Columnar self-assembly of novel benzylidenehydrazones and their difluoroboron complexes: Structure-property correlations

Article abstract of DOI:10.1039/c9nj01192g

Organoboron complexes are considered to be an important class of optically dynamic materials and because of their imperative properties, they have been unceasingly studied in a wide range of scientific areas. In this context, a new family of D-A-D′ archit

Full text of DOI:10.1039/c9nj01192g

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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DOI: 10.1039/C9NJ01192G
Columnar self-assembly of novel benzylidenehydrazones and
their difluoroboron complexes: structure-property correlations
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D. R. Vinayakumara,a K. Swamynathan,b Sandeep Kumar,b Airody Vasudeva Adhikari *a
a Organic Materials Laboratory, Department of Chemistry, National Institute of Technology Karnataka,
Surathkal, Mangalore-575 025, India. E-mail: avachem@gmail.com; avachem@nitk.ac.in
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b
SCM group, Raman Research Institute, C.V. Raman Avenue, Sadashivanagar, Bangalore-560 080,
India.
Abstract
Organoboron complexes are considered to be an important class of optically dynamic
materials and because of their imperative properties, they have been unceasingly studied in a
wide range of scientific areas. In this context, a new family of D-A-D΄ architectured boron
difluoride complexes FB1-4 derived from arylhydrazones HZ1-4 has been synthesized and
characterized. The work also deliberates detailed studies on their thermal, self-assembling and
optoelectronic properties with respect to structural modifications. The systematic studies reveal
that, the new arylhydrazone ligands and their BF2 complexes exhibit prospective columnar
mesomorphism with different 2D lattice geometries at room temperature, owing to their critical
structural features. Interestingly, an unusual morphological cross-over from columnar
hexagonal to rectangular mesophase has been confirmed in one of the investigated pseudo-
discoidal boron complexes, FB4. Their optoelectronic studies showcased their ICT dominating
dye properties. Finally, the experimental results have been consciously correlated with
quantum chemical simulations.
Introduction
Since the discovery, columnar self-assembly of disc-like molecules have attracted great
scientific attention because of their potential utility in the wide range of electronic devices.1–6
Typically, disc-like (discotic) or properly tailored non-discotic aromatic molecules are known
to self-segregate into one-dimensional columnar stacking by the dint of various noncovalent
interactions in the average intra- and inter-columnar spacings of 0.3-0.4 and 2-4 nm,
respectively.7 Due to this fact, discotic liquid crystals (DLCs) are considered as soft one-
dimensional conducting molecular wires and they are identified to be better alternatives for the
highly expensive organic single crystals or insulating amorphous conjugated polymers. In
addition to their intrinsic charge transporting ability, they own inherent anisotropic
photoconductivity, controlled molecular order, easy processability, good self-healing, excellent
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optical absorption, intense fluorescence and great response to various external stiDmOuI: l1i0..10T3h9/eCs9eNJ01192G
advantageous features render DLCs as promising candidates in several commercial
applications.8,9
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In recent years, enormous research is progressing in the design and development of
functional DLCs to meet the basic demands for the individual device fabrication. As a result, a
large variety of discotics have emerged in the last two decades. Particularly, discotic mesogens
with preserved solid-state luminescence have been an emerging class of advanced optical
materials for their promising candidature in organic light emitting diode (OLED), which is
known to be a next generation eco-friendly lighting technology. Till date, many fluorescent
columnar liquid crystalline (CLCs) materials have been investigated as potential emissive
components in OLEDs.10–17 One of the strategies to achieve luminescent CLC materials is the
introduction of donor-accepter configuration to the core system. Evidently, organoboron
complexes (OBCs) are imperious building blocks to achieve the highly fluorescent LC
compounds. These OBCs provide easy access for a variety of structural modulations in order
to tune for the desirable electro-optical properties.18 Generally, OBCs can be divided into tri-
coordinate and tetra-coordinate complexes, the latter complexes are known to be highly stable
than the former one.19 To construct the four-coordinated complexes, difluoroboron (BF2) has
been extensively used to lock configurationally in the three usual modes, viz. [O, O′],20–22 [N,
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23–26
27–30
O]
and [N, N′].
