A rational synthesis of a novel imidazo[4,5-f][1,10]phenanthroline templated Schiff base: Characterization, photoluminescence and DFT/TD-DFT study

Article abstract of DOI:10.4314/bcse.v31i1.12

A new imidazo[4,5-f][1,10]phenanthroline (imp) derivative imidazo-N⁵,N⁶-bis((4-(1H-imidazo[4,5-f][1,10]phenanthroline-2-yl)phenyl)methylene)-1,10-phenanthroline-5,6-diamine (impap) was synthesized in five steps starting from bare phenanthroline (phen) precursors. The novel compound was fully characterized by ¹H-NMR, IR, elemental analysis and electrospray ionization mass spectroscopy (ESI-MS) techniques. Solid state emission spectrum of impap showed two distinct strong emission maxima with large Stokes shifts. The ground state gas phase geometry of impap was predicted by DFT calculations. Excited state properties of the molecule were examined through TD-DFT calculations conducted at the optimized geometry. Responsible transitions for the strong fluorescence of impap were assigned to single component charge transfer transitions with large oscillator strengths based on the ground state calculated molecular orbital contributions.

Full text of DOI:10.4314/bcse.v31i1.12

Bull. Chem. Soc. Ethiop. 2017, 31(1), 137-147.
2017 Chemical Society of Ethiopia and The Authors
DOI: http://dx.doi.org/10.4314/bcse.v31i1.12
ISSN 1011-3924
Printed in Ethiopia

A RATIONAL SYNTHESIS OF A NOVEL IMIDAZO[4,5-f][1,10]PHENANTHROLINE
TEMPLATED SCHIFF BASE: CHARACTERIZATION, PHOTOLUMINESCENCE
AND DFT/TD-DFT STUDY
1
2
3*
4
S. Karslıoğlu , S. Demir , H. Yılmaz and S. Gördük
1
Department of Chemistry, Faculty of Science, Karadeniz Technical University,
Trabzon 61080, Turkey
2
Department of Industrial Engineering, Faculty of Engineering, Giresun University, Giresun
8200, Turkey
Department of Chemistry, Faculty of Arts and Science, Ondokuz Mayıs University, Samsun
5200, Turkey
Department of Chemistry, Faculty of Arts and Science, Yildiz Technical University, Istanbul
4210, Turkey
2
3
5
4
3
(
Received July 31, 2015; revised August 6, 2016)
5
6
ABSTRACT.
A
new imidazo[4,5-f][1,10]phenanthroline (imp) derivative imidazo-N ,N -bis((4-(1H-
(impap) was
imidazo[4,5-f][1,10]phenanthroline-2-yl)phenyl)methylene)-1,10-phenanthroline-5,6-diamine
synthesized in five steps starting from bare phenanthroline (phen) precursors. The novel compound was fully
1
characterized by H-NMR, IR, elemental analysis and electrospray ionization mass spectroscopy (ESI-MS)
techniques. Solid state emission spectrum of impap showed two distinct strong emission maxima with large
Stokes shifts. The ground state gas phase geometry of impap was predicted by DFT calculations. Excited state
properties of the molecule were examined through TD-DFT calculations conducted at the optimized geometry.
Responsible transitions for the strong fluorescence of impap were assigned to single component charge transfer
transitions with large oscillator strengths based on the ground state calculated molecular orbital contributions.
KEY WORDS: Phenanthroline, TD-DFT, Photoluminescence, Stokes shift, 1,10-phenanthroline-5,6-dione,
Imidazo[4,5-f][1,10]phenanthroline
INTRODUCTION
The syntheses of imidazo[4,5-f][1,10]phenanthroline (imp) (Scheme 1) and its derivatives
started with the syntheses of phenanthrimidazoles by Steck and Day [1]. The subsequent
explorations have gained an increasing prominence on the strength of their fruitful
implementations such as various luminescent sensors or stimulants, pharmacological agents,
photoactive materials of organic light emitting diode (OLED) devices and mesophasic
ingredients of liquid crystals. More precisely imp derivatives, associated with their large Stokes
shifts and luminescence lifetimes, have excellent photophysical applications such as efficient
hole blocker and electron transporters in OLED materials and molecular switches [2-9]. They
are available as multifunctional ligands in the construction of high order non-linear systems [10-
1
2]. They constitute versatile components of metallomesogens or behave as mesogenic phase
without metal component in the design of liquid crystals [13-16]. In addition, they play key
roles in spectroscopic biosensing of nucleic acids owing to site-selective and adaptive binding to
DNA in intercalative mode and in the development of unique chemotherapeutics where they
impact on the functions of DNA within cancerous cells [17-27]. Therefore, imp derivatives
seem to attract much more interest with many attempts to synthesize and design new materials
with the above-mentioned properties. These pivotal multifunctionalities stem essentially from
their interrelated rigid flat and extended π-aromatic structures displaying high conjugation
throughout the entire molecule. The researchers exploiting their planar natures are mainly
_
*
_________
Corresponding author. E-mail: hakan.yilmaz@omu.edu.tr

