Substituent effect on the electronic properties and morphologies of self-assembling bisphenazine derivatives

Article abstract of DOI:10.1002/chem.200802148

This paper reports the synthesis and characterization of novel self-assembling n-type organic semiconductors based on asymmetrically substituted bisphenazines with various functional groups of different size, electron-withdrawing ability, and conjugation

Full text of DOI:10.1002/chem.200802148

DOI: 10.1002/chem.200802148
Substituent Effect on the Electronic Properties and Morphologies of
Self-Assembling Bisphenazine Derivatives
Kelly K. McGrath, Kyoungmi Jang, Kathleen A. Robins, and Dong-Chan Lee*[a]
Abstract: This paper reports the syn-
thesis and characterization of novel
self-assembling n-type organic semi-
conductors based on asymmetrically
substituted bisphenazines with various
functional groups of different size, elec-
tron-withdrawing ability, and conjuga-
tion length. The overarching objective
of this research is to tune electronic
properties and morphologies of self-as-
sembled structures of this system si-
multaneously, which offers a potential-
ly useful platform for future optoelec-
tronic applications. The thermal, opti-
cal, and electrochemical properties as-
sociated with different substituents
were studied by differential scanning
calorimetry (DSC), UV-visible and
fluorescence spectroscopy, and cyclic
voltammetry (CV). Electronic proper-
ties were calculated using density func-
tional theory, and results were com-
pared to experimental HOMO,
LUMO, and energy gaps. The one-di-
mensional (1D) self-assembly proper-
ties of these new n-type molecules are
discussed in terms of the type of pe-
ripheral substituents, alkyl side group
length, and assembly conditions. This
study includes extensive investigations
by scanning electron microscopy
(SEM) and X-ray diffraction (XRD).
Keywords: electronic properties
morphology nanostructures
self-assembly · semiconductors
·
·
·
Introduction
Meanwhile, tuning the electronic properties of p-cores in-
cluding the band gap,[5] as well as the HOMO and LUMO
energy levels, by introducing peripheral substituents such as
cyano[6] or fluorine[7] is equally as important as morphology
control. These structural modifications are much more facile
and convenient than designing a new p-core, which likely
entails new and complex synthetic routes. Although periph-
eral substituent effects on some p-cores have been exten-
sively studied, in most cases, the major focus has been elec-
tronic properties. Several current studies demonstrate the
role of peripheral substituents on the molecular assembly,
however, they primarily concentrated on the crystalline
form[3c,6f] or surface based assembly.[8]
The 1D self-assembly of p-conjugated organic semiconduc-
tors[1] into well-defined superstructures with high aspect
ratios has been one of the most exciting research areas in
recent years. These systems are of interest primarily because
of their potential application in nanoscale electronic devi-
ces.[2] Diverse molecular systems have been reported, which
show a variety of 1D nanostructures. A popular design ap-
proach involves the synthesis of large p-cores with solubiliz-
ing side groups, maximizing intermolecular p-orbital overlap
as a driving force for assembly.[1a] Controlling the morpholo-
gies of nanostructures is particularly important as many
properties are dependent on molecular packing.[3] To that
end, the manipulation of solubilizing alkyl side groups
bonded to p-cores has been the most rigorously studied to
date.[4]
In a recent communication,[9] we reported an organogela-
tor based on a bisphenazine asymmetrically substituted with
acetylene and hexadecyloxy groups (compound
1 in
Scheme 1). We found that belt-shaped 1D nanofibers were
[a] K. K. McGrath, K. Jang, Prof. K. A. Robins, Prof. D.-C. Lee
Department of Chemistry
University of Nevada Las Vegas
4505 Maryland Parkway, Box 454003
Las Vegas, Nevada 89154-4003 (USA)
Fax : (+1)702-895-4072
E-mail: Dong-Chan.Lee@unlv.edu
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200802148.
Scheme 1. Asymmetrically substituted bisphenazine derivatives.
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FULL PAPER
grown in select solvents via the cooperative interplay of p–p
interaction, hydrogen bonding, and van der Waals interac-
tion. This design approach enabled us to introduce various
substituents directly onto the bisphenazine core. In this
paper, we aim to achieve not only well-defined 1D nano-
structures of the new n-type semiconductors, but also the
ability to tune the electronic properties of the heteroaromat-
ic moiety without compromising the assembling ability.
