Aryl substitution of pentacenes

Article abstract of DOI:10.3762/bjoc.10.178

A series of 11 new pentacene derivatives has been synthesized, with unsymmetrical substitution based on a trialkylsilylethynyl group at the 6-position and various aryl groups appended to the 13-position. The electronic and physical properties of the new pentacene chromophores have been analyzed by UV-vis spectroscopy (solution and thin films), thermoanalytical methods (DSC and TGA), cyclic voltammetry, as well as X-ray crystallography (for 8 derivatives). X-ray crystallography has been specifically used to study the influence of unsymmetrical substitution on the solid-state packing of the pentacene derivatives. The obtained results add to our ability to better predict substitution patterns that might be helpful for designing new semiconductors for use in solid-state devices.

Full text of DOI:10.3762/bjoc.10.178

Aryl substitution of pentacenes
Andreas R. Waterloo1, Anna-Chiara Sale1, Dan Lehnherr2, Frank Hampel1
and Rik R. Tykwinski*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 1692–1705.
1Department of Chemistry and Pharmacy & Interdisciplinary Center
for Molecular Materials (ICMM), University of Erlangen-Nürnberg
(FAU), Henkestrasse 42, 91054 Erlangen, Germany and 2Department
of Chemistry, University of Alberta, Edmonton, AB T6G 2G2, Canada
doi:10.3762/bjoc.10.178
Received: 06 February 2014
Accepted: 25 June 2014
Published: 28 July 2014
Email:
Rik R. Tykwinski* - rik.tykwinski@fau.de
This article is part of the Thematic Series "Functionalized
carbon-nanomaterials".
* Corresponding author
Guest Editor: A. Krueger
Keywords:
carbon-nanomaterials; organic semiconductor; pentacene; π-stacking;
polycyclic aromatic hydrocarbon; solid-state structure
© 2014 Waterloo et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
A series of 11 new pentacene derivatives has been synthesized, with unsymmetrical substitution based on a trialkylsilylethynyl
group at the 6-position and various aryl groups appended to the 13-position. The electronic and physical properties of the new
pentacene chromophores have been analyzed by UV–vis spectroscopy (solution and thin films), thermoanalytical methods (DSC
and TGA), cyclic voltammetry, as well as X-ray crystallography (for 8 derivatives). X-ray crystallography has been specifically
used to study the influence of unsymmetrical substitution on the solid-state packing of the pentacene derivatives. The obtained
results add to our ability to better predict substitution patterns that might be helpful for designing new semiconductors for use in
solid-state devices.
Introduction
Conjugated organic molecules are promising candidates for use nity. Substituted pentacenes can offer several advantages to
in optoelectronic applications including OLEDs [1], photo- other small molecule semiconductors, including stability,
voltaics [2], and OFETs [3]. Even though there is literally an processability, and semiconductor device performance. Inter-
infinite number of possibilities to chemically assemble small molecular π–π-interactions between chromophores can be
organic molecules for such applications, clever design and syn- dramatically improved upon functionalization at the 6- and
thesis have rapidly offered new materials for the nascent era of 13-positions of pentacene, as demonstrated by the two-dimen-
molecular electronic [4-11]. Prominent p-type organic semicon- sional (2D) bricklayer-packing motif for TIPSPc [13,14]. A
ductors include substituted acenes in general [12-15], and number of well-designed substitution patterns for the pentacene
specifically 6,13-(triisopropylsilylethynyl)pentacene (TIPSPc) skeleton have been realized to date [17-25], and pentacene
[13,14]. The latter was synthesized by Anthony over a decade derivatives that provide good semiconductor device perfor-
ago [16], but it is still a leading focus of the scientific commu- mance often show similar 6,13-substitution patterns (Figure 1a)
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[12,26]. The most reactive positions of the acene core are the 6- tion with SnCl2/H2SO4 without further purification, ultimately
and 13-positions [27-29], and these positions can be effectively yielding pentacene products 3a–h. While the isolation and char-
blocked by different trialkylsilylethynyl units [30]. Inspired by acterization of diol products resulting from nucleophilic addi-
previous studies in which we [18,31], and others [32-34], tions to pentacenequinone has been possible [17,18,34],
observed promising solid-state arrangement and properties of previous work has shown that aromatized products were more
aryl-substituted pentacenes, we were interested in the influence easily purified by column chromatography and recrystallization
of different aryl moieties directly linked to the pentacene core. following the last step [30,31,36]. Thus, it was deemed
In this work we present a simple synthetic approach to unsym- procedurally more efficient to eliminate the purification
metrically substituted pentacenes via stepwise functionalization step of the intermediate diols. Once formed, pentacenes 3a–h
of pentacenequinone, using a nucleophilic aryl group were obtained in moderate yield over two steps, as deep-blue
(Figure 1b). To determine the influence of aryl substitution, the solids.
