Ligand influenceversuselectronic configuration of d-metal ion in determining the fate of NIR emission from LnIIIions: a case study with CuII, NiIIand ZnIIcomplexes

Article abstract of DOI:10.1039/d0nj04020g

Lanthanides (Ln^III) are well known for their characteristic emission in the near-infrared region (NIR). However, direct excitation of lanthanides is not feasible as described by Laporte's parity selection rule. Here, we obtain NIR emission from

Full text of DOI:10.1039/d0nj04020g

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Ligand influence versus electronic configuration
of d-metal ion in determining the fate of NIR
emission from LnIII ions: a case study with CuII,
NiII and ZnII complexes†
Cite this: New J. Chem., 2021,
45, 2696
ab
c
b
Abhineet Verma,
S. K. Saddam Hossain,
Sailaja S. Sunkari,
*
Joseph Reibenspiesd and Satyen Saha
*
a
Lanthanides (LnIII) are well known for their characteristic emission in the near-infrared region (NIR).
However, direct excitation of lanthanides is not feasible as described by Laporte’s parity selection rule.
Here, we obtain NIR emission from [L–M–Ln] complexes (where, L = an organic ligand, M = a d-block
metal ion, and Ln = a lanthanide ion) in which the [L–M] moiety acts as an antenna to absorb the excita-
tion light to transfer to Ln energy levels. Based on fifteen lanthanide complexes that are presented here
in which Cu, Ni and Zn complexes with a Schiff base ligand act as an antenna, we have demonstrated
for the first time that electronic configuration of the d-block metal ion is very crucial for obtaining NIR
emission. With Ln ion and the same ligand, Cu and Ni complexes show completely different emission
than that of the corresponding NIR emitting [L–Zn–Ln] complexes.
Received 10th August 2020,
Accepted 28th December 2020
DOI: 10.1039/d0nj04020g
rsc.li/njc
scope of lanthanide photonics. In addition, due to forbidden
parity from the f–f transitions, the absorption coefficients of these
Introduction
In recent years electromagnetic radiation, specifically the near- lanthanide ions are normally very low (e E 1ꢀ10 Mꢀ1 cmꢀ1).1
infrared (NIR) region (650 to 1600 nm) has become popular This disadvantage is also eluded by coordinating with organic
among researchers working in materials and biosciences.1 ligands or by use of organic ligand–d-block metal complexes. It is
Lanthanide (LnIII) ions are well reported to emit in this region. postulated that when the LnIII ion is inserted into a chemical
Luminescent lanthanides can have emission bands ranging environment as in the case of Ln–organic ligand coordination
from the visible to NIR regions, controlled by the nature of complexes, several non-centrosymmetric interactions permit the
the LnIII ions. However, due to the fact that LnIII cannot be mixing of electronic states of opposite parity into the 4f wave
directly excited by an external source, the requirement for functions and induced electric dipole transitions become partially
indirect excitation arises.2 Back in 1942, Weissman was the allowed.3 Interestingly, it has been noted that the intensity of
first to show that lanthanide-centred luminescence in coordi- some of these bands is found to be sensitive to the nature of the
nation compounds can be sensitized upon ligand excitation.3 environment surrounding the metal-ion, and therefore the transi-
The excitation energy that is absorbed by the coordinating tions responsible for these bands are called ‘‘hypersensitive’’ and
ligand can subsequently be funnelled onto the excited states tunable.3
of lanthanide ion, which is now well known as the ‘‘antenna
Emission from LnIII ions is a fundamentally important
effect’’.2 This finding eventually broadens considerably the process as evident from a continuously expanding range of
applications starting from biology,4 to materials for various
purposes like in lasers,4 OLEDs,5 optical fibres,5 and WLEDs.5
a Department of Chemistry, Institute of Science, Banaras Hindu University,
The state of the art application has recently been reviewed
Varanasi, India. E-mail: satyen.saha@gmail.com
b Department of Chemistry, Mahila Maha Vidhyalaya, Banaras Hindu University,
extremely well by Bu¨nzil.6 Considering the understanding that
functional properties and structural aspects of living systems
are the key challenges in biology and medicine, NIR emitting
materials have recently been found to be promising in the
application of biological cell imaging. Besides the biological
imaging, NIR luminescent materials (particularly, Er, Nd and
Ho) are found to be useful for telecommunication, as the active
material for optical amplification of the NIR signal,7 and
Varanasi, India. E-mail: sunkari.s7@gmail.com
c School of Chemistry, University of Hyderabad, Hyderabad, India
d X-ray Diffraction Laboratory, Department of Chemistry, Texas A&M University,
USA. E-mail: reibenspies@chem.tamu.edu
† Electronic supplementary information (ESI) available. CCDC 1913571, 1891364,
2021619, 1891363, 1891365, 1480671, 1480672, 1480673, 1918470 and 1918455.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/d0nj04020g
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encrypted tags (‘‘barcodes’’) to recognize the identity of (PXRD) data were collected on a Bruker D8-ADVANCE diffract-
chemical or biological samples.5 It is important to understand ometer equipped with Cu Ka1 (l = 1.5406 Å; 1600 W, 40 kV, 40 mA)
that NIR luminescent materials possess several critical advan- at a scan speed of 51 minꢀ1. Rietveld’s method12 was successfully
tages over semiconductor nanocrystals,5,7,8 carbon nanotubes9 applied for determination of the quantitative phases of all [L–Ni–
and organic fluorophores7 due to having multiple sharp emis- Ln] complexes. Rietveld refinement has been done by adjusting
sion bands in addition to photostability and longer lumines- major parameters: scale factor, flat background, zero-point shift,
cence lifetimes. As already discussed, the wavelength of sharp lattice parameters, orientation parameters, peak width para-
emissions, as well as the intensities, can be tuned by varying meters, asymmetry parameter and peak shape. Initial guess
the environment in which LnIII ions are embedded. One of the parameters have been taken from corresponding single crystal
popular ways is to use organic ligands to make a complex with data of Zn complexes. Peak profiles were fitted with the pseudo-
LnIII ions. This organic ligand acts as an antenna to absorb the Voigt function.13 Elemental analyses were measured on a Vario
excitation light and then passes the absorbed energy onto the EL III elemental analyzer. A diffractometer was employed for
energy manifold of Ln ions. Since light of longer wavelength is crystal screening, unit cell determination, and data collection.
