Tuning of photoluminescence properties of functional phthalides for OLED applications

Article abstract of DOI:10.1016/j.jphotochem.2016.08.008

With an objective to develop simple organic systems and tailor their properties for optoelectronic applications, we have synthesized three functionalized phthalide derivatives and have investigated their electroluminescence and photophysical properties under different conditions. These derivatives showed good solubility in common organic solvents and exhibited strong absorption in the range 320–400?nm, having molar extinction coefficient values of ca. 10⁴?M^?1?cm^?1. The monomeric solution of these derivatives exhibited very low fluorescence quantum yields (Φ_F) of ca. 0.003–0.04 owing to their inherent structural features such as intramolecular free rotation and decay to the dark triplet states. However, upon complexation with Lewis acids, such as BCl₃, these derivatives showed increased fluorescence quantum yields up to ca. 0.21?±?0.01 and also exhibited aggregation induced emission (AIE) in water/acetonitrile mixtures with the emission yields in the range ca. 0.11–0.16. The morphological analysis of the aggregates through SEM and TEM showed the formation of rod-like structures in 90% water/acetonitrile mixture with an average size of ca. 100?nm. Supporting the observed aggregation induced enhancement in emission properties, these derivatives also exhibited significantly enhanced solid state fluorescence quantum yields of ca. 0.58–0.60. As a representative example, organic light emitting diode (OLED) fabricated using the derivative 3 as the emissive layer showed an efficient electroluminescence centered at 524?nm with a turn on voltage of 9?V, demonstrating thereby their potential use in optoelectronic applications.

Full text of DOI:10.1016/j.jphotochem.2016.08.008

Journal of Photochemistry and Photobiology A: Chemistry 330 (2016) 156–162
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Invited paper
Tuning of photoluminescence properties of functional phthalides for
OLED applications
Madhesh Shanmugasundarama,b, Joshy Joseph, Dr.a,b, Danaboyina Ramaiah, Prof.a,b,c,
Photosciences and Photonics Section, Chemical Sciences and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology
*
a
(CSIR-NIIST), Thiruvananthapuram 695019, Kerala, India
b
Academy of Scientific and Innovative Research (AcSIR), CSIR-NIIST Campus, Thiruvananthapuram 695019, Kerala, India
CSIR-North East Institute of Science and Technology (CSIR-NEIST), Jorhat 785006, Assam, India
c
A R T I C L E I N F O
A B S T R A C T
Article history:
With an objective to develop simple organic systems and tailor their properties for optoelectronic
applications, we have synthesized three functionalized phthalide derivatives and have investigated their
electroluminescence and photophysical properties under different conditions. These derivatives showed
good solubility in common organic solvents and exhibited strong absorption in the range 320–400 nm,
having molar extinction coefficient values of ca. 104 Mꢀ1 cmꢀ1. The monomeric solution of these
derivatives exhibited very low fluorescence quantum yields (FF) of ca. 0.003–0.04 owing to their
inherent structural features such as intramolecular free rotation and decay to the dark triplet states.
However, upon complexation with Lewis acids, such as BCl3, these derivatives showed increased
fluorescence quantum yields up to ca. 0.21 ꢁ0.01 and also exhibited aggregation induced emission (AIE)
in water/acetonitrile mixtures with the emission yields in the range ca. 0.11–0.16. The morphological
analysis of the aggregates through SEM and TEM showed the formation of rod-like structures in 90%
water/acetonitrile mixture with an average size of ca. 100 nm. Supporting the observed aggregation
induced enhancement in emission properties, these derivatives also exhibited significantly enhanced
solid state fluorescence quantum yields of ca. 0.58–0.60. As a representative example, organic light
emitting diode (OLED) fabricated using the derivative 3 as the emissive layer showed an efficient
electroluminescence centered at 524 nm with a turn on voltage of 9 V, demonstrating thereby their
potential use in optoelectronic applications.