obtained pentacenes have been studied by a variety of spectro-
scopic characterization methods as well as X-ray crystallog- To expand the π-system in a linear fashion along the short
raphy of eight derivatives. The studies reported herein offer an molecular axis of the pentacene core, the general procedure
opportunity to interpret various properties of acenes based on described above was changed slightly, and ketone 4a was
their substitution and should contribute toward the use of such treated with a solution of biphenylmagnesium bromide in THF.
derivatives in materials science.
After work-up and isolation of the intermediate diol 5i, reduc-
tive aromatization gave pentacene 3i in moderate yield over the
two steps. Elaborating on the general idea of lateral functional-
ization, iodoaryl pentacene 3g offered an opportunity to vary
the pendent substituent through a Pd-catalyzed cross-coupling
Results and Discussion
Synthesis and Characterization
Synthesis
The synthesis of arylpentacenes was developed based on the protocol (Scheme 2). Thus, pentacene 3g was treated under
known ketone derivatives 4a and 4b, formed through the addi- Suzuki–Miyaura coupling conditions with arylboronic acids,
tion of an acetylide to pentacenequinone (Scheme 1) [21,35-38]. and the desired pentacenes 3j,k were obtained in yields of 92
With these two ketones in hand, a second nucleophilic addition and 68%, respectively. Notably, anthracenyl-substituted
was initiated. Thus, commercially available aryl halides pentacene 3k was the least stable of all derivatives synthesized
dissolved in dry THF were subjected to lithium halogen in this study. It slowly decomposed in solution when exposed to
exchange at −78 °C using n-butyllithium. A substoichiometric ambient laboratory conditions and was unstable toward silica
amount of n-BuLi was used in each case to ensure complete gel chromatography. Compound 3k could, however, be puri-
consumption of the n-BuLi through Li–halogen exchange and fied by recrystallization from a mixture of MeOH and acetone.
thus avoid the possibility of competitive addition of the nucle-
ophilic n-Bu anion to the ketone group of either 4a or 4b. After UV–vis spectroscopy
reaction with the appropriate aryl lithium species, the reaction Aryl pentacenes 3a–k have been studied by UV–vis absorption
was quenched with a proton source, and the resulting diol inter- spectroscopy in CH2Cl2 solutions and as thin films cast from
mediates 5a–h were carried on directly to reductive aromatiza- CH2Cl2 onto quartz. Solution-state UV–vis spectra show char-
Figure 1: a) Examples of common pentacene functionalization patterns and b) unsymmetrically aryl-substituted pentacenes reported in this work.
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Scheme 1: Synthesis of unsymmetrically substituted pentacenes by nucleophilic addition (yields given are for the pentacene product 3, over the two
steps from either 4a or 4b).
Scheme 2: Functionalization of iodoaryl pentacene 3g using the Suzuki–Miyaura cross-coupling reaction.
acteristic absorptions in the high-energy region with a As can be seen in the representative spectra in Figure 2, the
maximum intensity absorption centered at ~310 nm, as well as nature and size of aryl substituents at the 13-position does not
low-energy absorptions at ~578 nm and ~622 nm. In compari- alter the basic absorption wavelengths as one progresses, for
son, unsubstituted pentacene (PEN) shows a λmax = 576 nm example, through the series of phenyl (3a), naphthyl (3b) and
(measured in benzene) [39], while the symmetrically substi- anthracenyl (3c), although some differences in molar absorp-
tuted analogue TIPSPc shows a low-energy absorption at tivity are observed. Similar trends are observed within the series
643 nm (measured in CH2Cl2) due to extended conjugation of pentacenes 3i, 3j, and 3k. Thus, these UV–vis spectra clearly
through the two alkyne substituents [11].
