Facile synthesis, sensor activity and logic behaviour of 4-aryloxy substituted 1,8-naphthalimide

Article abstract of DOI:10.1016/j.jphotochem.2013.01.008

A series of 4-phenoxy- and 4-alkoxy-1,8-naphthalimide fluorophores have been synthesized in high yields at room temperature applying novel facile synthesis. One of the compounds was designed as a fluorescent PET sensor according to the fluorophore-spacer-receptor model. The ability of this molecule to detect protons and transition metal cations with pronounced selectivity towards Fe³⁺ was demonstrated. Moreover the 4-phenoxy substituted 1,8-naphthalimides were highly solvent sensitive and able to detect small quantities of water. The potential of the 1,8-naphthalimide sensor to execute INHIBIT logic function at molecular level was also demonstrated.

Full text of DOI:10.1016/j.jphotochem.2013.01.008

Journal of Photochemistry and Photobiology A: Chemistry 254 (2013) 54–61
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Facile synthesis, sensor activity and logic behaviour of 4-aryloxy substituted
1,8-naphthalimide
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:
A series of 4-phenoxy- and 4-alkoxy-1,8-naphthalimide fluorophores have been synthesized in high
yields at room temperature applying novel facile synthesis. One of the compounds was designed as a
fluorescent PET sensor according to the “fluorophore-spacer-receptor” model. The ability of this molecule to
detect protons and transition metal cations with pronounced selectivity towards Fe3+ was demonstrated.
Moreover the 4-phenoxy substituted 1,8-naphthalimides were highly solvent sensitive and able to detect
small quantities of water. The potential of the 1,8-naphthalimide sensor to execute INHIBIT logic function
at molecular level was also demonstrated.
Received 31 October 2012
Received in revised form 9 January 2013
Accepted 12 January 2013
Available online 4 February 2013
Keywords:
1,8-Naphthalimide
Fluorescence
© 2013 Elsevier B.V. All rights reserved.
Fluorescent sensor
Photoinduced electron transfer (PET)
Molecular logic
1. Introduction
referent compounds was applied. The photophysical properties,
fluorescent characteristics, sensor activity and logic behaviour of
Fluorescent sensors are molecule systems, which are able to
detect instantly “guest” species at low concentration and provide
unique spectral response towards a specific “guest” in the presence
of interfering species [1–3]. Because of their simple design, princi-
ples and practically unlimited sensing capacities towards multitude
of different species, fluorescent sensors have attracted much atten-
tion in the last decades and the scope of their exploitation is
expanding constantly [4–6]. They can be designed according to a
few principles with emphasis on the mechanism of photoinduced
electron transfer (PET). The PET using the “fluorophore-spacer-
receptor” format is the most commonly exploited approach for the
design of the fluorescent sensors and switchers [7].
Naphthalimides are a class of environment-sensitive fluo-
rophores with excellent fluorescence characteristics which can be
easily tuned by appropriate molecule design. They have found
application in a number of areas including colouration of polymers
[8,9], laser active media [10,11], fluorescent markers in biology
[12,13], anticancer agents [14] and analgesics in medicine [15], flu-
orescent sensors and molecular switches [16–21], light emitting
diodes [22,23], electroluminescent materials [24,25], liquid crystal
displays [26] and molecular logic devices [27].
the compounds were studied. The target naphthalimide 1 as well
as the referent compounds 2 and 3, not possessing either a recep-
tor unit (naphthalimide 2) or both a receptor unit and a 4-phenoxy
moiety (naphthalimide 3) are depicted in Scheme 1. Compound 1
was designed according to the “fluorophore-spacer-receptor” model
where the 4-phenoxy-1,8-naphthalimide moiety is the fluorophore
and the tertiary amino group attached at N-position is the recep-
tor [28]. Thus we expected that PET from the amine receptor to
the fluorophore acceptor will effectively quench the fluorescence
of the latter representing the “off” state of the system. Upon recog-
nition of a guest (proton or transition metal cation), which binds
to the receptor, PET communication between the receptor and the
fluorophore would be cut off and the fluorescence of the system
will recover.
