The formation of hydrogen-bond facilitated salts with tunable optical properties: An experimental and theoretical study of 2,4,5-triphenylimidazole

Article abstract of DOI:10.1021/cg3014927

Adjusting and modifying the optical properties of organic fluorophores are of importance for the development of the next generation of luminescent materials. Herein, we report the tunable fluorescence of 2,4,5- triphenylimidazole (TPI) chromophore by the formation of multicomponent crystals with the supramolecular building blocks, 4-bromotetrafluorobenzene carboxylic acid and tetrafluorosuccinic acid, by means of hydrogen bonding. A change in packing fashions between two adjacent TPI molecules is observed from a staggered pattern in the pure TPI crystal to an antisymmetric parallel arrangement in the multicomponent crystals. Moreover, the obtained organic salts show red-shift emission and enhanced fluorescence lifetime compared with the pure TPI crystal. Periodic density functional theoretical (DFT) calculations suggest that the introduction of the coformers can influence the energy level structures and orbital distributions of the TPI chromophore in the multicomponent crystals. Therefore, by the combination of experimental and theoretical studies on the multicomponent organic solids, this work not only reports the supramolecular assembly of new types of photoactive solid-state materials but also provides a detailed understanding of the electronic structures of the luminescent materials.

Full text of DOI:10.1021/cg3014927

Article
pubs.acs.org/crystal
The Formation of Hydrogen-Bond Facilitated Salts with Tunable
Optical Properties: An Experimental and Theoretical Study of 2,4,5-
Triphenylimidazole
Dongpeng Yan,*,† Bhavnita Patel,‡ Amit Delori,‡ William Jones,‡ and Xue Duan†
†State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, 100029, P. R. China
‡Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U. K.
S
* Supporting Information
ABSTRACT: Adjusting and modifying the optical properties of organic
fluorophores are of importance for the development of the next generation
of luminescent materials. Herein, we report the tunable fluorescence of
2,4,5-triphenylimidazole (TPI) chromophore by the formation of multi-
component crystals with the supramolecular building blocks, 4-bromotetra-
fluorobenzene carboxylic acid and tetrafluorosuccinic acid, by means of
hydrogen bonding. A change in packing fashions between two adjacent TPI
molecules is observed from a staggered pattern in the pure TPI crystal to an
antisymmetric parallel arrangement in the multicomponent crystals.
Moreover, the obtained organic salts show red-shift emission and enhanced
fluorescence lifetime compared with the pure TPI crystal. Periodic density
functional theoretical (DFT) calculations suggest that the introduction of
the coformers can influence the energy level structures and orbital
distributions of the TPI chromophore in the multicomponent crystals. Therefore, by the combination of experimental and
theoretical studies on the multicomponent organic solids, this work not only reports the supramolecular assembly of new types of
photoactive solid-state materials but also provides a detailed understanding of the electronic structures of the luminescent
materials.
2,4,5-Triphenylimidazole (TPI, Scheme 1A) is a typical
fluorescent dye, which is well documented as a model system
for the study of organic solid-state luminescence.8 TPI contains
a dicoordinate nitrogen atom, which can potentially build
complexes assembled by hydrogen bonds with a molecule
containing hydrogen-bond donor, such as a carboxylic acid. By
the use of such a design strategy, we have herein developed two
new types of TPI-based multicomponent crystal systems using
liquid-assisted grinding (LAG) method,9 and such method has
been demonstrated to be able to overcome different solubilities
between the components.9b Two representative compounds
(BTFB (Scheme 1B) and TFS (Scheme 1C)) without visible
fluorescence were chosen as the coformers with TPI. The
obtained products show different light-emission properties
(such as emission wavelength, fluorescence lifetime, and
photoluminescence quantum yield (PLQY)) compared to
those of the pure TPI organic solid. Detailed structures and
interaction fashions between two components in the as-
obtained multicomponent crystals were further characterized
and analyzed by single crystal X-ray diffraction, solid-state
NMR, differential scanning calorimetry, and thermogravimetric
1. INTRODUCTION
Organic solid-state fluorescent materials have recently received
considerable attention, due to their extensive applications in the
optoelectronic fields of light-emitting diodes,1 lasers,2 solar
cells,3 and fluorescent sensors.4 In views of fundamental studies
and practical applications, the development of new solid-state
multicomponent crystals of chromophores5 based on supra-
molecular architecture is important to extend the type and
application field of fluorescent materials and to complement
organic synthetic chemistry. Moreover, by rational selection
and design of the interaction between the components
(supramolecular synthons),6 the interaction mode, stacking
states, and molecular recognition fashions between the
assembled building blocks in the solid state can be modulated
and controlled, which can further tailor the photophysical
properties of the resulted organic solid-state suparmolecular
materials.7 However, to date, examples of multicomponent
organic solids organized from two or more assembled units
with tunable fluorescence are still limited compared with those
of the polymorphism-dependent emission,4g,h probably because
the cocrystallization is relatively difficult particularly when the
components have very different solubilities.6c In addition, the
detailed electronic structure and interaction fashion (such as
energy/electronic transfer) between components within multi-
component crystals are seldom discussed.
