Influence of molecular structure on the photoluminescence of 2-methyl benzimidazolium picrate: A new fluorescent material

Article abstract of DOI:10.1016/j.saa.2013.09.096

This work reports the structural and photo luminescent characterization of the 2-methyl benzimidazolium picrate salt for the first time. This new material exhibits the π-π stacking of aromatic rings involving the benzimidazolium cation and picrate anion, which is additionally supported with a rare n → π interaction as well as with the extended networks of intermolecular hydrogen bonds in the solid state. The FT-IR and UV characterizations further support the solid state structural features of the salt. However, the spectroscopic results reveal that the stacking of ion pairs, which is observed in the solid state, is not maintained in solution. The DSC result indicates that the material is stable under ambient temperature conditions. Interestingly, upon photo excitation at 325 nm in the solid state, the material shows an unusual red emission around 615 nm, which is probably attributed to the supramolecular stacked nature of the ion pairs, along with a usual picrate centered green emission at 530 nm. However, the fluorescence measurement in solution, wherein the stacking of ion pairs are not maintained, shows only a single peak at lower wavelength. These observations highlight the influencing role of the supramolecular stacking interactions on the photoluminescence of the material, due to which, a chemical sensing action may be envisaged.

Full text of DOI:10.1016/j.saa.2013.09.096

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 861–866
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Influence of molecular structure on the photoluminescence of 2-methyl
benzimidazolium picrate: A new fluorescent material
⇑
M. Obulichetty, D. Saravanabharathi
Department of Chemistry and Applied Chemistry, PSG College of Technology, Coimbatore 641 004, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
2-Methyl benzimidazolium picrate
crystals were grown and
characterized.
ꢀ
Rare heteromolecular stacking of ion
pairs and n ?
p interaction were
identified.
ꢀ
ꢀ
Spectral characterizations support the
structural features.
Stacking of ions causes red emission
of the material upon UV excitation.
a r t i c l e i n f o
a b s t r a c t
Article history:
This work reports the structural and photo luminescent characterization of the 2-methyl benzimidazoli-
Received 30 June 2013
Received in revised form 10 September
2013
Accepted 26 September 2013
Available online 8 October 2013
um picrate salt for the first time. This new material exhibits the
the benzimidazolium cation and picrate anion, which is additionally supported with a rare n ?
p
–
p
stacking of aromatic rings involving
inter-
p
action as well as with the extended networks of intermolecular hydrogen bonds in the solid state. The FT-
IR and UV characterizations further support the solid state structural features of the salt. However, the
spectroscopic results reveal that the stacking of ion pairs, which is observed in the solid state, is not main-
tained in solution. The DSC result indicates that the material is stable under ambient temperature con-
ditions. Interestingly, upon photo excitation at 325 nm in the solid state, the material shows an
unusual red emission around 615 nm, which is probably attributed to the supramolecular stacked nature
of the ion pairs, along with a usual picrate centered green emission at 530 nm. However, the fluorescence
measurement in solution, wherein the stacking of ion pairs are not maintained, shows only a single peak
at lower wavelength. These observations highlight the influencing role of the supramolecular stacking
interactions on the photoluminescence of the material, due to which, a chemical sensing action may
be envisaged.
Keywords:
2 – Methyl benzimidazolium picrate
X –ray structure
Ion pairs
Supramolecular stacking
Photoluminescence
Ó 2013 Elsevier B.V. All rights reserved.
Introduction
[3], second harmonic generation [3], fluorescence emission [1,3]
solar energy harvesting [4] etc. It is well known that the picric acid,
Crystalline picrates of organic compounds [1–6] have attracted
a great deal of interest in the recent past owing to their applica-
tions in various fields such as molecular recognition, selective
guest absorption [1–3], bio – mimicking models [1–3], magnetism
apart from being a Lewis acidic electron acceptor, can also function
as a Brønsted acidic proton donor and hence its acidity could be in-
duced either way depending upon on the nature of the partners in-
volved in the reactions [1,6]. For example, with
aromatic hydrocarbons, like naphthalene, or with weakly basic
aromatic amines, particularly when the pKa of the reactants are
less than 3.8, only the Lewis acidity of the picric acid is exemplified
p donating
D
⇑
Corresponding author. Tel.: +91 9443220623.
E-mail address: dsbharathi@yahoo.com (D. Saravanabharathi).
