Specific spectral characteristics of some phenylquinoxaline derivatives

Article abstract of DOI:10.1016/j.molliq.2014.12.018

Absorption and emission spectral characteristics of a bis(hydroxyphenylquinoxaline) Q, and two aromatic polyethers containing phenylquinoxaline rings, Q-DBF and Ox-Q, were studied in various solvents. Q, Q-DBF and Ox-Q display two absorption bands, while a poly(1,3,4-oxadiazole-ether) Ox-BisA, exhibits a single absorption band. Their fluorescence spectra (2.5 × 10^{- 6} mol L^{- 1}) were characterized by an emission band between 380 and 520 nm. At slightly lower concentrations (6.4 × 10^{- 7} mol L^{- 1}) and under excitation with light corresponding to the highest energy absorption band, Q-DBF and Ox-Q in ethyl acetate and cyclohexane solutions exhibited simultaneously dual emission bands in the near-UV (≈ 355 nm) and blue region (≈ 432 nm). The emission characteristics were quantified by CIE indices (in x, y color coordinates) for the investigated systems. Values of fluorescence quantum yields ranging from 15.93 to 49.83% in chloroform media were obtained.

Full text of DOI:10.1016/j.molliq.2014.12.018

Journal of Molecular Liquids 202 (2015) 62–67
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Specific spectral characteristics of some phenylquinoxaline derivatives
⁎
Mihaela Homocianu , Alina-Mirela Ipate, Corneliu Hamciuc, Anton Airinei
“Petru Poni” Institute of Macromolecular Chemistry, 41A, Grigore Ghica Voda Alley, 700487 Iassy, Romania
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 October 2014
Received in revised form 12 December 2014
Accepted 13 December 2014
Available online 15 December 2014
Absorption and emission spectral characteristics of a bis(hydroxyphenylquinoxaline) Q, and two aromatic
polyethers containing phenylquinoxaline rings, Q-DBF and Ox-Q, were studied in various solvents. Q, Q-DBF
and Ox-Q display two absorption bands, while a poly(1,3,4-oxadiazole-ether) Ox-BisA, exhibits a single absorp-
tion band. Their fluorescence spectra (2.5 × 10−6 mol L−1) were characterized by an emission band between 380
and 520 nm. At slightly lower concentrations (6.4 × 10−7 mol L−1) and under excitation with light corresponding
to the highest energy absorption band, Q-DBF and Ox-Q in ethyl acetate and cyclohexane solutions exhibited si-
multaneously dual emission bands in the near-UV (≈355 nm) and blue region (≈432 nm). The emission char-
acteristics were quantified by CIE indices (in x, y color coordinates) for the investigated systems. Values of
fluorescence quantum yields ranging from 15.93 to 49.83% in chloroform media were obtained.
Keywords:
Phenylquinoxaline derivatives
Dual emission band
Fluorescence lifetime
Emission quantum yield
Chromaticity diagram
Crown Copyright © 2014 Published by Elsevier B.V. All rights reserved.
1. Introduction
stability and good mechanical properties, poly(oxadiazole ether)s play
an important role in materials science [7–9].
Poly(aryl-ether)s represent a class of polymers well known as engi-
neering thermoplastics with desirable properties, such as melt and
solution processability, chemical stability and high glass transition tem-
perature. The aromatic part of these polymers contributes to thermal
stability and good mechanical properties, and the ether linkages
facilitate the polymer processing while maintaining oxidative and ther-
mal stability [1]. The introduction of heterocyclic units, such as
phenylquinoxaline or 1,3,4-oxadiazole rings, into the main chains of ar-
omatic poly(aryl-ether)s leads to polymers with good solubility, in-
creased glass transition temperature and high thermal stability [2,3].
