Engineering fused coumarin dyes: A molecular level understanding of aggregation quenching and tuning electroluminescence via alkyl chain substitution

Article abstract of DOI:10.1039/c4tc00807c

Simple molecular structures capable of emitting over the entire visible range are still a challenge. Planar molecular structures have the drawback of fluorescence quenching in the solid state thus limiting their application fields. Combining long range excimer/exciplex emissions with a compound emission have been used to get white light. In this work, a series of new coumarin derivatives having a planar structure have been synthesized and characterized. The effects of systematic variation in alkyl chain functionalization providing morphological variations that permit interesting solid state emitting properties have been discussed simultaneously with electrochemical behavior and OLED (organic light emitting diode) device applications. Carbon chains containing 0-16 carbon atoms have been studied in order to conclude the results that systematic changes in alkyl group substitution can be utilized as a tool to tune the emitting color of these planar coumarins. Alkyl chains were introduced by O-acylation and O-benzoylation reaction on the hydroxyl group of parent coumarin 5. Thus the present strategy is also helpful in establishing a template to control the unproductive interchromophore electronic couplings. Solid state fluorescence properties support the crystal studies. Theoretical studies are also in agreement with experimental data. Electroluminescence of Device 2 with a turn on voltage (V_on) around 5-6 V having s-CBP doped with 1% of 8 having alkyl substitution of 2-carbons is found to exhibit white emission with CIE co-ordinates of (0.29, 0.34) which is close to white emission while the alkyl substitution of 14-carbons (compound 17) in Device 7 (V_on = 7 V) exhibited green emission. Thus a strategy helpful to tune the electroluminescence has been discussed. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4tc00807c

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Materials Chemistry C
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Engineering fused coumarin dyes: a molecular level
understanding of aggregation quenching and
tuning electroluminescence via alkyl chain
substitution†
Cite this: J. Mater. Chem. C, 2014, 2,
6637
Sunil Kumar,a Punita Singh,b Ritu Srivastava,b Rik Rani Koner,a Avijit Pramanik,c
Jomon Mathew, Sougata Sinha, Madhu Rawat, R. S. Anand and Subrata Ghosh
d
a
e
e
a
*
Simple molecular structures capable of emitting over the entire visible range are still a challenge. Planar
molecular structures have the drawback of fluorescence quenching in the solid state thus limiting their
application fields. Combining long range excimer/exciplex emissions with a compound emission have
been used to get white light. In this work, a series of new coumarin derivatives having a planar structure
have been synthesized and characterized. The effects of systematic variation in alkyl chain
functionalization providing morphological variations that permit interesting solid state emitting properties
have been discussed simultaneously with electrochemical behavior and OLED (organic light emitting
diode) device applications. Carbon chains containing 0–16 carbon atoms have been studied in order to
conclude the results that systematic changes in alkyl group substitution can be utilized as a tool to tune
the emitting color of these planar coumarins. Alkyl chains were introduced by O-acylation and
O-benzoylation reaction on the hydroxyl group of parent coumarin 5. Thus the present strategy is also
helpful in establishing a template to control the unproductive interchromophore electronic couplings.
Solid state fluorescence properties support the crystal studies. Theoretical studies are also in agreement
with experimental data. Electroluminescence of Device 2 with a turn on voltage (Von) around 5–6 V
having s-CBP doped with 1% of 8 having alkyl substitution of 2-carbons is found to exhibit white
emission with CIE co-ordinates of (0.29, 0.34) which is close to white emission while the alkyl
substitution of 14-carbons (compound 17) in Device 7 (Von ¼ 7 V) exhibited green emission. Thus a
strategy helpful to tune the electroluminescence has been discussed.