138
S. Karslıoğlu et al.
focused on the stereo-selective binding affinity towards double stranded-DNA by an
intercalative mode. Since almost emission-silent triplet excited states in organic moieties or
MLCT states in metal complexes, are strongly perturbed upon intercalation of planar imp parts
between base pairs of DNA. These interactions produce efficient luminescence signals leading
them to be advertised as molecular light switches for nucleic acids [26, 28, 29]. The studies with
respect to their extended conjugate-π aromatic characters have been mostly on light emitting
displays. Since their extended highest occupied molecular orbital (HOMO) - lowest unoccupied
molecular orbital (LUMO) gap with relatively low LUMO energy as crucial characteristics
make them promising candidates especially for modern OLED’s [3-5, 30, 31].
N
N
R
N
H
N
R: Aliphatic or aromatic groups
Scheme 1. General structure of imp derivatives.
There are numerous studies on organic compounds or metal complexes containing imp
derivatives as functionalities that mainly account for the property of interest together with
different groups or ancillary ligands. In this study, we report the synthesis of a new bulky and
versatile member of imp family. The extended conjugate structure consists of imp moieties and
5
,6-diamino-1,10-phenanthroline body. These imp moieties were symmetrically attached via
terephthalaldehyde (tpa) connectors to 5,6-diamino-1,10-phenanthroline (dap) body which was
also synthesized from 1,10-phenanthroline-5,6-dione as the same precursor with that of imp.
1
The main compound was successfully characterized by ESI-MS, elemental analysis, H-NMR
and IR spectroscopy techniques. The solid state photoluminescence measurement was carried
out to simply show strong photoluminescent character of impap with an expected large Stokes
shift and further probed excited states on theoretical viewpoint by means of DFT/TD-DFT
calculations.
EXPERIMENTAL
Materials and measurements
All reagents and solvents were obtained from commercial suppliers and used without further
purification unless otherwise specified.
Infrared spectra were measured with a Perkin-Elmer Spectrum-One FT-IR Spectrometer
-
1
1
within 400-4000 cm range using KBr pellets. H-NMR spectra were recorded in DMSO-d
6
medium with a Varian Mercury 200 MHz Spectrometer. Elemental analysis measurement was
performed with a LECO CHNS Elemental Analyzer. ESI-MS spectrum was recorded in
methanol with an AB SCIEX QTRAP® 5500 LC/MS/MS System Mass Spectrometer. The
sample was dissolved in hyper grade DMF stirring in a Banderin Sonorex ultrasonic bath, and
infusion (continuous signal) method with a flow rate of 5-20 m/min was used for mass
scanning. The photoluminescence spectrum for the solid sample was taken with a Perkin-Elmer
LS-55 Spectrophotometer.
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A rational synthesis of a novel imidazo[4,5-f][1,10]phenanthroline templated Schiff base 139
Computational protocol
All computations were carried out using Gaussian 09 suit of programs running under Unix or
Windows [32]. Geometry optimization was performed by using hybrid DFT/B3LYP functional
within C2 symmetry constrain and subsequent TD-DFT calculation at the optimized geometry
was performed for the first 70 vertical singlet excitations [31, 33, 34]. Double zeta 6-31G(d,p)