Received 29 May 2016
Received in revised form 4 August 2016
Accepted 6 August 2016
Available online 8 August 2016
Keywords:
Phthalides
Absorption
Fluorescence
Aggregation-induced emission
Organic light emitting diode
ã 2016 Elsevier B.V. All rights reserved.
1. Introduction
from the detrimental effects such as fluorescence quenching due to
the free rotation, aggregation induced quenching (ACQ) and decay
Study of the conjugated organic molecules has been the subject
of active research due to their favourable photophysical properties
and wide range of applications in organic light-emitting diodes [1–
6], non-linear optics [7,8] photovoltaic devices [9–12] and as
fluorescent probes [13–20]. Among the various conjugated
systems, small organic molecules possess advantages like ease
of synthesis and solution processability over macromolecules or
inorganic systems [21–23]. However, the applications of such
systems in light emitting materials and as fluorescent probes and
reporters require tunable luminescence properties [24]. The vast
majority of these small organic systems have been known to suffer
through dark triplet states resulting in the loss of favourable
luminescence properties [24]. One of the approaches adopted to
mitigate the effects of various deactivation pathways was through
the aggregation induced emission (AIE) phenomenon as proposed
by Tang and coworkers [24–26] These authors have demonstrated
through
a propeller shaped conjugated molecule, 1-methyl-
1,2,3,4,5-pentaphenylsilole, which exhibited aggregation induced
high fluorescence intensity due to the restriction of the intramo-
lecular rotation (RIR) of the phenyl substituent [24]. These
observations led to the design of interesting AIE active chromo-
phores as luminophores for a variety of applications [25–27].
However, a majority of the reported systems do suffer from issues
such as structural complexity, synthetic difficulty and failure in
real-time applications. In this context, development of small
molecules with tunable excited state and desirable optoelectronic
properties are in high demand.
* Corresponding author at:CSIR-North East Institute of Science and Technology
(CSIR-NEIST), Jorhat 785006, Assam, India.
E-mail addresses: d.ramaiah@gmail.com, rama@niist.res.in (D. Ramaiah).
http://dx.doi.org/10.1016/j.jphotochem.2016.08.008
1010-6030/ã 2016 Elsevier B.V. All rights reserved.

M. Shanmugasundaram et al. / Journal of Photochemistry and Photobiology A: Chemistry 330 (2016) 156–162
157
the refractive indices of the standard and unknown solvents,
respectively. Fs and Ff are the fluorescence quantum yields of the
standard and unknown, respectively.
Photoluminescence quantum yield of the powder samples were
measured using a calibrated integrating sphere in a SPEX Fluorolog
Spectrofluorimeter. Samples were excited at the absorption
maximum using a Xe-arc lamp as the excitation source. The
absolute quantum yield was calculated on the basis of the de Mello
method [42]. Fluorescence lifetimes were measured on an IBH
picoseconds single photon counting system using a 635-nm IBH
NanoLED source and a Hamamatsu C4878-02 micro channel plate
(MCP) detector. The fluorescence decay profiles were de-convo-
luted using IBH data station software V2.1, fitted with a mono- or
biexponential decay and minimizing the x2 values of the fit
to 1 ꢁ0.1.
Chart 1. Structures of the phthalide derivatives 1–3 investigated in the present
study.