2. Experimental
2.1. Materials
The starting 4-nitro-1,8-naphthalic anhydride
4 was pre-
pared according to the reported procedure [29]. Phenol, KOH
(Fluka, Merck), 2-(dimethylamino)ethylamine and n-butylamine
(Aldrich), were used without purification. All solvents (Fluka,
Merck) were pure or of spectroscopy grade. Water used was dis-
tilled. Zn(NO3)2, Cu(NO3)2, Ni(NO3)2, Co(NO3)2 and Fe(NO3)3 salts
were the sources for metal cations. The effect of the metal cations
upon the fluorescence intensity was examined by adding portions
In this work a novel facile synthesis for the preparation of a
new fluorescent 4-phenoxy-1,8-naphthalimide PET sensor and two
∗
Corresponding author. Tel.: +359 2 8163 169/206.
E-mail address: vlbojin@uctm.edu (V.B. Bojinov).
1010-6030/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotochem.2013.01.008

N.V. Marinova et al. / Journal of Photochemistry and Photobiology A: Chemistry 254 (2013) 54–61
55
was added dropwise. The reaction mixture was stirred for 1.5 h at
room temperature and the target naphthalimide 1 that precipitated
was filtered off, washed with water and dried (0.28 g, 56%., m.p.
151–153 ◦C).
O
N
O
N
FT-IR (KBr) cm−1
(ꢀsCH2); 1695 (ꢀasN
:
3059 (ꢀAr-H); 2968 (ꢀаsCH2); 2850
O); 1655 (ꢀsN O). 1H NMR (CDCl3-d,
O
C
C
250.13 MHz) ppm: 8.68 (dd, 1H, J = 8.4 Hz, J = 1.2 Hz, naphthalim-
ide H-7); 8.65 (dd, 1H, J = 7.3 Hz, J = 1.2 Hz, naphthalimide H-5);
8.45 (d, 1H, J = 8.2 Hz, naphthalimide H-2); 7.77 (dd, 1H, J = 8.4 Hz,
J = 7.3 Hz, naphthalimide H-6); 7.48 (m, 2H, Ph H-3, H-5); 7.33 (m,
1H, Ph H-4); 7.19 (m, 2H, Ph H-2, H-6); 6.90 (d, 1H, J = 8.2 Hz, naph-
thalimide H-3); 4.32 (t, 2H, J = 7.1 Hz, CONCH2CH2); 2.65 (t, 2H,
J = 7.1 Hz, CONCH2CH2); 2.35 (s, 6H, 2 × CH3). 13C NMR (CDCl3-d,
250.13 MHz) ppm: 164.4; 163.7; 159.9; 154.8; 132.8; 131.9; 130.4;
129.7; 128.6; 126.5; 125.6; 124.0; 122.6; 120.8; 116.6; 110.6; 57.0;
45.8; 38.1. Elemental analysis: Calculated for C22H20N2O3 (MW
360.41) C 73.32, H 5.59, N 7.77%; Found C 73.04, H 5.57, N 7.80%.
1
O
N
O
O
2
2.3.2. Synthesis of 4-phenoxy-N-butyl-1,8-naphthalimide 2
A suspension of 4-nitro-1,8-naphthalic anhydride 4 (3.2 g,
13 mmol) in 30 ml of ethanol and equimolar quantity of
n-butylamine (1.4 ml) was heated under reflux for 2 h. The hot reac-
tion mixture was filtered from the undissolved starting product and
the filtrate was let to cool. The yellow-brown crystals of 4-nitro-N-
butyl-1,8-naphthalimide 6 that precipitated at room temperature
was filtered off, washed with water and dried. To a solution of 0.51 g
(1.7 mmol) of compound 6 in 2 ml of DMF a solution of 0.19 g KOH
(3.4 mmol) and 0.32 g phenol (3.4 mmol) in 4 ml of DMF was added
dropwise. The reaction mixture was stirred for 1.5 h at room tem-
perature then poured into 10 ml of water. The target naphthalimide
2 that precipitated was filtered off, washed with water and dried
(0.47 g, 80%., m.p. 126–127 ◦C). No further purification was needed.
O
N
O
H3C
O
3
Scheme 1.
of the metal cations stock solution (2 × 10−3 M) to a known vol-
ume of the fluorophore solution (10 ml). The addition was limited
to 100 ␮l so that dilution remains insignificant.
