Induction of Mesomorphism through Supramolecular Assembly in Metal Coordination Compounds of “salphen”-Type Schiff Bases: Photoluminescence and Solvatochromism

Article abstract of DOI:10.1002/ejic.201600513

Mesomorphism induced by metal (VO²⁺, Ni²⁺, Cu²⁺and Zn²⁺) coordination and its influence on the photophysical properties of a new series of symmetric “salphen”-type N,N-bis(4-n-alkoxysalicylidene)-4,5-dichloro-1,2-phenylenediamine ligands (n = 12, 14, 16) bearing highly electronegative spacer substituents and alkoxy arms of varied length have been investigated. The mesomorphism has been probed by polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and variable-temperature powder X-ray diffraction (XRD). The Schiff base ligands are non-mesomorphic; however, coordination to Ni^IIand Zn^IIions induces columnar mesomorphism, this being stable down to room temperature in the latter case. Interestingly, the complexes of Cu^IIand VO^IVdid not show any mesomorphism. The free ligands and the Zn^IIcomplexes are luminescent both in the condensed state and in solution, with emission maxima in the condensed state being significantly red-shifted with respect to those in solution. The energy-optimized structure of a representative Zn^IIcomplex was ascertained to be distorted square planar in a DFT study employing the GAUSSIAN 09 program. The Zn^IIand VO^IVcomplexes exhibited solvatochromic effects, showing interesting aggregation behavior in the former case.

Full text of DOI:10.1002/ejic.201600513

DOI: 10.1002/ejic.201600513
Full Paper
Mesomorphic Assemblies
Induction of Mesomorphism through Supramolecular Assembly
in Metal Coordination Compounds of “salphen”-Type Schiff
Bases: Photoluminescence and Solvatochromism
Sutapa Chakraborty,[a] Paritosh Mondal,[a] S. Krishna Prasad,[b] D. S. Shankar Rao,[b] and
Chira R. Bhattacharjee*[a]
Abstract: Mesomorphism induced by metal (VO2+, Ni2+, Cu2+ being stable down to room temperature in the latter case. Inter-
and Zn2+) coordination and its influence on the photophysical estingly, the complexes of CuII and VOIV did not show any meso-
properties of a new series of symmetric “salphen”-type N,N- morphism. The free ligands and the ZnII complexes are lumines-
bis(4-n-alkoxysalicylidene)-4,5-dichloro-1,2-phenylenediamine cent both in the condensed state and in solution, with emission
ligands (n = 12, 14, 16) bearing highly electronegative spacer maxima in the condensed state being significantly red-shifted
substituents and alkoxy arms of varied length have been inves- with respect to those in solution. The energy-optimized struc-
tigated. The mesomorphism has been probed by polarizing op- ture of a representative ZnII complex was ascertained to be dis-
tical microscopy (POM), differential scanning calorimetry (DSC), torted square planar in a DFT study employing the GAUS-
and variable-temperature powder X-ray diffraction (XRD). The SIAN 09 program. The ZnII and VOIV complexes exhibited solva-
Schiff base ligands are non-mesomorphic; however, coordina- tochromic effects, showing interesting aggregation behavior in
tion to NiII and ZnII ions induces columnar mesomorphism, this the former case.
Introduction
vators in the synthesis of cyclic carbonates from terminal epox-
ides and carbon dioxide[22] etc.
Ligands based on the π-conjugated “salphen” framework have
attracted enormous attention in recent times, attributable pri-
marily to their interesting photophysical and supramolecular
properties and reactivity.[1] The extended conjugation offered
by the phenyl spacer in a salphen-ligated metal system offers
immense possibilities for the fabrication of photoresponsive
materials, supramolecular assemblies, nonlinear optical materi-
als, and liquid crystalline entities.[2–9] The electronic properties
and steric bulk of the ligand framework can be easily varied in
these systems by careful use of substituents to tune the overall
potential of the resulting salphen–metal complexes.[1,10–12]
Metal coordination often enhances the efficiency of salphen
ligands as effective catalysts in many homogeneous and heter-
ogeneous transformations including asymmetric ring-opening
of epoxides,[13] oxidation,[14,15] epoxidation of olefins,[16,17] po-
lymerization reactions,[18,19] carbonylation of aniline,[20] and de-
alkylation of organophosphates,[21] and also as Lewis acid acti-
Metallomesogens based on salphen ligands are yet another
major area of application of these systems, providing access to
exotic liquid crystalline materials.[8,9] The rigidity of the exten-
sively conjugated ligand offers the required anisotropy for the
construction of supramolecular structures favoring mesomor-
phism. Modification of components either in the salicyliden-
imine units or in the aryl bridging group often tunes the meso-
phases exhibited by these complexes, including disappearance
of the mesomorphism.[8,23] Incorporating metal in a square-pla-
nar geometry is another challenging aspect because tetrahedral
distortions away from planar geometry, in some cases, disrupt
the required anisotropy of the core, in turn frustrating meso-
morphism.[24] In the past few years, we have explored the me-
somorphism of salphen-based metal complexes with different
electron-donating or -withdrawing substituents across the
phenylene spacer and have studied the effects of the substitu-
ents and or the nature of the central metal ion on mesomor-
phism.[23,25–32]
[a] Department of Chemistry, Assam University,
Silchar 788011, Assam, India
E-mail: crbhattacharjee@rediffmail.com
www.aus.ac.in
[b] Centre for Nano and Soft Matter Sciences,
Jalahalli, Bangalore 560013, India
It is pertinent to mention here that the unusual square-pla-
nar geometry around the ZnII center generated by the rigid
ligand framework in these complexes leaves a vacant axial coor-
dination site, leading to high Lewis acidity of the Zn2+ ion.[33]
Bulky substituents at the 3,3′-positions of the salicyldehyde
fragments in these ligands are known to lead to the isolation
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejic.201600513.
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of unique square-planar complexes,[34] precluding chelation to alkoxysalicylidene)-4,5-dichloro-1,2-diaminobenzene], abbrevi-
suitable Lewis bases/donor ligands or the formation of aggre- ated as nc2pd (number of carbon atoms in alkyl chains n = 12,
gates/self-assembled structures.[1,33] The Lewis acidity of the 14, and 16) and the mononuclear complexes (VO-nc2pd, Ni-
ZnII–salphen complexes also makes them versatile supramolec- nc2pd, Cu-nc2pd, or Zn-nc2pd) is summarized in Scheme 1. The
ular building blocks,[5] chromogenic and fluorogenic chemosen- structure of the ligands and the corresponding complexes were
1
sors for detection of biologically relevant molecules,[35–37] se- ascertained by elemental analyses and UV/Visible, FTIR, and H
lective receptors for tertiary amines,[38] and catalysts,[22,39] and NMR (for ligands and NiII and ZnII complexes only) spectro-
has motivated exploration of their unique second-order nonlin- scopy.
ear optical properties.[6,7] Further, the square-pyramidal geome-
try of the vanadyl ion in oxovanadium(IV)–salphen complexes
renders the coordination sphere unsaturated. The structure can
be related to an octahedron with an axial vacant site, which
can be filled quite readily through intermolecular dative interac-
tions or coordination to neutral or anionic donors or Lewis
bases.[24,40] Dative interactions are of particular interest in the
case of the square-pyramidal vanadyl complexes because they
produce linear chain-like structures, (···V=O···V=O···) creating a
polar order. This offers possibilities for the generation of new
ferroelectric/piezoelectric materials.[41]
Against this backdrop, we report a series of “salphen”-type
symmetric
N,N-bis(4-n-alkoxysalicylidene)-4,5-dichloro-1,2-
phenylenediamine ligands (n = 12, 14, 16) and their complexes
with VOIV, NiII, CuII, and ZnII metal ions. The “dichloro-phenylene
spacer” was chosen in this work to allow exploration both of
the steric and the electronic effects of the two vicinal electro-
negative chloro groups on the overall mesomorphic and photo-
physical properties of the newly synthesized compounds. The
effects of metal coordination on mesomorphism and on the
photophysical properties of the salphen ligands have been in-
vestigated. Except in one instance, in which a long (C-12) alk-
oxy-substituted spacer has been employed to access some me-
somorphic MnIII complexes there appear to be no other exam-
ples of metallomesogens based on salphen systems with vicinal
disubstituted spacer groups in the literature.[8] The free ligands
are non-mesomorphic; however, coordination to NiII and ZnII
ions induces columnar mesomorphism in the resulting com-
plexes, although the VOIV and CuII complexes are non-meso-
morphic. Interestingly, for the ZnII complexes, the mesophase
formed is observed to be stable down to room temperature.
