Dually switchable heterotetracenes: Addressing the photophysical properties and self-organization of the P-S system

Article abstract of DOI:10.1021/ja108081b

New ladder-type, phosphorus- and sulfur-based heterotetracenes were synthesized, which allowed the engineering of the materials′ properties by exploitation of the different reactivities between sulfur and phosphorus. ³¹P NMR spectroscopy and X-

Full text of DOI:10.1021/ja108081b

ARTICLE
pubs.acs.org/JACS
Dually Switchable Heterotetracenes: Addressing the Photophysical
Properties and Self-Organization of the P-S System
Yi Ren and Thomas Baumgartner*
Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada
S Supporting Information
b
ABSTRACT: New ladder-type, phosphorus- and sulfur-based heterotetracenes were
synthesized, which allowed the engineering of the materials’ properties by exploitation
of the different reactivities between sulfur and phosphorus. 31P NMR spectroscopy and
X-ray crystallographic studies revealed that the different electronic effects of the
secondary heteroatom, sulfur, influence not only the conjugation in the heterotetra-
cene core but also the behavior of the phosphorus center. UV-vis and fluorescence
spectroscopy showed that the scaffold’s band gap is mainly controlled by the electronic
nature of sulfur, while the fluorescence quantum yield highly depends on the electronic
nature of phosphorus. Cyclic voltammetry indicated that the redox properties of the system could be altered by selective
modification of the respective heteroatom (oxidation of sulfur and/or functionalization of trivalent phosphorus). Importantly,
oxidation of the phosphorus center results in enhanced reduction features of the heterotetracene system, and oxidation of the sulfur
center further enhances the electron acceptor character of the core. Theoretical calculations provided insights on both selectivity of
phosphorus chemistry and communication between the two heteroatoms (sulfur and phosphorus). Macroscopic self-organization
of the heterotetracenes was observed when the tetracene core is functionalized with pendant functional groups. Preliminary results
showed that extension of the molecule with an alkyl chain along the long axis of molecules induces the formation of 1D microfibers,
which was confirmed by fluorescence microscopy and scanning electron microscopy.
thiophene materials.5,6 However, full oxidation of oligothiophenes,
1. INTRODUCTION
and fused thienoacenes in particular, is still a synthetic challenge
Ladder-type π-conjugated molecules exhibit intriguing prop-
erties, such as high fluorescence and/or high charge mobility,
which have been intensively studied in the context of organic
and requires rather harsh conditions due to the strong electron-
withdrawing effect from a neighboring sulfur dioxide group.6
Unlike the sulfur atom, boron shows significant electronic coupling
with a π-conjugated system, due to the empty p-orbital of the
light-emitting diodes (OLEDs), organic field-effect transistors
(OFETs), and photovoltaic cells.1 Although oligoacenes (e.g.:
boron center.7 Differently but similarly efficiently, both silicon
pentacene, tetracene, Chart 1) and their derivatives were the first
and phosphorus interact with a π-conjugated system through
ladder-type π-conjugated molecules employed in high perfor-
hyperconjugation between the σ*-orbital of exocyclic P/Si-C
mance OFETs, their instability toward oxidation sometimes
bonds and the endocyclic π-system of the butadiene moiety that
hampers their practical application.1 The incorporation of main
significantly lowers the LUMO energy level.8,9
group elements (B, Si, P, and S, Chart 1) into ladder-type π-
The diverse chemistry and intriguing electronic properties of
conjugated molecules provides an intriguing strategy in terms of
phosphorus make it a unique member among the main group
electronic stabilization and has therefore recently drawn signifi-
elements. Rꢀeau's and our group were among the first to success-
cant attention.2 Moreover, the different electronic natures of the
fully incorporate phospholes, the P-analogues of thiophenes,
main group elements can also endow π-conjugated molecules
into π-conjugated systems in a systematic fashion;9b,c,10 the
with quite diverse and interesting properties. As the first candi-
phosphorus chemistry (oxidation, methylation, and metalation)
date, sulfur has successfully been introduced in oligoacenes in the
allows modifying the photophysical properties of these systems
form of thiophene units.3 The high aromaticity of the thiophene
efficiently. Recently, the groups of Yamaguchi, Matano, and our
ring enhances benzenoid character and, thus, lowers the HOMO
group have demonstrated that the electron-accepting character
energy level with wider HOMO-LUMO band gap, which
of heteroacenes can be significantly enhanced via incorporation
enhances the photostability and thermal stability of the systems.
of phosphorus centers (see, e.g., IV, Chart 1).11 Phosphorus has
In addition, weak intermolecular S-S interactions induce pre-
also been incorporated into π-conjugated systems with other
ferred face-to-face π-stacking motifs in the solid state, which can
heteroatoms in order to take advantage of the different electronic
increase the electronic coupling between molecules and facilitate
charge and energy transport.4 To further optimize these features,
Yamaguchi and others have found that oxidation of sulfur is a
very efficient way to enhance the electron-accepting character of
Received: June 17, 2010
Published: January 6, 2011
r
2011 American Chemical Society
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Chart 1. Tetracenes and Heteroelement-Based Ladder-Type
Acenes
consistent with theoretical calculations (vide infra) and earlier
observations.10a,11f,13b
Further functionalization of the phosphorus center (oxidation,
methylation, complexation with borane) was carried out follow-
ing established procedures,10 providing the products 2b-d in
good yields (Scheme 2). The low-field-shifted 31P NMR reso-
nances of δ = 21.3 ppm for 2b, 26.1 ppm for 2c, and 14.2 ppm for
2d clearly supported the formation of the functionalized species.
The values are again somewhat low-field-shifted, when compared
to the dithienophosphole system (cf. IV, A = null, E = BH3, δ =
13.5 ppm; IV, A = null, E = O, δ = 18.8 ppm; IV, A = null, E =
Meþ, δ = 12.2 ppm; IV, A = C4H4, E = BH3 16.9 ppm; IV, A =
C4H4, E = O, δ = 16.7 ppm; IV, A = C4H4, E = Meþ, δ = 11.4
ppm),10,11f likely due to the less pronounced donor character of
benzene compared to thiophene. In order to enhance the electron-
accepting character of the system, the sulfur center in 2c was
subsequently oxidized with 3 equiv of mCPBA to afford 79% of
the sulfur dioxide derivative 3c with further low-field-shifted 31P
NMR resonance (δ = 29.9 ppm). Compared to other ring-fused
oligothiophene systems,5,6 this easy oxidation of the sulfur center
in thiophene-phosphole 2c is probably due to the small electron
density change at sulfur upon oxidation of the phosphorus center
(vide infra).
