Spectroscopic investigation, photophysical parameters and DFT calculations of 4,4′-(1E,1′E)-2,2′-(pyrazine-2,5-diyl)bis(ethene-2,1-diyl)bis(N,N-dimethylaniline) (PENDA) in different solvents

Article abstract of DOI:10.1016/j.saa.2015.05.018

A comprehensive investigation on the photophysics of a π-conjugated potential push-pull chromophore system 4,4′-(1E,1′E)-2,2′-(Pyrazine-2,5-diyl)bis(ethene-2,1-diyl)bis(N,N-dimethylaniline) (PENDA) has been carried out spectroscopically. The optical absorption and emission properties of this molecule have been studied in different solvents. The molecule PENDA shows strong solvatochromic emission upon changing the solvent polarity from nonpolar to polar; indicating that emission state is of intramolecular charge transfer (ICT) character. The solvent effect on the spectral properties such as singlet absorption, molar absorptivity, oscillator strength, dipole moment and fluorescence quantum yield of PENDA have been studied in detail. Lippert-Mataga and Reichardt correlations were used to estimate the difference between the excited and ground state dipole moments (Δμ). Ground and electronic excited states geometric optimizations were performed using density functional theory (DFT) and time-dependent density functional theory (TDDFT), respectively, with the Gaussian 09 package. A solution of (8 × 10^-5 M) PENDA in THF, dioxane, CH₃CN and CHCl₃ gives laser emission when pumped by a nitrogen laser pulse (λ_ex = 337.1 nm) of 800 ps duration and 1.48 mJ pulse energy. PENDA dye displays fluorescence quenching by colloidal silver nanoparticles (Ag NPs) in ethanol. The fluorescence data reveal that dynamic quenching and energy transfer play a major role in the fluorescence quenching mechanism.

Full text of DOI:10.1016/j.saa.2015.05.018

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 722–730
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Spectroscopic investigation, photophysical parameters and DFT
calculations of 4,40-(1E,10E)-2,20-(pyrazine-2,5-diyl)bis(ethene-2,1-
diyl)bis(N,N-dimethylaniline) (PENDA) in different solvents
Abdullah M. Asiri a,b, Khalid A. Alamry a, Mehboobali Pannipara a, Abdullah G. Al-Sehemi c,
Samy A. El-Daly a,
⇑
a Department of Chemistry, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
b Center of Excellence for Advanced Materials Research, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
c Research Center for Advanced Materials Science (RCAMS), King Khalid University, P.O. Box 9004, Abha 61413, Saudi Arabia
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The graphical presentation of the highest occupied and lowest unoccupied molecular orbitals of title
compound at B3LYP/6-31G⁄ level of theory.
ꢀ
ꢀ
ꢀ
ꢀ
New solvatochromic diolifinic dye.
Highly photostable laser dye.
DFT study.
Fluorescence quenching by silver
nanoparticle.
a r t i c l e i n f o
a b s t r a c t
Article history:
A comprehensive investigation on the photophysics of a p-conjugated potential push–pull chromophore
system 4,40-(1E,10E)-2,20-(Pyrazine-2,5-diyl)bis(ethene-2,1-diyl)bis(N,N-dimethylaniline) (PENDA) has
been carried out spectroscopically. The optical absorption and emission properties of this molecule have
been studied in different solvents. The molecule PENDA shows strong solvatochromic emission upon
changing the solvent polarity from nonpolar to polar; indicating that emission state is of intramolecular
charge transfer (ICT) character. The solvent effect on the spectral properties such as singlet absorption,
molar absorptivity, oscillator strength, dipole moment and fluorescence quantum yield of PENDA have
been studied in detail. Lippert–Mataga and Reichardt correlations were used to estimate the difference
Received 11 February 2015
Received in revised form 22 April 2015
Accepted 4 May 2015
Available online 9 May 2015
Keywords:
Styryl pyrazine
Fluorescence quantum yield
Effect of solvent
DFT
between the excited and ground state dipole moments (Dl). Ground and electronic excited states geo-
metric optimizations were performed using density functional theory (DFT) and time-dependent density
functional theory (TDDFT), respectively, with the Gaussian 09 package. A solution of (8 ꢁ 10ꢂ5 M) PENDA
in THF, dioxane, CH3CN and CHCl3 gives laser emission when pumped by a nitrogen laser pulse
(kex = 337.1 nm) of 800 ps duration and 1.48 mJ pulse energy. PENDA dye displays fluorescence
Silver nanoparticles
⇑
Corresponding author at: King Abdulaziz University, P.O. Box 80302, Jeddah
21589, Saudi Arabia. Tel.: +966 554953522.
