Linear Coumarinacenes Beyond Benzo[g]coumarins: Synthesis and Promising Characteristics

Article abstract of DOI:10.1002/ejoc.202001025

New heteroacenes, named coumarinacenes, have been designed, synthesized and characterized. They possess visible absorption and orange-red emission that are far beyond those of benzo[g]coumarins. They exhibit deep HOMO energy levels, and reveal excellent thermal, photochemical and air stabilities as compared to tetracene/pentacene. Their preliminary thin-film devices display encouraging transistor characteristics, moderate mobility and considerable environmental stability paving way into electronics.

Full text of DOI:10.1002/ejoc.202001025

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Linear Coumarinacenes Beyond Benzo[g]coumarins: Synthesis
and Promising Characteristics
Authors: Abhinav Kumar, Anuj Rajpoot, Fiheon Imroze, Sudhakar
Maddala, Soumya Dutta, and Parthasarathy Venkatakrishnan
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202001025
Link to VoR: https://doi.org/10.1002/ejoc.202001025
01/2020

10.1002/ejoc.202001025
European Journal of Organic Chemistry
COMMUNICATION
Linear Coumarinacenes Beyond Benzo[g]coumarins: Synthesis
and Promising Characteristics
Abhinav Kumar,[a] Anuj Rajpoot,[b] Fiheon Imroze,[b] Sudhakar Maddala,[a] Soumya Dutta,*[b] and
Parthasarathy Venkatakrishnan*[a]
[a]
Mr. Abhinav Kumar, Mr. Sudhakar Maddala, Dr. P. Venkatakrishnan
Department of Chemistry
Indian Institute of Technology Madras
Chennai – 600 036, Tamil Nadu, India
E-mail: pvenkat@iitm.ac.in, http://chem.iitm.ac.in/faculty/venkat/
Mr. Anuj Rajpoot, Mr. Fiheon Imroze, Dr. Soumya Dutta
Department of Electrical Engineering
[b]
Indian Institute of Technology Madras
Chennai – 600 036, Tamil Nadu, India
E-mail: s.dutta@iitm.ac.in, http://www.ee.iitm.ac.in/user/s.dutta/
Supporting information for this article is given via a link at the end of the document.
Abstract: New heteroacenes, named coumarinacenes, have been
designed, synthesized and characterized. They possess visible
absorption and orange-red emission that are far beyond those of
benzo[g]coumarins'. They exhibit deep HOMO energy levels and
reveal excellent thermal, photochemical and air stabilities as
compared to tetracene/pentacene. Their preliminary thin-film devices
display encouraging transistor characteristics, moderate mobility and
considerable environmental stability paving way into electronics.
appeared on the helical,8 vertical9 and lateral10 π-expansion of
coumarins/benzocoumarins, and their photophysical properties
have been investigated carefully.
However, linear coumarins, beyond benzo[g]coumarins, have
not been achieved so far and their optical properties remain yet
unexplored. In continuation to our recent efforts on coumarin
and its π-expansion,8c,10,11 to build on its chemistry further we
have herein targeted the synthesis of novel coumarinacenes
(Scheme 1). Coumarinacenes are linearly π-expanded coumarin
analogues. In other words, they are anthracene-fused coumarins
(i.e., anthra[2,3-g]coumarins). We envisaged (i) by linearly
expanding the cyclic π-conjugation (such as anthra-annulation)
of coumarin, the absorption and emission properties as well as
the electronic properties can be drastically improved, (ii) by
introducing appropriate substituent at 3-/4-position of the
coumarin skeleton, its solubility and stability characteristics can
be fine-tuned, and (iii) the acene-annulation of coumarins may
Coumarins are pyranone-annulated benzenes (2H-benzopyran-
2-ones) that are ubiquitous in nature.1 They are actually popular
for their wide spectrum of biological and pharmacological
activities,2a
such
as,
antioxidant,2b
antidepressant,2c
anticoagulant,2d anti-inflammatory,2e antifungal,2f antitumor,2g
anti-HIV,2h antibiotic,2i etc., and for their excellent
biocompatibility.1 Structurally simple derivatives of coumarin
exhibit rich photophysical and photochemical properties (high
photoluminescence quantum yields, large Stokes shifts and
good photostability) paving their entry into modern photonic
applications; laser dyes,3a,b non-linear optics,3c two-photon
bioimaging3d and sensing3e are some examples. Moreover,
coumarin derivatives are recently being exploited as functional
materials in organic light emitting devices,4a,b liquid crystals,4c
dye-sensitized solar cells,4d,e electron and energy transfer
systems,4f,g and fluorescent probes.4h Despite the broad utility of
coumarins, the low-lying absorption and emission wavelengths
(due to shorter conjugation lengths) are primarily the bottlenecks
in furthering their applications.
