Elastic orange emissive single crystals of 1,3-diamino-2,4,5,6-tetrabromobenzene as flexible optical waveguides

Article abstract of DOI:10.1039/d1tc01841h

Single crystals of monoaromatic compounds exhibiting both mechanical softness and optical properties have attracted significant scientific interest in recent years, but they are very scarce. Herein, single crystals of 1,3-diamino-2,4,5,6-tetrabromobenzene

Full text of DOI:10.1039/d1tc01841h

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Materials Chemistry C
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PAPER
Elastic orange emissive single crystals of
1,3-diamino-2,4,5,6-tetrabromobenzene
as flexible optical waveguides†
Cite this: J. Mater. Chem. C, 2021,
9, 9465
a
c
Venkatesh Gude,
Shivakiran Bhaktha B. N.,bd C. Malla Reddy *c and Kumar Biradha
Priyanka S. Choubey,b Susobhan Das,
a
*
Single crystals of monoaromatic compounds exhibiting both mechanical softness and optical properties
have attracted significant scientific interest in recent years, but they are very scarce. Herein, single
crystals of 1,3-diamino-2,4,5,6-tetrabromobenzene (compound 1) were shown to exhibit excellent and
unexpected elastic behaviour upon mechanical deformation. The crystal structure analysis of 1 illustrates
the intermolecular interactions (BrÁ Á ÁBr, NÁ Á ÁBr and N–HÁ Á ÁBr) that are responsible for such flexible beha-
viour. Furthermore, the elastic modulus of crystals of 1 is found to be around 12 GPa, which was deter-
mined through nanoindentation experiments on the bendable major (100) face. In addition, interestingly,
compound 1 was also found to exhibit optical properties in the longer wavelength region (orange emis-
sion) both in the solid and solution states despite not having any significant extended conjugation. To
rationalize the experimentally observed optical behaviour, time-dependent density functional theory
(TDDFT) calculations have been performed. The potential use of single crystals of 1 has been tested in
optical waveguide applications. The present work demonstrates the significance of soft interaction
guided packing in the origin of both mechanical and optical properties of organic single crystals.
Received 21st April 2021,
Accepted 4th June 2021
DOI: 10.1039/d1tc01841h
rsc.li/materials-c
interesting owing to their significant impact in developing
flexible optical waveguides7–10/electronics11–18 and mechano-
Introduction
The mechanical properties such as plasticity and elasticity of an photonic applications.19–23
organic single crystal are a manifestation of non-covalent
At the fundamental level, the molecular packing features
interaction guided molecular packing.1–3 Most of the crystals which are essential for single crystals to exhibit plasticity,
undergo brittle fracture when an external load is applied. elasticity and brittle behaviour are well documented, yet design
Therefore, the search for crystals with compliant mechanical principles are still evolving.2 In principle, the significant degree
properties is extremely important in order to develop flexible of strength of interaction gradients in orthogonal directions of
device technologies.4,5 Both qualitative and quantitative studies a crystal leads to an irreversible migration (plastic behaviour) of
which are performed on flexible crystals have incredible molecules along the slip planes when the load is applied in a
potential in developing pharmaceutical solids.6 Furthermore, perpendicular direction.24 Similarly, crystals with soft interac-
single crystals of organic compounds possessing a combination tions generally with an interlocked structure lead to a reversible
of fluorescence and flexible mechanical properties are highly migration (elastic behaviour) of molecules along the outer and
inner arcs when subjected to loading–unloading cycles.25–27
a Department of Chemistry, Indian Institute of Technology-Kharagpur, Kharagpur,
West Bengal, 721302, India. E-mail: kbiradha@chem.iitkgp.ac.in
b School of Nano-Science and Technology, Indian Institute of Technology-Kharagpur,
Kharagpur, West Bengal, 721302, India
Furthermore, upon mechanical deformation, the fracture of a
crystal (brittle behaviour) is observed when there is isotropic
strength of interactions in all directions and in the absence of
facile slip planes.2
c Department of Chemical Sciences, Indian Institute of Science Education and
Recently, the optical and mechanical properties of monoaro-
matic compounds have gained significant scientific interest due to
their potential application in flexible optical waveguides.28,29 It
is important to note that achieving both optical and flexible
mechanical properties in single crystals of monoaromatic com-
pounds is rare and challenging,30 and our work on 1,3-diamino-
2,4,5,6-tetrabromobenzene (compound 1, Scheme 1) aims to fill
Research Kolkata, Nadia, West Bengal, 741246, India.
