7-Alkoxy-appended coumarin derivatives: synthesis, photo-physical properties, aggregation behaviours and current-voltage (I-V) characteristic studies on thin films

Article abstract of DOI:10.1039/d1ra00762a

In this study, we designed and synthesised a series of coumarin derivatives appended with a long alkoxy chain on the seventh position of the coumarin-3-carboxylate/carboxylic acid core to make thin film materials. Synthesised compounds were characterized by their UV and fluorescence spectra in solutions as well as their films prepared by both LB and spin-coated methods. The surface morphology and electrical behaviour of thin films were judged by AFM, SEM andI-Vcharacteristic mapping respectively. Isotherm, UV-Vis absorption and fluorescence spectroscopic investigations revealed the formation of aggregates on thin films. The result of SEM and AFM images provides the information about the length and height of aggregates on the thin films of coumarin derivatives. FromI-Vcharacteristics, it was found that at room temperature, the spin-coated films of coumarin derivatives containing an ester functional group exhibited a threshold switching behaviour, whereas derivatives containing the carboxylic acid functional group showed both threshold and bipolar switching behaviours. We also noticed that theI-Vcharacteristic features of synthesized materials depended on the length of the alkyl chain of individual series.

Full text of DOI:10.1039/d1ra00762a

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7
-Alkoxy-appended coumarin derivatives:
synthesis, photo-physical properties, aggregation
behaviours and current–voltage (I–V)
characteristic studies on thin films†
Cite this: RSC Adv., 2021, 11, 10212
a
b
b
b
Abhijit Rudra Paul, Bapi Dey, Sudip Suklabaidya, Syed Arshad Hussain
a
and Swapan Majumdar
*
In this study, we designed and synthesised a series of coumarin derivatives appended with a long alkoxy
chain on the seventh position of the coumarin-3-carboxylate/carboxylic acid core to make thin film
materials. Synthesised compounds were characterized by their UV and fluorescence spectra in solutions
as well as their films prepared by both LB and spin-coated methods. The surface morphology and
electrical behaviour of thin films were judged by AFM, SEM and I–V characteristic mapping respectively.
Isotherm, UV-Vis absorption and fluorescence spectroscopic investigations revealed the formation of
aggregates on thin films. The result of SEM and AFM images provides the information about the length
and height of aggregates on the thin films of coumarin derivatives. From I–V characteristics, it was found
that at room temperature, the spin-coated films of coumarin derivatives containing an ester functional
group exhibited a threshold switching behaviour, whereas derivatives containing the carboxylic acid
functional group showed both threshold and bipolar switching behaviours. We also noticed that the I–V
characteristic features of synthesized materials depended on the length of the alkyl chain of individual
series.
Received 29th January 2021
Accepted 19th February 2021
DOI: 10.1039/d1ra00762a
rsc.li/rsc-advances
these innovative organic nanomaterials exhibit a variety of
important properties such as optical, electrical, photoelectrical
Introduction
Electronic and optoelectronic devices are used in various conducting, semiconducting, memory, storage and magnetic
4
applications ranging from simple household appliances to properties. Recent extensive studies have shown that organic
communications, computing, and medical instruments, and materials have distinctive advantages such as the low cost of
therefore, they are considered as basic building blocks for processing, lightweight, mechanical exibility, ambient pro-
modern civilization. Given the demand for ever more compact cessing, printability, and a variety of materials, which have
5
and powerful systems, there is growing interest in the devel- attracted specic attention towards organic electronics. In
opment of nanoscale devices that could enable new functions addition, several organic materials have been used as principal
and/or greatly enhance the performance. During the last components to design various electronic and optoelectronic
decade, organic electronics have been widely adopted in various devices such as diodes, sensors, organic light-emitting diodes
1
electrical, electronic and optoelectronic applications. They (OLED), organic eld effect transistors (OFET), solar cells,
6
provide alternative options in terms of the ease of processing, lasers, detectors, memory-switching devices, and logic gates.
low cost, better exibility, and superior electronic/ Nowadays, molecular electronics has emerged as an important
2
optoelectronic properties. Taking the advantage of solution- technology. It deals with the design, processing and device
processing, self-assembly and surface-engineering, organic application of organic molecules at the molecular level/
7
materials could serve as new building blocks for the low-cost nanoscale level. Thin-lm-fabricated switching devices of
3
manufacturing of exible and large-area devices. Organic organic molecules are promising candidates for the next
electronics deals with various interesting organic materials and generation of non-volatile memories due to their simple struc-
ture, low cost, excellent performance and great scale-down
8
potential.
a
Coumarins structurally belong to a lactone family con-
structed with a benzene ring fused to an a-pyrone ring and
essentially possess a conjugated system with electron rich and
Department of Chemistry, Tripura University, Suryamaninagar, 799 022, India.
E-mail: smajumdar@tripurauniv.in; Fax: +91-381-2374802; Tel: +91-381-237-9070
b
Department of Physics, Tripura University, Suryamaninagar, 799 022, India
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d1ra00762a
9
good charge transport properties. The structural simplicity and
1
10212 | RSC Adv., 2021, 11, 10212–10223
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versatility of the coumarin scaffold make them interesting via hydrophobic–hydrophobic interactions, and electron-
starting point for a wide range of applications. They have been withdrawing groups at C-3 can modulate the electronic behav-
found to show versatile pharmacological and biological activi- iour of coumarin derivatives. Our study reveals that the aggre-
10–15
ties,
which can display anti-cancer, anti-HIV, anti-tumor, gation behaviour depends on the length of the alkyl chain as
anti-microbial, antibacterial, anti-inammatory, anti-oxidant, well as the nature of the group at C-3. Electrical switching
anti-diabetic, anti-coagulant functions, etc. Despite tremen- behaviours via I–V characteristic mapping of spin-coated lms
dous biological activities of coumarin derivatives, only very of coumarin derivatives containing an ester group at C-3 show
limited studies have been undertaken on their photo-physical threshold switching, whereas a carboxylic acid group shows
16–18
and spectroscopic studies.
The uorescence of coumarin both threshold and bipolar memory switching. I–V character-
can be tuned by introducing different substituents with varied istic features also depend on the length of the alkyl chain of
electron-donating/withdrawing abilities, which is widely used individual series.
in the design of coumarin derivatives. The substituent-
dependent uorescence properties of the coumarin moiety
19,20
Experimental section
may result from intramolecular charge transfer.
