Unsymmetrical diimine chelation to M(II) (M = Zn, Cd, Pd): Atropisomerism, pi-pi stacking and photoluminescence

Article abstract of DOI:10.1039/c1dt10270b

Three types of atropisomeric unsymmetrical diimine complexes, tetrahedral (L_R)MX₂ (M = Zn, Cd; X = Cl, Br; R = Me, CMe₃, OH, OMe, Cl; 1a-k, type-I), tetrahedral (L_Me2)ZnBr₂ (2, type-II) and square planar (L_OH)PdCl₂ (3, type-III) with different photoluminescence properties, have been reported (L_R = (E)-4-R-N-(pyridine-2-ylmethylene)aniline; = dihedral angle between the diimine unit including the pyridine ring and the phenyl ring planes). In crystals, = 0°for type-I, 90°for type-II and 63°for type-III atropisomers have been confirmed by single crystal X-ray structure determinations of 1c, 1e, 2 and 3H₂O isomers. Optimizations of geometries in methanol have established = 28-32°for type-I, 90.83°for type-II and 43.44°for type-III isomers. In solids, type-I atropisomers with = 0, behave as conjugated 14πe systems facilitating π-π stacking and are brightly luminescent at room temperature while type-II and type-III isomers in solid and type-I isomers in solutions are more like non-conjugated 8πe + 6πe systems and non-emissive. Frozen glasses of acetonitrile, methanol and dichloromethane- toluene mixture at 77 K of type-I isomers are emissive and display structured excitation and emission spectra for R = Me, CMe₃, OMe species. Excitation and emission maxima of frozen glasses (λ_ex = 320-380 nm; λ_em = 440-485 nm) are red shifted in the solid (λ_ex = 390-455 nm; λ_em = 470-550 nm). TD-DFT calculations on 1b, 1d, 1f and stacked (1b)₂ isomers and luminescence lifetime measurements have elucidated that an excited ¹ILCT state has been the origin of emission of the type-I isomers and delocalizations of the photoactive π_diimine and π_diimine^* orbitals of the L_R over the stacked layers shift the λ_ext and λ_em of solids to lower energies than those in frozen glasses. The trends of diimine ligand based electron transfer events of the complexes in DMF have been investigated by cyclic voltammetry at 298 K.

Full text of DOI:10.1039/c1dt10270b

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PAPER
Unsymmetrical diimine chelation to M(II) (M = Zn, Cd, Pd): atropisomerism,
pi–pi stacking and photoluminescence†
Amit Saha Roy,a Pinaki Saha,a Partha Mitra,b Shyam Sundar Maity,c Sanjib Ghoshc and Prasanta Ghosh*a
Received 18th February 2011, Accepted 28th April 2011
DOI: 10.1039/c1dt10270b
Three types of atropisomeric unsymmetrical diimine complexes, tetrahedral (LRf)MX2 (M = Zn, Cd;
X = Cl, Br; R = Me, CMe3, OH, OMe, Cl; 1a–k, type-I), tetrahedral (LMe2f)ZnBr2 (2, type-II) and
square planar (LOHf)PdCl2 (3, type-III) with different photoluminescence properties, have been reported
f
(LR = (E)-4-R-N-(pyridine-2-ylmethylene)aniline; f = dihedral angle between the diimine unit
including the pyridine ring and the phenyl ring planes). In crystals, f = 0◦ for type-I, 90◦ for type-II and
63◦ for type-III atropisomers have been confirmed by single crystal X-ray structure determinations of
1c, 1e, 2 and 3·H2O isomers. Optimizations of geometries in methanol have established f = 28–32◦ for
type-I, 90.83◦ for type-II and 43.44◦ for type-III isomers. In solids, type-I atropisomers with f = 0,
behave as conjugated 14pe systems facilitating p–p stacking and are brightly luminescent at room
temperature while type-II and type-III isomers in solid and type-I isomers in solutions are more like
non-conjugated 8pe + 6pe systems and non-emissive. Frozen glasses of acetonitrile, methanol and
dichloromethane–toluene mixture at 77 K of type-I isomers are emissive and display structured
excitation and emission spectra for R = Me, CMe3, OMe species. Excitation and emission maxima of
frozen glasses (lex = 320–380 nm; lem = 440–485 nm) are red shifted in the solid (lex = 390–455 nm; lem
=
470–550 nm). TD–DFT calculations on 1b, 1d, 1f and stacked (1b)2 isomers and luminescence lifetime
measurements have elucidated that an excited 1ILCT state has been the origin of emission of the type-I
*
f
isomers and delocalizations of the photoactive pdiimine and pdiimine orbitals of the LR over the stacked
layers shift the lext and lem of solids to lower energies than those in frozen glasses. The trends of diimine
ligand based electron transfer events of the complexes in DMF have been investigated by cyclic
voltammetry at 298 K.
complexes, the non-planar tetrahedral and octahedral coordina-
tion spheres generally forbid this possibility.
Introduction
p-Conjugated organic molecules like naphthalene (10pe), an-
thracene (14pe) exhibit remarkable photoluminescence properties
both in solid and solutions.1 These molecules possess rigid
planar structural features. The effect of p–p stacking on the
photoluminescence properties of these aromatics has also been
evidenced in many aspects.2 Although, the electronic effect of p–p
stacking has been under investigation so far,3 it is important to fix
all the molecules in the lattice in a similar orientation, one of the
prerequisites of a luminophore. In the case of vast coordination
The possibility of generating non-rigid (unlike 2,2¢-bipyridine or
1,10-phenanthroline) conjugated luminophores with the support
of a redox innocent metal ion4–7 have been explored here. Recently,
significant features of photoluminescence of the coordinated
unsymmetrical diimine with 14pe have been reported.8 Unsym-
metrical diimines (LRf) with an N–Ar function coordinated to
a redox innocent metal centre as shown in Chart 1, potentially
can have atropisomers due to rotation of the Ar group around
the single N–Ar bond. The isomers are defined by the dihedral
angle, f (f = dihedral angle between the diimine unit including
the pyridine ring and the phenyl ring planes). It is to be noted
that with f = 0, isomers having a maximum energy gap between
the pdiimine (HOMO) and pdiimine* (LUMO) are more like conjugated
14pe systems (p iso-electronic with anthracene molecule), which
display numerous photoactivities.8 These features are absent in
heterocyclic symmetrical rigid diimines, like 2,2¢-bipyridine or
1,10-phenanthroline incorporating 12pe only.
