Novel photoluminescent lanthanidomesogens forming bilayer smectic phase derived from blue light emitting liquid crystalline, one ring O-donor Schiff-base ligands

Article abstract of DOI:10.1016/j.poly.2011.01.015

A series of new mononuclear lanthanide(III)-salicylaldimine complexes of the type [Ln(LH)₃(NO₃)₃] (Ln = La, Pr, Sm and Gd; LH = N-(2-hydroxyethyl)-4n-alkoxysalicylaldimine, n = 14, 18) have been synthesized and characterized by FT-IR, ¹H NMR, ¹³C NMR, UV-Vis, FAB-mass and magnetic susceptibility measurements. The ligand (LH) coordinate to lanthanide ions in zwitterionic form via the phenolic-oxygen with the proton shifted to the imine-nitrogen. The nitrato groups occurring in chelated bidentate fashion complete a nine-coordinate geometry. Polarized optical microscopy (POM) and differential scanning calorimetry (DSC) show that the ligands are monotropic and their complexes exhibit enantiotropic highly viscous smectic A (SmA) mesophase in the temperature range 60-185 °C. A bilayer self organized assembly of the molecules in the mesophase are proposed on the basis of the small angle XRD study. The ligands are blue light emitters with a broad emission maxima at ～447 nm while the lanthanide complexes show intense emission in the visible range (～465-679 nm) at 350 nm excitation. The samarium(III) complex, [Sm(LH)₃(NO₃)₃] is distinct from the rest in emitting bright orange light (～660 nm, Φ = 48%). The S_o-S₁ excitation band being stronger than the direct f-f excitation in the samarium complex clearly suggests that the Schiff-base ligands efficiently sensitize the luminescence of Sm³⁺. DFT calculations have been performed using DMol3 program at BLYP/DNP level to obtain the stable electronic structure of the ligand and complex.

Full text of DOI:10.1016/j.poly.2011.01.015

Polyhedron 30 (2011) 1040–1047
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Novel photoluminescent lanthanidomesogens forming bilayer smectic
phase derived from blue light emitting liquid crystalline, one ring O-donor
Schiff-base ligands
a
b
a
c
Chira R. Bhattacharjee a, , Gobinda Das , Pankaj Goswami , Paritosh Mondal , Subbarao Krishna Prasad ,
⇑
D.S. Shankar Rao c
a Department of chemistry, Assam University, Silchar 788011, Assam, India
b Department of chemistry, Silchar Polytechnic, Silchar 788015, Assam, India
c Centre for Soft Matter Research, Jalahalli, Bangalore 560013, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of new mononuclear lanthanide(III)-salicylaldimine complexes of the type [Ln(LH)3(NO3)3]
(Ln = La, Pr, Sm and Gd; LH = N-(2-hydroxyethyl)-4n-alkoxysalicylaldimine, n = 14, 18) have been synthe-
sized and characterized by FT-IR, 1H NMR, 13C NMR, UV–Vis, FAB-mass and magnetic susceptibility mea-
surements. The ligand (LH) coordinate to lanthanide ions in zwitterionic form via the phenolic-oxygen
with the proton shifted to the imine–nitrogen. The nitrato groups occurring in chelated bidentate fashion
complete a nine-coordinate geometry. Polarized optical microscopy (POM) and differential scanning cal-
orimetry (DSC) show that the ligands are monotropic and their complexes exhibit enantiotropic highly
viscous smectic A (SmA) mesophase in the temperature range 60–185 °C. A bilayer self organized assem-
bly of the molecules in the mesophase are proposed on the basis of the small angle XRD study. The
ligands are blue light emitters with a broad emission maxima at ꢀ447 nm while the lanthanide com-
plexes show intense emission in the visible range (ꢀ465–679 nm) at 350 nm excitation. The samar-
ium(III) complex, [Sm(LH)3(NO3)3] is distinct from the rest in emitting bright orange light (ꢀ660 nm,
Received 27 November 2010
Accepted 5 January 2011
Available online 31 January 2011
Keywords:
Lanthanide
Luminescent
Metallomesogen
XRD
DFT
U
= 48%). The So–S1 excitation band being stronger than the direct f–f excitation in the samarium com-
plex clearly suggests that the Schiff-base ligands efficiently sensitize the luminescence of Sm3+. DFT cal-
culations have been performed using DMol3 program at BLYP/DNP level to obtain the stable electronic
structure of the ligand and complex.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
fluoroimmunoassays, catalysts for the selective cleavage of RNA
and DNA and cancer radiotherapeutic agents [10,11]. This requires
Metallomesogen has continued to be one of the frontier areas of
research for the last few decades [1]. Numerous metallomesogenic
compounds exhibiting novel mesomorphic properties have been re-
ported and their physical properties studied [2]. Compounds that
combine both luminescence and liquid crystallinity fast emerging
as one of the most promising area for the applications in OLEDs,
information storage, sensors, and enhanced contrast displays
[3,4]. The trivalent lanthanide ions are most sought after for the core
of metal-containing liquid crystals because of their interesting opti-
cal and magnetic properties. Due to large magnetic anisotropy,
lanthanide compounds are of interest in the design of magnetic
liquid crystal [5–9]. Further, lanthanide(III) complexes are well
known for their wide range of applications such as contrast agents
for magnetic resonance imagining (MRI), luminescent stains for
delicate and tunable control of the coordination sphere to enhance
specific structural and electronic properties [12,13]. However, to
date rather few examples of lanthanidomesogens have been
described in the literature [14–23]. The first mesomorphic lantha-
nide compounds are the columnar bis-(phthalocyaninato)
lutetium(III) complexes, described by Piechocki et al. [24]. Lantha-
nides have very rich coordination chemistry, exhibiting coordina-
tion numbers 6–12, with 8 or 9 being ubiquitous. Such high
coordination numbers seem to be incompatible with the structural
anisotropy which is prerequisite for liquid crystalline behavior [6].