and these complexes were often derived from different chelating
ligands such as α, β-diketonates,31–34 β-diketiminates,35,36 formazanates,37–40 and quinolone-β-
ketones.41,42 Consequently, such complexes often possess remarkable thermal and
photochemical stability; their low lying LUMO intensify the fluorescence; they have good
electron transport ability. Hence, these metalloid complexes have been successfully employed
in various electro-optical applications such as OLEDs, OFET, solar cells, photosensitizers,
sensors, optical imaging and other. 18,19,43–45
A number of BF2 complexes with [N,N] and [O,O] chelating ligands have been reported
as mesogenic fluorophores.46–50 However, only two BF2-[N,O] complexes were known, one
with heterocyclic benzoxazoles 51 and the other with salicylideneamines ligands.52 Yet, liquid
crystalline BF2 complexes from hydrazones have not been known. In continuing our research
on luminescent columnar liquid crystals,17,53,54 we explore a new series of columnar mesogenic
boron difluoride complexes derived from benzylidenehydrazones for the first time. The new
molecules comprise non-symmetrical conjugative attachment of alkoxyaryl groups to 2,2-
difluoro-1,3,4,2-oxadiazaborol-3-ium-2-uide ring. The number and length of peripheral chains
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of one of the arms were kept intact and both the functions were varied on the otheDrOaIr: 1m0..10T39h/Ce9iNrJ01192G
self-assembling, photophysical and electrochemical properties were studied as a function of
the structural modifications. Interestingly, the key-precursors, i.e. non-symmetrical
polycatenar arylhydrazones were also found to be mesomorphic. Finally, the report deals with
the structure-property correlations of arylhydrazones and their BF2 complexes with respect to
their mesogenic and optoelectronic properties.
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Results and discussion
Synthesis and characterization
The synthetic route for the preparation of polycatenar arylhydrazones (HZ1-4) and the
corresponding difluoroboron complexes (FB1-4) is outlined in Scheme 1. The synthesis of key
precursors, i.e. substituted arylhydrazones is often straightforward which involves simple acid
catalyzed condensation of hydrazine derivative H12 with different alkoxy substituted
benzaldehydes (A1-4) by refluxing in ethanol for two hours. The hydrazine Schiff bases were
purified by repeated recrystallization in methanol. The required aldehydes (A1-4) and
phenylhydrazine (H12) were synthesized by adopting reported procedures with slight
modifications and their synthetic pathway is summarized in Scheme S1.55,56
R4
O
R4
R3
CHO
R1
H2N
OC12H25
OC12H25
R3
OC12H25
OC12H25
OC12H25
N
a
H
+
N NH
O
R2
R1
R2
OC12H25
A1-A4
H12
HZ1-HZ4
A1:
R1 = R2 = H, R3 = n-OC12H25,R4 = H
R1 = R2 = H, R3 = R4 = n-OC12H25
R1 = R2 = R3 = n-OC12H25,R4 = H
R1 = H, R2 = R3 = R4 n-OC12H25
HZ1:
HZ2:
HZ3:
HZ4:
R1 = R2 = H, R3 = n-OC12H25,R4 = H
R1 = R2 = H, R3 = R4 = n-OC12H25
R1 = R2 = R3 = n-OC12H25,R4 = H
R1 = H, R2 = R3 = R4 = n-OC12H25
A2:
A3:
A4:
b
C12H25
O
O
OC12H25
OC12H25
R4
R3
FB1:
FB2:
FB3:
FB4:
R1 = R2 = H, R3 = n-OC12H25,R4 = H
R1 = R2 = H, R3 = R4 = n-OC12H25
R1 = R2 = R3 = n-OC12H25,R4 = H
R1 = H, R2 = R3 = R4 = n-OC12H25
R2
N
N
R1
B
F
F
FB1-FB4
Scheme-1. Synthetic routes of new organoboron mesogens; reagents and conditions: (a) cat.
AcOH, ethanol, 70 ℃, 2 hrs, 69-75 %; (b) DIPEA, BF3OEt2, DCE, overnight, 42-60%
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The obtained new hydrazone derivatives, HZ1-4 exist in keto-enol forms as shown in
Scheme 2; the coordination with the boron source prefers the enolic tautomer form.57 On
boronation with boron trifluoride etherate in presence of N,N-Diisopropylethylamine,
hydrazones yielded the corresponding difluoroboron complexes in 42-60%. The chemical
structures of all key-precursors, and organoboron complexes were established using FTIR, 1H-
NMR 13C-NMR spectral and elemental analyses; the associated structural data are furnished in
supporting information.