Aromatic poly(aryl-ether)s containing these heterocyclic units show
improved solution and melt processing characteristics as compared
to their counterparts containing only aromatic rings. These polymers
can be prepared by nucleophilic displacement to form ether linkages
during the polymerization reaction. For example, poly(aryl-ether
phenylquinoxaline)s can be prepared by polymerization of bisphenols
with bis(fluorophenylquinoxalines) [3], by reacting self-polymerizable
phenylquinoxaline monomers [4,5], or by the reaction of bisphenols
containing phenylquinoxaline rings with aromatic electrophiles
such as 4,4′-difluorobenzophenone or 2,5-bis(4-fluorophenyl)-1,3,4-
oxadiazole [6].
Generally, dual fluorescence of different compounds is of great sig-
nificance in their application in multifunctional sensor microarrays
and other microscale sensor systems [10]. The dual fluorescence of
many chromophores can be attributed to different processes occurring
in the system, such as tautomerism [11], formation of two different ex-
cited states [12–15], conformational changes, excimer/exciplex forma-
tion [16], excited-state intramolecular proton transfer (ESIPT) [17],
and twisted intramolecular charge transfer (TICT) [18].
The present work focuses on the characterization of a series of
phenylquinoxaline derivatives, one of them containing 1,3,4-oxadiazole
rings, by using optical absorption, fluorescence, emission quantum
yields and emission lifetime measurements. The photophysical proper-
ties of the phenylquinoxaline derivatives have been investigated in di-
lute solutions of various solvents (cyclohexane, toluene, chloroform
and ethyl acetate) and in solid state. Their emission properties in chlo-
roform were compared using a chromaticity diagram.
2. Experimental
2.1. Materials
Aromatic poly(oxadiazole ether)s are of special interest due to their
various physico-chemical properties and their potential use as electron-
transporting materials or electroluminescence materials in organic light
emitting diodes. Due to their optoelectronic properties, high thermal
Bis(hydroxyphenylquinoxaline), Q, was synthesized by the reaction
of 4-hydroxybenzil with an aromatic tetraamine, 3,3′-diaminobenzi-
dine, according to a previously published method [19]. 2,5-Bis(4-
fluorophenyl)-1,3,4-oxadiazole was prepared from 4-fluorobenzoic
acid and hydrazine hydrate, in polyphosphoric acid, according to the lit-
erature [20].
Poly(phenylquinoxaline ether ketone), Q-DBF, and poly(phenyl-
quinoxaline-1,3,4-oxadiazole ether), Ox-Q, were synthesized by the
⁎
Corresponding author.
E-mail address: michalupu@yahoo.co.uk (M. Homocianu).
http://dx.doi.org/10.1016/j.molliq.2014.12.018
0167-7322/Crown Copyright © 2014 Published by Elsevier B.V. All rights reserved.

M. Homocianu et al. / Journal of Molecular Liquids 202 (2015) 62–67
63
conventional aromatic nucleophilic substitution polymerization tech-
nique of bis(hydroxyphenylquinoxaline) with 4,4′-difluorodiphenyl ke-
tone or with 2,5-bis(4-fluorophenyl)-1,3,4-oxadiazole, respectively.
Poly(1,3,4-oxadiazole ether), Ox-BisA was prepared by the same syn-
thetic procedure from 2,5-bis(4-fluorophenyl)-1,3,4-oxadiazole and
2,2-bis(4-hydroxyphenyl)propane [21].
were carried out with polymer solutions in chloroform using chloro-
form as eluent. Polystyrene standards of known molecular weight
were used for calibration.
Fourier transform infrared (FTIR) spectra were recorded on an FTIR
Bruker Vertex 70 spectrometer at frequencies ranging from 400 to
4000 cm−1. UV–Vis absorption spectra were obtained with a Shimadzu
3600 spectrophotometer. Steady state fluorescence emission and exci-
tation spectra were measured with a Perkin Elmer LS55 spectrofluorim-
eter. Fluorescence emission and excitation spectra were recorded by
exciting at the corresponding wavelengths of the absorption and emis-
sion maxima, respectively. All the solvents used were of spectrophoto-
metric grade. Thin films for photophysical studies were prepared by
casting chloroform solutions of the polymers onto glass substrates.
