Dual fluorescence in a sehiff base derived from an acridinedione dye. excited state intramolecular proton transfer

Article abstract of DOI:10.1246/bcsj.80.1307

Newly synthesized nonconjugated, but covalently linked, bichromophoric systems (ADDSA) contain a salicyli-deneaniline (SA) moiety linked to an acridinedione (ADD) fluorophore via a covalent bond. The absorption and fluorescence characteristics of the ADDSA systems in various solvents reveal that the dual fluorescence originates from two different chromophores in the same molecule. The solvent polarity independent, long wavelength anomalous emission originates from the keto form of SA moiety via excited state intramolecular proton transfer (ESIPT). The ESIPT pathway was further confirmed by the benzylideneaniline (BA) and ADD moieties coupled non-proton transfer model system (ADDBA). Absence of the longer wavelength anomalous emission in the steady state as well as the shorter lifetime component in the time-resolved study for ADDBA system supports ESIPT mechanism for long wavelength anomalous emission in ADDSA system.

Full text of DOI:10.1246/bcsj.80.1307

ꢀ 2007 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 80, No. 7, 1307–1315 (2007) 1307
Dual Fluorescence in a Schiff Base Derived from an Acridinedione Dye.
Excited State Intramolecular Proton Transfer
y
ꢀ
Viruthachalam Thiagarajan and Perumal Ramamurthy
National Centre for Ultrafast Processes, University of Madras, Taramani Campus, Chennai-600 113, India
Department of Inorganic Chemistry, University of Madras, Guindy Campus, Chennai-600 025, India
Received October 11, 2006; E-mail: prm60@hotmail.com
Newly synthesized nonconjugated, but covalently linked, bichromophoric systems (ADDSA) contain a salicyli-
deneaniline (SA) moiety linked to an acridinedione (ADD) fluorophore via a covalent bond. The absorption and fluores-
cence characteristics of the ADDSA systems in various solvents reveal that the dual fluorescence originates from two
different chromophores in the same molecule. The solvent polarity independent, long wavelength anomalous emission
originates from the keto form of SA moiety via excited state intramolecular proton transfer (ESIPT). The ESIPT pathway
was further confirmed by the benzylideneaniline (BA) and ADD moieties coupled non-proton transfer model system
(ADDBA). Absence of the longer wavelength anomalous emission in the steady state as well as the shorter lifetime com-
ponent in the time-resolved study for ADDBA system supports ESIPT mechanism for long wavelength anomalous emis-
sion in ADDSA system.
ESIPT, discovered by Weller in 1955, has been the subject
of continuous interest of researchers for more than five dec-
ades.1 Photoinduced proton transfer, which is a representative
of a limited number of adiabatic photochemical processes giv-
ing rise to fluorescence with an abnormally large Stokes shift,2
plays an important role in biophysics,3 and has practical appli-
cation in technology, for example, in organic scintillators.4
Among the molecular systems characterised by excited state
proton transfer, both efficient organic photochromes5 and poly-
mer ultraviolet stabilisers6 are of much interest. Prospects for
the use of ESIPT systems in luminescent solar energy concen-
trators have also been discussed in literature.7 For the ESIPT
reaction to occur, it is necessary that proton-donor and pro-
ton-acceptor moieties be in close proximity and conjugated
to one another in an organic molecule. ESIPT is considered
as the driving force for the concerted enhancement of their
acidity or basicity upon the transition to an excited state.8 If
a molecule carries its own base, excited-state proton transfer
can occur intramolecularly and becomes more or less inde-
pendent of the surrounding solvent. ESIPT reaction is extreme-
ly fast (subpicosecond kinetics) and also occurs in rigid glasses
and at very low temperatures.9 Very often, only the ESIPT
pyridyls,17 and malonic aldehyde.18 Recently, Chou et al. have
studied the solvent polarity tuned excited state, charge-trans-
fer-coupled proton transfer reaction in p-N,N-ditolylaminosali-
cylaldehyde- and 3-hydroxyflavone-based systems.19
Orthohydroxy Schiff bases appear to be interesting com-
pounds due to their thermochromic and photochromic proper-
ties related to intramolecular proton-transfer reactions. It
makes them potential materials for optical memory and optical
switches.20 Considerable interest has been focused on salicyli-
deneaniline (SA), which is the simplest of the anil compounds.
On the basis of X-ray crystallography, and electronic and vi-
brational spectroscopy, SA in the crystalline state is predomi-
nantly in the enol form.21 It is generally accepted that in such
photoexcited, internally hydrogen-bonded systems, the proton
is quickly translocated from the oxygen towards the nitrogen
atom and a keto-like species is formed in the singlet excited
state. Studies of salicylideneanilines have been extended to
molecules possessing two identical proton-transfer centres
(BSP, BSD) by Grabowska et al.22 Substituent effects on the
ground state properties of naphthalene analogues of salicyli-
deneaniline have been studied by Ohshima et al.23
Acridinedione (ADD) dyes have been developed recently,
as a family of efficient laser dyes24 and these dyes are structur-
ally similar to NADH.25 ADD is a bifunctional molecule, and
due to this nature, it acts both as an electron donor and accep-
tor. It has been observed that the spectral properties and the
photophysics of the ADD dyes are largely influenced by the
nature of the substituents at the para position of the phenyl
group.26,27 ADD dyes have also been used as photoinitiators
for polymerization of acrylates and methyl acrylates.28 These
dyes have also been used as a photosensitizer for the onium
salt decomposition.29
ꢀ
product (P ) fluoresces, and this is the source of extremely
large Stokes shift, which are fairly independent of medium
and temperature. Thus, ESIPT dyes are ideal candidates for
use as fluorescence labels in order to avoid interference from
other fluorescent material present in the sample to be analyzed.
