Evidence of acid mediated enhancement of photoinduced charge transfer reaction in 2-methoxy-4-(N,N-dimethylamino)benzaldehyde: Spectroscopic and quantum chemical study

Article abstract of DOI:10.1016/j.jphotochem.2010.04.011

The photophysical behavior of 2-methoxy-4-(N,N-dimethylamino)benzaldehyde (2-MDMABA) has been investigated by steady state absorption and emission spectroscopy, time resolved emission spectroscopy and quantum chemical calculations. The molecule 2-MDMABA h

Full text of DOI:10.1016/j.jphotochem.2010.04.011

Journal of Photochemistry and Photobiology A: Chemistry 212 (2010) 161–169
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Evidence of acid mediated enhancement of photoinduced charge transfer
reaction in 2-methoxy-4-(N,N-dimethylamino)benzaldehyde: Spectroscopic and
quantum chemical study
Anuva Samanta, Bijan Kumar Paul, Subrata Mahanta, Rupashree Balia Singh,
Samiran Kar, Nikhil Guchhait∗
Department of Chemistry, University of Calcutta, 92 A.P.C. Road, Kolkata 700009, West Bengal, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The photophysical behavior of 2-methoxy-4-(N,N-dimethylamino)benzaldehyde (2-MDMABA) has been
investigated by steady state absorption and emission spectroscopy, time resolved emission spectroscopy
and quantum chemical calculations. The molecule 2-MDMABA having a weak acceptor aldehyde group
shows strong local emission in all solvents and a weak solvent polarity dependent red shifted emission in
polar aprotic solvents for the charge transfer (CT) state which follows well the Lippert–Mataga equation.
Instead of usual protonation at the donor site with inhibition of CT process, protonation at the acceptor
aldehyde site enhances acceptor properties and hence increases excited state charge transfer reaction.
The ground state structural calculation at Density Functional Theory (DFT) level with B3LYP functional
and 6-31++ G** basis set and potential energy surfaces (PESs) following twisted intramolecular charge
transfer model by using DFT method in the gas phase and in acetonitrile solvent have been performed
for the neutral and cation to correlate with experimental findings.
Received 19 January 2010
Received in revised form 30 March 2010
Accepted 19 April 2010
Available online 24 April 2010
Keywords:
2-Methoxy-4-(N,N-
dimethylamino)benzaldehyde
Fluorescence
Charge transfer
DFT
TDDFT–PCM
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
unit are able to rotate around this single bond, thereby stabilizing
the ICT state in solvents of large dielectric. According to the
Aromatic systems with electron donor and acceptor sub-
stituents often exhibit characteristics solvent-dependent spectral
properties due to intramolecular charge transfer (ICT) process in
the excited states. Molecules possessing an ICT state were used in
polymer science in order to get useful insight into the dynamic
behavior of the macromolecular media as well as for the study
of kinetics of free radical polymerization [1–5]. If the degrees of
electron transfer and molecular geometry can be controlled by
using the methodology of molecular recognition chemistry, ICT
molecules can be used as an excellent tool for detecting ions
and molecules and examining microscopic molecular environ-
ments.
Since Lippert’s first report [6] one of the most extensively
studied molecule capable for charge transfer process is the dual flu-
orescence of 4-(N,N-dimethylamino)benzonitrile (DMABN) [7–14].
Several experimental and theoretical investigations postulate
emission from an ICT state twisted about the donor dialkylamino
group as well as rotation of the acceptor group [15–26]. Molecules
with a flexible single bond between the donor and acceptor sub-
twisted intramolecular charge transfer (TICT) hypothesis proposed
by Grabowski and co-workers [7,27], 90◦ internal rotation around a
bond connecting a donor group and an acceptor group can decouple
the orbital of these two groups and this orbital decoupling provides
nearly complete electron transfer from the donor to the acceptor.
Consequently, charge separation occurs between the acceptor and
the donor, with the acceptor group containing a full negative charge
and the donor group containing a full positive charge which results
a highly polar twisted excited state. So the ICT molecules show
short wavelength emission band for coplanar locally excited state,
while the long wavelength band originates from a state with a per-
pendicular conformation which, under appropriate conditions, can
be preferentially stabilized with respect to the planar local excited
state. It is found that the emission properties from the charge sepa-
rated state of the molecule depend on solvent polarity and viscosity
[28–30]. Several other hypothesis have been developed in order to
explain the unusual emission behavior like solute–solvent exciplex
formation [31], planar intramolecular charge transfer (PICT) [32],
rehybridized intramolecular charge transfer (RICT) [33,34], wag-
ging intramolecular charge transfer (WICT) [35] and so on. Out of
all mechanisms, the TICT model is, nevertheless, the most widely
accepted explanation for the dual fluorescence phenomenon of
DMABN and other aromatic molecules having donor–acceptor
groups [7,15–26].