The different electronic natures of the sulfur and phosphorus
centers drew our attention to the selective reduction of the
oxidized phosphole (Scheme 3). Using the same method we have
reported before,10d phosphole oxide 3c was transferred into
phosphole-borane adduct 3b (δ31P = 27.9 ppm) by reacting
natures of both heteroatoms. Phosphonium- and borate-bridged
stilbene systems (III, Chart 1), which exhibit very interesting
nonlinear optical properties, were reported and systematically
studied by Yamaguchi’s group.12 Our recent study on ladder-type
phosphinine systems (V, Chart 1) has provided insights into the
molecular level fine-tuning of the photophysical and electroche-
mical properties, as well as organization in the solid state caused by
geometric and electronic effects of additional heteroatoms (Si, S,
and P) in conjunction with phosphorus.13
However, the diversity of π-conjugated organophosphorus
materials is still limited to date, due to the synthetic challenge
when dealing with these compounds; phosphorus behaves
fundamentally different from its pnictogen congener nitrogen.9b
Importantly, the influence of secondary heteroatoms on the
phosphorus chemistry and properties of organic materials has
not been studied in detail. Herein, we now report the synthesis
and structure-property studies of a new phosphorus-based asym-
metric heterotetracene system. The photophysical and electro-
chemical properties of these phosphorus-based heterotetracenes
were investigated systematically and correlated with the different
electronic natures of the heteroatoms (S and P). The new
phosphorus-based heterotetracenes were shown to self-organize
into 1D micro-/nanofibers upon functionalization of the core with
long alkyl chains along the long axis of the molecular scaffold.
it with an excess of BH3 SMe2. Subsequent treatment with an
3
excess of NEt3 provided the trivalent phosphole 3a that shows
a typical high-field shift at δ31P = -11.6 ppm that is comparable
to the sulfur(II) species 2a (δ31P = -16.5 ppm). The slight
difference can be explained by the electron-withdrawing sulfur
dioxide center in 3a. Notably, the sulfur dioxide center is not
reduced during the reduction even with 5 equiv of BH3 SMe2, as
3
1
verified by H and 31P NMR spectroscopy. The regenerated
trivalent phosphole 3a could also further be reacted with methyl
triflate,10c,11f to afford the strongly emissive compound 3d. It is
worth mentioning that the 31P NMR shift difference between
phosphonium 3d (δ = 28.6 ppm) and phosphole oxide 3c (δ =
29.9 ppm) is smaller than the difference between phosphonium
2d (δ = 14.2 ppm) and phosphole oxide 2c (δ = 26.1 ppm),
which could be attributed to the electron-withdrawing character
of the sulfur dioxide center decreasing the polar character of the
exocyclic PdO double bond and therefore shielding the phos-
phorus center. Compounds 2a-d, 3a, 3c, and 3d were studied by
means of Differential Scanning Calorimetry (DSC) and showed
good thermal stability without decomposition up to their re-
spective melting points (Tm = 131.6 °C for 2a, 168.0 °C for 2b,
164.7 °C for 2c; 193.4 °C for 2d; 190.6 °C for 3a, 208.9 °C for 3c,
and 209.2 °C for 3d).
2. RESULTS AND DISCUSSION
2.1. Synthesis. The starting material 1 was obtained via a
Suzuki coupling reaction between 2,3-dibromobenzo[b]thiophene,
which selectively reacts at the 2-position, and 2-bromophenylboronic
acid in 73% yield (Scheme 1). Phosphole synthesis following our
typical protocol toward dithieno[3,2-d:20,30-b]phospholes10 gave
the asymmetric phosphorus-based heterotetracene 2a in 70% yield.
Compared to the parent dithienophosphole (IV, A = null, E = lp,
Chart 1; δ = -21.5 ppm) and corresponding phosphorus-based
heteropentacene (IV, A = C4H4, E = lp, Chart 1; δ = -25.1 ppm),
2a shows a low-field shifted 31P NMR resonance at δ = -16.5
ppm indicating a relatively higher delocalization of the phos-
phorus lone pair within the heterotetracene system, which is
2.2. Molecular Structures and Organization in the Solid
State. We were able to obtain single crystals of compounds 2c,
2d, 3a, and 3d. Due to the asymmetric nature of the scaffold, the
new heterotetracenes are chiral at the pyramidal phosphorus center.
However, both enantiomers are generally found in the unit cells,
as determined by X-ray crystallography. Similar to the phos-
phole-based pentacene systems (IV, A = C4H4, Chart 1) we have
reported before,11f the five-membered phosphole ring in 2c shows
only a minor deviation from planarity (Figure 1). The sum of the
angles around phosphorus is 305.2°, which is typical for phosphole
systems.10 Compared to [1]benzothieno[3,2-b]benzothiophene
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Scheme 1. Synthesis of the Asymmetric Tetracene 2a
consistent with the low-field shift of the ipso-13C NMR resonance
of the thiophene in 2d. Furthermore, 2d arranges in dimers in the
solid state (see Supporting Information). In addition to a relatively
weak π-π interaction (distance: ca. 3.8 Å), a stronger π-π
interaction (distance: ca. 3.4 Å) is also observed intradimer, which is
probably due to the decreased electron density of the backbone.
As can be seen in the structure of 3a, oxidation of the sulfur
center significantly elongates the sulfur-carbon bonds (S1-C5:
1.774(2) Å, S1-C8: 1.772(2) Å) compared with those in 2c (cf.:
S1-C5: 1.758(3) Å, S1-C8: 1.722(3) Å), which is consistent
with the loss of aromaticity upon oxidation of sulfur (Figure 3).
The increased endohedral C-S-C angle of 94.2° around sulfur
and shortened C7-C8 bond length of 1.344(3) Å further
support the significant structural changes in 3a when compared
to 2c. As for the five-membered phosphole ring, the decreased
endohedral C-P-C angle (88.9°) and sum of the angles around
phosphorus (297.7°) confirm a more pyramidal geometry. The
bonds lengths of the phosphole ring are similar to those of other
π-conjugated phospholes9-11 supporting reduced electronic
delocalization within the phosphole ring of 3a. These changes
are consistent with the red-shifted absorption and emission
wavelengths of 3a compared with 2a, as the conjugation of
backbone increases (vide infra). In the solid state, only a very
weak intermolecular π-π interaction (distance: ca. 4.0 Å; see
Supporting Information) is observed.
Similar to 2d, the methylation of the phosphorus center
increases the sum of angles around phosphorus in 3d (314.0°;
Figure 4). Compared to 3a, the phosphorus-carbon bonds in 3d
are shortened supporting an increasing communication between
the phosphorus center and the π-system, as well as a more rigid
backbone. The electron-withdrawing nature of the phosphonium
center decreases the electron density of the π-system and, as a
result, increases the π-π interactions between molecules in the
solid state (distance ca. 3.5 Å; see Supporting Information). In
addition, the solid-state organization of 3d also involves several
intermolecular hydrogen bonds between the aromatic ring and
the SdO oxygen atoms (distances: 2.47 and 2.51 Å; see Supporting
Information).
Scheme 2. Functionalization of the Tetracene 2a
(BTBT, III, E1 = E2 = S, Chart 1),14a,b 2c exhibits similar bond
lengths and angles within the thiophene ring, which indicates that
the thiophene ring still maintains its aromaticity.
