Phosphole modified pentathienoacene: Synthesis, electronic properties and self-assembly

Article abstract of DOI:10.1039/c1ob06584j

A series of new ladder π-conjugated materials, phosphole modified pentathienoacene (PO-PTA), are synthesized and characterized. Single-crystal X-ray results demonstrate that methyl-disubstituted PO-PTA forms a face-to-face dimer structure driven by π-π interactions. The investigations of optical properties showed that the oxidized phosphole moiety in this ladder system can effectively narrow the band gap. PO-PTA is a promising building block in π-conjugated polymers and oligomers for optoelectronic applications. The derivative of PO-PTA, obtained by introducing four long alkyl chains, can self-assemble into one-dimensional (1D) fibers based on intermolecular π-π interactions, dipole-dipole interactions and van der Waals interactions. Interestingly, the uniform and well-ordered monolayers were also obtained for PO-PTA derivative on a HOPG (highly oriented pyrolytic graphite) surface.

Full text of DOI:10.1039/c1ob06584j

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PAPER
Phosphole modified pentathienoacene: Synthesis, electronic properties and
self-assembly†
Jun-Hua Wan,*a Wei-Fen Fang,a Yi-Bao Li,b,c Xu-Qiong Xiao,a Li-Hong Zhang,a Zheng Xu,a Jia-Jian Penga
and Guo-Qiao Lai*a
Received 16th September 2011, Accepted 17th November 2011
DOI: 10.1039/c1ob06584j
A series of new ladder p-conjugated materials, phosphole modified pentathienoacene (PO-PTA), are
synthesized and characterized. Single-crystal X-ray results demonstrate that methyl-disubstituted
PO-PTA forms a face-to-face dimer structure driven by p–p interactions. The investigations of optical
properties showed that the oxidized phosphole moiety in this ladder system can effectively narrow the
band gap. PO-PTA is a promising building block in p-conjugated polymers and oligomers for
optoelectronic applications. The derivative of PO-PTA, obtained by introducing four long alkyl chains,
can self-assemble into one-dimensional (1D) fibers based on intermolecular p–p interactions,
dipole–dipole interactions and van der Waals interactions. Interestingly, the uniform and well-ordered
monolayers were also obtained for PO-PTA derivative on a HOPG (highly oriented pyrolytic graphite)
surface.
heteroacene (P-PTA: 2a and 2b) based on phosphorus-containing
pentathienoacene by replacing the central sulfur atom of PTA with
Introduction
Ladder p-conjugated systems with fused polycyclic skeletons
are an important class of compounds for organic-electronic
materials because of their effective extension of the p-conjugation
without any conformational disorder.1 Their rigid p-conjugated
frameworks are suitable for forming dense molecular packing
in the condensed phase, which can lead to intriguing electronic
properties, such as high carrier mobility.2 Furthermore, the
fused aromatics as building blocks were usually introduced into
p-conjugated polymers toward organic field-effect transistors
(OFETs) and photovoltaic applications.3
a P atom (see Scheme 1). The formed phosphole, a phosphorus-
bridged cyclic butadiene, has a low-lying LUMO due to the
effective s* (P–R)–p*(1,3-diene) hyperconjugation,6 giving rise
Pentathienoacene (PTA) was attractive because it combined
the molecular shape of pentacene, which leads to a favorable
crystal packing geometry and orientation, with a thiophene
monomer, which would increase stability and nuclear aromatic-
ity and also provide points of attachment for solubilizing
substituents.2a,4 Very recently, a silole modified pentathienoacene
(sila-pentathienoacene, Si-PTA) had been synthesized by us
through replacing the central sulfur atom of PTA with a silylene
moiety.5 In this paper, we reported the synthesis of new conjugated
aKey Laboratory of Organosilicon Chemistry and Material Technology of
Ministry of Education, Hangzhou Normal University, Hangzhou, 310012,
P. R. China. E-mail: wan_junhua@hznu.edu.cn, gqlai@hznu.edu.cn;
Fax: 86-571-28868081; Tel: 86-571-28868081
bNational Center for Nanoscience and Technology, Beijing, 100080, China
cKey Laboratory of Organo-pharmaceutical Chemistry of Jiangxi Province,
Gannan Normal University, Ganzhou 341000, Jiangxi Province, P. R. China
† Electronic supplementary information (ESI) available: The POM, OM
and SEM images of gel, 1H NMR spectra. CCDC reference number
822000. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c1ob06584j
Scheme 1 The synthesis for compounds 3 and 6.
