Intramolecular charge transfer emission of a new ketocyanine dye: Effects of hydrogen bonding and electrolyte

Article abstract of DOI:10.1016/j.jphotochem.2011.09.004

Photophysical and intramolecular charge transfer characteristics of a new ketocyanine dye namely; 2-[3-(4-dimethylaminophenyl)-allylidene]-tetralone (DMAPT) have been studied in organic solvents having different polarities and hydrogen bond donating ability by using steady-state absorption and emission techniques. Spectral data show that increasing solvent polarity results in a significant solvatochromic shift in the absorption maxima and drastic red shifts in the fluorescence maxima, indicating an intramolecular charge transfer character (ICT) of the excited state, and that the solvent-induced dipole moment change is much greater in the excited state than in the ground state. Semiempirical calculations (PM3 and ZINDO/S) were employed to understand the geometric and electronic structure of DMAPT in both ground and excited state. It reveals twisted intramolecular charge transfer character of the excited state due to photoinduced electron transfer from the dimethyl amino group to the carbonyl oxygen. Also, in order to test the sensing properties and control the intramolecular charge transfer interaction of DMAPT, the complexation behavior of DMAPT in ground and excited states with Li⁺, Mg²⁺ and Ca²⁺, has been studied in acetonitrile solutions. The results indicate ground state complex formation between metal ions and the dye. Values of the equilibrium constants for the interaction of metal ions with DMAPT in the S₁ state have also been estimated.

Full text of DOI:10.1016/j.jphotochem.2011.09.004

Journal of Photochemistry and Photobiology A: Chemistry 224 (2011) 38–45
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Intramolecular charge transfer emission of a new ketocyanine dye: Effects of
hydrogen bonding and electrolyte
Tarek A. Fayed∗, Mohammed A. El-morsi, Marwa N. El-Nahass
Department of Chemistry, Faculty of Science, Tanta University, 31527 Tanta, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 April 2011
Received in revised form 22 August 2011
Accepted 10 September 2011
Available online 17 September 2011
Photophysical and intramolecular charge transfer characteristics of a new ketocyanine dye namely; 2-[3-
(4-dimethylaminophenyl)-allylidene]-tetralone (DMAPT) have been studied in organic solvents having
different polarities and hydrogen bond donating ability by using steady-state absorption and emission
techniques. Spectral data show that increasing solvent polarity results in a significant solvatochromic shift
in the absorption maxima and drastic red shifts in the fluorescence maxima, indicating an intramolec-
ular charge transfer character (ICT) of the excited state, and that the solvent-induced dipole moment
change is much greater in the excited state than in the ground state. Semiempirical calculations (PM3
and ZINDO/S) were employed to understand the geometric and electronic structure of DMAPT in both
ground and excited state. It reveals twisted intramolecular charge transfer character of the excited state
due to photoinduced electron transfer from the dimethyl amino group to the carbonyl oxygen. Also, in
order to test the sensing properties and control the intramolecular charge transfer interaction of DMAPT,
the complexation behavior of DMAPT in ground and excited states with Li+, Mg2+ and Ca2+, has been
studied in acetonitrile solutions. The results indicate ground state complex formation between metal
ions and the dye. Values of the equilibrium constants for the interaction of metal ions with DMAPT in the
S1 state have also been estimated.
Keywords:
Ketocyanine
Solvatochromic probe
Semiemperical calculation
Metal ion
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
the ability to chelate alkali and alkaline earth cations, which offers
prospects for studying the binding of ions on the surface of bio-
Fluorescent probes have been intensively used for many pur-
poses in physics, chemistry, biology, and medicine, since such
probes make it possible to solve a number of complicated and
important problems [1]. Fluorescent probes containing distinct
electron donors and acceptors, most often amino and carbonyl
groups have been previously synthesized. Ketocyanine dyes have
been used widely as fluorescent probes [2]. The presence of
electron-donor (amino) and electron-acceptor (carbonyl) groups
in the molecule lead to an increase in charge separation upon elec-
tronic excitation, and as a result, the electronic transitions depend
strongly on the interaction of the dye molecule with its microenvi-
ronment. Spectroscopic and photophysical properties of such dyes
have been the subject of intensive investigations in recent years
[3–5]. The solvatochromic/fluorosolvatochromic behavior makes
such dyes good probes for monitoring micropolarity, hydrogen
bonding interactions, and for investigation of microenvironmen-
tal characteristics of biological systems [6]. Some ketocyanines are
also used as laser dyes and have several industrial applications in
photopolymer imaging systems [7]. In addition, these dyes have
logical membranes [8–10]. Recently, the effect of electrolytes on
ground- and excited-state properties of ketocyanine dyes has been
studied [11]. It was observed that Li+ ions in aprotic solvents bring
about a significant change in the spectral properties. Further, a
complex is formed between the cation and the dye molecule in
the ground (S0) and the excited (S1) states. So, the goal of the
present study is to report on the photophysical characteristics, of
2-[3-(4-dimethylamino-phenyl)-allylidene]-tetralone (DMAPT) in
different solvents. To give insights into the solvatochromic behavior
of DMATP, its ground and excited states ICT interactions have been
investigated by both experimental and theoretical methods. Also, in
order to test the sensing properties and control the intramolecular
charge transfer, the interaction of DMAPT with alkaline earth met-
als like Li+, Mg2+ and Ca2+ in acetonitrile has been reported and the
spectroscopic properties for DMAPT–metal complexes have been
determined.
