Photophysical investigation of 3-substituted 4-alkyl and/or 7-acetoxy coumarin derivatives-A study of the effect of substituents on fluorescence

Article abstract of DOI:10.1016/j.saa.2010.02.027

In the present work an array of novel substituted 2H-chromen-2-one (coumarin) derivatives has been subjected to photophysical analysis. Though the influence of the electron-donating groups such as amino, substituted amino, hydroxyl, alkoxy groups, etc. at position 7 of the coumarin ring system has been extensively studied, the luminescent properties of the coumarin moieties with an acetoxy substituent have not been explored. Herein it is attempted to study the variation of fluorescence behavior of substituted coumarin derivatives with change of nature and position of the substituents on the 2H-chromen-2-one skeleton. Effect of a methyl substituent at position 4 which imposes abnormal photophysical behavior to the chromenone unit has also been briefly described.

Full text of DOI:10.1016/j.saa.2010.02.027

Spectrochimica Acta Part A 75 (2010) 1610–1616
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Photophysical investigation of 3-substituted 4-alkyl and/or 7-acetoxy coumarin
derivatives—A study of the effect of substituents on fluorescence
Choorikkat Ranjith , K.K. Vijayan , Vakayil K. Praveena,b, N.S. Saleesh Kumara,b
a,∗
a
a
Department of Chemistry, University of Calicut, Calicut University (PO), Tenjipalam Malappuram, Calicut, Kerala 673635, India
Photosciences and Photonics Group, Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science and Technology (NIIST), Kerala 695019, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
In the present work an array of novel substituted 2H-chromen-2-one (coumarin) derivatives has been
subjected to photophysical analysis. Though the influence of the electron-donating groups such as amino,
substituted amino, hydroxyl, alkoxy groups, etc. at position 7 of the coumarin ring system has been exten-
sively studied, the luminescent properties of the coumarin moieties with an acetoxy substituent have
not been explored. Herein it is attempted to study the variation of fluorescence behavior of substituted
coumarin derivatives with change of nature and position of the substituents on the 2H-chromen-2-one
skeleton. Effect of a methyl substituent at position 4 which imposes abnormal photophysical behavior to
the chromenone unit has also been briefly described.
Received 5 November 2009
Received in revised form 3 February 2010
Accepted 16 February 2010
Keywords:
Acetoxy coumarin
Photophysical
Methyl coumarin
Fluorescent lifetime
Stokes shift
©
2010 Elsevier B.V. All rights reserved.
Quantum yield
1
. Introduction
probes. Substituted 7-aminocoumarins also form an important
class of laser dyes for the blue-green region. The photophysical
properties of these compounds depend on the nature and posi-
tion of a substituent group in the parent molecule and also vary
with surrounding media. Coumarins are used as non-linear opti-
cal chromophores and as an excellent probe to study solvation
dynamics in homogeneous solutions as well as organized media
[2–8]. In the recent past, numerous coumarin heteroderivatives
have been synthesized and the possibility of their applications as
laser dyes, organic scintillators and triplet sensitizers have been
explored [9–13]. In a series of earlier works the effect of sol-
vents, substituents and temperature on the various photophysical
properties of coumarin compounds have been reported [14–19].
It is found that the nature of solvents and substituents bring
about changes in the values of fluorescence wavelength max-
ima, quantum yield, lifetime, polarization and excited-state dipole
moment of the coumarins [18]. A systematic study of fluores-
cence quenching of 4- and 7-substituted coumarins by halide
ions in aqueous media has also been conducted previously [3,19].
Recently, photochromic and redox properties of a series of 2H-
pyrano[3,2-c]chromen-5-one derivatives were investigated by the
UV/vis absorption spectroscopy [20].
