Excited state charge transfer reaction with dual emission from 5-(4-dimethylamino-phenyl)-penta-2,4-dienenitrile: Spectral measurement and theoretical density functional theory calculation

Article abstract of DOI:10.1016/j.molstruc.2011.05.022

The excited state intramolecular charge transfer process in donor-chromophore-acceptor system 5-(4-dimethylamino-phenyl)-penta-2,4- dienenitrile (DMAPPDN) has been investigated by steady state absorption and emission spectroscopy in combination with Densi

Full text of DOI:10.1016/j.molstruc.2011.05.022

Journal of Molecular Structure 998 (2011) 136–143
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Excited State Charge Transfer reaction with dual emission
from 5-(4-dimethylamino-phenyl)-penta-2,4-dienenitrile: Spectral measurement
and theoretical density functional theory calculation
⇑
Sankar Jana, Sasanka Dalapati, Shalini Ghosh, Samiran Kar, Nikhil Guchhait
Department of Chemistry, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The excited state intramolecular charge transfer process in donor–chromophore–acceptor system 5-(4-
dimethylamino-phenyl)-penta-2,4-dienenitrile (DMAPPDN) has been investigated by steady state
absorption and emission spectroscopy in combination with Density Functional Theory (DFT) calculations.
This flexible donor acceptor molecule DMAPPDN shows dual fluorescence corresponding to emission
from locally excited and charge transfer state in polar solvent. Large solvatochromic emission shift, effect
of variation of pH and HOMO–LUMO molecular orbital pictures support excited state intramolecular
charge transfer process. The experimental findings have been correlated with the calculated structure
and potential energy surfaces based on the Twisted Intramolecular Charge Transfer (TICT) model
obtained at DFT level using B3LYP functional and 6-31+G(d,p) basis set. The theoretical potential energy
surfaces for the excited states have been generated in vacuo and acetonitrile solvent using Time Depen-
dent Density Functional Theory (TDDFT) and Time Dependent Density Functional Theory Polarized Con-
tinuum Model (TDDFT-PCM) method, respectively. All the theoretical results show well agreement with
the experimental observations.
Received 16 March 2011
Received in revised form 13 May 2011
Accepted 13 May 2011
Available online 19 May 2011
Keywords:
ICT reaction
5-(4-Dimethylamino-phenyl)-penta-2,4-
dienenitrile
Dual fluorescence
Solvatochromism
DFT
TDDFT-PCM
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
Intramolecular Charge Transfer (RICT) model [11], the Planarized
Intramolecular Charge Transfer (PICT) model [12,13], and the Wag-
Photoinduced intramolecular charge transfer (ICT) processes
and subsequent dual fluorescence in molecules with acceptor
and donor groups find importance in various applications such as
electro-optical switches, pH and ion detectors, creation of new
optoelectronic devices such as electroluminescence devices, solar
cells and thin film transistor [1–3], chemical sensors for free vol-
ume measurement in polymers, probes for the study of micro-het-
erogeneous environments [4,5], degree of water penetration into
the surfactant aggregates and sensing the local polarity of the
microenvironment at binding sites on proteins [6]. These charge
transfer molecules also have a crucial role in biological light har-
vesting processes such as photosynthesis [7]. The phenomenon of
dual fluorescence was first reported by Lippert et al. [8] in 4-
(N,N-dimethylaminobenzonitrile) (DMABN). Various models have
been proposed so far for explaining the dual fluorescence in
DMABN and in similar types of molecules with donor acceptor
moiety attached to the chromophore group. The Twisted Intramo-
lecular Charge Transfer (TICT) model [7,9,10], the Rehybridized
ging Intramolecular Charge Transfer (WICT) model [14] are some
of the most discussed among them. Twisted Intra-molecular
Charge Transfer (TICT) model, proposed by Grabowski et al.
[15,16] is the most acceptable one for explaining dual fluorescence
in DMABN and its analogous molecules [17]. According to TICT
model, after absorption, the molecule from the promoted state
transforms to the twisted structure which is a local minimum on
the excited state potential energy surface. This transformation is
performed through a radiationless process. In the final conforma-
tion, the donor group is typically perpendicular to the rest of the
molecular frame, thus generates a complete electronic decoupling
state. The lower energy emission band is then thought to arise
from this charge transfer (CT) state and higher energy emission
arises from a locally excited (LE) state.
Till date, many compounds have been synthesized and reported
which are capable for charge transfer reaction with varying donor
such as primary, secondary and tertiary amino groups and acceptor
such as nitrile [18,19], acid [20,21], aldehyde [22], ester [23,24],
etc. attached to the chromophoric benzene [23–25] and naphtha-
lene [26,27] rings. Recently, we have reported some new donor
acceptor systems having flexible ethylenic double bond between
the acceptor and chromophore [28,29] and it is found that this type
⇑
Corresponding author. Tel.: +91 33 2432 4159; fax: +91 33 2351 9755.