document the lack of communication between the aryl
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substituent and the pentacene unit, as a result of hindered rota- results in rather significant scattering versus the formation of
tion about the aryl–pentacene C–C bond and a preferred con- films by spin-casting, only milligram quantities of material are
formation in which the π-system of the aryl group is orthogonal required and the results are qualitatively informative (i.e.,
to that of the pentacene. The orthogonal orientation of the aryl absorption wavelengths can be readily discerned, while
groups is also well-documented in the solid state by X-ray crys- determination of molar absorptivity is not possible). As can be
tallographic analysis (vide infra). The fluorescence characteris- seen in Figure 3, spectra from thin films show absorption
tics of 3a–k are unremarkable, showing only minor variances in profiles similar to those from solution-state measurements,
emission wavelength ranging from 652–671 nm, as measured in although signals are broadened and absorptivities vary dramati-
CH2Cl2 (see Supporting Information File 1 for details and cally due to scattering. In the solid state, there are no observed
spectra).
absorptions at wavelengths beyond ca. 660 nm, and absorptions
in the lower energy region show a red shift (7 to 33 nm) in com-
parison to analogous absorptions in solution. A red shift in the
absorption features of samples in the solid state relative to those
in solution is typically ascribed to a local electronic interaction
Figure 2: UV–vis spectra of pentacenes a) 3a–c and b) 3i–k
(measured in CH2Cl2).
The major electronic absorptions found in solution are also
reproduced to a large extent in spectra obtained from solid-state
films. Pentacene samples 3a–k were drop-cast from a concen-
trated CH2Cl2 solution onto a quartz surface, and after air-
drying, the absorption spectra were measured by UV–vis spec-
troscopy (Figure 3 and Table 1). While this method sometimes
Figure 3: UV–vis spectra of thin films (drop cast on quartz from a
CH2Cl2 solution) for pentacenes a) 3a–c and b) 3i–k.
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Table 1: Optical properties of pentacenes 3a–k, unsubstituted pentacene (PEN), and TIPSPc.
Compound
λmax (in CH2Cl2)a [nm]
λmax (film)b [nm]
red shift [nm] (meV)
Egap,opt [eV]c
3a
621
621
623
623
621
622
621
622
623
623
623
576d
643f
637
635
637
637
634
655
628
638
641
633
635
673e
705g
16 (50)
14 (44)
14 (44)
14 (44)
13 (41)
33 (100)
7 (22)
1.89
1.91
1.92
1.91
1.89
1.87
1.90
1.91
1.88
1.89
1.89
2.15
1.84f
3b
3c
3d
3e
3f
3g
3h
16 (50)
18 (56)
10 (31)
12 (38)
97 (310)
62 (170)
3i
3j
3k
PEN
TIPSPc
aLowest energy absorption maxima. bCast from CH2Cl2 onto quartz. cDetermined from solution-state spectra, based on a tangent line applied to the
lower edge of the longest wavelength absorption peak and the intercept with the x-axis. dMeasured in benzene and data taken from [39]. eData taken
from [41,42]. fData taken from [11]. gData taken from [35].
between the respective pentacene molecules in the solid state. groups, as well as the absence of strong π-stacking for both
Of the aryl pentacenes studied here, veratrole derivative 3f derivatives in the films, even though X-ray crystallographic
shows the largest red shift (33 nm) as a film compared to its analysis suggests that strong interactions might be possible for
solution-state UV–vis spectrum, although the origin of change some derivatives in the solid state (vide infra).
in the solid state is not understood. It is worth noting, however,
that more significant red-shifted λmax-values are often observed Thermal analysis
for samples which give solution cast films that result in signifi- The thermal stability of selected aryl pentacenes has been
cant π-stacking between molecules, such as TIPSPc, in which explored by traditional melting point analysis (MPA) in open
λmax shifts from 643 nm in solution to ca. 705 nm in the solid capillary tubes, thermal gravimetric analysis (TGA), and differ-
film [11,35,40]. This same logic also suggests that the minimal ential scanning calorimetry (DSC) measurements; the results
difference between the absorption characteristics of 3c and 3h are summarized in Table 2. TGA shows clearly that significant
results from both a lack of influence from the different silyl mass loss occurs in the range of 400 °C, which is also common
Table 2: Thermal properties of a representative selection of pentacenes.
