Design and synthesis of novel fluorescence sensing perylene diimides based on photoinduced electron transfer

Article abstract of DOI:10.1016/j.dyepig.2011.04.015

Two novel tetraester- and PAMAM-branched perylene diimides were synthesized and configured as "fluorophore-spacer-receptor" systems based on photoinduced electron transfer. Due to their long alkylester and alkylamine terminal groups the examined compounds were well soluble in organic solvents. Photophysical characteristics of the dyes were investigated in DMF and water/DMF (1:1, v/v) solution. The ability of the synthesized perylene diimides to detect cations was evaluated by the changes in their fluorescence intensity in the presence of metal ions (Zn²⁺, Co²⁺, Cu²⁺, Fe³⁺, Pb²⁺, Hg²⁺, Ag⁺ and Ni ²⁺) and protons. The dyes under study displayed "off-on" switching in its fluorescence as a function of pH, which is attributed to disallowing photoinduced electron transfer from the receptor moiety to the fluorophore. PAMAM-branched dye displayed a good pH sensor activity (FE = 6.4), however the pH sensing ability of tetraester was substantially higher (FE = 184). In the presence of Cu²⁺ and Pb²⁺ ions tetraester quenched its fluorescence intensity (FQ = 22 and 12 respectively), while PAMAM-branched dye enhanced its fluorescence intensity with pronounced selectivity to Cu²⁺ and Fe³⁺ (FE = 3.2 and 4.9, respectively). The results obtained indicate the potential of the novel compounds as fluorescent detectors for metal ions with pronounced selectivity towards Cu²⁺, Pb²⁺ and Fe³⁺ ions and highly efficient "off-on" pH switches, especially a tetraester-branched perylene diimide.

Full text of DOI:10.1016/j.dyepig.2011.04.015

Dyes and Pigments 91 (2011) 332e339
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Design and synthesis of novel fluorescence sensing perylene diimides based
on photoinduced electron transfer
,
,
*
Nikolai I. Georgieva, Alaa R. Sakr a b, Vladimir B. Bojinov a
a Department of Organic Synthesis, University of Chemical Technology and Metallurgy, 8 Kliment Ohridsky Str., 1756 Sofia, Bulgaria
b Department of Chemistry, Faculty of Science, Zagazig University, Zagazig City, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel tetraester- and PAMAM-branched perylene diimides were synthesized and configured as
“fluorophore-spacer-receptor” systems based on photoinduced electron transfer. Due to their long
alkylester and alkylamine terminal groups the examined compounds were well soluble in organic
solvents. Photophysical characteristics of the dyes were investigated in DMF and water/DMF (1:1, v/v)
solution. The ability of the synthesized perylene diimides to detect cations was evaluated by the changes
in their fluorescence intensity in the presence of metal ions (Zn2þ, Co2þ, Cu2þ, Fe3þ, Pb2þ, Hg2þ, Agþ and
Ni2þ) and protons. The dyes under study displayed “offeon” switching in its fluorescence as a function of
pH, which is attributed to disallowing photoinduced electron transfer from the receptor moiety to the
fluorophore. PAMAM-branched dye displayed a good pH sensor activity (FE ¼ 6.4), however the pH
sensing ability of tetraester was substantially higher (FE ¼ 184). In the presence of Cu2þ and Pb2þ ions
tetraester quenched its fluorescence intensity (FQ ¼ 22 and 12 respectively), while PAMAM-branched
dye enhanced its fluorescence intensity with pronounced selectivity to Cu2þ and Fe3þ (FE ¼ 3.2 and
4.9, respectively). The results obtained indicate the potential of the novel compounds as fluorescent
detectors for metal ions with pronounced selectivity towards Cu2þ, Pb2þ and Fe3þ ions and highly effi-
cient “offeon” pH switches, especially a tetraester-branched perylene diimide.
Received 21 March 2011
Received in revised form
25 April 2011
Accepted 28 April 2011
Available online 5 May 2011
Keywords:
Perylene-3,4,9,10-tetracarboxylic diimide
(PDI)
Polyamidoamine (PAMAM)
Fluorescence
pH sensors
Metal ion sensors
Photoinduced electron transfer (PET)
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
[23,24]. The usage of perylene diimides as dendrimer core would
increase their solubility in organic solvents due to the dendritic
Perylene-3,4,9,10-tetracarboxylic diimides exhibit
a
unique
swallow-tail conformation [25,26]. The polyamidoamine (PAMAM)
is a class of commercial dendrimers. Considerable interest in pol-
yamidoamine (PAMAM) dendritic macromolecules has arisen
because of their novel structural properties and wide range of
potential applications [27e31]. Constructing a PAMAM dendrimer
around a luminescent group could profitably alter the lumines-
cence signals in the macromolecular structure and amplify the
signals for sensing purposes [32e36].
