Exceptional dual fluorescent, excited-state intramolecular proton-transfer (ESIPT) columnar liquid crystals characterized by J-stacking and large Stokes shifts

Article abstract of DOI:10.1016/j.molliq.2021.115879

Excited-State Intramolecular Proton-Transfer (ESIPT) fluorophores are emerging as promising future materials for electronic and biotechnological applications. ESIPT columnar (Col) liquid crystals (LCs) have been especially projected as the apt materials for advanced technological endeavors. But, there are hardly any explorations in this direction and thus, needing immediate attention. Herein we report on the synthesis, characterization, and ESIPT activity of a homologous series of novel phasmidic bis(N-salicylideneaniline) Col LCs. Optical microscopic, calorimetric and powder X-ray diffraction (XRD) studies evidence the occurrence of hexagonal columnar (Col_h) phase having p6mm symmetry where the constituent slices result from the self-assembly of a pair of mesogens in a side-by-side manner facilitated by intense longitudinal π-π interactions. X-ray data confirm the absence of both directionally correlated tilting of the slices and transverse core-core interactions within the columns. Fluorescence probing clearly evidence the ESIPT occurring not only in DCM solution of the mesogens but also in their three-condensed states viz., solid, liquid crystal, and isotropic liquid phase; in general, two archetypal emission bands at ~430 nm (weak) and ~ 630 nm (strong) with large Stokes shifts (250–275 nm) of ESIPT phenomenon have been observed. The slow shift of emission maxima of the ESIPT fluorescence as a function of decreasing temperature without photoluminescence quenching coupled with the estimated tilt angle (?) of the slices normal to the columnar axis (37 to 42^o), from the XRD data, confirm the formation of so-called Scheibe or J-aggregates. The redox activity, metal ion sensing ability, and solvatochromism of the mesogens have also been investigated. The study suggests that these ESIPT Col LCs with band-gap of about 3 eV can be regarded as wide-bandgap semiconducting materials having the electronic characteristics falling between those of conventional semiconductors and insulators.

Full text of DOI:10.1016/j.molliq.2021.115879

Journal of Molecular Liquids 332 (2021) 115879
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Exceptional dual fluorescent, excited-state intramolecular
proton-transfer (ESIPT) columnar liquid crystals characterized
by J-stacking and large Stokes shifts
⁎
Madhu Babu Kanakala, Channabasaveshwar V. Yelamaggad
Centre for Nano and Soft Matter Sciences(CeNS), Arkavathi, Shivanapura, Dasanapura Hobli, Bangalore - 562162, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Excited-State Intramolecular Proton-Transfer (ESIPT) fluorophores are emerging as promising future materials
for electronic and biotechnological applications. ESIPT columnar (Col) liquid crystals (LCs) have been especially
projected as the apt materials for advanced technological endeavors. But, there are hardly any explorations in this
direction and thus, needing immediate attention. Herein we report on the synthesis, characterization, and ESIPT
activity of a homologous series of novel phasmidic bis(N-salicylideneaniline) Col LCs. Optical microscopic, calo-
rimetric and powder X-ray diffraction (XRD) studies evidence the occurrence of hexagonal columnar (Colh)
phase having p6mm symmetry where the constituent slices result from the self-assembly of a pair of mesogens
in a side-by-side manner facilitated by intense longitudinal π-π interactions. X-ray data confirm the absence of
both directionally correlated tilting of the slices and transverse core-core interactions within the columns. Fluo-
rescence probing clearly evidence the ESIPT occurring not only in DCM solution of the mesogens but also in their
three-condensed states viz., solid, liquid crystal, and isotropic liquid phase; in general, two archetypal emission
bands at ~430 nm (weak) and ~ 630 nm (strong) with large Stokes shifts (250–275 nm) of ESIPT phenomenon
have been observed. The slow shift of emission maxima of the ESIPT fluorescence as a function of decreasing tem-
perature without photoluminescence quenching coupled with the estimated tilt angle (ϴ) of the slices normal to
the columnar axis (37 to 42o), from the XRD data, confirm the formation of so-called Scheibe or J-aggregates. The
redox activity, metal ion sensing ability, and solvatochromism of the mesogens have also been investigated. The
study suggests that these ESIPT Col LCs with band-gap of about 3 eV can be regarded as wide-bandgap semicon-
ducting materials having the electronic characteristics falling between those of conventional semiconductors and
insulators.
Received 18 December 2020
Received in revised form 3 March 2021
Accepted 8 March 2021
Available online 11 March 2021
Keywords:
Phasmids
Columar hexagonal phase
ESIPT activity
Dual-fluorescence
J-aggregates
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
also structural defect eliminating property through the natural self-
healing process. It has been well demonstrated over the years that the
Functional materials belong to a special class of man-made sub-
stances that are mostly organic or inorganic or hybrid/composite sub-
stances ranging from small molecular (low molar mass) compounds
to polymers. [1–9] They, being hard [1–8] or soft [9] in nature, possess
the ability to display predetermined physicochemical properties/phe-
nomenon under the influence of an external stimulus such as light, tem-
perature, sound, electric field, magnetic field, mechanical stress,
substrate surface anchoring conditions, etc. Among wide varieties of
soft functional materials reported hitherto, liquid crystals (LCs) have se-
cured a prominent position in materials science encompassing the as-
pects of both basic research and applied science. [10–12] They are
self-organized fluid (soft) superstructures and thus, possess the inher-
ent ability of not only responding to the aforesaid external stimuli but
exceptional characteristics of LCs, which were so far associated mainly
with the LC display technology, can be well-exploited in devices such
as semiconductors, sensors, elastomer actuators, photovoltaics, organic
light-emitting diodes (OLEDs), organic field-effect transistors (OFETs),
etc. [11,13,14]
For example, such technological endeavors are attainable by utiliz-
ing the exceptional characteristics of columnar (Col) LC phases formed
by the parallel packing of infinitely long columns organized in two-
dimensional (2D) lattices; their intrinsic electronic features originate
purely from the disk-like (discotic) aromatic or heteroaromatic cores,
substituted peripherally with three to several flexible aliphatic tails,
stacked co-facially in columns through intense π-π associations/interac-
tions. [15–21] While the insulating alkyl chains prevent the closer inter-
columnar interactions, the π-π interaction of discotic mesogens in each
column creates 1D charge carrier pathway along the long column axis.
The average π-π stacking distance of the discotic cores in a column is
⁎
Corresponding author.
E-mail address: yelamaggad@cens.res.in (C.V. Yelamaggad).
https://doi.org/10.1016/j.molliq.2021.115879
0167-7322/© 2021 Elsevier B.V. All rights reserved.

