Synthesis and electronic properties of isotruxene-derived star-shaped ladder-type oligophenylenes: Bandgap tuning with two-dimensional conjugation

Article abstract of DOI:10.1021/ol9021035

The synthesis of star-shaped ladder-type oligophenylenes Sn (n = 4, 7, 10, 13, and 16), where n is the number of π-conjugated phenylene rings, is reported. The two-dimensional conjugation interactions in Sn result in a lower bandgap for S16 than the curre

Full text of DOI:10.1021/ol9021035

ORGANIC
LETTERS
2009
Vol. 11, No. 21
4942-4945
Synthesis and Electronic Properties of
Isotruxene-Derived Star-Shaped
Ladder-Type Oligophenylenes: Bandgap
Tuning with Two-Dimensional
Conjugation
Jye-Shane Yang,* Hsin-Hau Huang, Yi-Hung Liu, and Shie-Ming Peng
Department of Chemistry, National Taiwan UniVersity, Taipei, Taiwan 10617
jsyang@ntu.edu.tw
Received September 10, 2009
ABSTRACT
The synthesis of star-shaped ladder-type oligophenylenes Sn (n ) 4, 7, 10, 13, and 16), where n is the number of π-conjugated phenylene
rings, is reported. The two-dimensional conjugation interactions in Sn result in a lower bandgap for S16 than the currently known ladder-type
poly(p-phenylene)s (LPPPs).
Phenylene-based oligomers and polymers are potential
electronic materials for use in light-emitting diodes and
polymer lasers.1-4 A variety of phenylene scaffolds, differing
in dimensions and rigidity of the π-backbones, have been
explored1-13 to construct the structure-property relation-
ships. Nevertheless, examples of star-shaped ladder-type (SL)
systems are rare.9 A SL phenylene system demands a core
unit such as truxene (1) and isotruxene (2). We have recently
shown that the para-ortho-branched isotruxene core allows
strong electronic couplings among the branched arms.12,13
We report herein the synthesis and characterization of a series
of isotruxene-based SL oligophenylenes Sn, where n is the
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10.1021/ol9021035 CCC: $40.75
Published on Web 10/05/2009
 2009 American Chemical Society

number of π-conjugated phenylene rings (Figure 1). Our
results show explicitly that exciton in Sn can delocalize over
the whole π-backbone up to 16 phenylene rings, a size that
is beyond the effective conjugation length (ECL) of 11-12
rings defined3,5 by the rod-shaped ladder-type (RL) coun-
terparts (Rn).
ing spiro-fluorene substituted RL systems R′3-R′5 in
CH2Cl2 or toluene.8 This indicates that the larger system R′8
(λabs ) 433 nm and λfl ) 441 nm) reported in the same paper8
can also be used for comparison (see Figures 2b and 2c).
The most important findings of these spectral data are that
(a) Sn (n ) 4, 7, 10, 13, 16) and Rn (n ) 2-5 and 8) fall
in the same linear correlation (r2 > 0.99) for the 0-0 band
energies of either absorption or fluorescence against the
reciprocal of number of π-conjugated phenylene rings (i.e.,
1/n) and that (b) there is no sign of saturation in the plots
even with S13 and S16 (Figure 2b and 2c). The former
Scheme 1. Synthesis of S7 from S4 and Structures of A1-A4
Figure 1. Schematic representation of the structures of oligophe-
nylenes Rn and Sn, where the black dots represent the π-conjugated
phenylene rings with a number of n.
The synthesis of Sn adopts the commonly used strategy
for the synthesis of RL systems.5,6 A typical example by
the case of S7 is shown in Scheme 1, in which the
Suzuki-Miyaura reaction between S4BA and A1 elongates
the π-backbone and the subsequent nucleophilic carbonyl
addition and intramolecular Friedel-Crafts alkylation reac-
tions form the ladder scaffold. The synthesis of building
blocks isotruxene S4 and isotruxene tribromide S4Br was
recently reported.13,14 This synthetic protocol also applied
to S10, S13, and S16 simply by replacing A1 with the longer
arm precursors A2, A3, and A4, respectively. The synthetic
details can be found in the Supporting Information.
Figure 2a shows the electronic absorption and fluorescence
spectra for S4, S7, S10, S13, and S16 in dichloromethane.
Except for the fluorescence spectrum of S4, all the spectra
display well-defined 0-0 vibronic bands, and they undergo
a bathochromic shift with increasing number of phenylene
rings. Oligophenylenes Sn retain the features of high
fluorescence quantum yield (Φfl) generally observed for RL
systems. The corresponding photophysical data are shown
in Table 1. For better comparison, the known RL systems
R2-R56 were prepared and investigated under the same
experimental conditions. We found that the wavelengths of
the 0-0 absorption and fluorescence bands for R3-R5 in
CH2Cl2 are essentially the same as those for the correspond-
observation suggests that excitation is effectively delocalized
over the two-dimensional (2D) π-backbone of Sn in both
the ground- and excited-state configurations. To the best of
our knowledge, similar behavior has not been reported for
the other star-shaped conjugated oligomers.9-12 We could
ascribe the strong electronic couplings among the arms of
(14) Yang, J.-S.; Huang, H.-H.; Lin, S.-H. J. Org. Chem. 2009, 74, 3974–
3977
.
Org. Lett., Vol. 11, No. 21, 2009
4943