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.09.096

862
M. Obulichetty, D. Saravanabharathi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 861–866
Table 1
with the formations of p–p stacked charge transfer (CT) molecular
Crystallographic data for 2-methyl benzimidazolium picrate.
complexes [1,5]. On contrary, with the strong organic heterocyclic
bases, the Brønsted acidity of the picric acid is readily shown up
with the proton transfer from the –OH of the acid to the basic
nitrogen centers, resulting in the formations of ionic salts with dis-
tinct anion–cation pairs [1,6–8]. It is to be noted that the organic
picrates, although invariably exhibit extensive network of hydro-
gen bonds in the solid state, do not show the overall planar stack-
ing interactions of the anion–cation pairs very regularly, except in
few cases of ionic heterocyclic picrates [7–11]. Such materials have
become attractive, not only due to their fascinating structural
topologies [12], but also to their interesting optical functions,
which result from the delocalized nature of orbitals and the non
centrosymmetric manner of the crystal packing in the solid state
[13–15]. In this context, the picrates of the benzimidazole family
of compounds are also interesting and need to be scrutinized.
As the crystalline benzimidazolium picrates are scanty in the
literature [7,16], this paper reports the synthesis, growth and char-
acterizations of 2-methyl benzimidazolium picrate crystal. In fact,
this is the first report, which highlights the influencing role of the
supramolecular structure on the photoluminescence of the crystal-
line benzimidazolium picrates.
Empirical formula
Formula weight (g molꢂ1
Temperature
C14ꢁH11N5O7
)
361.28
293(2) K
0
Wavelength
0.71073 ÅA
Crystal system, space group
Unit cell dimensions
Monoclinic, P21/n
0
a = 9.147(5) ÅA, a(°) = 90.000(5)
0
b = 13.916(5) ÅA b(°) = 100.321(5)
0
c = 11.961(5) ÅA
c
(°) = 90.000(5)
0
Volume
1497.9(12) ÅA3
4, 1.602 Mg/m3
0.132 mmꢂ1
744
Z, Calculated density
Absorption coefficient
F(000)
Crystal size
Theta range for data collection
Limiting indices
0.20 ꢃ 0.20 ꢃ 0.20 mm
2.27 to 25.00 deg.
ꢂ10<=h<=10, ꢂ16<=k<=16, ꢂ12<=l<=14
13865/2634 [R(int) = 0.0306]
100.0%
Reflections collected/unique
Completeness to theta = 25.00
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2sigma(I)]
R indices (all data)
Semi-empirical from equivalents
0.9979 and 0.9310
Full-matrix least-squares on F2
2634/0/245
1.019
R1 = 0.0424, wR2 = 0.1071
R1 = 0.0541, wR2 = 0.1172
0.0089(12)
Extinction coefficient
Experimental
Synthesis and characterizations
Results and discussion
2-Methylbenzimidazole was prepared and purified according to
the known procedure [17] using commercially available 1,2 phen-
ylenediamine (Thomas Backer) and acetic acid (Ranbaxy). Picric
acid (Ranbaxy) was purchased and used without further purifica-
tion. The salt was obtained as a yellow precipitate by mixing the
methanolic solutions of the precursors in 1:1 stoichiometric ratio
(0.003 mol in 20 ml). The precipitated compound was filtered off,
air-dried and dissolved in a minimum amount of acetone, which
on slow evaporation yield bulkier crystals of centimeter dimen-
sion. The 1H NMR of the material in acetone showing signals at
d = 8.78 (s, 2H, picrate), 7.91 (m, 2H, benzimidazolium – aromatic),
7.57 (m, 2H, benzimidazolium – aromatic), 3.06 (s, 3H, benzimi-
dazolium – methyl)] confirms the occurrence of a neat reaction.
Recrystallization of the powdered sample in methanol has offered
the X-ray quality single crystals. The purity of the single crystals
was ascertained with the elemental analysis. [Calculated for
C14ꢁH11N5O7 is C = 46.53%, H = 3.04%, N = 19.39%; Found:
C = 46.50%, H = 3.09%, N = 19.54%].
The FT-IR spectra was recorded on a Shimadzu 8400S FT-IR
spectrophotometer as a KBr pellet. The UV–Visible spectra was re-
corded using Shimadzu UV -1700 spectrophotometer. The 1H NMR
was examined using Bruker AV III – 400 MHz instrument. The ele-
mental analysis was performed on Thermo finnigan, Flash EA 1112
CHN analyzer. Fluorescence measurement was carried out on the
powdered sample using the Jobin Yvon Fluorimeter instrument.
The DSC study was performed in a Pyris 6 DSC instrument with a
heating rate of 10 °C/min.