Wide variety of ligands such as Schiff bases [6,17,25–32], porphy-
rins [33,34], phthalocyanines [24,35–38], b-enaminoketones [39],
1,2, phenanthrolines [40], b-diketonates [41], crown ethers
[42–44] have been employed for the synthesis of new classes of
lanthanidomesogen. The lanthanidomesogen also have enjoyed lot
of interests owing to unusual geometries and novel properties such
as spin crossover, ferroelectricity, photorefractivity, non-linear
⇑
Corresponding author.
E-mail address: crbhattacharjee@rediffmail.com (C.R. Bhattacharjee).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.01.015

C.R. Bhattacharjee et al. / Polyhedron 30 (2011) 1040–1047
1041
optical properties, large magnetic anisotropy and luminescence
2.3. Synthesis and analysis
[24–30,45,46]. Galyametdinov et al. synthesized the first calamitic
lanthanide containing liquid crystal with Schiff base ligands that
contain two aromatic rings in 1991 [47]. These compounds con-
sisted of a central trivalent lanthanide ion surrounded by three
monodentate Schiff’s base ligands. The complexes were shown to
exhibit a highly viscous SmA mesophase. Simpler one-ring Schiff
bases, albeit nonmesogenic, has been complexed to lanthanides to
produce ordered mesophases [14,15,45]. Interestingly there ap-
pears to be no record of one-ring containing Schiff base mesogenic-
ity. Lanthanide metallomesogens containing neutral nonmesogenic
Schiff base ligands have been reviewed [48]. Nitrate is often shown
to be coordinated in a bidentate chelate fashion, allowing the
lanthanide ion to easily achieve high coordination number [8]. Lan-
thanide complexes with salicylaldimine Schiff base, in particular,
have been the subject of intense current research [48–59]. Poten-
tially bidentate though, common binding mode of these type Schiff
base ligands involves zwitterionic monodentate coordination via
deprotonated phenolic-oxygen with the proton shifted to imine–
nitrogen [5–8,56].
2.3.1. Synthesis of n-alkoxysalicyldehyde (n = 14, 18)
Alkoxysalicyldehyde derivatives were prepared following the
general method reported in literature [3]. 2,4-Dihydroxybenzalde-
hyde (10 cm3, 1.38 g), KHCO3 (10 cm3, 1 g), KI (catalytic amount)
and 1-bromotetradecane (10 cm3, 2.5 g) or 1-bromooctadecane
(10 cm3, 3 g) were mixed in 250 cm3 of dry acetone. The mixture
was heated under reflux for 24 h, and then filtered, while hot, to re-
move any insoluble solids. Dilute HCl was added to neutralize the
warm solution, which was then extracted with chloroform
(100 cm3). The combined chloroform extract was concentrated to
give a purple solid. The solid was purified by column chromatogra-
phy using a mixture of chloroform and hexane (v/v, 1/1) as eluent.
Evaporation of the solvents afforded a white solid product.
2.3.2. N-(2-hydroxyethyl)-4-octadecyloxysalicylaldimine (18-ea)
An ethanolic solution of 2-hydroxy-(4-octadecyloxy)-salicylal-
dehyde (0.39 g, 1 mmol) was added to an ethanol solution of
2-amino-1-ethanol (0.06 g, 1 mmol) The solution mixture was
refluxed with a few drops of acetic acid as catalyst for 3 h to yield
the yellow Schiff base N-(2-hydroxyethyl)-4-octadecyloxysalicyl-
aldimine. The compound was collected by filtration and recrystal-
lised from absolute ethanol to obtain a pure compound. Yield:
0.345 g, 75%. Yellow colored solid; m.p. 76 °C. Anal. Calc. for
In this paper, we report synthesis of a new low molecular
weight one ring O-donor Schiff base derived from amino alcohol
and their lanthanide complexes that are both mesogenic and lumi-
nescent. Both the ligand and intense light emitting nine-coordinate
complexes, [Ln(LH)3(NO3)3] (Ln = La, Pr, Sm, and Gd), exhibited
smecticA mesomorphism.
