Synthesis and sensor activity of photostable blue emitting 1,8-naphthalimides containing s-triazine UV absorber and HALS fragments

Article abstract of DOI:10.1016/j.jphotochem.2010.01.015

Two novel highly photostable blue emitting 1,8-naphthalimides, containing active fragments of both a hindered amine radical scavenger (HALS) and an s-triazine UV absorber were synthesized for the first time. They showed substantially higher photostability in respect to other similar fluorescent brighteners, not containing either UV absorber or HALS component in their molecules. Novel compounds were also configured as fluorophore-spacer-receptor systems based on photoinduced electron transfer by incorporation a cation receptor at the C-4 position of the 1,8-naphthalimide fluorophore. The ability of the new compounds to detect cations was evaluated by the changes in their fluorescence intensity in the presence of metal ions and protons. The presence of metal ions and protons was found to disallow a photoinduced electron transfer leading to an enhancement in the 1,8-naphthalimide fluorescence intensity. The results obtained indicate the potential of the novel compounds as highly photostable and efficient off-on pH switchers and fluorescent detectors for metal ions with pronounced selectivity towards Zn²⁺ and Cu²⁺ ions.

Full text of DOI:10.1016/j.jphotochem.2010.01.015

Journal of Photochemistry and Photobiology A: Chemistry 210 (2010) 89–99
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Synthesis and sensor activity of photostable blue emitting 1,8-naphthalimides
containing s-triazine UV absorber and HALS fragments
Vladimir B. Bojinova,∗, Ionka P. Panovaa, Danail B. Simeonovb, Nikolai I. Georgieva
a Department of Organic Synthesis, University of Chemical Technology and Metallurgy, 8 Kliment Ohridsky Str., 1756 Sofia, Bulgaria
b Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel highly photostable blue emitting 1,8-naphthalimides, containing active fragments of both a
hindered amine radical scavenger (HALS) and an s-triazine UV absorber were synthesized for the first
time. They showed substantially higher photostability in respect to other similar fluorescent brighten-
ers, not containing either UV absorber or HALS component in their molecules. Novel compounds were
also configured as “fluorophore–spacer–receptor” systems based on photoinduced electron transfer by
incorporation a cation receptor at the C-4 position of the 1,8-naphthalimide fluorophore. The ability of
the new compounds to detect cations was evaluated by the changes in their fluorescence intensity in the
presence of metal ions and protons. The presence of metal ions and protons was found to disallow a pho-
toinduced electron transfer leading to an enhancement in the 1,8-naphthalimide fluorescence intensity.
The results obtained indicate the potential of the novel compounds as highly photostable and efficient
“off–on” pH switchers and fluorescent detectors for metal ions with pronounced selectivity towards Zn2+
and Cu2+ ions.
Received 7 September 2009
Received in revised form 22 January 2010
Accepted 27 January 2010
Available online 2 February 2010
Keywords:
1,8-Naphthalimide
Fluorescent brightener
Hindered amine light stabilizer (HALS)
s-Triazine UV absorber
Sensors
Photoinduced electron transfer (PET)
Phase transfer catalysis
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
switchers [8,9]. The components are selected so that PET from
the receptor to the fluorophore quenches the fluorescence of the
During the last years, much attention has been paid to the
design, synthesis and characterization of photochemical molecular
devices. The extension of the concept of a device to the molecu-
lar level is of interest, not only for basic research, but also for the
growth of nanoscience and the development of nanotechnology
[1]. Supramolecular devices that show large changes in their so-
called “off” and “on” states are currently of great interest as these
can be modulated, or tuned, by employing external sources such
as ions, molecules, light, etc. [1–3]. The “off” and “on” states of the
molecular-level devices refer to their luminescence, magnetic or
electronic properties. Luminescence is one of the most useful tech-
niques to monitor the operation of molecular devices. A part of this
rapidly emerging field is the development of fluorescent sensors
where the fluorescence is switched “off” or “on” as a function of
the analyte [4–7]. They can be designed according to a few prin-
ciples with emphasis on the mechanism of photoinduced electron
transfer (PET).
system. However, in the presence of a guest, which binds to the
receptor engaging its lone pair of electrons, PET communication
between the receptor and the fluorophore gets cut off and the
fluorescence of the system is recovered. In other words, the
presence of a guest is signaled by fluorescence enhancement of
the system [10–12].
Naphthalimide derivatives are a special class of environmentally
sensitive fluorophores [13–15]. Because of their strong fluores-
cence and good photostability, the 1,8-naphthalimide derivatives
enjoy application in a number of areas including coloration and
brightening of polymers [16–22], laser active media [23,24], poten-
tial photosensitive biologically units [25], fluorescent markers in
biology [26], light emitting diodes [27,28], fluorescence sensors and
switchers [29–36], electroluminescent materials [37–39], liquid
crystal displays [40] and ion probes [41]. Moreover, these prop-
erties are essential when employing such devices in real-time and
on-line analysis.
The photoinduced electron transfer (PET) using the
“fluorophore–spacer–receptor” format is the most commonly
exploited approach for the design of fluorescent sensors and
In the last years, a number of papers devoted to prob-
lem of synthesis of combined stabilizers containing fragments
able to act according to different stabilizing mechanisms have
been published. Thus, the hindered amine fragments have
been combined with either 2-hydroxybenzophenone [42,43] or
2-hydroxyphenylbenzotriazole UV absorbers [19,44–50]. A signif-
icant synergistic stabilizing effect against photodegradation has
∗
Corresponding author. Tel.: +359 2 8163206.
E-mail address: vlbojin@uctm.edu (V.B. Bojinov).
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.01.015

90
V.B. Bojinov et al. / Journal of Photochemistry and Photobiology A: Chemistry 210 (2010) 89–99
1,2,2,6,6-pentamethylpiperidin-4-ol 10 [45] were synthe-
sized and purified according to the procedures described
before. 2,2,6,6-Tetramethylpiperidin-4-ol 9, cyanuric chlo-
ride 11, 2,2,6,6-tetramethylpiperidine 12, allyl alcohol 14,
4-bromo-1,8-naphthalic anhydride 15, 3-amonophenol 16,
4-amino-2,2,6,6-tetramethylpiperidine 17, 18-crown-6 and alu-
minium (III) chloride (Aldrich, Merck), p.a. grade, was used without
purification. All solvents (Merck, Fluka) were of p.a. or analytical
grade. Zn(NO3)2, Cu(NO3)2, Ni(NO3)2, Co(NO3)2 and Pb(NO3)2
salts were the sources for metal cations. To adjust the pH, very
small volumes of sulphuric acid and sodium hydroxide were used.
The effect of the metal cations and protons upon the fluorescence
intensity was examined by adding 5 ␮l portions of the metal
cations stock solution (8.336 × 10−7 mol l−1) to a known volume
of the fluorophore solution (3 ml). The addition was limited to
0.045 ml so that dilution remains insignificant.
2.2. Methods
FT-IR spectra were recorded on a Bruker IFS-113 spectrome-
ter at 2 cm−1 resolution using KBr discs. The NMR spectra were
recorded on a Bruker DRX-250 spectrometer, operating at 250.13
and 62.90 MHz for 1H and 13C, respectively, using a dual 5 mm
probe head. The measurements were carried out in DMSO-d6 solu-
tion at ambient temperature. The chemical shifts (given as ı in
ppm) were referenced to tetramethylsilane (TMS) standard. Exper-
iments with 30◦ pulses, 1 s relaxation delays, 16K time domain
points, zero-filled to 64K for protons and 32K for carbons were
performed. The distortionless enhancement by polarization trans-
fer (DEPT) spectra were recorded under the conditions used for
Scheme 1.
the 13C NMR spectra at ꢀ = (21JCH −1 = 3.45 ␮s. UV/vis spectra were
)
been determined [47]. Recently we have synthesized new poly-
merizable yellow-green emitting 1,8-naphthalimides, containing
built-in s-triazine UV absorber and a hindered amine light stabi-
lizer (HALS) fragments, designed as highly photostable additives for
“one-step” fluorescent dyeing and photostabilization of polymers
[51,52].
