Comparative studies by using spectroscopic tools on the charge transfer (CT) band of a novel synthesized short-chain dyad in isotropic media and in a gel (P123)

Article abstract of DOI:10.1016/j.cplett.2009.09.054

In the present investigation the photophysics of the synthesized short-chain organic dyad, 1-(4-chloro-phenyl)-3-(4-methoxy-naphthalen-1-yl)-propenone (MNCA) has been studied both in isotropic media and gel (P123) environment by using steady state, time-resolved spectroscopic techniques and fluorescence anisotropy decay. From the NMR and time-resolved spectroscopic studies E-isomeric form (elongated nature) of the charge-transfer species of the dyad MNCA appears to be the only isomeric form in the ground state and this conformation retains even after photoexcitation whatever be the nature of the environment, the isotropic solution or micro-heterogeneous medium (gel phase of P123).

Full text of DOI:10.1016/j.cplett.2009.09.054

Chemical Physics Letters 481 (2009) 142–148
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Comparative studies by using spectroscopic tools on the charge transfer (CT) band
of a novel synthesized short-chain dyad in isotropic media and in a gel (P123)
1
2
*
Munmun Bardhan, Tapas Misra , Joydeep Chowdhury , Tapan Ganguly
Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 July 2009
In final form 15 September 2009
Available online 18 September 2009
In the present investigation the photophysics of the synthesized short-chain organic dyad, 1-(4-chloro-
phenyl)-3-(4-methoxy-naphthalen-1-yl)-propenone (MNCA) has been studied both in isotropic media
and gel (P123) environment by using steady state, time-resolved spectroscopic techniques and fluores-
cence anisotropy decay. From the NMR and time-resolved spectroscopic studies E-isomeric form (elon-
gated nature) of the charge-transfer species of the dyad MNCA appears to be the only isomeric form in
the ground state and this conformation retains even after photoexcitation whatever be the nature of
the environment, the isotropic solution or micro-heterogeneous medium (gel phase of P123).
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
[14–18]. Photosensitized electron transfer reactions through
charge-transfer (CT) are of substantial interest as a means of solar
Recently water soluble triblock copolymers have received a lot
of attention for their interesting self-assembly, rich phase diversity
and their unique solution behaviors [1–7]. Poloxamers or pluronics
(BASF) are nonionic of the type polyxyethylene–polyxypropylene
(PEO–PPO–PEO). The co-polymer (PEO)20–(PPE)70–(PEO)20 is
known as pluoronic P123 (P123). The aggregation and phase
behavior of aqueous solution of P123 copolymer have been well
studied in absence and presence of additives [8,9]. A 30% w/v solu-
tion of P123 copolymer has a gelation temperature of 289 K [9].
The hydrophobicity of the PPO block is above 289 K and conse-
quent hydration leads to the formation of a micelle phase with
hydrophobic PPO core and hydrophilic PEO corona. At sufficiently
high concentration, successive dehydration of the PPO blocks of
the P123 triblock copolymer results in a close packed crystalline
gel phase [10–13]. The quest of novel materials for building artifi-
cial light energy conversion devices has prompted the scientists to
focus their attention not only on the synthesis of chemically versa-
tile organic molecules but also to examine the effect of host matri-
ces on the conformations of these compounds. To build these types
of devices one has to understand the supramolecular photophysics.
An electron donor and an acceptor located close to each other or
separated by a flexible spacers form a light emitting charge-trans-
fer (CT) state under photoexcitation. This is a well-known phenom-
enon for intramolecular (covalently linked) systems in solutions
or light energy conversion and storage. In liquid phase isotropic
medium (both aqueous and non-aqueous) the reorganization of
the solvent sphere around a donor–acceptor pair on CT complex
is often believed to play a crucial role in affecting the efficiency
of ground or photoinduced charge separation and recombination
processes. CT reactions in supramolecular assemblies such as
cyclodextrins (CDs) have been attractive research subject in terms
of artificial photosynthetic reaction center [19–21]. To understand
better, the thermodynamics and other properties such as oscillator
strength of CT transition, charge-transfer coupling matrix element
etc. of the charge-transfer process, one needs to get rid of the sol-
vent [22–24].