Received 21st April 2014
Accepted 9th June 2014
DOI: 10.1039/c4tc00807c
www.rsc.org/MaterialsC
are lacking in the literature and only a few reports are avail-
able.3–6 The most commonly exploited approaches for white
light emissions7 include multilayered device fabrication by
Introduction
Obtaining white light emission from organic molecules has
been an area of interest for researchers because of their
particular potential in the solid-state lighting industry as well as
in the back light application of liquid-crystal displays (LCDs)1
and full color OLED2 displays. Organic compounds having
emission covering the whole visible region (400 nm to 700 nm)
consecutive evaporation of different emitting compounds,7–11
spin coating of a blend of different soluble emitters,12 utilizing
excimer or exciplex emission,13 use of organic–metal complexes
in a hybrid device structure5,14 and phosphorescent emitters
with a suitable host.15 The device structure also plays an
important role for white emission. Phosphorescent emitters
have the drawback of their high cost and complicated device
structures. So to be practically useful in WOLEDs, emission
from a single compound is always desired for simple device
fabrication. White organic light emitting diodes (WOLEDs)
having a single emitting component have advantages such as
reproducibility, better device stability and simple device fabri-
cation process. Kim16 and colleagues have recently illustrated
the production of white light utilizing excited-state intra-
molecular proton transfer (ESIPT). In this frame, the area of
synthesizing new uorescent organic moieties showing emis-
sions over a wide range in their solid state is still interesting and
aSchool of Basic Sciences, Indian Institute of Technology Mandi, Mandi-175001, H.P.,
India. E-mail: subrata@iitmandi.ac.in; Tel: +91-1905-237926
bPhysics of Energy Harvesting Division, National Physical Laboratory, New Delhi, India
cDepartment of Chemistry and Biochemistry, Jackson State University, Jackson,
Mississippi 39217, USA
dSchulich Faculty of Chemistry, the Lise Meitner Minerva Center for Computational
Quantum Chemistry, Technion-Israel Institute of Technology, Haifa, Israel
eElectrical Engineering Department, Indian Institute of Technology Kanpur, Kanpur-
208016, India
† Electronic supplementary information (ESI) available. CCDC 941840 and
941841. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c4tc00807c
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challenging for researchers. Coumarins have been compounds spectra were recorded using a Perkin Elmer Spectrum 2 spec-
of interest for decades because of their wide range of applica- trophotometer. Mass analysis was carried out using a micrO-
tions in electronics and photonics due to their good stabilities TOF-QII mass spectrometer. Thermal behaviour studies were
and ease of synthesis and thereby nding applications as done using a NETZCH STA449 F1 instrument. Melting points
sensors17 and emitters for solid state dye lasers and OLEDs.18–20 were recorded using a Stuart SMP30 instrument. The electro-
But, due to aggregation and intermolecular interactions (p–p chemical cyclic voltammetry (CV) was performed using a
interactions) in planar coumarin structures, they easily suffer EPSILON electrochemical workstation in 0.1 M tetrabuty-
from self-quenching, thus limiting their applicability in lammonium hexauorophosphate (Bu4NPF6) with a scan speed
OLEDs.21 In this regard, the discovery of a host–guest doped at 0.1 Vs–1. A Pt wire and Ag/AgCl were used as the counter and
emitter system has proved a boon to the emissive display reference electrodes, respectively, for the CV measurements.
technology.22 Simultaneously, work has been done to reduce The EL spectrum has been measured with a high resolution
aggregation23/dye–dye interactions21 of planar luminophores. As spectrometer (Ocean Optics HR-2000CG UV-NIR). The current–
controlled p–p interactions hold key to give origin to long range voltage–luminescence (I–V–L) characteristics have been
emissions (excimer/exciplex formation) and hence has been measured with a luminance meter (LMT 1-1009) interfaced with
used as a boon for light generation especially white electrolu- a Keithley 2400 programmable voltage–current digital source
minescence (EL).6,24 Therefore the design of new uorescent meter.
materials for OLEDs as well as the development of a simple
device structure has received much attention over the past
decade.
Synthesis and characterization
The synthesis and characterization of synthesized coumarin
It is known that simple alkyl substitutions can change the
morphology of organic compounds by reducing the inter-
luminophore alignment. Researchers have used alkyl substitu-
ents as a tool to reduce intermolecular interactions25 and
modify transport properties.26 This strategy has also been used
to tune the optical properties of luminophores.27–30 But a
systematic study of the alkyl substitution effect on the
morphology and photoluminescence properties which make
the alkyl substitution effect useful in OLEDs remained mostly
unexploited. In this context the search for new electrolumi-
nescent materials for white emission lead us to synthesize
planar coumarin derivatives with extended p-conjugation and
we wish to report a simple three-step synthesis procedure of
novel coumarin dopants, PL performance with alkyl chain
substitution and preliminary results on the potential utility of
these compounds in a doped OLED device structure. To the best
of our knowledge, this is the rst attempt to explain the alkyl
chain length effect on PL behavior of luminophores and it was
observed that small alkyl chain substitution (compounds 8
and 11) produced white EL while longer chain substitution
showed green EL for these coumarin dopants (compounds 16
and 17). The physical properties such as UV-vis absorption,
uorescence (both in solution and solid states), crystal struc-
ture, electrochemical behavior and thermal properties were also
studied.
derivatives can be found in the ESI.†
Device fabrication
Indium-tin oxide (ITO) coated glass substrates with a sheet
resistance of 20 U ,ꢀ1 were used as anodes which were
patterned and cleaned by a conventional solution cleaning
process. The substrates were further exposed to oxygen plasma
to enhance the work function. All the layers were deposited by
vacuum deposition at a base pressure of 1 ꢁ 10ꢀ6 Torr with a
deposition rate of 0.1 A sꢀ1. The doped lms were deposited by a
˚
co-evaporation process. The thicknesses of all the lms were
measured in situ by a quartz crystal thickness monitor. The size
of each pixel was 4 ꢁ 4 mm2. All further measurements were
performed at room temperature and under an ambient atmo-
sphere, without any encapsulation.
