Balanced π-π Interactions directing the self-assembly of indolocarbazole-based low molecular mass organogelators

Article abstract of DOI:10.1039/c4ob00873a

New indolocarbazole derivatives emitting strong blue light have been synthesized. It is found that the strong π-π interactions between indolocarbazoles 4-6 without long carbon chains lead to the formation of crystal or crystal-like aggregates. We have pre

Full text of DOI:10.1039/c4ob00873a

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Balanced π–π interactions directing the
self-assembly of indolocarbazole-based
low molecular mass organogelators†
Cite this: Org. Biomol. Chem., 2014,
12, 6134
Peng Gong, Pengchong Xue, Chong Qian, Zhenqi Zhang and Ran Lu*
New indolocarbazole derivatives emitting strong blue light have been synthesized. It is found that the
strong π–π interactions between indolocarbazoles 4–6 without long carbon chains lead to the formation
of crystal or crystal-like aggregates. We have previously found that tert-butyl could tune the strength of
π–π interactions between carbazole units and the organogels were obtained from tert-butyl substituted
carbazoles directed by balanced π–π interactions. Herein, hexadecyl groups were introduced into N-positions
of indolocarbazoles in order to reduce the strength of π–π interactions between indolocarbazoles, and
compounds 7–9 were prepared. It is interesting that compounds 8 and 9 could form stable organogels in
alcohols, acetone, DMSO, and so on, upon ultrasound stimulation. Combined with the results of electronic
spectra, XRD patterns and the optimized molecular length based on the semiempirical (AM1) calculations,
we suggested the molecular packing modes in gel states, in which the lamellar structures were involved.
Although the fluorescence emission of indolocarbazoles 8 and 9 decreased during the gel formation to
some extent, the obtained gel nanofibers still emitted strong blue light and the fluorescence emission of
the film based on xerogel 9 decreased significantly upon exposure to gaseous TNT. It meant that the xero-
gels based on indolocarbazoles could be used as fluorescent sensors for detecting vapors of explosives.
Received 29th April 2014,
Accepted 26th June 2014
DOI: 10.1039/c4ob00873a
www.rsc.org/obc
cene, which has been used as a benchmark in organic field-
Introduction
effect transistors (OFET) with a high mobility of 3 cm2 V−1 s−1
,
In recent years, the low molecular mass organogelators have received much attention due to their extended π-conju-
(LMOGs) bearing rigid π-conjugated units have drawn enor- gated systems. However, they usually exhibit poor solubility in
mous attention because they can act as building blocks of common organic solvents. The high crystallization directed by
functional supramolecular assemblies with well-ordered nano- strong π–π interactions would decrease the stability of the con-
structures, leading to special optoelectronic properties.1–7 It is cerned devices and limit their practical applications.28,29 Till
known that π-organogels have potential applications in the now, the gelation of low molecular mass organic π-gelators has
fields of optoelectronic devices, fluorescent sensors, light been proven as an important strategy to arrange the chromo-
harvesting antennas, drug delivery, catalysis and template phores into functional soft materials, in which the carrier
synthesis.8–21 Although numerous conjugated skeletons, such mobility or the energy (or electron) transfer efficiency would be
as oligophenylenevinylene, perylene bisimide, β-diketone- improved. Notably, as another kind of polycyclic aromatic
boron difluoride, salene, benzoxazole, and cyanostilbene, have systems, indolocarbazole derivatives have found applications
been introduced into π-gelators, the construction of new in medicine and organic optoelectronic devices.30–36 For
π-organogels with enhanced performance in optoelectronic example, Beng S. Ong et al. have synthesized 5,11-bis(4-octyl-
devices is still challenging.22–27 Notably, acenes, such as penta- phenyl)indolo[3,2-b]carbazole, which could be used as a
p-channel semiconductor with good environmental stability
for organic electronic applications owing to the relatively low-
lying HOMO and large band gap. The mobility and the current
State Key Laboratory of Supramolecular Structure and Materials, College of
on/off ratio of the indolo[3,2-b]carbazole-based OTFT (organic
thin film transistor) were up to 0.12 cm2 V−1 s−1 and 107,
respectively.32 However, to the best of our knowledge, no
report on the π-gelators based on indolocarbazole derivatives
has been found. We anticipate that the unique properties of
indolocarbazole derivatives will be acquired if ordered nano-
Chemistry, Jilin University, Changchun 130012, P. R. China.
E-mail: luran@mail.jlu.edu.cn; Fax: +86-431-88499179
†Electronic supplementary information (ESI) available: Analytical data and
spectra (1H and 13C NMR), mass spectrometry data, CV curves and DFT calcu-
lation of configuration optimization materials, deposition numbers of single
crystal of compound 4. CCDC 993542. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c4ob00873a
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structures are gained via organogelation. In order to generate to gaseous TNT, suggesting that the xerogels based on indolo-
novel soft materials we synthesized new indolocarbazole carbazoles could become candidates as fluorescent sensory
derivatives via the Cadogan reactions (Scheme 1).37–39 In the materials to detect vapors of explosives.
case of indolocarbazoles 4–6 without long carbon chains, the
planar configuration of the polycyclic heterocycles would lead
to extended π-conjugation, so strong π–π interactions would
occur during the aggregation, which would not be favorable
Results and discussion
Synthesis
for the formation of the gel. As a result, crystals and crystal-
like aggregates were formed from compounds 4–6. In addition,
we have previously found that tert-butyl could tune the
strength of π–π interactions between carbazole units, and
organogel could be obtained from tert-butyl substituted
carbazoles directed by the balanced π–π interactions.12,16 In
order to decrease the strength of the π–π interactions between
rigid indolocarbazole moieties, hexadecyl groups, which might
enlarge the distance between conjugated units in the aggre-
gates, were introduced. It is interesting that indolocarbazole
derivatives with long alkyl groups 8–9 could form gels in alco-
hols (such as tert-amyl alcohol, n-octyl alcohol), acetone,
DMSO, DMF and the mixed solvents containing methanol.
