Irreversible Thermochromism in Organic Salts of Sulfonated Anils

Article abstract of DOI:10.1021/acs.cgd.0c00188

Condensation of p-amino benzenesulfonic acids with salicylaldehyde and o-vanillin and that of o-aminobenzenesulfonic acid with o-vanillin via mechanochemical grinding followed by heating under reflux yielded 4-[(2-hydroxy-benzylidene)-amino]-benzenesulfonic acid [4-SA-BA-2H] (1a), 4-[(2-hydroxy-3-methoxy-benzylidene)-amino]-benzenesulfonic acid [4-SA-oV-2H] (2a), and 2-[(2-hydroxy-3-methoxy-benzylidene)-amino]-benzenesulfonic acid [2-SA-oV-2H] (3a), respectively. Anticipating the formation of sulfonate···pyridinium supramolecular synthon, organic salts [(4,4′-BPY-2H)2+(4-SA-BA-H)ˉ 2-] (1), [(4,4′-BPY-2H)2+ (4-SA-oV-H)ˉ 2-·4H2O] (2), and [(4,4′-BPY-2H)2+(2-SA-oV-H)ˉ 2-·2H2O] (3) have been obtained by in situ as well as postsynthetic reaction of 1a-3a with 4,4′-bipyridine. Crystallographic investigations of the molecular salts reveal intriguing structural features and indicate the ubiquitous formation of hydroxylate···pyridinium interactions over otherwise robust sulfonate···pyridinium synthons when the hydroxyl group has an adjacent methoxy group. 1 crystallizes as an anhydrous salt with direct charge-assisted sulfonate···pyridinium interactions between crystal formers forming two-dimensional layers which π-interlock to form a rare three-dimensional yellow solid. Crystal formers in hydrated salts 2 and 3 associate through ionic hydroxylate···pyridinium interactions instead of anticipated sulfonate···pyridinium synthons. The organosulfonate ions in 2 and 3 associate with the help of octameric and tetrameric water clusters into two-dimensional and one-dimensional assemblies, respectively, which are further bridged by bis-pyridinium ions into three-dimensional colored ionic solids. The enhanced optical behavior of 1-3 on formation from 1a-3a is investigated through diffuse reflectance studies. Schiff base 1a and its salt 1 exhibit redshift on heating, both 2a and 2, as well as 3a do not respond notably to heat, while as 3 undergoes a remarkable blue shift on heating. Optical studies vis-à-vis crystallographic investigations rule out the conventional and general mechanism, i.e., enol ? cis-keto isomerization and instead strongly indicate packing changes to be the cause of irreversible thermochromism observed in 1 and 3. Furthermore, the resultant molecular salts exhibit significant thermal and chemical stability, and their formation is also substantiated by spectroscopic and analytical methods.

Full text of DOI:10.1021/acs.cgd.0c00188

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Article
Irreversible Thermochromism in Organic Salts of Sulfonated Anils
Aijaz A. Dar, and Arshid A. Ganie
Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 30 Apr 2020
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Irreversible Thermochromism in Organic Salts of Sulfonated
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Aijaz A. Dar, †,* Arshid A. Ganie †
†Department of Chemistry, Inorganic Section, University of Kashmir, Hazratbal, Srinagar-190006, India.
KEYWORDS: Supramolecular synthon, structure-property relationship, cocrystal
ABSTRACT:Condensation of p-amino benzene sulfonic
acids with salicylaldehyde and o-vanillin and that of o-
aminobenzene sulfonic acid with o-vanillin via mechano-
chemical grinding followed by heating under reflux yielded
4-[(2-Hydroxy-benzylidene)-amino]-benzenesulfonic
acid [4-SA-BA-2H (1a)], 4-[(2-Hydroxy-3-methoxy-
benzylidene)-amino]-benzenesulfonic acid [4-SA-oV-
2H (2a)] and 2-[(2-Hydroxy-3-methoxy-benzylidene)-
amino]-benzenesulfonic acid [2-SA-oV-2H (3a)], re-
spectively. Anticipating the formation of sul-
fonate∙∙∙pyridinium supramolecular synthon, organic salts
̄
[(4,4'-BPY-2H)2+(4-SA-BA-H) 2] (1), [(4,4'-BPY-2H)2+ (4-
SA-oV-H) 2
H) 2.2H2O] (3) have been obtained by in-situ as well as post-synthetic reaction of 1a-3a with 4,4'-bipyridine. Crystallographic investiga-
̄
.4H2O] (2) and [(4,4'-BPY-2H)2+(2-SA-oV-
̄
tions of the molecular salts reveal intriguing structural features and indicate the ubiquitous formation of hydroxylate∙∙∙pyridinium interac-
tion over otherwise robust sulfonate∙∙∙pyridinium synthon when hydroxyl group has adjacent methoxy group. 1 crystallizes as anhydrous
salt with direct charge assisted sulfonate∙∙∙pyridinium interaction between crystal formers forming 2-dimensional layers which π-interlock
to form rare 3-dimensional yellow solid. Crystal formers in hydrated salts2 and 3 associate through ionic hydroxylate∙∙∙pyridinium interac-
tion instead of anticipated sulfonate∙∙∙pyridinium synthon. The organo-sulfonate ionsin 2 and 3associate with help of octameric and
tetrameric water clusters into 2-dimensional and 1-dimensional assemblies, respectively, which are further bridged by bis-pyridinium ions
into 3-dimensional colored ionic solids. The enhanced optical behavior of 1-3 on formation from 1a-3a is investigated through diffuse
reflectance studies. Schiff base 1a and it's salt 1 exhibit redshift on heating, both 2a and 2, as well as, 3a do not respond notably to heat
while as 3 undergoes a remarkable blue shift on heating. Optical studies vis-à-vis crystallographic investigations rule out the conventional
and general mechanism i.e. enol↔cis-keto isomerization, and instead strongly indicate packing changes to be the cause of irreversible
thermochromism observed in 1 and 3. Furthermore, the resultant molecular salts exhibit significant thermal and chemical stability and their
formation is also substantiated by spectroscopic and analytical methods.