E-mail address: samyeldaly@yahoo.com (S.A. El-Daly).
http://dx.doi.org/10.1016/j.saa.2015.05.018
1386-1425/Ó 2015 Elsevier B.V. All rights reserved.

A.M. Asiri et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 722–730
723
quenching by colloidal silver nanoparticles (Ag NPs) in ethanol. The fluorescence data reveal that
dynamic quenching and energy transfer play a major role in the fluorescence quenching mechanism.
Ó 2015 Elsevier B.V. All rights reserved.
Introduction
nanoparticles and these phenomena can be used to probe the
micro environment of the fluorophore. The emission behaviour of
The development of new organic
p
-conjugated luminophores
a dye can be altered by using metallic nanoparticles and quenching
or enhancement of photoluminescence of a dye by silver nanopar-
ticles (Ag NPs) depending upon the distance between the dye
molecule and NPs [20–21]. The quenching processes are of three
types: static, dynamic and by electron/energy transfer. In static
quenching, the decrease in emission intensity is caused by adsorp-
tion of the dye on the surface of the metallic NPs, forming a non
fluorescent complex between the fluorophore and quencher
whereas in dynamic quenching, the reduction of emission intensity
is due to direct interaction or collision of the excited fluorophore
with a quencher during the excited state life time. The third type
of quenching occurs by non-radiative energy/electron transfer
between the dye molecule and NPs [22,23]. The quenching of fluo-
rescence dominates over enhancement at shorter distances and it
is attributed to the efficient non-radiative energy transfer between
a dye and the metallic NP [23].
has been of great interest over the last few decades, due to their
wide applications in various fields such as optoelectronics,
bio-imaging, and optical storage devices [1–4]. Organic molecules
containing both electron donating (D) and accepting (A)
substituents in a single molecule exhibit interesting optical and
spectral properties due to intramolecular charge transfer (ICT).
This phenomenon led to substantial research owing to potential
applications in photoelectronic and nonlinear optical devices
[5,6], chemical sensing [7], and understanding photochemical
and photobiological processes [8]. Compounds of this class show
very strong solvent polarity dependent changes in their photo-
physical characteristics, specifically, large red shifts in their emis-
sion spectra with increasing solvent polarity, exceedingly high
solvent polarity dependent changes in the Stokes shifts between
absorption and fluorescence spectra, significant reduction in the
fluorescence quantum yields (/f) and lifetimes (
the solvent polarity [9–11].
s
f) on increasing
In continuation of our research in the area of synthesis and pho-
tophysical properties of organic molecules having donor–acceptor
chromophores [24,25], we report here the spectral, and photophys-
Organic molecules with large delocalised
p-electron systems
along their back bone have attracted significant interest due to
potential applications associated with strong emission behaviour
and large nonlinear optical properties [12–14].The presence and
nature of electron-donating and electron-accepting groups play
significant roles in such properties. Pyrazines [15], have nitrogen
atoms at the 1, 4 positions of a six-membered aromatic ring. Due
to their highly electron-withdrawing character, pyrazines are
excellent candidates for incorporation as electron-withdrawing
groups in push–pull scaffolds favouring intramolecular charge
transfer (ICT). Diazines are of great importance since protonation,
hydrogen-bond formation, and chelation through the nitrogen
atoms of the heterocycle may be used for formation of supramolec-
ular assemblies and therefore sensors.
ical properties of a D-
group, A an electron-accepting group, and
p
-A-
p
-D type (where D is an electron-donating
p
a conjugating moiety)
in effect a linear styryl pyrazine bearing electron donating groups
at opposite ends of the pyrazine core. The aim of the present work
is to perform a detailed investigation on the spectral behaviours
and photophysical properties of this donor–acceptor type styryl
pyrazine derivative in different media. To explore the effect of
metallic nanoparticles on the synthesized fluorophore, we also
investigate fluorescence quenching by colloidal silver nanoparticles
in ethanol using steady state emission measurements.
Experimental
Metallic nanoparticles due to their small size and high surface
to volume ratios, possess unique spectroscopic, electronic and
chemical properties that are different from those of the individual
atoms as well as their bulk counterparts. The optical properties of
noble metal nanoparticles have received considerable attention
because the surface plasmon absorption band of noble metal
nanoparticles appears in the visible region of the spectrum thus
providing important contributions towards sensing and
bio-medical applications [16,17]. Interaction of metallic nanoparti-
cles (NPs) with fluorophores has been an active area of research
over the last two decades with applications ranging from material
to biomedical science [18,19]. The fluorescence of a dye molecule is
quenched or enhanced in the close proximity of metallic
Materials and methods
Spectroscopic grade solvents and chemicals were obtained from
Sigma Aldrich and used without further purification. Silver nitrate,
the metal precursor and the reducing agent trisodium citrate were
used to prepare the silver nanofluid in doubly distilled water.
