Synthesis and characterization of cross-conjugated cruciforms with varied functional groups

Article abstract of DOI:10.1016/j.tetlet.2009.06.119

A series of novel oligo(p-phenylenes) (OPPs) derivatives with bisdiene side-chains are synthesized and fully characterized. These cruciforms are soluble in common organic solvents and show good thermal stability. Absorption and emission spectra are record

Full text of DOI:10.1016/j.tetlet.2009.06.119

Tetrahedron Letters 50 (2009) 5311–5314
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and characterization of cross-conjugated cruciforms with varied
functional groups
*
Hairong Li, Suresh Valiyaveettil
Department of Chemistry, 3 Science Drive 3, National University of Singapore, Singapore 117 543, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel oligo(p-phenylenes) (OPPs) derivatives with bisdiene side-chains are synthesized and
fully characterized. These cruciforms are soluble in common organic solvents and show good thermal sta-
bility. Absorption and emission spectra are recorded to study the effect of different electron-withdrawing
and electron-donating groups on photophysical properties. Powder XRD shows crystalline structures for
all the cruciforms. Single crystal XRD studies show tight-packing along the planar bisdiene axis but sig-
nificant twisting along phenylene units, which correlate the role of functionalization along the bisdiene
axis in tuning the band gaps of the cruciforms.
Received 7 April 2009
Revised 16 June 2009
Accepted 26 June 2009
Available online 2 July 2009
Keywords:
Cruciform
Cross-conjugated
Single crystal
Liquid crystal
Ó 2009 Elsevier Ltd. All rights reserved.
Conjugated organic molecules have gained significant interest
due to the possibility of developing functional materials by intro-
ducing different functional groups.1 Besides linear conjugation, re-
search efforts are being focused on exploring extended cross-
conjugation and omni-conjugation, which offer interesting photo-
physical, electrochemical, and self-assembly properties.2 However,
conjugated polymers often suffer from the inevitable defects incor-
porated during synthesis, which interrupt the effective conjugation
length. Furthermore, due to strong aggregation, it can be difficult to
elicit accurate information regarding the role of individual conju-
gated side-chains as their effect can be masked by the whole poly-
meric system. Hence, we are interested in designing structurally
well-defined and soluble cross-conjugated molecules. It is interest-
ing to study the effects of various functional groups, introduced on
different segments, on the overall photophysical properties of the
molecule. Herein, the design, synthesis, and characterization of a
series of cruciforms with different electron-donating and elec-
tron-withdrawing groups on both segments are reported.
In order to design two cross-conjugated segments, polyaromatic
derivatives as one chain and bisdienes as another chain were cho-
sen along with the incorporation of alkyl chains to improve solubil-
ity. The bisdiene segments were synthesized via Wittig–Horner
reactions between trans-cinnamaldehyde or 3-(2-pyridyl)acrolein
(Scheme 1).3
The Pd(0)-catalyzed Suzuki coupling between 4 or 5 and the
corresponding boronic acids 6–10 (Table 1) in DMF/H2O (3:2) sol-
vent mixture afforded cruciforms O1–O10 in low yields (Scheme 2,
Table 1). We found that protonation of the pyridyl group by an
equal amount of TFA made 5 soluble in DMF, so a weaker base,
NaHCO3 (aq 2 M), was used then for the synthesis of O1–O5, to
minimize deprotonation and to obtain the target compounds O6–
O10 in reasonable yields. THF/H2O (3:2) as the solvent system gave
similar yields but toluene/H2O (3:2) failed to afford the products.
All the cruciforms were readily soluble in common organic sol-
vents such as toluene, CH2Cl2, CHCl3, THF, and DMF but not in hex-
ane, MeOH, or EtOH. The bisdienes all existed in the trans
conformation as observed by NMR and single crystal XRD. All the
products were thermally stable up to 310 °C (TG in Table 2). The
results indicated that the electron-withdrawing nitro and pyridine
groups tended to influence the stability.
Br
Br
NBS, CCl4
reflux, 2 h
PO(OEt)2
Br
P(OEt)3
150 OC, 3 h
Br
O
Br
Br
Br
Br
40%
95%
(EtO)2OP
1
2
Ph3P=CHCHO
54 %
O
N
N
3
X
t-BuOK, DMF
r.t. 6 h
or
O
2
Br
Br
3
70-80%
4
5
X = CH
X = N
X
* Corresponding author. Tel.: +65 6516 4327; fax: +65 6779 1691.
E-mail address: chmsv@nus.edu.sg (S. Valiyaveettil).
Scheme 1. Synthesis of phenyl and pyridyl-substituted bisdiene segments.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.06.119