2. Experimental details
2.1. Materials and methods
∗
The investigated ketocyanine namely; 2-[3-(4 dimethy
laminophenyl)-allylidene]-tetralone (DMAPT) was synthesized
Corresponding author. Tel.: +20 403120708; fax: +20 403350804.
E-mail address: tfayed2003@yahoo.co.uk (T.A. Fayed).
1010-6030/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2011.09.004

T.A. Fayed et al. / Journal of Photochemistry and Photobiology A: Chemistry 224 (2011) 38–45
39
by condensation of 4-(dimethylamino)-cinnamaldehyde with
␣-tetralone in ethanol–water mixture (30%, v/v) containing 6%
NaOH under stirring over night [12]. The formed violet product
was crystallized from ethanol and characterized by IR, 1H NMR,
UV–visible and fluorescence spectral measurements. The used sol-
vents like; methanol (MeOH), methanol-d (MeOD), ethanol (EtOH)
n-propanol (PrOH), n-butanol (BuOH), n-pentanol (PenOH), ace-
tonitrile (ACN), N,N-dimethylformamide (DMF), dichloromethane
(CH2Cl2), chloroform (CHCl3), benzene and n-heptane (Hep) were
of spectroscopic grade from Aldrich, except for glycerol (Gly) was
from Merck and used as received.
Steady-state absorption and emission spectral measurements
were carried out using a Shimadzu UV-3101PC scanning spec-
trophotometer and a Perkin-Elmer LS 50B spectrofluorometer,
respectively. In all experiments, 2 × 10−5 M solutions were used.
All measurements were carried out under dim light at room tem-
perature using freshly prepared solutions. For fluorescence spectral
measurements, the samples were excited at 430 nm except for Hep
solution where the excitation wavelength was 415 nm. For mea-
surement of the fluorescence quantum yield (ꢀf) diluted solutions
were used (optical densities at the excitation wavelength is less
than 0.2), and an ethanolic solution of Rhodamine 6G was used as
a standard (ꢀf = 0.95) [13].
Complex formation was studied using 2 × 10−5 M dye concen-
tration while the concentrations of Li+, Mg2+ and Ca2+ are varied
within the range 0.016–2.7 M.
2.2. Semiempirical quantum calculations
Geometrical optimization along with electronic structure and
dipole moment calculations of both the ground and excited state
of DMAPT has been performed with the help of ArgusLab 4.0 soft-
ware (Mark A. Thompson, Planaria Software LLC, Seattle, WA) [14].
Precise geometry optimization was obtained by PM3 Hamiltonian,
while electronic structure and dipole moments were calculated by
using PM3 and ZINDO/S methods. All calculations were performed
using the default parameters.
Fig. 1. (a) Optimized geometry of ground state of DMAPT using PM3 calculations.
(b) HOMO–LUMO representation.
3. Results and discussion
3.1. Theoretical calculations
acceptor, via the butadiene bridge. Confirmation for this fact comes
from charge density calculations where the charge densities on
the dimethyl aniline moiety, butadiene-bridge, carbonyl group
and tetralone moiety increases from 0.0233, 0.0512, −0.2149 and
−0.0714e, respectively, to 0.1974, 0.2385, −0.4719 and −0.291e
upon excitation. The photoinduced charge redistribution over the
whole molecular skeleton is well represented by plot of the HOMO
and LUMO molecular orbitals as illustrated in Fig. 1(b). Semiempir-
ical calculations using ZINDO/S methods were extended to provide
the ground and excited singlet state dipole moments as well as
some spectral properties in different solvents, Table 1. The sol-
vents were treated as a continuum dielectric model taking into
consideration only the value of the dielectric constant of the sol-
vent [15]. These calculations reveal a great enhancement of the
dipole moment of the excited singlet state which is characteristic
for TICT transition. Furthermore, it is found that the ground and
excited state dipole moments increases slightly as the polarity of
the solvent increases indicating the enhanced stability of DMATP in
polar solvents. However, the calculations show that the transition
dipole and oscillator strength of the HOMO–LUMO transition are
insensitive to the solvent polarity, see Table 2.