Melting points were determined on a Gallenkamp melting point
apparatus and infrared (IR) spectra were recorded on Shimadzu
FT-IR 8400S infrared spectrophotometer using the KBr pellet tech-
nique. NMR (1H and 13C) spectra were recorded on a Bruker
DPX-600 at 600 MHz and 150 MHz, respectively, using tetram-
ethylsilane as the internal standard. The chemical shift values are
Scheme 1. The reaction of 2,5-dimethylpyrazine with 4-(dimethylamino)benzaldehyde.

724
A.M. Asiri et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 722–730
Fig. 1. TEM image and electronic absorption spectrum of Ag NPs (kmax = 450 nm).
Procedure for the synthesis of 4,40-(1E,10E)-2,20-(Pyrazine-2,5-
diyl)bis(ethene-2,1-diyl)bis(N,N-dimethylaniline) (PENDA)
The title compound was synthesized by the reaction of
2,5-dimethylpyrazine
with
4-(dimethylamino)benzaldehyde
(Scheme 1). A solution of 2,5-dimethylpyrazine (0.5 g, 4.62 mmol)
and the 4-(dimethylamino)benzaldehyde (2.3 g, 13 mmol) were
dissolved in DMF and the mixture was cooled to 0 °C. To this
mixture potassium-tert-butanolate (1.55 g, 13 mmol) was added
in small portions and the mixture was brought to ambient temper-
ature. Stirring was continued until all starting material had been
consumed (TLC). After the completion of the reaction, water was
added, and the product was isolated by extraction with chloroform.
The organic layer was concentrated in vacuum and the solid mate-
rial obtained was recrystallized to give the product (1.4 g, 52%) as a
red solid. The structure of the compound was confirmed by IR, 1H
NMR and 13C NMR.
1H NMR (CDCl3, 600 MHz): d = 3.03 (s,12H), 6.74 (d,4H,
J = 8.9 Hz), 6.94 (d,2H, J = 16.1 Hz), 7.49 (d,4H, J = 8.9 Hz), 7.64
Fig. 2. Electronic absorption spectra of PENDA in different solvents.
recorded on d scale and coupling constants (J) in Hz. Splitting
patterns were designated as follows: s: singlet; d: doublet; m:
multiplet. UV–Vis electronic absorption spectra was recorded on
a Shimadzu UV-160A spectrophotometer, and the steady-state
fluorescence spectra were measured using Shimadzu RF 5300 spec-
trofluorophotometer using a rectangular quartz cell of dimensions
0.2 cm ꢁ 3 cm. The emission was monitored at right angle. The flu-
orescence quantum yields (/f) in different solvents were measured
using a comparative method where quinine sulfate (/f = 0.55
sf = 20 ns) in 0.5 M H2SO4 [26] solution was used as the reference
according to Eq. (1):
Iu As n2u
/u ¼ /s ꢁ
ꢁ
ꢁ
ð1Þ
Is Au n2s
where /u, /s are the fluorescence quantum yields of the unknown
and standard, respectively, I is the integrated emission intensity;
A is the absorbance at excitation wavelength, and n is the refractive
index of the solvent. The subscripts u and s refer to unknown and
standard, respectively.
Fig. 3. Emission spectra of PENDA in different solvents (kex = 380 nm).

A.M. Asiri et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 722–730
725
Table 1
Spectral and Photophysical data of PENDA in different solvents.
ꢂ1
Solvents
kabs (nm)
kem (nm)
D
m
(cm
)
e
Mꢂ1 cmꢂ1
6480
Uf
f
l12 Debye
ET(30) K cal molꢂ1
Df (D, n)
ETN
ꢀ
Hexane
434
450
435
464
446
450
445
450
457
459
458
457
458
457
492
514
483
562
514
529
523
559
548
555
562
563
565
566
2716
2767
2285
3758
2966
3319
3351
4333
3634
3768
4040
4120
4135
4214
0.64
0.475
0.38
0.24
0.55
0.45
0.43
0.27
0.43
0.31
0.19
0.15
0.07
0.01
0.11
0.80
0.26
0.82
0.83
0.77
0.80
0.88
0.79
0.79
0.79
0.79
0.80
0.81
3.16
8.71
4.91
8.96
8.86
8.55
8.66
9.17
8.75
8.78
8.78
8.72
8.78
8.86
31.1
33.9
31.1
45.1
36
37.4
38.1
45.6
39.1
43.8
50.2
49.2
51.9
55.4
0.0014
0.0132
0.0004
0.263
0.021
0.210
0.199
0.304
0.148
0.274
0.263
0.274
0.288
0.308
0.006
0.099
0.006
0.441
0.164
0.210
0.230
0.472
0.259
0.404
0.506
0.570
0.654
0.762
Toluene
Heptane
DMSO
Dioxane
THF
Ethyl Acetate
Acetonitrile
Chloroform
DMF
Butanol
Propanol
Ethanol
Methanol
48,070
15,280
47,440
48,770
45,150
45,790
49,830
44,970
46,420
44,270
44,240
44,530
30,500
Fig. 4. Plot of energy of absorption and emission (Cmꢂ1) versus
solvents.