These results encouraged our efforts towards the design
and synthesis of novel highly photostable blue emitting 1,8-
naphthalimide fluorescence sensors. Hence, fluorescent brighten-
ers 1 and 2 (Scheme 1), containing at the 1,8-naphthalimide C-4
position a secondary (2,2,6,6-tetramethylpiperidine) or a tertiary
(1,2,2,6,6-pentamethylpiperidine) amine receptor, respectively,
were synthesized and investigated by electronic absorption and
emission spectroscopy as potential PET sensors for protons and
transition metal ions. This paper takes on added significance given
the growing body of sensors and other optical devices which
employ 1,8-naphthalimide fluorophores.
It was also of interest to investigate photostability of the flu-
orescent sensors 7 and 8. In order to receive a more complete
comparative picture for the influence of both 2-hydroxyphenyl-
s-triazine and 2,2,6,6-tetramethylpiperidine fragments on the
properties of the examined compounds, previously synthesized
1,8-naphthalimide 4, not containing a UV absorber moiety [17,30],
as well as newly synthesized 1,8-naphthalimides 3 and 5, not
containing a hindered amine moiety (3) or not containing both
stabilizer fragments (5), were involved in the present study as ref-
erence compounds (Scheme 2).
2. Experimental
2.1. Materials
The reference 4-allyloxy-N-(2,2,6,6-tetramethylpiperidin-4-
yl)-1,8-naphthalimide 4 [16,17], starting 2-allyloxy-4-(2,2,6,6-
tetramethylpiperidin-1-yl)-6-chloro-1,3,5-triazine
7
[44] and
Scheme 2.

V.B. Bojinov et al. / Journal of Photochemistry and Photobiology A: Chemistry 210 (2010) 89–99
91
recorded on a Hewlett Packard 8452A spectrophotometer with
2 nm resolution at room temperature. The corrected excitation and
fluorescence spectra were taken on a PerkinElmer LS 55 fluores-
cence spectrophotometer. The fluorescence quantum yields (ФF)
were measured relatively to diphenylanthracene (Фref = 0.90) [53].
TLC was performed on silica gel, Fluka F60 254, 20 × 20, 0.2 mm,
using as eluant the solvent systems chloroform/methanol = (9:1).
The melting points were determined by means of a Kofler melting
point microscope.
7.63 (d, 1H, J = 8.0 Hz, phenyl 5-H); 7.59 (dd, 1H, J = 8.4 Hz, J = 7.2 Hz,
naphthalimide 6-H); 7.25 (m, 2H, phenyl 2-H and 6-H); 7.03 (d,
1H, J = 8.1 Hz, naphthalimide 2-H); 6.11 (m, 1H, allyl CH ); 5.30
(d, 1H, Jtrans = 17.1 Hz, allyl HCH ); 5.22 (d, 1H, Jcis = 10.4 Hz, allyl
HCH ); 5.06 (m, 1H, piperidine CH); 4.88 (d, 2H, J = 4.8 Hz, allyl
OCH2); 2.92 (br.s, 1H, piperidine NH); 2.34 (dd, 2H, J = 11.9 Hz,
J = 3.8 Hz, piperidine CH2); 1.82 (m, 6H, piperidine 3 × CH2); 1.66 (t,
2H, J = 11.8 Hz, piperidine CH2); 1.46 (s, 12H, piperidine 4 × CH3);
1.29 (s, 6H, piperidine 2 × CH3); 1.13 (s, 6H, piperidine 2 × CH3).
13C NMR (62.90 MHz, DMSO-d6) ı (ppm): 171.8 (triazine Ar–C N),
165.2 (triazine N C–O), 161.8 (triazine N C–N), 159.1 (Ar–C–OH),
157.9 (2 × C O), 154.9 (Ar–C–O), 142.2 (Ar–C–N–C O), 136.8 and
124.6 (Ar–C), 133.8 (allyl CH ), 131.7, 129.9, 129.1, 122.8, 118.7,
114.9, 106.6 and 97.6 (Ar–CH), 118.8 (Ar–C–C N), 114.7 (allyl
CH2 ), 111.5 (Ar 2 × C–C O), 74.3 (allyl OCH2), 64.2 (piperidine
C–O), 53.8 (piperidine 2 × CH2), 51.9 (piperidine 2 × N–C–Me),
47.5 (piperidine 2 × NH–C–Me), 39.3 (piperidine 2 × CH2), 31.1 and
29.6 (piperidine 4 × CH3), 28.2 and 26.7 (piperidine 4 × CH3), 16.6
(piperidine CH2). Elemental analysis: Calculated for C42H50N6O5
(MW 718.88) C 70.17, H 7.01, N 11.69%; Found C 69.82, H 6.93, N
11.81%.
2.3. Synthesis
2.3.1. Synthesis of intermediate (8)
To
a
solution
of
3.11 g
of
2-allyloxy-4-(2,2,6,6-
(10 mmol)
tetramethylpiperidin-1-yl)-6-chloro-1,3,5-triazine
7
and 1.34 g of AlCl3 (10 mmol) in 100 ml of xylene, 3.68 g of 4-bromo-
N-(3-hydroxyphenyl)-1,8-naphthalimide 6 (10 mmol) were added
at room temperature. The resulting mixture was heated slowly
to 60 ◦C and stirred at this temperature for 3 h. Then the reaction
mixture was heated slowly to 90 ◦C, held at this temperature for
5 h and finally, the mixture was stirred under reflux for 8 h. After
cooling the resulting solution was poured into 150 g of ice. Organic
phase (xylene) was removed and the water phase was acidified
to pH 2 with 10% hydrochloric acid. The solid was collected by
filtration, washed with fresh water and dried. Silica gel chro-
matography (chloroform/methanol = 9:1) afforded 3.92 g (61%) of
4-bromo-N-{4-[4-allyloxy-6-(2,2,6,6-tetramethylpiperidin-1-yl)-
4-(1,2,2,6,6-Pentamethylpiperidin-4-yloxy)-N-{4-[4-allyloxy-
6-(2,2,6,6-tetramethylpiperidin-1-yl)-1,3,5-triazin-2-yl]-3-
hydroxyphenyl}-1,8-naphthalimide (2). FT-IR (KBr), cm−1: 3082
(ꢁOH· · ·N); 3068 (ꢁCH ); 2920 and 2878 (ꢁCH3); 1700 (ꢁasC O);
1664 (ꢁsC O); 1598 (ꢁC C); 1334 (ꢁN–C–N). 1H NMR (250.13 MHz,
DMSO-d6) ı (ppm): 10.13 (s, 1H, OH); 8.66 (dd, 1H, J = 8.5 Hz,
J = 1.2 Hz, naphthalimide 5-H); 8.45 (dd, 1H, J = 7.4 Hz, J = 1.2 Hz,
naphthalimide 7-H); 8.19 (d, 1H, J = 8.3 Hz, naphthalimide 2-H);
7.61 (dd, 1H, J = 8.5 Hz, J = 7.4 Hz, naphthalimide 6-H); 7.49 (d, 1H,
J = 7.9 Hz, phenyl 5-H); 7.21 (m, 2H, phenyl 2-H and 6-H); 7.05 (d,
1H, J = 8.3 Hz, naphthalimide 2-H); 6.10 (m, 1H, allyl CH ); 5.32
(d, 1H, Jtrans = 17.2 Hz, allyl HCH ); 5.23 (d, 1H, Jcis = 10.1 Hz, allyl
HCH ); 5.13 (m, 1H, piperidine CH); 4.92 (d, 2H, J = 5.2 Hz, allyl
OCH2); 2.46 (dd, 2H, J = 12.0 Hz, J = 3.6 Hz, piperidine CH2); 2.34 (s,
3H, piperidine N–CH3); 1.79 (m, 6H, piperidine 3 × CH2); 1.66 (t,
2H, J = 11.8 Hz, piperidine CH2); 1.48 (s, 12H, piperidine 4 × CH3);
1.31 (s, 6H, piperidine 2 × CH3); 1.14 (s, 6H, piperidine 2 × CH3).