Fluorescence lifetime and emission quantum yields were measured
using an FLS 980 Edinburg Instruments spectrofluorimeter. Fluores-
cence lifetime measurements were carried out by the standard time-
correlated single-photon counting method. The excitation was provided
by an Eddinburgh Picosecond Pulsed Diode Laser (EPL-375), at 375 nm.
The criteria for a good fit for lifetimes were estimated by statistical pa-
rameters namely: χ2 values close to unity and residual trace which
was symmetrical about the zero axes. Fluorescence quantum yields
were measured using an integrating sphere. The absorbance of the solu-
tions was maintained around 0.1. All measurements were made at room
temperature, in a quartz cuvette, with 1 cm optical path length.
Q-DBF Yield: 90%. FTIR (film, cm−1): 3060 (C–H aromatic), 1653
(C = O), 1592 (C–C aromatic), 1344 and 1310 (phenylquinoxaline
ring), 1236 (aromatic ether group). 1H NMR (400 MHz, CDCl3, δ): 8.60
(2H, s) 8.31 (2H, d), 8.26 (2H, d), 7.81 (4H, d), 7.55 (8H, m), 7.37
(6H, d), 7.05 (8H, m).
Ox-Q Yield: 92%; FTIR (film, cm−1): 3057 (C–H aromatic), 1598 (C–C
aromatic), 1342 (phenylquinoxaline ring), 1241 (aromatic ether
group), 1012 and 977 (oxadiazole ring). 1H NMR (400 MHz, CDCl3, δ):
8.61 (2H, s), 8.32 (2H, d), 8.25 (2H, d), 8.11 (4H, d), 7.61 (8H, d), 7.42
(6H, s), 7.16 (4H, d), 7.07 (4H, d).
Ox-BisA Yield: 87%. FTIR (film, cm−1): 3060 (C–H aromatic), 2980
and 2880 (isopropylidene group), 1600 (C–C aromatic), 1240 (aromatic
ether group), 1020 and 980 (oxadiazole ring). 1H NMR (400 MHz, CDCl3,
δ): 8.06 (2H, d), 7.27 (2H, d), 7.10 (2H, d), 7.00 (2H, d), 1.55 (6H, s).
2.2. Methods
The inherent viscosities (ηinh) of the polymers were determined
with an Ubbelohde viscometer, by using polymer solutions in N-
methyl-2-pyrrolidone (NMP), at a concentration of 0.5 g dL−1, at 20 °C.
The 1H NMR spectra were recorded on a Bruker DRX 400 spectrom-
eter, equipped with a 5 mm multinuclear inverse detection probe, oper-
ating at 400 MHz. 1H chemical shifts are reported in δ units (ppm)
relative to the residual peak of the solvent (ref. 1H, CDCl3: 7.26 ppm).
The molecular weight was determined by gel permeation chroma-
tography (GPC) using UV and SLS detectors and PLgel 5micro Mixed C
Agilent and PLgel 5micro Mixed D Agilent Columns. Measurements
3. Results and discussion
3.1. Synthesis
The polycondensation reactions were carried out at high tempera-
ture in NMP, in the presence of anhydrous potassium carbonate. The nu-
cleophilic aromatic substitution of an aryl halide with a phenoxide is the
most common route to high performance, high temperature poly(aryl
Fig. 1. Chemical structure of bis(hydroxyphenylquinoxaline), Q and polymers Q-DBF, Ox-Q and Ox-BisA.