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to intriguing electronic structures. More importantly, the LUMO
level was further reduced through oxidation of the central
phosphorus center of P-PTA and achieved their oxidized relatives
(PO-PTA: 3a and 3b) with quite high thermal stability. Compared
to benzo-condensed dithieno[3,2-b:2¢,3¢-d]phospholes,6c two ter-
minal thiophene rings of PO-PTA provide an opportunity to be
chemically modified for functionalization.
nescence spectra were measured on a Hitachi F-2700 fluorescence
spectrophotometer (the path length of the quartz cell is 1 cm).
The emission band-width was 3 nm. UV absorption spectra were
obtained using a scinco S-3150 UV–vis spectrophotometer. POM
images were recorded on an E600POL (Nikon Corp). FE-SEM
images of xerogels were obtained using FEI Sirion-100 (Philips)
with accelerating voltage 25.0 kV. The samples were prepared by
dropping the gels on a flat surface of a cylindrical brass substrate
and coating with Au. STM measurements were performed by using
a Nanoscope IIIa (Veeco Metrology, USA) with mechanically
formed Pt/Ir (80/20) tips. A droplet (2 mL) of their 1-phenyloctane
solutions was deposited on freshly cleaved HOPG surface and
immediately observed by STM with constant current mode. The
proposed models of molecular patterns were constructed using
HyperChem. HOPG (grade ZYB) was purchased from Veeco
Metrology (USA). Diffraction data of single crystal were obtained
with MoKa (l = 0.71073 nm) at 293 K using an Oxford Diffraction
X-Calibur CCD system.
Current demands in the area of organic semiconductor design
focus on both electronic properties and self-assembling ability
in order to create well-defined nanostructures. Self-assembly
presents a process for controlling the supramolecular packing
and orientation of molecules to obtain the ordered aggregates
in materials sciences.7 The optical and electronic properties of
organic functional materials, which depend to a certain extent
on the molecular packing arrangements through excitonic inter-
actions, can be effectively modulated by self-assembly.8 Recently,
low molecular weight organogelators (LMOGs) as a novel class
of self-assembled materials have attracted much attention, which
produce one-dimensional (1D) stacks of functional molecules
and are easily processable without using advanced techniques.9,10
Self-organized 1D supramolecular structures offer great potential
in optoelectronic applications, such as field effect transistors
(FET)11 and photovoltaics (PV),12 because of their enhanced
device performance. This demand inspired us to introduce four
long alkyl chains into phosphorus-containing pentathienoacene
to obtain the PO-PTA derivative (6). The self-assembly behaviors
for compound 6 in different conditions (in organic solvent and
liquid-solid interface) were investigated.
The obtained interesting ladder p-conjugated materials (PO-
PTA) have a much smaller band gap (2.48 eV) than those of
PTA (3.20 eV) and Si-PTA (2.76 eV). The fused polycyclic
skeletons of this system are inclined to form a face-to-face
stacked structure through p–p interaction and dipole–dipole
interaction. Like DTS3a,d and b-alkylsubstituted fused thiophene,3g
PO-PTA is also a promising building block in p-conjugated
polymers and oligomers for FET and photovoltaic applications.
In addition, PO-PTA bearing long alkyl chains can self-assembly
into different aggregates, such as one-dimensional (1D) fibers
and a two-dimensional (2D) molecular adlayer. This serves as an
excellent example, demonstrating the fabricating of well-defined
nanostructure from p-conjugated ladder system.
∑
Crystal data for 3a. C20H13OPS4 CHCl3, M = 547.88, or-
˚
˚
˚
thorhombic, a = 20.6692(4) A, b = 7.93121(18) A, c = 29.3363(8) A,
◦
◦
◦
3
˚
a = 90.00 , b = 90.00 , g = 90.00 , V = 4809.15(19) A , T =
293(2)K, space group Pca21, Z = 8, 35391 reflections measured,
8740 independent reflections (Rint = 0.0452). The final R1 values
were 0.0576 (I > 2s(I)). The final wR(F2) values were 0.1410 (I >
2s(I)). The final R1 values were 0.0737 (all data). The final wR(F2)
values were 0.1509 (all data). The goodness of fit on F2 was 1.025.
