Synthesis and characterization of NADH model compound modified β-cyclodextrin and its role as an energy donor in FRET

Article abstract of DOI:10.1016/j.jphotochem.2011.12.007

β-Cyclodextrin is chemically modified to selectively introduce an acridinedione moiety on the primary face. The synthesis involves the substitution of one of the primary hydroxyl groups of β-cyclodextrin by a tosyl group which facilitates the introduction of an ethylene diamine modification which is finally condensed to a tetraketone. The acridinedione modified β-cyclodextrin thus obtained was fully characterized by IR, NMR and Mass spectrometry. The photophysical properties of the compound were also analyzed. The potential of the modified β-cyclodextrin to act as an energy donor in FRET was investigated with a suitable acceptor.

Full text of DOI:10.1016/j.jphotochem.2011.12.007

Journal of Photochemistry and Photobiology A: Chemistry 229 (2012) 60–68
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Synthesis and characterization of NADH model compound modified
ˇ-cyclodextrin and its role as an energy donor in FRET
∗
R. Krishnaveni, P. Ramamurthy , E.J. Padma Malar, P. Divya
National Centre for Ultrafast Processes, University of Madras, Taramani Campus, Chennai – 600 113, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 June 2011
Received in revised form 25 October 2011
Accepted 14 December 2011
Available online 22 December 2011
ˇ-Cyclodextrin is chemically modified to selectively introduce an acridinedione moiety on the primary
face. The synthesis involves the substitution of one of the primary hydroxyl groups of ˇ-cyclodextrin
by a tosyl group which facilitates the introduction of an ethylene diamine modification which is finally
condensed to a tetraketone. The acridinedione modified ˇ-cyclodextrin thus obtained was fully char-
acterized by IR, NMR and Mass spectrometry. The photophysical properties of the compound were also
analyzed. The potential of the modified ˇ-cyclodextrin to act as an energy donor in FRET was investigated
with a suitable acceptor.
Keywords:
Acridinedione
ˇ-Cyclodextrin
Modification
FRET
©
2011 Elsevier B.V. All rights reserved.
Acceptor
1
. Introduction
ranging from achieving solubility in a desired solvent to analyzing
the mechanisms of enzyme-catalyzed reactions [5,6]. For synthetic
chemists, CDs are of interest because they are chemically stable
and can be modified in a regioselective manner [7]. Such modi-
fications produce enormous opportunities and at the same time
more challenges to the chemists. Opportunities are provided by
the fact that, through modifications, CDs can produce exquisite
molecules that can be invaluable in investigations at the frontiers
of chemistry like enzyme like catalytic activity and antibody like
binding activity to aesthetically pleasing molecules. But the pres-
ence of hydrophobic cavity and a large number of hydroxyl groups
provide a great challenge to the selective modification of CD [8].
There always seems to occur a competition between the hydroxyl
groups at positions 2, 3 and 6 for the reagent and makes the
selective modification extremely difficult. In recent days, suitably
modified CD derivatives acting as fluorescent sensors for detect-
ing a variety of organic molecules including biologically important
substances have been indicated, e.g., pyrene, naphthalene and dan-
syl modified CDs [9]. Reports are also available on the increased
stability of inclusion complexes using bridged bis (ˇ-CD)s. It has
been observed that the cooperative binding of one guest by two
CD moieties in a single host molecule is the reason for such a
multifold enhancement in the stability of inclusion compounds
[10,11].
Cyclodextrins (CDs) are a family of cyclic oligosaccharides that
are composed of ␣-1,4-linked glucopyranose subunits existing in a
chair conformation [1]. CDs are synthetically obtained from starch
by enzymatic degradation. The three most common types of CDs
well known are ˛-CD, ˇ-CD and ꢀ-CD having 6, 7 and 8 glyco-
syl units, respectively. These are referred to as parent, native or
first generation CDs among which ˇ-CD is the most accessible, the
lowest priced and generally the most useful.
CDs possess a doughnut-like structure with wide and narrow
hydrophilic tops delineated by O(2)H and O(3)H secondary and
O(6)H primary hydroxyl groups respectively and by a hydropho-
bic annular core lined with H(3), H(5) and H(6) hydrogen atoms
and O(4) ether oxygen atoms [2]. It is the central hydrophobic cav-
ity of ˇ-CD with an inner diameter of 6–6.5 A˚ units, which makes
these molecules capable of forming inclusion complexes with many
organic and inorganic guest molecules [3]. Interesting strategies
are emerging in this field and a wide range of organic molecules
has been found to form complexes with CDs and hence they find
apt application ranging from solubilization of pharmaceutical com-
pounds to chiral chromatography [4].
CDs and their chemically modified derivatives have been
the subject of numerous investigations for a variety of reasons
CDs are essentially inert to photochemical excitation but their
chemical modifications with chromophoric moieties may convert
them into a spectroscopically active molecule. This property of
modified CDs could be extensively used to construct host–guest
sensory systems [12,13] with which a number of molecular species
∗ Corresponding author. Tel.: +91 44 24547190; fax: +91 44 24546707.