Development of new sensors with improved characteristics
(selectivity, sensitivity etc.) is a challenging and necessary task in the
21st century, marked by the technological booms consequences
[37e41]. Because of the high sensitivity, high speed, and safety the
fluorescent chemosensors and switches have been actively investi-
gated in the recent years. The photoinduced electron transfer (PET)
using the “fluorophore-spacer-receptor” format is the most commonly
exploited approach for the design of the fluorescent sensors and
switchers [42]. The components are chosen so that PET from the
receptor (usually an amino group) to the fluorophore quenches the
fluorescence of the system. However, in the presence of a guest, PET
communication between the receptor and the fluorophore gets cut off
combination of electrooptical and redox properties. They are highly
stable and widely used n-type materials for organic electronic
devices such as solar cells [1e3], field-effect transistors [4e6], light-
emitting diodes [7e9], in liquid crystal displays [10,11], as chemo-
sensing materials [12e14], light-harvesting materials [15,16] and as
dyes in photodynamic therapy [17]. However, most of the syn-
thesised dyes possess low solubility in organic solvents; this is
a serious drawback for their processing and material science
applications [18,19]. Usually the solubility is improved with the
long-tail or swallow-tail conformation that is obtained by long alkyl
substitutions in N-position of the dye [20e22].
Dendrimers are well defined macromolecules exhibiting
a three-dimensional structure that is roughly spherical or globular.
A characteristic of dendritic macromolecules is the presence of
numerous peripheral chain ends that all surround a single core
* Corresponding author. Tel.: þ 359 2 8163169.
E-mail address: vlbojin@uctm.edu (V.B. Bojinov).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.04.015

N.I. Georgiev et al. / Dyes and Pigments 91 (2011) 332e339
333
acid dianhydride 1 (Aldrich), p.a. grade, were used without purifi-
cation. All solvents (Fluka, Merck) were pure or of spectroscopy
grade. Zn(NO3)2, Cu(NO3)2, Ni(NO3)2, Co(NO3)2, Pb(NO3)2, Fe(NO3)3,
Hg(NO3)2 and AgNO3 salts were the sources for metal cations.
2.2. Methods
FT-IR spectra were recorded on a Varian Scimitar 1000 spec-
trometer. The 1H NMR spectra (chemical shifts are given as
d in
ppm) were recorded on a Bruker DRX-250 spectrometer, oper-
ating at 250.13 MHz. TLC was performed on silica-gel, Fluka F60
254, 20 ꢀ 20, 0.2 mm. The melting points were determined by
means of
a Kofler melting point microscope. The UVeVIS
absorption spectra were recorded on a spectrophotometer Hew-
lett Packard 8452A. The corrected fluorescence spectra were
taken on a Scinco FS-2 spectrofluorimeter at room temperature
(25 ꢁC). To adjust the pH values very small volumes of hydro-
chloric acid and sodium hydroxide solution were used. The pH
was determined with a pH meter Metrohm 704, which was
standardized with Aldrich buffers. The effect of the metal cations
and protons upon the fluorescence intensity was examined
Scheme 1.
and the fluorescence of the system is recovered. In other words, the
presence of a guest is signaled by fluorescence enhancement of the
system [43e46].
In this work we were focused on the divergent construction of
a novel perylene diimide core functionalized PAMAM dendrimer 5
and tetraester 4 together with their photophysical and fluorescence
chemosensing properties. Hence, compounds 4 and 5 (Scheme 1)
were synthesized and investigated by electronic absorption and
emission spectroscopy as potential PET sensors for protons and
transition metal ions.
by adding 30
ml portions of the metal cations stock solution
(2.0 ꢀ 10ꢂ6 mol Lꢂ1) to a known volume of the fluorophore
solution (3 ml).
2.3. Synthesis of amino functionalized perylene-3,4,9,10-
tetracarboxylic diimide (3)
To
a suspension of 2 g (5 mmol) perylene-3,4,9,10-
tetracarboxylic acid dianhydride in 25 ml of benzene, 6 ml
ethylenediamine (90 mmol) was added and the resulting
mixture was heated under reflux for 5 h. After cooling to room
temperature the precipitate was collected by filtration, washed
with benzene and dried. Then the crude solid was treated with
50 ml of 5% aqueous sodium hydroxide to give after filtration,
washing with water and drying 2.04 g (84%) of pure diimide 3
In order to receive a more complete comparative picture for the
influence of the polyamidoamine backbone to the perylene diimide
fluorophore, perylene diimide 2, not containing tertiary amine in its
molecule, was involved in the present study (Scheme 1).