X-ray crystallography
Compound 5 crystallized as light brown crystals utilizing a slow
crystallization method from the DMF–water mixture while
compound 8 gave needle like yellow crystals in the THF solvent,
which were then used for XRD analysis. X-ray crystallography
data was collected using
a Bruker SMART APEX CCD
diffractometer.†
Results and discussion
Synthesis and characterization
Experimental
General information
Compounds 5, 6 and 7 were synthesized, starting from aromatic
All the starting reactants were purchased from available aldehydes and ethyl cyanoacetate, utilizing a previously repor-
commercial sources (Sigma-Aldrich, Merck and Sd-ne Chem- ted procedure31 and the same was followed to obtain compound
icals) and were used as received. Spectroscopic solvents were 4 (Scheme 1). All products were obtained as precipitates and
utilized for photophysical studies. 1H and 13C NMR spectra were were obtained in pure form by simple ethanol washing avoiding
recorded using a Jeol ECX 500 MHz, Jeol ECS 400 MHz, Bruker a tedious and time consuming column purication technique.
Avance 600 MHz, Bruker AV 400 MHz spectrometers in CDCl3 Further derivatization of 5, 6 and 7 was carried out utilizing
and DMSO-d6. UV-vis and uorescence spectra were recorded triethyl amine and required acid chlorides to obtain
using a Shimadzu UV-2450 and Cary Eclipse uorescence compounds 8–21 and ethylchloroformate for 22, respectively
spectrophotometer (Agilent Technologies), respectively. FT-IR (Scheme 1). All derivatives were obtained as a single spot on TLC
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Scheme 1 Schematic representation of a three-step synthesis procedure acquired to obtain coumarin derivatives.
and further puried by water washing. The synthesized THF solvent at room temperature (Fig. 1). Absorption proles
compounds were characterized by 1H/13C-NMR, FT-IR and mass for compounds 9, 10, 20 and 21 are shown in the ESI (Fig. S1†).
spectroscopy. Structural identities of parent compound 5 and All compounds showed absorbance in the region of 270–460 nm
one of daughter compounds 8 have been investigated and and an absorption prole were almost similar due to the
authenticated by making use of single crystal X-ray common coumarin core. Peaks in the region of 280–340 nm
crystallography.
were characteristic of a naphthalene ring and assigned to single
UV-vis absorption and uorescence spectra of all newly electron p–p* transitions in the aromatic ring.32 Long wave-
developed coumarins (4–22) (Scheme 1) were recorded in the length absorption was expected due to extended p-conjugation
Fig. 1 (a) Normalized absorbance and PL spectra of 4, 5, 6 and 7. (b) UV spectra of compounds 8, 11–19 and 22. (c) PL spectra of compounds 8,
11–19 and 22. (d) Comparison of PL spectra of compounds 5–10, 19–21. (All spectra were recorded in 5 mM THF solution).
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of the molecule. Compound 4 showed a sharp peak at 284 nm, continuum of states that disturb the vibronic pattern, leading to
multiple peaks in the region of 320–340 nm, and a broad the observed broadening and red shied emissions33 and (c) the
absorbance at around 394 nm. The introduction of hydroxy change in the dielectric constant of media on going from
(–OH) functionality to 4 resulting in coumarin 5 shied the solution to quartz can also contribute to the red shi, both in PL
UV-vis spectrum bathochromically as expected by 4–14 nm and absorption proles. Of all these compounds, 8 and 11
(Fig. 1) which could be assigned to the formation of a push–pull exhibited interesting vibronically dened PL behavior in the
system (D–p–A) due to the electron donating nature of a solid state (thin lm). A red shi of 35 nm with peak emission at
hydroxyl group and the electron withdrawing nature of a 496 nm and a hump around 532 nm tailing up to 650 nm was
cyanide group. We also studied the effect of electron donor observed for 8 and similar behavior was observed for compound
(methoxy) and electron acceptor (chloro) substitution on 11. The remaining compounds (12–18) gave a broad structure
absorption and uorescence properties of 5. Almost a negligible with single emission proles (Fig. 2). But when PL was recorded
shi in the absorption spectrum was observed for 6, but a red for powdered compound 8, interestingly the hump around
shi of 3 nm was observed for 7 as compared to 5. A reasonable 532 nm became pronounced (Fig. 2). These dramatic changes of
explanation for this shi as compared to 4 is that a combination PL in the solid state encouraged us to investigate the origin of a
of a donor (hydroxy) and two acceptors (chloro and cyano) greenish yellow band. Additional broad emissions may be
contributes to a good intramolecular charge transfer (ICT) caused by the formation of an excimer, phosphorescence or by
process in 7. To conrm the existence of a D–p–A system in aggregation. Careful perusal of absorption and emission
these coumarin derivatives, we converted a donor group (–OH) proles of 8 in both solution and solid states helped us to
into an acceptor by ester (8–21)/carbamate (22) protection. As conclude that the emission at 532 nm come into existence only
expected a blue shi in the absorption prole was observed in in the excited state since no new absorption for this emission
8–22 because of the reduction of electron density on the was detected in the thin lm. So we can cast away the idea of
aromatic ring. These results further demonstrated that UV-vis formation of the aggregate state between solid subunits as the
spectra of the compound 4 depend on the substituents, for source of the additional peak in the emission prole. Moreover,
example, with the introduction of the electron donor (5–7) the the experimental results validate that an efficient excimer
spectra were found to be red shied and, on the other hand, formation process may occur in the molecular aggregate
converting a donor into an acceptor (8–22) caused a blue shi in leading to an additional emission band in luminescence and
absorption spectra.