It is well
21
known that p–p* transitions can form the excited states via General
excitation and usually transitions of coumarin are related to the
intra-molecular charge transfer from the benzene ring to the
pyranone moiety. Thus, an electron-donating group at position
NMR spectra were recorded using a Bruker Ascend 400 spec-
1
13
trophotometer. H NMR and C NMR spectra were recorded at
ambient temperature using 400 MHz spectrometers (400 MHz
7
and/or an electron-withdrawing group at position 3 can
1
13
for H and 100 MHz for C). Chemical shis are expressed in
parts per million from the tetramethylsilane internal reference,
and coupling constants are expressed in Hertz. Proton multi-
plicities are represented as s (singlet), d (doublet), dd (double
doublet), t (triplet), q (quartet), and m (multiplet). Infrared
spectra were recorded using a Fourier transform infrared (FT-
IR, Model: Spectrum 100) spectrometer with KBr pellets for
thin lms. UV-vis absorption and uorescence spectra of pure
solutions as well as spin-coated lms in quartz slide were
recorded using a Perkin Elmer Lambda-25 Spectrophotometer
and Perkin Elmer LS-55 uorescence spectrophotometer
respectively. A commercially available Langmuir–Blodgett (LB)
promote the process, thereby enhancing the uorescence
intensity of coumarin [shown in Fig. 1(a)]. Coumarins
substituted at position 7 with an electron-donating group are
known to exhibit strong uorescence as optical brighteners,
22–28
solar energy collectors, laser dyes and uorescent probes.
Accordingly this type of photo and electro-active organic mate-
rials have been the subject of current research including
organic semiconductors, organic metals including supercon-
ductors, organic photoconductors, organic photoactive mate-
rials for solar cells, organic non-linear optical materials, and
29–32
liquid crystals.
In this respect, the I–V characteristics for
resistive bipolar and threshold switching in thin lms are very
important due to their promising application as active compo-
nents for the memory device, logic circuits, etc., to realize the

lm deposition instrument (Apex 2006C, Apex Instruments Co.,
India) was used for isotherm measurement. The spin-coated
lms were prepared onto ITO-coated glass slides using a spin

33,34
molecular electronic structure.
To this end, ve and six-
coating lm deposition instrument SUC2005A (Apex instrument
company). Spin-coated lms of study materials were prepared
using a spin coater unit, Model: EZ spin-SD, Apex Instruments
Co., India. An atomic force microscopic (AFM) image in the
intermittent contact (tapping) mode of spin-coated lms was
taken in air (AFM system: Bruker Innova). The typical scan area
was different for different compounds. A silicon-wafer substrate
was used for the AFM measurement. The surface morphology of
the aggregate was estimated using a Field Emission Scanning
Electron Microscope Model – Sigma 300, Carl Zeiss instrument.
The current–voltage (I–V) characteristics were measured using
a Keithley 2401 source meter. The reported melting points were
uncorrected. High-resolution mass spectrometry (HR-MS) data
were acquired by electron spray ionization using a Q-ToF-micro
quadrupole mass spectrometer.
membered heterocycles, in particular coumarins, seem to be
suitable parent p-conjugated backbones as they represent
a robust and stable heterocycle, and can easily be synthesized in
high purities and further functionalized at suitable positions by
a simple chemistry to make them useful for devices. Given the
advantages of small molecules of well-dened molecular
structures, it is worthwhile to explore the physicochemical
behaviour of novel coumarin derivatives. In this work, the
synthesis, and spectroscopic, morphological and electrical
behaviours via I–V characteristic mapping of a series of
coumarin derivatives appended with a long alkoxy chain at the
C-7 position of coumarin-3-carboxylates/carboxylic acids [Fig.
1(b)] are reported. The alkoxy group is chosen because it can act
as a donor as well as help in aggregation for making thin lms
Materials
All reagents were purchased from commercial suppliers and
used without further purication, unless otherwise specied.
Commercially supplied ethyl acetate and petroleum ether (60–
ꢀ
8
0 C) were distilled before use. Column chromatography was
Fig. 1 (a) Intramolecular charge transfer and (b) general structures of
the synthesized coumarin derivatives 2a–e (R ¼ Et) and 3a–e (R ¼ H)
used in the present study.
performed on silica gel (60–120 mesh, 0.12–0.25 mm). Analyt-
ical thin-layer chromatography (TLC) was performed on
©
2021 The Author(s). Published by the Royal Society of Chemistry
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0
.25 mm extra-hard silica gel plates with a UV254 uorescent 353 nm; IR (KBr) nmax 2917, 2851, 1752, 1627, 1578, 1416,
ꢁ1 1
indicator. 2,4-Dihydroxy benzaldehyde was purchased from 1262, 1242, 1022 cm ; H NMR (400 MHz, CDCl ) d 8.52
3
Sigma Aldrich chemical Co., USA, and used as received. Diethyl (s, 1H), 7.50 (d, J ¼ 8.8 Hz, 1H), 6.88 (dd, J ¼ 2.0, 8.8 Hz,
malonate, anhydrous K CO , and DMF were purchased from 1H), 6.80 (d, J ¼ 2.4 Hz, 1H), 4.40 (q, J ¼ 7.2, 14.4 Hz, 2H),
2
3
SRL India. Dry CH
2
Cl
2
and dry DMF were prepared according to 4.04 (t, J ¼ 6.4 Hz, 2H), 1.86–1.79 (m, 2H), 1.47–1.45 (m,
the standard procedures. Spectroscopy-grade chloroform and 2H), 1.40 (t, J ¼ 7.2 Hz, 3H), 1.26 (bs, 24H), 0.88 (t, J ¼
1
3
ultra-pure Milli-Q water (triple distilled deionized resistivity ¼ 6.8 Hz, 3H); C NMR (100 MHz, CDCl
3
) d 164.8, 163.5,
18.2 MU cm) were used throughout the study. The synthesized 157.6, 157.3, 149.1, 130.7, 114.1, 113.8, 111.4, 100.8, 69.0,
1
product was characterized by melting point, IR data, H NMR, 61.7, 31.9, 29.7, 29.6, 29.5, 29.4, 29.3, 28.9, 25.9, 22.7,
1
3
+
+
C NMR and mass spectral analysis.
14.3, 14.1; HRMS calcd for C28H42O5 + H (M + H )
4
59.3112; found 459.3103.
Ethyl 7-tetradecyloxy-2-oxo-2H-chromene-3-carboxylate (2c).