Received: October 11, 2012
Revised: November 17, 2012
Published: December 5, 2012
© 2012 American Chemical Society
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Scheme 1. Chemical Structures of the Chromophore Molecule (A, 2,4,5-Triphenylimidazole (TPI)) and Coformers (B, 4-
Bromotetrafluorobenzoic Acid (BTFB); C, Tetrafluorosuccinic Acid (TFS))
at 5 kHz under a static magnetic field of 9.4 T. Morphologies of the
crystal and powder samples were investigated by using a scanning
electron microscope (SEM Zeiss Supra 55) with the accelerating
voltage of 20 kV and an Olympus U-RFLT50 fluorescence
microscope. Periodic density functional theory (DFT) calculations
were performed using the Dmol310 module in the Material Studio
software package.11 The molecular structure of A and the single crystal
structures of A, A.B, and 2A.C(H2O) were selected as the initial
models, which were optimized by the Perdew−Wang (PW91)12
generalized gradient approximation (GGA) method with the double
numerical basis sets plus polarization function (DNP). The SCF
convergence criterion was within 1.0 × 10−5 hartree/atom, and the
convergence criterion of structure optimization was 1.0 × 10−3
hartree/bohr. The Brillouin zone (BZ) was sampled by 1 × 1 × 1
k-points, and test calculations revealed that an increased number of k-
points does not influence the results.
analysis. Moreover, periodic density functional theoretical
(DFT) calculations were performed to explore detained
electronic structures of the multicomponent crystals, to show
the differences in energy level structures and orbital
distributions of the TPI chromophore in the organic solids,
which may in turn influence the photoexcitation/emission
process. Therefore, by virtue of combining experimental and
computational methods on the multicomponent luminescent
systems, this work not only confirms that the construction of
multicomponent crystals can serve as a facile way to tune the
luminescent properties of an organic fluorescent dye through
rational control of the interaction and arrangement fashions of
chromophores within the crystal structures, but also provides a
deep insight into how different coformers influence the
electronic structures in these multicomponent crystals.
2.4. Single Crystal Structures. Crystal data for A.B (CCDC:
900559) and 2A.C(H2O) (CCDC: 900560) are tabulated in Table 1.
2. EXPERIMENTAL SECTION
Table 1. Crystal Structure Data for A.B and 2A.C(H2O)
2.1. Reagents. 2,4,5-Triphenylimidazole (TPI, A), 4-bromotetra-
fluorobenzoic acid (BTFB, B), and tetrafluorosuccinic acid (TFS, C)
were purchased from Sigma Chemical Co. Ltd. and used without
further purification.
salts
A.B
2A.C(H2O)
mol formula
mol wt
C28H17BrF4N2O2,
569.35
C46H38F4N4O6
818.80
2.2. Preparation of the Multicomponent Systems. In typical
liquid-assisted grinding (LAG) experiments, 40 μL of a 3:1 (v/v)
chloroform/water mixture was added to 200 mg of a solid reactant
mixture (A + B; A + C) with the initial ratio of the two compositions
of 1:1 (A:B) and 2:1 (A:C), respectively. The mixtures were ground
for 30 min in a Retsch Mixer Mill MM200 mill at a frequency of 30 Hz
using stainless steel jars (15 mL) and balls. In standard experiments,
two stainless steel balls of 7 mm diameter (1.4 g) were used for
grinding. Single crystals were obtained by adding 50 mL of chloroform
solutions to the LAG products, which was allowed to evaporate over a
period of one week.