basis set was used for geometry optimization while flexible LANL2DZ basis set was used for
TD-DFT calculation [35-37]. Molecular orbital (MO) contributions of each transition were
analyzed using Gaussian version 2.2 software package [38]. Simulated excitation profile was
obtained using Swizard program version 4.7 [39, 40] with the Gaussian type fit (or lineshape or
-
1
function) and taking the half-bandwidths, Δ1/2,I, equal to 1500 cm .
Synthesis of the compound
The main compound was synthesized in a stepwise manner modifying the literature procedures
as outlined. Step by step reactions of the compound are depicted in Scheme 2.
Synthesis of 1,10-phenanthroline-5,6-dione (Step 1). A well-ground mixture of 10.0 g (50
mmol) phenanthroline monohydrate and 9.05 g (76.0 mmol) KBr in a round bottom flask fitting
with a dropping funnel was mounted in a cooling bath at -10 °C. A mixture of H
2 4
SO (91 mL,
9
6%) and HNO (42 mL, 65%) at -78 °C maintained by ducking into liquid nitrogen was gently
3
added to the first mixture during 30 min. The resulting reddish-orange suspension was left
undisturbed for cooling to room temperature (rt) and refluxed at 150 °C until bromine evolution
to cease (5-7 h). The obtained yellow suspension was cooled to rt, gradually poured into 1 kg
ice, and pH of the mixture was raised up to 6 by adding 30% NaOH. The final yellow mixture
2 4
was extracted with dichloromethane, dried on anhydrous Na SO , recrystallized from hot
methanol, and dried in air [41]. Yield: 7.15 g (68%), mp 257 °C.
Synthesis of 1,10-phenanthroline-5,6-dioxime (Step 2). A mixture of 1,10-phenanthroline-5,6-
dione (3.15 g, 15 mmol), hydroxylammonium chloride (3.65 g, 52.5 mmol) and barium
carbonate (4.44 g, 22.5 mmol) in 200 mL ethanol was refluxed for 12 h. 300 mL 0.2 M HCl was
added to the residue remained after the removal of the solvent and the mixture was further
stirred for 30 min. The resulting mixture was filtered and the pale yellow precipitate was washed
with water, ethanol and ether respectively and dried in a vacuum oven at 80 °C [42]. Yield: 1.80
g (50%), mp 231 °C.
Synthesis of dap (Step 3). 2.0 g 1,10-phenanthroline-5,6-dioxime (8.32 mmol) and 2.0 g Pd/C
(
10%) were slurried in 250 mL anhydrous ethanol and after degassing with nitrogen, the mixture
was refluxed for 1 h. A mixture of hydrazine monohydrate (18 mL) and ethanol (40 mL) was
added dropwise for 1 h to this solution and refluxed for 12 h. The crude reaction mixture was
filtered through celite using a crucible (4 μm) to remove catalyst and washed four times with hot
ethanol. The filtrate was brought to dryness with a vacuum evaporator and 120 mL water was
added to the remainder. The solution was left for a night at 4 °C. The resulting well-formed
yellowish-brown precipitate was filtered, washed with cold water and dried in vacuum [42].
Yield: 0.77 g (44%), mp > 300 °C.
Synthesis of impal (Step 4). To a formerly prepared stirring mixture of 1.68 g 1,10-
phenanthroline-5,6-dione (8 mmol), and 12.33 g ammonium acetate (160 mmol) in 80 mL
glacial acetic acid, terephthalaldehyde in acetic acid was added very gently to prevent instant
excess diimine formation and the mixture was refluxed for 2 h. The mixture was filtered while
hot to separate slurry diimine by-product, the filtrate was cooled to rt and diluted with 300 mL
Bull. Chem. Soc. Ethiop. 2017, 31(1)