Of the various small molecules investigated, the phthalide
(isobenzofuran-1(3H)-one) chromophore has attracted consider-
able attention in recent years [28–32]. The 3-arylidene phthalide
derivatives, in particular, have been used extensively as inter-
mediates for the synthesis of a variety of drugs and photochromic
materials [28–33]. Some of the phthalides derivatives were also
found to be biologically active as pesticidal, herbicidal, insecticidal,
antispasmodic and cytotoxic agents [34–37]. These derivatives
exhibited enhanced stability and interesting biological applica-
tions compared to the stilbene analogues because of their
structural backbone fragment diarylethene connected to an
isobenzofuran unit [36–39]. Despite these derivatives are biocom-
patible, photostable and easy to synthesize, the material applica-
tions of such systems have not been explored owing to their
Fluorescence microscopic images were obtained using a Nikon
HFX 35A Optiphot-2 polarized light optical microscope, equipped
with a Linkam THMS 600. Scanning Electron Microscopic (SEM)
analysis was performed using drop casted and air dried on flat
surface of cylindrical brass stubs and subjected to thin gold coating
using JOEL JFC-1200 fine coater, which further was inserted into
Zeiss EVO 18 Cryo SCM for taking the images. The Transmission
Electron Microscopic (TEM) analysis was performed on JEOL
100 kV high resolution TEM. The compounds were dissolved in
acetonitrile to prepare the stock solution. Then by increasing the
water percentages of acetonitrile solution up to 90%, we have drop
casted on the top of carbon-coated Cu grid. The grids were
mounted on reverse tweezers and the samples were dried by a
vacuum pump under reduced pressure for 1 h at 25 ꢂC. The
accelerating voltage of the TEM was 100 kV and the beam current
was 65 A. Samples were imaged using a Hamamatsu ORCA CCD
camera.
radiationless deactivations as
a result of the free rotation,
intersystem crossing to triplet states and Z-E isomerization [38–
40]. Herein, we report the synthesis of the three functionalized
phthalide systems 1–3 (Chart 1) and tuning of their luminescent
properties through adopting aggregation and Lewis acid complex-
ation approaches. In solution and monomeric state, these
derivatives exhibited negligible fluorescence quantum yields.
However, upon self assembly in water/acetonitrile mixtures,
complexation with Lewis acids such as BCl3, and in the solid state,
these systems showed significantly enhanced luminescence
quantum yields, due to the AIE phenomenon, molecular packing
and restricted intramolecular rotation. Uniquely, these small
molecular organic systems, showed exceptional thermal and
photostability, solution processability and efficient green electro-
luminescence at 524 nm, thereby indicating their use as optoelec-
tronic materials.
The organic light emitting device (OLED) was fabricated on
glass substrates pre-coated with a layer of indium tin oxide (ITO)
having a sheet resistance of 10
V per square. The ITO substrates
were ultrasonically cleaned with detergent, deionized water,
acetone and isopropanol, and then dried by blowing nitrogen. A
layer of PEDOT:PSS was spin-coated onto the pre-cleaned ITO
substrates, and then dried in a vacuum oven at room temperature
for 30 min to extract residual water. Then the samples were
prepared in a glove box under a nitrogen protected environment
(oxygen and water contents less than 1 ppm), and the emissive
layers (EMLs) were spin-coated on top of PEDOT:PSS from toluene
and then annealed at 120 ꢂC in a vacuum oven for 30 min to remove
the residual solvent. Subsequently, the samples were transferred to
a thermal evaporator chamber (pressure less than 5 ꢃ10ꢀ4 Pa)
connected to the glove box without exposure to the atmosphere.
50 nm Tris(8-hydroxyquinoline) aluminium (AlQ3) [1,43], 1 nm
LiF, and 150 nm Al were deposited sequentially by thermal
evaporation and then electroluminescence measurements were
performed.
2. Experimental section
2.1. Methods
The melting points were determined on a Mel-Temp II melting
point apparatus. The electronic absorption spectra were recorded
on
a Shimadzu UV-3101 or 2401 PC UV–vis-NIR scanning
spectrophotometer. The fluorescence spectra were recorded on a
SPEX-Fluorolog F112X spectrofluorimeter. 1H and 13C NMR were
recorded on a 500 MHz Bruker advanced DPX spectrometer. The
mass spectra were recorded on a Thermo Scientific Exactive ESI–
MS spectrophotometer. Reactions were carried out using MAS-II
microwave synthesizer. All the solvents were purified and distilled
before use. Quantum yields of fluorescence were determined by
the relative methods using optically dilute solutions. Quinine
sulphate in 0.1 M H2SO4, with a quantum yield of 0.54 [41], was
used as the standard and the quantum yields of fluorescence were
calculated using the Eq. (1).