In an effort to overcome the limitations, considerable attention is
devoted in recent times for the development of novel π-
expanded coumarin dyes.5 Benzocoumarins – the simplest π-
expanded benzene-fused coumarins – indeed absorb and emit
light in the longer wavelength region when compared to parent
coumarins (Scheme 1).6 Among the other isomeric benzo-
annulated coumarins (benzo[c]/[f]/[h]coumarins), the linearly
extended
benzo[g]coumarins
demonstrate
superior
photophysical properties (more polarized and large dipole
moments). By appropriately placing the substituents within the
molecular structure of benzo[g]coumarin, one can in principle
achieve a bathochromic shift further down (due to ICT) in the
absorption and emission spectra, allowing them for bioimaging
applications.7 Inspired by this result, several reports have
Scheme 1. Structures of linear coumarinacenes reported earlier and in this
report (top), and similar structures of acenes and modified acenes (bottom)
known.
1
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10.1002/ejoc.202001025
European Journal of Organic Chemistry
COMMUNICATION
bring in attractive charge transport characteristics (heretofore
unrevealed) to the coumarin family, as acenes (Scheme 1) are
valuable candidates in electronics.12
With this idea in mind, the synthesis, photophysical,
electrochemical and thermal characterization, and evaluation of
the charge transport properties via fabrication of TFT devices of
coumarinacenes, that are unprecedented till date, are targeted
in this paper.
The coumarinacenes 1-2 were synthesized, utilizing Diels-Alder
cycloaddition and Meerwein-Ponndorf-Verley-type reduction
reactions as shown in Scheme 2.13 Initially, the precursors 6,7-
dimethylcoumarin 1a and 4,6,7-trimethylcoumarin 2a were
prepared by a Pechmann condensation reaction from the
corresponding phenols.14 These coumarins were independently
subjected to benzylic bromination under the optimized conditions
CH3CN).5-7 In other words, the absorption of coumarinacenes 1
and 2 was found to fall between the absorption of tetracene
(300–480 nm) and pentacene (500–580 nm).18 Noteworthy is,
the pyranone-annulation of tetracene does contribute to the red-
shift (ca. 80–100 nm) in the absorption. A set of analogous
helical pyranones exhibited absorption maximum in the range
(360–415 nm).8 It highlights the importance of linear-fusion as
well as planarity, as in 1 and 2, in improving the absorption
characteristics of coumarins. Interestingly, the absorption
maximum of 1-2 is not influenced much by the solvent polarity
(Figures 1 and S1), which suggests their weak dipolar character,
opt
similar to other modified acenes.19 Optical energy gap (Eg
)
values of 1-2 determined from the onset of the absorption
spectrum (solution) were in the range 2.11‒2.16 eV. The energy
gap values are much smaller than or comparable to that of
tetracene (2.54 eV)20 or pentacene (2.15 eV)21 or thienoacene
(1.96 eV)13a reported earlier, signifying the effect of extensive π-
conjugation via pyranone-annulation in these systems.
to afford
6,7-bis(bromomethyl)coumarins 1b and 2b
exclusively.15 Further, the brominated products were exposed to
the Diels•Alder reaction with 1,4-napthoquinone to obtain the
corresponding quinones 1c and 2c.16 These acenequinones
were then subjected to aromatization and deoxygenation
reactions to provide the desired coumarinacenes (1 and 2) as
dark-red to maroon colour solids in moderate yields (up to 62%)
after recrystallization.