The geometrical and electronic structures of DMAPT, in both the
ground and excited state, have been elucidated using PM3 semiem-
pirical quantum calculations. The optimized molecular structure is
shown in Fig. 1(a). The full optimized geometry of DMAPT reflects
incomplete twisting of the single bond between the dimethyl ani-
line moiety relative to the rest of the molecular skeleton twist angle
for C13, C14, C15 and C16 is 39.99◦ which is much lower than 90◦,
while the dimethyl amino group is in plane with the phenyl ring.
This contributes to stabilization of the separated charges and shows
the twisted nature of the intramolecular charge transfer excited
state (TICT). Table 1 summarizes the calculated bond orders and
charge densities on the molecular skeleton. The bond orders cal-
culated after ground state optimization are consistent with the
expected ones and indicate extension of the conjugation over the
whole molecular skeleton. Meanwhile, the values calculated for
the excited state show an inverted alternation where the bonds
characterized by single bond characters in the ground state (such
as C1–C2, C12–C13, C14–C15, and C18–N21) acquire double bond
characters in the excited state and vice versa for C1–O11, C2–C12
and C13–C14 bonds. However, the incomplete twist can allow for
the bond C14–C15 to acquire double bond characters upon exci-
tation. These changes indicate a coupling of the dimethyl aniline
group as an electron donor with the carbonyl group as an electron
Due to ICT interactions following excitation, the structure of
the excited state DMATP can be presumably represented by a res-
onance hybrid having contributions from the charge transferred
“enol” form, and to a small extent the “keto” form (Scheme 1).

40
T.A. Fayed et al. / Journal of Photochemistry and Photobiology A: Chemistry 224 (2011) 38–45
Table 1
Calculated bond orders and charge densities (using PM3 method) on some selected atoms in the ground and first excited singlet states of DMAPT.
Bond
Bond order
S0
Unit
Charge density
S0
S1
S1
C1–C2
C1–O11
C2–C12
1.058
1.675
1.769
1.107
1.787
1.083
1.385
1.377
1.448
1.405
1.0034
1.125
1.473
1.419
1.28
1.405
1.215
1.265
1.272
1.5009
1.347
1.017
Butadiene bridge
0.0512
0.2385
C12–C13
C13–C14
C14–C15
C15–C16
C15–C20
C16–C17
C17–C18
C18–N21
Carbonyl group
Dimethylaniline moiety
−0.2149
−0.4719
0.0233
0.1974
Cyclohexanone moeity
Tetralone moeity
−0.1252
−0.0714
−0.3029
−0.291
O
(a)
(b)
O
NMe2
NMe2
Scheme 1. Resonating structures of DMAPT, “keto” form (a) and the charge-separated “enol” form (b).
The relative weights of the two resonating structures have been
found to depend on the solvent polarity [16]. Thus the interaction
of the dye molecule with the surrounding polar solvent molecules
is greater in the S1 state and the spectra undergo red shifts with
increasing the solvent polarity.
exhibit large red shifts in highly polar solvents. Alcoholic solu-
tions of DMAPT exhibit dual emission as shown in Fig. 3. In fact,
the fluorescence excitation spectra recorded at the two respec-
tive emission maxima are not identical and do not match exactly
with the absorption spectrum, Fig. 4(a). However, the excitation
spectrum, monitored at the long-wavelength emission maximum,
exhibits a broad band around 452 nm and a shoulder around
406 nm. The excitation maximum at 452 nm corresponds to the
long-wavelength absorption band in PrOH, while the shoulder cor-
responds to the higher energy tail of the absorption band. So, it
was concluded that the dual emission in alcohols is due to contri-
bution of two different species present in the ground state. This
conclusion was confirmed by recording the excitation spectrum in
aprotic solvents like DMF, where only one band which matches the
long wavelength absorption band was observed, Fig. 4(b). Based on
the foregoing results, these two species are the bare and hydrogen
bonded DMAPT molecules with the latter absorbing and emitting
at lower energies [19,20].