C
27H47NO3 (433.4): C, 74.8; H, 10.9; N, 3.2. Found: C, 73.8; H,
11.3; N, 3.4%. 1H NMR (400 MHz, CDCl3): d 0.89 (t, J = 6.4 Hz, –
CH3, 3H), 1.26–1.84 (m, –CH2 of methylene proton in side chain),
3.61 (t, J = 4.0 Hz, –CH2N@C, 2H), 3.70 (t, J = 4.0 Hz, –CH2OH, 2H),
3.94 (t, J = 8 Hz, –OCH2, 2H), 7.09 (d, J = 8.3 Hz, –C6H4, 1H), 8.23
(s, –N@CH, 1H), 13.94(s,1H, OH). FAB Mass (m/e, fragment): m/z:
2. Experimental section
2.1. Physical measurements
calc. 433; found: 434 [M+H+]; IR (
2925(
2850(
m
max, cmÀ1,KBr): 3458(
as(C–H), CH2), 2873( s(C–H),CH3),
C@N), 1298( C–O).
mOH),
m
as(C–H), CH3), 2921(
m
m
The C, H and N analysis was carried out using PE2400 elemental
analyzer. The 1H NMR were recorded on Bruker DPX-400MHz
spectrometer in CDCl3 (chemical shift in d) solution with TMS as
internal standard. UV–Vis absorption spectra of the compounds
in CH2Cl2 were recorded on a Shimadzu UV-160PC spectrophotom-
eter. Infrared spectra were recorded on a Perkin–Elmer L 120-000A
spectrometer on KBr disk. The mass spectra were recorded on JEOL
SX-102 FAB mass spectrometer. Magnetic susceptibility measure-
ments were made at room temperature on a Cahn–Faraday
balance. The optical textures of the different phase of the com-
pounds were studied using a polarizing microscope (Nikon optip-
hot-2-pol) attached with Instec hot and cold stage HCS302, with
STC200 temperature controller with 0.1 °C accuracies. The thermal
behavior of the compounds were studied using a Perkin–Elmer of
differential scanning calorimeter (DSC) Pyris-1 spectrometer with
a heating or cooling rate 5 °C/min. Variable temperature powder
X-ray diffraction PXRD studies were carried out using an image
plate detector (Mac Science, Japan) equipped with double mirror
focusing optics, with the sample contained in a Lindmann capillary
tube. Photoluminescence spectra were recorded on Shimadzu
RF-53D1 PC. The fluorescence quantum yield was determined by
using quinine bisulfate in 0.1 N H2SO4) as standard. Quantum
chemical calculation on 14-ea and Gd-14-ea was carried out
using density functional theory (DFT) as implemented in DMol3
package.
m
as(C–H), CH2), 1629(
m
m
2.3.3. N-(2-hydroxyethyl)-4-tetradecyloxysalicylaldimine (14-ea)
The Schiff-base (14-ea) was synthesized by similar procedure as
in 18-ea
Yield: 0.308 g (78%) yellow colored solid; m.p. 77 °C. Anal. Calc.
for C23H39NO3 (377.3): C, 73.1; H, 10.4; N 3.71. Found: C, 73.2; H,
10.39; N, 3.6%. 1H NMR (400 MHz, CDCl3): d 0.87 (t, J = 6.4 Hz, –
CH3, 3H), 1.27–1.83 (m, –CH2 of methylene proton in side chain),
3.60 (t, J = 4.1 Hz, –CH2N@C, 2H), 3.71 (t, J = 4.0 Hz, –CH2OH, 2H),
3.91 (t, J = 8.1 Hz, –OCH2, 2H), 7.09 (d, J = 8.6 Hz, –C6H4, 1H), 8.53
(s, 3Â –N@CH, 1H), 13.8 (s, 1H, OH); 13C NMR (75.45 MHz; CDCl3;
Me4Si at 25 °C, ppm) d = 161.3 (–C1), 101.3 (–C2), 161.9 (–C3),
116.6 (–C4), 107.3 (–C5), 132.7 (–C6), 162.1 (–C7), 69.6 (–NCH2),
68.4 (–OCH2) FAB Mass (m/e, fragment): m/z: calc. 377; found:
378 [M+H+]; IR
CH3), 2919( as(C–H), CH2), 2849(
CH2), 1626( C@N), 1287( C–O).
(mmax
,
cmÀ1
,
KBr):3433(
m
OH), 2918(
m
m
as(C–H),
as(C–H),
m
m
ms(C–H), CH3), 2850(
m
2.3.4. Syntheses of the lanthanide (III) complexes [Ln (LH)3(NO3)3],
LH = 14-ea, 18-ea
A solution of Ln(NO3)3Á6H20, [Ln = La(1 mmol, 0.43 g), Pr(1
mmol, 0.43 g), Sm(1 mmol, 0.44 g) and Gd(1 mmol, 0.51 g)] in a
minimum volume of acetonitrile was added dropwise to an etha-
nolic solution of the 4–14-ea (3 mmol, 1.13 g) or 4–18-ea(3 mmol,
1.29 g) at room temperature. The reaction mixture was left to stir
for 3 h. A white cream colored solid formed was filtered and
washed with absolute ethanol and dried in vacuo.
2.2. Materials
All solvents were purified and dried using standard procedures.
The materials were procured from Tokyo Kasei and Lancaster
Chemicals. Silica (60–120 mesh) from Spectrochem was used for
chromatographic separation. Silica gel G (E-Merck, India) was used
for TLC.
