Photoinduced properties of methyl ester of 2,6-dimethyl-1-amino-4-benzoic acid: Evidence of charge transfer emission from a weak primary amino donor

Article abstract of DOI:10.1016/j.saa.2009.10.033

Photophysical properties of methyl ester of 2,6-dimethyl-1-amino-4-benzoic acid (M26DMB) have been investigated using steady state absorption and emission spectroscopy and time-resolved emission spectroscopy. Interestingly, not only in polar solvents, the molecule M26DMB having a weak primary amino donor group shows broad red-shifted emission band even in non-polar environment which in all probability arises from closely spaced local and charge transfer (CT) states. Clear dual fluorescence in polar protic solvents comprises of less intense local emission band and strong red-shifted CT band. The position of the red-shifted emission band is dependent on both the polarity and the hydrogen-bonding ability of the solvent.

Full text of DOI:10.1016/j.saa.2009.10.033

Spectrochimica Acta Part A 75 (2010) 320–324
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Photoinduced properties of methyl ester of 2,6-dimethyl-1-amino-4-benzoic
acid: Evidence of charge transfer emission from a weak primary amino donor
Shalini Ghosh, Amrita Chakraborty, Samiran Kar, Nikhil Guchhait∗
Department of Chemistry, University of Calcutta, 92, A.P.C. Road, Kolkata 700009, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Photophysical properties of methyl ester of 2,6-dimethyl-1-amino-4-benzoic acid (M26DMB) have been
investigated using steady state absorption and emission spectroscopy and time-resolved emission spec-
troscopy. Interestingly, not only in polar solvents, the molecule M26DMB having a weak primary amino
donor group shows broad red-shifted emission band even in non-polar environment which in all prob-
ability arises from closely spaced local and charge transfer (CT) states. Clear dual fluorescence in polar
protic solvents comprises of less intense local emission band and strong red-shifted CT band. The position
of the red-shifted emission band is dependent on both the polarity and the hydrogen-bonding ability of
the solvent.
Received 12 August 2009
Received in revised form 12 October 2009
Accepted 26 October 2009
Keywords:
2,6-Dimethyl-1-amino-4-benzoic acid
Absorption
Fluorescence
Charge transfer
© 2009 Elsevier B.V. All rights reserved.
Lippert–Mataga equation
1. Introduction
to be out of plane of the chromophore ring and this insists ICT
reaction in the excited state [1]. Zachariasse proposed that with
Dual emission arising from intramolecular charge transfer (ICT)
process has been a much explored photophysical phenomenon in
molecules with suitable donor and acceptor groups [1–7]. Since the
first observation of dual emission in dimethylamino benzonitrile
(DMABN) by Lippert and Ludder [8] this unusual spectral property
has attracted a lot of interest over the years. Various theoretical
models such as twisted intramolecular charge transfer, planarized
intramolecular charge transfer and wagging intramolecular charge
transfer have been suggested to explain this photoinduced ICT pro-
cess in systems with substituted donor–acceptor moieties [9–12].
Systems exhibiting the ICT process have been used extensively
as probes for investigating the local environment of different
microheterogeneous media like proteins, micelles, cyclodextrin
nano-cavities, as optical switches and as fluorescent sensors of
metal ions [1,13–19]. These systems find wide applications even
today and further research on their applicability continues with
considerable interest.
By far and large, it has been found that tertiary amine group is
the most common donor in the ICT systems reported and photoin-
duced CT process occurs in the polar solvents due to stabilization
of the CT state in polar medium [1]. Charge transfer reaction from
a weak secondary or primary amino donor is very rare. There
are very few reports where primary and secondary amino donor
systems with bulky group near the donor site forces the donor
decreasing donor strengths the energy gap between the first and
second excited states becomes so large that it can then no longer
be tuned by any solvent [20]. Very recently, we have reported
excited state CT process in some secondary amino donor systems
without steric crowding near the donor site [2–4]. So far, the ICT
process in some fluoro substituted 2-aminobenzonitrile deriva-
tives have been reported by Zachariasse and co-workers [21]. Stalin
et al. reported the occurrence of ICT in 2-amino benzoic acid, 3-
amino-benzoic acid and p-amino benzoic acid [22–24]. Mikami
et al. reported the ICT process of p-cyanophenyldisilane in non-
polar solvents and in the isolated cooled jet [25]. Recently, we
have reported the CT phenomenon in 4-amino-3-methyl benzoic
acid methyl ester (AMBME) where an ortho methyl group insists
steric crowding at the donor site [5]. In the present study we have
intentionally introduced two methyl groups near the donor site and
have attempted to explore the photoinduced intramolecular charge
transfer phenomenon in the molecule 2,6-dimethyl-1-amino-4-
benzoic acid (M26DMB) where primary amino group is the charge
donor (Scheme 1). Observed pronounced dual fluorescence from
M26DMB in polar solvents and broad red-shifted CT emission in
non-polar solvents have been explored using steady state and time-
resolved spectroscopy.