photochromism due to prototropic and isomeric forms.13-17
Determination of the relationship between structure and opti-
INTRODUCTION
cal properties to understand the switching mechanism in solid-
In material chemistry, it is well perceived that properties fol-
state has been of special focus since their discovery. The
low from structure i.e. “function follows form”.1 However, the
relationship between molecular packing and consequent prop-
erties is not well understood and to design bespoke materials it
photochromism of these molecules is well explored and pri-
marily attributed to cis↔trans-keto isomerism while thermo-
chromism, attributed to enol↔cis-keto prototrophism, is less
explored, Chart 1. Empirical rules indicate that planner or
is imperative to further explore the structure-property relation-
ship.2-4 Thermochromism isan uncommon property which
nearly planner anils (ϕ < 20 °) in close-packed environments
involves reversible or irreversible color change with tempera-
exhibit thermochromic behavior and otherwise photochrom-
ism is exhibited.18 Rational attempts have been made to modu-
turevariation and the phenomenon finds utility as thermome-
ters, thermo-sensitive recording paper, temperature-indicating
late the structural and packing behavior of these molecules
labels, sun-protecting glazing, battery testers, temperature
through structural modifications, introduction of bulky groups
sensors, and temperature-controlled light filters.5-9Inorganic
complexes provide more examples of thermochromic materi-
and crystal engineering.19-22
als than organic molecules.6,10Thermochromism of inorganic
Crystal engineering or supramolecular approach is rapidly
complexes isgenerally triggered by local crystal field altera-
growing research domain in designing the optical mate-
tion or polarization induced charge transfer phenome-
rials.21-25Crystal engineering has been pursued to modulate
non.10,11,12N-Salicylidene aniline and its derivatives (anils)
environment or create free volumes around anil molecules to
represent an important but less explored class of organic com-
induce photo- and thermochromism.18, 21-22Cocrystallization
pounds with a tendency to exhibit solid-state thermo- and
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provides a convenient opportunity to modulate the envi-
Methods and Materials
ronment around anil molecules and hence their behavior
towards heat and light.26-29 Moreover astute selection of
the crystal coformer may lead to analogous solid forms,
but with different properties. Various studies involving
modulation of the crystal structure through packing effect
and/or size of the coformer on the chromic properties of
anils have been reported.28,29
4-Aminobenzene sulfonic acid (4-ABSA-H) (99%, Sigma-
Aldrich), 2-aminobenzene sulfonic acid (2-ABSA-H )(99%,
Sigma-Aldrich), Salicyldehyde (99%, Sigma-Aldrich), o-
vanilin (99%, Sigma-Aldrich), p-vanilin (99%, Sigma-
Aldrich) and 4,4'-bipyridyl (>99.9%, Sigma-Aldrich) were
used as procured. Single distilled H2O and methanol were used
as solvents. Melting points were determined on the MP70
Melting Point System capillary apparatus (Mettler Toledo). pH
was determined on Labtronics Micro Processor pH meter (LT-
49) at 44.1 °C. Infrared spectroscopic data of the sulfonate
anils 1a-3a and their molecular salts 1-3 were obtained using
Agilent Technologies Cary 630 FT-IR (4000-650cm-1) in ATR
mode. Ground crystals of 1a-3a and 1-3 were placed on the crys-
tal plate of the infrared instrument and the spectrum was recorded.
1H NMR spectra were recorded on Bruker 400 MHz spectrometer
in DMSO-d6 as solvent. Thermal gravimetric analyses and diffe-
rential thermal analyses of these samples were performed on a
Simultaneous Thermal Analyser-STA (LINSEIS, USA
6807/8835/16) using alumina crucible at heating rate 10°C. Dif-
fuse reflectance measurements of the products were recorded on
Schimadzu-2600 spectrometer as BaSO4 discs and converted to
Kubelka-Munk and Tauc plots in origin using appropriate conver-
sion formula. The free volume Vfree was evaluated by considering
the unit cell volume (Vunit cell), the anion (Vanion), the cation (Vca-
tion) and the solvent (Vsolvent) volumes as well as Z, the number of
molecules in the unit cell, as discussed previously,28using formu-
la:
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The supramolecular synthon approach30 has been utilized with
notable success to develop bespoke multicomponent crystals
with utility in pharmacy31, agriculture32 and industry33-35. A
robust supramolecular synthon should exhibit facile ma-
nifestation in addition to significant strength and directio-
nality. Robust supramolecular interactionspreferentially
lead to the isolation of high-energykinetic productsover
stable but elusive thermodynamic products, which how-
ever, are stabilized by numerous favorable secondary inte-
ractions. Together with tecton theory, supramolecular
synthon approach has been utilized to design crystals and
has been finitely successful in structural predic-
tion.36Nevertheless, the design and cogent structure pre-
diction still remain a conundrum for crystal engineers and
the area is being perceived emphatically. The discovery of
newer robust supramolecular synthons can be a salient
approach to develop the-state-of-art materials and under-
stand the structure-property relationship.
Vfree = [Vunit cell – Z * (Vanion + Vcation + Vsolvent)]/Z.
The anion, cation and solvent volumes were calculated online
using Molinspiration Galaxy 3D Structure Generator v2018.01
beta. Molecular structure and packing diagrams have been plotted
in Diamond software with aid of Mercury software.
Chart 1.Conventional mechanism of thermo-and pho-
tochromism in anils with their absorption regions in typi-
cal diffuse reflectance spectra.
Chart 2. Starting materials used for synthesis of 1-3.
SO3H
SO3H
N
N
HO3S
N
N
N
OH
O
Of recent, we have describedthe rational utility of lesser re-
ported sulfonate···pyridinium synthon and advocated its case
HO
HO
O
for consideration as
a
robust supramolecular syn-
3a
1a
2a
thon.37,38Extending the scope of utility of thisstable secondary
interaction we are reporting cogent derivatization of the aryl-
sulfonic acids into sulfonated anils which are furtherdeployed
for the development of molecular crystals. As mentioned ap-
propriate crystal coformers can attune molecular packing and
help in achieving larger free volumes in the crystal structures
of anils. 4,4'-bipyridyl has been utilized as a coformer with the
aim to form sulfonate···pyridinium synthon based molecular
complexes with modulated packing. Interestingly, the molecu-
lar complexes obtained from the precursors, depicted in Chart
2, do respond to heat but not to light, and they do not obey the
conventional mechanism of thermochromism depicted in
Chart 1. Crystallographic and optical studies of the reported
complexes provide comprehensibleunderstanding to rule out
the conventional mechanism of thermochromism and instead
validate attribution of the phenomenontochange in the crystal
packing.
SC-XRD Studies
Single-crystal X-ray data were collected on a Rigaku Saturn CCD
diffractometer using a graphite monochromator (Mo Kα, λ =
0.71073 Å). The selected crystals were mounted on the tip of a
glass pin using mineral oil and placed in the cold flow produced
with anCryo-cooling device. Complete hemispheres of data were
collected using ω and φ scans (0.3°, 16 s perframe). Integrated
intensities were obtained with Rigaku Crystal Clear-SM Expert
2.1 software, and they were corrected for absorption correction.