E-mail addresses: prm60@hotmail.com, perumal ramamurthy@yahoo.co.in
P. Ramamurthy).
(
1
010-6030/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2011.12.007

R. Krishnaveni et al. / Journal of Photochemistry and Photobiology A: Chemistry 229 (2012) 60–68
61
can be detected and hence-forth they can be considered as
biological markers or sensors for the detection volatile organic
compounds [14]. Detailed studies on pyrollinone modified CDs
have been carried out in this context wherein the pyrollinone
appended to the ˇ-CD acting as a hydrophobic cap to increase
the binding ability of the host have been already reported [15].
Such a chemical modification paves a convenient method for
constructing a fluorescence sensory system wherein even spec-
troscopically inert guest molecules can be examined directly
before use. Cyclohexane-1,3-dione for synthesis was obtained from
Sigma–Aldrich Chemicals Pvt. Ltd. and used as received. Toluene,
P2O5 used were also obtained from Qualigens India Ltd. Molecular
sieves with pore sizes of 4 A˚ were used to store DMF.
Absorption spectra were recorded in an Agilent 8453 diode array
spectrophotometer. Fluorescence spectra were recorded using a
Horiba Jobin Yvon Fluoromax 4P Spectrofluorimeter. Fluorescence
quantum yields were obtained from the corrected fluorescence
spectrum using quinine sulphate in 0.1 N H SO [27]. Time resolved
2
4
[
16].
Other interesting fields in which the modified CDs acquire
fluorescence decays were obtained by the time-correlated-single-
photon-counting (TCSPC) method. The excitation source used was
a 375 nm LED (model N-375 LH) with a pulse width of 600 ps
purchased from Horiba Jobin-Yvon. The emission intensity were
counted by a MCP PMT (Hamamatsu R 3809) and processed through
CFD, TAC and MCA. The fluorescence decay was analyzed by using
the software provided by IBH (DAS-6).
importance are the molecular modeling [17] and mechanism
of excitation energy transport [18] and Fluorescence Resonance
Energy Transfer (FRET). Extensive investigations have been already
carried out in the case of multichromophoric CDs being used
in the interpretation of photophysics involved in energy hop-
ping [19]. FRET is a fast emerging field owing to its selectivity
and sensitivity in providing information about the structure and
dynamics of macromolecules as well as molecular assemblies
2.1. Synthesis of
Mono-6-deoxy-6-aminoethylaminoacridinedione –
ˇ-CD:(ˇ-CDen-ADR)
[
20].
In this paper, our attention is focused on the synthesis and
a preliminary investigation on the photophysical properties of a
chromophoric CD viz., ˇ-CD linked to an acridinedione, which has
been reported as a class of laser dyes [21]. Apart from having a high
fluorescence quantum yield and lasing action comparable to that
of coumarin-103, the acridinedione chromophores also acquire
importance in biological systems due to their structural similar-
ity to 1,4-dihydropyridines and NADH which are active biological
co-enzymes [22].
Starting from ˇ-CD, one of the primary hydroxyl groups was
modified into a tosyl group by following reported procedures [28].
The mono-6-deoxy-6-tosyl modified ˇ-CD was further modified
to obtain mono-6-deoxy-6-aminoethylamino modified ˇ-CD (ˇ-
CDen) by treating it with freshly distilled ethylene diamine at high
temperature [29] In parallel, the required tetraketone was also
prepared by treating cylohexane1,3-dione with formaldehyde in
a medium of methanol [30,31].
The modified compound ˇ-CDen-ADR was obtained in the final
step as shown in Scheme 1. 2.3 g (2 mmol) of ˇ-CDen was treated
with 0.40 g (2 mmol) of tetraketone in 60 mL of toluene and refluxed
in a Dean-Stark apparatus for 48 h. The completion of reaction
was checked with TLC; the solvent was removed by distillation.
The obtained crude product was purified through multiple crystal-
lization from 1:1 MeOH:CHCl3 mixture. The resultant dark brown
Hence, with the synthesis of such an acridinedione modified ˇ-
CD, we further aim at the study of potential application of these dyes
as photosensitizers [23], fluorescent sensors [24,25] and probes
[
26], initiators in photopolymerization reactions and also suitable
substitutions on the acridinedione molecule which would further
develop the fluorescent behaviors of such compounds. Also, such
modified ˇ-CD provides an opening into an area of further research,
wherein, another chromophore of suitable size can be included into
the cavity of ˇ-CD and hence a bifluorophoric study can be initiated.
i.e., they can be considered as a variety of chemosensors and hence
a study of its signal transduction properties can be carried out.
colored crystals were collected and dried. Yield: 0.8 g, 40%, M.p:
◦
2
02–04 C.
3. Results and discussion
2
. Materials and methods
The attachment of fluorophores to natural or synthetic receptors
has received increased interest over last few years in endeavors to
furnish new fluorescent sensors. In particular, fluorescent CDs have
generated considerable interest among the synthetic chemists as it
is witnessed in recent articles dealing on sensing properties.