here this process existed efficiently in powder form.34
Similarly, upon excitation 5 mM THF solution of compounds
Compound 11 showed a similar type of behavior in powder
showed blue emission with an emission wavelength in the form (Fig. S2†), but as the chain length of ester protection
region of 461–477 nm. Parent compound 4 showed a maximum increased from 4-carbon to 16-carbon atoms (compounds
emission at 464 nm with a full width at half maximum (fwhm) 12–18), it seemed that the excimer formation process was slaked
of 73 nm which was red shied by 7 nm, 4 nm and 13 nm with a and structure less emissions were observed as these chains
fwhm of 67 nm, 64 nm and 70 nm for the compounds 5, 6 and 7 might not let molecules come close enough to interact with
respectively. The observed red shi was expected due to the each other which disfavored the formation of new emissions.
introduction of a hydroxyl group. On conversion of the hydroxyl Moreover long chain substitution plays a different role in
into an ester functionality, the compounds 8–21 showed blue molecular packing in aggregate form because they are known to
shied uorescence as compared to parent compounds 5–7. increase the intermolecular distance between neighboring
This behavior is similar to the UV-vis data for these compounds. molecules which can reduce ACQ (aggregation caused
Compound 22 behaved similarly as ester protected compounds quenching). But they can also induce disorder in molecular
(8–21). Quantum yields (F) for compounds could be found in arrangement and reduce the emission process. Therefore the
the ESI (Table S2†). Compound 8, 11–18 showed a quantum optimized size of an alkyl side chain for better emission in the
yield in the range of 0.03–0.08 while compound 5 showed a solid state is difficult to get theoretically. In our case we got
higher quantum yield of 0.11. In order to study the photo- maximum uorescence intensity for compound 17 having alkyl
physical properties of these coumarins more precisely, substitution of 14-carbons while a decrease in uorescence
compounds 8–18 were selected for thin lm studies. Photo- intensity was observed for alkyl substitution of 16-carbons
luminescence (Fig. 2) experiments were carried out for their (Fig. S2†). It is known that introduction of donor and/or
thin lms prepared on a quartz plate using a spin coater. The PL acceptor groups into a uorogen not only give an ICT (intra-
of compound 8 and 11–18 showed good uorescence in the molecular charge transfer) property but typically also lead to a
solid state compared to compound 5. The PL spectra for 8 in the strong solvatochromic effect35 and thus the behavior of uo-
thin lm showed red shis from their PL in solution. Similar rescence and absorbance as a function of solvent polarity was
results were obtained for compounds 11–18. Such a kind of red studied. As anticipated, uorescence experiment data indicated
shi in PL emission in the solid state is common and has been that these coumarin derivatives exhibit little solvatochromic
observed by others for organic small molecules as well as behaviour. As shown in Fig. 3, changing the solvent system from
polymeric materials33 possibly due to (a) the exciton hopping DCM to DMSO was accompanied by a small bathochromic shi
process in the solid state, (b) the intermolecular interaction in of 14 nm in the emission wavelength (lem) of 8. Compound 4
the solid state i.e. adjacent molecules may come close enough to also showed a positive solvatochromism employing a shi of
have charge transfer in the excited state which manifests as a 18 nm in lem from 464 nm (DCM) to 482 nm (DMSO).
6640 | J. Mater. Chem. C, 2014, 2, 6637–6647
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Fig. 2 (a) Comparison of shifts in the emission profile for compound 8 in dichloromethane solution, thin film and powder form and (b)
comparison of thin film emission profiles for compounds 8 and 11–18.