E-mail address: nguchhait@yahoo.com (N. Guchhait).
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.05.022

S. Jana et al. / Journal of Molecular Structure 998 (2011) 136–143
137
of extra flexibility favors CT reaction [30]. To this end we are inter-
2.3. Steady state spectral measurements
ested to follow how addition of additional ethylenic double bond
effects the CT reaction. Therefore, in this work, we have synthe-
All the spectral measurements were done at ꢁ10ꢀ5–10ꢀ6
M
sized
5-(4-dimethylamino-phenyl)-penta-2,4-dienenitrile
concentration of solute in order to avoid aggregation and self
quenching. All absorption and emission spectra of DMAPPDN were
recorded by Hitachi UV–Vis U-3501 spectrophotometer and Perkin
Elmer LS55 fluorescence spectrophotometer, respectively.
(DMAPPDN) where two ethylenic double bonds are tagged be-
tween the acceptor and chromophore and investigated its excited
state ICT process using steady state absorption, fluorescence spec-
troscopy, and theoretical calculations. Theoretical calculations
have been performed at Density Functional Theory (DFT) level
using B3LYP hybrid functional and 6-31+G(d,p) basis set. The
ground and excited states potential energy curves (PECs) along
the donor and acceptor twisted coordinates in vacuo have been
evaluated using Density Functional Theory (DFT) and Time Depen-
dent Density Functional Theory (TDDFT) methods, respectively. On
the other hand, Time Dependent Density Functional Theory Polar-
ized Continuum Model (TDDFT-PCM) methods have been used to
construct the PECs when solvent was included for both the donor
and acceptor twisted path.
The fluorescence quantum yield for the CT and LE state of
DMAPPDN in solvents of different polarity were measured relative
to quinine sulfate in 0.1 M sulfuric acid (Uf = 0.57) as secondary
standard and calculated on the basis of the following equation:
R
n2A0 If ðkf Þdkf
Uf
¼
Uf0
ð1Þ
R
n20A I0ðkf Þdkf
f
Where n0 and n are the refractive index of the solvents, A0 and A
are the absorbances, U0f and Uf are the quantum yields, and the
integrals denote the area of the fluorescence band for the standard
and the sample, respectively.
2. Experimental details
2.4. Computational details
2.1. Materials
All theoretical calculations were performed using Gaussian
03 W software [31]. Global minimum structure of DMAPPDN was
obtained by optimizing the geometry at DFT level using B3LYP
functional and 6-31+G(d,p) basis set. The ground state, first and
second excited singlet state potential energy curves (PECs) have
been calculated following TICT model [32–34]. Potential energy
surfaces were calculated by varying the twisting angle h1 for donor
group (ANMe2) and h2 for acceptor group (A(CH@CH)2ACN)
(Scheme 2) independently. The PECs of the first and second excited
singlet states were obtained using TDDFT method with same func-
tional and basis set. The TDDFT-PCM model was used for the con-
struction of PECs in acetonitrile solvent. We have used DFT, TDDFT
and TDDFT-PCM methods because the result obtain in these meth-
ods correlate well with the experimental data of the ICT processes
and also these methods have been applied in several systems in re-
cent times [35–39]. One limitation of our study is that the potential
energy surface is computed only along the twisting angle without
geometry optimization for various excited states. The validity of
such an approach that uses the ground state optimized geometry
as a basis of representation of the excited structure has been al-
ready successfully established in many recent scientific publica-
tions [35–39]. The difference in energy between the S0 and S1 or
S2 states of the calculated PECs without optimization of the excited
states at the twisted geometry is considered to be the emission en-
ergy. This emission energy value is just an estimation of emission
energy, but not the theoretical estimate of vertical emission from
the excited state, since the excited state optimization has not been
considered. HOMO, LUMO diagrams were also constructed from
the optimized structure and twisted form of DMAPPDN in vacuo
and acetonitrile solvent.
4-N,N-dimethylaminocinnamaldehyde was purchased from Al-
drich Chemicals, USA and used as received. Spectroscopic grade
solvents were purchased from Spectrochem and were used after
proper distillation. Ethanol and sulfuric acid from E. Merck were
used as received. Triple distilled water was used for the prepara-
tion of all aqueous solutions.
