes[22,24] by using nBuLi in the pres-
ence of N,N,N,N-tetramethylethylenediamine to lithiate the
3,3’-positions, followed by treatment of the dilithio inter-
mediate with phenyldichlorophosphane in diethyl ether at
À788C (Scheme 1).
Compared to parent dithienophosphole B (R=Ph, R’=
H, E=lone pair),[22a] extension of the dithieno backbone
does not have a great impact on the 31P NMR shift of 3, and
a singlet at d=À25.1 ppm comparable to the signal for the
parent compound (d=À21.5 ppm) is observed. This penta-
cene-analogous, rigid annelated ring system has a high
we were able to confirm
a
doping function of the phospho-
rus center, rather than its being
an integral part of the p-conju-
gated system.[19e,22] The materi-
als obtained in our studies dis-
play highly advantageous, un-
precedented
photophysical Scheme 1. Synthesis of benzannelated phosphole 3.
9880
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 9878 – 9889

Phosphorus-Based Heteropentacenes
FULL PAPER
degree of p conjugation that is evident in bright blue fluo-
rescence emission at lem =440 nm in solution. Compared
with the parent dithienophosphole (lem =415 nm), the band
gap of 3 is narrower, as expected;[22] its quantum yield is
also reasonably high (63%). Remarkably, phosphole 3
shows extensive fluorescence, not only in solution but also
in the solid state (lex =450, lem =512 nm), for which a bright
green emission can be observed that suggests some intermo-
lecular interactions in the solid state (vide infra).
As mentioned in the introduction, the central phosphorus
atom provides a means to efficiently manipulate the materi-
als by simple chemical modifications that provide conven-
ient access to a family of materials with significantly altered
optoelectronic properties.[15,19e] Oxidation of the phosphorus
center in 3 with hydrogen peroxide and sulfur gives 4 and 5,
respectively, as air-stable solids in nearly quantitative yields
(Scheme 2). The phosphorus(V) 31P NMR resonances show
that the electronic nature of the phosphorus atom is very
similar to that of oxidized species 4 (d
(31P)=16.6 ppm). The
G
excitation and emission maxima (lex =407 nm, lem =461 nm)
lie between those observed for phosphole 3 and oxidized
species 4, in accordance with the observations made for the
parent dithienophospholes and their oxidized relatives (B:
R=Ph, R’=H, SiR3, E=O). The broad 11B NMR signal at
d=À39.3 ppm further indicates formation of 6, by analogy
with related boron dithienophosphole compounds (d
À40Æ1 ppm).[22,24e]
N
N
phosphole 3 in dichloromethane at room temperature gave
gold complex 7 in 56% yield. Again, the low-field shift of
the 31P NMR resonance is in accordance with increased elec-
tron-acceptor character (d=2.4 ppm) and is comparable to
the 31P NMR resonances of related dithienophosphole gold
complexes (d=1.9–6.5 ppm).[24b,e] The fluorescence spectrum
for complex 7 also shows red-
shifted wavelengths of excita-
tion and emission (lex =398 nm,
lem =471 nm). However, low-
energy transitions that indicate
[21]
aurophilicinteratcions
could
not be observed in the fluores-
cence spectra of this compound.
This observation is consistent
with other dithienophosphole
gold complexes reported by us
earlier.[24d,e]
Methylation of the phospho-
rus center with methyl triflate
generates air- and water-stable
cationic phospholium com-
pound 8. Reaction of 3 with
methyl iodide as methylating
agent was significantly slower.
The 31P NMR spectrum of 8
shows a singlet at d=13.2 ppm
that is significantly shifted from
that of the starting material 3
Scheme 2. Derivatization of benzannelated phosphole 3.
(d=À25.1 ppm) and is charac-
[24d]
the typical low-field shift (4: d=16.6 ppm; 5: d=23.1 ppm)
relative to the trivalent phosphole 3.[19e,22,24] The optoelec-
tronicproperties of 4 and 5 are very similar, but in the solid
state the relative intensity of 5 is significantly weaker, which
is consistent with other related thio derivatives, due to the
softness of sulfur (versus oxygen) quenching the luminescen-
ce.[22b] In solution, both compounds show excitation maxima
at lex =424 nm (4) and lex =435 nm (5) and emission at
lem =483 nm (4) and lem =473 nm (5) that are redshifted
compared to trivalent phosphole 3, as is commonly observed
for dithienophospholes.[19e,22,24] This redshift is caused by the
lowered LUMO level in the oxidized, pentavalent phospho-
rus form, as we showed in earlier studies on related dithie-
nophospholes.[22b,24e]
teristicfor dithienophospholium compounds.
Compound
8 exhibits yellow fluorescence in solution (lex =433, lem
=
520 nm) with the largest redshift of this family of materials,
due to the cationic nature of the phosphorus atom, which
makes it an exceptionally strong acceptor center. The higher
wavelength emission maximum of 8 with respect to dithie-
nophospholium
salt B (R=Ph, R’=H, E=Me+),[24d] and derivative A
(E=Me+) investigated by RØau et al.[20c] indicates further
lowering of the LUMO level in combination with an exten-
sively delocalized p system in 8.
All benzannelated compounds 3–8 show fluorescence in
the solid state. The common quenching effect of neighboring
molecules in the solid state is not observed.[26] This can be
attributed to the bulky structure around the tetrahedral
phosphorus center, which prevents extended p overlap of
Borane adduct 6 was formed by addition of BH3 to phos-
phole 3. The 31P NMR resonance at d=16.9 ppm indicates
Chem. Eur. J. 2008, 14, 9878 – 9889
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9881

T. Baumgartner, L. Nyulµszi et al.
neighboring molecules. The solid-state excitation and fluo-
rescence emission of all compounds are further redshifted
from solution (lex =436–506 nm, lem =491–554 nm; for de-
tails, see Table 1), which is indicative of some intermolecular
impact on potential performance in OFET devices, the
solid-state structures of heteropentacenes 4, 5, and 8 were
investigated. These air- and moisture-stable materials are
soluble in a variety of organicsolvents, so solution process-
ing for fabrication of OFET de-
vices may be possible. More-
over, preliminary differential
Table 1. Experimental and theoretical optical spectroscopic data for 3–8.
lex [nm][a]
lem [nm][b]
fPL [%]
lex [nm][e]
lem [nm][e]
scanning calorimetry (DSC)
studies on 4 and 5 showed sig-
nificant thermal stability for
both compounds, which decom-
pose only at temperatures
above 4008C (4: 4548C; 5:
4308C) and have a much lower
melting/sublimation point (4:
2528C; 5: 2328C), which poten-
tially would also allow vapor-
phase deposition onto OFET
devices.
3
385
440
63[c]
290, 420, 450
512
calcd
4
calcd
5
calcd
6
7
8
315, 325, 375 (0.39[f])
347, 424
483
473
68[c]
72[c]
419, 452
509
498
321, 325, 407 (0.29[f])
370, 435
456
324, 374, 422 (0.14[f])
407
461
471
520
51[c]
63[c]
31[d]
421, 444
356, 436
495
491
502
554
338, 398
275, 362, 426
calcd
363, 365, 445 (0.20[f])
[a] lmax for excitation in CH2Cl2(p–p* HOMO–LUMO transition in italics). [b] lmax for emission in CH2Cl2.
[c] Fluorescence quantum yield relative to quinine sulfate (0.1m H2SO4 solution), Æ10%, excitation at 365 nm.
[d] Fluorescence quantum yield, relative to rhodamine 101 (0.1 mm EtOH solution), Æ10%, excitation at the
excitation maximum (426 nm). [e] Excitation (p–p* HOMO–LUMO transition in italics) and emission in the
solid state. Calculated values are vertical excitation energies at the TDDFT B3LYP/6-31G* level. [f] Intensity.