Most of the publications on ESIPT deal with derivatives of
salicylic acid, its esters and amides,10 salicylaldehyde, aceto-
phenone and related compounds,11 o-hydroxybenzoxazoles,12
benzothiazoles,13 triazoles,14 flavonols,15 anthraquinones,16 bi-
We are particularly interested in the photophysical proper-
ties of nonconjugated but covalently linked, bichromophoric
y Present address: Department of Chemistry, Graduate School of
Science, Tohoku University, Sendai 980-8578
Published on the web July 11, 2007; doi:10.1246/bcsj.80.1307

1308 Bull. Chem. Soc. Jpn. Vol. 80, No. 7 (2007)
Dual Fluorescence
systems, which show dual fluorescence due to the occurrence
of one or two of the following three mechanisms: photoin-
duced electron transfer (PET), photoinduced charge transfer
(PCT), and ESIPT.26 Transition-metal ion or anion recognition
induces a change in the photophysical properties of the fluoro-
phore due to change in the oxidation potential of the donor
group, which results in fluorescence enhancement or quench-
ing.26,30 This study will help to understand the photophysical
properties of dual emitting ADD dyes and their interaction
with ions.
One of the most important issues of current interest is
ESIPT reactions associated with intramolecular charge transfer
(ICT) within the same molecule. Newly synthesized bichromo-
phoric ADDSA systems contain salicylideneaniline (SA) moi-
ety connected to the ADD fluorophore via covalent bond.
ADDSA systems show dual fluorescence in aprotic solvents,
and in order to elucidate the exact mechanism for the dual fluo-
rescence, the following ADD dyes were selected (Scheme 1).
Results and Discussion
Steady State Spectral Studies. The absorption spectral
studies of ADDSA-1 in a series of protic and aprotic solvents
have been carried out. Table 1 summarizes the absorption
characteristics of ADDSA-1 in different solvents. The absorp-
tion spectra of ADDSA-1 in both protic and aprotic solvents
were similar, and they showed a strong absorption band at
350 nm. The absorption spectrum of ADDSA-1 in methanol
and acetonitrile are presented in Fig. 1.
The emission spectra were recorded in various solvents after
exciting the dye at its longer wavelength absorption maximum
(350 nm). The emission spectrum of ADDSA-1 recorded in
different solvents is shown in Fig. 2. The emission spectrum
of ADDSA-1 showed a single peak in protic solvents whereas
in aprotic solvents it shows two distinct peaks: a shorter wave-
length local excited fluorescence around 425 nm and a new
anomalous longer wavelength emission around 528 nm. Table
1 lists the emission characteristics of ADDSA-1 in various sol-
vents. The shorter wavelength emission around 425 nm was as-
signed to the CT from the ring nitrogen to ring carbonyl oxy-
gen center within the ADD fluorophore (local excited, LE),
which was found to shift towards lower energies with increas-
ing solvent polarity and this shift is due to the stabilization of
the CT state.31 In contrast to the lower wavelength emission,
the anomalous longer wavelength emission maximum was in-
dependent of solvent polarity, and it was more intensed in
aprotic nonpolar solvents. The concentration independent na-
ture of the ratio of 425 and 528 nm emission intensity shoes
that an excimer or exciplex did not form.
O
H
O
H
CH
CH
N
N
O
O
O
O
N
N
H
CH3
ADDSA-2
0.4
ADDSA-1
ACN
MeOH
0.3
0.2
0.1
0.0
CH
N
O
O
O
O
N
N
250
300
350
400
450
CH3
CH3
Wavelength / nm
ADDBA
ADD-1
Fig. 1. Absorption spectrum of ADDSA-1 in acetonitrile
and methanol.
Scheme 1.
Table 1. Absorption and Fluorescence Spectral Data of ADDSA-1, ADDSA-2, and ADDBA in Various Solvents
Fluorescence, ꢀmax/nma)
Absorption, ꢀmax/nm
Solvent
ADDSA-1
ADDSA-2
ADDBA
ADDSA-1
ADDSA-2
ADDBA
Toluene
Dichloromethane
Acetone
Acetonitrile
DMSO
Methanol
350
349
348
348
350
348
350
351
348
349
350
348
340
340
328
334
337
325
404, 531
419, 527
416, 527
425, 528
430, 527
435
430, 530(s)
438, 530(s)
440, 530(s)
441, 530(s)
440, 530(s)
452
426
432
434
439
438
451
a) (s), shoulder.

V. Thiagarajan et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 7 (2007) 1309
1.2
1.0
0.8
0.6
0.4
0.2
ADD-1
ADDSA-2
ADDBA
1.0
0.8
0.6
0.4
0.2
0.0
Toluene
ACN
DMSO
MeOH
0.0
400
450
500
550
600
650
400
450
500
550
600
650
Wavelength(nm)
Wavelength / nm
Fig. 4. Emission spectrum of ADD-1, ADDSA-2, and
ADDBA dyes (16 mM) in acetonitrile.
Fig. 2. Emission spectrum of ADDSA-1 in toluene, aceto-
nitrile, DMSO, and methanol.
1.25
0.5
420 nm
520 nm
1.00
ADD-1
ADDSA-2
ADDBA
0.4
0.3
0.2
0.1
0.0
0.75
0.50
0.25
0.00
280
300
320
340
360
380
400
250
300
350
400
450
Wavelength / nm
Wavelength / nm
Fig. 3. Absorption spectrum of ADD-1, ADDSA-2, and
ADDBA dyes (16 mM) in acetonitrile.
Fig. 5. Fluorescence excitation spectrum of ADDSA-1 in
acetonitrile monitored at two different emission wave-
lengths.
It is known that the deprotonated form of the ADD dyes
emits in the region of 500 to 520 nm.32 To account for the
anomalous emission at 528 nm, ADDSA-2 was prepared by
substituting the methyl group in the place of ADD ring amino
hydrogen. Table 1 lists the absorption and emission spectral
characteristics of ADDSA-2 in various solvents. The absorp-
tion and emission spectrum of ADDSA-2 in acetonitrile is
shown in Figs. 3 and 4, respectively. The presence of the
anomalous longer wavelength emission peak for ADDSA-2
in aprotic solvents rules out the possibility of anomalous emis-
sion due to ADD ring amino hydrogen deprotonation. The
presence of methyl group at the 10th position stabilizes the CT
within the ADD moiety, which results in the unresolved LE
and anomalous emission in aprotic solvents.
sion.26 The CT state has larger dipole moments than that of the
S0 and LE states; therefore, the relative energy of the CT state
against that of the LE state depends upon the solvent polarity.
In ADDSA systems, the anomalous longer wavelength emis-
sion maximum is independent of solvent polarity. This clearly
rules out the possibility that the longer wavelength emission is
due to the CT state formed via PET between SA and ADD
moieties.
Is Dual Fluorescence Originating from Two Different
Chromophores?