M.B. Kanakala and C.V. Yelamaggad
Journal of Molecular Liquids 332 (2021) 115879
typically of the order of 3.5 Å, and thus, an overlap of the π-π* lowest un-
occupied molecular orbitals (LUMOs) should be possible. [16,17,21]
Thus, the Col phases, which have been aptly regarded as molecular
wires or quasi-one-dimensional (Q1D) conductors, hold huge promise
for the field of organic electronics due to their intrinsic ability to serve
as the active media, in the form of thin-films, to perform the essential
functions such as carrier-injection or blocking, and electron-hole re-
combination or separation. [16–21] However, in recent years, consider-
able efforts have been devoted not only to control the vital parameters
such as oxidation potential and electron affinity but also to incorporate
the light absorption and luminescent properties to enable Col phase to
serve as the emissive layers. [16–22] Thus, fluorescent Col LCs have
not only the ability to replace light-emitting amorphous/crystalline
solids [23–25] but also possess promising technological aspects such
as long-range order [16], charge-transport property / anisotropic con-
ductivity, [16,19,26] long exciton diffusion lengths, [27,28] self-healing
ability [ 16,17], low-cost solution processability [16] etc. The Col phases
are generally classified, depending upon the degree of order in the mo-
lecular stacking, the alignment of the discogens along the columnar axis,
and the symmetry of the 2D lattice, into seven different classes. Of these,
Col hexagonal (Colh) and/or Col rectangular (Colr) phases are often en-
countered in many discotic LCs. [16,17] While four different lattice sym-
metries viz., p2mm, c2mm, p2gg, and p2mg are assigned for the Colr
phase, the Colh phase, wherein the columns are arranged on a 2D-
hexagonal lattice, is generally described by the plane group p6mm. [16]
As is well known, a special type of Colh phase is formed by the self-
assembly of simple non-discoid LCs, called phasmidic or polycatenar
mesogens. They comprise a long rod-like core ending on either side
with lipophilic segments comprising two or three flexible tails.
[12,16,29–44] They have been categorized according to the total num-
ber of n-alkoxy tails they possess; for example, polycatenars substituted
with four, five and six terminal paraffinic tails have been respectively re-
ferred to as tetracatenars, pentacatenars and hexacatenars [ 29–32].
These relatively new class of thermotropic LCs bridge the gap between
conventional rod-like and disk-shaped LC molecular architectures, and
thus, they exhibit a rich polymesomorphism. That is, experimentally,
it has been evidenced that these hybrid mesogens not only show the ne-
matic (N), smectic (lamellar) as well as bicontinuous/micellar cubic, te-
tragonal and orthorhombic three-dimensional phases but also stabilize
a unique Colh phase in a series of compounds or pure material.
[12,16,29–33] However, such phase transitional behavior largely de-
pends on the number of alkoxy tails they possess on either side of the
central calamitic core. Of the variety of polycatenar LCs reported hith-
erto, the hexacatenars (phasmids) have been attracting attention not
only because of their inherent ability to self-assemble into Col phase
but also they provide the opportunity to incorporate functional charac-
teristics through the selection of predetermined molecular structural
segments. Thus, a wide variety of functional phasmids have been real-
ized, which include heterocycles, [33–38] metallomesogens, [38] ionic
LCs, [40,41] photo-responsive mesogens, [42] intra-molecular H-
bonded systems, [43,44] thermochromic LCs, [45] bent-core com-
pounds, [46,47] fluorescent materials etc. [48–64]
Among the various types of ESIPT active molecules reported hitherto,
[43,44,66–73] salicylideneanilines attracted our attention owing to
their potential for various technological endeavors. [67] In particular,
the investigations on phasmidic LCs by Hiremath [43,44] clearly hinted
that the mesomoprism can be induced in bis(N-salicylideneaniline)s
(BSANs). It was observed in this detailed study that out of the three con-
stitutional isomeric bis(N-salicylideneaniline)s, viz., BSAN-I, BSAN-II &
BSAN-III (Fig. 1), the isomers belonging to BSAN-I category derived
from 3,6-diformylcatechol shows columnar behavior. These conformers
(BSAN-I) can undergo a reversible proton-transfer process to furnish
two tautomers, namely, enol-imine (OH) (BSAN-I(OH)), and keto-
enamine (NH) (BSAN-I(NH)) forms (Fig. 1). That is, these compounds
have the intrinsic ability to display ESIPT fluorescence owing to their
novel structural composition where the intramolecular hydrogen (H)-
bonding correlation among a hydrogen bond donor (–OH) group and
a hydrogen bond acceptor (-C=N, imine) linking group exists. Despite
these exciting feature, the investigations on the photophysical proper-
ties of BSAN-I columnar LCs have not been revealed hitherto. Thus, we
set out our goal to realize the analogs of BSAN-I hexacatenars and eval-
uate their ESIPT activity. Herein, we report on the synthesis and detailed
characterization of
a series of ESIPT active phasmidic bis(N-
salicylideneaniline)s (PBSANs) existing in Enol-Imine form, which
upon photoexcitation transforms into Keto-Enamaine tautomeric form
(Fig. 1).
2. Results and discussion
2.1. Synthesis and characterization
The conventional synthetic steps employed for the preparation
of target PBSAN materials have been outlined in Scheme 1. Gallic
acid was treated with ethanol in the presence of an acid catalyst
to obtain ethylgallate (1) [51], which was subjected to O-alkylation
with n-alkylbromides under the reaction conditions of Williamson
ether synthesis protocol to get ethyl 3,4,5-tris(n-alkyloxy)benzoates
(2a-j) [52]. Alkaline hydrolysis of esters 2a-j using aqueous sodium
hydroxide (5%) in ethanol provided the corresponding 3,4,5-tris
(n-alkyloxy)benzoic acids (3a-j) [53] which were esterfied with 4-
nitrophenol in the presence of N,N′-dicyclohexylcarbodiimide (DCC)
and 4-(dimethylamino)pyridine (DMAP) to obtain 4-nitrophenyl
3,4,5-tris(n-alkyloxy)benzoates (4a-j) [53]. Hydrogenation of these
nitro compounds 4a-j in the presence of Pd\\C (10%) and under hydro-
gen (H2) balloon (1 atm) conditions afforded 4-aminophenyl 3,4,5-tris
(n-alkyloxy)benzoates (5a-j) [54]. The central core of the target phas-
mids namely 2,3-dihydroxyterephthalaldehyde (7) was synthesized
starting from 1,2-dimethoxybenzene. It was subjected to lithiation
using n-butyllithium (n-BuLi) in the presence of tetramethylethylenedi-
amine (TMEDA), and the treatment of mettallated intermediate with N,
N-dimethylformamide (DMF) yielded 2,3-dimethoxyterephthala-
ldehyde (6). [55]
O-Demethylation facilitated by boron tribromide (BBr3) of com-
pound 6 afforded the vital intermediate 7. [56,57] Finally, an acid-
catalyzed condensation of amines (5a-j) with aldehyde 7 afforded the
desired materials in good yields (69–75%). The molecular structures of
all synthesized liquid crystals were ascertained unambiguously using
the data derived from spectroscopic techniques viz., UV–visible, FTIR,
1H NMR & 13C NMR, see Figs. S3 to S20 of supporting information (SI),
and elemental analyses. These characterization data, along with the syn-
thetic procedure of these final mesogens, are given in the experimental
section (II.5). However, the intermediates, 2a-j, 3a-j, 4a-j, 5a-j, 6 & 7, ac-
complished here are known compounds 51–57, and they have been ex-
amined for their molecular structures with the aid of FTIR, 1H NMR,
and elemental analyses only. The synthetic procedures and characteri-
zation data of these precursors have been presented in supporting
information (SI).
However, the fluorescent phasmidics, owing to their technological
relevance, have been studied intensively through the rational design
and synthesis of diverse series of materials derived from functional
fluorophores [48–65]. Of these, there has been a thought-provoking
work by Park and co-workers reporting that phasmidic LCs, possessing
intra-molecular proton-transfer (ESIPT) activity, form tilted (J-type)
stacking in the column of the LC phases; interestingly, this assembly
augments the fluorescence emission of the mesophase with a quantum
yield of 34% and a large Stokes' shift (173 nm) [ 65]. It is speculated that
the ESIPT fluorescence coupled with Col behavior make the phasmidics
ideal candidates for applications in emissive LCDs, polarized organic la-
sers and anisotropic OLEDs. [34,65] Thus, we became interested in
working on this topic of research and accordingly, searched for the
ESIPT active molecules for incorporating Col mesomorphic behavior.
2

M.B. Kanakala and C.V. Yelamaggad
Journal of Molecular Liquids 332 (2021) 115879
Fig. 1. Skeletal structures of the previously studied hexacatenar LCs (BSAN-I, II & III; Refs. 43 & 44) - the source of present work. Structures of the phasmidic bis(N-salicylideneaniline)s,
(PBSANs) investigated in the present work.
2.2. Thermal properties and mesomorphic behavior
characteristically displaying a fluid and homogeneous birefringent tex-
ture that remains unaltered until the isotropization temperature. The
fluidity of the phase was evident from the fact that the coverslip of the
specimen could be moved with great ease using a pair of tongue or a
paper pin. Upon cooling, a transition from the isotropic liquid state
into mesophase occurs, with the texture showing typical defects associ-
ated with the Colh phase.
The optical texture of the Col phase appears to vary notably from one
homolog to another. For example, as shown in Figs. 2a-c, the POM tex-
tures of the Col phase formed by mesogens PBSAN-10, PBSAN-11, and
PBSAN-14 differ considerably from each other. The texture of the Col
phase, emanating from the dark background of isotropic liquid of com-
pound PBSAN-10, appears initially as tiny batonnets that eventually co-
alesces to a pseudo focal-conic fan-like texture as shown in Fig. 2a. The
Col phases formed while cooling from the isotropic phase of
PBSAN-11 and PBSAN-14 exhibit pseudo-isotropic (homeotropic) tex-
ture, respectively comprising linear birefringent defects (Fig. 2b) and
flower-like texture/spherulitic domains (Fig. 2c). However, some of
The thermal and mesomorphic properties of the newly synthesized
PBSAN materials were evaluated by a combination of polarizing optical
microscopy (POM), differential scanning calorimetry (DSC), and
variable-temperature X-ray diffraction (XRD). To figure out the phase
transitions/temperatures, a minute quantity (~1 mg) of the pristine
sample, held between a glass slide and a coverslip, was subjected to re-
peated heating-cooling scans using a hot stage and viewed under POM.
DSC thermograms for the heating-cooling scans of the mesogens were
recorded at a rate of 5 °C / min to confirm the transition temperatures
and enthalpy changes of phase transitions. The data derived from
these studies are given in Table 1. It is apparent from the collected
data that the first (lowest) member of the series is non-mesomorphic,
whereas the remaining members are liquid crystalline exhibiting an
identical, thermodynamically stable mesophase. Upon heating, the pris-
tine samples PBSAN-6 to PBSAN-12, PBSAN-14 and PBSAN-16 undergo
an endothermic phase transition from solid to mesomorphic state,
3

M.B. Kanakala and C.V. Yelamaggad
Journal of Molecular Liquids 332 (2021) 115879
Scheme 1.
the phasmids displayed uniform (dark) domain of homeotropically
aligned Colh phase where column axes are vertical to the glass surface;
this is notable given the fact that the homeotropic alignment of
Col phase can be used in solar cells or OLEDs. Needless to say, the
large homeotropic domains appearing dark under POM reveal that
these LC phases are optically uniaxial, which is typical for the Colh
phase. Thus, the Col phase formed by these compounds remains
unaltered nearly up to room temperature (RT), and eventually, it trans-
forms into a solid-state with an exception for the member PBSAN-9
where the Col phase freezes (Table 1 & Fig. S21). The phase transitions
and the corresponding temperatures were found to be highly reproduc-
ible for any number of heating/cooling scans during POM study, imply-
ing that the synthesized phasmids possess excellent thermal stability
and hydrolytic resistance. Fig. 3a and b depict the representative DSC
traces obtained respectively for PBSAN-9 and PBSAN-14 where the
peak positions of the Col-I/I-Col phase transitions of the two consecutive
heating-cooling scans remain almost unaffected. Thus, as envisaged,
DSC profiles of the LCs corroborate the inference described above of
the POM study.
The indication from the POM textural investigations that the newly
prepared phasmidic LCs exhibit a columnar hexagonal phase was sub-
stantiated with variable-temperature powder X-ray diffraction (XRD)
experiments. XRD patterns were obtained for three compounds viz.,
PBSAN-11, PBSAN-12, and PBSAN-14 as representative cases. The sam-
ples were heated to their isotropic liquid state and filled into capillary
tubes (1 mm) of Lindemann glass. They were cooled into the Col
phase at a rate of 5 °C / min, and the diffraction patterns of the LC
phase were recorded at two different temperatures for each selected
sample. Figs. 4a, S22 & 4b respectively portray the 1D intensity vs dif-
fraction angle (2θ) profiles of mesogens PBSAN-11, PBSAN-12, and
PBSAN-14. Table 2 provides the Bragg reflections along with the results
pertaining to the detailed indexation of the mesophases of each
phasmid LC. It is thus evident from the XRD patterns (Figs. 4a-b &
S22) and the corresponding data collected in Table 2 that compounds
4