TGA Td
/°Ca
MPA
mp /°Cb
DSC mp
(DSC dp) /°Cc
Compound
R
Aryl
177
(178/179)
3a
iPr
370
370
162–165
244–246
3b
iPr
248
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Table 2: Thermal properties of a representative selection of pentacenes. (continued)
3c
3d
3h
3i
iPr
iPr
Et
375
372
410
380
306–308d
316–318d
291–293
211–214
197e
–
(206/247)
287
(288/290)
iPr
225
3j
iPr
380
233–235
220f
aMeasured under a nitrogen atmosphere. Td = decomposition temperature, see Supporting Information File 1 for details. bTraditional open capillary
melting point analysis (MPA), measured under ambient conditions; uncorrected. cMeasured under a nitrogen atmosphere; dp = decomposition point,
shown as onset/peak temperatures. dDecomposition observed in that temperature range, with no indication of melting or decomposition at lower
temperature. eEndotherm, although apparently not a true mp based on traditional mp analysis. Exotherm at 286 °C (peak) likely corresponds to dp in
DSC. fThe strongest of several endotherms.
for ethynylated pentacenes such as 1 and 2 [17,18]. There Fc/Fc+). Aryl-substituted pentacenes 3a–k each show a one-
appears to be little evidence of a trend based on the size of the electron reversible oxidation event in the range between
aryl group versus the temperature of observed mass loss in the 0.30–0.37 V (Table 3), and a second oxidation process (quasi-
TGA. By comparing the TGA data with that of MPA made in reversible) in the range of 0.80–0.99 V. There is, unfortunately,
open capillary tubes, however, it is clear that all pentacene no clear trend observed for the oxidation potentials based on the
derivatives undergo either a phase change or decomposition substitution pattern of the aryl moieties, although both oxida-
prior to the mass loss observed in the TGA. This premise is also tions appear somewhat easier for pentacene 3f as a result of the
confirmed by DSC analyses, which show a melting point in all two electron-donating methoxy groups attached to the pendent
cases except for thienyl derivative 3d (which decomposed phenyl ring.
directly in the solid state). In the case of 3a and 3h, melting is
followed immediately by decomposition (DSC and TGA scans Aryl-substituted pentacenes 3a–k each show one reversible
are provided in Supporting Information File 1).
reduction event in a rather narrow range of −1.59 to −1.68 V.
Similar to that observed for the oxidation potentials, there is no
While no correlation between pendent substituent and stability obvious trend that can be identified in the reduction potentials
emerges from this analysis, an important point is nevertheless based on substitution pattern, aside from the observation that
noted, as exemplified by the examination of 3c and 3d. Tradi- the silyl substituent might have a slight impact on reduction (3h
tional MPA is often insufficient for characterization of is slightly harder to reduce than 3c), and the reduction of thienyl
pentacene derivatives, in which subtle changes in the samples derivative 3d (−1.59 V) stands out as lower than the others.
can be difficult to discern because of the deep, dark color of the
sample, and conflicting results are often observed between Substituted pentacenes 3a–k are slightly easier to oxidize than
MPA and DSC.
