Solvatochromic and solid-state emissive azlactone-based AIEE-active organic dye: Synthesis, photophysical properties and color-conversion LED application

Article abstract of DOI:10.1016/j.molliq.2020.113482

Here, we report one-pot synthesis of a new azlactone-based dye bearing isoxazole moiety as a donor group (i.e., Dye-Iso), which displays fluorescence emission both in solution and solid-state, and a systematic study of its photophysical properties. The an

Full text of DOI:10.1016/j.molliq.2020.113482

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Solvatochromic and solid-state emissive azlactone-based AIEE-
active organic dye: Synthesis, photophysical properties and color-
conversion LED application
Murat Olutas, Akın Sagırlı
PII:
S0167-7322(20)31733-5
DOI:
https://doi.org/10.1016/j.molliq.2020.113482
MOLLIQ 113482
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
22 March 2020
15 May 2020
28 May 2020
Please cite this article as: M. Olutas and A. Sagırlı, Solvatochromic and solid-state
emissive azlactone-based AIEE-active organic dye: Synthesis, photophysical properties
and color-conversion LED application, Journal of Molecular Liquids (2020),
https://doi.org/10.1016/j.molliq.2020.113482
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Solvatochromic and Solid-State Emissive Azlactone-Based
AIEE-Active Organic Dye: Synthesis, Photophysical Properties
and Color-Conversion LED Application
Murat Olutas1*and Akın Sagırlı2
1 Department of Physics, Bolu Abant Izzet Baysal University, Bolu 14030, Turkey
2 Department of Chemistry, Bolu Abant Izzet Baysal University, Bolu 14030, Turkey
*Corresponding Author:
Email: olutas_m@ibu.edu.tr
Phone.: +90 374 254-1000 int:1334
Fax: +90 374 253-4642
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Abstract
Here, we report one-pot synthesis of a new azlactone-based dye bearing isoxazole moiety as a
donor group (i.e., Dye-Iso), which displays fluorescence emission both in solution and solid-
state, and a systematic study of its photophysical properties. The analyses of steady-state
photoluminescence (PL) spectroscopy measurements reveal that Dye-Iso exhibits positive
solvatochromic behavior accompanied by the decreasing trend in PL quantum yield (QY), as the
polarity of solvent increases. In addition, time-resolved fluorescence (TRF) spectroscopy shows
that the PL lifetime is shortening and overall effective nonradiative decay rate becomes
accelerated with increasing polarity of the medium, which explain well the decrease in PL QY.
These findings strongly suggest that the dipole-dipole interactions between the dye molecule
having intramolecular charge transfer (ICT) characteristic and solvent are responsible for the
redshift in PL emission with the increase in solvent polarity. Furthermore, the electronically
ground- and excited-state dipole moments were determined by using linear correlation methods
having different solvent polarity functions. The results depict that the Dye-Iso has a larger
excited-state dipole moment as against that in the ground-state, which reflects the ICT
mechanism. The spectroscopic studies show that the Dye-Iso exhibits aggregation-induced
emission enhancement (AIEE) under aggregate formation and is AIEE-active compound. The PL
QYs of the Dye-Iso in aggregation- and solid-state were also found to be higher in comparison
with those in solutions, which has been elucidated by the increase in the radiative decay rate due
to reduced restriction of intramolecular motions (RIM). Finally, we have demonstrated that the
AIEE-active Dye-Iso can be used as a color-conversion material for solid-state lighting
applications by employing that on a commercial near-ultraviolet LED.
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KEYWORDS: Azlactone, solvatochromic fluorescence, intramolecular charge transfer,
aggregation-induced emission enhancement, solid-state emission, dipole moment
Introduction
Fluorescent organic compounds from past to date are always one of the most studied optical
material groups having considerable interest due to their photophysical properties as well as
some advantages like easy synthesis, low cost and toxicity [1,2]. Their fluorescence emission
kinetics are typically governed by electronic transition delocalized over the whole chromophore
under photoexcitation in conventional organic dyes such as rhodamine and fluorescein, whereas
it is originated from intramolecular charge transfer transitions (ICT) in the fluorescent
compounds comprising electron-donating and -accepting groups [3–5]. The charge-separated
structure provides tunable photophysical properties such as spectral positions, fluorescence
quantum yield and lifetime with structural modification of the attached donor and acceptor
groups [6–11]. In addition, their photophysical properties are affected by environmental
parameters such as viscosity [12,13] and polarity [8,14] due to the redistribution of charges in the
excited state under optical excitation, which makes them well environment probe [15,16].
Although organic fluorescence materials display well fluorescence emission in their dilute
solutions, they suffer from the solid states such as crystals and thin films in which emission
either decreases or completely disappears due to the molecular aggregation, which is called as
aggregation-caused quenching (ACQ) [17,18]. This phenomenon generally originates from
strong intermolecular interactions (e.g., π-π stacking) that favors the nonradiative relaxation
processes [19,20]. However, in practice, this is an undesirable and problematic condition to
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incorporate the fluorescent organic materials especially into optoelectronic and biological
applications that require the aggregate or solid molecular ensemble [21–24]. Therefore,
overcoming the ACQ effect in the solid phase of fluorescent organic materials is crucial, and in
the literature, there are extensive studies to reduce or suppress this effect and develop the solid-
state emissive organic material [25–27]. Early in the 2000s, Tang et al. reported an unusual
phenomenon with a series of studies based on π-conjugated organic compounds displaying
emission in solid-state but not in solution. The phenomenon is called aggregation-induced
emission (AIE), which is generally explained by restriction of intramolecular motions (RIM) that
includes intramolecular rotations and vibrations [28–31]. Based on this result, the color
modulation of the emission in the solid phases of some functionalized boron diiminates has been
also reported with unique optical properties [32].