M.B. Kanakala and C.V. Yelamaggad
Journal of Molecular Liquids 332 (2021) 115879
Table 1
diffractograms of the Col phase is characterized by the halo scattering
corresponding to spacing (d) in the range of 4.3–4.56 Å, which stems
due to the slow mobility of the molten n-alkoxy tails located on either
side of the central five-ring aromatic rigid core. The Col phase of
phasmidic LCs may as well display a relatively sharp scattering in the
high-angle region (at Bragg angles 2θ ~ 25 deg) due to an intermolecular
co-facial π-π interaction between the constituent mesogenic cores of a
column. The lack of such an intracolumnar reflection, precisely the sig-
nature due to core-core separation, in the XRD profiles implies the like-
lihood of disordered arrangement of the hexacatenar molecules within
the columnar structure of samples under discussion. The low angle re-
gion (0 < 2θ < 5 o) of all the diffraction scans comprises a strong reflec-
tion along with three weak peaks (Fig. 4a; Table 2).
Phase transition temperatures (°C)a and associated enthalpies (kJ/mol) of transitions ob-
tained for the phasmidic mesogens of PBSAN–n series, Cr: Crystal; Colh/p6mm: Hexagonal
columnar phase having p6mm symmetry; I: isotropic liquid state.
LCs
Phase sequence
Heating; Cooling
PBSAN-5
PBSAN-6
Cr 151.7 (41.9) I; I 114 (1.27) Cr
Cr 112.9 (3.1) Colh/p6mm 118.9 (1.6) I; I 114.5 (1.5) Colh/p6mm 61.8
(3.18) Cr
Cr 61.8 (3.9) Colh /p6mm 120 (1.6) I; I 116.4 (1.5) Colh/p6mm 44.0
(0.2) Cr
PBSAN-7
PBSAN-8
PBSAN-9
Cr 55.8 (5.5) Colh/p6mm 128.9 (2.2) I; I 125.2 (2.17) Colh/p6mm 39.5
(0.7) Cr
Cr 51.9 (2.4) Colh/p6mm 133.6 (2.3) I; I 128.8 (2.17) Colh/p6mm b
PBSAN-10 Cr 49.1 (2.4) Colh/p6mm 133.3 (2.2) I; I 130.18 (2.18) Colh/p6mm 61.8
It is known that all interplanar spacing distances, and thus, the re-
flection peaks seen in the diffractograms will be in archetypal ratio
values with reference to the first indexed peak. Thus, the spacings of
these reflections were found to be in the ratio of 1:1/√3:1/√4:1/√7,
which could be indexed to (100), (110), (200), and (210) reflections
of a quasi 2D hexagonal lattice. Thus, the investigated compounds
show a columnar phase featuring hexagonal 2D lattice. It may be
recalled here that the Colh phase formed by phasmidic LCs has the
same symmetry as that of the discotic LCs. As is generally known, the
indexing of a Colh phase, belonging to the D6h point group symmetry,
is comparatively straightforward because of the high symmetry of the
p6mm plane group, which is equivalent to the p6/mmm space group. Ac-
cordingly, the Colh phase exhibited by the present PBSAN series of com-
pounds can be assigned with p6mm symmetry. The lattice parameter
(a) determined for the Colh phase at the chosen temperatures of the
mesogens has been collected in Table 2. As envisaged, the value of ‘a’ in-
creases with the elongation of long terminal tails, meaning yet again
(0.3) Cr
PBSAN-11 Cr 112.3 (4.4) Colh/p6mm 119.5 (2) I; I 115.1 (1.9) Colh/p6mm 62.3
(4.5) Cr
PBSAN-12 Cr 51.6 (8.9) Colh/p6mm 130.2 (1.8) I; I 125.4 (1.6) Colh/p6mm 53.1
(1.0) Cr
PBSAN-14 Cr 82.9 (9.7) Colh/p6mm 127.8 (2.4) I; I 125.6 (2.2) Colh /p6mm 23.5
(19.2) Cr
PBSAN-16 Cr 51.6 (62.0) Colh/p6mm 120.2 (1.0) I; I 117.5 (0.9) Colh/p6mm 24.4
(3.6) Cr
a
Transition temperatures determined by both polarizing optical microscope (POM) and
peak values of the DSC thermograms during the first heating/cooling cycles at 5 °C /min
rate. b The mesophase freezes at about 65 °C which remains unchanged upto - 60 °C (limi-
tation of the instrument).
PBSAN-11, PBSAN-12 and PBSAN-14 display an identical Col phase re-
gardless of variations in their terminal chain length or the measurement
temperatures. The high-angle area (for 2θ > ca 12-15o) of all the
Fig. 2. POM images of the thin-films of Colh phase realized by cooling the representative samples from their isotropic phase contained between two untreated glass substrates: (a) The
pseudo focal-conic fan-like (planar) texture of PBSAN-10 (at 90 °C), (b) the pseudo isotropic (homeotropic) pattern accompanied by linear birefringent defects seen for PBSAN-11 at
110 °C and (c) the spherulitic /fan-shaped birefringent pattern along with the homeotropic texture shown by PBSAN-14 at 105 °C.
Fig. 3. DSC traces recorded for the two successive heating (h1,h2)-cooling (c1, c2) cycles at a rate of 5 °C for (a) PBSAN-9 and (b) PBSAN-14. Note that the peak positions of all the traces
match implying that the PBSAN LCs are heat and moisture resistant. Notice that compound PBSAN-14 crystallizes during the second heating cycle.
5

M.B. Kanakala and C.V. Yelamaggad
Journal of Molecular Liquids 332 (2021) 115879
Fig. 4. 1D intensity vs 2θ profiles obtained for the Colh /p6mm phase of compounds PBSAN-11at 90 °C & 110 °C (a), and PBSAN-14 at 80 °C & 105 °C (b). Front view of a slice (disk-like
columnar core) resulting from the spontaneous self-assembly of a pair of phasmidic mesogens in a side-by-side fashion via π-π interaction (c). Schematic diagram of the Colh/p6mm
phase where the directionally correlated tilting of the slices within the columns is non-existent (d).
that the fundamental Col structure of all the compounds studied here is
indistinguishable.
in the wide-angle area of XRD profile, was calculated from the experi-
mental value of the unit cell parameter (a) and assuming a density of
ρ = 1 g cm−3. According to the standard expression Z = (√3 a2 hNA
ρ)/2Mw, where MW is the molecular weight and NA is Avogadro's num-
ber, about 2.1 to 2.7 phasmidic mesogens constitute each slice of the col-
umns. That is, on an average, 2 to 3 molecules arranged in a side-by-side
manner via π-π interaction yield a disk-like columnar core (slice) where
the self-assembled five-ring aromatic cores act as central rigid structure
(hard region) and the peripheral tails not only fill the space but also
serve as the fluid (soft) region of the disk (Fig. 4c). Thus, the attractive
π-π interactions coupled with nanophase segregation among the in-
compatible rigid and flexible domains of the molecules drive the forma-
tion of hexagonal columnar structure (Fig. 4d). It is important to discuss
the correlation between the columnar structure and the optical textures
observed under the POM. As mentioned above, the pairs of molecules
(slices) tilt within the columns, and the average tilt angle is expected
to be nearly the same in different columns. This directionally correlated
tilting obviously reduces the symmetry of the macroscopic structure,
meaning that the mesophase should be optically biaxial and thus, the
occurrence of homeotropic texture is ruled out. However, on the con-
trary, the Colh phase formed by the PBSANs show homeotropic patterns
predominantly, implying that the directionally correlated tilting among
the columns does not exist. That is, the tilt direction of the slices within
the columns is not correlated (Fig. 4c), and thus, the homeotropic tex-
ture is seen.
However, the Colh phase of hexacatenars differs from that of the con-
ventional discotic LCs in comparison to the molecular arrangements
within the columns. About two to several phasmidic molecules, driven
by the secondary forces (viz., H-bonding, π-π interaction, van der
Waals correlation, etc.), self-assemble in a side-by-side manner to
yield a slice (molecular disk/core or disk-like stratum), and the slices
thus generated stack on top of one another to yield indefinitely long col-
umns constituting a hexagonal 2D lattice. Precisely, the well-extended
columns are shaped or filled with slices comprising a minimum of two
phasmidic mesogens (Fig. 4c). However, the slices may have either up-
right or tilted orientation with respect to the columnar axis in the LC
phase. In order to figure out these aspects, the XRD results of the Colh
phase along along with the molecular mass (M) values of the materials
were used. As can be seen in Table 2, the intercolumnar distance, the lat-
tice parameter (a) derived, is notably smaller than the diameter (molec-
ular dimension) of the fully stretched all-trans conformation of the
hexacatenars. With the literature survey results on this subject in
hand, this feature can be obviously attributed to the tilting of slices
with respect to the cross section of each column. The tilt angle (θ) of
the 2D slice with respect to the normal to the columnar axis (i.e. along
the longitudinal slice direction) was estimated using an expression
θ = 90-cos−1(a/L) where a and L respectively being the lattice parame-
ter and length of the mesogens in their most extended geometrical form
with an all-trans conformation of the alkoxy chains. Accordingly, the
measured tilt angles of the stacked molecules in the Col phase are: 37o
for PBSAN-11 and 42o for both PBSAN-12 and PBSAN-14.
2.3. Photophysical studies
The average number of molecules (Z) constituting a slice of the col-
umns with a height of h = ~ 4.5 Å, which is the broad diffuse maximum
ESIPT is an exceptional four-level photochemical (E-E*-K*-K) pro-
cess wherein the photoexcited molecules relax their energy via
6

