Nonradiative inter- and intramolecular energy transfer from the aromatic donor anisole to a synthesized photoswitchable acceptor system

Article abstract of DOI:10.1016/j.saa.2009.11.034

We report steady state and time resolved fluorescence measurements on acetonitrile (ACN) solutions of the model compounds, energy donor anisole (A) and a photoswitchable acceptor N,N′-1,2-phenylene di-p-tosylamide (B) and the multichromophore (M) where A

Full text of DOI:10.1016/j.saa.2009.11.034

Spectrochimica Acta Part A 75 (2010) 647–655
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Nonradiative inter- and intramolecular energy transfer from the aromatic donor
anisole to a synthesized photoswitchable acceptor system
1
2
Munmun Bardhan, Sudeshna Bhattacharya, Tapas Misra , Rupa Mukhopadhyay ,
Asish De, Joydeep Chowdhury , Tapan Ganguly
3
∗
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 29 May 2009
Received in revised form
We report steady state and time resolved fluorescence measurements on acetonitrile (ACN) solutions
of the model compounds, energy donor anisole (A) and a photoswitchable acceptor N,N -1,2-phenylene
ꢀ
di-p-tosylamide (B) and the multichromophore (M) where A and B are connected by a spacer containing
both rigid triple (acetylenic) and flexible methylene bonds. Both steady state and time correlated single
photon counting measurements demonstrate that though intermolecular energy transfer, of Forster type,
1
2 November 2009
Accepted 17 November 2009
8
−1
between the donor and acceptor moieties occurs with rate 10 s but when these two reacting com-
Keywords:
Energy transfer
1
1
−1
ponents are linked by a spacer (multichromophore, M) the observed transfer rate (∼10
s ) enhances.
This seemingly indicates that the imposition of the spacer by inserting a triple bond may facilitate in the
propagation of electronic excitation energy through bond. The time resolved fluorescence measurements
along with the theoretical predictions using Configuration interaction singles (CIS) method by using 6-
31G (d,p) basis set, implemented in the Gaussian package indicate the formations of the two excited
conformers of B. The experimental findings made from the steady state and time resolved fluorescence
measurements demonstrate that, though two different isomeric species of the acceptor B are formed in
the excited singlet states, the prevailing singlet–singlet nonradiative energy transfer route was found
from the donor A to the relatively longer-lived isomeric species of B.
Photoswitchable system
Time resolved fluorescence
Excited conformers
Multichromophoric system
©
2009 Elsevier B.V. All rights reserved.
1
. Introduction
isomeric species) develops in the electronic excited states. Time
resolved fluorescence studies clearly reveal the conversions of
folded type or Z-isomeric structures, due to light absorption, from
the ground E-type isomeric species. In the present system, the
Considerable interest are devoted in recent years to investigate
intramolecular electronic energy transfer processes within various
organic energy donor and acceptor molecules being linked together
by various (rigid, semi-rigid or flexible) spacers [1–10]. These
studies would assist the understanding of electronic energy trans-
fer mechanisms within a variety of multichromophoric systems
including the light collection processes in natural photosynthetic
systems. However, so far our knowledge is concerned, not much
work was done before on the excitation energy transfer processes
within an energy donor and a photoswitchable acceptor. In many
organic dyad systems [11–13] we studied, generally extended type
conformation (or isomeric species) prevails in the ground state.
On photoexcitation, preponderance of folded structures (cis-type
ꢀ
photoswitchable molecule N,N -1,2-phenylene di-p-tosylamide (B)
(Fig. 1a) was used as an energy acceptor molecule in the presence
of the energy donor anisole (A). Though NMR studies demonstrate
the presence of only a particular conformation of B in the ground
state but time resolved fluorescence studies confirmed the pres-
ence of the two isomeric species (from the observed two different
fluorescence lifetimes) in the excited singlet state S . In the present
1
paper the detailed steady state and time resolved spectroscopic
investigations and the results obtained from therein to reveal the
nature of energy transfer mechanisms both in intermolecular and
intramolecular cases have been described. The efficiencies of the
excitation energy transfer processes for the free-encountering and
rigid systems are compared in the present investigation to exam-
ine whether the rigidity imposed in the spacer by inserting a triple
bond or transition dipole orientation in the donor and acceptor may
result in hindrance in propagation of electronic excitation energy.
Our primary aim of this study is to develop efficient photoswitch-
able devices through photophysical investigations on excitational
energy transfer from the organic donor to a photoswitchable accep-
∗ Corresponding author. Tel.: +91 33 2473 4971x253; fax: +91 33 2473 2805.
E-mail addresses: sptg@mahendra.iacs.res.in,
tapcla@rediffmail.com (T. Ganguly).
1
Address: Sabang S.K. Mahavidyalaya, Nutunia, West Mednapore, West Bengal,
India.
2
Address: Chemgenpharma International, Salt Lake, Kolkata 700091, India.
Address: Sammilani Mahavidyalaya Baghajatin Station, West Bengal, India.
3
1
386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.11.034

6
48
M. Bardhan et al. / Spectrochimica Acta Part A 75 (2010) 647–655
Fig. 1. (a) Molecular structures of A, B. (b) Molecular structures of M. (c) Two conformers present in the first excited singlet state of B molecule.
tor system. So far in our knowledge not much work was done before
using a photoswitchable system as an energy acceptor.
1596.1, 1498.8, 1415.3, 1396.9, 1326.1, 1278.0, 1148.0, 1087.4,
−
1
934.2, 810.2, 761.8 cm ; UV (CHCl ) ꢁmax 242.8 nm (log ε = 4.185);
3
1
H NMR (300 MHz, CDCl ) ı 3.61 (s, 6H, Ar–CH ), 8.15–8.86 (m,
2H, Ar–H), 9.93 (s, 2H,–NH–); C NMR (75 MHz, CDCl ) ı 21.73,
24.27, 126.22, 127.53, 129.70, 130.28, 136.48, 143.85. Anal. Calcd.