Synthesis
ꢀ
3
Yield: 89%; white solid, mp 74–76 C; UV (CHCl ) lmax 353 nm; IR
Synthesis of ethyl 7-hydroxy-2-oxo-chromene-3-carboxylate (KBr) nmax 2917, 2844, 1757, 1612, 1578, 1412, 1294, 1214,
ꢁ
1 1
(
1). A mixture of 2,4-dihydroxybenzaldehyde (1 g, 7.194 1141 cm ; H NMR (400 MHz, CDCl
3
) d 8.52 (s, 1H), 7.51 (d, J ¼
mmol), diethyl malonate (1.31 mL or 1.38 g, 8.615 mmol), 8.4 Hz, 1H), 6.88 (dd, J ¼ 2.4, 8.8 Hz, 1H), 6.80 (d, J ¼ 2.4 Hz, 1H),
absolute ethanol (5 mL), piperidine (15 drops) followed by 4.40 (q, J ¼ 7.2, 14.4 Hz, 2H), 4.04 (t, J ¼ 6.8 Hz, 2H), 1.86–1.79 (m,
ꢀ
glacial acetic acid (5 drops) was heated at reux at 80 C for 2H), 1.47–1.45 (m, 2H), 1.41 (t, J ¼ 7.2 Hz, 3H), 1.27 (bs, 20H), 0.89
13
1
.5 h. Aer the completion of reaction (TLC), the reaction (t, J ¼ 6.8 Hz, 3H); C NMR (100 MHz, CDCl
) d 164.8, 163.5, 157.6,
3
ꢀ
mixture was allowed to cool below 60 C and then water (15 mL) 157.3, 149.1, 130.7, 114.1, 113.8, 111.4, 100.8, 69.0, 61.7, 31.9, 29.7,
was added to the ask. The reaction mixture was then chilled in 29.6, 29.5, 29.4, 29.3, 28.88, 25.9, 22.7, 14.3, 14.1; HRMS calcd for
+
+
an ice bath. The solid product thus obtained was collected by
ltration, washed with a small amount of chilled 50% aqueous
26 38 5
C H O + H (M + H ) 431.2799; found 431.2701.

Ethyl 7-dodecyloxy-2-oxo-2H-chromene-3-carboxylate (2d).
ethanol and dried. The analytical sample was prepared by Yield: 88%; white solid, mp 64–66 C; UV (CHCl
ꢀ
3
) lmax
crystallization from water–ethanol. Yield: 85%, white solid, mp 353 nm; IR (KBr) nmax 1756, 1641, 1566, 1418, 1348, 1220,
ꢀ
35
ꢀ
ꢁ1
1
1
1
68–170 C; lit. mp 166–168 C; IR (KBr) nmax 3542, 2817, 1736, 1018 cm ; H NMR (400 MHz, CDCl
3
) d 8.53 (s, 1H), 7.51 (d, J
ꢁ1 1
3
612, 1531, 1450, 1226, 1032 cm ; H NMR (400 MHz, CDCl )
¼ 8.4 Hz, 1H), 6.89 (dd, J ¼ 2.4, 8.8 Hz, 1H), 6.81 (d, J ¼ 2.4 Hz,
d 11.14 (s, 1H), 8.66 (s, 1H), 7.74 (d, J ¼ 8.8 Hz, 1H), 6.80 (t, J ¼ 1H), 4.41 (q, J ¼ 7.2, 14.4 Hz, 2H), 4.05 (t, J ¼ 6.4 Hz, 2H),
1
3
8
.4 Hz, 2H), 4.25 (q, J ¼ 7.2 Hz, 2H), 1.29 (t, J ¼ 7.2 Hz, 3H);
C
1.85–1.82 (m, 2H), 1.47–1.45 (m, 2H), 1.42 (t, J ¼ 7.2 Hz, 3H),
13
NMR (100 MHz, CDCl ) d 164.5, 163.4, 157.5, 156.8, 149.9, 132.5, 1.28 (bs, 16H), 0.89 (t, J ¼ 6.4 Hz, 3H); C NMR (100 MHz,
3
1
14.4, 112.5, 110.8, 102.2, 61.3, 14.5.
3
CDCl ) d 164.7, 163.5, 157.6, 157.3, 149.1, 130.6, 114.0, 113.7,
General procedure for the synthesis of 2a–e by alkylation of 111.4, 100.7, 68.9, 61.7, 31.8, 29.6, 29.5, 29.4, 29.3, 28.8, 25.9,
+
+
34 5
ethyl 7-hydroxy-2-oxo-chromene-3-carboxylate. A mixture of 22.7, 14.3, 14.1; HRMS calcd for C24H O + H (M + H )
ethyl 7-hydroxy-2-oxo-chromene-3-carboxylate (1, 1 g, 4.27 403.2484; found 403.2370.
mmol), long-chain alkyl bromide (C
n
H
2n+2Br, n ¼ 18, 16, 14, 12 and
Ethyl 7-decyloxy-2-oxo-2H-chromene-3-carboxylate (2e).
ꢀ
1
0; 5.13 mmol), anhydrous potassium carbonate (8.54 mmol) and Yield: 97%; white solid, mp 58 C; UV (CHCl
3
) lmax 353 nm; IR
ꢀ
ꢁ1
1
DMF (5 mL) was stirred at 60 C for 1.5 h. Aer completion of the (KBr) nmax 1750, 1642, 1560, 1419, 1345, 1300, 1213 cm ; H
reaction (TLC), the reaction mixture was diluted with water (30 mL) NMR (400 MHz, CDCl
3
) d 8.52 (s, 1H), 7.50 (d, J ¼ 8.8 Hz, 1H),
and extracted with ethyl acetate (3 ꢂ 10 mL). The combined 6.88 (dd, J ¼ 2.4, 8.8 Hz, 1H), 6.81 (d, J ¼ 2.0 Hz, 1H), 4.41 (q, J ¼
organic layer was washed with water (3 ꢂ 10 mL), dried over 7.2, 14.4 Hz, 2H), 4.05 (t, J ¼ 6.8 Hz, 2H), 1.85–1.80 (m, 2H),
anhydrous sodium sulphate and concentrated under reduced 1.48–1.45 (m, 2H), 1.41 (t, J ¼ 7.2 Hz, 3H), 1.28 (bs, 12H), 0.89 (t,
1
3
pressure. The crude product was puried over silica-gel (60–120 J ¼ 6.4 Hz, 3H); C NMR (100 MHz, CDCl
) d 164.8, 163.5, 157.6,
157.3, 149.1, 130.7, 114.1, 113.8, 111.4, 100.8, 69.0, 61.7, 31.9,
Ethyl 7-octadecyloxy-2-oxo-2H-chromene-3-carboxylate (2a). 29.5, 29.3, 28.9, 25.9, 22.7, 14.3, 14.1; HRMS calcd for C22H O
30 5
3
mesh) using ethyl acetate–hexane (3 : 7) as the eluent.
ꢀ
+
+
Yield: 84%; white solid, mp 76–78 C; UV (CHCl
3
) lmax 353 nm; + H (M + H ) 375.2173; found 375.2046.
General procedure for the synthesis of coumarin 3-carboxylic
IR (KBr) n 2918, 2850, 1758, 1624, 1572, 1471, 1417, 1292,
max
1 1
ꢁ
1
8
1
1
2
226 cm ; H NMR (400 MHz, CDCl ) d 8.50 (s, 1H), 7.47 (d, J ¼ acid derivatives 3a–e. To a solution of alkylated coumarin 3-
3
.4 Hz, 1H), 6.87 (dd, J ¼ 2.0, 8.4 Hz, 1H), 6.78 (d, J ¼ 2.0 Hz, carboxylate (2a–e, 2.06 mmol) in ethanol (5.0 mL), 10% aq.