Compounds 5, 6 (Fig. 3) and 7 (Fig. S3†) exhibited uorescence suggested that a dipole moment in the ground state was not
emission wavelength shis of 36 nm, 29 nm and 39 nm, much affected by solvent polarity,36 while the red shi in uo-
respectively, indicating 7 to be an efficient solvatochromic rescence37 emission is more likely due to the absorption
compound. Compounds 9–22 also showed positive sol- occurring to a locally excited state which then interconverts into
vatochromic behavior as emission wavelengths were found to a charge-transfer state prior to emission.
be red shied with increase in solvent polarity (Fig. S3–S9†). But
Compounds 5, 8, 11 and 12 were selected to study typical AEE
an almost negligible effect on the absorption prole (Fig. S10 characteristic where they ensued the formation of aggregates.38
and S11†) was observed for all these compounds. This As the water content was increased from 0% to 20% in THF
Fig. 3 Solvatochromic behavior of compounds. The effect of solvent polarity on the emission wavelength of compounds (a) 4, (b) 5, (c) 6 and (d) 8.
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solution of compound 5, uorescence intensity started to layer, naphthalene aromatic rings are in one plane and a
decrease and kept on decreasing for further increment in water hydroxyl (–OH) group is chelated with the –C]O oxygen of the
content up to 90% (Fig. 4) which concluded that 5 do not lactone moiety through intermolecular O–H/O (O1–H/O3,
˚
possess AEE characteristics. In contrast, for compound 8 it was 2.821 A) interactions, which could open non-radiative pathways
observed that uorescence intensity started to increase from the for deactivation processes. In addition, the other supramolec-
˚
beginning with the addition of 20% water only and increased ular weak interactions e.g. C–H/O, (C18–H/O1, 3.421 A, C15–
˚
˚
until the water content reached 80% with a continuous red shi H/O1, 3.328 A) and C–H/N (C16–H/N1, 3.508 A, C19–H/N1,
˚
˚
in emission intensity, which can be ascribed to the continuous 3.596 A, C8–H/N1, 3.337 A) also play an essential role in
increase in polarity of solution and the change in the morphology building the zigzag network. Layers A–B and C–D layers are
of the aggregates formed or crystallization enhanced emission closely packed with partially overlapping benzo[h]chromen-2-
might be occurring.36,39,40 The concentration of the THF–water one polyaromatic cores through p–p stacking (paverage–paverage
,
˚
system was maintained at 5 mM always. The uorescence 3.24 A) interactions to form the slipped parallel structure
quantum yield exhibited an enhancement from 0.06 to 0.2. (Fig. 5e). Layers B and C are oriented in such a way that their
However, when the content of water was further increased to aromatic rings are not interacting with each other. Therefore,
90%, the uorescence decreased rapidly, but did not quench we can conclude that intermolecular H-bonding, strong p–p
fully. The reason could be the formation of agglomerates of stacking contact, and other supramolecular interactions play
solute molecules in a random way to form less emissive amor- key roles in quenching the PL signal of 5 in the solid state.42 On
phous particles or the p–p* stacking in nanoaggregates which the other hand, the crystal structure of 8 belonging to the
can also lead to the lowering of uorescence.41 Similar results orthorhombic Pca21 space group with an interplanar angle of
were observed for 11 and 12 (Fig. 4). Thus we expect that intro- 61.14ꢂ showed a zigzag pattern of the molecules in the crystal
duction of alkyl functionality into the coumarin structure (Fig. 5d). It can be noted that substituting the hydroxyl group
contribute signicantly to the enhanced solid state emission and with an acetyl group removed intermolecular H-bonding and
˚
˚
make the coumarins good candidates for doped OLED devices.
increased the interlayer distance from 3.24 A to 3.70 A.
Furthermore, the p–p interaction reduced between adjacent
layers and thus simultaneously increasing the uorescence
intensity.36 In addition, weak interactions e.g. C–H/O, (C3–H/
Molecular packing
The crystal structure analysis provides the most direct evidence
to understand the aggregation enhanced emission (AEE)
mechanism in the solid state. We have successfully obtained
two single crystals (5 and 8, Fig. 5) from a DMF–water mixture
and a THF solvent, respectively, that could be promising
evidence towards understanding the mechanisms in the solid
state. The two crystals have common contorted geometries and
adopt a twist-like conformation of the 2H-benzo[h] chromen-2-
one cores and phenyl moieties with torsion angles of 54.74ꢂ and
51.55ꢂ for 5 and angles of 58.15ꢂ and 55.45ꢂ for 8. The crystal
structure of 5 belongs to an orthorhombic system containing
˚
˚
˚
O2, 3.445 A, C12–H/O3, 3.2425 A), C–H/N, C2–H/N1, 3.381 A
˚
and the cofacial p–p stacking (paverage–paverage, 3.705 A) (Fig. 5f)
between adjacent layers were observed, but these interactions
may favor the formation of an excimer which can give rise to
new emissions in the PL spectra.
