Crystal structure, thermal, luminescent and terahertz time domain spectroscopy of magnesium N-phthaloyl-β-alaninate: A combined experimental and theoretical study

Article abstract of DOI:10.1016/j.molstruc.2018.05.063

A magnesium complex using N-phthaloyl-β-alanine (NPA) as ligand was synthesized and its crystal structure was characterized by single-crystal X-ray diffraction analysis. The Fourier transformation infrared spectroscopy (FTIR), thermogravimetry (TG) and differential thermal analysis (DTA), fluorescence spectroscopy and terahertz time-domain spectroscopy (THz-TDS) were performed for magnesium N-phthaloyl-β-alaninate (MgNPA). The coordination geometry around Mg (II) has found to be distorted octahedral and the coordination mode of NPA in MgNPA is monodentate i.e. η¹μ¹. The significant enhancement was observed in the emission intensity of MgNPA complex as compared to the ligand NPA during solid state photoluminescence spectroscopy. The computational investigations of the complex were carried out at B3LYP/6-31 + G (d,p) level in gas phase to support our experimental results. The small energy gap between frontier molecular orbitals (FMOs) manifested that MgNPA is a very reactive, chemically soft and optically active complex. The terahertz time domain spectroscopy of the complex and ligand was performed in order to characterize as well as to find out refractive index and absorption coefficient in 0.2–3.2 THz frequency range. Both the complex and ligand shows the characteristic absorption peaks in this frequency range. The decrease in refractive index is observed by the complexation of NPA with Mg.

Full text of DOI:10.1016/j.molstruc.2018.05.063

Accepted Manuscript
Crystal structure, thermal, luminescent and terahertz time domain spectroscopy of
magnesium N-phthaloyl-β-alaninate: A combined experimental and theoretical study
Muhammad Nadeem, Moazzam H. Bhatti, Koray Sayin, Uzma Yunus, Mazhar
Mehmood, Shoaib Mehboob, Ulrich Flörke
PII:
S0022-2860(18)30631-8
10.1016/j.molstruc.2018.05.063
MOLSTR 25236
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 24 December 2017
Revised Date: 4 May 2018
Accepted Date: 19 May 2018
Please cite this article as: M. Nadeem, M.H. Bhatti, K. Sayin, U. Yunus, M. Mehmood, S. Mehboob, U.
Flörke, Crystal structure, thermal, luminescent and terahertz time domain spectroscopy of magnesium
N-phthaloyl-β-alaninate: A combined experimental and theoretical study, Journal of Molecular Structure
(2018), doi: 10.1016/j.molstruc.2018.05.063.
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ACCEPTED MANUSCRIPT
Crystal structure, thermal, luminescent and terahertz time domain
spectroscopy of magnesium N-phthaloyl-β-alaninate: A combined
experimental and theoretical study
a, c
a
b
a
c
Muhammad Nadeem
Shoaib Mehboob
, Moazzam H. Bhatti *, Koray SAYIN , Uzma Yunus , Mazhar Mehmood ,
c
d
, Ulrich Flörke
a
Department of Chemistry, Allama Iqbal Open University, Islamabad, Pakistan
b
c
Department of Chemistry, Institute of Science, Cumhuriyet University 58140 Sivas – Turkey
National Center for Nanotechnology, Department of Metallurgy and Materials Engineering,
Pakistan Institute of Engineering and Applied Sciences (PIEAS), Nilore 45650, Islamabad,
Pakistan
d
Anorganische und Analytische Chemie, Fakultät für Naturwissenschaften, Universität
Paderborn, Warburgerstrasse 100, D-33098 Paderborn, Germany
E-mail of corresponding Author:
*
moazzamhussain_b@yahoo.com (M.H. Bhatti).