2. Experimental
2.1. Materials
∗
For the synthesis of M26DMB (Scheme 1), 4-amino-3-methyl
benzoic acid methyl ester prepared by standard procedure [5]
Corresponding author. Tel.: +91 33 2350 8386; fax: +91 33 2351 9755.
E-mail address: nguchhait@yahoo.com (N. Guchhait).
1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.10.033

S. Ghosh et al. / Spectrochimica Acta Part A 75 (2010) 320–324
321
Scheme 1. Structure of M26DMB.
was treated with dimethylsuphide (Me2S), triethylamine and N-
chlorosuccinimide in DCM medium. Product obtained was further
treated with Raney nickel in ethanol to get the desired molecule.
This was further recrystallized to obtain the pure compound.
Spectroscopic grade solvents from Spectrochem-UV and
E.Merck were used after checking for any fluorescence of their own
in the studied wavelength range. Sulphuric acid from E. Merck was
used as supplied. All aqueous solutions were prepared using triple
distilled water.
2.2. Spectroscopic measurements
Hitachi UV/Vis U-3501 spectrophotometer was used for record-
ing the absorption spectra. Emission spectra were recorded using a
PerkinElmer L5 50B fluorimeter. To avoid self-quenching or aggre-
gation of the fluorophore, the concentration of the solute was
maintained below 10−6 M during all the spectroscopic measure-
ments.
The laser system used for time-resolved experiments [26] con-
sists of a Coherent Mira 900F femtosecond laser system, the output
of which was pulsed picked (Coherent 9200 pulse picker) at a rate of
3–8 MHz and then frequency-tripled in an ultrafast harmonic gen-
eration system (INRAD 5-050). Fluorescence decays were recorded
using a Time Correlated Single Photon Counting (TCSPC) system
from Edinburgh Instruments (Lifespec-Red). This system has an
instrument response function (IRF) of 180 ps. The fluorescence
decays were deconvoluted using the iterative software provided
by the manufacturer.
Fig. 1. The room temperature (a) absorption and (b) emission spectra of M26DMB
in different solvents (ꢀext at the absorption band maxima).
3.2. Emission spectra
The emission spectra of M26DMB recorded in various solvents
by excitation at the respective absorption maxima are shown in
Fig. 1b and the position of the bands are presented in Table 1.
In cyclohexane, a broad emission band is observed with a maxi-
mum at 319 nm. The emission maximum is found to be gradually
red-shifted with increasing polarity of the solvent. This solva-
tochromic red-shift supports the polar character of the emissive
state. A clear dual fluorescence is recorded in methanol and water.
As was reported for other similar studied donor–acceptor CT sys-
tems [1–7], the high energy less intense band in methanol and
water is ascribed to local emission (LE) and the intense red-shifted
band to emission from the polar charge transfer (CT) state. Greater
stabilization of this polar photoexcited CT state in solvents of higher
polarity produces the observed solvatochromic red-shift of the CT
emission band. Compared to the high energy LE band maxima for
2.3. Theoretical calculation
The ground state structure of the molecule has been explored
at the Density Functional Theory (DFT) level with B3LYP functional
and 6–31 g (d, p) basis set using Gaussian 03 software [27].