Three major structural features are of importance in this
design. First, bisphenazine is a large, heteroaromatic mole-
cule that promotes self-assembly through its large flat p-
core.[10] In addition, the presence of four imine nitrogen
groups makes the ring electron-deficient[11] creating a poten-
tially useful electron transporting material. We are particu-
larly interested in developing new n-type organic semicon-
ductors, which are relatively scarce when compared to p-
type. The importance of self-assembling n-type molecules
has been substantiated by a number of studies.[3b,10,12] Sec-
ondly, the peripheral substituents (R1) added to one side of
the p-core vary in size, electron-withdrawing ability, and
conjugation length. These substituents will further tune the
electronic properties, such as electron-deficiency and
HOMO–LUMO energy gap, while modulating the morphol-
ogy of the assembly. Third, two long alkoxy chains (R2)
added to the other side of the p-core promote solubility and
assembly through cooperative van der Waals interactions.
Growth of the nanoclusters in the direction of the alkoxy
side group may be inhibited by the local interactions be-
tween solvent and side chain while the long axis growth will
be favored by p–p stacking. A thorough investigation of the
effect of the following parameters on either electronic or as-
sembly properties or both will be discussed: the type of pe-
ripheral substituents, the length of alkoxy side groups, as-
sembly methodology, and the types of solvent system used
for assembly.
Scheme 2. Stepwise cyclization (Routes 1 and 2) and sequential addition
(Route 3) to asymmetric bisphenazine derivatives.
yield when either Route 2 or 3 was used. Therefore, com-
pound 6 was synthesized by using Route 3. Compound 1
was synthesized from compound 4 according to the previous
report.[9]
The UV-visible absorption spectra for compounds 1–5 are
shown in Figure 1. All of the compounds showed lmax near
418 and 422 nm arising from the bisphenazine core. The
spectra of 1 and 4 have shoulders near l=450 nm, owing to
increased conjugation with the acetylene and iodine, respec-
tively. The lmax of compound 5 was slightly blue-shifted to
396 and 418 nm in comparison to the other compounds,
however, the shoulder was broadened and red-shifted to l=
468 nm. The greater shift of the shoulder in 5 may be a
Results and Discussion
The title compounds (2–6) were prepared by either stepwise
cyclization or sequential addition using 2,7-di-tert-butylpyr-
ene-4,5,9,10-tetraone and two different o-diaminoarenes to
generate asymmetric bisphenazine.
Compound 4 was tested with the stepwise cyclization
(Route 1 and 2 in Scheme 2) and sequential addition (Route
3). In Route 1, the insolubility of intermediate A hampered
its purification, thus resulting in a low yield in the second
cyclization. On the other hand, Route 2 produced the solu-
ble intermediate B which could successfully be purified with
silica gel column chromatography, generating a reasonable
yield for the two step reaction (ꢀ50%). As a result, Route
2 was used for the synthesis of final compounds 2–5, owing
to the flexibility of structural modification. Although Route
3 involved the least number of steps, the yields were incon-
sistent and low depending on the type of R1 if R2 was hexa-
decyloxy group (<35%). Interestingly compound 6, which
contains the dodecyloxy group, produced a comparable
Figure 1. UV-visible spectra of compounds 1–5 (absorption of compound
6 is identical to that of compound 5).
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D.-C. Lee et al.
result of more extensive conjugation with the NO2 groups.
We note that the absorption spectra of compounds 5 and 6
are nearly identical, which was anticipated because changing
the alkyl chain length of the side groups should not affect
the chromophore. The absorption of all the compounds, in-
cluding the shoulders at longer wavelengths, followed Beerꢁs
law, which ruled out any absorption induced by molecular
aggregation. The absorption edge was gradually shifted to
longer wavelengths as the peripheral substituent was
changed from H (2) to F (3) to I (4) to acetylene (1) to NO2
(5 and 6). As a result, the HOMO–LUMO energy gap cal-
culated from the tangent of the absorption edge[13] de-
creased in the same order (Table 1). The experimental esti-
ized at the emission maxima for comparison. The reduction
of the optical energy gap seen in the UV-visible spectra was
also manifested in fluorescence. As the HOMO–LUMO gap
narrowed, the fluorescence wavelength became longer,
which was clearly evident for these molecules. Note that the
emission maxima of compounds 5 and 6 showed significantly
large shifts to longer wavelengths compared to the other
compounds. This observation was predicted by theory
(Table 1).