Single crystals of heteropen-
tacene phosphole oxide 4 that
were suitable for an X-ray dif-
interactions, likely through p-stacking (vide infra). However,
the solid-state fluorescence data do not show a similar trend
to that in solution, in which shifts in the fluorescence data
depend on the nature of the phosphorus center. Apparently,
intermolecular interactions dominate the fluorescence prop-
erties in the solid state and surmount the impact of the
phosphorus center on the photophysics of the materials. The
experimental optoelectronic data of benzannelated deriva-
tives 3–8 and the computed values are summarized in
Table 1.
fraction study were obtained from a concentrated solution
of 4 in acetone at room temperature. Compound 4 has a
single molecule in the unit cell (Figure 2). All bond lengths
and angles of 4 are comparable to those observed for previ-
ously reported dithienophosphole derivatives,[22,24] and the
slight differences in bond-length alternation in 4 can be ex-
plained by further extension of the p-conjugated system
over five ring subunits. Compound 4 shows intermolecular
interactions between the oxygen and two sulfur atoms of
neighboring molecules (Figure 2, bottom; 2.78 and 2.80 Š).
This solid-state arrangement could prove useful for potential
application in OFETs. As carrier transport in organic solids
is governed by intermolecular overlap, it can also be en-
hanced by chalcogen–chalcogen contacts.[31] Likely as a
result of these O···S interactions, the solid-state packing
motif of 4 exhibits an edge-to-face herringbone-type ar-
rangement of molecules (Figure 2, middle). In addition, the
layers within the structure themselves show some p-stacking
interactions in which two/three carbon atoms of the terminal
benzene rings of adjacent molecules overlap (3.54 and
3.78 Š, see Supporting Information).
Heteropentacene phosphole sulfide 5, crystallized from a
concentrated solution of 5 in pentane at room temperature,
has two independent molecules in the unit cell (Figure 3,
top). Each has bond lengths and angles similar to those ob-
served for 4. Surprisingly, intermolecular chalcogen–chalco-
gen interactions are not observed in the solid-state arrange-
ment. This may be because the sulfide sulfur atom is too
large to fit between the two thiophene sulfur atoms of a
neighboring molecule, unlike the oxygen atom in 4. Howev-
er, the coplanar arrangement of the dithienophosphole
backbone in sulfur derivative 5 leads to p-stacking interac-
tions in two dimensions (interplane distance 3.6 Š) between
neighboring molecules of the same identity in the unit cell
(Figure 3, bottom).
Solid-state structures: The solid-state morphology of conju-
gated materials plays an important role in the performance
characteristics of electronic devices.[2,6a,9] If charge transport
occurs by a hopping mechanism, then the hopping rate con-
stant is also increased through a shorter distance and in-
creased overlap of electron density between neighboring
molecules.[27] Structural analysis of established OFET mate-
rials, such as pentacene and sexithiophene, show a herring-
bone motif in which the molecules are packed in an edge-
to-face two-dimensional layer.[1,2,28] However, p–p overlap
between adjacent molecules is minimized in this type of
solid-state arrangement. Intermolecular interaction is in-
creased by a face-to-face arrangement of the conjugated
molecules. The presence of p stacking can be inferred from
X-ray analysis; the typical interplanar distance for p stack-
ing is 3.4–3.6 Š.[2,29] Most strategies to alter the features of
pentacenes and their derivatives are based on changing the
solid-state order from an edge-to-face (herringbone) ar-
rangement to a face-to-face arrangement that allows p
stacking.[2] Recent studies have explored the effect of solid-
state order on the electronic properties of films and crys-
tals.[9] As a result, new and improved pentacene-based mate-
rials have been developed for FETs[1,2] and LEDs.[30] Since
packing properties and molecular interactions have a major
9882
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 9878 – 9889

Phosphorus-Based Heteropentacenes
FULL PAPER
Figure 3. Top: Molecular structure of 5 in the solid state (50% probabili-
ty level); hydrogen atoms are omitted for clarity; selected bond lengths
À
À
À
[Š] and angles [8]: P1 S1 1.9427(11), P1 C10 1.809(3), P1 C7 1.811(3),
À
À
À
À
À
P1 C21 1.824(3), S2 C8 1.728(3), S2 C5 1.759(3), S3 C9 1.731(3), S3
À
À
À
À
C12 1.749(3), C1 C2 1.376(4), C1 C6 1.406(4), C2 C3 1.407(4), C3 C4
1.384(4), C4 C5 1.387(4), C5 C6 1.420(4), C6 C7 1.431(4), C7 C8
1.376(4), C8 C9 1.454(4), C9 C10 1.366(4), C10 C11 1.433(4), C11 C12
1.423(4), C12 C13 1.402(4), C13 C14 1.383(5), C14 C15 1.412(5), C15
À
À
À
À
À
À
À
À
À
À
À
À
À
C16 1.374(4), C11 C16 1.409(4); C10-P1-C7 91.28(14), C10-P1-C21
107.03(13), C7-P1-C21 107.70(13), C10-P1-S1 117.08(10), C7-P1-S1
116.55(10), C21-P1-S1 114.56(10), C8-S2-C5 90.42(14), C9-S3-C12
90.13(15). Bottom: Molecular packing of 5.
À
compounds 4 and 5, the P C bonds in 8 are significantly
shorter due to the cationic nature of the phosphorus center.
À
Figure 2. Top: Molecular structure of 4 in the solid state (50% probabili-
ty level); hydrogen atoms are omitted for clarity. Selected bond lengths
The P Me bond length of 1.763(3) Š is somewhat shorter
than that of the few other structurally characterized phos-
pholium salts (1.775(3)–1.787(5) Š).[32] The bond lengths in
the dithienophosphole subunit are comparable to those in
phospholium salt B (R=Ph; R’=H, E=Me+).[24d] Again,
the slight differences can be explained by the more extend-
ed p system consisting of five rings. It is worth noting that
the ion pair of 8 is well separated in the solid state. Al-
though the arrangement of 8 in the solid state (Figure 4,
bottom) shows only weak interactions between the benzan-
nelated dithieno scaffold of neighboring molecules, distances
of 3.6 Š indicate that p-stacking interactions are present
here as well.
À
À
À
[Š] and angles [8]: P1 O1 1.4876(14), P1 C7 1.793(2), P1 C10 1.804(2),
À
À
À
À
À
P1 C21 1.797(2), S1 C8 1.729(2), S1 C5 1.746(2), S2 C9 1.736(2), S2
À
À
À
À
C12 1.745(2), C1 C2 1.381(3), C2 C3 1.411(3), C1 C6 1.407(3), C3 C4
1.383(3), C4 C5 1.393(3), C5 C6 1.416(3), C6 C7 1.429(3), C7 C8
1.381(3), C8 C9 1.475(3), C9 C10 1.375(3), C10 C11 1.429(3), C11 C12
1.411(3), C12 C13 1.404(3), C13 C14 1.380(3), C14 C15 1.409(3), C15
À
À
À
À
À
À
À
À
À
À
À
À
À
C16 1.379(3), C11 C16 1.409(3); O1-P1-C7 118.47(9), O1-P1-C21
110.97(8), C7-P1-C21 108.93(9), O1-P1-C10 116.98(9), C7-P1-C10
91.70(9), C21-P1-C10 108.07(9). Middle: Molecular packing of 4.
Bottom: Space-filling representation of the O···S interactions between
neighboring molecules in the structure of 4 (H atoms omitted for clarity).