2.3.5. [La(LH)3(NO3)3](La-14-ea)
Yield: 1.2 g, (78%) a cream colored solid; m.p. 62 °C Anal. Calc.
for LaC69H114N6O18 (1453.7): C, 56.9; H, 7.9; N, 5.7. Found: C,
56.7; H, 7.5; N, 5.5%. 1H NMR (400 MHz, CDCl3):
d 0.89(t,

1042
C.R. Bhattacharjee et al. / Polyhedron 30 (2011) 1040–1047
J = 6.9 Hz, 9H, 3CH3), 1.25–1.58 (m, –3CH2 of methylene proton in
side chain), 6.64 (d, J = 8.8 Hz, –3C6H4, 3H), 7.85 (s, 3H, 3CH@N),
12.31 (br, s, –3N+H); 3.71 (t, J = 4.0 Hz, –3CH2OH, 6H), FAB Mass
3. Results and discussion
3.1. Synthesis and structural assessment
(m/e, fragment): m/z: calc. 1453.7; found: 1454 [M+H+]; IR (mmax
,
cmÀ1, KBr): 3317 (
1475( 4(NO3)), 1292(
m
OH, –CH2OH), 1241(
m
C–O, ether), 1662(
mC@N),
The Schiff-base ligands, were synthesized by condensation of 4-
alkoxysubstituted aldehyde with a 2-amino-1-ethanol with slight
modification of the literature procedures [56]. The synthetic strat-
egy for the ligand[LH@N-(2-hydroxyethyl)-4-n-alkoxysalicylaldi-
mines), abbreviated as n-ea, where ea = 2-amino-1-ethanol]
mononuclear lanthanide complexes, [Ln(LH)3(NO3)3], (Ln-n-ea);
(Ln = La, Pr, Sm, Gd) are presented in Scheme 1. The compounds
were all characterized by elemental analyses, FAB mass spectrom-
etry, FT-IR, UV–Vis, 13C and 1H NMR studies. Complexes were ac-
cessed from a reaction of appropriate ligand with lanthanide
nitrate in ethanol and acetonitrile mixture in Ln:LH as 1:3 ratio
at ambient temperature and isolated as cream colored solid in
good yields. From the IR-spectra of the lanthanide complexes it
was observed that the C@N stretching frequency at 1626 cmÀ1 is
shifted to higher wave numbers 1655 cmÀ1 indicating the presence
of C@N+ group. A broad band at ca. 3317 cmÀ1 assigned to mN–H is
owing to the zwitterionic (–C@N+–HÁ Á ÁOÀ) form of the ligand. The
broadness of the band suggest that the hydrogen atom on –C@NH+
is engaged in intramolecular H-bonding with the phenolic oxygen.
The weak intensity bands centered at ca. 1228 cmÀ1 is due to the
m(C–O) phenolic stretch. The additional bands observed in the IR
spectra of the complexes at ca. 1475, 1290, 1135, and 855 cmÀ1
m
m
1(NO3)), 1135( 2(NO3)), 854(m
m
6(NO3)).
2.3.6. [Pr(LH)3(NO3)3] (Pr-14-ea)
Yield: 1.18 g, (76%) a cream colored solid; m.p. 65 °C Anal. Calc.
for PrC69H114N6O18 (1455.7): C, 56.9; H, 7.8; N, 5.7. Found: C, 56.4;
H, 7.6; N, 5.6%. FAB Mass (m/e, fragment): m/z: calc. 1455.7; found:
1456 [M+H+]; IR (
O, ether), 1655(
2(NO3)), 850( 6(NO3)).
m
m
max, cmÀ1, KBr): 3314 (
m
OH, –CH2OH), 1240(
mC–
C@N), 1474( 4(NO3)), 1289(m
m
1(NO3)), 113-
(m
m
2.3.7. [Sm(LH)3(NO3)3] (Sm-14-ea)
Yield: 1.17 g, (75%) a cream colored solid; m.p. 67 °C Anal. Calc.
for SmC69H114N6O18 (1468.7): C, 56.5; H, 7.8; N, 5.7. Found: C, 56.2;
H, 7.4; N, 5.5%. FAB Mass (m/e, fragment): m/z: calc. 1468.7; found:
1469 [M+H+]; IR (
O, ether), 1657(
(NO3)), 851( 6(NO3)).
m
max, cmÀ1, KBr): 3312 (
m
OH, –CH2OH), 1238(
mC–
mC@N), 1472( 4(NO3)), 1284(
m
m1(NO3)), 1130(m2-
m
2.3.8. [Gd(LH)3(NO3)3] (Gd-14-ea)
Yield: 1.14 g, (70%) a cream colored solid; m.p. 68 °C Anal. Calc.