Structure solution and refinement were performed with the
SHELX package. The structures were solved by direct methods
and completed by iterative cycles of ΔF syntheses and full-matrix
least-squares refinement against F. Crystal refinement parameters
of 1-3 are given in Table 1.
EXPERIMENTAL
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Hz), 7.36 (d, 1H, 3JH-H = 1.6 Hz), 7.24 (d, 1H, 3JH-H = 1.6 Hz), 7.12
Synthesis
(d, 1H, 3JH-H = 1.2 Hz), 6.90 (t, 1H, 3JH-H = 2.0 Hz), 3.80 (s, 3H).
4
4-((2-hydroxybenzylidene)amino)benzenesulfonic acid [4-SA-
BA-2H (1a)]. 4-ABSA-H (86 mg, 0.5 mmol) was dissolved in 10
mL of hot distilled water and salicylaldehyde (1173 µL, 0.5
mmol) was taken in 10 mL of methanol, two solutions were
mixed together to give yellow solution. The resultant solution was
refluxed for about an hour, filtered and kept for slow evaporation
to yield light yellow crystalline solid. Yield: 225 mg, 76%; M.P:
>250 °C; pH (of resultant solution at 141.1 °C): 2.68; IR, ν,
(neat): 3429 (br), 2981 (w), 2113 (br), 1647 (s), 1576 (s), 1464
(s), 1159 (vs), 1121 (vs), 1032 (s). 1H NMRδ, 400 MHz, DMSO-
5
6
7
8
2-((2-hydroxy-3-methoxybenzylidene)amino)benzenesulfonic
acid [2-SA-oV-2H (3a)]. 2-ABSA-H (86 mg, 0.5 mmol) and o-
vanillin (76 mg, 0.5 mmol) were dissolved separately in 10 mL of
hot distilled water and methanol, respectively. The solutions were
mixed together and the resultant solution was refluxed and stirred
for two hours, filtered and kept for slow evaporation. Yellow
crystals were obtained in 3-5 days. Yield: 221 mg; 68%;
MP.:>250 °C; pH: 2.71; IR, ν, (neat): 3391 (br), 2100 (br), 1636
(s), 1609 (s), 1512 (s), 1201 (vs), 1167 (vs), 1014 (s). 1H NMR,δ,
400 MHz, DMSO-d6, ppm: 10.27 (s, 1H), 8.23 (s, 1H), 7.82 (d,
1H, 3JH-H = 1.2 Hz), 7.65 (d, 1H, 3JH-H = 1.2 Hz), 7.46 (t, 1H, 3JH-H
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3
d6, ppm: 9.01 (s, 1H), 8.95 (s, 1H), 8.87 (d, 4H, JH-H = 1.2 Hz),
3
3
8.11 (d, 4H, JH-H = 1.2 Hz), 7.54 (d, 2H, JH-H = 1.6 Hz), 7.36 (t,
1H, 3JH-H = 1.6 Hz), 7.01 (d, 2H, 3JH-H = 1.2 Hz), 6.97 (t, 1H, 3JH-H
= 0.8 Hz).
3
3
= 1.6 Hz), 7.37 (d, 1H, JH-H = 1.2 Hz), 7.32 (t, 1H, JH-H = 1.2
Hz), 7.26 (t, 1H, 3JH-H = 1.2 Hz), 7.16 (d, 1H, 3JH-H = 1.2 Hz), 3.81
(s, 3H).
[(4,4'-BPY-2H)2+(4-SA-BA-H)
̄2] (1): 4-ABSA-H (86 mg, 0.5
mmol) and salicylaldehyde (1173 µL, 0.5 mmol) were dissolved
separately in 10 mL of hot distilled water and methanol, respec-
tively, and the solutions were mixed with stirring. To the resultant
solution 4,4'-BPY (78 mg, 0.5mmol), dissolved in 10 mL of me-
thanol, was added with stirring. The final solution was refluxed
for an hour, filtered and crystallized by slow evaporation at am-
bient temperature. Orange blocks were obtained in 3-4
days.Yield: 301 mg; 66%; MP.: 225 °C; pH: 3.24; IR, ν, (neat):
3421 (br), 2981 (w), 2109 (br), 1651 (s), 1505 (s), 1395 (s), 1170
(vs), 1118 (vs), 1028 (s). 1H NMR, δ, 400 MHz, DMSO-d6, ppm:
9.07 (s, 1H), 8.96 (s,1H), 8.92 (d, 4H, 3JH-H= 1.2Hz), 8.20 (d, 4H,
3JH-H= 1.2 Hz), 7.54 (d, 2H, 3JH-H= 1.6 Hz), 7.36 (d, 1H, 3JH-H= 1.2
Hz), 7.21 (d, 2H, 3JH-H= 1.6 Hz), 7.02 (t, 1H, 3JH-H= 1.6 Hz), 6.97
(t, 1H, 3JH-H= 0.8 Hz).
[(4,4'-BPY-2H)2+(2-SA-oV-H)
2̄ .2H2O] (3). 2-ABSA-H (86 mg,
0.5mmol) and o-vanillin (76 mg, 0.5 mmol) were dissolved sepa-
rately in 10 mL of hot distilled water and methanol, respectively.
The solutions were mixed with stirring and solution of 4,4'-BPY
(78 mg, 0.5 mmol) in methanol was added to the resultant solu-
tion. Final solution was stirred under reflux for two hours, filtered
and kept for slow evaporation. Intense brown (or apparently
black) crystals were obtained in 2-5 days. Yield: 349 mg; 67%;
MP.: 220 °C; pH: 3.39; IR, ν, (neat): 3488 (br), 3087 (w), 2120
(br), 1622 (s), 1588 (s), 1505 (s), 1194 (vs), 1132 (vs), 1001 (s).
1H NMR,δ, 400 MHz, DMSO-d6, ppm: 10.24 (s, 1H), 8.3 (s, 1H),
8.89 (d, 4H, 3JH-H = 1.2 Hz), 8.17 (d, 4H, 3JH-H = 1.2 Hz), 7.82 (d,
1H, 3JH-H = 0.8 Hz), 7.70 (d, 1H, 3JH-H = 0.8 Hz), 7.47 (d, 1H, 3JH-H
3
3
= 1.6 Hz), 7.37 (t, 1H, JH-H = 1.2 Hz), 7.25 (t, 1H, JH-H = 1.2
Hz),7.17 (d, 1H, 3JH-H = 1.2 Hz), 6.90 (t, 1H, 3JH-H = 1.2 Hz), 3.31
(s, 3H).