3.2. Solvent-induced spectral shifts of DMAPT
In order to explore the nature of solute–solvent interactions,
which control the spectral behavior and ICT interactions of DMAPT,
it is important to discuss the effects of solvents on its absorption
and fluorescence spectra.
Fig. 2 shows the normalized absorption spectra in some solvents.
The spectra exhibit a significant broadening (expressed as band
width at half maximum, ꢁꢂ1/2, Table 3) as the polarity or acidity (˛)
of the solvent increases. The intense long-wavelength absorption
band (εmax ranges from 2.25 to 2.89 × 104 L mole−1 cm−1) under-
goes pronounced red-shifts with increasing solvent polarity (ca.
24 nm on going from n-heptane to DMF, Table 3). These features
indicate a strongly allowed ␲–␲* transition with charge transfer
characters. Based on the molecular structure and charge density
calculations, this absorption band could be attributed to ICT from
the dimethylaniline moeity to the carbonyl oxygen in the ground
state [17,18].
To further confirm the presence of two ground state species in
alcoholic solvents, the effect of excitation energy on the emission
1.0
0.8
0.6
0.4
0.2
0.0
Fig. 2 displays representative emission spectra of DMAPT in
some solvents, while the corresponding fluorescence data are col-
lected in Table 3. In aprotic solvents, the emission spectra are mirror
images to the corresponding long-wavelength absorption band and
Table 2
Calculated dipole moments (ꢃ, D) and oscillator strength (f) of DMAPT calculated at
the ZINDO/S level in different solvents.
Solvent
ꢃg
ꢃe
ꢃ12
f
Gas phase
Iso-octane
Benzene
CH2Cl2
MeOH
6.56(6.5)a
6.7
6.8
7.04
7.11
7.11
7.12
12.72
13.1
9.33
9.33
9.33
9.33
9.33
9.33
9.33
1.294
1.289
1.288
1.28
1.28
1.28
13.18
13.59
13.71
13.72
13.74
400
500
600
700
Wavelength (nm)
ACN
H2O
Fig. 2. Normalized absorption (to the left) and emission (to the right) spectra of
DMAPT; the solvents in the direction of the arrow are absorption: Hep, benzene,
CHCl3 and PenOH, emission: Hep, benzene, CHCl3, ACN and EtOH.
1.27
a
Dipole moment calculated using PM3 method.

T.A. Fayed et al. / Journal of Photochemistry and Photobiology A: Chemistry 224 (2011) 38–45
41
Table 3
Spectral data of DMAPT in various solvents at 25 ◦C.
Solvent
ꢄamax (nm)
ε × 104 (l mol−1 cm−1
)
ꢄfmax (nm)
ꢁꢂ1/2 (cm−1
)
ꢃ12
ꢀf
Hep
Benzene
CHCl3
CH2Cl2
DMF
ACN
n-PenOH
n-BuOH
n-PrOH
EtOH
MeOD
MeOH
Glycerol
H2O
414
430
439
436
438
429
444
447
445
442
443
445
466
437
2.25
2.30
3.55
2.50
2.75
3.0
494
521
560
576
591
595
533, 610
532, 614
532, 615
531, 617
531, 626
531, 623
620
4488
4607
4677
5233
6006
5105
5014
5105
5555
5690
5076
5638
5437
5986
6.18
6.45
8.16
7.22
8.13
7.75
7.68
8.89
8.306
8.46
8.31
8.14
9.008
9.048
0.133
0.284
0.296
0.32
0.65
0.47
0.39
0.36
0.29
0.24
2.9
3.17
3.05
3.11
3.361
2.891
3.501
2.42
0.235
0.229
0.56
597
0.067
800
700
600
500
400
300
200
100
0
460 nm
430 nm
200
150
100
50
n-PenOH
n-BuOH
PrOH
500 nm
EtOH
MeOD
MeOH
0
550
600
650
700
750
800
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Fig. 5. Fluorescence spectra of DMAPT in n-PrOH recorded at different excitation
wavelengths.
Fig. 3. Fluorescence spectra of DMAPT in aliphatic alcohols.
spectrum in n-PrOH was investigated. Fig. 5 shows that excitation
of DMAPT at 430 nm, give rise to two structured emission with a
band at 611 nm and shoulder around 526 nm with a remarkable dif-
ference in intensities. However, excitation at longer wavelengths
leads to enhancement of the emission intensity at 611 nm on the
expense of the intensity of the short-wavelength shoulder. Quanti-
tatively, the relative intensity (If611/If526) increases from 3.7 to 4.26
respectively. These changes indicate that excitation at the longer
wavelength tail of the absorption band promotes emission from
both the free and hydrogen bonded DMAPT molecules.