ACCEPTED MANUSCRIPT
Abstract
A magnesium complex using N-phthaloyl-β-alanine (NPA) as ligand was synthesized and its
crystal structure was characterized by single-crystal X-ray diffraction analysis. The Fourier
transformation infrared spectroscopy (FTIR), thermogravimetry (TG) and differential thermal
analysis (DTA), fluorescence spectroscopy and terahertz time-domain spectroscopy (THz-TDS)
were performed for magnesium N-phthaloyl-β-alaninate (MgNPA). The coordination geometry
around Mg (II) has found to be distorted octahedral and the coordination mode of NPA in
1
1
MgNPA is monodentate i.e. η µ . The significant enhancement was observed in the emission
intensity of MgNPA complex as compared to the ligand NPA during solid state
photoluminescence spectroscopy. The computational investigations of the complex were carried
out at B3LYP/6-31+G(d,p) level in gas phase to support our experimental results. The small
energy gap between frontier molecular orbitals (FMOs) manifested that MgNPA is a very
reactive, chemically soft and optically active complex. The terahertz time domain spectroscopy
of the complex and ligand was performed in order to characterize as well as to find out refractive
index and absorption coefficient in 0.2-3.2 THz frequency range. Both the complex and ligand
shows the characteristic absorption peaks in this frequency range. The decrease in refractive
index is observed by the complexation of NPA with Mg.
Keywords: N-phthaloyl-ß-alanine, Single X-ray, Terahertz time-domain spectroscopy (THz TDS),
Fluorescence spectroscopy

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1
Introduction
The coordination compounds have attained considerable attention owing to their potential
applications like catalysis, as optical and magnetic materials [1-4]. Magnesium ion being most
abundant in living things plays essential role in many biological processes [5]. The polarizing
power of magnesium (II) is very dominant in s-block metals, therefore the coordination bond
between magnesium (II) and oxygen is very strong providing some hybrid magnesium
framework [6-9]. The complex solid structure architecture mainly depends upon the choice of
organic ligand. ß-alanine has appeared to build up the concentration of carnosine in muscles,
diminish weariness in athletes and enhance the efficiency of muscles [10]. The phthalimide
group helps to provide supramolecular host design and also act as a protecting group for amino
acids. Its fluorescence properties are highly environment sensitive [11]. N-phthaloyl-β-alanine
contains highly conjugated phthalimide aromatic cyclic moiety. Nonlinear optical properties
have been enhanced due to the presence phthalimide group in previously reported CaNPG
complex [12]. For the support of experimental work quantum chemical calculations are
significant research area for researchers [13-17].
In recent years, terahertz spectroscopy has attracted the attention of researchers for the study of
vibrational modes in far infrared frequency range. It is used for the nondestructive testing [18],
identification of drugs [19], and determination of dielectric properties of materials [20-24]. It is
also used for the characterization of different biological molecules such as amino acids [25],
aromatic carboxylic acids [26], proteins, DNA, bovine serum albumin and collagen [27], etc.
Different organometallic complexes show the signature in THz frequency range [28-30]. These
signatures can be used for the identification of the complex.

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In this work we have reported the synthesis of new single crystal magnesium N-phthaloyl-β-
alaninate (MGNPA) complex. This compound was characterized by Single-crystal X-ray
diffraction analysis, FTIR, TG-DTA, UV-visible spectroscopy, NMR and THz TDS. Optimized
compound structure, geometric parameters, vibration frequencies, contour diagram of frontier
molecular orbitals (FMOs), molecular electrostatic potential (MEP) maps, MEP contours, NMR
spectrum are investigated at B3LYP/6-31+G(d,p) level. The solid state photoluminescence of
complex and ligand was performed in order to find out the optical properties of the complex.
2
2
Experimental
.1
Synthesis of N-phthaloyl β-alanine
N-phthaloyl β-alanine was prepared by the reported methods [31]. In a Pyrex test tube, an
equimolar amount of beta alanine and phthalic anhydride were fused using previously heated oil
o
bath at 180-185 C for fifteen minutes. After this, the test tube was carefully removed and cooled
until the liquid mass was solidified. It was then inverted and the excess phthalic anhydride
sublimed on the walls was scrapped out. The crude product was recrystallized in ethanol solvent.
2
.2
Synthesis of MgNPA
An aqueous solution of sodium hydroxide (9.1mmol in 50ml) was added dropwise to a well-
stirred solution of N-phthaloyl-β-alanine (9.1mmol) in ethanol (40 ml) at room temperature. The
solution was stirred for next 2 h and then 4.5mmol of magnesium acetate [Mg(CH COO) ] were
3
2
added. The solution was heated to 80°C and stirred at this temperature for 2 h. The hot solution
was filtered and set aside for crystallization. After four weeks, ~2g (41% yield) of colorless
single crystals were obtained. The synthesis scheme, a) for ligand, and b) for complex is given
below.