M.B. Kanakala and C.V. Yelamaggad
Journal of Molecular Liquids 332 (2021) 115879
Table 2
The results of (hkl) indexation of XRD profiles recorded at the specified temperature (T) of the columnar hexagonal phase of representative phasmidic LCs viz., PBSAN-11, PBSAN-12 and
PBSAN-14.
Compound (L/Å)
Phase (T/°C)
dobs (Å)
dcal (Å)
Miller indices hkl
a: Lattice parameter (Å)
S: Lattice area (Å2)
V: Molecular volume (Å3)
Z: Number of molecules per column slice
PBSAN-11 (55.1)
Colh
90
33.21
19.17
16.57
12.55
4.47
33.20
19.17
16.60
11.67
100
110
200
210
a = 38.34
S = 1273.01
V = 5690.39
Z = 2.17
Colh
110
33.31
19.25
16.62
12.70
4.46
33.30
19.23
16.65
11.70
100
110
200
210
a = 38.46
S = 1281
V = 5713.26
Z = 2.18
PBSAN-12 (56.6)
Colh
80
37.94
21.87
19.02
14.46
4.34
37.93
21.90
18.96
14.33
100
110
200
210
a = 43.80
S = 1661.41
V = 7210.55
Z = 2.53
Colh
100
37.93
22.21
18.92
14.42
4.39
37.92
21.89
18.96
14.33
100
110
200
210
a = 43.79
S = 1660.6
V = 7290.21
Z = 2.51
PBSAN-14 (57.8)
Colh
80
41.03
24.79
20.56
15.60
4.45
41.02
23.68
20.51
15.52
100
110
200
210
a = 47.37
S = 1943.20
V = 8647.63
Z = 2.70
Colh
105
41.10
24.20
20.61
15.61
4.56
41.09
23.72
20.54
15.53
100
110
200
210
a = 47.45
S = 1949.85
V = 8891.35
Z = 2.73
L = Length of the polycatenar mesogen estimated in its all-trans conformation from Chem 3D Pro 8.0 (Molecular model software from Cambridge Soft). dobs: Spacing observed; dcal: spac-
ing calculated (deduced from the lattice parameter ‘a’ for the Colh phase).
tautomerization from Enol (E)-form to Keto (K)-form essentially involv-
ing intramolecular proton-transfer (PT) process. [73–82] As illustrated
in Fig. 5, PBSAN molecules, being ESIPT fluorophores, stay in thermody-
namically favored enol-imine (OH) tautomeric form at the ground (E,
S0) state due to the presence of H-bridged, the quasi six-membered
ring formed by the intramolecular H-bonding between the H-atom
and N-atom of the hydroxy and imine groups respectively (Fig. 5).
After light absorption, the singlet excited (S1, E*,) state of the enol-
form, due to photoexcitation, gets populated but without the relaxation
of the geometry, which is in accordance with the Franck–Condon prin-
ciple. The excited enolic state (E*), which may experience a radioactive
decay (emission) and revert to the original enolic form (E), necessarily
undergoes ESIPT process much ahead of fluorescence, on a time scale
ranging from femtoseconds (fs) to picoseconds (ps), yielding the ex-
cited ketone species (K*, S1’). This means that ESIPT invariably occurs
due to the keto tautomer. The excited ketone (K*) state decays to attain
ketone ground (K, S0`) state with the emission of light at lower energy
than the E* form; the large Stokes shift in fluorescence occurs due to
the significant reorganization of the geometry. Subsequently, a reverse
PT takes place to produce the fundamental enol-form. Thus, PBSAN
mesogens should display dual emission – the first (normal) one at
lower wavelengths is due to enol-emission and second one at higher
wavelengths with large Stokes shift resulting from the keto-tautomer
formed after PT (Fig. 5).
room temperature. The temperature-dependent photoluminescence
spectra for the Colh/p6mm phase were recorded for two representative
samples; in this mode, the emission spectra were also recorded in the
isotropic liquid state and solid/frozen Col phase of the samples. Given
the fact that all the members of the series vary structurally only with re-
spect to the length of terminal tails, they are expected to display identi-
cal photophysical properties. Thus, all the hexacatenars exhibited
analogous UV–Vis and fluorescence spectra. As shown in Fig. 6a, the
DCM solutions of the mesogens exhibit two distinct absorption maxima
viz., a strong band at ~360 nm and a weak peak at ~270 nm (Table 3)
possibly arising due to the π-π* and n-π* transitions respectively.
Given the fact that the hexacatenars are characterized by the strong in-
tramolecular H-bonding, the aforesaid UV–Vis bands arising due to ag-
gregation of the mesogens can be ruled out. As shown in Table 3, the
molar extinction coefficients (ε) determined for the higher wavelength
bandwere foundtobeintherange of1.8 ×104 to 5.7 × 104 L mol−1 cm−1
(Table 3).
Seemingly, the absorption bands fall in the UV region due to high ab-
sorption coefficients implying that these materials readily absorb pho-
tons. The corresponding emission spectra recorded for the excitation
wavelength of 365 nm showed an intense peak at ~610 nm with a
broad shoulder at ~660 nm; in addition, a weak absorbance band at
~430 nm exists in the spectra of some members of the series (Fig. 6b).
It may be mentioned here that the colour of these solutions, appearing
yellow under daylight (Fig. 6b - left-inset and Fig. S23a), turns intense
orange when exposed to 365 nm light (Fig. 6b- right-inset and
Fig. S23b). The strong band located at around 610 nm can be attributed
to ESIPT emission, whereas the less intense, blue-shifted band occurring
at ~430 nm can be ascribed to the normal emission. It is immediately ap-
parent from the data collected in Table 3 that the Stokes shift is in the
Thus, to illustrate the aforesaid novel four-level photochemical
phenomenon of the PBSANs realized in this study, photophysical
measurements were carried out. UV–Vis electronic absorption and
emission spectra of the solid samples, in the form of drop-casted thin-
films, and dilute solutions of the compounds in dichloromethane
(DCM) (c = 1 × 10−4 mol L−1) using quartz cells were recorded at
7

M.B. Kanakala and C.V. Yelamaggad
Journal of Molecular Liquids 332 (2021) 115879
Fig. 5. Schematic representation of the ESIPT (four-level photochemical) process proposed for the PBSAN materials studied in this work.
range of 250–256 nm; this is important given the fact that the large
Stokes shift prevents the self-absorption of the dyes or the inner filter
effect. Such a large Stokes shift observed here for the ESIPT chromo-
phores can be attributed not only to the geometry relaxation but also
to the drastic change in geometry of the keto-form when compared to
that of the enol-tautomer. It may be worth mentioning here that the
fluorescence peak position and/or intensity appears to depend on the
polarity of the solvent used, which is shown in Fig.S24.
The dependence of fluorescence intensity on the concentration was
probed for a representative material. The emission spectra recorded, for
the excitation wavelength of 365 nm light, at the different con-
centrations of PBSAN-12 in DCM revealed that the fluorescence inten-
sity indeed increases with the increase in the concentration of the
solute (Fig. S25a), as expected. Fig. S25b shows that the intensity of
ESIPT (keto) fluorescence intensity when compared to that of the
normal (enol) one, increases progressively with an increase in the
Fig. 6. UV–Vis absorption (a) and photoluminescence spectra (b) of PBSAN-n series of mesogens at c = 1 × 10−4 mol L−1 in DCM. Insets of (b) are the photographs of the solution of
PBSAN-8 in DCM under daylight (left) and that exposed to UV light at 365 nm (right).
8