Furthermore, the development of new organic compounds exhibiting fluorescence emissions,
both in dilute solution and solid-state, has attracted much attention and it is still a challenge [33–
36]. For instance, it was shown that a covalently modified anthracene derivative exhibits
persistent and strong blue emission in both solution and solid phases [35]. In another study,
fluorine-containing diphenylacetylenes having extended π-conjugated structure with a large
molecular dipole moment have been demonstrated as alternative candidates for organic
molecules exhibiting fluorescence characteristic both in the solid-state and their dilute solutions
[36]. Recently, Lu and co-workers have reported a series of triazolopyridine–thiophene donor–
acceptor–donor (D–A–D) type conjugated molecules with PL QYs reaching 89% in solution and
29% in solid-state [37].
One other candidate structure for organic emitters both in solution and solid phases is the
azlactone (or oxazolone) derivates are known as notable intermediates and modified to reach
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synthetically valuable heterocycles over the past decades [38]. These heterocycles not only
exhibit various biological activities [39] but also display promising photophysical and chemical
sensor properties [40,41], leading an increased interest for the development of new azlactone
compounds.. For instance, when the substituent on para-position of phenyl ring of azlactone is an
electron-donating one, such as OCH3 and N(CH3)2, it has been reported that they exhibit
remarkable photophysical properties as molecular photoswitches [42,43], a green fluorescent
protein chromophore [44] and an optical sensor for pH measurements [40]. In addition, the
azlactone derivatives have been also noticed as highly emissive in polymer matrices compared
with their solution phases [40,45,46]. However, a very limited number of studies have been
published associated with azlactone derivatives displaying fluorescence emission both in liquid
and solid phases, and to the best of our knowledge, no study including an application utilizing
their solid-state fluorescence properties have been reported to date.
To this end, here we designed and synthesized a new azlactone-based molecule (i.e., Dye-Iso),
which displays fluorescence properties both in solution and solid-state, by positioning the
heterocyclic aromatic isoxazole as an electron-donating group on the 4-position of the azlactone
ring. The structural and photophysical properties of the fluorescent Dye-Iso were systematically
investigated by using infrared (IR), nuclear magnetic resonance (NMR), mass (MS), UV-Vis
absorption, steady-state photoluminescence (PL) and time-resolved fluorescence (TRF)
spectroscopy. The strong solvent polarity dependent fluorescence behavior (i.e., bathochromic
shift) and the decrease in PL QY were simultaneously observed with increasing solvent polarity.
In addition, the significant shortening in fluorescence decay lifetime and acceleration in the
nonradiative decay rate with increasing medium polarity reveal the ICT character of the
fluorophore. The ICT characteristic of the Dye-Iso was also analyzed by different
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solvatochromic shift methods, which strongly suggest the excited-state dipole moment of the
fluorophore is greater than that of the ground-state. Besides, in aggregation state, the Dye-Iso has
displayed aggregation-induced emission enhancement (AIEE) in acetone/water mixture. Besides,
it was found that the PL QY of the Dye-Iso in the solid-state is higher than those in the solution
ones, which can be explained by restriction of intramolecular motion (RIM). The achieved
promising photophysical properties in solid-state can make this organic compound a suitable
candidate for solid-state lighting applications. With this motivation, as a proof-of-concept
demonstration, we employed the synthesized Dye-Iso compound as a color-converter material on
a light-emitting diode (LED).
Results and Discussion
The synthesis of fluorescent azlactone-based dye (i.e., Dye-Iso) was carried out using the
traditional Erlenmeyer-Plöchazlactone method. Figure 1 shows the molecular structure and
synthetic routes of target Dye-Iso molecule. Briefly, first, 1,3 dipolar cycloaddition reaction of
nitrile oxide formed in situ by the oxidation of p-(methoxy)benzaldoxime 1 with NaOCl to
propargyl alcohol 2 gave isoxazole substituted methanol 3 in a regioselective manner. Then, it
was readily oxidized to its corresponding aldehyde 4, key intermediate precursor, by pyridinium
chloro chromate (PCC) in good yield. Finally, acetic anhydride mediated condensation of
hippuric acid 5 with heterocyclic aldehyde 4 yielded only E isomer of target fluorescent
molecule (Dye-Iso) in one pot (see Figure 1b). Structural elucidation of all precursors and Dye-
Iso were performed by IR, NMR, Mass data and physical characteristics, i.e., melting point, (see
experimental section for details).
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Figure 1. (a) 4-isoxazole substituted oxazol-5(4H)-one (Dye-Iso). (b) Synthetic pathway of the Dye-Iso.