Df of different
Fig. 5. Plot of Stokes shift of PENDA versus
Df (D, n) of different solvents.
and 3, and the corresponding spectral data are summarized in
Table 1. As indicated in Fig. 2 and Table 1, absorption spectra of
PENDA show a red shift of 30 nm on changing the solvent polarity
from hexane to DMSO suggesting some polar character in the
ground state. The red shift is expected in compounds having
(d,2H, J = 16.1 Hz), 8.49 (s,2H,). 13C NMR (CDCl3, 150 MHz):
d = 40.3, 112.2, 120.7, 124.7, 127.5, 132.8, 141.7, 148.0, 150.7.
Synthesis and characterization of silver nanoparticles
Silver nanoparticles were prepared by a literature procedure
[27]. In a typical preparation, 100 ml of 1 ꢁ 10ꢂ3 M silver nitrate
solution was heated to boiling and 5 ml of 1% trisodium citrate
solution (as nucleating and reducing agent) was added quickly,
resulting in a color change from pale yellow to golden yellow indi-
cating the formation of Ag NPs; stirring was continued until cooled
to room temperature. The nanoparticles were characterized by
UV–Vis absorption spectrophotometer and Transmission Electron
Microscope (TEM). A typical solution of 40–45 nm diameter silver
nanoparticles having polygonal shape, exhibiting a characteristic
surface Plasmon band around 430 nm was obtained. A TEM image
of Ag NPs with an absorption spectrum shown as an inset, is shown
in Fig. 1.
Results and discussion
Solvent effect on absorption and emission spectra
The absorption and fluorescence spectra of (1 ꢁ 10ꢂ5 M) PENDA
were recorded in solvents of different polarity. The absorption and
emission spectra of PENDA in several solvents are shown in Figs. 2
Fig. 6. Plot of Stokes shift (Cmꢂ1) versus normalized ET(30) of different solvents.

726
A.M. Asiri et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 722–730
electron mobility and dipole moment of the solvent molecule. The
Onsager cavity radius (a) from molecular volume of molecules is
calculated using Suppan’s Eq. (4) [35],
donor–acceptor groups in a single molecule and is due to strong
intramolecular charge transfer (ICT) [28,29]. On excitation at
365 nm, the emission spectrum of PENDA correlates with increas-
ing polarity of the solvent (Fig. 3). It is evident that the emission
maxima undergo a red shift of 83 nm on increasing the polarity
from heptane to methanol, suggesting the fluorescent state is more
polar than the ground state. Thus the emission spectra are more
sensitive to solvent polarity than the absorption spectra, which
indicates that a larger charge transfer taking place in the excited
state in comparison to the ground state, confirming the presence
ꢀ
ꢁ
1=3
3M
dN
a ¼
ð4Þ
4
p
where d is the density of dye, M is the molecular weight of dye and
N is Avogadro’s number. Fig. 5 shows the plot of Stokes shift versus
the orientation polarization
(Df), The linear correlation of
Lippert–Mataga plot supports the occurrence of photoinduced
intramolecular charge transfer. The change in dipole moment calcu-
lated for PENDA from the slope of this plot and cavity radius (a) is
found to be 5.47 D. The data in polar protic solvents were excluded
from the plot of Fig 5, to avoid specific solute–solvent interactions
(hydrogen bonding).
The change in dipole moment (Dl) between the excited singlet
and ground state has been further investigated using the solva-
tochromic shift method introduced by Reichardt [36], making use
of the dimensionless microscopic solvent polarity parameters ETN
of
p–
p⁄ transitions in PENDA and stabilization of a highly dipolar
excited state in polar solvents. The absorption and fluorescence
peak energies (abs and fl respectively in cm-1) of PENDA has corre-
lated with solvent polarity parameter
abs and fl values follow reasonably good linear relationship with
f and the plot between fl and f show large negative slop, indicat-
Df. As seen from Fig. 4, the
D
D
ing that the emission state is strongly dipolar in nature due to
strong ICT from the donor to the acceptor moieties [30,31].