2. Experimental
(m.p. > 250 ꢁC). IR (KBr) cmꢂ1: 3368 (
n
NH2); 1684 (
n
asN-C¼O);
2.1. Material
1658 (
n
sN-C¼O). 1H NMR (CF3COOD-d, 250.13 MHz) ppm: 8.41
(m, 8H, perylene H); 4.39 (m, 4H, 2 ꢀ CH2N(CO)2); 3.41 (m, 4H,
2 ꢀ CH2NH2). Elemental analysis: Calculated for C28H20N4O4
(MW 476,5) C 70.58, H 4.23, N 11.76%; Found C 70.49, H 4.36, N
11.55%.
Reference perylene-3,4,9,10-tetracarboxylic diimide
synthesized according to the procedure described before [47]. Eth-
ylenediamine, methyl acrylate and perylene-3,4,9,10-tetracarboxylic
2 was
Scheme 2.

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N.I. Georgiev et al. / Dyes and Pigments 91 (2011) 332e339
2.5. Synthesis of core functionalized 3,4,9,10-
perylenebis(dicarboximide) PAMAM dendrimer (5)
To a solution of ethylenediamine (4 ml, 60 mmol) in 5 ml of
methanol, a suspension of perylene diimide 4 (0.82 g, 1 mmol) in
25 ml of methanol was added dropwise at 5 ꢁC for a period of
30 min. The reaction mixture was stirred for 168 h at room
temperature. Then the solvent and the ethylenediamine excess
were distilled under vacuum. Final traces of ethylenediamine were
removed azeotropically using a 9:1 toluene/methanol (v/v) solu-
tion. The aminofunctionalized dendrimer 5 was obtained as dark-
red crystals (0.86 g, 92%, m.p. 163e166 ꢁC). IR (KBr) cmꢂ1: (
nNH
and
n
NH2); 2943e2842 (
n
CH); 1668 (
n
asN-C¼O); 1646 (
n
sN-C¼O).
1O NMR (DMSO-d6, 250.13 MHz) ppm: 8.72 (d, 4H, J ¼ 8.0 Hz,
perylene H-3, H-4, H-9, H-10); 8.54 (d, 4H, J ¼ 8.2 Hz, perylene H-5,
H-6, H-7, H-8); 7.96 (br.s, 4H,
4
ꢀ
NHCO); 4.14 (m, 4H,
2 ꢀ (CO)2NCH2); 3.12 (m, 8H, 4 ꢀ NHCH2CH2NH2); 2.79 (m, 12H,
6
4
ꢀ
ꢀ
NCH2); 2.55 (m, 8H,
NHCH2CH2NH2). Elemental analysis: Calculated for
4
ꢀ
CH2CO); 2.31 (m, 8H,
C48H60N12O8 (MW 933.1) C 61.79, H 6.48, N 18.01%; Found C 61.58,
H 6.25, N 17.77%.
Scheme 3.
2.4. Synthesis of perylene-3,4,9,10-tetracarboxylic diimide
tetraester (4)
3. Results and discussion
3.1. Design of the dyes
A mixture of methyl acrylate (2.7 ml, 32 mmol) and 1.5 g of 3
(3.2 mmol) in 30 ml of methanol was stirred for 4 days at room
temperature, then the precipitate was collected by filtration,
washed with water and dried. The crude solid was extracted with
boiling chloroform to give after vacuum evaporation of the solvent
and silica-gel chromatography (methylene chloride) 0.74 g (28%)
pure tetraester 4 as dark-red crystals (m.p. 206e209 ꢁC). IR (KBr)
The dyes under study were designed as fluorescent core func-
tionalized tetraester and PAMAM dendrimer, respectively, for
determining transition metal cations and pH changes over a wider
pH scale. We chose perylene diimide chromophore as a fluorescent
core of the synthesized compounds, because of its chemical
stability, high fluorescent efficiency and high electron-accepting
ability.
cmꢂ1: 1730 (
n
COOCH3); 1688 (
n
asN-C¼O); 1660 (
n
sN-C¼O). 1H
NMR (CDCl3-d, 250.13 MHz) ppm: 8.50 (d, 4H, J ¼ 8.0 Hz, perylene
H-3, H-4, H-9, H-10); 8.36 (d, 4H, J ¼ 8.2 Hz, perylene H-5, H-6, H-7,
H-8); 4.27 (t, 4H, J ¼ 6.9 Hz, 2 ꢀ CH2N(CO)2); 3.56 (s, 12H,
4 ꢀ OCH3); 2.91 (t, 8H, J ¼ 7.0 Hz, 2 ꢀ CH2N(CH2)2); 2.83 (t, 4H,
J ¼ 6.9 Hz, 2 ꢀ CH2N(CH2)2); 2.48 (t, 8H, J ¼ 7.1 Hz, 4 ꢀ CH2COOCH3).
Elemental analysis: Calculated for C44H44N4O12 (MW 820.9) C
64.38, H 5.40, N 6.83%; Found C 64.19, H 5.29, N 6.70%.
