Spectroscopic Investigation and Photophysics of a D-π-A-π-D Type Styryl Pyrazine Derivative

Article abstract of DOI:10.1007/s10895-015-1698-7

E,E-2,5-bis(3,4-dimethoxystyryl)pyrazine (BDSP) has been prepared by aldol condensation between 2,5-dimethypyrazine and 3,4-dimthoxybenzaldehyde and characterized by IR, ¹H NMR, ¹³C NMR and X-ray crystallography. The electronic absor

Full text of DOI:10.1007/s10895-015-1698-7

J Fluoresc
DOI 10.1007/s10895-015-1698-7
ORIGINAL ARTICLE
Spectroscopic Investigation and Photophysics of a D-π-A-π-D
Type Styryl Pyrazine Derivative
1
1
Samy A. El-Daly & Khalid A. Alamry
Received: 24 August 2015 /Accepted: 14 October 2015
Springer Science+Business Media New York 2015
#
.
.
Abstract E,E-2,5-bis(3,4-dimethoxystyryl)pyrazine (BDSP)
has been prepared by aldol condensation between 2,5-
dimethypyrazine and 3,4-dimthoxybenzaldehyde and charac-
Keywords Styrylpyrazine Effectofsolvents DFT/TDDFT
and fluorescence quenching by Ag NPs
1
13
terized by IR, H NMR, C NMR and X-ray crystallography.
The electronic absorption and emission properties of BDSP
were studied in different solvents. BDSP displays a strong
solvatochromic effect of the emission spectrum that is
reflected by large red shifts of its fluorescence emission max-
imum on increasing the solvent polarity, indicating a large
change in dipole moment of BDSP upon excitation due to
photoinduced intramolecular charge transfer (PICT). Excited
state intermolecular hydrogen bonding affects the energy of
emission spectrum and fluorescence quantum yield of BDSP
Introduction
Organic molecules containing both electron donating (D) and
accepting (A) group in a single molecule exhibit interesting
optical and spectral properties due to intramolecular charge
transfer (ICT). This phenomenon led to substantiate research
owing to potential applications in photoelectronic and nonlin-
ear optical devices [1, 2], chemical sensing [3] and under-
standing photochemical and photobiological processes [4].
Compounds of this class show strong solvent polarity depen-
dent changes in their photophysical characteristics, specifical-
ly, large red shifts in their emission spectra, significant change
in the fluorescence quantum yields and excited state lifetimes
on increasing the solvent polarity [5–7]. The nature of
electron-donating and electron-accepting groups plays signif-
icant roles in such properties.
Organic molecules with large delocalized π-electron sys-
tems along their backbone have attracted significant interest
due to potential applications associated with strong emission
behavior and large nonlinear optical properties [8–10]. Pyr-
azine [11] has nitrogen atoms at the 1, 4 positions of a six-
membered aromatic ring. Due to the high electron withdraw-
ing character, pyrazines are excellent candidates for incorpo-
ration as electron-withdrawing groups in push-pull scaffolds
favoring intramolecular charge transfer (ICT). Some distyryl
pyrazines had been prepared [12] and investigated for opto-
electronic devices, as emitting materials for lasing purposes
−4
dye. The dye solutions (1×10 M) in DMSO, DMF, CH CN
3
and dioxane give laser emission in green region upon excita-
tion by a 337.1 nm nitrogen pulse (λ=337 nm). The tuning
range, gain coefficient (α), emission cross – section (σ ) and
e
half-life energy (E ) has been determined. Ground and elec-
1
/2
tronic excited states geometric optimization were performed
using density functional theory (DFT) and time-dependent
density functional theory (TD-DFT), respectively. A DFT nat-
ural bond analysis complemented the ICT. BDSP dye displays
fluorescence quenching by colloidal silver nanoparticles
(
AgNPs). The fluorescence data reveal that radiative and
non-radiative energy transfer play a major role in the fluores-
cence quenching mechanism.
*
Samy A. El-Daly
samyeldaly@Yahoo.com
[13, 14].
Interaction of metallic nanoparticles (NPs) with
fluorophores has been an active area of research over the last
two decades with applications ranging from material to
1
Department of Chemistry, Faculty of Science, King Abdulaziz
University, P.O. Box 80203, Jeddah, Saudi Arabia21589