E-mail: cmallareddy@gmail.com
d Department of Physics, Indian Institute of Technology-Kharagpur, Kharagpur,
West Bengal, 721302, India
† Electronic supplementary information (ESI) available: Details of synthesis,
1HNMR results, optical waveguide experiments, elastic strain, crystallographic
parameters and computational results. CCDC 2077700. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/d1tc01841h
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crystalline state with the exception of one example that also
shows polymorphism.28 Zhang and his co-workers contributed
extensively to obtaining single crystals which can exhibit both
efficient optical and elastic properties. Furthermore, they have
tested the potential of crystals in flexible optical waveguide
applications.28,29 To the best of our knowledge, there are two
reports available on monoaromatic compounds which can act
as elastically bendable optical waveguides (Scheme 1c).28,29
Herein, we report the synthesis, and optical and mechanical
(elastic) properties of single crystals of compound 1 and their
application in flexible optical waveguides. It is important to
note that the molecular structure of 1 is significantly different
from that of the two reported compounds, in which the phenyl
ring is in extended conjugation with other functional groups.
Results and discussion
Compound 1 was synthesized and characterized as detailed in
the ESI.† The recrystallized compound 1 was used to obtain
acicular single crystals by slow evaporation of the mixture of
solvents (THF & MeOH).
The mechanical properties of the single crystals of 1 have
been tested qualitatively by using a pair of forceps and metallic
needle at ambient temperature. When the (100) plane was
supported by forceps and the mechanical force was applied
from the opposite side (À100), the needle shaped crystal was
bent into a semicircular shape (Fig. 1a–d). The crystal can be
repeatedly bent into an arc shape without breaking (Video S1,
ESI†). Upon exceeding a certain threshold of force, the crystal
was fragmented into pieces (Fig. 1e). Interestingly, when a
fragmented piece was subjected to mechanical stress again, it
Scheme 1 Molecular structures representing the (a) mechanical (b) emis-
sive and (c) flexible optical waveguide behaviour of monoaromatic
compounds.
this gap. Furthermore, our focus in the present work is restricted to
monoaromatic compounds with mechanical softness.31,32 A few
compounds which are structurally relevant to the present work are
shown in Scheme 1a. For instance, hexachlorobenzene is known to
bend plastically at room temperature while hexabromobenzene
(isostructural) is brittle but bends at a higher temperature.31,33
Interestingly, the single crystals of compound 1 are found to exhibit
remarkable elastic flexibility at room temperature. Note that the
difference between compound 1 and hexabromobenzene is that the
two amino groups in the former are at the meta position instead of
Br atoms in the latter. The amino groups in compound 1
are expected to form strong hydrogen and halogen bonds (e.g.,
N–HÁÁÁN, NÁÁÁBr) compared to hexabromobenenzene which solely
exhibits BrÁÁÁBr interactions (moderate strength). In order to under-
stand the structure and mechanical property correlation, a detailed
study was carried out on single crystals of 1.
Furthermore, there are various types of fluorophores which
could exhibit superior optical properties in the solid-state and
are comparable to p-conjugated materials.34–41 To the best of
our knowledge these monoaromatic fluorophores exhibit an
emission maximum (Scheme 1b) which covers almost the
entire visible wavelength (blue to red region) indicating their
significance for practical applications.42–44 It is important to
note that monoaromatic compounds displaying an orange
emission are very rare. None of the reported emitters34–44
Fig. 1 Optical microscopy images demonstrating the qualitative elastic
bending behaviour of single crystals of 1 when subjected to mechanical
shown in Scheme 1b exhibit mechanical flexibility in the stress.
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showed the original elastic nature (Fig. 1f, g and Video S1,
ESI†). Furthermore, in order to calculate the maximum elastic
strain using the Euler–Bernoulli beam-bending theory,45 the
crystal of 1 was subjected to mechanical stress without crossing
the elastic limit (Fig. 1h–j) so as to form an arc or a semicircle
(Fig. 1k and Video S2, ESI†).
The calculated elastic strain was found to be over B15%
(Fig. S1, ESI†). This value is comparable to the previously
reported highest elastic strain bearing yellow needles of
ROY46 (B14.6%) and crystal fibers47 (B14.28%), indicating
the exceptional ability of the crystal to dissipate mechanical
stress (Fig. 1k and l). However, after exceeding the stress
beyond the elastic limit, the crystal undergoes brittle fracture
without leaving any significant plastic deformation in the
fragmented pieces (Video S2, ESI†). The pristine crystals and
their fragmented pieces did not exhibit any noticeable plastic
(permanent) deformation after several repeat loading–unload-
ing cycles, indicating the usefulness of the elasticity.