2.2. Materials
The starting materials and reagents were purchased from Sigma
Aldrich and used without further purification. Spectroscopic grade
solvents were purchased from Merck and used as received for
photophysical studies. The phthalide derivatives were synthesized
through a modified procedure and characterized as described in
the Supporting Experimental Section [1, mp 110–111 ꢂC (mixture
mp 110–112 ꢂC) [31]; 2, mp 128–129 ꢂC (mixture mp 128–130 ꢂC)
[31]; 3, mp 113–114 ꢂC (mixture mp 114–115 ꢂC) [31]]. The
phthalide-BCl3 complexes were prepared by dissolving the
phthalide derivatives in CH2Cl2 and the absorbance of the solution
2
As Fu nu
Au Fs ns
Ff
¼
2 Fs
ð1Þ
wherein As and Au are the absorbance of the standard and
unknown, respectively; Fs and Fu are the areas of fluorescence
peaks of the standard and unknown, respectively; and ns and nu are
was kept in between 0.3–0.5. Then 50
mL of boron trichloride (BCl3)
solution (1.0 M in CH2Cl2) was added to the phthalide solution and

158
M. Shanmugasundaram et al. / Journal of Photochemistry and Photobiology A: Chemistry 330 (2016) 156–162
the resultant mixture was allowed to equilibrate for 2 min. The UV–
vis absorption, fluorescence emission and other spectral measure-
ments were carried at 25 ꢂC.
B)
A) 1.2
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1
2
2
1.0
0.8
0.6
0.4
0.2
0.0
3
3
3. Results and discussion
3.1. Synthesis and photophysical properties
Synthesis of the phthalide derivatives 1–3 (Chart 1) was
achieved in ca. 90–95% yields by one step microwave assisted
modified Perkin condensation [29] of phthalic anhydride and the
corresponding arylacetic acids (Scheme S1, Supporting informa-
tion). The products obtained were purified through column
chromatography and characterized on the basis of spectral and
analytical evidence. The structure of the representative phthalide
derivative 3 was further confirmed through single crystal X-ray
analysis (Fig. 1). The derivative 3 was found to crystallize in
orthorhombic crystal system with Pbca space group and have four
molecules in its unit cell. Further, the derivative 3 exhibited a
250
300
350
400
450
400 450 500 550 600
Wavelength, nm
Wavelength, nm
Fig. 2. A) Absorption and B) fluorescence spectra of the phthalide derivatives 1, 2
and 3 in acetonitrile. lex = 335 and 360 nm for 2 and 3 respectively.
3 showed a red shifted absorption with a maximum of 355 nm. In
the emission spectra (Fig. 2B), all these derivatives showed weak
fluorescence in the range 400 nm–600 nm with a maximum at ca.
430 nm and 448 nm, for 2 and 3, respectively, whereas, the
derivative 1 showed negligible fluorescence emission under these
conditions. We have determined the fluorescence quantum yields
of these systems using quinine sulphate in 0.1 M sulphuric acid as
reference (FF = 0.54) [41]. We observed low fluorescence quantum
yield values of ca. 4 ꢃ10ꢀ2 and 3 ꢃ10ꢀ3 for the derivatives 2 and 3,
respectively in acetonitrile. This non-emissive behaviour of these
derivatives can be attributed to the predominant non-radiative
decay pathways such as free rotation, intermolecular hydrogen
bonding and intersystem crossing associated to their structure as
reported in the literature [38–40]. The photophysical parameters in
the solution state for all the derivatives are summarized in Table 1.
unique molecular stacking pattern in such a way that
p-stacking is
prevented. The molecules are arranged into two non-equivalent
stacks (Fig. 1B). The molecules in the first stack were arranged in
one plane; however, the molecules in the second stack were
packed in a perpendicular plane to the first stack. This unique style
of packing allowed the molecules of this phthalide family to show
interesting properties.