13C NMR (62.90 MHz, DMSO-d6) ı (ppm): 172.1 (triazine Ar–C N),
164.9 (triazine N C–O), 162.2 (triazine N C–N), 158.7 (Ar–C–OH),
157.6 (2 × C O), 155.3 (Ar–C–O), 141.8 (Ar–C–N–C O), 137.1 and
125.4 (Ar–C), 133.3 (allyl CH ), 132.4, 130.0, 129.6, 123.5, 117.8,
115.3, 107.4 and 98.2 (Ar–CH), 119.1 (Ar–C–C N), 114.2 (allyl
CH2 ), 110.9 (Ar 2 × C–C O), 73.8 (allyl OCH2), 64.8 (piperidine
C–O), 54.3 (piperidine 2 × CH2), 48.5 (piperidine 2 × N–C–Me), 44.6
(piperidine NMe), 42.8 (piperidine 2 × NH–C–Me), 40.2 (piperi-
dine 2 × CH2), 35.4 and 32.9 (piperidine 4 × CH3), 29.8 and 28.5
(piperidine 4 × CH3), 15.8 (piperidine CH2). Elemental analysis:
Calculated for C43H52N6O5 (MW 732.91) C 70.47, H 7.15, N 11.47%;
Found C 70.80, H 7.24, N 11.59%.
1,3,5-triazin-2-yl]-3-hydroxyphenyl}-1,8-naphthalimide
8 as a
pale brown solid. FT-IR (KBr), cm−1: 3080 (ꢁOH· · ·N); 3066 (ꢁCH );
1704 (ꢁasC O); 1670 (ꢁsC O); 1592 (ꢁC C); 1338 (ꢁN–C–N). 1H
NMR (250.13 MHz, DMSO-d6) ı (ppm): 10.23 (s, 1H, OH); 8.78
(dd, 1H, J = 8.5 Hz, J = 1.2 Hz, naphthalimide 5-H); 8.72 (dd, 1H,
J = 7.3 Hz, J = 1.2 Hz, naphthalimide 7-H); 8.63 (d, 1H, J = 8.1 Hz,
naphthalimide 3-H); 8.44 (d, 1H, J = 8.1 Hz, naphthalimide 2-H);
7.82 (dd, 1H, J = 8.5 Hz, J = 7.3 Hz, naphthalimide 6-H); 7.42 (d, 1H,
J = 7.9 Hz, phenyl 5-H); 7.28 (m, 2H, phenyl 2-H and 6-H); 6.12 (m,
1H, allyl CH ); 5.33 (d, 1H, Jtrans = 17.2 Hz, allyl HCH ); 5.21 (d,
1H, Jcis = 10.2 Hz, allyl HCH ); 4.90 (s, 2H, allyl OCH2); 1.83 (m, 6H,
piperidine 3 × CH2); 1.46 (s, 12H, piperidine 4 × CH3). Elemental
analysis: Calculated for C33H32BrN5O4 (MW 642.54) C 61.69, H
5.02, N 10.90%; Found C 62.01, H 4.93, N 11.03%.
2.3.2. General preparation procedure for fluorescent
1,8-naphthalimides (1) and (2)
To a mixture of 0.94 g of 2,2,6,6-tetramethylpiperidin-4-ol
9 (6 mmol) or 1.03 g of 1,2,2,6,6-pentamethylpiperidin-4-ol 10
(6 mmol) in 15 ml of DMF, 0.83 g of finely ground potassium car-
bonate (6 mmol) and 0.08 g of 18-crown-6 (0.3 mmol, 5 mol%), a
solution of intermediate 8 (3.86 g, 6 mmol) in 15 ml of DMF was
added at room temperature. The resulting mixture was vigorously
stirred and heated to 80 ◦C for 4 h, then cooled to room temperature
and the solution was poured into 300 ml of water. The precipitate
was filtered off and washed with water. The crude product was dis-
solved in a mixture solvent of minim water and ethanol (100 ml),
and the undissolved residue was filtered off and dried. The latter
then was extracted with chloroform to give after evaporation of
the solvent 2.85 g (66%) of 1,8-naphthalimide 1 or 3.12 g (71%) of
1,8-naphthalimide 2 as pale yellow solid.
2.3.3. Synthesis of reference 1,8-naphthalimides (3) and (5)
A suspension of 6.93 g of 4-bromo-1,8-naphthalic anhydride 15
(25 mmol) and 2.73 g of 3-aminophenol 16 (25 mmol) in 180 ml of
glacial acetic acid was stirred at 110 ◦C for 8 h. The crude prod-
uct that precipitated on cooling was filtered off, washed with
water and treated with 50 ml of 5% aqueous sodium carbon-
ate. The solid phase was filtered off, washed with water and
dried. Re-crystallization from acetic acid afforded 7.64 g (83%)
of 4-bromo-N-(3-hydroxyphenyl)-1,8-naphthalimide 6 as pale
yellow-brown solid.
To a mixture of 0.70 g of allyl alcohol 14 (d = 0.85, 12 mmol),
0.16 g of 18-crown-6 (0.6 mmol, 5 mol% to the allyl alcohol) and
0.67 g of finely ground potassium hydroxide (12 mmol) in 30 ml of
toluene a suspension of 1,8-naphthalimide 6 (3.68 g, 10 mmol) in
120 ml of toluene was added at room temperature. The resulting
4-(2,2,6,6-Tetramethylpiperidin-4-yloxy)-N-{4-[4-allyloxy-6-(2,
2,6,6-tetramethylpiperidin-1-yl)-1,3,5-triazin-2-yl]-3-
hydroxyphenyl}-1,8-naphthalimide (1). FT-IR (KBr), cm−1: 3378
(ꢁNH); 3078 (ꢁOH· · ·N); 3064 (ꢁCH ); 2918 (ꢁCH3); 1694 (ꢁasC O);
1656 (ꢁsC O); 1596 (ꢁC C); 1332 (ꢁN–C–N). 1H NMR (250.13 MHz,
DMSO-d6) ı (ppm): 10.08 (s, 1H, OH); 8.67 (dd, 1H, J = 8.4 Hz,
J = 1.1 Hz, naphthalimide 5-H); 8.44 (dd, 1H, J = 7.2 Hz, J = 1.2 Hz,
naphthalimide 7-H); 8.16 (d, 1H, J = 8.1 Hz, naphthalimide 2-H);

92
V.B. Bojinov et al. / Journal of Photochemistry and Photobiology A: Chemistry 210 (2010) 89–99
mixture was vigorously stirred and heated to 90 ◦C for 3 h, then
cooling to room temperature. The solid phase was filtered off, and
the toluene solution was washed with water and dried over anhy-
drous sodium sulphate. After evaporation of the toluene under
reduced pressure the crude product was dissolved in a mixture
solvent of minim water and ethanol (100 ml), and the undissolved
residue was filtered off. The filtrate then was diluted in 100 ml of
water and the precipitated product was filtered off and dried. Re-
crystallization from ethanol–water (50:50, vol.%) afforded 2.97 g
(86%) of 4-allyloxy-N-(3-hydroxyphenyl)-1,8-naphthalimide 5 as
pale yellow solid. FT-IR (KBr), cm−1: 3288 (ꢁOH); 3072 (ꢁArCH );
1702 (ꢁasC O); 1665 (ꢁsC O); 1596 (ꢁC C); 1340 (ꢁN–C–N). 1H
NMR (250.13 MHz, DMSO-d6) ı (ppm): 8.65 (dd, 1H, J = 7.4 Hz,
J = 1.2 Hz, naphthalimide 5-H); 8.52 (dd, 1H, J = 8.2 Hz, J = 1.2 Hz,
naphthalimide 7-H); 8.14 (d, 1H, J = 8.1 Hz, naphthalimide 3-H);
7.78 (dd, 1H, J = 8.2 Hz, J = 7.4 Hz, naphthalimide 6-H); 7.22 (m, 3H,
phenyl 2-H, 5-H and 6-H); 7.09 (d, 1H, J = 8.1 Hz, naphthalimide
3-H); 6.81 (dd, 1H, J = 8.0 Hz, J = 1.6 Hz, phenyl 4-H); 6.26 (br.s,
1H, OH); 6.02 (m, 1H, allyl CH ); 5.28 (d, 1H, Jtrans = 17.4 Hz, allyl
HCH ); 5.17 (d, 1H, Jcis = 10.2 Hz, allyl HCH ); 4.89 (d, 2H, J = 4.9 Hz,
allyl OCH2). Elemental analysis: Calculated for C21H15NO4 (MW
345.35) C 73.03, H 4.38, N 4.06%; Found C 69.67, H 4.46, N 3.99%.