2.3. Instrumentation. Characterization of solid materials using
powder X-ray diffraction was conducted on an X-Pert PRO MPD
powder X-ray diffractometer, using nickel-filtered Cu Kα radiation
(1.54 Å) generated at 40 kV and 40 mA with a scanning rate of 10°/
min, and a 2θ angle ranging from 3 to 50°. Single crystal X-ray
diffraction data were collected for salts A.B and 2A.C(H2O) on a
Nonius Kappa CCD diffractometer equipped with a graphite
monochromator and an Oxford cryostream, using Mo Kα radiation.
Differential scanning calorimetry (DSC) thermograms were measured
on a DSC822e calorimeter under nitrogen with a heating rate of 10
°C/min. Thermogravimetric analysis (TGA) was conducted on TGA/
SDTA851e thermobalance with a heating rate of 10 °C/min. The
solid-state fluorescence spectra were performed on RF-5301PC
fluorospectrophotometer, and the width of both the excitation and
emission slit was 3 nm. The fluorescence decays were measured using
a LifeSpec-ps spectrometer, and the lifetimes were calculated with the
F900 Edinburgh instruments software. Photoluminescence quantum
yield (PLQY) was measured using an HORIBA Jobin-Yvon
FluoroMax-4 spectrofluorimeter, equipped with an F-3018 integrating
sphere. Solid-state cross-polarization/magic angle spinning (CP/MAS)
13C and 15N nuclear magnetic resonance (NMR) spectra was recorded
by Bruker BioSpin AV 300 MHz at 20 °C with a 4 mm rotor spinning
color/shape
size (mm3)
cryst syst
space group
a (Å)
colorless/block
0.28 × 0.28 × 0.14
monoclinic
P2(1)/c
colorless/block
0.23× 0.21× 0.07
triclinic
P1
̅
8.1916(1)
23.5372(5)
12.5644(3)
90.00
8.6090(1)
10.7972(2)
12.1225(2)
109.500(1)
109.517(1)
94.035(1)
979.96(3)
1
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
104.577(1)
90.00
2344.53(8)
4
Z
Dc (g/cm3,)
R1 (I > 2σ(I))
wR2
1.613
1.387
0.0512
0.0481
0.1049
0.1030
GOF
1.022
1.031
3. RESULTS AND DISCUSSION
3.1. The Formation of Multicomponent Crystals. The
new types of multicomponent crystals were synthesized by
employing the liquid-assisted grinding (LAG) method.9
Powder X-ray diffraction (PXRD) patterns of the as-prepared
salts (1, A + B; 2, 2A + C), TPI, and the coformer precursors
are shown in Figure 1. It can be observed that the PXRD
patterns of the as-prepared multicomponent products are
different from that of pure TPI (sample A) and its coformer
precursors, demonstrating that new compositions have formed.
Upon crystallization from chloroform, the transparent single
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Figure 1. Powder XRD profiles of the multicomponent crystals (a) A.B and (b) 2A.C(H2O) and their assembled precursors, and simulated XRD
from single crystal results respectively; A, 2,4,5-triphenylimidazole (TPI), and its coformer compounds B, 4-bromotetrafluorobenzene carboxylic acid
(BTFB), and C, tetrafluorosuccinic acid (TFS).
Scheme 2. Crystal Structures of the Pure TPI and TPI-Based Multicomponent Crystals (A.B and 2A.C(H2O))
crystals of two complex systems can be obtained; the
morphologies of the crystals as well as their LAG powder
products are shown in Figure S1 in the Supporting Information.