J Fluoresc
biomedical science [15, 16]. The fluorescence of a dye mole-
cule is quenched or enhanced in the close proximity of metal-
lic nanoparticles and the phenomena can be used to probe the
microenvironment of the fluorophore. The emission behavior
of a dye can be altered using metallic nanoparticles and
quenching or enhancement of photoluminescence of a dye
by silver nanoparticles (Ag NPs) depends upon the distance
between dye molecule and NP [17, 18]. There are of three
types of quenching processes: static, dynamic and by electron
spectra were recorded on a Burker DPX-600 and 150 MHz,
respectively, using tetramethylsilane as the internal standard.
The chemical shift values are recorded on δ scale and coupling
constants (J) in Hz. Splitting patterns were designated as fol-
lows: s: singlet, d: doublet, m: multiplet. UV–vis electronic
absorption spectra were recorded on a Shimadzu UVis-1650-
pc spectrophotometer. Fluorescence spectra were taken on
Shimadzu RF 5300 spectrofluorometer using a rectangular
quartz cell. The emission was monitored at right angle ar-
/
energy transfer. In static quenching, the decrease in emission
rangement. Fluorescence quantum yields (ϕ ) were measured
f
intensity is caused by adsorption of the dye on the surface of
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 dur-
ing the excited state lifetime. The third type of
quenching occurs by radiative and non-radiative energy
transfer between the dye molecule and NPs [19, 20].
The quenching of fluorescence dominates over enhance-
ment at shorter distances and it is attributed to efficient
non-radiative energy transfer between a dye and the
metallic NPs [20]. The competition between radiative
and non-radiative decay energy affects the fluorescence
of fluorophore molecules located near the AgNPs due to
joule heating and surface Plasmon absorption.
In continuation of our research in the area of synthesis and
photophysical properties of organic molecules having donor-
acceptor chromophores [21–23], we report here the spectral
and photophysical properties of a D-π-A-π-D type in effect a
linear styryl pyrazine bearing electron-donating groups at op-
posite ends of the pyrazine core. The aim of the present work
is to perform detailed investigations on the spectral and
photophysical characteristics of the donor–acceptor-type
styryl pyrazine derivative in different media, together with
complementary DFT theoretical studies. In order to explore
the effect of metallic nanoparticles on the excited state
fluorophore, we also investigate fluorescence quenching of
synthesized pyrazine by colloidal AgNPs in ethanol using
steady state emission measurements.
relative to quinine sulphate as a reference standard (ϕ =0.55 in
f
0.5 M H SO ) [24]; low sample concentrations (≤0.1 absor-
2
4
bance unit) were used to minimize re-absorption effect. The
following relation was applied to calculate the fluorescence
quantum yields relative to those of quinine sulphate:
Z
Is
2
5
Ar
n
∅
f ðsÞ ¼ ∅f ðrÞ Â Z
Â
 n
ð1Þ
2
As
r
Ir
The integrals represent the corrected fluorescence peak
areas, A is the absorbance at the excitation wavelength, and
n is the refractive index of solvent used. The subscripts s and r
refer to sample and reference, respectively. The lasing action
was monitored at a dye laser (GL-302 Dye laser PTI) pumped
by a nitrogen laser (GL-3300 Nitrogen laser, PTI). The pump
laser (λex=337.1 nm) was operated at a repetition of 3 Hz with
a pulse energy of 1.48 mJ and pulse duration of 800 ps. The
output of the dye laser was measured with a Pyroelectric Joule
Meter(ED 100, Gen – Tec).
Procedure for the Synthesis
of E,E-2,5-bis(3,4-Dimethoxystyryl)Pyrazine
The title compound (BDSP) was synthesized by the reaction
of 2,5-dimethylpyrazine(0.5 gm, 0.00462 M) with 3,4-
dimethoxy benzaladehyde (2.3 g, 0.013 M) were dissolved
in DMF and the mixture was cooled to 0.0 °C (Scheme 1).
To this mixture potassium-tert-butanolate(1.55 g, 13 mM) was
added in small portions and the mixture was brought to ambi-
ent temperature. Stirring was continued until all starting ma-
terial 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 concentrat-
ed in vacuum and the solid material obtained was recrystal-
lized to give the product (1.4 g, 52 %) as a yellow solid. The
Experimental
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 the silver nanofulid in doubly
distilled water. Melting point was determined on a
Gallenkamp melting point apparatus and infrared (IR) spectra
recorded on Shimadzu FT-IR 8400S infrared spectrophotom-
1
structure of the compound was confirmed by H NMR and
1
3
C NMR.
1
H NMR (CDCl , 600 MHz):δ=3.58(s,6H, OCH ), 3.92(s,
3
3
6H, OCH3), 6.82(d,J=8.4 Hz, H, Ph), 6.96(d, J=15.9 Hz,
2H), 7.09–7.11(m,Ph), 7.6 (d, J=15.9 Hz, 2H), 8.50(s, 2H,
1
13
13
eter using the KBr pellet technique. NMR ( H and C)
Pyr)., C NMR(CDCl , 150 MHz): δ=55.83, 108.01,
3

J Fluoresc
Scheme 1 Reaction route of the synthesized BDSP
1
1
1
12.14, 121.13, 123.07, 129.27, 133.71, 142.85, 149.81,
50.06, 149.79. IR(KBr): 3422, 2941, 1617, 1588, 1524,
476, 1421, 1334, 1259, 1158, 1141, 1023, 972, 816.
1.2 Ueq(C) carbon atoms for aromatic carbon atoms and C-
H=0.96 Å and Uiso(H)=1.5 Ueq(C) for methyl carbon atoms.
The cif has been submitted to CCDC and obtained CCDC
number i.e., 1402546 for molecules BDSP. The data file can
be obtained free of charges on application to CCDC 12 Union
Road, Cambridge CB21 EZ, UK. (Fax: (+44) 1223 336–033;
e-mail: data_request@ccdc.cam. ac.uk).
Crystallography
Yellow rod-like crystals were screened out to select a suitable
sample crystal for data collection. The sample was fixed on
glass tip supported by magnetic base containing copper rod.
This was mounted on Agilent SuperNova (Dual source)
Agilent Technologies Diffractometer, equipped with
graphite-monochromatic Cu/Mo Kα radiation for data collec-
tion. The data collection was accomplished using CrysAlisPro
software [25] at 296 K under the Cu Kα (λ=1.45184Ắ) radi-
ation. The structure solution was performed using SHELXS–
Synthesis and Characterization of Silver Nanoparticles
Silver nanoparticles were prepared by a literature procedure
−3
[20]. In a typical preparation, 100 ml of 1×10 M AgNO3
solution was heated to boiling and 5 ml of 1 % trisodium
citrate solution (as reducing and nucleating agent) was added
quickly, resulting in a color change from yellow to golden
yellow, indicating the formation of AgNPs. Stirring was con-
tinued until the mixture cooled to room temperature. The
nanoparticles were characterized by UV–vis absorption spec-
trophotometer and Transmission Electron Microscope (TEM).
A typical solution of 40–45 nm diameters silver nanoparticles
having polygonal shape, exhibiting a characteristic surface
Plasmon band around 430 nm was obtained.
9
7 [26] and refined by full–matrix least–squares methods on
2
F using SHELXL–97 [26], in-built with WinGX [27]. All
non–hydrogen atoms were refined anisotropically by full–ma-
trix least squares methods [26]. Figures were drawn using
PLATON [28] and ORTEP-3 [29].
All the C-H hydrogen atoms were positioned geometrically
and treated as riding atoms with C–H=0.93 Å and Uiso(H)=
Fig. 1 Electronic absorption spectra of BDSP in different solvents
Fig. 2 Emission spectra of BDSP in different solvents (λex=380 nm)