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R4
R3
OC12H25
OC12H25
OC12H25
N NH
R2
R1
O
Keto
R4
R2
OC12H25
OC12H25
OC12H25
R3
N N
R1
HO
Enol
Scheme-2. Tautomer forms of new hydrazone derivatives
Thermal and self-assembling properties
The thermotropic phase behavior and the thermodynamic features of both hydrazones
and boron complexes were studied by using Differential Scanning Calorimetry (DSC) and
Polarized Optical Microscope (POM) techniques. The thermotropic phase transitions and their
corresponding enthalpy changes are summarized in Table 1. Further, nature and symmetry of
self-assembly were evaluated by the temperature variable wide-angle X-ray diffraction
experiments.
Mesomorphism of benzylidenehydrazones (HZ1-4)
The DSC thermogram of first member of the series HZ1 bearing four dodecyloxy
peripheral chains displayed single reversible transition owing to crystal to isotropic transition
without any mesomorphic behavior as depicted in Figure S31. In contrast, hydrazone HZ2 with
an increased number of alkyl chains exhibited an enantiotropic mesophase transition as
evidenced by its DSC scan (Figure 1c). On heating cycle, it showed two distinct crystalline
morphology and transformed into a mesophase (104.39 ℃) which clears at 145.5 ℃ with the
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energy change of 1.46 kJmol-1. The observed phases were certainly reappeared inDtOhI:e10c.1o0o39l/iCn9gNJ01192G
scan as well. These thermotropic transitions were further confirmed by POM; under cross
polarizers, HZ2 displayed a pseudo-focal conic fan-shaped texture with dark homeotropic
regions and it crystallized below 92 ℃ as visualized in Figures 1a-b indicating the existence
of optically uniaxial columnar phase.58
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Table 1. a Phase transition temperatures (℃) and the corresponding energy changes (kJmol-1)
Compd.
Phase sequence
Cooling
Heating
HZ1
HZ2
Cr 71.03 (82.076) I
I -14.39 (-6.14) Cr
Cr1 94.24 (54.50) Cr2 104.39 (24.55) I 141.45 (-1.29) Colh 92.01 (-24.79) Cr2
Colh 145.50 (1.46) I
82.29 (-59.91) Cr1
HZ3
Cr 8.47 (23.67) Colh 154.70 (2.74) I
150.44 (-2.94) Colh 0.0 (-27.79) Cr
Colh1 93.59 (3.12) Colh2 174.23 I 171.70 (-12.26) Colh2 92.25 (-2.66)
(13.79) I Colh1 0.13 (-37.40) Cr
HZ4
a
Peak temperatures/℃ (enthalpy/kJmol-1) obtained by second heating and first cooling at
the rate of 5 ℃/min; Colh = Columnar hexagonal phase; I = Isotropic liquid phase
To get further insight into the nature of columnar assembly of HZ2, its X-ray diffraction
patterns were obtained at various temperatures within the mesophase thermal region and they
were found to be similar as shown in Figure 1d. For instance, a pattern gained at 135 ℃ showed
a single reflection in its lower-angle region at the d-spacing of 23.0 Å, which could be indexed
to (10) plane of hexagonal symmetry with lattice parameter a = 26.55 Å. Additionally, a noticed
diffused halo in the wide-angle region at the maxima of 4.55 Å and the absence of core-core
reflection manifested the fluidic property of the mesophase. In fact, the occurrence of single
lower angle reflection can’t elucidate the hexagonal lattice unambiguously. Nevertheless, this
kind of patterns are commonly observed in various discotics. Indeed, the typical birefringent
textural features strappingly confirm the Colh mesophase. Further, the hexagonal lattice area,
S and its volume, V were estimated to be 611 Å2 and 2779 Å3, respectively.