To a solution of 1.48 g of cyanuric chloride 11 (8 mmol) and
1.07 g of AlCl3 (8 mmol) in 80 ml of xylene, 2.76 g of 4-allyloxy-
N-(3-hydroxyphenyl)-1,8-naphthalimide 5 (8 mmol) were added
at room temperature. The resulting mixture was heated slowly
to 60 ◦C and stirred at this temperature for 3 h. Then the
reaction mixture was heated slowly to 90 ◦C, held at this tem-
perature for 5 h and finally, the mixture was stirred under
reflux for 8 h. After cooling the resulting solution was poured
into 120 g of ice. Organic phase (xylene) was removed and the
water phase was acidified to pH 2 with 10% hydrochloric acid.
The solid was filtered off, washed with fresh water and dried.
Silica gel chromatography (chloroform/methanol = 9:1) afforded
2.37 g (60%) of 4-allyloxy-N-[4-(4,6-dichloro-1,3,5-triazin-2-yl)-3-
hydroxyphenyl]-1,8-naphthalimide 3 as a pale yellow solid. FT-IR
(KBr), cm−1: 3064 (ꢁCH ); 3018 (ꢁOH· · ·N); 1700 (ꢁasC O); 1668
(ꢁsC O); 1652 (ꢁC N); 1598 (ꢁC C). 1H NMR (250.13 MHz, DMSO-
d6) ı (ppm): 9.89 (s, 1H, OH); 8.68 (d, 1H, J = 8.0 Hz, naphthalimide
5-H); 8.56 (d, 1H, J = 7.3 Hz, naphthalimide 7-H); 8.20 (d, 1H,
J = 8.3 Hz, naphthalimide 2-H); 7.71 (t, 1H, J = 7.7 Hz, naphthal-
imide 6-H); 7.39 (d, 1H, J = 7.8 Hz, phenyl 5-H); 7.18 (m, 2H,
phenyl 2-H and 6-H); 7.08 (d, 1H, J = 8.3 Hz, naphthalimide 3-H);
6.05 (m, 1H, allyl CH ); 5.30 (d, 1H, Jtrans = 17.6 Hz, allyl HCH );
5.17 (d, 1H, Jcis = 10.3 Hz, allyl HCH ); 4.92 (d, 2H, J = 5.2 Hz, allyl
OCH2). 13C NMR (62.90 MHz, DMSO-d6) ı (ppm): 173.5 and 172.7
(2 × triazine N C–Cl), 169.4 (triazine Ar–C N), 159.2 (Ar–C–OH),
155.5 and 154.8 (2 × C O), 153.2 (Ar–C–O), 140.5 (Ar–C–N–C O),
136.4 (Ar–C–C N), 133.9 (allyl CH ), 131.7 (Ar–CH), 130.8 (Ar–C),
129.9 and 128.6 (2 × Ar–CH), 124.7 (Ar–C), 124.1 (Ar–CH), 121.1
(Ar–C–C O), 119.6 (Ar–CH), 115.4 (allyl CH2 ); 114.8 (Ar–CH),
112.8 (Ar 2 × C–C O), 106.3 and 101.2 (2 × Ar–CH), 72.7 (allyl
OCH2). Elemental analysis: Calculated for C24H14Cl2N4O4 (MW
493.30) C 58.43, H 2.86, N 11.36%; Found C 58.71, H 2.94, N 11.48%.
3. Results and discussion
3.1. Design of 1,8-naphthalimides (1) and (2)
The aim of the present study was to synthesize multifunctional
fluorescence sensing 1,8-naphthalimide brighteners, containing
both 2-(2-hydroxyphenyl)-1,3,5-triazine UV absorber and 2,2,6,6-
tetramethylpiperidine radical scavenger fragments as well as an
unsaturated allyl function usable for simultaneously chemically
fluorescent brightening and photostabilization of polymers.
The two 1,8-naphthalimides (1 and 2) were designed to
act as photostable detectors of environment pollution by tran-
sition metal cations and protons. They were configured on
the “fluorophore-spacer-receptor” model, where the 4-oxy-1,8-
naphthalimide moiety is the fluorophore and the piperidine amine
in the C-4 substituent is the analyte receptor. The hydrocarbon part
of the piperidine fragment serves as a spacer that covalently sepa-
rates the two units. In these particular cases, it was predicted that
a PET process (an electron transfer from the receptor to the excited
state of the fluorophore) would quench fluorescence emission of
the 1,8-naphthalimide unit. This would represent the “off-state” of
the system. The protonation or respective metal complex formation
of the piperidine amine would increase the oxidation potential of
the receptor, and as such, thermodynamically disallow the electron
transfer [54,55]. Consequently the emission would be “switched on”.
Thus, we expect the fluorescence to be strong in acidic media and
transition metal cations environment.
3.2. Synthesis of the target and reference fluorescent brighteners
The synthesis of the novel 1,8-naphthalimides 1 and 2 was
performed in two steps as it is shown in Scheme 3. First,
the intermediate 8 was obtained by Friedel–Crafts acylation
of 4-bromo-N-(3-hydroxyphenyl)-1,8-naphthalimide 6 with s-
triazinylpiperidine 7 in xylene.
In order to obtain the target fluorescent 1,8-naphthalimides
1 and 2 and functionalize them with an analyte receptor in one
step, the bromine in the intermediate 8 was substituted under
phase transfer catalysis conditions with the commercially available
2,2,6,6-tetramethylpiperidin-4-ol 9 (Aldrich) for compound 1 or
with the previously synthesized 1,2,2,6,6-pentamethylpiperidin-
4-ol 10 [45] for compound 2.
The starting s-triazinylpiperidine 7 was synthesized in two
steps as described before [44] by N-acylation of 2,2,6,6-
tetramethylpiperidine 12 with the commercially available cyanuric
chloride (Merck) in xylene at 110 ◦C, and subsequent nucleophilic
substitution of the second chlorine in the intermediate 13 with
allyl alcohol under phase transfer catalysis conditions (PTC) at room
temperature (Scheme 4).
Reference 1,8-naphthalimides 3, 4 and 5 were synthesized
following Scheme 5. Compound 4 was obtained in two steps
as described before [16,17] by acylation of 4-amino-2,2,6,6-
tetramethylpiperidine 17 with 4-bromo-1,8-naphthalic anhydride
15, and subsequent nucleophilic substitution of the bromine in
the intermediate 18 with the commercially available allyl alcohol
14.
2.4. Photodegradation of fluorescent 1,8-naphthalimides
4-Bromo-1,8-naphthalic anhydride 15 was also used as a start-
ing material for the preparation of reference 1,8-naphthalimides
3 and 5. First, 4-bromo-N-(3-hydroxyphenyl)-1,8-naphthalimide
6, the starting material for the synthesis of novel fluorescent
brighteners 1 and 2 (Scheme 3), was synthesized by N-acylation
of 3-aminophenol 16 with 4-bromo-1,8-naphthalic anhydride 15.
Reference 1,8-naphthalimide 5 was obtained by nucleophilic sub-
stitution of the bromine in the compound 6 with the commercially
available allyl alcohol 14 under phase transfer catalysis condi-
tions at room temperature by analogy with the method described
The study on the photodegradation of the fluorescent 1,8-
naphthalimides was conducted in a solar simulator SUNTEST
CPS equipment (Heraeus, Germany), supplied with an arc air-
cooled Xenon lamp (Hanau, 1.1 kW, 765 W m−2), at ambient
temperature. The irradiation of dyes was performed in DMF
solution at concentration 10−5 mol l−1
. The changes in the
1,8-naphthalimide concentration were followed spectrophotomet-
rically by the changes of the absorption maxima using the method
of a standard calibration curve.

V.B. Bojinov et al. / Journal of Photochemistry and Photobiology A: Chemistry 210 (2010) 89–99
93
Scheme 3.
before [56]. Reference 1,8-naphthalimide 3 was synthesizes by
Friedel–Crafts acylation of 4-allyloxy-N-(3-hydroxyphenyl)-1,8-
naphthalimide 5 with the commercially available cyanuric chloride
11 in xylene.
The synthesized compounds were fully characterized by their
melting points, TLC (Rf values) and UV/vis spectra (Table 1) and
identified by elemental analysis data, FT-IR, 1H and 13C NMR
spectra. The target 1,8-naphthalimides (1 and 2) and reference
compounds (3–5) were additionally characterized by fluorescence
maxima, Stokes shifts (ꢁA–ꢁF) and quantum yields of fluorescence
(˚F). The data are presented in Table 2.