TIPSPc (Eox1 = 0.39 V), and the Eox1 values of 3a–k fall into a
similar range as found for pentacene-based PAH dyads 2a–c in
which the pendant aryl groups are linked by an ethynyl spacer
Cyclic voltammetry
Electrochemical analysis by cyclic voltammetry (CV) was used that allows electronic communication between the two arenes
to investigate the electronic properties of pentacene derivatives [18]. The range of oxidation potentials between TIPSPc, 2a–c,
3a–k in CH2Cl2 (ca. 1.5 mM) using tetrabutylammonium hexa- and 3a–k is, however, quite narrow, suggesting that the pendent
fluorophosphate as supporting electrolyte and ferrocene as substituent offers little influence on the HOMO level. On the
internal standard (all potentials reported are thus given versus other hand, there is a marked difference in the observed reduc-
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Table 3: Electrochemical properties of 3a–k compared to TIPSPc and 2a–c.a
Compound
R
Aryl
Eox1 [V]
Eox2 [V]
Ered1 [V]
−1.63
Egap,el [eV]b
3a
iPr
0.34
0.87
1.97
3b
3c
3h
iPr
iPr
Et
0.37
0.36
0.32
0.99
0.93
0.91
−1.61
−1.65
−1.68
1.98
2.01
2.00
3d
3e
iPr
iPr
0.35
0.32
0.87
0.87
−1.59
−1.68
1.94
2.00
3f
iPr
0.30
0.80
−1.67
1.97
3g
3i
iPr
iPr
0.34
0.32
0.87
0.87
−1.65
−1.66
1.99
1.98
3j
iPr
0.32
0.93
−1.67
1.99
3k
iPr
iPr
0.35
0.39
0.88
−1.67
−1.44
2.02
1.83
2ac
–
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Table 3: Electrochemical properties of 3a–k compared to TIPSPc and 2a–c.a (continued)
2bc
iPr
0.39
–
−1.42
1.81
2cc
iPr
iPr
0.33
0.39
–
−1.38
−1.52
1.71
1.91
TIPSPc
0.99
aCyclic voltammetry was performed in CH2Cl2 solutions (1.5 mM) containing 0.1 M n-Bu4NPF6 as supporting electrolyte at a scan rate of 150 mV/s.
Platinum wire was used as counter electrode, Ag/AgNO3 as reference electrode, and Pt working electrode. The potential values (E) were calculated
using the following equation E = (Epc + Epa)/2, where Epc and Epa correspond to the cathodic and anodic peak potentials, respectively. Potentials are
referenced to the ferrocene/ferrocenium (Fc/Fc+) couple used as an internal standard. All potentials represent a reversible one-electron reduction or
oxidation event. bElectrochemical HOMO–LUMO gaps determined by Egap,el = Eox1 − Ered1. cData taken from [18].
tion potentials. Compounds 2a–c are most easily reduced while
3a–k are the most difficult, and the reduction of TIPSPc falls at
approximately a midpoint between the two other classes. Thus,
the biggest influence of the pendent substituent appears to be
related to the energy of the LUMO.
As suggested by the UV–vis analyses (vide infra), the
HOMO–LUMO gap estimated for pentacenes 3a–k by CV
(1.94–2.02 eV) is larger than the HOMO–LUMO gap of
Figure 4: Schematic classification of three common solid-state
TIPSPc (1.91 eV), while incorporation of the ethynyl spacer in
2a–c provides for the lowest HOMO–LUMO gap of the mole-
cules discussed here.
arrangements of pentacene derivatives a) herringbone packing, b) 1D
slipped-stack packing, and c) 2D “bricklayer” packing (as viewed side-
on, approximately down the short-molecular axis).
X-ray crystallographic analysis
With the understanding that the solid-state packing of acenes
Typically, three predominant solid-state packing patterns are can provide vital information about intermolecular interactions,
found by X-ray crystallographic analysis of pentacene and its single crystals of pentacenes 3a–d and 3g–j were grown and
derivatives [13]: a) a herringbone packing, b) a one-dimen- their solid-state arrangements determined by single crystal
sional (1D) slipped-stack, and c) a 2D “bricklayer” packing, as X-ray diffraction analysis (crystallographic details are provided
schematically summarized in Figure 4. While several poly- in Supporting Information File 1). Pentacene 3a crystallizes in
morphs have been reported for unsubstituted pentacene [43-46], the space group C2/c with eight molecules in the unit cell
the arrangement in the solid state is commonly the edge-to-face, (Figure 5). Within the solid-state structure, the pendant phenyl
herringbone motif [47]. This packing arrangement provides ring and the pentacene core are slightly twisted, with a torsion
strong electronic coupling in the solid state, and therefore angle of ~71°. Molecules of pentacene 3a arrange as dimeric
makes this material interesting as the active component for pairs, which then pack into a so-called sandwich herringbone
semiconducting devices [48]. It has been shown that functional- motif (Figure 5c) [50]. Each dimeric pair of pentacenes shows
ization of the pentacene framework, particular by trialkylsilyl- face-to-face π-stacking interactions with an interplanar distance
ethynyl groups, drastically alters the solid-state arrangements of of ~3.40 Å and a total overlap of nearly four of the aromatic
the acenes [30]. In certain cases, this leads to a 2D face-to-face pentacene rings.