3. Results and discussion
3.1. Absorption spectra
Absorption spectra of M26DMB recorded in various solvents
show single structureless band with a maximum near 290 nm
(Fig. 1a). In cyclohexane, the absorption maxima is, however,
slightly blue-shifted (ꢀmax = 273 nm). The spectral band maxima
observed in different solvents are presented in Table 1. This broad
absorption band is assigned to the S0 → S1 (␲ → ␲*) transition of
the benzene chromophore. Very similar absorption characteristics
were observed for methyl ester of 2-methyl-1-amino-4-benzoic
acid (AMBME) [5]. This indicates that the presence of an extra
methyl group does not largely influence the electronic transition
energy. The red-shift of the absorption maxima with respect to pure
benzene is due to the presence of substituted amino and the ester
groups. The position of the absorption band maxima is found to be
nearly independent of solvent polarity indicating weak polar nature
of the molecule in the ground state.
Table 1
Spectroscopic parameters of M26DMB obtained from absorption and emission mea-
surement at room temperature in different solvents.
Solvents
ET(30)
ꢀabs (nm)
ꢀflu (nm)
Fluorescence
quantum yield
Cyclohexane
Dioxane
Tetrahydrofuran
Chloroform
Acetonitrile
Ethanol
30.9
36.0
37.4
39.1
45.6
51.9
55.4
63.1
276
291
292
291
291
293
296
289
320
333
338
335
0.095
0.38
–
–
340
0.751
310,354
310, 355
310, 363
0.035 (LE), 0.169 (CT)
0.0032 (LE), 0.069 (CT)
0.0009 (LE), 0.014 (CT)
Methanol
Water

322
S. Ghosh et al. / Spectrochimica Acta Part A 75 (2010) 320–324
Table 2
Optimized Geometrical parameters for M26DMB in vacuo at DFT level (using theB3LYP functional and 6–31 g(d,p) basis set).
Bonds
Calculated values (Å)
Angles
Calculated values (◦)
Dihedral angles
Calculated values (◦)
C1–C2
C2–C3
C3–C4
C3–N10
N10–H15
C2–C9
C6–C11
C4–C12
C12–O14
C12–O13
1.416
1.390
1.400
1.389
1.008
1.509
1.509
1.479
1.359
1.218
C16–H10–C1
H16–C10–H17
N10–C1–C2
N10–C1–C6
C3–C4–C12
C4–C12–O14
C4–C12–O13
O14–C12–O13
116.0
112.8
119.7
119.7
122.0
112.6
125.0
122.4
C16–N10–C1–C2
C17–N10–C1–C6
C3–C4–C12–O14
O13–C12–C4–C5
−22.90
22.55
0.05
−0.15
other ICT systems in polar solvents [1–4], the position of the emis-
sion band of M26DMB in cyclohexane appears more red sided than
usually observed in non-polar solvents. Similar red-shifted, broad
emission maxima in non-polar solvent were observed in AMBME
[5] and in 2-amino, 3-amino and p-amino benzoic acids [22–24].
A comparison with similar systems [5,22–24], the observed red-
shift of emission maxima of M26DMB in cyclohexane may arise
in all probability due to some contribution from the CT state. This
assumption is further supported by the fact that the band maxi-
mum of M26DMB in cyclohexane is more red-shifted than the LE
band observed in water and methanol. This clearly points out that
the emission observed in alkane solvents does not arise only from
the LE state as was observed in most ICT systems [1]. A possible
overlap between very closely lying LE and CT states might be the
reason for the red-shift and broadness of the emission maxima of
M26DMB in non-polar solvents. The polar nature of the emissive CT
state is further highlighted by the linearity of the Lippert–Mataga
plot (Fig. 2a). The plot of Stokes’ shift (ꢁꢂ in cm−1) vs. solvent polar-
ity parameters (ꢁf) is represented by the following relations [1]:
ꢂ
ꢃ
ꢀ
ꢁ
2 ꢁf
a3
ε − 1
Á2 − 1
Á2 + 1
hc ꢁꢂ =
(ꢃ∗ − ꢃ)2,
ꢁf =
−
2ε + 1
where h, c, a, ε and Á are Planck’s constant, speed of light,
Onsager cavity radius, medium dielectric and refractive index of
the medium, respectively. The terms ꢃ* and ꢃ are the excited and
ground state dipole moment of the system. Ground state geom-
etry optimization has been performed using B3LYP functional and
6–31 g (d,p) basis set incorporated in the Gaussian 03 software [27].
