Design, synthesis and pH sensing properties of novel 1,8-naphtalimide-based bichromophoric system

Article abstract of DOI:10.1016/j.jphotochem.2011.05.012

In this work we report on the design, synthesis and sensor properties of a novel bichromophoric sensor system based on 1,8-naphthalimide fluorophores. The synthesized dyad was configured as a fluorescent wavelength-shifting energy transfer chromophore. The novel donor-acceptor system contains blue emitting 4-methoxy-1,8-naphthalimide donor dye, capable of both absorbing light and efficiently transferring the energy to yellow-green emitting 4-N-methylpiperazinyl-1,8-naphthalimide acceptor. The energy-transfer efficiency in the dyad system was calculated to be more than 99%. The acceptor unit in the donor-acceptor system was also designed as a PET based sensor according to the "fluorophore-spacer-receptor" model. The fluorescence behaviour of the bichromophoric system was investigated as a function of pH. The fluorescence enhancement of the novel dyad in acidic media was more than 29 times indicating the high ability of the system to act as an efficient pH chemosensor.

Full text of DOI:10.1016/j.jphotochem.2011.05.012

Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 132–140
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Design, synthesis and pH sensing properties of novel 1,8-naphtalimide-based
bichromophoric system
Nevena V. Marinova, Nikolai I. Georgiev, Vladimir B. Bojinov∗
Department of Organic Synthesis, University of Chemical Technology and Metallurgy, 8 Kliment Ohridsky Str., 1756 Sofia, Bulgaria
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work we report on the design, synthesis and sensor properties of a novel bichromophoric sensor
system based on 1,8-naphthalimide fluorophores. The synthesized dyad was configured as a fluorescent
wavelength-shifting energy transfer chromophore. The novel donor–acceptor system contains blue emit-
ting 4-methoxy-1,8-naphthalimide donor dye, capable of both absorbing light and efficiently transferring
the energy to yellow-green emitting 4-N-methylpiperazinyl-1,8-naphthalimide acceptor. The energy-
transfer efficiency in the dyad system was calculated to be more than 99%. The acceptor unit in the
donor–acceptor system was also designed as a PET based sensor according to the “fluorophore-spacer-
receptor” model. The fluorescence behaviour of the bichromophoric system was investigated as a function
of pH. The fluorescence enhancement of the novel dyad in acidic media was more than 29 times indicating
the high ability of the system to act as an efficient pH chemosensor.
Received 18 January 2011
Received in revised form 10 May 2011
Accepted 21 May 2011
Available online 12 June 2011
Keywords:
1,8-Naphthalimide
Fluorescence
Energy transfer
pH sensor
© 2011 Elsevier B.V. All rights reserved.
Photoinduced electron transfer (PET)
Wavelength-shifting chromophore
1. Introduction
assembled molecule looses its fluorescence due to photoinduced
electron transfer (PET) from receptor to the fluorophore. Upon
Fluorescent sensors are currently of great interest due to the
increasing need of fast and reliable sensing of chemical species
in many areas of human activity. With their intensive “naked
eye detectable” fluorescent signal and high sensitivity, they allow
immediate detection of protons [1–4], anions, cations and anions
in vivo or in the environment [5,6], as well as detection of potentially
dangerous substances like alkylation agents [7], organophospho-
rous compounds [8], chemical warfare [9], etc. Development of
new sensors with improved characteristics (selectivity, sensitiv-
ity, etc.) is a challenging and necessary task in the 21st century,
marked by the technological booms’ consequences [10,11]. Dif-
ferent design strategies are being employed in the development
of fluorescent sensors and variety of sensor systems which differ
in their operation principle exist, for instance PET (photoinduced
electron transfer) based sensors, CT (charge transfer) sensor, ET
(electron transfer) sensors [12,13]. Combination of two design
models in one single molecule could lead to profitable properties,
such as improved selectivity and sensitivity, facilitated interpreta-
tion of sensor information, etc. [14].
recognition of guest, which binds to the receptor, engaging its
lone-pair electrons, the PET process is no longer possible and the
fluorescence of the system is recovered [15].
1,8-Naphthalimide fluorophores are well studied class of
luminophores possessing high photostability, excellent photo-
physical characteristics and have already found application as
signal fragment in fluorescent sensor systems for protons [16–18],
ions [19–21] and neutral molecules [22]. Previously, various PET
naphthalimide based sensor systems, containing energy donor
naphthalimides and single acceptor naphthalimide fragment have
been reported [23–25]. The multichromophoric systems along with
the excellent sensor properties, exhibit improved absorption char-
acteristics (broader absorption band) and high energy transfer
efficiency.
Considering the importance of PET systems and their grow-
ing application in
a variety of areas, we were interested
in the design, synthesis and photophysical investigation of a
novel 1,8-naphthalimide PET based pH chemosensor. The new
compound was designed as bichromophoric dyad containing 1,8-
naphthalimide blue emitting donor dye which absorbs light and
transfers its excitation energy to a yellow green-emitting 1,8-
naphthalimide acceptor fluorophore. In addition, the acceptor dye
was designed according to the “fluorophore-spacer-receptor” model,
where the 4-amino-1,8-naphthalimide moiety is the fluorophore
and the 4-methylpiperazine N-amine at C-4 is the receptor. Thus
we expect that PET process from the tertiary piperazine amine
A PET based fluorescent chemosensor involves a signal fragment
(fluorophore) and a receptor, bound through a spacer (“fluorophore-
spacer-receptor” model). When the receptor is unbound, the so
∗
Corresponding author. Tel.: +359 2 8163169.
E-mail address: vlbojin@uctm.edu (V.B. Bojinov).
1010-6030/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2011.05.012

N.V. Marinova et al. / Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 132–140
133
360 nm
360 nm
ET
ET
e
e
O
N
O
O
N
O
H
O
N
O
N
N
O
N
O
N
NH
O
O
O
O
Scheme 1.
will effectively quench the yellow-green fluorescence of the accep-
tor representing the “off” state of the system. Upon recognition
of guest, which binds to the receptor engaging its lone pair of
electrons, PET communication between the receptor and the flu-
orophore would be cut off and the fluorescence of the system will
recover (Scheme 1).