The absorption spectra of the fluorescent brighteners (FBs) 1 and
2 in DMF solution (Fig. 1) clearly show the participation of both UV
absorber and 1,8-naphthalimide units in the combined molecules.
2-Hydroxyphenyl-s-triazine fragments absorb in the UV region at
ꢂA1 = 340–342 nm, while the 1,8-naphthalimide absorption is red
shifted to ꢂA2 = 360–362 nm. The dyes’ molar absorptivity (ε) in
the longest-wavelength band of the absorption spectra is higher
than 10,000 l mol−1 cm−1 (Table 1), indicating that this is a charge
transfer (CT) band, due to (␲,␲*) character of the S0 → S1 transition.
Basic fluorescent characteristics of novel 1,8-naphthalimides (1
and 2) and reference compounds (3–5) such as the fluorescence
(ꢂF) maxima, Stokes shift (ꢁA–ꢁF), oscillator strength (f), fluorescent
both quantum (˚F) and energy (EF) yields were measured in DMF
solution and presented in Table 2.
3.3. Photophysical characterization of fluorescent brighteners
(1–5)
In DMF solution all compounds under study displayed blue
fluorescence due to the charge transfer in the 1,8-naphthalimide
moieties from the electron-donating alkyloxy group at C-4 position
to the carbonyl groups. The FBs’ emission was observed in the visi-
ble region with well-pronounced maxima (ꢂF) at 440–444 nm. Fig. 2
displays the absorption and fluorescence spectra of the fluorescent
dye 1 as a typical example for the spectra of compounds under
study. The overlap between absorption and fluorescence bands is
The light absorption properties of the 4-alkoxy-1,8-
naphthalimides under study are basically related to the
polarization of the 1,8-naphthalimide molecule on irradiation,
resulting from the electron donor-acceptor interaction between
the substituent at C-4 position and the carbonyl groups of the chro-
mophoric system, and may be influenced by the environmental
effect of the media.
Scheme 4.

94
V.B. Bojinov et al. / Journal of Photochemistry and Photobiology A: Chemistry 210 (2010) 89–99
Scheme 5.
Table 1
Yields, melting points, retention factors and absorption data for novel compounds 1 and 2, starting materials 6 and 7, intermediate 8 and reference naphthalimides 3–5 in
DMF solution.
a
b
b
Compound
Yield (%)
Mp (◦C)
Rf
ꢂA1 (nm)
Log ε (l mol−1 cm−1
)
ꢂA2 (nm)
Log ε (l mol−1 cm−1
)
1
2
3
4
5
6
7
8
66
72
60
88
86
83
93
61
246–248
215–217
>260
178–180
195–196
>260
0.30
0.42
0.51
0.40
0.56
0.68
0.63
0.59
342
340
340
–
–
–
4.286
4.323
4.371
–
–
–
362
360
362
362
360
344
–
4.242
4.259
4.195
4.247
4.266
3.923
–
38–40
247–249
–
342
–
4.359
346
3.968
a
TLC in a solvent system chloroform/methanol = (9:1).
ꢂA1 and ꢂA2 represent the absorption maxima of 2-(2-hydroxyphenyl)-1,3,5-triazine and 1,8-naphthalimide units in the combined molecules, respectively.
b
small and an aggregation effect at concentration of 10−5 mol l−1 has
not been observed.
Eq. (1).
ꢀ
ꢁ
1
ꢂA
1
ꢂF
(ꢁA − ꢁF) =
−
× 107
(1)
The Stokes shift (ꢁA–ꢁF) and oscillator strength (f) are impor-
tant characteristics for the fluorescent compounds. The Stokes shift
is a parameter that indicates the difference in the properties and
structure of the fluorophores between the ground state S0 and the
first excited state S1. The Stokes shifts (cm−1) were calculated by
The oscillator strength (f) shows the effective number of elec-
trons whose transition from ground to excited state gives the
absorption area in the electron spectrum. Values of the oscillator
Table 2
Fluorescence characteristics of 1,8-naphthalimides 1–5 in DMF solution at concentration 10−5 mol l−1 (ꢂex = ꢂA2).
Compound
ꢂex (nm)
ꢂF (nm)
ꢁA–ꢁF (cm−1
)
f
˚
F
EF
1
2
3
4
5
362
360
362
362
360
440
442
442
444
440
4897
5153
5000
5102
5051
0.243
0.225
0.214
0.205
0.221
0.16
0.09
0.52
0.46
0.55
0.132
0.073
0.426
0.375
0.450

V.B. Bojinov et al. / Journal of Photochemistry and Photobiology A: Chemistry 210 (2010) 89–99
95
The energy yield of fluorescence EF (Table 1), calculated by Eq.
(4), could also be used instead of ˚F [60].
ꢂA
EF = ˚F
(4)
ꢂF
As can be seen (Table 2), the quantum yield of fluo-
rescence of 1,8-naphthalimides and 2, possessing 2,2,6,6-
1
tetramethylpiperidin-4-yloxy unit at C-4 position, are lower in
respect to those of traditional blue emitting 1,8-naphthalimides,
not containing PET receptor (reference compounds 3–5). On the
other hand, the quantum yield of fluorescence of 1,8-naphthalimide
2, containing tertiary amine receptor, was lower than those of
1,8-naphthalimide 1. This phenomenon might be caused by the
possible PET process from the piperidine amine donor (recep-
tor) to the 4-oxy-1,8-naphthalimide fluorophore through the
saturated piperidinyl ring. Thus the fluorescence of the 4-oxy-
1,8-naphthalimide fluorophore is quenched (Scheme 6). Upon
recognition of the analyte (guest proton or transition metal ion)
piperidine amine would increase the oxidation potential of the
receptor, and as such, thermodynamically disallow the electron
transfer and the emission would be “switched on” [54,55].
Fig. 1. Absorption spectra of novel 1,8-naphthalimides 1 and 2 in DMF solution at
concentration 10−5 mol l−1
.
Furthermore, as demonstrated experimentally by de Silva et al.,
only the receptor that is directly attached to the 1,8-naphthalimide
4-oxy moiety (the “lower” moiety) is capable of quenching the fluo-
rophore excited state [54]. This is due to the fact that molecules like
1,8-naphthalimides 1 and 2 have high exited state dipole moments
that arise from their internal charge transfer (ICT) excited state
nature. In this case the alkyloxy unit is acting as an electron donor,
whereas the imide functions as an electron acceptor. Consequently,
a push-pull mechanism is in operation, and due to charge repulsion,
disallows the “upper” piperidine amine (compound 4) to transfer
an electron to the naphthalimide excited state [61].
The results obtained suppose PET sensor properties of FBs 1 and
2, containing “lower” piperidine moiety at C-4 position of the 1,8-
naphthalimide fluorophore, which was the reason to investigate
their photophysical behaviour in water/DMF (4:1, v/v) at different
pH values and in the presence of transition metal ions.
strength were calculated using Eq. (2) where ꢃꢁ1/2 is the width of
the absorption band (cm−1) at 1/2 (εmax) [57].
f = 4.32 × 10−9ꢃꢁ1/2εmax
(2)
The Stokes shift (4897–5153 cm−1) and oscillator strength
(0.205–0.243) values for the FBs 1–5 (Table 2) were in accordance
with the data for other blue emitting 1,8-naphthalimide derivatives
[58,59].
The ability of the molecules to emit the absorbed light energy
is characterized quantitatively by the fluorescence quantum yield
(˚F). The quantum yields of fluorescence were calculated relatively
to diphenylanthracene (Фref = 0.90) as a reference compound [53]
according to Eq. (3), where Aref, Sref, nref and Asample, Ssample, nsample
represent the absorbance at the exited wavelength, the integrated
emission band area and the solvent refractive index of the standard
and the sample, respectively.
3.4. Influence of pH on the fluorescent properties of
ꢄ
ꢅ
1,8-naphthalimides (1–5)
ꢂ
ꢃꢂ
ꢃ
n2sample
n2ref
Ssample
Sref
Aref
˚
F
= ˚
(3)
ref
The photophysical characteristics of the dyes in distilled
water/DMF (4:1, v/v) solution are represented in Table 3. Some
bathochromic shift of the absorption and fluorescence maxima
of the compounds was observed if compared spectra to those
recorded in DMF. The oscillator strength values calculated in
water/DMF (4:1, v/v) were higher than those in DMF which is well
correlated with the increase in the extinction coefficient of the dyes
in this medium.