The free ligands and the ZnII complexes showed photolumines-
cence both in solution and in the condensed state. In addition,
the ZnII and VOIV complexes showed interesting solvatochro-
mism, with the former showing concomitant formation of ag-
gregates depending on the coordinating ability of the solvent.
Scheme 1. (i) CnH2n+1Br, KHCO3, KI, dry acetone, Δ, 24 h, (ii) glacial acetic
acid, absolute EtOH, Δ, 3 h, (iii) M(OAc)2·2 H2O (M = Ni, Cu, Zn)/VOSO4·H2O,
MeOH, stirring, 3 h.
The IR spectra of the ligands showed bands corresponding
to the νC=N and νO–H vibrations at ca. 1614 and 3436 cm–1,
respectively. For the corresponding metal complexes the νC=N
vibrations were observed at lower wavenumbers than for the
free ligands along with the absence of νO–H vibrations, suggest-
ing coordination of the imine nitrogen and phenolate oxygen
to the metal center. The VOIV complexes additionally showed
νV=O stretching at ca. 971 cm–1 indicating the absence of any
intermolecular (···V=O···V=O···) interaction, consistently with
the monomeric nature of the complexes.[25] The V=O stretching
frequency is an indicative test for oxygen bridging in five-coor-
dinate square-pyramidal vanadyl(IV) complexes. In general, for
polymeric complexes this band appears below 900 cm–1, due
to linear chain interactions, whereas for monomeric ones it is
located in the 900–1000 cm–1 region.[25,42]
1
The H NMR spectra of the ligands (nc2pd) showed charac-
teristic signals at δ = 13.17 ppm, corresponding to the OH pro-
tons, and ca. 8.52 ppm, for the symmetric imine protons. The
1H NMR spectra of the corresponding NiII and ZnII complexes
did not exhibit any signal for the phenolic –OH proton. How-
ever, upfield shifts in the peak positions of –N=CH protons with
respect to the free ligands were observed for both complexes,
confirming azomethine nitrogen coordination.
Results and Discussion
Synthesis and Characterization
Condensation of 4-n-alkoxy-substituted salicylaldehydes with
4,5-dichlorobenzene-1,2-diamine afforded the corresponding
salphen ligands. Complexes were obtained by treatment of the
Liquid Crystalline Properties
corresponding Schiff bases with vanadyl sulfate (VOSO4·H2O), The thermal behavior of the ligands and of the corresponding
nickel acetate [Ni(OAc)2·2 H2O], copper acetate [Cu(OAc)2· metal complexes was investigated by polarizing optical micros-
2 H2O], or zinc acetate [Zn(OAc)2·2 H2O] in 1:1 molar ratio. The copy (POM), differential scanning calorimetry (DSC), and varia-
strategy implemented for the synthesis of ligands [N,N-bis(4-n- ble-temperature powder X-ray diffraction (XRD). The ligands are
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non-mesomorphic, melting directly to isotropic liquids at 80– columnar phases (Figure 1, b) at 160 °C.[44] The optical texture
90 °C. However, coordination of the salphen ligands, particularly of the Zn-nc2pd complexes remained unaltered down to room
to ZnII and NiII ions, conferred liquid crystalline properties on temperature.
the resulting complexes. The CuII complexes decomposed just
at the onset of melting at 120 °C. The VOIV analogues, on the
contrary, decomposed prior to melting at temperatures higher
than 280 °C, precluding any mesomorphic study. This unusual
thermal behavior of the VOIV and CuII complexes can be ex-
plained by invoking axial interaction for the stability of the
mesophase.
Square-planar copper(II)–salen or –salphen complexes are
coordinatively unsaturated and the metal center usually
achieves full occupation through interaction with neighboring
molecules in the axial direction. This additional interaction fa-
Figure 1. POM photographs of (a) Ni-16c2pd at 150 °C and (b) Zn-16c2pd at
160 °C, on cooling.
vors the formation of a one-dimensional array of the mol-
ecules.[24] Tetrahedral distortions away from the square-planar
coordination in some cases prevent this axial interaction involv-
ing oxygen atoms of neighboring mesogens, preventing meso-
phase formation.[24,43] In the current salphen-CuII complexes,
the electron-withdrawing dichloro substitution on the phenyl-
ene spacer is considered to disturb the axial interaction, hence
resulting in decomposition of the complexes. Similar observa-
tions were noted earlier in the cases of analogous CuII–salphen
complexes with nitro-substituted phenylene spacers.[23]
The VOIV–salphen complexes, on the other hand, each adopt
a square-pyramidal geometry, which is nonplanar and less
anisotropic. The axial vanadyl oxygen atom is believed to form
a weak dative bond with the vanadium atom of a neighboring
mesogen, thus assembling the cores into an one-dimensional
array, favoring mesophase formation.[24] Again, too-rigid linear
V=O···V=O interactions may lead to polymeric complexes, caus-
ing loss of mesomorphism.[42] Therefore, the length of the V=O
bond (i.e., the extent of axial interaction) is crucial to the forma-
tion of mesophases. The relatively high temperatures of decom-
position of the VOIV complexes in this study can thus be ac-
counted for by invoking (i) the presence of the dichloro substit-
uents on the phenyl spacer, which might somewhat obscure
the extruding oxo group, or (ii) possible trigonal-bipyramidal
distortion in the metal coordination geometry.[24]
The DSC thermogram of Ni-16c2pd displayed four endother-
mic peaks in the first heating cycle, due to two crystal–crystal
transitions, one crystal–mesophase transition, and one meso-
phase–isotropic liquid transition (Figure 2, a). In the cooling
cycle, four exothermic peaks were observed, corresponding to
isotropic liquid–mesophase, mesophase–crystal, and two crys-
tal–crystal transitions. The endothermic peak observed at
101 °C in the first heating run appeared as an exothermic peak
at 98 °C in the second heating run (Figure 2, a). This is surmised
to be originating from a low-energy crystal packing at higher
temperatures.[45] The DSC traces for Ni-12c2pd and Ni-14c2pd,
however, each consisted of only two peaks both in the heating
and in the cooling cycle (Figures S1 and S2 in the Supporting
Information and Table 1). The DSC thermogram of Zn-nc2pd
with n = 12 (Figure S3 in the Supporting Information), 14 (Fig-
ure S4 in the Supporting Information), and 16 (Figure 2, b) each
displayed two endothermic peaks in the heating cycle, corre-
sponding to crystal–mesophase and mesophase–isotropic liq-
uid transitions. In the cooling cycle, the isotropic liquid–meso-
phase transition as observed in the POM could not be identified
by DSC. The absence of this peak in the cooling cycle of the
DSC thermogram, accompanied by the low enthalpy value for
the mesophase–isotropic liquid transition in the heating scan,
suggests that the mesophase is highly disordered.[46] A gradual
decrease in the clearing temperature was noted on increasing
The complexes Ni-nc2pd and Zn-nc2pd exhibited enantio- the number of carbon atoms in the alkoxy arms. The isotropic
tropic mesomorphism (Table 1). The NiII complexes displayed a liquid-to-mesophase transition temperature also displayed simi-
somewhat atypical texture (Figure 1, a) at 150 °C upon slow lar trends with increasing chain length. The reversibility of the
cooling from the isotropic melt. The ZnII complexes, on the thermal behavior was verified by DSC in subsequent heating/
other hand, showed a broken fan-like texture characteristic of cooling cycles.