In the present investigation 1-(4-chloro-phenyl)-3-(4-methoxy-
naphthalen-1-yl)-propenone (MNCA) is synthesized where the do-
nor 1-Methoxy-naphthalene (MNT) is connected with the acceptor
p-choroacetophenone (PCA) by an unsaturated bond (Fig. 1). From
NMR and the time-resolved spectroscopic investigations a unique
property of this dyad was observed. It possesses only one (elon-
gated i.e. E) isomeric species in the ground state and even upon
photoexcitation. In the present investigation, special attention
was given to examine whether the nature of the surrounding med-
ia (after the inclusion within the gel matrix from the isotropic
aqueous and non-aqueous media) has any significant effect on
the extended conformational geometry of the dyad. This investiga-
tion would help to reveal its versatility as a candidate of light en-
ergy conversion device. The present Letter comprises the
investigations made by steady state, time-resolved spectroscopic
techniques and fluorescence anisotropy decay and the analysis of
the results obtained from the measurements.
* Corresponding author. Fax: +91 33 2473 2805.
E-mail addresses: tapcla@rediffmail.com, sptg@iacs.res.in (T. Ganguly).
Sabang S K Mahavidyalaya, Nutunia, West Mednapore, West Bengal, India.
Sammilani Mahavidyalaya Baghajatin Station, West Bengal, India.
1
2
0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2009.09.054

M. Bardhan et al. / Chemical Physics Letters 481 (2009) 142–148
143
a
COCH
3
OMe
b
Cl
MNT
PCA
OMe
O
Cl
MNCA
Fig. 1. (a) Structure of P123 in (A) micellar form and in (B) gel form. (b) Molecular structures of MNT, PCA and MNCA.
70 °C for 1 h and then cooled and extracted with ether. The ether
layer was washed with water and dried over Na2SO4.
2. Experimental section
(b) Synthesis of 4-methoxy-naphthalene-1-carbaldehyde (3): To
dry DMF (295 mg, 4 mmol) freshly distilled POCl3 was added drop
wise with stirring and cooling at such a rate that temperature did
not exceed 5 °C. After 30 min 2 (164 mg, 1 mmol) was added drop
wise to the resulting solution at 0–5 °C and the reaction mixture
was stirred for half an hour at 0 °C and then at room temperature.
The reaction mixture was then continuously heated until a vigor-
ous reaction ensured at 110 °C (bath temp). After initial reaction
has subsided, the reaction mixture was heated on oil bath at
90 °C for 2 h. At the end, the mixture was cooled, neutralized with
sodium acetate solution (25%) with proper cooling and extracted
with ether. The etheral layer was washed with water and dried.
2.1. Materials
2.1.1. Synthesis and characterization
The synthesis and characterization of the donor 1-methoxy-
naphthalene (MNT) and dyad 1-(4-chloro-phenyl)-3-(4-methoxy-
naphthalen-1-yl)-propenone (MNCA) are discussed below in
detail.
(a) Synthesis of 1-methoxy-naphthalene (2) (MNT): To a stirred
solution of naphthalene-1-ol (1) (1 equiv) in 20% aqueous NaOH
solution kept at room temperature, dimethyl sulphate (3 equiv)
was added through a pressure equalizing dropping funnel. After
stirring for 45 min, the reaction mixture was warmed to 60–

144
M. Bardhan et al. / Chemical Physics Letters 481 (2009) 142–148
where G is an instrumental factor (polarization characteristic of the
photometric system) and is measured by using the relation
NMR (300 MHz, CDCl3): dH 10(1H, s, CHO), 9.28(1H, d,
2-H, J = 8.5 Hz), 8.28(1H, d, 3-H, J = 8.37 Hz), 6.81(1H, dd, 5-H,
J = 7.74 Hz), 7.68–7.5(m, 2H), 4.02(3H, s, -OMe) IR tmax
I
I
?