J Fluoresc
Results and Discussion
Solvent Effect on Absorption and Emission Spectra
of BDSP
Figures 1 and 2 show the electronic absorption and emission
spectra of a 1×10− M BDSP solution were measured in sol-
vents of different polarity. The corresponding spectral data are
summarized in Table 1. As indicated in Fig. 1 and Table 1, the
solvent polarity shows a slight effect on the position of elec-
tronic absorption spectral maxima (ca. 10 nm). This indicates
a less polar character of BDSP in the ground state. On the
other hand, the emission spectrum undergoes a strong
bathochromic shift by ca. 45 nm as the solvent polarity in-
creases. This exemplified by the decrease of emission maxi-
mum from 455 nm in heptane to 500 nm in methanol, observ-
ably indicating the intensively photoinduced intramolecular
charge transfer taking place from methoxy groups to pyrazinyl
moiety within the BDSP molecule in the singlet excited state.
The absorption occurring with high molar absorptivities (ε=
5
T
Fig. 3 Plot of fluorescence quantum yield of BDSP versus E (30) of
different solvents
prominent in the dye (positive solvatokinetic behavior) as well
as efficient internal conversion and/or intersystem crossing by
3
−1
−1
1
2010–53450 dm mol cm ) and high oscillator strength
1
1
(f=0.22–0.94) at lower energy is due to intramolecular charge
extensive mixing between the close-lying (π-π*) and (n-π*)
states[31–35]. In addition the non-radiative rate increases
markedly in solvents with strong hydrogen character such as
methanol and competes with fluorescence emission due to the
enhancement of intersystem crossing, internal conversion and
vibrational deactivation [36–40].
transfer (ICT) characteristics of strong allowed π –π* transi-
tion in the dye. The fluorescence quantum yield (ϕ ) of BDSP
f
is affected strongly by solvent polarity. As shown in Fig. 3, the
value of ϕ decreases with increasing solvent polarity E (30)
f
T
(
empirical Dimroth polarity parameters), where E (30) is the
T
solvent polarity parameters that considers interaction such as
solvent polarizability and hydrogen bonding besides those of a
specific nature [30]. The decrease in fluorescence as solvent
polarity increases could be attributed to a highly polar-excited
state population (ICT) state and a non-radiative decay was
Solid crystals of BDSP exhibit intense yellow emission with
maximum at 550 nm and bandwidth equal 50 nm upon excita-
tion at wavelength 420 nm (Fig. 4). With the change in the
excitation wavelength from 340 to 420 nm, the position of the
emission remains unchanged, which suggests that this band is a
Table 1 Summary of spectral data of BDSP in different solvents
(30) K cal mol−
1
Δf(D,n)
E
T
N
Solvents
λabs (nm)
λem (nm)
ε
M
ϕ
f
f
μ
12
E
T
−
1
Δν (cm )
-1cm−1
Debye
Hexane
Toluene
Heptane
DMSO
402
412
405
416
412
411
408
406
410
412
410
410
409
408
456
466
455
486
467
469
466
481
478
479
496
498
500
504
2946
2813
2713
3462
2859
3009
3051
3841
3470
3395
4229
4310
4450
4669
12010
49110
10440
46190
55470
49290
53450
52310
42910
47950
42680
48280
44530
48880
0.98
0.625
0.99
0.4
0.22
0.86
0.19
0.82
0.98
0.87
0.94
0.93
0.77
0.86
0.71
0.85
0.81
0.89
4.33
8.65
4.01
8.53
9.24
8.70
9.01
8.94
8.16
8.68
7.87
8.60
8.39
8.75
31.1
33.9
31.1
45.1
36
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
Dioxane
THF
0.7
0.66
0.7
37.4
38.1
45.6
39.1
43.8
50.2
49.2
51.9
55.4
Ethyl Acetate
3
CH CN
0.51
0.52
0.5
CHCl
3
DMF
Butanol
Propanol
Ethanol
Methanol
0.44
0.39
0.37
0.27