The electrochemical properties of 1–6 were studied with
cyclic voltammetry. All of the compounds studied showed
quasi-reversible (DEp ꢁ90 mVnÀ1) first reduction waves
with peak splitting of 140 (1), 125 (2), 102 (3), 79 (5), and
86 mV (6). The peak splitting in compound 4 could not be
calculated because the peak potentials were not clearly re-
solved.
Table 1. Electronic properties of compounds 1–6 from experiments and
theoretical calculation.
Although the reductions of 5 and 6 appeared to be quasi-
reversible processes, compound 6 exhibited a more reversi-
ble reduction, given that the cathodic and anodic peak cur-
rents were similar (Figure 3). A second reduction wave was
peak
onset
[a]
[b]
[c]
[d]
[d]
[d]
Ered
[V]
Ered
[V]
ELUMO
[eV]
EHOMO
[eV]
Egap
ELUMO
EHOMO
[eV]
Egap
[eV] [eV]
[eV]
1
2
3
4
5
6
À1.68 À1.53 À3.27
À1.81 À1.68 À3.12
À1.76 À1.63 À3.17
À5.84
À5.86
À5.88
n/a[e]
2.57 À2.66
2.74 À2.39
2.71 À2.61
2.58 n/a[f]
n/a[g] À3.39
n/a[g] À3.39
À5.75
À5.69
À5.80
n/a[f]
3.09
3.30
3.19
n/a[f]
2.71
2.71
n/a[e]
n/a[e]
n/a[e]
À1.14 À1.03 À3.77
n/a[g]
À6.10
À1.14 À1.03 À3.77
n/a[g]
À6.10
[a] ELUMO =À(Eredonset +4.8 eV). [b] EHOMO =ELUMOÀEgapoptical. [c] Optical
HOMO–LUMO energy. [d] Theoretical calculation. [e] Peak potentials
for 4 were not clearly resolved. [f] Theoretical treatment for 4 was not
carried out, owing to limitations in suitably packaged basis sets for
iodine. [g] Evaluation of experimental Egap from UV-visible spectroscopy
was not clearly resolved, owing to long tailing of the absorption edge. As
a result, experimental EHOMO was not estimated.
mation of HOMO–LUMO energy gaps of compounds 5 and
6 was not clearly resolved, owing to long tailing of the ab-
sorption edge. However, our early estimation indicated that
the energy gaps for 5 and 6 are identical to each other, and
are smaller than those of the other compounds listed in
Table 1. The UV-visible absorption results showed that the
electronic properties of bisphenazine can be modulated with
the type of peripheral substituent introduced.
Figure 3. Cyclic voltammogram for the reduction of 6. Scan rate=
100 mVsÀ1
.
observed in the voltammograms of both 5 and 6 since the
presence of electron-withdrawing NO2 groups increased the
electron affinity of the molecules allowing the second wave
to be easily seen. Compounds 1–3 may also have second re-
duction waves, however, they were not clearly observed
owing to overlap with the solvent background. The onset of
the reduction potential can be directly related to the elec-
tron affinity, or LUMO energy, of the molecule. Thus the
onset of the first reduction wave was used to approximate
the LUMO level (Table 1). The LUMO level was consistent-
ly lowered as the electron-withdrawing ability of the periph-
eral substituent increased from 2<3<1<5=6. This result
illustrates that further increase in the electron affinity of the
whole system is possible by introducing a proper peripheral
substituent to the bisphenazine core. To be a useful elec-
tron-transporting material, the electron-affinity of the mole-
cule should be in the range of 2.7–3.4 eV for organic light-
emitting diodes, and 3.8–4.5 eV for organic photovoltaics
and field-effect transistors.[14] The CV results imply that
these compounds are potentially applicable for such opto-
The fluorescence emission spectra for compounds 1–5 are
shown in Figure 2. The fluorescence spectra were normal-
Figure 2. Photoluminescence spectra of compounds 1–5 (emission of com-
pound 6 is identical to that of compound 5). Excitation l: 422 nm (1 and
4), 423 nm (2 and 3), and 418 nm (5).