Estimation of dipole moments using solvatochromic methods
given by the Eqs. (5) and (6),
ET ðsol
v
32:4
entÞ ꢂ 30:7
ETN
¼
ð5Þ
ð6Þ
The solvatochromic method based on linear correlation
between the absorption, fluorescence maxima and solvent polarity
functions [32,33], is well-known for determining excited state and
ground state dipole moment. Lippert–Mataga’s Eqs. (2) and (3)
[32,34], which is based on the correlation of energy difference
between the ground and excited states (Stokes’ shift) with the sol-
vent orientation polarizability (Df), can be used to investigate the
change in dipole moment between the excited singlet state and
ground state.
28; 591
kmaxðnmÞ
ET ðsolventÞ ¼
where kmax corresponds to the peak wavelength in the red region
of the intramolecular charge transfer absorption of the
betaine dye. The main advantage of this method over the
Lippert–Mataga method is the incorporation of hydrogen bonding
in addition to the solvent polarity into the solvent parameter. In
this method, change in dipole moment is calculated by correlating
2
2ðle
ꢂ
lg
Þ
ꢀ
D
D
mst
¼
D
f þ Const:
ð2Þ
ð3Þ
hca3
the Stokes shift of the fluorophore to ENT (Fig. 6) according to
Eq. (7),
e
2
ꢂ 1
n2 ꢂ 1
ꢀ
ꢁ
f ¼
ꢂ
2
ꢂ
ꢃ
3
e
þ 1 2n2 þ 1
D
l
aD
a
D
m
¼ 11307:6
ENT þ Const:
ð7Þ
D
lD
ꢀ
where
D
mst is the difference between the absorption and emission
maxima expressed in wave numbers (cmꢂ1), respectively, h is
where
D
l
is the difference between the excited and ground state
Planck’s constant, c is the speed of light in vacuum, a is the
dipole moments of the probe molecule and
D
lD is the change in
Onsager cavity radius,
tive index of the solvent, respectively, le and lg are the dipole
moments in the excited and ground state, respectively, and f is
e and n are the dielectric constant and refrac-
the dipole moment of the betaine dye; a (taken as 4.34 Å) and aD
are the Onsager cavity radius of the probe molecule and betaine
molecule, respectively. Since the values of aD and lD are known
(6.2 Å and 9 Debye, respectively) the change in dipole moment is
calculated using Eq. (8),
D
the orientation polarizability of the solvent which measures both
"
#
1=2
81m
3
D
l
¼
ð8Þ
ð6:2=aÞ ꢁ 11307:6
where, m is the slope of linear plot of ENT vs Stokes shift (Fig. 6) and
the value of is found to be 5.58 D. The values of obtained by
both methods are found to be very close and the higher value of
g) suggests that the emissive state of PENDA is of strong
D
l
Dl
(
le
ꢂ
l
ICT character.
The ground to excited state transition dipole moment (l12) of
PENDA in different solvents was calculated using Eq. (9) [37]:
f
l212
¼
ð9Þ
4:72 ꢁ 10ꢂ7Emax
where Emax is the energy maximum absorption in cmꢂ1 and f is the
oscillator strength which shows the effective number of electrons
whose transition from ground to excited state gives the absorption
area of the electronic spectrum. The experimental oscillator
strength values were calculated using Eq. (10) [38],
Z
f ¼ 4:32 ꢁ 10ꢂ9
eð
m
Þd
m
ð10Þ
ꢀ
ꢀ
Fig. 7. Plot of fluorescence quantum yield of PENDA versus ET(30) of different
solvents.

A.M. Asiri et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 722–730
727
Table 2
Selected optimized geometrical parameters (bond length in Å and dihedral angle in o) and dipole moment (in Debye) computed for PENDA in gas phase and solvents at the ground
(S0) and first excited state (S1) using B3LYP/6-31G⁄. For labeling, refer to Fig. 8.
