A multifunctional porous organic schottky barrier diode

Article abstract of DOI:10.1002/anie.201205439

Mesoporous materials: A multifunctional porous organic material (ANPPIT; see picture) has been synthesized and characterized. Multifunctionality of the compound has been determined from nitrogen adsorption, guest-dependent luminescence, and electrical conductivity measurements. Copyright

Full text of DOI:10.1002/anie.201205439

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Angewandte
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DOI: 10.1002/anie.201205439
Multifunctional Organic Materials
A Multifunctional Porous Organic Schottky Barrier Diode**
Sasanka Dalapati, Rajat Saha, Sankar Jana, Astam K. Patra, Asim Bhaumik, Sanjay Kumar, and
Nikhil Guchhait*
For the past two decades scientists have been engaged in
designing and synthesizing porous metal–organic frameworks
(MOFs) capable for gas and solvent adsorption.[1,2] From the
middle of the last decade, researchers have tried to include
many functional properties in a single framework. Thus
multifunctional MOFs[3–5] were developed as materials with
a set of well-defined properties, for example, porosity and
magnetism, porosity and electrical conductivity, or porosity
and optical properties. This synergism, resulting from the
combination of two different functionalities, opens up the way
to new multifunctional materials.
In the last two years, a large number of porous covalent
organic frameworks were reported by different research
groups worldwide and they have shown that in porous organic
materials (POMs) a degree of porosity can be achieved which
is higher than in MOFs.[6–8] In the case of 3D MOFs, several
modifications are needed to achieve higher porosity, such as
generation of unsaturated metal centers (UMCs) and for-
mation of hydrophobic channels. In contrast to 3D MOFs,
adsorption by supramolecular 3D POMs occurs in several
steps because of their structural flexibility.[8–10] Regarding
materials with a similar degree of porosity, the molecular
weight of POMs is lower than that of MOFs and zeolites.
Thus, it can be said that the adsorption properties of porous
organic materials (POMs) are better than those of other
porous materials like MOFs and zeolites because of their high
flexibility, multistep adsorption process, and low molecular
weight.
studies. This material is porous after removal of the dime-
thylformamide (DMF) and the water guest molecules. The
porosity of this material has been studied by N2 adsorption at
77 K. Further, to show the multifunctional character of the
material, we have studied its I-V characteristic and guest-
dependent luminescence property.
Slow Et2O vapor diffusion in DMF solution of ANPPIT
affords yellow rhombic crystal suitable for X-ray diffraction at
room temperature (see the Supporting Information).[16] The
diffraction analysis indicates that the compound crystallizes in
an achiral C2/c space group (see Table S1 in the Supporting
Information). Each asymmetric unit contains half of the
ANPPIT moiety with one DMF and 1.5 water guest mole-
cules. Within the molecular structure, p-nitroaniline (PNA)
units are nearly perpendicular to the basic pyrrolo[3,4-
f]isoindole-1,3,5,7-tetraone moiety (Figure 1), that forms
Figure 1. ORTEP view of ANPPIT without guest molecules (DMF and
water) at a 30% probability.
Recently, some research groups were engaged in estab-
lishing multifunctional properties in POMs.[11–15] Here, we also
synthesized a multifunctional porous organic material, 2,6-
bis-(2-amino-5-nitro-phenyl)-pyrrolo[3,4-f]isoindole-1,3,5,7-
tetraone (ANPPIT, see the Supporting Information), and
characterized this material by single-crystal X-ray diffraction
analysis, NMR and IR spectroscopy, PXRD, and thermal
a 3D architecture by H-bonding and p interactions (Figur-
es S1, S2 and Tables S2, S3 in the Supporting Information).
Each molecular unit is connected to four other units through
À
N1 H2···O4 hydrogen-bonding interactions and thus a 3D
supramolecular architecture is formed (Figure S1). During
the packing process, a 1D channel is generated along the
crystallographic c axis (Figure 2). The diameter of the channel
is 17.3 ꢀ 7.0 ꢁ2. Water and DMF guest molecules persist
within the channels through supramolecular hydrogen-bond-
ing interactions (Figure S3). The guest water molecules are
connected by hydrogen-bonding interactions leading to the
formation of a 1D helical chain (Figure S4).