64
M. Homocianu et al. / Journal of Molecular Liquids 202 (2015) 62–67
Table 2
Solubility of polymers.
ether)s. It has been demonstrated that heterocycles, as well as the ke-
tone group, can activate aryl halides towards nucleophilic aromatic sub-
stitution polymerization generating high molecular weight poly(aryl-
ether)s. The oxadiazole moiety and the ketone group in 2,5-bis(4-
fluorophenyl)-1,3,4-oxadiazole or 4,4′-difluorodiphenyl ketone, respec-
tively, act as activating groups: they can accept a negative charge and
lower the activating energy for the displacement of the p-substituted
fluoro group through a Meisenheimer complex [6,21,22]. The structures
of monomer Q and of polymers Q-DBF, Ox-Q and Ox-BisA are illustrated
in Fig. 1.
Polymer
Solvent
NMP
DMAc
DMF
Chloroform
DCE
Acetone
DMSO
Q-DBF
Ox-Q
Ox-BisA
+
+
+−
+
+
+
+−
+
+
+
+
+
+
+
+
−
−
−
−
−
−
NMP = N-methyl-2-pyrrolidone; DMAc = N,N-dimethylacetamide; DMF = N,N-dimeth-
yl-formamide; DCE = dichloroethane; DMSO = dimethyl sulfoxide; + soluble; − insolu-
ble; +− partial soluble.
The inherent viscosity values of Ox-BisA, Ox-Q and Q-DBF, deter-
mined in NMP, were of 0.39, 0.54 and 0.48 dL g−1, respectively. The
values of weight-average molecular weight (Mw) ranged between
8800 and 21,000 g mol−1, the number-average molecular weight
(Mn) values between 16,800 and 66,000 g mol−1 and the polydispersity
Mw/Mn between 1.91 and 3.14 (Table 1).
The polymers were soluble in polar solvents, such as NMP, N,N-
dimethylacetamide, and N,N-dimethylformamide, and in chlorinated
solvents, such as chloroform or dichloroethane. They were insoluble in
acetone or even dimethyl sulfoxide. Polymer Q-DBF was partially solu-
ble in DMF (Table 2). The good solubility of these polymers was due
to the presence of flexible ether linkages which determined an increase
in macromolecular chain flexibility. Also, in the case of polymers Ox-Q
and Q-DBF, the bulky phenylquinoxaline units disturbed the packing
of the macromolecular chains, thus facilitating the diffusion of small
molecules of solvent which led to a better solubility.
higher degree of conjugation provided by the oxadiazole unit compared
to that imparted by the diphenyl ketone group. In contrast with the
spectral pattern of phenylquinoxaline derivatives, Ox-BisA presents
only an absorption band around 300 nm (Table 3) due to the oxadiazole
chromophore, confirming that the second absorption band originates
from the phenylquinoxaline core. Some photophysical characteristics
of the investigated compounds in various solvents are listed in Table 3.
3.3. Steady-state emission spectra and fluorescence lifetime decays
The emission spectra of compounds Q, Q-DBF and Ox-Q in chloro-
form solution are presented in Fig. 3. As may be noted from Fig. 3 and
Table 3, it can observe that these compounds show an emission maxi-
mum ranging from 410 to 432 nm, depending on the solvent. The fluo-
rescence intensity of these bands decreased when the samples were
excited with the wavelength corresponding to the absorption maxi-
mum of Band II, as compared with the emission intensity obtained
under excitation with the wavelength of Band I. As in the case of the ab-
sorption spectra, the emission maxima of Q-DBF and Ox-Q are blue-
shifted in comparison with that of Q, and the fluorescence maximum
of Q-DBF is also blue-shifted, as opposed to that of Ox-Q (Fig. 3). The
3.2. UV–visible spectral characterization
The optical properties of bis(hydroxyphenylquinoxaline), Q, and
polymers Q-DBF, Ox-Q and Ox-BisA, were investigated by UV–Vis ab-
sorption and photoluminescence measurements. The electronic absorp-
tion spectra of phenylquinoxaline compounds in chloroform solution
are shown in Fig. 2a. Bis(hydroxyphenylquinoxaline), Q, and polymers
Q-DBF and Ox-Q displayed two main absorption bands an intense
band (Band I) in the wavelength range 250–325 nm and the second
less intense band (Band II) located around 325–425 nm (Fig. 2a). The
higher energy bands (Band I) are assigned to the absorption of phenyl
rings from the phenylquinoxaline unit, diphenyl ketone group (from
Q-DBF) or diphenyl-1,3,4-oxadiazole unit (from Ox-Q sample), while
the low-energy bands (Band II) are due to the π–π* transition of the
phenylquinoxaline moiety [23].