Figure 2 depicts the UV-Visible absorption and steady-state photoluminescence (PL) emission
profile of the Dye-Iso dissolved in some aprotic solvents having different polarities, and all
photophysical properties are summarized in Table 1. All the measurements were performed at
room temperature. As can be seen in the absorption spectra (Figure 2a), the two main absorption
peaks have emerged for each dye solution having different polarity, where the first absorption
maximum is positioned between 263 and 265 nm, whereas the second one having the highest
absorbance is centered between 365 and 370 nm at the lower-energy side of the spectrum. In
addition, the shape of the absorption band of the Dye-Iso remains almost unchanged with the
increasing solvent polarity although position of that changes with slight redshift of a few
nanometers. As seen in the inset of Figure 2a, the maximum spectral shifts in the main
absorption peaks were found to be 2 and 5 nm for the higher- and lower-energy sides of the
spectrum, respectively, with increasing solvent polarity. This nearly solvent polarity independent
behavior in the absorption spectra indicates that the ground-state of the dye is not affected
dramatically in the mean of structural and electronic properties with increasing solvent polarity
[47]. To better analyze the absorption behavior of the Dye-Iso, the density functional theory
(DFT) and the time-dependent density functional theory (TD-DFT) calculations were carried out
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by using Gaussian 09W software [48]. The hybrid exchange-correlation B3LYP functional with
the 6-311G(d) basis set was employed during the calculations [49,50]. Figure 2b shows the
localizations of the highest occupied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) on the optimized structure of the Dye-Iso for the gas phase. The
HOMO density is mainly localized on the isoxazole moiety with bearing phenyl ring, whereas
LUMO density is mainly located on the azlactone moiety. The energies of HOMO and LUMO
are associated with the ionization potential and the electron affinity of a molecule, respectively.
Hence, the HOMO and LUMO localizations on the Dye-Iso molecule depict that the isoxazole
and azlactone moieties have the ability to donate and accept electrons, respectively, and indicate
the intramolecular charge transfer (ICT) character of the Dye-Iso. The computed UV-Vis spectra
of the Dye-Iso for different solvent mediums are also represented in Figure 2c. The absorption
maxima at the higher-energy side of the spectrum were located between 258 and 260 nm,
whereas the dominant absorption maxima were found to emerge between 397 and 399 in the
computed absorption bands for different solvents. The results show that the simulated absorption
bands were in very close agreement with the experimentally obtained ones. In addition,
according to the results of the TD-DFT calculations performed for the gas phase (see Figure S8),
the dominant absorption band was found to correspond to HOMO-1 → LUMO transition having
94.20 % contribution with the oscillator strength of 1.1396 and had an ICT character. The
computed parameters characterizing the absorption features of the Dye-Iso, such as excitation
energy, wavelength, oscillator strength, and the major component in terms of the percentage
contribution of the molecular orbital transitions are summarized in Table S2 (see supporting
information).
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Figure 2. (a) UV-Vis absorption spectra of the Dye-Iso in solvents having different polarity. (b)
Localizations of the HOMO and LUMO on the optimized structure. (c) Computed UV-Vis absorption
spectra of the Dye-Iso in solvents having different polarity employing TD-DFT calculation. (d)
Normalized photoluminescence spectra of the Dye-Iso in solvents having different polarity. The inset in
(a) and (c) show the zoom-in of the normalized curves belonging to same absorption data. DCM and DCE
are represent the abbreviation for solvents of Dichloromethane and 1,2-Dichloroethane, respectively.
On the other hand, the PL emission spectrum of the Dye-Iso exhibits significant solvent polarity
dependent behavior contrary to its absorption spectrum (see Figure 2b). All the steady-state PL
measurements were carried out under the excitation at 375 nm by a continuous-wave pump. As
the polarity of solvent is increased, the PL emission maximum of the Dye-Iso shifts to the longer
wavelength of the spectrum (i.e., bathochromic shift). The maximum shift in the peak position of
PL emission due to solvent polarity was found to be ~62 nm (i.e., from 510 nm in 1,4-dioxane to
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572 nm in 1,2-dicholoraethane). Owing to the nearly unchanged absorption features despite the
strong change in the PL emission profiles with the increase in solvent polarity, the Stokes shift in
a polar solvent is larger than that in relatively non-polar solvent (see Table 1). It is well known
that the intermolecular interactions between the solute and solvent molecules can affect some
photophysical properties of the environment-sensitive molecules such as absorption and
fluorescence features via hydrogen bonds and dipole-dipole interactions [51]. Here, the observed
bathochromic behavior, or positive solvatochromism, in the PL emission curve with increasing
polarity can be elucidated to dipole-dipole interactions between the dye and the host, due to the
fact that aprotic solvents cannot make any hydrogen bonding with the dye. The redshifted PL
behavior indicates the excited-state dipole moment (µe) of the Dye-Iso is larger than that in its
ground state (µg), owing to redistributed charge separation under excitation between the electron-
donating isoxazole ring and the electron-accepting azlactone ring (i.e., intramolecular charge
transfer, ICT). Therefore, this is resulted with lowering in excited state energy of the Dye-Iso,
inducing from rotationally re-oriented solvent molecules surrounding the dye [52].
In order to deeply analyze the effect of solvent polarity on the fluorescence properties of the
Dye-Iso, the PL QY measurements were also conducted. Here, the de Mello methodology [53]
was utilized to determine the PL QYs of the Dye-Iso for different solvent media, and all the
measurements were carried out using a calibrated integrating sphere at 375 nm excitation. During
the measurements, dilute solutions of the dye were studied to eliminate the probability of re-
absorption effect. The evolution of the PL QY of the Dye-Iso as a function of the solvent polarity
is given in Figure 3a. The PL QY of the Dye-Iso exhibits decreasing trend with increasing
solvent polarity. The highest value for the PL QY was found to be 40.5% in 1,4-dioxane,
whereas it decreases down to 12.6% in 1,2-dicholoraethane (see Table 1). This significant
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change in PL QY clearly indicates the solvent polarity dependence of fluorescence mechanism of
the Dye-Iso, and a change in the nonradiative decay rate can be responsible for the decrease in
the PL QY.