“bricklayer” arrangement, which can potentially facilitate
charge transport in an electric device by several orders of Pentacene 3b crystallizes in the space group P-1 with two mole-
magnitude [49].
cules in the unit cell (Figure 6). Notably, the naphthyl unit is
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Figure 5: X-ray crystallographic analysis of 3a showing a) molecular structure and b) packing motif (triisopropylsilyl groups omitted for clarity);
ORTEP drawn at 50% probability level. c) Schematic representation of “sandwich herringbone” packing arrangement.
significantly disordered over two unique positions in the solid pentacene rings overlapping and interplanar distances of
state but is nearly perpendicular to the pentacene core with a ~3.52 Å and 3.46 Å.
twist angle of ~81°. Pentacene 3b assembles in dimeric pairs,
which then arrange in a 1D π-slipped stack motif, and the Pentacene 3d crystallizes in the space group P21/n with four
pentacene molecules are separated by interplanar distances of molecules in each unit cell (Figure 8), and the thienyl and
~3.42 Å and ~3.52 Å.
trialkylsilyl groups show disorder in the structure. With a twist
angle of ~90°, the thienyl unit is essentially perpendicular to the
Pentacene 3c crystallizes in the space group P21/c with four pentacene skeleton. Centrosymmetric dimeric pairs of
molecules in the unit cell (Figure 7) [31]. The pentacene pentacene 3d pack with an interplanar distance of 3.52 Å and
skeleton and the anthracenyl substituent are nearly perpendic- these pairs then arrange into a sandwich herringbone stacking
ular to each other with a twist angle of ~90°. This motif also pattern, similar to phenyl derivative 3a.
places the anthracene moieties in a face-to-face packing 1D
slipped stack arrangement, although the interplanar distance of Pentacene 3g crystallizes in the space group P21/n with four
~3.61 Å is sizable. The aromatic pentacene cores pack in a face- molecules in the unit cell (Figure 9). The phenyl substituent is
to-face 2D bricklayer arrangement, with approximately two twisted with an angle of ~70° relative to the pentacene skeleton.
Figure 6: X-ray crystallographic analysis of 3b showing a) molecular structure and b) packing motif (triisopropylsilyl groups omitted for clarity);
ORTEP drawn at 50% probability level.
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Figure 7: X-ray crystallographic analysis of 3c showing a) molecular structure and b) packing motif (triisopropylsilyl groups omitted for clarity);
ORTEP drawn at 50% probability level.
Figure 8: X-ray crystallographic analysis of 3d showing a) molecular structure, and b) packing motif (triisopropylsilyl groups omitted for clarity);
ORTEP drawn at 50% probability level.
Two neighboring molecules of 3g arrange into a dimeric pair pentacene molecules form channels along the crystallographic
with an interplanar distance of 3.42 Å, and these pairs then pack a-axis, which are composed of only two tiers of a brick wall
in a sandwich herringbone arrangement. The overall solid-state structure. The pentacene molecules within these channels are
arrangement is similar to that observed for 3a and 3d.
stacked with an interplanar distance of 3.57 Å. These channels
are macroscopically arranged as staircases, dictated by the
Pentacene 3h crystallizes in the space group P21/c with four anthracenyl moieties that are oriented such that CH–π interac-
molecules in each unit cell (Figure 10). The anthracenyl tions of ~2.90 Å likely play a role in directing the packing (see
substituent is twisted relative to the pentacene skeleton with an Supporting Information File 1, Figure S12).
angle of ~74°, less than that found for 3c (90°). Interestingly,
pentacene 3h shows an unusual solid-state arrangement not Pentacene 3i crystallizes in the space group P-1 with two mole-
typically observed for pentacene derivatives. Namely, the cules in each unit cell (Figure 11). The benzene ring directly at-
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Figure 9: X-ray crystallographic analysis of 3g showing a) molecular structure and b) packing motif; ORTEP drawn at 50% probability level.