4-((2-hydroxy-3-methoxybenzylidene)amino)benzenesulfonic
acid [4-SA-oV-2H(2a)].4-ABSA-H(86 mg, 0.5 mmol) and o-
vanillin (78 mg, 0.5 mmol) were dissolved separately in 10 mL of
hot distilled water and methanol, respectively and the solutions
were mixed together with stirring. The resultant solution was
refluxed and stirred for an hour, filtered hot and kept for slow
evaporation to yield orange crystalline solid in 4
RESULTS
days. Yield: 225 mg; 69 %; MP.: 240°C; pH:
2.89; IR, ν, (neat): 3421 (br), 2981 (w), 2109
Table 1.Crystal data details of molecular salts1-3.
(br), 1657 (s), 1595 (s), 1505 (s), 1291 (s), 1173
(vs), 1118 (vs), 1008 (s). 1H NMR,δ, 400 MHz,
DMSO-d6, ppm: 10.28 (s, 1H), 9.08 (s, 1H),
Compound
1
2
3
1981821
C18H15N2O4S C19H25N2O9S
355.38
293(2)
Triclinic
P-1
1981822
1981823
C19H21N2O7S
421.44
293(2)
Triclinic
CCDC No.
Empirical formula
Fw
Temp [K]
Crystal system
Space group
a, [Å]
b, [Å]
c, [Å]
, [°]
, [°]
7.74(d, 2H, 3JH-H = 1.6 Hz), 7.49 (d, 2H, 3JH-H
=
3
1.6 Hz), 7.37 (d, 1H, JH-H = 0.8 Hz), 7.26 (d,
1H, 3JH-H = 1.6 Hz), 6.97 (t, 1H, 3JH-H = 2.0 Hz),
3.85 (t, 3H).
457.47
293(2)
Triclinic
P-1
[(4,4'-BPY-2H)2+(4-SA-oV-H)
2̄ .4H2O] (2). 4-
P-1
ABSA-H (86 mg, 0.5 mmol) and o-vanillin (76
mg, 0.5 mmol) were dissolved separately in 10
mL of hot distilled water and methanol, respec-
tively and mixed with stirring. To the resultant
solution 4,4'-BPY (78 mg, 0.5 mmol) in metha-
nol was added and the final solution was ref-
luxed with stirring for an hour. The solution
obtained was filtered and crystallized by slow
evaporation to yield red blocks in 3-5 days.
Yield: 356 mg; 64%; MP.: 114 °C; pH: 3.55; IR,
ν, (neat): 3425 (br), 3011 (w), 2094 (br), 1638
(s), 1601 (s), 1374 (s), 1173 (vs), 1118 (vs),
1028 (s). 1H NMR,δ, 400 MHz, DMSO-d6, ppm:
10.21 (s, 1H), 8.94 (d, 4H, 3JH-H = 1.6 Hz), 8.24
5.773(7)
7.613(8)
18.38(2)
86.89(3)
87.270(19)
83.52(2)
800.8(16)
2
9.513(7)
10.309(6)
12.079(8)
71.52(2)
85.22(2)
76.044(18)
1090.3(13)
2
8.8556(5)
11.3511(6)
11.5730(7)
117.113(6)
102.490(5)
98.873(4)
967.26(9)
2
, [°]
V, [Å3]
Z
D(calcd)
µ [mm-1]
Θ range [o]
Reflns collected
Indep. Reflns
GOF
1.474
1.393
1.447
0.229
0.201
0.213
25.500
2935
25.000
3844
25.000
3419
3
3
2454
3174
2948
(d, 4H, JH-H = 1.6 Hz), 7.66 (d, 2H, JH-H = 1.6
1.065
1.030
1.136
0.0468
0.1468
0.0519
0.1592
R1(I0>2(I0)
wR2(all data)
0.0407
0.1472
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cated by shorterimine C4-N1and longer hydroxyl C9-O4 bond
Solvent assisted mechanical grinding of amino-arylsulfonic
acids: 4-aminobenzene sulfonic acid (4-ABSA-2H) and 2-
aminobenzene sulfonic acid (2-ABSA-2H), with correspond-
ing benzaldehydes, followed by heating under reflux in me-
thanol yielded sulfonated Schiff bases 4-[(2-Hydroxy-
length value of 1.277(2) and 1.346(1) Å, respectively. The sul-
fonate sulphur is in distorted tetrahedral geometry with average
O-S-O and C-S-O bond angle of 112.12° and 106.18°, respec-
tively. Two crystal structural analogues of anil sulfonate anion in
1 have been reported previously in the cationic matrix of
[Cu(phen)3]2+ and diallyl hexyl ammonium cations.21,29 In both
reported forms the sulfonated anil anion exists in nearly planer en-
ol form as confirmed from torsion angle and bond length values of
imine (C=N) and C-O (hydroxyl). The 4,4'-BPYcoformer is pro-
tonated on both ends with N···N linear distance of 6.993(1)Å and
dihedral angle value of 1.06(2)° between pyridinium
rings.Protonation at nitrogen centersofbis-pyridinium [4,4′-BPY-
2H]2+is substantiated by relaxed C14-N2-C16 bond angle value of
121.44(1)°. Bis-pyridinium centers bridge twoSchiff base anions
through anticipated sulfonate···pyridiniumsupramolecular interac-
tions to form non-stoichiometric salt [(4,4'-BPY-2H)2+(4-SA-BA-
benzylidene)-amino]-benzenesulfonic
acid
[4-SA-BA-
2H](1a), 4-[(2-Hydroxy-3-methoxy-benzylidene)-amino]-
benzenesulfonic acid [4-SA-oV-2H](2a) and 2-[(2-Hydroxy-3-
methoxy-benzylidene)-amino]-benzenesulfonic acid [2-SA-
oV-2H] (3a)as colored amorphous solids. The molecular com-
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plexes [(4,4'-BPY-2H)2+(4-SA-BA-H)
2̄ ] (1), [(4,4'-BPY-
2H)2+(4-SA-oV-H)2ˉ.4H2O] (2) and [(4,4'-BPY-2H)2+(2-SA-
oV-H)2ˉ.2H2O] (3) have been obtained by reflux of 1a-3a with
4,4'-BPY as well as by in-situ mechano-reflux reaction of the
organo-sulfonic acids with the benzaldehydes in presence of
4,4'-BPY. Products 1a-3a and 1-3 have been obtained in good
yields and their formation is substantiated by various spectros-
copic, analytical and thermal methods of analyses.