Table 1. Photophysical properties of the Dye-Iso in different aprotic solvents at room temperature as a
function of the solvent polarity scale of ET(30).
b
d
i
j
ϕ g
Solvent
ET(30) a
λa
ε c
λPL
fwhm e
Δλ f
τ h
kr
knr
(nm)
(x104 M-1cm-1)
(nm)
(nm)
(nm)
(%)
(ns)
x107(s-1)
x107 (s-1)
Dioxane
36.0
39.1
263 / 365
264 / 368
1.03 / 2.62
1.15 / 1.92
510
529
132.01
137.31
145
161
40.5
29.4
7.96
6.22
5.09
4.73
7.47
Chloroform
11.35
DCM
40.7
265 / 369
0.98 / 1.90
561
164.75
192
21.7
5.22
4.15
14.94
DCE
41.3
265 / 370
1.04 / 2.00
572
165.73
202
12.6
4.83
2.62
18.09
d
g
a The solvent polarity scales [16].b Absorption maxima for two main peaks. c Molar extinction coefficients (absorptivities) for two main peaks.
e
Photoluminescence maxima. Full-width half-maximum of the PL emission peak. f Stokes shift according to the dominant absorption peak.
Photoluminescence quantum yield. h Fluorescence lifetime, i Radiative decay rate, j Nonradiative decay rate. DCM and DCE are represent the
abbreviations for solvents of Dichloromethane and 1,2-Dichloroethane, respectively.
To better understand the underlying reason in decreased PL QY with increasing solvent polarity,
time-resolved fluorescence (TRF) measurements were also carried out using the time-correlated
single photon counting system with picosecond pulsed-pump laser at 375 nm. All the
fluorescence decay curves for different solvent medium were collected at the maximum emission
wavelength of the Dye-Iso in corresponding solvent. Figure 3b depicts the fluorescence decay
profiles of the Dye-Iso in solvents with different polarity. As it is seen from Figure 3b,
fluorescence decay curves become faster as the solvent polarity increases. This accelerated decay
behavior is a significant indication that overall nonradiative decay pathways could be more
dominated with increasing polarity as the decreasing PL QY trend is also considered. In order to
analyze the fluorescence decay curves, the data were numerically fitted by using following
single-exponential decay function [51],
⁄
( )
(1)
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where A and τ are the amplitude of the exponential decay and the fluorescence lifetime,
respectively. The obtained fluorescence lifetimes are represented in Figure 3c as a function
solvent polarity and summarized in Table 1. It is clearly seen that the fluorescence lifetime of the
Dye-Iso exhibits strong solvent polarity dependent behavior. As the solvent polarity is increased,
the fluorescence decay lifetime was found to be decreased from 7.96 (1,4-dioxane) to 4.82 ns
(1,2-dicholoraethane). In addition, the radiative and nonradiative decay rates of the Dye-Iso for
all solvents were analyzed using the fitted fluorescence decay parameters and the PL QYs. The
decay rates were computed by the following relations [51],
(2)
and
⁄
( )
(3)
where kr, knr, ϕ and τ are the radiative and nonradiative decay rate constants, PL QY and
fluorescence lifetime, respectively. Figure 3d represents the radiative and nonradiative decay
rates as a function of solvent polarity. As seen in Figure 3d, an increase in solvent polarity results
in a decrease in the radiative decay rate whereas an increase in the nonradiative decay rate. The
increasing trend in the nonradiative decay rate with the increasing solvent polarity is an
indication to the decreasing behavior in the PL QY of the Dye-Iso. The interaction between
solute having ICT characteristic and solvent becomes generally more prominent with increasing
solvent polarity. This would feed the nonradiative decay processes and gives rise to a decrease in
the PL QY. Therefore, the further increase of nonradiative processes in the fluorescence
mechanism of Dye-Iso with increasing solvent polarity can be attributed to the ICT processes
[54–56].
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Figure 3. (a) Evolution of the photoluminescence quantum yields (PL QY) of the Dye-Iso in different
solvent medium. (b) Time-resolved fluorescence decays, (c) fluorescence lifetimes and (d) decay rates of
the Dye-Iso as a function of ET(30) solvent polarity scale. The inset in (b) shows the zoom-in of the same
decay curves. The dotted lines are a guide for the eyes.
In order to investigate the solvatochromic behavior of the Dye-Iso, the Stokes shifts between the
absorption and fluorescence spectra were analyzed by using linear correlation methods having
different orientation polarizability functions proposed by Lippert–Mataga [57,58], Bakhshiev
[59], Kawski–Chamma–Viallet [60,61]. The methods, which are most accepted and used
techniques in the literature, also enable to estimate the ground- and excited-state dipole moments
of the Dye-Iso as well as change between them. The three independent linear correlation
equations proposed by Lippert–Mataga, Bakhshiev, Kawski–Chamma–Viallet are given
respectively as follow,
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(
)
̅
̅
(4)
(5)
( )
̅ ̅
̅ ̅
(
)
(6)
where
̅
and
̅
are wavenumbers of absorption and fluorescence maximums in cm-1,
(
)
(
)
(
)
respectively. The , and characterize the orientation polarizability
functions corresponding to related methods. The polarity functions are as follows,
(
)
[
]
(7)
(8)
(9)
(
)
[
] [
]
( )
(
)
[
)] [
] [
]
)
(
(
where the ε and n are the dielectric constant and refractive index of solvent, respectively. The
computed polarizability functions for each method are given in Table 2. The m1, m2 and m3 in the
( ) ( )
equations given above define the slopes of ( ̅ ̅ ) vs , ( ̅ ̅ ) vs and
(
)
(
̅
̅
) vs graphs represented in Figure 4, respectively and are given as follows
( )
(10)
(11)
( )
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(
)
(12)
Here, the h, c and a are Planck’s constant, speed of light in vacuum and Onsager cavity radius,
respectively. In this study, the dielectric constants and the refractive indices of the solvents were
taken from the literature [11,62], whereas the Onsager cavity radius of the molecule was
theoretically determined as 5.61 Ǻ by using density functional theory (DFT-B3LYP/6-311G(d))
calculation.