The spectral shape and band positions of the compound Q are differ-
ent from those of its derivatives, Q-DBF and Ox-Q, in chloroform solu-
tion. The absorption characteristics of these derivatives present the
strongest differences in the region of Band I. It is noted that this absorp-
tion band exhibits a prominent maximum (centered on 275 nm for Q,
for example) accompanied by a shoulder on both the left and right
sides of the spectrum (Fig. 2a). The I- and II-absorption bands were
fitted using the deconvolution of the initially overlapped absorption
spectrum (Fig. 2b). The spectral differences (position of the shoulders)
can be the consequence of possible intermolecular interactions within
the systems and the electron delocalization. The longest wavelength ab-
sorption band of Ox-Q and Q-DBF (Band II) is blue-shifted relative in re-
lation to that of Q. As expected, a hypsochromic shift in the absorption
maximum of Q-DBF occurred, as opposed to that of Ox-Q, due to a
Table 1
Inherent viscosity and GPC data of the polymers.
Polymer
ηinh
Mn
Mw
Mw/Mn
(dL g−1
)
(g mol−1
)
(g mol−1
)
Q-DBF
Ox-Q
Ox-BisA
0.48
0.54
0.39
19,700
21,000
8800
56,000
66,000
16,800
2.84
3.14
1.91
Fig. 2. (a) Absorption spectra of Q, Q-DBF and Ox-Q in CHCl3 solution. (b) Gaussian decom-
ηinh = inherent viscosity (20 °C, 0.5 g/100 mL, NMP).
position of the absorption spectra of Q-DBF, in chloroform solution.

M. Homocianu et al. / Journal of Molecular Liquids 202 (2015) 62–67
65
Table 3
Spectral data and Stokes shifts of studied compounds in various solvents.
Code
Solvent
λ
abs, nm
λ#em, nm
Stokes
shifts, cm−1
Band I
Band II
a
Q
CHX
275
–
390
390
390
390
364
370
370
365
–
–
Toluene
CHCl3
EtAc
424, 445(sh)
443
2056.12
3067.66
2599.55
–
275
271
275
–
434
a
Q-DBF
Ox-Q
CHX
–
Toluene
CHCl3
EtAc
412
417
415
2755.18
3046.21
3300.87
–
260, 292(sh)
264, 282(sh)
b
b
CHX
–
–
–
toluene
CHCl3
EtAc
CHX
toluene
–
384
387
384
–
422
425
432
355
338(sh), 357,
370, 422(sh)
340(sh), 360,
374, 398(sh)
335(sh), 355,
370
2344.98
2310.38
2893.51
5388.03
–
278(sh), 303
275
298
–
Ox-BisA
383
Fig. 4. Excitation spectra for Ox-BisA in CHX and Q, Ox-Q in toluene solutions.
CHCl3
EtAc
307
300
–
–
4795.51
5164.31
In the case of polymer Ox-BisA, the absorption spectrum consists of
one band (inset in Fig. 5, Table 3), while the emission spectra (Fig. 5)
present one band with two pronounced peaks, accompanied by two
shoulders in CHCl3, ethyl acetate, and toluene (Table 3), except the cy-
clohexane (where only one emission band is exhibited). The fluores-
cence intensity of Ox-BisA in CHCl3 solution gradually decreased when
the excitation light wavelength values increased (Fig. 5). It is obvious
that the shape and band position of the emission spectra did not change
with the excitation wavelength, indicating the presence of only one type
of emitter in the conjugated chains [24].