for GdC69H114N6O18 (1472.7): C, 56.2; H, 7.8; N, 5.7. Found: C, 56.1;
H, 7.2; N, 5.4%. FAB Mass (m/e, fragment): m/z: calc. 1472.7; found:
can be assigned to m4, m1, m2, and m6 vibrational mode of the coordi-
nated nitrate, respectively. The difference (ca. 185 cmÀ1) between
the m4 and m1 peak positions is typical of bidentate nitrate groups
[7,56,60]. The FAB-mass spectra of the ligand and their Ln(III) com-
plexes are concordant with their formula weights. The 1H NMR
spectrum of the diamagnetic lanthanum(III) complex showed a
broadened imine (CH@N) proton signal at 7.8d compared to 8.5d
in the uncoordinated ligand and conspicuous by the absence of sig-
nal due to phenolic-OH proton. In the ligand, the phenolic-proton
appears at 13.8d. In fact, appearance of a new signal at 12.3d char-
acteristic of –N+H resonance in the La-complex unequivocally sug-
gest the occurrence of the Schiff base ligand in zwitterionic form
with the phenolic-H shifted to imine–N. Notably, the ethanolic-H
which appeared, expectedly, as a triplet at 3.74d in the ligand re-
mained almost unaffected in the complex. The 1H NMR spectral
features corroborates the FT-IR observations (vide infra) and fur-
nishes further evidence for the zwitterionic Schiff base ligand. Thus
with three chelated nitrato groups and an equal number of uniden-
tate O-coordinated Schiff base ligands, a nine coordinate structure
for the lanthanide(III) complex is envisaged. Pertinent here is to
1473 [M+H+]; IR (
O, ether), 1652(
(NO3)), 856( 6(NO3)).
m
max, cmÀ1, KBr): 3316 (
m
OH, –CH2OH), 1234(
mC–
mC@N), 1470( 4(NO3)), 1281(
m
m
1(NO3)), 1132(m2-
m
2.3.9. [La(LH)3(NO3)3](La-18-ea)
Yield: 1.2 g, (75%) a cream colored solid; m.p. 62 °C Anal. Calc.
for LaC81H138N6O18 (1621.9): C, 59.9; H, 8.5; N, 5.1. Found: C,
59.7; H, 8.4; N, 5.5%. 1H NMR (400 MHz, CDCl3):
d 0.88(t,
J = 6.5 Hz, 9H, 3CH3), 1.21–1.54 (m, –3CH2 of methylene proton
in side chain), 6.63 (d, J = 8.7 Hz, –3C6H4, 3H), 7.84 (s, 3H, 3CH@N),
12.32 (br, s, –3N+H); 3.73 (t, J = 4.0 Hz, –3CH2OH, 6H); FAB Mass
(m/e, fragment): m/z: calc. 1621.9; found: 1622 [M+H+]; IR (mmax
,
cmÀ1, KBr): 3314 (
1475( 4(NO3)), 1292(
m
OH, –CH2OH), 1242(
m
C–O, ether), 1659(
mC@N),
m
m
1(NO3)), 1131( 2(NO3)), 851(m
m
6(NO3)).
2.3.10. [Pr(LH)3(NO3)3] (Pr-18-ea)
Yield: 1 g, (78%) a cream colored solid; m.p. 60 °C Anal. Calc. for
PrC81H138N6O18 (1623.9): C, 59.8; H, 8.5; N, 5.1. Found: C, 59.7; H,
8.3; N, 5.4%. FAB Mass (m/e, fragment): m/z: calc. 1623.9; found:
1624 [M+H+]; IR (
C–O, ether), 1654(
1131( 2(NO3)), 853( 6(NO3)).
m
max, cmÀ1, KBr): 3304 (
m
OH, –CH2OH), 1242-
(m
mC@N), 1477(m4(NO3)), 1287( 1(NO3)),
m
m
m
i
H2n+1CnO
CHO
HO
CHO
2.3.11. [Sm(LH)3(NO3)3] (Sm-18-ea)
n=14,18.
Yield: 1.31 g, (76%) a cream colored solid; m.p. 63 °C Anal. Calc.
for SmC81H138N6O18 (1636.9): C, 59.5; H, 8.5; N, 5.1. Found: C, 59.6;
H, 8.4; N, 5.2%. FAB Mass (m/e, fragment): m/z: calc. 1636.9; found:
OH
OH
ii
H2N
OH
1637[M+H+]; IR (
O, ether), 1659(
2(NO3)), 851( 6(NO3)).
m
max, cmÀ1, KBr): 3310 (
m
OH, –CH2OH), 1235(
mC–
m
C@N), 1473( 4(NO3)), 1288(m
m
1(NO3)), 1133-
7
(m
m
N
OH
6
4
iii
2.3.12. [Gd(LH)3(NO3)3] (Gd-18-ea)
[Ln(LH)3(NO3)3]
OH
R.T
5
Yield: 1.29 g, (72%) a cream colored solid; m.p. 60 °C Anal. Calc.
for GdC81H138N6O18 (1640.9): C, 59.5; H, 8.5; N, 5.1. Found: C, 59.6;
H, 8.3; N, 5.0%. FAB Mass (m/e, fragment): m/z: calc. 1640.9; found:
3
1
2
H2n+1CnO
Ln=La, Pr, Sm, Gd.
1641 [M+H+]; IR (
O, ether), 1651(
2(NO3)), 853( 6(NO3)).
m
m
max, cmÀ1, KBr): 3314 (
m
OH, –CH2OH), 1241(
mC–
C@N), 1471( 4(NO3)), 1281(m
m
1(NO3)), 1129-
Scheme 1. i. CnH2n + 1Br, KHCO3, KI, dry acetone,
D, 40 h, and ii. glacial AcOH,
(m
m
absolute EtOH
D
, 4 h iii. M(NO3)3Á6H2O, acetonitrile, stir, 3 h.