The phthalide derivatives 1–3 under investigation, showed
good solubility in all common organic solvents such as acetonitrile,
chloroform, dichloromethane, tetrahydrofuran and ethanol. The
absorption and fluorescence spectra of 1–3 in acetonitrile are
shown in Fig. 2. These derivatives showed characteristic absorp-
tion of the phthalide chromophore and exhibited two structured
bands in the region 280–310 nm and 330–380 nm having extinc-
tion coefficient values in the range ꢄ104 Mꢀ1 cmꢀ1 (Fig. 2A). The
derivatives 1 and 2 showed similar absorption spectra with the
long wavelength band centered at ca. 335 nm, while the derivative
3.2. Aggregation induced emission properties
To understand the propensity of the phthalide systems to
undergo aggregation, we have carried out their absorption and
emission properties in various mixtures of water and acetonitrile.
Fig. 1. A) Single crystal X-ray structure showing ORTEP diagram and B) unit cell structure showing the crystal packing of the phthalide derivative 3.
Table 1
Photophysical properties of the phthalide derivatives 1–3 in solution.e
a,c
b
a
b
b
Compound
lab (nm)
labs (nm)
lem (nm)
lem (nm)
FFa
FF
d
d
1
2
3
335 (1.9 ꢁ 0.2 ꢃ104 Mꢀ1 cmꢀ1
)
)
340
337
361
432
486
478
0.007
0.21
0.16
336 (3.8 ꢁ 0.2 ꢃ104 Mꢀ1 cmꢀ1
446
438
0.004
0.003
355 (2.6 ꢁ 0.1 ꢃ104 Mꢀ1 cmꢀ1
)
a
Acetonitrile.
90% water-acetonitrile mixture.
Molar extinction coefficients are given in parentheses.
Negligible emission.
b
c
d
e
Average of more than three experiments, the error is ca. 5%; labs = absorption maximum; lem = emission maximum; FF = fluorescence quantum yields.

M. Shanmugasundaram et al. / Journal of Photochemistry and Photobiology A: Chemistry 330 (2016) 156–162
159
A)0.40
A)
12
B)
B)
a
j
15
12
9
j
j
a
104
a
j
90%
60%
30%
ACN
0.32
0.24
0.16
0.08
0.00
103
102
101
100
a
9
6
3
0
6
3
0
225 300 375 450 525
W avelength, nm
400
500
600
700
0
20 40 60 80 100
0
5
10 15 20 25 30 35 40
Time, ns
W avelength, nm
fw
Fig. 3. Changes in A) absorption and B) emission spectra of 3 (15
m
M) in various
water-acetonitrile mixtures. Percentage of water in acetonitrile (fw); a) 0 and j) 90%.
lex 370 nm. The inset shows the visual changes in fluorescence intensity of 3 in
acetonitrile and in fw ca. 90%.
Fig. 5. A) Changes in fluorescence intensity and B) fluorescence decay profiles of 3
(15 M) monitored at 475 nm, with varying water fraction in acetonitrile (fw). lex
375 nm.
m
Interestingly, the phthalide derivatives 1–3 showed significant
variation in their absorption and emission features with increase in
percentage of water in acetonitrile. For example, with an
increasing percentage of water in acetonitrile (fw), we have
observed a gradual decrease in the absorbance of the derivative
3 at 355 nm, with a bathochromic shift of 6 nm (Fig. 3A). Similar
observations were made for the phthalide derivatives with a
bathochromic shift of ca. 5 nm and 6 nm was observed, respectively
for 1 and 2. Further, no intense red shifted peak formation was
observed in the absorption spectrum of the phthalide derivatives,
which corroborates to the fact that there is negligible probablity for
the formation of J-type aggregates [44] upon increasing the water
content in acetonitrile.
However, in the fluorescence spectrum of the derivative 3, we
observed significant changes with increase in percentage of water
(fw). Upon increasing water fraction from 0 to 90%, we observed ca.