Table 1. Photophysical and Electrochemical Data for Sn and
Rn in CH2Cl2
a
c
d
e
λabs
λfl
∆νst
Eox1
(V)
compd (nm) logꢀb (nm) (cm-1
)
Φfl
τfl (ns)
S4
S7
352
412
439
452
458
305
347
375
396
4.19
4.81
5.03
5.21
5.27
3.85
4.63
4.88
5.04
382f
428
453
464
470
312
353
382
403
2231
907
704
572
557
736
490
489
439
0.55
0.73
0.87
0.87
0.89
3.22
1.97
1.38
1.16
1.03
4.04
2.01
1.36
1.12
0.79
0.57
0.48
0.44
0.43
S10
S13
S16
R2
R3
R4
R5
0.41
0.72
0.75
1.02
0.83
0.75
a Maximum of the 0-0 absorption band. b Extinction coefficient of the
0-0 absorption band. c Maximum of the 0-0 fluorescence band. d Stokes
shift defined by the difference of λabs and λfl. e Potentials were recorded vs
ferrocene/ferrocenium. f Estimated from the blue edge of the fluorescence
plateau.
(9.8°) between the two ortho-branched arms, as shown
by the X-ray crystal structure (Figure 4a). A smaller
deviation for S4 in the fluorescence plot (Figure 2c) is
consistent with the expected structural relaxation (pla-
narization) in the excited state. This steric effect is
negligible for S7 and the larger Sn systems, although the
twist is even larger (e.g., 18.6° for S7, Figure 4b) due to
the tolyl substituents. This reflects a dilution of wave
function in the isotruxene core. The increased Stokes shift
(∆νst) on going from Rn to Sn and from S16 to S4 (Table
1) also reflects the steric and the chain length effects,
respectively.
It should also be noted that the absorption spectra of Sn
display more bands at shorter wavelengths (e.g., 300-375
nm for S7) in addition to the 0-0 and 0-1 absorption bands
that mirror the fluorescence spectra. These short-wavelength
absorption bands can be attributed to localized excitation of
the ortho-branched arm of Sn, which contains (n + 2)/3
phenylene rings, as those of S4, S7, S10, and S13 match
reasonably well in positions (306, 356, 386, and 402 vs 305,
347, 375, and 396 nm) with the absorption bands of R2,
R3, R4, and R5, respectively.
Figure 2. (a) Normalized absorption (black) and fluorescence (red)
spectra for S4, S7, S10, S13, and S16, and linear plots of (b)
absorption and (c) fluorescence 0-0 transition energy (eV) and (d)
HOMO and LUMO energy levels (eV) versus the reciprocal of
number of π-conjugated phenylene rings (1/n) for both Sn (red
squares) and Rn (black squares). The red open circle specifically
denotes S4 for the purpose of discussion.
Sn to both the ortho-para-branched isotruxene core and the
torsion-constrained π-scaffold. The latter observation further
indicates that the ECL of 11-12 phenylene rings defined
by RL systems4,5,15 describes only one dimension of the
region allowed for exciton delocalization. We tentatively
propose that the maximum chromophore size of effective
conjugation interactions covers a cyclic region with a
diameter equivalent to the one-dimensional (1D) ECL (Figure
3).16 As a result of the second dimensional conjugation, S16
possesses a fluorescence maximum at longer wavelength (470
vs 464 nm)17 than a recently reported long-chain RL polymer
(i.e., a ladder-type poly(p-phenylene) (LPPP) of Mw ) 62.0
kDa).7
It is noted that with the same number of phenylene rings
for S4 and R4 the former deviate from the line toward a
higher absorption energy (Figure 2b). This could be
attributed to a less planar π-backbone due to a small twist
Table 1 also shows the first oxidation potentials (Eox1) of
Sn (n ) 4, 7, 10, 13, 16) and Rn (n ) 3-5) in dichlo-
romethane determined by cyclic and differential pulse
voltammetries (Figure S1, Supporting Information). A linear
correlation between Eox1 and 1/n was observed (Figure S2,
Supporting Information). Figure 2d shows the corresponding
(15) The exact ECL of ladder-type polyphenylenes (LPPP) is not well
defined. It has also been suggested1 to be 14-15 phenylene rings based on
the fluorescence wavelength determined by single molecule spectroscopy.
It should also be noted that the ECL of nonladder-type poly(p-phenylene)s
was suggested to be larger (n ∼ 20) than that of LPPP.2
(16) According to the model of Figure 3, a saturation of transition energy
might be reached at S19 or S22. However, attempts to prepare them were
unsuccessful, presumably due to poor solubility of the intermediates. New
design and synthesis toward larger and soluble 2D ladder-type polyphe-
nylenes are in progress.
(17) Values reported for compounds in chloroform. The maximum of
fluorescence 0-0 band for S16 in chloroform is the same as that in
dichloromethane.
Figure 3. Model proposed for the effective conjugation size (a two-
dimensional region covered by the yellow circle) of conjugated
polymers illustrated with R16 and S16.
4944
Org. Lett., Vol. 11, No. 21, 2009