During the last few years there has been a remarkable growth
in the use of fluorescence in biological sciences especially in bio-
chemistry and biophysics. Fluorescence also finds application in
environmental monitoring, clinical chemistry, DNA sequencing and
genetic analysis by fluorescence in situ hybridization (FISH). In
molecular biology, fluorescence is used for cell identification and
sorting in flow cytometry, and in cellular imaging to reveal the
localization and movement of intracellular substances by means
of fluorescence microscopy. Because of the high sensitivity of flu-
orescence detection, there is continuing development of medical
tests based on the phenomenon of fluorescence. These tests include
the widely used enzyme linked immunoassays (ELISA) and fluores-
cence polarization immunoassays.
Coumarin derivatives are the subject of photophysical stud-
ies during the last few decades as they are highly fluorescent
molecules. The nature and position of the substituents on coumarin
ring has profound importance in deciding the photophysical behav-
ior of the substituted coumarin compounds. Coumarins substituted
at position 7 with an electron-donating group are known to exhibit
strong fluorescence [1]. As 7-aminocoumarins are highly fluores-
cent, they have been used as optical brighteners and fluorescent
The influence of the alkoxy substituent at position 7 and alkyl
group at position 4 of the coumarins has been investigated by
Diehl et al. [15]. The spectroscopic properties of 7-dialkylamino
and 3-styryl substituted coumarins have been studied by Raju and
Varadarajan [16]. The solvent effect on the absorption and fluores-
cent spectra of some 6-alkylamino-7-alkyl coumarin derivatives
∗ Corresponding author. Tel.: +91 9160009322.
E-mail addresses: ranjunbr@yahoo.co.in (C. Ranjith), prof.kkvijayan@yahoo.co.in
(
K.K. Vijayan).
1
386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.02.027

C. Ranjith et al. / Spectrochimica Acta Part A 75 (2010) 1610–1616
1611
Fig. 1. Substituted coumarin derivatives under study.
have been reported recently [17]. The spectroscopic analyses of
coumarins with bulky groups such as phenyl, phenylthio, ben-
zylthio, etc. substituted at position 3 were reported in solvents
of different viscosity and polymer matrices [2]. The depen-
dence of various photophysical parameters on concentration [18],
polarity and viscosity of the solvents and effect of various sub-
stituents on some biscoumarins [21], cyclopentacoumarins [11],
aminocoumarins [22], etc. have been reported in the subsequent
years. Recent reports by Zhao et al., on coumarin chromophores
demonstrated with the support of theoretical and experimental
studies that the hydrogen-bonding dynamics can strongly facilitate
the ultrafast radiationless deactivation process, such as internal
conversion and intermolecular photoinduced electron transfer in
fluorophores [23–25]. In a different study, Liu and co-workers
investigated theoretically the electronic excited-state hydrogen-
bonding dynamics of photoexcited coumarin chromophore in
hydrogen-donating aniline solvent [26].
a modified microwave assisted reacting system, MARS 1505 (CEM
corporation, USA).
2.2. General synthetic procedure for coumarin derivatives [31]
Substituted 2-hydroxybenzaldehydes/2-hydroxyacetophenone
(10 mmol), substituted acetic acid/derivatives (10 mmol), triethy-
lamine (0.7 ml, 5 mmol) and acetic anhydride (5 ml) are taken in
a 50 ml stoppered flask, mixed well and irradiated under 120 W
microwave for 5 min intermittently. The reaction mixture is poured
into ice-cold water, stirred for half an hour at low temperature
◦
(0–10 C), the separated solid is washed repeatedly with dilute
NaHCO3 solution and distilled water, dried and re-crystallized to
more than 90% purity from ethanol. The final purification is done
with chromatography using a 100–200 mesh silica gel column with
light petroleum-ethyl acetate as the eluent by gradient elution
technique. The yield is 65–88%.
In the present work an array of substituted 2H-chromen-2-one
derivatives has been subjected to photophysical analysis. Although
the influence of the electron-donating groups such as amino, sub-
stituted amino, hydroxy, alkoxy groups, etc. at position 7 of the
coumarin ring system has been extensively studied, the lumi-
nescent properties of the coumarin moieties with an acetoxy
substituent have not been explored. Hence our interest was to
exploit the luminescence of these coumarins to find application
in industries and biological field. Nowadays coumarins are used
in dye-laser techniques [27,28], transcription assays [29], intrin-
sic probe for labeling peptides [30], etc. To know the applicability
based on fluorescence of any molecule, the knowledge of pho-
tophysical parameters and the factors governing it are essential.