The literature data offer several methods for the modification
of CDs. Among the hydroxyl groups in ˇ-CD, the primary ones are
more nucleophilic and hence they can be easily modified into other
Resorcinol and p-Toluenesulfonyl chloride used for the
synthesis were obtained from S.D. Fine Chemicals Pvt. Ltd.
Ethylenediamine also obtained from S.D. Fine chemicals was dou-
bly distilled before use. N,N-Dimethyl formamide and methanol
used were of HPLC grade and were obtained from Qualigens India
Ltd. ˇ-CD received from Cyclolab was recrystallized from water
O
O
N
N
H
NHCH CH NH
2
2
2
O
O
+
Toluene
Δ
O O
Scheme 1. Synthesis of ˇ-CDen–ADR.

6
2
R. Krishnaveni et al. / Journal of Photochemistry and Photobiology A: Chemistry 229 (2012) 60–68
functional groups than their secondary counterparts. Tosylation at
the 6th position is the most popular method for mono-modification
of CDs, and is found to offer to be a very good precursor for a
variety of modifications. This is due to the nature of the tosyl
substitution which behaves as a good leaving group, especially in
presence of nucleophiles [32]. As illustrated in the scheme, native
ˇ-CD was mono modified as aminoethylamino-acridinedione-ˇ-
CD (ˇ-CDen-ADR). The purity of the intermediate products were
verified and the results were found to be in good agreement with
the reported values [24,33]. The final compound was well charac-
terized at using IR, NMR and Mass analysis.
1.0x10
8.0x10
367nm
462 nm
Excitation
6.0x10
4.0x10
2.0x10
Time (ns)
Emission
3.1. Characterization
3.1.1. Mono-6-deoxy-6-aminoethylaminoacridinedione – ˇ-CD
IR (KBr) spectrum showed absorption peaks corresponding to
0.0
−1
acridinedione-like moiety, one at 1716 cm , characteristic of the
250
300
350
400
450
500
550
600
650
700
−
1
carbonyl groups and the other one at 3275 cm
indicating the
Wavelength(nm)
interaction between the carbonyl and hydroxyl groups.
1
H NMR (500 MHz, DMSO-d ) showed the signals of both the
Fig. 1. Excitation spectrum (monitored at 450 nm) and Emission spectrum (excited
at 375 nm) of ˇ-CDen-ADR. Inset shows the decay curve of ␤-CDen-ADR when
excited at 375 nm.
6
entities of the new compound namely, acridinedione and the native
ˇ-CD. The acridinedione couplings were found to appear between
2
.3 and 1.9 in the spectrum and the signals were assigned to four
2,7
sets of protons viz., ı 2.65(CH ), ı 2.37–2.27 (t, 4H, C ), ı 2.25–2.15
the other hand, the fluorescence decay of the modified ␤-CD is
non-exponential and requires three exponential decay terms for
adequate fitting with the dominant component exhibiting a decay
time of 7.4 ns. The observation of triple exponential fluorescence
decay for the modified dye gave us an indication for the possi-
ble existence of the molecule in more than one conformation. This
may be due to the rotation of the acridinedione moiety about the
–CH –CH – axis. This has been ascertained by studying the inten-
sity and lifetime of the molecule by changing the viscosity of the
medium. It was observed that with an increase in viscosity, both
the emission intensity (Fig. S5) and the average life time (Fig. S6) of
the molecule increased, which is attributed to the arresting of the
free rotation of the acridinedione moiety along the axis.
We have also carried out the TRES and TRANES analysis for the
modified compound and the spectra are as shown in Figs. 2 and 3.
The spectral shift of 5 ns and the observation of two isoemissive
points in these figures reveal the presence of three different emit-
ting species in the system. This was further supported by our
theoretical calculations. As per the quantum chemical computa-
tions using GAUSSIAN 03 (G03W) software calculations carried out
2
t, 4H, C4 ), ı 1.3–0.9 (q, 4H, C ). The signals of cyclodextrin moiety
,5
3,6
(
were found to appear at ı 8.0 (s, 1H (N–H), ı 5.85–5.60 (m, 14H
2,3
1
6
OH ), ı 4.95–4.8 (m, 7H, H ), ␦ (4.6–4.4 (m, 6H, OH ), ı 3.8–3.6
3
,5,6
2,4
(
m, 35H, H-
), ı 3.45–3.2 (m, H, ) overlap with HOD) (Fig. S1).
13
C NMR (125 MHz, DMSO-d ) showed the signals correspond-
6
ing to the carbonyl and olefinic groups of acridinedione at 195 ppm,
45 ppm respectively. The signals corresponding to aliphatic car-
1
2
2
bons appeared at 46, 39, 36, 21 ppm and 9 ppm respectively
1
(
1
Fig. S2). The signals of ˇ-CD ring were assigned as follows: C at
01 ppm, C at 82 ppm, C
4
2,3,5
at 73,72.8 and 72.4, C6 at 60 ppm,
respectively (Fig. S2).
An electron impact mass spectrum was recorded for the com-
pound which showed the molecular weight as 1376, which was in
good agreement with the calculated value (Fig. S3).