Asample
Family of fluorescence emission spectra of the novel 1,8-
naphthalimides 1 and 2 and reference compound 4 as a function
of pH were recorded in water/DMF (4:1, v/v) and plotted in Fig. 3.
Reference 4-allyloxy-1,8-naphthalimide 4, which lacks the amine
receptor at the 4-alkyloxy moiety, did not show any observable
changes in the emission properties as a function of pH. The fluores-
cent enhancement (FE) was insignificant (FE = 1.13).
The fluorescent enhancement (FE) for other reference 1,8-
naphthalimides (3 and 5), not containing tetramethylpiperidine
substituent, as could be expected, was practically missing (FE = 1.04
for 1,8-naphthalimide 3 and FE = 1.01 for 1,8-naphthalimide 5).
In contrast, 1,8-naphthalimides 1 and 2, containing amine
receptor at their 4-alkyloxy moieties, showed higher fluorescence
sensitivity as a function of pH. However, after careful titration from
ca. pH 12 to 2 the fluorescence enhancement of 1,8-naphthalimide
2, containing tertiary amine receptor, was considerable higher in
respect to that of 1,8-naphthalimide 1, containing secondary amine
Fig. 2. Normalized absorption (ꢂA
naphthalimide 1.
) and fluorescence (ꢂF) spectra of 1,8-

96
V.B. Bojinov et al. / Journal of Photochemistry and Photobiology A: Chemistry 210 (2010) 89–99
Scheme 6.
Table 3
Absorption and fluorescence characteristics of 1,8-naphthalimides 1–5 in water/DMF (4:1, v/v) at concentration 10−5 mol l−1 (ꢂex = ꢂA).
Compound
ꢂA (nm)
Log ε (l mol−1 cm−1
)
ꢂF (nm)
ꢁA–ꢁF (cm−1
)
f
FEa
pKa
1
2
3
4
5
366
366
368
366
364
4.357
4.365
4.307
4.363
4.364
446
448
446
450
446
4902
5001
4752
5100
5051
0.323
0.308
0.316
0.318
0.332
2.34
8.11
1.04
1.13
1.01
8.08
7.96
–
–
–
a
Factor for proton-induced fluorescence enhancement FE = IFmax/IFmin (IF: fluorescence intensity, arbitrary units).
receptor (Fig. 3). This shows that the protonation of the outer rim
(amine receptor) is responsible for the main part of the fluorescence
enhancement. Fig. 4 presents as an example fluorescence changes
of the fluorescent brightener 2 over a wider pH scale. As seen pro-
tonation of the alkylated amine donor (receptor) drastically alters
the electron-donating properties and consequently switching of
the PET path from the alkylated amine donor to the 4-oxy-1,8-
naphthalimide moiety. The typical naphthalimide emission band
of 2 in water/DMF (4:1, v/v) is blue-shifted upon-protonation
and red shifted upon deprotonation (Fig. 4). Compared with the
fluorescence in the basic medium, protonation of the alkylated
amine of compound 2 results in the fluorescence enhancement
of the 4-oxy-1,8-naphthalimide fluorophore by 8.11 times. At the
same time the fluorescence enhancement of compound 1 was only
FE = 2.34. Obviously, in the case of compound 1 the oxidation poten-
tial of the receptor (secondary amine) increases less than that of
compound 2 after the protonation of the amino moiety, and as
such, thermodynamically disallows the electron transfer to a lower
extent.
The changes in the fluorescence intensity of the fluorescent
brighteners 1 and 2 as a function of pH are of such magnitude
that they can be considered as representing two different “states”,
where the fluorescence emission is “switched off” in alkaline solu-
tion and “switched on” in acidic solution. This switching process was
also found to be reversible.
It can be concluded that novel 1,8-naphthalimides 1 and 2, espe-
cially compound 2, are efficient “off–on” switchers for pH. Taking
the part of the graphs located between pH 5.0 and 11.0, the pH
influence on the fluorescence intensity (IF) has been calculated by
Eq. (5) [55].
ꢆ
IFmax − IF
log
= pH − pKa
(5)
IF − IFmin
pKa values of 8.08 for 1,8-naphthalimide 1 and 7.96 for
1,8-naphthalimide 2 have been found. The results obtained are con-
Fig. 3. Effect of pH on the fluorescence intensity of compounds 1, 2 and 4 in
Fig. 4. Changes in the fluorescence spectra of 2 as a function of pH in water/DMF
water/DMF (4:1, v/v).
(4:1, v/v). The pH range was from 11.21 to 2.44.

V.B. Bojinov et al. / Journal of Photochemistry and Photobiology A: Chemistry 210 (2010) 89–99
97
sistent with those for compounds of similar nature that have been
developed before [30,55]. These pKa values also indicate that the
novel sensors 1 and 2 would be well suited to monitor changes in
the physiological pH range.
3.5. Influence of metal cations on the fluorescence intensity of
1,8-naphthalimides (1) and (2)
The signalling fluorescent properties of novel 1,8-
naphthalimides 1 and 2 in the presence of transition metal
ions have been investigated 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 fluo-
rescence switching by PET process. Also DMF guarantees a good
solubility of the dye ligands, used metal salts and the respective
complexes.
The fluorescence behaviour of the novel compounds (1 and
2) throughout the coordination process with different metal ions
(Cu2+, Pb2+, Zn2+, Ni2+, Co2+) was, as expected, approximately the
same as that in the presence of protons. It was found that the coor-
dination of the receptor with the metal ion results in fluorescence
enhancement, as it is demonstrated as an example in Fig. 5 for the
2,2,6,6-tetramethylpiperidine substituted 1,8-naphthalimide 1 in
the presence of Zn2+ ions.
The sensor capacity of the fluorescent brighteners 1 and 2 in
respect to different metal ions and their concentration was evalu-
ated on the basis of the fluorescence enhancement values (Fig. 6).
The FE = I/I0 was calculated using minimal (I0) and maximal (I) fluo-
rescence intensity recorded before and after addition of metal ions.
The highest fluorescence enhancement for 1,8-naphthalimides 1
and 2 has been observed in the presence of Zn2+ ions (FE = 3.17 and
10.06, respectively). As it is seen (Fig. 6), the FE values for compound
2 in the presence of all types of metal cations are considerably
higher as compared to those of compound 1, which could be related
to the best increase in the oxidation potential of the tertiary amine
receptor as discussed above (Section 3.4).
Fig. 6. Fluorescence enhancement (FE) of 1,8-naphthalimides 1 and 2 (10−5 mol l−1
)
in the presence of different metal cations at concentration 7.503 × 10−6 mol l−1 in
DMF solution.
also induced an increase in the fluorescence. Further augmenta-
tion of the metal ions concentration up to 7.503 × 10−6 mol l−1 had
a small impact on the alteration of fluorescence intensity.
As discussed above, the fluorescence intensity increased due to
the complexation between the piperidine amine receptor and Zn2+
ions. Titration plots (Fig. 7) suppose a 1:2 metal/ligand complex for-
mation as it is shown in Scheme 7. Thus the PET process is “switched
off” and the fluorescence emission of fluorescent brighteners is
recovered.
The results obtained reveal a good sensor activity of the novel
blue emitting fluorophores, especially possessing tertiary amine
receptor fluorescent brightener 2, indicating their potential as
highly efficient “off–on” switchers for transition metal ions with
pronounced selectivity towards Zn2+ and Cu2+ ions.
3.6. Photostability of fluorescent brighteners (1–5)
The increase in fluorescence intensity was occurred after addi-
tion of Zn2+ ions in the concentration range of 8.336 × 10−7 to
1.084 × 10−5 mol l−1. Noticeable fluorescence intensity enhance-
ment was observed at Zn2+ concentration of 2.5 × 10−6 mol l−1
which showed good sensitivity of the investigated compounds
(Fig. 7). Raising the cation concentration up to 5.835 × 10−6 mol l−1
The fluorescent sensors’ photostability is a very important
characteristic with regard to their practical usage. To study
the influence of both 2,2,6,6-tetramethylpiperidine and 2-(2-
hydroxyphenyl)-1,3,5-triazine fragments on the photostability of
the novel compounds 1 and 2, DMF solutions of the compounds
were subjected to irradiation in a SUNTEST equipment for 10 h.