The branching perylene diimides 4 and 5 were configured on
the “fluorophore-spacer-receptor” model, where the perylene dii-
mides core is the fluorophore and the tertiary amines are the
proton receptors. The ethylene part between fluorophore and the
branching amines serves as spacer that covalently separates the
two units. In these particular cases, it was predicted that a PET
process (an electron transfer from the receptors to the excited state
Scheme 4.

N.I. Georgiev et al. / Dyes and Pigments 91 (2011) 332e339
335
Table 2
Photophysical characteristics of 2,
4
and
5
in DMF solution at concentration
10ꢂ5 mol Lꢂ1
(lex ¼ 460 nm).
Compound
DMF
(L molꢂ1 cmꢂ1
)
lF (nm)
n
AenF (cmꢂ1
)
VF
a
lA (nm)
3
2
4
5
524
524
524
85,497
81,280
78,846
540
539
542
565
531
634
1
0.04
0.03
a
VF is relative quantum yield of fluorescence calculated using compound 2 as
standard.
absorption bands at 1690e1680 cmꢂ1 and 1660e1650 cmꢂ1
resulting from the imide ring formation. At the same time
absorption maxima in range of 1750e1800 cmꢂ1 corresponding to
the anhydride carbonyl functions are absent. This is solid evidence
that the starting perylene anhydride 1 was completely converted in
1,8-naphthalimides 2 and 3. Analogously, in the FT-IR spectra of 4
a typical for the ester compounds absorption band at 1730 cmꢂ1
appears and no absorption above 3000 cmꢂ1 corresponding to N-H
primary and secondary amine groups were observed. Furthermore
in the 1H NMR (250.13 MHz) spectrum of compound 4 resonance
triplets at 2.91 ppm for 8 protons and 2.83 ppm for 4 protons were
observed (Fig. 1), suggesting that the two primary amine groups of
3 were completely branched to tetraester 4. The structure of
compound 5 was confirmed using the same logic. In the FT-IR
spectrum of 5 the lack of ester absorption at 1730 cmꢂ1 was
accompanied with the appearance of novel bands in range of
3187e3268 cmꢂ1 for amine groups that is well correlated with the
1H NMR spectrum of the compound.
Fig. 1. 1H NMR (CDCl3-d, 250.13 MHz) spectrum of tetraester 4 in range of about
3.00e2.80 ppm.
of the fluorophore) would quench fluorescence emission of the
perylene diimide unit. This would represent the “off-state” of the
system. The protonation or the respective metal coordination of the
amine receptors would increase their oxidation potential, and as
such, thermodynamically disallows the electron transfer [48].
Consequently the emission would be “switched on”. Thus we expect
the fluorescence to be amplified in acidic media (Scheme 2).
3.3. Photophysical characterization of the dyes
The solubility of the perylene derivatives in different solvents
was tested at concentration 0.1 mg mLꢂ1 at room temperature (see
Table 1). As can be seen from the data inTable 1 the starting perylene
diimide 3 is insoluble in common organic solvents while the novel
compounds 4 and 5 are well soluble. The improved solubility of the
perylene derivatives 4 and 5 may be attributed to the introduction of
swallow-tail branches that reduced the interaction between per-
ylene rings with the solvent. Furthermore, as can be seen from the
data presented in Table 1, the different terminal groups gave strong
influence on the solubility of novel compounds 4 and 5. The tet-
raester 4 was well soluble in chlorinated hydrocarbons (chloroform,
3.2. Synthesis
The synthesis of perylene diimides 2 and 3 was achieved as
shown in Scheme 3. The reference compound 2 was synthesized as
described before [47] by refluxing perylene-3,4,9,10-tetracarboxy
anhydride 1 and n-butylamine in water/propanol. The amino func-
tionalized core 3 was prepared by analogy with the previously
reported procedure [49] by refluxing perylene-3,4,9,10-tetracarboxy
anhydride 1 with ethylenediamine in benzene media. The pure dye 3
was obtained after removing the unreacted anhydride 1 under
boiling the crude solid in 5% aqueous sodium hydroxide.
The novel PAMAM perylene tetracarboxydiimide
5 was
synthesized via PAMAM divergent strategy, involves initial Michael
addition of amino functionalized core 3 with methyl acrylate fol-
lowed by exhaustive amidation of the resulting tetraester 4 with
a large excess of ethylenediamine to afford the fluorescent den-
drimer 5 with amine ending groups of its periphery (Scheme 4).
The structure and purity of the desired products were confirmed
by conventional techniques (elemental analysis data, UVeVIS,
fluorescence, FT-IR and 1H NMR spectroscopy). For instance, the FT-
IR spectra of the synthesized perylene diimides 2 and 3 showed
Table 1
Solubility of 3, 4 and 5 in different organic solvents and water.