Table 2. Room temperature spectroscopic properties of the Dye-Iso and the solvent polarity functions for
different solvatochromic methods.
Solvents
1,4-Dioxane
Chloroform
DCM
DCE
ε a
2.22
4.81
9.08
10.42
n b
1.4224
1.4454
1.4241
1.4448
̅
(cm-1) c
(cm-1) d
27397.26
19607.84
7789.42
23502.55
0.02150
0.04361
0.30848
0.12288
0.16193
27173.91
18903.59
8270.32
23038.75
0.14842
0.37201
0.48742
0.25359
0.42629
27100.27
17825.31
9274.96
22462.79
0.21838
0.59500
0.58529
0.32005
0.60163
27027.03
17482.52
9544.51
22254.77
0.22118
0.62341
0.61314
0.32625
0.62543
̅
e
̅ ̅
(
̅ ̅
)(cm-1)
f
g
h
i
(
)
)
)
)
)
(
(
j
(
k
(
a
b
c
d
e
f
Dielectric constant. Refractive index. Absorption maxima. Fluorescence maxima. Stokes shift. Arithmetic mean of absorption and
fluorescence maximum. Lippert-Mataga solvent polarity functions. h Bakhshiev solvent polarity functions. i Kawski–Chamma–Viallet solvent
g
polarity functions. j Weller solvent polarity functions. k Retting solvent polarity function.
Figures 4a and 4b depict the variation of Stokes shift as a function of solvent polarity functions
proposed by Lippert–Mataga and Bakhshiev. The data were fitted to a straight line and all the
fitting parameters are given in Table 3. The data show the linear behavior with correlation
coefficients of 0.809 and 0.873 for Lippert–Mataga’s and Bakhshiev’s plots, respectively. The
deviation in the linearity might be related to the specific interactions between the solute and the
solvents. The change between the ground- and excited-state dipole moments (Δµ) was calculated
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by utilizing the slope of the Lippert–Mataga’s plot and found to be 12.04D. In addition, the
variation of arithmetic means of the Stokes shift as a function of solvent polarity function
proposed by Kawski–Chamma–Viallet is represented in Figure 4c. The data exhibit good
linearity with correlation coefficients of 0.925.
Table 3. The calculated fitting parameters and correlation coefficients for different solvatochromic
models.
Slope
Intercept
Correlation coefficient
(cm-1)
(cm-1)
Method
Lippert–Mataga
8268.86
2955.14
7459.86
7512.60
0.809
0.873
Bakhshiev
Kawski–Chamma–Viallet
4005.19
9834.33
4435.68
24811.64
20969.38
20467.83
0.925
0.852
0.910
Weller
Retting
Utilizing the slopes of the Bakhshiev and the Kawski–Chamma–Viallet plots, the ground- (µg)
and excited-state (µe) dipole moments were also estimated. To this end, the following equations
can be derived using equations (11) and (12) in the case of assumptions that the solute geometry
does not change during the electronic transition and electric dipole moments (i.e., µg and µe) are
in parallel [11].
(
)
√
(13)
(
)
√
(14)
(15)
[
] for
The ground- (µg) and excited-state (µe) dipole moments were found to be 1.29D and 8.49D,
respectively, using the equations (13) and (14). The ratio of the ground- and excited-state dipole
⁄
moments ( ) was also estimated as 6.59. The findings support the observed spectral shift in
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PL emission with increasing solvent polarity and strongly indicate that redistribution of the
charge densities in the excited-state is the main responsible mechanism for the large difference
between the µg and µe. This makes the excited-state more polar than the ground-state and it is
also a reflection of the ICT mechanism as well [8]. However, owing to the fact that the emission
arising from the charge transfer state (CTS) is not attainable by direct excitation and the
absorption characteristic of the CTS is not known, the excited-state CT characteristic of the Dye-
Iso was also analyzed by Weller's method [52,63]. The equation, which only associated with the
shift in emission spectrum, proposed by Weller is as follows.
(
)
̅
(16)
(17)
(
)
[
]
(18)
Figure 4d shows Weller’s plot with a linear regression coefficient of 0.852 and indicates the
excited-state ICT characteristic of dye. The excited-state dipole moment of the Dye-Iso was also
estimated by using the slope of the Weller’s plot and found to be 13.13D.
Table 4. Ground- and excited-state dipole moments of the Dye-Iso.
(D)a
(D)b
(D)c
(D)e
(D)f
d
⁄
12.04
1.29
8.49
6.59
13.13
8.82
a
c
The change in dipole moment calculated from Equation (10). b The ground-state dipole moment calculated from Equation (13). The excited-
state dipole moment calculated from Equation (14). d The ratio of excited-state dipole moment and ground-state dipole moment calculated from
Equation (15). The excited-state dipole moment calculated from Equation (18). The excited-state dipole moment calculated from Equation
(21). In the calculations, 1 Debye was taken as 3.33564 x 10-30 cm.
e
f
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The large change in electric dipole moments between the ground- and excited-states is a direct
indication of the ICT characteristic and points to the possible existence of another mechanism
known as twisted intramolecular charge transfer (TICT) originated from structural conformation.