used as an excitation source for various applications, it is The crystal mounted on a nylon loop was then placed in a cold
necessary to find chromophores that absorb at a longer wave- nitrogen stream (Oxford) maintained at 110 K. 45 data frames
length. Due to scarcity of suitable chromophores that can were taken at widths of 11. These reflections were used to
coordinate with LnIII ions, a popular method has been developed determine the unit cell. The unit cell was verified by examination
to use organic ligand–d-block metal complexes as the ‘antenna’. of the hkl overlays on several frames of data. After careful
It can be seen that especially metal ions having d10 configuration examination of the unit cell, an extended data collection proce-
leads to the formation of complexes with a large molar absorption dure (5 sets) was initiated using omega and phi scans. The
coefficient, resulting in better-sensitized emission.10 It can be seen goniometer was controlled using the APEX3 software suite.14 The
in the literature that ZnII is quite frequently used for this type of sample was optically centered with the aid of a video camera
complex formation over any other d-block metal ion.11,42 Limited such that no translations were observed as the crystal was rotated
literature is available which depicts that complexes of d-block through all positions. The X-ray radiation employed was generated
metal ions having configurations other than d10, do not from a Mo–I ms X-ray tube (Ka = 0.71073 Å). Integrated intensity
show good emission intensities.7,41 Among them, CuII, in parti- information for each reflection was obtained by reduction of the
cular, is reported for its malign behaviour as it quenches the data frames using the program APEX3.14 The integration method
fluorescence.7,43 However, to the best of our knowledge, there is employed a three-dimensional profiling algorithm, and all data
no literature where systematic studies have been performed with were corrected for Lorentz and polarization factors, as well as for
the same set of organic ligands complexed to various d-block crystal decay effects. Finally, the data were merged and scaled to
metal ions including ZnII ion, as an antenna for LnIII ion emission, produce a suitable data set. The absorption correction program
to study the effect of d-configuration on the photophysical SADABS15 was employed to correct the data for absorption effects.
responses of LnIII complexes.
Crystal data for complexes [C1 to C5] were collected on a Bruker
To understand the importance of ZnII ion on the antenna CMOS area detector with a microfocus sealed X-ray tube as a source
effect, we have synthesized three series of CuII, NiII and ZnII ion using graphite monochromatized Mo-Ka radiation at 173 K, for
complexes with the same organic ligand and used those complexes [C1], at 110 K for complexes [C2 to C4] and at 100 K for [C5]. Data
as an antenna to complex with several LnIII ions (Nd, Pr, Sm, Dy, for complexes [Z1 to Z3] were collected on an OXFORD DIFFRAC-
and Er). All the complexes, except for the NiII series (due to TION XCALIBER EoS diffractometer using graphite monochroma-
nonformation of single crystals) were structurally characterized tized Mo-Ka radiation at 298 K and for [Z4 and Z5], on a Rigaku
by the single-crystal X-ray diffraction (SCXRD) technique. Rietveld’s Saturn724 using a rotating anode as a X-ray generator, and Mo-Ka
method12 was successfully applied for determination of the struc- radiation at 293 K. Electronic spectra were recorded on a CARY 100
ture for all NiII complexes from the PXRD data. Comparative NIR BIO UV-vis spectrophotometer. Fluorescence spectra were recorded
luminescence studies of these complexes are reported herein with on a Horiba spectrofluorometer (Fluoromax).
an intention to understand the effect of d electronic configuration
on the final luminescence of several Ln-complexes.
Results and discussion
Preparation of lanthanide complexes
Experimental section
Material and methods
All the complexes were synthesized in a similar manner
(Scheme 1), except for the change in metal ions. A typical
3-Methoxysalicylaldehyde, 1,4-diamino-butane, Ni(CH3COO)2ꢁ procedure is detailed here, for complex [L–Ni] with Nd, while
2H2O, Cu(CH3COO)2ꢁH2O, Zn(CH3COO)2ꢁ2H2O and Ln(NO3)3ꢁ only analytical details are provided for the rest.
6H2O were purchased from Sigma Aldrich and were used with-
Ligand synthesis (L). The synthesis of ligand L is done
out further purification. Infrared spectra were recorded on a following the procedure described elsewhere,16 where 1 mole
PerkinElmer Spectrum-2 FTIR spectrometer using KBr pellets of 1,4-diamino butane was condensed with 2 moles of o-vanillin
in the region of 400–4000 cmꢀ1. The powder X-ray diffraction under reflux conditions in ethanol.
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Scheme 1 Synthetic route for the preparation of the complexes. M stands for d-block metal ion, while Ln stands for lanthanide ion. MeOH represents
methanol.
[N1]. 20 mL of a methanolic solution of ligand (L) (0.38 g, calcd for C20H22N5O13CuEr (M.W. = 771.19): C 31.15; H 2.88; N
1.0 mmol) was added to 15 mL of methanolic solution of 9.08. Found C 31.21; H 3.03; N 8.92.
Ni(CH3COO)2ꢁ2H2O (0.43 g, 1.0 mmol) while stirring, followed
[Z1]. 0.89 g (1.1 mmol, 89.3%); M.P. 261.8 (ꢂ 1) 1C; anal.
by dropwise addition of 15 mL of a methanolic solution of calcd for C23H29N4O13ZnNd (M.W. = 778.11): C 35.50; H 3.63;
Nd(NO3)3ꢁ6H2O (0.10 g, 1.0 mmol). The mixture was refluxed N 7.20. Found C 35.65; H 3.72; N 7.25. [Z2]: 0.85 g (1.1 mmol,
for 3 hours, cooled to room temperature and filtered. Layer 85.5%); M.P. 269.4 (ꢂ1) 1C; anal. calcd for C23H29N4O13ZnPr
diffusion of ether into the methanolic filtrate at moderate (M.W. = 774.77): C 35.66; H 3.64; N 7.23. Found C 35.75; H 2.70;
temperature (B15 to 20 1C) afforded (i) dark greenish-black N 7.30. [Z3]: 0.76 g (1.0 mmol, 76.9%); M.P. 267.3 (ꢂ1) 1C; anal.
coloured powder for [L–Ni–Ln] complexes; (ii) shiny black calcd for C23H29N4O13ZnSm (M.W. = 785.23): C 35.18; H 3.72;
coloured plate-shaped crystals for [L–Cu–Ln] and (iii) light N 7.13. Found C 36.47; H 3.75; N 7.24. [Z4]: 0.80 g (0.5 mmol,
yellow blocks for [L–Zn–Ln] complexes, suitable for SCXRD 80.5%); M.P. 4 350.0 (ꢂ1) 1C; anal. calcd for C22H25N4O12ZnDy
measurements.