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Figure 1. POM texture of HZ2 at (a) 137 ℃ and (b) 85 ℃; (c) DSC thermogram of HZ2 (red
and blue traces represent heating and cooling scan, respectively); (d) XRD pattern of HZ2 at
various temperature points
The hexacatenar hydrazone, HZ3 was easily be sheared between the two glass slides
and it looked birefringent under POM indicating that, the compound is already in LC state at
RT. Seemingly, the optical transitions were well consistent with its DSC results and it also
exhibits the pseudo-focal conic texture on slow cooling below its isotropic temperature 150.44
℃, confirming the columnar hexagonal mesophase (Figures 2a-c). As noticed in case of HZ2,
the XRD patterns of HZ3 also showed a single reflection at lower-angle region (23.90 Å at 55
℃) with a wide-angle broad hump at 4.55 Å which is attributed to the fluidic assembly of alkyl
chains as depicted in Figure 3a. The first intense peek can be indexed into a hexagonal lattice
with a = 27.59 Å and the calculated mesophase parameters are summarized in Table 2. The
compound HZ4 behaves thermotropically different from HZ3, although they are regioisomeric
derivatives. HZ4 has two distinct mesophases with the isotropic transition at 174.23 ℃, which
is the highest clearing temperature among the four investigated hydrazones and it shows
crystallization at 0.13 ℃ on cooling below lower temperature phase (Figure 2d). As can be seen
in Figures 2e-f, the material displayed fan type of texture below I phase, which on further
cooling slight increase in brightness of the birefringent domains was realized. To identify the
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higher and lower temperature mesophase ranges on cooling from its liquid phase. The patterns
obtained in both the mesophase regions were apparently similar and their lower angle
reflections were indexed to (10) plane of the hexagonal lattice (Figure 3b). To differentiate
between these two mesophases, we named lower temperature mesophase as Colh1 and Colh2 for
higher temperature mesophase, since there is a clear thermodynamic transition with
considerable enthalpy change (3.12 kJmol-1) associated with it. This kind of transition is
frequently observed in compounds with non-discoidal molecular geometry.
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Figure 2. (a) DSC thermogram of HZ3 (red and blue traces represent heating and cooling scan,
respectively); POM texture of HZ3 captured at (b) 145 ℃ and (c) 25 ℃; (d) DSC thermogram
of HZ4 (red and blue traces represent heating and cooling scan, respectively); POM texture of
HZ4 at (e) 150 ℃ and (f) 70 ℃
Figure 3. (a) XRD pattern obtained for Colh phase of HZ3 and (b) Colh1 and Colh2 phases of
HZ4 at various temperatures
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To understand the possible molecular packing in these hydrazone deriDvOaIt:i1v0.e10s3,9/tCh9eNJ01192G
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number of molecules accompanied per each columnar stratum Z was calculated by assuming
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the density ρ = 1 g cm-3. Subsequently, the Z was found to be 1.5, 1.35 and 1.4 for HZ2,
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HZ3 and HZ4, respectively. The observed non-integral values suggest that, there may be
folding and/or intercalation of peripheral tails in the columnar cross-section. It is reported that,
this kind of benzylidenehydrazones stabilize the columnar phase by the formation of helical
superstructures wherein the H-bonded chain of N-H···O=C adopts a helical conformation.
Because of this reason, they embrace more than one molecules per columnar unit in order to
maintain the average circular cross-section for organizing in a hexagonal lattice.58
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Table 2. XRD data of new hydrazones
Compd.
dobs (Å)
dcal (Å)
Miller
Indices
(hk)
Lattice parameter
a (Å), Lattice area
S (Å2), Molecular
volume V (Å3)
Phase (T/℃)
HZ2
Colh (135)
23.0
4.55 (ha)
23.0
(10)
(10)
(10)
(10)
a = 26.55
S = 610.83
V = 2779.3
Z = 1.46
HZ3
HZ4
HZ4
Colh (55)
23.90
4.55 (ha)
23.90
24.67
23.82
a = 27.59
S = 659.6
V = 3001.1
Z = 1.35
Colh1 (60)
Colh2 (150)
24.67
4.43(ha)
a = 28.48
S = 702.76
V = 3113.23
Z = 1.40
23.82
4.56(ha)
a = 27.50
S = 655.16
V = 2992.0
Z = 1.35
dobs , experimental lattice spacing; dcal, calculated from the lattice parameter, a for the
hexagonal lattice; ha, alkyl chains correlation peak value; hc, core-core spacing; Z,
number of molecules per columnar slice; S, lattice area; V, molecular volume
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Mesomorphism of boron difluoride complexes (FB1-4)
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The thermotropic phase behavior of all the boron difluoride complexes was also
investigated systematically. Evidently, upon coordination with BF2 the mesomorphism of
hydrazones HZ1-4 was completely altered. The thermal phase sequence and corresponding
thermodynamic parameters of FB1-4 are tabulated in Table 3. The first member of the series
bearing four alkyl tails was crystalline in nature, it melted at the same temperature (~71.0 ℃)
as its ligand HZ1. However, the enthalpy change for the melting transition of FB1 was found
to be much higher than HZ1 indicating the greater thermal stability of BF1 complex in the solid
state (Figure S32). Unlike HZ2, its boron complex FB2 was characterized to be non-
mesogenic, instead it showed three distinct crystalline morphology as evidenced by its DSC
thermogram (Figure S33). The non-mesomorphism of FB2 may be due to the lack of space
occupation by only five alkyl chains around the central aromatic segment. While its ligand
molecules (HZ2) segregated into columnar phase because of co-operative intermolecular
hydrogen bonding as discussed earlier.