3
3
13
1
1
3
2
2
2
. Experimental
for C20H20N S O : C, 57.69, H, 4.80, N, 6.73. Found: C, 57.70, H, 4.80,
2
2
4
.1. Materials
N, 6.65.
.1.1. Synthesis and characterization
The synthesis [14,15] and characterization of the N,N -1,2-
ꢀ
ꢀ
ꢀ
ꢀ
2
-
.1.1.2. N-(prop-2 -ynyl)-N,N -1,2-phenylene di-p-tosylamide. N,N
1,2-phenylene di-p-tosylamide (2.0 g, 0.48 mmol) was dissolved
in dry acetone (50 ml) and anhydrous potassium carbonate (1.33 g,
.96 mmol) was added to it. After stirring overnight at room
ꢀ
phenylene di-p-tosylamide (B) and N-[(3 -4-methoxyphenyl)prop-
ꢀ
ꢀ
2
-ynyl]-N,N -1,2-phenylene di-p-tosylamide (M) (Fig. 1a and b) are
discussed below in detail.
0
temperature, propargyl bromide (0.514 g, 0.43 mmol) was added
drop-wise to the stirred solution over half an hour. The reaction
mixture was stirred at room temperature for 1 h, followed by heat-
ꢀ
2
.1.1.1. N,N -1,2-phenylene di-p-tosylamide (B). To a solution of o-
phenylenediamine (1.0 g, 0.92 mmol) in dichloromethane (25 ml),
p-toluene sulfonyl chloride (3.53 g, 1.84 mmol) and pyridine
◦
ing at 80 C for 16 h. The reaction mixture was filtered after this
(
1.46 g, 1.84 mmol) were added. After stirring the reaction mix-
period and the residue was washed with dry acetone (3× 50 ml).
The filtrate and the washings were collected and the solvent evap-
orated. The residue was dissolved in chloroform (50 ml) and the
chloroform solution was washed with water (3× 25 ml) and then
dried over anhydrous sodium sulfate. The crude residue obtained
after the removal of solvent was purified by column chromatogra-
phy using silica gel column (60–120 mesh) and 50% chloroform
ture for 24 h, the organic layer was separated and washed with
water (3× 25 ml) and dried with anhydrous sodium sulfate. After
the removal of the solvent the crude product was purified by
column chromatography using silica gel (60–120 mesh). Elution
with 2% ethyl acetate in chloroform afforded N,N -1,2-phenylene
di-p-tosylamide (3.2 g, 83%) as a white solid, which was crystal-
ꢀ
◦
◦ ꢀ
in petroleum ether (60–80 C) as the eluent. N-(prop-2 -ynyl)-
lized from chloroform. m.p. 205 C; IR (KBr) ꢀmax 3318.6, 3220.1,

M. Bardhan et al. / Spectrochimica Acta Part A 75 (2010) 647–655
649
ꢀ
N,N -1,2-phenylene di-p-tosylamide (1.96 g, 90%) was obtained as
white crystalline solid which was further purified by crystalliza-
◦
tion from chloroform and petroleum ether: m.p. 175 C; IR (KBr),
ꢀ
1
max 3289.6, 3262.2, 2118.0, 1596.1, 1493.6, 1413.2, 1343.6, 1185.6,
−1
163.2, 1090.0, 905.1, 717.4, 669.7 cm ; UV (CHCl ) ꢁmax 242.6 nm
3
1
(
log ε = 4.181); H NMR, ı 2.11 (t, J = 2 Hz, 1H, ≡CH), 2.38 (s, 3H,
Ar–CH ), 2.42 (s, 3H, Ar–CH ), 3.35 (d, J = 18 Hz, 1 H, –CH ), 4.32 (d,
3
3
2
13
J = 18 Hz, 1H, –CH ), 6.73–7.82 (m, 12H, Ar–H); C NMR (75 MHz,
2
CDCl ), ı 21.9, 22.1, (Ar–CH ), 41.7 (–CH ), 75.0 (≡CH), 122.5, 125.1,
3
3
2
1
1
2
1
6
27.8, 129.1, 129.2, 129.3, 129.6, 129.7, 130.1, 130.4, 134.2, 136.6,
1
3
37.2, 144.4, 145.1; C NMR (75 MHz, CDCl , DEPT–135) ı 21.6,
3
1.8, 41.4 (inverted), 122.2, 124.7, 127.6, 128.8, 129.0, 129.5, 129.8,
30.1, 131.3. Anal. Calcd. for C23H22N S O : C, 60.79, H, 4.84, N,
2
2
4
.16. Found: C, 60.49, H, 4.92, N, 6.13.
ꢀ
ꢀ
ꢀ
2
.1.1.3. N-[(3 -4-methoxyphenyl)prop-2 -ynyl]-N,N -1,2-phenylene
di-p-tosylamide (M). A mixture of 4-iodo anisole (0.18 g,
.08 mmol), (PPh ) PdCl (0.018 g, 3 mol%), copper(I) iodide
0
3
2
2
(
0.013 g, 8 mol%) and triethylamine (0.446 g, 0.55 mmol) was
stirred in acetonitrile (10 ml) for half an hour under nitrogen
Fig. 2. Steady state UV–vis absorption spectra of
A
(dotted line) (conc.