The investigation of the gelation process revealed that the gela-
tion ability of indolocarbazoles could be tuned by the struc-
tures of the π-skeletons. Herein, we provided a strategy to
design π-gelators via tuning the π–π interactions between rigid
conjugated systems. On the other hand, blue emitting gels are
relatively rare, while in this work we fabricated the nanofibers
with intense blue-light emission via organogelation of new
indolocarbazoles. Notably, the fluorescence emission of the
film based on xerogel 9 decreased significantly upon exposure
The synthetic routes for the indolocarbazole derivatives
without or with long carbon chains 4–9 are shown in
Scheme 1. The reactions were performed under a N2 atmos-
phere unless otherwise stated. Firstly, the boronic acids,
including naphthalene-1,4-diyldiboronic acid, naphthalen-1-
ylboronic acid and (4-(diphenylamino)phenyl)boronic acid, as
well as 1,5-dibromo-2,4-dinitrobenzene were prepared accord-
ing to the reported methods.40–43 The double Suzuki–Miyaura
cross coupling reaction between naphthalene-1,4-diyldiboronic
acid and 1-bromo-2-nitrobenzene catalyzed by Pd(PPh3)4 in
toluene–H2O afforded compound 1 in a yield of 87%. Simi-
larly, compounds 2 and 3 were synthesized via Suzuki–Miyaura
reactions of 1,5-dibromo-2,4-dinitrobenzene with naphthalen-
1-ylboronic acid or (4-(diphenylamino)phenyl)boronic acid in
yields of 71% and 80%, respectively. Then, compounds 4–6
were obtained by Cadogan reactions through triethyl phos-
phate mediated reductive cyclization reactions of compounds
1–3 in yields of 42%, 12% and 13%, respectively. Finally, com-
pounds 7–9 were obtained by the alkylation reactions of com-
pounds 4–6 with 1-bromohexadecane catalyzed by NaH in
yields of 85%, 76% and 72%, respectively. The obtained
indolocarbazole derivatives were characterized by 1H NMR,
13C NMR, FT-IR and MALDI-TOF mass spectrometry. In
addition, we attempted to obtain single crystals of compounds
4–6 by means of slow solvent diffusion. We found that after
n-hexane was slowly diffused into the solutions of compounds
4–6 in THF, long rod-like crystals were obtained from com-
pound 4, needle-like crystals were generated from compound
5, and thin plate crystals were obtained from compound 6.
Unfortunately, we only obtained the single crystal from com-
pound 4, while the X-ray diffraction signals of compounds 5
and 6 were too weak to be detected. As shown in Fig. 1, the
Scheme 1 Synthetic routes for the indolocarbazole derivatives.
Fig. 1 Packing arrangement in the crystal structure of compound 4.
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molecule 4 exhibited good planarity and we could find a face electronic transitions from naphthalene and carbazole moi-
to face stacking mode, in which the distance between two adja- eties. Similarly, for compound 5, five absorption bands
cent molecular planes was 3.355 Å. Additionally, we also found appeared in the UV region, such as 260 nm, 300 nm, 352 nm,
a vertically oriented stacking mode, in which the distances 370 nm and 390 nm. The absorption at 300 nm and 352 nm
between H atoms and the vertical molecular plane were 0.646, could be attributed to π–π* transitions, and the absorption at
1.221, 1.720, 2.463, 2.902, and 4.615 Å, and the two N atoms in 370 nm and 390 nm could be attributed to n–π* transitions. It
the aromatic ring were away from the molecular plane of 2.451 should be noted that the absorption band at the long wave-
and 3.322 Å. The distances between H atoms connected to the length for compound 5 (390 nm) red-shifted compared with
N atom and the vertical molecular plane were 1.720 and that for compound 4 (362 nm), indicating the enlarged conju-
2.463 Å. It should be noted that the distance of 3.355 Å gation length of 5. Because of the introduction of triphenyl-
between the two indolocarbazole rings revealed that strong π–π amine units, the absorption band at the long wavelength for
interactions occurred in the single crystal of 4.
compound 6 (389 nm) red-shifted compared with compound
4. The absorption bands for compound 6 at 289 nm and
321 nm could be attributed to π–π* transitions, while the
absorption bands at 371 nm and 389 nm could be attributed
UV-vis absorption and fluorescence emission spectra in
solutions
The UV-vis absorption and fluorescence emission spectra of to n–π* transitions. When long carbon chains were introduced
indolocarbazole derivatives 4–9 in THF (5 × 10−6 M) are shown into the N-positions of indolocarbazole derivatives, com-
in Fig. 2, and the data are listed in Table S1 (ESI†). The absorp- pounds 7–9 showed similar absorption behaviors to those of
tion behaviors of compounds 4–6 without long carbon chains 4–6, respectively, besides a small red-shift (less than 10 nm,
are first discussed (Fig. 2a). It was clear that several well- Fig. 2b). For example, the absorption bands at 345 nm and
resolved structured absorption bands appeared in the range of 362 nm for compound 4 red-shifted to 352 nm and 369 nm for
250–420 nm for each compound. In detail, as to compound 4, compound 7 because of the weak electron donating effect of
the absorption bands appeared in the range of 250–280 nm carbon chains. As shown in Fig. 2c, compound 4 gave two
might be due to the B bands of benzene rings, and the absorp- strong emission bands at 367 nm and 386 nm, as well as a
tion emerged at 345 nm and 362 nm might be ascribed to the weak emission at 406 nm in THF (5 × 10−6 M) when excited at
Fig. 2 Normalized UV-vis absorption (a, b) and fluorescence emission spectra (c, d) of compounds 4–9 in THF (5 × 10−6 M, λex = 345 nm for
compound 4, λex = 365 nm for compounds 5–9).