The fluorescence excitation spectra of
ADDSA-1 in acetonitrile monitored at two different emission
maximum (420 and 520 nm) are shown in Fig. 5. Two distinct
excitation bands were observed, one at 370 nm and the other at
363 nm, corresponding to LE state and anomalous emission,
respectively. The excitation spectrum recorded at the longer
wavelength emission maximum also shows a broad shoulder
around 340 nm, which was not seen in the excitation spectrum
recorded at the shorter wavelength emission maximum. The
above result indicates that the dual emission bands originate
from two different chromophores of the same molecule. In
order to confirm the above result, the 3D emission spectral
Is Photoinduced Electron Transfer (PET) Responsible
for the Anomalous Longer Wavelength Emission?
In
our earlier photophysical studies on the bichromophoric
donor–acceptor coupled 9-(4-dimethylamino-phenyl)-3,3,6,6-
tetramethyl-3,4,6,7,9,10-hexahydro-2H,5H-acridine-1,8-dione
(DMAADD) system, PET from the donor moiety (N,N-dieth-
ylaniline) to the ADD acceptor causes an emissive CT state
that is responsible for the longer wavelength anomalous emis-

1310 Bull. Chem. Soc. Jpn. Vol. 80, No. 7 (2007)
Dual Fluorescence
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
O
H
O
H
C
C
H
O
H
O
N
N
ESIPT
reaction
O
O
400
450
500
550
600
N
R
N
R
Wavelength / nm
Enol form (I)
Keto form (II)
Scheme 2.
400
450
500
Wavelength / nm
550
600
studies were performed. The 3D spectrum recorded for
ADDSA-1 in acetonitrile is shown in Fig. S1. In acetonitrile,
two contours were observed, and the corresponding excitation
spectrum showed two bands at 370 and 363 nm. The excitation
band observed at 370 nm corresponds to LE emission from
ADD. The excitation band observed at 363 nm corresponds
to anomalous emission. The excitation and 3D spectra confirm
that the dual fluorescence in ADDSA-1 originates from two
different chromophores. Due to the above reason, the excita-
tion spectrum recorded at two different emission maxima
was different from the absorption spectrum.
Fig. 6. Emission spectrum of ADDSA-1 in acetonitrile at
various excitation wavelengths. ꢀexc is (from top to bot-
tom) 310, 330, 350, and 380 nm. Inset the normalised
emission spectra for the same set [ꢀexc is (from top to bot-
tom) 310, 330, 350, and 380 nm].
and emission spectrum of ADDBA in acetonitrile are shown
in Figs. 3 and 4. The emission spectrum of ADDBA was sim-
ilar to the emission spectrum of ADD dye without the SA moi-
ety (ADD-1). The absorption and emission spectral character-
istics of ADDBA are presented in Table 1. Unlike ADDSA
systems, ADDBA exhibited a single fluorescence band around
440 nm in all protic and aprotic solvents studied. In ADDBA,
the observed emission around 430 nm is only due to the intra-
molecular CT from the ring nitrogen to the ring carbonyl oxy-
gen centre within the ADD moiety. The emission around
430 nm was found to shift towards lower energies with an in-
crease in the solvent polarity, and this shift is due to the stabi-
lization of CT state. In contrast to the emission, the absorption
spectra are due to the overlap of electronic transitions from
two different chromophores of the same molecule (both BA
and ADD moieties). In protic solvents, the hydrogen-bonding
interaction of solvent proton with the imine proton may desta-
bilize the electronic transition of the BA moiety. In order to
confirm that, a detailed study is needed for ADDBA system.
Because in ADDBA itself, two different processes are opera-
tive (CT within the ADD moiety and electronic transition
within the BA moiety).
Absorption and emission spectral studies for ADDSA-1 in
acetonitrile and methanol mixture were carried out. There was
no significant change in the absorption spectrum of ADDSA-1
upon increasing the percentage of methanol. In contrast to the
absorption spectrum, the normalized fluorescence spectrum
presented in Fig. S2 (Supporting Information) showed a de-
crease in the fluorescence intensity of longer wavelength band
along with a red shift in the emission maximum of LE as the
percentage of methanol was increased. This clearly indicates
that there is intermolecular interaction between the SA moiety
with the protic solvents weakening the ESIPT pathway.
Time-Resolved Fluorescence Studies. In the previous
time-resolved studies of the SA compound, time-domain or
frequency domain techniques with pico- or femtosecond reso-
Is Anomalous Emission Due to an ESIPT Reaction with
in the SA Moiety?
The non-conjugated, but covalently
linked, bichromophoric ADDSA systems consist of ADD
and SA chromophores. ADD dyes show absorption maximum
around 360 nm due to the CT from the ring nitrogen to ring
carbonyl oxygen center.31 SA compounds are characterized
by a broad intense band at 340 nm.33 The 350 nm absorption
band of ADDSA-1 system is the overlap of two electronic tran-
sitions, that is, one CT absorption within ADD moiety (around
360 nm) and one electronic transition involving the SA moiety
(around 340 nm) (Scheme 2).
Overlap of both the electronic transitions result in the ab-
sorption maximum of ADDSA systems around 350 nm.
Figure 6 shows the emission spectra of ADDSA-1 in acetoni-
trile at different excitation wavelengths. The overlapping of
electronic absorption of two chromophores caused the dual
fluorescence at all the wavelengths of excitation in the region
from 300 to 400 nm. At the blue edge of excitation anomalous
fluorescence was dominant, while at the red edge of excitation,
LE state emission was dominant.
The fluorescence spectra of SA compounds are character-
ized by a weak emission band around 530 nm region. The ori-
gin of this emission band has been previously attributed to ex-
cited keto tautomer species generated by a very fast enol-
imine to cis-keto amine ESIPT.1,34 From the above results,
the longer wavelength anomalous emission of ADDSA sys-
tems at 530 nm was attributed to the excited state intramolec-
ular proton-transfer reaction in the excited state of SA chromo-
phore. Fluorescence quantum yield of ADDSA systems were
extremely low (ꢁ10ꢂ4), which indicates that ESIPT acts as a
main nonradiative channel for deactivation in the excited state.