S0
S1
Gas phase
THF
n-hexane
Methanol
Gas phase
THF
n-hexane
Methanol
C1–N1
N1–C2
C2–C6
C3–N2
N2–C4
C4–C5
C5–C8
C6–C7
C7–C10
C8–C9
C13–N3
N3–C22
N3–C21
1.3266
1.3555
1.4566
1.3266
1.3555
1.4566
1.3531
1.3531
1.4574
1.4574
1.3855
1.4531
1.4527
1.3854
1.4527
1.4531
ꢂ179.8
ꢂ179.7
179.9
1.3276
1.3575
1.4554
1.3276
1.3575
1.4554
1.3554
1.3554
1.4555
1.4555
1.3761
1.4553
1.4551
1.3761
1.4550
1.4553
1.3055
1.3487
1.4543
1.3054
1.3486
1.4544
1.3507
1.3509
1.4559
1.4562
1.3796
1.4536
1.4532
1.3823
1.4541
1.4545
ꢂ180.0
ꢂ180.0
179.9
1.3278
1.3578
1.4552
1.3278
1.3578
1.4552
1.3559
1.3559
1.4551
1.4551
1.3748
1.4560
1.4557
1.3748
1.4557
1.4560
ꢂ180.0
ꢂ180.0
180.0
1.2978
1.3985
1.3910
1.2978
1.3984
1.3910
1.4161
1.4161
1.4274
1.4274
1.3852
1.4535
1.4532
1.3852
1.4531
1.4534
ꢂ179.9
ꢂ180.0
179.8
1.3120
1.3856
1.4188
1.3120
1.3856
1.4188
1.3865
1.3865
1.4326
1.4326
1.3733
1.4565
1.4565
1.3733
1.4565
1.4565
1.3128
1.3810
1.4227
1.3128
1.3810
1.4227
1.3820
1.3820
1.4354
1.4354
1.3781
1.4541
1.4540
1.3781
1.4540
1.4541
ꢂ180.0
ꢂ180.0
180.0
180.0
180.0
0.005
ꢂ180.0
ꢂ0.035
ꢂ180.0
ꢂ0.008
180.0
1.3117
1.3874
1.4174
1.3117
1.3874
1.4174
1.3878
1.3878
1.4316
1.4316
1.3715
1.4574
1.4573
1.3715
1.4573
1.4574
ꢂ180.0
ꢂ180.0
180.0
180.0
179.95
0.061
ꢂ179.9
ꢂ0.056
ꢂ179.93
ꢂ0.073
179.93
0.073
C
18–N4
N4–C23
N4–C24
N2–C4–C5–C8
ꢂ180.0
ꢂ180.0
180.0
180.0
178.4
1.8
ꢂ180.0
ꢂ180.0
180.0
C
20–C9–C8–C5
N1–C2–C6–C7
C
C
C
C
C
C
C
C
C
15–C10–C7–C6
12–C13–N3–C21
12–C13–N3–C22
14–C13–N3–C22
14–C13–N3–C21
17–C18–N4–C23
17–C18–N4–C24
19–C18–N4–C24
19–C18–N4–C23
179.7
172.1
8.8
180.0
176.5
3.6
180.0
179.9
0.10
180.0
172.2
8.3
180.0
179.09
ꢂ0.07
ꢂ179.9
ꢂ0.11
ꢂ179.9
ꢂ0.06
179.9
ꢂ171.5
ꢂ178.3
ꢂ176.6
ꢂ179.9
ꢂ172.0
ꢂ8.2
ꢂ1.7
ꢂ3.7
ꢂ0.09
ꢂ8.1
ꢂ172.2
ꢂ8.7
ꢂ178.5
ꢂ1.6
ꢂ172.5
ꢂ8.3
ꢂ179.9
ꢂ0.12
ꢂ172.3
ꢂ8.3
171.6
178.5
1.5
172.0
179.9
172.1
8.2
7.7
0.12
8.1
0.10
0.037
Fig. 8. The optimized molecular structure of PENDA compound.
where
e
is the numerical value for molar absorption coefficient
correlated with the empirical Dimroth polarity parameters ET(30)
of solvent (Fig. 7). A linear correlation was obtained, implying
potential application of PENDA to probe the polarity of the medium
[11,39–42]. The decrease in /f (positive solvatokinetic effect) in
highly polar protic solvents is because the photoexcited dye with
ICT state undergoes a conversion to a highly polar TICT state by
twisting of the molecule around a C–C single bond, making a com-
plete transfer of electrons from donor to acceptor moiety.
measured in dm3 molꢂ1 cmꢂ1 and
m
is the numerical value of the
ꢀ
wave number measured in cmꢂ1. The values of f and l12 are listed
in Table 1 and indicate that the So ? S1 transition is the allowed
transition.
Fluorescence quantum yield of PENDA in different solvents
The fluorescence quantum yield (/f) of PENDA is strongly influ-
enced by the polarity and hydrogen bonding ability of the solvent
(Table 1). As shown in Table 1, the value of /f decreases with
increasing solvent polarity ET(30), where ET(30) is the solvent
polarity parameter that considers interactions such as solvent
polarizability and hydrogen bonding besides those of a specific
nature. This could be attributed to efficient internal conversion
Theoretical calculations
The ground and electronic excited states geometric optimiza-
tions of PENDA are performed using density functional theory
(DFT) and time-dependent density functional theory (TDDFT)
methods, respectively, with the Gaussian 09 package [43]. The
DFT was treated according to Becke’s three parameter
gradient-corrected exchange potential and the Lee–Yang–Parr
gradient-corrected correlation potential (B3LYP) [44–46], and all
calculations were performed by using 6-31G⁄ basis set [47]. This
model has been widely used for geometry optimization and the
determination of electronic properties [24]. TDDFT has been used
to investigate absorption spectra and has proved to be an efficient
and/or intersystem crossing by extensive mixing between the close
1
– lying
(p–
p⁄) and 1(n ꢂ p⁄) states. In addition the non-radiative
rate increases markedly in solvents with strong hydrogen bond
character such as alcoholic solvents and competes with fluores-
cence emission due to the enhancement of intersystem crossing,
strong internal conversion and vibrational deactivation. The fluo-
rescence quantum yield of PENDA in different solvents is

728
A.M. Asiri et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 722–730
Fig. 9. The graphical presentation of the highest occupied and lowest unoccupied molecular orbitals of title compound at B3LYP/6-31G⁄ level of theory.