To get information about the various modes of solvation (e.g.
specific or non-specific) which determine the absorption, E(A),
and the fluorescence energies, E(F), of DMAPT. E(A) and E(F) were
plotted as a function of the Dimorth–Reichardt emperical sol-
vent polarity parameter ET(30) [21], Fig. 6. The correlations have a
when the excitation wavelength is changed from 430 to 500 nm,
1.2
0.7
(a)
(b)
0.6
0.5
0.4
0.3
0.2
0.1
0.0
1.0
0.8
0.6
0.4
0.2
0.0
300
350
400
450
500
300
350
400
450
500
550
Wavelength (nm)
Wavelength (nm)
Fig. 4. Excitation spectra of DMAPT in (a) PrOH monitored at 611 nm (—) and 526 nm (. . .) and (b) DMF, monitored at 591 nm. The dashed line is the corresponding absorption
spectrum.

42
T.A. Fayed et al. / Journal of Photochemistry and Photobiology A: Chemistry 224 (2011) 38–45
Table 4
80
70
60
50
40
30
20
Experimental data of ground (ꢃg, D), excited (ꢃe, D) dipole moments, slope and
correlation factor of DMAPT by Bilot method.
ꢃg (D)
ꢃe (D)
ꢁꢃ (D)
ꢃe/ꢃg
m1 (cm−1
)
m2 (cm−1
)
(r)
EA
4.92
14.11
9.19
2.87
2826.0
5850.2
0.98a0.92b
a
Is the slope using the function f(ε, n).
Is the slope in the case of using f(ε, n) + 2g(n).
b
h being Planck’s constant and c is the velocity of light in vacuum, ꢃg
and ꢃe are the dipole moments of the ground and excited states. The
parameters m1 and m2 can be determined from the plots of absorp-
E
F
tion and fluorescence band shifts (
−
em and
+ em) with the
abs
abs
solvent polarity parameters according to the above equations. If the
ground and excited state dipole moments are parallel, the following
expressions are obtained on the basis of above equations [25].
ꢀ
ꢁ
1/2
m2 − m1 hca3
ꢃg
ꢃe
=
(6)
(7)
2
2m1
30
35
40
45
50
55
E T(30)/ kcal mol-1
ꢀ
ꢁ
1/2
m2 + m1 hca3
=
2
2m1
Fig. 6. Dependence of EA and EF on the empirical solvent polarity parameter ET(30)
of DMAPT.
And
m1 + m2
m2 − m1
ꢃe
=
ꢃg; m2 > m1
(8)
negative slope indicating that both absorption and emission max-
imum is red shifted in highly polar solvents. Also, the enhanced
solvent sensitivity of E(F) compared to E(A) is explained in terms
of increased solute–solvent interaction in the excited state due to
increased dipole moment upon excitation. For alcohols, in addition
to dipolar interactions, stronger hydrogen bonding with the solvent
molecules is expected in the excited state as a result of increased
charge density on the carbonyl oxygen from −0.2149 in the ground
state to −0.4719 in the excited state, Section 3.1.
The cavity radius was taken as 40% from the distance between
the two farthest atoms in the direction of charge separation within
the molecule [26], which was estimated following geometry opti-
˚
mization of DMAPT and comes out to be 6.7 A. The slopes m1 and
m2 of the fitted data (according to Eqs. (3) and (4)), as well as the
dipole moment values are given in Table 4. The calculated ground
and excited state dipole moments are 4.92 and 14.11D, respectively.
The values are in good agreement with those obtained theoretically
by using PM3 semiemperical calculations (6.5 and 12.72D, in gas
phase).