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Table 3. a Phase transition temperatures (℃) and the corresponding energy changes (kJmol-1)
Compd.
Phase sequence
Cooling
Heating
FB1
FB2
Cr 71.25 (71.91) I
I -14.39 (-54.36) Cr
Cr1 44.19 (14.31) Cr2 66.83 (10.57) I 86.50 (-52.19) Cr3 61.99 (-9.14) Cr2
Cr3 96.98 (50.45) I 41.48 (-12.44) Cr1
FB3
Cr1 10.48 (7.39) Cr2 31.03 (23.92) I 32.08 (-1.09) Colh 2.20 (-22.10) Cr
Colh 35.01 (0.62) I
FB4
Colh 66.19 (23.83) Colr 96.65 (15.82) I 87.78 (-15.97) Colr 61.11 (-23.61) Colh
I
a
Peak temperatures/℃ (enthalpy/kJmol-1) obtained by second heating and first cooling at
the rate of 5 ℃/min; Colr = Columnar rectangular phase; Colh = Columnar hexagonal phase;
I = Isotropic liquid phase
The two regioisomers, FB3 and FB4 bearing six peripheral chains exhibit entirely
different thermotropic phase transitions from each other. The isomer FB3 having dodecyloxy
chains at 2nd, 3rd and 4th positions shows a trivial endothermic transition at 35.01 ℃ (ΔH = 0.62
kJmol-1) which could be assigned to an LC to I transformation. On cooling scan, the transition
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occurred at 32.08 ℃ (ΔH = -1.09 kJmol-1) and the phase gets crystallized at 2.20 ℃ as shown
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in Figure 4a. The perceived mosaic pattern was the typical indication of presence of columnar
mesomorphism (Figures 4b-c).
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Figure 4. (a) DSC thermogram of FB3 (red and blue traces represent heating and cooling scan,
respectively); POM texture of FB3 captured at (b) 30 ℃ and (c) 25 ℃; (d) DSC thermogram
of FB4 (red and blue traces represent heating and cooling scan, respectively); POM texture of
FB4 at (e) 83 ℃ and (f) 60 ℃
Further, XRD pattern of FB3 confirms the hexagonal lattice structure with the lattice
parameter a = 28.39 Å (Figure 5a). Whereas, its isomeric derivative FB4 having dodecyloxy
chains at 3rd, 4th and 5th positions exhibited two distinct enantiotropic mesophases as
demonstrated in DSC trace (Figure 4d). It displayed two reversible transitions with slightly
larger enthalpy values (ΔH = 24 kJmol-1 for LC-LC and ΔH = 16 kJmol-1 for LC-I). The
observed ΔH is much higher than that of its isomer FB3, this could be due to the compact
arrangement of FB4 in the mesophase. These results reveal that, changing the position of the
peripheral chains in the non-discoidal molecules significantly stabilizes the mesophase. The
lower temperature phase of FB4 was assigned to Colh mesophase (Figure 5b). A number of
XRD reflections in the lower-angle region at higher temperature mesophase have been indexed
to a centre rectangular columnar geometry with space group symmetry of C2mm as the peak
indexation follows the extinction rule h+k = even number (Figure 5c). The unit cell parameters
calculated from the diffraction pattern are a = 30.34 A and b = 62.38 A. This unusual transition
from hexagonal to rectangular columnar results in doubling of the lattice volume and hence the
number of constituent molecules also doubles in each unit cell. This kind of hexagonal to
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rectangular phase transition was previously observed in case of triphenylene dDeOriI:v1a0.t1i0v3e9/sC.96N0J01192G
Based on the XRD results, we propose a model to demonstrate the self-assembly of FB4
molecules as illustrated in Figure 6.
