Fluorescence resonance energy transfer from a bio-active imidazole derivative 2-(1-phenyl-1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)phenol to a bioactive indoloquinolizine system

Article abstract of DOI:10.1016/j.saa.2011.02.049

Novel donor imidazole derivative, 2-(1-phenyl-1H-imidazo [4,5-f][1,10]phenanthrolin-2-yl)-phenol (PIPP) was screened as highly sensitive chemisensor for transition metal ions and it can be used as a "multi-way" optically switchable material. Solvatochromi

Full text of DOI:10.1016/j.saa.2011.02.049

Spectrochimica Acta Part A 79 (2011) 236–244
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Fluorescence resonance energy transfer from a bio-active imidazole derivative
2
-(1-phenyl-1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)phenol to a bioactive
indoloquinolizine system
∗
Jayaraman Jayabharathi , Venugopal Thanikachalam, Marimuthu Venkatesh Perumal, Natesan Srinivasan
Department of Chemistry, Annamalai University, Annamalainagar, Tamilnadu 608002, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 January 2011
Received in revised form 18 February 2011
Accepted 25 February 2011
Novel donor imidazole derivative, 2-(1-phenyl-1H-imidazo [4,5-f][1,10]phenanthrolin-2-yl)-phenol
(PIPP) was screened as highly sensitive chemisensor for transition metal ions and it can be used as a
“
multi-way” optically switchable material. Solvatochromic effects on the fluorescence behaviour of PIPP
were studied in different solvents. The fluorescence of PIPP was highly sensitive to both the polarity
as well as protic nature of the solvent. Fluorescence (Forster) resonance energy transfer (FRET) process
from PIPP to a potent bioactive indoloquinolizine molecule was studied and it is argued that long-range
dipole–dipole interaction is operating for the energy transfer mechanism. The energy transfer efficiency
Keywords:
Chemisensor
FRET
Forester distance
Stern–Volmer plot
Lippert–Mataga plot
DFT
(E) and the distance between the acceptor and the donor (r0) have been determined.
©
2011 Elsevier B.V. All rights reserved.
1
. Introduction
Recently, heterocyclic imidazole derivatives have attracted
the indole nucleus are now well established as potent bioac-
tive molecules. ␤-Carbolines are known to be efficient cancer cell
photosensitizers and are used in photodynamic therapy (PDT).
Since FRET causes a number of important biological phenomena,
much interest lies in FRET studies between two potent bioac-
tive molecules in vitro. Given the broad-range biological activities
of imidazoles and indoloquinolizines, we became interested in
the FRET studies between PIPP and AODIQ. The present FRET
study has been performed considering its future application in
PDT.
considerable attention because of their unique optical prop-
erties [1] and used for preparing functionalized materials [2].
Imidazole nucleus forms the main structure of human organ-
isms, i.e., the amino acid histidine, Vitamin B12, a component
of DNA base structure and also has significant analytical
applications utilizing their fluorescene and chemiluminescence
properties [3].
The present article deals with the solvent dependent fluores-
cence behaviour of a newly designed imidazole derivative, 2-(1-
phenyl-1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)-phenol (PIPP)
due to the presence of phenanthroline function and it behaves as a
Lewis base due to the presence of the two diiminic nitrogen atoms.
We have also exploited the fluorescence resonance energy transfer
An important property that makes imidazole derivatives more
attractive as a chelator is the appreciable change in its fluores-
cence upon metal binding. Therefore, imidazole derivatives have
been used to construct highly sensitive fluorescent chemisensors
for sensing and imaging of metal ions and its chelates in par-
ticular those with Ir3+ are major components for organic light
emitting diodes [1] and are promising candidates for fluorescent
chemisensors for metal ions. In continuation of our recent research
experimental results [6–15], in the present paper we summarized a
comprehensive study of the imidazole derivative and its structure
was characterized by NMR spectra. Photophysical, photochemical
studies, its chemisensor behaviour and FRET studies were ana-
lyzed and discussed. Theoretical calculations were carried out by
using Gaussian-03 program [16] to supplement the experimental
results.
(
FRET) from PIPP to the bioactive nitrogen heterocycle namely 3-
acetyl-4-oxo-6,7-dihydro-12H-indolo-[2,3-a]quinolizine (AODIQ)
4]. The indole nucleus appears to be a promising basis for the
[
design and synthesis of new derivatives that can effectively pro-
tect the nervous system [5]. Carbazoles and ␤-carbolines containing
∗ Corresponding author. Tel.: +91 9443940735.