36-fold enhancement of the fluorescence intensity with a bath-
ochromic shift of ca. 40 nm (Fig. 3B). Similar observations were
made with the derivatives 1 and 2 (Supporting information,
Figs. S1 and S2), wherein we obtained ca. 28-fold and 7-fold
enhancement in the fluorescence intensity, respectively. We have
calculated the fluorescence quantum yields at 90% of water in
acetonitrile and the values are found to be ca. 0.07, 0.21, 0.16 ꢁ 0.01,
for the derivatives 1, 2 and 3, respectively. Interestingly, the
changes in the emission can be visually monitored through the
appearance of blue fluorescence emission from colorless for the
thiophene derivative 3 under UV illumination with the addition of
water as shown in Fig. 4.
Fig. 5A shows the changes in fluorescence intensity of the
derivative
3 at 475 nm with variation of water fraction in
acetonitrile (fw). We observed slow increase in intensity initially,
but observed significant increase after ca. 50% water fraction in
acetonitrile. This clearly suggests the fact that these derivatives
undergo molecular aggregation under these conditions, which in
turn lead to restriction of the freedom of degree of rotation. As a
result we observed increased radiative decay processes and hence
fluorescence yields as reported in the literature for several such
systems [22–27]. The properties of the systems 1–3 in 90% water-
acetonitrile mixture are summarized in Table 1.
To get better insights about the excited state properties of these
systems, we have carried out the picosecond time-resolved
fluorescence measurements. Fig. 5B shows the fluorescence decay
profile of the derivative 3 monitored at 475 nm after the excitation
at 375 nm. In acetonitrile, we observed a very short lifetime of
<0.01 ns owing to its negligible fluorescence quantum yield.
However, by gradually increasing the water fraction to ca. 30%, the
derivative 3 decayed bi-exponentially with fluorescence lifetimes
of ca. 0.14 ꢁ 0.01 ns (95%) and 0.78 ꢁ 0.02 ns (5%), which can be
Fig. 4. Photographs showing the visual changes in fluorescence intensity of 3 (15
mM) by varying amounts of water in acetonitrile under normal light (top row) and under UV
light (365 nm) (bottom row). Percentage of water (fw): a) 0, b) 15, c) 30, d) 45, e) 60, f) 75 and g) 90%. lex 365 nm.

160
M. Shanmugasundaram et al. / Journal of Photochemistry and Photobiology A: Chemistry 330 (2016) 156–162
attributed to the monomer and aggregated species, respectively.
On further increase in the water content, the contribution from the
long-lived species was found to be increasing with a concomitant
decrease in the short-lived species. Interestingly, at ca. 90% water-
acetonitrile, the conjugate 3 showed monoexponential decay with
a fluorescence lifetime value of 0.90 ꢁ 0.01 ns. These results further
corroborate the fact that the aggregates formed from these systems
are the predominant emitting species present in 90% water-
acetonitrile mixture.
With a view to understand the stability of the aggregates
formed, we have carried out the temperature dependent studies by
employing both UV–vis absorption and fluorescence techniques. At
ca. 90% water-acetonitrile mixture, the derivative 3, showed
significant quenching of the fluorescence intensity with the
increase in temperature from 20 ꢂC to 70 ꢂC (Fig. S3, Supporting
information). Notably, in the cooling cycle, from 70 ꢂC to 20 ꢂC, we
observed the reverse trend in the emission intensity along with the
negligible changes in the absorption spectra. Furthermore, we have
investigated the effect of the viscosity on the aggregation
properties of these phthalide systems. By varying the amounts
of glycerol, a viscous solvent, in ethanolic solution of the derivative
3, we observed significant enhancement in the emission intensity
similar to that of the water-acetonitrile mixtures (Fig. S4,
Supporting information). These observations can be attributed
to the restriction of the degree of free rotation around the axis of
A)0.6
B)
3
10
8
0.5
0.4
0.3
0.2
0.1
0.0
3+BCl3
3+BCl3
6
4
3
2
0
250 300 350 400 450 500 550
Wavelength, nm
400 450 500 550 600 650
Wavelength, nm
Fig. 7. Changes in the A) absorption and B) fluorescence spectra of the phthalide 3
(15 M) with the addition of BCl3 (16 M) in dichloromethane. lex = 390 nm.
m
m
3.4. Interaction with BCl3
The aromatic ketones generally exhibit weak fluorescence
because, the lowest singlet excited states of these systems
normally involve 1(n-
p
*) or
(
p
–
p
*) transitions, and undergo
*) states [45,46].