aDepartment of Chemistry, R. K. Mission Residential College, Narendrapur,
Kolkata, 103, India. E-mail: ghoshp_chem@yahoo.co.in; Fax: +91-33-2477-
3597; Tel: +91-33-2428-7017
bDepartment of Inorganic Chemistry, IACS, Kol-32
cDepartment of Chemistry, Presidency College, Kol-73, India
† Electronic supplementary information (ESI) available: Syntheses, ana-
lytical and spectral data of 1b–1k. Optimized coordinates, calculated bond
parameters and isodensity surfaces of HOMOs and LUMOs of 1b, 1f, 2a,
3, 1f+, 1f- and (1b)2 unit. Full crystallographic tables of 1c (CCDC reference
number, 753978), 2 (813732) and 3·H2O (813733). CCDC reference
numbers 753978, 813732 and 813733. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c1dt10270b
In this effort, three types of atropisomeric unsymmetrical di-
imine (LRf) complexes of zinc, cadmium and palladium, i.e., tetra-
hedral (LRf)MX2 (M = Zn, Cd; type-I), tetrahedral (LMe2f)ZnBr2
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scopic grade solvents were used for spectroscopic measurements.
The C, H, N contents of the compounds were obtained from
Perkin–Elmer 2400 series II elemental analyzer. Infrared spectra of
the samples were measured from 4000 to 400 cm-1 as KBr pellets at
room temperature on a Perkin–Elmer FT–IR-Spectrophotometer
1
Spectrum RX 1. H and 13C NMR spectral measurement were
carried out on a ‘Bruker DPX-300¢ MHz spectrometer with
tetramethylsilane (TMS) as an internal reference. ESI mass spectra
were recorded on a micro mass Q-TOF mass spectrometer.
Electronic absorption spectra in solution at 298 K were measured
on a Perkin–Elmer Lambda 25 spectrophotometer in the range
200–1100 nm.
f
Chart 1 Atropisomerism of the coordinated LR ligand.
(type-II) and square planar (LOHf)PdCl2 (type-III) have been
reported (Scheme 1). Photoluminescence of these species depends
on their atropisomerism. It is observed that in solids, type-I
isomers with f = 0◦, act as conjugated 14pe systems assisting
p–p stacking and are brightly luminescent while type-II and
type-III atropisomers with f = 90◦ and 63◦ are non-conjugated
8pe + 6pe systems and are non-luminescent. In this article,
syntheses, structures, spectra of these atropisomers and the effect
of orientation (f) and p–p stacking on photoluminescence have
been reported.
Syntheses
We failed to pre-isolate the ligands in absence of metal ions except
the LOH which has been isolated by the reported procedure.8 Unless
otherwise stated all the compounds have been prepared by the
same procedures. Details of the procedure have been outlined for
the synthesis of 1a and it has been followed to synthesize all other
type-I species.
φ
(LMe )ZnCl2 (1a). In a round bottom flask, 4-methylaniline
(214 mg, 2 mmol) and pyridine-2-carbaldehyde (214 mg, 2 mmol)
were mixed well to form a paste. To this paste anhydrous zinc
chloride (275 mg, 2 mmol) and methanol (25 ml) were added
successively and stirred for a few minutes. The resulting mixture
was heated with stirring at 40 ◦C for another 15 min. Immediately
a yellow crystalline solid separated out. The reaction mixture was
◦
cooled at 20 C, filtered and the yellow residue was dried in air.
Yield: 584 mg (87% with respect to zinc). MS (ESI, positive ion
+
CH3CN): m/z, 297.06, {1a-Cl} ; 1H NMR (DMSO-d6, 300 MHz):
d (ppm) = 8.87 (1 H, d, 6-H), 8.69 (1 H, s, -N CH), 8.11 (2 H, m, 3-
H & 4-H), 7.72 (1 H, t, 5-H), 7.22 (4 H, m, 10-H, 11-H, 13-H & 14-
H) and 2.31(3 H, s, -Me); elemental anal. (%) for C13H12N2ZnCl2:
calcd C 46.96, H 3.64, N 8.42. Found: C 45.02, H 3.40, N 8.31.
IR(KBr)(nmax, cm-1): 1602(vs), 1560(m), 1509(s), 1475(s), 1446(s),
1304(w), 1239(w), 1024(s), 820(vs), 774(vs), 742(w), 648(w) and
532(s). UV-Vis: lmax (e, 104 M-1 cm-1) nm-1, CH3CN, 349(2.77),
246(2.082): CH2Cl2, 360(2.29), 249(1.42).
Syntheses, analytical and spectral data of (LC(Me)3f)ZnCl2
(1b), (LC(Me)3f)ZnBr2 (1c), (LOHf)ZnCl2 (1d), (LOHf)ZnBr2
(1e), (LOMef)ZnCl2 (1f), (LClf)ZnCl2 (1g), (LCH3f)CdCl2 (1h),
(LC(Me)3f)CdCl2 (1i), (LOHf)CdCl2 (1j) and (LOMef)CdCl2 (1k) have
been given as supplementary information.†
φ
(LMe2 )ZnBr2 (2). It was prepared by the same procedure as
1a using pyridine-2-carbaldehyde (214 mg, 2 mmol), 2, 6-xylidine
(242 mg, 2 mmol) and anhydrous ZnBr2 (450 mg, 2 mmol). The
products have been isolated as single crystals those are used for
X-ray diffractions study. Yield: 217 mg (50% with respect to zinc).
+
Mass spectrum (ESI, positive ion, CH3CN): m/z, 355, {2-Br} .