C.R. Bhattacharjee et al. / Polyhedron 30 (2011) 1040–1047
1043
mention that salicylaldimine complexes of lanthanides prepared at
a relatively higher temperature (>40 °C) affords [Ln(LH)2LX2] while
those accessed at ambient temperature or below 40 °C have been
found to possess [Ln(LH)3X3] stoichiometry [48]. At higher temper-
ature, the dissociation of the phenolic proton is believed to be en-
hanced [48]. Considering that the complexes in the present case
have been synthesised at ambient temperature (<40 °C) the inter-
pretation of stoichiometry and coordination mode appeared con-
sistent with earlier reports [48,59] on analogous 9-coordinate
lanthanide-complexes involving monobasic, zwitterionic Schiff
base ligands. Schiff base ligands with one aromatic ring are be-
lieved to lack the structural anisotropy to show liquid crystallinity
[8]. At least, two aromatic rings are generally required to stabilize
thermotropic mesophase of rodlike molecules [6–9]. The Schiff
base ligands, under discussion here, are believed to be the first
example of one ring system exhibiting mesogenicity. When com-
plexed to lanthanide ions, rotation of the ligands around Ln–O
bond axis becomes feasible though some restrictions may be im-
posed due to relatively bulkier nitrate groups [8]. The spatial loca-
tion of the phenyl rings in the complex relative to other is not yet
known with certainty. Density functional theory (DFT) study (vide
supra) was carried out to ascertain this aspect alongside the energy
optimized structure of the complex. With two-ring Schiff base li-
gands, composition of the type [Ln(LH)2L(NO3)2] with coordination
number 7 are often formed [8]. The room-temperature magnetic
moments of 3.68, 1.55 and 7.86 BM obtained for the Ln-complexes
(Ln = Pr, Sm, and Gd), respectively, are in accord with van Vleck
values [56].
Fig. 1. Emission spectrum of Sm-14-ea at room temperature in solid state.
caused by electronic density distribution from the molecular orbi-
tal to the ion orbital of Pr3+. The emission spectrum of the Gd(III)-
complex showed a strong blue-light emission (Supplementary
information, S5), with a wavelength maxima of 469 nm upon
photo-excitation at 350 nm. This emission occurs from the 1pp⁄
excited state of the ligand [64].
3.2. Photophysical properties
3.3. Mesomorphic behavior: POM and DSC studies
Lanthanide(III) complexes have no significant absorption in the
visible region. The electronic absorption spectra of n-ea, (n = 14,
18) (Supplementary information, S1), recorded in chloroform
The liquid crystalline behavior of the compounds was studied by
polarizing optical microscope (POM) and differential scanning cal-
orimetry (DSC). The ligands, N-(2-hydroxyethyl)-4-n-alkoxysalicyl-
aldimines are all very viscous and despite repeated scans at slow
heating rates, only broad peaks were observed. These ligands are
not simple calamitic species, the presence of the alcoholic –OH
group tend to induce attractive intermolecular interactions involv-
ing HO. . . HO... resulting in liquid crystalline behavior. The DSC ther-
mogram (Supplementary information, S6), of 14-ea showed a broad
peak at ꢀ77.5 °C in the heating run and two very weak peaks at
ꢀ77.2 and 75 °C in the cooling cycle. The DSC trace Table 2, for
(10À4 M), showed three bands attributed to a p⁄ transition local-
p–
ized on the aromatic rings in the region 254–309 nm. The band at
388 nm is due to the n–p⁄ excitation of the lone pair on the imine–
nitrogen atom to the p⁄ orbital on the C@N fragment.