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Self-Assembling Bisphenazine Derivatives
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ACHTUNGTRENNUNGelectronics for n-channel function. The onset of the first oxi-
dation wave can similarly be used to approximate the
HOMO level of the molecule; however, oxidation potentials
were not resolved for these compounds owing to irreprodu-
cibility and overlap with the solvent background. Thus,
HOMO levels could not be directly estimated from CV. In-
stead, the HOMO values were calculated using the LUMO
energies from CV and the optical energy gap (Egap) values
obtained from UV-visible spectroscopy[15] (Table 1). In the
case of compounds 5 and 6, experimental HOMO energies
were not determined by this method, as the energy gaps
from UV-visible spectra were not resolved. Theoretical cal-
culations were performed to further corroborate the experi-
mental HOMO and LUMO energies and energy gap results
obtained from cyclic voltammetry and UV-visible spectros-
copy. Optimum geometries were calculated by using density
functional theory (DFT)[16] at the B3LYP/6-31G* level,[17]
and HOMO and LUMO energies and energy gaps were pre-
dicted by single point B3LYP/6-31+G*//B3LYP/6-31G* cal-
culations. Compound 4 was not considered for comparable
theoretical treatment because of limitations in suitably pack-
aged basis sets for the element, iodine. Theoretical values
are summarized and compared with experimental values in
Table 1.
Although discrepancies between the experimental and
theoretical energy levels were observed and expected, it is
important to note that the theoretical calculations predicted
that the LUMO levels would be lowered in the order 2<3<
1<5=6 which corresponded to the order seen experimen-
tally. In addition, the lowering of the energy gap predicted
from theoretical calculations was in the order 2<3<1<5=
6. This order was observed from UV-visible experiments
(2<3<4<1<5=6).
The thermal properties of the final products were investi-
gated by DSC. Compounds 2–4 (Figure 4 and Figure 5) had
similar patterns, showing one endotherm (melting) and one
exotherm (crystallization). The melting temperature showed
a gradual increase from 83.18C to 105.48C to 121.98C from
2 to 4 to 3. These results indicate that the intermolecular in-
teractions become stronger in the series 2<4<3. Com-
pound 5 exhibited one endotherm at 140.58C and one exo-
therm at 115.08C with an additional small endotherm and
exotherm at 168.98C and 157.58C, respectively, which may
Figure 5. DSC thermograms of compounds 1–6 (second cooling scan).
indicate the existence of a liquid crystalline (LC) phase. The
thermogram of compound 6 exhibited an endotherm at
144.28C and an exotherm at 117.58C similar to compound 5
with the absence of the exotherm/endotherm at higher tem-
peratures. However, there was an additional exotherm at
65.58C. The heat of melting was equal to the sum of the
heats of crystallization for the two exotherms. One possible
explanation is that two different crystallites are being
formed upon cooling, and both melt at the same tempera-
ture. The higher melting points of compounds 5 and 6 at
140.58C and 144.28C, respectively, could be attributed to
the additional intermolecular interaction present between
the NO2 groups.[18] The DSC scan of compound 1 showed
one exotherm at 90.18C and the corresponding endotherm
at 102.48C in a similar manner to compounds 2–4. In addi-
tion, there was a second higher temperature endotherm at
124.88C. The second endotherm indicates the possible pres-
ence of an LC phase. The presence of a functional group,
such as acetylene[19] or NO2,[18] which can participate in an
additional intermolecular interaction, may lead to the mole-
cules arranging in such a way as to create a larger mesogen
which could introduce an LC phase. We note that all of the
transitions were reproducible over three heating/cooling
scans.
In conjunction with molecular design approaches, assem-
bly methodologies beyond simple spin-coating or casting
have also been developed to fine-tune 1D structures includ-
ing phase transfer,[3b,20] solvent vapor annealing,[3b,12n] injec-
tion,[12j,21] precipitation,[22] and organogelation.[23] The 1D
self-assembly of compounds 2–6 was studied using recrystal-
lization from CH2Cl2 and a phase transfer (PT) method
using two different binary solvent systems: CH2Cl2/hexane
and CH2Cl2/methanol. The 1D assembly of compound 1 was
not examined by these methods because detailed investiga-
tion of 1D nanostructure formation through organogelation
was already demonstrated in the previous report.[9] In the
case of the recrystallization method, only compounds 5 and
6 formed self-assembled clusters. On the other hand, in the
recrystallization of compound 2, 3, and 4, a precipitation or
a partial gelation with some precipitation was observed. The
morphologies of the assembled clusters and their size distri-
bution were compared based on the peripheral substituent
(R1), the alkyl chain length (R2 =dodecyloxy or hexadecy-
Figure 4. DSC thermograms of compounds 1–6 (second heating scan).