3.2. Photophysical studies
For all spectroscopic measurements of ˇ-CDen-ADR, 6%
methanol was used as the solvent.
The absorption and emission spectra of all acridinedione dyes
show a maximum around and 395 nm and 460 nm, respectively
in 6% methanol solution. This band has been assigned to an
intramolecular charge transfer from the ring nitrogen to the ring
carbonyl oxygen rich centre within the ADR fluorophore [34].
In order to establish the photophysical behavior of the mod-
ified molecule, we made a comparison between the absorption
and emission characteristics of free ADR and ˇ-CDen-ADR. The
absorption spectrum of the free ADR molecule is found to show
maxima at 250, 273 and 395 nm and when excited at 395 nm shows
a characteristic emission with maximum intensity at 465 nm. The
absorption spectrum of the ˇ-CDen-ADR recorded in the UV–vis
region showed a similar observation; the absorption maxima were
found to occur at 247, 263 and a shoulder around 390 nm (Fig. S4).
Fig. 1 depicts the excitation and emission spectra of the acridine-
dione modified ˇ-CD. The maximum absorbance of the excitation
spectrum was observed at 367 nm and when excited, the flu-
orescence spectrum showed a characteristic emission with the
maximum intensity centered at 462 nm. The inset shows the life
time profile of the molecule when excited at 375 nm.
1
.2
5 ns
0 ns
1
0.8
0.6
0.4
0.2
0
18500
19500
20500
21500
22500
23500
24500
The lifetime profiles of the free ADR and ˇ-CDen-ADR molecules
were also analyzed. The free ADR is found to possess a lifetime
of 5.2 ns [35] and is well fit by a single exponential function. On
Wavenumber(cm-1)
Fig. 2. TRES spectrum of ˇ-CDen-ADR.

R. Krishnaveni et al. / Journal of Photochemistry and Photobiology A: Chemistry 229 (2012) 60–68
63
2.50E-04
2.00E-04
1.50E-04
1.00E-04
5.00E-05
A series of ADR dyes, both free and with substitutions have been
already synthesized, photophysical processes were studied, quan-
tum yields were determined and reported by Srividya et al. [35].
Lasing actions of these dyes also have been studied in various sol-
vents and have been reported [36–38].
As far as the quantum yields the ADR dyes are concerned, the
substitution on the N atom at the 10th position is found to play
a great role. Such a reduction in quantum yields of the acridine-
dione dye series due to the substitution of bulky groups like phenyl,
chlorophenyl, benzyl, phenyl naphthalimides etc., on the nitrogen
atom in the 10th position have been already reported [35]. On a
similar note, the substitution of a ˇ-CDen moiety on the N is found
to have decreased the quantum yield of the molecule almost to
1
5%. This decrease in quantum yield of the ˇ-CDen-ADR is once
again attributed to the free rotation of the ˇ-CD moiety linked to
the ADR which in turn would lead to an increase in rate of decay
of the molecule in the non-radiative modes. Whereas the free ADR
molecule with a rigid structure is reported to have a higher rate
constant for radiative decay than that of a non-radiative one.
0
.00E+00
1
5000 16000 17000 18000 19000 20000 21000 22000 23000 24000 25000
Wavenumber (cm-1).
Fig. 3. TRANES spectrum of ˇ-CDen-ADR.
3.3. ˇ-CDen-ADR in FRET
the three structural configurations of the modified molecule is as
shown in Fig. 4. Among the conformers, ‘III’ is the one with highest
energy which corresponds to the partial inclusion of the acridine-
dione moiety into the cavity of ␤-CD, ‘II’ is the one with lowest
energy and ‘I’ with intermediate energy, where in the acridine-
dione moiety is outside the cavity and shows a slight change in
FRET occurs when the electronic excitation energy of
donor chromophore is transferred to an acceptor molecule
nearby (10–90 A˚ ) via a dipole–dipole interaction between the
a
donor–acceptor pair [39]. The process of FRET seems to be more
efficient when there is an appreciable overlap between the emis-
sion spectrum of the donor and the absorption spectrum of the
acceptor [40].
Owing to the reasonable quantum yield, narrow absorption and
emission spectra obtained for the chromophore modified ˇ-CD
derivative synthesized, we set to investigate the potential of this
dye to act as a donor in order to initiate a photoinduced energy
transfer to an acceptor molecule.
The acceptor that we have chosen for the present investiga-
tion is safranine. The choice of the acceptor was made for the
following reasons: (i) it is hydrophobic, (ii) a good spectral over-
lap of the absorption spectrum with that of emission spectrum the
donor ˇ-CDen-ADR was found to occur, (iii) the approximate size
of the acceptor matched with that of the nano-cavity of the donor.