Therefore the aim of the present study is to determine the basic
spectroscopic and photophysical data for the compounds given in
Fig. 1 and analyze the variation of their properties in the presence
and absence of certain functional groups.
2.3. Electronic spectral measurements
Electronic absorption spectra were recorded on a Shimadzu UV-
3101 PC UV/VIS/NIR scanning spectrophotometer or SYSTRONIC
double beam UV/VIS spectrophotometer. The emission spectra
were recorded on a SPEX-Fluorolog F112x spectrofluorimeter. The
absorption measurements were carried out using 1 mm cuvette
and fluorescence measurements were carried out using 1 cm × 1 cm
cuvette. The fluorescence quantum yields of all the coumarin
derivatives in DMSO were estimated by comparison with quinine
sulfate in 0.1 M sulfuric acid (˚ = 0.546) as the standard reference
F
[32].
2.4. Fluorescence lifetime measurements
Fluorescence lifetimes were measured using IBH (FluoroCube)
time-correlated picoseconds single-photon counting (TCSPC) sys-
tem. Solutions were excited with a pulsed diode laser (<100 ps
pulse duration) at a wavelength of 375 nm (NanoLED-11) with a
repetition rate of 1 MHz. The detection system consisted of a micro-
channel plate photomultiplier (5000U-09B, Hamamatsu) with a
2
. Materials and methods
2.1. General
3
8.6 ps response time coupled to a monochromator (5000 M) and
All the chemicals used are of synthetic grade and purified before
TCSPC electronics (Data station Hub including Hub-NL, NanoLED
controller and preinstalled fluorescence measurement and analy-
sis studio (FMAS) software). The fluorescence lifetime values were
obtained using DAS6 decay analysis software. The quality of the fit
has been judged by the fitting parameters such as ꢀ2 (<1.3) as well
as the visual inspection of the residuals.
use. The melting points are determined using a GUNF melting point
apparatus and are uncorrected. IR spectra are measured on a JASCO
FT/IR-4100 spectrophotometer by KBr pellet method, NMR spectra
recorded on a BRUKER AVANCE DPX 300 MHz spectrometer using
TMS as internal standard and mass spectra on a SHIMADZU GC-MS-
QP 2010 mass spectrometer. Microwave irradiation is done using

1
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C. Ranjith et al. / Spectrochimica Acta Part A 75 (2010) 1610–1616
Table 1
Photophysical parameters of coumarin derivatives.
−
−1
−1
kR (107
−1
kNR (107
s )
−1
Entries
C11
ꢁA (nm)
εA (M 1 cm
)
ꢁF (nm)
Stokes Shift ꢂS (cm
)
˚F
ꢃav (ns)
s
)
304
11,601
404
421
6922
0.060
0.95
1.36
6.32
4.41
98.94
95.59
326
12,431
1573
C12
C13
358
326
435
4944
5984
0.006
0.153
3.74
0.16
99.83
14,938
13,300
405
423
0.50
0.41
30.41
37.05
168.39
205.08
304
C15
300
9800
6200
435
6862
0.003
3.66
0.08
27.24
345
C18
C21
343
285
19,657
435
406
6166
7115
0.002
0.003
2.48
3.09
0.08
0.11
40.32
32.25
11,600
9980
317
C23
C31
286
14,600
12,500
403
431
6932
8344
0.007
0.006
0.98
2.56
0.72
0.23
101.85
38.83
319
288
14,136
11,900
317
C41
C42
328
298
13,400
428
408
7124
9048
0.250
0.024
2.22
0.80
11.26
3.00
33.78
7700
6800
122.00
329
C45
301
8600
8400
466
11,764
0.040
3.25
1.23
29.53
339
3
. Results and discussions
The optical properties are investigated in detail by measur-
ing the UV/vis absorption (as expressed by the molar absorption
coefficient, ε ), steady state and time resolved fluorescence. The
A
photophysical parameters like extinction coefficient (ε ), quan-
A
tum yield (˚F), lifetime values (ꢃF) radiative (kR) and non-radiative
(
kNR) decay constants form a basic set of data characterizing a
luminescent molecule. An analysis on the correlation between the
optical properties and structural characteristics of newly synthe-
sized coumarin derivatives reveals some interesting results.