1678 cmꢀ1
.
G ¼
k
(c) Synthesis of 1-(4-chloro-phenyl)-3-(4-methoxy-naphthalen-
1-yl)-propenone (4) (MNCA): To the ethanolic solution (65 ml)
of 3 and p-chloroacetophenone (PCA) (0.3 g, 0.002 mol), 40%
KOH solution was added drop wise at 10 °C. The reaction mix-
ture was stirred for 15 min at that temperature and allowed to
attain room temperature and stirred overnight. A yellow precip-
itate appeared and the reaction mixture was acidified with dilute
HCl. Upon pouring into 100 ml ice-cold water the separated solid
was filtered which was washed with water until free from acid
and dried in open air, obtained as yellow solid (4), Yield ꢁ 62%,
m.p. 140 °C.
where I is the emission intensity with the polarizer set at 90° (hor-
izontal in the lab-axis) and analyzer set at 0° (vertical in the lab-
axis) while I is the emission intensity with the polarizer set at
?
k
90° (horizontal in the lab-axis) and analyzer set at 90° (horizontal
in the lab-axis). In fluorescence decay analysis, the quality of fit
was assessed over the entire decay, including the rising edge, and
tested with a plot of weighted residuals and other statistical param-
eters e.g. the reduced v
2 and the Durbin–Watson (DW) parameters.
All the solutions for room temperature measurements were deoxy-
genated by purging with argon gas stream for about 30 min.
NMR(300 MHz, CDCl3): dH 8.64[1H, d, 3-H(trans), J = 15 Hz,
8.33(1H, dd, 500-H, J = 8.28 Hz), 7.92(2H, d, 20- and 60-H,
J = 8.16 Hz), 6.88(2H, d, 30- and 500-H, J = 8.18 Hz), 7.65–7.48(6H,
m), 8.03–7.99(2H, m), 8.23(1H, dd, 70-H, J = 8.37 Hz), 4.07(3H, -
2.4. Theoretical method of calculations
The geometry optimizations for the isomers of MNCA were
done by ab initio method Density Functional Theory (DFT) with
B3LYP/6-311 G(d,p) basis function set separately.
OMe), 7.26[1H, 20-H(trans)] IR tmax 1627.8 cmꢀ1
.
3. Results and discussion
2.2. Other chemicals used
3.1. Steady state measurements
The solvents acetonitrile (ACN), benzonitrile, chloroform, etha-
nol, toluene purchased from Sisco Research Laboratory (SRL) of
spectroscopic grade were distilled under vacuum following the
standard procedure and tested before use to detect any impurity
emission in the concerned wavelength region. The copolymer
P123 of Aldrich was used as such. Water was deionized using a
Millipore Milli-Q system. In a 30 wt.% aqueous solution, P123 ex-
ists in the gel phase (cubic phase, I1) over a wide range of temper-
atures (17–45 °C) [25,26]. The gel was prepared by mixing a proper
amount of P123 with 100 ml of water. The solution was kept at a
low temperature (ꢁ0 °C) and stirred for 8–10 h using a magnetic
stirrer in a sealed container. Subsequently, the solution was kept
in a refrigerator for a week.
The UV–vis spectra of the mixture of the electron donor MNT
and the electron acceptor PCA in highly polar ACN (
e
ꢁ 37.5) fluid
solution at ambient temperature appear to be just a superposition
of the corresponding spectra of individual components (Fig. 2).
Even at higher conc. (up to 10ꢀ2 M) of the donor and acceptor,
the similar superposition exists. This result indicates the slim pos-
sibility of formation of the intermolecular charge-transfer (CT)
complex in the ground state in case of mixture of the donor and
the present acceptor. But in the case of the dyad MNCA where
the donor and acceptor moieties are connected by a short unsatu-
rated spacer, the absorption spectrum is completely changed
(Fig. 2). A broad band ranging from 325 nm to 475 nm is developed
in that ACN solvent indicating the possibility of the formation of
ground state CT complex between the two redox components, do-
nor and acceptor.