2.2. Synthesis of DMAPPDN
4-N,N-dimethylaminocinnamaldehyde (A) and triphenyl-
phosphanylideneacetonitrile (B) in dry dichloromethane were
stirred at room temperature for 24 h (Scheme 1). After the solvent
was removed over vacuum, the crude product was purified by sil-
ica gel column chromatography and repeated crystallization with
minimum amount of methanol to get pure nitrile compound
DMAPPDN (C). The compound DMAPPDN contains mixture of E
and Z isomer. After repeated column chromatography we obtained
the ratio of E:Z as 2.65:1. Further separation is not possible due to
their almost similar Rf values. M.P. 155 °C. 1H NMR: d(CDCl3,
300 MHz) (E:Z 2.65:1) 3.03(6H, s, Me2N); 5.01(d, JZ, 10.2 Hz, 1H);
5.21(d, JE, 15.9 Hz, 1H); 6.56–7.88(m, 7H, ArH). 13C NMR: d (CDCl3,
75 MHz) 40.05, 92.44, 94.25, 111.89, 117.50, 119.22, 119.69,
120.72, 123.10, 128.92, 129.12, 130.75, 141.97, 142.29, 150.16,
151.20, 162.22. IR
m
(cmꢀ1) 989, 1371, 1600, 2206, and 2920.
CHO
Ph3PCHCN
N
+
B
DCM
A
RT, 24 hrs
CN
θ2
θ1
N
C
Scheme 2. Optimized structure of 5-(4-dimethylamino-phenyl)-penta-2,4-diene-
nitrile (DMAPPDN) with numbering of atoms. h1 and h2 are the torsional angles at
the donor and acceptor sides, respectively.
Scheme 1. Synthetic scheme of DMAPPDN.

138
S. Jana et al. / Journal of Molecular Structure 998 (2011) 136–143
Table 1
3. Results and discussion
3.1. Absorption spectra
Spectroscopic parameters and quantum yield of 5-(4-dimethylamino-phenyl)-penta-
2,4-dienenitrile (DMAPPDN) in different solvents at room temperature.
Solvent
kabs
(nm)
kflu
(nm)
Stokes shift
(cmꢀ1
U
ꢂ 103
D
m
)
The absorption spectra of DMAPPDN (ꢁ10ꢀ6 M) in nonpolar, po-
lar protic and polar aprotic solvents are shown in Fig. 1a and the
spectral band maxima are presented in Table 1. From the absorp-
tion spectra, it is clear that DMAPPDN shows band at ꢁ390 nm in
polar protic and polar aprotic solvents, except for water and aceto-
nitrile, and in nonpolar solvents at ꢁ380 nm. These absorption
bands arise from the S0 ? S1 transition of benzene chromophore.
Compared to our previous reported similar type of substituted ben-
zene chromophoric systems [27,39], the absorption band maxima
is found to be red shifted due to the presence of two conjugated
double bond on the acceptor side and tertiary amine group as do-
nor. The absorption band maxima is slightly blue shifted in case of
n-hept
CY
MCH
n-hex
CCl4
DCM
THF
DMF
DOX
DMSO
CHCl3
ACN
BuOH
iPrOH
MeOH
EtOH
H2O
378
381
380
377
387
394
389
393
386
398
395
387
392
389
388
390
372
435, 415
439, 418
441, 420
435, 413
448
488
482, 415
507
462
514, 420
476
503
477
3466
3433
3605
3536
3485
4856
4927
5689
4228
5670
4276
5925
4545
4753
5898
5232
8083
7.59
6.55
9.6
7.72
7.36
11.43
12.84
28.58
10.57
21.33
10.09
16.13
8.67
478
504
490
533, 425
9.13
10.31
8.42
13.01
H2O
(a)
ACN
iPrOH
0.6
hydrogen bonding solvents probably due to the formation of
hydrogen bonded solvated clusters [40,41] in the ground state
and it is prominent in water, where ꢁ15 nm blue shift was ob-
served. Moderate solvent dependency of absorption band maxima
may be due to relatively high ground state dipole moment (10.8 D
calculated) of DMAPPDN.
BuOH
DMF
DMSO
n-hex
CHCl3
0.4
THF
DOX
0.2
3.2. Emission spectra
The steady state fluorescence spectra of DMAPPDN were re-
corded in various solvents by exciting at the respective absorption
maxima and shown in Fig 1b. The spectral band maxima are pro-
vided in Table 1. Interestingly, in non polar alkane type solvents,
emission spectra show structured emissions with two peak heads
at ꢁ415 and ꢁ435 nm. Compared to other commonly reported do-
nor acceptor systems the emission band in the present case is
found to be broad in nature in nonpolar solvents. In general re-
ported similar benzene derivative shows single, sharp local emis-
sion in non polar alkane type of solvent [42,43]. In the present
case, the observed structured emissions at ꢁ415 and ꢁ435 nm
may arise due to incorporation of extra flexibility by introducing
an extra double bond in DMAPPDN molecule. In polar solvents like
water, DMSO and THF the emission spectra show dual emission
bands. The higher energy emission at ꢁ420 nm corresponds to LE
band and lower energy solvent dependent bands to the CT band
[39,44]. From emission spectra it is clear that the position of the
short wavelength emission band depends slightly on the polarity
of the solvent but the long wavelength emission band exhibits sol-
vatochromic shift with increasing solvent polarity. As shown in
Fig. 1c, the excitation spectra in all solvents monitored at both
the high and low wavelength emission band matches with the cor-
responding absorption spectrum. This indicates that the molecule
exists as a single species in the ground state and excited state pho-
toinduced process is responsible for dual emission.