J = 0.9 Hz, naphthalimide H-5); 8.54 (m, 2H, naphthalimide H-2,
H-3); 8.05 (dd, 1H, J = 8.6 Hz, J = 7.5 Hz, naphthalimide H-6); 4.85
(m, 1H, OH); 4.14 (t, 2H, J = 6.4 Hz, CH2CH2OH); 3.64 (m, 2H,
CH2CH2OH). Elemental analysis: calculated for C14H10N2O5 (MW
286.24) C 58.74, H 3.52, N 9.79%; found C 59.02, H 3.61, N 9.64%.
This article takes on added significance given the growing
body of sensors and other optical devices which employ 1,8-
naphthalimide fluorophores. Also we expect the distinguishing
features of energy transfer wavelength-shifting chromophores that
affords simultaneous recording of two emission intensities at dif-
ferent wavelengths in the presence and absence of analyte and
provides a facile method for visualizing complex biological pro-
cesses at the molecular level to be combined with the properties of
classical PET systems.
2.3.2. Synthesis of
4-methoxy-N-(2-hydroxyethyl)-1,8-naphthalimide 3
To a solution of 4-nitro-1,8-naphthalimide 2 (1 g, 0.0035 mol)
in 4 ml of DMF a solution of 0.22 g KOH (0.004 mol) in 2 ml of
methanol was added dropwise under stirring. The resulting mix-
ture was stirred at room temperature for 2 h then poured into 10 ml
of water. The precipitate was collected by filtration, washed with
water and dried. The pure product was obtained after recrystal-
lization from acetone (0.89 g, 75%, m.p. 184–186 ◦C). FT-IR (KBr)
cm−1: 3464 (ꢀOH); 2957 (ꢀCH3); 2888 (ꢀCH2); 1693 (ꢀasN–C O);
1649 (ꢀsN–C O). 1H NMR (CDCl3-d, 250.13 MHz) ppm: 8.58 (m,
3H, naphthalimide H-7, H-5, H-2); 7.71 (dd, 1H, J = 8.4 Hz, J = 7.3 Hz,
naphthalimide H-6); 7.07 (d, 1H, J = 8.3 Hz, naphthalimide H-
3); 4.46 (m, 2H, CH2CH2OH); 4.15 (s, 3H, OCH3); 4.00 (m, 2H,
CH2CH2OH); 2.46 (m, 1H, OH). Elemental analysis: calculated for
2. Experimental
2.1. Materials
The starting 4-nitro-1,8-naphthalic anhydride
1 was pre-
C
15H13NO4 (MW 271.27) C 66.41, H 4.83, N 5.16%; found C 66.15,
pared according to the reported procedure [26]. Ethanolamine,
triphenylphosphine, tetrabromomethane, p-aminophenol and N-
methylpiperazine (Fluka, Merck), p.a. grade, were used without
purification. All solvents (Fluka, Merck) were pure or of spec-
troscopy grade.
H 4.90, N 5.05%.
2.3.3. Synthesis of
4-methoxy-N-(2-bromoethyl)-1,8-naphthalimide 4
To
a solution of 4-methoxy-1,8-naphthalimide 3 (0.35 g,
0.0013 mol) and tetrabromomethane (0.5 g, 0.0016 mol) in 5 ml of
DMF 0.4 g (0.0016 mol) of triphenylphosphine were added. The
mixture was stirred at room temperature for an hour then poured
into 7 ml of water. The solid was filtered off, washed with water
and dried (0.45 g, 74%, m.p. 190–192 ◦C). FT-IR (KBr) cm−1: 2970
(ꢀCH3); 2933 (ꢀCH2); 1696 (ꢀasN–C O); 1646 (ꢀsN–C O). 1H NMR
(CDCl3-d, 250.13 MHz) ppm: 8.60 (m, 2H, naphthalimide H-7, H-5);
8.56 (d, 1H, J = 8.3 Hz, naphthalimide H-2); 7.72 (dd, 1H, J = 8.4 Hz,
J = 7.3 Hz, naphthalimide H-6); 7.06 (d, 1H, J = 8.3 Hz, naphthalim-
ide H-3); 4.6 (t, 2H, J = 7.2 Hz, CH2CH2OH); 4.15 (s, 3H, OCH3);
3.67 (t, 2H, J = 7.2 Hz, CH2CH2Br). Elemental analysis: calculated for
C15H12BrNO3 (MW 334.16) C 53.91, H 3.62, N 4.19%; found C 54.19,
H 3.54, N 4.31%.
2.2. Methods
FT-IR spectra were recorded on a Varian Scimitar 1000 spec-
trometer. The 1H NMR spectra (chemical shifts are given as ı in
ppm) were recorded on a Bruker DRX-250 spectrometer operat-
ing 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 UV–vis absorption
spectra were recorded on a spectrophotometer Hewlett Packard
8452A. The fluorescence spectra were taken on a Scinco FS-2
spectrofluorimeter. The fluorescence quantum yields (ФF) were
measured relatively to Coumarin 6 (ФF = 0.78 in ethanol) [27] and
p-methoxybenzylidenephthalide (˚F = 0.14 in ethanol) [28].
2.3. Synthesis of blue emitting 1,8-naphthalimide donor dye 4
2.4. Synthesis of yellow-green emitting 1,8-naphthalimide
acceptor dye 6
2.3.1. Synthesis of
4-nitro-N-(2-hydroxyethyl)-1,8-naphthalimide 2
A
solution of 4-nitro-1,8-naphthalic anhydride (2.08 g,
Suspension of 4-nitro 1,8-naphthalic anhydride (5.3 g,
0.022 mol) and equimolar quantity of ethanolamine in ethanol
was heated under reflux for 2 h. The hot reaction mixture was
filtered to remove the unreacted starting anhydride and the filtrate
was cooled to room temperature. The solid that precipitates
was filtered off, washed with water and dried (5.01 g, 80%, m.p.