H) 2̄ ]. Furthermore, the supramolecular synthon in 1 manifests
2
itself in cyclic R2 (7) form, consisting of N2-H2N···O2
[2.701(1)Å; 170.92(2)°] and weak C16-H16···O3 [3.193Å;
131.21°] interactions. Molecular structure diagram of 1 is de-
picted in Figure 1.
Fourier transform infrared (FT-IR) spectroscopic studies of 1a-3a
and 1-3 support their formation. The absorption bands at around
1205-1115 and 1350-1300 cm-1 correspond to S=O stretching of
sulfonic group while as absorption bands at 1660-1620 cm-1 and
1600-1505 cm-1 corresponding to C=N and C=C stretching, re-
spectively. Figure ESI-1-6. Compounds 1-3 exhibit stability to-
wards non-polar and less polar solvents and are soluble in sol-
vents of high polarity only, Table ESI-1. The 1H NMR spectra of
1a-3a and 1-3, reported in DMSO-d6, are consistent with their
formation in terms of both the observed chemical shift and peak
1
intensity ratio. H NMR spectra with peak assignments are pro-
vided in supported information, Figure ESI-7-12. All the products
have been further characterized by thermo-gravimetry and their
definitive solid-state structures and crystal packing has been es-
tablished by single crystal-X-ray diffraction (SC-XRD) method.
Furthermore, the intriguing optical properties of the thermochro-
mic products are reported.
Figure 1. Molecular structure diagram of
[(4,4'-BPY-2H)2+(4-SA-BA-H) 2
̄ ] (1).
Each bis-pyridinium crystal former in 1 exhibits intriguing hydro-
gen bonding behaviour by coordinating six different molecules of
acid co-formers forming centro-symmetric [(4,4′-BPY-2H)2+(4-
Structural Studies
4-
1
SA-BA-H) 6̄ ] cluster, stabilized by two R2 (7) synthons: consist-
ing of C15-H15···O1 [3.343(1) Å; 172.30(7)°] and C17-H17···O1
Structure description of 1.Molecular salt 1, obtained by in-situ
reaction of 4-ABSA-2H and salicylaldehyde with 4,4'-BPY, crys-
tallizes in triclinic centro-symmetric space group P-1. 1 is non-
solvated with onemolecule of anionic sulfonatedSchiff base 1a
and half molecule of bis-pyridinium in the asymmetric unit. The
anionic Schiff base is nearly planar with torsion angle of 3.93(1)°
between aryl rings and is stabilized by cis-enol intramolecular
hydrogen bondinginteraction O4-H4O···N1 [D···A: 2.625(1) Å;
D-H···A: 153.88(2)°]. Existence of 1 in enol form is also indi-
[3.376(2) Å; 174.09(2)°], and two C14-H14···O3 [2.985(3) Å;
2
108.76(6)°]
interactions,
and
two
R2 (7)
sul-
fonate···pyridiniumsynthons, Figure ESI-13. Furthermore, the
sulfonate end of each Schiff base anion interacts with three differ-
ent molecules of bis-pyridinium ions to form2-D layered tapes,
Figure ESI-14, which are also stabilized by slipped π-π interac-
tions between the aryl rings of Schiff base anions and bis-
pyridinium cations, FigureESI-15. The 2-D layeredtapes further
Figure 2. Packing diagram of [(4,4'-BPY-2H)2+(4-SA-BA-H) 2
̄
] (1), depicting assembly of tri-layers (depicted in different colors
for clarity) into rare 3-D π-interlocked supramolecular framework.
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aggregate to form rare π-interlocked 3-D aggregate, stabilized by
π-stackingof the Schiffbase aryl centers and weak C3-H3···O4
[3.621(5)Å; 153.17(1)°] interactions. The packing diagram of 1a
is depicted in Figure 2, in which different colors represent differ-
ent 2-D layered tapes.
Molecular salt 2 is significantly hydrated and the four lattice wa-
ter molecules residue within the triclinic unitcell while as the
crystal formers occupy cell periphery, Figure ESI-17. The water
molecules undergo aggregation to formrare octameric cluster with
cyclic tetrameric core.Water clustersin crystal lattices areof great
interest for understanding the role of solvent molecules in crystal
packing and their presence in biological systems makes them
more significant. Based on the computational data various
tetrameric water clusters have been predicted. Tetrameric core of
water octamer in 2, formed by donating and accepting one hydro-
gen bond by each water molecule is in good agreement with the
one predicted by Vaitheeswaran et. al.,39 except for the orientation
of dangling hydrogen atoms, which are further engaged in the
present case. The water cluster existing in the lattice of 2 and its
interaction with the crystal formers is depicted in Figure 4.Cyclic
tetrameric cores in the octameric water cluster W8in 2and metha-
nol-water solvent hexamer W4M2 (W = water and M = methanol),
reported by us earlier,37 are similar. The comparative structures
and bond parameters ofW8 and W4M2 are given in Figure ESI-18.
Each of the diagonal water molecules (O7) in the cyclic core of
2further associates with two water molecules, by donating a hy-
drogen bond,O7-H7A···O9 [2.758(3) Å; 173.91(1)°] and accept-
ing a hydrogen bond O7···H6B-O6 [2.959(1) Å; 171.11(8)°]. The
peripheral water molecules (O6 and O9) and a water molecule
(O8) in the tetrameric cyclic core aggregate eight sulfonated anil
anions via O-H···O interactions.Reoccurrence of the W4 cyclic
cluster indicates tendency of its facile manifestation and may be a
robust supramolecular solvent-cluster-binder.
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Structure description of 2.Red crystals of the molecular salt 2
were obtained by in-situreaction of 4-ABSA-2H, o-vanillin and
4,4'-BPY in water-methanol solvent system. 2crystallizes in
centro-symmetric triclinic lattice: P-1. One anion of sulfonated
Schiff base and half of bis-pyridinum cation, in addition to four
lattice water molecules constitute itsasymmetric unit. The sul-
fonated Schiff base exists in plannar-cis-keto form with dihedral
angle of 4.77(1)° between aryl rings. Stabilized by intramolecular
interaction N1-H1N···O4 [2.599(1) Å; 139.24(2)°], cis-keto form
of Schiff base in 2 is corroborated by longer imine C7-
N1[1.306(3) Å], shorter C9-O4 [1.301(2) Å] bond distance and
relaxed C4-N1-C7 [127.21(2)°] bond angleas compared to similar
structural parameters of value 1.277(2) Å, 1.346(2) Å, and
122.43(1)°, respectively in the en-ol form of the sulfonated Schiff
base anion in 1.The sulfonate sulfur center has distorted tetrahe-
dral geometry with nearly identical S-O bond length ranging from
1.445(1) to 1.453(2) Å and average O-S-O bond angle value of
112.25°. As compared to gentle yellow 1, the intense red colour-
ation of 2 may be attributed to existence of acid former in keto-
form. Molecular structure diagram of 2 is depicted inFigure 3.