140
S. Karslıoğlu et al.
Scheme 2. Synthesis reactions of impap.
Bull. Chem. Soc. Ethiop. 2017, 31(1)

A rational synthesis of a novel imidazo[4,5-f][1,10]phenanthroline templated Schiff base 141
distilled water. A yellow precipitate formed upon dropwise addition of concentrated ammonia to
the solution was filtered, washed with water, dried in air and resolved in hot ethanol. The
crystalline product formed upon cooling was filtered, washed with ethanol, water and ether,
respectively and dried in vacuum [1, 12]. Yield: 1.94 g (75%), mp > 300 °C.
Synthesis of impap (Step 5). A 25 mL ethanol solution of dap (0.6 g, 2.85 mmol) was gradually
added into a degassed stirring solution of impal (1.85 g, 5.7 mmol) in ethanol. The mixture was
refluxed for 4 h under nitrogen atmosphere, cooled to rt, and filtered. The precipitate formed
upon cooling was washed several times with hot ethanol and ether, and dried in a vacuum oven
-
1
at 80 °C. Yield: 1.52 g (65%), brown powder, mp > 300 °C. IR spectrum, ν, cm : 3401 ν(N-H);
1
3
065 ν(C-H); 1637 νimine(C=N); 1606 νphen(C=N). H-NMR spectrum (200 MHz; DMSO-d
6
),
+
+
+
δ/ppm: 13.9 (2H, s); 7.8-9.1 (28H, m). ESI-MS spectrum m/z: 619 [a] , 605 [b] , 543 [c] , 516
+
+
+
+
+
+
+
+
[
d] , 501 [e] , 323 [f] , 310 [g] , 296 [h] , 234 [i] , 220 [j] , 180 [k] . Found, %: C 70.38; N
1
8.55; H, 4.98. Calculated (tetrahydrated form), %: C 69.79; N 18.78; H 4.28.
RESULTS AND DISCUSSION
Characterization
-
1
Characteristic ‒NH stretching vibration of dap at 3371-3264 cm and ‒C=O stretching
2
-1
vibration of impal at 1697 cm were not observed in IR spectrum of impap. Besides, appearance
of ‒C=N‒ stretching vibration at 1637 cm instead of them clearly denotes to the formation of
impap.1
-
1
In H-NMR spectrum of impap, a broad singlet at 13.9 ppm related to imidazolic protons
and another peaks related to CH protons between 7.8-9.1 ppm were observed. The proximities
of chemical shifts of CH protons and low frequency power of the instrument complicate the
discrimination of them. On the other hand, integration rate (13.73) of imidazolic and CH protons
indicates the formation of impap (NH, 2H; CH, 28H). Also disappearance of aldehydic proton
1
(
s, 10.05 ppm) of impal in H-NMR spectrum of impap additionally supports the formation of
the related product.
ESI-MS spectrum of impap most clearly verifies the formation of impap. Since peaks at m/z:
6
19, 605, 543, 516, 501, 323, 310, 296, 234, 220, 180 notated respectively with a, b, c, d, e, f, g,
h, i, j, k belong to fragmentation products of impap.
Ground state structure and MO analysis
The geometry of impap was optimized and verified for global minimum by subsequently
performed frequency analysis having no imaginary frequency. Optimized structure of impap
with C2 constrained symmetry is depicted in Figure 1. 2-(4-formylphenyl)imidazo-[4,5-
f][1,10]phenanthroline (impal) fragments of the molecule are trans oriented reciprocally at the
optimized geometry and in these units, imp and tpa fragments are placed coplanar solely with a
torsion of 0.713°. It can be said that the planarity of impal units is the major effect which
produces large conjugation that also reflects strong fluorescence with large Stokes shift. A
complete planarity of impap seems impossible owing to steric repulsion of imine hydrogens.
Ground state MO contributions of frontier, the first two low and high lying MOs from the
groups are given in Table 1 and their contourplots are depicted in Figure 2. As inferred from
Table 1, HOMO is mainly localized on phen moiety and comprises equal contributions from tpa
and imp parts while the LUMO comprises the largest contribution from tpa. HOMO-1 and
HOMO-2 are composed mainly of imp and HOMO-3 completely resides on phen with none of
contribution from the others. LUMO+1 spreads over the entire molecule with the largest
Bull. Chem. Soc. Ethiop. 2017, 31(1)