Photoluminescence study of the ligand, 14-ea and its corre-
sponding complexes Ln-14-ea have been carried out in solid state
at room temperature. The free ligand exhibit a broad emission at
(Supplementary information, S2), under UV irradiation at
350 nm. In case of complexes, the most intense emission was ob-
served from the Sm3+, which glows bright orange under similar
UV irradiation condition. The room temperature emission and exci-
tation spectra of the samarium complex are shown in (Fig. 1),
(Supplementary information, S3). The emission spectrum for
Sm3+ which showed four characteristic emission peaks, originate
the ligand 14-ea at 77.2 °C (D
H = 0.7 kJ molÀ1) is due to I-SmA
phase transition. In POM study, no phase transition could be de-
tected in the heating run with the crystalline sample directly trans-
forming to isotropic phase at 80 °C. Upon cooling the ligand
samples from isotropic liquid, typical batonnets are formed which
coalesce to give rise to a highly birefringent fanlike texture, charac-
teristic of the smectic A phase (Fig. 2) at ꢀ76 °C, which quickly re-
verts to crystalline phase at ꢀ74.8 °C. For 18-ea, the DSC profile
recorded showed high hysteresis and broadening of peaks preclud-
ing any meaningful interpretation. In POM study, however, a rather
similar behavior is observed for 18-ea showing Cr ? I transition at
76.6 °C and I ? SmA at ꢀ73 °C. The DSC profiles for the complex,
[Pr-14-ea], (Fig. 3) showed two transitions in the heating and two
in the cooling cycle. The complex exhibited I–SmA transition at
155.8 °C, with a rather large associated enthalpy of 31.7 kJ molÀ1
Table 1. The complexes, as evident from the DSC data, exhibited a
fairly wide mesophase stability range (ꢀ100 °C). When observed
under microscope, the complexes showed a focal conic textures
(Fig. 4) reminiscent of a SmA mesophase, which tend to decompose
on further heating after reaching the isotropic phase. Like ligands,
the complexes were also found to be highly viscous imposing se-
vere restriction in the molecular mobility. However, on pressing
the sample with a needle on glass cover slips, fluidity could be
4
from the Sm3+ lowest emitting state G5/2 and correspond to tran-
6
6
6
6
sitions to H11/2 (679 nm), H9/2(642 nm), H7/2(601 nm), and H5/
2(564 nm) levels [61,62]. The bright luminescence observed for
the Sm(III) complex is reflective of a good match between the li-
gand centerd triplet state and the Sm3+ emissive states. The ligand
molecules are excited to their singlet state, and through intersys-
tem crossing, transit to their triplet state. The energy of the singlet
and triplet states is intramolecularly transferred to the energy level
of the Sm3+ ion. Luminescence is observed during radiative deacti-
vation to the ground state [61]. The excitation bands, for Sm(III)
complex monitored with orange emission of 601 nm show four
peaks at 357, 398, 467, 495 nm. The intense broad band corre-
sponds to the excitation of the organic chromophore (S0 ? S1)
and other weaker peaks due to intra-configurational f–f transitions
are from the ground state of Sm3+ [62]. The Pr-complex exhibited a
broad weaker emission band (Supplementary information, S4), at
595 nm ascribed to the 1D2 ? 3H4 transition [63]. The 1S0 ? 1I6
(459 nm) transition originates from the charge transfer state,

1044
C.R. Bhattacharjee et al. / Polyhedron 30 (2011) 1040–1047
Fig. 2. Optical texture of the ligand 14-ea at 74 °C.
Fig. 4. Optical texture of the complex Pr-14-ea at 155 °C.
40
30
20
10
of the mesophase decreases along the lanthanide series, which
matches with a recent observation on related lanthanide complexes
[45]. Relatively larger melting enthalpies of the complexes reflect
stronger crystallization tendency [45]. Quite unusual thermal
behavior (pronounced hysteresis) observed for all the complexes
is concordant with earlier observation on related lanthanide com-
plexes [65]. Considering the anti-parallel packing of such nonlinear
molecules, showing strong intermolecular dipole–dipole interac-
tions, this kind of thermal behavior is not unexpected [66].
3.4. X-ray diffraction study of 14-ea and Pr-14-ea
The XRD studies were carried out using an image plate detector
(Mac Science, Japan) equipped with double mirror focusing optics,
with the sample contained in a Lindmann capillary tube. The
diffraction patterns obtained in the SmA phase of the ligand
14-ea at 74 °C (Fig. 5), and its praseodymium complex Pr-14-ea
at 155 °C, are shown in the (Fig. 6) exhibit one sharp reflection in
the low angle region corresponding to d-spacing of 41.7 and
55.3 Å for the ligand and complex respectively. For both the mate-
rials, in the wide angle region, a diffuse intensity maximum
60
80
100
120
140
160
180
Temperature(0C)
Fig. 3. DSC thermogram of Pr-14-ea.
Table 1
Mesophase transition temperatures (T, °C) and associated enthalpies (
D
H, kJ molÀ1) of
n-ea and their complexes Ln-n-ea.
Compounds Heating
Cooling
14-ea
Cr 77.5 (54.7) I
Cr 76.5 I
Cr 62.1 (42.4) SmA 183.7
(48.4) I
I 77.2 (0.7) SmA 75(0.6) Cr
I 73 SmA 72.2 Cr
I 159.3(35.8) SmA 57.6(17.4)
Cr
18-eaa
La-14ea
Pr-14-ea
Sm-14-ea
Gd-14-ea
La-18-ea
Pr-18-ea
Sm-18-ea
Gd-18-ea
Cr 65.7 (41.6) SmA 182.3
(52.5) I
Cr 67.3 (43.2) SmA 180.8
(52.0) I
Cr 68.8 (38.6) SmA 179.7
(47.9) I
Cr 62.0 (44.4) SmA 185.7
(49.3) I
Cr 60.8 (32.5) SmA 170.9
(18.4) I
Cr 63.6 (56.0) SmA 160.1
(27.8) I
Cr 60.8 (32.5) SmA 159.8
(33.6) I
I 155.8 (31.7) SmA 61.9(20.2)
Cr
I 159.5 (37.8) SmA 63.6(20.8)
Cr
I 155.1 (31.2) SmA 31.2(12.3)
Cr
I 155.3 (33.8) SmA 55.6(16.8)
Cr
I 143.5 (24.9) SmA 54.6(15.7)
Cr
I 126.1 (17.0) SmA 42.4(23.4)
Cr
I 147.6 (24.0) SmA 51.7(18.6)
Cr
a
POM data.
observed. No dependence of melting or clearing points on the size
of the lanthanide ions is noticed. The temperature difference be-
tween the melting point of the crystal phase and the clearing point
Fig. 5. X-ray diffractogram of 14-ea at 74 °C (The inset shows the wide angle diffuse
peak on an enlarged scale).