Perhaps the strongest support for this ESIPT is given by the
non-proton transfer model system ADDBA. The absorption
ꢀ
ꢀ

V. Thiagarajan et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 7 (2007) 1311
lution have been used.33–36 In our time-resolved study, we used
time-domain technique with both picosecond and femtosecond
resolution. Figure 7 presents the fluorescence decay profiles of
ADDSA-1 in acetonitrile at both the emission maximum. The
fluorescence decay parameters of ADD systems in acetonitrile
are listed in Table 2. In aprotic solvents, both the ADDSA-1
and ADDSA-2 systems had a shorter lifetime component
(ꢁ34 ps) in the larger fraction. Another component having
longer lifetime (2:57 ꢃ 0:06 ns for ADDSA-1 and 7:80 ꢃ
0:06 ns for ADDSA-2) with smaller contribution was also ob-
tained. Due to the limitation of picosecond instrument re-
sponse function (ꢁ52 ps), femto-upconversion studies were
carried out to find the exact lifetime of short-lived component.
The lifetime for ADDSA-1 (Fig. S3) and ADDSA-2 was 13 ꢃ
2 ps. Our shorter lifetime components are in good agreement
with ꢁ ¼ 10 ꢃ 3 ps in acetonitrile, found by Ziolek et al. for
N,N0-bis(salicylidene)-p-phenylenediamine (BSP).35 Recently,
Vergas has reported the biexponential decay for the substituted
SA compounds in cyclohexane and alcoholic solution. The
shorter lifetime component is about 0.02 ns, and the longer
lifetime component is about 1.5 ns. His model considers the
cay time of the longer wavelength state is smaller than that
of the shorter wavelength state and when the longer wave-
length state is formed out of the shorter wavelength state, the
pre-exponential factor of the longer wavelength state should be
negative (rise time) at all wavelengths. Therefore, the absence
of such a term in the time-resolved decay also supports the hy-
pothesis that the two-state decay is independent in nature and
that there is no mother–daughter relationship between the two
states. The former conclusion was supported by the absence
of short-lived component (13 ps) in the non-proton transfer
model system ADDBA. ADDBA showed biexponential decay
with a lifetime of 0:68 ꢃ 0:02 ns (96%) and 1:73 ꢃ 0:02 ns
(4%). PET from the BA to ADD acceptor is responsible for the
shorter component, and LE state decay (CT within ADD
moiety) is responsible for the longer component. Similar
biexponential decay has been observed in non-proton transfer
bichromophoric DMAADD system (0.66 and 1.98 ns in aceto-
nitrile).26 In contrast to ADDSA and ADDBA systems, ADD-1
showed single-exponential fluorescence decay (7:54 ꢃ 0:02 ns)
in acetonitrile due to the absence of both ESIPT and PET path-
ways. The ꢂf values of ADDBA and ADD-1 in acetonitrile
were found to be ꢁ0:0092 (ꢃ5%) and 0.63 (ꢃ0:02), respec-
tively. PET is quite efficient in ADDBA, and this is evident
from its low fluorescent quantum yield in comparison with that
of ADD-1 without the donor moiety.
Effect of Zn2þ on the Dual Fluorescence. In the first row
transition metal ions, Zn2þ is photophysically inactive. It does
not display any electron-transfer and energy-transfer processes
due to completely filled d orbital. In order to confirm that the
longer wavelength emission is due to an ESIPT reaction, pho-
tophysical studies were carried out in the presence of Zn2þ in
acetonitrile for ADDSA systems. When Zn2þ was added to the
ADDSA systems in acetonitrile, in the absorption spectrum,
the absorbance at 350 nm decreased, and a new shoulder
around 425 nm appeared. Figure 8 shows the effect of Zn2þ
on the absorption spectra of ADDSA-1 in acetonitrile. The de-
crease in the absorbance at 350 nm in the presence of Zn2þ
suggests that there is an interaction between metal ion and
the SA moiety in the ground state. Unlike the absorption spec-
trum, the emission spectrum showed fluorescence enhance-
ment in the presence of Zn2þ along with a red shift of 10 nm
as depicted in Fig. 9. The water molecules present in the hy-
drated Zn2þ increase the solvent polarity around the ADDSA
systems resulting in the stabilisation of CT state. We carried
out blank experiments by taking the same amount of water
molecules present in the hydrated form of Zn(ClO4)2, which
gave less than one fold increase in the fluorescence intensity
of the LE state ruling out the possibility of enhancement due
ꢀ
cis-keto tautomer associated with the shorter lifetime compo-
nent in a larger fraction.33 From the earlier literature, the short-
er component observed for the ADDSA systems corresponds
to the decay of proton-transferred keto form. CT within the
ADD moiety is responsible for the decay of longer component
with smaller fraction. From the upconversion results, it should
be noted that no rise component was observed. When the de-
105
Lamp profile
420 nm
530 nm
104
103
102
4
5
6
7
8
Time / ns
Fig. 7. Fluorescence decay profiles of ADDSA-1 moni-
tored at 420 and 530 nm in acetonitrile. ꢀexc ¼ 375 nm.
Table 2. Fluorescence Lifetime of ADDSA-1, ADDSA-2, ADDBA, and ADD-1 in Acetonitrilea)
LE statea)
530 nm
Dyes
ꢁ1/ns
ꢁ2/ns
ꢁ1/ns
ꢁ2/ns
ADDSA-1
ADDSA-2
ADDBA
ADD-1
ꢁ0:034 (95)
ꢁ0:034 (96)
2:57 ꢃ 0:06 (5)
7:80 ꢃ 0:06 (4)
1:73 ꢃ 0:02 (4)
—
ꢁ0:034 (98)
ꢁ0:034 (97)
0:68 ꢃ 0:02
7:54 ꢃ 0:02
2:57 ꢃ 0:06 (2)
7:80 ꢃ 0:06 (3)
—
0:68 ꢃ 0:02 (96)
7:54 ꢃ 0:02
a) Excitation wavelength 375 nm and fluorescence decay were monitored at 420 nm (ADDSA-1) and
440 nm (ADDSA-2, ADDBA, and ADD-1) for LE state. Amplitude (in parentheses).