The `benzo’ C-C bond lengths of both, the phosphole and
thiophene subunits, are identical (C5-C6: 1.409(4) Å and
C9-C14: 1.410(3) Å) indicating no significant distortion of
the structural parameters of both benzene rings. It should be
mentioned in this context that molecular organization in the solid
state is also an important feature for both energy and charge
transfer in the practical application of molecular materials. BTBT
(III, E1 = E2 = S, Chart 1) derivatives with long alkyl chains have
been reported to crystallize in a herringbone motif and exhibit
CH-π and S-S interactions.14a,b With the exocyclic phenyl
ring at the phosphorus center, 2c also organizes in a herringbone
arrangement between layers; however, π-π stacking interactions
(distance: ca. 3.5 Å) are observed within each layer (see Supporting
Information). A similar molecular organization is also observed
in the structure of a classical tetracene derivative, rubrene (II,
Chart 1), with excellent semiconducting properties.14c,d
2.3. Photophysical Properties. Absorption and fluorescence
properties of the heterotetracene series 2a-d and 3a-d are
summarized in Table 1. Compared to the absorption spectrum of
BTBT, 2a shows similar absorption features in the region λmax
=
In the phosphonium species 2d, both the endohedral C-P-C
angle (94.2°) and sum of the angles around phosphorus (316.4°)
increase due to the phosphorus center approaching tetrahedral
geometry (Figure 2). The P-C bond lengths of P1-C8
(1.779(2) Å) and P1-C15 (1.7975(15) Å) are shorter than
those in 2c (P1-C7: 1.792(3) Å, P1-C14: 1.816(2) Å)
indicating that the positive phosphonium center polarizes the
central C8-C9 `stilbene’ double bond more efficiently, which is
280-360 nm but a less resolved structure with the maximum at
λmax = 317 nm.14a,15 Functionalization of the phosphorus center
modifies the HOMO-LUMO gap of the molecule very efficiently,
as evidenced in the red-shifted absorption of the compounds
2a-d (λonset = 365 nm for 2a, 376 nm for 2b, 394 nm for 2c, and
410 nm for 2d). The single absorption peaks of 2a within λmax
=
290-370 nm split up into two broad bands in the functionalized
phosphorus species 2b, 2c, and 2d indicating that the electronic
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Scheme 3. Selective Functionalization of the Phosphorus Center
Figure 1. Molecular structure of 2c in the solid state (50% probability
level); hydrogen atoms are omitted for clarity. Selected bond lengths [Å]
and angles [deg]: P1-O1: 1.4826(17); P1-C7: 1.792(3); P1-C14:
1.816(2); P2-C21: 1.808(2); C7-C8: 1.365(4); S1-C5: 1.758(3);
S1-C8: 1.722(3); C5-C6: 1.409(4); C9-C14: 1.410(3); C5-S1-
C8: 90.95(13); C7-P1-C14: 91.74(12); C7-P1-C21: 106.47(12);
C14-P1-C21: 106.97(10).
Figure 3. Molecular structure of 3a in the solid state (50% probability
level); hydrogen atoms are omitted for clarity. Selected bond lengths [Å]
and angles [deg]: P1-C7: 1.809(2); P1-C14: 1.832(3); P1-C21:
1.826(2); C7-C8: 1.344(3); C5-C6: 1.396(3); C9-C14: 1.414(3);
S1-C5: 1.774(2); S1-C8: 1.772(2); S1-O1: 1.4401(18); S1-O2:
1.4435(19); C7-P1-C14: 88.87(11); C7-P1-C21: 103.77(10);
C14-P1-C21: 105.8(10); C5-S1-C8: 92.18(11).
Figure 2. Molecular structure of 2d in the solid state (50% probability
level); hydrogen atoms are omitted for clarity. Selected bond lengths [Å]
and angles [deg]: P1-C1: 1.780(2); P1-C8: 1.779(2); P1-C15:
1.7975(19); P1-C21: 1.7954(19); C8-C9: 1.372(3); C6-C7:
1.414(3); C10-C15: 1.408(3); S1-C6: 1.750(2); S1-C9: 1.717(2);
C8-P1-C15: 94.20(9); C8-P1-C21: 111.06(9); C15-P1-C21:
111.10(9); C6-S1-C9: 90.60(10).
Figure 4. Molecular structure of 3d in the solid state (50% probability
level); hydrogen atoms and one acetone solvent molecule are omitted
for clarity. Selected bond lengths [Å] and angles [deg]: P1-C1:
1.773(2); P1-C8: 1.799(2); P1-C15: 1.800(2); P1-C21: 1.785(2);
C8-C9: 1.345(3); C6-C7: 1.405(3); C10-C15: 1.408(3); S1-C6:
1.769(2); S1-C9: 1.771(2); S1-O1: 1.435(2); S1-O2: 1.436(2);
C8-P1-C15: 92.83(10); C8-P1-C21: 106.65(10); C15-P1-C21:
114.46(10); C6-S1-C9: 92.35(11).
structure of the system changes significantly (Figure 5, top).
Furthermore, as observed in the related trivalent phosphole
system by Yamaguchi et al.,11a the trivalent phosphole 2a shows
a high molar absorption coefficient of log ε = 4.45, that is
comparable with that of BTBT (cf.: log ε = 4.06 at 332 nm,
4.41 at 308 nm, 4.41 at 226 nm, and 4.35 at 258 nm in CHCl3).15
This absorption splitting can be correlated with the elimination
of the orbital symmetry in the P-based tetracene cores by
functionalization of the phosphorus center, as verified by NMR
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Table 1. Photophysical Data for the Heterotetracenes
λabs
[nm]a
Eop
[eV]d
λem
Δλ
τ
Kr
Knr
(10-7 s-1
)
A
[Å]h
log εc
[nm]
[nm]
φPL
[ns]g
(10-7 s-1
)
f
Compd
2a
317b
325b
329b
340b
357b
386b
395b
4.45
3.40
3.30
3.15
3.02
2.95
2.84
2.74
421a
428e
400a
400e
428a
434e
445a
445e
453a
471e
466a
465e
483a
485e
428a
104
75
0.24
0.38
0.71
0.36
0.14
0.98
0.99
0.15
0.3 (70%)
11.2 (30%)
3.0
6.7
12.7
6.3
2.4
1.2
7.6
7.8
21.3
20.7
2.6
3.44
4.11
5.48
4.02
2.75
8.24
8.96
2b
2c
2d
3a
3c
3d
4a
3.43
3.86
3.42
4.21
3.73
4.33
99
11.3
14.8
105
96
4.3
0.7 (14%)
13.0 (86%)
12.9
7.6
80
0.2
88
5.1 (3%)
0.1
13.0 (97%)
343
359b
4.06
3.94
2.55
4.78
5.30
3.43
4.27
4.65
3.75
375 (sh)
4b
5a
334
359b
427a
427a
0.13
0.29
380 (sh)
349
368b
381 (sh)
a Absorption measured in CH2Cl2. b The lowest energy absorption maxima. c ε: Molar absorption coefficient [L mol-1 cm-1]. d Eop: optical band gap
3
3
from absorption onset. e Solid state. Fluorescence quantum yield was determined by a calibrated integrating sphere system. g Fluorescence
f
lifetime. h A (Å) = ln(Kr/Knr) þ 4.6.