Cationicphospholium speices
8 was obtained as single
These very different solid-state structures of the three rel-
atives of the same heteropentacene dithienophosphole
family nicely illustrate how the versatile reactivity of the
phosphorus center can efficiently be utilized to tune the or-
crystals from a concentrated solution of 8 in acetone at
room temperature and has a single molecule in the unit cell
(Figure 4, top). Compared to the oxidized and sulfurized
Chem. Eur. J. 2008, 14, 9878 – 9889
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9883

T. Baumgartner, L. Nyulµszi et al.
phospholes to begin with. In general, the electron affinity of
molecules with heavier elements is higher than that of their
carbon analogues. For example, a P=C bond is very effective
in this respect: triphosphabenzene has a high electron affini-
ty, while that of benzene is negative (it repels electrons).[33]
Naphthalene is a better electron acceptor than benzene; its
electron affinity is À0.2 eV (ꢀ0).[34] Thus, larger p systems
are also better electron acceptors, and with increasing
number of phosphorus atoms in the system the stability of
the anion further increases. These features led us to investi-
gate the electrochemical behavior of the heteropentacene
dithienophospholes, in order to determine their preferred
conduction channel. Our recent studies suggested that di-
thienophospholes may be good candidates for ambipolar
semiconductors, or may even show dominant n-channel be-
havior.[22,24]
For the electrochemical studies, we chose trivalent phosp-
hole 3, oxidized/sulfurized species 4/5, and phospholium
cation 8 as a representative set of compounds. We found
that the electrochemical behavior strongly depends on the
nature of the central phosphorus atom (Table 2). Trivalent
Table 2. Cyclovoltammetric data (E1/2 vs. Ag/AgCl) for 3, 4, and 8.
Eox [V]
Ered [V]
DE [V]
3[a]
4[b]
8[b]
1.42[c]
1.61[c]
–
À1.95[d]
À1.45[e]
À1.25[d]
3.37
3.06
–
[a] In THF. [b] In CH2Cl2. [c] Irreversible. [d] Quasireversible. [e] Rever-
sible (reduction peak separation ca. 60 mV).
Figure 4. Top: Molecular structure of 8 in the solid state; hydrogen atoms
are omitted for clarity (probability level 50%); selected bond lengths
À
À
À
À
[Š]: P1 C1 1.763(3), P1 C8 1.777(2), P1 C11 1.773(2), P1 C21 1.783(2),
À
À
À
À
À
S1 C6 1.753(1), S1 C9 1.712(1), S2 C10 1.715(1), S2 C13 1.745(1), C2
À
À
À
À
C3 1.367(4), C2 C7 1.395(3), C3 C4 1.396(4), C4 C5 1.381(4), C5 C6
1.387(3), C6 C7 1.412(3), C7 C8 1.422(3), C8 C9 1.370(3), C9 C10
1.456(3), C10 C11 1.369(3), C11 C12 1.422(3), C12 C13 1.410(3), C12
C17 1.399(3), C13 C14 1.384(3), C14 C15 1.376(3). Bottom: Molecular
phosphole 3 indeed shows ambipolar behavior with an irre-
versible oxidation at Eox =1.42 V (vs. Ag+/AgCl) that is
shifted somewhat from that of the parent system (Eox =
1.20 V vs. SCE),[22b] but more importantly also shows a qua-
À
À
À
À
À
À
À
À
À
À
packing of 8.
sireversible reduction at
E
red =À1.95 V, consistent with
strong electron affinity in this heteropentacene. Oxidation
of the phosphorus center in 4 significantly affects the redox
properties of the system. Phosphole oxide 4 still shows some
ambipolar behavior, but reduction is significantly more
dominant in this material. Due to the increased electron-ac-
ceptor character of the phosphorus center, oxidation (irre-
versible) is disfavored, as is evident in the further shift of
the oxidation potential (Eox =1.61 V) compared to the
ganization in the solid state. Even the simplest variations,
such as replacing an oxygen atom with sulfur, have tremen-
dous impact on the packing motif and possibly on the per-
formance of corresponding OFET devices.
Electrochemical properties: Although organic semiconduc-
tors should intrinsically be able to conduct charge either
through their valence band (p-channel) or their conduction
band (n-channel), that is, they should show ambipolar be-
havior, in many materials the p-channel dominates.[1b] How-
ever, it is much more desirable to generate n-channel (or n-
type) materials, as this conduction mechanism is more effi-
cient in devices.[1] Much effort has therefore been dedicated
to the development of new organicn-type semiocndu-c
tors.[1,2] As already indicated by the fluorescence properties
of the various members of this dithienophosphole family,
modification of the phosphorus center also has a significant
impact on the electronic structure. Increasing the acceptor
character of the phosphorus center significantly affects the
LUMO energy level,[15,17] which is already fairly low in
parent dithienophosphole
B (R=Ph, R’=H, E=lone
pair)[22b] and phosphole 3. Reduction in 4 is ideally reversi-
ble at Ered =À1.45 V, consistent with increased electron af-
finity and a very stable radical anion (Figure 5). Unfortu-
nately, the electrochemistry of the sulfurized species 5 could
not be determined under the conditions available to us.
Studies at scan rates of 100 and 500 mVsÀ1 revealed that,
likely due to the presence of the thio group at phosphorus, 5
was irreversibly adsorbed on the Pt electrode used for these
studies and that this process was rather slow. Neither oxida-
tion nor reduction potentials could be obtained. However,
judging from the photophysical properties of 5, as well as
from theoretical investigations, it can be assumed that the
9884
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 9878 – 9889

Phosphorus-Based Heteropentacenes
FULL PAPER
X-ray and computed structures, respectively), that is, rota-
À
tion of the phenyl group about the P C bond has a small
effect on the photophysical properties of the system. The
computed TD spectra are in good agreement with the mea-
sured absorption maxima in solution (Table 1).
To investigate the S···O(=P) interactions in more detail, a
dimer of 4 was also optimized. At the B3LYP/3-21G* level
the dimer has a structure similar to that found by X-ray dif-
fraction with S···O distances of 2.820 and 2.818 Š and rota-
tion of the phenyl substituent at phosphorus from the sym-
metrical position. At the B3LYP/6-31G* level, however, the
S···O distance increased to 3.272 and 3.222 Š. Similar distan-
ces were obtained for the dimer of B (R=Ph, R’=H, E=
O). The B3LYP/6-31G*//B3LYP/3-21G(*) electron density,
analyzed by the atoms in molecules (AIM) method of
Bader,[37] revealed two bond critical points, each between
the O and one of the two S atoms of the neighboring mole-
cule with an electron density of 0.015. Similar electron den-
sities were reported for bond critical points in molecular as-
semblies with S···O interactions.[31d,e] A ring critical point
Figure 5. Cyclic voltammogram of 4 at 293 K. Conditions: Pt wire as
working electrode; solution (ca. 0.1 mm) in CH2Cl2 with NBu4PF6 as sup-
porting electrolyte; scan rate 100 mVsÀ1; potentials are referred to an
Ag/AgCl/KCl 3m electrode.
redox properties are likely similar to those of oxide 4. Me-
thylated phospholium compound 8 could be investigated
without further complications. Its even stronger electron-ac-
ceptor character, due to the presences of a cationic charge,
exclusively leads to characteristics necessary for n-type be-
havior; an oxidation process cannot be observed within the
available window (Eꢁ2.0 V), and the reduction process is
strongly favored (Ered =À1.25 V). We found reduction to be
rather slow. Investigation of the reduction potential of 8 at
different scan rates (10, 100, and 500 mVsÀ1) showed no
true reversibility; that is, the process is kinetically controlled
and quasireversible.[35] Notably, the preferred one-electron
reduction of a series of related phospholium compounds was
recently reported by Le Floch and co-workers,[32] who
showed that reduction of these cationic species provides
very stable neutral radicals. The values observed for 3, 4,
and 8 are also lower than those recently reported by
Matano et al. for somewhat related bithiophene-fused ben-
zo[c]phospholes.[36]
À À
À À
was located for the S C=C S O fragment. The dimer was
more stable than two monomers by 2.1 kcalmolÀ1 (uncor-
rected for basis-set superposition error). To analyze the
effect of this interaction on the photophysical properties,
TDDFT calculations at the B3LYP/6-31G*//B3LYP/3-
21G(*) level were carried out on the dimer of 4. The com-
puted transition energy of the dimer of 509.9 nm (albeit
with low intensity) contrasts with the value of 406 nm
(Table 1) obtained for monomeric 4. This significant shift is
in accordance with the observed difference between the so-
lution and crystal-phase absorption spectra (Table 1).