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345 nm. On account of the larger conjugation length of com- band gaps, and they were −2.24 eV, −2.25 eV and −1.90 eV,
pound 5, its emission in dilute solution red-shifted to 412 nm respectively.
and 435 nm with a shoulder at ca. 460 nm (λex = 365 nm) com-
Moreover, the molecular orbitals and the optimized con-
pared with compound 4. In addition, a strong emission band figurations of the synthesized indolocarbazole derivatives were
at 400 nm with a shoulder at 418 nm was detected for com- studied by the B3LYP/6-31G level using the Gaussian 09w
pound 6. Moreover, the fluorescence emission spectra of com- program. Based on the optimized molecular configurations we
pounds 7–9 bearing long carbon chains were also similar to found that 4–6 without alkyl groups were planar molecules.
those of 4–7, respectively, besides the red-shift (Fig. 2d). For When long alkyl groups were introduced to compounds 5 and
instance, the strong emission bands of compound 8 shifted to 6, the π-conjugated skeletons of compounds 8 and 9 were still
420 nm and 445 nm from 412 nm and 435 nm for compound in one plane because no interaction occurred between the two
5 resulted from the weak electron donating ability of alkyl carbon chains (see Fig. S3–S4†). It meant that π–π interactions
groups. The fluorescence quantum yields (ΦF) of the syn- would occur during the aggregation of 8 or 9, which might
thesized indolocarbazole derivatives were determined in THF generate well-ordered arrangements. However, twist configur-
using quinine sulfate as the standard at room temperature (see ation of the aromatic heterocyclic ring was shown for com-
Table S1†). The ΦF values for compounds 4–9 were 0.44, 0.39, pound 7, in which the two long carbon chains extended to the
0.66, 0.69, 0.38 and 0.45 respectively, meaning that they were front and back of the aromatic heterocyclic ring, respectively.
high emissive in solutions.
We deemed that the steric hindrance of long carbon chains
would affect its self-assembling properties. As shown in Fig. 3,
the HOMO and LUMO for compounds 7–9 were mainly deloca-
lized through the entire molecule besides the LUMO of 9 was
delocalized over the indolocarbazole ring. In addition, the
HOMO and LUMO energy levels obtained from the calcu-
lations are depicted in Table 1. It was clear that the calculated
HOMO energy levels of compounds 7–9 were similar to those
based on electrochemical results, and the HOMO energy level
of 7 was lower than compounds 8 and 9. As to the calculated
LUMO energy levels for compounds 7–9, although they were
higher than the data from electrochemical results, they were in
the same order of 9 > 7 > 8 as that based on electrochemical
data.
Electrochemical properties and DFT calculations
Because the conjugated skeletons of indolocarbazole deriva-
tives 4–6 were the same as those in 7–9, respectively, the
electrochemical properties of the selected compounds 7–9
were investigated using cyclic voltammetry with Ag/Ag+ as the
reference electrode in CH2Cl2. The electrochemical data and
HOMO/LUMO energy levels are summarized in Table 1. Com-
pounds 7 and 8 showed one reversible oxidation peak with
onset potential around 1.2 V and 0.87 V, respectively, while
compound 9 showed three reversible oxidation peaks with oxi-
dation potentials around 0.69 V, 0.99 V and 1.33 V (see
Fig. S2†). We deduced that the three oxidation peaks of com-
pound 9 were originated from the oxidation processes of the
Gelation properties
indolocarbazole ring and two triphenylamine units, respecti- We found that the indolocarbazole derivatives 4–6 could not
vely, and the single oxidation peak for 7 and 8 without the tri- form any organogels due to their strong π–π interactions
phenylamine moiety came from the oxidation of the during aggregation. In order to gain new indolocarbazole-
indolocarbazole. The HOMO energy levels of compounds 7–9 based organogels, long carbon chains were introduced to com-
calculated from the oxidation onset potential using the ferro- pounds 4–6, which would adjust the balance between the solu-
ox
onset
cene calibrated formula of EHOMO = −(E
+ 4.19) eV were bility and precipitation in organic liquids. As shown in
−5.39 eV, −5.06 eV and −4.9 eV, respectively, indicating strong Table 2, compounds 8 and 9 could form opacity gels in polar
electron donor abilities of the three compounds. The fluo- solvents, including acetone, tert-amyl alcohol, n-octyl alcohol,
rescence quenching of compound 9 in the xerogel-based film DMSO, DMF and the mixed solvents containing methanol,
when exposed to gaseous TNT could support this point, which such as dichloromethane–methanol (v/v = 1/1), 1,2-dichloro-
will be discussed below. Since the reduction peaks were out of ethane–methanol (v/v = 1/1), chloroform–methanol (v/v = 1/1),
our scan range, the LUMO energy levels of compounds 7–9 tetrahydrofuran–methanol (v/v
= 1/1), toluene–methanol,
were calculated from HOMO energy levels and the absorption diethyl ether–methanol, ethyl acetate–methanol under ultra-
Table 1 Electrochemical data and HOMO/LUMO energy levels of compounds 7–9
ox
onset
Compound
E
(V)a(eV)
HOMOb (eV)
LUMOb (eV)
Eg c (eV)
HOMOd (eV)
LUMOd (eV)
7
8
9
1.20
0.87
0.69
−5.39
−5.06
−4.90
−2.24
−2.25
−1.90
3.15
2.81
3.00
−5.32
−4.74
−4.72
−1.10
−1.14
−0.79
a
ox
E
(V) = onset oxidation potential; potentials versus Ag/Ag+, working electrode Pt, 0.1 M Bu4NPF6–CH2Cl2, scan rate 50 mV/S, Fc+/Fc was used
b ox
onset
as an external reference. EHOMO = −(E
+ 4.19) eV; LUMO = HOMO + Eg. c Determined from the onset of the absorption at the lower energy
onset
band edge (Eg = 1240/λonset). d Calculated data.