and UV-vis spectroscopy, as well as theoretical calculations
(vide infra). The molar absorption coefficients for the P-functio-
nalized derivates, however, are lower than that of 2a (log ε = 3.43
for 2b, 3.86 for 2c, 3.42 for 2d), which is in line with earlier
observations on similar phosphorus-based tetracenes.11a As
found in other bis-heteroatom-bridged stilbenes, oxidation of the
sulfur bathochromically shifts and broadens the absorption spectra
(λonset = 421 nm for 3a, 437 nm for 3c, 453 nm for 3d), due to the
electron-withdrawing character of the SO2 group and the con-
comitant increase in conjugation within the system (Figure 5,
bottom). Importantly, the change of absorption through oxidation
of the sulfur (Δλonset = 56 nm between 3a and 2a) is more
significant than functionalization of the phosphorus center
(Δλonset = 32 nm in 3a-d, 45 nm in 2a-d) suggesting that
the band gap of the system is more efficiently controlled by the
electronic nature of sulfur (vide infra).
In the nonoxidized sulfur(II) series 2a-d, functionalization of
phosphorus does not significantly change the emission wave-
lengths of the system (λem = 421 nm for 2a, 400 nm for 2b, 428 nm
for 2c, and 445 nm for 2d) (Figure 6). Similar to the absorption
spectra, oxidation of sulfur has a significantly larger effect on the
emission wavelengths of the system (Δλem = 32 nm between 3a
and 2a, Δλem = 24 nm between 2d and 2a), which indicates that
the decreasing aromaticity of the thiophene ring enhances the π-
conjugation throughout the whole scaffold, essentially convert-
ing the system into a heteroatom-bridged stilbene, instead of an
oligoacene (Chart 2).
62 nm for 2b, 69 nm for 2c, and 71 nm for 2d), which further
supports the delocalization of the phosphorus lone pair in the
excited state (Figure 6).13b,16 Compared to 2a, the narrow
and blue-shifted emission of 2b is probably due to both, its
weaker electron donor and acceptor character that is consistent
with the theoretical calculations. Moreover, the larger Stokes shift of
2a also implies the existence of structural relaxation in the excited
state.16
In addition, functionalization of the phosphorus center in the
sulfur dioxide series 3, results in further red-shifted emission
features (3a: λem = 453 nm, 3c: λem = 466 nm, 3d: λem = 483 nm)
due to the increasing electron-withdrawing character of the
phosphorus center 3a f 3d (Figure 6). All compounds in the
sulfur dioxide series 3 show similar full widths at half-maximum
(fwhm = 82 nm for 3a, fwhm = 78 nm for 3c, fwhm = 83 nm for
3d). In general, the Stokes shift of the phosphole oxide species in
both the sulfur(II) series 2a-d and the sulfur dioxide series
3a-d are smaller than those of the nonoxidized species indicat-
ing a rigidified system upon oxidation of phosphorus resulting in
less rotational freedom for the exocyclic phenyl group due to the
partial cationic charge at the pentavalent phosphorus center.
Since oxidation and/or functionalization of the sulfur and
phosphorus centers were found to change the electronic nature
of the whole π-conjugated scaffold significantly, we decided to
also study the solvatochromism of the heterotetracenes (see
Supporting Information). However, not all of the compounds do
show significant solvatochromism in their in absorption spectra
(Δλmax = 5 nm for 2a; 2 nm for 2b; 2 nm for 2c; 5 nm for 2d,
8 nm for 3a; 6 nm for 3c). In the nonoxidized sulfur(II) series,
In CH2Cl2, the emission of 2a is very broad (full width at half-
maximum (fwhm) = 95 nm) relative to its congeners (cf.: fwhm =
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Figure 5. UV-vis absorption spectra of the sulfur(II) series 2 (top) and
the sulfur dioxide series 3 (bottom) in CH2Cl2.
Figure 6. Normalized emission of the heterotetracenes in CH2Cl2 (top:
sulfur(II) series 2; bottom: sulfur dioxide series 3).
2c shows the strongest solvent dependent emission properties
(λem = 417 nm in cyclohexane; 428 nm in CH2Cl2; 427 nm in
MeCN, and 434 nm in EtOH), which is consistent with the
formation of a dipole between the sulfur (donor) and the
phosphorus oxide (acceptor) centers (vide infra). Remarkably,
compared to the somewhat related trivalent 1,4-dihydro-1,4-
thiaphosphinine (V, E1 = S, E2 = lp, Chart 1; λem = 476 nm
in CH2Cl2 and 446 nm in EtOH),13b trivalent phosphole 2a
does not show significant solvatochromism in either absorption
(λabs = 319 nm in cyclohexane, 317 nm in CH2Cl2, 314 nm in
EtOH, and 314 nm in CH3CN) or emission (λem = 416 nm in
cyclohexane, 421 nm in CH2Cl2, 421 nm in EtOH, and 425 nm
in CH3CN), probably due to a stronger delocalization of the
phosphorus lone pair within the system (see theoretical section).
The sulfur dioxide compounds 3a and 3d show stronger solva-
tochromism in their fluorescence emission relative to the non-
oxidized sulfur(II) species 2a,d (see Supporting Information).
The comparatively stronger solvatochromism in the emission of
3a and 3d (relative to the absorption) indicates that oxidation of
sulfur induces a more polarized structure in the excited state than
in the ground state.16 It needs to be mentioned, however, that 3d
is not stable in EtOH, which is probably due to the weakening
of the phosphonium center and central CdC bond by the
oxidation of sulfur allowing the EtOH to nucleophilically attack
the phosphorus center.10c,17
Chart 2. Conjugation-Path Change upon Modification of the
Phosphorus and Oxidation of the Sulfur Center
In the series 2a-d and 3a-d, the phosphorus oxide and
phosphonium species (2c: φPL = 0.71; 2d: φPL = 0.36, 3c: φPL
=
0.98, and 3d: φPL = 0.99) generally show higher photolumines-
cence quantum yields than their congeners. Most of the com-
pounds show lifetimes between τ = 10-15 ns, except for 2a and
2b that have lifetimes τ = 3.6 ns and τ = 2.6 ns, respectively. It
should be noted that compounds 2a, 3a, and 3d show biexpo-
nential-decay profiles (see Supporting Information) that were
confirmed through excitation at varying wavelengths. Likely causes
for this feature could be intramolecular charge transfer processes
involving the exocyclic P-phenyl substituent and the main
scaffold, or conformational changes involving the P-center and/or
its phenyl substituent; in either case one lifetime component was
clearly dominant (Table 2). The quantum yields and lifetimes
allowed for the determination of relaxation processes from the
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Table 2. Electrochemical Data for the Heterotetracenes
E
red [V]a
E
ox [V]a
(Epa)
HOMO
[eV]c
Compd
(Epc1/pc2
)
EA [eV]b LUMO [eV]c
2a
-
0.88
-
-
-
-1.35
-1.77
-1.74
-5.53
-5.90
-5.80
2b
2cd
-1.45
-1.56 f
-
-3.04
(-1.60/
-1.52)
2dd,e
3ad
-1.35
-1.84 f
-
-
-2.64
-2.11
-6.64
-6.07
0.64
-2.76
(-2.01/
-1.67)
3c
-1.44 f
-
-3.16
-2.97
-6.65
Figure 7. Rate constants of the tetracene series 2 and 3 in CH2Cl2.