H), 4.38 (q, J ¼ 7.2, 14.4 Hz, 2H), 4.03 (t, J ¼ 6.4 Hz, 2H), 1.84– KOH solution (10 mL) was added at once. The ask was then
.78 (m, 2H), 1.47–1.41 (m, 2H), 1.39 (t, J ¼ 7.2 Hz, 3H), 1.25 (bs, tted with a condenser and the solution was heated in a water
1
3
ꢀ
bath at 80 C for 30 minutes. Cold HCl (6 M) was added to the
8H), 0.87 (t, J ¼ 6.4 Hz, 3H); C NMR (100 MHz, CDCl
3
)
d 164.7, 163.5, 157.6, 157.2, 149.0, 130.6, 114.0, 113.8, 111.4, reaction mixture under ice cold conditions for the precipi-
1
2
4
00.7, 68.6, 61.7, 31.9, 29.7, 29.5, 29.4, 29.3, 29.2, 28.8, 25.9, tation of the product. The product was then collected by
+
+
2.7, 14.2, 14.1; HRMS calcd for C H O + H (M + H ) ltration, washed thoroughly with cold water, dried and
3
0
46 5
87.3425; found 487.3466.
nally crystallized from ethanol–water. Identities of the
1
13
Ethyl 7-hexadecyloxy-2-oxo-2H-chromene-3-carboxylate (2b). compounds were established by IR data, H NMR, C NMR
Yield: 88%; white solid, mp 80–82 C; UV (CHCl
ꢀ
3
) lmax and mass spectral analysis.
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Ethyl 7-octadecyloxy-2-oxo-2H-chromene-3-carboxylic acid
RSC Advances
Results and discussions
Synthesis
ꢀ
(
3a). Yield: 99%; white solid, mp 120–122 C; UV (CHCl ) lmax
3
3
1
57 and 374 nm; IR (KBr) nmax 2917, 2849, 1735, 1692, 1622,
ꢁ1 1
508, 1428, 1384, 1258, 1144, 815 cm ; H NMR (400 MHz, A series of coumarin derivatives (2a–e and 3a–e) containing
) d 12.21 (bs, 1H), 8.85 (s, 1H), 7.62 (d, J ¼ 8.8 Hz, 1H), a long alkoxy chain and an electron-withdrawing group at
.00 (dd, J ¼ 2.0, 8.8 Hz, 1H), 6.90 (d, J ¼ 2.0 Hz, 1H), 4.08 (t, J ¼ position 3 were prepared for our study by a straightforward two-
.4 Hz, 2H), 1.88–1.81 (m, 2H), 1.47–1.43 (m, 2H), 1.25 (bs, step process from ethyl 7-hydroxy-coumarin-3-carboxylate (1),
CDCl
3
7
6
2
1
3
8H), 1.03 (t, J ¼ 6.4 Hz, 3H); C NMR (100 MHz, CDCl3) which was synthesized from 2,4-dihydroxybenzaldehyde and
35
d 165.9, 164.6, 163.2, 157.1, 151.2, 131.6, 115.5, 112.1, 110.6, diethylmalonate by a known protocol. Treatment of 1 with
1
1
4
01.2, 69.4, 31.9, 29.7, 29.6, 29.5, 29.3, 29.2, 28.8, 25.9, 22.7, alkyl bromide of variable chain length in the presence of
+
+
4.1; HRMS calcd for C28
59.3110.
H
42
O
5
+ H (M + H ) 459.3112; found anhydrous potassium carbonate in DMF afforded the corre-
sponding O-alkylated products (2), which upon base-catalysed
Ethyl 7-hexadecyloxy-2-oxo-2H-chromene-3-carboxylic acid hydrolysis followed by acidication produced coumarin deriv-
ꢀ
(
3b). Yield: 90%; white solid, mp 124 C; UV (CHCl ) l
357 atives (3) in excellent yields (Scheme 1).
max
3
and 374 nm; IR (KBr) n
2917, 2851, 1752, 1627, 1578, 1416,
max
ꢁ1 1
1
8
1
262, 1242 cm ; H NMR (400 MHz, CDCl ) d 12.24 (bs, 1H),
3
UV-vis absorption spectroscopy measurement
.87 (s, 1H), 7.65 (d, J ¼ 8.8 Hz, 1H), 7.02 (dd, J ¼ 2.4, 8.8 Hz,
Initially, we began with the study of absorption spectroscopy of
the synthesized molecules to understand their absorption
behaviour in solutions as well as in thin lms. Fig. 2[A] and [B]
H), 6.92 (d, J ¼ 2.0 Hz, 1H), 4.10 (t, J ¼ 6.4 Hz, 2H), 1.90–1.82
(m, 2H), 1.52–1.46 (m, 2H), 1.41 (t, J ¼ 7.2 Hz, 3H), 1.26 (bs,
1
3
20H), 0.88 (t, J ¼ 6.8 Hz, 3H); C NMR (100 MHz, CDCl
3
)
represents the absorption spectra of compounds 2a–e in CHCl
d 164.8, 163.5, 157.6, 157.3, 149.1, 130.7, 114.1, 113.8, 111.4,
3
ꢁ5
ꢁ1
solution (c 1.0 ꢂ 10 mol L ) and spin-coated quartz lms
respectively. Compounds 2a–e exhibit a strong absorption band
at 353 nm in the solution and at 324 nm in the spin-coated lm.
The absorption bands of spin-coated quartz lms are blue-
shied with respect to the absorption bands of compounds
1
2
4
00.8, 69.0, 61.7, 31.9, 29.7, 29.6, 29.5, 29.4, 29.3, 28.88, 25.9,
+
+
2.7, 14.3, 14.1; HRMS calcd for C H O + H (M + H )
2
6
38 5
31.2799; found 431.2701.
Ethyl 7-tetradecyloxy-2-oxo-2H-chromene-3-carboxylic acid
ꢀ
(
3c). Yield: 98%; white solid, mp 126 C; UV (CHCl
3
) lmax 357
(
2a–e) in the chloroform solution. Similarly, compounds 3a–e in
the solution (Fig. 2[C]) exhibit a strong absorption band at
57 nm with a weak shoulder at around 374 nm and in the spin-
and 374 nm; IR (KBr) nmax 2910, 2844, 1751, 1625, 1572, 1413,
ꢁ1 1
1
8
1
3
254, 1201 cm ; H NMR (400 MHz, CDCl ) d 12.25 (bs, 1H),
3
.87 (s, 1H), 7.64 (d, J ¼ 8.8 Hz, 1H), 6.88 (dd, J ¼ 2.4, 8.8 Hz,
coated lm show a absorption band at 335 nm (Fig. 2[D]).
Compounds 3a–e also show a hypso-chromic shi (blue-shi)
in spin-coated quartz lms with respect to the solution
absorption spectra. For both series, the absorption spectra of
spin-coated lms are shied to a shorter wavelength or higher
energy with respect to solution absorption bands. This kind of
absorption shi indicated that in the lm, the molecules were
organized as H-aggregates via hydrophobic–hydrophobic
interactions with long alkyl chains and p–p stacking between
electron-rich coumarin cores. The intensities of absorption
bands also depend on the length of the alkyl chain of the
individual series.