160–161 ◦C). FT-IR (KBr) cm−1: 3416 (ꢀOH); 2956 (ꢀаsCH2); 2883
(ꢀsCH2); 1703 (ꢀasN–C O); 1662 (ꢀsN–C O); 1521 (ꢀasNO2);
1336 (ꢀsNO2). 1H NMR (DMSO-d, 250.13 MHz) ppm: 8.66 (dd, 1H,
J = 8.7 Hz, J = 0.9 Hz, naphthalimide H-7); 8.58 (dd, 1H, J = 7.5 Hz,
0.0086 mol) and p-aminophenol (9.3 g, 0.0086 mol) in 50 ml
of glacial acetic acid was stirred under reflux for 8 h. After cooling
the precipitate was filtered off, washed with water and dried to give
1.93 g of pure 4-nitro-N-(4-hydroxyphenyl)-1,8-naphthalimide
as brown crystals (1.93 g, 76%, m.p. >260 ◦C). FT-IR (KBr) cm−1
:
3413 (ꢀOH); 1709 (ꢀasN–C O); 1661 (ꢀsN–C O); 1514 (ꢀasNO2);
1346 (ꢀsNO2). Elemental analysis: calculated for C18H10N2O5 (MW
334.28) C 64.67, H 3.02, N 8.38%; found C 64.92, H 2.95, N 8.50%.
To a solution of 0.96 g of 1,8-naphthalimide 5 (0.0029 mol)
in 20 ml of DMF 0.29 g of N-methylpiperazine (0.0029 mol) were

134
N.V. Marinova et al. / Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 132–140
e-
e-
O
N
O
O
N
O
N
N
N
N
H
HO
HO
H
hν1
hν2 (fluorescence)
hν1
hν2 (fluorescence)
fluorophore
fluorophore
spacer receptor
spacer receptor
6
6
Scheme 2.
added dropwise at room temperature. The resulting mixture was
stirred for 168 h then poured into 30 ml of water. The crude product
that precipitated was collected by filtration and dried. Recrystal-
lization from chloroform afforded 0.56 g of yellow-green emitting
naphthalimide 6 (0.56 g, 50%, m.p. >260◦ C). FT-IR (KBr) cm−1: 3316
(ꢀOH); 2807 (ꢀCH2); 1701 (ꢀasN–C O); 1655 (ꢀsN–C O), 1374
(ꢀC–N). 1H NMR (DMSO-d, 250.13 MHz) ppm: 9.63 (s, 1H, OH); 8.46
(m, 2H, naphthalimide H-7, H-5); 8.37 (d, 1H, J = 8.1 Hz, naphthal-
imide H-2); 7.78 (t, 1H, J = 7.9 Hz, naphthalimide H-6); 7.33 (d, 1H,
J = 8.2 Hz, naphthalimide H-3); 7.09 (d, 2H, J = 8.8 Hz, Ph H-2, H-6);
6.82 (d, 2H, J = 8.7 Hz, Ph H-3, H-5); 3.24 (s, 4H, 2 × ArNCH2CH2N);
2.65 (s, 4H, 2 × ArNCH2CH2N); 2.31 (s, 3H, NCH3). Elemental anal-
ysis: calculated for C23H21N3O3 (MW 387.43) C 71.30, H 5.46, N
10.85%; found C 71.56, H 5.53, N 11.01%.
the donor dye and the absorption of the acceptor dye is required.
1,8-Naphthalimide fluorophores poses high photostability and
excellent photophysical characteristics which can be easily mod-
ified depending on the substituent nature at C-4 position. 4-Alkoxy
substituted 1,8-naphthalimides have an absorption band around
360 nm and emission in the blue region where the absorption of
the 4-amino substituted 1,8-naphthalimides appears [29–34]. Thus
the combination of 4-alkoxy-1,8-naphthalimide as a donor and
4-amino-1,8-naphthalimides as an acceptor dye could provide sat-
isfactory spectral overlap and an efficient energy transfer can be
expected. Moreover, variety of such 1,8-naphthalimide-based sys-
tems with high energy transfer efficiency has already been reported
[35,36]. Additionally, the acceptor unit in the novel dyad system
was designed according to the “fluorophore-spacer-receptor” model,
where the 4-amino-1,8-naphthalimide moiety is the fluorophore
and the 4-methylpiperazine N-amine at C-4 position is the recep-
tor. Thus we expect that photoinduced electron transfer (PET) from
the tertiary amine in the piperazine substituent will effectively
quench the fluorescence of the yellow-green emitting acceptor
representing the “off” state of the system (Scheme 2). Upon recog-
nition of guest, which binds to the receptor engaging its lone pair
of electrons, PET communication between the receptor and the flu-
orophore would be cut off and the fluorescence of the system will
be recovered [37].
For both donor and acceptor dyes the starting product was
4-nitro-1,8-naphthalic anhydride 1. First, the blue emitting fluo-
rophore 3 was obtained by reaction of 1,8-naphthalic anhydride
with ethanolamine and subsequent substitution of the nitro group
at C-4 position with methoxy one. The hydroxyethyl group was
then converted to bromoethyl function upon treatment with Ph3P
and CBr4 to afford fluorophore 4 (Scheme 3).
The acceptor dye 6 was prepared by reaction of 1 with
p-aminophenol under reflux in acetic acid and subsequent intro-
duction of a spacer-receptor fragment at 1,8-naphthalimide C-4
position (Scheme 4).
2.5. Synthesis of donor–acceptor dyad 7
A mixture of 0.1 g of 1,8-napthalimide 6 (0.00025 mol), 0.086 g of
blue-emitting 1,8-napthalimide 4 (0.00025 mol), dried potassium
carbonate (0.018 g, 0.00013 mol) and TBAB (tetrabutylammonium
bromide) was heated at 100 ◦C in 6 ml of dry DMF for 20 h under PTC
(phase transfer catalysis) conditions. The solution was then poured
into 15 ml of water and the precipitate was collected by filtration.
Silica gel chromatography (chloroform:methanol = 9:1) afforded
0.025 g (15%) of dyad 7 as yellow crystals (m.p. 167–170 ◦C). 1H
NMR (CDCl3-d, 250.13 MHz) ppm: 8.74 (dd, 1H, J = 8.4 Hz, J = 1.2 Hz,
donor naphthalimide H-7); 8.69 (dd, 1H, J = 7.3 Hz, J = 1.2 Hz, donor
naphthalimide H-5); 8.65 (dd, 1H, J = 7.3 Hz, J = 1.2 Hz, acceptor
naphthalimide H-5); 8.58 (d, 1H, J = 8.1 Hz, donor naphthalimide
H-2); 8.53 (d, 1H, J = 8.3 Hz, acceptor naphthalimide H-2); 8.49
(dd, 1H, J = 8.6 Hz, J = 1.2 Hz, acceptor naphthalimide H-7); 7.82
(dd, 1H, J = 8.4 Hz, J = 7.3 Hz, donor naphthalimide H-6); 8.75 (dd,
1H, J = 8.6 Hz, J = 7.3 Hz, acceptor naphthalimide H-6); 7.43 (d, 2H,
J = 9 Hz, Ph H-2, H-6); 7.34 (d, 2H, J = 9 Hz, Ph H-3, H-5); 7.29 (d,
1H, J = 8.2 Hz, acceptor naphthalimide H-3); 7.16 (d, 1H, J = 8.2 Hz,
donor naphthalimide H-3); 4.48 (m, 2H, CH2CH2O); 3.67 (m, 2H,
CH2CH2O); 3.36 (m, 4H, ArNCH2CH2N); 2.77 (m, 4H, ArNCH2
CH2N); 2.46 (s, 3H, OCH3); 1.25 (s, 3H, NCH3). Elemental analysis:
calculated for C38H32N4O6 (MW 640.68) C 71.24, H 5.03, N 8.74%;
found C 70.93, H 5.10, N 8.61%.