The final reason leads to an interesting discussion on whether the
orientation along the –CH –CH2 axis. Accordingly, the conformer
2
associated with the lowest energy is attributed to the component
with the highest population and the longest life time (7.4 ns, ∼84%)
which is close to that of a free acridinedione dye and the one
with highest energy is attributed to the lowest populated compo-
nent obtained from the lifetime decay analysis (0.53 ns, ∼3%). The
quenching observed in the lifetime of this component is justified
by the hydrogen bond formation between the carboxyl group of the
acridine dione moiety and the hydroxyl group of the cyclodextrin
moiety as evidenced from the Fig. 4III.
From the corrected fluorescence spectrum, we also have cal-
culated the quantum yield of the ˇ-CDen-ADR molecule as 0.13.
This can be compared with those of many free ADR dyes which are
reported to have a good quantum yield i.e. in the range of 0.5–0.9.
Fig. 4. Energy minimized conformers of ␤-CDen-ADR. Hydrogen bonding interactions (green dashed lines) between the aciridinedione fragment and CD unit in the HF/3-21G
optimized conformers of ␤-CDen-ADR. Color code: O – red; N – blue; C – cyan; acridinedione fragment is shown in orange. The hydrogen bond lengths are in Å and the
◦
hydrogen bond angles are in . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

6
4
R. Krishnaveni et al. / Journal of Photochemistry and Photobiology A: Chemistry 229 (2012) 60–68
acceptor is in simple interaction with the donor or it gets into the
nano-cavity of the donor.
4.0
3.5
3.0
The efficiency of energy transfer by FRET mechanism is defined
as “the fraction of photons absorbed by the donor that is transferred
to the acceptor.” This parameter can be experimentally determined
by comparing the fluorescence intensities of the donor (D) in the
presence and absence of acceptors (A) as given by Eq. (1).
2.5
2.0
1.5
1.0
IDA
E = 1 −
(1)
ID
e
a
Since we encounter many interfering parameters in fluorescence
emission measurements like inner-filter effects, background noise
etc., we rely more on lifetime measurements to calculate the energy
transfer efficiency as given by Eq. (2).
3
90 nm
ꢁDA
E = 1 − ꢁ
(2)
0.5
0.0
D
where ꢁDA, ꢁD and IDA, ID are the lifetimes and intensities of the
donor in the presence and absence of acceptors, respectively. Alter-
natively, energy transfer efficiency can also be calculated as:
200
400
600
Wavelength(nm)
1
E =
(3)
Fig. 5. Absorption spectrum of ␤-CDen-ADR in the presence of increasing concen-
−5
−5
1
+ (r/R◦)6
trations of safranine [ˇ-CDen-ADR] – 2 × 10 M, [safranine]: (a) 0 M, (b) 2 × 10 M,
−5 −5 −5
(
c) 4 × 10 M, (d) 6 × 10 M, and (e) 8 × 10 M.
◦ꢀ
where ‘r’ is the distance between D and A chromophores, ‘R is
the so-called Förster distance, at which the energy transfer effi-
ciency is 50%. Thus R is the critical distance, at which energy
transfer and spontaneous decay of the excited donor are equally
probable, and it may be experimentally determined from the spec-
troscopic data and is given by Eq. (4) as
◦
12
−1 −1
M s . Such
Stern–Volmer plot and was found to be 1.5 × 10
values of rate constants in the order of 1011–10
12
M
−1 −1
s were very
much greater than those expected for a radiative or a collisional
energy transfer. Hence, a non-radiative phenomenon is proposed
as the possible mechanism of energy transfer for the chosen pair of
donor and acceptor.
The fluorescence decay curves and lifetime profiles of the ˇ-
CDen-ADR with the acceptor provided more information regarding
the interaction between the D and A in the excited state. The lifetime
of the safranine molecules in their free state was found to be 3.1 ns
and fits into a single exponential decay function when excited at
375 nm (Fig. S7).
When the ˇ-CDen-ADR/safranine was excited at 375 nm, the
decay curves were recorded at the emission wavelengths of both
the donor and the acceptor. Fig. 7 reveals that, a quenching was
observed in the lifetime of the donor molecule to the extent of
1.87 ns at the donor decay wavelength (Table 2). Such a decrease in
◦
D
2
9
000 (ln 10)k ꢂ
6
R
◦
=
J(ꢅ)
(4)
5
1
28ꢃ N ꢄ
A
4
where ꢆ2 is an orientation factor and has a value of 2/3 for ran-
◦
domly oriented donor–acceptor dipoles, ꢂ D is the quantum yield
of the donor in the absence of acceptor, NA is the Avogadro number,
ꢄ is the refractive index of the medium and J(ꢅ) is a quantitative
measure of the donor–acceptor overlap [41]. Among the factors
appearing in the equation, it is the spectral overlap integral J(ꢅ)
which makes a difference with regard to efficiency of the process
since all the others remain the same for a typical system.