0.0
360
390
420
450
Wavelength (nm)
1.2
0.9
0.6
0.3
0.0
ACN
(b)
(c)
MeOH
BuOH
DOX
CHCl
3
CCl
4
MCH
H O
2
DMSO
400
450
500
550
600
Wavelength (nm)
200
150
1. CHCl3 (476)
1
2. DOX (462)
3. DMF (507)
4. DMSO (514)
5. BuOH (476)
2
3
4
5
6. iPrOH (477)
7. n-hex (435)
6
The high dipolar character of the emissive species can be ratio-
nalized by solvatochromic shift of the CT band and by the large
change in the dipole moment of DMPPDN from ground state to
the excited state. With increasing solvent polarity, the CT band
shifted more to the red because the solvent dipoles orient them-
selves around the fluorophore to attain an energetically favorable
arrangement, thereby stabilizing the polar CT state. The excited
state dipole moment has been calculated from the slope of the well
100 8. ACN (503)
7
9. THF (482)
8
10. H2O (533)
9
50
10
320
360
400
440
Wavelength (nm)
known Lippert–Mataga plot (Fig. 2a), i.e. Stokes shift (
vent parameter f( r, n). According to the following Lippert–Mat-
aga relation [45]
Dm) vs. sol-
Fig. 1. (a) Absorption (b) fluorescence emission and (c) excitation spectra (mon-
itored at corresponding emission maxima given in parenthesis) of DMAPPDN in
different solvents at room temperature.
D
e

S. Jana et al. / Journal of Molecular Structure 998 (2011) 136–143
139
Lippert–Mataga plot shows linearity for nonpolar and polar aprotic
solvents. The value of Onsagar cavity radius ( ) was calculated to be
(a)
q
5.05 Å by volume test of the optimized structure of DMAPPDN at
DFT level using B3LYP functional and 6-31+G(d,p) basis set. From
the ratio of the slope of the Lippert plot and calculated values of
q
and
23.2 D. This large difference in the dipole moment (
from ground state (
= 10.8 D) to the excited state (l⁄ = 23.2 D)
l
, the excited state dipole moment was calculated to be
lD
= 12.4 D)
l
could only be possible by redistribution of charge in the excited
state surface by intramolecular charge transfer process from the
electron rich tertiary amino donor to the nitrile acceptor group
upon photoexcitation. However, a deviation from linearity was ob-
served in the Lippert–Mataga plot in case of the protic solvents,
which indicates that hydrogen-bonding solvents have a different
type of influence on the nature of the charge transfer state [46].
We have plotted the position of the emission band (mf cmꢀ1) vs
Δ
(b)
hydrogen bonding parameter
a [17,18,39,47] for protic solvents
(Fig. 2b). The linear nature of the plot again supports that the red
shifted CT band in protic solvents is influenced by the hydrogen
bonding interaction. As seen in Fig. 2c, the plot of Stokes’ shift vs.
solvent polarity parameter ET(30) [48] generates two separate
straight lines with different slope, one for the non-polar, polar
aprotic solvents and the other for the polar protic solvents. This
observation clearly infers that two types of interaction are present.
Only dipolar interaction is effective in aprotic solvents, where both
dipolar and hydrogen-bonding interactions are present in the prot-
ic solvents.
3.3. Effect of binary solvent mixture
α
To know the origin of CT state, we have measured the emission
spectra of DMAPPDN in the binary solvent mixture of MCH and
ethanol and are shown in Fig. 3a. With gradual addition of ethanol
H2O
Aprotic solvent
Protic solvent
8000
7000
6000
5000
4000
3000
2000
(c)
to MCH solution of DMAPPDA the emission maxima show a red
max
shift from ꢁ435 nm to ꢁ485 nm (k
of DMAPPDN in EtOH
em
490 nm) with initial decreasing and then increasing of intensity
after 30% of added ethanol. This observation indicates that the
red shifted emission (CT) band originates from the LE state [19].
The emission intensity in both these solvents is almost equal due
to comparable fluorescence quantum yield of DMAPPDN in both
ethanol and MCH solvents (Table 1).