Synthesis and X-ray structural investigations
The salt was readily prepared as a yellow powder in a near
quantitative yield by simply mixing the methanolic solutions of
the precursors in 1:1 stoichiometry (Scheme 1). It has also been
found that the powdered sample can be easily grown either as
bulkier crystals of centimeter dimension (Fig. S1 A) or as fine single
crystals, by simply changing the medium from acetone to metha-
nol respectively. The integrity of re crystallized sample was ascer-
tained through elemental analysis and by its 1H NMR spectrum in
acetone-d6.
The molecular structure of the salt is shown in Fig. 1. The con-
stituents of the asymmetric unit possess all the relevant structural
features of a picrate anion and a protonated benzimidazolium cat-
ion, indicating an acid–base reaction between the precursors. As
seen in the other structural reports [19], the delocalized nature
of the constituents is discernible through the bond lengths, angles
of the near planar aromatic rings of the system (Table S1). Interest-
ingly, the pair of ions present in the asymmetric unit shows the
p–p stacking interactions involving the six membered rings of pic-
rate anion and the benzimidazolium moiety (Fig. 1 and (Fig. S2)
such that the inter planar centroid–centroid distance between
0
the stacking is in the range of 3.6 ÅA, which is well within the char-
acteristic magnitude of the CT complexes [20]. The rings of the
stacks are twisted against each other with an angle of 22° and
resemble the usual values of the stacked aromatic rings [5,20].
In addition, the overlap of the imidazolium five membered ring
of the cation with the para nitro group of the picrate anion
deserves attention (Fig. S3). It is interesting to note that the carbon
atom (C8) of the five membered segment of the cation overlaps the
Crystallographic studies
X-ray diffraction and data collection was performed on a Bruker
Axs Kappa Apex 2 CCD Diffractometer. The structure was solved by
direct method with SHELXS-97 and refined using SHELXL-97 [18].
The non-hydrogen atoms were refined with anisotropic displace-
ment parameters. The C bound hydrogen atoms were placed at cal-
culated positions and were treated as riding on their parent atoms.
A summary of crystallographic data and refinement parameters are
given in Table 1.
nitrogen (N1) of the para nitro group of the anion, with a non
0
bonded C8---N1 distance at 3.38 ÅA, which is very similar in pattern
to those found in the structures of serotonin picrate [21] and in the
supramolecular tapes of bis–benzimidazoles [22], wherein such
interactions play crucial roles in the assembly of solid state struc-
tures. The observed non bonded distance, although is not within
the sum of the van der Waals radii of the atoms involved, is

M. Obulichetty, D. Saravanabharathi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 861–866
863
O
H
OH
O2N
NO2
N
N
+
O2N
NO2
CH3
.
CH3
N
H
+
N
H
NO2
NO2
Scheme 1. Synthesis of the 2-methyl benzimidazolium picrate.
moiety of the imidazolium group and the symmetry related O
atoms of the picrate group (Figs. S5 and S6) in the solid state.
As reviewed in the introduction, the ionic picrate crystals with
the
p–p stacking interactions of the aromatic rings are not very
recurring in nature and therefore the present structure represents
one such example of the ionic picrates, in which the stacking is fur-
ther facilitated with a rare n ?
p interaction as well as by the inter-
molecular hydrogen bonds in the solid state. The spectral studies
also substantiate the observed solid state structural features and
are discussed in the following sections.
Spectroscopic analysis
The FT-IR spectral appearance of the product (Fig. 2 and
(Fig. S7), differing from those of its precursors, points out a clean
ionic reaction. Particularly, the sensitivity associated with the mas
N–O vibration is worth discussing. While it occurs as a single band
at 1525 cmꢂ1 for the free picric acid, it shows splitting with one
higher wave number band at 1560 cmꢂ1 and another as a broad-
ened line in the region between 1548 cmꢂ1 and 1528 cmꢂ1 for
the product. In addition, the aromatic C–H out of plane bending
vibrations of the product, occurring at 790 cmꢂ1 is also slightly
higher in wave numbers when compared to the value of
783 cmꢂ1 shown by the picric acid. These observations, fitting well
with the IR spectral classification of organic picrates by the Kross
et al. [23] and further resembling the IR spectral outcome of the
earlier reports on the molecular picrates containing both the
n ?
existence of a similar n ?
the imidazolium ring to the vacant orbital of the nitro group, which
are oriented due to the stacking of the aromatic rings in the
solid state, thus supporting the X-ray structural features.
p
and
p
–
p
interactions [24], provide clear evidences for the
Fig. 1. The molecular structure of 2-methyl benzimidazolium picrate with atom
labeling. (Hydrogen atoms were omitted).
p
interaction involving a donor site of
p–p
comparable to the ranges found in those structures, suggesting
that the nitro group does indeed interact with the five membered
segment of the imidazolium cation. In fact, by combining these
X-ray structural outcomes, with the FT-IR results (discussed in
the next section), it is possible to infer an n ?