The absorption spectrum for ˇ-CDen-ADR for increasing con-
centrations of safranine acceptor is as shown in Fig. 5. Fig. 6 shows
the steady-state emission of ˇ-CDen-ADR for increasing concen-
trations of the safranine acceptor. The excitation wavelength of
3
75 nm was absorbed dominantly by the donor. Upon increasing
the concentration of the acceptor, the donor fluorescence intensity
decreased with concomitant increase in the fluorescence intensity
of the acceptor. This gives the evidence for energy transfer from
ˇ-CDen-ADR as there is no significant contribution of safranine flu-
orescence by direct excitation at 375 nm. A red shift of ≈11 nm and
a blue shift of ≈5 nm in the acceptor fluorescence maximum and
donor fluorescence maximum, respectively, were observed with
increase in the acceptor concentration. The red shift is attributed
to radiative migration due to the effect of self-absorption among
acceptors and the blue shift is due to the radiative transfer from
donor to acceptor [41]. A decrease in intensity of the donor emis-
sion peak at 448 nm was observed with increase in concentration
of safranine solution. Also a new acceptor emission peak appeared
at 567 nm and its emission intensity was found to increase with
continuous additions of the safranine solution.
We were able to calculate the spectral overlap integral from the
overlay spectrum of the absorption spectrum of the safranine and
emission spectrum of ˇ-CDen-ADR. Based on the intensity mea-
surements of the donor in the presence and absence of safranine,
the Förster radius and efficiency of energy transfer were calculated
Fig. 6. Steady-state emission of ␤-CDen-ADR/safranine system [ˇ-CDen-ADR]:
−5
−5
−5
−5
−5
2
× 10 M, [safranine]: (a) 1 × 10 M, (b) 2 × 10 M, (c) 3 × 10 M, (d) 4 × 10 M.
(
Table 1). The rate constant for the process was obtained from
Inset: Stern–Volmer plot of ˇ-CDen-ADR/safranine system. ꢅexc = 375 nm.

R. Krishnaveni et al. / Journal of Photochemistry and Photobiology A: Chemistry 229 (2012) 60–68
65
Table 1
Spectral parameters calculated from steady-state emission studies.
J (ꢅ) in M 1 cm nm4
−
−1
RDA (Å)
R
◦
DA (Å)
Transfer efficiency (%)
Rate constant (M
−1 −1
s
)
Donor
5.63 × 10−
8.54 × 10−
15
25.42
37.68
23.49
34.83
39
38
1.5 × 10
12
ˇ-CDen-ADR
ADR
15
11
4.2 × 10
Table 2
Lifetime measurements of ˇ-CDen-ADR/safranine system at 448 nm.
Safranine (10 5 M)
−
Donor life time (ns)
Relative amplitude
ꢇ2
ꢁ
1
ꢁ2
ꢁ3
B1
B2
B3
0
2
4
6
8
2.42
2.25
2.37
3.18
2.26
7.45
7.08
7.04
6.82
5.36
0.528
0.574
0.83
1.19
0.602
12.52
12.51
12.93
13.43
17.54
84.24
83.34
82.15
79.41
77.84
3.24
2.74
2.92
7.16
4.62
1.24
1.09
1.12
1.06
1.23
fluorescence lifetime in presence of acceptor reveals it clearly that
the mechanism operating is a non-radiative one.
Whereas the decay observed at the acceptor wavelength also
fits into a tri-exponential decay function, but showed a rise time
which indicated a formation in the excited state which is also a
characteristic occurrence for a FRET process (Fig. 8). Two lifetimes
results that invariably in all cases, steady state FRET efficiencies are
larger than those extracted from the time-resolved measurements
[43].
Also the rate constant for the energy transfer processes were
obtained from Stern–Volmer plot on the basis of lifetime measure-
ments of donor–acceptor combination. Similar to the observation
with the free ADR as donor, with the ˇ-CDen-ADR donor, the rate
constant for the energy transfer was found to be in the order of
were recorded at the acceptor wavelength, one longer (ꢁ ) and
l
one shorter (ꢁs), which indicated that the D and A exist in the
excited state in two different configurations; may be one bound
and the other one unbound. Hence, the longer lifetime component
11
−1 −1
10
M s , which supported the proposed mode of non-radiative
energy transfer occurring in these systems.
(
ꢁl) is attributed to the free ˇ-CDen-ADR molecules and the shorter
In an attempt to check the efficiency of the acridinedione mod-
ified ˇ-CDen-ADR as an energy donor in FRET analysis, same set of
experiments were carried out with free ADR. A good spectral over-
lap was observed between the absorption and emission spectra of
the acceptor and donor respectively, from which the spectral over-
lap integral and Förster radii were calculated and are tabulated in
Table 1 The absorption spectrum of free ADR in the presence of
increasing concentrations of safranine is shown in Fig. 9. Fig. 10
shows the steady state spectrum of free ADR in the presence of
safranine. A blue shift of ≈8 nm and a red shift of ≈13 nm were
observed at donor and acceptor fluorescence wavelengths respec-
tively. From the intensity values of the free ADR in the presence
and absence of donor, the efficiency of energy transfer process and
the rate constant of the same were also calculated (Table 1). The
one (ꢁs) with a negative amplitude to the ˇ-CDen-ADR bound to
the acceptor molecules (Table 3). It was also observed that as the
acceptor concentration was increased, the rise time decreased. This
indicated that the transfer rate became faster on increasing the
acceptor concentration which was due to the closer proximity of
the D–A molecules. Such a decrease in rise time with increase in
acceptor concentrations have been already reported by Auerbach
et al. theoretically [42].