2.3. Spectroscopic apparatus
Steady state UV–vis and fluorescence emission spectra of dilute
solutions (10ꢀ2 to 5 ꢂ 10ꢀ5 mol dmꢀ3) of the samples were re-
corded at ambient temperature (296 K) using 1 cm path length
rectangular quartz cells by means of an absorption spectrophotom-
eter (Shimadzu UV–VIS 2101PC) and F-4500 fluorescence spectro-
photometer (Hitachi), respectively. Fluorescence lifetime
measurements were carried out by the time correlated single pho-
ton counting (TCSPC) method using Horiba Jobin Yvon Fluorocube.
For fluorescence lifetime measurements the samples were excited
at 405 nm using a LASER diode. The time correlated single photon
counting (TCSPC) set up consists of an ortec 9327 CFD and a Tenn-
elec TC 863 TAC. The data are collected with a PCA3 card (Oxford)
as a multichannel analyzer. The typical FWHM of the system re-
sponse is about 40 ps and Instrumental Response Function (IRF)
is 120 ps (for LASER diode). The channel width is 12 ps/channel.
With this TCSPC lifetime system, the shortest and longest possible
fluorescence lifetimes of the order of 40 ps to several microseconds
could be measured.
To know the true nature of this newly appeared long wave-
length absorption band (ranging from 325 nm to 475 nm), this
2.5
2.0
1.5
1.0
0.5
0.0
To study fluorescence anisotropy decay (time dependent anisot-
ropy), the analyzer was rotated at regular interval to get perpen-
dicular (\) and parallel (k) components. Then the anisotropy
function, r(t) was calculated using the formula.
200
250
300
350
400
450
Wavelength / nm
Fig. 2. Steady state UV–vis absorption spectra of MNT (dotted line), PCA (solid line)
and MNCA (dashed line) and mixture of MNT & PCA (dash dotted line) (conc. of all
compounds ꢁ5 ꢂ 10ꢀ5 mol dmꢀ3) in ACN.
I ðtÞ ꢀ GI ðtÞ
k
?
rðtÞ ¼
I ðtÞ þ 2GI ðtÞ
k
?

M. Bardhan et al. / Chemical Physics Letters 481 (2009) 142–148
145
band was measured in non-polar (toluene, chloroform), polar apro-
tic (ACN) and in polar protic (ethanol and water) solvents having
different polarity (ranging from of dielectric constant 2.4 to 80).
The normalized absorption spectra in different solvents are
reproduced in the Fig. 3a. The absorption maximum suffers a con-
siderable gradual bathochromic shift with the increasing of the sol-
vent polarity (es ꢀ dielectric constant). From non-polar toluene
Now to elucidate the solvent effect, the fluorescence of the
MNCA was studied in various aprotic solvents of different polari-
ties (normalized fluorescence spectra are shown in Fig. 4). In case
of aprotic solvents, with the increase of the polarity of the solvent,
the fluorescence maxima are continuously red-shifted. In aqueous
medium, the fluorescence peak shifts to far long-wavelength
(ꢁ518 nm) region (ꢁnearly 60 nm shift from the situation in
non-polar toluene). These observations further corroborate our
views that the fluorescence of the dyad MNCA mainly originates
from the CT state. The similar trend was observed in the case of
the other bichromophores (or dyads) used in our earlier observa-
tions [27,28]. So, we can use this dyad MNCA as a polarity-sensitive
fluorescent probe in these studies.
(
es ꢁ 2.4) to polar water (es ꢁ 80), the absorption shift was nearly
30 nm and there is significant change in absorption band width.
These results indicate in favor of the formation of intramolecular
CT complex in ground state of the dyad MNCA.