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Scheme 1. (a) Synthesis of NPA and (b) MgNPA.
2
.3
Computational Method
GaussView 5.0.9 [32] and Gaussian 09 AS64L-G09RevD.01 [33-37] package programs were
used to do quantum chemical calculations. The graphs and figures were prepared by using
different softwares as ChemBioDraw Ultra Version (13.0.0.3015), Gabedit, Chemissian and
Chemcraft [38, 39]. B3LYP which is one of the hybrid density functional theory functions was
selected as method with basis set, 6-31+G(d,p) [40, 41] for MgNPA. No imaginary frequency
was obtained from the calculations.
2
.4
The intensity data for all compounds were recorded using a Bruker SMART CCD area-
detector diffractometer with graphite monochromated Mo K radiation ( = 0.71073 Å) at
Crystal structure determination
α
λ
T = 130(2) K. Semi-empirical absorption correction applied from equivalents i.e.
structure solutions by direct methods [42], full-matrix least squares refinements [42]
2
based on
F
. All but H-atoms were refined an-isotropically, hydrogen atoms were clearly
located from difference Fourier maps, refined at idealized positions riding on the parent
atoms with isotropic displacement parameters U (H) = 1.2U (C/O) and C-H 0.95-0.98
iso
eq
Å.

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2
.5
Measurements
Infrared spectra were recorded for the free ligand and title complex on Nicolet iS 10 FTIR
-
1
Spectrophotometer (4000-400 cm ) in KBr discs. Melting points were determined via
Gallen Kamp electro thermal apparatus, Cat No.MPD350, sanyo, UK. UV-visible spectra
of ligand solution and MgNPA solution in deionized water were recorded on UV-1800
Shimadzu using UV probe version 2.32 control software for Shimadzu UV-Vis(-NIR)
1
13
spectrophotometers. The H and C nuclear magnetic resonance spectrum were recorded
at room temperature in DMSO-d6 on a Bruker Avance Digital 300 MHz and 75 MHz
spectrometer (Switzerland), respectively. Chemical shifts are given in ppm and coupling
constant (
J) values are given in Hz. Fluorescent emission spectra were recorded on F-
7
000 FL spectrophotometer 2133-007. Thermal decomposition patterns (TG and DTA) of
the title compound was carried out under nitrogen atmosphere up to 800°C utilizing a
heating rate of 10°C/min on a DTG-60H simultaneous DTA-TG Analyzer.
2
.6
THz-Time Domain Spectroscopy
The THz measurements were performed using Femto Fiber Pro laser system from TOPTICA
Photonics. The 100 fs laser pulses centered at 780 nm with average power of
~45mWweregenerated. Beam splitter was used to divide the laser beam into two parts. The THz
pulses are generated and detected by focusing the laser power of ~30 mW and ~15 mW on
photoconductive THz emitter and detector, respectively. All the measurements were performed
in transmission mode. In order to reduce the effect of humidity, the measurement chamber was
purged with N gas.
2
The ground powder of MgNPA and NPA was placed in a cell with HDPE windows. HDPE
windows are used because of low absorption of THz radiations in HDPE. The time domain

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reference signal was recorded by passing THz radiations through the empty cell and the time
domain sample signal was recorded by passing THz radiations through the cell filled with the
powder. These time domain signals were fourier transformed to get complex frequency domain
signals which were then used to calculate the refractive index and absorption coefficient of
MgNPA and NPA. The detail of calculations can be found elsewhere [21].
3
3
Results and Discussions
Specific structural details
.1
Single-crystal X-ray diffraction analysis reveals the formation of the magnesium complex [C44
H N O Mg]. It was found MgNPA crystallized in monoclinic system with C2/c space group.
4
2
4
20
The unit cell has dimensions a = 20.8884(17) Å, b = 19.8335(17) Å, c = 13.4979(11) Å, α=
3
9
0°, β = 129.319(2) °, γ = 90° with volume i.e. V= 4326.2(6) Å and Z=4. The most relevant
crystal data and structure refinement details are given in Table 1. Each magnesium atom is
coordinated by six oxygen atoms i.e. two carboxylate oxygen atoms [O(4), O(4A)] in a
1
1
monodentate manner (η µ ) and four with labeled oxygen atoms [O(9),O(9A),O(10), O(10A)] of
water molecules. Out of four ligand molecules, the carbonyl groups of two n-phthaloyl ß-alanine
molecules remained uncoordinated. The selected bond angles (deg) and bond lengths suggest the
formation of distorted octahedral geometry around magnesium. The schematic of optimized and
experimental structure with atomic labeling is shown in Fig. 1. The calculated thermo-dynamic
parameters including total energy, enthalpy and Gibbs free energy of the complex are -
2
065.028007, -2065.027063 and -2065.143392 a.u., respectively. The selected experimental and
calculated geometric parameters of magnesium complex are given in Table 2.