Table 1. Thermodynamic data for the NiII and ZnII complexes. Transition temperatures are given in °C, and the corresponding enthalpy changes are in
parentheses (ΔH, J g–1). Cr, Cr1, and Cr2 refer to phases that are crystalline or solid. Colo: oblique columnar phase. Colr: rectangular columnar phase.
Heating cycle[a]
Cooling cycle[a]
Ni-12c2pd
Ni-14c2pd
Ni-16c2pd[b]
Ni-16c2pd[c]
Zn-12c2pd
Zn-14c2pd
Zn-16c2pd
Cr 105(1.3) Colo 192(21.3) I
Cr 98(59.3) Colo 184.5(9.9) I
Cr 75.7(20.5) Cr1 100.6(6.9) Cr2 128(45.7) Colo 167(6.8)I
Cr 81.6(20.4) Cr1 98.3(–3.7) Cr2 128(29.8) Colo 169(7.4)I
Cr 144(15.4) Colr 253(1.4) I
I 185(22.9) Colr 99(1.3) Cr
I 172(25.1) Colo 79(3.5) Cr
I 127(12.3) Colo 104(1.3) Cr2 73(13.3) Cr1 68(5.5) Cr
I 127(11.5) Colo 103(1.6) Cr2 73.2(13.1) Cr1 68(5.3) Cr
I 247.3[d] (–) Colr
Cr 122(20.6) Colr 235(1.7) I
Cr 111(23.2) Colr 183(1.2) I
I 225.4[d] (–) Colr
I 177.2[d] (–) Colr
[a] DSC peak temperature. [b] First heating. [c] Second heating. [d] Optical microscopy data.
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angle region: a broad one at 4.7 Å for the lateral short-range
order of molten alkyl chains and another, less broad halo at
3.7 Å, probably due to the liquid-like correlation between adja-
cent cores. X-ray diffraction profiles obtained over a wide tem-
perature range (126–80 °C) were very similar (Figures S5 and S6
in the Supporting Information). Profile fitting of the data
matched well with those for a two-dimensional oblique colum-
nar phase (Colo) with a p1 space group.[47,48] The lattice parame-
ters a, b, and γ were found to be 57.39 Å, 99.38 Å, and 69.8°,
respectively (Table 2). In a columnar oblique phase, the elemen-
tary cell is made up of two columns (Figure 5a, below).[47,48] If
a density of 1 g cm–3 is assumed for these half-discoid mol-
ecules[48] and a periodic stacking value of 3.7 Å of the molecular
cores within the column is estimated, we can infer that the
columnar cross-section is made up of six molecules (Ncol = 6).
Table 2. X-ray diffraction data for Ni-16c2pd and Zn-16c2pd.
Temp.
[°C]
dmeas.
dcalcd.
Miller
Mesophase
[Å][a,b] [Å][a]
indices
parameters[d]
(hkl)[c]
Ni-16c2pd 126
39.59s 39.59
22.92s 22.92
19.80s 19.80
16.21s 16.21
13.17s 13.20
11.96s 12.13
11.41s 11.44
10.63s 10.72
110
130
100
250
330
450
030
570
200
Colo, p1
ao = 57.32 Å
bo = 99.38 Å
γ = 69.8°
S
o = 5346.05 Å2
V
S
cell = 20052.23 Å3
col = 2673.03 Å2
mol = 1629.86 Å3
cell = 12.3, Ncol
6.15
V
N
9.92s
9.90
=
9.58s
8.87m
8.44m
8.08m
7.77m
6.90m
5.80m
4.74d
4.01
9.54
8.73
8.58
8.08
7.77
6.86
5.77
4.67
4.01
–
380
270
680
430
210
¯
050
530
–
620
–
100
–
100
010
200
300
020
–
Figure 2. DSC thermogram of (a) Ni-16c2pd during first and second heating
and cooling cycles, and (b) Zn-16c2pd during first heating and cooling cycles.
3.75
Zn-16c2pd 140
25
44.78s 45.83
highly disordered LC
4.5d
–
52.12s 51.35
30.97s 31.00
24.56s 25.67
16.47m 17.12
15.56m 15.50
Colr, p2mm
The XRD pattern of Ni-16c2pd at 126 °C (Figure 3) consisted
of a number of Bragg diffraction peaks in the small-angle re-
gion. In addition, two diffuse halos were observed in the wide-
ar = 51.35 Å
br = 31.0 Å
Sr = 1591.85 Å2
V
V
N
cell = 6717.6 Å3
4.22d
–
mol = 1543.20 Å3
cell/Ncol = 4.35
[a] dmeas. and dcalcd. are the experimentally measured and calculated diffrac-
tion spacings. [b] Intensities of the reflections, s: sharp, m: medium, d: diffuse.
The distances are given in Å. [c] [hkl] are the Miller indices of the reflections.
[d] Phase parameters, Colo phase: 1/dhk = 1/sinγ √[(h/ao)2 + (k/bo)2 – (2hk
cosγ/aobo)], γ is columnar tilt angle; lattice area, So = aobosinγ; columnar
cross-section, Scol = aobo/2; Colr phase: ar = d10; 1/dhk = √[(h/ar)2 + (k/br)2];
columnar cross-section in the rectangular cell, Sr = arbr. Vcell, volume of an
elementary cell: Colo phase, Vcell = aobosinγ × h; Colr phase, Vcell = arbr×h,
where h is intracolumnar repeating distance. Vmol (molecular volume) is deter-
mined by considering a density of 1 g cm–3 according to the relationship
MW/λρNA, where MW is the molecular weight, NA is the Avogadro number,
and λ(T) is a temperature correction coefficient at the temperature of the
experiment (T), λ(T) = VCH (To)/VCH (T), where VCH (T) = 26.5616 + 0.02023 T
is the volume of a methyl2ene group (in Å3) at a 2given temperature (in °C),
and T° = 25 °C. Ncell is the number of molecules per unit cell, and Ncol is the
number of molecules per columnar cross-section.
2
Figure 3. X-ray diffraction pattern of Ni-16c2pd at 126 °C.
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To account for the supramolecular organization in the meso- complex is rather disordered at high temperatures, relative to
phase, it is assumed that the molecules of the nickel complexes the NiII complexes, so indexation of the PXRD pattern was only
self-assemble into dimers in an antiparallel fashion (Figure 5, c, possible at room temperature, in a frozen state. Size of the
below).[8,23] Three such dimers stack side by side to cover the metal ion, in addition to the geometry of the core, is therefore
surface area of a thin slice of a disk.[8] The lattice constant a in an important factor and any significant variation in the ionic
this case is considerably greater than the DFT-optimized radius radii also affects the thermal stability of the resulting com-
of a fully extended molecule (vide infra), thus further support- plexes.[50]
ing the proposed arrangement.