ACN
MeOH
EtOH
DMF
DCM
DMSO
iPrOH
BuOH
CHCl3
3.4. Effect of acid on steady state spectra of DMAPPDA
n-hex
Cyclohexane
To support the phenomenon of intramolecular charge transfer
reaction [18,20,25,44,47], the absorption and emission spectra
were recorded in presence of acid. Addition of dilute sulfuric acid
to the aqueous solution of DMAPPDN produces a blue shifted
absorption band at ꢁ302 nm with simultaneous decrease in inten-
sity of the original absorption band at ꢁ372 nm through an isos-
bestic point at ꢁ332 nm (Fig. 3b). The blue shifted band at
ꢁ302 nm is generated from the protonated species of DMAPPDN.
The emission spectral profile (Fig. 3c) with increasing sulfuric acid
concentration produces a blue shifted emission band at ꢁ425 nm
with progressive decrease in intensity of the original CT band
and generates an isoemissive point at ꢁ450 nm. The emission band
at 425 nm is thus assigned to the local emission of the protonated
species of DMAPPDN. When acid was added, H+ ion prefers to bind
to the lone pair of nitrogen of ANMe2 group, which is then no long-
er available for charge transfer process in the excited state. As a re-
sult the intensity of the CT band decreases. However, even with
high concentration of acid there is some intensity of the CT band.
This may be due to the excited state deprotonation of the proton-
ated DMAPPDN and generation of CT state in the excited state sur-
face which can show normal red shifted CT emission. These
observations support that the CT emission band is mainly due to
30
35 40
45
ET (30 )
50
55
60
65
Fig. 2. Plot of (a) Stokes’ shift (
band maxima against hydrogen bonding parameter (
against Reichardt solvent polarity parameter ET(30).
D
m
) against solvent parameter (
D
f), (b) emission
a
), and (c) Stokes’ shift (Dm)
2
ð
lꢃ
ꢀ Þ
l
ma
ꢀ
mf
¼
ꢂ fðer; nÞ
2p
e
0hcq3
ꢀ
ꢁ
ꢀ
ꢁ
er ꢀ 1
n2 ꢀ 1
where fð
er; nÞ ¼
ꢀ
2
er þ 1
2n2 þ 1
m
a and
m
f are the corresponding absorption and emission band posi-
r is dielectric constant and n is refractive
tion in cmꢀ1, respectively,
e
index of the medium. The terms h, e0, c, q in the above equation are
Planck’s constant (6.626 ꢂ 10ꢀ34 J s), permittivity of vacuum
(8.85 ꢂ 10ꢀ34 V Cꢀ1 mꢀ1), velocity of light (3 ꢂ 108 m sꢀ1
)
and
Onsagar cavity radius, respectively. The terms l⁄ and
l are the
ground and excited state dipole moments. It is found that

140
S. Jana et al. / Journal of Molecular Structure 998 (2011) 136–143
3.5. Fluorescence quantum yield measurement
Increasing EtOH Conc.
(a)
150
The observed fluorescence quantum yields of DMAPPDA at
room temperature with variation of solvent polarity are presented
in Table 1. The quantum yields for the CT emission in polar aprotic
solvents are found to be higher than in the polar protic solvents.
This observation indicates the existence of nonradiative decay
channels by intermolecular hydrogen bonding interaction with
protic solvents. We have also measured the fluorescence life time
of the emissive species in different solvents by picosecond TCSPC
instrument. It is found that the fluorescence decay times in differ-
ent pure solvents are found to be too short to be measured with the
picosecond TCSPC instrument. Extra flexibility may be the reason
for so fast decay of the excited state.
100
50
0
9
1
400
450
500
550
Wavelength (nm)
3.6. Theoretical calculations
Theoretical computations have been performed at DFT level
using B3LYP hybrid functional and 6-31+G(d,p) basis set to get
the global minimum structure of the molecules. The optimized
global minimum structure of DMAPPDN is shown in Scheme 2. In
the ground state global minimum structure of DMAPPDN, the
donor ANMe2 [C13AN7AC1AC2 = ꢀ1.0893 (h1)] and acceptor
A(HC@CH)2ACN [C21AC10AC4AC3 = ꢀ0.0410 (h2)] are coplanar
with that of the benzene ring. As a result there occurs an extensive
resonance delocalization in the ground state. High ground state di-
pole moment (10.8 D) of the optimized structure suggests the
unsymmetrical charge distribution in the ground state of
DMAPPDN.