We also investigated the redox properties of 3–5 and 8 by
means of theoretical calculations. Thus, we calculated ioniza-
tion energies of 3–5 and the electron-attachment energies of
3–5 and 8 by optimizing the radical anion and cation. These
calculations were carried out at the B3LYP/6-31+G* level
of theory, which was shown to give reliable values for anions
that have electron affinities more positive than about
À1 eV.[33] In each case the electron affinity of the molecules
is positive, and thus a stable anion is formed. The computed
ionization energies and electron affinities are compiled in
Table 3. From the data it is evident that the electron affini-
ties are larger for molecules with more extended p delocali-
The electrochemical studies on heteropentacenes 3, 4, and
8 nicely show that the versatile reactivity of phosphorus can
also be used to effectively manipulate the electronic proper-
ties of the materials. More importantly, the presence of the
phosphorus center intrinsically gives rise to features impor-
tant for n-channel materials that can be easily tuned by
means of simple modifications.
Table 3. B3LYP/6-31+G* adiabaticionization energies (aIE) [aklc
molÀ1], electron affinities (aEA) [kcalmolÀ1], and Kohn–Sham HOMO
and LUMO energies [eV] of 3–5, 8 and related derivatives of B.
Theoretical calculations: The B3LYP/6-31G* optimized
structures are similar to those obtained by X-ray crystallog-
raphy. In the case of 4, however, the phenyl group was opti-
mized to be in the mirror plane of the molecule, while the
X-ray structure of the molecule is asymmetric. Apparently
the interaction between molecules in the unit cell makes the
asymmetricarrangement favorable; the potential-energy
aIE
aEA
D(IE,EA) HOMO LUMO
A
B (R’=H, R=Ph, E=lp[a]
B (R’=H, R=Ph, E=O)
)
162.1
170.1 20.2 149.9
168.3 21.8 146.5
8.1 154.0
À5.67
À6.03
À5.98
À9.44
À1.62
À2.15
À2.16
À5.66
B (R’=H, R=Ph, E=S)
B+ (R’=H, R=Ph,
–
101.7
–
E=Me)
3
4
155.6 19.9 135.7
161.4 30.3 131.1
164.5 32.5 132.0
À5.60
À5.85
À5.88
À8.94
À1.95
À2.39
À2.44
À5.66
À
surface for rotation about P C bond in the gas phase
proved to be quite flat. Interestingly, the computed TD
spectra of the two structures are similar (406 and 326 nm vs.
405 and 324 nm are predicted for the first two bands for the
5
8[b]
–
105.8
–
[a] lp=lone pair. [b] Cation part only.
Chem. Eur. J. 2008, 14, 9878 – 9889
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9885

T. Baumgartner, L. Nyulµszi et al.
niques. Solvents were dried over appropriate drying agents and then dis-
tilled. CuCl2, nBuLi (2.5m in hexane), H2O2 (30% in H2O), sulfur,
BH3·SMe2 (1m in CH2Cl2), and methyl triflate were used as received.
N,N,N,N-Tetramethylethylenediamine (TMEDA) and phenyldichloro-
phosphane were distilled prior to use. 2,3-Dibromobenzo[b]thiophene[38]
zation (3–5 and 8 have larger electron affinities than the cor-
responding B-type compounds). The largest electron affinity
is exhibited by cationic species 8. The P-oxide and P-sulfide
also have slightly higher electron affinity than their parent
phosphole. All these findings are in agreement with the re-
duction potentials observed for these compounds, that is, 4,
5, and especially 8 are good candidates for exhibiting n-type
behavior. To investigate the effect of other heteroatoms on
the electron-acceptor properties, we also computed the
redox properties of the arsenicanalogues of 3 and 8. The As
compounds exhibit similar redox properties to their phos-
phorus analogues, and hence no further improvement of the
n-type conductivity can be expected by replacing phospho-
rus with its heavier analogue.
and [Au
(tht)Cl][39] were prepared by literature methods. 1H NMR,
A
13C{1H} NMR, 31P{1H} NMR, and 11B NMR spectra were recorded on a
Bruker DRX 400, Varian Mercury 200, or Unity 500 MHz spectrometer.
Chemical shifts were referenced to external 85% H3PO4 (31P), BF3·Et2O
(11B), or TMS (13C, 1H). Elemental analyses were performed at the Mi-
croanalytical Laboratory of the Institut für Anorganische und Analyti-
sche Chemie, Johannes Gutenberg-Universität, Mainz; the Institut für
Organische Chemie, RWTH-Aachen University; and the Department of
Chemistry at the University of Calgary. Crystal data and details of data
collection are provided in Table 4. Diffraction data for 4, 5, and 8 were
collected on a Bruker SMART D8 goniometer with APEX CCD detec-
tor by using graphite-monochromated MoKa radiation (l=0.71073 Š).
The structures were solved by direct methods (SHELXTL) and refined
on F2 by full-matrix least-squares techniques. Hydrogen atoms were in-
cluded by using a riding model. EI mass spectra were recorded on a Fini-
gan SSQ 7000 spectrometer. Fluorescence spectra were recorded on a
JASCO FP6500 spectrofluorometer; for the solid-state measurements, a
polycrystalline sample was placed in the corresponding solid-state acces-
sory. Electrochemical studies were performed on an Autolab
PGSTAT302 instrument with a Pt wire electrode (flamed in a torch) as
working electrode, Pt mesh as counterelectrode, and an Ag/AgCl/KCl 3m
reference electrode; supporting electrolyte was NBu4PF6; standard scan
rates were 100 mVsÀ1 for 4, 5, and 8 and 200 mVsÀ1 for 3.
Conclusion
We have synthesized a series of phosphorus-based hetero-
pentacenes by simple chemical modifications at the trivalent
phosphorus center. This signature of organophosphorus p-
conjugated materials allows a whole family of derivatives to
be generated from just one precursor material. Chemical
modification of the central phosphorus atom provides mate-
rials with significantly altered properties, including organiza-
tion in the solid state, photoluminescence, and redox behav-
ior, all of which are of fundamental importance for organic
semiconductors. Even the simplest variations, such as replac-
ing an oxygen atom by a sulfur atom at the phosphorus
center, have tremendous impact on the solid-state packing
of the compounds, particularly the phosphole oxide, which
shows an intriguing intermolecular S···O interaction that
may improve the charge carrier mobility of the materials in
devices. Furthermore, intense photoluminescence of the
pentacene-analogous dithienophospholes in the solid state
suggests potential application as active materials in light-
emitting field-effect transistors (LEFETs). Most important-
ly, theoretical and electrochemical studies on representative
members of this family revealed that the phosphorus atom
intrinsically furnishes the materials with features important
for n-channel (or n-type) or ambipolar semiconductor be-
havior, which can again be efficiently manipulated by means
of the electronic nature of the phosphorus center. Our elec-
trochemical studies showed that phosphole oxide 4 exhibits
the best reversible reduction, whereas cationic phospholium
species 8 has the lowest reduction potential. These observa-
tions suggest that both materials may be excellent candi-
dates for n-type semiconductors. Studies on implementation
of 4 into an OFET device are underway and will also be ex-
tended to the other members of this heteropentacene
family.
Synthesis of 2: 2,3-Dibromobenzo[b]thiophene (1, 20 mmol, 5.84 g) was
dissolved in diethyl ether (250 mL) and nBuLi (21 mmol, 8.4 mL) was
added at À788C. The reaction mixture was stirred for 20 min at this tem-
perature, then for 1 h at À208C, during which a yellow suspension was
obtained. After cooling to À788C, CuCl2 (30 mmol, 4 g) was added and
the reaction mixture was allowed to warm to room temperature and was
further stirred at this temperature for 10 h. The brown precipitate was fil-
tered off and HCl (5n, 300 mL) was added at 08C. The product was ex-
tracted with CHCl3 and diethyl ether, and the red organicphase was
washed with HCl (5n) and H2O and dried with MgSO4. After evaporat-
ing all volatile materials the crude product was obtained as dark oil. The
pure product could be obtained as a pink-tinted solid after washing with
acetone. Needle-shaped crystals of 2 could be obtained by crystallization
from pentane (2.31 g, 54.5% yield). 1H NMR (200 MHz, CDCl3): d=
7.95–7.78 (m, 4H), 7.40–7.55 ppm (m, 4H); 13C{1H} NMR (125 MHz,
CDCl3): d=139.1, 137.94, 129.34, 126.30 (s; benz), 125.42 (s; benz),
123.99 (s; benz), 122.23 (s; benz), 110.83 ppm (s; ArBr).