E-mail address: jtchalam2005@yahoo.co.in (J. Jayabharathi).
1
386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.02.049

J. Jayabharathi et al. / Spectrochimica Acta Part A 79 (2011) 236–244
237
2
. Experimental
129.05, 129.72, 130.67, 135.20, 136.40, 137.40, 149.4, 150.00, 154.1,
1
56.02. MS: m/z 388.13,calcd 388.42.
2.1. Materials and methods
3. Results and discussion
1
,10-Phenanthroline-5,6-dione (Sigma–Aldrich Ltd.), salicy-
laldehyde (S.D. fine.), aniline (S.D. fine) and all the other reagents
used without further purification. 3-Acetyl-4-oxo-6,7-dihydro-
3.1. Photo physical properties of imidazole derivative (PIPP) in
solution
1
2H indolo-[2,3-a]quinolizine (AODIQ) was synthesized in the
laboratory using the method mentioned elsewhere [16]. It was
purified by column chromatography and the purity of the com-
pound was checked by thin layer chromatography (TLC). Further,
the compound was vacuum sublimed before use.
Absorption and emission properties of the imidazole derivative,
PIPP have been studied in various solvents. The absorption band
maxima (ꢀabs), emission band maxima (ꢀemi) and the associated
stokes shift (ꢁE) are shown in Table 1.
The imidazole derivative (PIPP) fluoresces strongly in solution
at room temperature. The luminescence excitations spectra of
PIPP are in coincide with their absorption spectra and differ from
their emission spectra. Therefore, the fluorescence of the imidazole
derivative was taken for discussion.
2
.2. Optical measurements and composition analysis
NMR spectra were recorded for PIPP on a Bruker 500 MHz.
The ultraviolet–visible (UV–vis) spectra were measured on UV–Vis
spectrophotometer (Perkin Elmer, Lambda 35) and corrected for
background due to solvent absorption. Photoluminescence (PL)
spectra were recorded on a (Perkin Elmer LS55) fluorescence spec-
trometer. MS spectra were recorded on a Varian Saturn 2200 GCMS
spectrometer.
3.2. Effect of solvent on the absorption spectra
The shift of absorption with different solvents observed for PIPP
is shown in Fig. 1. The shift can be described as hypsocromic or
bathochromic depending on whether the absorption maximum
occurs at a lower or higher wavelength respectively (Table 1). In
the present study, the absorption spectra shifts to higher wave-
length with increasing solvent polarity, indicating a bathochromic
shift. Normally, the bathochromic shift happens when the dipole
moment of the compound increases during the electronic tran-
sition i.e., the ground state dipole moment is lesser than the
excited state dipole moment (ꢂg < ꢂe) and the excited state is
formed in solvent cage of already partly oriented solvent molecules
2.3. Computational details
Quantum mechanical calculations were used to carry out
the optimized geometry, HOMO–LUMO energies and MEP with
Gaussian-03 program using the Becke3-Lee-Yang-Parr (B3LYP)
functional supplemented with the standard 6-311++G(d,p) basis
set [17,18].
[
19,20].
Thus more polar solvents favour the stabilisation of excitation of
2
.4. General procedure for the synthesis of 2-(1-phenyl-1H-
imidazo[4,5-f][1,10]phenanthro-lin-2-yl)phenol (PIPP)
the imidazole derivative. By examination of the structure of imida-
zole derivative, the electrons are expected to delocalise between
the nitrogen atoms. In addition hydrogen bonding between the
solvent and nitrogen atom of the imidazole may induce electron
The experimental procedure was used as the same as described
in our recent papers [6–15]. The imidazole derivative, PIPP was
synthesized from an unusual four components assembling of
1
,10-phenanthroline-5,6-dione, ammonium acetate, aniline and
salicylaldehyde (Scheme 1).
Table 1
Photophysical properties of the imidazole derivative PIPP.