FT-IR (KBr) cm−1
(ꢀsCH2); 1696 (ꢀasN
:
3055 (ꢀAr-H); 2955 (ꢀаsCH2); 2869
C
O); 1656 (ꢀsN O). 1H NMR (CDCl3-d,
C
250.13 MHz) ppm: 8.67 (m, 2H, naphthalimide H-5, H-7); 8.45 (d,
1H, J = 8.2 Hz, naphthalimide H-2); 7.77 (t, 1H, J = 7.8 Hz, naphthal-
imide H-6); 7.48 (m, 2H, Ph H-3, H-5); 7.3 (m, 1H, Ph H-4); 7.19
(d, 2H, J = 7.7 Hz, Ph H-2, H-6); 6.91 (d, 1H, J = 8.2 Hz, naphthal-
imide H-3); 4.18 (t, 2H, J = 7.5 Hz, CONCH2CH2CH2 CH3); 1.72 (m,
2H, NCH2CH2CH2CH3); 1.45 (m, 2H, NCH2CH2CH2CH3); 0.98 (t, 3H,
J = 7.3 Hz, CH3). 13C NMR (CDCl3-d, 250.13 MHz) ppm: 164.4; 163.7;
159.8; 154.9; 132.7; 131.8; 130.4; 129.7; 128.5; 126.5; 125.6; 124.0;
122.7; 120.7; 116.7; 110.7; 40.2; 30.3; 20.4; 13.8. Elemental analy-
sis: Calculated for C22H19NO3 (MW 345.39) C 76.50, H 5.54, N 4.06%;
Found C 76.23, H 5.48, N 3.99%.
2.2. Methods
FT-IR spectra were recorded on a Varian Scimitar 1000 spec-
trometer. The 1H NMR and 13C NMR spectra (chemical shifts are
given as ı in ppm) were recorded on a Bruker DRX-250 spectrom-
eter operating at 250.13 MHz. TLC was performed on silica gel,
Fluka F60 254, 20 × 20, 0.2 mm. The melting points were deter-
mined by means of a BÜCHI 535 capillary type digital melting
point apparatus. The UV–vis absorption spectra were recorded
on a spectrophotometer Hewlett Packard 8452A. The fluorescence
spectra were taken on a Scinco FS-2 spectrofluorimeter. All the
experiments were performed at room temperature (25.0 ◦C). A
1 cm × 1 cm quartz cuvette was used for all spectroscopic analysis.
The fluorescence quantum yields (ФF) were measured relatively to
p-methoxybenzylidenephthalide (˚F = 0.14 in ethanol) [30].
2.3.3. Synthesis of 4-methoxy-N-butyl-1,8-naphthalimide 3
To a solution of 4-nitro-N-butyl-1,8-naphthalimide 6 (0.51 g,
17 mmol) in 2 ml of DMF a solution of 0.11 g of KOH (2 mol)
in 2 ml of methanol was added dropwise under stirring. The
resulting mixture was stirred at room temperature for 1.5 h
then poured into 10 ml of water. The precipitate was collected
by filtration, washed with water and dried to give 0.4 g of
4-methoxy-N-butyl-1,8-naphthalimide 3 (84%, m.p. 111–113 ◦C).