Therefore, the TICT characteristic of the dye was analyzed by utilizing Retting method [52,64].
The Retting’s equation is as follows,
(
)
̅
(19)
(20)
(
)
[
]
(21)
Figure 4e represents the Retting’s plot and the data show good linearity with a correlation
coefficient of 0.910. This suggests that the redshift in the PL emission spectrum with the large
Stokes shift might be predominantly associated with the TICT process. With increasing solvent
polarity, the broadening in full-width half-maximum (fwhm) of the PL emission band (see
Table1), the large Stoke shift, the decreasing PL QY and the large change between the ground-
and excited-states dipole moments strongly point out the TICT mechanism in the Dye-Iso [65].
The excited-state dipole moment of the Dye-Iso was also found to be 8.83D by using the slope of
the Retting’s plot.
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Figure 4. Solvatochromic linear plots of the Dye-Iso as a function of (a) Lippert-Mataga, (b) Bakhshiev,
(c) Kawski–Chamma–Viallet, (d) Weller and (e) Retting solvent polarity functions. The dotted lines show
the corresponding linear fittings.
To understand the Dye-Iso is whether AIE-active or not, the effect of aggregation on its
fluorescence behavior was also investigated. To this end, a series of colloidal suspensions was
prepared using acetone and Milli-Q water, where the Dye-Iso is dissolved well in acetone,
whereas it is insoluble in water. The aggregation experiment was carried out by varying the
fractional ratios of water in the colloidal suspensions, where the existing form of the Dye-Iso
molecules can be changed with increasing water fraction from solution to aggregation state. The
concentration of the all suspensions were kept in 5.0x10-5 M during the measurements. Figure 5a
depicts the UV-Vis absorption spectra of the Dye-Iso in varying volume to volume (v/v) ratio of
acetone/water mixtures. The inset in Figure 5a also shows the molar extinction coefficients
(absorptivities) of the mixtures corresponding to 365 nm and summarized in Table S2 (see
supporting information). As can be seen in Figure 5a, with increasing content of water in the
mixture, up to 60%, no dramatic change is observed in the absorptivity of the mixture, whereas it
is significantly decreased between 70 and 95% of water content. The decrease in absorptivity
with the further increase of water fraction in acetone/water mixture is an indication of aggregate
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formation resulting from the degraded solvating power of the aqueous mixture. In addition, an
increase in the tails of absorption curves after 70% water fraction is started to appear. In
literature, such tails have been observed, which originates from the formation of nano-aggregates
in colloidal suspensions comprising organic systems with different water fractions, and they are
explained by Mie scattering [66,67]. Furthermore, the PL spectroscopy measurements were
carried out to investigate the emission characteristic of the Dye-Iso in the aggregation phase.
Figure 5b shows the PL emission spectra of the Dye-Iso as a function of the acetone/water ratio
(v/v) in the mixture. The Dye-Iso exhibits very weak fluorescence emission in dry acetone at
about 573 nm because of the strongly polarized ICT state. However, due to the polarity of
medium is increased with the addition of an amount of water, the ICT effect becomes more
prominent that enables nonradiative decay processes. Therefore, it is observed that the PL
emission is quenched up to 4:6 v/v ratio of acetone/water. In this case, the molecular dissolution
of the Dye-Iso can continue through still sufficient solvation power of the acetone/water mixture.
With the further increase in water fraction, AIE characteristic have been started to be observed in
the PL emission spectrum. The PL spectra exhibit blue-shifted behavior with a gradually
enhanced PL maxima as the water fraction increases and it reaches the maximum level in the
suspension comprising 95% of water. The enhancement in PL emission accompanied by a blue-
shifting emission maximum can be attributed to the limitation of ICT effect. The evolution of the
PL maxima with increasing water content is depicted in Figure 5c. In addition, to have a better
insight into the increase in the emission characteristic of the Dye-Iso, the PL QY measurements
were performed (Figure 5d). It was found that the PL QY was in the tendency to an increase,
where the PL QYs were found to be 12.9, 18.9, and 54.1 % for water fraction of 80, 90, and 95
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% in the mixtures, respectively. These results reveal that the Dye-Iso displays aggregation-
induced emission enhancement (AIEE) characteristics and it is an AIEE-active compound.
Figure 5. (a) UV-Vis absorption and (b) photoluminescence spectra of the Dye-Iso in varying volume to
volume ratio of acetone/water mixtures. Evolution of (c) the photoluminescence emission maxima and (d)
photoluminescence quantum yield of the Dye-Iso as a function of water fraction in the mixtures. The
insets in (c) are the images of the Dye-Iso dissolved in solvent mixtures having different water content
under day-light and UV-illumination with a 365 nm UV-lamp. The red dashed line in (b) shows the zero-
point, whereas the black dotted lines are a guide for the eyes.