3.2. Steady state fluorescence studies
The steady state fluorescence emission band of MNT in ACN was
found to reside within 310 nm to 450 nm region (Fig. 3b). In case of
dyad MNCA, the fluorescence emission band between 310 nm and
450 nm which corresponds to donor MNT, was found to be
strongly quenched. Moreover, a new long wavelength broad emis-
sion band peaking at 500 nm appeared (Fig. 3b). Thus when the
electron donor moiety MNT is present within the MNCA dyad with
the acceptor PCA, its fluorescence is considerably quenched
(Fig. 3b). Again when CT absorption band of MNCA was excited, a
fluorescence band at the same energy position (500 nm) was
found. It is apparent from this observation that this band at
500 nm should be assigned as the CT emission one. It reveals from
the steady state measurements that when the donor and acceptor
moieties are connected by the unsaturated bond like short spacer,
the mutual alignment of the two redox centers would be such that
it will facilitate the formations of charge transfer (CT) or ion-pair
complex both in the ground as well as in the excited electronic
state.
3.2.1. Steady state characteristics in heterogeneous organized
assembly
P123, the co-polymer used in the present study, turns into gel at
289 K [9]. When such a sol-gel transition takes place, the viscosity
of the solution increases significantly and it ceases to flow. Never-
theless, at microscopic level, the gel is essentially made up of high
concentration of micelles. Fig. 1a represents the formation of mi-
celle phase with hydrophobic PPO core and a hydrophilic PEO cor-
ona. At sufficiently high concentration, successive dehydration of
PPO block of the P123 triblock co-polymer results in a close pack
crystalline gel phase (Fig. 1a). In the gel phase around 48% of the
total space is void and is occupied by water [29–31].
From detail studies, fluorescence behavior and red edge excita-
tion shift (REES) of steady state CT fluorescence were observed in
gel-phase of P123. Fig. 5b shows the emission spectrum of MNCA
(CT fluorescence region only) in P123 gel for different excitation
wavelengths (kex). The variations of the CT emission maximum of
MNCA in P123 gel as a function of kex are shown in Fig. 5c. Sigmoi-
dal shape of the curve (Fig. 5c) suggests that the microenvironment
of P123 gel is highly heterogeneous and there are broadly two
1
5
environments,
a
hydrophilic
peripheral
shell
(where
a
1.0
0.8
0.6
0.4
0.2
0.0
kem ꢁ 491 nm) and a hydrophobic core (with kem ꢁ 487 nm). At
very short kex (ꢁ350 nm) molecules in the core region are prefer-
entially excited and exhibits blue shifted emission spectrum
(kem ꢁ 487 nm). On the contrary, excitation at the red end
(kex ꢁ 420 nm), MNCA molecules in the peripheral hydrophilic re-
gion are selectively excited that give rise to a red shifted emission
max
spectrum (
c
ꢁ 491 nm). However at an intermediate kex, both
em
these two regions are excited and emission maximum of MNCA
are found in between 487 nm and 491 nm.
From Fig. 5c it is clear that the emission maximum of CT spectra
of MNCA undergoes a small (ꢁ4 nm) but definite red-shift with
change in kex. This red-shift of the emission spectra of MNCA sug-
gests that at the different position of the solute (or dyad), the
300
350
400
450
500
550
Wavelength / nm
1
b
1
5
450
300
150
0
1.0
0.8
0.6
0.4
0.2
0.0
3
2
2
300
400
500
600
450
500
550
600
650
Wavelength / nm
Wavelength / nm
Fig. 3. (a) Normalized absorption spectra of MNCA in (1) Toluene, (2) P123, (3)
Chloroform, (4) EtOH, (5) H2O. (b) Fluorescence emission spectra of MNT (conc.
Fig. 4. Normalized emission spectra of MNCA in (1) Toluene (kex ꢁ 383 nm), (2)
P123 (kex ꢁ 390 nm), (3) Chloroform (kex ꢁ 391 nm), (4) EtOH (kex ꢁ 390 nm), (5)
H2O (kex ꢁ 415 nm).
ꢁ5 ꢂ 10ꢀ5 mol dmꢀ3
)
with
excitation
ꢁ280 nm
(1),
MNCA
(conc.