ꢀ
ꢀ
∼1 × 10−4 mol dm−3), B (solid line) (conc. ∼1 × 10−5 mol dm−3), and mixture of A and
atmosphere. The acetylenic compound N-(prop-2 -ynyl)-N,N -1,2-
phenylene di-p-tosylamide (0.48 g, 0.1 mmol) in acetonitrile (3 ml)
was slowly added from a syringe to the reaction mixture followed
by stirring at room temperature for 24 h. After removal of the
solvent under reduced pressure, water (50 ml) was added to the
residue followed by extraction with chloroform (3× 25 ml). The
combined chloroform layer was washed with water (3× 25 ml) and
was then dried over anhydrous sodium sulfate. The residue left
after removal of solvent was purified by column chromatography
using neutral alumina column and 25% ethyl acetate in petroleum
−
4
−3
−5
−3
B (dashed line) (conc. of A ∼1 × 10 mol dm and conc. of B ∼1 × 10 mol dm
−3
)
−
5
in mixture) in ACN. Inset: Absorption spectra of B (conc. ∼1 × 10 mol dm
−4
−3
(
solid line) and mixture of A and B (conc. of A ∼1 × 10 mol dm and conc. of
B ∼1 × 10−5 mol dm−3 in mixture) (dashed line) from 296 nm to 305 nm.
and tested with a plot of weighted residuals and other statisti-
cal parameters, e.g., the reduced ꢂ2 and the Durbin–Watson (DW)
parameters. All the solutions for room temperature measurements
were deoxygenated by purging with argon gas stream for about
3
3
3
0 min.
ꢀ
ꢀ
ether as eluent to afford N-[(3 -4-methoxyphenyl)prop-2 -ynyl]-
ꢀ
◦
N,N -1,2-phenylene di-p-tosylamide as white solid m.p. 145 C
crystallized from ethyl acetate–petroleum ether); IR (KBr), ꢀmax
. Results and discussion
(
3
1
(
286.0, 1601.6, 1509.1, 1503.1, 1400.4, 1347.9, 1295.3, 1247.1,
.1. Steady state UV–vis studies
−1
165.4, 1090.5, 1049.0, 932.9 cm ; UV (CHCl ) ꢁ max 245.8 nm
3
log ε = 4.399) 255.4 nm (log ε = 4.407); 1H NMR (300 MHz, CDCl3)
Fig. 2 shows the UV–vis absorption spectra of the molecules,
anisole (A) and the molecule B (Fig. 1a), in highly polar ACN
ı 2.36 (s, 6H, Ar–CH ), 3.48 (d, J = 18 Hz, 1H, –CH , H␣), 3.80 (s, 3H,
3
2
–
OCH ), 4.56 (d, J = 18 Hz, 1H, –CH , H ), 6.77–7.8 (m, 16 H, Ar–H),
3
2
␤
(
εs ∼ 37.5) fluid solution. In this medium, it was found that the
1
3
C NMR (75 MHz, CDCl ) ı 21.5, 21.6, 21.7, 42.3, 55.3, 81.3, 86.1,
3
spectra of the mixture of A and B are just the superposition of
the absorption bands of the individual components. This obser-
vation excludes the possibility of formation of any intermolecular
ground state charge transfer (CT) complex within the reactants
1
1
13.9, 114.0, 122.4, 124.7, 127.4, 127.6, 128.4, 128.7, 128.7, 129.3,
29.7, 129.8, 130.5, 132.9, 134.3, 136.2, 143.9, 144.4, 159.9, 13
C
NMR, (75 MHz, CDCl , DEPT–135) ı 21.7, 42.47 (inverted), 55.4,
3
1
14.0, 122.5, 124.8, 127.5, 128.8, 129.0, 129.4, 129.8, 130.0, 133.1.
Anal. Calcd. for C30H28N S O5: C, 64.28, H, 5.0, N, 5.0. Found: C,
2
2
6
4.27, H, 5.14, N, 4.92.
.2. Other chemicals used
The solvents acetonitrile (ACN) (SRL) and toluene (SRL) of spec-
2
troscopic grade was distilled under vacuum following the standard
procedure and tested before use to detect any impurity emission in
the concerned wavelength region.
2.3. Spectroscopic apparatus
Steady state UV–vis and fluorescence emission spectra of dilute
−4
−6
−3
solutions (10 to 10 mol dm ) of the samples were recorded
at ambient temperature (296 K) using 1 cm path length rectangular
quartz cells with the absorption spectrophotometer (Shimadzu UV-
VIS 2101PC) and F-4500 fluorescence spectrophotometer (Hitachi),
respectively. Fluorescence lifetime measurements were carried
out with the help of the time correlated single photon counting
(
TCSPC) method using HORIBA JOBIN YVON FLUOROCUBE. The typ-
−
4
−3
)
Fig. 3. Steady state UV–vis absorption spectra of A (conc. ∼1 × 10 mol dm
ical FWHM of the system response is about 40 ps. The quality of
fit was assessed over the entire decay, including the rising edge,
−5
−3
)
(
dotted line),
B
(conc. ∼6.37 × 10 mol dm
(solid line) and M (conc.
−
5
−3
∼2.4 × 10 mol dm ) (dashed line) in ACN.

6
50
M. Bardhan et al. / Spectrochimica Acta Part A 75 (2010) 647–655
Fig. 4. Fluorescence emission spectra of A (conc. ∼1 × 10− mol dm ) with exci-
4
−3
−4
−3
Fig. 5. Fluorescence emission spectra of A (conc. ∼1 × 10 mol dm ) in the pres-
−
5
−3
tation wavelength ∼280 nm (dotted line), B (conc. ∼6.37 × 10 mol dm ) with
ence of different conc. of B (ꢁ ∼ 277 nm) in ACN at the ambient temperature. conc.
ex
−
5
−3
−6
−3
−6
−3
−6
−3
.
excitation wavelength ∼280 nm (solid line) and M (conc. ∼2.4 × 10 mol dm
with excitation wavelength ∼280 nm (dashed line) in ACN.