At the global minimum structure of M26DMB, the amino group is
found to be slightly out of plane of the benzene ring. This could
be due to steric effect of two ortho methyl groups near the donor
site. The acceptor group is, however, planar. The selected optimized
geometry parameters are represented in Table 2. According to the
most accepted twisted intramolecular charge transfer (TICT) model
it is proposed that more the donor group is out of plane of the ben-
zene ring more is the possibility of excited state CT reaction [1].
Thus, the excited state CT process is expected from the structural
point of view according to TICT model [1]. For the global mini-
mum structure, the values of ‘a’ and ‘ꢃ’ are found to be 4.56 Å and
3.6608 D, respectively. The excited state dipole moment is calcu-
lated to be 8.1 D. The increase in dipole moment of M26DMB from
the ground to the excited state further highlights the enhanced
polarity of the emissive CT state and hence justifies the red-shift
of the emission maxima with polar solvent.
The deviation from linearity in the Lippert plot for polar protic
solvents (not shown) indicates that intramolecular solute solvent
hydrogen-bonding interactions have some effect on the red-shifted
emission band. In this context it is found that the plot of the emis-
sion maxima against hydrogen-bonding parameter ‘˛’ is found to be
linear (Fig. 2b). Linearity in this plot suggests that intermolecular
hydrogen-bonding interactions have some influence on the red-
Fig. 2. (a) Lippert plot of M26DMB; (b) plot of emission maxima against hydrogen-
bonding parameter (˛); (c) plot of Stokes’ shift against ET(30) values.

S. Ghosh et al. / Spectrochimica Acta Part A 75 (2010) 320–324
323
Table 3
Fluorescence lifetime of M26DMB in various solvents (ꢀext = 280 nm) (ꢄ represents fluorescence lifetime).
2
Solvent
Wavelength (nm)
320
ꢄ1(a1) (ps)
ꢄ2(a2) (ps)
ꢄ3(a3) (ps)
Cyclohexane
Isopropanol
280 (0.95)
1300 (0.05)
1.2
320
345
94 (0.17)
266 (0.26)
690 (0.66)
774 (0.69)
1300 (0.17)
2000(0.05)
1.0
1.2
Acetonitrile
320
340
419 (0.69)
432 (0.76)
727 (0.31)
1000 (0.24)
1.1
1.0
shifted ICT emission of M26DMB [2–5,28]. This might also open up
non-radiative decay paths via intermolecular hydrogen-bonding in
protic solvents. This possibility is indeed reflected in the lowering
of quantum yields observed in solvents of increasing hydrogen-
bonding characters (Table 1). The correlation of Stokes shift with
the solvent dependent ET(30) parameter is shown in Fig. 2c [29].
Two distinct lines are observed, one for the non-hydrogen-bonding
solvents and the other for the hydrogen-bonding solvents. This
indicates that both electrostatic dipolar and hydrogen-bonding
interactions influence the CT emission.
The measured fluorescence quantum yields [30] of M26DMB
are presented in Table 3. As expected, the quantum yield steadily
increases with increasing polarity of aprotic polar solvents which
indicates that photoinduced ICT occurs in our system and it
increases with the polarity of the solvent. However, as mentioned
before increasing hydrogen-bonding ability of the solvents makes
way for the accessibility of some non-radiative decay channels
which deplete the quantum yield of M26DMB. The lowering of
quantum yield of both the LE and the CT emission from ethanol to
water is hence explained on the basis of the increasing hydrogen-
bonding strengths of these solvents.