(-1.58/
-1.29)
and 3d (Aπ = 8.96) relative to the others (Aπ = 3.44 for 2a, 4.11
for 2b, 4.02 for 2d, 2.75 for 3c) also indicates that a positive
phosphorus center (i.e., phosphorus oxide or phosphonium)
increases the conjugation in the S1 state of the heterotetracenes
and provides a higher quantum yield (see theoretical section).18
2.4. Electrochemistry. The redox properties of the hetero-
tetracenes can also provide valuable information on how the
functionalized phosphorus and sulfur centers change the electro-
nic properties of the system as a whole.13b The electrochemical
properties of compounds 2a-d and 3a,b,d are summarized in
Table 2 (see Supporting Information for CV spectra). Compared
to BTBT that shows a reversible oxidation at Eox = þ0.89 V (vs
Fc/Fcþ),14a 2a shows an irreversible oxidation Eox = þ0.88 V (vs
Fc/Fcþ) supporting a certain degree of delocalization of the
phosphorus electron lone pair. By contrast, borane adduct 2b
only shows an irreversible reduction at Ered = -1.45 V, due to
decreasing charge density at the phosphorus center that switches
the redox properties of heteroacene core. Phosphole oxide 2c
exhibits a reversible reduction potential at Ered = -1.56 V, which
is in line with other phosphole oxides reported by us earlier.11f
The methylation affords a more positive phosphorus center,
resulting in an irreversible reduction at Ered = -1.35 V that is also
consistent with the lower LUMO energy of 2d obtained from the
theoretical calculations (vide infra). As provided by the photophy-
sical studies, the system is switched to a diheteroatom-bridged
stilbene system upon oxidation of the sulfur center. Sulfur dioxide 3a
shows a quasi-reversible reduction potential at Ered = -1.84 V
and an irreversible oxidation at Ered = þ0.64 V. The reduction
process of 3a could be rationalized by the electron-accepting
character of the sulfur dioxide center, which is consistent with the
more positive sulfur center (vide infra). The oxidation of phosphorus
further enhances the electron-accepting character of the system,
which is evident in a quasi-reversible reduction at Ered = -1.44 V
for 3c. As for 3d, two reduction processes were observed, one at
Ered = -1.11 V and another at Ered = -1.53 V. Compared to 3c,
the first more positive reduction of 3d indicates that the methylation
of phosphorus further enhances the electron-accepting character
of the system as a whole.
3dd,e
4a
-1.11, -1.53
-
-
-
-3.51
-2.34
-7.11
-6.15
f
-2.07
-2.53
(-2.20/
-1.93)
-2.14 f
(-2.29/
-1.99)
-1.78 f
(-1.89/
-1.68)
4b
5a
-
-
-2.46
-2.82
-2.22
-2.91
-6.13
-6.57
a Measured in CH2Cl2, with a Pt rod as working electrode, Pt wire as
counter electrode, and Ag/AgCl/KCl 3 M as reference electrode;
supporting electrolyte was NBu4PF6; Standard scan rate was 50 mV
s-1, Ered(Eox) = 1/2(Epc þ Epa) for reversible or quasi-reversible
process, otherwise Ered(Eox) = (Epa). b Electron affinities were
calculated according ref 20. c Calculated at the B3LYP-6/31G(d)-
level of theory. d X-ray crystallographic data were used as input files
(counteranions were not included). e Polarizable continuum model
(PCM, solvent = dichloromethane) was used. f Reversible or quasi-
reversible redox process.
excited state (Figure 7). Since the fluorescence emission spectra
that are governed by the main scaffold do not show any discernible
fine structure that would result from competing processes, the
average lifetimes were used for the calculation of the relaxation
processes of 2a, 3a, and 3d.16 In 2a, the large nonradiative rate
constant Knr = 21.3 ꢀ 10-7 s-1 is the major contributor to the
low quantum yield. P-complexation with borane increases the
radiative rate constant in 2b, thus increasing the quantum yield.
The both oxidation and methylation of the phosphorus center
decrease the nonradiative rate constants significantly (2c: Knr
=
3.6 ꢀ 10-7 s-1, 2d: 4.3 ꢀ 10-7 s-1), which are likely the major
cause for the higher quantum yield in the series 2a-d. As for the
sulfur dioxide series 3a-d, the higher quantum yields of 3c
and 3d are mainly due to both larger radiative constants (3c: Kr =
7.6 ꢀ 10-7 s-1, 3d: 7.8 ꢀ 10-7 s-1) and smaller nonradiative
constants (3c: Knr = 0.2 ꢀ 10-7 s-1, 3d: 0.1 ꢀ 10-7 s-1). An
interesting trend is that, starting from 2a (φPL = 0.24) with
sulfur(II) and phosphorus(III) centers, the fluorescence quan-
tum yield increases more than three times when the phosphorus
center is oxidized (2c: φPL = 0.71), while the quantum yield
The redox potential (next to thin film morphologies19) further-
more provides an indication on the potential charge injection
character of the new materials that is important for practical
device applications. Since all of the P-oxidized phosphole derivatives
show reversible or quasi-reversible reduction, the solid-state
electron affinity (EA) of the compounds was estimated from the
electrochemically obtained electron injection (Table 2).20 3c
shows the highest EA value (ca. -3.2 eV) approaching that of the
actually decreases upon oxidation of the sulfur center (3a: φPL
=
0.14). Subsequent oxidation of the phosphorus center in 3c, in
turn, increases the quantum yield again (φPL = 0.98). The larger
extent of π-conjugation (Aπ) of 2c (Aπ = 5.48), 3c (Aπ = 8.24),
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Figure 8. Frontier orbitals of BTBT, 2a, 2c, and 3c (B3LYP/6-31G(d) level of theory).
well-established electron-transport material C60 (ca. -3.7 eV).21
Furthermore, 2c and 3c show higher EA values (EEA = ca. -2.8
eV for 2c, ca. -3.0 eV for 3c) than that of Alq3 (-2.3 eV)21 that
is widely used as an electron-transport material in organic light-
emitting diodes.