Synthesis of 3: nBuLi (2.83 mL, 7.08 mmol) was added dropwise to a so-
lution of 2 (1.80 g, 2 mmol) and TMEDA (2 mL, 10 mmol) in diethyl
ether (200 mL) at À788C. After stirring for 10 min, PhPCl2 (0.63 g,
3.54 mmol) was added and the resulting suspension was allowed to warm
quickly to room temperature. The solvent was then removed under
vacuum, and the residue taken up in CH2Cl2 (100 mL) and filtered
through neutral alumina. After evaporation of the solvent the residue
was washed with pentane and diethyl ether to give 3 as light yellow
powder (yield 72%). 31P{1H} NMR (80.9 MHz, CDCl3): d=À25.1 ppm
(s); 1H NMR (200 MHz, CDCl3): d=7.91–7.84 (br, 2H) 7.76–7.68 (br,
2H), 7.48–7.40 (br, 3H) 7.36–7.24 ppm (br, 6H); 13C{1H} NMR
(100 MHz, CDCl3): d=143.7 (d, 3J
9.58 Hz; Ar), 142.3 (d, 4J(C,P)=3.4 Hz; p-Ph), 138.3 (d, 1J
17.3 Hz; ipso-Ar), 133.5 (d, 2J(C,P)=21.1 Hz; o-Ph), 132.2 (d, 2J
13.4 Hz, o-Ph), 130.3 (s) 129.5 (d, 3J
(C,P)=7.7 Hz; m-Ph), 125.7 (s;
A
ACHTREUNG
A
ACHTREUNG
A
ACHTREUNG
AHCTREUNG
benzo), 124.9 (s; benzo), 124.1 (s; benzo), 122.5 ppm (s; benzo); MS
(70 eV, EI): m/z (%): 372 (100) [M+], 340 (70) [M+ÀS], 295 (40) [M+
ÀPh]; HRMS calcd for C22H13PS2 [M+]: 372.0196; found: 372.0180; ele-
mental analysis calcd (%) for C22H13PS2·0.5LiCl/Et2O (430.70 gmolÀ1): C
66.93, H 4.21; found: C 67.27, H 4.35.
Experimental Section
Synthesis of 4: Phosphole 3 (60 mg, 0.16 mmol) was dissolved in CH2Cl2
(25 mL), an excess of H2O2 (2 mL, 30% aqueous solution) was added,
and the mixture was then stirred for 2 h at room temperature. After
General procedures: Reactions were carried out in dry glassware under
an inert atmosphere of purified argon or nitrogen by using Schlenk tech-
9886
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 9878 – 9889

Phosphorus-Based Heteropentacenes
FULL PAPER
Table 4. Crystal data and structure refinement for 4, 5, and 8.[a]
1J
analysis calcd (%) for C22H13S3P
(403.99 gmolÀ1):
65.32, 3.24,
N
4
5
8
formula
Mr
C22H13OPS2
388.41
C22H13SPS2
404.47
C24H16F3OPS3
536.52
130(2)
0.71073
triclinic
23.78; found: C 65.09, H 3.53, S 23.42.
Synthesis of 6: An excess of BH3·SMe2
(2 mL, 1m in CH2Cl2) was added to
T [K]
153(2)
110(2)
l [Š]
0.71073
monoclinic
P21/n
11.151(2)
10.998(2)
15.705(3)
90
93.90(3)
90
1749.2(6)
4
0.71073
triclinic
P1
phosphole
3 (372 mg, 1 mmol) in
crystal system
space group
a [Š]
b [Š]
c [Š]
CH2Cl2 (30 mL) and the mixture was
stirred for 5 h at room temperature.
After evaporation of the solvent the
crude product could be obtained as a
yellow powder, which was washed
with n-pentane and recrystallized from
toluene (243 mg, 63% yield). 31P{1H}
¯
¯
P1
8.292(2)
13.855(3)
17.940(4)
110.977(5)
91.980(5)
103.165(5)
1858.5(7)
4
9.3146(13)
10.6391(15)
12.5703(18)
113.546(2)
93.590(2)
93.141(2)
1135.4(3)
2
a [8]
b [8]
g [8]
NMR
16.9 ppm
(80.9 MHz,
(s);
CDCl3):
11B{1H}
d=
V [Š3]
Z
NMR
(160.3 MHz, CDCl3): d=À39.3 ppm
(brs; BH3); 1H NMR (200 MHz,
CDCl3): d=7.92–7.76 (m, 6H; Ar)
7.55–7.30 (m, 7H; Ar); 1.28 ppm (s,
3H, BH3); 13C{1H} NMR (100 MHz,
1calcd [MgmÀ3
]
1.475
0.404
800
1.446
0.488
832
1.569
0.448
548
m [mmÀ1
(000)
]
F
A
crystal size [mm]
q range [8]
index ranges
0.47”0.24”0.06
2.26–30.12
À14ꢁhꢁ13
À15ꢁkꢁ15
À22ꢁlꢁ21
22115
0.24”0.20”0.09
1.23–27.17
À10ꢁhꢁ10
À17ꢁkꢁ17
À22 lꢁ22
23875
8210 (R
27.17 (99.4%)
empirical
0.28”0.18”0.09
1.77–25.65
À11ꢁhꢁ11
À12ꢁkꢁ12
À15ꢁlꢁ15
12527
4292 (R(int)=0.0520)
25.65 (99.7%)
empirical
CDCl3):
d=145.7
(d,
3J
ACHTREUNG
10.2 Hz; Ar), 143.3 (d, 3J
10.7 Hz; Ar), 136.4 (d, 3J
14.4 Hz; Ar), 134.2 (d, 1J
66.0 Hz; ipso-Ar), 133.1 (d, 2J
21.5 Hz; o-Ph), 132.2 (d, 4J
2.5 Hz; p-Ph), 132.04 (d, 3J
11.1 Hz; m-Ph), 129.3 (d, 3J
ACHTREUNG
ACHTREUNG
ACHTREUNG
reflections collected
independent reflections
completeness to q [8]
absorption correction
max/min transmission
data/restraints/parameters
GoF on F2
ACHTREUNG
5038 (R
A
U
G
ACHTREUNG
30.12 (97.6%)
empirical
0.9762/0.8327
5038/0/235
1.139
ACHTREUNG
ACHTREUNG
0.9574/0.89191
82102/0/469
0.932
0.9608/0.8848
4292/0/371
0.931
12.5 Hz; Ar), 126.1 (s; benzo), 125.4
(s; benzo), 123.7 (s; benzo), 122.7 ppm
(s; benzo).
final R indices [I>2s(I)]
R indices (all data)
R1 =0.0520, wR2 =0.1148 R1 =0.0433, wR2 =0.0864 R1 =0.0370, wR2 =0.0739
R1 =0.0632, wR2 =0.1195 R1 =0.0654, wR2 =0.1123 R1 =0.0550, wR2 =0.0789
Synthesis of 7: [Au(tht)Cl] (320 mg,
R
largest diff. peak/hole [eŠ3] 0.575/À0.331
0.530/À0.311
0.368/À0.328
1 mmol) was added to a solution of 3
(372 mg, 1 mmol) in CH2Cl2 (20 mL).
After stirring for 2 h at room tempera-
ture, the yellow precipitate was fil-
tered off, washed with n-pentane, and
[a] CCDC-693332 (4), CCDC-693333 (5) and CCDC-693334 (8) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
dried in vacuo (338 mg, 56% yield).