1H NMR (DMSO-d6, 300 MHz): d (ppm) = 8.73 (s, 1H), 8.42 (s,
1H), 8.20 (d, 1H), 8.08(t, 1H), 7.66(s, 1H), 7.11(d, 2H), 7.00(t,
1H), 2.09(s, 7H). Elemental anal. (%) for C14H14N2ZnBr2: calcd
C 38.63, H 3.24, N 6.43; found: C 38.40, H 3.05, N 6.29. IR
(KBr, nmax/cm-1): = 2954(m),1678(m), 1596(s), 1567(s), 1478(vs),
1442(vs), 1356(s),1302(vs), 1273(s), 1181(vs), 1024(vs), 784(vs),
648(m), 627(m), 412(m).
Scheme 1 Type-I, type-II and type-III atropisomers.
Experimental
Materials and physical measurements
Reagents or analytical grade materials were obtained from com-
mercial suppliers and used without further purification. Spectro-
7376 | Dalton Trans., 2011, 40, 7375–7384
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Table 1 Crystallographic data for 1c, 2 and 3·H2O
1c
2
3·H2O
Formula
Fw
C16H18N2Zn Br2
C14 H14 N2 Zn Br2
435.46
Yellow
Monoclinic
P21/c
14.411(2)
8.452(2)
13.411(2)
105.942(5)
1570.7(5)
4
150(2)
1.841
2715
1774
0.71073/6.639
848
0.0924
1.090
0.1257
C12 H12 N2 O2 PdCl2
393.54
Yellow
Orthorhombic
Pbca
10.068(4)
15.311(6)
18.234(8)
90.00
2811(2)
8
293(2)
1.860
2467
1859
0.71073/1.698
1552
0.0344
1.089
0.0540
0.0821
173/0
0.835
463.51
Yellow
Monoclinic
P21/m
9.253(1)
7.621(1)
12.375(1)
97.059(2)◦
866.18(17)
2
293(2)
1.777
1645
1534
0.71073/6.025
456
0.0281
1.133
0.0306
0.0726
167/0
Crystal colour
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
b/◦
˚
V, A3
Z
T/K
Calcd/g cm-3
Uniq. reflections
Ref (I > 2s(I))
-1
˚
l/A/m/mm
F(000)
a
R1 [I > 2s(I))]
GoFb
a
R1 (all data)
c
wR2 (I > 2s(I))
0.2717
175/0
1.584
Param./restr.
-3
˚
Residual density/e A )
0.549
Observation criterion: I > 2s(I).a R1 = RꢀFo| - |Fcꢀ/R|Fo|. b GooF = {R[w(Fo - Fc2)2]/(n - p)}
,
c wR2 = [R[w(Fo - Fc2)2]/R[w(Fo2)2]]1/2 where w =
2
1/2
2
2
1/[s (Fo2)+(aP)2+bP], P = (Fo2+2Fc2)/3.
φ
(LOH )PdCl2·H2O (3·H2O). It was prepared by the same
procedure as 1a using pyridine-2-carbaldehyde (214 mg, 2 mmol),
4-aminophenol (218 mg, 2 mmol) and PdCl2 (354 mg, 2 mmol).
The products have been isolated as single crystals those are used
for X-ray data collections. Yield: 225 mg (60% with respect to
palladium). Mass spectrum (ESI, positive ion, CH3CN): m/z, 375,
Density functional theory (DFT) calculations
All calculations reported in this article were done with the
Gaussian 03W10 programme package supported by GaussView
4.1. The DFT11 and TD–DFT12 calculations have been performed
at the level of Becke three parameter hybrid functional with the
non-local correlation functional of Lee–Yang–Parr (B3LYP)13 The
geometries of 1b, 1d, 1f (LMe2f)ZnBr2 (2a) and 3 in methanol
with CPCM model have been optimized using Pualy’s Direct
Inversion14 in the Iterative Subspace (DIIS), ‘tight’ convergent
SCF procedure15 ignoring symmetry. To analyze the effect of
p–p stacking in solids, geometry of stacked (1b)2 unit has been
optimized. In all calculation, a LANL2DZ basis set16–18 along
with the corresponding effective core potential (ECP) was used
{3} . 1H NMR (DMSO-d6, 300 MHz): d (ppm) = 9.92 (s, 1H), 9.03
+
(d, 1H), 8.63 (s, 1H), 8.36(t, 1H), 8.16(d, 1H), 7.89(t, 1H), 7.29(d,
2H), 6.78(d, 2H). Elemental anal. (%) for C12H12N2O2PdCl2: calcd
C 36.63, H 3.06, N 7.12; found: C 36.18, H 2.85, N 7.22. IR (KBr,
n
max/cm-1): = 3543(s), 3412(s), 3060(s), 1594(s), 1505(s), 1475(m),
1452(m), 1264(m), 1232(s), 839(s), 777(s), 584(m).
X-Ray crystallographic data collections and refinement of the
structures
for zinc metal atom. Valence Triple Zeta with polarization and
19,20
diffuse functional basis set, 6-311++G**
have been used for
C, H, N, O and Cl atoms in all calculations. The percentage
contributions of metal, chloride and ligands to the frontier orbitals
of the optimized geometries have been calculated using GaussSum
programme package.21 The seventy five lowest singlet excitation
energies on the optimized geometries of 1b and 1c in methanol
using CPCM model22 and (1b)2 in gas phase have been calculated
by TD–DFT method.
Yellow crystals of 1c, 1e, 2 and 3·H2O were picked up with
nylon loops and were mounted in the nitrogen cold stream of
the diffractometer. All data were collected on a Bruker SMART
APEX diffractometer, equipped with graphite monochromator
˚
(Mo-Ka, l = 0.71073 A). Final cell constants were obtained
from least squares fits of all measured reflections. Intensity data
were corrected for absorption using intensities of redundant
reflections. The structures were readily solved by direct methods
and subsequent difference Fourier techniques. Crystals of 1e are
weakly diffracting. Similar to 1c, the diimine ligand of 1e is
planar with f = 0 and display p–p stacking interactions. The
crystallographic data of 1e have not been listed in Table 1.