The dyad finally was assembled by Williamson reaction of the
blue emitting 1,8-naphthalimide 4 and the acceptor dye 6 under
PTC conditions (Scheme 5).
The synthesized compounds were characterized and identified
by their melting points, TLC, elemental analysis data, UV–vis, fluo-
rescence, FT-IR and 1H NMR spectroscopy.
3. Results and discussion
3.2. Photophysical characterization of the dyes
3.1. Design and synthesis of bichromophoric sensor dyad
Photophysical properties of the 1,8-naphthalimide derivates are
basically related to the polarization of their chromophoric sys-
tem. Light absorption in this molecule generates a charge transfer
interaction between the substituents at C-4 position and the imide
carbonyl groups. In general, the derivatives with alkoxy groups
are colorless and show blue emission, while the amino substituted
The new compound was designed as bichromophoric dyad con-
taining 1,8-naphthalimide blue emitting donor dye which absorbs
light and transfers its excitation energy to a yellow green-emitting
1,8-naphthalimide acceptor fluorophore. In order to obtain efficient
energy transfer, maximal spectral overlap between the emission of

N.V. Marinova et al. / Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 132–140
135
OH
OH
O
Br
O
OH
O
O
O
O
N
Ph3P, Br4C
DMF, 20oC
N
N
O
MeOH, KOH
DMF, 20oC
H2N
EtOH, Reflux
O
O
O2N
O
O
O2N
2
1
3
4
Scheme 3.
OH
OH
OH
NH
O
O
O
N
O
N
N
H2N
O
O
O
DMF, 20oC
CH3COOH, Reflux
O2N
O2N
N
1
5
6
N
Scheme 4.
OH
Br
O
O
O
N
N
K2CO3 ,TBAB
DMF, 90oC
+
O
N
O
O
N
O
O
N
O
N
N
N
O
6
4
O
7
Scheme 5.
1,8-naphthalimides are yellow colored and display green fluores-
cence [29–34]. The absorption data of the examined compounds
are listed in Table 1.
Compound 4 in chloroform, methanol and DMF is colorless with
a typical for the 4-alkoxy substituted 1,8-naphthalimides maximal
absorption at about 366 nm [38,39].
Acceptor fluorophore 6 exhibit in chloroform solution atypi-
cally blue shifted for the 4-aminosubstitued 1,8-naphthalimides
absorption maximum at 388 nm. This phenomena has already
been observed for some 4-aminoethylpipezinyl substituted 1,8-
naphthalimides and it is possibly due to a repulsive interaction
between the receptor tertiary amino group in the piperazine ring
and the 4-amino moiety which destabilize the ICT excited state
nature of this compound to a larger extent, resulting in more energy
required to access the excited state [40,41]. Compound 6 also shows
typical for ICT fluorophores bathochromic shift of the longest-
wavelength band upon increasing the solvent polarity (ꢁA = 396 nm
in methanol and ꢁA = 398 nm in DMF).
Dyad 7 shows a more complex absorption spectrum which as
expected contains absorption bands corresponding to the absorp-
tions of donor and acceptor dyes 4 and 6 (Fig. 1). Therefore it can
be assumed that there is no interaction between the chromophores
in the system and the absorption properties of the fluorophores in
the dyad do not differ from single donor and acceptor fluorophores’
properties.
All of the compounds under study showed in the longest-
wavelength absorption band typical for the 1,8-naphthalimide dyes
molar extinction coefficients (ε), higher than 10,000 l mol−1 cm−1
,
indicating that this is a charge transfer (CT) band [42,43].
Table 1
Absorption data of dyad 7 and 1,8-naphthalimides 4 and 6 in different solvents.
The fluorescent characteristics of the 1,8-naphthalimides 4, 6
and dyad 7 in different solvents such as fluorescence maxima (ꢁF),
Stokes shift (ꢀA–ꢀF) and fluorescence quantum yield (ФF), are pre-
sented in Tables 2 and 3.
Compound
Chloroform
Methanol
DMF
ꢁA (nm)
ꢁA (nm)
ꢁA (nm)
ε (l mol−1 cm−1
)
Donor dye 4
Acceptor dye 6
Dyad 7
366
388
366
366
396
372
364
398
374
16,902
15,981
21,457
In solution compound 4 display blue fluorescence due to
the charge transfer from the electron-donating group at 1,8-
naphthalimide C-4 position to the carbonyl groups. Compound 6

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N.V. Marinova et al. / Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 132–140
Table 2
Fluorescence characteristics of 1,8-naphthalimides 4 (ꢁex = 360 nm) and 6 (ꢁex = 420 nm) in different solvents.
Compound
Chloroform
Methanol
DMF
ꢁF (nm)
ꢀA–ꢀF (cm−1
)
ФF
ꢁF (nm)
ꢀA–ꢀF (cm−1
)
ФF
ꢁF (nm)
ꢀA–ꢀF (cm−1
)
ФF
Donor dye 4
Acceptor dye 6
430
505
4067
5971
0.521a
0.185b
449
543
5050
6836
0.453a
0.012b
440
529
4745
6222
0.423a
0.005b
a
Quantum yields calculated using and p-methoxybenzylidenephthalide (˚F = 0.14 in ethanol).
Quantum yields calculated using Coumarin 6 (ФF = 0.78 in ethanol).
b
Fig. 1. Normalized absorption spectra of 1,8-naphthalimides 4, 6 and dyad 7 in DMF.