With the lifetime values obtained for the donor molecule in the
presence and absence of safranine at the donor decay wavelength,
the efficiency of energy transfer was calculated using Eq. (2) and
found to be as 29% which is less than that observed based on inten-
sity measurements. This is in accordance with the earlier observed
Fig. 7. Lifetime decay profile of ˇ-CDen-ADR/safranine system at 448 nm [ˇ-
−
5
−5
−5
−5
CDen-ADR]: 2 × 10 M, [safranine]: (a) 2 × 10 M, (b) 4 × 10 M, (c) 6 × 10 M,
Fig. 8. Lifetime decay profile of ˇ-CDen-ADR/safranine system at 567 nm [ˇ-CDen-
−5
−5
−5
−5
−5
(
d) 8 × 10 M. Inset: Stern–Volmer plot of ˇ-CDen-ADR/safranine system.
ADR]: 2 × 10 M, [safranine]: (a) 2 × 10 M, (b) 4 × 10 M, (c) 6 × 10 M, and (d)
−5
ꢅexc = 375 nm.
8 × 10 M.

6
6
R. Krishnaveni et al. / Journal of Photochemistry and Photobiology A: Chemistry 229 (2012) 60–68
Table 3
Life-time measurements of ˇ-CDen-ADR/safranine system at 567 nm.
Safranine (10 5 M)
−
Acceptor life time (ns)
Relative amplitude
ꢇ2
ꢁ
1
ꢁ2
ꢁ3
B1
B2
B3
2
4
6
8
2.27
2.34
2.39
2.41
0.27
5.36
6.16
5.69
6.15
83.67
71.45
83.73
76.33
−10.81
−5.35
−2.18
−1.67
27.64
33.90
18.45
25.34
1.23
1.01
1.07
1.07
0.193
0.155
0.146
Table 4
Life-time measurements of free ADR/safranine system at 467 nm and 567 nm.
Safranine (10 5 M)
−
Donor decay
Acceptor decay
A(ns)
ꢁ
Relative amplitude
ꢁ
D (ns)
ꢇ2
ꢁ1
ꢁ2
B1
B2
ꢇ2
0
2
4
6
8
4.95
4.48
4.13
3.87
3.63
1.18
1.11
1.17
1.13
1.19
2.34
2.39
2.64
2.74
4.58
4.45
3.93
3.66
−78.28
−86.70
178.28
186.70
218.96
240.95
1.19
1.18
1.20
1.29
−118.96
−140.95
lifetime decay profiles (Figs. 11 and 12) at donor emission wave-
length (467 nm) and acceptor emission wavelength (567 nm) are
illustrated in Table 4. From the lifetime values of the donor in the
presence and absence of acceptor, the rate constant and efficiency
additions of ␤-CDen-ADR are as shown in Figs. 13 and 14 respec-
tively.
Fixing up the concentration of the acceptor, the addition of ␤-
CDen-ADR, showed no appreciable change in the absorbance of
safranine at its ꢅmax (Fig. 13). This may be attributed to the weak
binding of the acceptor into the cavity of ␤-CDen-ADR. An isos-
bestic point was observed at 476 nm which indicated the formation
of 1:1 inclusion complex between the ␤-CDen-ADR and the safra-
nine. The emission behavior was studied by exciting the system
at 476 nm and here it was observed that there was an increase
in the emission intensity of the acceptor without any change in
the emission maximum (Fig. 14). And from the intensity values,
the Benesi–Hildebrand plot was constructed and the complexation
12
−1 −1
of the FRET process were calculated as 1.04 × 10
M s and 24%.
The observation of an isoemissive point at 555 nm in the emis-
sion spectrum of ˇ-CDen-ADR in the presence of safranine is an
evidence for the existence of an equilibrium between the donor
and acceptor [44]. In this donor–acceptor pair, a possible inclusion
of safranine into the cavity of ˇ-CD is expected due to the match-
ing sizes of the cavity of ˇ-CD and that of the acceptor and also due
to the hydrophobicity of the CH3 substituents. Reports are already
available on such inclusion complexation of ˇ-CD and safranine
3
−1
[
45].
We have also carried out the experiments to find out the com-
constant was found as 3.5 × 10 M (inset in Fig. 14). This comes as
an evidence for our proposed model of binding of safranine acceptor
with the ␤-CD cavity.
plexation constant of the acceptor with the ␤-CDen-ADR. The
absorption and emission spectrum of safranine with continuous
Hence, such a FRET system with the donor and acceptor located
outside and inside the cyclodextrin cavity, respectively, can keep
the donor and acceptor within the effective distance of the Förster
4
3
d
2
a
397 nm
1
0
200
300
400
500
600
700
Wavelength(nm)
−
5
Fig. 9. Absorption spectrum of free ADR in the presence of increasing concentra-
Fig. 10. Steady-state emission of free ADR/safranine system [ADR]: 2 × 10 M,
−5 −5 −5 −5
−5
−5
−5
tions of safranine [ADR] – 2 × 10 M, [safranine]: (a) 2 × 10 M, (b) 4 × 10 M, (c)
[safranine]: (a) 1 × 10 M, (b) 2 × 10 M, (c) 3 × 10 M, (d) 4 × 10 M. Inset:
Stern–Volmer plot of free ADR/safranine system. ꢅexc = 375 nm.