Furthermore, the Dye-Iso not only exhibit fluorescence emission in solution but also in solid-
state. Figure 5a shows the PL emission profile of the Dye-Iso in the solid-state under 405 nm
excitation and the data were collected via Ocean Optics spectrometer. In the case of the solid-
state, the shape of the PL emission band exhibit slight change, and the nearly symmetric
behavior of the emission in solution phases is disappeared. The shape of the PL emission in
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mechanofluorochromic (MFC) materials typically depends on the aggregation condition in the
solid-state [68]. Therefore, the change in the shape of the PL emission profile can be an
indication for the MFC behavior in the solid phase of the sample. The PL emission maximum in
the solid-state is located at 527 nm and exhibits a shift to the lower energy side of the spectrum
as compared to that of its solution form in 1,4-Dioxane (i.e., 510 nm). Besides, the PL emission
band in the solid-state becomes narrower (i.e., fwhm~101.37 nm) than those in solutions (see
Table 1). The redshifting and narrowing behaviors in the emission line of the Dye-Iso is an
indication of the AIE characteristic govern by formation of aggregates in solid form [69]. These
spectral changes in the emission spectrum are due to possible specific molecular interactions,
such as intermolecular stacking and π-π interactions, caused by near-field strong dipole-dipole
interactions in the solid-state [70]. In the case of the fluorescent organic molecules having planar
structure, specific molecular interactions give rise to a decrease in PL QY due to the increasing
nonradiative recombination of the excited-state electrons in the aggregate forms of the condensed
state [25]. However, in our case, by using the calibrated integrating sphere, the PL QY of the dye
in the solid-state was measured to be 61.3% which is high as compared to those in the solutions.
The higher PL QY behavior in aggregate- or solid-state than that in solution is a very common
property of the AIEE-active chromophores, which is explained by restriction of intramolecular
motions (RIM). In solution, the intramolecular motions (i.e., intramolecular rotations and
vibrations) such as fast rotation of phenyl rings give rise to dissipate the energy of absorbed
photons. This leads to deactivate the radiative relaxation pathways of the excited state electrons
and reduces the PL QY. In solid-state, the intramolecular motions are restricted by the physical
constraints and results in enhanced PL emission due to the blocking of the nonradiative decay
channels. In addition, another proposed mechanism is suppressing intermolecular π-π stacking.
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This mechanism also leads to a decrease in the nonradiative decay rate and enhanced the PL QY.
To analyze decay kinetics of the excited state electrons in solid-state, time-resolved fluorescence
measurement was also performed. Figure 6b represents the fluorescence decay curve of the Dye-
Iso in solid-state. The decay curve was numerically fitted by using single-exponential decay
function and the fluorescence decay lifetime was found to be 7.78 ns. The radiative and
nonradiative decay rate constants were computed as 8.10x107 and 4.97x107 s-1, respectively. The
decrease in the nonradiative decay rate as compared to those in the solutions indicates that
restriction of intramolecular motions and suppression of strong intermolecular interactions can
be responsible for the increased PL QY in aggregation- and solid-state.
Finally, as a proof of concept demonstration, the Dye-Iso displaying efficient solid-state
fluorescence emission was tested as a color-converting material for solid-state lighting
applications. To this end, a monolith including the dense Dye-Iso was prepared in a pellet form.
In the process, first, the dye in solid was grinded with the KBr salt, which has a large
transmission window in the visible and infrared spectra, to get a well-structured monolith and
then it was pressed for 10 min under 60 kN. The prepared monolith was employed on a high
power near-UV-LED emitting at 405 nm. Figure 6c depicts the spectrum of the resulting dye-
monolith with an integrated LED. The emission spectrum of the dye-monolith exhibits redshifted
behavior (from 527 to 535 nm) as compared to that of the pristine solid (see Figures 5a and 5c).
The redshifted emission resulting from the external mechanical effect implies that the Dye-Iso
shows a typical MFC property. The shifts in the solid-state emission peak wavelength can be
associated with the change of molecular packing mode as a response to external physical effects
(e.g., grinding and pressing) [71,72]. Besides, the solid-state emission line of the Dye-Iso in the
form of the monolith shows slight broadening (i.e., fwhm~106.81 nm) with respect to that in the
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powder form. This spectral broadening of 5.44 nm in the emission band can be ascribed to a
change of the dielectric medium surrounding the dye [73] due to KBr. After analyzing the PL
emission spectrum of the dye-monolith, in order to the characterize the shade of the monolith
integrated LED emission, Commission Internationale de l’Enclairage (CIE) color coordinates
(x,y) and correlated color temperature (CCT) of the emission of the monolith integrated LED
were determined by using the spectrum given in Figure 6c. The corresponding color coordinates
were found to be (0.1716, 0.0087) for the only-near-UV LED and (0.3784, 0.5660) for the dye-
monolith integrated LED and they are represented on the CIE 1931 chromaticity diagram in
Figure 6d. The CCT value for the dye-monolith integrated LED was also determined as 4780 K.
The findings show that the Dye-Iso in solid-state can be used as color-conversion material in
solid-state lighting applications.
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Figure 6. (a) Normalized photoluminescence spectrum and (b) time-resolved fluorescence decays of the
Dye-Iso in solid-state. The insets in (a) and (b) show the photographs of the synthesized dye sample in
solid under day-light (left) and UV-excitation (right), and the zoom-in of the same decay curves,
respectively. (c) Normalized emission spectrum of the dye-monolith integrated near-UV LED. (d) CIE
coordinates of the only-near-UV LED and dye-monolith integrated near-UV LED. The inset in (c) and (d)
show the zoom-in of photographs of the dye-monolith under day-light (top) and UV-excitation (right),
and dye-monolith integrated near-UV LED, respectively.