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Fig. 1. (a) View of single X ray crystal Structure for MgNPA, anisotropic displacement ellipsoids are drawn at the
5
0% probability level (color codes: Mg, dark green; C, grey; O, red; N, blue; H, light green), symmetry
transformations used to generate equivalent atoms for 2a: #1 -x,y,-z+1/2. (b) optimized structure of MgNPA at
B3LYP/6-31+G(d,p) level.
Table 1. Crystal data and structure refinement details for MgNPA.
Parameters
Empirical formula
Formula weight
T (K)
MgNPA
C44 H Mg N O
20
42
4
971.13
130(2) K
0.71073
Monoclinic
C2/c
Wavelength (Å)
Crystal system
Space group
a (Å)
20.8884(17)
19.8335(17)
13.4979(11)
90
b (Å)
c (Å)
α (°)
β (°)
129.319(2)
90
γ (°)
3
Volume (Å )
4326.2(6)
4
Z
3
D calc (Mg/m )
1.491
-1
Absorption coefficient(mm )
F(000)
0.132
2024
3
Crystal size (mm )
0.38 x 0.21 x 0.18
1.63 to 27.88
20410
θ(°) range
Reflections collected
Independent reflections
R(int)
5159
0.0339
Completeness to θ = 27.88°
Max. and min. transmission
99.90%
0.9767 and 0.9516

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Table 2. Experimental and theoretical bond lengths (Å) and bond angles (°) for MgNPA.
Assignments
Experimental
Calculated
Bond Lengths (Å)
Mg1-O6 O4A
Mg1-O7 O4
2.0489
2.0489
2.0699
2.0699
2.0561
2.0560
1.9760
1.9763
2.2364
2.2364
2.1377
2.1368
Mg1-O8 O9A
Mg1-O9 O9
Mg1-O10 O10A
Mg1-O11 O10
Bond Angles (deg.)
O6-Mg1-O7
93.72
167.07
90.36
83.13
96.21
90.36
167.07
96.21
83.13
88.34
84.23
96.46
96.46
84.23
179.04
107.37
175.92
76.22
97.01
99.08
76.17
175.97
98.98
97.18
100.32
80.28
82.31
82.24
80.27
152.61
O6-Mg1-O8
O6-Mg1-O9
O6-Mg1-O10
O6-Mg1-O11
O7-Mg1-O8
O7-Mg1-O9
O7-Mg1-O10
O7-Mg1-O11
O8-Mg1-O9
O8-Mg1-O10
O8-Mg1-O11
O9-Mg1-O10
O9-Mg1-O11
O10-Mg1-O11
Hydrogen Bond Lengths (Å)
O8-H····O7
-
-
-
-
1.9127
1.9138
1.7565
1.7558
O9-H····O6
O10-H····O2
O11-H····O1
The four intramolecular hydrogen bonds present in the complex discussed in the Table 2 and
graphically presented in Fig. 1 (b). The experimental and calculated geometric parameters were
compared through distribution graph as shown in the Fig. 2. It was found that there is a good
agreement between experimental and calculated structures.