This supposition is supported by the origin of a new long
range emission in the PL. As supported by these observations,
one can expect similar behavior in the EL of devices.
Thermal properties and HOMO–LUMO levels
the P21212 space group with an interplanar angle between two Utilizing the results of solid state uorescence, structurally
planes of 55.45ꢂ (Fig. S13†). As shown in the Fig. 5c, in each related compounds 5, 8, 11, 12, 16 and 17 were selected for
Fig. 4 (a) Comparison of the PL intensity change on the increasing water content in the THF–water system for 5 mM solution of (a) 5 and 8 and (b)
11 and 12.
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Fig. 5 Ortep diagrams of molecules (a) 5 and (b) 8. (c) Side view of the crystal lattice of 5 showing a hydrogen bond and short range contacts. (d)
Side view of the crystal lattice of 8 showing a zigzag pattern and an interplanar distance. (e) Top view showing an overlap of aromatic rings
between layer A and layer B in 5. (f) Top view showing an interaction between adjacent layers of 8.
further studies including device fabrication. Thermal behavior
of these organic compounds was investigated by DSC and TGA
(Fig. 6a). Parent compound 5 was found to be stable upto 300
ꢂC. Compound 8 exhibited a decomposition temperature (Td ¼
5% weight loss) around 304 ꢂC which is quite high, whereas 16
and 17 exhibited Td around 261 ꢂC and 268 ꢂC, respectively.
Melting points were observed in the rst heating scan of all the
compounds without crystallization, which is expected to yield
better stabilities of these uorophores as electroluminescent
materials.43 Solution electrochemistry studies were carried out
to ascertain the redox behavior of these coumarin derivatives,
and the results obtained were employed to evaluate the LUMO
and the HOMO levels. All measurements were performed in a
three electrode setup using Ag/AgCl as a reference electrode and
a Pt disc as a working electrode in acetonitrile for 8 and
dichloromethane for 16 and 17, respectively. Ferrocene was
used as the external standard. The LUMO for compound 8 was
assessed at ꢀ3.52 eV from the onset reduction (Ered), using the
equation: ELUMO ¼ ꢀ(Ered + 4.8) eV, where Ered is the onset
reduction potential relative to the (Fc/Fc+) couple (Fig. 6b).44
From the lowest energy absorption edge of the UV-vis absorp-
tion spectra in solution, the band gap (Eg) was calculated to be
2.88 eV, so EHOMO was assessed at ꢀ6.40 eV. Similarly, onset
reduction potentials for long chain ester derivatives 16 and 17
in their CV voltammograms were at ꢀ0.98 V and ꢀ1.00 V,
respectively. LUMO and HOMO energy levels for 16 and 17 were
estimated to be lying at ꢀ6.22 eV, ꢀ3.34 eV and ꢀ6.20 eV, and
ꢀ3.32 eV, respectively. Therefore long alkyl chains do not have
noticeable inuence on HOMO–LUMO energy levels unlike the
effect on PL.
Computational studies
Density functional theory calculations have been carried out by
employing the B3LYP45 hybrid functional and 6-311G(d,p) basis
set to understand the spectral properties and for the analysis of
molecular orbitals. Gaussian 09 suite of programs is used for all
calculations.46 A representative set of compounds comprising 5,
8, 11, 12 and 19 are used for the computational analysis. Opti-
mized structures of compounds 5 and 8 along with their cor-
responding HOMO and LUMO are depicted in Fig. 7 (structure
and frontier molecular orbitals of other compounds are given in
Fig. S22†) and Table 1 summarizes the energy values of the
HOMO, LUMO and band gap of all calculated compounds. The
HOMO in both 5 and 8 are formed by the p-orbitals of the
central ring system, lone pairs on oxygen atoms and the
p-orbitals of carbonyl and cyanide groups and LUMO of both
compounds are the corresponding p*-orbitals. The phenyl
group is nearly perpendicular to the plane of the naphthyl
moiety in both compounds and is not contributing to the
HOMO. The HOMO–LUMO energy gap in 5 is 3.59 eV and the
replacement of the –OH group by an ester group in 8 lowers the
HOMO energy and increases the HOMO–LUMO energy gap to
3.66 eV which explains the observed blue shi in the absorption
wavelength. The computed LUMO energy and the HOMO–
LUMO energy gap in 8 is slightly different from the experi-
mental values which is expected since the solvent effect and
other possible interactions of 8 in the solvent is not included in
the computation. The theoretical calculations that include the
solvent effect [both Polarizable Continuum Model (PCM) and
the conductor like polarizable continuum model (CPCM)] do
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Fig. 7 Theoretically calculated optimized geometries and frontier
molecular orbitals. (a) Optimized structure and frontier molecular
orbitals of 5. (b) Optimized structure and frontier molecular
orbitals of 8.