M.B. Kanakala and C.V. Yelamaggad
Journal of Molecular Liquids 332 (2021) 115879
Table 3
Photophysical data derived from the UV–Vis and photoluminescence spectra of PBSAN-n series of LCs recorded for their dilute DCM solutions and the solid (thin-film) states.
LCs
Solution
Stoke
shiftc
Extinction coefficient
(L/mol.cm)
Solid (Thin-film) d
Stoke
shiftc
Absorptiona
Emission a,b
Absorptiond
Emission b
[λabs, (nm)]
[λem, (nm)]
[λabs, (nm)]
[λem, (nm)]
PBSAN-5
PBSAN-6
PBSAN-7
PBSAN-8
PBSAN-9
PBSAN-10
PBSAN-11
PBSAN-12
PBSAN-14
PBSAN-16
280,363
265, 360
286, 359
278, 362
280, 364
281, 364
279, 364
286, 361
267, 360
279, 364
614
614
615
614
251
254
256
252
250
250
250
254
255
250
5.08 × 104
2.43 × 104
2.95 × 104
5.40 × 104
5.77 × 104
4.59 × 104
5.46 × 104
3.14 × 104
1.84 × 104
3.83 × 104
275, 354
273, 353
276, 359
278, 356
278, 360
278, 355
273, 353
278, 360
277, 357
278, 361
629
629
628
275
276
269
273
266
271
272
264
268
268
629+
626+
626+
625
614
614*
614*
615*
615*
614*
624+
625+
629
⁎
A less intense, broad band exists at 430 nm (see Fig. 6b).
+A less intense but broad peak occurs at 435 nm (see Fig. 7b).
aAbsorption and fluorescence spectra were obtained for the DCM solution of the samples. b The excitation wavelength was set at 360 nm. c Stoke shift = Difference between the position
of the peak maxima of the absorption band and the maximum of the fluorescence emission. d Drop-casted thin-films were made by placing dilute DCM solution of the samples on a
quartz plate.
concentration of the dye. The band maximum of the emission peak as
well as fluorescence (photoluminescence) quantum yield (ΦFL) of the
keto-tautomer fluorescence band, unlike that of the normal band, de-
pend on several vital parameters, including the surrounding medium
(environment). Thus, relative ΦFL of compound PBSAN-12, as a repre-
sentative case, was measured by the comparative method where Nile
red dissolved in DMSO was used as an internal standard (Fig. S26).
The expression employed for the calculation of ΦFL along with the rele-
vant details, has been given in SI. The quantum efficiency of this
mesogenic dye for the longer emission was determined to be 0.12.
These results are in complete agreement with the fact that emission
from the ESIPT state is characterized by a large Stokes' shift that enables
longer wavelength fluorescence by UV excitation and has the tendency
to possess low ΦFL because of non-radiative deactivation events taking
place at the keto-form excited (K*) state. [76]
The majority of the organic fluorophores fluoresce highly in their di-
lute solution with high fluorescence quantum efficiencies. However, the
emissive behavior in their two condensed states viz., solid-state and liq-
uid phase is generally feeble, and even remains absent, owing to the
aggregation-caused quenching (ACQ) process; this is also applicable to
another condensed state, namely LC state of the mesogenic dyes. In
order to circumvent the ACQ concern, a number of strategies have
been explored to contain the other radiationless deactivation processes
occurring at the excited state; of these, aggregation-induced emission
(AIE) and aggregation-induced enhanced emission (AIEE) appear to
be interesting. [76] Thus, the three condensed states viz., solid, Col LC
phase, and isotropic liquid state of synthesized materials were probed
for their ESIPT fluorescence competence. The solid samples drop-
casted, using their DCM solutions, onto quartz substrate affording the
thin-films were used to collect both UV
−
Visible and
photoluminescence spectra. Fig. 7a and b respectively portray the ab-
sorption and emission spectra of the thin-films of mesogens. The thin-
film pictures of PBSAN-12 as seen in the daylight and after exposure
to UV light (365 nm) are shown as insets in Fig. 7b. UV–Vis spectra of
the films possess two distinct absorption maxima, one being weak in
the range of 273–278 nm and another strong band in the range of
353–361 nm. These longer and shorter wavelength bands of the sam-
ples possibly arise due to n-π* and the π-π* transitions, respectively.
The photoluminescence spectra recorded for an excitation wave-
length of 360 nm, the wavelength of maximum absorbance of the
thin-films, are presented in Fig. 7b. Apparently, some of the thin-films
possess two emission bands at ~435 nm (weak) and ~ 625 nm (strong)
arising due to intramolecular proton transfer process via an excited
state. The occurrence of a strong longer wavelength fluorescence band
in all the films with large Stokes shift (266 to 276 nm) suggests that
ESIPT process takes over the relaxation of the excited enolic (E*) state
to the original enolic form (E). The temperature-dependent
photoluminescence measurements on the fluid / frozen Col LC phase
along with the isotropic liquid state were performed for the two repre-
sentative materials namely, PBSAN-9 and PBSAN-12; their respective
Fig. 7. The electronic absorption and photoluminescent spectra (a and b) of the drop-casted thin-films of the PBSAN-n series of compounds. The thin-film pictures of PBSAN-12 as seen
before, inset of (b)-top, and after the illumination with UV light (365 nm), inset of (b)-bottom.
9

M.B. Kanakala and C.V. Yelamaggad
Journal of Molecular Liquids 332 (2021) 115879
phase sequences and transition temperatures (in parenthesis, oC) noted
during the cooling cycle are: I (128.8) Colh – freezes at about 65, and I
(125.4) Colh (53) Cr. The samples held between quartz plates in minute
quantities were slowly heated to 150 °C (well above their isotropization
temperature) using a programmable hot stage where the top quartz
substrate of each cell was pressed hard to spread the sample evenly.
Fig. 8a and c respectively depict the fluorescence spectra recorded at
varying temperatures for the samples (excited at 360 nm) PBSAN-9
and PBSAN-12 at 10 °C interval while cooling them from 150 °C to 40 °C.
The spectral signatures of both samples in their liquid state (in the
temperature range of 130 to 150 °C) and fluid/frozen Colh phase (50
to 120 °C), were found to be virtually similar, except for peak intensities,
displaying dual-fluorescence emission; indeed, the analogous spectral
pattern was seen for the solid-state (40 and 50 °C) of PBSAN-12 (Ta-
ble S1). The weak emission appearing around 465 nm could be ascribed
to enol-form (E*-E), whereas the emission band at a longer wavelength,
existing around 630 nm, can be convincingly assigned to keto-tautomer
(K*-K).
As mentioned earlier, whereas the pattern of the
temperature-dependent fluorescence spectra of the samples remains
indistinguishable, their ESIPT photoluminescence (K*-K) intensity in-
creases gradually with the decrease in temperature; this also true for
the intensity of normal emission. However, as shown in Fig. 8b and d,
the magnitude of ESIPT fluorescence intensity of the Col phase belong-
ing to both the mesogens, when compared to that of the normal (E*-
E) fluorescence emission, depends remarkably on the temperature
that can be attributed to the breaking of larger aggregates or thermally
activated non-radiative transitions [65,83–87]. Interestingly, the emis-
sion maxima of the ESIPT fluorescence spectra, recorded while progres-
sive cooling of the Col phase of the compounds PBSAN-9 and PBSAN-12,
Fig. 8. Fluorescence spectra recorded as a function of temperature in the Colh phase which includes isotropic liquid and solid states of compounds PBSAN-9 (a) and PBSAN-12 (c); notice
here that the spectral signatures are identical in all the three condensed states, i.e. liquid state, fluid/frozen Colh LC phase and solid state. The temperature-dependence of the magnitude of
ESIPT fluorescence emission intensity and the normal (E*-E) fluorescence intensity of PBSAN-9 (b) and PBSAN-12 (d). The bathocromic shift of ESIPT fluorescence emission maxima
observed while cooling PBSAN-9 (e) and PBSAN-12 (f) from 150 °C to 40 °C implying J-aggregation.
10

M.B. Kanakala and C.V. Yelamaggad
Journal of Molecular Liquids 332 (2021) 115879
seem to shift marginally, by and large, towards red (longer wavelength)
end of the spectrum (Fig. 8e and f).
LUMO for the enol-imine (E)/(OH) and keto-enamine (K)/(NH) states
of a representative material PBSAN-12, were obtained by the Hartree
Fock (HF) and Density-functional theory (DFT)-based calculations. The
HF calculations were performed using RHF single point energy func-
tional and DFT calculations using B3LYP functional in conjunction with
the def2-TZVP basis set in ORCA 4.01 software [95]. The optimized
structures along with the HOMO and LUMO electron distribution and
the energy values/levels obtained from these (DFT and HF) studies
have been depicted in Fig. 9 (also see Figs. S27 and S28), and the rele-
vant data are collected in Table 4. The optimized structures of both
enol and keto states (Fig. S28), the former being thermodynamically
stable than the latter, reveal the coplanar conformation between hy-
droxyl and imine/keto and enamine groups that can be ascribed to
the presence of strong intramolecular H-bonding, which accounts for
the stability of different tautomeric forms of mesogens. That is, the in-
tramolecularly H-bonded cis-enol and cis-keto forms exist in the opti-
mized structures.
Apparently, the band gaps and electron densities are different for
enol and keto forms. According to the data presented in Table 4
(Fig. 9), the HOMO energy level of keto-enamine form is higher than
that of the enol state. In the enol-imine (OH) form, the HOMO orbitals
are evenly localized on the central three benzene-rings whereas the
LUMO orbitals are delocalized mostly on the central benzene-ring
where both hydroxyl and imine groups are located; it is especially
This demonstration of narrow bathocromic shift of the emission
maxima upon decreasing the temperature without photoluminescence
quenching of the Col phase (formed by the self-assembly of mesogens in
the manner mentioned above) imply the formation of so-called Scheibe
or J-aggregates [88–90] where the π-π stacked aromatic cores within
the columns, oriented in a head-to-tail manner, tilt with respect to the
column axis of the column (Fig. 4c); this behavior completely agrees
with the Col LCs reported earlier. [65,91,92] The occurrence of
J-aggregates can be figured out by determining the tilted angle between
the molecular plane and the aggregation (packing) direction. As per
the exciton-coupling theory [93,94], in the J-aggregates, wherein the
dipoles maintain a head-to-tail orientation of the constituent fluorop-
hores, and the tilted angle should be less than 54.7 degrees. The results
of X-ray diffraction analysis revealed that the tilted stacking angle mea-
sured for PBSAN-12 is less than the aforesaid theoretical value viz. 42
degrees. This means that the phasmidic fluorophores spontaneously
self-assemble, through the strong π-π interactions (but not through in-
termolecular H-bonding), into indefinitely long J-aggregates (columns),
which in turn arrange on a 2D hexagonal lattice (Fig. 4d).
To corroborate the aforementioned photophysical results, the struc-
tural and energy parameters viz., electron distribution and band-gaps
(energy levels) of the HOMO (highest occupied molecular orbital) and
Fig. 9. Theoretical parameters of ESIPT active mesogen PBSAN-12 (a representative case) derived from the DFT calculations: The optimized conformation structures, electron density
distributions in the HOMO and LUMO states of enol-imine (OH)/(E) and keto-enamine (NH)/(K) forms, and HOMO-LUMO energy levels (band-gaps).
11