The ShelXL979a software package was used for solution of the
structures. ShelXL979b was used for the refinement. All non-
hydrogen atoms were refined anisotropically. Hydrogen atoms
were placed at the calculated positions and refined as riding atoms
with isotropic displacement parameters.
Recording of excitation and emission spectra and lifetimes
measurements
Emission spectra at 77 K were recorded in MeOH, MeCN and
CH2Cl2–toluene glasses using quartz sample tube on Perkin–
Elmer LS 55 luminescence spectrophotometer equipped with a
Perkin–Elmer low-temperature luminescence accessory. Emission
spectra of the solid sample at 298 K were also recorded on the
same luminescence spectrometer, equipped with a Perkin–Elmer
solid-state luminescence accessory. Data of the solid samples
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◦
˚
Table 2 Experimental bond distances (A) and angles ( ) of 1c, 2 and
3·H2O
were collected by using 2% sample of KBr pellet with the
emission slit 3.0 & 3.3 to have minimum scattering effect. Singlet
state lifetime was measured by Time Master Fluorimeter from
Photon Technology International (PTI). The system measures the
luminescence lifetime using PTI’s patented strobe technique and
gated detection. The software Felix 32 controls all acquisition
modes and data analysis of the Time Master system. The sample
was excited using a thyratron gated nitrogen flash lamp (full width
at half-maximum 1.2 ns) that is capable of measuring luminescence
time-resolved acquisition at a flash rate of 25 kHz as well as with
Nano LED (295 nm, full width at half-maximum 1.0 ns). Lamp
profiles were measured at the respective excitation wavelengths,
namely, 337 and 356 nm using Ludox as the scatterer. Decays at
77 K were made using a Dewar system having a 5 mm o.d. quartz
tube.
Isomers
1c
2
3·H2O
M = Zn, X = Br M = Zn, X = Br M = Pd, X = Cl
M(1)–N(1)
M(1)–N(8)
M(1)–X(1)
M(1)–X(2)
N(1)– C(2)
N(8)–C(7)
C(2)–C(7)
2.040(3)
2.082(3)
2.323(1)
2.323(1)
1.341(5)
1.273(5)
1.462(5)
2.049(9)
2.087(9)
2.345(2)
2.329(2)
1.347(13)
1.285(15)
1.468(15)
81.1(4)
110.6(3)
107.1(3)
118.03(7)
90
2.026(4)
2.028(4)
2.294(2)
2.289(2)
1.364(6)
1.286(6)
1.450(6)
81.14(15)
175.38(11)
175.51(11)
89.97(5)
63
N(1)–M(1)–N(8) 80.58(13)
N(1)–M(1)–Cl(2) 112.61(3)
N(8)–M(1)–Cl(1) 112.53(3)
Cl(1)–M(1)–Cl(2) 112.53(3)
f
0
Results and discussion
Syntheses and characterization
Three types of atropisomeric unsymmetrical diimine (LRf) com-
plexes of zinc(II), cadmiun(II) and palladium(II) isolated in this
work have been listed in Scheme 1. Different isomers of these com-
plexes are defined by f. Thus, tetrahedral (LRf)MIIX2, tetrahedral
(LMe2f)ZnIIBr2 and square planar (LOHf)PdIICl2 complexes having
different orientations of the ligands with different f are respectively
abbreviated as type-I, type-II and type-III atropisomers. No
ligands have been successfully isolated except the LOH ligand.8
Most of the complexes have been prepared in high yields by
stoichiometric reactions of pyridine 2-carbaldeyde, corresponding
amines and metal dihalide. 3 has been isolated as 3·H2O.
Fig. 1 Molecular geometry of 1c in crystals (f = 0◦).
The nC stretching frequency of the coordinated species span
N
the range 1590–1600 cm-1. The nO–H stretching frequency of 1d, 1e,
1j and 3·H2O appear respectively at 3379, 3410, 3285 and 3411,
1
3543 (OH2) cm-1. H NMR data of all the species in DMSO-d6
solvent including 13C NMR spectra of 1d, 1f, 1j and 1k have been
assigned and are listed in the experimental section.
Fig. 2 p–p stacking in the lattice of 1c (Br atoms have been omitted).
similar to that of 1c with j = 0◦ facilitating p–p stacking as shown
in Fig. 2.
Molecular geometries
Solid state geometries.
Type-II atropisomers. Single crystal X-ray structure determina-
tion of 2 has confirmed the molecular geometry of the type-II
isomer. 2 crystallizes in the P21/c, space group. In contrast to 1c
and 1e, the substituted phenyl ring with two ortho-methyl groups
in type II isomer is orthogonal to the diimine moiety, i.e., f = 90◦.
The tetrahedral coordination sphere of 2 in crystals with the atom
labelling scheme is shown in Fig. 3. The relevant bond parameters
are listed in Table 2. In the orthogonal orientation of the pendent
phenyl ring (f = 90◦), the diimine ligand in 2 behaves more like a
non-conjugated 8p + 6pe systems and does not display any p–p
stacking interactions as shown by type-I isomers in crystals.
Type-III atropisomers. To have a different orientation of the
diimine fragment, a square planar coordination sphere with the
palladium(II) ion has been enforced in type-III complexes. Single
crystal X-ray structure determination has confirmed the molecular
orientation in 3·H2O. It crystallizes in Pbca space group. The
molecular geometry of 3·H2O in crystals with the atom labelling
scheme is shown in Fig. 4. The significant bond parameters are
summarized in Table 2. Because of the square planar coordination
Type-I atropisomers. Geometrical features of type-I isomers
have been confirmed by single crystal X-ray structure determi-
nations of 1c and 1e. 1c crystallizes in centrosymmetric space
group, P21/m. The molecular structure with the atom labelling
scheme is depicted in Fig. 1. The relevant bond parameters
are summarized in Table 2. Bond parameters have revealed the
tetrahedral coordination sphere around the metal centre. The
substituted phenyl ring in 1c is coplanar with the ligand with f = 0◦.