Stokes shift (cm 1
−
)
2
.5. 2-(1-Phenyl-1H-imidazo[4,5-f]
Solvents
Hexane
ꢀabs (nm)
ꢀemi (nm)
[1,10]phenanthro-lin-2-yl)phenol (PIPP)
276
290
286
287
285
292
290
289
288
293
392
427
10,722
11,064
11,682
11,587
11,886
11,311
11,652
11,875
12,072
11,505
1,4-Dioxane
◦
Yield: 55%. mp = 289 C, Anal. calcd. for C25
H
N O: C, 77.3; H,
Benzene
429.5
430.5
431.0
436.0
438.0
440.0
441.5
442.0
16
4
1
Chloroform
Ethyl acetate
Dichloromethane
4
.15; N, 14.42. Found: C, 76.65; H, 4.08; N, 13.98. H NMR (500 MHz,
CDCl ): ␦ 4.90 (S, 1H), 6.63 (d, 1H), 6.81 (m, 2H), 7.01–7.16 (m,
1
3
H), 7.29 (d, 2H) [o-hydroxy phenyl ring], 7.59–7.75 (Aromatic pro-
1-Butanol
tons of aniline ring), 9.04 (d, 2H, H-aryl, J = 4.5 Hz), 9.10 (d, 2H,
Ethanol
Methanol
Acetonitrile
J = 8.0 Hz), 9.20 (d, 2H, J = 5.6 Hz). 13C (100 MHz, CDCl ): ı 115.86,
3
1
19.42, 121.50, 121.85, 122.40, 122.91, 127.53, 127.61, 128.30,
Scheme 1. Synthesis route for IPP and PIPP.

2
38
J. Jayabharathi et al. / Spectrochimica Acta Part A 79 (2011) 236–244
Fig. 2. Schematic representation of halochromism.
−
the loss of phenolic proton forming [IPP-H] and the second depro-
tonation step occurs at pH 11.0 is attributed to the formation of
doubly deprotonated compound [IPP-2H]2 . The pH effect of PIPP
reveal that the deprotonation step of PIPP is almost identical to the
first deprotonation step of IPP, supporting our assumption that the
deprotonation step of phenolic proton takes place at lower basic
pH 8.5 [7].
−
3.4. Effect of solvent on the fluorescence spectra
Fig. 1. Absorption solvatochromism of PIPP in various solvents.
Fluorescence spectral studies of PIPP were carried out in dif-
ferent solvents (Fig. 3), the peak maximum shifts to the right
bathochromic shift) as the solvent polarity increases (hexane to
methanol). The position of the peak maximum (ꢀemi) was plotted
against the solvent dielectric constant (␧) and the observed linear
correlation (Fig. S3) reveal that with increasing solvent polarity,
the ꢀmax also increases. To better understand the solvent polarity
effect, Lippert–Mataga relation (Fig. S4) was applied. This relation
has been widely used to correlate the energy difference between
the absorption maximum and the emission maximum (stokes
distributions. This might be the reason in protic solvents with
strong hydrogen bond ability shifts to higher wavelength, but it
remains fixed at lower wavelength in hexane where the formation
of hydrogen bond is prevented.
Measurement of absorption solvatochromism have been inter-
preted with Marcus and Reichardt–Dimroth solvent functions to
estimate the transition dipoles associated with low lying excited
state. The linear correlation (Fig. S1) of solvent shifts of absorp-
tion band position of PIPP with Reichardt–Dimroth solvent ET
parameters is indicative of the fact that the dielectric solute sol-
vent interactions are responsible for the observed solvatochromic
shift for the imidazole derivative. The observed linear correlation
(
shift) (Table 1) with solvent polarity represented by ꢃ . Stokes
f
shift linearly increases with ꢃ implies that specific solute–solvent
f
interactions are involved, which is known to consist of sev-
eral effects i.e., dipole–dipole interactions, hydrogen bonding and
dipole induced dipole interactions. Relative contribution of individ-
ual effect depends on the type of solvent used. In protic-hydrogen
bonding solvents, the molecule can be stabilized through hydrogen
bonding with the more electronegative nitrogen atom (N22 of the
imidazole derivative). The more electronegative nature of N22 is
confirmed by molecular electrostatic potential (MEP) study which
is discussed in the next section. In aprotic solvents, dipole–dipole
and dipole-induced dipole forces are assumed to be the predomi-
nant interactions. Decrease in total dipole moment of the solvent
molecules (non-polar solvents) will result in the change of the
molecular charge distributions of the imidazole derivative, PIPP.