Compound
DMF
DMSO
Water
ROH
R-X
3
4
5
ꢂ
þ
þ
ꢂ
þ
þ
ꢂ
ꢂ
þ
ꢂ
ꢂ
þ
ꢂ
þ
ꢂ
þ Soluble; ꢂ Insoluble; ROH ¼ Methanol, Ethanol; R-X ¼ Chloroform, Dichloro-
methane, Dichloroethane.
Fig. 2. Normalized absorption spectra of compounds 2 and 4 in DMF solution.

336
N.I. Georgiev et al. / Dyes and Pigments 91 (2011) 332e339
Fig. 4. Normalized to the optical density fluorescence spectra of compounds 2 and 4 in
DMF solution.
Fig. 3. Normalized absorption and fluorescence spectra of compound
solution.
2 in DMF
The Stoke’s shift (nAenF) is important parameter for the fluo-
dichloromethane and dichloroethane), while the amine terminated
compound 5 was insoluble in these solvents. Also, unlike compound
5, tetraester 4 was not soluble in water and alcohols.
rescent compounds that indicates the differences in the properties
and structure of the fluorophores between the ground state S0 and
the first excited state S1. The Stoke’s shifts (cmꢂ1) were calculated
by Eq. (1).
The basic photophysical characteristics of the examined
compounds 2, 4 and 5 such as absorption (lA) and fluorescence (lF
maxima, Stokes shift ( AenF) and quantum yield of fluorescence
(VF) were measured in DMF and presented in Table 2.
)
n
ꢀ
ꢁ
1
1
ð
nA
ꢂ
nFÞ ¼
ꢂ
ꢀ 107
(1)
lA lF
Data presented in Table 2 and Fig. 2 showed that the different
alkyl substituents at the perylene tetracarboxydiimide N-position
(compounds 2e5) did not affect the energy and the shape of the
dyes’ absorption bands.
The Stoke’s shift values are between 531 and 634 cmꢂ1, which
corresponds to the results for other perylene tetracarboxydiimide
derivatives [50] and these values do not indicate remarkable
changes in the geometry of the first singlet excited state due to the
excitation.
As can be seen from the data in Table 2, the quantum yield of
fluorescence of perylene diimides 4 and 5 are lower in comparison
with the data for compound 2, not containing tertiary amine
moiety. This phenomenon might be caused by the possible PET
process from the amine receptor to the perylene diimide fluo-
rophore through the ethylene spacer. Thus the fluorescence of the
perylene diimide fluorophores 4 and 5 is quenched (Scheme 5,
Fig. 4), which was the reason to investigate their photophysical
The compounds under study showed typical for the perylene
tetracarboxydiimides fluorescence spectra (lex ¼ 460 nm) with
three bands at about 540, 578 and 620 nm, as the highest intensity
was observed at the lowest wavelength band (lF ¼ 540 nm) [50].
The substituents at the N-position did not affect the energy of the
dyes’ fluorescence maximum. In all cases the shape and the
maximum of the fluorescence band did not depend on the excita-
tion wavelength. Fig. 3 presents the normalized absorption and
emission spectra of perylene diimide 2 in DMF solution as a typical
example for the spectra of all compounds under study. The fluo-
rescence band is the mirror image of the absorption and no
concentration effects on the shape of the fluorescence bands were
observed for C < 10ꢂ5 mol Lꢂ1
.
Fig. 5. Compound 4 in DMF solution (left sample) and in DMF after addition of HCl
Scheme 5.
(right sample).

N.I. Georgiev et al. / Dyes and Pigments 91 (2011) 332e339
337
Fig. 6. Changes in the absorption intensity of 4 in water/DMF (1:1, v/v) as a function of
pH.
Fig. 8. Changes in the fluorescence intensity (lF ¼ 545 nm) of 4 in water/DMF (1:1, v/v)
as a function of pH.
behaviour at different pH values and in the presence of transition
metal ions.
case of 4 and 5, significant pH-dependent changes in absorption band
intensities were observed (Fig. 6), which suggest reversible aggrega-
tion processes at alkaline pH values.
Due to the aggregation the absorption in alkaline media was
decreased 4 times for 4 and 1.3 times for 5, in comparison with their
absorptions in acid media. Analysis of these absorbance (A) changes
according to Eq. (2) [12] gives the pKa values 3.1 and 1.9, respectively.
3.4. Influence of pH on the fluorescence properties of the dyes
The PET process can be disallowed after protonation of the
electron donating tertiary amine receptor in acid media. Because of
this, compound 4 shows weak fluorescence emission in DMF
solution (Fig. 5, left sample) and strong emission after acidification
of the solution (Fig. 5, right sample). The last experiment clearly
illustrates the potential of compound 4 as highly efficient “offeon”
pH chemosensing material.
Photophysical characteristics of perylene diimides 4 and 5 as
a function of pH were investigated in water/DMF (1:1, v/v) solution.