J Fluoresc
600
500
400
300
200
100
Exc. 420 nm
Exc. 380 mn
Exc. 365 nm
Exc. 340 nm
0
4
50 475 500 525 550 575 600 625 650
Wavelength (nm)
Fig. 4 Emission spectra of BDSP crystalline solids at different excitation
wavelengths
Fig. 5 Plot of Stokes shift of BDSP versus Δf (D, n) of different solvents
true emission band and not due to any second order diffraction of
the excitation light [41–43]. Such behavior was attributed to
emission from exciton state whose fluorescence is 94 nm red
shifted compared to emission from 1×10−5 M in n-hexane. This
indicates that the crystal structure of BDSP belongs to β-type of
Steven classification [44]. Solid-state photo luminescent
substances are very interesting for several high-
technology applications such as the fabrication of light
emitting diodes [45].
to avoid specific solute-solvent interactions (hydrogen bond-
ing). The change of dipole moment of BDSP upon excitation
was calculated from the slope of the plot and the cavity radius
was found to be 5.4 Debye. This change in dipole moment is
caused by redistribution of atomic charges as a result of charge
transfer from the electron-rich methoxy groups to the electron
acceptor pyrazinyl moiety. The change in dipole moment
(Δμ) between the excited singlet and ground state has been
further investigated using the solvatochromic shift method
introduced by Reichardt [30] making use of the dimensionless
N
T
microscopic solvent polarity parameters E given by the
Eqs. (4 and 5):
Determination of Dipole Moment of BDSP
Analysis of the solvatochromic effect allows the estimation of
E ðsolventÞ−30:7
N
T
the dipole moment difference (Δμ=μ – μ ) between the ex-
e
g
E ¼
ð4Þ
ð5Þ
T
3
2:4
cited singlet and the ground state. This was achieved by ap-
plying the simplified Lippert–Mataga Eq. (2) [46, 47].
28591
ET ðsolventÞ ¼ λmax
ꢀ
ꢁ
2
ðnmÞ
2
μe−μg
hca3
va−vf ¼
Δf þ constsnt
ð2Þ
where λmax 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
Where
2
ðε−1Þ
ðn −1Þ
Δf ðε; nÞ ¼
−
ð3Þ
2
ð2ε þ 1Þ ð2n þ 1Þ
parameter. In this method, dipole moment change is calculated
by correlating the Stokes shift of the fluorophore to E (Fig. 6)
N
T
where Δνst (ν –ν ) is the Stokes shift which increases with
a
f
increasing solvent polarity. It points to a stronger stabilization
of the excited state in polar solvents; h is Planck’s constant; c
is the speed of light in vacuum; a is the Onsager cavity radius;
ε and n are the dielectric constant and refractive index of the
solvent, respectively. The constant represents higher order
terms which are usually neglected; μ and μ are the dipole
moments in the excited and ground state, respectively. The
Onsager cavity radius was taken as 4.2Ặ which is comparable
to the radius of a typical aromatic fluorophore [48]. Fig. 5
shows the plot of Stokes shifts versus the orientation polari-
zations (Δƒ). The data in polar protic solvents were excluded
according to Eq. (6),
ꢂ
ꢃ
2
ꢀ
ꢁ
3
Δμ
aD
a
N
T
Δv ¼ 11307:6
E þ constant
ð6Þ
ΔμD
where Δμ is the difference between the excited and
ground state dipole moments of the probe molecule
and ΔμD is the change in the dipole moment of the
betaine dye; a (taken as 4.2 Å) and aD are the Onsager
cavity radius of the probe molecule and betaine mole-
cule respectively. Since the values of aD and μD are
e
g

J Fluoresc
Laser Activity of BDSP
BDSP is characterized by a high fluorescence quantum
yield, high molar absorptivity, Stokes shift in the range
−1
(
2713 – 4669 cm ), in most organic solvents. In addition,
DBSP dye is free from molecular aggregation either in
ground or in excited state since no shift in the emission
−4
spectrum as the concentration is raised up to 1×10 M.
Such properties make BDSP a sutible candidate for dye
−
4
lasing. A 1×10 M solution of BDSP dye in DMSO,
DMF, CH CN and dioxane gave laser emission when ex-
3
cited by a nitrogen pulsed laser (λ =337.1 nm) of 800 ps
ex
pulse duration and 1.48 mJ pulse energy. The solution was
placed in oscillator and amplifier cuvettes of 10 mm path
length. The output energy of the laser dye was measured as
a function of wavelength to determine the lasing range in
different solvents. The maximum gain coefficient (α) was
calculated at the maximum laser emission by measuring the
−1
T
Fig. 6 Plot of Stokes shift (Cm ) versus normalized E (30) of different
solvents
intensity I of laser emission from the entire cell length L
and the intensity from the cell half length IL/2. One can
known (6.2 Å and 9 Debye, respectively) the change in
dipole moment is calculated using Eq. (7),
L
calculate the laser gain emission from Eq. (10) [52].
2
3
1
2
ꢄ
ꢅ
8
1m
2
IL
6
7
5
Δμ ¼ 4ꢀ . ꢁ
ð7Þ
αðλÞ
¼
ln
− 1
ð10Þ
3
L
IL=2
6
:2 a  11307:6
It is well known that a gain occurs when the stimulated
emission of photons exceeds the reabsorption or loss due to
scattering. Therefore, the gain is the increase in the number of
emitted photons and is dependent on both wavelength and
incident light intensity. The cross section for the stimulated
N
T
where, m is the slope of linear plot of E vs Stokes shift
Fig. 6) and the value of Δμ was found to be 2.56 D. The
value of Δμ obtained by application of the Lippert–Mataga
equation is higher than that obtained by dimensionless micro-
(
N
T
scopic solvent polarity parameters E , because the Lippert
laser emission (σ ) of the dye was calculated at the laser emis-
e
equation is an approximation, i.e., higher-order terms are
neglected. These terms would account for second-order ef-
fects, such as the dipole moments induced in the solvent mol-
ecules by the excited fluorophore and vice verse [49]. The
ground to excited state transition dipole moment (μ ) of
sion maximum using Eq. (11) [53]
4
λ FðλÞdλ
σe ¼
ð11Þ
2
8
πcn τ f
1
2
BDSP in different solvents was calculated using Eq. (8) [50]:
Here λ is the emission wavelength, n is the refractive index
of the solvent, c is the velocity of light and F(λ) is the nor-
malized fluorescence intensity where ∫F(λ)dλ=1. The fluores-
f
2
μ
¼
ð8Þ
1
2
−7
4
:72 Â 10 Emax
Where, Emax is the absorption maximum energy in cm−1 and f
cence lifetime (τ ) values of the BDSP were calculated using
f
the Strickler–Berg equation [38, 54–57]. This equation is a
modification of the Einstein fundamental relationship between
the probability of absorption and emission, which is applica-
ble to polyatomic molecules in solution. Through this equa-
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 experimen-
tal oscillator strength values were calculated using Eq. (9)
tion, k can be obtained from the absorption and fluorescence
r
spectra using Eq. (12)
[51].
Z
Z
ꢀ ꢁ
ꢀ ꢁ
ꢀ
ꢁ
−9
F ν~ dν Z
f ¼ 4:32 Â 10
ε v dv
ð9Þ
ε ν~
1
−
9 2
kr ¼
kr ¼
¼ 2:88 Â 10 n Z ꢀ ꢁ
d ν~ ð12Þ
ð13Þ
−
3
τ0
ν
F ν~ ν~ dν
Where, ε is the numerical value for molar absorption coef-
ficient measured in dm mol cm and ν is the numerical
value of the wavenumber measured in cm . The values of f
and μ12 are listed in Table 1.
3
−1
−1
Z
D E
ꢀ ꢁ
−1
2
1
−9 2
¼ 2:88 Â 10 n ν~
ε ν~ d ν~
τ0
av