)
of B is (0) 0, (1) 3 × 10 mol dm , (2) 5 × 10 mol dm , (3) 7 × 10 mol dm
Inset: The growth of B is shown in the mixture of A and B in magnified form.
at ambient temperature. Even at relatively higher concentration
−
3
−3
(
∼10 mol dm ) of A and B, the similar spectral superposition of
the individual components was observed.
using excitation at 277 nm which corresponds to the S1 state of A.
In this case decrement in the fluorescence intensity of A was appar-
ent accompanied by the gradual augmentation of the fluorescence
band of B (Fig. 5, inset). It is to be mentioned here that the fixed con-
centration of A (0.13, OD at 277 nm) and various concentrations of
B (Fig. 6a) were chosen in such a way that at the maximum concen-
tration of B used the OD at excitation wavelength of 277 nm is only
0.01. Thus, by excitation with 277 nm, most of the energy would be
absorbed by A, so that energy transfer could occur from S1 level of
A to the corresponding electronic level of B. To be more sure that B
is not actively involved in competitive absorption with A at 277 nm
position, an attempt was made to measure fluorescence spectra of
the solution of only B molecules, having the same concentration
as above, in the same solvent using 277 nm excitation. No signifi-
cant emission was found. This finding clearly demonstrates in favor
of the occurrence of nonradiative energy transfer from S1 level of
A to produce the corresponding energy state of B via resonance
interaction.
On the other hand, the significant changes occur in the elec-
tronic absorption spectra of the multichromophore (M) (Fig. 3).
In this spectra both the absorption spectra of the chromophores
A and B part appeared as a broad structure, but both peak posi-
tions are largely shifted. In the region 225–245 nm, there may be
the overlapping of the higher absorption bands of A and B chro-
mophores and considerable red shift also be present. Again the
long wavelength absorption band of M (250–300 nm) appearing
to be mainly due to the overlapping of the long wavelength band
of chromophores A and B are found to be largely blue shifted.
Here in case of multichromophoric molecule M, no new long wave-
length band was found. This observation excludes any ground state
charge transfer between the chromophores (A and B) which are
linked by a short unsaturated acetylenic spacer in the molecule
M.
3
3
.2. Steady state fluorescence measurements
Moreover, from the comparison between the fluorescence exci-
tation spectra of B and the mixture of A and B, monitoring only
the emission at 360 nm (Fig. 6b), it is appeared that the difference
between the two spectra is the absorption of A. This experiment
clearly demonstrates the occurrence of excitation energy transfer
from A to B.
.2.1. Intermolecular case
The steady state fluorescence spectra of the three compounds (A,
B and M) in polar ACN medium were measured at ambient temper-
ature (Fig. 4). To prevent any nonlinearity of fluorescence intensity,
the isosbestic point of the absorption spectra (∼280 nm, Fig. 3) was
chosen as the excitation wavelength. The two molecules A and B
exhibit the fluorescence band maxima at separate positions (band
maximum for A ∼ 290 nm and band maximum for B ∼ 340 nm), but
there are very small amount of superposition between the fluores-
cence bands of A and B.
As from the electronic absorption spectra (Fig. 2) it appears that
the lowest lying excited singlet (S1) of A (0,0 band at 277 nm)
lies in higher energy position relative to the corresponding level
of B (band maxima ∼300 nm), the former chromophore should
play the role of the energy donor and the later one the acceptor
in energy transfer mechanism. Further, observed spectral overlap-
ping of the fluorescence emission of the energy donor A with long
wavelength absorption spectra of the energy acceptor B molecule
fulfils the requirements for the occurrence of nonradiative exci-
tational energy transfer process within the donor and acceptor
systems.
Now B was excited using 300 nm (inset of Fig. 2), which corre-
sponds to its S –S transition. The concentration of B remains fixed
1
0
and A was gradually added to the mixture. It was found that there is
no significant change in the fluorescence of B. From the above two
observations, it seems improbable that the two molecules A and B
could form any excited complex like exciplex, etc. because had it
been formed then the fluorescence emission of B in the presence of
A should have been affected. Hence, drop in the fluorescence emis-
sion intensity of A accompanied by the enhancement of B emission,
as observed in the first case, should be attributed to the nonradia-
tive energy transfer process from the lowest excited state S1 of A
to the S1 state of B.
As the singlet–singlet energy transfer is apparent, the mecha-
nism in all probability would be of Förster type.
From the spectral overlapping consideration between the donor
emission and the absorption spectra of the acceptor, the value of R0,
the Förster’s critical transfer distance for long range dipole–dipole
interactions was computed using the well known expression
The fluorescence spectrum of fixed concentration of A in ACN
was measured in the presence of varying concentrations of B by

M. Bardhan et al. / Spectrochimica Acta Part A 75 (2010) 647–655
651
sition moments on a plane perpendicular to the line joining the
centers. Iv refers to the spectral overlap between donor emission
and acceptor absorption bands and is given by the expression:
ꢀ
∞
FD(ꢀ)ε (ꢀ)dꢀ
A
Iꢀ =
(2)
ꢀ4
0
where ꢀ and ε are the wave numbers in the overlapping region and
A
molar decadic extinction coefficient of the acceptor, respectively.
FD(ꢀ) is the spectral distribution of the donor emission in quanta
and normalized to unity so that
ꢀ
∞
FD(ꢀ)dꢀ = 1
0
The values of fluorescence quantum yield and the overlap inte-
−
14
3
−1
gral Iꢀ are 0.29 [20] and 0.53 × 10
cm M respectively.