0.3
1
2
0.2
2
0.1
0.0
400
600
800
Wavelength (nm)
Figure 1. UV-vis absorption (bold lines) and photoluminescence (dotted lines)
spectrum of coumarinacenes 1 and 2 in chloroform. Inset photographs refer to
solutions of 2 under room (left) and UV (right) light.
Further, the photoluminescence spectrum of 1 and 2 recorded in
chloroform solutions (ca. 10-5 M, Figure 1) revealed a strong
emission in the orange-to-red region (475–750 nm) with vibronic
features similar to acenes.18b,21 Such red emission in
coumarinacenes is exciting, as they may allow deeper tissue
penetration and be suitable candidates for bioimaging
applications. For a comparison, their shorter linear analogue,
benzo[g]coumarin and non-planar helical coumarins exhibited
blue emission at 459 nm and 452-486 nm, respectively.6-8
Emission spectrum of 1 and 2 recorded in solvents of varying
polarity pointed no obvious change in the emission maximum
(Figure S1) indicating a minimal or no drastic change of their
dipole moment in the excited state. As coumarins are brilliant
fluorophores, the photoluminescence quantum yields (Φf) of the
Scheme 2. Synthesis of coumarinacenes. i) NBS (3.0 equiv.), AIBN (0.1
equiv.), chlorobenzene, hν, 90 oC, 18-21 h, 80-85%; ii) 1,4-naphthoquinone,
NaI, DMF, 120 °C, 40-45 h, 56-61%; iii) Al (30.0 equiv.), HgCl2 (0.06 equiv.),
CBr4 (0.24 equiv.), cyclohexanol, reflux at 160 °C, 6-18 h, 60-62%.
Acenes usually suffer from solubility issues. The synthesized
coumarinacenes 1 and 2 have better solubility (ca. 2 mg/mL in
chloroform at RT, ca. 1.5 mg/mL in toluene at RT and ca. 3.0
mg/mL in o-DCB at 130 °C) when compared to pentacene (0.21
mg/mL in CHCl3 at RT) or tetracene (1.0 mg/mL in toluene at
RT) or thienoacene (<1.4 mg/mL in o-DCB at 130 oC).17
Incorporating substituents at the end-pyranone unit (3rd/4th
position) indeed improves the solubility.
The role of anthra-fusion on to coumarins was contemplated by
evaluating their optical properties. The UV-vis absorption
spectrum of chloroform solutions (ca. 10-4 M, Figure 1) of 1 and
2 revealed absorption in the visible region (ca. 380–580 nm, π-
π*) with a typical vibronic pattern as that of acenes.18 This
characteristic is suggestive of a planar, structurally-rigid nature
and an extended π-conjugation provided by the electron-
new anthra-fused red-emissive coumarins
1 and 2 were
estimated, and they were 24 and 19%, respectively. Gratifyingly,
these values are found to be significantly higher than that of
simple benzo[g]coumarin, helical coumarins, tetracene and
pentacene.6,8,18b,22
Tetracene and pentacene are highly photoreactive.23 They
undergo photochemical [4π + 4π] dimerization at the central
carbons and form endoperoxide with atmospheric oxygen.23
Moreover, simple coumarins are known to undergo [2π + 2π]
photodimerization at the pyranone double bond in the solution
state whereas they are relatively photostable in solid.24 To
assess the photostability of acene incorporated coumarins
withdrawing
conjugated
carbonyl
functionality
in
coumarinacenes. A remarkable red-shift (ca. 180 nm) in the
absorption maximum, due to anthra-fusion, is discernible on
comparing them with simple benzo[g]coumarin (λabs = 321 nm,
2
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10.1002/ejoc.202001025
European Journal of Organic Chemistry
COMMUNICATION