∗
Corresponding author. Tel.: +91 33 2350 8386; fax: +91 33 2351 9755.
E-mail addresses: nguchhait@yahoo.com, nikhil.guchhait@rediffmail.com
(N. Guchhait).
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.04.011

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A. Samanta et al. / Journal of Photochemistry and Photobiology A: Chemistry 212 (2010) 161–169
Molecules exhibiting the presence of an ICT state contain mainly
iso-propanol (i-PrOH), methanol (MeOH) and dimethyl sulfoxide
(DMSO) of spectroscopic grade from E-Merck were used as sup-
plied, but only after checking the purity fluorimetrically in the
wavelength range of interest. De-ionized water was used for mak-
ing aqueous solution. Trifluoroacetic acid (TFA) and triethylamine
(TEA) from Spectrochem were used as supplied.
a strong donor dialkylamino group linked by a ␴-bond to an aro-
matic ring having electron-withdrawing residues such as cyano,
formyl, carboxyl, ester, sulfonamide groups. Systems with weak
charge donor such as primary and secondary amino group and with
weak acceptor such as aldehyde group are very uncommon for CT
reaction [36]. It is also found that CT reaction is suppressed in pres-
ence of acid by preferential binding of H+ ion at the donor site than
at the acceptor site due to more negative charge at the donor site
than the acceptor site. In general, DMABN and similar systems show
weak LE emission and strong CT emission in highly polar solvents
due to facile excited state CT reaction [7,19–26]. On the contrary,
it is found that the molecule 4-(N,N-dimethylamino)benzaldehyde
(DMABA), a weak acceptor system, shows very weak CT emission
[36] and reported spectral data from different groups which are
inconsistent [36–41]. Therefore, we have synthesized a substituted
DMABA namely 2-methoxy-4-(N,N-dimethylamino)benzaldehyde
(2-MDMABA) to throw light on CT reaction of a weakly acceptor
system. Purposefully we have tagged –OMe group near the accep-
tor group to modify the acceptor property and steric effect at the
acceptor side. Structurally the presence of –OMe group near the
acceptor site increases the electron density at the acceptor site
compared to the usual donor site. And thereby insisting the H+
ion to bind to the acceptor group and consequently increasing
the acceptor strength. We have studied the photophysical prop-
erties of the neutral molecules in the acidified condition by steady
state absorption and emission and time resolved emission spec-
troscopy. Theoretical studies including solvent effects for the bare
molecule and its protonated species have been explored to get a
better understanding of the experimental findings. We have per-
formed quantum chemical calculations at DFT level with B3LYP
functional and 6-31++G** basis set in the gas phase to find the
ground state structural information of the molecule. Considering
TICT model the ground and excited state potential energy surfaces
for 2-MDMABA and its cation in the gas phase and in acetoni-
trile solvent have been explored using Density Functional Theory
(DFT), Time Dependent Density Functional Theory (TDDFT) and
Time Dependent Density Functional Theory–Polarized Continuum
Model (TDDFT–PCM) method using same functional and basis set
to correlate with experimental results.
2.3. Steady state spectral measurements
The absorption and emission spectra of 2-MDMABA were
recorded in different solvents using Hitachi UV/VIS U-3501 spec-
trophotometer and PerkinElmer (Model LS50B), respectively. For all
spectral measurements, the sample concentration was maintained
at 10−5 to 10−6 M in different solvents in order to avoid aggregation
and self-quenching.
2.4. Fluorescence lifetime measurements
Fluorescence lifetime measurement has been done by Time
Correlated Single Photon Counting (TCSPC) technique using
nanoLED-07 (IBH UK) [42]. The light source used in the TCSPC set
up is a nanoLED of wavelength 340 nm.
2.5. Computational details
For the structural calculations, the optimized geometry of 2-
MDMABA in vacuum at DFT level and in acetonitrile solvent with
TDDFT–PCM model, respectively, have been obtained with B3LYP
functional and 6-31++G** basis set using Gaussian 03 software [43].
The molecule forms different low energy structures with respect to
orientation of flexible substitutions in the ground state [Scheme 1].