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Figure 3. Molecular structure diagram of [(4,4'-BPY-2H)2+(2-SA-
oV-H) 2̄ .4H2O] (2).
4,4'-BPY is protonated on both ends with linear N···N distance of
6.988Å.The relaxed C17-N2-C19 bond angle value of
120.86(3)°substantiatesprotonation of pyridyl nitrogen
centers. Although it is difficult to ascertain whether
sulfonic or hydroxyl hydrogen has protonated the
imine nitrogen, however apparently the hydroxyl hy-
drogen is involved in the charge assisted hydroxy-
late···pyridinium interaction. Interestingly, instead of
the anticipated direct charge assisted sul-
fonate···pyridinium interaction the bis-pyridnium
cations bridge two molecules of anil anions by hydro-
gen bonding interaction with keto oxygen:N2-
H2N···O4 [2.601(2)Å, 153.62(1)°], with assistance
from the adjacent methoxy-pyridinium bifurcated
interaction N2-H2N···O5 [3.138(2) Å; 128.49(1)°].
Furthermore, as in 1, all the bis-pyridinium hydrogens
areinvolved in weak C-H···O interactions with two
sulfonated Schiff bases through C19-H19···O3
[3.367(2) Å; 133.09(1)°], C18-H18···O1 [3.414(1) Å;
170.80(2)°] and C16-H16···O1 [3.479(3) Å;
174.71(2)°], and two molecules of water via C17-
H17···O8 [3.377(2) Å; 134.04(2)°] interaction form-
Figure 4. Rare water cluster W8 in the lattice of [(4,4'-BPY-2H)2+
̄
Figure 5. Packing diagram of [(4,4'-BPY-2H)2+ (2-SA-oV-H) 2
̄ .4H2O] (2). The
hydrogen bonded water channels are depicted as pink spheres and bis-pyridinium
linkers are highlighted in blue.
ing [(4,4'-BPY-2H)2+(4-SA-oV-H)4ˉ.2H2O]2ˉ cluster,
Figure ESI-16.
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(4-SA-oV-H) ̄2.4H2O]
(2).
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The water molecules
associate the sulfinilate
anils into 2-D networks
4
withalternateR4 (8) and
4
R4 (10)hydrogen
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bonded rings, Figure
ESI-19. The 2-D layers
are bridged by bis-
pyridnium linkers into
3-D
supramolecule
depicted in Figure 5.
Molecular structure
description of 3.Dark
brown crystals of 3 are
obtained
by
slow
evaporation of reaction
mixture obtained by
three component in-situ
reaction. The molecu-
Figure 7. Packing diagram of [(4,4'-BPY-2H)2+(2-SA-oV-H)
2̄ .2H2O] (3), bis-pyridinium cations are hig-
hlighted in blue.
Sulfonated Schiff base anions assemble with help of the lattice
water molecules into 1-Dtapes through O6-H6A···O7
[2.874(2) Å; 162.21(1)°], O6-H6B···O1 [2.819(1) Å;
161.97(2)°], O7-H7A···O3 [2.813(2) Å; 166.73(1)°] and O7-
H7B···O2 [2.339(1) Å; 164.23(1)°] interactions. Similar to H-
bonded tapes reported for organo phosphates and
phosphonates,40 the 1-D anionic tapes of 3 consist of alternate
lar salt crystallizes in centro-symmetric triclinic crystal system: P-
1, with an anion of Schiff base 3a, half of pyridinium ion and two
molecules of lattice water in the asymmetric unit. Sulfonated base
exists in non-planer keto form with C1-C6-C8-C13 torsion angle
value of 21.2° between the aryl rings. As has been previously
reported for the free ligand3a(see discussion), the sulfonic hydro-
gen is shifted to imine nitrogenN1 and forms the bifurcated hy-
drogen bond with sulfonate and keto oxygen centers: O1···H1N-
N1 [2.883(1) Å; 132.50(1)°] and O4···H1N-N1 [2.717(1)Å;
130.86(2)°], respectively. Hydroxyl hydrogen of o-vanilin shifts
to the pyridyl nitrogen leading to ionichydroxylate···pyridinium
interaction N2-H2N···O4 [2.640(2) Å; 159.23(1)°] instead of
anticipated sulfonate···pyridinium synthon formation. The hy-
droxylate···pyridinium interaction in 3 is assisted by bifurcated
interaction of pyridinium hydrogen with adjacent methoxy oxy-
gen: N2-H2N···O5 [3.111(2) Å; 123.32(1)°]thereby forming
4
orthogonally displaced hexagonal [R4 (4)] and octagonal
6
[R6 (6)] H-bonded rings in chair conformations, Figure ESI-
20. The resultant 1-D tapes are bridged by bis-pyridinium ions
into ionic 2-D reticular sheets (Figure ESI-21), which are
further assembled by weak dispersive H···H [2.224(1) Å; C-
H···H 130.09°] interactions into 3-D solid, Figure 7.H···H
London dispersive forces have been identified recently and
utilization of theses interaction to design or modulate crystal
structures remains an incredible challenge.41As in3, the weak
dispersive H···H interactions have also been reported by us to be
the only supramolecular interaction associating 2-D sheets in an
organic complex into 3-D solid.38
2
R1 (2) synthon, similar to that in 2. Protonation of pyridyl nitro-
gen N2 in 4,4'-BPY and existence ofSchiff base in keto form is
substantiated by relaxed C14-N2-C18 bond angle value of
120.48(2) °and shorter C13-O4 and longer C7-N1bond distance of
1.280(2) and 1.310(1)Å, respectively. The molecular structure
diagram of 3 is depicted in Figure 6.