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Figure 5. XRD pattern obtained for (a) Colh phase of FB3; XRD traces recorded within (b)
lower and (c) high-temperature mesophase ranges of FB4
Figure 6. Schematic illustration of phase transition of FB4
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DOI: 10.1039/C9NJ01192G
Table 4. XRD characterization data of new organoboron complexes
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6
7
8
9
Compounds
dobs (Å)
dcal
(Å)
Miller Lattice parameter a
Indices (Å), Lattice area S
Phase (T/℃)
Colh (27)
Colh (25)
Colr (65)
(hk)
(Å2),
Molecular
volume V (Å3)
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58
59
60
FB3
24.59
14.26
4.42 (ha)
24.59 (10)
14.19 (11)
a = 28.39
S = 698.2
V = 3086.1
Z = 1.34
FB4
28.24
16.04
4.23 (ha)
3.96 (hc)
28.24
16.30
(10)
(11)
a = 32.08
S = 920.0
V = 3646.7
Z = 1.73
31.19
27.27
15.15
13.61
11.56
10.42
9.95
31.19
27.27
15.16
13.63
11.53
10.87
9.97
(02)
(11)
(20)
(22)
(15)
(24)
(31)
a = 30.32
b = 62.38
S = 1891.7
V = 8322.0
Z = 3.60
4.40 (ha)
dobs ,dcal, are observed and calculated value of d spacings. Lattice parameter, a for the
hexagonal lattice; ha, alkyl chains correlation peak value; hc, core-core spacing; Z,
number of molecules per unit cell; S, lattice area; V, molecular volume
Photophysical studies
The optical properties of newly synthesized D-A-D΄ configured boron difluoride
complexes were studied by recording their steady-state absorption and emission spectra in
DCM solution (c = 2×10-5 M) at room temperature. The obtained spectra are depicted in Figure
7a-b, and the subsequent key parameters are summarized in Table 5. Seemingly, a variation in
number of alkoxy chains has the least influence on their absorption property; whereas they
exert a slight impact on their emission behavior. In their absorption spectra, the observed higher
energy bands at ~230 nm are due to localized π-π* electronic transition of alkoxyphenyl rings
and the intense longer wavelength bands (377-385 nm) are assigned to delocalized
intramolecular charge transfer (ICT) transitions from π (D) to π* (A) of the molecules. On
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exciting at their absorption maxima, they show single featureless broad emission bVaienwdAsrticle Online
DOI: 10.1039/C9NJ01192G
indicating the floppy nature of the excited state.61 Their emission maxima were noticed in the
range of 492-524 nm; interestingly, FB1 bearing four alkoxy donors displayed red-shifted
emission compared to FB2 and FB3 having five and six respective alkoxy substitutions,
respectively. This may be related to the structural reorganization of the molecules in the excited
state. Certainly, they exhibit larger Stokes shift in the range of 5514 -7331 cm-1 confirming the
pronounced ICT behavior of these unsymmetrical dyad systems and the property is one of the
key factors in the fabrication of OLED devices. Further, to understand the emission behavior
of the complexes in the solid state, the PL spectra were recorded for their drop-casted thin films
and the resulting spectra are shown in Figure 7c. Evidently, they showed blue-shifted emission
maxima when compared to their respective peaks in solution. The fact may be due to the H-
type aggregation of the molecules in the solid state. Furthermore, the optical band gap values
of these materials were estimated by considering red-edge of their respective absorption
spectrum and they were found to in the range of 2.81-2.90 eV.