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Table 2 Gelation abilities of compounds 7–9 in organic solvents
Solvent
7
8 (CGCa/mM)
9 (CGC/mM)
Methanol
Ethanol
Acetone
Acetonitrile
tert-Amyl alcohol
Glycol dimethyl ether
Glycol
n-Octyl alcohol
DMSO
P
P
S
P
P
P
P
P
S
S
S
PG
PG
G (4.1)
P
G (12.4)
S
P
P
G (2.1)
P
G (1.4)
S
P
P
G (5.0)
G (9.9)
G (7.1)
(v/v = 1/1)
G (3.1)
(v/v = 1/1)
G (3.3)
(v/v = 1/1)
G (4.2)
(v/v = 1/1)
G (4.9)
(v/v = 2/1)
G (5.5)
(v/v = 1/1)
G (3.0)
(v/v = 2/1)
G (5.6)
G (1.9)
G (2.1)
G (1.1)
(v/v = 1/1)
G (2.7)
(v/v = 1/1)
G (1.9)
(v/v = 1/2)
G (0.9)
(v/v = 1/1)
G (3.8)
(v/v = 1/1)
G (1.5)
(v/v = 1/1)
G (2.6)
(v/v = 1/1)
G (0.6)
DMF
Dichloromethane–methanol
1,2-Dichloroethane–methanol
Toluene–methanol
S
S
S
S
S
S
Chloroform–methanol
Diethyl ether–methanol
THF–methanol
Ethyl acetate–methanol
P = precipitate; PG = partly gel; S = soluble; G = gel. a CGC: critical
gelation concentration.
easily dissolved in most common solvents, including dichloro-
methane, chloroform, ethyl acetate, THF, DMF, DMSO, etc. In
the cases of compounds 8 and 9, the long carbon chains
extended in the same plane of indolocarbazoles, so the conju-
gated moieties could approach each other and π–π interactions
might become the driving force for the gel formation, which
was confirmed by the UV-vis absorption and fluorescence
emission spectral changes during the gelation processes.
The morphologies of xerogels 8 and 9 obtained from DMSO
were investigated by SEM. As shown in Fig. 4, lots of long
fibers with diameters of 6–18 µm were formed from 8 in
DMSO gel, while the formed fibers in xerogel 9 were more
slender, whose diameter was in the range of 1.5–4.1 µm. We
Fig. 3 The molecular orbital surfaces of HOMO and LUMO for com-
pounds 7–9 calculated by the B3LYP/6-31G method using the Gaussian
09w program.
sound stimulation. The low critical gelation concentrations also found that most of the fibers in xerogel 9 were made up of
(CGC) for compounds 8 and 9 in the above solvents were in several thinner fibers, which twined together to form many
the range of 3.1–12.4 mM and 0.6–3.8 mM, respectively. There- junctions.
fore, we could deduce that the gel formation ability of com-
In order to investigate the process of gel formation, the
pound 9 was stronger than 8, and 9 could be considered as a time-dependent UV-vis absorption and fluorescence emission
supergelator to some extent.13 The obtained gels were stable spectra of compounds 8 and 9 in DMSO upon cooling the hot
for several months at room temperature and could be solutions, which were stimulated with ultrasound for 5 s, to
destroyed when heated. Moreover, the organogels could be room temperature are shown in Fig. 5. It was found that
recovered after the hot solutions were stimulated by ultra- during the gel formation of compound 8 the absorption bands
sound. Although the indolocarbazole ring was involved in decreased gradually. It should be noted that the n–π* tran-
compound 7, no organogel was obtained because the two long sition band at 398 nm in hot solution red-shifted to 402 nm in
carbon chains extended to the front and back of the slight the gel state, indicating the formation of J-aggregates in the gel
twist aromatic heterocyclic ring, respectively (Fig. S4†). The state. However, during the gel formation of compound 9 the
steric hindrance of long carbon chains in compound 7 would UV-vis absorption spectra showed abnormal changes. When
prevent the occurrence of π–π interactions between indolo- the hot solution was naturally cooled down within 40 s (the gel
carbazole units, which would be unfavorable for the gelation. was not formed), the absorption bands decreased gradually
Therefore, it was understandable that compound 7 could be because the solution became a bit of opaque. Further prolong-
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during the cooling process, and shifted to 403 nm in the gel
state. It suggested that the H-aggregates were formed in the gel
state of compound 9. From the time-dependent fluorescence
emission spectra we could find that the emission intensities of
compounds 8 and 9 decreased significantly upon cooling the
hot solutions to room temperature. Meanwhile, the emission
band for compound 9 blue-shifted from 418 nm in solution to
416 nm in the gel state, and no shift was detected during the
gel formation of 8. The above changes of the emission spectra
indicated that π–π interactions played a key role in the gel for-
mation. Notably, although the fluorescence emission intensity
of compounds 8 and 9 decreased during the gel formation to
some extent, the organogels still emitted strong blue light
(Fig. 4c). As a result, the nanofibers with strong blue-light
emission were obtained via the organogelation of the indolo-
carbazole derivative, which might become candidates as
fluorescent sensory and emitting materials.