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200
150
100
R² = 0.9617
5
0
0
0
50
100
150
200
Experimental Values
Fig. 2. Distribution graph between experimental and calculated geometric parameters.
3
.2. FTIR spectroscopy
The FT-IR spectra for MgNPA and NPA along with theoretical IR spectrum of MgNPA are
presented in Fig. 3. The modes assigned to the calculated and experimental vibrational
frequencies are given in Table S1. The medium broad absorption in the experimental spectrum of
-1
-1
MgNPA at 3448 cm and 3397 cm proved the presence of coordinated water molecule [43, 44].
The corresponding theoretical symmetric and asymmetric H O stretching vibrations are present
2
-1
-1
at 3576 cm and 3462 cm respectively. The characteristic stretching vibrations of cyclic
-1
carbonyl related to the Phthalimide group appeared at 1763 and 1706 cm in experimental
-1
spectrum, while in calculated spectrum these vibrations were found at 1740 and 1703 cm . The
asymmetric (ma) and symmetric (ms), due to the carboxylate group appeared at 1456 and 1232
-1
cm in experimental spectrum respectively. The phthalimide C-C ring stretching vibrations were
-1
-1
observed experimentally and theoretically at 1363 cm and 1352 cm , respectively. In the low
-1
frequency region, the absorption bands at 519 and 460 cm in experimental IR and at 528, 478,
-1
4
66 cm in theoretical IR are related to Mg-O vibrations.

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Fig. 3. FTIR spectrum of NPA and MgNPA in KBr disc alongwith calculated IR spectrum at B3LYP/6-31+G(d,p)
level in vacuo of MgNPA.
3
.3
Frontier molecular orbitals, molecular electrostatic potential maps and contours
Frontier molecular orbitals which are the highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular orbitals (LUMO) play prominent role in chemical process. The
selected portion of the energy diagram for molecular orbitals of MgNPA calculated at B3LYP
level with basis set 6-31+G(d,p) is presented in Fig. 4. In this diagram, degeneracy tolerance is
determined as 0.002 a.u.
Fig. 4. A certain section of energy diagram of studied complex at B3LYP/6-31+G(d,p) level in gas phase.

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The difference of energy between HOMO-1 and HOMO orbital was found very small which
determines the fact that both HOMO and HOMO-1 are important to determine chemical
reactivity of the complex. As for the unoccupied molecular orbitals, energy of LUMO+1 is close
to energy of LUMO. Thus, the activity of LUMO+1 is similar to that of LUMO. According to
these results, contour diagrams of related molecular orbitals are calculated and represented in
Fig. 5.
Fig. 5. Contour diagrams for HOMO, HOMO-1, HOMO, LUMO and LUMO+1 molecular orbitals at B3LYP/6-
3
1+G(d,p) level in gas phase for the title complex.

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The electron density for molecular orbitals of MgNPA was mapped through color scale which
implies electron dense region (between red and green) or electron scarce region (between green
and dark blue). There are some balloons in HOMO and LUMO of studied complex. The
electrons in HOMO are delocalized in these balloons. If studied complex give electrons, these
regions are active in chemical process. As for the LUMO, there is no electrons in this molecular
orbital. If mentioned complex accept electrons from appropriate chemical species, these regions
are active in this interaction. Therefore, these regions are active in chemical process.
Molecular electrostatic potential (MEP) maps are related to electron density on molecule surface.
This map can be used to predict the active sites of complex. MEP map and contour of mentioned
complex are calculated at B3LYP/6-31+G(d,p) level and represented in Fig. 6.
Fig. 6. MEP maps and contours of MgNPA at B3LYP/6-31+G(d,p) level in gas phase
According to color scale, red implies the electron rich region and blue is electronically poor
region. These results are related with electrophilic and nucleophilic reactivities. So, colors
between red and green in MEP maps are related to electrophilic reactivity. Additionally, the
colors between green and blue in MEP maps are related to nucleophilic reactivity. There are red
regions around the oxygen atoms on MEP map and it means that environment of oxygen atoms
are electrophilic active regions. In MEP contour, there are yellow and red lines. Red lines imply