In order to receive a more complete comparative picture of this
Fig. 5. Fluorescence spectra of 1 (10−5 mol l−1) in DMF solution at various concentra-
Fig. 7. Changes in the fluorescence maxima of FBs 1 and 2 (10−5 mol l−1) upon
tions of Zn2+. The concentrations of Zn2+ cations are in order of increasing intensity
addition of Zn2+ cations with a concentration step of 8.336 × 10−7 mol l−1
.
from 0 to 7.503 × 10−6 mol l−1
.

98
V.B. Bojinov et al. / Journal of Photochemistry and Photobiology A: Chemistry 210 (2010) 89–99
Scheme 7.
influence, previously synthesized 1,8-naphthalimide derivatives
(Scheme 2), not containing a hindered amine 3 or UV absorber 4
moieties as well as not containing both stabilizer fragments 5 in
their molecules were involved in the present study as reference
compounds.
The results obtained show that the novel compounds 1 and 2 are
highly photostable fluorescent brighteners with high potential for
use as effective fluorescence detectors of environmental pollution
by metal ions and protons.
The kinetics of the fluorophores’ photodegradation was mon-
itored colorimetrically. As no changes were observed in the dyes
absorption maxima (ꢂA2) during the irradiation, the correlation
between the dye concentration and the time of irradiation was
monitored using the method of the standard calibration curve
(Fig. 8).
4. Conclusions
In this paper we have given a comprehensive account of the
design and synthesis of two multifunctional 1,8-naphthalimide
fluorescent brighteners as highly photostable PET fluorescence
sensors for metal ions and protons, containing active fragments
of a hindered amine radical scavenger, s-triazine UV absorber
and a polymerizable allyl function. It was shown that the pres-
ence of both a hindered amine and an UV absorber fragments in
the fluorophores’ molecules considerably improved their photo-
stability in respect to other similar fluorescent brighteners, not
containing either an UV absorber or a HALS component. The pho-
tophysical properties of novel compounds were studied in DMF
and water/DMF (4:1, v/v) solution and discussed. In the pres-
ence of metal ions (Cu2+, Pb2+, Zn2+, Co2+, Ni2+) and protons
these molecules sustained appreciable changes in their fluores-
cence intensity. These changes can be attributed to coordination
of their piperidine amine receptor with the analyte, whereupon
the receptor increases its oxidation potential, and as such, ther-
modynamically disallow the electron transfer and the emission is
“switched on”. It was found that the detecting ability of compound
2 is considerably higher in respect to that of fluorophore 1, which
could be related to the better increase of the tertiary amine oxida-
tion potential in the presence of transition metal ions and protons.
The results indicate the potential of the novel compounds, espe-
cially that of fluorescent brightener 2, as highly photostable and
efficient “off–on” pH switchers and fluorescent detectors for metal
ions with pronounced selectivity towards Zn2+ and Cu2+ ions.
As seen (Fig. 8), in DMF solution photostability of the novel
compounds 1 and 2, containing both HALS and UV absorber
fragments, was higher than those of reference compounds 3
and 4, not containing either 2,2,6,6-tetramethylpiperidine or 2-
hydroxyphenyl-s-triazine fragment in their molecules, which could
be explained by a synergistic interaction of the UV absorber
and HALS moieties combined in one molecule. The study also
showed that the photostability of 1,8-naphthalimide 2, containing
N-substituted piperidine moiety, is relatively lower in respect to
that of the piperidine N-free analogue 1 probably due to the reduced
ability of the piperidine nitrogen to form N-oxyl radicals.
Photodegradation of reference compound 5, not containing sta-
bilizer fragments was considerably faster if compared to those of
reference dyes 3 and 4, especially to those of novel fluorophores 1
and 2. Within 2 h the solution of compound 5 looses significant part
of its absorption capacity as a result of photodegradation of the dye
chromophoric system.
Acknowledgements
This work was supported by the National Science Foundation
of Bulgaria (project VU-X-201/06). Authors also acknowledge the
Science Foundation at the University of Chemical Technology and
Metallurgy (Sofia, Bulgaria).
References
[1] V. Balzani, Photochemical molecular devices, Photochem. Photobiol. Sci. 2
(2003) 459–476.
[2] A.P. de Silva, D. Fox, A. Huxley, T. Moody, Combining luminescence, coordina-
tion and electron transfer for signalling purposes, Coord. Chem. Rev. 205 (2000)
41–57.
[3] K. Rurack, U. Resch-Gender, Rigidization, preorientation and electronic decou-
pling -the magic triangle for the design of highly efficient fluorescent sensors
and switches, Chem. Soc. Rev. 31 (2002) 116–127.
[4] V. Balzani, A. Credi, F. Raymo, J. Stoddart, Artificial molecular machines, Angew.
Chem. Int. Ed. 39 (2000) 3348–3391.
Fig. 8. Photodegradation of 1,8-naphthalimides 1–5 in DMF solution.

V.B. Bojinov et al. / Journal of Photochemistry and Photobiology A: Chemistry 210 (2010) 89–99
99
[5] F. Raymo, Digital processing and communication with molecular switches, Adv.
[34] L. Jia, Y. Zhang, X. Guo, X. Qian,
A
novel chromatism switcher with
Mater. 14 (2002) 401–414.
double receptors selectively for Ag+ in neutral aqueous solution: 4,5-
diaminoalkeneamino-N-alkyl-l,8-naphthalimides, Tetrahedron Lett. 45 (2004)
3969–3973.
[6] H. He, M. Mortellaro, M. Leiner, S. Young, R. Fraatz, J. Tusa, A fluorescent
chemosensor for sodium based on photoinduced electron transfer, Anal. Chem.
75 (2003) 549–555.
[7] T. Gunnlaugsson, B. Bichell, C. Nolan, A novel fluorescent photoinduced electron
transfer (PET) sensor for lithium, Tetrahedron Lett. 43 (2002) 4989–4992.
[8] R. Bissell, A.P. de Silva, H. Gunaratne, P. Lynch, G. Maguire, C. McCoy, K. San-
danayake, Molecular fluorescent signalling with fluorophore–spacer–receptor
systems: approaches to sensing and switching devices via supramolecular pho-
tophysics, Chem. Soc. Rev. 21 (1992) 187–196.
[9] A.P. de Silva, H. Gunaratne, T. Gunnlaugsson, A. Huxley, C. McCoy, J.
Rademacher, T. Rice, Signaling recognition events with fluorescent sensors and
switches, Chem. Rev. 97 (1997) 1515–1566.
[10] B. Valeur, I. Leray, Design principles of fluorescent molecular sensors for cation
recognition, Coord. Chem. Rev. 205 (2000) 3–40.
[35] E. Tamanini, A. Katewa, L. Sedger, M. Todd, M. Watkinson, A synthetically
simple, click-generated cyclam-based zinc(II) sensor, Inorg. Chem. 48 (2009)
319–324.
[36] 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.
[37] W. Zhu, M. Hu, R. Yao, H. Tian, A novel family of twisted molecular luminescent
materials containing carbazole unit for single-layer organic electroluminescent
devices, J. Photochem. Photobiol. A Chem. 154 (2003) 169–177.
[38] H. Facoetti, P. Robin, P. Le Barny, M. Schott, C.M. Bouche, P. Berdague, Side-chain
electroluminescent polymers, Synth. Met. 81 (1996) 191–195.
[39] W. Zhu, N. Minami, S. Kazaoui, Y. Kim, Fluorescent chromophores func-
tionalized single-wall carbon nanotubes with minimal alteration to their
characteristic one-dimensional electronic states, J. Mater. Chem. 13 (2003)
2196–2201.
[11] A.P. de Silva, D. Fox, A. Huxley, N. McClenaghan, J. Roiron, Metal complexes as
components of luminescent signalling systems, Coord. Chem. Rev. 186 (1999)
297–306.
[12] A.P. de Silva, B. McCaughan, B. McKinney, M. Querol, Newer optical based
molecular devices from older coordination chemistry, Dalton Trans. 10 (2003)
1902–1913.
[40] 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.
[13] J. Callan, A.P. de Silva, D. Magri, Luminescent sensors and switches in the early
21st century, Tetrahedron 61 (2005) 8551–8588.
[14] 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.