In order to reveal the stacking mode of compounds 8 and 9
in gel phases, we showed the XRD patterns of the xerogels in
Fig. 6. The xerogel 8 gave three diffraction peaks in small angle
area, and the corresponding d-spacing values were 2.2 nm,
1.1 nm and 0.87 nm, respectively, corresponding to a ratio of
1 : 1/2 : 1/3 (Fig. 6a). It indicated that a layered structure with a
long period of 2.2 nm was generated from molecule 8 in the
gel state. Combined with the optimized molecular length of
molecule 8 (2.3 nm) based on the semiempirical (AM1) calcu-
lations (see Fig. S1a†) we suggested that the molecule 8
arranged into a lamellar structure using the molecular length
as the long period in the gel state. Moreover, in the wide angle
region of the XRD pattern for xerogel 8 we could detect the
diffraction peaks with d-spacings of 0.41 nm and 0.37 nm (see
the inset in Fig. 6a), and the latter one was the typical distance
between aromatic rings in π-aggregates. It could further
confirm the formation of π-aggregates in the gel phase of 8,
which was in accordance with the results of time-dependent
UV-vis absorption and emission spectral data. In addition, the
diffraction peak with a d-spacing of 0.41 nm was suggested as
the distance between the neighboring long carbon chains.
Therefore, we proposed the possible molecular packing model
for compound 8 in the gel phase as shown in Fig. 6b, in which
the molecular plane and alkyl chain adopted a parallel stack-
ing mode and J-aggregates were involved. Additionally, the
xerogel 9 showed two diffraction peaks in the small angle area
and the corresponding d-spacings were 3.1 nm and 1.0 nm
with a ratio of ca. 1 : 1/3 (see Fig. 6c). Although the second
diffraction peak could not be detected, we could deduce that
molecule 9 adopted a layered structure with a long period of
3.1 nm in the gel state. The result of the semiempirical (AM1)
calculations showed that the length of molecule 9 was 2.1 nm
(see Fig. S1b†), so we suggested that the stacking mode in
Fig. 4 SEM images for the xerogels 8 (b, 9.9 × 10−3 M) and 9 (a, 6.4 ×
10−3 M), and fluorescence microscopy for xerogel 9 (c, 6.4 × 10−3 M,
λex = 365 nm) obtained from DMSO.
ing the cooling time, the absorption band at 298 nm increased xerogel 9 adopted a bimolecular layered structure. The wide
gradually, and the two bands at 330 nm and 356 nm combined angle XRD pattern of xerogel 9 showed two diffraction peaks
into one at 350 nm, which also increased during the gel for- with d-spacings of 0.42 nm and 0.39 nm (see the inset in
mation. Such spectral changes indicated the occurrence of the Fig. 6c). The latter one could be ascribed to the distance
π–π interaction in the gelation process. In addition, the absorp- between aromatic planes and the former one corresponded to
tion band at 411 nm in hot solution blue-shifted obviously the distance between long carbon chains. Therefore, a layered
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Fig. 5 UV-vis absorption (a for 8, b for 9) and fluorescence emission (c for 8, d for 9) spectra of 8 (9.9 × 10−3 M) and 9 (6.4 × 10−3 M) during the
gelation process in DMSO.
Fig. 6 XRD patterns of the xerogels 8 (a) and 9 (c) obtained from DMSO, and the proposed molecular packing models of 8 (b) and 9 (d) in gel
phases.
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structure was proposed for the molecular packing model for
compound 9 in the gel state as shown in Fig. 6d, in which
Conclusions
H-aggregates were formed, according to the results based on In summary, new indolocarbazole derivatives with extended
the absorption spectral changes during sol–gel transition. It π-conjugated systems 4–9 have been synthesized. It was found
should be noted that the distance between aromatic rings in that the indolocarbazole derivatives could emit strong blue
xerogel 8 was shorter than that in 9, meaning that stronger π–π light in solutions and in gel states. The indolocarbazoles
interactions would happen in gel 8 than in 9. The stronger the without long carbon chains 4–6 tended to form crystal or
π–π interactions, the easier the crystallization of the conju- crystal-like aggregates due to strong π–π interactions. It should
gated molecule. In our case, the gelation ability of compound be noted that the strength of π–π interactions could be
9 was better than compounds 7 and 8 might be attributed to reduced to some extent via the introduction of long alkyl
the balanced π–π interactions. Therefore, it is useful for the groups into N-positions of indolocarbazoles. As a result, the
fabrication of π-gels with desired functionality by tuning the organogels based on indolocarbazoles were gained in alcohols
strength of π–π interactions in the aggregated states.
(such as tert-amyl alcohol, n-octyl alcohol and the mixed sol-
vents containing methanol), acetone and DMSO, directed by
balanced π–π interactions. The molecular packing models of
indolocarbazoles were proposed, and the layered structures
were involved in the nanostructures. Notably, the xerogel-
based film of compound 9 could sense gaseous TNT with high
sensitivity via fluorescence quenching because the amplified
fluorescence quenching could be induced by the enhanced
intermolecular exciton diffusion along the long axis of the 1D
nanofibers and the high surface-to-volume ratio and large
interspace in the 3D networks favored the adsorption, accumu-
lation and diffusion of gaseous molecules. It is believed that
the gelation of polycyclic aromatic heterocyclic compounds
is useful for the construction of novel functional soft materials,
exhibiting unique photophysical and optoelectronic properties.