J Fluoresc
Table 2 Laser parameters of
DBSP in some solvents
2
e(cm )
α(cm−1)
Solvent
λ
ex(max)
Lasing range
nm)
σ
τ
f
(ns)
E
1/2
(
(
nm)
(MJ/mol)
2.4×10−
2.2×10−
2.2×10−
1.8×10−
16
16
16
16
0.98
0.76
1.12.
1.24
1.65
1.60
1.45
1.15
14.52
14.42
13.26
12.89
DMSO
DMF
485
480
480
465
470–500
468–496
465–495
452–476
3
CH CN
Dioxane
ꢂ
ꢃ
where F ð ν~ Þ is the emission spectrum plotted on wave-
M J
mol
−
1
−1
−1
Normalized photostability
number scale(cm ), ε ð ν~ Þ (in units of M cm ) is the
decadic molar absorption coefficient at wavenumber (in
Epulse  N
1=2
−
1
units of cm ) and n is the refractive index of the me-
¼
ð15Þ
2
6
π  r  l  c  10
dium. The integrals are calculated over S ↔S absorp-
o
1
tion and the emission spectrum. For the ideal case of a
negligible Stokes shift and a perfect mirror-image rela-
tionship of absorption and fluorescence spectrum,
Eq. (12) simplifies to Eq. (13) [58, 59]; h ν~ i av is the
average wave number corresponding to the 0–0 transi-
tion and the fluorescence lifetime can be calculated
using Eq. (14) [60]
where Epulse is the pulse energy (in Joules), N1/2 is the number
of pulses to get half the initial emission intensity, r is
the radius of laser beam on the surface of the sample
and l is the sample thickness (in centimeters) and c is
the dye concentration(mol dm ). The obtained values
of E1/2 indicate a good photostability of BDSP dye in
comparison with the values of 1, 4-bis (5-phenyl-2-
oxazolyl)benzene (POPOP) lasing dye [62].
−
3
τ f ¼ τo∅f
ð14Þ
The lasing parameters of BDSP and excited state lifetime
are listed in Table 2. It seems that the dye laser emission given
at fluorescence maximum is comparable to the steady state
emission maximum and the cross-section values are close to
those of the best lasing dyes [53]. The photochemical stability
Fluorescence Quenching Study by Silver Nanoparticles
Quenching studies are applied as tools for investigating the
response of fluorophores to quenchers. This implies that the
nature of molecular interaction and microenvironment of
fluorophores in solution can be studied by fluorescence
quenching experiments. The fluorescence emission of BDSP
was studied in presence of various concentrations of silver
nanoparticles (AgNPs) in ethanol at room temperature by in-
vestigating the interaction of AgNPs with BDSP. The emis-
of BDSP dye was determined as the half-life energy(E1/2
)
(
Table 2) which is the amount of total pump energy absorbed
when the dye laser energy dropped to 50 % of its initial output
value using the relation [52, 61]:
−5
sion spectrum (1×10 M) of BDSP in ethanol exhibits a
Fig. 7 Emission spectra of 1×10−5 M of BDSP in ethanol at different
concentrations of AgNPs. The concentrations of AgNPs at decreasing
emission intensity are 0.0, 0.04, 0.08, 0.12, 0.16, 0.0.20, 0.27 and
Fig. 8 Stern–Volmer plot for fluorescence quenching of BDSP by
0
.31 nM (λex=340 nm)
AgNPs in ethanol