H), 6.80 (d, J ¼ 2.4 Hz, 1H), 4.04 (t, J ¼ 6.8 Hz, 2H), 1.90–1.82
(m, 2H), 1.52–1.45 (m, 2H), 1.41 (t, J ¼ 7.2 Hz, 3H), 1.26 (bs,
1
3
20H), 0.88 (t, J ¼ 6.8 Hz, 3H); C NMR (100 MHz, CDCl )
3
d 164.8, 163.5, 157.6, 157.3, 149.1, 130.7, 114.1, 113.8, 111.4,
1
2
4
00.8, 69.0, 61.7, 31.9, 29.7, 29.6, 29.5, 29.4, 29.3, 28.88, 25.9,
+
+
2.7, 14.3, 14.1; HRMS calcd for C24
03.2486; found 403.2488.
34 5
H O + H (M + H )
Ethyl 7-dodecyloxy-2-oxo-2H-chromene-3-carboxylic acid
ꢀ
(
3d). Yield: 98%; white solid, mp 128–130 C; UV (CHCl ) l
3
max
3
1
1
4
1
1
3
57 and 374 nm; IR (KBr) n
1641, 1562, 1414, 1348,
max
ꢁ
1 1
018 cm ; H NMR (400 MHz, CDCl ) d 12.27 (bs, 1H), 8.88 (s,
3
H), 7.63 (d, J ¼ 8.8 Hz, 1H), 7.02 (d, J ¼ 8.4 Hz, 1H), 6.93 (s, 1H),
.11 (t, J ¼ 6.4 Hz, 2H), 1.88–1.84 (m, 2H), 1.49 (bs, 2H), 1.28 (bs,
13
2H), 0.88 (t, J ¼ 6.4 Hz, 3H); C NMR (100 MHz, CDCl
3
) d 165.9,
Fluorescence measurement
64.7, 163.2, 157.1, 151.3, 131.7, 115.5, 112.2, 110.6, 101.2, 69.4,
1.9, 29.7, 29.6, 29.5, 29.4, 29.3, 28.8, 25.9, 22.7, 14.1; HRMS calcd
To understand the electronic excitation behaviour, we recorded
the uorescence spectra of compounds 2a–e and 3a–e in
+
+
for C H O + H (M + H ) 375.2173; found 375.1967.
22 30 5
Ethyl 7-decyloxy-2-oxo-2H-chromene-3-carboxylic acid (3e).
ꢀ
Yield: 91%; white solid, mp 132–134 C; UV (CHCl ) l
357
max
3
ꢁ
1
and 374 nm; IR (KBr) nmax 1642, 1565, 1419, 1345, 1017, 930 cm
;
1
H NMR (400 MHz, CDCl
3
) d 12.25 (bs, 1H), 8.87 (s, 1H), 7.65 (d, J ¼
8
4
1
.8 Hz, 1H), 7.02 (dd, J ¼ 2.4, 8.8 Hz, 1H), 6.92 (d, J ¼ 2.4 Hz, 1H),
.10 (t, J ¼ 6.4 Hz, 2H), 1.90–1.83 (m, 2H), 1.53–1.45 (m, 2H), 1.36–
1
3
.29 (m, 12H), 0.90 (t, J ¼ 6.4 Hz, 3H); C NMR (100 MHz, CDCl
3
)
d 165.9, 164.7, 163.2, 157.1, 151.3, 131.7, 115.5, 112.1, 110.6, 101.2,
9.4, 31.9, 29.5, 29.3, 28.8, 25.9, 22.7, 14.2; HRMS calcd for
Scheme 1 Synthesis of coumarin derivatives 2a–e and 3a–e. Reaction
6
conditions: (a) anhydrous K
2
CO
3
, long-chain alkyl bromide (n ¼ 18, 16,
+
+
C H O + H (M + H ) 347.1860; found 347.1866.
14, 12 and 10), DMF, 60 ꢀC, 1.5 h, 84–97%; (b) 10% aq. KOH, 80 ꢀC,
20 26 5
EtOH, 30 min, then 6 M HCl, 90–99%.
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Fig. 2 UV-Vis absorption spectra: Panels [A] and [B] for 2a–e in
solutions and quartz films; Panels [C] and [D] for 3a–e in solutions and
quartz films.
ꢁ
5
ꢁ1
a chloroform solution at a concentration of 1.0 ꢂ 10 mol L
as well as in spin-coated thin lms. The uorescence spectra
were recorded with excitation into the maximum of the longest
wavelength absorption. Fig. 3[A]–[D] shows the excitation
spectra of 2a–e and 3a–e in their solution and thin lms. The
excitation spectra were recorded for compounds 2a–e in solu-
tions at 358 nm and the maximum absorption band at 353 nm,
and for quartz lms, it was excited at 280 nm and the absorption
band appeared at 324 nm. Similarly, for compounds 3a–e, in the
solution, the maximum excitation was recorded at 385 nm and
maximum absorption band at 357 nm, and for quartz lms, it
was excited at 302 nm and the absorption band appeared at
Fig. 3 Fluorescence spectra for compounds (2a–e) and (3a–e): Panels
335 nm. A longer excitation spectrum was recorded in solutions
[
A], [B] and [C], [D] represents excitation spectra for compounds 2a–e
and 3a–e in solutions and thin films respectively; Panels [E], [F] and [G],
H] represents emission spectra for compounds 2a–e and 3a–e in
due to the presence of electron-withdrawing and -donating
substituents that possess a strong push–pull system to exhibit
[
high uorescence intensities. The lower excitation of 3a–e in solutions and films respectively.

lms may be attributed due to the compactness of the mole-
cules via intermolecular hydrogen bonding between carboxylic
acid functional groups.
Isotherm measurement/monolayer characteristics
The emission spectra showed signicant dependence on the
excitation wavelength. The emission spectra of 2a–e and 3a–e in
In order to have an idea about monolayer properties, two
extreme members [2a (n ¼ 16, R ¼ Et), 2e (n ¼ 8, R ¼ Et), 3a
ꢁ
5
ꢁ1
the chloroform solution (1.0 ꢂ 10 mol L ) and quartz lm at
different excitation wavelengths are displayed in Fig. 3[E]–[H].
The emission spectrum for compounds 2a–e obtained by exci-
tation at 358 nm has an emission maximum of 405 nm in the
chloroform solution, whereas in spin-coated lms, the excita-
tion wavelength is 280 nm and emission maximum is 427 nm.
However, the emission spectrum for compounds 3a–e obtained
by excitation at 385 nm has an emission maximum of 406 nm in
(
n ¼ 16, R ¼ COOH), and 3e (n ¼ 8, R ¼ COOH) in Fig. 1] of each
series of the coumarin derivatives were selected and then
surface pressure–area per molecule isotherm (p–A) at the air–
water interface using the Langmuir–Blodgett (LB) techniques
was
1.0 ꢂ 10 mol L ) of 100 mL of each material was spread by
a micro syringe on the water sub-phase of pure Milli-Q water
18.2 MU cm) at room temperature and allowed to evaporate.
Aer complete evaporation of the volatile solvent, the barrier
recorded.