Upon excitation, molecular charge distribution is changed having
less polar structure, as a result the extent of the specific hydrogen
bonding might decrease [22].
(
Fig. S2) of solvent shift of absorption band positions of PIPP with
Marcus [21] optical dielectric solvent function [(1 − Dop)/(2Dop + 1)]
reveal that transition dipoles associated with absorption and the
direction of excited dipole is opposite to that of the ground state-
dipole.
3
.3. Halochromisim of the imidazole derivative PIPP
We have measured the absorption spectra of imidazole deriva-
tive (PIPP) in aprotic solvent at different pH value (Fig. 2) and
it was found that the imidazole compound show halochromic
behaviour dependent on the pH. In the present study, for
the comparison purpose, the halochromic behaviour of 2-
(
1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)phenol IPP was also
studied. The absorption band of IPP was red shifted with increasing
pH and the absorption peak (ꢀmax) was located at 270 and 402 nm
i.e., as the pH of the solution increases, the absorption band disap-
peared and new absorption band appeared at 402 nm. There are two
isobestic points observed approximately at 320 and 370 nm which
may be due to the two deprotonation steps shown in Scheme 2.
The first deprotonation step of IPP around pH 8.5 is attributed to
3.5. Quantum yield, emission kinetics and thermodynamic
properties of PIPP
The fluorescence quantum yield for PIPP (Table 2) were mea-
sured in acetonitrile, using coumarin 47 in ethanol as a standard

J. Jayabharathi et al. / Spectrochimica Acta Part A 79 (2011) 236–244
239
Scheme 2. Deprotonation steps of IPP and PIPP.
according to the equation,
ꢀ
ꢁꢀ
ꢁꢀ
ꢁ
2
Iunk
Istd
Astd
ꢄunk
ꢄstd
˚unk = ˚std
(1)
Aunk
where Фunk, Ф , I , Istd, A , Ф
and Фstd are the fluores-
std unk
unk
unk
cent quantum yields, the integration of the emission intensities,
the absorbances at the excitation wavelength and the refrac-
tive indexes of the corresponding solution for the sample and
the standard, respectively. The radiative and non-radiative decay
was calculated by these equations kr = ꢅp/ꢆ, knr = 1/ꢆ − ˚p/ꢆ,
−
1
ꢆ = (kr + knr) , where kr, knr are the radiative and non-radiative
deactivation, ꢆ is the life time of the S1 excited state.
f
3.6. HOMO–LUMO energies of imidazole derivative PIPP by DFT
method
The frontier orbitals (HOMO and LUMO) determined the way
in which the molecule interacts with other species. The frontier
orbital gap (Table 2) helps to characterize the chemical reactivity
and kinetic stability of the molecule. A molecule with a small fron-
tier orbital gap is more polarizable and is generally associated with
high chemical reactivity, low kinetic stability and is also termed as
soft molecule [7]. The HOMO is the orbital that primarily acts as
an electron donor and the LUMO is the orbital that largely acts as
the electron acceptor. The 3D plots of the frontier orbitals HOMO
and LUMO of PIPP is shown in Fig. S5. The HOMO is located on the
imidazole ring (not on the phenanthroline ring) and the o-hydroxy
phenyl ring attached to the C34 carbon of the imidazole ring and
the LUMO located on the phenanthroline ring and not on the two
phenyl rings attached to the N21 and C34 carbons of the imidazole
rings). The HOMO → LUMO transition implies that intramolecular
charge transfer takes place [23] within the molecule.
Fig. 3. Emission solvatochromism of PIPP in various solvents.

2
40
J. Jayabharathi et al. / Spectrochimica Acta Part A 79 (2011) 236–244
Table 2
Electronic properties of PIPP.
ꢂ
HOMO−1 (ev)
HOMO (ev)
LUMO (ev)
0.083
LUMO+1 (ev)
0.090
Eg (EHOMO−1 − ELUMO) (ev)
−0.378
Eg (EHOMO − ELUMO) (ev)
−0.338
0
.62
−0.25
−0.255
Therefore, introduction of an electron donating substituent into
the molecular structure with its physiochemical property relation-
ships [25], the three-dimensional distribution of its MEP is helpful.