Table 1 Summary of molecular orbital calculations at the B3LYP/6-
311G (d,p) level and on cyclic voltammetry
Compound
EHOMO [eV]
ELUMO [eV]
Eg [eV]
Fig. 6 (a) TGA graphs for selected compounds and (b) CV graphs for
compounds 8, 16 and 17.
5
8
ꢀ6.30a
ꢀ2.71a
3.59a
ꢀ6.40b/ꢀ6.49a
ꢀ6.49a
ꢀ3.52b/ꢀ2.83a
ꢀ2.82a
2.88b/3.66a
3.67a
11
12
16
17
19
ꢀ6.47a
ꢀ2.81a
3.66a
not make a signicant change to the LUMO energy value (not
greater than 0.1 eV). Therefore, the orbital energy values given
in the text are calculated from gas phase calculations. The band
gap of 8, 11, 12 and 19 are nearly the same showing that the
length of the alkyl chain and even the replacement of the alkyl
group by a phenyl group do not signicantly change the
HOMO–LUMO gap which can be rationalized from the fact that
the substituents on ester carbonyl do not contribute to HOMO
and LUMO in any of these systems and the inductive effect by
the substituent on the naphthyl ring is almost negligible.
UV-vis absorption properties of all optimized compounds
have been calculated at the TD-PBE0/6-311G(d,p) level of DFT.47
The solvent effect (THF) is treated with a conductor like polar-
izable continuum model (CPCM) implemented in Gaussian
09.48 The absorbance of all compounds are in the region of 240–
390 nm which are in good agreement with the experimentally
ꢀ6.22b
ꢀ3.34b
2.88c
ꢀ6.20b
ꢀ3.32b
2.88c
ꢀ6.47a
ꢀ2.82a
3.65a
a
b
c
B3LYP/6-311G(d,p) level. Using cyclic voltammetry. Band gap
calculated from edge of the absorption prole.
(ESI†). The HOMO–LUMO energy gap (PBE0/6-311G(d,p)-
CPCM) in 5 is 3.84 eV and the replacement of the –OH group by
an ester group in 8 lowers the HOMO energy and increases the
HOMO–LUMO energy gap to 3.93 eV which explains the
observed blue shi in the absorption wavelength for the p–p*
transition.
Electroluminescent devices
recorded absorption prole (TD-DFT/CPCM spectra all Based upon the PL behavior of coumarin derivatives 8, 11–18 in
compounds along with the molecular orbitals involved in elec- the solid state, coumarins 8, 11, 12, 16 and 17 were selected to
tronic transition are given in the ESI, Fig. S23†). Peaks in the investigate the change in their electroluminescence behavior as
region of 380–390 nm are characterized by the p–p* (HOMO / these selected compounds differ only in the chain length of
LUMO) transition in all computed compounds while the ester functionality. Aer fabrication of the devices, the EL
absorption in 240–290 nm are resulted from the electronic spectrum (Fig. 8a) and current–voltage–luminescence (J–V–L)
transitions from HOMO and HOMO ꢀ 1 to LUMO, LUMO + 1 characteristics (ESI, Fig. S14–S21†) were recorded and simulta-
and LUMO + 2, depending on the orbital energy and symmetry neously current and power efficiencies (ESI, Fig. S14–S21†) were
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also calculated for all devices. Since the PL spectrum of 8 generating anomalous EL. To verify the presence of exciplex
covered a wide range of the visible region, Device 1: ITO/a-NPD formation, PL spectra of the thin lm of pristine a-NPD and its
(30 nm)/EML (35 nm)/BCP (6 nm)/Alq3 (28 nm)/LiF (1 nm)/Al mixture with dye derivative 8 were recorded (Fig. 8c).
(150 nm) was fabricated utilizing 8 as an emissive layer (EML).