As seen in Fig. 3a and b, addition of dilute sulphuric acid to a
solution of M26DMB in water produces prominent changes in the
absorption and fluorescence spectra. The absorption band origi-
nally observed at 290 nm in water disappears with addition of acid
and a new blue-shifted band is generated at 280 nm. This new band
at 280 nm can be assigned to the protonated species of M26DMB
produced by addition of acid. Protonation of M26DMB by acid is also
mirrored in the changes in the emission spectra. The red-shifted CT
band is found to be quenched and simultaneous growth of another
new blue-shifted band is observed on gradual addition of acid. Acid
in all probability protonates the amino group of M26DMB and the
lone-pair of electrons of nitrogen atom is then no longer available
to produce charge transfer process. The emission from the proto-
nated species is consequently blue-shifted as now it has a structure
very close to the LE state. At very high concentrations of acid the
presence of some intensity of the CT band indicates the probability
of excited state deprotonation reaction. This may be due to the for-
mation of CT state by deprotonation reaction in the excited state
surface and hence CT emission is observed even at high acid concen-
tration. This observation clearly predicts that photoinduced charge
transfer occurs from the donor amino to the acceptor group in our
system [1–5].
Fig. 3. Effect of acid on the (a) absorption and (b) emission spectra of M26DMB in
water (arrow indicates increasing acid concentrations) (ꢀext at the absorption band
maxima).
The emission spectrum of M26DMB has been recorded in
ethanol glass matrix at 77 K and its comparison with the room
temperature emission spectra in ethanol is shown in Fig. 4. A clear
blue shift of the emission maxima is observed in the ethanol glass
matrix. Spectral features in the glass matrix indicate that the local
emission is favored over the CT emission, i.e., excited state LE to CT
conversion is hindered in this highly viscous medium. A number of
factors like changes in solvent polarity, polarizability and viscosity
produced in the low temperature glass matrix may be responsible
for this [31]. The increased rigidity of the medium in all probability
also affects the emission characteristics, thus contributing to the
observed blue shift of the fluorescence maxima. According to the
Fig. 4. The emission spectra of M26DMB in ethanol at room temperature (1) and at
77 K (2).

324
S. Ghosh et al. / Spectrochimica Acta Part A 75 (2010) 320–324
most accepted TICT model, viscosity might hinder rotation of the
donor group and hence influences the CT process.
Acknowledgements
NG acknowledges CSIR, India (Project no. 01(2161)07/EMR-II)
for financial support. The authors would like to thank Prof. T.
Ganguly of IACS, Kolkata for allowing them the low temperature
measurement. They also thank Dr. P.K. Gupta and Dr. Kaustuv Das
of CAT, Indore for providing and helping with the lifetime mea-
surements. SG thanks CSIR for Junior Research Fellowship under
the above project.
3.3. Lifetime measurements
Fluorescence lifetimes of M26DMB in different solvents are pre-
sented in Table 3. Interestingly, fluorescence lifetime of 320 nm
emission band in cyclohexane is found to be double exponential
and the lifetime values are 280 ps and 1300 ps. This observation
supports our assumption that CT and LE emissions overlap in
alkanes for M26DMB. As per the assignment of other reported
donor–acceptor CT systems [1], the component with the long life-
time is assigned to the LE state and the one with the short lifetime
but greater abundance is assigned to the CT state. In acetonitrile, the
emission has been monitored at 320 nm and 340 nm. In both cases
again, two lifetime values were recorded. Here also, the longer life-
time and less abundant component is assigned to the LE state and
the shorter lifetime, major component generates from the CT state.
As is expected, at the lower wavelength (320 nm) the abundance
of the LE state is greater than at the higher wavelength of emission
(340 nm). Proximity between lifetimes of the LE state in cyclo-
hexane and acetonitrile (1300 ps and 1000 ps, respectively) points
towards a common origin of the emissive species. In isopropanol,
for both 320 nm and 345 nm emission, the longer lifetime compo-
nent is assigned to the LE state and the shorter lifetime component
is assigned to the CT state. The lifetime of the CT state in polar protic
isopropanol (94 ps at 320 nm and 266 ps at 345 nm) is found to be
much lower than the corresponding CT state lifetimes in acetoni-
trile (419 ps at 320 nm, 432 ps at 340 nm). This can be explained
on the basis of the non-radiative decay paths operative in polar
protic solvents via formation of intermolecular hydrogen-bonding
between M26DMB and the solvent which leads to depletion of the
fluorescent lifetime.
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amine donor, exhibits the photoinduced intramolecular charge
transfer phenomenon in all solvents. The emission maximum is
progressively red-shifted in more polar solvents and pronounced
dual fluorescence is observed in protic solvents hinting at a polar
excited state generation. The occurrence of ICT and generation
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