2.5. Theoretical Calculations. Theoretical calculations were
carried out at the B3LYP/6-31G(d) level of theory22 to reveal
further details on the electronic nature of the new heterotetra-
cenes; the energies of the HOMO and LUMO levels are summarized
in Table 2. As reported previously, the frontier orbital energy
levels can efficiently be modified by functionalization of the
phosphorus center. Unlike in the dithienophosphole system,10,11f an
interaction between the phosphorus lone pair and the π-system
can be observed in the HOMO of 2a (Figure 8). A similar
interaction between sulfur and the π-system that is also observed
in BTBT (III, E1 = E2 = S, Chart 1) supports the high degree of
delocalization within the ring and could explain the similar
absorption spectra of BTBT and 2a.
However, the symmetry of the HOMO and LUMO orbitals of
the phosphole analogues are distorted upon functionalization of
the phosphorus center (BH3, O, and Meþ), which is very likely
the reason for the different absorption features of 2b, 2c, and 2d
relative to 2a. Oxidation of sulfur converts this center into an
electron acceptor, which in turn reestablishes the symmetry of
the HOMO and LUMO orbitals in the sulfur dioxide series 3
(Figure 8; full details in the Supporting Information). Although
the HOMO of 3a also shows an interaction between the lone pair
of phosphorus and the π-system, absorption and emission features
support a more stilbene-like structure that is also consistent with
the molecular structure of 3a in the solid state. Moreover, the
rigidified stilbene architecture enhances conjugation and narrows
the band gap of the system. TD-DFT calculations were carried
out in order to elucidate the transitions observed in the UV-vis
spectra (see Supporting Information). All of the compounds
were optimized at the B3LYP/6-31G(d) level theory and, where
applicable, using the X-ray data as input files (2c, 2d,23 3c, and
3d;23 see Supporting Information). The trend in the calculated
S0fS1 transition correlates well with the optical band gaps (see
Supporting Information), except for 3a, which is probably due to
different conformations involving the pyramidalization of the
phosphorus center and/or the exocyclic phenyl ring that exist in
solution.13b As a consequence of the distorted orbital symmetry
in the sulfur(II) series 2, the S0fS1 transition of 2a only originates
from HOMO f LUMO, while the S0fS1 transitions of 2b, 2c,
and 2d also involve other energy levels (for details, see Supporting
Information), highlighting the impact from the distorted sym-
metry of the molecular orbitals.
Other important information that could be obtained from the
theoretical calculations relates to the charge distribution within
the various derivatives. As expected, the phosphorus center becomes
more positive through increasing its valency and concomitantly
its coordination number (Table 3). The oxidation of sulfur
decreases the electron density at this center significantly; as
observed in the optical and the calculated band gaps, the more
positive the sulfur center, the smaller the band gap of the P,S
tetracenes. Surprisingly, in contrast to the nonoxidized sulfur(II)
system, 3c actually exhibits a higher electron density at phos-
phorus when compared to that of 3d. These differences could be
rationalized in terms of the oxidation of sulfur enhancing the
communication between the sulfur and phosphorus centers due
to the resulting relatively stronger electron acceptor character,
i.e., S(VI) vs P(V), creating an inverted push-pull system. These
features transform the oxidized phosphorus center into a donor,
while the sulfur center remains the acceptor component. To
counterbalance the electronic deficit at phosphorus, therefore,
the generally very polar nature of the PO double bond (i.e.,
Pþ;O-) is reduced leading to a less polar form (i.e., PdO) with
increased electron density at P, which is consistent with the NMR
spectroscopy (vide supra). On the other hand, the spread between
the Mulliken charges at the sulfur and phosphorus atoms is
increased by the oxidation of sulfur, which is consistent with the
solvatochromism in the emission of these compounds. In addi-
tion to the `rigidifying’ effect of functionalized phosphorus, the
electronic nature of phosphorus shows a similar trend as the
fluorescence quantum yield; i.e., a more positive phosphorus
center affords a higher quantum yield (Figure 9).
2.6. Functionalization of the Core. The high EA values of
the heteroacene cores encouraged us to further improve the
materials’ properties, such as solubility, solid state organization, and
film morphologies, which also play important roles in the fabrication
and properties of organic electronics.19 Alkylcarbonyl and per-
fluorophenylcarbonyl substituted oligothiophenes have been
shown before to not only exhibit high charge carrier mobilities
(p-type and n-type) but also facilitate solution-based device fabrica-
tion, which is very desirable with a view to low-cost processing.24
Therefore, we were interested if the new heterotetracences could
also be functionalized in a similar way and how the functionalization
would influence the electronic and self-organization features of
the core. Corresponding Friedel-Crafts acylation reactions were
carried out between 2c and suitable acyl chlorides (Scheme 4).
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Table 3. Mulliken Charge Distributions; B3LYP-6/31G(d)
Level of Theory
Scheme 4. Functionalization of the Core of 2c via Friedel-
Crafts Acylation
Mulliken Mulliken charge of
Mulliken
charge
charge
phosphorus
(sulfur)
Compound of sulfur
of oxygen
Δ|P - S|a
BTBT
2a
0.234
0.224
0.242
0.236
0.314
1.133
0.234
0.409
0.572
0.820
0.669
0.399
NA
0
NA
0.185
0.330
0.584
0.325
0.734
2b
2cb
2db,c
3ab
NA
-0.545 (OdP)
NA
-0.488 (OdS)
-0.502 (OdS)
-0.352 (OdS)
-0.371 (OdS)
-0.474 (OdP)
-0.462 (OdS)
-0.465 (OdS)
3c
1.867
0.417
1.45
3db,c
1.160
0.698
0.484
a Dipole strength calculated from the difference between Mulliken
charge of sulfur and phosphorus. b X-ray crystallographic data were used
as input files (counteranions were not included). c Polarizable conti-
nuum model (PCM, solvent = dichloromethane) was used.
related dithienophosphole system.25 As expected, the carbonyl
group influences the photophysical properties in the extended
heterotetracenes (Table 1). The lowest energy absorption max-
ima of the carbonyl compounds (4a: λmax = 359 nm, 4b: λmax
=
359 nm, 5: λmax = 368 nm) are red-shifted with respect to those
of 2c (Figure 10), which is consistent with the decreased
HOMO-LUMO gaps from the theoretical calculations (ΔE =
3.81 eV for 4a, 3.91 eV for 4b, 3.66 eV for 5). Notably, in the
“ortho” isomer 4b, the higher energy transition (at λmax
=
290-320 nm) is blue-shifted compared to that of 2c and shows
a smaller molar absorptivity than the low energy transition, while
the opposite is true for the “para” isomer 4a.