(M.W. = 765.33): C 34.52; H 3.29; N 7.31. Found C 34.36; H 3.32;
Yields for [N1]: 0.80 g (1.5 mmol, 87.9%); M.P. 270.0 (ꢂ1) 1C; N 7.28. [Z5]: 0.75 g (0.5 mmol, 75.3%); M.P. 4 350.0 (ꢂ1) 1C;
anal. calcd for C23H29N4O13NiNd (M.W. = 771.39): C 35.76; H anal. calcd for C22H25N4O12ZnEr (M.W. = 770.09): C 34.31;
3.78; N 7.25. Found C 35.52; H 3.41; N 7.43. [N2]: 0.65 g H 3.27; N 7.28. Found C 34.12; H 3.31; N 7.32. FT-IR spectra
(0.9 mmol, 71.4%); M.P. 273.9 (ꢂ1) 1C; anal. calcd for of all the complexes are given in the ESI† (Fig. S1).
C
23H29N4O13NiPr (M.W. = 768.06): C 35.92; H 3.80; N 7.28.
Found C 35.91; H 3.42; N 7.32. [N3]: 0.35 g (0.5 mmol, 38.5%);
M.P. 274.3 (ꢂ1) 1C; anal. calcd for C23H29N4O13NiSm (M.W. =
General characterization
778.52): C 35.48; H 3.75; N 7.20. Found C 35.18; H 3.42; N 7.23. Heterobimetallic complexes have been synthesized in situ
[N4]: 0.46 g (0.6 mmol, 50.5%); M.P. 283.1 (ꢂ1) 1C; anal. calcd (three different series) in a stepwise manner without the isola-
for C22H25N4O12NiDy (M.W. = 758.62): C 34.83; H 3.32; N 7.39. tion of mononuclear d-block complexes. FT-IR spectra (ESI,†
Found C 35.12; H 3.39; N 7.12. [N5]: 0.31 g (0.4 mmol, 34.1%); Fig. S1), for all fifteen complexes, exhibit a sharp peak due to
M.P. 286.3 (ꢂ1) 1C; anal. calcd for C22H25N4O12NiEr (M.W. = the azomethine (CQN) group of the Schiff base in the range
763.38): C 34.61; H 3.30; N 7.34. Found C 34.51; H 3.68; N 7.12. of 1615–1630 cmꢀ1 17
The phenolic C–O stretching band at
.
[C1]. 0.76 g (1.4 mmol, 83.5%); M.P. 275.3 (ꢂ1) 1C; anal. 1249 cmꢀ1 in the spectra of L was shifted to 1217 cmꢀ1 on
calcd for C20H22N5O13CuNd (M.W. = 748.17): C 32.10; H 2.96; complexation, supporting the deprotonation and coordination
N 9.36. Found C 32.21; H 3.01; N 10.10. [C2]: 0.70 g (0.9 mmol, of the phenolic oxygen donor to the metal centre. The counter
76.9%); M.P. 271.2 (ꢂ1) 1C; anal. calcd for C20H22N5O13CuPr anions show their characteristic vibrational band in FT-IR
(M.W. = 744.87): C 32.25; H 2.98; N 9.40. Found C 32.31; H 2.83; spectra. Sharp bands appearing at 526 and 421 cmꢀ1 corre-
N 9.83. [C3]: 0.66 g (0.9 mmol, 72.5%); M.P. 271.8 (ꢂ1) 1C; anal. spond to the M–N and M–O stretching frequencies, respec-
calcd for C20H22N5O13CuSm (M.W. = 754.29): C 31.84; H 2.94; tively. [L–Zn–Ln] and [L–Ni–Ln] series have an extra band at
N 9.28. Found C 32.11; H 3.31; N 9.10. [C4]: 0.60 g (0.8 mmol, about 1495 cmꢀ1 showing the presence of the acetate group in
65.93%); M.P. 276.5 (ꢂ 1) 1C; anal. calcd for C20H22N5O13CuDy these complexes. Other high energy bands such as the one at
(M.W. = 766.43): C 31.34; H 2.89; N 9.14. Found C 31.10; H 2.99; 920 cmꢀ1 for C–C stretching in the –(O)2C–CH3 group can also
N 9.57. [C5]: 0.72 g (0.9 mmol, 79.1%); M.P. 270.0 (ꢂ1) 1C; anal. be noticed.
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was carried out using the reliability index parameter, Rwp
(weighted residual error), RB (Bragg factor) and Rexp (expected
error).20 The determined structures are presented in the ESI,†
Fig. S5. The goodness of fitting (w2) for [N1] to [N5] are 1.27, 1.73,
1.33 1.28 and 1.29, respectively.
SCXRD studies
Single crystal structures are solved by direct methods and
refined by full matrix least-squares on F2 using SHELX-2018.21
The non-hydrogen atoms were refined with anisotropic thermal
parameters. A disordered carbon atom of the methoxy group
(C19) of complexes in [L–Zn–Nd], [L–Zn–Pr] and [L–Zn–Sm] is
modeled by splitting into two halves with positions refined by FVAR
refinement and thermal parameters by anisotropic refinement. All
the hydrogen atoms are geometrically fixed and allowed to ride
upon the atom to which they are attached. Important crystal data
for Cu and Zn containing complexes are presented in Table 1
whereas important bond angles and bond distances are presented
in Table 2. In spite of several efforts (at different temperature, with
different solvent combinations as well as emplying diffusion tech-
nique) to grow the single crystals, none of the [L–Ni–Ln] complexes
could be grown as a single crystal suitable for SCXRD measure-
ments. However, all other crystals could be grown by employing a
diffusion technique using a combination of methanol and ether
solvents at laboratory temperature. Drawings are made using
OLEX II,22 ORTEP-III and MERCURY.23,24 The refinement con-
verged to final values of R1 & wR2 = 0.0291 & 0.0664 for [C1]; 0.0325
& 0.0734 for [C2]; 0.0310 & 0.0755 for [C3]; 0.0244 & 0.0526 [C4];
0.0194 & 0.0412 for [C5]; 0.0809 & 0.1809 for [Z1]; 0.0663 & 0.1167
for [Z2]; 0.1067 & 0.1944 for [Z3]; 0.0359 & 0.0935 for [Z4]; and
0.0274 & 0.0705 for [Z5], respectively.