[15] A.P. de Silva, A. Goligher, H. Gunaratne, T. Rice, The pH-dependent fluorescence
of pyridylmethyl-4-amino-1,8-naphthalimides, Arkivoc 7 (2003) 229–243.
[16] V. Bojinov, Novel adducts of a hindered amine and a blue emitting fluorophore
for “one-step” fluorescent brightening and stabilization of polymer materials,
J. Photochem. Photobiol. A Chem. 162 (2004) 207–212.
[17] V. Bojinov, G. Ivanova, D. Simeonov, Synthesis and photophysical investigations
of new polymerizable blue emitting fluorophores, a combination of a hindered
amine with a benzo[de]isoquinoline-1,3-dione, Macromol. Chem. Phys. 205
(2004) 1259–1268.
[41] F. Cosnard, V. Wintgens, A new fluoroionophore derived from 4-amino-N-
methyl-1,8-naphthalimide, Tetrahedron Lett. 39 (1998) 2751–2754.
[42] V. Bojinov, I. Grabchev, Synthesis and application of new combined 2,2,6,6-
tetramethylpiperidine-2-hydroxybenzophenone 1,3,5-triazine derivatives as
photostabilizers for polymer materials, J. Photochem. Photobiol. A Chem. 146
(2002) 199–205.
[43] J. Zakrzewski, J. Szymanowski, Thiophosgene, a building block for stabilizers
bearing different types of protecting fragments, Polym. Degrad. Stab. 87 (2005)
17–23.
[44] V. Bojinov, I. Grabchev, Synthesis and properties of new adducts of
2,2,6,6-tetramethylpiperidine and 2-hydroxyphenylbenzotriazole as polymer
photostabilizers, J. Photochem. Photobiol. A Chem. 150 (2002) 223–231.
[45] V. Bojinov, I. Grabchev, Synthesis of combined 2,2,6,6-tetramethylpiperidine-
2-hydroxyphenylbenzotriazole 1,3,5-triazine derivatives as stabilizers for
polymer materials, Polym. Degrad. Stab. 74 (2001) 543–550.
[46] V. Bojinov, Synthesis and properties of adducts of a hindered amine and
2-hydroxyphenylbenzotriazole as novel polymer stabilizers, Photochem. Pho-
tobiol. Sci. 1 (2002) 340–346.
[18] V. Bojinov, I. Panova, I. Grabchev, Novel adducts of a 2-(2-hydroxyphenyl)-
benzotriazole and a blue emitting benzo[de]isoquinoline-1,3-dione for “one-
step” fluorescent brightening and stabilization of polymers, Polym. Degrad.
Stab. 88 (2005) 420–427.
[19] V. Bojinov, I. Panova, Synthesis and absorption properties of new yellow-green
emitting benzo[de]isoquinoline-1,3-diones containing a hindered amine and a
2-hydroxyphenylbenzotriazole fragments, Dyes Pigments 74 (2007) 551–560.
[47] V. Bojinov, Synergistic efficiency of combined HALS-UV absorber polymerizable
stabilizers, J. Appl. Polym. Sci. 102 (2006) 2408–2415.
[48] V. Bojinov, Synthesis of novel bifunctional HALS-UV absorber polymer stabiliz-
ers, Polym. Degrad. Stab. 91 (2006) 128–135.
ˇ
[20] P. Hrdlovicˇ, S. Chmela, M. Danko, Spectral characteristics and photochemical
stability of fluorescence probes based on 1,8-naphthaleneimide in solution and
[49] V. Bojinov, D. Simeonov, Synthesis of novel bifunctional polymer stabilizers—a
combination of HALS and UV absorber, J. Photochem. Photobiol. A Chem. 180
(2006) 205–212.
in polymer matrix, J. Photochem. Photobiol. A Chem. 112 (1998) 197–203.
ˇ
[21] J. Kollar, P. Hrdlovicˇ, S. Chmela, Synthesis and spectral characteristics of di-
substituted 1,8-naphthalimides: bi-radical formation, J. Photochem. Photobiol.
[50] V. Bojinov, I. Panova, Synthesis of novel fluorophores—combination of hindered
amine and UV absorber in the molecule of benzo[de]isoquinoline-1,3-dione, J.
Univ. Chem. Technol. Metall. (Sofia) 41 (2006) 277–284.
A Chem. 195 (2008) 64–71.
ˇ
[22] P. Hrdlovicˇ, S. Chmela, M. Danko, M. Sarakha, G. Guyot, Spectral properties
of probes containing benzothioxanthene chromophore linked with hindered
amine in solution and in polymer matrices, J. Fluoresc. 18 (2008) 393–402.
[23] E. Martin, R. Weigand, A. Pardo, Solvent dependence of the inhibition of
intramolecular charge-transfer in N-substituted 1,8-naphthalimide derivatives
as dye lasers, J. Lumin. 68 (1996) 157–164.
[24] V. Gruzinskii, A. Kukhto, G. Shakkah, Spectra of lasing efficiency in lasers with
solutions of complex organic compounds, J. Appl. Spectrosc. 65 (1998) 463–465.
[25] Z.F. Tao, X. Qian, Naphthalimide hydroperoxides as photonucleases: sub-
stituent effects and structural basis, Dyes Pigments 43 (1999) 139–145.
[26] W. Stewart, Synthesis of 3,6-disulfonated 4-aminonaphthalimides, J. Am.
Chem. Soc. 103 (1981) 7615–7620.
[27] J. Morgado, J. Gruner, S.P. Walcott, T.M. Yong, R. Cervini, S.C. Moratti, A.B.
Holmes, R.H. Friend, 4-AcNI—a new polymer for light-emitting diodes, Synth.
Met. 95 (1998) 113–117.
[28] W. Zhu, C. Hu, K. Chen, H. Tian, Luminescent properties of copolymeric dyad
compounds containing 1,8-naphthalimide and 1,3,4-oxadiazole, Synth. Met. 96
(1998) 151–154.
[29] H. Tian, J. Gan, K. Chen, J. He, Q. Song, X. Hou, Positive and negative fluores-
cent imaging induced by naphthalimide polymers, J. Mater. Chem. 12 (2002)
1262–1267.
[30] 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.
[31] 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)
545–549.
[32] V. Bojinov, I. 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.
[33] X. Poteau, A. Brown, R. Brown, C. Holmes, D. Matthew, Fluorescence switching
in 4-amino-1,8-naphthalimides: “on–off–on” operation controlled by solvent
and cations, Dyes Pigments 47 (2000) 91–105.
[51] V. Bojinov, I. Panova, D. Simeonov, Design and synthesis of polymerizable
yellow-green emitting 1,8-naphthalimides containing built-in s-triazine UV
absorber and hindered amine stabilizer fragments, Dyes Pigments 78 (2008)
101–110.
[52] V. Bojinov, I. Panova, Photostability of yellow-green emitting 1,8-
naphthalimides containing built-in s-triazine UV absorber and HALS
fragments and their acrylonitrile copolymers, Polym. Degrad. Stab. 93 (2008)
1142–1150.
[53] S. Hamai, F. Hirayama, Actinometric determination of absolute fluorescence
quantum yields, J. Phys. Chem. 87 (1983) 83–89.
[54] A.P. de Silva, H. Gunaratne, C. McCoy, A molecular photoionic AND gate based
on fluorescent signaling, Nature 364 (1993) 42–44.
[55] 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 molecular electric field in the excited state, Angew.
Chem. Int. Ed. 34 (1995) 1728–1731.
[56] V. Bojinov, I. Grabchev, A new method for the synthesis of 4-allyloxy-1,8-
naphthalimide derivatives for use as fluorescent brighteners, Dyes Pigments
51 (2001) 57–61.
[57] P. Gordon, P. Gregory, Organic Chemistry in Colour, Springer-Verlag, Berlin,
1987.
[58] J. Mata, Developments in the chemistry of long wavelength fluorescent dyes,
Adv. Colour. Sci. Technol. 2 (1999) 162–166.
[59] I. Grabchev, H. Petkov, V. Bojinov, 1,8-Naphthalimides as blue emitting fluo-
rophores for polymer materials, Macromol. Mater. Eng. 287 (2002) 904–908.
[60] A. Terenin, Photonica of Dyes and Related Organic Compounds, Science,
Leningrad, 1967 (in Russian).
[61] T. Gunnlaugsson, C. McCoy, R. 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.