3.1. Absorption and emission properties of coumarin derivatives
All the measurements have been made in the polar solvent,
dimethyl sulfoxide (DMSO), as it is the only solvent in which all
the compounds are soluble. The absorption maxima (ꢁ ), and the
A
molar extinction coefficients (ε ) and absorption spectra are shown
A
in Table 1 and Fig. 2 respectively.
It is known that the spectral shifts result from the general
effect of solvent polarity whereby the energy of the excited-state
decreases with increasing solvent polarity. However, spectral shifts
also occur due to specific fluorophore–solvent interactions caused
by charge separation in the excited state. Hence emission from flu-
orophores generally occurs at wavelengths, which are longer than
those at which absorption takes place. This loss of energy is also
due to various dynamic processes, which occurs following the light
absorption [32].
Coumarins are polar in nature and hence display large sen-
sitivity to the solvent polarity. Solvent polarity and the local
environment have profound effects on the emission spectra of
polar fluorophores. Previous studies demonstrated that substituted
coumarin molecules exhibit solvatochromic effect in different sol-
vents with varying polarity due to internal charge transfer (ICT)
[
16,17]. However, in the current investigation the solvatochromic
effect of all the coumarin derivatives in different solvents could not
be studied due to the solubility problem. The emission maxima (ꢁF)
and emission spectra are shown in Table 1 and Fig. 3 respectively.
The Stokes shift (ꢂ ) calculated for all the coumarin derivatives
are given in Table 1. The Stokes shifts are expressed in wave number
S
Fig. 2. Absorption spectra of coumarin derivatives.

C. Ranjith et al. / Spectrochimica Acta Part A 75 (2010) 1610–1616
1613
the solvent molecules to reorient around the excited-state dipole,
which lowers its energy and shifts the emission to longer wave-
lengths. This process of solvent relaxation results in substantial
Stokes shift.
3.2. Quantum yield
Quantum yield of a molecule is a direct measure of its lumi-
nescent property. If the absolute fluorescence efficiency of one
substance (reference ˚r) is known, then that of the other (sam-
ple ˚s) can simply be calculated. Quinine sulfate in dilute H SO4
2
(0.1 M) has been used as a standard substance for quantum yield
measurements. The quantum yield of a solution of quinine sul-
fate in 0.1 M H SO4 is known to be 0.546 and hence the quantum
2
yield values of the coumarin derivatives are calculated knowing the
other parameters [32]. The experiments were done using optically
matching solutions and the quantum yield is calculated using the
following equation:
ꢂ
ꢃ
ꢀ
ꢁ
ArFs
AsFr
Ás2
Ár2
˚s = ˚r
where Ar and As are the absorbance of the ‘reference standard’ and
‘sample’ respectively at the excitation wavelength, Fr and Fs are the
relative integrated fluorescent intensities (area under the fluores-
cence curve, peak area) of the reference and samples respectively.
Ár and Ás are respectively the refractive indices of the solvents in
which the reference standard and samples are prepared.
A general trend of decrease in quantum yield is observed for
all these compounds in DMSO (Table 1). This is presumably due
to the solvent relaxation resulting from the excited-state internal
charge transfer (ICT) of fluorophores. The large Stokes shift of these
fluorophores also point towards an ICT emission in highly polar
solvents. In addition to specific solvent–fluorophore interactions
and ICT state formation, the fluorophores can also form a twisted
internal charge-transfer (TICT) state.