[*] S. Dalapati, S. Jana, Dr. N. Guchhait
Department of Chemistry, University of Calcutta
92 A.P.C. Road, Kolkata (India)
E-mail: nguchhait@yahoo.com
R. Saha, Dr. S. Kumar
Department of Physics, Jadavpur University, Kolkata (India)
Thermal analysis reveals that at 1208C all DMF and water
guest molecules are removed and the evacuated framework is
stable up to 2308C without any further loss of weight
(Figure S5). The powder X-ray diffraction (PXRD) pattern,
obtained after removal of the guest molecules by heating at
1208C for 3 h, indicates that the evacuated framework
remains crystalline (Figure S6). However, there is a change
in the PXRD patterns of different materials which leads to the
conclusion that a structural transformation occurs upon
removal of the guest molecules followed by the formation
A. K. Patra, Dr. A. Bhaumik
Department of Materials Science, Indian Association for the
Cultivation of Science, Kolkata (India)
[**] N.G. acknowledges the DST (project number SR/S1/PC/26/2008).
S.D. and S.J. acknowledge UGC for a fellowship. A.K.P. and R.S. is
thankful to the CSIR, New Delhi. We thank S.K.’s colleague, Dr. P. P.
Ray, for valuable discussion.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201205439.
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monolayer adsorption at the void surfaces. However, at
higher pressure (in the P/P0 range of 0.01–0.80) the quantity
of adsorbed gas increases monotonically. After P/P0 = 0.80,
the quantity of adsorbed gas (178 mL) sharply increases
without saturation at P/P0 = 1.
This adsorption phenomenon can be explained through
the occurrence of “stepwise adsorption”. Stepwise adsorption
has been observed for 3D supramolecular MOFs which are
hosting the adsorbate molecules in their framework.[5a,18] The
supramolecular interactions induce flexibility within the
framework. Up to a certain pressure, adsorbate molecules
cannot enter into the void spaces but when a certain pressure
is reached, the pores are open and at this pressure a large
amount of adsorbate molecules enters into the void spaces.
Supramolecular interactions are rather weak forces. The
discrete units packed by supramolecular interactions can be
moved by the external pressure and thus a “gate-opening”
process is observed.[8] For POMs, which show extrinsic
porosity, the discrete molecules/building blocks are packed
through supramolecular interactions. Thus, POMs with
“framework flexibility” are mostly associated with “stepwise
adsorption”.[8] In the present case, there is a stiff increase in
the adsorption of N2 from 70 mL at P/P0 = 0.80 to 178 mL at
P/P0 = 1. Thus, it can be said that after P/P0 = 0.8 this
supramolecular and flexible framework undergoes structural
transformation to achieve a higher porosity which may be
indicative of “gate opening”.
Figure 2. 3D supramolecular structure with a 1D channel along the
c axis (DMF and water guest molecules persist within the channels).
of a new framework with a diffraction peak at 7.168 for 2q.
PLATON[17] analysis suggests that the compound contains
a potential solvent-accessible void space of 1583.3 ꢁ3 (46.2%,
Figure 2). The high stability and flexibility of the supramolec-
ular framework after loss of the guest molecules allows us to
study the adsorption behavior of ANPPIT.