(coumarinacenes), at first, the CHCl3 solutions of 1 and 2 were
exposed to room light. The changes in the absorption spectrum
were monitored with time (Figures 2 and S2). The absorbance in
the range 450–575 nm was found to decrease with an increase
in the absorbance below 300 nm. Notably, the absorption
intensity dropped down to ca. 50% only around 4 h. After 22 h,
the colour of the solution turned pale yellow and the absorption
in the region 450–575 nm was almost negligible. On the contrary,
when the solids of 2 were exposed to room light, they revealed
exciting photostability up to 3 months (1H NMR spectrum, Figure
S3), as noted with simple coumarin. The above studies evinced
an encouraging photostability of coumarinacenes.21,25,26 Next,
LUMO
-2.57 eV
+1.40
+1.05
+0.51
2.49 eV
-5.06 eV
0.0
0.5
1.0
+ 1.5
HOMO
Potential (V vs Ag/Ag )
Figure 3. Left: Cyclic voltammograms of coumarinacene 2 in presence of
ferrocene in DCM. Right: DFT calculated HOMO (below) and LUMO (above)
pictures and energy levels of 2.
the solids of
2
stored in dark exhibited extraordinary
1
photostability up to 9 months (by H NMR spectrum, Figure S3).
Thus, it is evident that the attempted modification, i.e., linear π-
expansion, does not significantly alter the photostability of
coumarin.
Table 1. Optical and electrochemical data of coumarinacenes 1 and 2.
Entry
λmax, abs[a]
nm
λmax, em[a]
nm / ϕf[b]
λonset
,
[a] nm
HOMO[c]
eV
LUMO[c]
eV
Egopt / Eg[c], eV
1
2
399, 424, 500
397, 422, 502
518, 560,
606 / 0.24
588, 2.11 /
2.15
-5.08
-5.34
-2.93
-3.01
0 min
5 min
10 min
15 min
20 min
30 min
40 min
50 min
300 C
1
2
100
518, 570,
609 / 0.19
574, 2.16 /
2.33
80
60
1 h
3 h
[a] in CHCl3. [b] determined using Rhodamine 6G in ethanol as the standard
(Φf = 0.95), λex = 500 nm. [c] calculated from CV using EHOMO = [Eox – E1/2(Fc))
+ 4.8] eV, ELUMO = [Ered – E1/2(Fc)) + 4.8] eV and Eg = [EHOMO – ELUMO] eV.
3.30 h
4 h
22 h
40
400
500
600
200
400
Temperature, C
600
800
Wavelength (nm)
Acenes, such as, tetracenes and pentacenes are excellent
organic semiconductors (OS)12 that are shown to exhibit high
charge carrier mobilities29 in organic thin-film transistor (OTFT)
devices, despite their high photoreactivity. To put such reactive
molecules into use, several structural modifications have been
attempted by Bao,13a,30a Anthony,30b,c Swager,20 Wudl,30d and
others.30e-j As coumarinacenes are end-β-pyrone annulated
tetracenes, the charge transporting ability of 2 and its potential in
electronics were examined by fabricating thin-film transistor
(TFT) devices, consisting of interdigitated bottom-gate bottom-
contact architecture. The schematics of the same is shown in
Figure 4a. To the best of our knowledge, this is the first-ever
TFT device fabricated employing a coumarin skeleton as organic
semiconductor. Our initial OTFT characteristic of 2 on SiO2 gate
dielectric showed an obvious p-channel accumulation mode
behaviour. The transfer characteristics (Figures S5 (a) and (b))
of the device measured in the saturation mode at VD = -80 V
exhibited a good ON/OFF ratio (~ 103). Encouraged by this initial
result, we then studied the effect of substrate heating on the
channel layer morphology and OTFT device parameters, as the
Figure 2. Left: UV-vis absorption spectral changes of CHCl3 solution of 2 upon
exposure to room light. Right: TGA traces of 1 and 2.