M.B. Kanakala and C.V. Yelamaggad
Journal of Molecular Liquids 332 (2021) 115879
Table 4
Table 5
Electrochemical properties a of representative compounds.
Theoretically calculated HOMO−LUMO energy levels and band gap values derived from
both theory and experimental measurements.
Mesogens
Eb
EHOMO (eV) Eb
ELUMO (eV) ΔEcg.CV (eV) Edg.opt (eV)
oxd
red
Parameters of PBSAN-12
Enol-Imine (OH)
form
Keto-Enamine (NH)
state
PBSAN -9
PBSAN - 11 0.96 −5.76
PBSAN - 16 1.00 −5.80
1.04 −5.84
−0.18 −4.62
−0.20 −4.59
−0.17 −4.63
1.22
1.17
1.17
2.40
2.54
2.53
HF
DFT
HF
DFT
a
HOMO (eV)
LUMO (eV)
ΔE (eV)
ΔE (eV) (Experimental)
−7.8
1.0
6.8
−5.6
−2.3
3.3
−6.9
0.9
6.0
−4.4
−2.4
2.0
Experimental conditions: Room temperature measurements with a scanning rate of
0.5 mVs−1; Ag/AgCl reference electrode, Pt-rod working electrode, Pt-wire counter elec-
trode and TBAPF6 (0.1 M) in HPLC grade acetonitrile as a supporting electrolyte.
b
3.3 (~ 376 nm)
2.0 (~ 621)
Estimated from the onset oxidation and reduction potential (vs Ag/Ag+) plus 4.8.
Electrochemical band gaps were calculated from EHOMO and ELUMO.
Optical band-gaps were determined from the red edge of the longest wavelength
c
d
from the UV–Vis absorption of mesogens.
visible that in the LUMO state the electronic density at the nitrogen
atoms relatively more and thus they act as proton acceptor implying
that, the proton transfer is favored after the absorption of photons. It
is clear from the Fig. 9 that, in the keto-state, the HOMO is concentrated
on three benzene-rings and the LUMO is localized on the keto and en-
amine groups. The HOMO-LUMO energy gaps derived from the DFT cal-
culations for enol and keto states are 3.3 eV (376 nm) and 2.0 eV
(621 nm), respectively; the corresponding experimentally measured
values were found to be 2.8 eV (430 nm) and 2.0 eV (621 nm). Seem-
ingly, the theoretical and experimental HOMO-LUMO band gap values
of the keto-form are in good agreement with each other. Overall, this
theoretical study undertaken here clearly substantiates the experimen-
tally observed photo-excitation induced ESIPT occurring in the
phasmidic LCs synthesized.
bandgap value viz.,3.3 eV derived from theoretical calculations for
their homolog PBSAN-11. Therefore, one can assume that these
mesogens may have bandgap of about 3.0 eV and thus, they can be
regarded as wide-bandgap semiconducting columnar LC materials hav-
ing the electronic characteristics falling between those of conventional
semiconductors and insulators. This hypothesis is also supported by
the fact that the electronic absorption and ESIPT phenomenon of these
novel mesogens respectively take place in the near-ultraviolet and visi-
ble spectral ranges.
As described earlier, in the ESIPT phenomenon, the photoexcited
mesogens relax their energy by converting themselves from enol-state
to keto-form wherein intramolecular PT occurs. Thus, the presence of
hydroxyl (-OH) group enabling intramolecular PT is essential for the
ESIPT process to happen. This means that the ESIPT activity is basically
governed by the electronic environment of both imine (proton
acceptor) and oxygen atom of the hydroxyl group (proton donor) of
the cis-enol tautomer. Needless to say, by varying the electron density
at the nitrogen (acceptor) atom and/or oxygen (donor) atom, the pro-
ton transfer process leading to dual emission property of ESIPT active
molecules can be modulated. One of the well-known methods of
accomplishing this task is to treat such fluorophores with metals
forming cations (metal-ions). From another standpoint, one can say
that with the help of such binding studies, the occurrence of ESIPT pro-
cess in functional materials can be evidenced with great ease. Hence, the
phasmidic LCs, having two binding (chelating) sites, each consisting of a
hydroxyl group and an imine unit, were screened for their binding abil-
2.4. Electrochemical properties
The cyclic voltammetry (CV) technique was employed to study the
reduction and oxidation characteristics of the ESIPT active mesogens
synthesized. As representative cases, PBSAN-9, PBSAN-11, and
PBSAN-16 were examined for their redox behavior by using a conven-
tional three-electrode cell comprising a set of working electrode, coun-
ter electrode, and reference electrode. While the reference electrode
determines the applied potential, the current flows between the coun-
ter and working electrodes. Precisely, Ag/AgCl (reference electrode),
platinum rod (working electrode), and platinum wire (counter
electrode) were employed. TBAPF6 (tetrabutylammonium hexaflu-
orophosphate, 0.1 M) in acetonitrile (HPLC grade) was used as a
supporting electrolyte. The room temperature measurements were
conducted at a scanning rate of 0.5 mVs−1. Ferrocene /ferrocenium
ion (Fc/Fc+) couple was employed as the internal reference, and all
the potentials were calibrated with the same system; here, it assumed
that the energy level of the Fc/Fc + pair is 4.8 eV below the vacuum
level. The samples used for analysis were the same as those for UV–
Vis experiments. As depicted in Fig. S29, all the mesogens functional
groups such as hydroxyl and imine segments in their molecular struc-
ture. The energy levels of the HOMO and LUMO were determined
ities in solutions with different metal ions such as Cr6+, Mn2+, Hg2+
,
Au3+, Zn2+, Cu2+, Ni2+and Fe3+. The binding (chelating) ability of a
representative ligand PBSAN-16 for the aforesaid metal ions was tested
by monitoring changes in light emission properties. The titrated solu-
tions, obtained after the drop-wise addition of metal salt (CrO3, MnCl2,
Hg(OAc)2, HAuCl4, Zn(OAc)2, Cu(OAc)2, NiCl2, FeCl3) solutions (50 μL)
in DMF (c = 0.5 M) to the 10 mL of PBSAN-16 in DCM (c = 10−4 M),
were subjected to their photoluminescence behavior. The photographs
obtained for the probe PBSAN-16 in DCM and those of the titrated solu-
tions under 365 nm light are shown in Fig. 10a and Fig. 10b, respec-
tively. The emission spectra recorded for these titrated solutions,
excited at 365 nm, are shown in Fig. 11.
using the formulas EHOMO = −(4.8 + Eox,onset) eV and ELUMO
=
−(4.8 + Ered,onset) eV. The electrochemical band-gap (ΔEg.CV) between
the frontier orbitals was determined using the expression ΔEg.CV
=
E
HOMO - ELUMO eV. The optical band gap (ΔEg.opt) values were figured
out from the longest wavelength absorption onset of the absorption
spectra of mesogens by using the expression E = 1240/λonset. The elec-
trochemical band gap values of these mesogens determined using their
voltammograms and absorption spectra are in the range of 1.17–1.22 eV
and 2.40–2.54 eV respectively (Table 5). The comparison reveals that
the electrochemical band gaps are lesser than those derived from the
optical method and such discrepancy can be ascribed to several factors
such as the electrode characteristics (size, surface, heterogeneity),
media (the status of electrolyte –oxygen content), pre-adsorbed gases,
molecular adsorption/desorption, double layer charging/discharging,
2D surface diffusion etc. However, it appears that the electrochemical
band gaps of the compounds (ca 2.5 eV) are relatively closer to the
Fig. 10. Photographs captured under a UV lamp (365 nm) for the mesogen PBSAN-16 in
DCM before (a) and after treating with the various metal salts in DMF (b).
12