Each series of complexes is found to be isostructural. Com-
plexes [C1 to C5] are isostructural, crystallizing in the mono-
clinic system with a space group P21/n as shown in Fig. 2. The
crystallographically independent unit contains one neutral CuII
and LnIII heterodinuclear unit, [Cu(m-L)Ln(NO3)2] [LnIII = Nd for
[C1]; Pr for [C2]; Sm for [C3]; Dy for [C4] & Er for [C5]; L = N,N0-
bis(3-methoxysalicylidene)-1,4-diaminobutane]. On the other
hand, complexes [Z1 to Z5] are also isostructural, crystallizing
in the monoclinic system but with different space groups. P21/c
Fig. 1 PXRD patterns for the complexes (a) [L–Cu–Ln] and (b) [L–Zn–Ln].
For comparison, the simulated patterns generated from SCXRD data are
also given. The patterns for [L–Ni–Ln] complexes are discussed separately.
Powder X-ray diffraction (PXRD) patterns (Fig. 1) confirm the for [Z1] and [Z2], P21/a for [Z3] and P21/n for [Z4] and [Z5] (Fig. 2).
bulk purity of the complexes. For [L–Cu–Ln] and [L–Zn–Ln] The crystallographically independent unit contains one neutral ZnII
complexes, the experimental PXRD patterns are in good agree- and LnIII heterodinuclear unit, [Zn(m-L)(m-CH3COO)SLn(NO3)2] for
ment with the simulated patterns obtained from the SCXRD LnIII = Nd for [Z1], Pr for [Z2] and Sm for [Z3] S = CH3OH, whereas
data, indicating crystal structures are the true representative of complexes [Z4] and [Z5] have two crystallographically independent
the bulk. However, in addition, experimental PXRD data have units where [Zn(m-L)(m-CH3COO)Ln(NO3)3] for [LnIII = Dy for
been used to elucidate the structures for [L–Ni–Ln] complexes [Z4] and Er for [Z5] L = N,N-bis(3-methoxysalicylidene)-1,4-
for which SCXRD data are not available. In order to determine diaminobutane].
lattice parameters and to quantify the phases present in [L–Ni–
[L–Cu–Ln] complexes [C1 to C5] are iso-structural. Here CuII
Ln] complexes, Rietveld refinement was carried out.12 Fig. 4 ions adopt the distorted square planar geometry occupied by
and Fig. S4, Table S1 (ESI†) show detailed information obtained two nitrogen atoms (N1 and N2) and two phenoxo oxygen (O1
after Rietveld refinement of all [L–Ni–Ln] complexes. Considering and O2) atoms from the ligand; whereas the LnIII ions are ten
the integrated intensity of the peaks as a function of structural coordinated adopting a bicapped square antiprismatic geometry
parameters only, the Marquardt least-squares18 procedure was with two phenoxo oxygens (O1 and O2) and two methoxy oxygens
used for minimization of the difference between the observed (O3 and O4) from the ligand, while the remaining six positions
and simulated powder diffraction patterns.19 The minimization are occupied by six oxygens from the three bidentate nitrate
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groups (O5, O7, O8, O10, O11 and O13) as clearly shown in the
ORTEP diagrams Fig. 2(a–e). The iso-structurality of [L–Cu–Ln]
complexes is also supported by their similar coordination
environment (Fig. 3(d)) and packing diagram, which have similar
packing in all the five complexes presented in Fig. 3(a), whereas
respective individual packing diagrams of each complex are
presented in ESI,† Fig. S3(a–e).
[L–Zn–Ln] complexes fall into two categories. [Z1–Z3], which
have already been discussed in our previous report,16 have
similar coordination with ZnII ions adopting the distorted
square pyramidal geometry, in which the planar positions are
occupied by two phenoxo oxygen (O1 and O2) and two nitrogen
(N1 and N2) atoms, while the fifth axial position is occupied by
the bridged acetate oxygen (O11) atom, Fig. 2(f–h). The LnIII
ions with ten coordination number adopt bicapped square
antiprismatic geometry. The coordination numbers are fulfilled
by ten oxygen atoms; four from the ligand (two phenoxo
oxygens (O1 and O2) and two methoxy oxygens (O3 and O4)),
while the remaining six oxygens are from two bidentate nitrate
units (O5, O7, O8 and O10), one from acetate oxygen (O12) and
one from solvent methanolic oxygen (O13) atom as presented in
Fig. 2(f–h). These three complexes exhibit a similar coordina-
tion environment (Fig. 3(e)) and therefore are expected to have
similar packing as presented in Fig. 3(b), whereas individual
packing diagrams are presented in ESI,† Fig. S3(f–h). In the
other two complexes [Z4] and [Z5], the ZnII coordination
environment is similar to that above, with square pyramidal
geometry where the planar position is occupied by two phenoxo
oxygen (O1 and O2) along with two nitrogen (N1 and N2) atoms
and the fifth axial position is occupied by an acetate oxygen (O11)
(Fig. 2(i and j)). Here the LnIII ions are only nine coordinated with
capped square antiprismatic geometry as opposed to the ten
coordination of the previous series. Four oxygens from the tetra
coordinated ligand (two phenoxo oxygens (O1 and O2) and two
methoxy oxygens (O3 and O4)) along with two bi coordinated
nitrate groups (O5, O7, O8 and O10) and the ninth position is
occupied by the acetate oxygen (O12) fulfilling the coordination
requirements of the lanthanide ion as presented in the ORTEP
diagrams in Fig. 2(i and j). These two have a similar coordination
environment (Fig. 3(f)) and are expected to have similar packing
as presented in Fig. 3(c), whereas individual packing diagrams
are presented in the ESI,† Fig. S3(i and j).