In order to understand the molecular packing features that
are responsible for the elastic behaviour of compound 1, the
single crystal X-ray diffraction (SCXRD) analysis was performed.
Compound 1 crystallizes in the P21/c space group with one
molecule in the asymmetric unit. The pertinent crystallo-
graphic parameters for compound 1 are given in Table S1
(ESI†). The experimental powder XRD pattern of compound 1
is found to be identical to its simulated XRD pattern (Fig. S2,
ESI†), indicating the phase purity of 1. The differential scan-
ning calorimetry (DSC) thermogram also indicates no signifi-
cant phase change prior to melting of 1 (Fig. S3, ESI†).
The computed electrostatic potential (ESP) coloured mole-
cular van der Waals (vdW) surface map (Fig. 2a) indicates that
there are three chemically different bromine atoms in com-
pound 1. The blue and red regions correspond to the negative
and positive ESP surfaces. The strength of the s-holes of Br
atoms follows the decreasing order: Br-I (20.66 kcal molÀ1) 4
Br-II (16.8 kcal molÀ1) 4 Br-III (14.9 kcal molÀ1).48,49 The Br-II
Fig. 2 (a) Computed ESP coloured molecular vdW surface map (iso value =
0.001) of compound 1. Numerical numbers represent the energy in kcal molÀ1
.
(b), (c) and (d) Crystal morphology with face indices and packing for down (010),
up (001) and (100) faces, respectively. (e) Schematic representation of the
changes in crystal packing along the outer (left) and inner (right) arcs upon
deformation. For clarity, the substituents over the benzene are omitted.
and Br-IV are chemically equivalent with the same strength of expected to bear the strain to accommodate local molecular
s-holes. In the crystal structure, the molecules of 1 form dimers rotations (Fig. 2d).
(Fig. 2b) via BrÁ Á ÁBr (type-II) interactions (3.61 Å, y1 = 170.11,
We would like to note here that BrÁ Á ÁBr interactions
y2 = 128.61). These dimers are connected to each other through are known to provide such ‘‘structural buffering’’ during
halogen bonds (NÁ Á ÁBr, 3.33 Å) and N–HÁ Á ÁBr (HÁ Á ÁBr, 3.03 Å, deformation.52 In other words, they are soft enough to accom-
173.91) hydrogen bonds to form an infinite 1D-chain. The modate the mechanical strain during the flexure and act as
adjacent 1D-chains are connected through BrÁ Á ÁBr (3.66 Å, restoring forces upon withdrawal of the load. As the plastic
y1 = y2 = 146.91) type-I interactions and N–HÁ Á ÁN (HÁ Á ÁN, deformation in crystals of 1 was not observed, we suggest that
2.24 Å, 164.11) hydrogen bonds to form a corrugated 2D-sheet the corrugated packing pattern with isotropic interactions does
in the ac-plane (Fig. 2b) which are p-stacked to form a crystal not favour the long-range molecular movement during defor-
lattice. Overall, the non-covalent interactions between the mation of the crystal, hence the crystals manifest the elastic
molecules such as slipped p–p stacking (3.59 Å) along [010], properties and directly fracture at higher strains.2 Therefore, it
C–NÁ Á ÁBr, N–HÁ Á ÁBr along [100] and BrÁ Á ÁBr along [001] create could be anticipated that the BrÁ Á ÁBr interactions will experi-
near isotropic and corrugated/interlocked packing patterns in ence the strain upon bending of the crystal. Furthermore, the
mutually perpendicular directions (Fig. 2c).2,50 In the structure length of elastic crystals increases in the outer arc (tensile
analysis of 1, among the four bromine atoms, three (II, III, and stress) and decreases in the inner arc (compressive stress) upon
IV) are involved in type-II (less than 2 kcal molÀ1; Fig. S4, bending.2,25,27 Recently, Worthy et al. experimentally confirmed
ESI†)48,51 and one (I) in type-I halogen–halogen interactions. that during the course of elastic bending, the molecular rota-
Furthermore, when these crystals are subjected to mechanical tions facilitate the compression along the interior of the arc
stress in the (100) face, the type-II BrÁ Á ÁBr interactions are with simultaneous expansion of the outer arc.26 Therefore,
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higher strain in the crystals is expected to cause not only the weak absorption band is broadened and the maximum is
splaying (rotations) of molecules but also change in molecular red-shifted when moving from non-polar to polar solvents
distances when compared to thermodynamic equilibrium posi- (Fig. S5a, ESI†). To rationalize the experimental solid and
tions at the centre of the crystal (Fig. 2e).19,26,27,53
solution absorption properties and to understand the nature
The nanoindentation technique was employed in order to of transitions, single point energy calculations were performed