(b)
Without acid
With acid
0.12
0.09
0.06
0.03
0.00
0.04
0.02
0.00
2
4
6
8
pH
The molecules having twisted geometry of the amino donor
group in their ground states prefer intense charge transfer emission
[19]. The calculated geometry for DMAPPDN is found to be planer in
the ground state. The angular dependency of the ground and two
excited states energies can be obtained by the rotation of the donor
and acceptor groups around the benzene plane. The ground state,
first and second excited states potential energy curves along the
twisted angle at the donor (h1) and acceptor (h2) groups both in va-
cuo and in ACN solvent are given in Figs. 4 and 5. The first two ex-
cited singlet states potential energy curves (PECs) were constructed
using TDDFT and TDDFT-PCM model with B3LYP functional and 6-
31+G(d,p) basis set in vacuo and ACN solvent, respectively. As seen
in Fig. 4, the potential energy of the singlet ground state (S0) in va-
cuo both for donor (Fig. 4a) and acceptor twisting (Fig. 4b) increase
with increasing twist angle and reach a maxima at h1 ꢁ 90°. This is
due to loss of delocalization of nitrogen lone pair through the ben-
zene ring in the twist conformer. As seen in Fig. 4a, for donor twist-
ing in vacuo, the nature of the S1 and S2 state surface are such that
after excitation, the molecule easily transform from locally excited
state to lowest energy charge transfer state through small barrier of
2.74 kcal/mol for S1 and 6.68 kcal/mol for S2 surface. Similarly, for
acceptor twisting in vacuo, the potential energies barrier for trans-
formation from LE to CT states are 0.83 kcal/mol and 14.17 kcal/mol
for S1 and S2 states, respectively (Fig. 4b). In vacuo, for the conver-
sion of LE to CT state, the acceptor twisting in the S1 state is more
favorable than donor twisting, due to less energy barrier for accep-
tor twisting in vacuo (0.83 kcal/mol) for S1 state compared to the
donor twisting in vacuo (2.74 kcal/mol) (Table 2). PECs has also
been constructed in ACN solvent (Fig. 5) using the TDDFT-PCM
model with same basis set and functional as mention before. The
nature of PECs for the ground and excited states are similar with
that of the vacuo except there is some difference in the energy bar-
rier. All the absorption and emission energy and oscillator strength
for experimental and theoretical calculations are presented in Ta-
ble 3. For acceptor twist in vacuo, the computed absorption and
emission energy [kabs = 378.7, kem = 449.4 nm] are well matched
with that of the experimental values in MCH solvent [kabs = 380,
300
350
400
Wavelength (nm)
40
(c)
60
40
20
0
30
20
10
0
1
2 3 4 5
pH
without acid
With acid
400
450
500
550
600
Wavelength (nm)
Fig. 3. (a) Emission spectra (kex = 380 nm) of DMAPPDN in MCH + ethanol mixed
solvent, (1 ? 9, % of EtOH = 0, 10, 15, 30, 40, 60, 70, 80, 90 respectively), Effect of
dilute sulfuric acid on the (b) absorption spectra (inset shows absorbance vs pH
plot) and (c) emission spectra (inset shows emission intensity vs pH plot)
(kex = 372 nm) of DMAPPDN in water at room temperature.
n ? p⁄ type of electronic transition. We have measured the absorp-
tion and emission spectra by using H3PO4ANaOH mixture at differ-
ent pH and the spectral behavior was found to be similar to that of
the effect of acid to DMAPPDN in aqueous solvent. Both the absorp-
tion and emission spectra were found to be invariant after pH 6.91,
which indicates that base has no effect on the ICT process in this
molecule. The variation of absorbance and emission intensity vs.
pH are shown in inset of Fig. 3b and c, respectively. From both
the graphs, it is clear that in acidic medium the absorbance and
emission intensity increases linearly with the increase of pH of
the medium. This infers that this molecule can be used as pH sen-
sor in the acidic range.

S. Jana et al. / Journal of Molecular Structure 998 (2011) 136–143
141
(a)
(a) 100
100
S2
S2
S1
80
S1
80
53.62 kcal/mol
[533.2nm]
57.66 kcal/mol
[495.8nm]
S0
S0
68.17 kcal/mol
10
0
[416.1nm]
75.50 kcal/mol
[378.7nm]
1.0
10
0
1.0
0.5
0.0
0.5
0.0
0
30 60 90
0
30 60 90
Twisted Angle
Twisted Angle
0
30
60
90
0
20
40
60
80
100
Twisted Angle (θ1 , deg)
Twisted Angle (θ1 , deg)
(b)
(b)
S2
100
80
S2
100
S1
S0
S1
80
58.31 kcal/mol
[490.3nm]
63.62 kcal/mol
[449.4nm]
S0
10
5
68.17 kcal/mol
[416.1nm]
10
5
75.50 kcal/mol
[378.7nm]
1.2
0.6
0.0
1.0
0.5
0.0
0
0
30 60 90
0
40 80 120
Twisted Angle
Twisted Angle
0
-5
0
30
60
90
0
30
60
90
Twisted Angle (θ2 , deg)
Twisted Angle (θ2 , deg)
Fig. 5. Potential energy surface for the ground state, first and second excited singlet
states of DMAPPDN with variation of twist angle (a) h1 for donor group rotation (b)
h2 for acceptor rotation in ACN solvent. Inset shows the plot of variation of oscillator
strength along twist coordinates.