To determine the individual contribution of different modes of
solute–solvent interactions that controls the solvent induced spec-
tral shifts a multiparametric approach, known as solvatochromic
comparison method (SCM) proposed by Kamlet et al. [22] has been
used. This approach separates the dielectric effects of solvents (ꢅ*),
hydrogen-bond donor (˛) and acceptor (ˇ) abilities of the solvents
on the spectral properties. Excluding the data in water, the absorp-
tion and fluorescence energies have been analyzed using the SCM
according to the following correlations:
The emission of incomplete TICT excited DMAPT exhibits signif-
icant yield (ꢀf) which is highly sensitive to the solvent polarity and
acidity, Table 3. The ꢀf values decreases steadily as the hydrogen
bond donating ability of the solvents increases (ꢀf value decreases
from 0.47 in ACN to 0.23 in MeOH) indicating that hydrogen
bonding interactions are controlling strongly the decay of excited
DMAPT. As illustrated in Fig. 7, the ꢀf value increases largely with
increasing the solvent polarity (expressed as ET(30)) in aprotic sol-
vents reaching a maximum value in DMF. Further increase in the
ET(30) in protic solvents leads to a decrease in the fluorescence
quantum yield. Several mechanisms such as biradicaloid charge
transfer, proximity effect (due to coupling between close-lying
␲–␲* and n–␲* excited states) conformational changes have been
proposed to explain the increase of ꢀf with a suitable enhancement
of the intramolecular charge transfer character (negative solvatoki-
netic effect) [27]. The enhancement of ꢀf on going from benzene to
DMF can be attributed to the strong ICT interaction. The population
probability of ICT state depends on the electron donor–acceptor
capabilities of the involved partners as well as on the solvent polar-
ity. Polar DMF solvent would stabilize the highly polar ICT state
to become below the n–␲* state thus decreasing the probabil-
ity of population of the non-radiative n–␲* transition. In contrast,
the ꢀf decreases (positive solvatokinetic effect) strongly in highly
polar protic solvents like alcohols due to hydrogen-bonding inter-
actions between solvent molecules and the carbonyl group of the
fluorophore, which enhance the radiationless internal conversion.
To confirm this conclusion, fluorescence quenching of the inves-
tigated dye was studied in DMF using some alcohols like MeOH,
MeOD, EtOH, n-PrOH, n-BuOH and n-PenOH, as quenchers. The
Stern–Volmer constants (KSV) as calculated from the Stern–Volmer
E(A) = 67.79 − 2.22˛ − 0.32ˇ − 2.3ꢅ∗ r = 0.87
E(F) = 60.63 − 6.10˛ − 3.13ˇ − 12.32ꢅ∗ r = 0.94
(1)
(2)
From Eqs. (1) and (2), it could be observed that the absorption
and fluorescence energies are controlled by the cooperative effects
of all modes of solvation. The higher coefficients of ˛, ˇ and ꢅ* in the
case of E(F) relative to E(A) reflect the stronger solute–solvent inter-
actions in the excited state and the enhanced hydrogen bonding
interaction and higher stabilization of the ICT excited state.
The ground and excited state dipole moments of DMAPT are
determined by using the solvent perturbation method [23] based on
the absorption and fluorescence shifts in various solvents, accord-
ing to:
−
+
= m f (ε, n) + const
(3)
(4)
em
em
1
abs
abs
= −m [f (ε, n) + 2g(n)] + const
2
where abs and em are the absorption and fluorescence band max-
ima in solvents of varying permittivity (ε) and refractive index (n)
[24]. f(ε, n) = 2n2 + 1/n2 + 2[(ε − 1/ε + 2) − (n2 − 1/n2 + 2)] is the sol-
vent polarity parameter and g(n) = 3/2[n4 − 1/(n2 + 2)2] with
2(ꢃe − ꢃg)2
2(ꢃ2e − ꢃ2g )
m1
=
and m2
=
(5)
hca3
hca3

T.A. Fayed et al. / Journal of Photochemistry and Photobiology A: Chemistry 224 (2011) 38–45
43
0.8
0.6
0.4
0.2
0.0
3.3. Spectral features of DMAPT in electrolytes of some alkali
metals
In order to test the sensing properties of the present probe to
metal ions, the absorption and emission characteristics of diverse
cations binded to DMAPT was investigated. Readily detectable
cation-induced effects were observed on adding Li+, Mg2+ and Ca2+
ions to DMAPT in aprotic dipolar solvent like ACN. Fig. 8 shows the
electronic absorption spectra of the interacting DMAPT with Li+,
Mg2+ and Ca2+ ions in ACN. As shown, on adding Li+ ions the absorp-
tion band maximum of DMAPT appearing at 431 nm decreases and
a new lower energy-band appears and it becomes prominent as the
concentration of Li+ increases. Only Li+ cations have been found to
cause pronounced spectral change with an isosbestic point appears
at 447 nm. Such results point clearly to the existence of two species
in equilibrium. While the lower wavelength band at 431 nm cor-
responds to the solvated DMAPT, the longer wavelength band (at
458 nm) is presumably due to the Li+-DMAPT complex. For the
interaction of DMAPT with Mg2+ and Ca2+ ions in ACN, the band
shifts only to a small extent with appearance of two isosbestic
points at 393, 516 nm for Mg2+, and at 408, 524 nm for Ca2+. All
these results confirm the formation of complex between the used
metal ions and the investigated dye.