quantify the observed mechanical properties, namely elastic on ground state (S0) optimized geometry with the PBEPBE
modulus (E) and hardness (H) of single crystals. The indenta- functional56 and double zeta valence plus polarization function
tion experiments were performed on the bendable major face (DZVP) basis set57,58 using the TDDFT method.59 All the calcu-
(100) of single crystals using a Berkovich indenter diamond lations were performed using the Gaussian 09 package.60
tip.27,54 During the experiments, a load (P) of 6 mN is applied
The experimentally observed absorption maximum is
and the corresponding depth of penetration (h) of the indenter significantly overlapped with the calculated absorption maxi-
is monitored. The absence of pop-in events on the loading part mum (Fig. S6a–c, ESI†). Upon excitation at 480 nm both in solid
of the P–h curve (Fig. 3a) indicates that a sudden elastic to and DCM, compound 1 exhibited orange emission with the
plastic transition is not observed.6 Furthermore, the smooth maximum at 590 and 575 nm, respectively. Furthermore, these
nature of the loading part of the P–h curve and moderate depth emission bands are extended into the red region till 700 nm
recovery during unloading suggest that this crystal face is soft. (Fig. 4a and b), whereas, in MeOH the emission maximum is
This is reasonable as the indentation direction is parallel to the 40 nm red-shifted. This redshift of absorption and emission
plane intersecting BrÁ Á ÁBr interactions. The E and H values maximum in MeOH when compared to that in DCM is attrib-
extracted from the indentation experiments are found to be uted to the solvent polarity effect. The recorded excitation
12.3 Æ 0.3 and 0.48 Æ 0.01 GPa, respectively. These values are spectra (Fig. S6d, ESI†) exhibit similar absorption features
comparable to some of the reported elastically bendable (peak positions) of compound 1, indicating that no new excited
organic single crystals.55 The in situ scanning probe images species are formed during optical excitation. It is important to
(Fig. 3b–d) of nanoindenter impressions describe the height note that the origin of the orange emission of compound 1 is
profiles for the major face (100). No significant pile-up was significantly different when compared with that of the recently
observed surrounding the indent impression (Fig. 3c and d). reported monoaromatic fluorophore,29 in which, the excited
The results for this face indicate its compressible nature which state intramolecular proton transfer (ESIPT) process is respon-
is consistent with its soft nature.
sible for the observed green emission (lmax = 520 nm). Further-
In order to investigate the optical properties, steady state more, upon careful observation of each monoaromatic
diffuse reflectance spectra (DRS) and photoluminescence spec- chromophore structure reported so far (Scheme 1b and c),34–44
tra (PLS) of compound 1 were recorded in the solid state it can be understood that the benzenoid ring is in conjugation
(Fig. 4a). Compound 1 possesses two absorption bands with a with the substituted functional group such as carbonyl, suplhonyl,
maximum at 315 nm and a weak broad shoulder in the region alkene, etc. However, in the present work, the benzenoid ring of
of 400–600 nm. Similar absorption features (Fig. 4b) of 1 were compound 1 is not in conjugation with any substituted functional
noticed in hexane, dichloromethane (DCM) and MeOH. But, (amino and bromine) groups. It is well known that conjugation
shifts the absorption and emission positions to a longer wave-
length. From the optical point of view, without any significant
extended conjugation in compound 1, the exhibited optical
properties in the longer wavelength region are fascinating and
useful for practical applications.
Our computed (under vacuum and in DCM) vertical excita-
tion energies (VEEs) and nature transition orbitals (NTOs) for
various excited electronic states of 1 indicate that the absorp-
tion maximum (S0–S4) and longer wavelength band (S0–S1)
correspond to p, p* and n, p* transitions (Fig. S7 and S8, ESI†).
The fluctuation in the electron density during these electronic
transitions can be seen in the provided NTOs (Fig. 4d). Further-
more, the excitation maximum (at 530 nm) of 1 also supports
the existence of the lowest excited state (S1) in the longer
wavelength region (Fig. S6d, ESI†). The crystals of 1 exhibit a
single emission peak around 590 nm after (Fig. S9a and b, ESI†)
and before (Fig. S9c, ESI†) 400 nm excitation in the solid state.