A
dilute
solution
in
chloroform
ꢁ
5
ꢁ1
(
(
ꢁ
5
ꢁ1
the chloroform solution (1.0 ꢂ 10 mol L ), whereas in quartz

lms, the excitation wavelength is 302 nm and the emission
ꢁ1
was compressed at a rate of 5 mm min to record the surface
pressure–area per molecule isotherms. The surface pressure (p)
versus average area (A) available for one molecule was measured
maximum is 605 nm. The uorescence excitation and emission
data of 2a–e and 3a–e are summarized in Table 1.
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Table 1 UV-Vis absorption and fluorescence excitation/emission measurement in a chloroform solution (c 1.0 ꢂ 10ꢁ5 mol L ) and a thin film
ꢁ1
Compounds
UV-Vis absorption/uorescence excitation/emission
2a–e
3a–e
UV-vis absorption maxima in solution
UV-vis absorption maxima of thin lm
Fluorescence excitation maxima in solution
Fluorescence emission maxima in CHCl solution
3
Fluorescence excitation maxima of thin lm (spin coated)
Fluorescence emission maxima of thin lm (spin coated)
353 nm
324 nm
358 nm
405 nm
280 nm
427 nm
357 nm
335 nm
385 nm
406 nm
302 nm
605 nm
36
by a Wilhelmy plate arrangement. Each isotherm was repeated of compounds 2a, 2e, 3a and 3e in thin lms deposited onto the
a number of times and data for surface pressure-area per smooth silicon substrate. The AFM images of spin-coated lms
molecule isotherms were obtained by a computer interfaced revealed the sphere-like structures of aggregated 2a, 2e, 3a and
with the LB instrument. Before each isotherm measurement, 3e in the lms, but the AFM line spectra of lms of 2a and 2e
the trough and barrier were cleaned with ethanol and then indicated the clustering nature of the aggregates (see ESI†).
rinsed with Milli-Q water. The compound (2a, 2e, 3a and 3e) Interestingly, all the sphere-like aggregates are uniformly
monolayer formed at the air–water interface was found to be distributed throughout the lms, albeit they exhibited a small
stable and easily transferable onto the solid substrate to variation in dimensions as well as in heights. Since the
monolayer LB lms. Fig. 4[A] shows the corresponding (p–A) dimension of the individual molecules is beyond the scope of
isotherm curves of compounds 2a and 2e. The isotherm of resolution, it is not possible to distinguish individual mole-
2
compound 2a starts rising with an initial li off area of 2.2 nm
cules. Aer the analysis of height prole and line spectra (see
2
and for compound 2e it was observed at 2.3 nm . It was found ESI†) of AFM images (with respect to the empty substrate), it is
ꢁ
1
that the isotherm of compound 2a gets collapsed at 45 mN m
,
estimated that the dimensions of sphere-like particles for
whereas the collapse occurred at the lower surface pressure at compounds 2a, 2e, 3a and 3e are 200–250 nm, 150–250 nm,
ꢁ
1
3
8 mN m for compound 2e, i.e. 2a forms a more stable lm 200–300 nm and 120–170 nm respectively and heights are 10–
than 2e probably due to the presence of longer alkyl chains in 12 nm, 10–12 nm, 12–14 nm and 7–8 nm respectively. As
a. However, isotherms of compounds 3a and 3e started rising a whole, AFM images give us visual information about the
2
2
2
with li off areas 4.2 nm and 6.3 nm and gets collapsed aer formation of dened nano-structure aggregates (spherical in
ꢁ
1
ꢁ1
surface pressure 29 mN m and 15 mN m respectively (Fig. 4 shape) of the compounds in the lms.
B]). The isotherm proles of compounds 3 (a, e) indicate To get the insight into the surface morphology of the lms
a higher li off area than the isotherm proles of compounds 2 prepared by spin-coated techniques, scanning electron micro-
a, e). The differences in li off areas as well as collapse pres- scopic (SEM) studies were carried out for four selected
[
(
sures for compounds 2 (a, e) and 3 (a, e) are probably due to the compounds by taking two from each series. Fig. 6 shows the
hydrophobicity and polarity of the compounds. Amongst all the SEM images of the compounds 2a, 2e, 3a and 3e. These images
compounds, 2a is more hydrophobic due to the long alkyl chain
and ester group. In contrast, compound 3e is more polarizable
in nature due to the presence of a carboxylic acid (COOH) group
and a small alkyl chain than others.
In order to have a deep idea about the surface morphologies
of the thin lms, we investigated the atomic force microscopic
(AFM) images of spin-coated lms. Fig. 5 shows the AFM images
Fig. 4 Surface pressure vs. area per molecule isotherms: [A] for (2a,
e) and [B] for (3a, 3e). Inset shows the molecular structure of the Fig. 5 AFM images of spin-coated films for compounds 2a, 2e, 3a and
compounds. 3e.
2
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Fig. 6 Scanning electron microscopic (SEM) images of compounds 2a, 2e, 3a and 3e.
reveal that the molecules of the compounds 2a and 2e aggre- of mechanisms are responsible for the electrical switching
gated to form a cluster-like morphology, whereas 3a and 3e behaviour (bipolar and threshold switching) of organic
show a sphere-type surface morphology. We anticipated that the compounds, such as conformational changing, rotation of
possible additional interaction via H-bonding in 3a and 3e functional group, charge transfer, oxidation–reduction process,
aggregated in such a way that they appeared as spherical in lamentary conduction, space charge and traps, and ionic
42–44
shape but in other series (2a and 2e) due to the presence of the conduction.
From UV, uorescence and morphological
ester group, they aggregated to form a cluster-like morphology. studies of the synthesized compounds 2a–e and 3a–e indicated
that they formed H-aggregates, i.e. in lms they associated by
face-to-face assembly through hydrophobic–hydrophobic
interactions between long alkyl chains and p–p stacking
between aromatic cores. These aggregation behaviours prompt
us to conduct the electrical switching behaviour of compounds
I–V characteristic studies of the compounds 2a–e and 3a–e
Switching behaviour. On the basis of current–voltage (I–V)
characteristics, there are two types of switching, namely, volatile
and non-volatile switching. According to the volatility, there are
2a–e and 3a–e assembled onto spin-coated lms. In this
two types of resistance switching: threshold and memory
switching and based on the non-volatility, these are bipolar and
unipolar switching. On the survey of the literature, it was found
that, based on the polarity of the compounds and applied
purpose, by the variation of compounds (2a–e and 3a–e), we
designed various (ten types) switching devices: (i) Au/2x/ITO
(
where x ¼ a, b, c, d and e) and (ii) Au/3y/ITO (where y ¼ a, b, c,
d and e) and studied their resistive bipolar memory switching
and threshold switching behaviour by adjusting the bias
voltage, compliance current, sweep direction, etc. The cong-
uration of the devices and details of initial sweep voltages and
compliances current are shown in Fig. 7–12.