In order to receive a more complete comparative picture for the
influence of the nitrogen donor (amine receptor) to the perylene
diimide fluorophore, the reference compound 2, not containing
amine receptor in its molecule, was involved in the present study.
The absorption spectra of 2, 4 and 5 did not show any pH-
dependent band shifts as the perylene-3,4,9,10-tetracarboxydiimide
fluorophore does not give rise to ICT excited states. However, in the
log½ðAmax ꢂ AÞ=ðA ꢂ AminÞꢃ ¼ pH ꢂ EKa
(2)
The fluorescence spectra of compounds 2, 4 and 5 were also
recorded in water/DMF (4:1, v/v) solution at different pH values.
N,N’-(n-Butyl)-perylene diimide 2, which lacks the amine receptor,
did not show any changes in emission properties as a function of
pH. In contrast to the above result, in alkaline solution for
compounds 4 and 5 only a weak emission was observed between
500 and 700 nm, with lF at 545 nm. However, upon acidification
the emission was gradually increased as demonstrated in Figs. 7
and 8. After careful titration to pH ca. 2.0, the emission intensity
of perylene diimides had enhanced more than hundred times
(FE ¼ 184). This fact is due to the two effects e reversible aggre-
gation and PET quenching at alkaline pH values, which do not take
Fig. 7. Fluorescence spectra of 4 in water/DMF (1:1, v/v) in pH range ca. 1.5e13.0
Fig. 9. Fluorescence spectra of 5 in water/DMF (1:1, v/v) in pH range ca. 1.0e10.0
(
lex ¼ 460 nm).
(lex ¼ 465 nm).

338
N.I. Georgiev et al. / Dyes and Pigments 91 (2011) 332e339
Fig. 12. Effect of the metal cations at concentration c ¼ 2 ꢀ 10ꢂ6 mol Lꢂ1 on the
fluorescence of 5 (c ¼ 10ꢂ6 mol Lꢂ1) in DMF solution.
Fig. 10. Changes in the fluorescence intensity (lF ¼ 546 nm) of 5 in water/DMF (1:1,
v/v) as a function of pH.
The determined pKa values of 1.8 and 4.1 for 5 and 3.4 for 4
indicate that the amine branching perylene diimides would be able
to act as efficient “offeon” switchers for pH. The lower pKa value of
1.8 detected for 5 is similar to the value calculated using Eq. (2) and
probably is characteristic of the aggregation.
place in acid media. As the aggregation quenches 4 times the
fluorescence of 4 in alkaline media (Fig. 6), consequently the PET
quenching effect is 46 times.
The pH dependence of the fluorescence properties of dendrimer
5 as a function of pH in water/DMF (1:1, v/v) is illustrated in Figs. 9
and 10. As can be seen in acid media the fluorescence intensity of
dendrimer 5 is 6.4 times higher than in alkaline media. These
changes are of such magnitude that they can be considered as
representing two different “states”, where the fluorescence emis-
sion is “switched off” in alkaline solution and “switched on” in acidic
solution. The lower value of FE for compound 5 as compared to that
for 4 probably is due to the conformational differences, which could
be related to the increase the distance between the tertiary amine
(receptor) and fluorophore in 5. Also, the more flexible PAMAM
scaffold is able to induce energy loses thus reducing the quantum
yield of fluorescence in “on-state” of PAMAM compound 5, which
results in lower FE value.
3.5. Influence of metal cations on the fluorescence intensity of the
dyes
The signaling fluorescent properties of novel perylene diimides
4 and 5 in the presence of transition metal ions have been inves-
tigated spectrophotometrically in DMF with regard to their
potential application as PET sensors. DMF has been chosen in all
measurements since it is able as a polar solvent to stabilize the dye
charge separated state thus favouring the fluorescence switching
by PET process. Also DMF guarantees a good solubility of the dye
ligands, used metal salts and the respective complexes. Experi-
ments have been performed in the presence of different metal
Taking the part of the graphs located between pH 2 and 6, the
pKa values of compounds 4 and 5 have been calculated by the Eq.
(3) [12].
cations: Zn2þ, Co2þ, Cu2þ, Fe3þ, Pb2þ, Hg2þ, Agþ and Ni2þ
.
The above metal cations induced negligible changes in the
absorption spectra of 4 and 5. The enhancement (FE) and the
quenching (FQ) of the fluorescence emission have been used as
a qualitative parameter. The FE ¼ I/Io is determined as a ratio
between the maximum fluorescence intensity (I - after the metal
ion addition) and the minimum fluorescence intensity (Io - free of
metal cations solution). The FQ ¼ Io/I has been determined from the
ratio between the maximum fluorescence intensity (Io - solution
free of metal cations) and the fluorescence intensity in the presence
of metal cations (I). Figs. 11 and 12 present the calculated FE and FQ
values for perylene diimides 4 and 5.
log½ðIFmax ꢂ IFÞ=ðIF ꢂ IFminÞꢃ ¼ pH ꢂ pKa
(3)
As can be seen the novel perylene diimides 4 and 5 exhibit
sensor selectivity. In DMF solution of tetraester 4, the fluorescence
quenching was observed only upon the addition of Cu2þ and Pb2þ
(FQ ¼ 22 and 12 respectively), while the other metal ions produced
a negligible effect (Fig.11). In contrast of the fluorescence behaviour
of tetraester 4, after addition of metal ions to the DMF solution of
PAMAM dendrimer 5 (Fig. 12), the fluorescence of the latter was
enhanced with pronounced selectivity to Cu2þ and Fe3þ (FE ¼ 3.2
and 4.9, respectively).