Fig. 2. Normalized fluorescent spectra of naphthalimide
(ꢁex = 420 nm) and dyad 7 (ꢁex = 360 nm) in DMF.
4 (ꢁex = 360 nm), 6
showed typical for the 4-piperazinyl-1,8-naphthalimides yellow-
green fluorescence [44] due to the stronger electron-donating
ability of the C-4 substituent.
The Stoke’s shift (ꢀA–ꢀF) is important parameter for the fluores-
cent 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 shift values (cm−1) were calculated
by Eq. (1).
single fluorophores 4 and 6 indicating no interaction between the
chromophores in the molecule.
The fluorescence spectra of the dyad 7 in DMF solution, obtained
after excitation within the spectral region of maximal absorption of
the donor fluorophore (ꢁex = 360 nm), showed two emission bands,
corresponding to the emission bands of donor and acceptor 1,8-
naphthalimide fragments in the donor–acceptor systems (Fig. 2).
As a result of the energy transfer, the emission intensity of
the donor (blue emitting donor 1,8-naphthalimide fragment) in
the systems was strongly decreased in comparison to the single
blue emitting fluorophore 4. This is illustrated in Fig. 3, where are
presented the recorded at the same optical density fluorescence
spectra of dyad 7 and donor dye 4, excited within the maximal
absorption of the donor (ꢁex = 360 nm).
ꢀ
ꢁ
1
ꢁA
1
ꢁF
(ꢀA − ꢀF ) =
−
× 107
(1)
The Stoke’s shift values of compound 4 are between 4067 cm−1
and 5050 cm−1 in different solvents and correspond to the results
for other 4-alkoxy-1,8-naphthalimide derivatives [29–31].
The Stoke’s shift values for acceptor 1,8-naphthalimide 6
(5971–6836 cm−1) are relatively higher but also typical for the 4-
piperazine substituted 1,8-naphthalimides [40,41]. As the dipole
moment of the molecule is enhanced upon excitation due to elec-
tron density redistribution, the excited molecule is better stabilized
in polar solvents, such as DMF and methanol, because of stronger
interaction with the solvent dipoles [40,45]. This effect causes the
red shift in the fluorescence maxima resulting in large-scale aug-
mentation of the Stoke’s shift. The Stokes shift values of the dyes
under study do not indicate remarkable changes in the geometry of
the first singlet excited state in comparison with other naphthalim-
ide fluorophores. Due to considerable absorption overlap between
the donor and acceptor units in dyad 7 it is difficult to discuss the
Stoke’s shift values of the latter. However the absorption spectrum
of 7 consists of two bands, corresponding to the absorptions of
Table 3
Fluorescence characteristics of dyad 7 in different solvents.
Chloroform
Methanol
DMF
ꢁF (nm)
ꢁF (nm)
ꢁF (nm)
ФF
Fig. 3. Normalized to the optical density fluorescence spectra of naphthalimide 4
and dyad 7 in DMF excited at 360 nm.
504
525
527
0.01

N.V. Marinova et al. / Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 132–140
137
The energy-transfer efficiency (ET) in the donor–acceptor sys-
tem was calculated to be more than 99% using Eq. (2) [46], where
FD is the normalized to the optical density fluorescence intensity of
donor model compound 4 and FDA is the normalized to the optical
density fluorescence intensity of the donor chromophore in dyad
7. Since the energy transfer efficiency is strongly dependent on dis-
tance between donor and acceptor fragments, such a high energy
transfer in dyad 7 was predictable.
ꢀ
ꢁ
FDA
FD
ET = 1 −
(2)
The ability of molecules to emit the absorbed light is charac-
terized quantitatively by the fluorescence quantum yield (ФF). The
quantum yields of fluorescence were calculated using Coumarin
6
(ФF = 0.78 in ethanol) for the yellow-green emitting 1,8-
napthalimide 6 and dyad 7 and p-methoxybenzylidenephthalide
(˚F = 0.14 in ethanol) for the blue emitting dye 4, as a standards
according to Eq. (3) [47]. Aref, Sref, nref and Asample Ssample, nsample
represent the absorbance at the excitation wavelength, the inte-
grated emission band area and the solvent refractive index of the
standard and the sample, respectively.
Fig. 4. Absorption spectra of compound 4 in water/DMF (4:1 v/v) at different pH
values.
reduced due to the protonation of the aromatic 4-amino moiety
[40].
ꢄ
ꢅ
ꢂ
ꢃꢂ
ꢃ
n2sample
n2ref
Similar to the absorption properties, the fluorescent character-
istics of blue-emitting fluorophore 4 in water/DMF (4:1 v/v) remain
unchanged in a wide pH interval. In contrast to 4 compounds 6 and 7
showed significant changes in the emission intensity at different pH
values. Family of fluorescent spectra (ꢁex = 420 nm) of compound
6 at different pH values in water/DMF (4:1 v/v) are presented in
Fig. 6. As expected in alkaline media the compound showed very
weak fluorescence which was increasing gradually upon acidifica-
tion. The increment of the fluorescent intensity together with the
lower quantum yield in polar media than in nonpolar one (Table 2)
indicates that the fluorescence of compound 6 is being quenched
by PET process from the tertiary amino group to the fluorophore.
Upon recognition of the analyte (protons) the oxidation potential
of the receptor is being increased thermodynamically disallowing
PET process and the fluorescence of the compound is recovered.
The changes of the fluorescence intensity (ꢁex = 420 nm) of com-
pound 6 as a function of pH in water/DMF (4:1 v/v) are plotted in
Fig. 7. After careful titration from pH ca. 10 to pH ca. 2 the fluores-
cence of the yellow-green emitting naphthalimide 6 had enhanced
more than 50 times (FE = 52). The results show that the fluorophore
switches between “off” and “on” state in the interval of pH ca.
8.5–6.5.