The [L–Cu–Ln] complexes have similar interactions as they
all have similar coordination environments. Thus [C1 to C5]
also have similar packing (Fig. 3) stabilized by C–Hꢁ ꢁ ꢁO hydro-
gen bonding between oxygen atoms of nitrate and hydrogen
atoms of the butyl group, aromatic rings and imine group. This
weak interaction gives extra stability to the crystal packing. It is
to be noted that the interactions in the corresponding series are
almost similar, leading to similar packing. In addition to this
weak interaction, the [L–Cu–Ln] complex also exhibits Cꢁ ꢁ ꢁp
interactions, which are prominent in the ORTEP representation
shown in the last column of Table 3.
On the contrary, [L–Zn–Ln] complexes (i.e., [Z1 to Z5]) are
also stabilized by intermolecular C–Hꢁ ꢁ ꢁO hydrogen bonding
between oxygen atoms of nitrate and the hydrogen atoms of the
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Table 2 Important bond angles (1), bond lengths (Å) and dihedral angles (1) for [L–Cu–Ln] and [L–Zn–Ln] complexes. Pictorial depictions of the bond
angles and lengths are given below the tabulated data
Nd
Cu
Pr
Sm
Cu
Dy
Cu
Er
Metals
Zn
Cu
Zn
Zn
Zn
Cu
Zn
Angles (1)
N1–M–N2
N1–M–O3
O3–M–O2
O2–M–N1
O2–Ln–O3
O3–Ln–O4
O4–Ln–O1
O1–Ln–O2
100.19
90.97
78.85
92.58
60.65
62.19
144.45
62.20
96.43
88.03
77.43
86.73
64.52
59.46
134.02
59.32
100.47
92.28
78.87
90.93
60.43
61.87
114.81
61.94
96.73
88.33
77.02
87.03
64.32
59.95
134.02
59.64
100.33
90.78
79.02
92.42
60.33
61.85
114.96
62.04
96.86
87.55
76.43
87.14
65.13
60.53
134.03
59.93
100.73
92.52
78.00
91.02
61.83
63.56
113.62
63.36
96.02
87.26
75.04
88.95
66.89
63.52
143.66
64.77
100.85
90.89
77.58
92.67
62.25
64.30
142.97
63.70
96.18
88.92
74.95
87.23
67.26
63.81
143.19
65.27
Distance (Å)
M–N1
M–N2
M–O1
M–O2
M–O5
Ln–O1
Ln–O2
Ln–O3
Ln–O4
Ln–O6
Ln–O7
1.958
1.997
1.938
1.943
—
2.612
2.406
2.475
2.589
—
2.062
2.071
2.052
2.076
1.996
2.740
2.414
2.422
2.716
2.409
2.520
22.015
2.005
1.964
1.953
1.941
—
2.619
2.424
2.490
2.595
—
2.048
2.057
2.038
2.075
2.005
2.743
2.402
2.408
2.736
2.417
2.520
21.562
1.963
2.007
1.946
1.950
—
2.602
2.500
2.432
2.626
—
2.043
2.068
2.038
2.078
1.988
2.725
2.365
2.367
2.731
2.361
2.362
21.023
1.960
2.000
1.938
1.935
—
2.542
2.409
2.331
2.567
—
2.067
2.058
2.053
2.066
2.007
2.527
2.268
2.312
2.526
2.312
—
1.999
1.954
1.933
1.938
—
2.526
2.390
2.304
2.542
—
2.055
2.065
2.057
2.063
2.146
2.512
2.285
2.254
2.494
2.392
—
—
17.345
—
17.359
—
17.233
—
16.353
—
15.377
Dihedral angle (N1–N2–O3–O2)
17.297
17.222
Six membered ring formation with M (d-block metal)
Plane angle
Centroid
26.367
3.733
52.494
3.649
26.151
3.747
52.941
3.626
26.055
3.751
52.931
3.621
25.364
3.729
46.316
3.676
24.639
3.730
46.303
3.673
butyl group, imine group, methoxy group, and acetate anion calculated intensities are shown as solid black lines. The bottom
and also coordinated methanol hydrogens of a neighbouring blue lines represent the difference between the measured and
complex moiety (vide Table 3). In addition, the crystal structure is the calculated intensities. The allowed Braggs positions for the
further stabilized by C–Hꢁꢁꢁp interactions between the centroids of space group are marked as vertical green lines. The space groups
the methoxy-substituted phenyl rings of neighbouring molecules are: P21/c for [N1] and [N2], P21/a for [N3] and P21/n for [N4] and
as tabulated and depicted in Table 3. Here complexes [Z4 and Z5] [N5]. We have observed that all the experimental peaks are
have different packing because they have two crystallographically allowed Bragg 2y positions for the corresponding space group.
independent units, leading to extra stability to the crystal system
because of more packing interactions.
In the refinement, the initial parameters were taken from the
corresponding [L–Zn–Ln] complexes. Parameters such as lattice
constants are taken for the monoclinic system, occupancies are
kept as 1.0, scale factors, isothermal parameters, and shape
parameters have been kept unconstrained. The background
Structural determination of [L–Ni–Ln] complexes: powder X-ray
diffraction study using Rietveld refinement
The details of powder X-ray diffraction patterns along with has been corrected by pseudo-Voigt function.13 The refined
Rietveld refined data are shown in Fig. 4 for [L–Ni–Nd], while PXRD patterns show that the samples are in single phase form.
for the rest of the Ni complexes they are presented in the ESI,†
The various R factors are listed in the ESI,† Table S1.
Fig. S4. The experimental data are shown as red circles, and The values of R factors such as Rp are found to be large. Similar
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Fig. 2 ORTEP diagrams of [L–Cu–Ln] and [L–Zn–Ln] complexes at 30% probability ellipsoids. (a) For [L–Cu–Nd], [C1]: O6 atom is split and one split
component is represented for better presentation; (b) for [L–Cu–Pr], [C2]; (c) for [L–Cu–Sm], [C3]; (d) for [L–Cu–Dy], [C4]; (e) for [L–Cu–Er], [C5];
(f) for [L–Zn–Nd], [Z1]; (g) for [L–Zn–Pr], [Z2]; (h) for [L–Zn–Sm], [Z3]; (i) for [L–Cu–Dy], [Z4]; and (j) for [L–Zn–Er], [Z5].
high values of R factors for crystalline materials have been experimental data (CHN, PXRD, IR, and photophysical studies).