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the electron dense regions. According to these data, electron dense region is environment of
oxygen atoms.
3
.4
TG-DTA
The TG and DTA curves of NPA and MgNPA are shown in Fig. 7 and the thermo analytical data
were summarized in Table 3. The DTA curve of NPA shows two endothermic peaks. The first
peak at ~157⁰C associated with zero mass loss is due to the melting of NPA. The melting point
of NPA is 149-151⁰C [45]. The second endothermic peak at ~328⁰C is due to the boiling of
NPA. This is also supported by 100 % weight loss in 207-352⁰C temperature range. Finally, the
decomposition of NPA vapors start at ~360⁰C. The shoulders in DTA curve after this
temperature are due to the release of different moieties from NPA at different temperatures. The
TGA of MgNPA showed that the decomposition was taken in five stages. The first stage
occurred with endothermic peak at a maximum temperature of 103ºC. This stage is due to the
loss of four water molecules with a weight loss of 7.31 % while the theoretical weight loss 7.41
%. The rest four stages in the range of 111-770ºC showing the decomposition of four N-
phthaloyl-ß-alanine. In DTA of MgNPA, the exothermic peak at ~218⁰C is shown. The removal
of four water molecules in first stage leaves the dangling bonds of Mg. The peak at 218⁰C may
be due to bonding of uncoordinated ligand molecules with Mg. The final exothermic peak at
5
41⁰C is associated with decomposition of the complex along with the oxidation of Mg. The
decomposition is an endothermic process while oxidation of magnesium is an exothermic
process. The four stages were associated with a weight loss of 84.77 % and the calculated weight
loss value in these four stages was 88.36 %. The found total weight loss value was 93.22 % with
a residue equal to 6.78 % which is corresponding to magnesium oxide MgO. These results are in
a good agreement with the theoretical values, total loss equal 95.77 % and residual 4.12 %.

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Table 3. Thermal Data of Compounds NPA and MgNPA
Thermogravimetry(TG)
Number
of
Stages
o
Decomposition
Species
Compounds/
Codes
Temp.
range ( C)
T
max ( C)
o
Mass Loss (%) Mass Loss (%)
Observed
calculated
2
130-206
157 (+)
328 (+)
0.985 99.39
98.405
100
NPA
NPA
207-352
7
0-110
103(+)
134(+)
218(-)
332(+)
541(-)
7.306
15.947
7.41
4H
2
O
111-200
MgNPA
5
201-290
0.598 84.77
26.266
88.36
4NPA
2
91-425
26-770
4
41.957
Fig. 7. TG-DTA curves of compounds (a) for NPA and (b) for MgNPA.
3
.5
THz spectroscopy measurements
The refractive index and absorption coefficient of MgNPA and NPA are depicted in the Fig. 8.
-1
-1
The absorption spectra of NPA shows peaks at 1.84 THz (61.5 cm ), 2.32 THz (77.6 cm ), 2.88
-1
-1
THz (96 cm ) and a shoulder at 1.29 THz (43 cm ). Wei Ning et. al. [46] has observed a
shoulder at 1.23 THz and 1.99 THz for phenylalanine which were absent in the spectra of
alanine. This suggests that the shoulder at 1.29 THz and peak at 1.84 THz is due to association of
-1
-1
aromatic groups with alanine. Two peaks at 2.23 THz (74 cm ) and 2.56 THz (85 cm ) are
reported by some authors [46, 47] for L- alanine while these peaks are absent in NPA. We have
observed two new peaks at 2.32 THz and 2.88 THz for NPA. The absorption spectra of MgNPA
-1
-1
-1
complex shows peaks at 1.26 THz (42 cm ), 1.82 THz (60.7 cm ) and 2.82 THz (94 cm ) which

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-1
are slightly shifted to lower frequency than that of NPA. Two new peaks at 1.02 THz (34 cm )
-1
and 1.52 THz (50.7 cm ) are also observed in MgNPA which are not present in NPA. These
peaks may arise due to different bonding motility in MgNPA.
The refractive index in terahertz frequency range of NPA increased from 1.96 at 0.2 THz to 2.21
at 1.88 THz and then decreases to 2.08 at 3.2 THz. The changes in refractive index
corresponding to the absorption peaks can also be observed. The refractive index of MgNPA
changes from 1.91 at 0.2 THz to 1.94 at 3.2 THz. The refractive index is higher for the
frequencies corresponding to the absorption peaks of MgNPA. The higher refractive index for
ligand (NPA) than the complex (MgNPA) is also observed by Turan et. al. [48] for optical
frequencies. According to Lorentz-Lorenz equation [49] (B), the higher polarizability of material
results in higher refractive index. This suggests that the polarizability of MgNPA is lower than
that of NPA. This may be due to stronger bonding in MgNPA that of NPA as was also indicated
by different authors. [50, 51]
Fig. 8. Refractive index and absorption coefficient of MgNPA and NPA measured using THz-TDS.