It displays 14pe–14pe stacking orienting two adjacent molecules
and the metal chelates trans to each other but perfectly overlaying
two imine functions as shown in Fig. 2. The distance between
two imine carbon centres of the two adjacent molecules, 3.88 A,
is minimum. The distances between the centres of two overlaying
phenyl rings are around 3.95 A. Unfortunately, single crystals of
1e with the space group P1 containing eight molecules in the unit
cell, are weakly diffracting and the final refinement ended with
Rw = 22.01. The crystal data and bond parameters have not been
added in this article. The gross molecular geometry of 1e is very
˚
˚
¯
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Table 3 UV-Vis spectral data of 1a–1k, 2 & 3·H2O in MeOH
Compound
lmax (nm) (e (104 M-1 cm-1))
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
2
333(0.71), 278(1.13), 241(1.14)
339(1.66), 288(1.45), 241(1.96)
333(0.62), 279(1.00), 240(1.05)
360(1.48), 284(9.94), 249(1.44)
353(1.02), 285(0.71), 246(0.98)
353(1.75), 285(1.25), 239(1.89)
330(0.53), 284(0.78), 249(1.36)
333(1.81), 288(1.63), 241(2.15)
337(2.17), 288(1.87), 241(2.53)
360(1.23), 286(0.74), 249(1.13)
352(1.17), 285(0.85), 246(1.21)
381(0.43), 262 (0.6), 221(1.59)
382 (0.22), 261(0.44), 221(1.19)
Fig. 3 Molecular geometry of 2 in crystals (f = 90◦).
3.H2O
In both gas phase and methanol solvent, the dihedral angle, f,
of the type-I isomers span the range 28–32◦. Thus, p–p stacking
enforces the planarity of the diimine ligand in type-I isomers
in solid state that results versatile photoactivities (see below).
The geometries of 2a and 3 in methanol are similar to those
found in crystal lattice of 2 and 3·H2O respectively with f = 90◦
and 43◦.
Fig. 4 Molecular geometry of 3·H2O in crystals (f = 63◦; H2O has been
omitted for clarity).
sphere, the plane of the pendent aryl group in 3·H2O is rotated by
63◦ with respect to the diimine fragment including the pyridine
ring, i.e., f = 63◦ to minimize the non-bonding interactions.
In this orientation, the diimine ligand similar to that in type-II
atropisomer, behaves more like a non-conjugated 8p + 6pe system
and does not display any p–p stacking interactions as observed in
type-I isomers.
Electronic spectra
The electronic spectra of 1a–1k (type-I atropisomers) in methanol,
acetonitrile and dichloromethane solvents are similar and well
structured displaying three strong absorption bands at 360–325,
288–278 and 250–235 nm ranges. It is found that the lowest energy
absorption band at around 330–360 nm is red shifted with the
decrease of the polarity of the solvent.
Gas phase and solution geometries
The electronic spectra of 2 (type-II atropisomer) and 3·H2O
(type-III atropisomer) have been recorded in methanol. Repre-
sentative spectra of type-I family (1c and 1e), 2 and 3·H2O for
comparison are shown in Fig. 6 and data of MeOH solutions have
been summarized in Table 3. Data in other solvents have been
listed under experimental section of the respective compounds.
The absorption features of 2 and 3·H2O are similar. The absorption
spectra of type-I complexes interestingly are different from type-II
and type-III isomers. The red shifted low energy absorption peaks
of type-II and type-III atropisomers are comparatively weaker and
broad.
Gas phase and solution geometries of type-I, type-II and type-III
isomers have been investigated by geometry optimizations of 1b,
1d, 1f, (LMe2f)ZnIICl2 (2a) and 3 at the restricted B3LYP/DFT level
with the singlet spin state without any symmetry constraint. Effect
of p–p stacking interactions in solid state have been calibrated
by geometry optimization of stacked (1b)2 unit in gas phase.
Optimized geometries in methanol solvent are shown in Fig. 5.
The bond parameters of the optimized structures in gas phase
and solutions are very similar with those obtained from single
crystal X-ray structure determinations and are listed in supporting
information.†
Fig. 5 Optimized geometries of 1b, 1f, 2a and 3 in methanol solvent
respectively with f = 29.58, 27.53, 90.83, 43.44◦.
Fig. 6 Electronic spectra of 1c (green), 1e (blue), 2 (red) and 3·H2O (black)
in MeOH.
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Table 4 Calculated excitation energies (nm), oscillator strength (f ) and
electronic transitions of 1b in MeOH and (1b)2 in gas phase
Table 5 Excitation (lex) and emission maxima (lem) of (LRf)ZnCl2 species
in solid state at 298 K and in frozen glasses at 77 K
l/nm (f )
Exp. l (nm) Significant contributions Transition type
Frozen Glasses at 77 K
Solid (RT)
MeOH
MeCN
lem lex
CH2Cl2
lem lex
1b
Compound lext
lem
lex
lem
*
382 (0.601)
290 (0.601)
367
288
H→L (88%)
pdiimine→p
ppyridine→p
diimine
*
H-4→L (46%)
H-3→L (40%)
diimine
1a
1b
1c
1d
1e
1f
415
405
390
453
440
448
400
486
479
470
546
547
532
474
350
326
325
375
375
375
320
442 390
450 367
457 367
485 390
466 380
466 380
460
446 370
426 370
424 370
465 370
478 388
478 388
430
430
430
430
475
475
*
pCl→p
diimine
(1b)2
*
*
395.2 (0.065) 405
H-3 → L + 1 (25%)
H-2 → L (31%)
H → L + 1 (41%)
H-5 → L (31%)
H-4 → L + 1 (31%)
H-5 → L (19%)
H-4 → L + 1 (17%)
H-2 → L (13%)
H-1 → L (18%)
H → L + 1 (19%)
H-3 → L + 1 (54%)
H-2→ L (39%)
pCl →p
pCl →p
diimine
diimine
*
pdiimine→p
diimine
*
383.0 (0.145)
375.7 (0.418)
pCl →p
diimine
1g
*
*
*
*
pCl →p
pCl →p
pCl →p
pCl →p
diimine
diimine
diimine
been optimized in gas-phase. Significant transitions in (1b)2 have
been summarized in Table 4 which shows the number of low
energy transitions in (1b)2 are more than the isolated 1b molecule.