[12,21–25] involving a donor site of the imidazolium ring with
the vacant orbital of the nitro group. Probably, to maximize such
p interactions
p
interactions, the nitrogen of the para nitro group (N1) is situated
approximately in between the imidazolium nitrogen atoms of the
rings (the non bonded distances of N1---N4, and N1---N5
0
0
respectively are at 3.58 ÅA and 3.77 ÅA (Fig. S4).
The stacked sets of ion pairs generate an extended network of
intermolecular hydrogen bonds (Table 2) through the –N+–H
Table 2
0
Intermolecular hydrogen bonding details (AÅ, °) for the 2-methyl benzimidazolium
picrate.
D–H. . .A
d(D–H)
d(H. . .A)
d(D. . .A)
<(DHA)
N(5)–H(5A). . .O(7)#1
N(5)–H(5A). . .O(4)#1
N(4)–H(4A). . .O(5)#2
0.89(2)
0.89(2)
0.85(2)
1.89(2)
2.38(2)
2.10(3)
2.696(2)
3.049(3)
2.937(3)
150(2)
131.7(18)
169(2)
1700
1650
1600
1550
cm-1
1500
1450
1400
Symmetry transformations used to generate equivalent atoms: #1 ꢂx + 3/2, y + 1/2,
ꢂz + 5/2 #2 x + 1, y, z.
Fig. 2. The FT-IR showing the higher wave number shift of the
the salt (solid line) when compared to the free picric acid (dashed line).
m as N–O vibration of

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M. Obulichetty, D. Saravanabharathi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 861–866
the strong hydrogen bonding interactions, thus showing downfield
shifts, the methyl protons on the other hand, may get placed in the
core of the shielding zone, due to which they could exhibit upfield
shift upon lowering the temperature. Although this VT NMR study,
does not pinpoint the through–space interactions, it provides con-
vincing evidences to realize that the stacking of the ion pairs can
occur readily in the absences of any destabilizing forces and thus
go in line with the inference derived from the UV–Visible
experiments.
Thermal studies
In order to check the influence of temperature on this material,
a DSC investigation has been carried out. The result (Fig. 4) indi-
cates that the material is stable up to 150 °C. A sharp melting of
the crystal occurs around 212 °C, coinciding with the observation
made through the melting point apparatus. The endothermic event
around 150 °C may be possibly attributed to a transition of the sys-
tem from the state of ionic salt to the molecular CT complex [1] as
there is a characteristic color change from yellow to red, which is a
well known phenomenon associated with this type of transitions
[1]. In general, the DSC results point out the stability of the mate-
rial for ambient temperature applications.
350
400
450
500
550
600
Wavelength (nm)
Fig. 3. Electronic absorbance spectrum of the material as a transparent film.
The electronic spectrum of the material also gives direct evi-
dence to the solid state structural features. Although as an acetone
solution it shows only a single absorption with a kmax at 377 nm
due to the picrate group (Fig. S8), as a transparent solid film (which
was obtained after the evaporation of the solvent), it shows two
absorption maxima (Fig. 3), one at 369 nm and the other at
412 nm. Since the absorption at 369 nm is due to the picrate moi-
ety, the additional lower energy absorption of the film could be as-
cribed to the formation of the
[26–28], which indicate that the supramolecular stacking interac-
tions are inevitable in the solid state.
Since the variable temperature NMR (VT NMR) is a powerful
technique in exploring the solution dynamics of the
systems [29,30], it has been utilized in the present study as well
to examine the nature of the stacking in solution, by carrying out
the experiments in the range from 25 C to ꢂ50 C in acetone-d6. It
is quite clear from the results (Table 3 and (Fig. S9) that upon low-
ering the temperature, the singlet of the methyl protons show a
slight but gradual upfield shift, whereas, the singlet of the picrate
protons and the multiplet from the set of benzimidazolium protons
show definite downfield shifts. These results, indeed confirm the
existence of a dynamic motion of the ion pairs in the solution.
Since the charges are located almost entirely on the rings of the io-
nic species, the self association phenomenon among the identical
ions is highly improbable [31] and therefore the observed temper-
ature dependence of the chemical shifts, could only be attributed
to some loose stacking of the ion pairs, which could make the pro-
tons at different parts of the molecule to feel the magnetic anisot-
ropy effect at different levels [30], particularly when there is a
reduction in the dynamic motion at lower temperatures.