From expression (1), the estimated R0 value was found to be
large (∼25.5 Å). This indicates the energy transfer through Förster
mechanism may be operative here and there is a slim possibility of
occurrence of Dexter’s type short range energy transfer process.
This observation made from the steady state measurements
indicates in favor of the occurrence of efficient intermolecular
dipole–dipole energy transfer process within A and B in ACN.
3.2.2. Intramolecular case
To investigate the intramolecular excitational energy transfer
process within the chromophoric units A and B present in M and
linked together by the insulating spacer, the A chromophore of
the linked chromophore M was excited in ACN. On excitation
of the multichromophore (M) at the isosbestic point (∼280 nm)
it was surprisingly observed that the fluorescence band which
observed clearly for A (anisole), between 285 nm to 340 nm, was
strongly quenched, and a broad band of large intensity peaking
about 340 nm appeared in highly polar ACN medium where the
fluorescence band maximum of the chromophore B resides (Fig. 4).
Thus, both in intermolecular and intramolecular cases, nonradia-
tive energy transfer process appears to be operative.
The fluorescence lifetime data further corroborates the views
made from the steady state measurements that the occurrence of
efficient intramolecular and intermolecular energy transfer process
within A and B in polar ACN solution is highly probable.
−
4
−3
Fig. 6. (a) Absorption spectra of A (conc. 1 × 10 mol dm ) (dotted line) and B
−
5
−3
(
conc. 1 × 10 mol dm ) (solid line). (b) Fluorescence excitation spectra of B (conc.
−
5
−3
−4
−3
1
× 10 mol dm ) (solid line) and the mixture of A (conc. 10 mol dm ) and B
−5 −3
(
conc. 1 × 10 mol dm ) (dotted line) monitoring at 360 nm.
3.3. Time resolved fluorescence studies
[
16–19a]:
From Table 1 it was observed that the fluorescence decay
2
0
of anisole (A) is of single exponential nature (ꢅ ∼ 22.8 ns). But
the decay of the other chromophore B exhibits two lifetimes,
one short (ꢅ ∼ 1.47 ns) and the other relatively longer component
(ꢅ ∼ 6.93 ns). These are mainly the monomer emissions of the indi-
vidual molecules. Two different lifetimes observed for the molecule
B may be due to the fluorescence emissions originating from the
two different excited conformers of this molecule. Though NMR
and single crystal X-ray diffraction [15] studies clearly indicate
the formation of only one type of conformation (exact geometry
is not known) in the ground state, but time resolved fluorescence
measurements indicate in favor of formation of the two different
(
9000 ln 10Ä ꢃ )Iꢀ
6
0
f
R
=
(1)
5
4
1
28ꢄ N n
A
where the symbols have their usual meanings.
2
Here Ä = 2/3 as random orientation occurs in inter-
molecular
infra),
case
[19b].
In
intramolecular
case
(vide
2
Ä
should be computed from the expression [7],
2
2
Ä = (sin ÂD sin ÂA cos ꢃ − 2 cos ÂD cos  ) , where ÂD and ÂA are
A
the angles sustained by the donor and the acceptor transition
moments with the line joining the centers of these transition
moments and ꢃ is the angle between the projections of the tran-
Table 1
Time resolved fluorescence data in ACN (fi ∼ fractional contribution) at the ambient temperature.
System
ꢁex (nm)
ꢁem (nm)
ꢅ1 (ns)
f1
ꢅ2 (ns)
f2
ꢅ3 (ns)
f3
ꢂ2
A
B
M
M
280
280
280
280
300
340
320
340
22.8
1.47
11.4
11.4
1
–
6.93
6.8
7.2
–
–
–
1.31
–
–
0.11
0.35
1.08
1.11
1.16
1.06
0.43
0.33
0.05
0.57
0.56
0.60
1.43
The
monoexponential
decay:
I(ꢁ, t) = A exp(−t/ꢅ).
The
biexponential
decay:
I(ꢁ, t) = A1 exp(−t/ꢅ1) + A2 exp(−t/ꢅ2).
The
triple
exponential
decay:
ꢁ
I(ꢁ, t) = A1 exp(−t/ꢅ1) + A2 exp(−t/ꢅ2) + A3 exp(−t/ꢅ3), fi = Aiꢅi/(
jAjꢅj). (f ∼ fractional contributions, A ∼ normalized pre-exponential factors).

6
52
M. Bardhan et al. / Spectrochimica Acta Part A 75 (2010) 647–655
Table 2
Time resolved fluorescence data in ACN (fi ∼ fractional contribution) of mixture of A (conc. ∼10 mol dm−3) and B at the ambient temperature (ꢁex ∼ 280 nm).
−4
Concentration of B (mol dm−3)
ꢁem (nm)
ꢅ1 (ns)
f1
ꢅ2 (ns)
f2
ꢅ3 (ns)
f3
ꢂ2
0
292
292
360a
22.8
3.75
4.2
1
–
0.61
–
–
1.5
–
–
0.07
–
–
5.06
–
–
0.32
1.09
1.03
1.08
−
−
5
1
1
× 10
× 10
5
a
Two rise times (1 ns and ∼5 ns) (with a negative pre-exponential factor) are obtained when the data are taken in short decay range.
species (possibly different conformers) in the excited electronic
states, as evidenced from the observations of the two different
fluorescence lifetimes. There are some intermolecular hydrogen
bonding between the oxygen atom of the SO2 and NH groups but
the molecule exists as essentially single conformer. The existence
of two conformers, one short and one long-lived, in the excited
state is therefore somewhat intriguing. In the course of our ear-
lier work [13] we observed that when a molecule has possibility
of cis–trans-isomerism, the ground state exists predominantly as
trans-isomer but there is an increase in the population of cis-isomer
in the excited state on photoexcitation. In the present case, how-
ever, there is no scope for geometrical isomerism. In our opinion,
the only way the present observation can be rationalized is in the
shows the two lifetimes even without undergoing energy transfer
process. Thus the origin of the two lifetimes of B should be in high
probability due to the formations of its two excited conformers.