Fluorescent sensory properties of compound 9 in the xerogel-
based film for probing gaseous TNT
On the one hand, the detection of explosives is important in
military operations and public security, and some emitting
electron-rich conjugated systems can sense aromatic nitro
compounds via fluorescence quenching.44,45 On the other
hand, we have found that the synthesized indolocarbazoles
were electron donors and strongly emissive in the xerogel-
based film. Therefore, we selected xerogel 9 as an example to
study its fluorescent sensory properties for detecting TNT
(2,4,6-trinitrotoluene). As shown in Fig. 7, the fluorescence
emission intensity of compound 9 in xerogel-based film
decreased significantly upon exposure to gaseous TNT at
40 °C, and the fluorescence quenching efficiency reached 62%
when the exposure time was 5 min. We deduced that the good
sensitivity of the xerogel-based film of compound 9 towards
TNT resulted from the amplified fluorescence quenching
induced by the enhanced intermolecular exciton diffusion
along the long axis of the 1D nanofibers as well as the high
surface-to-volume ratio and the large interspace in 3D
networks.13,15
Experimental
Instruments
1H NMR spectra were recorded with a Bruker avance III 500 or
avance III 400 MHz using CDCl3 and DMSO-d6 as the solvents.
13C NMR spectra were recorded on a mercury plus 125 MHz or
100 MHz using CDCl3 and DMSO-d6 as the solvents. Mass
spectra were performed on Agilent 1100 MS series and AXIMA
CFR MALDI/TOF (matrix assisted laser desorption ionization/
time-of-flight) MS (COMPACT). IR spectra were measured with
a Nicolet-360 FT-IR spectrometer by incorporation of samples
in KBr disks. UV-vis absorption spectra were determined on a
Shimadzu UV-1601PC Spectrophotometer. Fluorescence emis-
sion spectra were carried out on a Shimadzu RF-5301 Lumine-
scence Spectrometer. Fluorescence microscopy images were
obtained on a fluorescence microscope (Olympus Reflected
Fluorescence System BX51, Olympus, Japan). SEM images were
obtained through the HITACHI-SU8020. XRD spectra were
determined on the PANalytical-Empyrean. Cyclic voltammetry
(CV) was performed using a CHI 604C voltammetric analyzer
and measurements were carried out in CH2Cl2 (refluxed with
CaH2 overnight) containing 0.1 M Bu4NBF4 as a supporting
electrolyte. A platinum button was used as a working electrode
and a platinum plate as a counter electrode. All potentials
were recorded versus Ag/Ag+ as a reference electrode (using
ferrocene as an internal standard). The scan rate was main-
tained at 50 mV s−1. The optimized configurations and HOMO
Fig. 7 Time-dependent fluorescence emission spectra of compound 9
in xerogel-based film upon exposure to the saturated vapor of TNT at
40 °C (λex = 365 nm).
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Organic & Biomolecular Chemistry
and LUMO molecular orbitals were calculated by the DFT
(B3LYP/6-31G) method on the Gaussian 09 software.
4′,6′-Dinitro-N4,N4,N4″,N4″-tetraphenyl-[1,1′:3′,1″-terphenyl]-
4,4″-diamine (3). By following the synthetic procedure for
compound 1 except (4-(diphenylamino)phenyl)boronic acid
(2.00 g, 6.90 mmol) and 1,5-dibromo-2,4-dinitrobenzene
(1.20 g, 3.68 mmol) as reagents. The crude product was puri-
fied by column chromatography using petroleum ether–ethyl
acetate (v/v = 4/1) as an eluent to afford 3 (1.80 g, 80%) as red
Fabrication of organogel and xerogel-based film
The solutions of indolocarbazoles in selected organic solvents
were obtained by heating. After the hot solutions were sub-
jected to sonication for 5 s, followed by aging at room tempera-
ture, the organogels were formed. The xerogel-based film was
generated by dropping the organogel 9 (obtained from DMSO)
dispersed in ethanol on a glass slide, followed by the natural
evaporation of the solvent.
1
powder. mp > 200 °C. H NMR (400 MHz, CDCl3) (ppm): 8.36
(s, 1H, ArH), 7.55 (s, 1H, ArH), 7.30 (t, 8H, J = 7.30 Hz), 7.16
(m, 12H, ArH), 7.08 (m, 8H, ArH) (see Fig. S9†). IR (KBr, cm−1):
3039, 2924, 1589, 1520, 1493, 1335, 1282, 1184, 831, 754, 698,
517. MS, m/z: calc.: 654.7, found: 655.3 [M+ + H] (see
Fig. S10†).
Sensing properties of compound 9 in the xerogel-based film
13,14-Dihydrobenzo[c]indolo[2,3-a]carbazole (4). A solution
of 1 (2.00 g, 5.4 mmol) in triethyl phosphite (20 mL) was
refluxed for 2 days under a N2 atmosphere. After triethyl phos-
phite was evaporated under vacuum, the residue was purified
by column chromatography using dichloromethane as an
eluent to afford 4 (0.70 g, 42%) as a light yellow solid. mp >
The sensing performance of compound 9 in the xerogel-based
film towards vapor of TNT was studied in a way adopted by
Swager.46 The film to be tested was inserted into a sealed vial
containing solid TNT and cotton gauze, which could prevent
the direct contact of the film and the analyte, at 40 °C. The
time-dependent fluorescence spectra of the film were
measured after immersing the film into the sealed vial for a
certain time.