In MEP diagram negative regions can be regarded as nucleophilic
centres, whereas regions with positive electrostatic potential are
potential electrophilic sites. The molecular electrostatic potential
surface (MEP) is a plot of electrostatic potential map into the iso-
electron density surface simultaneously displays molecular shape,
size and electrostatic potential values and has been plotted for PIPP
(Fig. 4). The MEP map of PIPP clearly suggests that oxygen and nitro-
gen atoms represent the most negative potential region (dark red)
but the nitrogen atom seems to exert comparatively small nega-
tive potential as compared to oxygen atom. The hydrogen atom
attached to the six membered ring bear the maximum positive
charge (blue region). The MEP clearly confirms the existence of
the electrophyllic active centres characterized by red colour. The
predominance of green region in the MEP surface corresponds to
a potential halfway between the two extremes red and dark blue
colour.
the phenyl ring attached to the C34 carbon raises the energy
of the HOMO resulting in a red-shift of the emission. On the
other hand, the introduction of the electron withdrawing sub-
stituent into the aldehydic phenyl ring lowers the energy of the
HOMO, leading to the blue shift of the emission. The calculated
energy gap explains the eventual charge transfer interactions
within the molecule. The optimized HOMO−1, HOMO, LUMO and
LUMO+1 molecular orbitals of PIPP reveal that HOMO−1 and
HOMO have identical nodal patterns i.e., nodal between the C34
o-hydroxy phenyl ring (partly) and the imidazole ring. In the case
of LUMO and LUMO+1orbitals, no nodal between the imidazole
ring and the C34 o-hydroxy phenyl ring. Therefore the ␲ inter-
action between the two rings is anti-bonding in character. From
Table 2 it was concluded that the lowest energy transition (S → S )
0
1
corresponds to HOMO → LUMO transition having higher oscillator
strength and the higher energy transition (S → S ) corresponds to
0
2
(
HOMO−1 → LUMO) transition having lower oscillator strength.
The energy gap (Eg) of PIPP was calculated from the HOMO
The charge distribution of PIPP was calculated by the NBO and
Mulliken methods (Fig. 5). These two methods predict the same
tendencies i.e., compared to nitrogen atoms N21 and N22, oxygen
atom O45 is more basic. Among the nitrogen atoms N21 and N22,
N22 is considered as more basic site [26]. The charge distribution
shows that the more negative charge is concentrated on oxygen
atom whereas the partial positive charge resides at hydrogens.
and LUMO levels. The decrease in the HOMO and LUMO energy
gap explains the probable charge transfer (CD) taking place inside
the chromophore. The energy gap law predicts that the rate of
non-radiative decay increases when the energy gap separating the
ground and excited state decreases. This relation is based on the
vibrational overlap between the ground state and excited state and
knr is a function of Frank–Condon overlap integral. In the case of
compounds having similar excited states and vibrational coupling,
a simplified form of the energy gap law is obtained that predicts a
linear relationship between ln(knr) and the energy gap. This corre-
lation suggests that decay rate constant is due to the energy gap of
the compound [24] and from the emission spectra, it is observed
that the emission of PIPP is due to ␲–␲* transition only.
3.8. Chemosensor fluorescent
Stokes shift is important for a fluorescent sensor. The higher
−
1
stokes shift value (11,311 cm ) supplies very low background sig-
nals and resultantly allows the usage of the material in construction
of a fluorescence sensor [27]. Imidazole derivatives have been used
to construct highly sensitive fluorescent chemisensors for sens-
ing and imaging of metal ions and its chelates with Ir3+ are the
major components for organic light emitting diodes [6,7] and are
promising candidates for fluorescent chemisensor for metal ions, if
their radiationless channel could be blocked by metal binding. The
changes in emission spectrum of PIPP depending on different con-
centrations of Co2+ (Fig. 6a). According to these spectra, when the
3
.7. Molecular electrostatic potential map (MEP)
In order to prove the higher electron density at the four nitrogen
atoms in PIPP, we have performed DFT calculation to get the molec-
ular electrostatic potential (MEP) for PIPP. For the consideration
of the reactive behaviour of a chemical system and to investigate
Fig. 4. Molecular electrostatic potential (MEP) diagram of PIPP.

J. Jayabharathi et al. / Spectrochimica Acta Part A 79 (2011) 236–244
241
Fig. 5. Bar diagram of NBO and Mulliken atomic charges of PIPP.