The X-ray diffractogram of Zn-16c2pd at 140 °C (Figure 4, a)
consisted of a single diffraction maximum at low angles that
corresponds to Miller index (100). At wide angles, a broad halo
was observed at 4.6 Å, corresponding to liquid-like correlation
of the molten alkyl chains. The absence of additional character-
istic features in the diffraction pattern precluded unambiguous
assignment of the mesophase at higher temperatures. The dif-
fraction pattern at 25 °C (Figure 4, b), however, consisted of five
reflections in the small-angle region, at 52.12, 30.97, 24.56,
16.47, and 15.56 Å (Table 2). In fact, spacing for the second
reflection, from the lowest-angle side, is far from being a sec-
ond harmonic of the first peak. Thus, a lamellar structure is
ruled out. Accordingly, these were assigned as (100), (010),
(200), (300), and (020) reflections of a two-dimensional rectan-
gular lattice. Application of extinction rules to the estimated
Figure 5. Schematic illustration of (a) unit cell of a two-dimensional columnar
oblique lattice (p1), (b) unit cell of a two-dimensional rectangular columnar
indexations excluded the possibility of the c2mm and p2gg
plane groups (for c2mm, hk: h + k = 2n, h0: h = 2n, 0k: k = 2n;
for p2gg, hk: no conditions, h0: h = 2n, 0k: k = 2n) that are
commonly observed.[44,49] Also, the absence of reflections with
a set of restrictions; hk: no conditions, h0: no conditions, 0k:
k = 2n, ruled out the possibility of p2mg symmetry.[44,49] In this
scenario, because no specific symmetry rules are followed it is
presumed that the columns are packed in accordance with an
overall p2mm symmetry (Figure 5, b).[49] In addition to the
small-angle reflections, a broad halo was observed in the wide-
angle region at 4.2 Å, corresponding to the liquid-like nature of
molten alkyl chains. However, no second diffuse peak such as
would be expected for intracolumnar stacking was observed,
thus suggesting high disorder along each column, which would
also justify small enthalpy values associated with the clearing
process.[46] If a stacking periodicity of 4.2 Å for the molecular
cores is assumed, it could be deduced that the columnar cross-
section is formed by four molecules. In this case, the columnar
cross-section is formed by lateral arrangement of two dimers
(Figure 5, c). The lattice parameter ar (51.35 Å) in this case is
smaller than that of the NiII complex, consistently with the pro-
posed organization. Also, the columnar mesophase of the ZnII
p2mm lattice, and (c) model for dimeric association of the molecules and
molecular organization in the mesophase.
A rich mesomorphic diversity has also been found in struc-
turally related salphen systems containing unsubstituted or
monosubstituted phenylenediamine spacers, containing either
electron-donating methyl or electron-withdrawing chloro or
nitro substituents. A critical comparison of the metal complexes
of all such ligand systems with regard to mesomorphism is pre-
sented in Table 3. In cases of salphen systems with unsubsti-
tuted or electron-donating methyl-substituted spacers, al-
though the complexes are all mesomorphic, the thermal stabil-
ity and/or the nature of the two-dimensional packing of the
columns in the lattices are quite different.[23,25,26,29,31,32] For
nitro-substituted salphen ligands, whereas the NiII complexes
are mesomorphic, the CuII complexes undergo decomposi-
tion.[23,26] Similar behavior has been noted for the CuII com-
plexes studied here. CuII complexes of unsubstituted or methyl-
substituted ligands were, however, mesomorphic, displaying
Colrp or Colr phase, respectively.[23]
The previously reported NiII complexes all exhibited rectan-
gular columnar mesophases (c2 mm), though the phase transi-
tion is monotropic for nitro- and methyl-substituted derivatives
and enantiotropic for unsubstituted ones.[26] The NiII complexes
currently under study, however, show enantiotropic oblique co-
lumnar phases.
Of the differently substituted ZnII complexes, those without
any substituent or with an electron-donating methyl group in
the spacer displayed monotropic columnar mesomorphism.
Mesophases exhibited by these complexes were either rectan-
gular/oblique columnar (P222/P112) or hexagonal columnar of
varied symmetries.[31,32] In contrast, with the electron-withdraw-
ing Cl substituent in the spacer, enantiotropic phase behavior
is observed for the chloro-substituted ZnII complexes, which
exhibit rectangular columnar phases with p2gg symmetry.[27]
Figure 4. X-ray diffraction pattern of Zn-16c2pd: (a) at 140 °C, and (b) at room
temperature (25 °C).
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Table 3. Mesomorphism and photoluminescence of metal complexes with
different “salphen” spacer substituents (PL: photoluminescence, +: PL-active.
solution (conc. 10–5
and 7). The spectroscopic data for the compounds are listed in
Table 4.
M
) and in the solid state at 298 K (Figures 6
Figure 6. UV/Visible spectra of 16c2pd and its metal complexes in dichloro-
methane solution (1 × 10–5
M, room temp.).
In the case of VOIV complexes, whereas the unsubstituted
ones exhibited columnar lamellar mesomorphism, a cross-over
between two rectangular columnar phases has been observed
in the methyl-substituted representatives.[25,29] In each case the
mesophase is stable down to room temperature and the nature
of the phase transition is enantiotropic. The VOIV complexes
under investigation here, however, undergo high-temperature
decomposition, frustrating mesomorphism in turn.
Thus, a minor modification of the steric or electronic proper-
ties in the spacer can have a major influence on the thermal
properties of the salphen systems and the resulting complexes.
The role of the metal center is also crucial. A subtle interplay of
electronic factors originating from the phenylene spacer and
distortions in coordination geometry associated with the nature
of metal ions might therefore be responsible for the observed
mesomorphic diversity.
Figure 7. (a) Absorption (left) and normalized fluorescence emission (right)
spectra of 16c2pd, and (b) absorption (left) and normalized fluorescence
Photophysical Properties
emission (right) spectra of Zn-16c2pd in dichloromethane (1 × 10–5
M, room
temp.) solution (–), solid state (----) and mesophase (···). Side panels show
photoluminescence pictures of solution (top), solid state (middle), and meso-
phase [bottom, (b) only], respectively, as seen after irradiation with 366 nm
light.
The photophysical properties of the salphen ligands nc2pd and
their corresponding metal complexes – VO-nc2pd, Ni-nc2pd,
Cu-nc2pd, and Zn-nc2pd – were recorded in dichloromethane
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Table 4. Absorption data for the ligands (n = 12, 14, 16) and their metal complexes in various solvents, molar absorption coefficient (ε) in
M
photoluminescence data for ligands and ZnII complexes in solution and the solid state at 298 K. (-do- stands for a value similar to that of the preceding data
in the column.)
–1 cm–1, and
max
em
max
[a]
[b]
[c]
[d]
Solvent
λ
abs(ε/104) nm
λ
[nm]
λ
[nm]
em
Φ
F(solution)
Φ
F(solid)
(solution)
(solid)
12c2pd
14c2pd
16c2pd
CH2Cl2
-do-
-do-
-do-
-do-
-do-
-do-
-do-
-do-
-do-
-do-
-do-
-do-
300(2.2), 336(2.9), 358(2.5)
300(2.2), 336(3.1), 358(2.4)
300(2.3), 336(3.0), 358(2.5)
460
-do-
-do-
–
538
-do-
-do-
–
0.09
0.10
0.10
–
0.04
0.03
0.04
–
VO-12c2pd
VO-14c2pd
VO-16c2pd
Ni-12c2pd
Ni-14c2pd
Ni-16c2pd
Cu-12c2pd
Cu-14c2pd
Cu-16c2pd
Zn-12c2pd
Zn-14c2pd
Zn-16c2pd
334(2.8), 410(3.1), 427sh (2.6)
334(2.7), 410(3.1), 427sh (2.5)
334(2.8), 410(3.2), 427sh (2.6)
317(2.5), 396(2.9), 475 (1.4)
317(2.7), 396(3.0), 475 (1.7)
317(2.6), 396(2.9), 475 (1.5)
328(2.2), 350(1.9), 407 (3.2) 433sh (2.4)
328(2.3), 350(1.8), 407 (3.2) 433sh (2.3)
328(2.3), 350(1.8), 407 (3.1) 433sh (2.3)
327(3.2), 386(2.5), 411sh (2.1)
327(3.4), 386(2.6), 411sh (2.3)
327(3.3), 386(2.6), 411sh (2.2)
327(3.5), 386(2.8), 413sh (2.3)
327(3.3), 391(2.9), 415sh (2.4)
323(3.5), 403(3.4), 426sh (2.6)
–
–
–
–
–
–
–
–
468
-do-
468
468
468
469
504
-do-
-do-
0.19
0.19
0.20
0.20
0.21
0.30
0.07
0.07
0.08
-do-
CH2Cl2
CHCl3
toluene
THF
[a] λex = 336 nm (ligand), 320 nm (Zn complex). [b] λex = 386 nm (ligand), 320 nm (Zn complex). [c] Estimated error: 5 %. [d] Estimated error: 10 %.