observed by other authors.25,26 This could be due to the Here in [L–Ni–Ln], the Ni ion is found to be in a square pyramidal
large noise-to-signal ratio of PXRD patterns for crystalline structure with coordination number of 5 and the lanthanide ion is
materials. However, we have observed an appropriate (close to in a bicapped square antiprismatic geometry with coordination
1) value of goodness of fit (w2), which justifies the goodness number of 10, which is structurally similar to [L–Zn–Ln] complexes.
of refinement. In the powder diffraction pattern of crystal- The inset of Fig. 4 shows the structure of the [L–Ni–Nd] complexes
line materials, diffuse scattering is more dominant than in obtained from PXRD data which is as expected.
those of bulk crystalline materials, and it is due to the large
Photophysical studies
surface to volume ratio of the atoms. The diffuse scattering
becomes significant at the nanoscale, while Bragg scattering There is considerable interest in the photophysical properties of
gets diminished, which leads to a decline in crystallinity and lanthanide complexes because of the various fascinating properties
large R factors.27
as described in the introduction. Particularly, the long-wavelength
From the derived structure it is found that [L–Ni–Ln] structurally narrow band with a longer lifetime28 makes it attractive for many
follows [L–Zn–Ln] complexes, which is also expected from the other applications like telecommunication and in biology.4,5 Here, we
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Fig. 3 Packing diagrams of complexes viewed down the a-axis. (a) All [L–Cu–Ln] complexes, ([L–Cu–Nd] presented here) in this series show the same
packing. (b) Packing in [L–Zn–Nd], and complexes [Z1]–[Z3], show the same type of packing. (c) Packing in [L–Zn–Dy]:[Z4], and [L–Zn–Er]:[Z5] also
shows the same packing. (d–f) Show the polyhedral environment around the metal ions.
Table 3 The important intermolecular p interactions common in the corresponding series. Cꢁ ꢁ ꢁp for [L–Cu–Ln] complexes and C–Hꢁ ꢁ ꢁp for [L–Zn–Ln]
complexes. The last column shows the ORTEP pictorial representation of the intermolecular interaction
Complex
Cꢁ ꢁ ꢁp (Å)
C–Hꢁ ꢁ ꢁp (Å)
[C1]
[C2]
[C3]
[C4]
[C5]
[Z1]
[Z2]
[Z3]
[Z4]
[Z5]
3.322
3.303
3.310
3.301
Absent
Absent
Absent
Absent
Absent
2.743
2.758
2.729
2.964
3.049
3.399
Absent
Absent
Absent
Absent
Absent
intend to find the effect of d-block metal ion influence on the Ln–ligand complexation is also reported in the literature.28 On the
lanthanide emission. The photophysical properties have been other hand, interestingly, we have also come across several reports
studied in MeOH. The UV-vis absorption spectra of Schiff base where redshift is mentioned on complexation.29 This discrepancy
ligand (L) alone and its complexes [L–Ni–Ln], [L–Cu–Ln] and is presumably due to the fact that the dilute solution of lanthanide
[L–Zn–Ln] are presented in Fig. 5.
metal complex does not provide the lowest energy band with
The free ligand exhibits the lowest energy band at 420 nm, in appreciable optical density to detect and thereby is missed in
addition to higher energy bands at 221, 260, 292 and 330 nm. spectra published in several reports.1,4,8,16 All complexes here have
While the 420 nm band is assigned to n–p* transitions in the displayed three characteristic bands at 229 nm, 274 nm and
ligand, the higher energy bands are attributed to the p–p* transi- 353 nm, which are due to intra ligand charge transfer (229, and
tions in the benzene ring and azomethine group.16 Interestingly, 274 nm) and ligand to metal charge transfer (353 nm), respec-
all the metal ion complexes show blue-shifted absorption spectra tively. However, the d–d transition band in [L–Cu–Ln] complexes is
upon complexation. Such blue shifting of absorption bands on not observed at a low concentration (1 ꢃ 10ꢀ5 M, in Fig. 5a), but at
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Fig. 4 Rietveld refined XRD patterns for the [L–Ni–Nd], [N1] complex. The red circles represent experimental points, and the solid black line represents
Rietveld refined data. The bottom blue line shows the difference between the experimental and refined data. The marked 2y positions are the allowed
Bragg peaks. The inset shows the expected structure derived from the Rietveld refinement.
a higher concentration (B2 ꢃ 10ꢀ4 M) this band shows up in the [Ar]3d94s1, whereas CuII is a d9 system having [Ar]3d94s0 electronic
range of 500–800 nm (vide ESI,† Fig. S2). On the other hand, configuration; in both the cases the d orbital is not completely
[L–Ni–Ln] complexes show a prominent d–d transition band at filled and thus under ligand field strength, the d orbital gets split
around 420 nm, even at low concentration (B1 ꢃ 10ꢀ5 M) into t2g and eg energy levels between which transition takes place.
(Fig. 5b). NiII is a d8 system with the electronic configuration Many reports are available for d–d transition in CuII and NiII
Fig. 5 Absorption spectra for the (a) ligand (L) and complexes [L–Zn–Ln] (in solid lines) and [L–Cu–Ln] complexes (in dotted lines) (at B1 ꢃ 10ꢀ5 M).
(b) Absorption spectra for [L–Ni–Ln] complexes, showing the d–d transition band (B1 ꢃ 10ꢀ5 M).
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complexes and their significance.30 On the other hand, the trinuclear complexes.32–35 The emission spectra of ligands (L)
[L–Zn–Ln] complexes did not show any d–d transitions as d10 and all the complexes have been studied in methanol and are
systems (of ZnII) have no inter-electronic transitions between the presented in Fig. 6. The emission spectrum of the ligand
split d-levels (t2g 3 eg) as they are fully filled.31 These absorption exhibits a broad (400–600 nm) emission band with a maximum
spectra of lanthanide complexes are found to match well with at 486 nm. The emission spectra of all the complexes exhibit a
those of similar complexes reported in the literature.32
broad emission band covering 400–600 nm (visible region) with
Recently, attention has also been focused particularly on the an emission maximum at around 450 nm (for [L–Zn–Ln]) which
complex of LnIII with the near-infrared (NIR) emission (B900– is blue-shifted compared to the free ligands (486 nm). The blue-
1700 nm), which is transparent for the biological tissues shift may originate from the ligand-to-metal-charge transfer
making it possible to be applied in bioassays and bio- (LMCT) emission and has already been reported.14,15 Interestingly,
luminescent probes.9 Photophysical properties of LnIII ions in addition to the ligand centred band, one can observe several
depend markedly on their coordination environment that sharp emission bands in the visible range for [L–Zn–Ln] com-
includes chromophore ligands, and d-block metal ions. Emis- plexes. These peaks correspond to the f–f transitions. Table 4
sion studies of lanthanide complexes with d-block metal ions, tabulates all transition data along with their term symbols and
as a sensitizer for Ln emission, are often found to be interesting their assignments that could be observed in these complexes.