Thermal studies
Sulfonated Schiff bases 1a-3a exhibit high melting points, while
their salts melt at relatively lower temperatures, Table ESI-2.The
melting points of the products 1a-3a and 1-3are also substantiated
by endothermic peaks in DTA curves, Figure ESI-22-27. TGA
curves of 1a-3a indicate presence ofsolvent molecules and their
high thermal stability upto 250 °C. It is pertinent to highlight that
sodium salt of 1a, reported earlier, exhibits higher thermal stabili-
ty than 1a.21Molecular salt 1doesnot undergo early weight loss
and remains stable till 230 °C, 2and 3 undergoes early weight loss
of nearly 8 and 5%, corresponding to loss of four and two water
molecules in their crystal lattice, respectively.Organic salts 1 and
2 exhibit slightly lower thermal stability than corresponding sul-
fonatedSchiff bases while as 3 exhibits augmented stability than
itsSchiff base former 3a. Comparative normalized TGA curves of
1a-3a and 1-3 are depicted in Figure 8, while the separate
TGA/DTA curves of the products are depicted in Figure ESI-22-
27.
Figure 6. Molecular structure diagram of [(4,4'-BPY-2H)2+(2-SA-
oV-H) 2̄ .2H2O] (3).
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provided in Chart ESI-1. 1and 2 behavealmost similar to that
of 1a and 2a when heated at110°C, by undergoingcolor
intensification which is also reflected in their diffuse
refluctance spectrum.However, the molecular salt 3responds
strikingly to heat as indicated by color change from intense
brown to brick red. The blue shift in the absorption of 3 on
heating
is
also
substantiated
by
K-Mfunctions.
Interestingly,besides strong blue shift on heating, 3 does also
undergo observable color change on grinding. The
comperative K-M plots of 3, its grinded form 3G and heated
form 3' are further discussed in next section. Band gapvalues
inthe molecular salts 1-3 are low compared to the sulfonated
anils 1a-3a. Lowering of band gap on the salt formation may
be attributed to strong π-π interactions, as observed in the
crystal structures of 1-3. Moreover, band gap change in 1and
2is mild and pronounced in 3, when the molecular salts are
heated, indicating heat induced packing changes may be
responsible for the the striking thermochromism in 3. Tauc
plots of 1-3 are given as Figure ESI-37-39.
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Figure 8.Comparative normalized TGA curves of sulfonated
Schiff bases 1a-3a and their organic salts 1-3.
Optical studies
DISCUSSION
Our recent results aboutthe development of molecular complexes
based on sulfonate-pyridinium syntnon, ubiquitously in salt
form,encouraged us to further utilize the supramolecular interac-
tion for designing new materials. Anil derivatives of 4-and 2-
aminobenzenesulfonic acids 1a-3awere synthesised to explore
their anticipated optical behaviour towards heat and light. How-
ever, 1aonly exhibits notable optical response to heat. Defining
criteria to exhibit thermo- and photochromism in anils has been
attributed to enol-cis-keto (prototropism) and cis-trans-keto
(isomerism) transformations, respectively. Lack ofthermochrom-
ism in 1a-3a may be attributed to protonation of the imine nitro-
gen by sulfonic hydrogen, as has been confirmed by previously
reported crystal structure of 3a,42 Figure 10, hence ruling out the
enol-cis-keto prototropism.Diffuse reflectance studies of 1a-3a
also rule out enol-keto transformation. And the absence of photo-
chromism in these anils may be due to close packing or planer
configuration or both.Failure to crystallize 1a and 2a and aim to
modulate molecular packing in 1a-3aprompted us to co-crystallize
the sulfonated anils with bipyridyl.Interestingly, besides undergo-
ingredshift on salt formation,the molecular salts 1 and 3 exhibit
markable irreversible thermochromism as well.
To understand the optical behavior and its response towards
heat in the sulfonated anils 1a-3a and their organic slats 1-3,
diffuse reflectance (DR-UV-Vis) studies have been carried
out. 1a shows notable color change from light to intense yel-
low when heated at 85 °C while 2aand 3a do not undergo
noteworthychange in colorwhen heated upto 150 °C. Compar-
ative Kubelka-Munk (K-M) function of 1a-3a and their heated
forms 1a'-3a' are depicted in Figure 9a and Figure ESI-28-30.
Thermochromism in anil type molecules is a famous pheno-
menon and generally attributed to proton dynamics between
enol and cis-keto forms. Sulfonated anils with lower packing
coefficients have been reported to exhibit both opto- and
thermochromism due to enol↔cis-keto↔trans-keto isomer-
ism. However, as indicated by K-M plots the color transforma-
tion in 1a-3a cannot be accounted tosuch isomerization in
these molecules.Tauc plots of 1a-3aindicate shift in the band
gap on heating, Figure ESI-31-33.The sulfonated anils do not
exhibit photochromism.
Organic salts 1-3, obtained by reaction of the sulfonated anils
with bipyridyl also respond to the heat as indicated by color
changesin 1'-3'. The K-M plots of 1-3 are depicted in Figure
9b and Figure ESI-34-36.Optical Images of 1a-3a and 1-3 are
(a)
(b)
Figure 9.(a) Comparative Kubelka-Munk plots of sulfonated Schiff bases 1a-3a and their heated forms 1a'-3a'.(b) Comparative Kubelka-
Munk function of organic salts 1-3 and their heated forms 1'-3'.
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the absorption is continuously blue-shifted with grinding and
heating the band gap increases on grinding in 3G and heating in
3’ to 1.76 and 1.88 eV, respectively, compared to 1.70 eV in 3,
strongly indicating change in the molecular packing in the solid.
Hence ruling out the prototropism and lattice solvent loss as plau-
sible explanation for thermochromism, the phenomenon plausibly
originates due to change in crystal packing of the products. K.M.
functions and Tauc plots of 3, 3G and 3’ are depicted in Figure
11a and 11b.
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Figure 10. Molecular structure of 3a (CCDC No. VUQLIE), lat-
tice water molecules are omitted for clarity.42
a)
b)
Crystal formers in non-solvated molecular salt 1 associate via
anticipated charge assistedsulfonate···pyridinium interaction and
aggregate into rare π-interlocked supramolecule. In 2 and 3 the
water-arylsulfonate aggregates are bridged by bis-pyridinium ions
throughmethoxy assisted hydroxylate∙∙∙pyridinium interactions
into 3-D solids, leading to a significant observation: in presence of
adjacent/ortho methoxy groups, phenolic hydroxalate∙∙∙pyridinium
interaction dominates over otherwise robustsulfonate∙∙∙pyridinium
synthon formation. As has been reported by usformation of sul-
fonate∙∙∙pyridinium synthon (or transfer of sulfonate hydrogen to
pyridyl nitrogen)is ubiquitous interaction as observed in 5-
sulfosalicylic acid bearing carboxylic, hydroxyl as well as sul-
fonate groups, indicating preferential formation of sul-
fonate···pyridinium interaction over hydroxylate···pyridinium
and carboxylate∙∙∙pyridinium synthon.36Structural analyses indi-
cate the driving force for formation of hydroxalate∙∙∙pyridinium
synthon in 2 and 3 possiblyis the bifurcated hydrogen bonding of
pyridinium hydrogen with phenoxy and ortho-methoxy group,
Chart 3.
c)
Figure 11. Kubelka-Munk and Tau plots of 3, 3G and 3' (a and b)
and optical images of 3, 3G and 3' (c).