31P{1H} NMR (80 MHz, CDCl3): d=
2.44 ppm (s); 1H NMR (200 MHz,
quenching with water, the organic layer was separated and dried with
MgSO4, and all volatile materials were removed in vacuum. The product
4 was obtained as a light yellow solid and could be recrystallized from
acetone to give needle-shaped crystals (53 mg, 84% yield). 31P{1H} NMR
(80.9 MHz, CDCl3): d=À16.7 ppm; 1H NMR (200 MHz, CDCl3): d=
7.92–7.76 (m, 6H; Ar) 7.55–7.23 ppm (m, 7H; Ar); 13C{1H} NMR
CDCl3): d=7.99–7.96 (m, 2H; Ar) 7.83–7.79 (m, 2H; Ar) 7.77–7.71 (m,
2H; Ar) 7.58–7.53 (m, 1H; Ar) 7.48–7.41 ppm (m, 6H; Ar); 13C{1H}
3
ACHTREUNG
3
NMR (100 MHz, CDCl3): d=143.4 (d, J
(C,P)=15.7 Hz; Ar), 133.6 (d, 3J(C,P)=18.9 Hz; Ar), 133.1 (d, 4J
2.5 Hz; p-Ph), 129.8 (d, 3J
(C,P)=13.1 Hz; m-Ph), 126.6 (s; benzo), 125.8
A
N
ACHTREUNG
AHCTREUNG
(s; benzo), 123.9 (s; benzo), 122.3 ppm (s; benzo).
(100 MHz, CDCl3): d=146.7 (d, 2J
(C,P)=13.4 Hz; Ar), 136.1 (d, 3J(C,P)=13.4 Hz; Ar), 133.9 (d, 1J
111.2 Hz; ipso-Ar), 132.5 (d, 3J(C,P)=2.9 Hz; benzo), 130.2 (d, 2J
11.5 Hz, o-Ph), 129.2 (d, 3J
(C,P)=12.5 Hz; m-Ph), 126.3 (s; p-Ph), 125.3
(s; benzo), 123.5 (s; benzo), 123.1 (s; benzo), 121.8 ppm (d, J(C,P)=
ACHTREUNG
Synthesis of 8: An excess of methyl triflate (200 mg, 1.2 mmol) was
added to a solution of phosphole 3 (406 mg, 1.09 mmol) in CH2Cl2
(25 mL) and the mixture was stirred for 24 h at room temperature. The
yellow suspension became dark orange. After evaporating all volatile ma-
terials in vacuo the remaining solid was washed with n-pentane, diethyl
ether, and acetone. The product 8 was obtained as a yellow-orange solid.
Recrystallization from acetone gave single crystals suitable for X-ray
analysis (400 mg, 74% yield). 31P{1H} NMR (80.9 MHz, CDCl3): d=
11.4 ppm (s); 1H NMR (200 MHz, CDCl3): d=8.01 (m, 2H; Ar) 7.91 (m,
A
G
ACHTREUNG
U
ACHTREUNG
AHCTREUNG
AHCTREUNG
96.8 Hz, ipso-Ph); elemental analysis calcd (%) for C22H13OPS2
(388.44 gmolÀ1): C 68.02, H 3.37, S 16.51; found: C 68.68, H 2.80, S 16.98.
Synthesis of 5: Phosphole 3 (93 mg, 0.25 mmol) was dissolved in CH2Cl2
(30 mL) and sulfur (excess) was added. The mixture was stirred at room
temperature for 5 h until no further shift of the fluorescence could be ob-
served. All volatile materials were removed in vacuo. The product 5 was
obtained as a light yellow solid. The residue was taken up in n-pentane,
from a concentrated solution in which crystals formed (73 mg, 73%
yield). 31P{1H} NMR (80.9 MHz, CDCl3): d=23.1 ppm (s); 1H NMR
(200 MHz, CDCl3): d=7.97–7.78 (m, 6H; Ar) 7.55–7.30 ppm (m, 7H;
4H), 7.69 (m, 1H; p-Ph), 7.61 (m, 2H), 7.54 (2H), 7.45 (2H), 3.01 ppm
3
(d, J
A
2J
58.3 Hz; ipso-Ar), 136.3 (d, 4J
14.2 Hz; benzo), 132.7 (d, 2J(C,P)=13.2 Hz, o-Ph), 131.3 (d, 3J
14.2 Hz; m-Ph), 128.5 (s; p-Ph), 127.4 (s; benzo), 124.3 (s; benzo), 122.9
(s; benzo), 121.7 (d, 1J
(C,P)=81.5 Hz, ipso-Ph), 122.8 (s; Ar), 122.0 (s;
Ar), 6.9 ppm (d, 1J
A
1
ACHTREUNG
Ar); 13C{1H} NMR (100 MHz, CDCl3): d=145.1 (d, 2J
(C,P)=20.1 Hz;
for C24H16F3O3PS3 (563.55 gmolÀ1): C 53.72, H 3.01; found: C 53.80, H
3.24.
Ar), 143.4 (d,3J
135.5 (d, 3J
(d, 2J(C,P)=12.5 Hz, o-Ph), 129.1 (d, 3J
p-Ph), 125.4 (s; benzo), 123.6 (s; benzo), 1232.5 (s; benzo), 123.3 ppm (d,
A
G
1
A
N
A
ACHTREUNG
Calculations: Theoretical calculations were carried out at the B3LYP/6-
31G* level[40] by using the Gaussian 03 suite of programs.[41] This level of
Chem. Eur. J. 2008, 14, 9878 – 9889
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9887

T. Baumgartner, L. Nyulµszi et al.
the theory has provided satisfactory results for phosphole–thiophene oli-
gomers.[20c,22b,24e] The geometries were fully optimized, and calculation of
second derivatives at the resulting structures revealed only positive ei-
genvalues of the Hessian matrix. The vertical excitation energies were
calculated at the optimized structures (and in case of 4 also at the X-ray
structure) by the time-dependent (TD) DFT method with the B3LYP
functional and the 6-31G* basis set. Further calculations were carried out
for 3–5 and the corresponding radical cations and radical anions at the
B3LYP/6-31+G* level, to properly account for the anionic states.
18; c) K. Walzer, B. Maennig, M. Pfeiffer, K. Leo, Chem. Rev. 2007,
107, 1233–1271.
[10] a) Y. Shirota, H. Kageyama, Chem. Rev. 2007, 107, 953–1010; b) V.
Coropceanu, J. Cornil, D. A. da Silva Filho, Y. Olivier, R. Sibey, J.-
L. BrØdas, Chem. Rev. 2007, 107, 926–952; c) M. D. Curtis, J. Cao,
J. W. Kampf, J. Am. Chem. Soc. 2004, 126, 4318–4328; d) T. M. Pap-
penfus, R. J. Chesterfield, C. D. Frisbie, K. R. Mann, J. Casado, J. D.
Raff, L. L. Miller, J. Am. Chem. Soc. 2002, 124, 4184–4185; e) R. J.
Chesterfield, C. R. Newman, T. M. Pappenfus, P. C. Ewbank, M. H.
Haukaas, K. R. Mann, L. L. Miller, C. D. Frisbie, Adv. Mater. 2003,
15, 1278–1282; f) T. M. Pappenfus, J. D. Raff, E. J. Hukkanen, J. R.
Burney, J. Casado, S. M. Drew, L. L. Miller, K. R. Mann, J. Org.
Chem. 2002, 67, 6015–6024; g) X. C. Li, H. Sirringhaus, F. Garnier,
A. B. Holmes, S. C. Moratti, N. Feeder, W. Clegg, S. J. Teat, R. H.
Friend, J. Am. Chem. Soc. 1998, 120, 2206–2207.
Acknowledgements
We thank Prof. J. Okuda for his generous support of this work. Prof. U.
Englert is acknowledged for his help with the X-ray data collection. Fi-
nancial support of the Fonds der Chemischen Industrie, BMBF, Deutsche
Forschungsgemeinschaft (DFG), Natural Sciences and Engineering Re-
search Council (NSERC) of Canada, and from the Hungarian Scientific
Research Fund (OTKA Grant No T 049258) is gratefully acknowledged.