DMF
ACN
n-BuOH
CH2Cl2
CHCl3
n-BuOH
n-PrOH
MeOH
EtOH
HO
2
Hep
30
35
40
45
50
55
60
65
ET(30), kcal mol-1
Fig. 7. Variation of the fluorescence yield of DMAPT with the solvent polarity.
Spectral observations can be rationalized in terms of the exis-
tence of the following equilibrium in solution:
plots are 0.447, 0.38, 0.181, 0.17, 0.129 and 0.09 M−1 for the pre-
vious alcohols, respectively, and increase linearly with the solvent
acidity (˛) according to the following equation;
DMAPT . . . S + M = DMAPT . . . M + S
(10)
Here (DMAPT . . . S) and (DMAPT . . . Mn+) represent solvated and
complexed dye, respectively, the equilibrium constant (K) can be
found out by assuming that the observed maximum energy of a
spectroscopic transition, E(A) is a mole fraction average of those of
the two species, the solvated and complexed form of the dye [29].
Thus, one gets:
KSV = −0.649 + 1.079˛ r = 0.90
(9)
This indicates that the quenching mechanism is governed by hydro-
gen bond formation between the solvent molecules and the excited
DMAPT.
The solvent dependence of the fluorescence yield has been
also analyzed in terms of
a multiple solvent parameters
CSES + CCEC
approach collecting the solvent acidity (SA) and basisity (SB), sol-
vent polarity–polarizability (SPP), solvent viscosity (Á) and the
dipolarity–polarizability (SdP) [28]. Accordingly, the following
equation has been obtained (r = 0.98):
E(A) =
(11)
CS + CC
where E(A) is the maximum energy of absorption for a metal ion
concentration CM, ES and EC are that for the solvated and the com-
plexed dye, respectively. Eq. (11) can be rearranged to give:
ꢀf = −1.013 − 0.467SA + 0.162SB
+ 1.765SPP − 0.0158Á − 0.108SdP
E(A) = ES + ECK CM − KE(A)CM
(12)
Values of ES, EC and K that fit Eq. (12) can thus be determined by a
linear regression analysis. Values of spectroscopic parameters for
the dye metal ion complexes have been summarized in Table 5. The
value of ES, as obtained from the linear regression analysis equals
It shows clearly that the solvent dependence of the fluorescence
yield is mainly due to solvent acidity and polarity–polarizability
effects, while the solvent viscosity has virtually no contribution.
1.8
(a)
(b)
1.6
447 nm
1.0
0.8
0.6
0.4
0.2
0.0
1.4
408 nm
1.2
1.0
0.8
0.6
0.4
524 nm
0.2
0.0
300
400
500
600
300
400
500
600
700
Wavelength (nm)
Fig. 8. Absorption spectra of DMAPT containing different concentrations of (a) Li+ and (b) Ca2+ in ACN.

44
T.A. Fayed et al. / Journal of Photochemistry and Photobiology A: Chemistry 224 (2011) 38–45
Table 5
Spectroscopic parameters for ground and excited state complexation of DMAPT with various metal ions in ACN.
Metal ion
Ionization potentiala (eV)
ES (kcal mole−1
EC kcal mole−1
K l mole−1
ES1 kcal mole−1
EC1 kcal mole−1
K1 l mole−1
Li+
5.34
7.64
6.11
64.1
64.48
64.04
75.18
92.5
57.03
0.133
0.28
0.82
48.17
47.73
47.9
49.3
46.0
45.06
0.22
1.6
2.26
Mg2+
Ca2+
a
Values were taken from [33].
64.4 kcal mole−1 for DMAPT which agrees with the value measured
in ACN (66.3 kcal mole−1).
This is also reflected by the significantly higher values of the equi-
librium constant for complexation involving the S1 state of DMAPT,
Table 5. The nature of the interaction in the weak molecular com-
plex formed between the dye and the metal ion is predominantly
electrostatic in nature as reflected from the little variation of E(A)
and E(F) with the ionization potential of the metal.
As shown in Fig. 10, the fluorescence of DMAPT was effectively
quenched by Li+, Mg2+ and Ca2+ ions in ACN, and a new structur-
less red-shifted emission band is observed at sufficiently higher
concentration of the salt, confirming the formation of an emit-
ting complex under these experimental conditions. Representative
Stern–Volmer plots are illustrated in Supplementary Fig. S2. The
plots show an upward curvature at higher concentrations of metal
ions, indicating that static quenching mechanism via ground state
complexation is predominant. In such cases, the KSV constants were
calculated from the linear part of the plots, where a dynamic pro-
cess in which quenching is mainly due to collision is taking place.