Molecular orbitals analyses have shown that photoactive pdiimine
diimine
*
pdiimine→p
pdiimine→p
diimine
*
diimine
*
364.1 (0.057)
pCl →p
diimine
diimine
diimine
diimine
diimine
diimine
diimine
*
*
*
*
*
*
pCl →p
pCl →p
pCl →p
pCl →p
pCl →p
pCl →p
*
and pdiimine orbitals in (1b)2 unit are delocalized over the two
351.9 (0.038)
342.7(0.084)
H-6 → L (54%)
H-10 → L (42%)
H-9 → L (26%)
H-13 → L +1 (41%)
H-12 → L (54%)
stacked layers as HOMO and HOMO-1 (OPMOs) and LUMO
and LUMO+1 (UPMOs) as given in Fig. 7. Similarly pCl MOs are
delocalized over the two layers. This is the effect of p–p stacking
of type-I isomers in crystals shifting all the excitations to lower
energies. In the stacked lattice the contributions of halides to the
OPMOs and UPMOs are greater. The calculated lowest excitation
energy (lext) of the 1b molecule in methanol is 382 nm that shifts
to 395 nm in the stacked (1b)2 unit considering delocalization only
over two layers. The experimental value (for multilayers stacking
with f = 0) of lext is 405 nm (Table 5). Similar extent of red shifts in
solids have been observed in other members of the type-I isomers
(Table 5). The red shift of the excitation energies in the stacked
molecule compared to those in the isolated molecule have also
been evidenced in other cases too.2 The work disclosed that p–p
stacking results in the delocalization of the photoactive pdiimine and
302.4(0.042)
H = HOMO, L = LUMO
Occupied photoactive molecular orbitals (OPMOs) and
unoccupied photoactive molecular orbitals (UPMOs) and excited
states
The origins of the absorptions of these isomers in solutions and
crystals have been elucidated by TD–DFT calculations on the
optimized geometries of 1b, 1d and 1f in methanol and (1b)2
in the gas phase. The calculated excitation energies, oscillator
strengths and transition probabilities in 1b and (1b)2 are given
in Table 4. The calculations show that in all cases ligand based
LUMO and LUMO+1 are the most significant UPMOs. For
1b and (1b)2, ligand based HOMO, HOMO-4, HOMO-6 and
HOMO-7 are responsible for the electronic transitions (with an
oscillator strength of more than 0.035), absorbing strongly in the
UV region, and are defined as OPMOs.
pdiimine orbitals of the (LRf) ligands over the stacked layers and
lowers the excitation and emission energies.
*
*
Analyses have shown that in solution, the LUMO is the pdiimine
orbital of the ligand while the HOMO is the pdiimine orbital. HOMO-
1 is a pphenyl orbital of the ligand and HOMO-4, HOMO-6 and
HOMO-7 are composed of ppyridine orbitals of the ligand. All the
absorptions in the UV region of these species have been assigned as
the intra ligand charge transfer (ILCT) transitions. Similar features
have been established for 1d. The lowest energy excitation of these
complexes in solutions is purely due to the HOMO (pdiimine) to
LUMO (pdiimine*) transition with the probability more than 85%.
Fig. 7 Delocalizations of photoactive pdiimine (HOMO and HOMO-1) and
pdiimine (LUMO and LUMO+1) over two stacked layers of the (1b)2 unit.
*
*
p–p Stacking and delocalizations of photoactive pdiimine and pdiimine
orbitals over the stacked layers
Emission spectra
Although, the ILCT transitions as the origin of absorptions of the
type-I species in solutions have been established, in solids both
the excitation and emission wave lengths (Table 5) are red shifted
compared to those in frozen glasses. To analyze these red shift of
excitation or emission energies in solids, TD–DFT calculations
have also been carried out on the stacked (1b)2 unit that has
Type-I isomers. Single crystal X-ray structure determinations
have confirmed that in crystals, the 14pe diimine ligands in
type-I isomers are planar and exhibit p–p stacking (Fig. 2).
In this orientation, the ligands are like conjugated aromatics
with 14pe (p isoelectronic with the anthracene molecule) and
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likewise, are brightly emissive in solid state. In contrast, due to
the orthogonal orientation of the substituted phenyl ring with the
diimine fragment including the pyridine ring, type-II and type-III
atropisomers are more like non-conjugated 8pe + 6pe systems and
are non-emissive.
excitation and emission spectra of 1a in dichloromethane and in
acetonitrile solvents at 77 K have been depicted in Fig. 9. These
species either in solid state or in methanol glasses do not display
such structures of the excitation or emission spectra. The presence
of different possible atropisomers in such less polar solvents may
be one of the reasons of such structured emission spectra.
The emission spectra of type-I isomers in solid state have been
recorded at room temperature using KBr pellet of 2% sample with
the emission slit 3.0 to minimize the scattering signals. Because
of different orientations, the compounds are non-emissive in
solution at room temperature but the frozen glasses in methanol,
acetonitrile and dichloromethane–toluene mixture at 77 K are
emissive. All the experimental data of the zinc species are listed
in Table 5 which shows the effect of R on both the excitation and
emission maxima. The effect is more pronounced in solids than in
frozen glasses at 77 K. By changing R, the emission maxima of
solids have been tuned in a wide range of 470–550 nm. The free
ligand LOH is not emissive in solid or solution even at 77 K.
The electronic effect of the substituent (R) on the emission
profile of the isomers is reflected both in the solid and in solution.
It is noted that solids of (LRf)ZnCl2 species, 1a–1c and 1g emit
blue light (lem = 470–486 nm) upon excitation with 390–415 nm
(Table 5) while the species, 1d–f, with R = OH and OMe when
excited at 440–453 nm emit only green light at 532–546 nm. It is
observed that the excitation maxima of the solids are red shifted
compared to the absorption maxima in solutions. The effect of
R on the emission maxima of the type-I species in solid at room
temperature and in acetonitrile at 77 K has been shown in Fig. 8(a).