The theoretical predictions using configuration interaction sin-
gles (CIS) method also support the above conclusion. The geometry
optimizations of the B molecule in the first excited state were done
by CIS method by using 6-31G (d,p) basis set, implemented in the
Gaussian package [21]. The calculation indicates the possibility of
two isomers present in the first excited state. The optimized excited
state geometries of two isomers of B are shown in Fig. 1c. The opti-
mized energy of the two isomers of B in the first excited state is
−53622.27 eV and −53576.71 eV respectively. The observed two
fluorescence lifetimes (Table 1) in case of B appear to be responsi-
ble for these two isomers which primarily differ in the geometry of
inversion of the geometry of the O
S O group upon photoexcita-
tion. Thus the molecule B behaves like a photoswitchable system
which possesses though a single ground state conformation but
on photoexcitation, some amount of this molecules convert into
another type of isomeric species. Another possibility of formations
of intramolecular dimer in case of B could be ruled out as the two
end toluene groups being linked by a rigid short spacer would not
be able to approach each other within the distance of <3 Å. Thus
the possibility of splitting of the first singlet excited state into two
different energy levels due to dimer formations is very slim.
The reconvolution analysis of the decay of the multichro-
mophoric system M obtained by exciting at 280 nm where both
reacting components A and B absorb, shows three lifetimes
the O S O bond.
The fluorescence lifetime study was also made in case of inter-
molecular quenching experiment. The data are shown in Table 2
and Fig. 7a. Monitoring the emission at 360 nm the decay was col-
lected on the short time scale (Fig. 7b). Two clear rise times, ∼1 ns
and ∼5 ns (accompanied by negative pre-exponential factors) were
observed followed by decay. As one of the rise times is similar to the
lifetime of the quenched energy donor A and the other is shorter,
this observation demonstrates that the excited energy donor A
directly relaxes to the two singlet states of B which correspond to
the two different conformers of the latter. It could be inferred that
the decay of the energy donor molecule A occurs concurrently with
the rise of fluorescence from the two singlet conformeric states of B.
Moreover time resolved measurements demonstrate that the non-
radiative energy transfer process from the reacting species A to B,
results larger populations of the particular singlet conformer of the
latter which possesses longer lifetime (Tables 1 and 2), whatever
be the nature of the reactions, intermolecular or intramolecular.
Thus, it appears that, as the transfer probability depends upon the
extent of overlapping between the donor emission and the acceptor
absorption, this longer-lived isomer predominates in the ground
state and on photoexcitation, these ground species are converted
into excited state of this isomeric species followed by formations of
some other isomeric singlet state due to its photoswitchable char-
acter. In high probability this isomer corresponds to the excited
state shorter-lived species, the presence of which is apparent from
time resolved fluorescence studies.
(
Table 1), one short (∼1.31 ns) and the other two long compo-
nents (∼6.8 ns and 11.4 ns) at monitoring wavelength of 320 nm
where the monomeric emissions of both the energy donor A and the
acceptor B overlap. As it was apparent from the steady state fluores-
cence spectra of the multichromophore (Fig. 4), the long component
(
∼11.4 ns) should be attributed to the quenched lifetime of the
energy donor A. The other components (ꢅ = 1.31 ns and ꢅ = 6.8 ns)
should be due to the energy acceptor B, as observed above. Again
exciting the same wavelength (M, ꢁex ∼ 280 nm), but monitoring at
3
40 nm where mostly the energy acceptor B emits, the decay shows
here also three components, one is short lifetime (∼1.43 ns) and the
other two are long (∼7.2 ns and 11.4 ns). Here the long component,
1
1.4 ns, which is due to the quenched donor species A, possesses
a very small contribution of this species (5% of the total emission,
Table 1). The contribution of the emission of A at this position, as
expected, is thus nearly absent. So the emissions of the multichro-
mophoric molecule M where the chromophores A and B are present
Formations of the two excited species due to excitation energy
transfer process may be responsible for the observed two rise times
from the time resolved fluorescence measurements.
linked by –CH bond, are mainly due to the emissions of B originat-
2
ing from the two proposed excited state conformers. However, an
alternative approach could be considered for describing the exis-
tence of the two characteristic times. As an alternative mechanism,
it may be presumed that initially, after excitation with 280 nm,
Another important findings are that the quenched fluorescence
lifetime of the donor A becomes much shorter (∼4 ns, Table 2) in the
intermolecular energy transfer process than the situation observed
for intramolecular case (∼11.4 ns, Table 1). The intermolecular
quenching rate has been computed by using the Stern–Volmer rela-
tion [22–25]:
the lowest excited singlet state (S ) of A undergoes energy trans-
1
fer to populate an initial high excited state Sini of B which relaxes
to the first excited singlet S1 (monitoring at 340 nm) by internal
conversion. The singlet state S1 then decays to the ground state.
This mechanism provides two apparent lifetimes for the decay of
S1. One is due to the decay of S1 itself and the other may originate
from the processes which populate S1 namely the decay and inter-
nal conversion of Sini. However, the possibility of this alternative
mechanism is very slim in the present case as the molecule B itself
F0
=
1 + kqꢅ0 [Q]
F
where F0 and F represent the steady state fluorescence intensity
of the energy donor A without and in the presence of the different
concentrations [Q] of the energy acceptor B.