ꢁ5 ꢂ 10ꢀ5 mol dmꢀ3) with excitations ꢁ280 nm (2) and ꢁ370 nm (3) in ACN.

146
M. Bardhan et al. / Chemical Physics Letters 481 (2009) 142–148
wavelength in highly polar water solution, which corresponds to
0.60
0.45
0.30
0.15
0.00
CT fluorescence emission decay time in polar environment. Single
exponential nature of fluorescence decay shows that only one type
of species is involved in this decay. On the other hand, the fluores-
cence lifetime in the non-polar toluene was observed to be 2.5 ns
(Table 1). With increasing the solvent polarity it is observed from
the time-resolved fluorescence measurements that the lifetime de-
creases (Table 1).
Time-resolved fluorescence data of MNCA in P123 gel exhibit
two different time constants when excited at 390 nm. From the
fractional contributions (fi) shown in the Table 1 it is apparent that
nearly 84% of the dyad present in aqueous phase i.e. water filled
tunnel (void) of P123 system and 16% in hydrophobic core (non-
polar environment) PPO block. From this observation it can be in-
ferred that in gel phase distribution of the dyad is maximum in po-
lar (i.e. in pore) region. This faster decay of the excited CT
(intramolecular) species within the relatively polar isotropic fluid
solution corresponds with the fluorescence decays nature of in-
fra-red CT emission of porphyrin-fullerene dyad [33].
In this gel phase the location of the solutes must be ascertained
for proper understanding of the experimental results. In micro-het-
erogeneous systems, the solutes can be solubilized either in core or
palisade layer or even in aqueous phase (pore (void) region) unlike
in homogeneous liquids. The probe MNCA, though sparingly solu-
ble in water gives sufficient fluorescence counts under laser excita-
tion. In bulk water, the reorientation time of fluorescence
anisotropy decay of MNCA is 0.17 ns at 298 K. The fluorescence
anisotropy decay of MNCA in P123 gel (Fig. 6) is found to be much
slower than the measured reorientation time of MNCA in water at
this temperature. The fluorescence anisotropy decay is fitted to a
biexponential function. For kex ꢁ 405 nm the anisotropy decay is
described by two decay components of 2.03 ns (99%) and 0.18 ns
(1%) but the fractional contribution of the fast component is so
small that we can neglect it. The above mentioned facts indicate
the possibility that relatively rigid dyad MNCA is distributed in
the restricted medium P123 gel.
a
250
300
350
400
450
Wavelength / nm
2
1
b
1.0
0.8
0.6
0.4
0.2
0.0
3
450
500
550
600
Wavelength / nm
490.7
490.0
489.3
488.6
487.9
487.2
c
3.4. NMR analysis to reveal the nature of the ground state species
In the present investigation alkoxynaphthalene (MNT) as donor
moiety and acceptor PCA is connected by a short unsaturated
spacer in the MNCA dyad. In our earlier work, it was found [27]
for both 4MBA and 4MBAS, at least two geometrical isomers of
E- (elongated) and Z-type (folded) are expected. Indeed, the 1H
NMR spectra of 4MBA and 4MBAS revealed that although most of
the molecules in the ground state exist in the E-form, a trace
amount of Z-isomer was also present. The widely different cou-
pling constants of Z- and E-olefinic protons helped to identify the
two isomers. The percentage of the Z-isomer increased signifi-
cantly upon photoexcitation.
350 360 370 380 390 400 410 420
λex(nm)
Fig. 5. (a) Absorption spectra of MNCA in P123 gel indicating the exciting
wavelength. (b) Normalized emission spectra of MNCA at kex ꢁ (1) 350 nm, (2)
420 nm in P123 gel and (3) in H2O (kex ꢁ 415 nm). (c) Plot of emission maximum of
MNCA in gel phase (30 wt.%) as a function of exciting wavelength.