The ground and excited state potential energy curves along the
donor twist coordinate have been performed by DFT, TDDFT and
TDDFT–PCM for gas and solvated phase, respectively, using the
same functional and basis set. There are several reported methods
for the evaluation of excited properties of donor–acceptor systems
[20–26]. The TDDFT method with approximate functional has some
restrictions during the evaluation of vertical excitation energies to
Rydberg states [44], for the dissociating molecules [45], long range
CT excitations for large systems [46], etc. In spite of those limita-
tions, this TDDFT method has been used successfully to ascertain
the excited state properties [20–26]. In the ground state, the geom-
etry at different donor twist angles has been kept frozen for the
evaluation of PE for the S0 state. In case of S1 and S2 states, vertical
transition energies have been calculated with respect to the ground
state optimized structure using TDDFT method. For the study of
solvent effect on the charge transfer state, the non-relaxed scan
has been performed using TDDFT–PCM model. Similar calculations
have also been done for the cation too. Though this method of cal-
culation (non-relaxed model) has some restrictions regarding the
consideration of frozen geometry, however it has been already used
by many scientific groups.
2. Experimental
2.1. Materials
The molecule 2-methoxy-4-(N,N-dimethylamino) benzalde-
hyde was synthesized by the reaction of N,N-dimethyl-m-anisidine
with N,N-dimethylformamide and phosphorus oxychloride. Phos-
phorus oxychloride (1 eqv.) was added slowly in dry DMF at 0 ◦C
and stirred for 30 min. Then N,N-dimethyl-m-anisidine (1 eqv.) in
dry DMF was added to the reaction mixture at 0 ◦C and then slowly
warmed to room temperature and stirred for another 2 h. The reac-
tion mixture was poured into ice cold water and neutralized by
dropwise addition of aqueous sodium acetate with vigorous stirring
and extracted with dichloromethane. The organic layer was washed
with water and brine and dried over anhydrous sodium sulfate.
The solvent was removed under vacuum to give 2-methoxy-4-
(N,N-dimethylamino)benzaldehyde. The compound was purified
by column chromatography and repeated crystallization. [1H NMR
(300 MHz, CDCl3) ı: 10.14 (s, 1H, CHO), 7.68 (d, J = 8.7 Hz, 1H, ArH),
6.26 (d, J = 8.7 Hz, 1H, ArH), 6.00 (d, J = 2.4 Hz, 1H, ArH), 3.87 (s, 3H,
OMe), 3.06 (s, 6H, NMe2)].
3. Results and discussions
3.1. Absorption spectra
The molecule 2-MDMABA is basically similar to that of DMABA
and the ␲␲* transition in the absorption spectra is expected to be
same in nature. However, this molecule shows two bands in the
absorption spectra instead of a single absorption band (∼340 nm)
for the parent molecule DMABA [36–41]. As seen in Fig. 1a, the
observed peaks are at ∼300 nm (weak) and ∼340 nm (strong). The
␲␲* transition of 2-MDMABA is shifted to the red with respect to
benzene due to the presence of substitutions such as –NMe2, –CHO
and–OMe groups capable of extra resonance stabilization. As seen
in Fig. 1a, the higher energy ∼300 nm band is practically unchanged
2.2. Solvents
The solvents methylcyclohexane (MCH), acetonitrile (ACN),
chloroform (CHCl3), dioxane (DOX), dimethylformamide (DMF),

A. Samanta et al. / Journal of Photochemistry and Photobiology A: Chemistry 212 (2010) 161–169
163
Scheme 1. Different ground state low energy conformers of 2-MDMABA molecule. Calculated energies at DFT (B3LYP/6-31++G**) level are with respect to conformer I.
*
with the solvent polarity whereas the position of the lower energy
peak changes with the dielectric constants and the hydrogen bond-
ing ability of the solvents. The long wavelength absorption band
is red shifted with increasing polarity of the solvents and is sim-
ilar to that of DMABA [36–37]. But the extent of shifting of the
absorption peak with solvent polarity is more pronounced in case of
DMABA than 2-MDMABA. Comparing the absorption spectra of 2-
MDMABA with DMABA and assuming the presence of single species
in the ground state, these two peaks are assigned to transition from
ground to two excited states. The high absorption intensity for the
low energy band may indicate that this could be a ␲–␲ type of
transition.