CCDC number CCDC 822000. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre at
www.ccdc.cam.ac.uk/data_ request/cif.
Synthesis of compound 3a. To a solution of 1a (0.24 g,
0.52 mmol) in 150 mL THF, n-BuLi (0.44 mL, 1.09 mmol) was
added dropwise at -78 ◦C and stirred for 20min. Then, PPhCl2
(0.10 g, 0.57 mmol) was added dropwise to the solution at this
temperature, and the solution was quickly warmed to room tem-
perature and stirred for 2 h. After evaporating most of the THF,
the mixture was dissolved into diethyl ether and washed with water
and dried over anhydrous MgSO4. The solvent was removed under
vacuum, the obtained residue taken up in dichloromethane and
filtered over neutral alumina. The filtrate was dried under vacuum
to afford orange amorphous powder. The obtained mixture was
oxidized with H2O2 and stirred at room temperature for another
hour, and the solvent was removed under vacuum. The residue was
extracted with dichloromethane and the organic layer was dried
with MgSO4, and the solvent was removed under vacuum. The
solid was taken up in n-hexane, heated for 10 min, and filtered.
The ◦product 3a was obtained as a red solid in 35% yield. mp:178–
Experimental
General methods
All starting materials were obtained from commercial suppliers
and used as received. Reagent grade solvents were dried and
purified according to the following procedures: THF was dried
over sodium metal and distilled. Compounds 14,5 and 513 were
synthesized according to reported methods.
NMR spectra were recorded on a Bruker DPX 400 (1H NMR
400 MHz and 13C NMR 100 MHz) spectrometer. Matrix-assisted
laser desorption ionization time-of-flight (MALDI-TOF) mass
spectra were obtained using 2, 5-dihydroxybenzoic acid (DHB) as
the matrix on a Perseptive Biosystems Voyager DESTR MALDI-
TOF mass spectrometer. Cyclic voltammetry was carried on a CHI
600A potentiostat. The working and reference electrodes were disk
Pt and Ag/AgNO3 (0.1 M in acetonitrile), respectively. The lumi-
1
180 C. H NMR (400 Hz, CDCl3): d 7.83 (q, J = 7.2 Hz, 2H),
7.53 (t, J = 6.0 Hz, 1H), 7.45–7.25 (m, 2H), 6.99 (s, 2H), 2.37
(s, 6H, –CH3); 13C NMR (100 Hz, CDCl3): d 147.5, 147.2, 143.5,
136.4, 132.8, 130.8, 129.9, 129.2, 192.1, 123.7, 14.7 ppm. 31P NMR
(CDCl3): d 17.85 ppm. MALDI-TOF/DHB-HRMS calcd for
C20H13OPS4: m/z (+H+): 428.9660. Found: m/z (+H+): 428.9661.
Synthesis of compound 3b. Compound 3b was prepared ac-
cording to a similar procedure to 3a. The p◦roduct 3b was obtained
as a red solid in 46% yield.mp:133–135 C.1H NMR (400 Hz,
CDCl3):d 7.85 (q, J = 7.2 Hz, 2H), 7.53 (t, J = 6.0 Hz, 1H),
7.46–7.26 (m, 2H), 6.99 (s, 2H), 2.70 (t, J = 7.2 Hz, 4H), 1.74
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(t, J = 7.2 Hz, 4H), 1.34–1.33 (m, 12H), 0.89 (s, 6H, CH3); 13C
NMR (100 Hz, CDCl3): d 147.2, 142.4, 136.4, 135.3, 132.8, 130.8,
129.7, 129.2, 128.5, 122.9, 31.4, 29.7, 29.0, 29.6, 22.6, 14.1 ppm.
31P NMR (CDCl3): d 18.05 ppm. MALDI-TOF/DHB-HRMS
calcd for C30H33OPS4: m/z (+H+): 569.1225. Found: m/z (+H+):
569.1217.
was obtained. However, as well as for comparison, the other one
with two methyl groups (3a) was also synthesized to get a single
crystal for explaining the molecular packing structure of the p-
conjugated ladder system in solid state.
Generally, the synthesis of the phosphole derivatives (6) in-
volves three procedures, i.e., synthesis of corresponding diiodo-
funtionalized compound 4, synthesis of the 3,4-dialkoxyphenyl
pinacolboronic ester 5, and coupling reaction (see Scheme 1).