1312 Bull. Chem. Soc. Jpn. Vol. 80, No. 7 (2007)
Dual Fluorescence
to hydrated water molecules present in the Zn2þ. In ADDSA
systems, ESIPT is the main nonradiative decay pathway prob-
ably due to the presence of SA moiety. Binding of Zn2þ with
SA moiety suppresses the ESIPT pathway, resulting in the
fluorescence enhancement with an increase in the concentra-
tion of metal ion.
systems in acetonitrile was studied. Figure 10 presents the
fluorescence decay profiles of ADDSA-1 at different concen-
tration of Zn2þ in acetonitrile. The shorter component ampli-
tude decreased gradually with an increase in the amplitude
of longer component (2.57 ns for ADDSA-1 and 7.80 ns for
ADDSA-2) upon increasing the concentration of Zn2þ in
acetonitrile at both emission maximum. An increase in the life-
time of the longer component in the presence of Zn2þ and the
disappearance of the short component is in accordance with
the metal ion-induced suppression of ESIPT in the ADDSA
systems (Scheme 3).
The steady state and time-resolved characteristics of
ADDSA system suggests that the dual fluorescence originates
from two different chromophores of the same molecule. The
shorter wavelength emission around 430 nm is due to the CT
within the ADD fluorophore. The solvent polarity independent
longer wavelength anomalous emission originates from the
keto form of the SA moiety via ESIPT in the excited state.
The effect of Zn2þ on the fluorescence decay of ADDSA
0.4
0.3
0.2
0.05 M of Zn(ClO4)2
0
0.1
0.0
Conclusion
Nonconjugated, but covalently linked, bichromophoric sys-
tems ADDSA-1 and ADDSA-2 showed dual fluorescence in
aprotic solvents. The shorter wavelength emission around
430 nm is due to the CT within the ADD fluorophore, which
250
300
350
400
450
500
Wavelength / nm
Fig. 8. Absorption spectrum of ADDSA-1 (16 mM) in the
presence and absence of Zn(ClO4)2 in acetonitrile.
105
104
3.5
0.05 M of Zn(ClO4)2
2.8
0
f
103
2.1
1.4
0.7
0.0
102
b
a
c
3
6
9
12
15
Time / ns
400
450
500
550
600
650
Wavelength / nm
Fig. 10. Fluorescence decay profiles of ADDSA-1 (16 mM)
in the absence (b) and presence of Zn(ClO4)2 in acetoni-
trile. ꢀexc ¼ 375 nm and ꢀem ¼ 420 nm. (a) is the laser
profile; (c–f) 8 to 469 mM of Zn(ClO4)2.
Fig. 9. Emission spectrum of ADDSA-1 (16 mM) in aceto-
nitrile in the absence and presence of Zn(ClO4)2.
ꢀexc ¼ 378 nm.
H
O
O
H
O
C
H
C
C
H
O
H
O
H
Zn
N
N
N
ESIPT
reaction
Zn2+
O
O
O
O
N
R
N
R
N
R
Scheme 3.

V. Thiagarajan et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 7 (2007) 1313
lished work.30
was found to shift towards lower energies with an increase
in the solvent polarity and is due to the stabilisation of the
CT state. In contrast to the lower wavelength emission, the
anomalous longer wavelength emission maximum was inde-
pendent of solvent polarity, and it was more pronounced in
aprotic nonpolar solvents. Steady state and time-resolved
measurement showed that the longer wavelength anomalous
emission around 528 nm is due to the excited state intramolec-
ular proton-transfer reaction in the SA chromophore. The fluo-
rescence lifetime of the longer wavelength (530 nm) keto tau-
tomer was found to be the same order as the shorter component
in the shorter wavelength (420 nm). In presence of Zn2þ, the
decrease in the relative amplitude of the shorter component
confirms the metal ion-induced suppression of ESIPT in the
ADDSA systems.
Synthesis of 9-(4-Salicylideneaminophenyl)-3,3,6,6-tetrameth-
yl-3,4,6,7,9,10-hexahydro-1,8(2H,5H)-acridinedione (ADDSA-1):
A mixture of the aminophenylacridinedione AADD-1 (1.0 g,
2.7 mmol) and salicylaldehyde (0.33 g, 2.7 mmol) in methanol
(20 mL) was stirred at room temperature for 12 h. After comple-
tion, the reaction mixture was concentrated and cooled. The sep-
arated solid was collected by filtration and dried under vacuum.
The product was purified by column chromatography over silica
gel using 5% methanol in chloroform as eluent.
Yield: 1.10 g (86%); mp: 260–262 ꢄC; IR (KBR): 3463, 3286,
3193, 2950, 1624, 1485 cmꢂ1; 1H NMR (400 MHz, CDCl3, TMS):
ꢃ 0.96 and 1.07 (2s, 12H, gem-dimethyl), 2.13–2.37 (m, 8H, C2,
C4, C5, C7–CH2), 5.12 (s, 1H, C9–H), 6.89–7.47 (m, 8H, Ar–H),
7.77 (s, 1H, –N=CH), 8.52 (s, 1H, NH), 13.35 (s, 1H, –OH);
13C NMR (100 MHz, CDCl3, TMS) ꢃ 27.14, 29.54, 32.64, 33.49,
40.83, 50.84, 113.11, 117.11, 119.04, 119.27, 120.81, 129.09,
132.20, 132.91, 145.73, 146.16, 148.97, 161.04, 161.87, 195.74;
MS, m=z (%) 468 (Mþ, 49); Analysis calculated for C30H32N2O3:
C, 76.89; H, 6.88; N, 5.98%. Found: C, 76.98; H, 7.05; N, 6.17%.
Synthesis of 9-(4-Salicylideneaminophenyl)-3,3,6,6,10-
pentamethyl-3,4,6,7,9,10-hexahydro-1,8(2H,5H)-acridinedione
(ADDSA-2): By following the above procedure, reaction of the
aminoacridinedione AADD-2 (1.0 g, 2.6 mmol) with salicylalde-
hyde (0.32 g, 2.6 mmol) afforded the Schiff base ADDSA-2.
Yield: 1.18 g (93%); mp: 280–282 ꢄC; IR (KBR): 3436, 2950,
The above ESIPT state was confirmed by the ADDBA
molecule (the absence of hydroxy group blocs the ESIPT path-
way). In this molecule the absence of shorter component clear-
ly reveals that in ADDSA series ESIPT mechanism plays a key
role and results in the longer wavelength anomalous emission.