The absorption spectra of 2-MDMABA undergoes a striking
change in presence of acid. As seen in the Fig. 1b, gradual addition
of strong acid TFA to the acetonitrile solution of 2-MDMABA gener-
ates two bands by the expense of the long wavelength absorption
band of the neutral species. One of these two bands is red shifted
(∼375 nm) and the other is blue shifted (∼315 nm) with respect to
that of the ∼340 nm band of the neutral molecule. This may indicate
that these two absorption bands may arise due to protonation of
the neutral species. Structurally the molecule 2-MDMABA has two
protonation sites – one is the nitrogen atom of –NMe2 group and
the other is the oxygen atom of –CHO group. Due to the presence of
–OMe group at the ortho position of the acceptor group, the charge
density at the oxygen atom of –CHO group increases compared to
that of the nitrogen atom of –NMe2 group. The –NMe2 group is
expected to be coplanar with the aromatic ring in the ground state.
Consequently, the lone pair electron of –NMe2 group can delocalize
with the ␲ cloud of the aromatic ring. This may decrease the charge
density at the –NMe2 group, but increase the same on the oxygen
atom of –CHO group. That is why the oxygen atom may prefer pro-
tonation first and the large red shifted absorption band could be
assigned to the monocation (I) as the generated monocation can be
stabilized by resonance (Scheme 2). The addition of another proton
to –NMe2 group will shift the spectra to the blue in comparison to
that of the neutral molecule. In this case the usual resonance sta-
bilization of the nitrogen lone pair disappears and this observation
is common for donor–acceptor systems [20–26].
3.2. Emission spectra
Room temperature fluorescence emission spectra of 2-
MDMABA measured in non-polar, polar and hydrogen bonding
solvents are shown in Fig. 2 and the corresponding band max-
ima are reported in Table 1. In case of non-polar solvents like
methyl cyclohexane, on excitation of 2-MDMABA at 330 nm, a sin-
gle emission at ∼388 nm is observed with very low fluorescence
intensity. It is noteworthy to point out here that DMABA is prac-
Fig. 1. (a) Absorption spectra of 2-MDMABA in different solvents at room temper-
ature and (b) effect of acid TFA on the absorption spectra of 2-MDMABA in ACN.
Scheme 2. Possible protonated species formed when TFA is added in ACN solution
of 2-MDMABA.

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A. Samanta et al. / Journal of Photochemistry and Photobiology A: Chemistry 212 (2010) 161–169
Table 1
Spectroscopic parameters obtained from absorption and emission spectra of 2-MDMABA in different solvents at room temperature.
Solvents
ꢀabs (nm)
ꢀem (nm)
Stokes’ shift (ꢁꢂ) (cm−1
)
Quantum yielda
ϕf
ϕf
LE
CT
MCH
CHCl3
DCM
DOX
330
342
343
343
387
384
384
381
510
4464
3164
3079
2908
9547
–
–
–
–
–
11.04
–
0.16
ACN
DMF
341
343
387
517
3486
9983
0.62
1.75
0.13
0.45
387
521
3315
9961
i-PrOH
EtOH
MeOH
Water
DMSO
345
347
348
351
347
380
383
390
410
391
529
2670
2709
3094
4100
3243
9915
3.42
1.82
1.55
0.29
0.70
–
–
–
–
0.09
Monocation in ACN
Dication in ACN
375
315
455
500
360
4689
6667
3968
1.16
0.50
4.78
–
a
Quantum yield × 10−3
.
tically non-fluorescent in non-polar solvents. The signature of the
spectra predicts that this single emission band is nothing but emis-
sion from the LE state of the molecule. Dual emission is observed
in polar aprotic solvents upon excitation at 340 nm. The high-
energy strong emission band at ∼385 nm is assigned to LE of the
molecule as its position is nearly insensitive to the polarity of the
solvents. But the other low energy weakly intense red shifted band
at ∼515 nm shows strong dependency on solvent polarity. Compar-
ing this emissive behavior with DMABA and similar type of systems
[15–26], the long wavelength emission band with low intensity is
ascribed as charge transfer emission. Due to the presence of the
–OMe group, the charge transfer ability as well as solvent depen-
dency of the emission band of 2-MDMABA are further diminished
compared to DMABA. The excitation spectra (Fig. 3a) of 2-MDMABA
monitored at 385 nm and 515 nm resembles the absorption spec-
tra. Therefore, both the LE and CT bands correspond to the same
ground state species which absorbs at ∼340 nm.
In case of hydroxylic polar solvents, single emission band is
observed at ∼385 nm. In addition to the decrease in energy of the CT
state by dielectric stabilization, the intermolecular hydrogen bond-
ing interaction with protic solvents opens up usual non-radiative
channels. That may be the reason of observing only LE emission
in protic solvents. The absence of CT emission of DMABA in protic
solvents was explained by considering stabilization of the CT state
[47]. It is proposed that the stabilization of the CT state reduces
the energy gap between the ICT state and the Franck–Condon state
resulting in a rapid non-radiative decay of the ICT state to the low-
lying state [47]. In ACN, 2-MDMABA shows two emission bands
corresponding to LE and CT state of the molecule. When methanol
was added to that solution, a decrease of intensities of both the
bands is noticed (Fig. 3b). This result is consistent with the facts
that increase in polarity and H-bonding parameter increases the
non-radiative rate and thus decreases the fluorescence intensity of
both the bands.