M.B. Kanakala and C.V. Yelamaggad
Journal of Molecular Liquids 332 (2021) 115879
Fig. 11. (a) Photoluminescence spectra recorded at room temperature for the solutions formed by treating the probe PBSAN-16 in DCM with different metal salts in DMF; (b) fluorescence
spectra of ligand PBSAN-16 in DCM (red-trace) and after treating with Zn2+ ions (green-trace) (Excitation wavelength is 360 nm).
The presence of Cr6+and Fe3+ ions alters drastically the
photoluminescence profile of the ligand PBSAN-16 where the bands
due to dual fluorescence pertaining to the normal and ESIPT emissions
vanish completely (Fig. 11a) that can be ascribed to the collisional
quenching coupled with the formation of coordination complexes. The
complex formation especially disrupts ESIPT activity of the probe mole-
cules. The addition of Cu2+ and Ni2+ also perturbs the emission prop-
erty of the ligands to the extent that the ESIPT band (~ 600 nm)
remains absent that can be attributed yet again to the formation of com-
plexes; however, the existence of three emission bands at ca 410, 430
and 460 nm, suggests that the coordination compounds generated are
photoluminescent. More or less, no changes in the emission spectra
were noticed when Mn2+, Hg2+ and Au3+ ions were treated with the
ligand, implying that these metal ions do not chemically interact with
probe molecules. Interestingly, as shown in Fig. 11b, the interaction of
Zn2+ ions with PBSAN-16 molecules not only augments the fluores-
cence emission intensity but also shifts the ESIPT peak towards a shorter
wavelength (blue shift). In essence, these experiments clearly substan-
tiate the fact that the phasmidic molecules studied here not only are
ESIPT active but also can serve as chemosensors.
HOMO and LUMO for the enol and keto states determined by HF and
DFT-based calculations support the photophysical experimental obser-
vations. The evaluation of metal ions chelating property reveals that a
mesogen (as a representative case) selectively binds to certain metal
ions to form metal complexes; notably, the complex formation of
mesogen with Zn2+ ions not only enhances the emission intensity but
also shifts the peak towards shorter wavelength. The electrochemical
band gap values determined from the CV experiments appear to
match with those derived from the theoretical calculations closely. In
essence, these phasmidic columnar LCs exhibiting ESIPT phenomenon
and other physicochemical properties may serve as functional materials
for various technological applications.
4. Experimental
4.1. IGeneral remarks
4.1.1. General information (materials and methods)
Gallicacid, n-alkylbromides, 4-nitrophenol and 1,2-dimethox-
ybenzene procured from Sigma-Aldrich Company were utilized as re-
ceived. Organic solvents employed for various general purposes were
distilled prior to use. Silica gel thin layer chromatography (TLC) was
used to monitor the progress of the organic reactions and also assess
the purity of the compounds synthesized, especially the intermediates.
In particular, TLC plates consisting of a thin layer of silica gel (Merck,
Kieselgel60, F254) on an aluminium foil backing were employed. The
column chromatography was performed using glass columns packed,
by the wet method, with either neutral alumina or silica gel (60–120/
100–200 mesh) as the solid support. UV–Vis spectra of the samples
were recorded using Perkin–Elmer's Lambda 750, 2015 NIR spectropho-
tometer. FTIR spectra of the samples were obtained from Perkin-Elmer
Spectrum 1000 FT-IR spectrometer, and the typical unit used in the
spectra is wave numbers (which is cm−1). Fluorescence emission
(photoluminescence) spectra of all the ESIPT active molecules synthe-
sized were collected, in their solid, solution amd LC phases, using a spec-
trofluorometer Flurolog-3, Horiba Jobin Yvon. Nuclear magnetic
resonance (NMR) spectra were recorded with the help of a Bruker
AMX-400 spectrometer operating at 400 MHz for 1H and 100 MHz for
13C. The 1H NMR chemical shifts (δ) are presented in parts per million
(ppm) relative to tetramethylsilane (TMS) peak (δ 0.0). The spectra
were obtained at room temperature from solutions of the samples dis-
solved in deuterated chloroform (CDCl3) with residual solvent serving
as internal standard (7.26 ppm for 1H and 77.16 ppm for 13C). The
data (the nature of the proton NMR peaks) are reported as s (singlet),
d (doublet), t (triplet), and m (multiplet). The coupling constants
(J values), calculated directly from the spectra, are given in Hertz (Hz).
3. Conclusion
In this article, we have presented the synthesis, structural character-
ization, mesomorphism, photophysical behavior, redox activity, metal
ion sensing ability and solvatochromism of ten new, ESIPT active,
phasmidic bis(N-salicylideneaniline) LCs belonging to a homologous se-
ries. They have been readily synthesized with notable yields by the acid-
catalyzed condensation of two moles of lipophilic (half-disk shaped)
amines with one mole of 2,3-dihydroxyterephthalaldehyde. These o-
hydroxy Schiff bases, being resistant to heat and moisture, which can
be attributed due to the presence of hydrogen (H)-bridged quasi six-
membered rings, display columnar LC phase. The optical textural pat-
terns combined with XRD data analysis confirm the presence of Colh
having p6mm symmetry. XRD results imply that the slices (disk-like co-
lumnar core), formed by the self-assembly of two molecules in a side-
to-side fashion, tilt with respect to the columnar axis and tilt direction
among the columns is not correlated; the profiles also show that the
core-core (slice-to-slice) interactions are too weak. Photophysical mea-
surements clearly revealed the intrinsic ESIPT phenomenon not only in
their dilute solutions but also in their solid, liquid crystal (Col) phase
and isotropic liquid state. In the Colh phase, the magnitude of ESIPT fluo-
rescence emission intensity display temperature-dependence; the
emission maxima shifts to longer wavelengths upon decreasing the
temperature confirming the J-type aggregation in the mesophase. Elec-
tron distribution in the optimized structures and band-gaps of the
13