The higher is the electron donating effect of R, the higher is the
extent of red shift of both lext and lem maxima.
Fig. 9 Excitation (green) and emission spectra (red) of 1a in (a) CH2Cl2
and (b) CH3CN at 77 K.
Fig. 10 (a) Emission spectra of 1a and (b) 1f in solid (red) and frozen
MeOH glass (blue).
In contrast to solids, frozen solutions of all these species in
methanol, acetonitrile and dichloromethane irrespective of R, M
and X emit only blue light as given in Table 5. The shifting
of lem with different R is also observed but the effect is less
pronounced with respect to those in solid. The excitation maxima
span the range 325–378 nm in methanol while it is 370–390 nm
in dichloromethane and acetonitrile. Compared to solids, the
emission maxima of frozen glasses appear in the narrower range
of 440–485 nm only (Fig. 8(b)). Two important features of the
excitation and emission profile of the compounds in glasses have
been noted. One is the blue shift of both the lext and lem maxima of
all species in solution particularly in methanol and another is the
structure of the excitation and emission spectra of the compounds
in dichloromethane and acetonitrile frozen glasses. Comparative
spectra of solids and frozen glasses of 1a and 1f have been shown
in Fig. 10. Both the excitation and emission wavelengths are blue
shifted at least by 50–80 nm compared to the frozen glasses. The
To investigate the effect of metal ions (M) on the emission
properties, we have isolated cadmium analogues also, (LRf)CdCl2.
The emission spectra of the species, for comparison, have been
recorded in solid state at room temperature and in frozen methanol
glass at 77 K. The data have been summarized in Table 6 and the
representative spectra have been illustrated in Fig. 11. In these
species too, the electronic effect of R tunes the excitation and
emission maxima of the solids in a wide range like (LRf)ZnCl2
series. In solid states, (LRf)CdCl2 species, 1h with R = Me on
excitation at 340 nm emit blue light at 476 nm. Similar to zinc,
cadmium analogues 1j and 1k, with R = OH, OMe, emit only
green light at 532 and 520 nm. In contrast to zinc, the cadmium
species, 1i, with R = C(Me)3, emit green light at 544 nm.
The tuning of both lext and lem parameters in solids is notable
with the metal ions. It has been found that only zinc luminophores
with R = Me, C(Me)3, Cl emit blue light and all other species with
zinc and cadmium mainly emit green light.
Fig. 8 (a) Effect of R on emission maxima of solids (left) at 298 K and (b) frozen CH3CN glasses (right) at 77 K of (LRf)ZnCl2 species.
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Table 6 Excitation and emission maxima (nm) of (LRf)CdCl2 isomers
Table 7 Luminescence lifetimes of (LRf)MCl2 species in solid state at
77 K
Solid (RT)
Frozen glasses (MeOH) (77 K)
2
Lifetime (ns)
c
tav
Compound
lext
lem
lex
lem
Compound
t1, (%), t2, (%)
1h
1i
340
440
448
420
476
544
532
520
321
325
380
370
464
450
480
463
1a
1b
1c
1e
1f
t1 = 4.08(20.4), t2 = 0.42(79.6)
t1 = 4.18(37.7), t2 = 1.10(62.3)
t1 = 3.52(21.1), t2 = 1.06(78.9)
t1 = 4.05(65.0), t2 = 0.94(35.0)
t1 = 3.94(66.1), t2 = 0.79(33.9)
t1 = 3.95(69.8), t2 = 1.05(30.2)
1.18
0.90
1.16
1.29
1.40
1.31
1.67
2.26
1.58
2.96
2.87
3.07
1j
1k
1k
Table 8 Luminescence lifetimes of (LRf)ZnCl2 species in frozen MeOH
glasses at 77 K
2
Lifetime (ns)
c
tav
Compound
t1, (%), t2, (%)
1a
1b
1c
1e
1f
1j
t1 = 21.7(25.1), t2 = 11.8(74.9)
t1 = 17.4(73.3), t2 = 3.10(26.7)
t1 = 2.53(25.4), t2 = 0.48(74.6)
t1 = 3.87(25.4), t2 = 0.88(97.5)
t1 = 12.76(80), t2 = 3.78(20)
t1 = 8.70(46.0), t2 = 0.93(54.0)
1.13
0.96
1.11
1.05
1.44
1.08
14.29
13.58
1.00
0.95
10.97
4.50
Fig. 11 (a) Excitation (black), emission (red) spectra of 1j and (b)
comparison of emission spectra of 1d (purple) and 1j (black) in solid
state.
As the atomic number of the metal ions increases, the excitation
wave length of the species (LRf)MX2 with the same substituent
decreases. As for example, lext for (LOMef)MCl2 species are 450 nm
for M = Zn (1f) and 420 nm for M = Cd (1k), to emit at 538, and
520 respectively as given in Table 5 and Table 6. A similar trend has
also been observed for the species with R = OH. Emission maxima
are blue shifted with the increase of atomic number of the metal
with the same substituent on the diimine ligand. A comparison
of the emission spectra of (LOHf)ZnCl2 (1d) and (LOHf)CdCl2
(1j) is shown in Fig. 11(b). There is no significant effect of the
halogen (X) on either the excitation and emission maxima of the
photoluminescence spectra of the species in solid as well as in
solution except the lower intensity of the bromo analogues.
sponding bromo analogues. In bromo compounds, the enhanced
spin orbit coupling increases the singlet–triplet non-radiative rate.
Measurement of luminescence lifetime (t/ns, 3.95 and 1.05) of one
of the cadmium species, (LOMef)CdCl2 (1k), in solid has affirmed
the similar features of the emission profile of the cadmium and
zinc chromophores.