M. Bardhan et al. / Spectrochimica Acta Part A 75 (2010) 647–655
653
Fig. 7. (a) Instrument response function and fluorescence rise and decay profile of B after excitation of A. The triple exponential was found to be the best fit (ꢂ2 ∼ 1.08). (b)
The double exponential rise time function (measured in short decay range) of the system comprising the mixture of A and B. The monitoring wavelength was at 360 nm
(
emitting region of B) and the excitation wavelength was at 280 nm (mostly the energy donor A will be excited).
kq is the bimolecular rate constant for the observed quench-
using the another form of Stern–Volmer expression:
ing phenomena which primarily arises due to intermolecular
ꢅ0
ꢅ
energy transfer process. From the observed linear plot, kq has
= 1 + kqꢅ0 [Q],
12
3
−1 −1
s (Fig. 8). Nearly
been estimated to be 1.75 × 10 dm mol
similar bimolecular quenching rate (∼2.1 × 10 dm mol
was observed from time resolved fluorescence measurements also
12
3
−1 −1
s )
where the symbols have their usual meanings. This observation is
indicative of the dynamic nature of the quenching phenomena.

6
54
M. Bardhan et al. / Spectrochimica Acta Part A 75 (2010) 647–655
8
−1
which is much less than the cor-
value of kEET is around 10 s
responding value observed in intramolecular system.
In intramolecular case from the observed higher energy transfer
rate relative to the intermolecular situation, it seemingly indicates
that imposition of the spacer of acetylenic bond between the donor
and acceptor facilitates the excitation energy transfer process to
propagate. Another possibility is that due to a particular favorable
transition dipole orientation in the donor and acceptor within M
may result in rapid propagation of electronic excitation energy.
However, such possibility could be ruled out due to the observed
similar values of Ä2 both in inter- and intramolecular cases. This
seemingly indicates that the transition dipole orientation in the
donor and acceptor does not play significant role in modifying the
transfer rate in the case of the intramolecular energy transfer rela-
tive to the situation in intermolecular case. Possibly in the former
case the spacer acts as a favorable path for the electronic coupling
between the energy donor and the acceptor moieties. Thus, the
composition of the spacer possesses considerable effect in the prop-
agation of excitation energy. Further investigations on electronic
excitation energy by changing the nature of the spacers situated
between the energy donor and acceptor components are underway.
Fig. 8. Stern–Volmer (SV) plot, constructed from the steady state measurements, to
measure bimolecular quenching rate (see text).
4
. Conclusions
The observed large quenching rate, larger than the diffusion-
controlled limit, suggests in favor of occurrence of dipole–dipole
Forster type energy transfer process, as predicted above.
In intramolecular system, the energy transfer efficiency, E has
been estimated from the expression [19b,19c]:
The above experimental findings made from the steady state
and time resolved fluorescence measurements demonstrate that,
though two different isomeric species of the acceptor B are appar-
ent in the excited singlet state, the prevailing singlet–singlet
nonradiative energy transfer route was found from the donor A to
the relatively longer-lived isomeric species of B. It appears that,
as the transfer probability depends upon the extent of overlap-
ping between the donor emission and the acceptor absorption,
the longer-lived isomers which predominate in the ground state
are converted into excited state of this isomeric species followed
by formation of some other isomeric singlet state due to its pho-
toswitchable nature. In high probability this isomer corresponds
to the excited state shorter-lived species, the presence of which
is apparent from time resolved fluorescence studies. From the
observed results on intermolecular and intramolecular systems,
it is apparent that the energy transfer rate within the donor and
acceptor moieties could be enhanced by inserting a suitable spacer
between them. The transition dipole orientations in the donor and
acceptor molecules appear not to play significant role in modifying
the transfer rate in the case of the intramolecular energy trans-
fer process. The present study possesses significant implications in
developing photoswitchable devices.
6
(
R0/R)
E =
6
1
+ (R /R)
0
The above expression is only applicable to energy
donor–acceptor pairs which are separated by a fixed distance
R.
To determine the value of R , Ä2 was computed carefully from
0
the optimized geometry of M (obtained by CIS method by using
6
-31G (d, p) basis set, implemented in the Gaussian package) and
2
2
using the expression Ä = (sin ÂD sin  cos ꢃ − 2 cos ÂD cos  ) (the
A
A
meaning of the symbols is given above). The value was found to
be very similar (∼0.70) to the value used in intermolecular case
2
(
(
Ä ∼ 2/3 = 0.66). So, in the intramolecular system, using expression
1) the value of R0 was estimated to be ∼25.8 Å, very close to the
value obtained in the intermolecular system (25.5 Å).
From the optimized geometry of M, the center-to-center (sepa-
ration) distance, R, between the donor and acceptor molecules was
found to be 5.81 Å.
Using these two values of R and R, the transfer efficiency, E was
0
Acknowledgments
found to be 0.99. As R < R0 (R = 5.81 Å and R = 25.8 Å), the energy
0
transfer should dominate over the deactivation of the donor [19b].
The rate of intramolecular energy transfer, kEET, within the donor
and acceptor moieties present within M has been determined from
the relation:
T.G. gratefully acknowledges the financial assistances provided
by the Department of Science and Technology (DST), New Delhi,
India (Project No.: SR/S1/PC-17/2003) and the Council of Scientific
and Industrial Research (CSIR), New Delhi, India (Project No.: 01
(1883)/03/EMR-II). M.B. thanks CSIR for awarding the NET fellow-
ship.