Ssample
Sref
Aref
˚
F
= ˚
(3)
ref
Asample
The quantum yield data are presented in Tables 2 and 3. As
can be seen the sensor compounds 6 and 7 have low quantum
yield compared to the traditional 4-amino-1,8-naphthalimides,
not containing amine receptor [47]. This phenomenon might
be caused by the possible photoinduced electron transfer from
the N-methylpiperazine amine receptor to the 4-amino-1,8-
naphthalimide fluorophore through the saturated piperazinyl
ring (Scheme 2). Thus the fluorescence of the 4-amino-1,8-
naphthalimide fluorophore is quenched. On the other hand the
quantum yield of methylpiperazinyl substituted naphthalimide 6
is highly solvent dependent. In polar media the quantum yield is
very low (˚F = 0.005–0.012), while in non-polar media the quan-
tum yield is considerably higher (˚F = 0.185). This fact indicates PET
process which is quenched in non-polar media causing restoration
of the fluorescence emission [36]. Upon recognition of the analyte
(protons) N-methylpiperazine amine would increase the oxidation
potential of the receptor, and as such, thermodynamically disallow
the electron transfer and the emission would be “switched on”.
Taking part of the graph between pH 4 and pH 10 the pKa value of
sensor 6 was calculated using Eq. (4) [18]. The pKa value was calcu-
3.3. Influence of pH on absorption and fluorescent properties of
the dyes
The novel compound 7 was designed as bichromophoric sensor
system for detection of pH changes over a wide interval. Therefore
it was of interest to study the photophysical behaviour of the new
compound 7 in water/DMF (4:1 v/v) solution as a function of pH.
In order to receive a more complete comparative picture for the
properties of the bichromophoric dyad compounds 4 and 6, repre-
senting respectively the single donor and acceptor fragment, were
also included in this investigation as reference compounds.
The absorption spectra of blue emitting naphthalimide 4 in
water/DMF (4:1 v/v) at different pHs are presented in Fig. 4. As
expected the compound showed no changes in the absorption max-
imum and the absorption profile indicating no proton influence
on the absorption properties. In contrast to 4, sensor 6 showed a
blue shifted absorption maxima of about 20 nm upon acidification
(Fig. 5). One of the reasons for this effect is that the protonation
of the amine receptor exerts some weak charge repulsion on the
4-amino moiety of the fluorophores. On the other hand in very
acidic conditions the push–pull character of the ICT state is partially
Fig. 5. Absorption spectra of compound 6 in water/DMF (4:1 v/v) at different pH
values.

138
N.V. Marinova et al. / Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 132–140
Fig. 6. Fluorescence spectra of 6 in water/DMF (4:1 v/v) at different pH values
(ꢁex = 420 nm).
Fig. 9. Normalized to the same optical density fluorescence intensity of dyad 7
(ꢁem = 535 nm) in water/DMF (4:1 v/v) as a function of pH.
increases the solubility of the compound in the solvent system
water/DMF.
ꢆ
IF max − IF
log
= pH − pKa
(4)
IF − IF min
The changes of the fluorescence intensity of the acceptor fluo-
rophore in dyad 7 as a function of pH was recorded in water/DMF
(4:1 v/v) and are presented in Fig. 8A and B. In alkaline media the
fluorescence of the acceptor unit is quenched due to PET from the
tertiary amine in the receptor to the fluorophore. Upon recogni-
tion of guest, which binds to the receptor engaging its lone pair
of electrons, PET communication between the receptor and the
fluorophore is cut off and the fluorescence of the system is recov-
ered (Scheme 2). The acceptor fluorescence intensity of dyad 7 in
acidic media excited by the energy transfer from the donor unit
(ꢁex = 360 nm) is app. 1.5 times higher than the fluorescence inten-
sity of the compound excited directly in the acceptor (ꢁex = 420 nm).
This fact is a result of the higher light input in the system due to
higher absorption ability of dyad 7 at 360 nm and is very well cor-
related with the difference in the optical density of compound 7 at
360 and 420 nm.
Fig. 7. Effect of pH on the fluorescence intensity (ꢁex = 420 nm, ꢁem = 535 nm) of
compound 6 in water/DMF (4:1 v/v).
The changes in the fluorescence intensity of the receptor unit in
dyad 7 in water/DMF (4:1 v/v) as a function of pH after excitation at
360 and 420 nm at the same optical density are presented in Fig. 9.
The titration curves presented in figure are very similar, showing
that pH sensing properties of the dyad are related only to the sen-
sitivity of the acceptor fluorophore, which contains PET quenching
lated to be 7.45 which is in agreement with the data of previously
reported compounds of the same nature [44]. The pKa calculated
by means of the variation of the absorption spectra as a function
of pH comes up to 7.87, suggesting absorption changes caused
by the ionization of the phenoxy group in alkaline media, which
Fig. 8. (A) Fluorescence spectra of 7 in water/DMF (4:1 v/v) at different pH values (ꢁex = 360 nm). (B) Effect of pH on the fluorescence intensity of compound 7 (ꢁem = 535 nm)
in water/DMF (4:1 v/v).

N.V. Marinova et al. / Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 132–140
139
fragment. Upon acidification the tertiary amine (receptor) is being
protonated and the yellow-green fluorescence of the acceptor flu-
orophore is increasing gradually. Moreover the titration profile of
the curve in Fig. 8B, obtained after direct excitation of the accep-
tor fragment (ꢁex = 420 nm), is a very similar to that of the curve,
registered after normalization to the same optical density (Fig. 9),
which suppose lack of absorption changes during the titration. The
acceptor’s fluorescence enhancement of dyad 7 in acidic media was
calculated to be more than 29 times, demonstrating the ability of
the novel compound to act as a highly efficient pH sensor.
Taking part of the curves plotted in Fig. 9 between pH 4 and
pH 10 the pKa value for compound 7 was calculated by Eq. (4). The
calculated pKa value (pKa = 7.12) differs from the emission pKa value
of compound 6 (pKa = 7.45). The alkaline shifted value of 6 could be
related to the presence of aromatic hydroxyl group in the molecule
resulting in better solubility of the compound in alkaline media.
[7] S. Tal, H. Salman, Y. Abraham, M. Botoshansky, Y. Eichen, Sensitive and selective
photoinduced-electron-transfer-based sensing of alkylating agents, Chem. Eur.
J. 12 (2006) 4858–4864.
[8] J.T. Dale, J.J. Rebek, Fluorescent sensors for organophosphorus nerve agent
mimics, J. Am. Chem. Soc. 128 (2006) 4500–4501.
[9] S. Zhang, T.M. Swager, Fluorescent detection of chemical warfare agents: func-
tional group specific ratiometric chemosensors, J. Am. Chem. Soc. 125 (2003)
3420–3421.
[10] J.F. Callan, A.P. de Silva, D.C. Magri, Luminescent sensors and switches in the
early 21st century, Tetrahedron 61 (2005) 8551–8588.