CONCLUSION
Sulfonated anils 1a-3a have been synthesized in good yield byre-
fluxing the mechanically grinded paste of reactants in aqueous
methanol and were subsequently converted to organic slats 1a-3a
by reaction with 4,4'-BPY. 1a-3a and 1-3 do not exhibit antici-
pated enol↔cis-keto↔trans-keto isomerism induced by heat and
light as indicated by their Kubelka-Munk functions and supported
by previously reported crystal structure of 3a. However, the prod-
ucts respond to heat but not to light. Organic salts 1-3 exhibit red
shift on formation from 1a-3a indicated by their colors and sub-
stantiated by diffuse reflectance studies. Faint but notable thermo-
chromism observed in 1a and 2a is replicated in 1 and 2, while
unlike 3a, 3 exhibits remarkable thermochromism. Tauc plots of
1a-3a and 1-3 and their heated forms 1a’-3a’ and 1’-3’, respec-
tively, together with 3G (grinded form of 3) indicate change in
crystal packing causes the thermochromism. Currently we are
generating more data to investigate the phenomenon in greater
detail.
Chart 3. Depiction of possible interactions (I-IV) in functional-
ized arylsulfonates. Yes and No imply whether the interactions
are formed or not.
Diffuse reflectance studies of the molecular salts 1-3 substantiate
red shift in their absorption vis-a-vis1a-3a, observed in their opti-
cal images depicted in Chart ESI-1. The salts 1 and 2 nearly repli-
cate the optical behaviour of the corresponding Schiff bases to-
wards heat while as 3 exhibits striking blue shift in the absorption
on heating. Based onthe crystal structure studies and K-M func-
tions thermochromism in these salts cannot be attributed to proto-
tropism. The photochromism in 1-3 is not observedand may be
attributed to close packing of the components in the lattice as
indicated by Vfree value of 100, 142 and 119 Å3 for 1-3, respec-
tively, while as Vfree requirement for execution of the phenome-
non is >170 Å3.27 Infra red and thermo-gravametric investigations
of 1a-3a and 1-3indicate loss of lattice water when heated till
color change, however the phenomenon can’t be attributed to
solvent loss induced packing changes as it is observedeven in
non-solvated salt 1. Moreover, to our interest intense brown 3
undergoes color change even on grinding (3G)and the resultant
powder undergoes continuous color change on heating until it
attains brick red complexion (3’). Optical pictures of the three
forms:3, 3G and 3’, are depicted in Figure 11c. Tauc plots of 1a-
3a, Figure ESI-31-33, and 1-3,Figure ESI-37-39, confirm band
gap increase in the products on heating. Particularly in case of3,as
Crystallographic studies of the organic salts 1-3 reveal manifesta-
tion of anticipated direct sulfonate···pyridinium in 1, while as
crystal
formers
in2and3assemble
through
hydroxy-
late···pyridinium synthon. Organic salt 1aggregates as rare π-
interlocked solid while rare octameric water cluster W8 in 2 and
weak H···H London dispersive forces in 3 lead to formation of 3-
dimensional ionic solids. Unlike the ubiquitous formation of sul-
fonate···pyridinium synthon even in presence of carboxylate and
hydroxyl groups, as reported earlier, preference for the formation
of hydroxylate···pyridinium synthon in 2 and 3 may be attributed
to the presence of adjacent methoxy group which strengths the
2
R1 (2) bifurcated interaction.
ASSOCIATED CONTENT
Supporting Information.This material is available free of
charge via the Internet at http://pubs.acs.org.
FT-IR of 1a-3aand 1-3.1Hspectra of 1a-3a and 1-3.Structure
and packing diagrams of 1a-3a and 1-3.TGA-DTA thermo-
grams of 1a-3a and 1-3. Diffuse reflectanceUV-Vis spectra,
K-M plots and tauc plots of pristine components and 1a-3a
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in iron coordination compounds.Beilstein J. Org. Chem.
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CCDC 1981821-1981823contain the supplementary crystal-
lographic data for this paper. These data can be obtained free
of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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photochromism of tris-anilsRSC Adv., 2013, 3, 6466-6471.
18. Robert, F.; Naik A. D.; Tinant, B.; Garcia, Y. Insights into
the Origin of Solid-State Photochromism and Thermoch-
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AUTHOR INFORMATION
Corresponding Author
*(A.A.D.) E-mail: aijazpapers@gmail.com / daraijaz@uok.edu.in
Tel./Fax: +91-9596034885
ORCID
Aijaz Ahmad Dar: 0000-0002-6180-274X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
Authors are thankful to SERB-DST-New Delhi for ECRAgrant
(ECR/2017/000058) and Darshan Mahatre for assistance in crystal
refinement.D. A. A. also acknowledges host Prof. G. R. Desiraju
and funding agency INSA-New Delhi for Visiting Scientist
Award.
ABBREVIATIONS
4-ABSA-H, 4-aminobenzene sulfonic acid; 2-ABSA-H, 2-
aminobenzene sulfonic acid; 4,4'-BPY, 4,4'-bipyridyl; K-M
function, Kubelka-Munk function.
N-Salicylideneamino-1-alkylpyridinium
Derivatives
Through Formation of Tetra-Aryl Boxes. J. Phys. Chem. C.
2018, 122, 10999−11007.
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Irreversible Thermochromism in Organic Salts of Sulfonated
Anils
Aijaz A. Dar, †,* Arshid A. Ganie †
†Department of Chemistry, Inorganic Section, University of Kashmir, Hazratbal, Srinagar-190006, India.
TOC Graphic
Synopsis
Sulfonate···pyridinium synthon based molecular salts 1-3 obtained from sulfonated anils 1a-3a have
been reported to exhibit trivial to striking irreversible thermochromism. Optical studies vis-à-vis crystal-
lographic investigations rule out the conventional and general mechanism i.e. enol-cis-keto isomeriza-
tion, and instead indicate packing changes to be the cause of irreversible thermochromism.
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