37,38
voltage, bipolar switching shows bidirectional resistance.
At
a certain voltage, non-volatile memory switching, switched on
both the high-resistance and low-resistance states and at the
low voltage, threshold switching shows only the high-resistance
45
39
state. Nowadays, extensive research is carried out to exploit the
electrical switching resistance (low and high) as a potential
application in non-volatile memory devices for future genera-
I–V characteristic of molecules (2a–e) in spin-coated lms
40,41
tion.
Electrically induced resistive memory switching and To study the electrical switching behaviour of coumarin deriv-
threshold switching phenomena in organic compounds are of atives 2a–e, the device having congurations Au/2x/ITO (where x
great interest to develop novel electronic devices. It is possible ¼ a, b, c, d and e) was prepared by spreading 1–2 drops of
to have different switching modes by adjusting the device solutions on an ITO-coated glass substrate spun at a rotating
conguration and measurement protocols. Moreover, a variety speed of 1800 rpm. The resulting spin-coated lms are dried
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Fig. 7 I–V characteristic for threshold switching of 2a [A], 2b [B], 2c [C], 2d [D] and 2e [E] for compounds 2a–e of the Au/2a–e/ITO device based
on spin-cast films for two sweep direction. Initial sweep voltages (A–E) for all the devices were ꢃ3.5 V at 5 mA compliance current. Arrows
indicate the sweep direction of applied voltage.
4
1
sufficiently in vacuum at room temperature before the compu- in the OFF state is 5.11 ꢂ 10 U and the on state is 165.49 ꢂ 10
tation of I–V characteristics. The current–voltage behaviour was U respectively. The results of I–V characteristics of these devices
recorded by applying the voltage tuning from +Vmax to ꢁV
are listed in Table 2. From the results tabulated in Table 2, it is
and vice versa. The voltage was imposed on the top electrode clear that with the decrease in the alkoxy chain length of 2 in the
gold tip), whereas the bottom electrode (ITO-coated glass devices Au/2x/ITO, the threshold voltages increase with the
substrate) was held on ground potential. decrease in resistance in the OFF state, whereas in the ON state,
max
(
The electrical (I–V) measurement was performed by giving an the resistance increases. No bipolar switching was observed in
2
active area of 1 mm for measurement. Same/different this series probably due to the hydrophobic and non-polar
scanning/tuning voltages were applied to measure the I–V nature of 2a–e.
ꢁ1
curves for all the devices at a voltage scan rate of 2.5 mV s
.
Fig. 7[A]–[E] shows the I–V curves of 2a–e. The device Au/2a/ITO
4
I–V characteristic of molecules (3a–e) in spin-coated lms
exhibits a low conducting state (OFF state, 15.42 ꢂ 10 U) for
tuning the voltage range from +3.5 V to ꢁ3.5 V (scanning range To study the switching behaviour of the compounds 3a–e,
was 0 to +3.5 to 0 to ꢁ3.5 to 0) (Fig. 7[A]), but the current was a device having congurations Au/3y/ITO (where y ¼ a, b, c,
increased when the voltage approaches the threshold voltage d and e) was prepared by spin-coating techniques. The current–
(
about 2 V), showing its high conducting state (ON state, 138.72 voltage behaviour measurement was studied by tuning the
1
ꢂ 10 U). A similar procedure was followed for all other devices: voltage from +Vmax to ꢁVmax and vice versa (with an increment of
Au/2b/ITO, Au/2c/ITO and Au/2d/ITO. The device Au/2e/ITO 0.1 V for Vmax each time). The I–V computation curve of the
exhibits a threshold voltage of 3.0, and the resistance recorded device having the conguration Au/3a/ITO for tuning the
voltage range from +2.5 V to ꢁ2.5 V is shown in Fig. 8[A].
Table 2 Different parameters of I–V characteristics for Au/2a–e/ITO devices
Threshold switching
Threshold voltage (V)
Resistances at
Maximum compliance
current (mA)
Minimum compliance current
(mA)
Device conguration
OFF state (U)
ON state (U)
4
1
Au/(2a)/ITO
Au/(2b)/ITO
Au/(2c)/ITO
Au/(2d)/ITO
Au/(2e)/ITO
2
10
10
10
10
10
2
2
2
2
2
15.42 ꢂ 10
138.72 ꢂ 10
146.68 ꢂ 10
150.93 ꢂ 10
161.92 ꢂ 10
165.49 ꢂ 10
4
1
1
1
1
2.2
2.5
2.7
3
12.79 ꢂ 10
4
10.01 ꢂ 10
4
8.21 ꢂ 10
4
5.11 ꢂ 10
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Fig. 8 I–V characteristics for bipolar resistive switching (A, B and C)
and threshold switching (D) for compound 3a of the Au/3a/ITO device
based on spin-cast films for two sweep directions. Initial sweep volt-
ages were (A) ꢃ2.5 V, (B and C) ꢃ3.1 V at 20 mA compliance current
and (D) ꢃ5 V at 5 mA compliance current. Arrows indicate the sweep
direction of the applied voltage.
Fig. 9 I–V characteristics for bipolar resistive switching (A, B and C)
and threshold switching (D) for compound 3b of the Au/3b/ITO device
based on spin-cast films for two sweep directions. Initial sweep volt-
ages were (A) ꢃ3 V, (B and C) ꢃ3.5 V at 20 mA compliance current and
(
D) ꢃ5 V at 5 mA compliance current. Arrows indicate the sweep
direction of the applied voltage.
Initially, the above-mentioned device exhibits a low conducting
state, by tuning the voltage range from +Vmax to ꢁVmax, but
when the voltage closes to a threshold value of 1.5 V, it shows
Moreover, when applied to a higher voltage, i.e. 3.1 V in the
device, it gets short circuit and shows only one way switching, it
may be due to the organic layer between the electrodes in the
ON state and the OFF state. However, in threshold and bipolar
switching devices, the current in the OFF state is due to the low
leakage of current.
a high conducting state, and during scanning reverse i.e. ꢁV
max
46
to +V , the device shows transition from a high conducting
max
state to a low conducting state (ON to OFF state) aer reaching
the threshold voltage (Fig. 8[A]).