−7
concentrations of Co2+ are from 1 × 10 to 1 × 10 mol/L, the rel-
−4
equation is as follows.
ative intensity changes (I − I/I ) are 0.987–0.201, respectively. The
0
0
I − I
2+
0
= 1.150 + 0.1520 log[Co2+
quenching in fluorescence of PIPP during exposure to Co cation
]
(2)
I
0
2+
indicates the complexation of PIPP with Co . When the concen-
2
+
−4
−7
tration of Co ion increased from 1 × 10 to 1 × 10 mol/L the
2+
where I0 is the emission intensity of Co free PIPP solution and
relative intensity changes linearly decreases. The linear regression
2+
I is the emission intensity of Co containing PIPP solution. The
correlation co-efficient is 0.9907. According to the obtained results
PIPP can be used as a new fluorescence sensor to detect the quantity
of Co2+ in any sample solution depending on the relative intensity
changes [28].
3.9. Selective fluorescent chemosensor studies
In order to determine the selectivity of the new Co2+ sensor
the influence of a number of transition metal cations were investi-
−
5
2+
gated. The experiments were performed in 1 × 10 mol/L of Pb
,
2+
2+
2+
2+
2+
Mn , Ni , Hg , Cu and Co . Separate solutions of each con-
−
5
tain 1 × 10 M of PIPP in 1:1 (v:v) dichloromethane/deionized
water mixture. The results shows that PIPP has quite fluorescence
2+
quenching in the absence of Co and the relative intensity changes
with the tested ions of Pb2+, Mn , Ni , Hg , Cu and Co
Fig. 6b).
The selectivity results show that PIPP can be easily complex with
Co2+ ion. Also, as seen in Fig. 11a and b, the intensity of PIPP rela-
2+
2+
2+
2+
2+
(
2+
2+
tively decreases in presence of some other ions such as Pb , Mn
Ni , Hg , Cu and Co . These decreases may be affect the quan-
,
2+
2+
2+
2+
2+
titative measurement of Co ion in any sample if they presence
with too high concentrations. However, the known samples like sea
water usually contain little concentration of these cations those are
not expected to affect the quantitative measurement of Co ion.
As a result, PIPP can be used as an alternate Co2+ sensor by using
the fluorescence technique.
2+
3.10. Excited state dipole moment
For the determination of the difference in dipole moment of
the excited state and ground state (ꢁꢂ), we followed the method
as described by Ravi et al. [29]. According to this method, we have
N
Fig. 6. (a) Fluorescent chemosensor behaviour of PIPP with Co2+ and (b) fluorescent
chemosensor behaviour of PIPP with various metal ions.
plotted the Stokes shift of the fluorescence maximum against ET of
different polar aprotic solvents (Fig. S6). From the slope of the above

2
42
J. Jayabharathi et al. / Spectrochimica Acta Part A 79 (2011) 236–244
Fig. 8. FRET mechanism.
absorbs appreciably at 420 nm and emits at 455 nm [30]. Consid-
ering the fluorescence band of PIPP and the absorption band of
AODIQ, this pair is an excellent pair of donor–acceptor system for
the FRET study. Gradual addition of AODIQ to the PIPP solution in
dioxane associated with bathochromic shift of the emission maxi-
mum along with decrease in the fluorescence intensity at 427 nm
Fig. 7. Parallel and anti-parallel dipole alignments.
plot and Onsager’s radius (a) for the PIPP, we have determined the
ꢂ. The excited-state dipole moment has been estimated to be
5.21 D taking the value of the ground-state dipole moment as 7.92
D (corresponding to the optimized geometry of PIPP through DFT
calculation).
The ratio of the difference between the absorption and fluo-
rescence shift can be used to calculate the dipole moment of the
first excited singlet state assuming that the direction of the charge
transfer (ꢁꢂ) to be in the plane of the ␲ system. The simplified
expression is ꢂe = ꢂg (ꢁEemi/ꢁEabs), where ꢂe and ꢂg are dipole
moments and ꢁEabs and ꢁEemi are the solvatochromic shifts in
the absorption and fluorescence spectra in different solvents of
similar refractive indices. ꢂg of PIPP was calculated using the pro-
gram Gaussian-03. On the basis of these values, the ꢂe calculated
from the above equation is 14.99 D. The calculated higher ꢂe value
support that the excited states of PIPP is more polar than their
corresponding ground state.