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Figure 7. (a) UV-Visible absorption and (b) photoluminescence spectra of FB1-4 at c = 2×10-
5 M DCM solution; (c) drop-casted thin film photoluminescence spectra of FB1-4
Table 5. Photophysical characterization data of new complexes
a b
Compd. a λabs (nm) a λem (nm) Stokes shift
E
c λem
(nm)
468
g opt
(cm-1)
(eV)
2.90
FB1
FB2
FB3
FB4
228, 377
228, 387
230, 384
230, 385
521
492
508
524
7331
5514
6357
6890
2.85
2.84
2.81
488
494
501
a Determined form DCM solution at c = 2×10-5 M, b Optical band
gap estimated by red edge of absorption band in UV-visible
spectrum, c Estimated for thin films
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Theoretical studies
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The Density Functional Theory (DFT) calculations were performed for all the new
complexes to establish their structure-property correlation. The calculations were
accomplished using Gaussian 09 software by means of Becke’s three-parameter functions
hybridized with the Lee-Yang-Par functional and the 6-31G (d,p) basis set. The geometries of
all complexes were optimized under vacuum condition and computed energy minimized
structures are showcased in Figures 8a-b. As visualized in Figure 8b, central core-system
comprising three rings adopts entirely planar geometry which fundamentally helps for the
stabilization of columnar arrangement. Further, the alkyl chains surrounded by this planar core
moiety facilitate to acquire more or less disc-shaped molecules.
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Figure 8. (a) Energy minimized structures of new BF2 complexes and (b) their side view
The electronic distributions in the frontier molecular orbitals (FMOs) are displayed in
Figure 9 and the simulated electrochemical data are tabulated in Table 6. The HOMOs are
localized over the two alkoxyphenyl rings with a slight contribution from the boron-containing
central core. Contrary, LUMOs are concentrated on the central ring and the alkoxyphenyl ring
attached through imine linkage. The results clearly indicate the electronic migration from the
donor to acceptor units, which manifests the ICT behaviour of the complexes. The observed
fact has reflected in experimental absorption spectral measurements. The simulated HOMO
and LUMO energy levels were found to be in the range of -5.56 to 5.69 and -2.02 to -2.21 eV,
respectively. The calculated energy bang gap (Eg) was noticed to be slightly higher than that
of the optical band gap. However, the trend of Eg is in good agreement with the experimental
values.
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Figure 9. Electronic distributions in the FMOs of new BF2 complexes
Table 6.a Theoretical electrochemical data of new complexes
b
Compd. HOMO (eV) LUMO (eV) aEg DFT (eV) Eg Opt (eV)
FB1
FB2
FB3
FB4
-5.665
-5.565
-5.598
-5.695
-2.136
-2.078
-2.027
-2.212
3.529
3.487
3.571
3.483
2.90
2.85
2.84
2.81
b
aThe values obtained in DFT calculations to vacuum; optical band
gap obtained by UV-visible spectra
Conclusion
In summary, we have designed and synthesized a novel series of organoboron
complexes (FB1-4) featuring dyad structural motif with a critical chain length. They were
subjected to in-depth liquid crystalline and optoelectronic studies. Interestingly, the key-
precursors benzylidenehydrazones (HZ1-4) exhibited columnar mesophase with the hexagonal
structure mainly by means of co-operative intermolecular hydrogen bonding, at ambient
conditions. Contrary, their corresponding BF2 complexes showed different thermotropic phase
behaviour. The mesomorphic properties of these hemi-discoidal boron complexes are greatly
dependent on number and position of fluidic chains around the aromatic-core. Consequently,
the isomeric complexes FB3 and FB4 displayed dissimilar thermotropic phase behaviour and
showed distinct columnar mesophase at different temperature ranges. Their experimental
optoelectronic studies as well as theoretical simulations explored their significant ICT
behavior. Conclusively, these prospective π-conjugated materials possessing stable Col phase
would be the promising candidates for molecular electronic applications.
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Conflicts of interest
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6
There are no conflicts of interest to declare.
7
8
Acknowledgement
9
The authors are thankful to NITK, Surathkal and SCM group, RRI, Bangalore, India for
providing necessary laboratory facilities. The authors are also thankful to Dr Vijayaraghavan
for recording the NMR spectra and Mrs. N. Vasudha for their technical support. We are grateful
to Prof. V. A. Raghunathan for his valuable inputs on XRD analysis. VK is grateful to NITK
for providing fellowship.
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Table of Contents
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A series of prospective columnar liquid crystalline materials derived from novel organoboron
complexes has been developed in virtue of their application in organic electronic applications.
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52
53
54
55
56
57
58
59
60