Figure 10: X-ray crystallographic analysis of 3h showing a) molecular structure and b) packing motif (triethylsilyl groups omitted for clarity); ORTEP
drawn at 50% probability level.
tached to the pentacene framework is nearly perpendicular to slipped stack motif along the crystallographic c-axis, with
the pentacene core with an angle of ~81°, while the torsion π-stacking distances of 3.28 Å and 3.35 Å.
angle between the two rings of the biphenyl unit is 32°. The
biphenyl substituent is slightly bent from linearity with an angle Pentacene derivative 3j crystallizes in the space group P-1 with
of ~6° (as estimated from the three atoms designated with an two molecules in the unit cell (Figure 12). Interestingly, the
asterisk *). Biphenyl-substituted pentacene 3i arranges in a 1D pentacene core and the naphthyl group are nearly coplanar (4°),
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Figure 11: X-ray crystallographic analysis of 3i showing a) molecular structure and b) packing motif (triisopropylsilyl groups omitted for clarity);
ORTEP drawn at 50% probability level.
Figure 12: X-ray crystallographic analysis of 3j showing a) molecular structure and b) packing motif (triisopropylsilyl groups omitted for clarity);
ORTEP drawn at 50% probability level.
while the intervening benzene ring is nearly perpendicular to substituents is limited, as a result of the orthogonal orientation
both the pentacene skeleton (~90°) and the naphthyl group of the pentacene backbone and the pendent aryl moieties. Thus,
(~86°). Compound 3j arranges in a 1D slipped stack arrange- these results show that the nature of the substituent does not
ment along the crystallographic b-axis with two different inter- change the electronic properties of the pentacene skeleton itself.
planar distances of ~3.50 Å and 3.32 Å.
Aryl-substitution pattern does however, have a considerable
effect on solid-state arrangement of the molecules, and X-ray
crystallographic analysis afforded insight on the packing
Conclusion
In summary, a library of unsymmetrically substituted arrangements of the synthesized pentacenes. In spite of the large
pentacenes has been synthesized by a straightforward proce- number of crystallographic analyses that have been examined
dure that requires only one purification step. Optical spec- here, general trends are difficult to establish based on, for
troscopy and cyclic voltammetry reveal that electronic commu- example, either the number of π–π and CH–π interactions or the
nication between the pentacene core and the different size of the aromatic group appended to the pentacene core. It
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Beilstein J. Org. Chem. 2014, 10, 1692–1705.
16.Anthony, J. E.; Brooks, J. S.; Eaton, D. L.; Parkin, S. R.
does seem, however, that anthracenyl substitution (in 3c and
3h) affords the highest degree of π-stacking amongst the deriva-
tives examined.
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Supporting Information
Supporting Information File 1
19.Lehnherr, D.; Gao, J.; Hegmann, F. A.; Tykwinski, R. R. J. Org. Chem.
2009, 74, 5017–5024. doi:10.1021/jo9007089
Experimental procedures and characterization data for all
new compounds. Copies of 1H and 13C NMR, UV–vis, and
emission spectra; CV, DSC, and TGA scans.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-178-S1.pdf]
20.Lehnherr, D.; McDonald, R.; Ferguson, M. J.; Tykwinski, R. R.
Tetrahedron 2008, 64, 11449–11461. doi:10.1016/j.tet.2008.09.041
21.Lehnherr, D.; Gao, J.; Hegmann, F. A.; Tykwinski, R. R. Org. Lett.
2008, 10, 4779–4782. doi:10.1021/ol801886h
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23.Kuninobu, Y.; Seiki, T.; Kanamaru, S.; Nishina, Y.; Takai, K. Org. Lett.
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Acknowledgements
The authors acknowledge funding for this work from the
Energie Campus Nürnberg (EnCN), “Solar Technologies go
Hybrid” (an initiative of the Bavarian State Ministry of Science,
Research and Art), and the Deutsche Forschungsgemeinschaft
(DFG) through the Cluster of Excellence “Engineering of
Advanced Materials”. We thank Dr. Michael J. Ferguson for the
X-ray crystallographic analysis of ketone 4b.
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