[11] P. Pallavicini, Y. Diaz-Fernandez, L. Pasotti, Micelles as nanosized containers for
the self-assembly of multicomponent fluorescent sensors, Coord. Chem. Rev.
253 (2009) 2226–2240.
[12] A.P. de Silva, B. McCaughan, B.O.F. McKinney, M. Querol, Newer optical-based
molecular devices from older coordination chemistry, Dalton Trans. 10 (2003)
1902–1938.
[13] A.P. de Silva, P.T. Vance, M.E.S. West, G.D. Wright, Bright molecules with sense,
logic, numeracy and utility, Org. Biomol. Chem. 6 (2008) 2468–2480.
[14] V.B. Bojinov, A.I. Venkova, N.I. Georgiev, Synthesis and energy-transfer
properties of fluorescence sensing bichromophoric system based on Rho-
damine6G and 1,8-naphthalimide, Sens. Actuators B: Chem. 143 (2009)
42–49.
[15] B. Valeur, I. Leray, Design principles of fluorescent molecular sensors for cation
recognition, Coord. Chem. Rev. 205 (2000) 3–40.
[16] S. Trupp, P. Hoffmann, T. Henkel, G. Mohr, Novel pH indicator dyes for array
preparation via NHS ester activation or solid-phase organic synthesis, Org.
Biomol. Chem. 6 (2008) 4319–4322.
[17] Y. Hu, B. Wang, Z. Su, Synthesis and properties of a new green water-soluble
polymer for proton and metal–cation sensing, J. Appl. Polym. Sci. 111 (2009)
1931–1935.
4. Conclusion
A
novel fluorescent donor–acceptor system with PET
sensing properties, containing blue emitting 4-methoxy-
1,8-naphthalimide donor and yellow-green emitting
4-N-methylpiperazinyl-1,8-naphthalimide acceptor, was synthe-
sized. The novel compound was designed as a wavelength-shifting
energy transfer chromophore capable of absorbing light by its
donor chromophore and efficiently transferring the energy to the
acceptor chromophore of the system. The synthesized dyad system
showed highly efficient energy transfer (more than 99%) suggesting
that the selected 1,8-naphthalimides are suitable donor–acceptor
pair for energy transfer systems. The novel compound was also
designed according to the “fluorophore-spacer-receptor” model
and as such it would be able to act as a fluorescence PET based
chemosensor. The fluorescent enhancement of the dyad sys-
tem in acidic media is more than 29 times, demonstrating the
high potential of the compound to act as an effective “off–on”
switch for pH. Thus the distinguishing features of energy transfer
wavelength-shifting chromophores were successfully combined
with the properties of classical PET systems.
[18] A.P. de Silva, H. Gunaratne, J.-L. Habib-Jiwan, C. McCoy, T. Rice, J.-P. Soumillion,
New fluorescent model compounds for the study of photoinduced electron
transfer: the influence of a molecular electric field in the excited state, Angew.
Chem. Int. Ed. 34 (1995) 1728–1731.
[19] N. Singh, N. Kaur, B. McCaughan, J.F. Callan, Ratiometric fluorescent detection
of Cu(II) in semi-aqueous solution using a two-fluorophore approach, Tetrahe-
dron Lett. 51 (2010) 3385–3887.
[20] B. Ramachandram, Fluorescence sensor design for transition metal ions: the
role of the PIET interaction efficiency, J. Fluoresc. 15 (2005) 71–83.
[21] B. Ramachandram, N.B. Sankaran, R. Karmakar, S. Saha, A. Samanta, Flu-
orescence signalling of transition metal ions by multi-component systems
comprising 4-chloro-1,8-naphthalimide as fluorophore, Tetrahedron 56 (2000)
7041–7044.
[22] Y. Li, C. Li, F. Xu, Y. Zhou, Q. Xiao, A fluorescent chemosensor for cysteine based
on naphthalimide derivative in aqueous solution, Sens. Actuators B: Chem.
(2011), doi:10.1016/j.snb.2010.12.011.
[23] V.B. Bojinov, N.I. Georgiev, P.S. Nikolov, Design and synthesis of core and
peripherally functionalized with 1,8-naphthalimide units fluorescent PAMAM
dendron as light harvesting antenna, J. Photochem. Photobiol. A: Chem. 197
(2008) 281–289.
[24] N.I. Georgiev, V.B. Bojinov, The design and synthesis of
a novel 1,8-
naphthalimide PAMAM light-harvesting dendron with fluorescence “off–on”
switching core, Dyes Pigments 84 (2010) 249–256.
[25] N.I. Georgiev, V.B. Bojinov, Design, synthesis and photostability of novel
1,8-naphthalimide PAMAM light-harvesting dendrons, J. Fluoresc. (2010),
doi:10.1007/s10895-010-0689-y.
[26] K.K. Dubey, R.K. Singh, K. Misra, A novel bifunctional fluorescent tag for use in
molecular biology, Indian J. Chem. 34B (1995) 876–878.
[27] G. Reynolds, K. Drexhage, New coumarin dyes with rigidized structure for
flashlamp-pumped dye lasers, Opt. Commun. 13 (1975) 222–225.
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).
[28] P. Nikolov, F. Fratov, S. Minchev,
A new class of organic luminophores
with a stilbene chromophore: 3-phenylmethylene-1(3H)-Isobenzofuranones,
Z. Naturforsch. 38a (1983) 200–205.
References
[29] V. Bojinov, D. Simeonov, N. Georgiev, A novel blue fluorescent 4-(1,2,2,6,6-
pentamethylpiperidin-4-yloxy)-1,8-naphthalimide pH chemosensor based on
photoinduced electron transfer, Dyes Pigments 76 (2008) 41–46.
[30] B. Ramachandram, G. Saroja, N.B. Sankaran, A. Samanta, Unusually high flu-
orescence enhancement of some 1,8-naphthalimide derivatives induced by
transition metal salts, J. Phys. Chem. B 104 (2000) 11824–11832.
[31] N.I. Georgiev, V.B. Bojinov, P.S. Nikolov, Design and synthesis of a novel pH
sensitive core and peripherally 1,8-naphthalimide-labeled PAMAM dendron
as light harvesting antenna, Dyes Pigments 81 (2009) 18–26.