A keen examination of the quantum yields also reveals some
interesting results regarding the influence of substituents on the
pyrone ring as well as the aromatic ring. The presence of electron-
withdrawing groups (–CN, –COCH , p-NO -phenyl) at position 3 of
3
2
the benzopyrone ring is found to decrease the fluorescence quan-
tum efficiency whereas electron-donating functionalities (phenyl,
p-Cl-phenyl) cause an increase in quantum yield of the molecules.
Another observation is that an acetoxy group at position 7 of the
coumarin ring increases the fluorescence of the coumarins. These
results are in conjunction with those reported literatures [21,22].
3.3. Abnormal photophysical behavior of coumarin derivatives
Fig. 3. Emission spectra of coumarin derivatives.
with a methyl substituent at position 4
On a perusal of the photophysical values of the coumarin deriva-
tives under investigation (Table 1), the noteworthy observations
that could be made is that, when an alkyl group (methyl) is present
at position 4 of the benzopyrone ring, the fluorescence of the com-
pound is diminished to a great extent, which is different from
the earlier reports [15]. This is discussed in detail in view of the
photophysical properties of the representative compounds C41
and are calculated from the experimental parameters by taking the
difference between absorption and emission maxima expressed in
wave number using the equation [32]:
7
−1
,
Stokes shift, ꢂS = ( ꢂ¯ A − ꢂ¯ F) × 10 cm
where
1
(˚F = 0.25) and C31 (˚F = 0.006) (Table 1).
ꢂ¯ A = ꢁ (nm)
A
Compounds C41 and C31 differ only by a methyl group at posi-
tion 4 (Fig. 1). But the quantum yield of C31 is found to be 40 times
lesser than that of C41. One of the possible explanations for the
discrepancy could be that, the electron-donating ability of methyl
group on the benzopyrone ring of the compound C31 can lead to an
enhanced donor–acceptor capability compared to C41, which even-
tually helps the molecule to form a better internal charge transfer
state (ICT) upon excitation.
and
1
ꢂ¯ F =
(nm)
ꢁ
F
The moderately high Stokes shift observed for all coumarin
derivatives are ascribed partly to the polarity of the solvent, DMSO.
The relatively long timescale of fluorescence allows ample time for

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C. Ranjith et al. / Spectrochimica Acta Part A 75 (2010) 1610–1616
Fig. 4. Electron displacement in 7-hydroxy/7-acetoxy coumarin.
Another reason for a reduced fluorescence in the presence of a
Fig. 5. Electron displacement in 7-hydroxy/7-acetoxy coumarin in the presence of
a methyl group at position 4.
methyl group at position 4 could be suggested on the basis of the
following electron displacement mechanism that can happen in the
coumarin structure. Generally in 7-hydroxy/7-acetoxy coumarin,
the electron displacement that will results the polarity at carbonyl
carbon can be represented as in Fig. 4.
Similarly the electron interaction in the presence of a methyl
group at position 4 of the coumarin ring is as shown in Fig. 5.
In the case of 7-acetoxy-4-methyl coumarin derivatives the
decreased fluorescence observed could be attributed in part to the
competitive electronic interactions of the two substituents with the
lactone carbonyl group. Thus the effect of the acetoxy group at posi-
tion 7 is reduced and hence the fluorescence. In addition to these,
the methyl group at position 4 of the coumarin may have a major
impact on the conformation of phenyl ring at position 3. During
photo excitation, because of the steric effect of the methyl sub-
stituent, a geometric twist can occur by the rotation of phenyl ring
through C–C bond at position 3, which can make the molecule as
whole nonplanar. This twisted state may disrupt the ␲-conjugated
bridge that connects the donor with the acceptor and can subse-
quently result in a drastic change on their fluorescence quantum
yields presumably via a TICT mechanism. Under this condition, the
excited-state dipole moment of the compound C31 would be high
Fig. 6. Fluorescent lifetime decays of compounds C11, C12, C13, C15, and C18. ꢁEM is the emission wavelength at which fluorescent lifetime is monitored.