Experimental
General. Methanol, acetonitrile, dichloromethane, acetone,
DMSO, and toluene used in this investigation were of HPLC
grade purchased from Qualigens India Ltd. Zinc perchlorate used
in the photophysical studies was purchased from Aldrich. Dime-
done used for the preparation of ADD dyes was purchased from
Ubichem (India) Ltd. Salicylaldehyde, benzaldehyde, methyl-
amine, and p-nitrobenzaldehyde were purchased from E. Merck.
Synthesis of Acridinedione Dyes. ADD-1 was synthesized by
the procedure reported in the literature.37 The synthetic route for
ADDSA and ADDBA are given in Scheme 4. AADD systems
(AADD-1 and AADD-2) were synthesized according to the pub-
1635, 1473, 1369 cmꢂ1 1H NMR (400 MHz, CDCl3, TMS): ꢃ
;
1.04 and 1.09 (2s, 12H, gem-dimethyl), 2.21 (s, 4H, C2 and C7–
CH2), 2.36–2.65 (2d, 4H, J ¼ 16:8 Hz, C4 and C5–CH2), 3.30
(s, 3H, N–CH3), 5.28 (s, 1H, C9–H), 6.90–7.35 (m, 8H, Ar–H),
8.55 (s, 1H, –N=CH), 13.43 (s, 1H, –OH); 13C NMR (100 MHz,
CDCl3, TMS): ꢃ 28.69, 31.80, 32.71, 33.43, 40.68, 49.95,
114.95, 117.16, 118.94, 120.88, 128.70, 132.13, 132.80, 144.98,
151.01, 161.66, 195.31; MS, m=z (%) 482 (Mþ, 37); Analysis cal-
culated for C31H34N2O3: C, 77.15; H, 7.10; N, 5.80%. Found: C,
77.28; H, 7.33; N, 5.99%.
Synthesis of 9-(4-Benzylideneaminophenyl)-3,3,6,6,10-
pentamethyl-3,4,6,7,9,10-hexahydro-1,8(2H,5H)-acridinedione
(ADDBA): By following the above procedure, reaction of the
aminoacridinedione AADD-2 (1.0 g, 2.6 mmol) with benzalde-
hyde (0.29 g, 2.6 mmol) afforded the Schiff base ADDBA.
Yield: 1.08 g (89%); mp: 232–235 ꢄC; IR (KBR): 2956, 1629,
HO
O
CH
NH2
N
CHO
OH
O
O
O
1
1572, 1105 cmꢂ1; H NMR (400 MHz, CDCl3, TMS): ꢃ 1.02 and
1.07 (2s, 12H, gem-dimethyl), 2.23 (s, 4H, C2 and C7–CH2), 2.34–
2.63 (2d, 4H, J ¼ 16:7 Hz, C4 and C5–CH2), 3.28 (s, 3H, N–CH3),
5.28 (s, 1H, C9–H), 7.03–7.85 (m, 9H, Ar–H), 8.40 (s, 1H,
–N=CH); 13C NMR (100 MHz, CDCl3, TMS): ꢃ 28.69, 31.47,
32.65, 33.39, 40.59, 49.92, 114.95, 120.63, 128.32, 128.65,
131.02, 136.38, 143.84, 149.68, 150.98, 151.09, 159.53, 195.32;
MS, m=z (%) 466 (Mþ, 42); Analysis calculated for C31H34N2O2:
C, 79.79; H, 7.34; N, 6.00%. Found: C, 79.92; H, 7.52; N, 6.13%.
Techniques. Absorption spectra were recorded on an Agilent
8453 diode array spectrophotometer. Fluorescence spectral
measurements were carried out using a Perkin-Elmer MPF-44B
fluorescence spectrophotometer interfaced to a PC through a
RISHCOM-100 multimeter. IR spectra were recorded on a
MeOH
N
R
N
R
AADD-1 : R = H
ADDSA-1 : R = H
AADD-2 : R = CH3
ADDSA-2 : R = CH3
CH
N
NH2
CHO
O
O
O
O
1
FTIR-8300 Shimadzu spectrophotometer. H NMR and 13C NMR
spectra were recorded on Jeol-GSX 400 (400 MHz) instrument
with TMS as internal standard (chemical shift in ꢃ ppm). The mass
spectra were recorded with a Jeol-JMS-DX 303 HF instrument.
Fluorescence decays were recorded using the TCSPC method
using the following setup. A diode pumped millena CW laser
MeOH
N
N
CH3
CH3
AADD-2
ADDBA
Scheme 4.

1314 Bull. Chem. Soc. Jpn. Vol. 80, No. 7 (2007)
Dual Fluorescence
(Spectra Physics) at 532 nm was used to pump the Ti:sapphire rod
in Tsunami picosecond mode locked laser system (Spectra Phys-
ics). The 750 nm (80 MHz) light was taken from the Ti:sapphire
laser and passed through pulse picker (Spectra Physics, 3980 2S)
to generate 4 MHz pulses. The second harmonic output (375 nm)
was generated by a flexible harmonic generator (Spectra Physics,
GWU 23PS). The vertically polarized 375 nm laser was used
to excite sample. The fluorescence emission at a magic angle
(54.7ꢄ) was dispersed in a monochromator (f/3 aperture), counted
by an MCP PMT (Hamamatsu R 3809) and processed through
CFD, TAC, and MCA. The instrument response function for this
system was ꢁ52 ps. The fluorescence decay was analysed by using
the software provided by IBH (DAS-6) and PTI global analysis
software.
Shorter components of the fluorescence decay of the dyes were
recorded using a femtosecond upconversion spectrometer. The
sample solution was excited with frequency doubled laser pulse
(395 nm) generated by coupling FOG-100 (CDP, Russia) with a
60 fs Ti-sapphire Laser (Tsunami, Spectra Physics). The polariza-
tion of the excitation beam for magic angle condition was control-
led with a Berek compensator. The rotating sample cuvette was 1
mm thick. The horizontally polarized fluorescence emitted from
the sample was upconverted in a BBO crystal with residual IR
gate pulse passing through a variable delay line with a resolution
of 6.25 fs. Spectral resolution was achieved by dispersing the up-
converted light in a spectrometer with single monochromator and
a photon counter. The FOG-100 had a 350 fs (FWHM) cross cor-
relation function and maximum time delay of 1.7 ns. The fluores-
cence decay curves were fitted with windows based LUMEX 3.1
software (CDP), and the quality of fit was judged from the ꢄ2.