5312
H. Li, S. Valiyaveettil / Tetrahedron Letters 50 (2009) 5311–5314
Table 1
1.2
Cross-conjugated cruciforms O1–O10
O1
O2
O3
O4
O5
1.0
Entry
R
Cruciform, yield (%)
0.8
S
6
O1 (39), O6 (30)
0.6
0.4
7
8
O2 (60), O7 (48)
OC6H13
0.2
H13C6 C6H13
0.0
O3 (50), O8 (42)
300
350
400
450
500
Wavelength (nm)
C6H13
N
O1
O4
O6
O9
9
O4 (38), O9 (35)
1.2
1.0
0.8
NO2
10
O5 (43), O10 (27)
0.6
0.4
0.2
0.0
DMF/H 2O,
K2CO3, Pd(0)
70 oC, 48 h
350 400 450 500 550 600 650
O1-O5
6-10
4
5
Wavelength (nm)
DMF/TFA/H 2O,
NaHCO 3, Pd(0)
70 oC, 48 h
Figure 1. Normalized absorption (top) and emission spectra (bottom) of the
cruciforms in THF (10À5 M). Insets are photos of the fluorescence.
O6-O10
6-10
(nitro, pyridine, etc.) on the absorption maxima was limited. The
fluorescence spectra showed emission maxima within the range
442–469 nm for O1–O5, while the peak maxima of O6–O10 were
in the range 488–510 nm. The latter series with the pyridine moi-
ety on the bisdiene chain showed ca. 40–50 nm red shifts in the
emission maxima. We can only attribute this to the influence of
the electron-withdrawing pyridyl group. Also, O1–O5 showed blue
emissions while the remainder gave near-green emissions (Fig. 1).
These results indicated that different functional groups on the
2D-conjugated backbone of the cruciforms have moderate effects
on the overall photophysical properties. Cruciform O7 showed rel-
atively higher emission maxima, probably due to extended conju-
gation, less steric hindrance as well as the electron-donating
effects of the alkoxy groups. Products O3, O4, O8, and O9 with
bulky alkyl chains should influence the planarity of the molecular
structure and overall conjugation with a blue shift in emission
maxima. Since the two sets of cruciforms only differ in the struc-
ture of the bisdiene segment, it would be anticipated that the influ-
ence of this segment on the photophysical properties would be
more significant compared to the oligoarene segment. Quantum
yields (QY) were found to be low for all the products in solution,
and which made emissions very weak in the solid state (therefore
data were not accurate and not recorded). These were attributed to
strong intermolecular interactions and energy loss during reso-
nance energy transfer from an excited donor to a fluorescent
acceptor, providing large Stokes’ shifts, as is evident in Table 2.
Powder XRD analysis revealed that all the cruciforms were
highly crystalline in the solid state (Fig. 2). Products O1–O5 had
similar spectra to those of O6–O10 (see Supplementary data) with
no pretreatment of the samples. It was surprising that O3 and O8
bearing a bishexylfluorene group showed ordered packing. Cruci-
forms O4 and O9 bearing a hexylcarbazole group were less-orga-
nized in the powder state and the XRD peaks were not well
resolved, probably due to the spatial flexibility of the hexyl group.
N
R
R
R
R
O6-O10
O1-O5
N
Scheme 2.
Table 2
UV–vis (kabs), emission (kem), band gap (BG), quantum yield (QY), and decomposition
temperature (TG) of O1–O10
Cruciform
O1
O2
O3
O4
O5
kabs (nm)
kem (nm)
BG (eV)
QY (%)
394
467
3.14
3.9
405
469
3.06
7.7
388
447
3.19
9.4
392
442
3.16
7.3
399
454
3.10
4.8
TG (°C)
270
280
289
310
239
O6
O7
O8
O9
O10
kabs (nm)
kem (nm)
BG (eV)
QY (%)
415
510
2.98
3.6
426
509
2.91
8.1
412
501
3.00
10.3
288
414
488
2.99
6.6
425
498
2.91
5.0
TG (°C)
243
282
253
229
UV–vis and fluorescence spectra were recorded for all the cruci-
forms (Fig. 1) and the results are summarized in Table 2. Strong
absorptions were observed in the range 388–405 nm for O1–O5
and 412–426 nm for O6–O10. This indicates that the electron-
withdrawing pyridyl group reduced the band gap by lowering
the LUMO level.4 However, the effect of different functional groups