CHX-cyclohexane; CHCl3-chloroform; EtAc-ethyl acetate; # — excitation with wavelength
corresponding to the absorption maximum of Band II, at 2.5 × 10−6 mol L−1 concentra-
tion; sh — shoulder peak.
a
No fluorescence emission was exhibited under excitation with wavelength corre-
sponding to the absorption maximum of Band II.
b
Sample Ox-Q is insoluble.
shape of the emission spectra of sample Q and its derivatives (Q-DBF
and Ox-Q) in all studied solvents is similar. On the other hand, the
band positions and intensities of the emission spectra strongly depend
on the solvent nature. In the emission spectral data of the Q-DBF and
Ox-Q compounds (Table 3), shifts to longer wavelengths of the emission
bands are observed in toluene (412 nm, 422 nm), CHCl3 (417 nm,
425 nm) and EtAc (415 nm, 432 nm). The emission maximum of Ox-Q
(with diphenyl-1,3,4-oxadiazole) is more red-shifted as compared to
that of sample Q-DBF (which contains a ketone unit) in the solvents
used. The emissions of compounds Q, Q-DBF and Ox-Q in cyclohexane
are practically very small under the excitation was done with the wave-
length corresponding to the phenylquinoxaline ring.
The excitation spectra of the polymers under study were measured
at fixed emission wavelengths (as listed in Table 3). Fig. 4 presents ex-
emplified excitation spectra for Q, Ox-Q and Ox-BisA in toluene and cy-
clohexane solutions, respectively. For all samples, the shapes of the
absorption and excitation spectra are similar and the maxima appear
at around the same wavelengths.
At low concentration (1 × 10−7 mol L−1) and under excitation with
the wavelength corresponding to the absorption maximum of Band I, in
ethyl acetate and cyclohexane, the two emission bands of derivatives Q-
DBF and Ox-Q were observed. In order to understand the presence of
dual emissions, the effect of excitation wavelength and concentration
values on the emission spectra was investigated. Fig. 6 depicts the emis-
sion spectra for Ox-Q (in EtAc) and Q-DBF (in CHX), at 6.4 × 10−7 and
1.27 × 10−6 mol L−1 respectively, under different excitation wave-
lengths. At the excitation wavelength of Band II (from the absorption
spectra), Q-DBF in CHX displayed a single emission band situated at
409 nm (Fig. 6a). When Q-DBF in CHX was excited with λex
275 nm (Band I), dual emission bands were observed, one centered at
em = 330 nm and another band situated at 408 nm, with an I330/I408
=
λ
ratio of 0.336 (Fig. 6a). The first emission band around 330 nm is due
to the locally excited (LE) state and the longer wavelength emission
band arises from internal charge transfer (ICT). The fluorescence inten-
sities of these bands depend on the excitation wavelength, indicating
Fig. 3. Emission spectra of Q (-, 1.22 × 10−5 mol L−1), Ox-Q (—, 2.5 × 10−6 mol L−1) and
Q-DBF (•••, 6.03 × 10−6 mol L−1), in CHCl3 solution.
Fig. 5. Emission and absorption spectra (inset plot) of Ox-BisA, in CHCl3 solution, at differ-
ent excitation wavelengths.

66
M. Homocianu et al. / Journal of Molecular Liquids 202 (2015) 62–67
Fig. 8. Fluorescence decay profile and residuals for compound Q in CHCl3 solution, λex
294 nm; IRF = instrument response function.
=
shift values, for all samples in different solvents have been observed to
vary in the range 2056–5388 cm−1 (Table 3) and to increase with in-
creasing solvent polarity.
The emission spectra of the Q-DBF and Ox-Q derivatives in solid
state (thin films) are shown in Fig. 7. The maxima associated to the
emission band position (λem = 446 for Ox-Q and λem = 425 nm for
Q-DBF) are red shifted by 10–21 nm, as compared to their correspond-
ing dilute solution spectra, as a consequence of reduced rotational free-
dom of the polymer chains and a considerable intermolecular
interactions that occurred in the solid state.