X-ray single crystal diffraction measurements were performed
on the single crystal samples to obtain detailed crystal
structures of the TPI-containing complex samples. The
structures solved from the diffraction data confirm that these
new members of TPI-containing multicomponent crystals are
assembled by the hydrogen bond interaction between the
dicoordinate nitrogen atom in TPI and carboxylic acid groups
in the coformers with 1:1 (A.B) or 2:1 (2A.C) ratio, which is
consistent with the initial design principle for the formation of
the multicomponent crystals. The PXRD patterns are also
consistent with the powder patterns simulated from the single
crystal structures (Figure 1), illustrating a high purity of the
obtained multicomponent powder samples formed by the LAG
method. This further demonstrates that the LAG method can
serve as an alternative crystallization technique for the rapid
formation of large scale of multicomponent organic solids
compared with the typical solution process.
Based on the single crystal diffraction results, we can further
compare the differences in the assembled fashions between
these crystal samples. In the crystal structure of A (Scheme 2),
the molecules exhibit a staggered pattern assembled with the
aid of the N−H···N hydrogen bonding between two adjacent
TPI molecules. Upon formation of the A.B multicomponent
crystal, proton transfer occurs from the carboxylic acid in B to
the dicoordinate nitrogen atom of TPI, i.e., an ionic salt forms
within the A.B crystal. Moreover, the TPI molecule shows an
antisymmetric parallel arrangement assembled by the N−H···O
hydrogen-bonding between A and B, which further organizes
into an R4 (16) dimeric motif.13 Proton transfer was also
4
observed in the 2A.C system. The stacking fashion of TPI in
2A.C is similar to that found in the A.B system. Three types of
hydrogen bonds form between the A and H2O (N−H···O), C
and H2O (O−H···O), and A and C (N−H···O), as shown in
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Figure 2. DSC profiles of the salts A.B and 2A.C(H2O) and their coformer precursors.
Figure 3. TGA profiles of the salts A.B and 2A.C(H2O) and their coformer precursors.
Scheme 2. Therefore, by the formation of hydrogen-bonding
assembly of multicomponent organic crystals, the stacking and
interaction fashions between the TPI chromophore were
altered. The occurrence of H2O molecule as guest within the
2A.C host crystal structure may be attributed to the need of
geometric matching and minimizing energy within the crystal.
3.2. Thermal Behavior and Solid-State NMR Analysis.
To compare the thermal behavior of the multicomponent
crystals and their precursors, differential scanning calorimetry
(DSC) was performed on the pure A, B, C, A.B, and
2A.C(H2O) crystals. For the pure precursors A, B, and C, the
endotherms accompanying the melting endotherms appear at
ca. 276, 144, and 244 °C, whereas endotherms of salts A.B and
2A.C can be observed at 211.2 and 212.9 °C, respectively.
These endotherms illustrate that the dissociation of A.B and
2A.C occurs due to the difference in the melting points
between TPI (high temperature) and its corresponding
coformers B and C (low temperature). This phenomenon
further shows that the hydrogen-bond interactions in the salts
A.B and 2A.C are stronger than those in pure B and C organic
solids. Moreover, thermogravimetric analysis (TGA) profiles of
crystal A.B and 2A.C(H2O) are shown in Figure 3. The shape
weight losses occurring near the melting temperatures are
assigned to the sublimation and/or decomposition process of
the coformers within the multicomponent crystals, which
further confirm the content ratios of the coformers to A (1:1 or
2:1) within the salts. For salt A.B, TGA shows a weight loss of
ca. 49.4%, which occurs at 211 °C, this corresponds to the
expected percentage content (50.3%) of coformer B in the A.B
salt with a 1:1 ratio of A to B. For salt 2A.C(H2O), the first
step weight loss of ca. 4.5% can be assigned to the loss of guest
water molecules (theoretical value: 4.4%) within the host
matrix 2A.C; the second step is the weight loss (25.7%) of the
coformer C, which is also consistent with the percentage
content (23.2%) of coformer C in the 2A.C(H2O).