H. Li, S. Valiyaveettil / Tetrahedron Letters 50 (2009) 5311–5314
5313
The oligoarene unit was highly twisted due to the steric hindrance
between the ortho protons of neighboring rings (67° relative to the
central phenyl ring for O5, Fig. 3). The strong twisting along the oli-
goarene segment also affected the effective conjugation and led to
the large structure change from the ground state to the first excited
state upon excitation and therefore the large Stokes’ shifts. Never-
theless, these cross-conjugated cruciforms were shown to have
lower band gaps compared to linear oligoarenes and alkoxy-substi-
tuted oligo(p-phenylene vinylene)s.5
Packing inside the crystal lattice of O5 showed that molecular
planes were parallel to each other with similar segments being
face-to-face. The 3-nitrophenyl segments were highly twisted,
but parallel to each other. An interplanar distance of 3.7 Å indicates
O1
O2
O3
O4
O5
5
10
15
20
2θ
25
30
35
40
that the intermolecular p–p interaction could be the main driving
force for ordered packing; it is also one of the factors resulting in
the low QY. From powder XRD and single crystal studies, it appears
that such 2D-conjugated cruciforms might aggregate and organize
in the lattice.
Figure 2. Powder XRD spectra of O1–O5.
Differential scanning calorimetry (DSC) studies showed that
cruciforms O2 and O3 had obvious glass transition behavior and
the mesophases were also identified by polarized optical micros-
copy (POM). A nematic phase for O2 (Schlieren texture) and smec-
tic-A phase for O3 were observed (Fig. 4).6 The rigid mesogenic
units not only force the molecules to take on an extended confor-
Cruciforms O2, O3, O7, and O8 showed a sharp peak at a low angle
(6–7°), which was assigned to the intermolecular distance sepa-
rated by the long alkyl chain.
Understanding the molecular structure of the cruciforms is
imperative for investigating the structure–property relationships.
The single crystal X-ray diffraction study of O5 showed that it
adopted a cruciform shape and the trans-bisdiene bond showed
complete planarity relative to the central phenyl ring. This result
was different from a report2h where the styryl groups showed a
departure from planarity with a twisting of approximately 15°.
T = 83.4 ºC
g
O2
O3
T = 137.1 ºC
T = 49.7 ºC
g
T = 139.8 ºC
-30 -10 10 30 50 70 90 110 130 150 170 190
Temperature (ºC)
Figure 3. Molecular structure and packing of O5. CCDC deposition number:
731098.
Figure 4. POMs of the textures on cooling of O2 (top) and O3 (middle) at a cooling
rate of 0.5 °C/min and the DSC spectra (bottom).

5314
H. Li, S. Valiyaveettil / Tetrahedron Letters 50 (2009) 5311–5314
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Supplementary data
Experimental details, characterization data, and a detailed re-
port of the single crystal of O5. Supplementary data associated
with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2009.06.119.
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