Fig. 6. Emission spectra of Q-DBF (6.4 × 10−7 mol L−1) in CHX (a) and Ox-Q in EtAc
(1.27 × 10−6 mol L−1) (b), under different excitation wavelengths.
that these bands represent the emitting levels resulting from two differ-
ent electronic states [25]. At excitation wavelengths shorter than
275 nm (260 and 250 nm), the intensity of the emission band increases,
while the intensity of the second emission band decreases insignificant-
ly (Fig. 6a). In the case of sample Ox-Q in EtAc solution, the effect of ex-
citation wavelength values on the fluorescence intensity of the emission
band is less pronounced (Fig. 6b). The emission maximum observed
around 330 nm is assigned to the locally excited state [26]. The Stokes
The emission decay curves of the investigated compounds give the
information about lifetimes of the excited states of these systems.
Fig. 8 illustrates the emission decay curves for Q in CHCl3 solution. The
results are consistent with single exponential fluorescence decay in
each case. Luminescence lifetime values and chromaticity CIE coordi-
nates of the Q, Q-DBF, Ox-Q and Ox-BisA compounds in chloroform so-
lutions are listed in Table 4. Our results for fluorescence lifetimes, τ,
(Table 4) are comparable with those of the other quinoxaline deriva-
tives, e.g. for 2,3-bis(4-ethoxyphenyl)quinoxaline (EOQ) [27].
Photoluminescence quantum yields (QY) were measured in CHCl3
solution for all the studied compounds and were determined to range
between 15.93 and 49.83%, (Table 4). The quantum yield values
were strongly affected by the presence of oxadiazole or biphenyl
ketone units in the macromolecular chain. The CIE — Commission
Internationale de l'Eclairage, (x,y) coordinates [28] of the studied deriv-
atives in chloroform were determined based on the corresponding
emission spectra, in order to compare the emission characteristics
(Fig. 9). The chromaticity coordinates of phenylquinoxaline derivatives
Table 4
Fluorescence lifetime and chromaticity coordinates of phenylquinoxaline derivatives in
CHCl3 solution.
Sample
τ (ns)
χ2
CIEx
CIEy
QY (%)
Q
0.94
0.50
0.60
–
1.096
1.033
0.998
–
0.150
0.161
0.165
0.163
0.083
0.052
0.074
0.016
15.93
49.83
22.36
–
Q-DBF
Ox-Q
Ox-BisA
Fig. 7. Emission spectra for Ox-Q (–) and Q-DBF (-) thin films, with excitation at the low-
energy absorption maximum of each compound.

M. Homocianu et al. / Journal of Molecular Liquids 202 (2015) 62–67
67
spectral shifts. The intensity of the observed dual emission bands
changed with the excitation wavelength. The CIE chromaticity coordi-
nates evaluated from the emission spectra for all these compounds con-
firmed the blue-emitting characteristic of these materials. The strong
blue fluorescence of the compounds under study was influenced by
the presence of diphenyl-1,3,4-oxadiazole or diphenyl ketone units
and by the solvent nature.
Acknowledgments
The financial support of UEFISCDI through PN-II-PT-PCCA-2013-4
project, contract no. 43/2014, funded under Joint Applied Research
Projects — Parteneriate 2013 is gratefully acknowledged.
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—
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4. Conclusion
The spectral characteristics of some phenylquinoxaline derivatives
were investigated by determining the UV–visible absorption, emission
and excitation spectra, fluorescence lifetimes and emission quantum
yields. At low concentration and under excitation with light corre-
sponding to the absorption maximum of Band I, the phenylquinoxaline
derivatives show dual emission bands with more intense ICT band over
LE emission. In solvents with different polarities, these bands show
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