Fluorescence studies
As the supramolecular structural features of the material in the
solid state, such as stacking of the delocalized planar aromatic
units and coplanar arrangement of the picrate anion with respect
to the cation are well desired for a fluorescence emission character,
which is indeed witnessed upon exposing the material even under
an ordinary household UV lamp (Figs. S1B and S1C), an attempt has
been made to explore the photoluminescent character of the mate-
rial. When the powdered sample of the material was excited in the
absorption range of the picrate group at 325 nm in the solid state,
it shows an efficient luminescence in the longer wavelength ranges
(Fig. 5). A green emission around 520–550 nm, and a longer wave-
length red emission around 615 nm are clearly seen in the photo-
luminescence profile. While the green emission may be attributed
to originate from the picrate anion moiety of the system [33], the
occurrence of a red emission around 615 nm is an interesting phe-
nomenon and deserves attention. This additional red emission may
be attributed to the stacked nature of the ion pairs of the system in
similar line to that of the picrate–peptide complex [27], which was
p–p stacking of the aromatic rings
p–p stacked
characterized for its
p stacking related emissions.
It is to be noted that, no such red emission has been reported in
any of the benzimidazoles in their neutral or in their protonated
state [34–36] and in fact they are known to possess emission max-
ima only within 450 nm. Similarly, the plain imdiazolium picrate,
which cannot exhibit any stacking interaction as that of the pres-
ent compound, showed only in the 500–530 nm range and did
not exhibit the red emission [37]. Therefore, from these correla-
tions, it may be understood that the observed red emission is a
consequence of the supramolecular stacking interactions of the
ion pairs in the solid state. A shoulder around 575 nm may be
attributed for the shallow traps, which are known in the literature
[37].
While the aromatic protons may be located away [32] from the
molecular center of the ion pairs, possibly due to the influence of
Table 3
As the stacking interactions are not sustained in solution, one
would expect that the longer wavelength emission to disappear
in the solution state photoluminescence profile. In fact, the fluores-
cence measurement in the acetone has shown only a single and
shorter wavelength peak (Fig. S10A) around 470 nm. Further it
has been observed in the solid state that the nature of the emission
profile does not change significantly with respect to excitation
wavelength (Figs. S10B–10D). These results highlight the influenc-
ing role of supramolecular structural motifs on the luminescence
Temperature dependent chemical shift values of the different types of protons.
Temperature
(°C)
Chemical shift values
Picrate Benzimidazolium
[A]
Benzimidazolium
[B]
Methyl
25
0
8.78
8.81
8.82
8.83
7.91
7.92
7.93
7.94
7.57
7.57
7.58
7.58
3.06
3.06
3.05
3.04
ꢂ25
ꢂ50

M. Obulichetty, D. Saravanabharathi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 861–866
865
30
28
26
24
22
20
18
16
picrate moiety. The stacks are interlinked with extensive hydrogen
bonds. The FT-IR and UV–Visible characterizations of the material
also support the structural characterizations. This is the first re-
port, showing the photoluminescence of a crystalline benzimi-
dazolium family of picrates. The supramolecular stacking
interaction in the solid state influences the fluorescence character
of the material with an unusual longer wavelength red emission in
the solid state. The report highlights the need to explore the struc-
ture–luminescence relationship in several other compounds of this
family.
Acknowledgements
The authors wish to thank the Principal and Management of the
PSG College of Technology for the facilities and the Head – Depart-
ment of Chemistry and Applied Chemistry of PSG CT for his encour-
agement. Authors also wish to thank the SAIF – IIT Madras for the
X-ray data collection, solving and refining of the structure and for
fluorescence measurement. Authors are thankful to SAIF – IIT Bom-
bay and SIF – IISc Bangalore for the elemental analysis and for VT
1H NMR spectra respectively.
50
100
150
200
250
Temperature oC
Fig. 4. The DSC result of the material.
Appendix A. Supplementary material
530 nm
615 nm
CCDC 912688 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cam-
bridge Crystallographic Data Center, 12, Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033). Figs. S1–S10 and Table S1
are given in the electronic supplementary information.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2013.09.096.
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Conclusion
The synthesis, structural and photo luminescent characteriza-
tions of the 2-methyl benzimidazolium picrate have been per-
formed in this study. The solid-state structural characterization
has revealed the proton transfer reaction from picric acid to 2-
methyl benzimidazole resulting in the formation of ion pairs. The
ion pairs are found to possess a rather unusual heteromolecular
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