The salphen ligands each exhibited two broad absorption chelation provides additional rigidity to the salphen frame-
bands with maxima at 300 nm and 336 nm and a shoulder at work.[27]
The solid-state emission spectra of the ligands and the ZnII
complexes indicated a structure similar to that in solution (Fig-
ure 7, a and b). However, emission maxima both for the ligands
and for the corresponding ZnII complexes were substantially
red-shifted with respect to solution, with the associated shifts
for the ligands and for the ZnII complexes being 78 nm and
36 nm, respectively. The red shift of the emission maxima in
the solid state can be attributed to closer association of aro-
matic cores with a reduction in the local mobility, which redu-
ces deactivation through non-radiative channels.[27] As for the
ZnII complexes, because the mesophase is stable down to room
temperature, a photoluminescence study was conducted in this
state as well. The mesophase emission maximum is located at
518 nm (λexc. = 320 nm), bathochromically shifted with respect
to that observed in solution (468 nm) and in the solid state
(504 nm).
355 nm corresponding to π–π* transitions of the aromatic rings
and the imine unit (Figure 6). After coordination, all the absorp-
tion bands were shifted significantly to longer wavelengths,
with the longer-wavelength feature acquiring substantial
charge-transfer character. For CuII complexes, an additional
peak was observed at 350 nm in each case. For NiII complexes,
the higher-wavelength feature at 475 nm (Figure 6) is believed
to be of charge-transfer character. This is ascribed to the excita-
tion of filled d orbital electrons into the empty antibonding π
orbitals (d→π*) of the salphen ligand.[51,52] Moreover, the alkyl
chain length variation had virtually no effect on the spectral
behavior in all cases.
Photoluminescence study of the compounds was carried out
in dichloromethane solution (conc. 10–5
M) and in the solid state
at room temperature. Emission spectra in the solid state were
recorded by placing a uniform powder sheet of the sample be-
tween two quartz plates. The ligands and the ZnII complexes
are luminescent. As a representative example, Figure 7 shows
the photoluminescence spectra of 16c2pd and Zn-16c2pd, re-
spectively. Emission spectra of the ligands in dichloromethane
solution each consisted of a broad band with maxima at ca.
460 nm associated with intraligand π–π* electronic transitions.
Previously reported unsubstituted or monosubstituted ligands
were however, nonluminescent.[26,27,31,32] Further, whereas the
corresponding NiII complexes were both photoluminescent, the
NiII complexes with the dichloro-substituted phenylenediamine
spacer currently under investigation are not (Table 4). The emis-
sion maxima for the corresponding ZnII complexes were batho-
chromically shifted (ca. 8 nm) relative to the free ligands. The
observed fluorescence emission in the ZnII complexes origi-
nates from metal-perturbed π–π* ligand-centered transitions.
The emission intensity is increased on complexation because
In addition, a solvatochromic study of a representative com-
plex from each series – VO-16c2pd, Ni-16c2pd, Cu-16c2pd, and
Zn-16c2pd – in dilute solutions in solvents (10–5
M) of different
polarities was also carried out. For the CuII and the NiII com-
plexes, absorption features in coordinating solvent (THF) were
similar to those observed in the case of noncoordinating sol-
vents (Figure S8, Figure S9, and Table S1 in the Supporting In-
formation).
Absorption spectra of VO-16c2pd in noncoordinating sol-
vents consisted of two well-defined bands at 334 nm and
410 nm with a shoulder at 427 nm; however, in coordinating
solvent (THF) only two bands were observed, with substantial
broadening and red shift of the second absorption feature and
the shoulder at longer wavelength being overlapped by the
second band (Figure S7 and Table S1 in the Supporting Infor-
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mation). This observation can be related to the weak interaction tra of Zn-16c2pd in noncoordinating solvents (e.g., CH2Cl2,
of the solvent molecule trans to the vanadyl oxo ligand.[53] The CHCl3, toluene) showed two well-defined bands located at ca.
d level ordering in five-coordinate square-pyramidal complexes 328 nm and 386–391 nm, together with a shoulder at 413 nm
of vanadyl ion (VO2+) is:[54] dxy < dπ(dxz, dyz) < dx
< dz2. The (Figure 8, a), indicating the presence of aggregates.[56,57] On
2 – y2
lower-wavelength band is ascribed to the dxy → dx
transi- switching to a coordinating solvent (e.g., THF), solvent-induced
2 – y2
tion whereas the higher-wavelength band appears to be due to deaggregation caused substantial changes in the optical ab-
the dxy → dπ transition of vanadium. The antibonding dπ orbital sorption spectrum. Hyperchromism accompanied by a red shift
participates in the pπ → dπ donation from the vanadyl oxygen of the longer-wavelength feature was observed. The results are
atom, whereas the antibonding dx
orbital depends on the consistent with the formation of H-type aggregates, similarly to
2 – y2
strength of the in-plane ligand field. To minimize the accumula- what has been observed in related salphen systems.[56] Concen-
tion of charge on VIV, the π donation from the axial oxygen tration variations had no effect on these spectral features up to
decreases as the in-plane donor strength of the ligand in- a concentration of 1 × 10–4
M (Figure 8, b). Photoluminescence
creases.[55] It is speculated that the square-pyramidal structure spectra of Zn-16c2pd (Figure 8, a) in noncoordinating solvents
of the complex is subtly perturbed upon solvent coordination, each consisted of a broad band with a maximum at about
with the V=O group moving toward the equatorial plane of the 468 nm. This remained virtually unaltered in THF, though an
ligand, resulting in an enhancement of the ligand–vanadium enhancement in fluorescence intensity, indicating the formation
interaction. Coordination of a solvent molecule in the sixth po- of a 1:1 adduct, could be observed. The emission quantum yield
sition, trans to the axial oxygen, thus decreases the π-donation (EQY) values in noncoordinating solvents (e.g., CH2Cl2, CHCl3,
of the axial oxygen to the metal. As a result the antibonding dπ toluene) are close to 20 %; this is enhanced (EQY = 30 %) upon
orbital is stabilized whereas the dx
orbital is destabilized. deaggregation in THF. Quenching of fluorescence of face-to-
2 – y2
This accounts for the decrease in the molar absorptivity values face-stacked H-type dimer aggregates (sandwich-type dimers)
and the observed shift in THF.