The effect of the nature of the solvent on the energy difference
between the ground and excited states of a molecule depends on
the refractive index (n) and dielectric constant (εr) of the solvent
and is described by the following Lippert–Mataga equation [48].
(ꢃ∗ − ꢃ)2
Fig. 2. (a) Fluorescence emission spectra of 2-MDMABA in different solvents at
room temperature (ꢀext = 330 nm for non-polar and ꢀext = 350 nm for polar protic
solvents). (b) Normalized emission spectra of 2-MDMABA in polar aprotic solvents
(ꢀext = 340 nm) at room temperature.
ꢁꢂ = ꢂa − ꢂf =
3 f (εr, n)
2ꢄε0hc
where f (εr,n) = [(εr − 1)/(2␧r + 1)] − [(n2 − 1)/(2n2 + 1)].

A. Samanta et al. / Journal of Photochemistry and Photobiology A: Chemistry 212 (2010) 161–169
165
Fig. 4. (a) Plot of stokes shift (ꢁꢂ) versus solvent parameter (ꢁf).
In 2-MDMABA, due to the presence of –OMe group, the aldehyde
group is expected to be more basic than the tertiary amino group
and it is opposite to that of DMABA. Hence oxygen atom of –CHO is
expected to be protonated first before protonation at the nitrogen
lone pair of N,N-dimethylaniline group. As seen in the Fig. 5a, after
addition of TFA to the ACN solution of 2-MDMABA both the LE and
CT band disappear and a new broad band peaks at 500 nm. As dis-
cussed earlier in Section 3.1, strong acid TFA may insist protonation
at both the centers with the formation of both monocation and dica-
tion. The electron-pulling ability of the acceptor must increase after
first protonation at the aldehyde oxygen atom and the generated
monocation is expected to show a large red shift emission band for a
favorable charge transfer emission from the monocationic species.
Second protonation, i.e., protonation at the nitrogen centre insists
the absence of CT emission due to unavailability of the nitrogen lone
pair. When the protonated species (dication) is excited at 315 nm,
two emission bands are observed – one at ∼360 nm and broad band
∼480 nm (Fig. 5b). As seen in Fig. 6a, the excitation spectra moni-
tored at 360 nm and 480 nm resemble with the absorption spectra
of dication and monocation, respectively. Therefore, the emission
band at 360 nm is assigned to the local emission of doubly proto-
nated species whereas the broad emission band may arise from the
monocation form generated from the dication in the excited state
surface after deprotonation. Interestingly, a broad emission band
of the monocation appears at ∼455 nm on excitation at 375 nm. As
shown in inset of Fig. 5b, this broad emission band after decon-
volution using Gaussian fit shows two bands – one at ∼450 nm
and another at ∼500 nm. As the excitation spectra monitored at
450 and 500 nm are same, the low energy red shifted emission
band at 500 nm band is assigned to the charge transfer emission
of the monocation arising from the deprotonation of the dication.
The electron-pulling ability of the monocation is more than the
neutral molecule, that is why the charge transfer intensity of the
protonated species is very high. The relationship between the neu-
tral (N), monoprotonated (M) and doubly protonated (D) forms is
summarized in Scheme 3. When methanol is added, the rate of
deprotonation decreases, as a result of which the intensity of the
CT band of monocation is diminished (Fig. 6b). In a protic solvent
or an aprotic solvent mixed with a protic solvent, the excited state
deprotonation process does not occur. The excitation spectra mon-
itored at 400 nm in the presence of methanol is same as that of the
dication. In the presence of protic solvent, only dication is present.
The fluorescence quantum yields are measured relative to
␤-napthol in MCH. As shown in Table 1, it is found that the fluores-
cence quantum yield for the CT emission of 2-MDMABA in different
solvents is lower than the LE emission band. This may indicate that
Fig. 3. (a) Excitation spectra of 2-MDMABA in different solvents at room tem-
perature. (b) Effect of mixed solvent (ACN + MeOH) on the emission spectra
(ꢀext = 340 nm) of 2-MDMABA.