Fig. 4. Potential energy surface for the ground state, first and second excited singlet
states of DMAPPDN with variation of twist angle (a) h1 for donor group rotation (b)
h2 for acceptor rotation in vacuo. Inset shows the plot of variation of oscillator
strength along twist coordinates.
Table 2
kem = 441 nm] (considered to be same as vacuo) as shown in Table 3.
From the data presented in Table 3, it is clear that for ACN solvent
acceptor twisting is more favorable compared to donor twisting
motion. Theoretically, the more red shifted emission band in ACN
solvent (490.3 nm for acceptor (Fig. 5b) and 533.2 nm for donor
twist (Fig. 5a)) compared to the emission band in vacuo
(449.4 nm for acceptor and 495.8 nm for donor twist) also supports
that the emission band depends upon the solvent polarity which
was also observed experimentally.
Potential energy barrier for the transformation from LE to CT state on S0, S1, S2
surfaces in kcal/mol.
Medium
S0 surface
S1 surface
S2 surface
Donor rotation
Vacuo
ACN
13.27
15.06
2.74
4.56
6.69
5.27
Acceptor rotation
Vacuo
ACN
8.73
10.60
0.83
2.03
14.17
13.95
The HOMO–LUMO diagrams of DMAPPDN for optimized struc-
ture (normal form) and different twisted geometries in vacuo
and ACN solvent are shown in Fig 6. In the global minima structure
of DMAPPDN, the lone pair on the nitrogen atom of ANMe2 group
as that of the optimized structure (Fig. 6a), but LUMO is different.
Here HOMO to LUMO transition is pbenzene acceptor type and it is
?
p
a forbidden transition with oscillator strength 0.000. For donor
twist geometry in vacuo and normal form in ACN solvent, the
nitrogen lone pair is more localized (HOMO orbital) compared to
localized HOMO orbital for the optimized normal structure and
acceptor twisted structure in vacuo. In the donor twisted geometry
in vacuo, the localized lone pair favors the CT process in the
twisted decoupled state. In spite of symmetry forbidden nature
of an n ? p⁄ type transition, it is energetically generated through
Frank Condon excitation, where as the CT state is generated from
LE state through the crossing of a small potential energy barrier
on the S1 surface.
is more or less uniformly distributed over the entire
the benzene chromophore for both the HOMO ( ) as well as LUMO
p⁄). Therefore the HOMO to LUMO electronic transition is mainly
p-system of
p
(
p
?
p⁄ type and it is allowed transition with high oscillator
strength (1.1909) (Table 3). On the other hand, for the donor
twisted state (90° configuration) in vacuo, the electron density is
mainly on the nitrogen atom and the HOMO (n) is a nonbonding
orbital. On the other hand, in the twisted configuration, the elec-
tron density is located over acceptor group for the LUMO (p⁄) orbi-
tal of DMAPPDN. Therefore, this is a n ? p⁄ type forbidden
transition with 0.000 oscillator strength. Due to twisting of the do-
nor ANMe2 group in vacuo, the nitrogen lone pair accumulated on
the nitrogen atom due to non-conjugation with the benzene chro-
mophore, and is available for transfer to the acceptor group. In the
MO diagram for acceptor twisted form in vacuo, the HOMO is same
3.7. Effect of donor acceptor chain length on ICT process
Here the photophysical properties of DMAPPDN were com-
pared with its analogous earlier reported without ethylene double

142
S. Jana et al. / Journal of Molecular Structure 998 (2011) 136–143
Table 3
Comparison of experimental spectral data with theoretical values of vertical transition energies (kcal/mol) of DMAPPDN in vacuo and ACN solvent using B3LYP hybrid functional
and 6-31+G(d,p) basis set (ways of calculation are given in the experimental section).
Medium
Vacuo
ACN
States
Absorption(kcal/mol)
Emission(kcal/mol)
a
b
c
Eth
Eexp
Eth
Eth
Eexp
S1
S2
S1
S2
75.50(1.191)
[378.7]d
94.93(0.023)
[302.9]
68.71(1.390)
[416.1]
94.35(0.017)
[303.0]
75.24
[380]
57.66(0.000)
[495.8]
86.05(1.356)
[332.2]
53.62(0.000)
[533.2]
80.04(1.505)
[357.2]
63.62(0.000)
[449.4]
100.35(0.044)
64.83
[441]
73.88
[387]
[284.9]
58.31(0.000)
[490.3]
97.70(0.006)
[292.6]
56.84
[503]
Values in parenthesis are calculated oscillator strength.