plots with the HOMO and LUMO energy levels derived
according to eqs 1 and 2:18
EHOMO ) -(4.8 + Eox1
)
(1)
(2)
ELUMO ) EHOMO + E0,0
where E0,0 is the 0-0 transition energy obtained from the
intersection of normalized absorption and fluorescence 0-0
bands. The larger slope for the HOMO (-2.26) vs LUMO
(1.01) plot reveals that the chain length effect on optical
bandgap is mainly due to perturbations of the HOMO energy
levels. For S7 and the larger Sn systems, more than one
oxidation potential was recorded and the number increases
as the n value increases (Table S2, Supporting Information),
which indicates their ability to accomondate more than one
polarion (radical cation). This phenomenon has been ob-
served and elucidated for the stepladder analogues of Sn.12
In summary, we have synthesized a series of para-ortho-
branched SL oligophenylenes Sn, in which S16 is thus far the
largest monodisperse ladder-type oligophenylene. The observa-
tion of effective exciton delocalization over the 2D conjugated
backbones of Sn opened up a new venue for tuning the optical
bandgap of conjugated oligomers and polymers.
Figure 4. X-ray crystal structures of (a) S4 (top and side views)
and (b) S7 (top view).
Acknowledgment. We thank the National Science Council
of Taiwan, ROC, for financial support.
Supporting Information Available: Experimental meth-
ods, synthetic procedures, characterization data, cyclic and
differential pulse voltammograms, and the Eox1 - 1/n plot
for Sn and Rn and CIF for S4 and S7. This material is
available free of charge via the Internet at http://pubs.acs.org.
(18) Pommerehne, J.; Vestweber, H.; Guss, W.; Mahrt, R. F.; Ba¨ssler,
H.; Porsch, M.; Daub, J. AdV. Mater. 1995, 7, 551–554.
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