C.R. Bhattacharjee et al. / Polyhedron 30 (2011) 1040–1047
1045
Fig. 6. X-ray diffractogram of Pr-14-ea at 155 °C (The inset shows the wide angle
diffuse peak on an enlarged scale).
centered at ꢀ4.6 Å is observed. This is associated with the short
range liquid-like positional ordering of the hydrocarbon chains
within the layer plane. The low angle diffraction peak is attributed
to the layer ordering of the molecules in the mesophase. Both the
ligand and the complex self-assemble differently in the mesopha-
Fig. 8. Schematic representation of arrangements of molecules in complex.
within the layers; such a value is in perfect agreement with a
SmA phase [66]. Since the complexes bears three chains, the
molecular organization here is different than that of the ligand
and involves two complexes side-by-side, one with two chains
on one side, and the neighboring complex with one chain on the
same side (Fig. 8), and vice versa (A = 2420 Â 2/55.3 ꢁ 90 i.e
30 Å2 per chain) [67].
ses. In the case of ligand, the mean volume (Vm = Mw/NA
NA is the is Avogadro’s number and density
= 1.2 g mLÀ1) is found
to be 630 Å3, the cross-section area A which is equals to two times
q, where
q
the volume divided by the smectic periodicity (d = 1/Nhk(Rld00l).
This gives a value of about 30 Å2 (A = 630 Â 2/41.7 ꢁ 30 Å2)for
one chain suggesting a dimeric head-to-head association (Fig. 7)
3.5. DFT study of 14-ea and Gd-14-ea
As efforts to obtain single crystals of the complexes suitable for
X-ray studies were not fruitful, density functional theory (DFT) cal-
culation is performed to ascertain the electronic structure of a rep-
resentative ligand, 14-ea and its Gd(III) complex (Figs. 9 and 10).
Geometry optimizations and energy calculations were made based
on the generalized gradient approximation (GGA) level using the
most popular, the Becke exchange functional (B88) in conjunction
with the Lee–Yang–Parr correlation functional (BLYP) [68]. The ba-
sis set chosen in this study is DNP [69], which is double-numerical
atomic orbitals augmented by polarization functions, i.e., functions
with angular momentum one higher than that of the highest occu-
pied orbital in the free atom. Relativistic effects were taken into ac-
count with an all-electron scalar relativistic method based on the
Fig. 7. Schematic representation of arrangements of molecules in ligand.
Fig. 9. DFT optimized structure of 14-ea.

1046
C.R. Bhattacharjee et al. / Polyhedron 30 (2011) 1040–1047
Table 2
Selected bond lengths Å and bond angles (°) of
Gd-14-ea complex optimized at the BLYP/DNP
level.
Structure parameter
Gd-14-ea
Gd–O(phenolic)
Gd–O (nitrato)
O(1)–Gd–O(2)
O(2)–Gd–O(3)
O(1)–Gd–O(3)
2.46
2.53
72.9
131.7
148.7
perpendicular to each other with the peripheral alkoxy tail located
as far as possible. The DFT calculation revealed that the length of
the ligand, 14-ea and complex, Gd-14-ea to be 2.7 and 4.0 nm,
respectively. The HOMO and LUMO energies of the ligand 14-ea
is calculated to be À5.39 and À2.27 eV, respectively,
DE = 3.12 eV.
The HOMO (À4.98 eV)–LUMO (À2.76 eV) energy gap for the com-
plex Gd-14-ea was found to be 2.22 eV. Selected geometric param-
eters of Gd-14-ea complex are shown in Table 2.
4. Conclusion
Fig. 10. DFT optimized structure of Gd-14-ea.
First example of one ring containing Schiff base exhibiting mes-
ogenicity as well as photoluminescence is furnished. The lantha-
nide complexes, [Ln(LH)3(NO3)3] also are both liquid crystalline
and photoluminescent. The Schiff base occurring as zwitterions
binding through phenolic-oxygen and nitrate as bidentate ligand
complete nine coordination around the lanthanide ion. The mole-
cules of the complexes self assemble in head-to-head antiparallel
fashion in the mesophase. The Schiff bases ligands are blue light
emitters while the complexes exhibited bright photoluminescence
in the visible range at room temperature. The samarium (III) com-
plex show ligand sensitized bright orange emission at 350 nm exci-
tation with very high quantum yield of ꢀ48%. Pronounced
hysteresis in thermal behavior and high viscosity exhibited by
the lanthanide complexes are correlated with packing of non-
linear molecules with rather large intermolecular dipolar
interactions.
Acknowledgments
The authors thank Department of Science and Technology and
University Grants Commission, New Delhi, for financial support
and Prof. R.C. Deka (Tezpur University) for assistance with the the-
oretical calculations. SAIF,NEHU, Shillong, is acknowledged for
spectral results.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.poly.2011.01.015.
Fig. 11. Enlarged core structure of the complex(alkoxy-tails are omitted for clarity).
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