ꢂ
ꢃꢂ
ꢃ
6
1
ꢅ0
R0
R
kEET =
11
−1
s . ꢅ0 is the fluorescence lifetime of
and is found to be 3.3 × 10
References
the free donor. In intermolecular system, where the energy transfer
occurs through space, the value of energy transfer rate could be
determined by the relation
[
[
1] B. Jing, D. Zhu, Tetrahedron Lett. 45 (2004) 221–224.
2] T.A. Smith, N. Loken, N. Cabral, S.R. Davies, M.N. Paddon-Row, K.P. Ghiggino, J.
Photochem. Photobiol. A: Chem. 149 (2002) 55–69.
kEET = kq [Q]max
[3] M.N. Paddon-Row, Covalently-linked systems based on organic components,
in: V. Balzani (Ed.), Electron Transfer in Chemistry, vol. 3, Wiley/VCH, Wein-
heim, 2001, pp. 179–271, (Chapter 1, Part 2).
where [Q]max is 5.01 × 10−5 M.
[
4] M.N. Paddon-Row, Electron and energy transfer, in: M. Shibasaki, J.F. Stoddart,
F. Vogtle (Eds.), Stimulating Concepts in Chemistry, Wiley, Weinheim, 2001,
pp. 267–292.
As the value of kq is found from the quenching measurements
discussed above) around 2 × 1012 dm mol
3
−1 −1
s , the computed
(

M. Bardhan et al. / Spectrochimica Acta Part A 75 (2010) 647–655
655
[
[
[
5] G.D. Scholes, K.P. Ghiggino, A.M. Oliver, M.N. Paddon-Row, J. Phys. Chem. 97
1993) 11871.
6] C.H. Tung, L.P. Zhang, Y. Li, H. Cao, Y. Tanimoto, J. Am. Chem. Soc. 119 (1997)
348–5354.
7] P. Mandal, T. Misra, A. De, S. Ghosh, S. Roy Chowdhury, J. Chowdhury, T. Ganguly,
Spectrochim. Acta Part A 66 (2007) 534–545.
(b) N.J. Turro, Modern Molecular Photochemistry (1978) 305;
(
(c) K.K. Rohatgi-Mukherjee, Fundamentals of Photochemistry, New Age Inter-
national (P) Limited, Publishers (formerly Wiley Eastern Limited), New Delhi,
Bangalore, Kolkata, 2009, p. 195.
[20] C. Buck, F.X. Hartmann, D. Motta, S. Schoenert, Chem. Phys. Lett. 435 (2007)
252–256.
5
[
[
8] J. Bigman, Y. Karni, S. Speiser, Chem. Phys. 177 (1993) 601–617.
9] B. Jing, D. Zhang, D. Zhu, Tetrahedron Lett. 41 (2000) 8559–8563.
[21] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
V.G. Zakrzewski, J.A. Montgomery, R.E. Stratmann, J.C. Burant, S. Dapprich, J.M.
Millam, A.D. Daniels, K.N. Kudin, M.C. Strain, O. Farks, J. Tomasi, V. Barone,
M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochter-
ski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck,
K. Raghavachari, J. Foresman, J.V. BOrtiz, B.B. Stefanov, G. Liu, A. Liashenko,
P. Piskorz, J. Komaromi, I. Cioslowski, R. Gomperts, R.L. Martin, D.J. Fox, T.
Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe,
P.M.W. Gill, B.G. Johnson, W. Chen, M.W. Wong, J.L. Andres, M. Head-Gordon,
E.S. Replogle, J.A. Pople, Gaussian 98, Revision A. 7, Gaussian, Inc, Pittsburgh,
PA, 1998.
[
[
10] B. Yan, L. Zhao, J. Liu, J. Photochem. Photobiol. A: Chem. 199 (2008) 50–56.
11] S. Bhattacharya, T.K. Pradhan, A. De, S. Roy Chaudhury, A.K. De, T. Ganguly, J.
Phys. Chem. A 110 (2006) 5665–5673.
[
[
12] T. Misra, T. Bhattacharya, S.K. Pal, A. De, R.D. Saini, T. Ganguly, Chem. Phys. Lett.
3
82 (2003) 167–174.
13] M. Maiti, T. Misra, T. Bhattacharya, C. Basu, A. De, S.K. Sarkar, T. Ganguly, J.
Photochem. Photobiol. A: Chem. 152 (2002) 41–52.
14] R. Mukhopadhyay, N.G. Kundu, Synlett (2001) 1143–1145.
15] K. Chowdhury, M. Mukherjee, R. Mukhopadhyay, M. Helliwell, A.K. Mukherjee,
Z. Kristallogr. 222 (2007) 99–104.
[
[
[22] O. Stern, M. Volmer, Phys. Z 20 (1919) 183–188.
[
[
16] S. Sinha, R. De, T. Ganguly, J. Phys. Chem. A 101 (1997) 2852–2858.
17] K.P. Ghiggino, E.K.L. Yeow, D.J. Haines, G.D. Scholes, T.A. Smith, J. Photochem.
Photobiol. A: Chem. 102 (1996) 81–86.
18] T. Förster, Ann. Phys. 2 (1948) 55–75.
19] (a) T. Förster, O. Sinanoglu (Eds.), Modern Quantum Chemistry, vol. III,
Academic Press, New York, 1965;
[23] P.R. Bevington, Data Reduction Error Analysis for the Physical Sciences,
McGraw-Hill, New York, 1969.
[24] M. Maiti, T. Misra, S. Sinha, S.K. Pal, D. Mukherjee, R.D. Saini, T. Ganguly, J. Lumin.
93 (2001) 261–274.
[25] P. Mandal, T. Sahu, T. Misra, S.K. Pal, T. Ganguly, J. Photochem. Photobiol. A:
Chem. 188 (2007) 235–244.
[
[