Fluorescence quantum yield was obtained from the corrected
fluorescence spectrum using the expression;
1, 188. c) S. M. Ormson, R. G. Brown, Prog. React. Kinet. 1994,
19, 45. d) D. LeGourrierec, S. M. Ormson, R. G. Brown, Prog.
React. Kinet. 1994, 19, 211. e) S. J. Formosinho, L. G. Arnaut,
J. Photochem. Photobiol., A 1993, 75, 21.
2
F. Vollmer, W. Rettig, E. Birckner, J. Fluoresc. 1994, 4,
65.
3
A. B. Kotlyar, N. Borovok, S. Kiryati, E. Nachliel, M.
Gutman, Biochemistry 1994, 33, 873.
M. L. Martinez, W. C. Cooper, P.-T. Chou, Chem. Phys.
Lett. 1992, 193, 151.
4
5
M. E. Kletskii, A. A. Millov, A. V. Metelitsa, M. I.
Knyazhansky, J. Photochem. Photobiol., A 1997, 110, 267.
6
G. J. Stueber, M. Kieninger, H. Schettler, W. Busch, B.
Goeller, J. Franke, H. E. A. Kramer, H. Hoier, S. Henkel, P.
Fischer, H. Port, T. Hirsch, G. Rytz, J.-L. Birbaum, J. Phys. Chem.
1995, 99, 10097.
7 F. Vollmer, W. Rettig, J. Photochem. Photobiol., A 1996,
95, 143.
8
9
J. Catalan, J. Am. Chem. Soc. 2001, 123, 11940.
a) W. Rettig, R. Fritz, J. Springer, in Photochemical Proc-
esses in Organized Molecular Systems, ed. by K. Honda, Elsevier
Science Publishers, Amsterdam, 1991, p. 61. b) N. P. Ernsting, A.
Mordzinski, B. Dick, J. Phys. Chem. 1987, 91, 1404. c) B. Dick,
N. P. Ernsting, J. Phys. Chem. 1987, 91, 4261.
10 a) A. U. Acuna, F. Amat-Guerri, J. Catalan, F.
Gonzalez-Tablas, J. Phys. Chem. 1980, 84, 629. b) F. Toribio, J.
Catalan, F. Amat, A. U. Acuna, J. Phys. Chem. 1983, 87, 817.
c) D. Gormin, M. Kasha, Chem. Phys. Lett. 1988, 153, 574.
d) D. Gormin, J. Phys. Chem. 1989, 93, 5979. e) F. Lahmani,
A. Zehnacker-Rentien, J. Phys. Chem. A 1997, 101, 6141.
11 a) J. Catalan, F. Toribio, A. U. Acuna, J. Phys. Chem.
1982, 86, 303. b) S. Mitra, R. Das, S. Mukherjee, Chem. Phys.
Lett. 1993, 202, 549. c) P.-T. Chou, M. Chao, J. H. Clements,
M. L. Martinez, C.-P. Chang, Chem. Phys. Lett. 1994, 220, 229.
12 a) S. Lochbrunner, K. Stock, E. Riedle, J. Mol. Struct.
2
ꢂf ¼ ðAs=ArÞðar=asÞðns=nrÞ ꢅ 0:546;
ð1Þ
where, As and Ar are the area under the corrected fluorescence
spectrum, as and ar are the absorbances at the wavelength of ex-
citation (366 nm), ns and nr are the refractive indices of the solvent
for the sample and reference, respectively. The absorbance value
was adjusted to 0.02. The area under the spectra was obtained by
numerically integrating the area using Simpson’s method. Quinine
sulfate in 0.05 mol Lꢂ1 sulfuric acid was used as the reference for
quantum yield determination (ꢂf of quinine sulfate = 0.546).
´
2004, 700, 13. b) A. Mordzinski, A. Grabowska, W. Kuhnle,
A. Krowczynski, Chem. Phys. Lett. 1983, 101, 291. c) J. M.
¨
´
¨
Kaufmann, P. T. Litac, W. J. Boyko, J. Heterocycl. Chem.
1995, 32, 1541.
13 a) M. Rini, A. Kummrow, J. Dreyer, E. T. J. Nibbering, T.
Elsaesser, Faraday Discuss. 2003, 122, 27. b) M. Rini, J. Dreyer,
E. T. J. Nibbering, T. Elsaesser, Chem. Phys. Lett. 2003, 374, 13.
c) S. Lochbrunner, A. J. Wurzer, E. Riedle, J. Phys. Chem. A
2003, 107, 10580. d) R. de Vivie-Riedle, V. De Waele, L. Kurtz,
E. Riedle, J. Phys. Chem. A 2003, 107, 10591. e) A. Heller, D. L.
Williams, J. Phys. Chem. 1970, 74, 4473.
14 a) G. Woessner, G. Goeller, J. Rieker, H. Hoier, J. J.
Stezowski, E. Daltrozzo, M. Neureiter, H. E. A. Kramer, J. Phys.
Chem. 1985, 89, 3629. b) J. Rieker, E. Lemmert-Schmitt, G.
Goeller, M. Roessler, G. J. Stueber, H. Schettler, H. E. A. Kramer,
J. J. Stezowski, H. Hoier, S. Henkel, A. Schmidt, H. Port, M.
Wiechmann, J. Rody, G. Rytz, M. Slongo, J.-L. Birbaum, J. Phys.
Chem. 1992, 96, 10225.
15 a) A. J. G. Strandjord, D. E. Smith, P. F. Barbara, J. Phys.
Chem. 1985, 89, 2362. b) M. Itoh, Y. Fujiwara, M. Sumitani, K.
Yoshihara, J. Phys. Chem. 1986, 90, 5672. c) S. M. Ormson, R. G.
Brown, F. Vollmer, W. Rettig, J. Photochem. Photobiol., A 1994,
81, 65.
16 a) S. J. Schmidtke, D. F. Underwood, D. A. Blank, J. Am.
Chem. Soc. 2004, 126, 8620. b) F. V. R. Neuwahl, L. Bussotti,
R. Righini, G. Buntinx, Phys. Chem. Chem. Phys. 2001, 3,
1277. c) T. P. Smith, K. A. Zaklika, K. Thakur, P. F. Barbara,
The authors acknowledge Council of Scientific and Industri-
al Research (CSIR) and Department of Science and Technolo-
gy (DST), India for financial support. We also thank Dr. A. K.