because their responses can be tuned by varying the ligand, These emission bands are completely absent in Cu and Ni
and/or d-block metal ions. Liu and coworkers have made complexes. It is therefore fascinating to observe that despite
several significant contributions highlighting the interesting having Ln ions in these complexes (C1–C5 and N1–N5 series),
luminescence properties exhibited by various types of no emission from Ln could be observed. The complete quenching
Fig. 6 Emission spectra for bare ligand and complexes. Emission spectra are categorized into groups based on d-block metal ions in the complexes. The
top set shows the emission spectra of all Cu-containing Ln complexes (i.e., all the [L–Cu–Ln]). The C5 (corresponds to [L–Cu–Er]) shows exceptional
behaviour showing a broad emission band in the visible region as discussed in the text. The middle set shows emission spectra of all Ni-containing Ln
complexes (i.e., all the [L–Ni–Ln]). In both the sets, no emission from the Ln ion is observed. The lower panel shows the emission spectra of all Zn
ion-containing Ln complexes (i.e., all the [L–Zn–Ln]). Rich emission bands can be observed for the [L–Zn–Ln] complexes in the NIR region down to
1600 nm. Assignments for emission bands for Ln ions are also depicted on each peak. The excitation wavelengths were kept at 420 nm and 350 nm for
ligand and complexes, respectively.
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of emission throughout the range for CuII complexes (except [C5]) seen to be completely quenched.8 Herein these complexes, d-
is also noteworthy. The exception that the [C5] which is an ErIII block metal ion-organic ligand (Cu/Ni/Zn–L) complexes should
complex (i.e., [L–Cu–Er]) only shows emission in the visible region be the source of energy for transfer to any of the LnIII ion energy
among all CuII complexes is quite intriguing. However, a thorough manifolds.1,2,24 Since this [L–Cu/Ni] complex emission gets
literature search indicates the existence of a few reports on such quenched, there is no energy transfer to LnIII ion energy
exception, that copper–lanthanide complexes show some excep- manifold, making the complex non-emissive in the NIR region.
tional behaviours, particularly with the higher series of lanthanide Due to the absence of d–d transition in the [L–Zn] system,
ions such as Eu, Tb, and Er ions.36 Ohtsu et al. have reported the [L–Zn–Ln] complexes show usual emission in the NIR range.
photophysics of a macrocyclic trinuclear heterobimetallic complex It transpires from this study that [L–Cu] and [L–Ni] are not
with EuII (namely, [EuCu2L2(NO3)ꢁ3MeOH]) highlighting the suitable to function as ‘‘antenna’’ in such complexes while
quenching of all bands except one around the 720 nm region. [L–Zn] undoubtedly acts as a superior ‘‘antenna’’. However,
They have ascribed the quenching of bands due to the overlap questions may arise as to whether the different coordination
between the lanthanide emission with that of d–d transition in environment (ten-coordination in the [L–Cu–Ln] complex vs.
CuII.36 While Sakamoto and coworkers have observed only one nine-coordination in [L–Zn–Ln] complexes) affects the photo-
selective band for EuIII (at 625 nm) and TbIII (at 550 nm),37 physics of these complexes. Lanthanide coordination com-
McMohan and coworkers have reported a drastic decrease plexes consist of two electronic systems with little interaction
of intensity for TbIII bands on complexation with CuII, instead of between the ligand electronic system and the electronic
complete disappearance.38 Therefore, the selective appearance of configuration of the lanthanide ions. The latter consists of 4f
any specific band for copper complexes is intriguing and needs electrons which are shielded by the filled 5s2 5p6 subshells
further exclusive studies with the help of a time resolved fluores- allowing little mixing between the two systems (the ‘‘covalency’’
cence technique.
of the Ln–ligand bonds rarely exceeds 5%). As a consequence:39
However, all the NiII complexes [L–Ni–Ln], show emission in (a) the absorption/emission spectra of the ligands sustain little
the visible region but none in the NIR region. On the contrary, changes upon complexation and (b) on the other hand, the
all [L–Zn–Ln] complexes show emission in the range spanning absorption/emission spectra of the lanthanide ions (f–f-
400 nm to 1600 nm, having both broad and sharp bands. [Z5], transitions) will sustain large changes, not in energy (i.e.