However, when compound 1 is excited at the absorption max-
imum in the solution state (Fig. S5b, ESI†), in addition to the
strong emission peak in the region of 525–650 nm, a weak
Fig. 3 Nanoindentation results. (a) The representative load–depth (P–h)
curve obtained by indenting on the major (100) face. In situ scanning probe
emission peak centred at 425 nm is observed. The intense peak
is red-shifted but does not become a weak peak on increasing
images of post-nanoindentation impressions. (b) 2D indent impression,
(c) height profile and (d) their 3D projection.
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Fig. 4 Absorption and emission spectra of compound 1 in (a) solid and (b) solution. (c) The PL decay traces of 1 when monitored at 620 nm and
(d) schematic representation of the plausible photo-physical processes involved in the origin of orange emission of compound 1. The NTOs
corresponding to S1 and S4 states are shown here.
the polarity of the solvent. In the absorption and PL spectra, the charge-transfer)62 by BrÁ Á ÁBr, C–NÁ Á ÁBr play a significant role in
polarity sensitive and insensitive nature of higher and lower extending the absorption to a longer wavelength and hence
wavelength peaks indicate the n, p* and p, p* characters of resulted in orange emission. The relative quantum yield of
transitions. This experimental optical behaviour of compound compound 1 is found to be 9% from rhodamine 6G as the
1 is also supported by the computational calculations, in standard (Table S2, ESI†). The PL decay traces of compound 1
particular, by the nature of transitions (Fig. S7 and S8, ESI†). when monitored at 620 nm well fit with the single exponential
These observations indicate that compound 1 exhibits an equation (Fig. 4c). The fluorescence lifetime of compound 1 is
orange emission from the S1 state in both solid and solution found to be 2.1 (Æ 0.01) ns in DCM and 1.7 (Æ 0.01) ns in
states, even after excitation to the S4 state (Fig. S5b and S9c, MeOH, respectively. All photo-physical parameters such as
ESI†). This indicates the significance of the ultrafast internal radiative and non-radiative rate constants, etc., are shown in
conversion process prior to the emission from the S1 state Table S2 (ESI†).
(Fig. 4d).61 Furthermore, the NTOs corresponding to the S1
As single crystals of compound 1 exhibited interesting
state indicate the movement of electrons located at the amino elastic and optical properties, their potential application in
and Br groups to the aromatic ring during electronic excitation. optical waveguides was investigated in both unbent and bent
It is important to note that without having significant crystals. The details of the experimental setup and the
extended conjugation in compound 1, the developed stable S1 approach of bending the crystal prior to recording the emission
state in the longer wavelength region is due to the interaction of are shown in the ESI.† The single crystals under both condi-
nonbonding electrons/lone pairs located at the amino and Br tions were excited with a pulsed laser (lex = 532 nm) at different
groups with the neighbouring molecules. These interactions positions and the resulting emission was recorded at one end
reflected as a weak broad shoulder in the region of 400–600 nm of the crystal (Fig. 5b and e). The propagated orange emission
of the DRS spectrum (Fig. 4a). It is evident from the SCXRD to one end of the crystal in unbent and bent shaped crystals can
analysis (Fig. 2b) that compound 1 exhibits isotropic and be seen in Fig. 5a and b. As observed in both the shapes of the
weak dispersive interactions. These isotropic and weak (less crystal, the emission intensity at the end of the crystal gradually
than 2 kcal molÀ1) dispersive or soft interactions (a type of decreases on increasing the distance between the excited and
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intermolecular interactions can be controlled through suitable
design and substitution of functional groups. In this context,
our design and synthesis of compound 1 with bromine and
amino functional groups helped significantly in controlling the
required molecular packing features to achieve both the proper-
ties. The presence of a relatively large number of non-bonding
electrons surrounding the benzenoid ring facilitates the rota-
tion of molecules without affecting the continuity of crystal
packing. Furthermore, our computed NTOs in 1 suggest the
role of both amino and Br groups during excitation. The relaxed
molecule to the lowest vibrational state of S1 emits the orange
light in the unbent and bent shapes of crystals, which is also
supported by the lack of a considerable change in the peak
maxima of the emission spectra (Fig. 5b and e). These results
indicate that even after the elastic bending of the crystal, there
is no significant perturbation in the energy of the singlet
electronic states. Thus, the observed elastic flexibility and
orange emission by single crystals of 1 are linked to the
isotropic and weak dispersive (soft) intermolecular interaction
guided molecular packing.