1
200 °C. H NMR (400 MHz, DMSO-d6) δ (ppm): 11.56 (s, 2H,
NH), 8.91 (s, 2H, ArH), 8.64 (d, 2H, ArH, J = 8.64 Hz), 7.83 (d,
2H, ArH, J = 7.83 Hz), 7.66 (m, 2H, ArH), 7.45 (t, 2H, ArH, J =
7.45 Hz), 7.36 (t, 2H, ArH, J = 7.36 Hz) (see Fig. S11†). 13C NMR
(100 MHz, DMSO-d6) δ (ppm): 138.40, 126.45, 125.93, 123.95,
Materials
The precursors of naphthalene-1,4-diyldiboronic acid, 123.75, 123.59, 121.26, 120.12, 113.22, 112.19 (see Fig. S12†).
naphthalen-1-ylboronic acid,
(4-(diphenylamino)phenyl)- IR (KBr, cm−1): 3408, 3049, 1622, 1545, 1421, 1327, 1261, 744,
boronic acid and the 1,5-dibromo-2,4-dinitrobenzene were syn- 663, 428. MS, m/z: calc.: 306.4, found: 307.4 [M+ + H] (see
thesized according to the literature.40–43
Fig. S13†).
1,4-Bis(2-nitrophenyl)naphthalene
(1). A
mixture
of
7,9-Dihydrobenzo[g]benzo[4,5]indolo[2,3-b]carbazole (5). By
1-bromo-2-nitrobenzene (4.65 g, 23 mmol), naphthalene-1,4- following the synthetic procedure for compound 4 except
diyldiboronic acid (2.00 g, 9.27 mmol), sodium carbonate using compound 2 (2.00 g, 0.78 mmol) and triethyl phosphite
(9.00 g, 65 mmol), water (30 mL) and toluene (30 mL) was first as reagents. After triethyl phosphite was evaporated under
refluxed under a N2 atmosphere, and Pd(PPh3)4 (0.04 g, vacuum, the residue was recrystallized in THF and petroleum
0.03 mmol) was added rapidly. After refluxing for 2 days, the ether to afford the crude product as a dark brown solid, which
organic layer was separated. After drying over anhydrous mag- was further purified by column chromatography using
nesium sulfate, the solvent was removed. The crude product dichloromethane as an eluent to afford 5 (0.20 g, 12%) as a
1
was purified by column chromatography using petroleum light yellow solid, mp > 200 °C. H NMR (400 MHz, DMSO-d6)
ether–ethyl acetate (v/v = 5/1) as an eluent to afford 1 (3.00 g, δ (ppm): 11.67 (s, 2H, NH), 9.57 (s, 1H, ArH), 9.11 (d, 2H, ArH,
87%) as yellow powder. mp > 200 °C. 1H NMR (400 MHz, J = 9.10 Hz), 8.11 (d, 2H, ArH, J = 8.09 Hz), 7.94 (d, 2H, ArH, J =
CDCl3) (ppm): 8.10 (d, 2H, J = 8.1 Hz), 7.74 (d, 2H, J = 7.74 Hz), 7.93 Hz), 7.89 (t, 2H, ArH, J = 7.87 Hz), 7.82 (s, 1H, ArH), 7.79
7.65 (m, 2H), 7.59 (m, 2H, ArH), 7.51 (m, 2H, ArH), 7.41 (m, (d, 2H, ArH, J = 7.77 Hz), 7.53 (t, 2H, ArH, J = 7.51 Hz) (see
2H, ArH), 7.37 (s, 2H, ArH) (see Fig. S5†). IR (KBr, cm−1): 3057, Fig. S14†). 13C NMR (100 MHz, DMSO-d6) δ (ppm): 138.69,
2922, 2858, 1950, 1869, 1834, 1741, 1518, 1340, 849, 771. MS, 138.64, 129.73, 129.64, 128.91, 127.48, 126.60, 123.52, 122.77,
m/z: calc.: 370.1, found: 369.5 [M+] (see Fig. S6†).
119.62, 114.73, 113.71, 113.64 (see Fig. S15†). IR (KBr, cm−1):
1,1′-(4,6-Dinitro-1,3-phenylene)dinaphthalene (2). By follow- 3406, 3053, 2931, 1618, 1521, 1460, 1319, 1167, 1018, 810, 740,
ing the synthetic procedure for compound 1 except naphthal- 451. MS, m/z: calc.: 356.4, found: 357.6 [M+ + H] (see
en-1-ylboronic acid (3.00 g, 17.4 mmol) and 1,5-dibromo-2,4- Fig. S16†).
dinitrobenzene (2.84 g, 8.7 mmol) as reagents. The crude
N3,N3,N9,N9-Tetraphenyl-5,7-dihydroindolo[2,3-b]carbazole-
product was purified by column chromatography using pet- 3,9-diamine (6). By following the synthetic procedure for com-
roleum ether–ethyl acetate (v/v = 5/1) as an eluent to afford 2 pound 4 except using compound 3 (1.00 g, 1.53 mmol) and
(2.60 g, 71%) as yellow powder. mp > 200 °C. 1H NMR triethyl phosphite as reagents. After triethyl phosphite was
(400 MHz, CDCl3) (ppm): 8.85 (d, 1H, J = 8.85 Hz), 7.93 (t, 4H, evaporated under vacuum, the residue was purified by column
J = 7.94 Hz), 7.70 (d, 1H, ArH), 7.52 (m, 8H, ArH), 7.42 (dd, 2H, chromatography using dichloromethane as an eluent, followed
J = 7.43 Hz) (see Fig. S7†). IR (KBr, cm−1): 3049, 2862, 1936, by recrystallization from light petroleum to afford 6 (0.12 g,
1815, 1722, 1589, 1522, 1346, 918, 775. MS, m/z: calc.: 420.4, 13%) as a brown solid. mp > 200 °C. 1H NMR (400 MHz,
found: 419.6 [M+] (see Fig. S8†).