Although it is still not exactly clear what causes the spectral
splitting at this stage, it is inherently plausible that the different
locations of the acyl substituents are the reason for the altered
electronic structures in 4a/4b. On the other hand, the emission
features of all three acylated compounds (λem = 427/428 nm) are
similar to those of 2b, and their low photoluminescence quantum
yields (φPL = 0.13-0.29) are likely due to the quenching effect of
the carbonyl moieties that has also been observed for aldehyde
and ketone functionalized dithienophospholes.25,26 Similar to
the other phosphole oxides in the series 2a-d and 3a-d, all
carbonyl-substituted derivatives exhibit one quasireversible re-
duction process (see Supporting Information for CV spectra).
Compound 5 shows the most positive reduction at Ered = -1.78
V (vs Fc/Fcþ) due to the electron-poor perfluorophenyl group;
the hexanoyl isomers show reduction potentials at Ered = -2.07
V for 4a and -2.14 for 4b (vs Fc/Fcþ). EA values of the extended
cores (EEA = -2.53 eV for 4a, -2.46 eV for 4b, -2.82 eV for 5)
are again higher than that of Alq3 (-2.3 eV), albeit lower than the
nonextended tetracenes (vide supra).
We were able to obtain suitable single crystals for X-ray
crystallography of the acyl-extended compounds 4a/4b and 5
from dichloromethane. Unfortunately, the structural parameters
of 4b were not good enough for a satisfying quantitative analysis
of its molecular structure; however, the qualitative structural
information still supported the position of the acyl substituent.
To confirm the connectivity in 4b, 2D NMR spectroscopy was
carried out that further confirmed the position of the acyl
Figure 9. Trends between fluorescence quantum yield and the electro-
nic nature of phosphorus; red: DFT calculated Mulliken charge of
P/10-2; blue: 31P NMR shift (CDCl3 at 298 K)/ppm; green: fluorescence
quantum yield/%.
Surprisingly, the reaction between 2c and hexanoyl chloride
(even when applied in 3-fold excess) only resulted in the
monosubstituted isomers 4a/4b as the major products in a ratio
of 2:3 that could be isolated through column chromatography.
This reactivity is likely due to the electron-withdrawing effect of
the oxidized phosphorus center, which decreases the reactivity of
the benzene ring adjacent to the phosphole unit. The thienoben-
zene, on the other hand, is more electron-rich and thus activated
for an electrophilic attack. The reaction between 2c and 2 equiv
of perfluorobenzoyl chloride also gave monosubstituted 5 as the
only major product and further supported the deactivating effect
of phosphorus oxide; in either case, only the thienobenzene ring
can be acylated. Remarkably, monofunctionalization through
Friedel-Crafts acylation is also observed with the somewhat
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Figure 12. Molecular structure of 5 in the solid state (50% probability
level); hydrogen atoms are omitted for clarity. Selected bond lengths [Å]
and angles [deg]: P1-O1: 1.478(3); P1-C7: 1.797(5); P1-C14:
1.828(5); P1-C21: 1.809(5); C7-C8: 1.368(6); C5-C6: 1.411(6);
C9-C14: 1.392(7); S1-C5: 1.753(4); S1-C8: 1.720(5); C7-P1-
C14: 91.5(2); C7-P1-C21: 106.7(2); C14-P1-C21: 106.4(2); C5-
S1-C8: 90.4(2).
Figure 10. Absorption spectra of the extended acylated heterotetra-
cenes vs 2c.
long axis of the main tetracene scaffold precipitated from the
solvent mixture (acetone/hexane: 4/1) in the shape of fibers
upon slow evaporation at room temperature. Using optical and
fluorescence microscopy, microfibers with lengths of ca. 30-40 μm
and widths of up to 5 μm could indeed be observed (Figure 13a
and b). Scanning Electron Microscopy (SEM) of the precipitated
4a (Figure 13c and d) further confirmed the formation of 1D
microfibers in the solid state, but also that they consist of bundles
of smaller fibers. The red-shifted solid-state absorption of the
microfibers compared with that of 4a in CH2Cl2 solution
(Figure 14) suggests that π-π interactions play a role in the 1D
self-assembly process. Such 1D organic nano-/micromaterials
have recently attracted significant attention, due to their efficient
energy transfer and charge carrier transport features, as well as
high ionic conductivity.27 By contrast, the “ortho” hexanoyl isomer
4b does not show a similar self-organization process into 1D
microfibers. Under the same conditions (acetone/hexane: 4/1),
4b only precipitates as a crystalline powder, as verified via SEM
(Figure 13e and f).
Exposing the nonextended heterotetracene cores (2a-d
and 3a,b,d) to similar conditions (acetone/hexane: 4/1; room
temperature) did not afford the analogous precipitation of 1D
microfibers. Instead, amorphous powders or polycrystalline
solids were obtained only upon complete evaporation of the
solvents. To further investigate the self-organization process, 4a
and 4b were also exposed to a variety of different solvents (neat
acetone, dichloromethane, ethyl acetate, and THF), which also
did not result in the precipitation of similar 1D microfibers during
evaporation at room temperature, but a similar behavior as that
for the nonextended core systems (amorphous powders/poly-
crystalline solids only after complete evaporation of the solvents).
Notably, the 1H NMR spectrum of the dissolved fibers of 4a
reveals that acetone is part of the fibers, as a corresponding signal
at 2.17 pm can be observed that integrates to a ratio of 3:1 (4a/
acetone). To further support the presence of acetone, the X-ray
diffraction (XRD) pattern of the 4a microfibers shows distinctly
different structural features from the bulk crystals of 4a (see
Supporting Information). By contrast, the XRD of the crystalline
sample of 4b (from acetone/hexane: 4/1) shows an almost identical
structure pattern to that of the bulk crystals (see Supporting
Information). To gain some deeper insight into the interaction
Figure 11. Molecular structure of 4a in the solid state (50% probability
level); hydrogen atoms are omitted for clarity. Selected bond lengths [Å]
and angles [deg]: P1-O1: 1.478(18); P1-C7: 1.796(2); P1-C14:
1.807(3); P1-C21: 1.798(2); C7-C8: 1.364(3); C5-C6: 1.419(3);
C9-C14: 1.400(3); S1-C5: 1.753(2); S1-C8: 1.723(2); C7-P1-
C14: 92.02(12); C7-P1-C21: 109.12(11); C14-P1-C21: 107.61(11);
C5-S1-C8: 90.27(12).
substituent (see Supporting Information). The structural data for
4a and 5, on the other hand, were acceptable for quantitatively
determining their molecular structure in the solid state
(Figures 11 and 12).
The carbonyl group does not change the structure of hetero-
tetracene core very much. In 4a, P-C (P1-C7: 1.796, P1-C14:
1.807, and P1-C21: 1.798 Å) and S-C (S1-C5: 1.753 and
S1-C8: 1.723 Å) bonds are similar to those of 2c. However, the
pentafluorophenyl group in 5 is perpendicular (at 86.6°) to the
tetracene core, which explains the small absorption red shift of
this derivative relative to 2c. Due to the perpendicular penta-
fluorophenyl arrangement suppressing potential extended inter-
molecular π-stacking interactions, 5 only associates into dimers
inthesolidstate(π-πdistance: 3.6 Å; see Supporting Information).