Double iodination of phosphole 3b was achieved by treat-
ment with excess N-iodosuccinimide (NIS) to afford 4 in high
yields after purification through column chromatography. 3, 4-
dialkoxyphenyl pinacolboronic ester (5) was synthesized accord-
ing to the literature.13 Compound 4 then underwent 2-fold Suzuki
coupling reactions with the 3, 4-dialkoxyphenyl pinacolboronic
ester to afford the corresponding 2-fold substituted PO-PTA
derivatives 6 in acceptable yields. The structure and purity of the
phosphole 3 and its derivative 6 compound system were confirmed
by 1H NMR, 13C NMR and 31P NMR, MALDI-TOF-HRMS.
Synthesis of compound 4. To a solution of 3b (0.568 g, 1 mmol)
in dichloromethane/acetic acid (1 : 1, 25 mL) N-iodosuccinimide
(0.675 g, 3 mmol) was added at 0 ◦C. After 2.5 h sodium thiosulfate
(10% aq., 20 mL) was added and the product was extracted to
diethyl ether (20 mL), washed with additional sodium thiosulfate
(10%, aq., 20 mL), sodium carbonate (sat. aq., 20 mL) and finally
water (20 mL). The organic layer was dried with MgSO4 and the
solvent was removed under vacuum. The obtained red solid was
purified by column chromatography (silica gel, hexane/DCM =
3 : 1, v/v) to give pure 4 as a red solid in 95% yield. mp:200–
203 ◦C. 1H NMR (400 Hz, CDCl3):d 7.78 (q, J = 7.2 Hz, 2H), 7.56
(t, J = 6.0 Hz, 1H), 7.47–7.45 (m, 2H), 2.70 (t, J = 7.2 Hz, 4H),
1.67 (t, J = 7.2 Hz, 4H), 1.38–1.32 (m, 12H), 0.90 (t, 6H) ppm.13C
NMR (100 Hz, CDCl3): d146.6, 146.4, 140.5, 139.8, 138.9, 133.1,
130.7, 129.3, 31.7, 31.5, 29.0, 28.3, 22.6, 14.1. 31P NMR (CDCl3): d
17.34 ppm. MALDI-TOF/DHB-HRMS calcd for C30H33OPS4I2:
m/z (+H+): 820.9157. Found: m/z (+H+): 820.9165.
Crystal structure
Single crystals of 3a suitable for X-ray analysis were obtained
by slow evaporation from CHCl3 at room temperature. As
shown in Fig. 1, the molecular structure of 3a shows that
the ring skeleton is highly planar. In the crystal structure, two
inversion-related molecules associate through p–p interactions
Synthesis of compound 6. Compounds 4 (1.3 g, 1.58 mmol)
and 5 (2.8 g, 4.75 mmol) were dissolved in toluene (15 mL) and
K2CO3 (2 M, aq., 7 mL). After adding a catalytic amount of
Pd(PPh3)4 (0.03 g, 0.02 mmol), the reaction mixture was refluxed
at 75 ◦C for 24 h. The product was extracted with CH2Cl2 and the
combined organic layer was washed by water, brine, and dried over
MgSO4. The solvent was removed under vacuum and the residue
was purified by column chromatography (silica gel, hexane/ethyl
acetate = 6 : 1, v/v) to ◦give the title product as a red solid in
˚
(centroid . . . ..centroid = 3.63 A) to form a face-to-face dimer.
It is worth noting that the significant interactions between
divalent S atoms and substituting phenyl rings (S . . . p interaction),
usually occurring in protein crystal structures by promoting the
formation of a-helices, were recognized in this crystal structure.14
Therefore, it is convincible that the S . . . p interaction also plays an
important role in stabilizing molecular dimer structure for form-
ing above-mentioned face-to-face arrangement. Interestingly, the
molecular dimers interconnect pairwise through p–p interactions
1
37% yield. Mp: 76–78 C. H NMR (400 Hz, CDCl3): d 7.88
(q, J = 7.2 Hz, 2H), 7.54 (t, J = 6.0 Hz, 1H), 7.47–7.43 (m,
2H), 6.96–6.89 (m, 6H), 4.01 (t, J = 6.4 Hz, 8H), 2.78 (t, J =
7.2 Hz, 4H), 1.83–1.72 (m, 8H), 1.59–1.58 (m, 4H), 1.47–1.25
(m, 84H), 0.88 (t, 18H); 13C NMR (100 Hz, CDCl3): d149.28,
148.99, 143.59, 141.21, 134.24, 130.91, 130.80, 130.42, 129.24,
129.10, 126.96, 121.97, 114.92, 113.59, 69.37, 69.29, 31.93, 29.65,
29.08, 26.05, 22.69, 22.59, 14.06 ppm; 31P NMR (CDCl3): d 18.19
ppm. MALDI-TOF/DHB-HRMS calcd for C90H137O5PS4: m/z:
1456.9086. Found: m/z: 1456.9107.
Results and Discussion
Synthesis
Scheme
1
depicts the synthetic route to phosphorus-
pentathienoacenes (3) and their derivative (6). A key intermediate
1 was synthesized using the reported methodology. The corre-
sponding phospholes (2, P-PTA) were obtained according to the
classical method from compound 1. Thus, compound 1 was treated
with two equivalents of n-BuLi to produce the corresponding
dianion, which was then quenched with PhPCl2. The attained
air-unstable P-PTA were directly transferred to their oxidized
relatives (3, PO-PTA) by oxidation of the central phosphorus
atom with H2O2. By introducing hexyl chains on two terminal
thiophene rings, the soluble phosphorus-pentathienoacene (3b)
Fig. 1 The crystal packing of 3a. (a) side view; (b) side view for S . . . p
interactions; (c) molecular arrangement in single crystal.
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Table 1 Gelation properties of 6 in various solventsa
˚
(centroid . . . ..centroid = 3.85 A) interactions to form an infinite
chain along the axis. Intermolecular interaction is increased by
a face-to-face arrangement of the conjugated molecules thus
beneficial for achieving high carrier mobility.
6
Solvents
state
CGC (mg mL-1)
The packing structure of 3a in the solid state is obviously
different from that of oxidized benzo-condensed dithieno[3,2-
b:2¢,3¢-d]phospholes, which exhibits an edge-to-face herringbone-
type arrangement of molecules in its solid state.6c
hexane
chloroform
CH2Cl2
THF
toluene
P
S
S
S
S
acetone
ethanol
ethyl acetate
G
P
G
6
7
Gelation
Owing to bearing four long alkyl chains, compound 6 was
evaluated for gelation properties in various organic solvents at
a moderate concentration. The “stable to inversion of a test tube”
method was adapted to test their gelation properties.15 Generally,
the gelation process involves three steps. The first is mixing an
organogelator with a solvent at ambient temperature. The second
is warming the mixture to produce a clear solution. Finally, the
solution is cooled slowly to the ambient temperature. Gelation
typically occurred during the cooling step. Compound 6 is readily
soluble in chloroform, CH2Cl2, and THF, but it isinsoluble in some
alcohol and alkane solvents. Among various solvents investigated
here, only acetone and ethyl acetate were induced into gelation by
the above method (see Table 1). Like most reported organogels,
the organogels of 6 in acetone and ethyl acetate also exhibit
thermally reversible sol–gel transitions. The data of the critical
gelation concentration (CGC) for different solvents of the two
compounds are shown in Table 1. The driving forces for gelation
will be discussed below.
a G = gel, S = Solution, P = solution when heated but precipitation after
cooling.
Fig. 2, the xerogels of compounds 6 are composed of fibrous
structures. It is very interesting that the fibers of xerogels formed
many spherical structures with different sizes from the OM images
(Fig. 2b). The typical Maltese cross extinction (birefringence)
observed under POM reveals that the fibers further aggregate into
spherical crystallites (Fig. 2c). A more interesting possibility to
grow spherical crystallites from an organogel is rarely realized.16
The FE-SEM investigations provided the details of morphology of
the xerogels. As shown in Fig. 2d, the spherical domains composed
fibers can also be observed. The intertwisted and interlocked fibers
formed the 3D network structure, as revealed by the large scale FE-
SEM image (Fig. 2e), project from a central core. According to the
AFM image (Fig. 2f), an observed fiber bundle is approximately
0.5 mm wide and incorporates several small fibers. The small fiber
diameter ranged from 100 to more than 200 nm, suggesting that
the observed small fibers are actually bundles of smaller fibers. It is
obvious that the original fibers, aggregates formed from compound
6, proceed to self-assemble into small fibers then the observed fiber
bundles, which forms the backbone of the organogel. Based on
the obtained results and the available literature,17 the mechanisms
Aggregation structure of the gels
The morphology of xerogels of 6, which are prepared by slow
evaporation of acetone from the corresponding organogels, was
investigated by optical microscopy (OM), polarized optical mi-
croscopy (POM), field emission scanning electron microscopy
(FE-SEM) and atomic force microscopy (AFM). As shown in
Fig. 2 (a) Photographic images of a gel of 6 in acetone observed upon UV irradiation at 365 nm. (b) OM image and (c) POM image, (d and e) FE-SEM
images and (f) AFM image of a xerogel formed from 6 in acetone (6 mg mL-1).