From the data it was deduced that pristine a-NPD and
a-NPD was used as the hole transport layer (HTL), BCP as the coumarin derivative 8 do not possess long range emissions in
hole blocking layer (HBL) and Alq3 served as the electron their thin lm forms. However, a mixture of both shows the
transport layer (ETL). Electroluminescence (EL) spectra presence of long range (low energy) emissions justifying the
obtained were completely different from that obtained for the exciplex formation in Device 1. This coumarin derivative 8 was
thin lm (Fig. 8a). A completely new emission peak centered at than doped in s-CBP (host matrix) as the guest emitting mate-
668 nm appeared in EL which did not appear in PL spectra rial. To have the better emission properties, the doping
either in solutions or in the solid state. It is to be noted that EL concentration of the guest emitting material 8 was optimized
characteristics are sometimes affected by chemical and physical utilizing different percentages of dyes (0.5%, 1%, 1.5%, 2%, and
interaction at organic/organic interfaces in OLEDs. An interac- 3%) in the host matrix. It was observed that 1% (by weight) of
tion of organic materials at the interface forms a charge-transfer the compound revealed excellent performance with enhanced
excited state complex, known as the exciplex.2,49 Hence, emission (Fig. 8e).51 So, 1% of 8 doped in s-CBP was used as the
this could be probably due to the interaction of the dye EML and Device 2 was fabricated with conguration: ITO/a-
(compound 8) with the a-NPD or the BCP. Since BCP does not NPD (30 nm)/s-CBP: 1% 8 (35 nm)/BCP (6 nm)/Alq3 (28 nm)/LiF
have its own emission, it simply restricts the passage of holes (1 nm)/Al (150 nm) (Fig. 8d). Here, EL and PL spectra were
towards the ETL reducing the interaction of the EML with the almost similar (Fig. 8b), though, EL was much broader than PL
ETL. Thus, it is assumed that there is an interaction between and as expected it covered an almost whole visible spectrum
EML and a-NPD50 resulting in exciplex formation thereby resulting in white emission. The absence of host emission in
Fig. 8 (a) EL spectra of Devices 1, 2, 4, 5, 6, and 7. (b) Comparison of PL spectra of compound 8 in the solid state with EL spectra of Device
2 having (s-CBP: 1% 8) as the EML. (c) Comparison of PL spectra of pure thin films of a-NPD and premixed a-NPD–8 mixture. (d) Schematic
representation of the device used for study. (e) Luminance characteristics for devices having 0.5%, 1%, 1.5%, 2%, and 3% of dye doped on host
s-CBP. (f) CIE diagram for devices with device pictures.
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electroluminescence conrmed the full energy transfer from compared to those where multiple layers have been utilized for
the host to the guest dye in the s-CBP: coumarin device. The white light generation. The work to nd optimized layers (HTL,
turn-on voltage for the device was observed to be 5–6 V. ETL and host) for OLED devices for these series of novel
According to the Commision Internationale de L'Eclairage coumarin dyes is in progress.
(CIE), Device 2 resulted in white-light emission with color co-
ordinates (0.29, 0.34). To verify that emission originates from an
organic compound only and no interference come from Alq3,
Acknowledgements
Device 3 [ITO/a-NPD (30 nm)/s-CBP: 1% 8 (35 nm)/PBD (28 nm)/ Financial support was received from the Department of Science
LiF (1 nm)/Al (150 nm)] was fabricated by replacing BCP and the and Technology, India (Grant no. SERB/F/2408/2012-13 and SR/
ETL layer (Alq3) of Device 2 with 2-(4-biphenyl)-5-(4-tert-butyl- FT/CS-57/2010(G)). We are thankful to the Director, IIT Mandi
phenyl)-1,3,4-oxadiazole (PBD). EL obtained (Fig. S25†) was for research facilities. The support of sophisticated instrument
similar to that obtained from Device 2 which conrmed that no facility of Advanced Materials Research Center (AMRC), IIT
emission originated from the Alq3 layer. Thus the conguration Mandi, is thankfully acknowledged. SK greatly acknowledges
of Device 2 was selected for further device fabrications. Devices University Grant Commission for providing doctoral fellowship.
4, 5, 6 and 7 were fabricated utilizing 1% of 11, 12, 16 and 17, We warmly thank Prof. J.A. Gareth Williams for his valuable
respectively, as EML doped in s-CBP and their EL spectra were suggestions which helped us in improving the manuscript
recorded (Fig. 8a). As expected, Device 4 (Von ¼ 10 V) showed contents. We acknowledge the help of Pankaj Gaur and Anshul
white emission with violet tinges (CIE: 0.25, 0.26). But Device Sharma of IIT Mandi for their help during the manuscript
5 (Von ¼ 7 V), Device 6 (Von ¼ 6 V) and Device 7 gave EL shied preparation. One of the author (PS) thanks Department of
towards bluish-green/greenish emissions with CIE: (0.27, 0.49), Science and Technology (DST) for nancial aid.
(0.28, 0.49) and (0.31, 0.48), respectively (Fig. 8f). Thus it was
observed that the alkyl chain length is playing an important role
in tuning the electroluminescence of non-optimized device
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