The present dyad MNCA was found to exist only as a single iso-
mer of E-type in the ground state, clearly evident from the coupling
constants of the olefinic protons in the 1H NMR spectrum (Section
2). There are two olefinic protons and six ortho-coupled protons
microenvironment for that dyad gradually becomes more polar in
nature. Such type of small but regular interactions also is found for
coumarin derivatives in micro-heterogeneous media such as mi-
celle/gel etc. [32]. For all kex, the emission maximum of CT state
of MNCA in P123 gel is found to be blue-shifted from that
(518 nm) observed in the bulk water.
Table 1
Time-resolved fluorescence data of MNCA in gel P123 and in different solvents of
varying polarity (kex ꢁ 405 nm, fi ꢁ fractional contribution).
Solvent
kem s1
s2
f1
f2
v2
P123
H2O
ACN
EtOH
490
518
500
516
0.2 ns ( 0.1 ns) 2.4 ns ( 0.1 ns) 0.84 0.16 1.1
62 ps ( 1 ps)
66 ps ( 1 ps)
87 ps ( 1 ps)
–
–
–
–
–
1
1
1
1
1
–
–
–
–
–
1.02
1.02
1.17
1.16
1.2
3.3. Time-resolved fluorescence studies
Chloroform 480 147 ps ( 1 ps)
Toluene 445 2.5 ns ( 0.1 ns)
From the time-resolved fluorescence data, only a very fast com-
ponent (ꢁ62 ps) was observed when monitored at 500 nm

M. Bardhan et al. / Chemical Physics Letters 481 (2009) 142–148
147
Thus it is confirmed from time-resolved fluorescence spectra,
NMR and Theoretical calculation that MNCA dyad presents mostly
in E-isomeric form in ground as well as in excited state.
0.4
0.3
0.2
0.1
3.6. Coupling characteristics of CT spectra
In the present DA system (dyad), the electron donor (methoxy-
naphthalene; MNT) and acceptor groups (p-chloroacetophenone;
PCA) are separated by a short flexible olefinic unsaturated bond.
Due to this short bond separated dyad system, weak overlapping
between the donor and acceptor moieties present within the dyad
could be expected. It is logical to presume that this overlapping
causes the formation of ground state CT. From the UV absorption
and fluorescence spectral measurements, the formations of CT both
in the ground as well as in the excited states have been con-
firmed.From the CT band in these systems the electronic coupling
matrix element (HAD) for charge transfer can be estimated by using
the Hush relation [33,38–41]
0.0
-0.1
0.0
1.5
3.0
Time / ns
4.5
6.0
Fig. 6. Fluorescence anisotropy decay of MNCA along with fitted curve in P123 gel
(30 wt.%) at kex ꢁ 405 nm and kem ꢁ 490 nm.
beside four other aromatic protons in the molecule. The coupling
constant of the olefinic protons is 15 Hz, clearly indicating trans
(E-form) coupling. The coupling constants of all the ortho-protons
are ca. 8 Hz.
2:06 ꢂ 10ꢀ2
1=2
ꢀCT
ꢀCT
1=2
HDA ðcmꢀ1Þ ¼
ð
eCT
c
maxD
c
Þ
max
R
where R, separation between the centers of MNT nucleus and the
acceptor moiety in Å, emax is molar absorptivity at the absorbance
3.5. GC analysis
maximum if the CT absorption band in Mꢀ1 cmꢀ1 and
Dm is the full
width of the band at the half maximum in cmꢀ1
.
Further corroboration of the formation of only E-isomer came
from the GC analysis of the MNCA dyad. The analysis showed the
existence of only extended structured isomeric species in the
ground state similar to the observations made from the NMR anal-
ysis also.