an ErIII complex [L–Zn–Er] exhibits a strong emission band at transition occurs more or less at the same wavelengths), but
1520 nm, longest among all, followed by [Z2] and [Z1] at in crystal field splitting and intensity. This is presumably
1407 nm and 1333 nm, respectively. The reason for the com- because complexation modifies the symmetry around the metal
plete absence of LnIII emission in CuII and NiII containing ion. So it is expected to see some differences depending on the
complexes might be related to the existence of d–d transition coordination number, but they are sometimes difficult to
within the metal complex. It is well-known that in the case of a observe depending on the ion. The shape of absorption spectra
simple Cu–ligand complex, the fluorescence of a ligand is often for NdIII has been used in the past to determine the
Table 4 Emission data for the complexes. Bands are assigned based on the literature as cited on the top of the code of the complexes
Code
L
B(400–500) B(600–700)
B(800–1100)
B(1200–1300)
B(1400–1500)
491
NdIII
Visible
4I9/2–4I9/2
870; 904
4I9/2–4I11/2
1057
4I9/2–4I13/2
1333
[Z1]1,2,4,5,7,16 451
[C1]
[N1]
—
474
No emission
PrIII
Visible
450
—
1D2–3H4
600
No emission
3P0–3H6
623
1D2–3F4
1028
1G4–3H4
1338
1G4–3H5
1372
1G4–3H6
1407
[Z2]1,5,7,16
[C2]
[N2]
478
SmIII
Visible
452
—
4G5/2–6H5/2 4G5/2–6H7/2 G5/2–6H9/2 G5/2–6F1/2 G5/2–6F5/2 G5/2–6F7/2 G5/2–6F9/2
4
4
4
4
4
[Z3]1,5,7,16
[C3]
559
595
641
889
942
1021
1161
No emission
[N3]
477
DyIII
Visible
450
—
4F9/2–6H13/2
570
No emission
[Z4]1,5,7,11
[C4]
[N4]
478
ErIII
Visible
446
472
4S3/2–4I15/2 S3/2–4I13/2
917
4S3/2–4I11/2 S3/2–4I9/2 4I13/2–4I15/2
4
4
[Z5]1,2,5,7,11
[C5]
994
1119
1319
1520
No emission
[N5]
477
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4 (a) Y. Li, Y. Sun, J. Li, Q. Su, W. Yuan, Y. Dai, C. Han,
Q. Wang, W. Feng and F. Li, Ultrasensitive Near-Infrared
Fluorescence-Enhanced Probe for in Vivo Nitroreductase
Imaging, J. Am. Chem. Soc., 2015, 137(19), 6407–6416;
(b) E. S. Levy, C. A. Tajon, T. S. Bischof, J. Iafrati,
A. Fernandez-Bravo, D. J. Garfield, M. Chamanzar,
M. M. Maharbiz, V. S. Sohal, P. J. Schuck, B. E. Cohen and
E. M. Chan, Energy-Looping Nanoparticles: Harnessing
Excited-State Absorption for Deep-Tissue Imaging, ACS
Nano, 2016, 10(9), 8423–8433; (c) G. Jalani, R. Naccache,
D. H. Rosenzweig, L. Haglund, F. Vetrone and M. Cerruti,
Photocleavable Hydrogel-Coated Upconverting Nano-
particles: A Multifunctional Theranostic Platform for NIR
Imaging and On-Demand Macromolecular Delivery, J. Am.
Chem. Soc., 2016, 138(3), 1078–1083.
5 (a) J.-C. G. Bu¨nzli, S. Comby, A.-S. Chauvin and
C. D. B. Vandevyver, New Opportunities for Lanthanide
Luminescence, J. Rare Earths, 2007, 25(3), 257–274;
(b) J. Andres and A.-S. Chauvin, Lanthanides: Luminescence
Applications, Encycl. Inorg. Bioinorg. Chem., 2012, 1–18; (c) J.-
C. G. Bu¨nzli and S. V. Eliseeva, Lanthanide NIR Lumines-
cence for Telecommunications, Bioanalyses and Solar
Energy Conversion, J. Rare Earths, 2010, 28(6), 824–842.
6 J.-C. G. Bu¨nzli, Lanthanide Photonics: Shaping the Nano-
world, Trends Chem., 2019, 1(8), 751–762.
coordination number but found to be not reliable. Depending
on the ion, it is reported that a charge transfer band, such as
LMCT for Eu and Yb may appear.40 However, each case is a bit
special and generalizations may be incorrect.
Conclusions
Here in this paper, we report three new series of hetrodinuclear
d- and f-block metal complexes of NiII, CuII and ZnII with
f-block LnIII (Nd, Pr, Sm, Dy and Er) metal ions. All ten
complexes having Cu/Zn are structurally characterized by
SCXRD. In the absence of single crystals for [L–Ni–Ln], the
structures of Ni containing complexes have been established by
PXRD data (Rietveld refinement) and found to be similar to
those of [L–Zn–Ln] complexes. Crystal packing for all [L–Cu–Ln]
complexes is similar in which Ln is ten coordinated. However,
[L–Zn–Ln] complexes show two different types of packing due
to variation of the coordination number of LnIII ions (nine or
ten). Photophysical studies, however, reveal fascinating differ-
ences. While all the [L–Zn–Ln] complexes show prominent Ln
emission in the visible to NIR range (600 nm to 1600 nm),
[L–Ni–Ln] and [L–Cu–Ln] complexes do not exhibit any emis-
sion in the NIR region. Considering the fact that synthesis of
metal–ligand–lanthanide complexes has been an attractive field
to obtain the NIR emission, which can be used for various
purposes, our study clearly demonstrates that one must be
careful, while choosing the d-block metal ion. It is now clear
that L–Cu and L–Ni do function as ‘‘antenna’’, while L–Zn is
found to be an excellent antenna for transferring the absorbed
energy to Ln energy manifolds.
7 L. Sun and L. Shi, Lanthanides: Near-Infrared Materials,
Encycl. Inorg. Bioinorg. Chem., 2012, 1, 123–133.
8 (a) D. Maji, M. Zhou, P. Sarder and S. Achilefu, Near-
Infrared Fluorescence Quenching Properties of Copper(II)
Ions for Potential Applications in Biological Imaging, Repor-
ters, Markers, Dyes, Nanoparticles, and Molecular Probes
for Biomedical Applications VI, 2014; (b) Y. Rahimi,
A. Goulding, S. Shrestha, S. Mirpuri and S. K. Deo, Mecha-
nism Of Copper Induced Fluorescence Quenching of Red
Fluorescent Protein, DsRed, Biochem. Biophys. Res. Com-
mun., 2008, 370(1), 57–61.
Conflicts of interest
There are no conflicts to declare.
9 Y. Iizumi, M. Yudasaka, J. Kim, H. Sakakita, T. Takeuchi and
T. Okazaki, Oxygen-Doped Carbon Nanotubes for Near-
Infrared Fluorescent Labels and Imaging Probes, Sci. Rep.,
2018, 8(1), 1–6.
10 (a) J.-C. G. Bu¨nzli, On The Design of Highly Luminescent
Lanthanide Complexes, Coord. Chem. Rev., 2015, 293–294,
19–47; (b) J. B. S. Linden-Leibeszeit, S. V. Eliseeva,
N. Chaudhary, M. Zeller, S. Petoud and E. R. Trivedi,
Near-Infrared Emitting Heterobimetallic Zn-4F Schiff Base
Complexes With Visible Excitation Wavelength, Eur. J. Inorg.
Chem., 2020, 75–78.
Acknowledgements
Department of Chemistry, Institute of Science, Banaras Hindu
University is gratefully acknowledged for the infrastructure. AV
thanks CSIR, New Delhi, for providing a JRF-Fellowship. SSS
acknowledges the help in XRD data recording from Prof. H
Nishihara and the JSPS Bridge Fellowship.
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