The quenching effect of Cu2þ, Pb2þ and Ni2þ in DMF solution of 4
suggests that the tertiary amine is not involved in the coordination
with metal ions in contrast to 5 at the same conditions. This is
Fig. 11. Effect of the metal cations at concentration c ¼ 2 ꢀ 10ꢂ6 mol Lꢂ1 on the
fluorescence of 4 (c ¼ 10ꢂ6 mol Lꢂ1) in DMF solution.

N.I. Georgiev et al. / Dyes and Pigments 91 (2011) 332e339
339
probably due to that the ester groups in 4 hindered the tertiary
References
amine and in the presence of metal ions the PET quenching process
from the amine receptor to the fluorophore core is still there. Also,
the specific interaction between fluorophore 4 and Cu2þ, Pb2þ, Ni2þ
causes an additional quenching process with well pronounced
efficiency toward Cu2þ ions. Most likely the coordination occurring
in the ester branches of 4 with Cu2þ, Pb2þ and Ni2þ ions is followed
by an electron transfer or energy transfer from the fluorophore
excited state to the metal complex, which quenches the fluores-
cence without any change in the maxima of the fluorescence and
absorption spectra.
[1] Tian H, Liu P-H, Zhu W, Gao E, Wu D-J, Cai S. J Mater Chem 2000;10:2708.
[2] Zafer C, Karapire C, Sariciftci N, Icli S. Sol Energy Mater Sol Cells 2005;88:11.
[3] Fortage J, Sèverac M, Houarner-Rassin C, Pellegrin Y, Blart E, Odobel F.
J Photochem Photobiol A Chem 2008;197:156.
[4] Singh T, Erten S, GuËnes S, Zafer C, Turkmen G, Kuban B, et al. Org Electron
2006;7:480.
[5] Hosoi Y, Tsunami D, Ishii H, Furukawa Y. Chem Phys Lett 2007;436:139.
[6] Zhang Y, Seo H-S, An M-J, Choi J-H. Org Electron 2009;10:895.
[7] Zhang X, Wu Y, Li J, Li Fan, Li M. Dyes Pigm 2008;76:810.
[8] Angadi M, Gosztola D, Wasielewski M. Mater Sci Eng B 1999;63:191.
[9] Pan J, Zhu W, Li S, Zeng W, Cao Y, Tian H. Polymer 2005;46:7658.
[10] Sakong C, Kim Y, Choi J, Yoon C, Kim J. Dyes Pigm 2011;88:166.
[11] Choi J, Sakong C, Choi J-H, Yoon C, Kim J. Dyes Pigm 2011;90:82.
[12] Daffy L, de Silva A, Gunaratne H, Huber C, Lynch P, Werner T, et al. Chem A Eur
J 1998;4:1810.
4. Conclusions
[13] Feng L, Chen Z. Sensors Actuators B 2007;26:600.
[14] Yan L, Yang L, Lan J, You J. Sci China Ser B Chem 2009;52:518.
[15] Cotlet M, Vosch T, Habuchi S, Weil T, Müllen K, Hofkens J, et al. J Am Chem Soc
2005;127:9760.
In this paper, we have given a comprehensive account of the
design and synthesis of two new PET perylene diimide sensors for
protons and transition metal ions. The novel compounds are based of
branched perylene diimide core, containing tetraester and PAMAM
wedges. Due to their long alkylester and alkilamine terminal groups
the synthesized compounds show good solubility in organic
solvents. Their photophysical properties were studied in both DMF
and water/DMF (4:1, v/v) solution. The core emission intensity of
novel systems had enhanced in the pH range from 10 to 2 e FE ¼ 6.4
for PAMAM-branched dye 5 and FE ¼ 184 for tetraester compound 4.
The determined pKa values of 3.1 for 4 and 4.1 for 5 indicate that they
would be able to act as highly efficient “offeon” switches for pH.