The N2 adsorption study has been carried out with the
evacuated framework at 77 K (see the Supporting Informa-
tion). Following the general classification of adsorption
isotherms, the adsorption profile can be classified as type II
(Figure 3). The Brunauer–Emmett–Teller (BET) surface area
and the respective pore volume have been determined to be
93.2 m2 gÀ1 and 0.21 ccgÀ1, respectively. The pore size distri-
bution of the sample using the NLDFT model (N2 adsorption
on carbon as reference) suggested that ANPPIT has an
average pore width of about 4.9 nm, which could be attributed
to interparticle mesopores. The isotherm shows that at a low
pressure (P/P0 of about 0) the material adsorbs a certain
amount of gas (about 5 mL), which could be attributed to
To show a guest-dependent optical property, we have
studied the luminescence behavior of the synthesized com-
pound (ANPPITwith 3H2O and 2DMF guest molecules) and
the evacuated framework in the solid state. Both compounds
exhibit similar absorption behavior with bands at about 315
and 390 nm (Figure S7), which corresponds to the p–p*
transition of the benzene chromophore and the intermolec-
ularly conjugated p-stacked framework, respectively.[19,20a]
The guest-filled compound shows an extra broad band at
about 500 nm, which can be attributed to charge transfer from
the electron-rich NH2 to the electron-withdrawing NO2 group
or in the pyrrolo[3,4-f]isoindole-1,3,5,7-tetraone moiety in the
presence of polar guest molecules (DMF and H2O). Upon
excitation at 390 nm, the compound (ANPPITwith 3H2O and
2DMF guest molecules) shows well-separated emission bands
at about 450, 480, and 525 nm (Figure 4a). These bands
correspond to different p-electronic transition in the packed
molecular system.[19] Upon removal of guest molecules,
a remarkable emission enhancement was observed by keep-
ing the highly structured bands unaltered. Fluorescence
enhancement can be attributed to the inhibition of energy
transfer (nonradiative decay) through polar guest molecules.
However, both compounds (ANPPIT with and without guest
molecules) exhibit similar but less intense luminescence
spectra (Figure S8) upon excitation at 315 nm.
Recently, extended p-conjugated organic supramolecular
compounds have emerged as promising semiconducting
materials.[19] These organic semiconducting materials are
generally used for the fabrication of several electronic devices
such as field effect transistors (FETs), light-emitting diodes
(LEDs), photovoltaic cells (PCs), and Schottky barrier
diodes.[20–22] Metal–semiconductor rectifying contacts are
*
*
Figure 3. N2 adsorption ( )–desorption ( ) isotherm at 77 K (inset,
the pore size distribution of the sample calculated using the NLDFT
model). STP=standard temperature and pressure.
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Figure 4. a) Guest-dependent photoluminescence (lex =390 nm) and
b) I-V characteristics of ANPPIT at room temperature. I-V curve: a solid
black line indicates the fitting curve at a forward bias voltage and 4.7
is the I(2 V)/I(À2 V) ratio.
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active element” of the next generation of semiconducting
devices. The current–voltage (I-V) characteristic (Figure 4b
and Figure S9) for the “ITO/sample/Al” sandwich structure
(ITO = indium–tin oxide and sample = ANPPIT) and a curve
fitted to the simplest diode equation[23] are shown in Figure 4b
(fitting has been carried out in forward bias voltage only). The
I-V curve of the junction exhibits a nonlinear behavior and
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(ANPPIT) most likely originate from the extended p-
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Figure S2). The experimental band gap (Eopt = 2.88 eV, Fig-
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conduction (LUMO = lowest unoccupied molecular orbi-
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are working as Ohmic contact and Schottky barrier diode,
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[16] CCDC 886757 (ANPPIT) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
In summary, we have successfully designed and synthe-
sized a purely organic porous material {2,6-bis-(2-amino-5-
nitro-phenyl)-pyrrolo[3,4-f]isoindole-1,3,5,7-tetraone,
ANPPIT} of relatively low molecular weight. The single-
crystal X-ray diffraction analysis shows that its 3D architec-
ture is formed through H-bonding and p interactions which
undoubtedly characterize its porosity, guest-dependent lumi-
nescence, and electrical properties. This may be useful for the
future construction of multifunctional materials with low
molecular weight. The discovery of multifunctional materials
obtained by a simple low-cost method is a breakthrough in an
area of current chemical interest.
Received: July 10, 2012
Revised: September 29, 2012
Published online: November 5, 2012
Keywords: adsorption · electrical conductivity · luminescence ·
mesoporous materials · X-ray diffraction
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