The results can be best fitted by the following equations;
The fluorescence of DMAPT appears as a broad and structure-
less band with a maximum around 596 nm in ACN. On addition
of Li+, Mg2+ or Ca2+ ions, the band maximum is red shifted,
Fig. 9. The extent of the red shift is 9, 11 and 20 nm, for Ca2+
,
Mg2+ and Li+, respectively. The red-shifted band is due to the
dye–metal ion complex formation. This was rationalized in view
of increased charge density on the carbonyl oxygen in the S1 state,
leading to a stronger dye–metal ion interaction relative to that in
the ground state [30]. Supplementary Fig. S1 shows representa-
tive plots of the fluorescence energy, E(F), for the complexed dye
[E(F)/kcal mole−1 = 28590/(ꢄ/nm)] as a function of metal ion con-
centration.
The observed E(F) can be considered as a mole fraction average
of the solvated and the complexed dye, ES1 and EC1, respectively.
Thus, one gets [31]:
CSES1 + CCEC1
E(F) =
(13)
For Li+ F0/F = 1.2347 + 0.44[Q] + 0.57[Q]2 r = 0.994
(15)
CS + CC
For Mg2+ F0/F = 0.985 + 1.92[Q] + 69.27[Q]2 r = 0.997 (16)
Eq. (16) can be rearranged to give:
For Ca2+ F0/F = 1.09 + 2.95[Q] + 24.46[Q]2 r = 0.992
(17)
E(F) = ES1 + EC1K1 CM − K1E(F)CM
(14)
where K1 is the binding constant for the following process [32]:
where F0 and F are the fluorescence intensities in the absence and
presence of the quencher (Li+, Mg2+ and Ca2+), respectively, and
Q is the concentration of the metal ions. The results point to the
simultaneous existence of static and dynamic quenching. It is inter-
esting to note that the sum of the constants representing ground
and excited-state complexation of DMAPT (K and K1) equals 0.353,
1.88 and 3.08, for Li+, Mg2+ and Ca2+, respectively, which are in close
agreement with the coefficient of Q in Eqs. (15)–(17) confirming the
dynamic and static quenching modes. The Stern–Volmer constants
(KSV) as calculated from the Stern–Volmer plots are 0.45, 8.1 and
6.37 M−1 for Li+, Mg2+ and Ca2+, respectively. There was a satis-
factory correlation between KSV and the ionization potential of the
S . . . D(S1state) + M = M . . . D(S1state) + S
Thus, values of ES1, EC1 and K1 that fit Eq. (14) were determined
by a linear regression analysis and listed in Table 5. It appears that
the value of K1 is greater than that of K, indicating formation of a
relatively stronger complex in the S1 state.
Alkaline earth metal ions, when present in solution of DMAPT
in aprotic solvents, interact with the carbonyl group of the dye, and
the relative weight of the “enol” form (Scheme 1) in the ground state
as well as in the excited state increases. The interaction of Li+, Mg2+
and Ca2+ ions with the S1 state of DMAPT is expected to be stronger
in view of the greater relative weight of the “enol” like structure.
350
[Li+] mole/L
350
0.00
Mg2+
300
250
200
150
100
50
0.1
0.25
0.35
0.5
0.75
1
1.25
1.5
1.75
2
300
Li+
250
200
150
100
50
2.25
2.5
2.7
0
0
500
550
600
650
700
750
450
500
550
600
650
700
750
Wavelength (nm)
Wavelength (nm)
Fig. 9. Fluorescence spectra of DMAPT in ACN solution saturated with Li+ and Mg2+
The dotted line represents the fluorescence spectra in ACN.
.
Fig. 10. Changes of fluorescence spectra of DMAPT on adding different concentra-
tions of Li+ in ACN.

T.A. Fayed et al. / Journal of Photochemistry and Photobiology A: Chemistry 224 (2011) 38–45
45
metal [33], Supplementary Fig. S3. The correlation indicates contri-
bution of the electron transfer (from the carbonyl group of the dye
to the metal ion) to the quenching mechanism.
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4. Conclusion
DMAPT is very sensitive to the solvent polarity and hydro-
gen bond properties. Spectral data show that increasing solvent
polarity results in a significant shift in the absorption and fluores-
cence maxima, indicating an intramolecular charge transfer (ICT)
from the dimethyl amino group to the carbonyl oxygen which is
confirmed by theoretical calculations. Electronic absorption and
emission spectral characteristics of DMAPT with metal ions such
as, Li+, Mg2+ and Ca2+ have been studied in acetonitrile solution.
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