(Fig. 9) and finally a new band at 455 nm appears to correspond
ꢁ
1
to the acceptor molecule. Since the emission band position of the
donor (427 nm) and the acceptor (455 nm) are very close, isoemis-
sive point is not possible, which is a main characteristic of FRET
process. Absence of any additional absorption band for the mix-
ture of PIPP and AODIQ, exploited the ground state complex of the
donor–acceptor pair was not formed in the solution [33,34]. The flu-
orescence emission spectrum of the mixture of donor and acceptor
did not show any additional new broad peak at longer wavelength
confirmed the absence of exciplex formation between the excited
donor and the acceptor molecules. Thus, the decrease in fluores-
cence intensity of the donor with increasing acceptor concentration
indicates non-radiative energy transfer between the excited donor
and the acceptor. Furthermore, the fluorescence excitation spec-
tra in presence of donor and acceptor monitoring at 465 nm clearly
show that an efficient Forester’s type resonance energy transfer
from PIPP to AODIQ system (Fig. 10).
The overall polarity of the synthesized imidazole derivative is
small when their dipole moments aligned in a parallel fashion
Fig. 7). When the electric field is removed, the parallel alignment of
The quenching of the fluorescence of the donor (PIPP) with the
addition of acceptor (AODIQ) was followed by Stern–Volmer plot
(
the molecular dipole moment begins to deteriorate and eventually
the imidazole derivative loses its NLO activity. The ultimate goal in
the design of polar materials is to prepare compounds which have
their molecular dipole moments aligned in the same direction.
3.11. Fluorescence (Forster’s) resonance energy transfer from
PIPP to AODIQ
FRET is a distance-dependent interaction between the different
electronic excited states of molecules in which excitation energy is
transferred from one molecule (donor) to the other (acceptor) with-
out emission of a photon from the donor molecular system (Fig. 8).
According to Forster’s theory, the efficiency of FRET depends mainly
on the extent of overlap between donor emission and the accep-
tor absorption, the orientation of the transition dipole of donor
and acceptor and the distance between the donor and the accep-
tor [30–32]. In the present work, the origin of idea of studying
the FRET from PIPP to AODIQ stemmed from the consideration of
overlap of the absorption and fluorescence band positions of the
two fluorophores. In dioxane solvent, PIPP absorbs appreciably at
2
90 nm giving fluorescence at around 430 nm. The molecule AODIQ
Fig. 9. Fluorescence spectra of PIPP as a function of AODIQ.

J. Jayabharathi et al. / Spectrochimica Acta Part A 79 (2011) 236–244
243
Fig. 12. Overlap of fluorescence spectrum of PIPP and absorption spectrum of
AODIQ.
Fig. 10. Fluorescence spectra of donor and acceptor.
where r is the distance between the donor and acceptor, R is called
0
0
the Forster’s distance or critical energy transfer distance, at which
the efficiency of transfer is 50% and R can be calculated as follows:
0
0
6
0
23
2
−4
6
R
= 8 : 8 × 10 [K n ˚DJ(ꢀ)] in A
(5)
where k2 is the relative orientation of the donor and acceptor
molecule, n is the refractive index of the medium, ˚D is the quan-
tum yield of the donor in absence of the acceptor and J(ꢀ) is the
overlap integral of the fluorescence emission spectrum of the donor
and the absorption spectrum of the acceptor. The overlap spectrum
([PIPP]:[AODIQ] = 1:1) in dioxane is shown in Fig. 12. The overlap
integral J(ꢀ) for a donor–acceptor pair is defined as
ꢃ
∞
J(ꢀ) =
FD(ꢀ)ε (ꢀ)ꢀ4 dꢀ
(6)
A
0
where FD (ꢀ) is the corrected fluorescence intensity of the donor
at wavelength ꢀ to (ꢀ + ꢁꢀ), with the total intensity normalized to
unity and ␧A (ꢀ) is the molar extinction coefficient of the accep-
Fig. 11. Stern–Volmer plot of PIPP.
tor at wavelength ꢀ. The Forster distance (R ) has been calculated
0
assuming random orientation of the donor and acceptor molecules.
2
(
Fig. 11).