In this equation, ꢂa and ꢂf are the wavenumber (cm−1) of
absorptionand emissionbandmaxima, respectively. Thesymbol ε0,
h, c are permittivity of vacuum (8.85 × 10−12 V C−1 m−1), Planck’s
constant (6.626 × 10−34 J s) and velocity of light (3 × 108 m s−1),
respectively. The radius of cavity ( ) in which the fluorophore
resides and the ground state dipole moment (ꢃ) have been com-
puted for the global minimum structure of 2-MDMABA calculated
at DFT level using B3LYP functional and 6-31++G** basis set and
the corresponding values are 4.74 and 7.98 Debye (The values for
DMABA are 4.3 Å and 5.6 D, respectively, at the same level of cal-
culation.) [36]. The excited state dipole moment can be obtained
from the slope of the plot of Stokes shift (ꢁꢂ) versus solvent polar-
ity parameter (ꢁf) (Fig. 4). Though the changes in absorption and
emission peak with solvent dielectric are not so pronounced, but
their differences i.e., ꢁꢂ changes linearly with ꢁf values. The calcu-
lated dipole moment of the excited state from the solvatochromic
measurement is found to be 12.03 D (ꢃCeT = 12 D for DMABA) [36].
Therefore, high dipole moment due to atomic charge redistribu-
tion in the excited state by charge transfer from –NMe2 group to
–CHO group is responsible for the polarity dependent Stokes shifted
emission.

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A. Samanta et al. / Journal of Photochemistry and Photobiology A: Chemistry 212 (2010) 161–169
Fig. 6. (a) Excitation spectra of protonated species monitored at different emis-
sion wavelength and (b) effect of addition of methanol on the emission spectra of
protonated species of 2-MDMABA.
Fig. 5. (a) Effect of TFA on the emission spectra of 2-MDMABA in ACN (ꢀext = 340 nm)
and (b) normalized emission spectra of monocation and dication of 2-MDMABA
(Inset: deconvoluted figure of the protonated form of 2-MDMABA using Gaussian
fit).
6-31++G** basis set to correlate the observed excited state phe-
nomenon of the studied molecule. In the ground state the molecule
2-MDMABA shows three low energy conformers. Among these
three conformers, conformer I (Scheme 1) is found to be the most
stable structure due to stabilization by intramolecular H-bonding.
Two other structures are quite high-energy conformers due to
the possible steric interaction of the neighboring –OMe substitu-
tion. The selective optimized parameters for the lowest energy
conformer are presented in Table 2. It is to note that the global
LE to CT conversion is less favorable. It is worthwhile to note that
the fluorescence quantum yield for the CT band of DMABA is higher
than 2-MDMABA [37,41]. This is in accord with the expectation for
2-MDMABA with a slightly lower electron affinity of the accep-
tor due to the ortho substitution. The fluorescence quantum yield
in polar protic solvents is low due to activation of non-radiative
channels by intermoleculer H-bonding interaction.
The fluorescence lifetime of 2-MDMABA in acetonitrile sol-
vent shows that the fluorescence decay of LE band (390 nm)
is 0.0581 ns (ꢅ2 = 1.0963) and that of the CT band (510 nm) is
0.6179 ns (ꢅ2 = 1.1.1032) whereas mean fluorescence lifetime of
DMABA ranges from 0.45 to 1.74 ns [36]. On the other hand in protic
solvent water, the LE emission shows single exponential decay with
decay time 0.028 ns (ꢅ2 = 1.1032). The decrease in fluorescence life-
time in hydrogen bonding solvent may be due to channel out of
energy by hydrogen bonding interaction [49].
3.3. Quantum chemical calculation
The ground state structure of 2-MDMABA and potential energy
surfaces along the CT reaction path have been calculated at DFT
level with Becke’s three parameter hybrid functional (B3LYP) and
Scheme 3. The relationship between the neutral (N), the monocation (M) and the
dication (D) of 2-MDMABA in the ground state as well as excited state.

A. Samanta et al. / Journal of Photochemistry and Photobiology A: Chemistry 212 (2010) 161–169
167
Table 2
Structural parameters for the optimized ground state lowest energy conformer of
2-MDMABA at DFT level with B3LYP functional and 6-31++G** basis set.