Thus, by careful synthesis and using the techniques NMR and
GC, it was clearly found that MNCA dyad exists only in the form
of E-isomeric species and no Z-isomer of this dyad was noticeable,
not even in trace amount. Interestingly, upon photoexcitation also,
the elongated conformation of MNCA remains unaltered, as evi-
denced from time-resolved spectroscopic measurements. The lack
of formations of folded conformations in the excited state, which
are rather unusual for these types of dyad systems, indicates the
structural rigidity and stability of this dyad. The theoretical predic-
tions (TD-DFT) also show the similar trend as shown below:
The values of electronic coupling matrix element are given in
Table 2. The values of HDA in different isotropic and micro-hetero-
geneous media (Gel, P123) are observed to be low (ꢁ0.3 eV) and
remain unchanged. This lower coupling is indicative of the separa-
tion of the two redox components from each other (extended i.e.,
E-form) because if the redox components are in close proximity,
stronger coupling could be expected [42]. This observation is in ac-
cord to our expectation as the dyad MNCA appears to be of elon-
gated nature both in the ground, as confirmed from the NMR
studies, and electronic excited states as evidenced from the time-
resolved fluorescence studies. This lower value of coupling matrix
has been reported for perylene excimer (as 0.37 eV) by Tachiya et
al. [41] and Vehmanen et al. [33,43] for compact porphyrin-fuller-
ene dyad systems. Such a lower coupling matrix values are very
much suitable for various artificial photosynthetic devices [42,44].
Moreover, the magnitude of the electronic coupling matrix ele-
ment, HDA, remains unchanged (Table 2) when the dyad (MNCA) is
dissolved in various isotropic aqueous and non-aqueous fluid solu-
tion to gel phase (solid/crystalline phase) of P123 (micro-heteroge-
neous medium). This is in accord to our expectation. The
conformational geometry remains more or less unchanged even
after the inclusion within the gel matrix from the isotropic aqueous
and non-aqueous media. The present novel synthesized dyad
MNCA is unique in the respect that E-isomeric form (elongated
nature) of the charge-transfer species appears to be the only iso-
meric form in the ground state and this elongated structure remain
the same even after photoexcitation when they are in the excited
states even in the isotropic solution and micro-heterogeneous
medium (gel phase of P123).
3.5.1. Theoretical method of calculations
The geometry optimizations for the Z- and E-isomer of MNCA
were done by ab initio method Density Functional Theory (DFT)
with B3LYP/6-311 G(d,p) basis function [34–36] set separately.
All are implemented in the Gaussian package [37]. Molecular co-
ordinates were obtained from minimum energy geometry using
Chem-office molecular modeling software. For electronic absorp-
tion spectra of optimized isomer of molecule or vertical excitation
energy in ground state, Time Dependent Density Functional Theory
(TD-DFT) with B3LYP/6-311 G(d,p) basis function was employed.
3.5.2. Percentage of isomers in ground state
In case of E-isomer, the sum of electronic and thermal enthal-
pies is ꢀ1381.737093 hartree per particle and entropy is 139.019
of electronic and thermal enthalpies is ꢀ1381.727821 hartree per
particle and entropy is 149.203 cal/mol K. Thus, from the calcula-
tions of heat of formation and entropy term, the ground state equi-
librium constant between E- and Z-isomers is computed to be
101.133. Therefore, it is assigned that the percentage of E-isomer
is 99.02% in the ground state. Thus, the theoretical predictions cor-
relate well with the experimental findings that E-isomers are
mostly present in the ground state.
Table 2
Coupling matrix of MNCA in solvents having different polarity.
CT
CT
CT
Medium
e
HDA
(eV)
e
c
Dc
1=2
max
max
(Mꢀ1cmꢀ1
)
(cmꢀ1
)
(cmꢀ1
)
Toluene
Chloroform
EtOH
ACN
H2O
2.4
4.8
30
37.5
80
6100
9100
8900
5100
3200
7200
26 000
25 000
25 000
27 000
23 000
25 000
5100
5100
5200
6600
7300
6000
0.31
0.37
0.37
0.33
0.29
0.36
In case of E-isomer the ground state dipole moment in vacuum
is 5.7235 Debye, in ACN environment 7.1556 Debye whereas in tol-
uene environment it is 6.3820 Debye.
Gel (P123)

148
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Acknowledgment
M.B. acknowledges the Council of Scientific and Industrial
Research (CSIR), New Delhi, India for providing her the financial
assistances in the form of CSIR NET fellowship.
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