The ZnII complexes, however, exhibited an interesting sol-
vent-induced aggregation phenomenon. The absorption spec-
relative to monomer in the present case, as well as in previous
ones with asymmetrically methyl-substituted ZnII–salen[57] and
analogous ZnII–salphen complexes,[56] may be argued to be a
consequence of rapid energy relaxation of the lower excited
states.[58]
DFT Study
Because diffraction-quality single crystals could not be isolated,
density functional theory (DFT) was used to arrive at the en-
ergy-optimized electronic structures of the complexes. The rep-
resentative ZnII complex Zn-16c2pd was chosen for the pur-
pose. All the quantum chemical calculations were performed
with the GAUSSIAN 09 program package.[59] The ground-state
geometry optimization of the complex in the gas phase was
performed by use of the three-parameter fit of the Becke hybrid
functional combined with the Lee–Yang–Parr correlation func-
tional termed the B3LYP hybrid,[60,61] as well as generalized gra-
dient approximation (GGA) exchange along with the 6-
311+G(d,p), 6-31+G(d,p), 6-31G(d), and 6-31G basis sets[62] for
Zn, N and O, C and H, respectively, without imposition of any
Table 5. Selected interatomic distances [Å] and angles [°] in Zn-16c2pd opti-
mized at the B3LYP level.
Structural parameter
Interatomic distances [Å] and angles [°]
1.912
Zn–O1
Zn–O2
Zn–N1
Zn–N2
1.912
2.039
2.039
104.2
81.0
92.6
92.6
–33.3
30.9
42.6
O1–Zn–O2
N1–Zn–N2
N1–Zn–O1
N2–Zn–O2
N1–N2–O2–O2
N1–O1–O2–N2
molecular length
dipole moment
Figure 8. (a) UV/Visible (left) and fluorescence (right) spectra of Zn-16c2pd in
noncoordinating and coordinating solvents (1 × 10–5
M). (b) Concentration-
1.28 D
dependent UV/Visible absorption spectra of Zn-16c2pd (CH2Cl2, room temp.).
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an Elementar Vario EL III Carlo–Erba 1108 elemental analyzer. The
1H NMR spectra were recorded with a Bruker Avance II 400 MHz
spectrometer in CDCl3 (chemical shift in δ) solution with TMS as
internal standard. Infrared spectra were recorded with a Perkin–
Elmer BX series spectrometer on KBr discs in the 400–4000 cm–1
range. Absorption spectra of the compounds were recorded with
a Perkin–Elmer Lambda 35 spectrophotometer. Photoluminescence
spectra were recorded with a Hitachi F-4600 Fluorescence spectro-
photometer. The fluorescence emission quantum yields (EQYs) in
degassed dichloromethane solutions were determined by the stan-
dard optically dilute method with 9,10-diphenylanthracene (EQY =
0.96, in cyclohexane) as standard. Quantum yields in the solid state
were measured with a 50 mm integrating sphere, in which the solid
sample film was excited by a pulsed Xenon source (400 V, 150 W)
coupled with holographic concave grating monochromators for se-
lecting wavelengths. The resulting luminescence was acquired with
an intensified charge-coupled detector for subsequent analyses.
Optical textures of the different phases of the compounds were
recorded with a Nikon ECLIPSE LV100 POL polarizing microscope
together with Instec hot and cold stage HCS402, with STC200 tem-
perature controller of 0.1 °C accuracy. The thermal behavior of the
compounds was studied with a Pyris-1 system linked to a Perkin–
Elmer differential scanning calorimeter (DSC) with a heating or cool-
ing rate of 5 °C min–1. Variable-temperature powder X-ray diffrac-
tion (XRD) studies were carried out with samples filled in Linde-
mann capillaries. The apparatus essentially involved a high-resolu-
tion X-ray powder diffractometer (PANalytical X'Pert PRO) with a
high-resolution fast detector PIXCEL. Quantum chemical calculation
of a representative ZnII complex was performed by density func-
tional theory (DFT) as implemented in the GAUSSIAN 09 package.
symmetry constraint. The appropriate structure of the complex
was confirmed as an energy minimum by calculation of the
vibrational frequency and confirmation of the absence of any
imaginary frequencies. Key geometric parameters of the opti-
mized Zn-16c2pd complex, estimated by DFT at the B3LYP level,
are gathered in Table 5. Average bond angles about the metal
center suggest a distorted square-planar geometry for the ZnII
complex (Figure 9).
Figure 9. Optimized electronic structure of Zn-16c2pd.
Conclusions
In summary, we have presented syntheses of a new series of
symmetric “salphen”-type ligands and their 1:1 metal com-
plexes with VO2+, Ni2+, Cu2+, and Zn2+ metal ions. Interesting
metal-coordination-induced mesomorphic behavior is demon-
strated by otherwise non-mesomorphic ligands; the role of the
metal ion being crucial. Whereas the CuII complexes decompose
just at the onset of melting, VOIV complexes decompose prior
to melting at higher temperatures. The NiII and ZnII complexes,
however, show columnar oblique and columnar rectangular
phases, respectively, the latter being stable down to room tem-
perature, thus affording room temperature mesomorphism. The
ligands and their ZnII complexes are also luminescent entities
both in solution and in the solid state. The photoluminescence
properties of the ZnII complexes are preserved in the meso-
phase as well. Further, the vacant axial coordination sites in the
square-planar ZnII and the square-pyramidal VOIV centers in the
corresponding complexes facilitate solvatochromism, with the
former also exhibiting aggregate or monomer formation in sol-
vents of different coordinating ability. The CuII and NiII com-
plexes lack such solvatochromism. The metal-coordination-in-
duced mesomorphism, coupled with solvatochromic effects, in
these salphen-ligated systems are of particular relevance to the
design of functional entities with both thermo- and chemo-
sensing properties.
Synthesis and Analysis
Synthesis of 4-n-Alkoxysalicyldehydes (n = 12, 14, 16): These
alkoxysalicylaldehydes were prepared as described previously.[27]
Synthesis of Imine Ligands
N,N-Bis(4-n-alkoxysalicylidene)-4,5-dichloro-1,2-phenylenedi-
amines nc2pd (n = 12, 14 or 16): A solution of 4,5-dichloro-1,2-
phenylenediamine (0.09 g, 0.5 mmol) in ethanol was added to a
ethanolic solution of the corresponding 4-n-alkoxysalicylaldehyde
(1 mmol). The solution mixture was heated under reflux with a few
drops of acetic acid as catalyst for 3 h to yield the yellow Schiff
base. The product was collected by filtration and recrystallized from
absolute ethanol.
N,N-Bis(4-dodecyloxysalicylidene)-4,5-dichloro-1,2-phenylene-
diamine (12c2pd): Yellow solid, yield 0.34 g (85 %). 1H NMR
(400 MHz, CDCl3, Me4Si at 25 °C): δ = 13.17 (s, 2 H, OH), 8.52 (s, 2
3
H, H4), 7.32 (s, 2 H, H5), 7.27 (d, JH,H = 8.0 Hz, 2 H, H1), 6.54 (d,
4
4JH,H = 2.0 Hz, 2 H, H3), 6.51 (dd, 3JH,H = 8.0, JH,H = 2.0 Hz, 2 H, H2),
3
4.00 (t, JH,H = 6.5 Hz, 4 H, OCH2), 1.82 (m, 4 H, CH2), 1.25–1.61 (m,
3
36 H, CH2), 0.90 (t, JH,H = 6.7 Hz, 6 H, CH3) ppm. IR (KBr): ν = 3437
˜
(νOH), 2916 [νas(C–H), CH3], 2850 [νs(C–H), CH3], 1614 (νC=N), 1298
(νC–O) cm–1. C44H62Cl2N2O4 (753.88): calcd. C 70.10, H 8.29, N 3.72;
found C 70.14, H 8.31, N 3.74.