142
S. Karslıoğlu et al.
contribution from tpa. Intere
s
s
tingly, LUMO+2 and LUMO+3 are completely degenera tt e and
reside completely on impap.
Figure 1. Geometry optimized structure of impap.
Table 1. Ground state MO contributions in impap from the selected groups.
MO
E (eV)
imp
H
-5.51
20
H-1
-5.82
80
H-2
-5.94
63
H-3
-6.45
0
L
-2.41
19
L+1
-2.2
25
L+2
-1.5
100
0
L+3
-1.5
100
0
tpa
18
16
12
0
54
55
phen
62
4
25
100
27
20
0
0
H, HOMO; H-1, HOMO-1, etc. L, LUMO; L+1, LUMO+1, etc.
Emission spectrum and TD-DFT excited states
In order to specify the origin of the excited states responsible for the experimental e mm ission
spectrum of imp and to expre
were considered to take plac
impap as shown in Figure 3 dii
s
e
s percentage MO compositions of them, the calculated tran
between ground state Kohn-Sahm MOs. Emission spectrum of
splays two distinct emission maxima at 535 nm and 588 nm upon
excitation at 387 nm where th ee strongest absorption occurs.
The first broader emissio maximum cannot be attributed to any excimer formation ee ven in
solid state since it takes place
ss itions
nn
a
t lower wavelength. Calculated excited state parameters of impap
with non-zero oscillator stren
The transitions with the l
nm and are almost related to s
gg ths are given in Table 2.
aa rgest oscillator strengths occur at higher wavelengths abo vv e 360
i
state with an oscillator streng tt h of 0.3705 is of almost pure HOMO→LUMO transition ww ith a
contribution of 96% and designated as single charge transfer (CT) transition with
phen/π→tpa/π* character. Th
ee
most intense third excited state with the oscillator stre nn gth of
0
.7794 is of also single comp
o
o
nent HOMO-1→LUMO transition with imp/π→tpa/π* ch
aracter
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A rational synthesis of a nn ovel imidazo[4,5-f][1,10]phenanthroline templated Schiff bas ee
143
Figure 2. Ground state MOs o ff impap.
Figure 3. Solid state emission spectrum of impap.
Bull. Chem. Soc. Ethiop. 2017, 31(1)

144
S. Karslıoğlu et al.
Table 2. Excited state parameters of impap.
State
λ (nm)
501
457
393
376
f
Assign./Cont.
H→L/96%
H→L+1/91%
H-1→L/96%
H-1→L+1/96%
H-2→L+1/89%
1
2
3
6
7
0.3705
0.2765
0.7794
0.2165
0.5841
364
and most probably related to applied excitation (387 nm) for the emission spectrum. As a
consequence, all calculated transitions of reasonably non-zero oscillator strengths and of higher
wavelengths are assigned to strong CT transitions since they all originate from almost one MO
transitions each of that is with varied MO composition as can be understood from Tables 1 and
2
. The observed emission maxima are most probably related to vibration manifolds of the first
excited state transition calculated at 382.4 nm to which the applied 387 nm excitation is
assigned. Stokes shifts of 535 nm and 588 nm emission maxima pursuant to 387 nm excitation
maximum are 148 nm and 201 nm respectively and are larger than most of imp derivatives
previously reported [43-46].
CONCLUSION
A novel member of imidazo[4,5-f][1,10]phenanthroline family was prepared in five successive
steps and characterized successfully by spectroscopic measurements. The ground state structure
of impap was predicted by DFT/B3LYP/6-31G(d,p) level and the geometry was found
nonplanar because of torsion around imine bonds. However, rigid planarity of impal units is
thought to lead strong conjugation and fluorescence. Since both HOMO-1 and LUMO involving
in the most intense CT transition responsible for the fluorescence reside completely on impal
units that spread the entire unit. Stokes shifts of the two emission maxima were found to be
larger than most of the related structures previously reported. In comparison to many other
previously published imp derivatives, the compound prepared herein has a higher conjugative
character through the entire molecule that gives rise to efficient fluorescence. The complete
conjugation of the molecule may also result in high electrical properties. Furthermore, the
compound may be considered as a potential candidate for DNA intercalation applications due to
its larger planarity and aromaticity.
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