J Fluoresc
and the absorption of spectrum of AgNPs (Fig. 9) reveals the
possibility of radiative and non-radiative energy transfer from
excited state BDSP to ground state AgNPs, according to
Förster’s theory [65]. This occurs due to the coupling of elec-
tronic transition dipole moment of BDSP with surface Plas-
mon of AgNPs.
Crystal Structure Studies of BDSP
BDSP molecule was crystallized in DMF with the space group
P2 /a. The structural study along with the other physical stud-
1
ies was carried out, with the aim, to determine the three di-
mensional behavior and Vander Walls interactions of mole-
cules in the unit cell. The geometry around the olefin group
Fig. 9 The spectral overlap of (a) absorption of AgNPs with (b) emission
spectrum of BDSP in ethanol
[
C=C] is Trans. The molecule has a plane of symmetry
through the central pyrazine ring due to which half molecule
appears in the unit cell and other half being completed by
crystallographic inversion symmetry [66]. BDSP is not planar
since the dihedral angle between the aromatic and half pyr-
maximum at 500 nm upon excitation at 340 nm. As the concen-
tration of the quencher (AgNPs) increases, the position of BDSP
fluorescence bands remains unchanged despite the substantial
decrease in the fluorescence intensity as shown in as shown in
Fig. 7. This indicates the absence of molecular aggregation under
the prevailing experimental conditions. No such quenching of
BDSP was observed in the presence of low concentrations of a
capping agent, indicating that the AgNPs are responsible for the
o
azine ring is 6.37 (1) . The ORTEP diagram of title com-
pound is shown in Fig. 10, crystallographic data given in
Table 3, bond lengths and bond angles are given in Tables 4
and 5, respectively. No classical or non-classical hydrogen
bonding were observed in the molecule. The π-π interaction
has not been noticed in BDSP.
−5
fluorescence quenching. When the absorption spectra of 1×10
M BDSP were recorded after addition of AgNPs in ethanol, the
spectral pattern and absorption maxima remained unchanged.
This observation suggests that the absence of ground state inter-
action between BDSP and AgNPs.
The fluorescence quenching behavior can be analyzed by
the Stern-Volmer Eq. (16) to determine the Stern-Volmer
DFT Calculations
ab initio molecular orbital computations were applied using
the Gaussian09 programs [67]. The GaussView [68] programs
viewed the molecular geometries. The hybrid Becke’s three
parameter Lee-Young-Parr correlation functional (B3LYP) of
the density functional theory (DFT) with triple-zeta and polar-
ization functions on hydrogen and heavy atoms basis set [6-
311++G(d,p)] has been used to optimize the geometry of the
title compound (BDSP) to a minimum. The UV–vis. spectra
quenching constant (K ) for BDSP using AgNPs as a quench-
sv
er [63, 64]:
Io
¼
1 þ K ½AgŠ
ð16Þ
sv
I
where I and I are the fluorescence intensities in the absence
and presence of the quencher concentration[Ag ]. The Stern-
of BDSP in Tetrahydrofuran (THF), acetonitrile (CH CN),
o
3
o
dimethyl sulphoxide (DMSO), dichloromethane (CH Cl )
2
2
Volmer plot for BDSP (Fig. 8) was found to be linear with
chloroform (CHCl ) and carbon tetrachloride (CCl ) solvents
3 4
2
correlation R =0.96 in ethanol. The K value calculated from
were computed by applying Time-Dependent density func-
tional theory (TD-DTF) [69] and the polarizable continuum
model (PCM) method [70] with the 6-31G(d) basis set.
sv
9
−1
the slope of the linear plot was found to be 3.09×10 M . The
significant overlap between the emission spectrum of BDSP
Fig. 10 ORTEP diagram of
BDSP, thermal ellipsoids were
drawn at 50 % probability level