bond/dihedral angles for the structures are tabulated in Table 2.
In general, N1–C2, N2–C4, C5–C8, C6–C7, N3–C21, N3–C22, N4–
C23 and N4–C24 bond lengths increase from ground state to
excited state for all solvents used and the difference value 0.032,
˘
0.032, 0.035, 0.035, 0.0043, 0.0046, 0.0046 and 0.0043 A respec-
tively. On the other hand, C1–N1, C2–C6, C3–N3, C4–C5, C7–C10,
C8–C9, C13–N3 and C18–N24 bond lengths shortened from ground
to excited states, i.e., 0.015, 0.039, 0.015, 0.039, 0.026, 0.026, 0.014
˘
and 0.014 A, respectively.
In the PENDA molecule, the two benzene rings attached to the
pyrazine ring with bridge are planar and have dihedral angles of
N1–C4–C5–C8, C20–C9–C8–C5, N1–C2–C6–C7 and C15–C10–C7–C6,
between 179.7 to 180.0 in both ground and excited states
Table 2. Fig. 9 illustrates the frontier molecular orbital, highest
occupied molecular orbitals (HOMO) and lowest unoccupied
Fig. 10. Calculated absorption spectrum of PENDA in gas phase (in black), n-hexane
(in purple), THF (in blue) and methanol (in red). (For interpretation of the references
to colour in this figure legend, the reader is referred to the web version of this
article.)
approach [48–50]. The polarizable continuum model (PCM) [51,52]
was used for evaluating bulk solvent effects at all stages. The
PCM-TDB3LYP/6-31G⁄ level of theory was used to compute the
absorption spectra in THF, n-hexane and methanol.
The geometrical parameters of the ground and excited states
are presented in Table 2. The labeling scheme is shown in Fig. 8.
We applied the DFT method to obtain the ground state geometrical
and electronic structure of PENDA in the gas phase (as a control)
and in solvents. The dielectric constants used in the PCM calcula-
tion are the default values in GAUSSIAN 09. We use PCM with
n-hexane (
e
= 1.882) and tetrahydrofuran (THF) (
e = 7.426) and
methanol (
e
= 32.613) as solvents. TD-DFT was used to investigate
Fig. 11. Calculated emission spectrum of PENDA in gas phase (black), n-hexane
(purple), THF (blue) and methanol (red). (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this article.)
the excited state properties in the gas phase and solvents, together
with the 6-31G(d). The optimized values of bond lengths and

A.M. Asiri et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 722–730
729
Table 3
2.624, 2.564 and 2.527 eV, whereas the corresponding emission
values are 2.321, 2.334, 2.251 and 2.246 eV. The emission maxi-
mum wavelength (kmax) shows a red shift relative to the absorption
kmax as the polarity of the solvent strengthens. The pronounced
Photophysical parameters of laser emission of PENDA in some solvents.
Solvent
kex(max)
Tuning range (nm)
Gain coefficient (a
) cmꢂ1
THF
514
548
529
558
482–562
482–572
505–590
514–604
1.42
1.25
1.25
1.02
shift in the emission spectra is an indication of a p?
p⁄ transition.
Dioxane
CHCl3
CH3CN
The calculated absorption and emission spectra are in excellent
agreement with experimental observation.