−
5
−5
6
× 10 M, (d) 8 × 10 M.

R. Krishnaveni et al. / Journal of Photochemistry and Photobiology A: Chemistry 229 (2012) 60–68
67
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
a
f
f
a
4
76nm
500
4
00
450
550
600
650
700
Wavelength(nm)
Fig. 11. Lifetime decay profile of free ADR/safranine system at 467 nm [ADR]:
−
5
−5
−5
−5
−5
−4
2
× 10 M, [safranine]: (a) 2 × 10 M, (b) 4 × 10 M, (c) 6 × 10 M, (d) 8 × 10 M.
Fig. 13. Absorption spectrum of 2 × 10 M Safranine with sequential additions ␤-
−
5
−4
−4
Inset: Stern–Volmer plot of free ADR/safranine system. ꢅexc = 375 nm.
CDen-ADR of concentrations (a) 0 M, (b) 5 × 10 M, (c) 1 × 10 M, (d) 2 × 10 M,
−4 −4
(
e) 4 × 10 M, and (f) 8 × 10 M.
energy transfer and hence can be expected to provide higher effi-
ciency of the process. This was established from the efficiencies
of the energy transfer processes of the two donors as indicated in
Table 1. Though at the first sight it appears that both the donors
have the same efficiency, it is worth to mention here that the free
ADR provides an efficiency of 38% with a quantum yield of 0.92
6
1.8
8x10
1
0
.2
.6
0
6
7x10
0
.E+00 1.E+04 2.E+04
6
6x10
5x10
4x10
3x10
2x10
1x10
f
1/[β-CDen-ADR]
[
39] whereas the acridinedione modified ˇ-CD produces an effi-
ciency of 39% with a quantum yield of 0.13. As shown in Table 4,
in case of ADR/safranine system, it was observed that the rise
time increases with increase in acceptor concentration which is
the reverse of what was observed with ˇ-CDen-ADR as donor. This
further supports the predicted inclusion of the acceptor into the
cavity of cyclodextrin since in case of free ADR where such a cav-
ity is not available the donor and acceptor molecules are more at
random movements and hence the FRET process is rather slower
and hence longer is the rise time. Also from the lifetime profiles,
it was once again established that the FRET efficiency was greater
with ˇ-CDen-ADR (29%) than with free ADR (24%).
6
6
6
6
6
a
0
5
00
550
600
650
700
Wavelength (nm)
−
4
Fig. 14. Emission spectrum of 2 × 10 M Safranine with sequential additions ␤-
−
5
−4
−4
CDen-ADR of concentrations (a) 0 M, (b) 5 × 10 M, (c) 1 × 10 M, (d) 2 × 10 M,
−
4
−4
(
e) 4 × 10 M, and (f) 8 × 10 M. Inset: Benesi–Hildebrand plot of ␤-CDen-
ADR/safranine system. ꢅexc = 476 nm.
4. Conclusion
We have synthesized a modified cyclodextrin namely, mono-
6
-deoxy-6-aminoethylaminoacridinedione-ˇ-CD. The above said
compound was prepared via the synthesis of mono-6-deoxy-6-
tosyl-ˇ-CD and Mono-6-deoxy-6-aminoethylamino-ˇ-CD in the
preceding steps by following reported procedures. The interme-
diates and the compound were well characterized by IR, NMR and
mass techniques. The chromophore attached ˇ-CD thus obtained
was found to possess a good quantum yield. The absorption, exci-
tation and emission spectra recorded for the compound were found
to be in close resemblance to those characteristic of the acridine-
dione family. A decrease in donor fluorescence intensity with a
concomitant increase in acceptor intensity; shortening of decay
time of the donor with a parallel species formation in the excited
state as inferred from the rise time at the acceptor wavelength,
Fig. 12. Lifetime decay profile of free ADR/safranine system at 567 nm [ADR]:
−
5
−5
−5
−5
−5
2
× 10 M, [safranine]: (a) 2 × 10 M, (b) 4 × 10 M, (c) 6 × 10 M, (d) 8 × 10 M.
ꢅexc = 375 nm.

6
8
R. Krishnaveni et al. / Journal of Photochemistry and Photobiology A: Chemistry 229 (2012) 60–68
with increasing concentrations of the acceptors indicate that FRET
is happening between the ˇ-CDen-ADR and the acceptors. The FRET
efficiency of acridinedione modified ˇ-CDen-ADR was compared
with that of a free ADR and found it was more in case of the mod-
ified compound, the reason being the inclusion of acceptor into
the cavity of cyclodextrin. Thus, the ˇ-CD modified dye was found
to behave as a persuasive energy donor in the FRET experiments
carried out.
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