T.B. thanks Alberta Ingenuity for a New Faculty Award.
[11] K. Xiao, Y. Liu, T. Qi, W. Zhang, F. Wang, J. Gao, W. Qiu, Y. Ma,
G. Cui, S. Chen, X. Zhan, G. Yu, J. Qin, W. Hu, D. Zhu, J. Am.
Chem. Soc. 2005, 127, 13281–13286.
[12] M. L. Tang, T. Okamoto, Z. Bao, J. Am. Chem. Soc. 2006, 128,
16002–16003.
[13] J. G. Laquindanum, H. E. Katz, A. J. Lovinger, J. Am. Chem. Soc.
1998, 120, 664–672.
[14] J. Gao, R. Li, L. Li, Q. Meng, H. Jiang, H. Li, W. Hu, Adv. Mater.
2007, 19, 3008–3011.
[1] a) A. R. Murphy, J. M. J. FrØchet, Chem. Rev. 2007, 107, 1066–1096;
b) J. Zaumseil, H. Sirringhaus, Chem. Rev. 2007, 107, 1296–1323;
c) S. Allard, M. Forster, B. Souharce, H. Thiem, U. Scherf, Angew.
Chem. 2008, 120, 4138–4167; Angew. Chem. Int. Ed. 2008, 47, 4070–
4098; d) B. S. Ong, Y. Wu, Y. Li, P. Liu, H. Pan, Chem. Eur. J. 2008,
14, 4766–4778; e) H. E. Katz, Z. Bao, S. L. Gilat, Acc. Chem. Res.
2001, 34, 359–369; f) C. D. Dimitrakopoulos, P. R. L. Malenfant,
Adv. Mater. 2002, 14, 99–117.
[15] T. Baumgartner, R. RØau, Chem. Rev. 2006, 106, 4681–4727 (correc-
tion: T. Baumgartner, R. RØau, Chem. Rev. 2007, 107, 303).
[16] a) D. Delaere, M. T. Nguyen, L. G. Vanquickenborne, J. Phys. Chem.
A 2003, 107, 838–846; b) J. Ma, S. Li, Y. Jiang, Macromolecules,
2002, 35, 1109–1115; c) L. Nyulµszi, J. Phys. Chem. 1995, 99, 586–
591; d) L. Nyulµszi, Chem. Rev. 2001, 101, 1229–1246; e) F. Mathey,
Chem. Rev. 1988, 88, 429–453.
[2] a) J. E. Anthony, Chem. Rev. 2006, 106, 5028–5048; b) J. E. Antho-
ny, Angew. Chem. 2008, 120, 460–492; Angew. Chem. Int. Ed. 2008,
47, 452–483.
[3] Organic Light Emitting Devices (Eds.: K. Müllen, U. Scherf), Wiley-
VCH, Weinheim, 2006.
[17] a) W. Schäfer, A. Schweig, F. Mathey, J. Am. Chem. Soc. 1976, 98,
407–414; b) E. Mattmann, F. Mathey, A. Sevin, G. Frison, J. Org.
Chem. 2002, 67, 1208–1213; c) E. Mattmann, F. Mercier, L. Ricard,
F. Mathey, J. Org. Chem. 2002, 67, 5422–5425; d) L. Nyulµszi, O.
Holloczki, C. Lescop, M. Hissler, R. RØau, Org. Biomol. Chem.
2006, 4, 996–998; e) D. Delaere, P.-T. Nguyen-Nguyen, M. T.
Nguyen, Chem. Phys. Lett. 2004, 383, 138–142; f) E. Mattmann, D.
Simonutti, L. Ricard, F. Mercier, F. Mathey, J. Org. Chem. 2001, 66,
755–758.
[18] a) Phosphorus: The Carbon Copy (Eds.: K. Dillon, F. Mathey, J. F.
Nixon), Wiley, Chichester, 1998; b) Houben Weyl, Methods of Or-
ganic Chemistry, Vols. E1 & E2: Organic Phosphorus Compounds I
& II (Ed.: M. Regitz), Thieme, Stuttgart, 1982; c) F. Mathey, A.
Sevin, Molecular Chemistry of the Transition Metals, Wiley, Chiches-
ter, 1996.
[19] a) F. Mathey, Angew. Chem. 2003, 115, 1616–1643; Angew. Chem.
Int. Ed. 2003, 42, 1578–1604; b) D. P. Gates, Top. Curr. Chem. 2005,
250, 107–126; c) M. Hissler, P. W. Dyer, R. RØau, Top. Curr. Chem.
2005, 250, 127–163; d) Z. Jin, B. L. Lucht, J. Organomet. Chem.
2002, 653, 167–176; e) M. G. Hobbs, T. Baumgartner, Eur. J. Inorg.
Chem. 2007, 3611–3628.
[20] a) C. Hay, D. Le Vilain, V. Deborde, L. Toupet, R. RØau, Chem.
Commun. 1999, 345–346; b) C. Hay, C. Fischmeister, M. Hissler, L.
Toupet, R. RØau, Angew. Chem. 2000, 112, 1882–1885; Angew.
Chem. Int. Ed. 2000, 39, 1812–1815; c) C. Hay, M. Hissler, C. Fisch-
meister, J. Rault-Berthelot, L. Toupet, L. Nyulµszi, R. RØau, Chem.
Eur. J. 2001, 7, 4222–4236; d) C. Fave, M. Hissler, T. Kµrpµti, J.
Rault-Berthelot, V. Deborde, L. Toupet, L. Nyulµszi, R. RØau, J.
Am. Chem. Soc. 2004, 126, 6058–6063; e) V. Lemau de Talance, M.
Hissler, L.-Z. Zhang, T. Kµrpµti, L. Nyulµszi, D. Caras-Quintero, P.
Bäuerle, R. RØau, Chem. Commun. 2008, 2200–2202.
[4] a) A. Kraft, A. C. Grimsdale, A. B. Holmes, Angew. Chem. 1998,
110, 416–443; Angew. Chem. Int. Ed. 1998, 37, 402–428; b) B. W.
DꢁAndrade, S. R. Forrest, Adv. Mater. 2004, 16, 1585–1595; c) I. F.
Perepichka, D. F. Perepichka, H. Meng, F. Wudl, Adv. Mater. 2005,
17, 2281–2305.
[5] a) G. A. Chamberlain, Solar Cells 1983, 8, 47–83; b) P. Peumans, A.
Yakimov, S. R. Forrest, J. Appl. Phys. 2003, 93, 3693–3723; c) F. Pa-
dinger, R. S. Rittberger, N. S. Sariciftci, Adv. Funct. Mater. 2003, 13,
85–88; d) M. Svensson, F. L. Zhang, S. C. Veenstra, W. J. H. Ver-
hees, J. C. Hummelen, J. M. Kroon, O. Inganaes, M. R. Andersson,
Adv. Mater. 2003, 15, 988–991; e) B. C. Thompson, J. M. J. FrØchet,
Angew. Chem. 2008, 120, 62–82; Angew. Chem. Int. Ed. 2008, 47,
58–77; f) S. Günes, H. Neugebauer, N. S. Sariciftci, Chem. Rev. 2007,
107, 1324–1338.
[6] a) Handbook of Conducting Polymers, 3rd ed. (Eds.: T. A. Sko-
theim, J. R. Reynolds), CRC Press, Boca Raton, 2007; b) S. W.
Thomas, III, G. D. Joly, T. M. Swager, Chem. Rev. 2007, 107, 1339–
1386; c) D. T. McQuade, A. E. Pullen, T. M. Swager, Chem. Rev.
2000, 100, 2537–2574.
[7] a) J. A. Rogers, Z. Bao, M. Meier, A. Dodabalapur, O. J. A. Schuel-
ler, G. M. Whitesides, Synth. Met. 2000, 115, 5–11; b) J. A. Rogers,
Z. Bao, K. Baldwin, A. Dodabalapur, B. Crone, V. R. Raju, V. Kuck,
H. Katz, K. Amundson, J. Ewing, P. Drzaic, Proc. Natl. Acad. Sci.
USA 2001, 98, 4835–4840; c) H. Sirringhaus, T. Kawase, R. H.