Novel perylene diimides 4 and 5 showed sufficient sensor selectivity
towards metal ions. In the presence of Cu2þ and Pb2þ tetraester
quenched its fluorescence intensity (FQ ¼ 22 and 12 respectively),
while PAMAM-branched dye enhanced its fluorescence intensity
with pronounced selectivity to Cu2þ and Fe3þ (FE ¼ 3.2 and 4.9,
respectively). The quenching effect of Cu2þ, Pb2þ and Ni2þ in DMF
solution for dye 4 probably is due to that the ester groups hindered
the tertiary amine and in the presence of metal ions the PET
quenching process from the amine receptor to the fluorophore core
is still there. The other reason for fluorescence quenching in
compound 4 would be the electron or energy transfer from the flu-
orophore excited state to the metal complex. These results demon-
strate the potential of the novel perylene diimides to detect metal
[16] Pan J, Zhu W, Li S, Xu J, Tian H. Eur J Org Chem 2006;4:986.
[17] Yukruk F, Dogan A, Canpinar H, Guc D, Akkaya E. Org Lett 2005;7:2885.
[18] Würthner F. Chem Commun; 2004:1564.
[19] Yang L, Shi M, Wang M, Chen H. Tetrahedron 2008;64:5404.
[20] Sun R, Xue C, Owak M, Peetz R, Jin S. Tetrahedron Lett 2007;48:6696.
[21] Mayorova J, Troshin P, Peregudov A, Peregudova S, Kaplunova M,
Lyubovskaya R. Mendeleev Commun 2007;17:156.
[22] Zhang Y, Xu Z, Cai L, Lai G, Qiu H, Shen Y. J Photochem Photobiol A Chem
2008;200:334.
[23] Gillies E, Frèchet J. DDT 2005;10:35.
[24] Wurm F, Frey H. Prog Polym Sci 2011;36:1.
[25] Qu J, Pschirer N, Liu D, Stefan A, De Schryver F, Müllen K. Chem A Eur J 2004;
10:528.
[26] Dirksen A, De Cola L. C R Chim 2003;6:873.
[27] Betley T, Hessler J, Mecke A, Holl M, Orr B, Uppuluri S, et al. Langmuir 2002;
18:3127.
[28] Ghosh S, Banthia A, Chen Z. Tetrahedron 2005;61:2889.
[29] Lee D, Kim J, Park H, Jun Y, Hwang R, Lee W-Y, et al. Synth Met 2005;150:93.
[30] Grabchev I, Chovelon J-M, Bojinov V, Ivanova G. Tetrahedron 2003;59:9591.
[31] Staneva D, McKena M, Bosch P, Grabchev I. Spectrochim Acta Part A 2010;76:
150.
[32] Bojinov V, Georgiev N, Nikolov P. J Photochem Photobiol A Chem 2008;193:
129.
[33] Georgiev N, Bojinov V, Nikolov P. Dyes Pigm 2009;81:18.
[34] Georgiev N, Bojinov V. Dyes Pigm 2010;84:249.
[35] Georgiev N, Bojinov V, Nikolov P. Dyes Pigm 2011;88:350.
[36] Georgiev N, Bojinov V, Marinova N. Sensors Actuators B Chem 2010;150:655.
[37] Mohr G. Sensors Actuators B Chem 2005;107:2.
[38] de Silva A, McCaughan B, McKinney B, Querol M. Dalton Trans; 2003:1902.
[39] Callan J, de Silva A, Magri D. Tetrahedron 2005;61:8551.
[40] Magri D, Vance T, de Silva A. Inorg Chim Acta 2007;360:751.
[41] de Silva A, Vance T, West M, Wright G. Org Biomol Chem 2008;6:2468.
[42] de Silva A, Uchiyama S. Top Curr Chem; 2010. doi:10.1007/128_2010_96.
[43] Tian H, Gan J, Chen K, He J, Song Q, Hou X. J Mater Chem 2002;12:1262.
[44] Ramachandram B. J Fluoresc 2005;15:71.
ions with pronounced selectivity towards Cu2þ, Pb2þ and Fe3þ
.
[45] Xu Y, Zheng L, Huang X, Cheng Y, Zhu C. Polymer 2010;51:994.
[46] Lu X, Zhu W, Xie Y, Li X, Gao Y, Li F, et al. Chem Eur J 2010;16:8355.
[47] Lee H, Moon B, Ko H, Choi W, Kim S- H. Pyrrole derivative and photosensitive
film using the same; 2008, Nov 4. US 7446166.
Acknowledgements
This work was supported by the National Science Foundation of
Bulgaria (project DVU-10-0195). Authors also acknowledge the
Science Foundation at the University of Chemical Technology and
Metallurgy (Sofia, Bulgaria).
[48] de Silva A, Gunaratne H, Habib-Jiwan J-L, McCoy C, Rice T, Soumillion J-P.
Angew Chem Int Ed Engl 1995;34:1728.
ꢀ
[49] Lukác I, Langhals H. Chem Ber 1983;116:3524.
[50] Pasaogullari N, Icil H, Demuth M. Dyes Pigm 2006;69:118.