Thermal stability studies by thermogravimetry (TGA, Figure 2)
revealed that coumarinacenes 1 and 2 possessed excellent
thermal stability, i.e., >310 C. The decomposition temperature
(Td) of 1 and 2 at 5% weight loss were found to be 320 C and
318 C, respectively. These Td values are higher than that of
tetracene (260 C) and thienoacene (300 C), which actually
points to a considerable improvement in their thermal stabilities
via ring-annulation.17a,27
In order to unfurl the electronic characteristics, the HOMO and
LUMO energy levels of coumarinacenes 1-2 were extracted from
cyclic voltammetry (CV) experiments (Figures 3 and S4), and
the data are provided in Table 1. The HOMO energy levels of 1
and 2 calculated from the oxidation potentials (Eox) were -5.08
and -5.34 eV, respectively, which are close to the DFT-
calculated values of 2 (Figure 3). The corresponding LUMO
energy values were derived from their reduction potentials (Ered).
One may notice that the HOMO energy levels of 1-2 are
comparable to or slightly deeper (ca. 0.08–0.24 eV) than that of
tetracene/pentacene (5.26/5.10 eV)20,28 or thienoacene (5.17
eV).13a This indicated an improved air stability in these systems,
which supports our results from photo-/air stability studies. The
LUMO levels are lowered to a greater extent (up to 0.30 eV)
than the HOMOs. It may possibly be due to the annulated
electron-withdrawing conjugated carbonyl functionality. The Eg
from CV matches with the optical as well as the DFT calculated
Egs. The DFT calculated FMO pictures of 2 revealed a perfect
planarity and a uniform distribution of electron density all along
the acene skeleton with some differences in the pyranone
portion.
substrate
heating
is
known
to
improve
the
morphology/crystallinity.31 The channel layer of 2 was now
deposited on the substrates at 70 °C and 90 °C. The devices,
fabricated at substrate temperature 70 °C, demonstrated the
best performance (Figure S6, Table S1). The output and transfer
characteristics of the bottom-gate OTFT of 2 fabricated under
this condition are shown in Figures 4b and 4c. The saturation
mobility (µ) and threshold voltage (Vth) were extracted to be 2.92
× 10-6 cm2/V.s and -22.5 V, respectively. Indeed, substrate
heating during the channel layer deposition influenced the
device performance considerably (2-3 orders). In support of this
trend, AFM images (Figure S7) and out-of-plane XRD results
(Figure S8) revealed large crystalline grains and strong intensity
3
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10.1002/ejoc.202001025
European Journal of Organic Chemistry
COMMUNICATION
infrastructural facilities. A. K., A. R. and F. I. thank IIT Madras for
their HTRA, and S. M. thanks CSIR for a SRF.
Keywords: π-Expansion • Coumarin • Optical • Electrochemical
• Thin-film transistors
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the OTFT of 2 with channel length of 4 µm and W/L = 500 with substrate
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Linear coumarinacenes were synthesized by simple synthetic
methods. They exhibited strong absorption and orange-to-red
emission in the visible region of the electromagnetic spectrum
showing promise for bioimaging applications. They possess
deep HOMO energy levels when compared to their
unfunctionalized linear acene counterparts revealing good
air/light stability. Their thermally evaporated films are crystalline
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Acknowledgements
P. V. K. and S. D. gratefully acknowledge the financial support
provided by the IIT Madras and also the departmental
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Entry for the Table of Contents
Insert text for Table of Contents here. (Linearly π-expanded coumarins beyond benzo[g]coumarins, named coumarinacenes, have
been successfully synthesized for the first time. They reveal exciting visible absorption and bright orange-red emission. They exhibit
high thermal, photochemical and air stabilities. Such linear π-expansion indeed profits coumarin with encouraging charge transport
characteristics and good device stability.)
Institute and/or researcher Twitter usernames: (@iitmadras)
Key Topic: Coumarinacenes
6
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