Interestingly, Zhang and co workers [46–49] have demonstrated
that the 1(n-
*) state of these systems can be replaced with a
fluorescent singlet charge transfer (1CT) or
* excited states by
raising the n- * energy levels of the parent molecule through
1
rapid intersystem crossing to the dark 3(n-
p
p
p–p
p
C C double bond in the phthalide derivatives as reported for
similar cases in the literature [22–27].
¼
complexation with Lewis acids. To investigate the effect of Lewis
acids, we have monitored the absorption and fluorescence
properties of the phthalide derivatives 1–3 with the addition of
BCl3. For example, the thiophene derivative, 3 initially exhibited
absorption band centered at 360 nm and weak emission around
440 nm in dichloromethane (DCM). However, with the addition of
BCl3 (16 mM) in dichloromethane resulted in the formation of new
absorption bands centered at 284 and 430 nm with isosbestic
points at 305 and 390 nm (Fig. 7A). On the other hand, the emission
spectrum exhibited significant enhancement with a ca. 80 nm
bathochromic shift having maximum intensity at ca. 520 nm. By
the addition of BCl3 (16 mM), we observed ca.15-fold enhancement
in the fluorescence intensity having the quantum yield of ca. 0.11
for the complex (Fig. 7B). Similar observations were made for the
derivatives 1 and 2 with fluorescence quantum yield values of
0.21 ꢁ0.01 and 0.18 ꢁ 0.01, respectively. This formation of a new
broad signal in the absorption and enhancement in the fluores-
cence intensity can be attributed to the formation of a newly
3.3. Morphological analysis of the aggregates
To understand the nature of the aggregates formed by these
systems, we have carried out their morphological analysis
through various microscopic techniques such as fluorescence
microscopy, scanning electron microscopy (SEM), and transmis-
sion electron microscopy (TEM). The fluorescence microscopic
images of the derivative 3 showed the formation of rod-like
structures in acetonitrile (Fig. 6A) and extended rods in 90%
water-acetonitrile mixture (Fig. 6D). This was further confirmed
by SEM and TEM analyses under similar conditions, and observed
small rod-like structures with an average diameter of ca. 30–
50 nm in acetonitrile (Fig. 6B and C) and extended rods in 90%
water-acetonitrile mixture with an average size of ca. 100 nm
(Fig. 6E and F).
Fig. 6. A) Fluorescence, B) scanning electron (SEM) and C) transmission electron (TEM) microscopic images of the phthalide derivative 3 (100
are the corresponding images of the derivative 3 in 90% water-acetonitrile mixture.
mM) in acetonitrile and D)–F)

M. Shanmugasundaram et al. / Journal of Photochemistry and Photobiology A: Chemistry 330 (2016) 156–162
161
generated emissive charge transfer state as reported in the
literature [45–49] (Table S2, Supporting information).
B)
A)
4.0x10-2
3.0x10-2
2.0x10-2
4
3
2
1
0
3.5. Solid state properties
The aggregation induced emission (AIE) properties exhibited by
the phthalide derivatives led us to study their photophysical
properties in the solid and thin film states. In this regard, we have
monitored the absorption and fluorescence aspects of the
compounds in their powder as well as in thin film state. For
example, the compound 3 showed bathochromic shifted absorp-
tion at around 402 nm, when compared to its absorption (355 nm)
in the solution state. On the other hand, it showed significantly
enhanced fluorescence intensity centered at 490 nm with a
quantum yield value of ca. 0.54 ꢁ 0.01 (Fig. 8) in the powder form.
Similar observations were made with the other derivatives,
wherein we obtained quantum yield values of ca 0.03 ꢁ 0.01 and
0.53 ꢁ 0.01, for 1 and 2, respectively. Notably, these derivatives
showed visible yellowish green luminescence under UV illumina-
tion (Fig. S5 and Table S3, Supporting information). In the thin film
state, for example, the compound 3 showed absorption at around
403 nm. On the other hand, it showed enhanced fluorescence
intensity centered at around 480 nm with a quantum yield value of
ca. 0.60 ꢁ 0.01 (Fig. S6 and Table S4, Supporting information).