J Fluoresc
Table 3 Crystal data and structure refinement for BDSP
Table 5 Bond angles for BDSP
Identification code
15071
Atom Atom Atom Angle/°
Atom Atom Atom Angle/°
Empirical formula
C
24
H
24
N
2
O
4
C2
C2
C6
C1
C4
C3
O1
O1
C6
O2
O2
C1
C1
C1
C2
C3
C4
C4
C4
C5
C5
C5
C6
C7
C7
C3
C2
C5
C3
C5
C4
C4
C6
117.94(15) C5
118.95(15) C8
123.08(15) C7
C6
C7
C8
C1
C1
C9
C8
C8
121.04(15)
127.26(16)
123.94(16)
120.90(15)
119.29(15)
Formula weight
404.45
293(2)
Temperature/K
1
Crystal system
monoclinic
P2 /a
121.75(15) C10 C9
Space group
1
119.83(15) N1
119.43(15) N1
124.92(15) N1
C9
C9
1
a/Å
b/Å
c/Å
α/°
β/°
γ/°
12.2031(18)
C10 119.81(15)
1
7.0419(8)
C10 C9
115.63(14) C10 N1 C9
123.88(15)
116.31(15)
12.9851(18)
90
119.83(14) C4
115.25(14) C5
124.92(14)
O1
O2
C11 116.61(14)
C12 117.07(14)
115.766(18)
90
3
Volume/Å
1004.9(3)
1
-X,1-Y,1-Z
Z
2
3
ρ
calcg/cm
1.337
theory together with those determined by X-Ray technique are
listed in Table 6. Referring back to Fig. 11 and Table 6, it is
clear that there exists an excellent agreement between the ex-
perimental crystal and theoretical gas-phase geometries. This
is manifested by the small absolute errors between each of
them and their very small total average error. The two benzene
rings and the pyrazine ring together with the two vinyl groups
and the two oxygen atoms of the methoxy groups are nearly
-1
μ/mm
0.092
F(000)
428.0
3
Crystal size/mm
Radiation
0.48×0.28×0.15
MoKα (λ=0.7107)
6.682–59.246
2θ range for data collection/°
Index ranges
−16≤h≤16, −9≤k≤6, −17≤l≤17
5818
Reflections collected
Independent reflections
Data/restraints/parameters
2442 [Rint=0.0206, Rsigma=0.0288]
2442/0/136
o
collinear. This is indicated by having ca. 178 or 1.00 dihedral
angles over the entire backbone of the molecule. The crystal
experiment revealed a dihedral angle between the aromatic
ring and half of the pyrazine ring of 6.37°. This discrepancy
could be explained in terms of phase difference (gas versus
solid). The carbon-nitrogen; Carbon-carbon and oxygen
methyl-carbon or oxygen ring-carbon bonds are either doubly
bonded or partially multiply bonded (see the bond lengths of
C1-C2, C1-N6, C22-O36 and O35-C51). The computed bond
2
Goodness-of-fit on F
1.039
Final R indexes [I>=2σ (I)]
Final R indexes [all data]
Largest diff. peak/hole / e Å−
R
1
1
=0.0528, wR
2
2
=0.1386
=0.1583
R
=0.0775, wR
3
0.24/-0.27
Geometry
2
angles reflect the sp hybridization environment over the en-
Some selected bond lengths and angles of the optimized ge-
ometry of the title compound (BDSP) (Fig. 11) which have
been calculated by applying B3LYP/6-311++G(d,p) level of
tire compound. It is clear that this substrate could assume
many geometrical structures, but the one shown in Fig. 11
agreed satisfactorily with the crystal structure. Thus, it has
been adopted for studying the electronic and spectral proper-
ties of BDSP in gas-phase and in some selected solvents.
Table 4 Bond lengths for BDSP
Atom
Atom
Length/Å
Atom
Atom
Length/Å
UV-Visible Spectra
C1
C1
C1
C2
C3
C4
C4
C5
C5
C2
C6
C7
C3
C4
C5
O1
C6
O2
1.381(2)
1.407(2)
1.462(2)
1.389(2)
1.380(2)
1.410(2)
1.365(2)
1.374(2)
1.3679(19)
C7
C8
C9
1.327(3)
1.460(2)
1.394(2)
1.349(2)
1.394(2)
1.325(2)
1.428(2)
1.421(2)
The UV-Visible absorption spectra of π-conjugated organic
molecules arise from π→π* and n→π* transitions [71].
These transitions embrace electron motions between the fron-
tier orbitals, which are the higher occupied molecular orbitals
C8
1
C9
C10
C9
N1
1
C10
C10
C11
C12
C9
(
(
HOMOs) and the lower unoccupied molecular orbitals
LUMOs). The title compound BDSP has many double bonds
N1
O1
O2
together with lone pairs on the nitrogen and the oxygen atoms.
The PCM/TD-B3LYP/6-31G(d) theoretical UV-Visible exci-
tations of BDSP in CH Cl , CHCl , CCl , THF, CH CN and
2
2
3
4
3
1
-X,1-Y,1-Z
DMSO are listed in Table 7. It is clear that the estimated

J Fluoresc
Fig. 11 The atom numbering of the optimized geometry of BDSP which has been obtained by using B3LYP/6-311++G(d,p) level of theory
maximum absorption wavelengths became more and more
red-shifted with the increase of the solvent polarity. The oscil-
lator strengths of these transitions also increase with the in-
creasing solvent polarity. These theoretical predictions are in
excellent agreement with our experimental findings. The dif-
ferences in magnitude between the experimental (Table 1) and
the estimated (Table 7) wavelength maxima are thought to be
due to the solvent effects [72].
benzene rings, the two vinyl moieties and the pyrazinyl ring
as π-bonding orbitals. On the other hand, the LUMO is ex-
tended all over the rings as π*-antibonding orbital; whereas
the LUMO+1 is centered on the pyrazinyl ring as a π*-anti-
bonding orbital. These placements eased the intramolecular
charge transfer from the methoxy groups and the benzene
rings toward the pyrazinyl ring across the vinyl bridges.
The energies of the HOMOs and LUMOs of the solvated
BDSPs together with their energy gaps (E.G.) are listed in
Table 8. They can be used as gauges for the intermolecular
solute-solvent charge transfer. It is clear that the energy gaps
of 2.597–2.677 eV facilitate the π→π* charge transfer and
complement the UV-Visible spectra. It is of interest to note
that the energy gaps of the solvated BDSP molecules are in-
versely proportional to their polarities, which are estimated by
Frontier Orbitals
The frontier orbitals of the title molecule BDSP in the gas-
phase are shown in Fig. 12. They have been constructed by
using B3LYP/6-31G(d) level of theory. On the one hand, the
HOMO and HOMO-1 are delocalized over the entire two
Table 6 Some selected bond
Designation
Bond length (Å)
Designation
Bond angle (degree)
lengths (Å) and bond angles
degrees) for gas-phase BDSP
(
Theor.
Cryst.
│Error│
Theor.
Cryst.
│Error│
which have been calculated using
B3LYP/6-311G++(d,p) level of
theory. The X-Ray structure is
given for comparison purposes
C17-C18
C17-C19
C13-C17
C18-C20
C20-C24
C24-C22
C24-O35
C22-C19
C22-O36
C9-C13
C4-C9
1.400
1.409
1.459
1.393
1.395
1.415
1.361
1.383
1.372
1.348
1.457
1.410
1.348
1.323
1.423
1.433
1.381
1.407
1.462
1.389
1.380
1.410
1.365
1.374
1.368
1.327
1.460
1.394
1.349
1.325
1.428
1.421
0.019
0.002
0.003
0.004
0.015
0.005
0.004
0.009
0.004
0.021
0.003
0.016
0.001
0.002
0.005
0.012
0.008
C18-C17-C19
C19-C17-C13
C17-C18-C20
C20-C24-C22
O35-C24-C20
O35-C24-C22
C19-C22-C24
O37-C31-C28
O37-C31-C33
C22-C19-C17
C9-C13-C17
C13-C9-C4
C5-C4-C9
117.34
123.53
121.61
118.90
124.75
115.35
119.86
116.72
123.97
121.90
127.47
123.18
120.53
120.20
119.27
123.38
117.94
123.08
121.75
119.43
124.92
115.63
119.83
115.25
124.92
121.04
127.26
123.94
120.90
119.29
119.81
123.88
0.60
0.45
0.14
0.53
0.17
0.28
0.03
1.47
0.95
0.86
0.21
0.76
0.37
0.91
0.54
0.50
0.55
C1-C2
C1-N6
C2-N3
N3-C4-C9
C51-O35
C47-O36
Average
N6-C1-C2
N3-C2-C1
Average