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D.-C. Lee et al.
loxy) and assembly method, and on the binary solvent sys-
tems used to induce the assemblies. First, the peripheral sub-
stituent effect on the self-assembly of compounds 2–5 was
examined by comparing the morphologies of the nanoclus-
ters obtained from the PT method (Figure 6).
Figure 7. SEM images of a) 5 and b) 6 recrystallized from CH2Cl2. Scale
bar: 10 mm.
of 6 required overnight standing while 5 recrystallized
within an hour.
In general, similar morphologies to those obtained from
recrystallization were seen with the PT assemblies of 5 and 6
from CH2Cl2/hexane and CH2Cl2/methanol (Figure 8). The
Figure 6. SEM images of PT assemblies of a) 2 , b) 3, c) 4, and d) 5 in
CH2Cl2/methanol. Scale bar: 10 mm.
Compounds 2–4 produced assembled structures in
CH2Cl2/methanol, while the CH2Cl2/hexane system failed to
induce any assembly, which may be attributed to some solu-
bility of the compounds in the CH2Cl2/hexane binary solvent
system. However, compound 5 and 6 were assembled in
both binary solvent systems and the morphologies of the 1D
nanoclusters were somewhat dependent upon the solvent
system used. This will be discussed later. Compound 2
formed endless, flexible, and flat nanofibers. Fiber bundles
varied in size from ꢀ300 nm to 2 mm. In the case of com-
pound 3, fairly homogeneous nanofibers of approximately
600 nm in width were formed. The nanofibers formed were
much straighter and shorter than those formed from com-
pound 2. Meanwhile, compound 4 formed thin and short
fibers with very heterogeneous thickness and length distri-
bution. This behavior may be due to irregular halogen-bond-
ing interaction between iodine and imine nitrogen in neigh-
boring molecules[24] caused by steric tert-butyl side group,
which may interfere with p–p interaction. The nitro substi-
tuted bisphenazine 5 formed straight nanofibers ranging in
width from ꢀ900 nm to 3 mm. The fibers of 5 showed more
of a belt-like morphology than the other compounds and
were much less flexible than the nanofibers of 2 and 3.
Figure 8. SEM images of the PT assemblies of 5 (a and b) and 6 (c and
d). Binary solvent system for a and c: CH2Cl2/hexane, for b and d:
CH2Cl2/methanol. Scale bar: 10 mm.
1D nanostructures of
5 obtained from CH2Cl2/hexane
ranged in width from ꢀ600 nm to 3 mm and those from
CH2Cl2/methanol were ꢀ900 nm to 3 mm. In the case of
compound 6, the width of the 1D clusters from CH2Cl2/
hexane ranged from ꢀ250 nm to 500 nm and those from
CH2Cl2/methanol were ꢀ600 nm to 900 nm. Three observa-
tions can be made from these results: 1) in the PT method,
the nanoclusters formed from compound 5 are wider than
those of 6 as previously described with those from recrystal-
lization, 2) the width of the nanoclusters from the CH2Cl2/
hexane binary solvent system are smaller than those from
CH2Cl2/methanol, which is indicative of some solubility in
the CH2Cl2/hexane system, thus forming the more thermo-
dynamic product, and 3) the PT assemblies of both 5 and 6
showed less lateral growth than recrystallization.
Unlike the other compounds, 5 and 6 produced very ho-
mogeneous micro- or nanobelts by recrystallization from
CH2Cl2 (Figure 7). The width of the nanobelts obtained
from 6 is much smaller than those of 5. The widths of the
microbelts obtained from 5 varied from ꢀ1 mm to 8 mm
while those of 6 varied from ꢀ400 nm to 2 mm. The reason
why 6 produced narrower nanobelts than 5 can be attributed
in part to the higher solubility of 6 in CH2Cl2 leading to
slower aggregation. It should be noted that recrystallization
XRD studies were conducted on the 1D self-assembled
clusters of compounds 2-6. Although XRD alone is not suf-
ficient to deduce the molecular packing in the 1D nanoclus-
ters, the major purpose of this study was to determine if sig-
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Self-Assembling Bisphenazine Derivatives
FULL PAPER
nificant p–p interactions were present in the assembled clus-
ters. The typical distance for p–p stacking is 3.5 ꢂ.[25]
The XRD patterns of the nanofibers of 2, 3, and 4 ob-
tained from the PT method using CH2Cl2/methanol were
compared to determine the effect of the peripheral substitu-
ents on the 1D assembly. Although there are a number of
peaks present in the patterns, the peaks of interest are near
238 (2q) corresponding to the p–p stacking distance along
the fiber growth direction. In compound 2 (Figure 9a) the
Figure 11. XRD patterns of 6 a) recrystallized from CH2Cl2, and b) PT
from CH2Cl2/hexane and c) CH2Cl2/methanol.