DMSO-d6) δ (ppm): 10.9 (s, 2H, NH), 8.64 (s, 1H, ArH), 8.05 (d,
6142 | Org. Biomol. Chem., 2014, 12, 6134–6144
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Paper
2H, ArH, J = 8.04 Hz), 7.30 (t, 9H, ArH, J = 7.29 Hz), 7.04 (m, column chromatography using dichloromethane–petroleum
14H, ArH, J = 7.02 Hz), 6.86 (d, 2H, ArH, J = 6.85 Hz) (see (v/v = 1/7) as an eluent to afford 9 (0.25 g, 72%) as a light
1
Fig. S17†). 13C NMR (100 MHz, DMSO-d6) δ (ppm):148.41, yellow solid. mp = 121.0–123.0 °C. H NMR (400 MHz,CDCl3)
144.80, 141.77, 140.65, 129.83, 123.67, 122.72, 120.57, 120.04, δ (ppm): 8.55 (s, 1H, ArH), 8.01 (d, 2H, ArH, J = 8.01 Hz), 7.25
117.78, 116.71, 110.68, 107.23 (see Fig. S18†). IR (KBr, cm−1): (m, 8H, ArH, J = 7.25 Hz), 7.15 (m,10H, ArH, J = 7.15 Hz), 7.06
3400, 3030, 2955, 2862, 1601, 1477, 833, 746, 683, 472. MS, (s, 1H, ArH), 6.99 (t, 6H, ArH, J = 6.99 Hz), 4.19 (t, 4H,
m/z: calc.: 590.7, found: 592.1 [M+ + 2H] (see Fig. S19†).
NC16H33, J = 4.20 Hz), 1.81 (m, 4H, NC16H33, J = 1.81 Hz), 1.24
13,14-Dihexadecyl-13,14-dihydrobenzo[c]indolo[2,3-a]carb- (m, 52H, NC16H33, J = 1.24 Hz), 0.87 (m, 6H, NC16H33, J = 0.86
azole (7). A solution of compound 4 (0.20 g, 0.65 mmol) and Hz) (see Fig. S26†). 13C NMR (100 MHz, CDCl3) δ (ppm):
C16H33Br (0.50 g,1.63 mmol) in DMF was cooled by ice-water 148.46, 145.05, 142.05, 141.09, 129.11, 123.54, 122.09, 120.09,
to 0 °C and NaH (0.10 g, 4.2 mmol) was slowly added under a 119.73, 117.50, 117.22, 110.61, 105.47, 87.01, 43.08, 29.75,
N2 atmosphere. After stirring at room temperature for 2 hours, 29.71, 29.59, 29.54, 29.41, 22.74, 14.18 (see Fig. S27†). IR (KBr,
the mixture was poured into ice water and extracted by CH2Cl2. cm−1): 2920, 2854, 1601, 1456, 1321, 1259, 1117,746, 694. MS,
The organic phase was washed with brine and dried over anhy- m/z: calc.: 1038.8, found: 1039.9 [M+ + H] (see Fig. S28†).
drous magnesium sulfate. After CH2Cl2 was removed, the
crude product was purified by column chromatography using
dichloromethane–petroleum (v/v = 1/7) as an eluent to afford 7
(0.42 g, 85%) as a white solid. mp = 68.0–69.0 °C. 1H NMR
(400 MHz,CDCl3) δ (ppm): 8.95 (dd, 2H, ArH, J = 8.95 Hz), 8.66
(d, 2H, ArH, J = 8.66 Hz), 7.67 (dd, 4H, ArH, J = 7.68 Hz), 7.50
(t, 2H, ArH, J = 7.50 Hz), 7.43 (t, 2H, ArH, J = 7.43 Hz), 4.66 (t,
4H, NC16H33, J = 4.66 Hz), 1.77 (m, 4H, NC16H33, J = 1.8 Hz),
1.42 (m, 4H, NC16H33, J = 1.41 Hz), 1.26 (m, 48H, NC16H33),
0.88 (t, 6H, NC16H33, J = 0.87 Hz) (see Fig. S20†). 13C NMR
(100 MHz, CDCl3) δ (ppm): 143.02, 131.01, 127.05, 127.13,
124.03, 123.91, 123.88, 122.15, 121.09, 118.19, 112.48, 48.39,
31.96, 29.72, 29.69, 29.66, 29.60, 29.52, 29.40, 29.28, 28.89,
22.73, 14.16, 0.03 (see Fig. S21†). IR (KBr, cm−1): 2929, 2848,
1628, 1531, 1452, 1335, 748. MS, m/z: calc.: 754.6, found: 756.2
[M+ + H] (see Fig. S22†).
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (21374041) and the Open Project
of State Key Laboratory of Supramolecular Structure and
Materials (SKLSSM201407) and the Open Project of State
Key Laboratory of Theoretical and Computational Chemistry
(K2013-02).
Notes and references
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1
(0.34 g, 76%) as a brown solid. mp = 106.0–108.0 °C. H NMR
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J = 9.11 Hz), 8.07 (d, 2H, ArH, J = 8.07 Hz), 7.93 (d, 2H, ArH, J =
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