J Fluoresc
Table 7 The UV–vis. calculated
maximum wavelengths (λ/nm)
for BDSP in different solvents
Solvent
Wavelengths(λ/nm)
Energy/eV
Oscillator strength
Dielectric Constant
CCl
CHCl
THF
CH Cl
CH CN
DMSO
4
503.73
530.06
540.74
544.11
547.79
558.96
2.461
2.339
2.293
2.279
2.223
2.218
2.417
2.492
2.517
2.523
2.549
2.551
2.23
4.71
3
7.43
2
2
8.93
3
37.50
46.83
Their transition energies (eV), and oscillator strengths were facilitated by using TD-B3LYP/6-31G (d) level of
theory
Dielectric constants of solvents were also listed to estimate the polarity of solvents
their dielectric constants. This finding favours the intermolec-
ular charge transfer between the target molecule BDSP and
the chosen solvents.
and least reactive in CCl . However, the ω values showed that
when BDSP solvated in DMSO became the strongest electro-
phile amongst the solvated BDSPs, while being the weakest
4
The electronic chemical potential (μ) that deals the traffic of
electrons in a chemical system [73], the chemical hardness (η) as
a beneficial connotation for studying stability and reactivity of
chemical systems [74] and the global electrophilicity index (ω)
that estimates the stabilizing energy when a chemical species
accepts additional electronic charge from the environment [73]
are also listed in Table 8. It is obvious that the η and μ values
indicated that BDSP in DMSO is the softest, least stable and
most reactive among them, while being the harder, most stable
electrophile (or a nucleophile) when solvated in CCl [75].
4
Natural Bond Orbital Analysis
The Natural Bond Orbital (NBO) theory [76] has gained wide
acceptance in studying hyperconjugative interactions [77].
This is accomplished through applying second order pertur-
bation energies (E ):
(2)
À
Á
2
E
ð2Þ
¼ ΔEij ¼ q Fij =Δε
i
where q is the donor orbital occupancy, F is the off-diagonal
i
ij
elements of the NBO Kohn-Sham Matrix and Δε is the energy
difference between a donor orbital (i) and an acceptor orbital
(
j). The most influential second order perturbation (E )
(2)
LUMO+1
hyperconjugative energies of BDSP in the gas-phase which
were computed by using B3LYP/6-31G(d)/B3LYP/6-311++
G(d,p) level of theory are listed in Table 9. They are catego-
rized as π→π* and n→π* charge transfer interactions. A
one-side intramolecular charge transfer from the methoxy
group toward the pyrazinyl ring and across the vinyl bond is
exemplified by six consecutive interactions (see Table 9).
There is a strong charge transfer from the lone pair of the
equatorial methoxy oxygen atom, n2O35, toward π*C20-C24
LUMO
HOMO
Table 8 The HOMO (eV), LUMO (eV) the energy gap (E.G./eV), the
electronic chemical potential (μ/eV), the chemical hardness (η/eV) and
the global electrophilicity index (ω/eV) for BDSP in different solvents
Parameter CCl
4
CHCl
3
THF
CH Cl
2 2
3
CH CN DMSO
HOMO
LUMO
E.G.
μ
−4.894 −4.935 −4.952 −4.957 −4.978
−2.217 −2.297 −2.330 −2.340 −2.380
−4.980
−2.383
2.597
3.682
1.299
5.218
2.677
3.556
1.339
4.722
2.638
3.616
1.319
4.957
2.622
3.641
1.331
4.980
2.617
3.649
1.309
5.086
2.598
3.679
1.299
5.210
Η
HOMO-1
Ω
Fig. 12 The frontier orbitals of BDSP that have been computed by using
B3LYP/6-31G (d) level of theory
They have been calculated using TD-B3LYP/631G(d) level of theory

J Fluoresc
Table 9 Some selected most influential second order perturbation (E(2)
)
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