Bond
Calculated
values (Å)
Angle/dihedral angle
Calculated
values (◦)
RC1–C2
RC2–C3
RC3–C4
RC6–C1
1.417
1.395
1.418
1.405
1.467
1.228
1.365
1.422
1.377
1.454
1.457
<C13–N10–C14
<C13–N10–C4
<C1–C7–O22
<C1–C2–O8
<C3–C4–N10–C13
119.390
120.148
124.069
116.142
0.013
RC1–C7
RC7–O22
RC2–O8
RO8–C23
RC4–N10
RN10–C13
RN10–C14
minimum structure (Conformer I) of 2-MDMABA has a nearly pla-
nar geometry at the nitrogen centre where –NMe2 group is in the
plane of the benzene ring (<C3–C4–N10–C13 = 0.013). So, there is
a possibility of extensively delocalized of the lone pair of nitrogen
atom to the benzene ␲-ring in the ground state. Hence the calcu-
lated Mulliken charges over the nitrogen atom (−0.062) of –NMe2
is quite lower than the oxygen atom (−0.420) of –CHO group. In
case DMABA, the Mulliken charges over nitrogen and oxygen atoms
are −0.510 and −0.441, respectively at the same level of calcula-
tion [50]. Extensive delocalization of nitrogen lone pair electron
for 2-MDMABA reduce electron charge at nitrogen atom and the
presence of –OMe group enhances the electron density over the
oxygen atom of –CHO group. This may be the reason why first pro-
tonation occurs at the oxygen atom of aldehyde group instead of
usual protonation at the donor site like DMABA.
Fig. 7 shows the angular dependency of energy for the ground
and first excited state by rotational motion of the donor group
around the benzene plane in vacuum using DFT and TDDFT method
and in acetonitrile solvent using TDDFT–PCM model. The ground
state potential energy surface shows an increase in energy upon
rotation of the dimethylamino group. In vacuum and in ACN sol-
vent, an energy-stabilized twisted geometry (Â = 90◦) is found in the
S1 surface. In the S1 PES, the transition from planar to twisted state
occurs through a barrier and the height of the barrier in vacuum
is higher than in ACN solvent. The occurrence of TICT implies the
existence of two close lying energy states i.e., the LE and CT states.
The first emission takes place from the LE state and the low energy
emission occurs from the CT state after relaxation to a perpendicu-
lar structure from the LE state through a barrier. The energy barrier
for the transformation from LE to CT state is 5.76 kcal/mol in vac-
uum and 2.64 kcal/mol in ACN solvent. This barrier may arise from
the decrease of delocalization of nitrogen lone pair along the twist
coordinate compared to the planar conformation of 2-MDMABA
in the global minimum state with maximum delocalization of lone
pair of nitrogen atom with aromatic moiety. The low barrier energy
in ACN indicates the feasibility of CT in the excited state in polar sol-
vent. Comparing the calculated barrier energy of 2-MDMABA (5.76
and 2.64 kcal/mol in vacuum and in ACN, respectively) with DMABA
(4.64 and 0.38 kcal/mol in vacuum and in ACN, respectively) [50],
it is obvious that LE to CT transition is more feasible for DMABA
due to very low crossover energy than 2-MDMABA. The experi-
mentally and theoretically values of vertical transition energies in
vacuum and in ACN are presented in Table 3. It is clearly found that
the theoretical values are in well agreement with the respective
Fig. 7. Potential energy curves of 2-MDMABA for the S0 and S1 states along the donor
twisting coordinate in (a) vacuum and (b) ACN solvent using DFT (B3LYP/6-31++G**)
level (TDDFT for S1 state and TDDFT–PCM for ACN solvent).
strength value, the emission intensity of the CT state is low compare
to the LE state. From the HOMO–LUMO picture (Fig. 9), it is seen
that the ␲ cloud density is delocalized over the entire aromatic
ring for the HOMO (␲) and LUMO (␲*) due to planar configura-
*
tion of the global minimum structure. So ␲␲ transition occurs
for non-twisted geometry. During the twisting of the donor group
(Â = 90◦), the lone pair is localized over the nitrogen atom for HOMO
*
(n) and LUMO is of ␲ character. In the CT state, HOMO–LUMO
*
transition is n␲ type having very low oscillator strength value.
Very similar trend is observed in ACN solvent where calculated
oscillator strength decreases from 0.5038 for the global minimum
Table 3
Comparison between experimental and computed energy (eV) for different excited
states of 2-MDMABA and the monocation in vacuum and acetonitrile at DFT level
with B3LYP functional and 6-31++G** basis set.