[32] T. Gunnlaugsson, C.P. McCoy, R.J. Morrow, C. Phelan, F. Stomeo, Towards the
development of controllable and reversible “on–off’ luminescence switching in
soft-matter; synthesis and spectroscopic investigation of 1,8-naphthalimide-
based PET (photoinduced electron transfer) chemosensors for pH in water-
permeable hydrogels, ARKIVOC 7 (2003) 216–228.
[33] A.P. de Silva, A. Goligher, H.Q.N. Gunaratne, T.E. Rice, The pH-dependent flu-
orescence of pyridylmethyl-4-amino-1,8-naphthalimides, ARKIVOC 7 (2003)
229–243.
[34] D. Cui, X. Qian, F. Liu, R. Zhang, Novel fluorescent pH sensors based on
intramolecular hydrogen bonding ability of naphthalimide, Org. Lett. 16 (2004)
2757–2760.
[1] L.M. Daffy, A.P. de Silva, H.Q.N. Gunaratne, C. Huber, P.L.M. Lynch, T. Werner, O.S.
Wolfbeis, Arenedicarboximide building blocks for fluorescent photoinduced
electron transfer pH sensors applicable with different media and communica-
tion wavelengths, Chem. Eur. J. 4 (1998) 1810–1815.
[2] Y. Diaz-Fernandez, E. Mottini, L. Pasotti, E.F. Craparo, G. Giammona, G. Cavallaro,
P. Pallavicini, Multicomponent polymeric micelles based on polyaspartamide
as tunable fluorescent pH-window biosensors, Biosens. Bioelectron. 26 (2010)
29–35.
[3] F. Denat, Y. Diaz-Fernandez, L. Pasotti, N. Sok, A. Micellar, P. Pallavicini, A micel-
lar multitasking device: sensing pH windows and gauging the lipophilicity of
drugs with fluorescent signals, Chem. Eur. J. 16 (2010) 1289–1295.
[4] Y. Diaz-Fernandez, F. Foti, C. Mangano, P. Pallavicini, S. Patroni, A. Perez-
Gramatges, S. Rodriguez-Calvo, Micelles for the self-assembly of “off–on–off”
fluorescent sensors for pH windows, Chem. Eur. J. 12 (2006) 921–930.
[5] R. Parkesh, T.C. Lee, T. Gunnlaugsson, Highly selective 4-amino-1,8-
naphthalimide based fluorescent photoinduced electron transfer (PET)
chemosensors for Zn(II) under physiological pH conditions, Org. Biomol. Chem.
5 (2007) 310–317.
[35] M. McKenna, I. Grabchev, P. Bosch, The synthesis of a novel 1,8-naphthalimide
based PAMAM-type dendron and its potential for light-harvesting, Dyes Pig-
ments 81 (2009) 180–186.
[6] R.M. Duke, T. Gunnlaugsson, Selective fluorescent PET sensing of fluoride
(F−) using naphthalimide-thiourea and -urea conjugates, Tetrahedron Lett. 48
(2007) 8043–8047.

140
N.V. Marinova et al. / Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 132–140
[36] N.I. Georgiev, V.B. Bojinov, N.V. Marinova, Novel PAMAM light-harvesting
antennae based on 1,8-naphthalimide: synthesis, energy transfer, photophys-
ical and pH sensing properties, Sens. Actuators B: Chem. 150 (2010) 655–666.
[37] H. Tian, T. Xu, Y. Zhao, K. Chen, Two-path photo-induced electron transfer in
naphthalimide-based model compound, J. Chem. Soc. Perkin Trans. 2 (1999)
545–550.
[38] V.B. Bojinov, I.P. Panova, J.-M. Chovelon, Novel blue emitting tetra-and
pentamethylpiperidin-4-yloxy-1,8-naphthalimides as photoinduced electron
transfer based sensors for transition metal ions and protons, Sens. Actuators B:
Chem. 135 (2008) 172–180.
[42] I. Grabchev, P. Bosch, M. McKenna, D. Staneva, A new colorimetric and fluori-
metric sensor for metal cations based on poly(propilene amine) dendrimer
modified with 1,8-naphthalimide, J. Photochem. Photobiol. A: Chem. 201
(2009) 75–80.
[43] I. Grabchev, I. Moneva, V. Bojinov, S. Guittonneau, Synthesis and properties of
fluorescent 1,8-naphthalimide dyes for application in liquid crystal displays, J.
Mater. Chem. 10 (2000) 1291–1296.
[44] V.B. Bojinov, N.I. Georgiev, P. Bosh, Design and synthesis of highly photostable
yellow–green emitting 1,8-naphthalimides as fluorescent sensors for metal
cations and protons, J. Fluoresc. 19 (2009) 127–139.
[39] Y. Wang, X. Zhang, B. Han, J. Peng, S. Hou, Y. Huang, H. Sun, M. Xie, Z. Lu, The
synthesis and photoluminescence characteristics of novel blue light-emitting
naphthalimide derivatives, Dyes Pigments 86 (2010) 190–196.
[40] V.B. Bojinov, N.I. Georgiev, N.V. Marinova, Design and synthesis of highly
photostable fluorescence sensing 1,8-naphthalimide-based dyes containing s-
triazine UV absorber and HALS units, Sens. Actuators B: Chem. 148 (2010)
6–16.
[45] V. Bojinov, I. Panova, D. Simeonov, N. Georgiev, Synthesis and sensor activ-
ity of photostable blue emitting 1,8-naphthalimides containing s-triazine UV
absorber and HALS fragments, J. Photochem. Photobiol. A: Chem. 210 (2010)
89–99.
[46] W.-S. Li, M.-J. Teng, X.-R. Jia, B.-B. Wang, J.-M. Yeh, Synthesis and energy-
transfer properties of poly(amidoamine) dendrons modified with naphthyl and
dansyl groups, Tetrahedron Lett. 49 (2008) 1988–1992.
[41] J. Gan, K. Chen, C.-P. Chang, H. Tian, Luminescent properties and photo-induced
electron transfer of naphthalimides with piperazine substituent, Dyes Pig-
ments 57 (2003) 21–28.
[47] V. Bojinov, N. Georgiev, P. Nikolov, Synthesis and photophysical prop-
erties of fluorescence sensing ester- and amidoamine-functionalized 1,8-
naphthalimides, J. Photochem. Photobiol. A: Chem. 193 (2008) 129–138.