The results of the ON state and OFF state resistances (U)
calculated from the I–V curve are listed in Table 3. Adjusting the
measurement and all other protocols we have also investigated
the threshold switching from the observed bipolar resistive
switching of the same device. In case of both the forward and
the reverse bias, the device shows threshold switching at xed
compliance current (5 mA). For threshold switching having the
same conguration of the device (Au/3a/ITO) with scanning
range 0 ꢃ 5 to 0 V (Fig. 8[D]) showed a similar type of threshold
switching by lowering the compliance current up to 2 mA. It was
observed that there is no effect on the threshold switching if the
sweep voltage range is more than 3.1 V. It is because the current
increases besides the compliance level, which has no effect by
increasing the sweep voltage and the device do not get short-
The similar bipolar switch could be formed independently by
a maximum tuning voltage up to 3 V. It was found that the
device sweeps to its ON state during tuning the voltage from
+
ꢁ
Vmax to ꢁV
when the voltage exceeds 3 V, and during tuning
it does not return back to its low conducting
max
Vmax to +V
max
state (Fig. 8[B]). Similarly, it was also observed that, when the
voltage exceeds 3 V, then driving the device to its high con-
ducting state (scanning from ꢁVmax to +Vmax) does not return
back to its low conducting state (Fig. 8[C]) during scanning from
+
V
max to ꢁVmax. It was noticed that by tuning, the compliance
current transition from memory switching to threshold
switching becomes irreversible. The bipolar switching cannot
be recovered if the threshold switching is completely formed.
Table 3 Different parameters of I–V characteristics for Au/3a–e/ITO devices
Bipolar switching Threshold switching
Maximum allowed Shorted
Resistances at
Maximum
Minimum
Device
Threshold
scanning
scanning
Threshold voltage compliance
compliance
OFF state ON
conguration voltage (V)
voltage (V)
voltage (V)
(V)
current (mA) current (mA) (U)
state (U)
4
4
4
4
4
1
1
1
1
1
Au/(3a)/ITO
Au/(3b)/ITO
Au/(3c)/ITO
Au/(3d)/ITO
Au/(3e)/ITO
1.5
2
2.3
2.5
2.7
2.5
3
3.5
3.8
4
3.1
3.5
4
4.2
4.5
1.5
2
2.3
2.5
2.7
5
5
5
5
5
2
2
2
2
2
2.7 ꢂ 10
3.3 ꢂ 10
4.3 ꢂ 10
5.8 ꢂ 10
6.7 ꢂ 10
43.00 ꢂ 10
51.30 ꢂ 10
79.20 ꢂ 10
101.7 ꢂ 10
104.4 ꢂ 10
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Fig. 10 I–V characteristics for bipolar resistive switching (A, B and C)
and threshold switching (D) for compound 3c of the Au/3c/ITO device Fig. 11 I–V characteristics for bipolar resistive switching (A, B and C)
based on spin-cast films for two sweep direction. Initial sweep volt- and threshold switching (D) for compound 3d of the Au/3d/ITO device
ages were (A) ꢃ3.5 V, (B and C) ꢃ4 V at 20 mA compliance current and based on spin-cast films for two sweep direction. Initial sweep volt-
(
D) ꢃ5 V at 5 mA compliance current. Arrows indicate the sweep ages were (A) ꢃ3.8 V, (B and C) ꢃ4.2 V at 20 mA compliance current
direction of the applied voltage.
and (D) ꢃ5 V at 5 mA compliance current. Arrows indicate the sweep
direction of the applied voltage.
2
circuited. In all the devices, the active junction area was 1 mm
injection to the organic layer is favourable. Therefore, depend-
ing on the bias, the conjugation in the backbone of the mole-
cule is presumably extended and this results in a high-
conducting state. Moreover, due to the presence of the
for I–V measurement. Similar trends were also observed for
other devices Au/3b/ITO, Au/3c/ITO, Au/3d/ITO and Au/3e/ITO
(Fig. 9–12). The results of all the devices are listed in Table 3.
During the I–V measurement, a gold tip, soly touching the
–
COOH group, organizational behaviours of 3a–e are consid-
erably different from those of 2a–e, as the hydrogen bond
donor–acceptor phenomenon makes highly organized
surface of the prepared thin lms (spin-coated), was used as the
2
top electrode for an active junction area of 1 mm for
a
measurement. However, we also measured the I–V characteris-
tics for different devices by varying the active junction area, but
no signicant variation is observed.
assembly for 3a–e in the lm, which is also reected in the SEM
images. However, it cannot be ruled out that the coumarin
molecule may undergo ring-opening due to the presence of
water molecules around the device followed by oxidation of
From the study of the current–voltage measurement, it was
found that the synthesised coumarin derivatives 2a–e show
threshold switching, but compounds 3a–e show threshold as
well as bipolar resistive switching. The observed bipolar
switching behaviour may be explained due to polarizable
carboxylic acid groups (–COOH) and electron-donating long
alkoxy groups (–OC H , –OC H , –OC H , –OC H ,
1
8
37
16 33
14 29
12 25
–
OC10H21) at position 3 of the coumarin ring. When the devices
of coumarin derivatives 3a–e were scanned from a positive
voltage to a negative voltage, it shows a very low OFF state
leakage current. It may be due to the attractions between the
presence of p-electron clouds in the cyclic ring of the coumarin
core and the electron acceptor carboxylic acid group. The
leakage of current may be restored by giving the electrons in the
system by an electron-donating alkoxy group. Due to the pres-
ence of delocalized p-electrons in the entire molecules, it can be
responsible to bear a conduction pathway to result in a high-
conducting ON state. Moreover, when extra electrons were
added through an electron-withdrawing alkyl group, it can
oxidise the compound and the device returns to its OFF state
Fig. 12 I–V characteristics for bipolar resistive switching (A, B and C)
and threshold switching (D) for compound 3e of the Au/3e/ITO device
during scanning from ꢁV
to +Vmax. In case of threshold
max
switching, the difference in work functions between the metals based on spin-cast films for two sweep directions. Initial sweep volt-
ages were (A) ꢃ 4 V, (B, C) ꢃ 4.5 V at 20 mA compliance current and (D)
ꢃ 5 V at 5 mA compliance current. Arrows indicate the sweep direction
of the applied voltage.
used as electrodes and the resulting built-in internal eld
determine the bias direction as well as at which the electron
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2021 The Author(s). Published by the Royal Society of Chemistry
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47
phenolic compounds so generated to quinone. Therefore, as
the biases applied are very large, the possibility of such
Acknowledgements
quinone–hydroquinone oxidation–reduction process pathways The authors are grateful to Department of Science and Tech-
could be another acceptable mechanism in the present studies. nology (DST), Govt. of India for providing 400 MHz NMR facility
The carrier conduction involves both injection and trans- under DST-FIST programme (No SR/FST/CSI-263/2015) and
port. Hence, electro-reduction of 2a–e and 3a–e compounds Central Instrumentation Centre (CIC) Tripura University for
occurred and the device switches to its high state at a forward Instrumental facilities. We also thank to honourable referees
bias. I–V characteristic for bipolar resistive switching and for their critical comments.
threshold switching for two sweep direction of the spin coated

lm based Au/2a–e/ITO and Au/3a–e/ITO devices was prepared
by spin coating technique. Spin-coated lms are quite thick and
they have good control in assembling of molecules onto thin
Notes and references

lms. It was observed that the device get short circuited when
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