Solid-state 13C and 15N NMR were performed on both the
pure A and as-obtained multicomponent organic crystals. For
pure A, the resonances of the C1, C2, C3, and C4 in the phenyl
group appear at 124.3, 126.6, 128.0, and 130.1 ppm, and those
of C5, C6, and C7 in the imidazole group are located at 135.4,
136.7, and 148.1 ppm respectively. In the A.B system, new
peaks occurring at 120.6, 121.7, 142.9, and 161.6 ppm can be
attributed to C8, C9, and C10 in the benzene as well as C11 in
the carboxylate group of B. Moreover, the C2 and C3 peaks in
the A.B sample are shifted downfield by 0.5 and 0.8 ppm
compared with the pure A sample. Upon formation of the
2A.C(H2O), new peaks appear at 143.0 and 162.2 ppm, which
can be assigned to the C12 and C13 in C as shown in the inset of
Figure 4a. In addition, C2 (127.2 ppm) and C3 (128.6 ppm)
peaks show a downfield shift of 0.6 ppm relative to those of
pure A, suggesting that the coformers B and C containing
fluorine atoms can polarize and delocalize the electronic density
of the TPI molecule to some extent. Furthermore, resonances
in the 15N CP-MAS ssNMR spectrum of the pure A appeared
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3.3. Photophysical Properties. Figure 5 shows the
excitation and emission spectra of pure A, A.B, and
2A.C(H2O) salts, respectively. For the pure A sample, the
max
maximum excitation and emission wavelengths (λex
and
λemmax) are located at ca. 349 and 382 nm, respectively. Upon
formation of salts A.B and 2A.C(H2O), the λex and λem
max
max
are red-shifted systematically at 370, 362 and 387, 393 nm,
respectively. The change in the photophysical properties of
these organic solid compositions can be partially assigned to
alternative arrangement of the molecules of A from the
staggered pattern in pure A to antisymmetric parallel mode in
the A.B and 2A.C(H2O). This behavior further suggests that
the formation of the multicomponent crystals can alter the light
emission behaviors of the pure organic chromophores. To
obtain an insight into the excited-state information of
fluorescence for these solids, the fluorescence lifetimes were
measured and the corresponding fluorescence decay curves are
shown in Figure 6. The fluorescence lifetimes were obtained by
Figure 4. Solid 13C (a) and 15N NMR (b) profiles for both the pure A
and as-obtained salts A.B and 2A.C(H2O).
at 144.3 and 146.5 ppm as shown in Figure 4b, which
correspond to the nitrogen atoms with N−H and N···H
fashions in the TMP crystal structure. After the occurrence of
the assembly of A with B and C, two peaks move downfield to
144.5, 147.1, and 145.4, 147.3 ppm respectively. This illustrates
the formation of two types of H-bonding in the N···H units of
A.B and 2A.C(H2O).
Figure 6. Fluorescence decay curves of the pure TPI (A) and salts
(A.B and 2A.C(H2O)).
Figure 5. Fluorescence excitation and emission spectra for pure TPI (A) and salts (A.B and 2A.C (H2O)).
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Figure 7. (A) HOMO, (B) LUMO, and (C) total and partial electronic density of states (TDOS and PDOS) profiles for pure A molecule. The
Fermi energy level EF was set to zero.
fitting the decay profiles with one-exponential form. The
fluorescence lifetime of pure A with low-wavelength emission is
0.63 ns, whereas for A.B and 2A.C(H2O) salts with long-
wavelength emission the values are 1.17 and 2.05 ns, suggesting
that the formation of the multicomponent crystal can adjust the
luminescent excited-state properties of the pure chromophore
effectively. The enhancement of the fluorescence lifetimes can
be attributed to the deformation of the TPI aggregation by the
introduction of the coformers within the salts A.B and
2A.C(H2O), which is consistent with the red-shift of the
emission spectra for the salt samples. To detect the
fluorescence efficiency of the samples, photoluminescence
quantum yield (PLQY) was further measured. It was observed
that PLQY values are 10.2%, 10.9%, and 15.1% for pure A, A.B,
and 2A.C(H2O), respectively. Such observation is also
consistent with the trend of their fluorescence lifetime.