Friend, T. Shimoda, M. Inbasekaran, W. Wu, E. P. Woo, Science
2000, 290, 2123–2126; d) S. P. Speakman, G. G. Rozenburg, K. J.
Clay, W. I. Milne, A. Ille, I. A. Gardner, E. Bresler, J. H. G. Steinke,
Org. Electron. 2001, 2, 65–73.
[21] a) C. Fave, T.-Y. Cho, M. Hissler, C.-W. Chen, T.-Y. Luh, C.-C. Wu,
R. RØau, J. Am. Chem. Soc. 2003, 125, 9254–9255; b) H.-C. Su, O.
Fadhel, C.-J. Yang, T.-Y. Cho, C. Fave, M. Hissler, C.-C. Wu, R.
RØau, J. Am. Chem. Soc. 2006, 128, 983–995.
[22] a) T. Baumgartner, T. Neumann, B. Wirges, Angew. Chem. 2004,
116, 6323–6328; Angew. Chem. Int. Ed. 2004, 43, 6197–6201; b) T.
[8] a) J. Roncali, Chem. Rev. 1997, 97, 173–206; b) J. Roncali, Macro-
mol. Rapid Commun. 2007, 28, 1761–1775.
[9] a) Q. Miao, X. Chi, S. Xiao, R. Zeis, M. Lefenfeld, T. Siegrist, M. L.
Steigerwald, C. Nuckolls, J. Am. Chem. Soc. 2006, 128, 1340–1345;
b) J. E. Anthony, D. L. Eaton, S. R. Parkin, Org. Lett. 2002, 4, 15–
9888
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 9878 – 9889

Phosphorus-Based Heteropentacenes
FULL PAPER
Baumgartner, W. Bergmans, T. Kµrpµti, T. Neumann, M. Nieger, L.
Nyulµszi, Chem. Eur. J. 2005, 11, 4687–4699.
[23] a) T. Baumgartner, J. Inorg. Organomet. Polym. Mater. 2005, 15,
389–409; b) X. Zhang, A. J. Matzger, J. Org. Chem. 2003, 68, 9813–
9815; c) S. Y. Hong, J. M. Song, J. Chem. Phys. 1997, 107, 10607–
10615.
[24] a) T. Baumgartner, W. Wilk, Org. Lett. 2006, 8, 503–506; b) Y.
Dienes, M. Eggenstein, T. Neumann, U. Englert, T. Baumgartner,
Dalton Trans. 2006, 1424–1433; c) T. Neumann, Y. Dienes, T. Baum-
gartner, Org. Lett. 2006, 8, 495–497; d) S. Durben, Y. Dienes, T.
Baumgartner, Org. Lett. 2006, 8, 5893–5896; e) Y. Dienes, S.
Durben, T. Kµrpµti, T. Neumann, U. Englert, L. Nyulµszi, T. Baum-
gartner, Chem. Eur. J. 2007, 13, 7487–7500.
[25] U. Dahlmann, R. Neidlein, Helv. Chim. Acta 1997, 80, 111–120.
[26] J. R. Lakowicz, Principles of Fluorescence Spectroscopy, 3rd ed.,
Springer, New York 2006.
D. B. Werz, H. Kçppel, Inorg. Chem. 2007, 46, 2249–2260; h) R. E.
Rosenfeld, R. Parthasarathy, J. Dunitz, J. Am. Chem. Soc. 1977, 99,
4860–4862; i) R. A. Poirier, ꢂ. Kucsman, I. G. Csizmadia, J. Am.
Chem. Soc. 1987, 109, 2237–2245.
[32] P. Adkine, T. Cantat, E. Deschamps, L. Ricard, N. MØzailles, P. Le
Floch, M. Geoffroy, Phys. Chem. Chem. Phys. 2006, 8, 862–868.
[33] A. Modelli, B. Hajgatꢃ, J. F. Nixon, L. Nyulµszi, J. Phys. Chem. A
2004, 108, 7440–7447.
[34] Y. Xie, H. F. Schaefer III, F. A. Cotton, Chem. Commun. 2003, 102–
103.
[35] A. J. Bard, L. R. Faulkner, Electrochemical Methods: Fundamentals
and Applications, 2nd ed., Wiley, New York, 2001.
[36] T. Miyajima, Y. Matano, H. Imahori, Eur. J. Org. Chem. 2008, 255–
259.
[37] R. W. F. Bader, Acc. Chem. Res. 1985, 18, 9–15.
[38] G. Barbarella, L. Favaretto, A. Zanelli, G. Gigli, M. Mazzeo, M.
Anni, A. Bongini, Adv. Funct. Mater. 2005, 15, 664–670.
[39] R. Usꢃn, A. Laguna, M. Laguna, Inorg. Synth. 1989, 26, 85–91.
[40] a) A. D. Becke, J. Chem. Phys. 1993, 98, 5648; b) C. Lee, W. Yang,
R. G. Parr, Phys. Rev. B 1988, 37, 785–789.
[27] A. B. Koren, M. D. Curtis, A. H. Francis, J. W. Kampf, J. Am. Chem.
Soc. 2003, 125, 5040–5050.
[28] a) D. Holmes, S. Kumaraswamy, A. J. Matzger, K. P. C. Vollhardt,
Chem. Eur. J. 1999, 5, 3399–3412; b) G. Horowitz, B. Bachet, A.
Yassar, P. Lang, F. Demanze, J. L. Fave, F. Garnier, Chem. Mater.
1995, 7, 1337–1341; c) T. Siegrist, R. M. Fleming, R. C. Haddon,
R. A. Laudise, A. J. Lovinger, H. E. Katz, P. Bridenbaugh, D. D.
Davis, J. Mater. Res. 1995, 10, 2170–2173.
[29] a) T. Yamamoto, D. Komarudin, M. Arai, B. L. Lee, H. Suganuma,
N. Asakawa, Y. Inoue, K. Kubota, S. Sasaki, T. Fukuda, H. Matsuda,
J. Am. Chem. Soc. 1998, 120, 2047–2058; b) J. K. Politis, J. C.
Nemes, M. D. Curtis, J. Am. Chem. Soc. 2001, 123, 2537–2547.
[30] M. A. Wolak, J. H. Delcamp, C. A. Landis, P. A. Lane, J. E. Antho-
ny, Z. H. Kafafi, Adv. Funct. Mater. 2006, 16, 1943–1949.
[31] a) M. Turbiez, P. Fr›re, M. Allain, N. Gallego-Planas, J. Roncali,
Macromolecules 2005, 38, 6806–6812; b) M. Turbiez, P. Fr›re, J.
Roncali, J. Org. Chem, 2003, 68, 5357–5360; c) P. Leriche, M. Tur-
biez, V. Monroche, P. Fr›re, P. Blanchard, P. J. Skabara, J. Roncali,
Tetrahedron Lett. 2003, 44, 649–652; d) C. Rethore, A. Madalan, M.
Fourmigue, E. Canadell, E. B. Lopes, M. Almeida, R. Clerac, N.
Avarvari, New J. Chem. 2007, 31, 1468–1483; e) G. Raos, A. Famu-
lari, S. V. Meille, M. C. Gallazzi, G. Allegra, J. Phys. Chem. A 2004,
108, 691–698; f) C. Bleiholder, D. B. Werz, H. Kçppel, R. Gleiter, J.
Am. Chem. Soc. 2006, 128, 2666–2674; g) C. Bleiholder, R. Gleiter,
[41] Gaussian 03, Revision C.02, M. J. Frisch, G. W. Trucks, H. B. Schle-
gel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomer-
y, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyen-
gar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N.
Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K.
Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J.
Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C.
Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari,
J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cio-
slowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaro-
mi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng,
A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W.
Chen, M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian, Inc., Wall-
ingford, CT, 2004.
Received: July 29, 2008
Published online: October 9, 2008
Chem. Eur. J. 2008, 14, 9878 – 9889
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9889