N,N-Bis(4-tetradecyloxysalicylidene)-4,5-dichloro-1,2-phenyl-
enediamine (14c2pd): Yellow solid, yield 0.32 g (81 %). 1H NMR
(400 MHz, CDCl3, Me4Si at 25 °C): δ = 13.18 (s, 2 H, OH), 8.52 (s, 2
Experimental Section
Materials and Physical Measurements: The reagents were pro-
cured from Sigma–Aldrich, USA, and TCI Chemicals, Japan. All sol-
vents were purified and dried by standard procedures. Silica (60–
3
H, H4), 7.31 (s, 2 H, H5), 7.25 (d, JH,H = 8.0 Hz, 2 H, H1), 6.53 (d,
3
4
4JH,H = 2.0 Hz, 2 H, H3), 6.50 (dd, JH,H = 8.0, JH,H = 2.0 Hz, 2 H, H2),
3
120 mesh) from Spectrochem was used for chromatographic sepa- 4.00 (t, JH,H = 6.6 Hz, 4 H, OCH2), 1.80 (m, 4 H, CH2), 1.27–1.57 (m,
3
ration. Silica gel G (E. Merck, India) was used for TLC. Elemental
analyses for carbon, hydrogen, and nitrogen were carried out with
44 H, CH2), 0.89 (t, JH,H = 6.8 Hz, 6 H, CH3) ppm. IR (KBr): ν = 3435
˜
(νOH), 2919 [νas(C–H), CH3], 2849 [νs(C–H), CH3], 1614 (νC=N), 1297
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3
4
(νC–O) cm–1. C48H70Cl2N2O4 (809.99): calcd. C 71.18, H 8.71, N 3.46;
found C 71.21, H 8.74, N 3.50.
(d, JH,H = 8.0 Hz, 2 H, H1), 6.60 (d, JH,H = 2.0 Hz, 2 H, H3), 6.36 (dd,
3JH,H = 8.0, 4JH,H = 2.0 Hz, 2 H, H2), 3.98 (t, 3JH,H = 6.7 Hz, 4 H, OCH2),
1.79 (m, 4 H, CH2), 1.25–1.59 (m, 44 H, CH2), 0.90 (t, JH,H = 6.8 Hz,
3
N,N-Bis(4-hexadecyloxysalicylidene)-4,5-dichloro-1,2-phenyl-
enediamine (16c2pd): Yellow solid, yield 0.38 g (83 %). 1H NMR
(400 MHz, CDCl3, Me4Si at 25 °C): δ = 13.17 (s, 2 H, OH), 8.52 (s, 2
6 H, CH3) ppm. IR (KBr): ν = 2918 [νas(C–H), CH3], 2850 [νs(C–H), CH3],
˜
1612 (νC=N), 1314 (νC–O) cm–1. C48H68Cl2N2NiO4 (866.66): calcd. C
66.52, H 7.91, N 3.23; found C 66.55, H 7.93, N 3.27.
3
H, H4), 7.32 (s, 2 H, H5), 7.27 (d, JH,H = 8.0 Hz, 2 H, H1), 6.54 (d,
3
4
4JH,H = 2.0 Hz, 2 H, H3), 6.50 (dd, JH,H = 8.0, JH,H = 2.0 Hz, 2 H, H2),
Ni-16c2pd: Orange solid, yield 0.07 g (75 %). 1H NMR (400 MHz,
CDCl3, Me4Si at 25 °C): δ = 7.91 (s, 2 H, H4), 7.71 (s, 2 H, H5), 7.19
3
4.00 (t, JH,H = 6.5 Hz, 4 H, OCH2), 1.81 (m, 4 H, CH2), 1.28–1.58 (m,
52 H, CH2), 0.90 (t, JH,H = 6.7 Hz, 6 H, CH3) ppm. IR (KBr): ν = 3436
(νOH), 2916 [νas(C–H), CH3], 2850 [νs(C–H), CH3], 1613 (νC=N), 1299
(νC–O). C52H78Cl2N2O4 (866.09): calcd. C 72.11, H 9.08, N 3.23; found
C 71.12, H 9.11, N 3.25.
3
3
4
(d, JH,H = 8.0 Hz, 2 H, H1), 6.60 (d, JH,H = 2.0 Hz, 2 H, H3), 6.35 (dd,
˜
3JH,H = 8.0, 4JH,H = 2.0 Hz, 2 H, H2), 3.99 (t, 3JH,H = 6.7 Hz, 4 H, OCH2),
3
1.80 (m, 4 H, CH2), 1.28–1.58 (m, 52 H, CH2), 0.90 (t, JH,H = 6.5 Hz,
6 H, CH ) ppm. IR (KBr): ν = 2918 [νas(C–H), CH3], 2850 [νs(C–H), CH3],
˜
3
1611 (νC=N), 1319 (νC–O) cm–1. C52H76Cl2N2NiO4 (922.77): calcd. C
Synthesis of ZnII Complexes Zn-nc2pd (n = 12, 14, 16). General
Procedure: A methanolic solution of Zn(OAc)2·2 H2O (0.02 g,
0.1 mmol) was added slowly to a solution of imine ligand nc2pd
(0.1 mmol) in ethanol, and the mixture was stirred for 3 h at room
temperature. A bright yellow solid formed and was filtered, washed
with diethyl ether, and recrystallized from chloroform/ethanol (1:1).
67.68, H 8.30, N 3.04; found C 67.71, H 8.27, N 3.07.
Synthesis of CuII Complexes Cu-nc2pd (n = 12, 14, 16). General
Procedure: A methanolic solution of Cu(OAc)2·2 H2O (0.02 g,
0.1 mmol) was added to a solution of the ligand nc2pd (0.1 mmol)
in ethanol. This was immediately accompanied by a brown colora-
tion. The mixture was stirred for 3 h at room temperature, and the
brown solid was isolated by filtration, washed with diethyl ether,
and recrystallized from chloroform/ethanol (1:1).
Zn-12c2pd: Yellow solid, yield 0.06 g (82 %). 1H NMR (400 MHz,
CDCl3, Me4Si at 25 °C): δ = 8.49 (s, 2 H, H4), 7.29 (s, 2 H, H5), 7.23
3
4
(d, JH,H = 8.0 Hz, 2 H, H1), 6.52 (d, JH,H = 2.0 Hz, 2 H, H3), 6.47 (dd,
3JH,H = 8.0, 4JH,H = 2.0 Hz, 2 H, H2), 3.97 (t, 3JH,H = 6.5 Hz, 4 H, OCH2),
1.79 (m, 4 H, CH2), 1.25–1.60 (m, 36 H, CH2), 0.88 (t, JH,H = 6.7 Hz,
Cu-12c2pd: Brown solid, yield 0.07 g (86 %). IR (KBr): ν = 2920 [νas(C–H)
CH3], 2849 [νs(C–H)
C44H60Cl2CuN2O4 (815.41): calcd. C 64.81, H 7.42, N 3.44; found C
64.84, H 7.46, N 3.47.
,
.
˜
3
, CH3], 1600 (νC=N), 1314 (νC–O)
cm–1
6 H, CH3) ppm. IR (KBr): ν = 2919 [νas(C–H), CH3], 2848 [νs(C–H), CH3],
˜
1609 (νC=N), 1313 (νC–O) cm–1. C44H60Cl2N2O4Zn (817.25): calcd. C
64.66, H 7.40, N 3.43; found C 64.69, H 7.43, N 3.47.
Cu-14c2pd: Brown solid, yield 0.07 g (82 %). IR (KBr): ν = 2920 [νas(C–H)
CH3], 2849 [νs(C–H)
C48H68Cl2CuN2O4 (871.52): calcd. C 66.15, H 7.86, N 3.21; found C
66.21, H 7.89, N 3.25.
,
.