Fig. 5 Optical waveguides of the crystals of compound 1 with unbent
(a–c) and bent shaped crystals (d–f). (a and d) Optical microscopy images
of crystals upon excitation (lex = 532 nm) at one end and bent position,
describing the waveguide characteristics. (b and e) Emission spectra
recorded at one end of the crystal with various distances (0–2.5 mm)
between the end and excitation position. (c and f) Optical loss coefficient
(a) was determined when the emission decay is fitted with a single
exponential function.
Conclusions
In summary, single crystals of a newly designed and synthesized
compound 1 are found to exhibit excellent elastic flexibility and
high elastic strain at room temperature under external flexural
mechanical stress. The observed isotropic and weak dispersive
intermolecular interaction guided molecular packing is responsi-
ble for the origin of elastic properties. The computed NTOs
indicate that the S1 state has n, p* character. The abundant
non-bonding electrons present in compound 1 due to amino
and Br groups, and their interaction with the neighbouring
molecules resulted in the absorption/excitation band in the higher
wavelength region. The single crystals of 1 exhibited an orange
emission when excited at 530 nm. The fluorescence lifetime of
compound 1 is found to be 2.1 ns. The experimentally observed
optical behaviour was rationalized by the TDDFT studies. Given
the advantages of both mechanical and optical properties of 1, a
flexible optical waveguide has been realized. This work demon-
strates the significance of soft interactions in realizing the
mechanical flexibility and practically useful optical properties of
organic single crystals, which are important for the development
of flexible crystalline materials for future technologies.
the end position. Furthermore, the peak maximum (l = 610 nm)
in both the shapes is slightly red-shifted when compared with
the solid state emission (Fig. 4a). Upon fitting the data, the
optical loss coefficient (a) was found to be 0.69 dB mmÀ1 for
unbent (Fig. 5c) and 0.99 dB mmÀ1 for bent shape (Fig. 5f) of
the crystals. These a values are lower or comparable to those of
few organic crystals and representative polymers (Table S3,
ESI†), indicating the applicability of single crystals of com-
pound 1 in flexible optical waveguides. However, these a values
are higher when compared with those of a few monoaromatic
fluorophores28,29 and organic crystals (Table S3, ESI†). The
reason might be due to the significant re-absorption effect,
which can be observed because of the considerable overlap of
the absorption and emission spectrum of compound 1 (Fig. 4a)
in the wavelength region of 550–650 nm.
Overall, the mechanical and optical responses of single
crystals of compound 1 can be summarized as follows. Funda- Conflicts of interest
mentally, the approach of investigating the mechanical proper-
ties (mode of external stimuli is the mechanical force) and the
There are no conflicts to declare.
optical properties (mode of excitation source is photons) of
crystals is different. Therefore, controlling of one property by
the other and understanding its subsequent changes requires
Acknowledgements
the help of multiple instrumentation techniques. However, VG and PC acknowledge IIT-Kharagpur for a post-doctoral and
both the properties are connected to various types of strong doctoral research fellowship. SD thanks CSIR, India, for a
and weak non-covalent interaction driven molecular packings. research fellowship. We acknowledge DST (SERB) (EMR/2017/
Therefore, regulation of appropriate packing features is one 001499), New Delhi, India, for the financial support and
of the ways of achieving both the properties. Furthermore, DST-FIST for providing the single crystal X-ray diffractometer.
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23 M. P. Zhuo, G. P. He, Y. Yuan, Y. C. Tao, G. Q. Wei,
X. D. Wang, S. T. Lee and L. S. Liao, CCS Chem., 2020, 2,
413–424.
24 G. M. Krishna, R. Devarapalli, G. Lal and C. M. Reddy, J. Am.
Chem. Soc., 2016, 138, 13561–13567.
We gratefully acknowledge the Centre for Theoretical Studies
and Department of Chemistry, IIT-Kharagpur, for providing
computational facilities. CMR thanks DST (New Delhi) for the
Swarnajayanti Fellowship (DST/SJF/CSA-02/2014-15).
25 S. Ghosh and C. M. Reddy, Angew. Chem., Int. Ed., 2012, 51,
10319–10323.
26 A. Worthy, A. Grosjean, M. C. Pfrunder, Y. Xu, C. Yan,
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