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of gelation can be suggested: the gelation of compound 6 with
acetone arises from a multistep process involving (i) the formation
of nuclei, (ii) the growth of branched fibers from these nucleation
points, and (iii) the formation of a three-dimensional network via
the entanglement of dendritic-shaped crystallites.
observed for compound 3a with same comparison. The single
crystal structure of 3a shows there exists a significant p–p stacking.
Based on the exciton-coupling theory, H-aggregates with face-to-
face stacking result in a pronounced blue shift of the absorption
and remarkable spectrum broadening.18 Apparently, this point is
consistent with its crystal structure. There is no dependence of the
absorption maxima on the solvent polarity. The spectrum of 3a
and 3b in THF is the same as that in toluene.
Similarly, compound 6 in solution also showed two distinct
absorption bands with maxima at 321 and 462 nm (Fig. 4). The
remarkable red-shifts of absorptions compared with that of 3a
and 3b attributed to the presence of synthetic modification of the
chromophore. Compared with the absorption of CH2Cl2 solution,
the absorption of gel for 6 exhibits a significant blue-shift with two
absorption bands at 270 nm and 440 nm, respectively.
Optical properties
UV-vis absorption and fluorescence characteristics of the new
ladder compounds 3a and 3b were studied in both solution and
solid state. As for compound 6, the optical properties in the gel
state were also investigated besides those in solution.
UV/Vis absorption spectra of 3a and 3b in toluene solutions
and thin films prepared by a drop-casting method are shown in
Fig. 3. In solution, both 3a and 3b showed two distinct absorption
bands with maxima at 293 and 425 nm. The lower transition
(425 nm) of 3a and 3b are observed at longer wavelength than
the corresponding transition (392 nm) of Si-PTA implying that
oxidized phosphole moiety in compounds 3 effectively narrows
the HOMO–LUMO gaps of their fused p systems (Fig. 3a,b). The
band gap from the absorption edge is around 2.48 eV, which is
remarkably lower than those of PTA2a and Si-PTA5.
Fig. 4 (a) Absorption and photoluminescence spectra of 6 in different
environments, solution and gel. The concentration of the solutions is
kept to be 1 ¥ 10-4 M. The concentration of the gel is 6 mg mL-1. (b)
Photoluminescence spectra of 6 in CH2Cl2 solution and gel with different
concentration.
The large blue-shift for absorption with spectrum broadening
is the typical features of H-aggregates and strongly suggests
the existence of intermolecular p–p interaction in the gel state.
Therefore, the p-stacking as one of driving forces may play a key
role in the formation of organogels.
In toluene, compound 3a and 3b exhibit only one maximum
emission at around 514 nm with high fluorescence quantum yields
(0.58 for 3a and 0.51 for 3b). The emissions of films showed obvious
red shifts (31 nm) in comparison with those in solution. Generally,
the large red-shift of emission can be attributed to the extension of
p-conjugation due to planarizartion of the molecular skeleton or
the intermolecular interaction due to the aggregation (p-stacking),
Fig. 3 Absorption and photoluminescence spectra of (a) 3a and (b) 3b
in different environments. The concentration of all the solutions is kept to
be 1 ¥ 10-4 M. The films were prepared by drop-casting from a solution of
toluene.
The absorption spectra of the films of 3b are similar to the
corresponding spectra in toluene solution, but show a slight
red-shift and clear broadening compared to those in solution,
suggesting the existence of weak intermolecular interaction.