˜
Zn-14c2pd: Yellow solid, yield 0.07 g (87 %). 1H NMR (400 MHz,
CDCl3, Me4Si at 25 °C): δ = 8.49 (s, 2 H, H4), 7.29 (s, 2 H, H5), 7.23
, CH3], 1600 (νC=N), 1316 (νC–O)
cm–1
3
4
(d, JH,H = 8.0 Hz, 2 H, H1), 6.52 (d, JH,H = 2.0 Hz, 2 H, H3), 6.47 (dd,
3JH,H = 8.0, 4JH,H = 2.0 Hz, 2 H, H2), 3.97 (t, 3JH,H = 6.5 Hz, 4 H, OCH2),
1.79 (m, 4 H, CH2), 1.25–1.60 (m, 44 H, CH2), 0.88 (t, JH,H = 6.7 Hz,
Cu-16c2pd: Brown solid, yield 0.07 g (80 %). IR (KBr): ν = 2917 [νas(C–H)
CH3], 2850 [νs(C–H)
C52H76Cl2CuN2O4 (927.62): calcd. C 67.33, H 8.26, N 3.02; found C
67.38, H 8.31, N 3.05.
,
.
3
˜
, CH3], 1599 (νC=N), 1318 (νC–O)
cm–1
6 H, CH3) ppm. IR (KBr): ν = 2918 [νas(C–H), CH3], 2849 [νs(C–H), CH3],
˜
1608 (νC=N), 1315 (νC–O) cm–1. C48H68Cl2N2O4Zn (873.36): calcd. C
66.01, H 7.85, N 3.21; found C 66.03, H 7.91, N 3.26.
Synthesis of VOIV Complexes VO-nc2pd (n = 12, 14, 16). General
Procedure: VOSO4·H2O (0.02 g, 0.1 mmol) in methanol was added
with stirring to a solution of the ligand nc2pd (0.1 mmol) in ethanol.
The solution acquired a green color. Addition of a few drops of
triethylamine resulted in a green suspension, affording a microcrys-
talline solid on further stirring. The light green product was filtered
off, washed with diethyl ether, and recrystallized from chloroform/
ethanol (1:1).
Zn-16c2pd: Yellow solid, yield 0.08 g (85 %). 1H NMR (400 MHz,
CDCl3, Me4Si at 25 °C): δ = 8.49 (s, 2 H, H4), 7.30 (s, 2 H, H5), 7.25
3
4
(d, JH,H = 8.0 Hz, 2 H, H1), 6.52 (d, JH,H = 2.0 Hz, 2 H, H3), 6.48 (dd,
3JH,H = 8.0, 4JH,H = 2.0 Hz, 2 H, H2), 3.98 (t, 3JH,H = 6.5 Hz, 4 H, OCH2),
3
1.78 (m, 4 H, CH2), 1.25–1.58 (m, 52 H, CH2), 0.88 (t, JH,H = 6.7 Hz,
6 H, CH ) ppm. IR (KBr): ν = 2917 [νas(C–H), CH3], 2850 [νs(C–H), CH3],
˜
3
1608 (νC=N), 1314 (νC–O) cm–1. C52H76Cl2N2O4Zn (929.47): calcd. C
67.20, H 8.24, N 3.01; found C 67.17, H 8.29, N 3.05.
Synthesis of NiII Complexes Ni-nc2pd (n = 12, 14, 16)
VO-12c2pd: Light green solid, yield 0.06 g (75 %). IR (KBr): ν = 2918
˜
[νas(C–H), CH3], 2849 [νs(C–H), CH3], 1608 (νC=N), 1313 (νC–O), 971
(νV=O) cm–1. C44H60Cl2N2O4OV (818.81): calcd. C 64.54, H 7.39, N
3.42; found C 64.57, H 7.43, N 3.45.
General Procedure: The ligand nc2pd (0.1 mmol) was dissolved in
as small a volume as possible of ethanol. Addition of a methanolic
solution of Ni(OAc)2·2 H2O (0.02 g, 0.1 mmol) resulted in an orange
red suspension that turned into a microcrystalline solid on further
stirring. The fine, orange, crystalline precipitate was isolated by fil-
tration, washed with diethyl ether, and recrystallized from chloro-
form/ethanol (1:1).
VO-14c2pd: Light green solid, yield 0.06 g (77 %). IR (KBr): ν = 2919
˜
[νas(C–H), CH3], 2848 [νs(C–H), CH3], 1610 (νC=N), 1314 (νC–O), 971
(νV=O) cm–1. C48H68Cl2N2O4OV (874.91): calcd. C 65.89, H 7.83, N
3.20; found C 65.91, H 7.87, N 3.24.
Ni-12c2pd: Orange solid, yield 0.06 g (80 %). 1H NMR (400 MHz,
CDCl3, Me4Si at 25 °C): δ = 7.92 (s, 2 H, H4), 7.71 (s, 2 H, H5), 7.19
VO-16c2pd: Light green solid, yield 0.06 g (73 %). IR (KBr): ν = 2919
˜
[νas(C–H), CH3], 2850 [νs(C–H), CH3], 1609 (νC=N), 1311 (νC–O), 971
(νV=O) cm–1. C52H76Cl2N2O4OV (931.02): calcd. C 67.08, H 8.23, N
3.01; found C 67.13, H 8.27, N 3.06.
3
4
(d, JH,H = 8.0 Hz, 2 H, H1), 6.58 (d, JH,H = 2.0 Hz, 2 H, H3), 6.37 (dd,
3JH,H = 8.0, 4JH,H = 2.0 Hz, 2 H, H2), 3.99 (t, 3JH,H = 6.6 Hz, 4 H, OCH2),
3
1.80 (m, 4 H, CH2), 1.25–1.60 (m, 36 H, CH2), 0.89 (t, JH,H = 6.7 Hz,
6 H, CH3) ppm. IR (KBr): ν = 2919 [νas(C–H), CH3], 2851 [νs(C–H), CH3],
˜
Acknowledgments
1610 (νC=N), 1314 (νC–O) cm–1. C44H60Cl2N2NiO4 (810.56): calcd. C
65.20, H 7.46, N 3.46; found C 65.23, H 7.43, N 3.47.
The authors are grateful for financial support from the Depart-
ment of Science and Technology (DST)-FIST, Ministry of Science
Ni-14c2pd: Orange solid, yield 0.06 g (78 %). 1H NMR (400 MHz,
CDCl3, Me4Si at 25 °C): δ = 7.90 (s, 2 H, H4), 7.72 (s, 2 H, H5), 7.18 and Technology, New Delhi. S. C. acknowledges DST for an
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Keywords: Metallomesogens · Photoluminescence ·
Aggregation · Solvent effects · Density functional
calculations
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Received: May 3, 2016
Published Online: ■
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Full Paper
Mesomorphic Assemblies
S. Chakraborty, P. Mondal,
S. K. Prasad, D. S. S. Rao,
C. R. Bhattacharjee* ....................... 1–13
Induction
of
Mesomorphism
through Supramolecular Assembly
in Metal Coordination Compounds
of “salphen”-Type Schiff Bases:
Photoluminescence and Solvato-
chromism
The effects of metal coordination (M = fording room temperature mesomor-
VO2+, Ni2+, Cu2+ and Zn2+) on meso- phism. Further, the axial vacant coordi-
morphism and photophysical proper- nation sites in the square-planar ZnII
ties of a new series of salphen ligands and the square-pyramidal VO(IV) cen-
have been investigated. The NiII and ters facilitate solvatochromism, with
ZnII complexes show columnar oblique the former exhibiting formation of ag-
and columnar rectangular phases, re- gregates or monomers in solvents of
spectively, the latter being stable different coordinating ability.
down to room temperature thus af-
DOI: 10.1002/ejic.201600513
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