Mishra, IIT, Chennai for allowing access to the contour mea-
surement facilities.
Supporting Information
3D contour plot of ADDSA-1 in acetonitrile; Femto-upconver-
sion fluorescence decay of ADDSA-1; Fluorescence decay profiles
of ADDSA-2 in acetonitrile; Absorption and emission spectrum of
ADDSA-2 in the presence and absence of Zn(ClO4)2 in aceto-
nitrile; Fluorescence decay profiles of ADDSA-2 in the presence
of Zn(ClO4)2 in acetonitrile. This material is available free of
charge on the web at http://www.csj.jp/journals/bcsj.
References
1 a) T. Elsaesser, H. J. Bakker, Ultrafast Hydrogen Bonding
Dynamics and Proton Transfer Processes in the Condensed Phase,
Kluwer, Dordrecht, 2002. b) A. Weller, Prog. React. Kinet. 1961,

V. Thiagarajan et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 7 (2007) 1315
J. Am. Chem. Soc. 1991, 113, 4035. d) T. P. Smith, K. A. Zaklika,
K. Thakur, G. C. Walker, K. Tominaga, P. F. Barbara, J. Phys.
Chem. 1991, 95, 10465. e) T. P. Smith, K. A. Zaklika, K. Thakur,
G. C. Walker, K. Tominaga, P. F. Barbara, J. Photochem.
Photobiol., A 1992, 65, 165.
17 a) H. Bulska, A. Grabowska, Z. R. Grabowski, J. Lumin.
1986, 35, 189. b) P. Borowicz, A. Grabowska, A. Le, L.
Kaczmarek, B. Zagrodzki, Chem. Phys. Lett. 1998, 291, 351.
26 a) V. Thiagarajan, C. Selvaraju, E. J. Padma Malar, P.
Ramamurthy, ChemPhysChem 2004, 5, 1200. b) V. Thiagarajan,
P. Ramamurthy, ISRAPS Bull. 2005, 17, 18.
27 a) V. Thiagarajan, V. K. Indirapriyadarshini, P.
Ramamurthy, J. Inclusion Phenom. Macrocyclic Chem. 2006,
DOI: 10.1007/s10847-005-9043-4. b) R. Kumaran, P.
Ramamurthy, J. Phys. Chem. B 2006, 110, 23783.
28 H. J. Timpe, S. Ulrich, C. Decker, J. P. Fouassier,
Macromolecules 1993, 26, 4560.
29 H. J. Timpe, S. Ulrich, J. P. Fouassier, J. Photochem.
Photobiol., A 1993, 73, 139.
30 a) V. Thiagarajan, P. Ramamurthy, D. Thirumalai, V. T.
Ramakrishnan, Org. Lett. 2005, 7, 657. b) V. Thiagarajan, P.
Ramamurthy, Spectrochim. Acta, Part A 2006, doi:10.1016/
j.saa.2006.08.031.
31 a) N. Srividya, P. Ramamurthy, V. T. Ramakrishnan,
Spectrochim. Acta, Part A 1997, 53, 1743. b) N. Srividya, P.
Ramamurthy, V. T. Ramakrishnan, Spectrochim. Acta, Part A
1998, 54, 245.
32 B. Venkatachalapathy, P. Ramamurthy, V. T.
Ramakrishnan, J. Photochem. Photobiol., A 1997, 111, 163.
33 V. Vargas C., J. Phys. Chem. A 2004, 108, 281.
34 a) P. F. Barbara, P. M. Rentzepis, L. E. Brus, J. Am. Chem.
Soc. 1980, 102, 2786. b) T. Sekikawa, T. Kobayashi, T. Inabe,
J. Phys. Chem. A 1997, 101, 644.
ˇ
18 M. Cuma, C. Thompson, S. Scheiner, J. Comput. Chem.
1998, 19, 129.
19 a) P.-T. Chou, W.-S. Yu, Y.-M. Cheng, S.-C. Pu, Y.-C. Yu,
Y.-C. Lin, C.-H. Huang, C.-T. Chen, J. Phys. Chem. A 2004, 108,
6487. b) P.-T. Chou, C.-H. Huang, S.-C. Pu, Y.-H. Liu, Y. Wang,
C.-T. Chen, J. Phys. Chem. A 2004, 108, 6452.
20 T. Sekikawa, T. Kobayashi, T. Inabe, J. Phys. Chem. A
1997, 101, 644.
21 a) M. D. Cohen, G. M. J. Schmidt, J. Phys. Chem. 1962,
66, 2442. b) K. Kownacki, A. Mordzinski, R. Wilbrandt, A.
Grabowska, Chem. Phys. Lett. 1994, 227, 270.
22 a) A. Grabowska, K. Kownacki, J. Karpiuk, S. Dobrin, L.
Kaczmarek, Chem. Phys. Lett. 1997, 267, 132. b) K. Kownacki, A.
Mordzinski, R. Wilbrandt, A. Grabowska, Chem. Phys. Lett. 1994,
227, 270.
23 a) A. Ohshima, A. Momotake, T. Arai, Chem. Lett. 2005,
34, 1288. b) A. Ohshima, A. Momotake, T. Arai, Bull. Chem.
Soc. Jpn. 2006, 79, 305.
24 K. J. Prabahar, V. T. Ramakrishnan, D. Sastikumar, S.
Selladurai, V. Masilamani, Indian J. Pure Appl. Phys. 1991, 29,
382.
´
35 M. Ziołek, J. Kubicki, A. Maciejewski, R. Naskrc¸cki, A.
Grabowska, J. Chem. Phys. 2006, 124, 124518.
36 a) S. Mitra, N. Tamai, Chem. Phys. Lett. 1998, 282, 391.
b) S. Mitra, N. Tamai, Chem. Phys. 1999, 246, 463.
37 N. Srividya, P. Ramamurthy, P. Shanmugasundram, V. T.
Ramakrishnan, J. Org. Chem. 1996, 61, 5083.
25 a) C. Selvaraju, P. Ramamurthy, Chem. Eur. J. 2004, 10,
2253. b) S. Singh, S. Chhina, V. K. Sharma, S. S. Sachdev,
J. Chem. Soc., Chem. Commun. 1982, 453.