However, a large blue-shift (54 nm) and clear broadening were
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Table 2 Photophysical data and dipole moment for compounds 3a, 3b and 6
Absorption (nm)a
Emission (nm)a
Compound
Non-polar solvent
Polar solvent
Film/Gel
Non-polar solvent (Uf)b
Polar solvent
Film/Gel
m (Debye)c
3a
3b
6
293, 425
293, 425
321, 462
291, 420
290, 423
301, 371
303, 431
270, 440
514 (0.58)
514 (0.51)
559
511
511
542
542
567
6.46 d/5.19
5.50
3.59
a Measured in solution (1 ¥ 10-4 M) and in solid thin film on quartz plates prepared by spin-coating from toluene solution for 3a and 3b (non-polar
solvent: toluene and polar solvent: THF), measured in solution (non-polar solvent: CH2Cl2) (1 ¥ 10-4M) and in gel of acetone with 6 mg mL-1 for 6.
b Fluorescence quantum yield relative to 9,10-diphenylanthracene in cyclohexane. c The dipole moments were obtained on a full optimization calculated
on HF/6-31G(d,p) level based pm3 optimization geometry. d The dipole moment was obtained by carrying out a single point calculated at HF/6-31G(d,p)
level based on the geometry obtained from crystal structure.
maybe both. Ladder p-conjugated systems have fused polycyclic
skeletons, which are highly planar. It is reasonable to believe that
the intermolecular interactions play a dominating role in the red-
shift of emissions.
The dilute solution of 6 exhibited only one fluorescence
maximum with lmax at 559 nm. However, the slight difference is
detected for the fluorescence spectra between the gel and solution.
The emission for gel with lmax at 567 nm exhibited a small red
shift (8 nm) compared with that of the solution. As expected, the
intensity of the emission of 6 in the gel state was obviously reduced
as a result of the formation of H-aggregate. The relative absorption
and emission data were listed in Table 2.
The molecules have an oxidized phosphole ring, creating
the directional arrangement of its local dipole. The extensive
Fig. 5 Cyclic voltammograms of 3a in CH2Cl2 for oxidation. Scan rate:
calculations on dipole moment of model-ring systems, 3a, 3b
100 mV s-1.
and 6 were performed by using the Gaussian 03 program.19 The
dipole moment for 3a were obtained by carried out a single point
calculated at HF/6-31G(d,p) level based on the geometry obtained
On sweeping anodically, 3a undergoes two ireversible oxidation
Ox
peaks. The oxidation potential with E
is 0.73 V (vs Ag/Ag+)
onset
from the crystal structure. Similar data for 3b and 6 were obtained
on a full optimization calculated on HF/6-31G(d,p) level based
pm3 optimization geometry. The long alkyl group was displaced by
a hexyl group in order to reduce calculation time. The dipole mo-
ments are listed in Table 2. All model compounds have fairly large
dipole moments. Upon aggregating, the fused polycyclic skeletons
containing oxidized phosphole rings in this system tend to accept
the inverse and face-to-face array due to the local dipole interac-
tions. This fact is excellently in accordance with the single crystal
structure for compound 3a. Therefore, it is reasonable to consider
that dipole–dipole interactions also play an important role in the
formation of aggregates for all three compounds (3a, 3b, 6).20
In the case of the gel of compound 6, intermolecular hydrogen
bonding does not exist and other strong secondary interactions
are believed to serve as the main driving forces for gelation,
which contains intermolecular p–p interactions, dipole–dipole
interactions and van der Waals interactions between the outside
long alkyl chains.
and obviously shifted negatively compared with that of oxidized
benzo-condensed dithieno[3,2-b:2¢,3¢-d]phospholes, which shows
ambipolar behavior with an irreversible oxidation at Eox = 1.61 V
(vs. Ag/AgCl).6c The HOMO energy level of 3a is estimated from
the onset of oxidation wave to be -5.43 eV based on the referenced
energy level ofAg/Ag+ (4.7eV below the vacuum level). Compared
with the HOMO of PTA (-5.33 eV)2a and Si-PTA (-5.21 eV),5
the slight enhancement of HOMO level for 3a may be ascribed
to the increased electron-acceptor character of the phosphorus
center. Unexpectedly, no invisible reduction peak in 3a is detected.
The reduction potential of 3a was estimated from the difference
between oxidation potential and optical band gap (2.48 eV).
Therefore, the LUMO level can be obtained -2.95 eV, which is
clearly lower than that of Si-PTA (-2.38 eV). Apparently, the
oxidized phosphole moiety exerts a significant impact on LUMO
levels.