Electron transfer events (ETEs)
The electron transfer events (ETEs) of the luminescent type-I,
non-luminescent type-II and type-III complexes have been studied
at 298 K by cyclic voltammetry in DMF solutions containing
0.2 M tetra-butyl ammonium hexafluorophosphate as supporting
electrolyte. The cyclic voltammogram of 1f is shown in panel (a)
of Fig. 12 and the reduction potentials data referenced vs. ferroce-
nium/ferrocene, Fc+/Fc, couple have been summarized in Table 9.
In all cases, the experiments were performed with the multiple scan
rates to analyse the reversibility of the electron transfer waves.
It is observed that the brightly luminescent complexes display
two distinct electron transfer waves both of which are irreversible
expectedly in all scan rates. As the metal ions in all cases are redox
innocent, these electron transfer events have been respectively
assigned to the [(LRf)MCl2]+/[(LRf)MCl2] (i.e., formation of
coordinated diimine cation radical) and [(LRf)MCl2]/[(LRf)MCl2]-
(i.e., formation of the coordinated diimine anion radical) reduction
couples. The spin density distributions of one electron oxidized
cation radical (1f+) and one electron reduced anion radical (1f-)
elucidated by unrestricted DFT calculations at the B3LYP level
of the theory are illustrated in panels (b) and (c) of Fig. 12. Both
the electron transfers are ligand centered and the instabilities of
these states make these electron transfer events irreversible in the
experimental time scale and non-isolable restricting to correlate
these data with luminescence parameters in solutions. However,
Type-II and type-III isomers
In the solid with f = 90◦, the diimine ligand of type-II isomer
with 8pe + 6pe types of electron distributions failed to have
p delocalization and 2 is non-emissive. But it is very weakly
luminescent (lex = 385 nm; lem = 432 nm) in methanol solution at
room temperature, maybe due to restricted rotation. In contrast,
because of atropisomeric free rotation, type-I isomers in solutions
at room temperature are non emissive. Type-III isomer (3·H2O)
with f = 63◦, is non-emissive in solid, solution even in frozen glasses
at 77 K. The emission feature is totally quenched by palladium(II)
in the isomer 3·H2O.
Luminescence lifetimes
Two exponential models have been used to analyze the decay
of the luminescence intensity both in solid and frozen glasses at
77 K. Luminescence lifetimes of the solids of (LRf)ZnCl2 at 77 K
have been given in Table 7 and those of in frozen methanol glass
at 77 K have been listed in Table 8. The data of the frozen glasses
correlate well with those of in solids.
Multi-component lifetimes of these species are expected for
variable orientations of the chromophore. Lifetimes of the chloro
species irrespective of the metal centre are higher than the corre-
differences of peak potentials give an approximate estimate of
*
differences of energies of the HOMO (pdiimine) and LUMO (pdiimine
)
in DMF solutions (Table 9), which differ significantly from those
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with f = 0◦ and behave like conjugated aromatics with a 14pe
system, p iso-electronic with anthracene and likewise strongly
fluorescent in solid state. Both excitation and emission maxima
of the luminescence are affected by the substituent R. In solution,
in the absence of p–p stacking and planarity of the ligand, type-I
isomers are non-emissive but frozen glasses of these complexes
at 77 K are emissive, displaying blue shifted structured spectra.
While atropisomers with f = 90◦ (type-II complexes) are ineligible
to attain planarity via p–p stacking and are non-emissive in the
solid state but weakly fluorescent in solutions due to restricted
rotation. Type-III atropisomers with f = 63◦ are non-emissive
in the solid state and solution. DFT and TD–DFT calculations
on type-I atropisomers in the gas phase and solution have
elucidated that intra-ligand charge transfers (ILCT) have been
the origin of absorptions of these species. DFT calculations on
the stacked (1b)2 unit have inferred that p–p stacking results in the
delocalization of the two photoactive pdiimine and pdiimine* orbitals of
Table 9 Redox potential of the complexes in DMF determined by cyclic
voltammetry at 298 K
Complexes
Epa,V (+1/0)
Epb,V (0/-1)
1a
+0.32
—
0.00
—
+0.24
—
-0.05
-1.35
-1.43
-1.42
-1.57
-1.37
-1.47c
-1.20
1b
1c
1d
1f
2
3.H20
a cathodic peak, b anodic peak, c E1/2
f
the LR ligand over the layers shifting the excitation and emission
maxima to the higher wave lengths in crystals. The trends of
diimine ligand centered electron transfer events of the luminescent
complexes in DMF have been investigated by cyclic voltammetry
at 298 K and are compared with non-luminescent congeners.
The generation of photoactive chromophores like p-conjugated
organic molecules with unsymmetrical diimine ligands and redox
innocent metal ions have been encouraging to substantiate new
tunable luminophores in coordination chemistry.
Acknowledgements
We are thankful to the Council of Scientific and Industrial
Research (CSIR), New Delhi, (01(1957)/04/EMR-II) and DST
(SR/SI/IC/10/2008) for the financial support. Crystallography
was performed at the DST-funded National Single Crystal
Diffractometer Facility at the Department of Inorganic Chemistry,
IACS. We are grateful to Ramakrishna Mission, Belur Math
for funding the Luminescence Spectrometer. A. S. R gratefully
acknowledges CSIR for RA (8/531(005)2011-EMR-I) and P. S.
acknowledges RKMRC for fellowship.
Fig. 12 (a) Cyclic voltammogram of 1f in DMF at 298 K. Conditions:
0.10 M [N(n-Bu)4]PF6 supporting electrolyte; scan rates (black, 50; red,
100; green, 200 and blue, 400 mv s-1); glassy carbon working electrode.
Spin density distributions of (b) 1f+ and (c) 1f- ions.
obtained from the luminescence spectra in solid state or in frozen
glasses. From the electron transfer events, interestingly, it is
observed that those species undergo electron transfers both at the
cathode (reduction) and anode (oxidation) with higher difference
of peak potentials are brightly luminescent, e.g., 1a, 1f; those
species undergo electron transfers both in cathode and anode
with low difference of peak potentials are non luminescent, e.g., 3
and those species undergo electron transfer only at the cathode
(reduction) are either weakly luminescent or non-luminescent,
e.g., 2.
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