C. Ranjith et al. / Spectrochimica Acta Part A 75 (2010) 1610–1616
1615
compared to that of C41 and may result in a higher stabilization of
the excited state of C31 by solvent relaxation. This argument is sup-
ported by the higher Stokes shift and lower radiative decay values
pounds C13 (˚F = 0.153) and C23 (˚F = 0.007) (Fig. 1; Table 1).
Compounds C11 (˚F = 0.06) and C21 (˚F = 0.003) are considered
as less emissive species in DMSO on the basis of the quantum yield
values. Even in this case also a well defined trend in decrease of fluo-
rescence in the presence of methyl group is apparent. The quantum
yield value of the C21, which is having a methyl group at position
4 is nearly 20 times less than that of the compound C11 that lacks
it (Fig. 1; Table 1).
−1
7
−1
of C31 (ꢂ = 8344 cm , kR = 0.23 × 10 s ) when compared that of
S
−1
7
−1
C41 (ꢂ = 7124 cm , kR = 11.26 × 10 s ).
S
Apart from the above mentioned reasons, on the basis of the
recent literature [25], it can also be proposed that the fluores-
cence quenching of C31 is feasible by an intermolecular electron
transfer from the solvents to the fluorophore, especially when
the donor-acceptor capability of C31 became increased by an
electron-donating methyl group at position 4 of benzopyrone
ring (Fig. 5). Although it could not be established due to the
insolubility of present coumarin derivatives in protic solvents,
there is a possibility of a hydrogen-bonding dynamics in fluores-
cence quenching by the ultrafast radiationless deactivation process
3.4. Fluorescent lifetime studies
The fluorescent decay of all the substituted coumarin derivatives
have been measured and plotted in Figs. 6 and 7.
From these figures, it is obvious that all the compounds follow a
characteristic bi-exponential fluorescent decay, which reveals the
existence of two different emissive states for the molecule, which
could be the locally excited state (LE, Franck-Condon state) and
charge transfer state (CT). The ꢀ2 values, which are known as the
[
23–26].
Similarly, a marginal decrease in the fluorescent property with
the presence of a methyl group is observed in the case of com-
Fig. 7. Fluorescent lifetime decays of compounds C21, C23, C31, C41, C42, and C45. ꢁEM is the emission wavelength at which fluorescent lifetime is monitored.

1
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C. Ranjith et al. / Spectrochimica Acta Part A 75 (2010) 1610–1616
fitting parameter, determine fine fit for the bi-exponential decay
and are found to be <1.3 and the average lifetime (ꢃav) is calculated
using the following equation [32]:
of the acetoxy coumarin compounds in cell imaging and dye-laser
techniques is in progress.
Acknowledgements
2
1
2
2
˛1
ꢃ
+ ˛ ꢃ
2
ꢃav =
˛1
ꢃ + ˛ ꢃ
1
2
2
The authors are thankful to the Department of Science & Tech-
nology (DST) New Delhi, India for Fund for Improvement of Science
& Technology (FIST) and Department of Chemistry, University of
Calicut for providing laboratory facilities. Thanks are also due to
Dr. Suresh Das, Photosciences and Photonics Group, Chemical Sci-
ences and Technology Division for supporting the photophysical
investigation at National Institute of Interdisciplinary Science and
Technology (NIIST), Trivandrum and Prof. N.R. Krishnaswamy, Ban-
galore, Dr. Sankaran Nedumbamana and Dr. Suresh Velate, John F.
Welch Technology Centre, GE Global Research, Bangalore, India for
the valuable inputs to the work.
where ꢃ1 and ꢃ2 are the lifetime values of the two emissive states
and ˛1 and ˛2 are called the pre-exponential factors, which give
the abundance of each emissive state. The average lifetime values
of all coumarin derivatives are summarized in Table 1.
3.5. Radiative and non-radiative decay constants
The fluorescence emission is a random process and occurs by
a unimolecular process. The radiative lifetime of the excited state
may be defined in terms of a first order decay process. The radiative
decay constant (kR) and non-radiative decay constant (kNR) can be
calculated by knowing the quantum yield (˚F) and lifetime values
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. Conclusions
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