2D self-assembly
Electrochemical properties
Scanning tunneling microscopy (STM) is a powerful method to
investigate the 2D assembly of functionalized organic molecules
on surfaces, which provides access to supramolecular structures
at the single molecule level.21 For this purpose, compound 6 was
investigated at the liquid–solid interface.
Compound 6 self-assembled into well-ordered monolayers when
adsorbed from 1-phenyloctane on the HOPG (highly oriented
pyrolytic graphite) surface, as shown in Fig. 6. From the large scale
Due to the negligible effect on electronic properties from periph-
eral alkyl substitutes, only the redox properties of compound 3a
were investigated. The redox properties of 3a were determined by
cyclic voltammetry in solutions (1 mm) of tetrabutyl-ammonium
hexafluorophosphate (TBAPF6, 0.1 m) using a Pt wire as the
counter electrode and a Ag/AgNO3 (0.1 m) as the reference
electrode. The cyclic voltammograms are given in Fig. 5.
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2.48 eV, which is remarkably lower than those of PTA and Si-
PTA. The different investigations showed that the fused polycyclic
skeletons of this system inclined to form face-to-face stacked
structure through p–p interaction and dipole–dipole interaction.
The novel p-conjugated system is a good candidate as a building
block incorporated to p-conjugated polymers for FET and
photovoltaic applications. Interestingly, the derivatives of PO-
PTA, obtained by introducing long alkyl chains, can self-assembly
into different supramolecular structures, such as 1D fibers and
2D monolayers. This study serves as an excellent example,
demonstrating the fabricating of well-defined nanostructure from
ladder p-conjugated materials.
Acknowledgements
This Project was supported by Zhejiang Provincial Natural Science
Foundation of China (Y4110460) and MOE Key Laboratory
of Macromolecular Synthesis and Functionalization, Zhejiang
University (2010MFS03). We also thank Dr Lian-Bin Wu, Prof.
Hua-Yu Qiu and Prof. Li-Jin Shu (Hangzhou Normal University)
for his helpful discussion, Prof. Chen Wang (National Center for
Nanoscience and Technology) for his help of the STM measure-
ment, Prof. Wan-Zhi Chen (Zhejiang University) for his helpful
discussion of single crystal structure.
Fig. 6 (a) A large-scale STM image (55 nm ¥ 55 nm) of 6. (b) A
high-resolution STM image (20 nm ¥ 20 nm) of 6. (c) The proposed
structural model of the ordered array.
STM images of the adlayer, it is noticed that the remarkably bright
spots were arranged uniformly in the whole area. The details of the
adlayers were seen in the high-resolution STM images (Fig. 6b).
Since bright STM images usually stem from strong adsorption of
p-conjugated molecules on HOPG surface, the bright spots were
assigned to the adsorption of the fused polycyclic skeletons of
compound 6 with their molecular planes parallel to HOPG. The
length of each bright spot is about 2.0 0.2 nm, in agreement with
the theoretical value of the p-conjugated backbone of compound
6 (2.1 nm). The relatively darker parts corresponded to the parallel
alkyl chains with lower electronic density. Based on the high-
resolution images, two piles of consecutive alkyl chains at the
two ends of 6 aligned with a trans conformation, and the alkyl
chains are aligned in a comb-like fashion between neighboring
molecules and interdigitated with each other. On the basis of the
above analysis, the unit cells of the adlayers are outlined in Fig. 6b.
The lattice parameters were determined from the STM images to
be a = 2.8 0.1 nm, b = 4.2 0.2 nm, and a = (84 2)◦. The proposed
models for the molecular arrangement models are illustrated in
Fig. 6c.
It is well known that the formation of well-defined adlayer
comes from the balance between molecule-substrate matching-
interaction (adsorption energy) and molecule–molecule interac-
tions, such as alkyl–alkyl interdigitation and head-group interac-
tions. Generally, the interactions between long alkyl chains and
substrate are very strong.22 The formation of the adlayer for
compound 6, shown in Fig. 6, appears to be driven, primarily,
by both adsorption energy and alkyl chain interdigitation, which
determines the detailed assembly configuration of molecules.
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containing pentathienoacene (PO-PTA), are synthesized and
characterized. The band gap from the absorption edge is around
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