Medium/solvent
Absorption
Emission
State
a
a
State
Eth
Eex
Eth
Eex
Vacuo
ACN
S1
S2
3.66
4.09
3.75
4.20
S1
S2
2.83
–
3.20
–
*
experimental values. Theoretically it is expected that ␲␲ transi-
S1
S2
3.88
3.99
3.64
4.11
S1
S2
2.54
–
2.39
–
tion having high oscillator strength value is allowed transition and
n␲* transition is forbidden transition having low oscillator strength
value. It is clear from the plot of oscillator strength versus twist
Cation in ACN
S1
3.55
3.35
S1
1.29
2.48
*
a
angle (Fig. 8) that LE emission is of ␲␲ type (f = 0.1803) and CT
Emission from CT state. Eth and Eex are theoretical and experimental energies,
*
respectively.
emission is n␲ type (f = 0.0079). As the CT state has low oscillator

168
A. Samanta et al. / Journal of Photochemistry and Photobiology A: Chemistry 212 (2010) 161–169
Fig. 8. The variation of oscillator strength of 2-MDMABA with varying donor twist
angle in (a) vacuum and (b) ACN solvent.
structure to 0.0083 in the twisted form. The HOMO–LUMO picture
supports that localization of electron density over nitrogen atom in
the twisted form favors the CT process.
We have also performed potential energy surface calculation for
the monocationic species using the same method as is done for the
neutral molecule. The ground state behavior of the cation is quite
same as that of the neutral molecule. The barrier to the conversion
from LE to CT along the donor coordinate in the first excited state
is found to be 4.02 kcal/mol (Fig. 10a) and it is lower than the neu-
tral molecule (5.76 kcal/mol). On the other hand, while we consider
the solvent effect on the monocation, it is noticed that the barrier
energy for the first excited state (Fig. 10b) of the monocation is quite
Fig. 10. Potential energy curves of the monocation of 2-MDMABA for the S0 and
S1 states along the donor twisting coordinate in (a) vacuum and (b) ACN solvent
using DFT (B3LYP/6-31++G**) method (TDDFT for S1 state and TDDFT–PCM for ACN
solvent).
same as neutral molecule (2.77 and 2.64 kcal/mol for monocation
and neutral molecule, respectively). It is seen from Figs. 7 and 10
that the calculated vertical transition energy for the cation is low
than the corresponding neural species and this is in right trend
with the experimental observation. As seen in Fig. 10, the calculated
electronic excitation energy for the crossing from LE to CT transi-
tion is ∼84.7 kcal/mol for the cation both in vacuum and in ACN
solvent which can be accessible upon photoexcitation at ∼340 nm
(∼84.1 kcal/mol). But in case of neutral molecule, the same calcu-
lated electronic excitation energy for the conversion from LE to CT
is ∼90–92 kcal/mol which is not easily accessible by ∼340 nm exci-
tation. This also supports a favorable photoinduced charge transfer
reaction for the monocation than the neutral molecule. Calculated
oscillator strength for the monocation is found to be similar to that
of the neutral species where local excitation is of ␲␲* type with high
oscillator strength having the planar geometry and charge transfer
*
emission is of n-␲ type with low oscillator strength having 90◦
twisted geometry.
4. Conclusion
In this paper, donor–acceptor substituted aromatic system
namely 2-methoxy-4-(N,N-dimethylamino)benzaldehyde having
a weak acceptor group has been synthesized and the excited state
properties of its neutral, monocation and dication have been inves-
Fig. 9. HOMO and LUMO picture of 2-MDMABA in (a) vacuum (Â = 0◦), (b) vacuum
(Â = 90◦), and (c) ACN (Â = 90◦) solvent obtained from calculated structure at DFT
level.

A. Samanta et al. / Journal of Photochemistry and Photobiology A: Chemistry 212 (2010) 161–169
169
tigated by steady state and time resolved spectral measurement. In
addition to the local emission the molecule shows weak charge
transfer emission in polar aprotic solvents. The molecule favors
CT emission in presence of acid due to enhancement of acceptor
strength by protonation at the acceptor site instead usual protona-
tion at the donor site. The substituted methoxy group is responsible
for such acid induced enhancement of CT emission. Excited state
deprotonation of the dication produces CT emission from the
monocation. Mulliken charge distribution at the donor and accep-
tor sites for the calculated structure of neutral and monocation
at DFT (B3LYP/6-31++G**) level well predicts the enhancement of
acceptor strength by acidification. Theoretical PESs for the ground
and excited state along the donor twist coordinate using TICT model
for the neutral and monocation in the vacuum and ACN solvent
clearly predicte the red shifted CT emission in the S1 surface at 90◦
twisted geometry. The lowering of LE to CT barrier energy from
vacuum to ACN solvent and also from neutral to monocation well
correlates the experimental spectral observations.
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Acknowledgements
NG acknowledges CSIR, India (Project no. 01(2161)07/EMR-II)
and DST, India (Project No. SR/S1/PC/26/2008) for financial support.
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