Figure 9. XRD patterns of PT assemblies of a) 2, b) 3, and c) 4 from
CH2Cl2/methanol.
diffraction pattern showed sharp, intense peaks indicative of
crystallinity in the assembled clusters. The peaks of interest
correspond to d-spacings of 3.79, 3.73, and 3.61 ꢂ. Com-
pound 3 had a diffraction pattern similar to that of 2 with a
more intense peak at 3.83 ꢂ and a smaller peak at 3.57 ꢂ.
Compound 4 exhibited peaks similar to those found in 2 and
3; however, they were of lower intensity and less defined
and broader, indicating more amorphous character.
Figure 12. Space filling model of compound 5[26] optimized at the B3LYP/
6-31G* level of theory: a) top view; b) side view from the nitro group.
Color code: red-oxygen, violet-nitrogen, dark gray-carbon. Hydrogen
atoms were omitted for clarity.
The XRD patterns of the recrystallized samples (com-
pounds 5 and 6) exhibited sharp, well-defined diffraction
patterns indicative of high crystallinity. The peak of interest
is at 228 (2q) corresponding to d-spacings of 3.96 ꢂ for 5
(Figure 10a) and 4.02 ꢂ for 6 (Figure 11a) which are larger
than those of compounds 2–4. This may be due to the tilting
of the NO2 groups from the plane of the heteroaromatic
core which was predicted by theoretical calculations
(Figure 12).
An interesting observation from the PT assembly with dif-
ferent binary solvent systems is that the diffraction patterns
of the assembly from CH2Cl2/hexane (Figures 10b and 11b)
are more similar to those of the recrystallized ones (Fig-
ures 10a, and 11a) than the PT with CH2Cl2/methanol (Fig-
ures 10c and 11c). During the process of PT, the molecules
are forced to aggregate by a slow and continuous decrease
in solubility, owing to the diffusion of a poor solvent into
the solution. The solubility of the molecules in the binary
solvent system should affect the morphology of assembled
clusters. In the case of nitro substituted bisphenazines (5
and 6), the lower solubility of the compounds in methanol
forces faster aggregation thus producing the less thermody-
namic product.
The intermolecular p–p interaction in solid state was fur-
ther investigated with UV-visible spectroscopy. Absorptions
of two solid-state samples of compounds 2, 3, 5, and 6 (self-
assembled clusters by PT method from CH2Cl2/methanol
and cast films) were recorded and compared with those in
the solution state (Figure 13). In general, the self-assembled
clusters showed significantly red-shifted absorption maxima
compared to the absorption in the solution state. The cast
films showed similar behavior with a less pronounced shift
Figure 10. XRD patterns of 5 a) recrystallized from CH2Cl2, and b) PT
from CH2Cl2/hexane and c) CH2Cl2/methanol.
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D.-C. Lee et al.
Acknowledgements
This work was partially supported by NSF EPSCoR Ring True III Award
(EPS-0447416).
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Figure 13. UV-visible spectra of compound a) 2, b) 3, c) 5, and d) 6:
I) CHCl3, II) PT from CH2Cl2/methanol, and III) cast film.
than the self-assembled clusters. In the case of compound 6
in CHCl3, lmax =419 nm was red-shifted to l=428 nm in the
assembled clusters. In addition, the appearance of a new
peak at l=498 nm in the self-assembled clusters (which was
a shoulder in the solution state) suggests a possible J-aggre-
gate formation. This trend is consistent with the previous
observation from the absorption of nanofibers of compound
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Conclusions
In this paper, we demonstrated the effect of small peripheral
substituents on the electronic property of the bisphenazine
core. Among the substituents studied, the NO2 group gave
the most pronounced effect, lowering the LUMO energy
level by ꢀ0.6 eV, and narrowing the HOMO–LUMO gap
considerably when compared to H. Significant influence of
these peripheral substituents was also observed in the mor-
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method, and assembly condition influenced the fiber mor-
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sonable method for tailoring electronic and assembly prop-
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Chem. Eur. J. 2009, 15, 4070 – 4077

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