Laser activity of PENDA
molecular orbitals (LUMO). The HOMO is
p bonding while the
LUMO is of antibonding character with p⁄ distributed over the
whole molecule. The HOMO is delocalized over the whole molecule
and the lone pair of electrons on the nitrogen atoms. The HOMO–
LUMO energy gap of this compound was calculated at the
B3LYP/6-31G⁄ level of theory, (Fig. 9). Orbital energy level analysis
at the B3LYP/6-31G⁄ level and the resultant HOMO–LUMO energy
gap (Egap) has been used as an indicator of kinetic stability of a
molecule. A large HOMO–LUMO gap implies a high kinetic stability
and low chemical reactivity, because it is energetically unfavour-
able to add electrons to a high-lying LUMO or to remove electrons
from a low-lying HOMO. As shown in Fig. 9 the HOMO is delocal-
ized over the whole of the molecule; the four nitrogen atoms also
take part in the HOMO. The LUMOs are also distributed over the
whole of the molecule, but the pyrazine ring core contribution to
the LUMO is small. We also computed the HOMO–LUMO orbitals
energy and energy gap at the same level of theory in gas phase,
n-hexane and THF. In the solvent, THF lowers the HOMO and
LUMO energies by about 0.11–0.25 eV and n-hexane decreases
the HOMO and LUMO energies from 0.047 to 0.14 eV respectively.
The effect of solvents on the HOMO–LUMO energy gap are also
shown in Fig. 9, the larger Egap in THF revealing that the spectrum
would be red shifted.
A solution of PENDA (8 ꢁ 10ꢂ5 M) in THF, dioxane, CH3CN and
CHCl3 gave laser emission when pumped by a nitrogen laser pulse
(kex = 337.1 nm) of 800 ps duration and 1.48 mJ pulse energy. The
solutions were placed in oscillator and amplifier cuvettes of
10 mm path length. The output energy of the laser dye was mea-
sured as a function of wavelength to determine the lasing range
in different solvents. The dye gave laser emission at fluorescence
maximum comparable to steady state emission maximum. The
maximum gain coefficient (a) was calculated at maximum laser
emission by measuring the intensity IL of laser emission over the
entire cell length L and the intensity over half the cell length IL/2.
The laser gain emission was calculated using Eq. (11) [53],
ꢄ
ꢅ
2
ðkÞ ¼ ln
L
IL
IL=2
a
ꢂ 1
ð11Þ
Table 3 summarizes the laser emission data of dye in different
solvents. It is well known that gain occurs when the stimulated
emission of photons exceeds the re-absorption or loss due to scat-
tering. Hence, gain is the increase in the number of emitted pho-
tons and is dependent on both wavelength and incident intensity.
Fluorescence quenching study of PENDA by silver nanoparticles
In order to study the effect of solvents on the ground and
excited states, quantum chemical calculations were carried out in
gas phase (as a control), n-hexane, tetrahydrofuran (THF) and
methanol using the TDDFT/PCM method. The UV–Vis absorption
spectra of PENDA exhibit only one band of maximum absorption
between 446.6 and 490.7 nm, depending on the solvent, whereas
the corresponding emission values occur between 531and
552 nm, (Figs. 10 and 11). There is a gradual increase in the
calculated properties with an increase in dielectric constant of
the solvents. In addition, computed absorptions attain 2.776,
Quenching studies can be used as a tool to investigate response
of fluorophores to quenchers and hence the nature of molecular
interaction and microenvironment of fluorophores in solution
can be studied by fluorescence quenching experiments.
Fluorescence measurements were performed to investigate the
interaction of silver nanoparticles with PENDA with variable con-
centrations of Ag NPs in ethanol. The emission spectra
(1 ꢁ 10ꢂ5 M) of PENDA in ethanol at different concentrations of
Ag NPs are shown in Fig. 12. The emission spectrum of PENDA
remains unaltered in wavelength but a substantial decrease in
Fig. 12. Emission spectra of 1 ꢁ 10ꢂ5 M of PENDA in ethanol at different concen-
trations of Ag NPS. The concentrations of Ag Nps at decreasing emission intensity
are 0.0, 0.039, 0.0 78, 0.117, 0.156, 0.195, 0.273 and 0. 312 nM (kex = 340 nm).
Fig. 13. Stern–Volmer plot for fluorescence quenching of PENDA by Ag NPs in
ethanol.

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A.M. Asiri et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 149 (2015) 722–730
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ð12Þ
v
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Conclusion
A styryl pyrazine derivative (PENDA) having D-p-A-p-D type
structure has been synthesized and characterized by spectroscopic
methods. The photophysical properties were investigated in protic
and aprotic solvents. It is inferred from the absorption and emis-
sion spectra that emissive state of PENDA has intramolecular
charge transfer characteristics. The fluorescence quantum yield
(/f) of PENDA is strongly influenced by the polarity and hydrogen
bonding ability of the solvents and /f undergoes a sharp decrease
with increasing the polarity of the solvent. Further, from the
Lippert–Mataga and Reichardt correlations it is found that pho-
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excited state and the polarity of the dye increases on excitation.
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agreement with experimental observation. The interactions of
PENDA with colloidal silver nanoparticles in ethanol have also
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ing data suggest that radiative energy transfer from excited PENDA
to ground state Ag NPs play major roles in the fluorescence
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