M.B. Kanakala and C.V. Yelamaggad
Journal of Molecular Liquids 332 (2021) 115879
Integration has been determined as the relative number of atoms. An
elemental analyzer, namely, Elementar Vario MICRO Select, was
employed to determine the elemental compositions of the materials.
DFT calculations were performed with optimized structures using
B3LYP functional in conjunction with the def2-TZVP basis set in ORCA
4.01 software. The calculations were mainly used to evaluate the elec-
tronic structural properties of phasmidic mesogens in their cis-enol
and cis-keto forms. The HF calculations were performed using RHF sin-
gle point energy functional.
The mesogenic materials synthesized were examined for their ther-
mal / mesomorphic properties using an Olympus BX50 (Model BX50F4)
polarizing optical microscope (POM) equipped a digital camera and a
hot stage, Mettler FP82HT having FP90 central processor. Differential
scanning calorimeter (DSC) traces were registered at a scanning rate
of 5 °C/min using Perkin-Elmer DSC 8000 equipment. The calibration
of the instrument prior to use was performed using pure indium as a
standard. Powder X-ray diffraction measurements were performed
with the help of either Panalytical (Empyrean) or Xenocs, Genix Cu
MAR345 using Cu-Kα (1.54 Å) radiation. The samples held in
Lindemann capillaries (0.5 mm diameter) and carefully flame sealed
at both ends were employed.
CDCl3): 13.45 (s, 2H, OH), 8.68 (s, 2H, CHN), 7.42 (d, 8H, J = 8 Hz, Ar),
7.30 (d, 4H, J = 4 Hz, Ar), 7.03 (s, 2H, Ar), 4.07 (m, 12H, 6 × OCH2),
1.86 (m, 90H, 36 × CH2, 6 × CH3) 13C NMR (100 MHz): 165.13,
162.23, 153.10, 150.59, 150.29, 145.79, 143.21, 123.72, 122.98, 122.38,
121.45, 120.71, 108.66, 77.36, 77.11, 76.85, 73.70, 69.37, 31.82, 31.64,
30.38, 29.34, 25.84, 25.79, 22.77, 22.71, 14.12. Anal. calcd for
C
82H120N2O12: C, 74.28; H, 9.12; N, 2.11; Found: C, 74.48; H, 9.54;
N, 2.31.
PBSAN-9: A brown colour solid; yield: 0.125 g (72%); IR (KBr pellet):
max in cm−1 3442, 2921, 1734, 1618, 1585, 1186, 954, 857. UV - Vis:
λmax = 364 nm, ε = 5.77 × 104 L mol−1 cm−1 1H NMR (400 MHz,
ν
;
CDCl3): 13.47 (s, 2H, OH), 8.68 (s, 2H, CHN), 7.42 (d, 8H, J = 8 Hz, Ar),
7.30 (d, 4H, J = 8 Hz, Ar), 7.03 (s, 2H, Ar), 4.09 (m, 12H, 6 × OCH2),
1.87 (m, 102H, 42 × CH2, 6 × CH3) 13C NMR (100 MHz): 165.13,
162.23, 153.09, 150.59, 145.78, 143.21, 123.71, 122.98, 122.38, 121.45,
120.70, 108.66, 77.37, 77.11, 76.86, 73.70, 69.37, 31.99, 29.69, 29.50,
29.39, 26.18, 22.78, 14.21. Anal. calcd for C88H132N2O12: C, 74.96;
H, 9.44; N, 1.99; Found: C, 75.27; H, 9.73; N, 2.27.
PBSAN-10: A brown colour solid; yield: 0.128 g (73%); IR (KBr pel-
let): νmax in cm−1 3432, 2924, 1730, 1618, 1586, 1186, 959, 859. UV -
Vis: λmax = 368 nm, ε = 4.59 × 104 L mol−1 cm−1 1H NMR
;
(400 MHz, CDCl3): 13.46 (s, 2H, OH), 8.68 (s, 2H, CHN), 7.42 (d, 8H,
J = 8 Hz, Ar), 7.29 (d, 4H, J = 4 Hz, Ar), 7.03 (s, 2H, Ar), 4.07 (m, 12H,
6 × OCH2), 1.85 (m, 114H, 48 × CH2, 6 × CH3) 13C NMR (100 MHz):
165.13, 162.23, 153.10, 150.58, 150.31, 145.80, 143.28, 123.72, 122.98,
122.38, 121.44, 120.70, 108.63, 77.11, 76.86, 73.70, 69.37, 31.99, 29.69,
29.38, 26.17, 22.77, 14.20. Anal. calcd for C94H144N2O12: C, 75.56;
H, 9.71; N, 1.87; Found: C, 75.93; H, 10.02; N, 2.03.
4.2. General synthetic procedure for (((2,3-dihydroxy-1,4-phenylene) bis
(methanylylidene)) bis(azanylylidene)) bis(4,1-phenylene) bis(3,4,5-tris
(hexyloxy)benzoate)s (PBSAN-n series)
A mixture of 2,3-dihydroxyterephthalaldehyde (7) (0.4 mmol,1 eq.)
and freshly purified 4-aminophenyl 3,4,5-tris(alkyloxy)benzoate
(0.8 mmol, 2 eq.), a catalytic amount of acetic acid in ethanol (10 mL)
was refluxed under an inert atmosphere for 2 h. The pale brown solid
separated upon cooling the reaction mixture was collected by filtration,
washed with ethanol, and dried. The crude product was purified by re-
peated recrystallizations in absolute ethanol (Yield: 69–75%).
PBSAN-11: A brown colour solid; yield: 0.125 g (69%); IR (KBr pel-
let): νmax in cm−1 3433, 2929, 1729, 1615, 1585, 1187, 961, 859. UV -
Vis: λmax = 364 nm, ε = 5.46 × 104 L mol−1 cm−1 1H NMR
;
(400 MHz, CDCl3): 13.47 (s, 2H, OH), 8.68 (s, 2H, CHN), 7.42 (t, 8H,
J = 4 Hz, Ar), 7.29 (d, 4H, J = 4 Hz, Ar), 7.03 (s, 2H, Ar), 4.09 (m, 12H,
6 × OCH2), 1.86 (m, 126H, 54 × CH2, 6 × CH3) 13C NMR (100 MHz):
165.13, 162.23, 153.09, 150.589150.29, 145.79, 143.21, 123.72, 122.98,
122.38, 121.45, 120.71, 108.66, 77.36, 77.11, 76.85, 73.70, 69.37, 31.82,
31.64, 30.38, 29.34, 25.84, 25.79, 22.77, 22.71, 14.18, 14.12. Anal. calcd
for C100H156N2O12: C, 76.10; H, 9.96; N, 1.77; Found: C, 76.45;
H, 10.34; N, 1.94.
PBSAN-5: A brown colour solid; yield: 0.125 g (70%); IR (KBr pellet):
ν
max in cm−1 3434, 2955, 1729, 1615, 1585, 1188, 955, 863. UV - Vis:
λmax = 363 nm, ε = 5.08 × 104 L mol−1 cm−1 1H NMR (400 MHz,
;
CDCl3): 13.47 (s, 2H, OH), 8.68 (s, 2H, CHN), 7.42 (d, 8H, J = 8 Hz, Ar),
7.29 (d, 4H, J = 4 Hz, Ar), 7.03 (s, 2H, Ar), 4.09 (m, 12H, 6 × OCH2),
1.87 (m, 54H, 18 × CH2, 6 × CH3) 13C NMR (100 MHz): 165.13,
162.23, 153.10, 150.58, 150.29, 145.78, 143.20, 123.73, 122.98, 122.39,
121.45, 120.71, 108.66, 77.37, 77.12, 76.86, 73.66, 69.36, 30.09, 29.06,
PBSAN-12: A brown colour solid; yield: 0.110 g (70%); IR (KBr pel-
let): νmax in cm−1 3433, 2920, 1735, 1619, 1586, 1237, 1186, 948, 855.
28.33, 28.27, 22.63, 22.52, 14.18, 14.13. Anal. calcd for C64H84N2O12
C, 71.62; H, 7.89; N, 2.61; Found: C, 71.84; H, 8.03; N, 2.89.
:
UV - Vis: λmax = 359 nm, ε = 3.41 × 104 L mol−1 cm−1 1H NMR
;
(400 MHz, CDCl3): 13.47 (s, 2H, OH), 8.68 (s, 2H, CHN), 7.42 (t, 8H,
J = 4 Hz, Ar), 7.29 (d, 4H, J = 4 Hz, Ar), 7.03 (s, 2H, Ar), 4.08 (m, 12H,
6 × OCH2), 1.87 (m, 138H, 60 × CH2, 6 × CH3) 13C NMR (100 MHz):
165.13, 162.23, 153.09, 150.59, 150.29, 145.79, 143.21, 123.71, 122.98,
122.38, 121.45, 120.71, 108.66, 77.36, 77.11, 76.86, 73.70, 69.37, 32.02,
30.44, 29.80, 29.73, 29.49, 26.18, 22.79, 14.22. Anal. calcd for
PBSAN-6: A brown colour solid; yield: 0.100 g (72%); IR (KBr pellet):
ν
max in cm−1 3435, 2930, 1729, 1619, 1587, 1183, 947, 866. UV - Vis:
λmax = 350 nm, ε = 2.43 × 104 L mol−1 cm−1 1H NMR (400 MHz,
;
CDCl3): 13.47 (s, 2H, OH), 8.68 (s, 2H, CHN), 7.42 (d, 8H, J = 8 Hz, Ar),
7.29 (d, 4H, J = 4 Hz, Ar), 7.03 (s, 2H, Ar), 4.08 (m, 12H, 6 × OCH2),
1.86 (m, 66H, 24 × CH2, 6 × CH3) 13C NMR (100 MHz): 165.13,
162.23, 153.08, 150.57, 150.28, 145.78, 143.18, 123.71, 122.98, 122.38,
121.44, 120.70, 108.63, 77.34, 77.09, 76.84, 73.69, 69.36, 31.95, 30.43,
29.59, 29.38, 26.17, 22.77, 14.20. Anal. calcd forC70H96N2O12: C, 72.63;
H, 8.36; N, 2.42; Found: C, 72.91; H, 8.66; N, 2.51.
C
106H168N2O12: C, 76.58; H, 10.19; N, 1.69; Found: C, 76.90; H, 10.54;
N, 1.83.
PBSAN-14: A brown colour solid; yield: 0.128 g (75%); IR (KBr pel-
let): νmax in cm−1 3432, 2919, 1733, 1616, 1585, 1184, 949, 857. UV -
Vis: λmax = 360 nm, ε = 1.84 × 104 L mol−1 cm−1 1H NMR
;
PBSAN-7: A brown colour solid; yield: 0.105 g (74%); IR (KBr pellet):
(400 MHz, CDCl3): 13.47 (s, 2H, OH), 8.68 (s, 2H, CHN), 7.42 (t, 8H,
J = 4 Hz, Ar), 7.29 (d, 4H, J = 4 Hz, Ar), 7.03 (s, 2H, Ar), 4.08 (m, 12H,
6 × OCH2), 1.85 (m, 162H, 72 × CH2, 6 × CH3) 13C NMR (100 MHz):
165.13, 162.23, 153.09, 150.59, 150.29, 145.79, 143.21, 123.70, 122.99,
122.38, 121.45, 120.71, 108.66, 77.36, 77.10, 76.85, 73.71, 69.37, 32.03,
30.44, 29.85, 29.81, 29.74, 29.49, 26.18, 22.79, 14.22. Anal. calcd for
ν
max in cm−1 3432, 2926, 1731, 1617, 1586, 1186, 936, 857. UV - Vis:
λmax = 356 nm, ε = 2.95 × 104 L mol−1 cm−1 1H NMR (400 MHz,
;
CDCl3): 13.47 (s, 2H, OH), 8.68 (s, 2H, CHN), 7.42 (d, 8H, J = 8 Hz, Ar),
7.29 (d, 4H, J = 4 Hz, Ar), 7.03 (s, 2H, Ar), 4.08 (m, 12H, 6 × OCH2),
1.86 (m, 66H, 30 × CH2, 6 × CH3) 13C NMR (100 MHz): 165.10,
162.21, 153.08, 150.59, 150.28, 145.79, 143.19, 122.97, 122.36, 121.40,
120.66, 108.65, 77.36, 77.10, 76.82, 73.66, 69.34, 32.01, 31.87, 30.42,
C
118H192N2O12: C, 77.41; H, 10.57; N, 1.53; Found: C, 77.94; H, 10.77;
N, 1.93.
PBSAN-16: A brown colour solid; yield: 0.105 g (71%); IR (KBr pel-
let): νmax in cm−1 3435, 2919, 1731, 1618, 1587, 1187, 947, 866. UV -
Vis: λmax = 364 nm, ε = 3.83 × 104 L mol−1 cm−1 1H NMR
29.38, 29.12, 26.09, 22.69, 14.16. Anal. calcd for C76H108N2O12
C, 73.51; H, 8.77; N, 2.26; Found: C, 73.88; H, 9.09; N, 2.47.
:
PBSAN-8: A brown colour solid; yield: 0.120 g (71%); IR (KBr pellet):
;
ν
max in cm−1 3432, 2925, 1731, 1617, 1586, 1186, 950, 859. UV - Vis:
(400 MHz, CDCl3): 13.47 (s, 2H, OH), 8.68 (s, 2H, CHN), 7.42 (t, 8H,
J = 4 Hz, Ar), 7.29 (d, 4H, J = 4 Hz, Ar), 7.03 (s, 2H, Ar), 4.08 (m, 12H,
λmax = 362 nm, ε = 5.40 × 104 L mol−1 cm−1
;
1H NMR (400 MHz,
14

M.B. Kanakala and C.V. Yelamaggad
Journal of Molecular Liquids 332 (2021) 115879
6 × OCH2), 1.85 (m, 162H, 84 × CH2, 6 × CH3) 13C NMR (100 MHz):
165.11, 162.21, 153.10, 150.61, 150.31, 145.80, 143.28, 123.72, 122.97,
122.36, 121.42, 120.71, 108.72, 77.35, 77.09, 76.84, 73.70, 69.40, 32.02,
30.45, 29.84, 29.80 29.45, 26.18, 22.78, 14.20. Anal. calcd for
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