Similar observations were made with the other derivatives,
wherein we obtained quantum yield values of ca 0.10 ꢁ 0.01 and
0.58 ꢁ 0.01, for 1 and 2, respectively. The enhanced quantum yields
in the powder and thin film state can be explained on the basis of
rigidity provided by the solid environment, which prevents the
non-radiative relaxations like isomerization and free degree of
intramolecular rotation. We have further evaluated the thermal
stability of these derivatives using thermo gravimetric analysis by
heating samples from 30 to 500 ꢂC (Fig. S7, Supporting informa-
tion). All the three derivatives 1–3, exhibited good thermal
stability with insignificant weight loss up to 300 ꢂC, with thermal
decomposition temperature (Td) (5% mass loss) of 285, 285 and
390 ꢂC, respectively, which makes them suitable for various
optoelectronic applications.
1.0x10-2
0.0
400 480 560 640 720
Wavelength, nm
0
5
10 15 20 25
Voltage, V
Fig. 9. A) Electroluminescence spectrum of an light emitting diode (LED) device
fabricated through spin coating of the solution of the phthalide derivative 3 over an
ITO surface. B) Current density–voltage plot of the LED device. The inset shows the
visual luminescence of the device.
450–700 nm with a maximum at 524 nm, with a turn on voltage of
9 V (Fig. 9). The EL spectrum showed a red-shift of ca. 35 nm, when
compared to its solid-state photoluminescence, which can be
attributed due to the differences in its molecular packing in the
spin coated film and powder. These initial results suggest that
these molecules, which can be easily synthesized and show
favourable photo physical and electroluminescence properties and
can have potential use in optoelectronic applications.
4. Conclusion
In summary, we have synthesized three simple phthalide
derivatives 1–3 by employing microwave assisted synthesis and
have investigated their photophysical and electroluminescence
properties under different conditions. The synthesized molecules
exhibited absorption and fluorescence emission in the range 300–
400 nm and 400–600 nm, respectively. Interestingly, their fluores-
cence emission could be tuned from almost non-emissive state in
solution (FF = 0.003) to a strongly emissive state employing
aggregation and Lewis acid coordination and achieved the
maximum fluorescence quantum yield in the thin film state
(
FF = 0.60 ꢁ 0.01). To investigate their utility as light emitting
applications, we have fabricated an OLED using the representative
derivative 3 as an example, which exhibited efficient green
luminescence material with a turn on voltage of 9 V. Our results
demonstrate that these simple, thermally stabile and solution
processable molecular organic systems exhibit favourable photo-
physical and electroluminescence properties and hence can have
potential applications as optoelectronic materials.
3.6. Organic light emitting diode (OLED) measurements
As the phthalides under investigation showed strong emission
in solid state, it was of our interest to demonstrate their ability in
real OLED applications. In this regard, we have fabricated a
prototype using the compound 3 as the emissive layer. Indium tin
oxide (ITO) and aluminium were used as the anode and cathode,
respectively. The electroluminescence (EL) spectra of the fabricat-
ed light emitting diodes (LED) were recorded by applying voltages
in the range of 5–25 V. We observed an increase in the emission
intensity with the applied voltage. The electroluminescent
spectrum (EL) of the derivative 3 showed emission in the range
Acknowledgments
We acknowledge the financial support from CSIR (CSC 0134 &
NWP 55), University Grants Commission (Research Fellowship for
MS) and Department of Science and Technology (Ramanujan
Fellowship Grant RJN-19/2012 for JJ), Government of India.
Appendix A. Supplementary data
A)
B)
1
1.2
0.9
0.6
0.3
0.0
1
1.0
0.8
0.6
0.4
0.2
0.0
2
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.
jphotochem.2016.08.008.
2
3
3
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