3.4. Periodic Density Functional Theoretical Calcu-
lations. To obtain energy level and electronic structural
information of the TPI-based multicomponent systems,
periodic density functional theoretical (DFT) calculations
were performed on an individual molecule of A, a crystal of
A, and salts A.B and 2A.C(H2O). For the pure A molecule,
frontier orbital analysis (Figure 7C and Figure S2 in the
Supporting Information) shows that the electron densities of
the highest occupied molecular orbitals (HOMOs) and lowest
unoccupied molecular orbitals (LUMOs) are mainly located on
the C and N atoms, and distributed over the whole π-
conjugated A molecule (shown in Figures 7A and 7B). Similar
results can also be obtained for the A crystal (Figure S3 in the
Supporting Information), which indicates that the photo-
excitation/emission processes for the A crystal involve the same
mechanism as the free A molecule, and no intermolecular
energy transfer appears during the photoexcitation/emission
process as shown in Figure S4 in the Supporting Information.
For both A.B and 2A.C(H2O), total electronic densities of
states (TDOS) and partial electronic densities of states
(PDOS) analyses show that HOMOs are mainly populated
on the A molecule, while LUMOs are located on both the C
and N atoms in the A molecule and C atoms in its coformer B
and F atoms in C respectively (Figure 8C and Figures S5C, S6,
and S7 in the Supporting Information), which can also be
viewed directly in Figure 8A,B and Figures S5A and S5B in the
Supporting Information. These results show that the
introduction of different coformers can influence the frontier
orbital distribution and electronic structure in these multi-
component crystals, which can further influence the lumines-
cent properties of the TPI-based organic solids as shown in the
Experimental Section.
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Figure 8. (A) HOMO, (B) LUMO, and (C) total and partial electronic density of states (TDOS and PDOS) profiles for 2A.C(H2O) salts. The
Fermi energy level EF was set to zero.
crystal (Figure S3); TDOS and PDOS profiles for different
atoms in pure A crystal (Figure S4); HOMO, LUMO and
TDOS/PDOS profiles for A.B salt (Figure S5); TDOS and
PDOS profiles for different atoms in A.B salt (Figure S6);
TDOS and PDOS profiles for different atoms in 2A.C(H2O)
salt (Figure S7); crystallographic data (.cif) of A.B and
2A.C(H2O) systems. This material is available free of charge
via the Internet at http://pubs.acs.org.
4. CONCLUSION
In summary, TPI-based multicomponent crystals have been
synthesized by the use of mechanochemical methods. The
molecular arrangement transfers from a staggered pattern in
pure TPI crystal to an antisymmetric parallel arrangement
between two adjacent TPI molecules in the salts due to the
formation of the N−H···O hydrogen-bonding between TPI and
its coformers. Compared with the pure TPI crystal, the two
salts exhibit red-shift emission and enhanced fluorescence
lifetime, which demonstrates that the optical properties in an
organic solid chromophore can be adjusted and modulated by
multicomponent crystal formation. Of particular interest here is
the introduction of coformers to change the electronic
structures and orbital distributions of the chromophore in the
multicomponent crystals, which was confirmed by DFT
calculations. It can be expected that the strategy of multi-
component crystal formation can be extended to tune the
fluorescent properties of other organic chromophore systems
for developing new types of organic luminescent materials.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: yandongpeng001@163.com; yandp@mail.buct.edu.cn.
Fax: +86-10-64425385. Tel: +86-10-64412131.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China, the 973 Program (Grant No.
2011CBA00504), and the 111 Project (Grant No. B07004).
ASSOCIATED CONTENT
* Supporting Information
■
S
EPSRC and INTERREG IVA 2 Mers Seas Zeeen Cross-border
̈
Profiles for the morphology of the salt crystals (Figure S1).
Total and partial electronic density of states (TDOS and
PDOS) profiles for different atoms in pure TPI (A) (Figure
S2); HOMO/LUMO and TDOS/PDOS profiles for pure A
Cooperation Programme 2007-2013 (BP) are acknowledged.
Pfizer Institute of Pharmaceutical Materials Science is acknowl-
edged for providing fellowship for AD.
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(9) (a) Papaefstathiou, G. S.; Zhong, Z.; Geng, L.; MacGillivray, L. R.
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