Conclusion
In summary, we have described the synthesis and photophysical properties of the Dye-Iso
displaying fluorescence emissions both in solution and solid-state. In solution, the systematic
steady-state and time-resolved fluorescence studies shows that the fluorescent Dye-Iso exhibits
the ICT-based photophysical properties depending on the solvent polarity. The strong redshifted
behavior in the PL maximum was observed with the increasing solvent polarity in comparison to
that of the absorption, which enable to large Stokes shift of 202 nm in 1,2-Dichloroethane. The
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Dye-Iso also shows solvent dependent behavior in the PL QY, and the time-resolved
fluorescence analyses indicate that the nonradiative recombination processes become more
dominant when the polarity of medium surrounding the dye increases. These findings reveal that
an increase in polarity of solvent enables a more prominent dipole-dipole coupling between the
solvent and the dye molecules having ICT character. Furthermore, the methodological analyses
that are carried out by using linear correlations with different polarity functions show that the
excited-state dipole moment is about 6.7-fold higher than that of its ground-state. The higher
polarity in the excited-state is an indication for the ICT character of the Dye-Iso. Furthermore,
the spectroscopic measurements reveal that the Dye-Iso also exhibits AIEE behavior in its
colloidal suspensions containing acetone/water mixture. The PL QY in the solid-state was also
found to be increased up to 4.8-fold higher than those in the solutions. The enhancement in the
PL QY in aggregation- and solid-state was explained by in terms of the restriction of
intramolecular motions in view of the decrease in nonradiative decay rate. Considering the
photophysical properties including the high PL QY and large Stoke-shifted emission in the solid-
state, the Dye-Iso has become highly promising material for optoelectronic applications. Finally,
as a proof-of-concept demonstration, it has been shown that with employing the Dye-Iso on the
commercial LED it can be utilized as a color-conversion material for solid-state lighting
applications.
EXPERIMENTAL SECTION
General procedure for synthesis of 3-(4-methoxyphenyl)isoxazole-5-carbaldehyde (4):
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To a mixture of (3-(4-methoxyphenyl)isoxazol-5-yl)methanol (3) (0.810 g, 4 mmol) in CH2Cl2
(25 mL) was added pyridinium chlorochromate (PCC) (2.150 g, 10 mmol) and stirred at room
temperature for 24 h. The reaction progress was monitored by TLC. After completion of the
reaction, the reaction mixture was washed with water and extracted with CH2Cl2 through
multiple extractions. The combined organic layers were dried over Na2SO4 and evaporated under
vacuum to give a crude product which was then purified by column chromatography on silica gel
100 (eluent EtOAc - Petroleum ether 4:1 (v/v) to furnish the desired aldehyde as yellow solid.
3-(4-methoxyphenyl)isoxazole-5-carbaldehyde:
o
Yellow solid (0.552 g, 68%). m.p. 94-96 C. IR (KBr): ν = 3474, 1697 (C=O), 1616, 1527,
1450, 1301, 1253, 1172, 1028, 916, 831, 744 cm-1. 1H NMR (400 MHz, CDCl3) δ 10.01 (s, 1H),
7.79 (d, J = 8.9 Hz, 2H), 7.22 (s, 1H), 7.00 (d, J = 8.9 Hz, 2H), 3.87 (s, 3H).
General procedure for synthesis of (E)-4-((3-(4-methoxyphenyl)isoxazol-5-yl)methylene)-2-
phenyloxazol-5(4H)-one (Dye-Iso):
A mixture of 3-(4-methoxyphenyl)isoxazole-5-carbaldehyde (4) (0.438 g, 2.16 mmol), hippuric
acid (5) (0.440 g, 2.46 mmol), and sodium acetate (0.089 g, 1.08 mmol) in acetic anhydride (0.6
◦
mL) was stirred at 60 C for 3h. After cooling the solution to room temperature, water (30 mL)
was added, and the precipitated crude fluorescent azlactone dye was filtered and washed with
cold water. The product was dried under vacuum for several hours and it was recrystallized from
acetone.
(E)-4-((3-(4-methoxyphenyl)isoxazol-5-yl)methylene)-2-phenyloxazol-5(4H)-one (Dye-Iso):
o
Yellow solid (0.583 g, 78%). m.p. 160-162 C. IR (KBr): ν = 2922, 2849 (aliphatic CH), 1804
(ester C=O), 1728 (C=N azlactone), 1656 (C=N isoxazole), 1609, 1428, 1248, 1178, 808, 702cm-
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1
1; H NMR (400 MHz, CDCl3) δ 8.21 (d, J = 7.0 Hz, 2H), 7.85 (d, J = 8.6 Hz, 2H), 7.68 (t, J =
13
6.9 Hz, 1H), 7.59 – 7.55 (m, 3H), 7.23 (s, 1H), 7.01 (d, J = 8.7 Hz, 2H), 3.87 (s, 3H).; C NMR
(101 MHz, CDCl3) δ 165.90, 165.05, 163.02, 161.33, 158.03, 136.18, 134.52, 129.26, 129.06,
128.56, 128.48, 124.86, 114.49, 114.01, 107.38, 55.50; HRMS (+APCl) calcd for C20H15N2O4
[M+H]+ 347.1032, found 347.1035.
ASSOCIATED CONTENT
Supporting Information
Infrared (IR), Nuclear Magnetic Resonance (NMR), Time-of-flight mass spectrometry (TOF-
MS) data, computed IR and UV-Vis spectra for gas phase using DFT and TD-DFT calculations.
AUTHOR INFORMATION
Corresponding Author
*olutas_m@ibu.edu.tr
ACKNOWLEDGMENT
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