Eth is the calculated energy (Eexcited ꢀ Eground) at DFT level [B3LYP/6-31+G(d,p)].
Eexp the experimental value.
a
Emission energy due to twisting of ANMe2 group.
Emission energy due to twisting of A(CH@CH)2ACN group.
Eexp, the experimental value in methylcyclohexane solvent.
Data with third bracket are absorption and emission energy in wavelength (nm).
b
c
d
Table 4
HOMO
LUMO
Spectroscopic parameters, quantum yield, dipole moment and life time of DMABN,
DMACA and DMAPPDN in different solvents at room temperature for comparison of
the ICT process with increasing donor acceptor chain length.
(a)
(b)
(c)
Solvent
DMABN
DMACA
DMAPPDN
D
l
(Debye)
9.00 [49]
320 [50]
320 [50]
320 [50]
345 [50]
423 [50]
469 [53]
0.130 [50]
0.046 [50]
0.022 [50]
280 [51]
329 [52]
9.23 [54]
353 [54]
363 [54]
364 [21]
391 [54]
435 [54]
460 [54]
0.065 [54]
0.026 [54]
0.020 [21]
100 [54]
110 [54]
12.4
378
389
387
435
kabs (nm)
kem (nm)
U
n-hept
THF
ACN
n-hept
THF
ACN
n-hept
THF
ACN
H2O
ACN
482
503
0.007
0.013
0.016
<100
<100
s(ps)
D
l
=
l
⁄ ꢀ
l
,
where l⁄ and
l are ground and excited state dipole moments,
respectively.
U
is the fluorescence quantum yield and
s is the fluorescence life time. References
are given in parenthesis of each data.
(d)
shift and favorable ICT process was observed for DMAPPDN than
its lower analogs.
Fig. 6. Molecular orbital pictures (HOMO and LUMO) of DMAPPDN for the (a)
normal, (b) donor twisted, (c) acceptor twisted geometry in vacuo and (d) normal
form in ACN solvent.
4. Conclusion
In this work, the molecule DMAPPDN was synthesized, charac-
terized and its photophysical behavior have been investigated by
steady state spectroscopy in combination with quantum chemical
calculations. The molecule DMAPPDN shows dual fluorescence cor-
responding to local and solvent polarity dependent CT emission.
Additional flexibility favors Excited State Charge Transfer reaction
and extremely fast excited state decay process. The high dipole
moment of the excited state calculated by solvatochromic plot sup-
ports the charge transfer process in the excited state. The LE state
relaxes to the twisted CT state, from which red-shifted emission
band was generated and it is sensitive towards polarity and pH
of the medium, hydrogen bonding ability of solvents. The large
Stokes shift (ꢁ8083 cmꢀ1), and large change in dipole moment
from ground to excited states and highly favorable photoinduced
intramolecular charge transfer process in DMAPPDA make a way
for its use as pH sensor, electroluminescence devices, solar cells,
chemical sensor and electro optical switches. The potential energy
surfaces in vacuo and solvent along both the donor and acceptor
bond molecules 4-(N,N-dimethylaminobenzonitrile) (DMABN)
[49–53] and with a single ethylene double bond molecules 4-
(N,N-dimethylaminocinnamic acid) (DMACA) [21,54]. Till date
there are no reports about the photophysical properties of 3-(4-
dimethylamino-phenyl)-acrylonitrile (DMAPAN). Therefore we
have compared the ICT process in DMAPPDN with DMACA mole-
cule instead of DMAPAN. In general with increasing distance be-
tween donor–acceptor group the absorption and emission bands
become more red shifted from DMABN to DMAPPDN with one
exception for emission spectra of DMACA in ACN solvents (Table.
4). Also the quantum yield and fluorescence life time decrease.
This decrease in quantum yield and life time may be due to in-
crease of nonradiative decay channel with increasing flexibility
with in the molecule. The difference in dipole moment between
the ground and excited state increases which mainly depends on
the donor acceptor distance. As a result of more solvatochromic

S. Jana et al. / Journal of Molecular Structure 998 (2011) 136–143
143
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twist coordinates have been constructed using DFT methods. The-
oretical calculations also predict well the polarity dependence of
the red shifted emission band.
Acknowledgements
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N.G. acknowledges DST, India (Project No. SR/S1/PC/26/2008).
S.J. and S.D. would like to acknowledge UGC for Fellowship.
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