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3
.6
Absorption and Emission Spectroscopy
The transition metal coordination compounds are often used as photoluminescent materials. [52]
Alexey et al reported the good luminescent behavior magnesium coordination complexes. [53]
The UV-Vis spectrum of NPA and MgNPA recorded in water solution showed two absorption
bands in the UV region (Fig. 9a). The bands at λ max range 220-250 nm and 260-300 nm were
assigned to ligand-centered (π-π*) and (n-π*) transitions respectively[54, 55]. The optical
g
bandgap (E ) estimated from the Tauc plot is 4.85eV, the corresponding wavelength λ= 256 nm
was used as excitation wavelength to take the fluorescence emission spectra of both ligand and
complex MgNPA in solid state, resulted emission at 404 nm and 473 nm. The Tauc’ plot and
emission spectra are depicted in Fig. 9b and Fig. S1. The ligand emission peaks may be assigned
*
to the π*→π and π → n transitions. The emission peak positions of ligand and complex are the
same. The intensities of emission for both NPA and MgNPA compounds were different. It was
observed that no emission originates from the metal-centered MLCT/LMCT excited states
because no red or blue shift was observed. Thus the emission bands of both ligand and complex
can be attributed to the intra-ligand fluorescent emission. The strong enhancement of the
luminescence intensity for MgNPA as compared to the free ligand is perhaps the result of metal–
ligand coordination, which effectively increases the rigidity of the ligand and the reduction of
energy loss by nonradiative decay. [56-59]

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-6
-3
Fig. 9. (a) UV spectra of compounds MgNPA and NPA at concentration of 1×10 mol dm in water solution at 25.0
±
0.1 ºC, (b) Photoluminescence spectra in solid state for title complex at 25.0 ± 0.1 ºC.
4
Conclusions
The magnesium complex of N-phthaloyl-β-alanine (MgNPA) was synthesized and its crystal
structure analyzed by both experimental and computational method. In MgNPA the coordination
number of magnesium atom is six with octahedral geometry. The two NPA molecules of ligand
1
1
coordinated with magnesium atom in a monodentate manner (η µ ). The calculated and
experimental results of FT-IR for MgNPA had shown good agreement. MEP maps showed
electrophilic active zone is mainly due to the oxygen atoms. The result of TG-DTA exhibited
that the decomposition of complex is an endothermic process while oxidation of magnesium is
an exothermic process. The THz-TDS shows that the characteristic absorption peaks at different
frequencies for both ligand and complex. The complexation of NPA with MgNPA results the
disappearance of peak at 2.32 THz and emergence of two new peaks at 1.02 THz and 1.52 THz.
Further, the refractive index in THz frequency range is also lower for complex than that of
ligand. In the fluorescence emission spectra the strong enhancement of the luminescence
intensity for MgNPA as compared to the free ligand is the result of metal–ligand coordination.
Acknowledgments

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The numerical calculations reported in this paper are performed at TUBITAK ULAKBIM, High
Performance and Grid Computing Center (TRUBA Resources). The authors also thank
CHEMCRAFT and GABEDIT for allowing using their free internet version. The authors are
thankful to Allama Iqbal Open University and Pakistan Institute of Engineering and Applied
Sciences (PIEAS) for experimental work and characterization. The authors are also thankful to
Mr. Muhammad Mumtaz (NILOP) for his kind cooperation.
Supplementary data
Crystallographic data (excluding structure factors) for the MgNPA had been deposited in the
Cambridge Crystallographic Data Center as supplementary publication, CCDC No. 876494. The
data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2
1
EZ, UK (fax: (internet) +44 1223 336 033; e-mail: deposit@ccdc.cam.ac.uk). or www at
http://www.ccdc.cam.ac.uk].
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Crystal structure, thermal, luminescent and terahertz time domain spectroscopy of
magnesium N-phthaloyl-β-alaninate: A combined experimental and theoretical study
Research Highlights
•
•
•
•
Synthesis and growth of single crystals for MgNPA.
Optimization of complex carried by B3LYP with basis set, 6-31+G(d,p).
TD-THz spectroscopy characterized well MgNPA and NPA.
Decrease in refractive index for NPA due to complexation of NPA with Mg.