Interestingly, the position of the hexanoyl group also has a
significant effect on the macroscopic self-organization of 4a and
4b in the solid state. When attempting to grow crystals for X-ray
crystallography, compound 4a with the hexanoyl group along the
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Figure 13. Images of the microstructures of 4a ((a) polarized optical microscopy; (b) fluorescence confocal miscroscopy; (c and d) SEM at different
scales) and 4b ((e and f) SEM at different scales).
between 4a and acetone in the microfibers, we have performed
additional IR and 13C NMR spectroscopy experiments, but
potential electrostatic or hydrogen-bonding interactions could
not be detected. In turn however, slow evaporation of acetone
transition in the second heating scan (see Supporting Information).
The “ortho” isomer 4b, on the other hand, shows a significantly
different thermal transition behavior. In the first and second
heating scan, a sharp endothermic transition (melting) could be
observed at Tm = 195.8 °C. But unlike 4a, the first cooling process
results in a sharp shape exothermic transition (Tc = 140.5 °C)
indicating the crystallization of 4b (see Supporting Information).
Based on these initial studies, the exclusive formation of 1D
microfibers from 4a can be rationalized by three common factors
generally employed for self-organizing acenes:27,28 (1) the polar
moieties (PdO and CdO) and solubilizing group (alkyl chain)
in heterotetracenes could direct molecules to self-assemble along
a certain direction through intermolecular interactions (dipole-
dipole interaction, π-π stacking, and H/π edge-to-face inter-
actions). Similar dipole-dipole effects have been found in dialkoxyl
substituted acene systems; (2) the correct solvent mixture containing
“good” (acetone) and “bad” (hexane) solvents helping the forma-
tion of 1D microfibers through solution-based self-organization
processes; and (3) the appropriate linear rod-coil structure of
1
from the fibers could be detected by H NMR, showing only
about 12% of residual acetone after about a month. The latter
suggests that acetone may simply be trapped in the voids of the
solid-state organization between the molecules of 4b without any
directed polar interactions. This feature could also be supported
by Differential Scanning Calorimetry (DSC) of the microfibers
of 4a. DSC also confirmed the presence of acetone, showing not
only a sharp melting transition at Tm = 169.0 °C (see Supporting
Information) but also a broad endothermal transition around 87 °C
(the evaporation of acetone) in the first heating scan; thermo-
gravimetric analysis (TGA) corroborates the loss of acetone at
67.2 °C. In the second heating scan, only a broad glass transition
at Tg = 46.2 °C can be observed, which is similar to the DSC results
for the nonfunctionalized cores (2b,c,d and 3a,c,d) showing a
sharp melting transition in the first heating scan and a broad glass
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chains provides access to intriguing 1D-microstructures and thus
opens the door for phosphole chemistry to be used in self-
assembling organic π-conjugated materials. Systematic studies
toward corresponding phosphole-based systems are therefore
currently underway in our group and will be reported in due
course.
’ ASSOCIATED CONTENT
S
Supporting Information. Full experimental details for the
b
synthesis of all new compounds. CIF files of all structures
determined by X-ray crystallography and intermolecular packing
interactions of all crystallographically characterized compounds.
Solvatochromism data and TD-DFT data for compounds 2a-d,
3a,c,d. Frontier orbitals for compounds 2a-c, 3a,b,d, 4a,b, and 5
(B3LYP/6-31G(d)-level of theory). Cyclic voltammograms for
compounds 2a-d, 3a,c,d, 4a,b, and 5. Thermal (DSC and TGA)
traces of compounds 4a,b and 5. SEM images of the microfibers
of 4a. XRD and IR spectra of 4a and 4b. Fluorescence lifetime
measurements for compounds 2a-d and 3a,c,d. This material is
available free of charge via the Internet at http://pubs.acs.org.
Figure 14. Absorption of the fibers of 4a, as solid and dissolved in
CH2Cl2.
4a (as opposed to the “nondirectional”, skewed molecular
structure of 4b) for different favorable 1D-directional intermo-
lecular interactions. In order to generate further well-defined
micro-/nanostructures from this new heterotetracene system,
detailed investigations on the specific molecular design (degree
of conjugation, alkyl chain length), solvent effects (polarity and
concentration), and temperature effect during the self-organiza-
tion processes are now being carried out in our laboratory.
’ AUTHOR INFORMATION
Corresponding Author
Thomas.baumgartner@ucalgary.ca
’ ACKNOWLEDGMENT
Financial support by NSERC of Canada and the Canada
Foundation for Innovation (CFI) is gratefully acknowledged.
We also thank Alberta Ingenuity, now part of Alberta Innovates -
Technology Futures for a graduate scholarship (Y.R.) and a New
Faculty Award (T.B.). Thanks is given to Prof. C. P. Berlinguette
for access to fluorescence instruments and Prof. T. Sutherland for
his help with the electrochemical studies. The authors would also
like to thank Dr. T. Gordon and P. G. Bomben for help with
fluorescence quantum yield and lifetime experiments, Prof. S.
Trudel for help with the SEM of 4a, and Prof. V. Thangadurai as
well as J. Kan for the access to powder X-ray diffraction.
3. CONCLUSIONS
In summary, we have synthesized a new phosphorus-based
heterotetracene core as a new member of the oligoacene family
and have extended the system through acylation. This structure-
property study further revealed the effect of incorporation of
phosphorus centers into ladder-type π-conjugated systems. For
the first time, we could demonstrate that sulfur as the secondary
heteroatom can also be oxidized readily. The different electronic
natures of the heteroatoms allowed independent functionaliza-
tion of the phosphorus and sulfur centers, respectively. Although
the oxidation of the sulfur center is not reversible under the
employed conditions, the phosphorus center can reversibly be
modified in both directions (oxidation and reduction). There-
fore, the reactivity of the latter de facto provides for full control
over the oxidation states of both heteroatom centers. UV-vis
spectroscopy and fluorescence studies showed that the oxidation
of sulfur not only influences the electronic contribution of
phosphorus but also alters the general conjugation pathway of
the heterotetracene system (from benzothienobenzophosphole
to stilbene), which is supported by 31P NMR spectroscopy and
X-ray crystallography, as well as theoretical calculations. More
importantly, the theoretical studies allowed us to correlate the
photophysical properties with the different electronic natures of
sulfur and phosphorus. In this dually switchable system, the band
gap of the system is mainly controlled by the electronic nature of
sulfur, while the photoluminescence quantum yield highly depends
on the electronic nature of phosphorus. Cyclic voltammetry
revealed that the redox properties of these S,P heterotetracenes
can be altered by the electronic nature of heteroatoms. Particu-
larly, the electron-accepting character and higher electron affinity
of the oxidized S/P species make them promising candidates for
n-type semiconducting materials. Furthermore, the functional-
ization of the phosphorus-based tetracene core with long alkyl
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