In the present case, k = 2/3, n = 1:422, ˚ = 0.62 and with the help
D
of the relation (5), we have calculated R0 as 24.0 A˚ . The sum of col-
Fo
=
1 + kqꢆ [Q] = 1 + KSV[Q]
(3)
˚
0
lision radii of donor (PIPP) and acceptor (AODIQ) is about 12–13 A.
The significant difference between the values of the collision radii
F
From this figure, it is evident that at high concentration of
(
12–14 A˚ ) and theoretical radius of energy transfer (24.0 A˚ ) suggest
acceptor it undergoes a positive deviation from the linearity. The
nonlinearity was attributed to the fact that at high quencher con-
centration, a fraction of the fluorophore is just adjacent to the
acceptor at the moment of excitation and thus immediately deac-
tivated upon excitation. From the linear portion of the curve, we
have determined the quenching rate constant (KSV) value to be
that for PIPP–AODIQ pair, long-range dipole–dipole interaction is
responsible for the energy transfer mechanism consistent with the
earlier proposition [33,34]. According to the Forster’s non-radiative
energy transfer theory, the energy transfer efficiency (E) is given by
the following relation:
4
−1
1 − F
3
.8 × 10 ML . This determined KSV value falls in the normal range
E =
(7)
F
0
reported earlier for FRET study [33,34].
where F is the fluorescence intensity of the donor in the presence of
the acceptor and F0 is the fluorescence intensity in the absence of
the acceptor molecule. The energy transfer efficiency (E) for the pair
of PIPP and AODIQ in case of 1:1 composition has been calculated
and is found to be 0.29. With the knowledge of the energy trans-
3
.12. Energy transfer efficiency and distance
To understand the energy transfer efficiency between PIPP and
AODIQ, we have determined the probable distance between the
reacting species for the maximum energy transfer. According to
the Forster non-radiative energy transfer theory [35,36], the energy
transfer efficiency (E) can be calculated by the following relation:
fer efficiency (E) and the critical energy transfer distance (R ) for
0
1:1 composition, the distance (r ) between the donor and acceptor
0
molecule has been calculated to be 18 A˚ using relation (4). With an
6
R
+ r06
energy transfer efficiency 0.29 (<50%), one should expect a value of
r0 greater than the critical radius. Thus, our determined value of r0
is rationally acceptable. The binding distance r = 1.8 nm is less than
0
E =
(4)
6
0
R

2
44
J. Jayabharathi et al. / Spectrochimica Acta Part A 79 (2011) 236–244
8
nm which indicates that the energy transfer from PIPP to AODIQ
[8] P. Gayathri, J. Jayabharathi, N. Srinivasan, A. Thiruvalluvar, R.J. Butcher, Acta
Cryst. E 66 (2010) o1703.
occurs with high probability [31,32].
[
9] P. Gayathri, A. Thiruvalluvar, K. Saravanan, J. Jayabharathi, R.J. Butcher, Acta
Cryst. E 66 (2010) o2219.
4
. Conclusion
[10] P. Gayathri, J. Jayabharathi, N. Srinivasan, A. Thiruvalluvar, R.J. Butcher, Acta
Cryst. E 66 (2010) o2519.
[
11] J. Jayabharathi, V. Thanikachalam, N. Srinivasan, K. Saravanan, J. Fluoresc. 2010,
doi:10.1007/s10895-010-0747-5.
The present study reports the solvatochromic effect on the
fluorometric behaviour of an imidazole derivative fluorophore in
different solvents of varying polarity. The study reveals that flu-
orescence property of the molecule is very much sensitive to the
polarity and the protic character of the solvent. The excited-state
dipole moment has been estimated following the solvatochromic
behaviour of the fluorophore with solvent polarity. Steady-state
FRET from the fluorophore to a bioactive indoloquinolizine molec-
ular system (AODIQ) has been studied in detail. KSV and R0 values
suggest that a long-range dipole–dipole interaction is responsible
for the energy transfer mechanism. We have developed imidazole
derivative as a new set of fluorescent chemisensor for transition
[12] J. Jayabharathi, V. Thanikachalam, K. Saravanan, N. Srinivasan, M. Venkatesh
Perumal, Spectrochim. Acta A 2010, doi:10.1016/j.saa.2010.12.026.
[
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[14] P. Gayathri, J. Jayabharathi, N. Srinivasan, A. Thiruvalluvar, R.J. Butcher, Acta
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[
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