2.3. Synthesis of blue emitting 1,8-naphthalimides
2.3.1. Synthesis of
4-phenoxy-N-(2-dimethylaminoethyl)-1,8-naphthalimide 1
A suspension of 4-nitro-1,8-naphthalic anhydride 4 (2.9 g,
12 mmol) in 30 ml of ethanol and equimolar quantity of 2-
(dimethylamino)ethylamine (1.31 ml) was heated under reflux
for 2 h. The hot reaction mixture was let to cool and fil-
tered from the solid particles. The filtrate was evaporated under
reduced pressure to give yellow-brown crystals of 4-nitro-N-(2-
dimethylaminoethyl)-1,8-naphthalimide 5. The crude product was
thoroughly washed with water and dried. To a solution of 0.45 g
(1.4 mmol) of compound 5 in 2 ml of DMF a solution of 0.16 g of
KOH (2.8 mmol) and 0.26 g of phenol (2.8 mmol) in 4 ml of DMF
FT-IR (KBr) cm−1
(ꢀsCH3); 1691 (ꢀasN
:
C
2957 (ꢀasCH3); 2927 (ꢀCH2); 2869
O); 1649 (ꢀsN O). 1H NMR (CDCl3-d,
C
250.13 MHz) ppm: 8.55 (d, 1H, J = 7.3 Hz, naphthalimide H-5); 8.45
(d, 2H, J = 8.3 Hz, naphthalimide H-2 and H-7); 7.65 (t, J = 7.8 Hz, 1H,
naphthalimide H-6); 6.99 (d, 1H, J = 8.3 Hz, naphthalimide H-3);
4.15 (t, 2H, J = 7.6 Hz, CONCH2CH2CH2CH3); 4.01 (s, 3H, OCH3); 1.75
(m, 2H, CONCH2CH2CH2CH3); 1.44 (m, 2H, CONCH2CH2CH2CH3);
0.96 (t, 3H, J = 7.3 Hz, CH3). 13C NMR (CDCl3-d, 250.13 MHz)
ppm: 164.4; 163.9; 160.7; 133.3; 131.4; 129.3; 128.5; 125.8;
123.4; 122.4; 115.1; 105.1; 56.1; 40.0; 30.2; 20.4; 13.8. Elemental

56
N.V. Marinova et al. / Journal of Photochemistry and Photobiology A: Chemistry 254 (2013) 54–61
analysis: Calculated for C17H17NO3 (MW 283.32) C 72.07, H 6.05,
N 4.94%; Found C 72.28, H 6.11, N 4.88%.
system. Light absorption in this molecule generates a charge trans-
fer interaction between the substituents at C-4 position and the
imide carbonyl functions. In general, the derivatives with alkoxy or
phenoxy groups are colourless with maximal absorption at about
360 nm. The absorption data of the examined compounds are listed
in Table 1.
3. Results and discussion
3.1. Synthesis of naphthalimides 1, 2 and 3
All compounds of the series are colourless in solution with
typical for the 4-alkoxy substituted 1,8-naphthalimides maxi-
mal absorption at about 366 nm [25,36]. The naphthalimides also
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 due to (␲,␲*) character of the S0 → S1 transition [26,37].
In solution, compounds 1, 2 and 3 display blue fluorescence
due to the charge transfer in the 1,8-naphthalimide moieties from
the electron-donating group at C-4 position to the carbonyl group
[38]. The fluorescence quantum yield (ФF) of the compounds was
measured in solvents with different polarity and nature and cal-
culated using Eq. (1). As expected the quantum yield of sensor 1
was strongly decreased in contrast to those of the referent 1,8-
naphthalimides, especially compound 3. This phenomenon might
be caused by the possible PET process from the tertiary amino group
to the fluorophore moiety which quenches the fluorescent emission
of the molecule (Scheme 3).
The fluorescent quantum yield of 1 is low in toluene compared
with chloroform which is not consistent with the behaviour of
other 1,8-naphthalimides based on “fluorophore-spacer-receptor”
in organic solvents with different polarity. Unusually in polar
organic solvents the quantum yields of 1,8-napththalimide PET sys-
tems are low, while in non-polar media the respective quantum
yields are higher. Probably this remarkable behaviour is due to the
lower solubility and rapid aggregation of compound 1 in toluene.
The target compounds were obtained by reaction of 4-nitro-
1,8-naphthalic anhydride 4 with primary amines and subsequent
substitution of the nitro group at C-4 position by different hydroxyl
derivatives (Scheme 2).
4-Alkoxy-1,8-naphthalimides are usually prepared from naph-
thalimides substituted at the C-4 position with electron-
withdrawing groups. Most of the synthetic methods for preparation
of 4-alkoxy-1,8-naphthalimides are based on nucleophilic aromatic
substitution of halogens or a nitro group at the C-4 position by
alkoxylating agents [31]. When a halogen is being substituted
heating and sometimes a catalyst are needed [28,32,33]. The sub-
stitution of the nitro group at C-4 position by a methoxy or an
ethoxy group could be performed under milder conditions when
ultrasound is applied [34]. A drawback of this method is that too
long reaction time may result in degradation of the desired product.
It is well-known that the nitro group at C-4 naphthalimide posi-
tion would be easily substituted by amino group in DMF at room
temperature, which is one of the most favourable methods for
synthesis of 4-amino-1,8-naphthalimides [35]. Therefore it was of
interest to prepare the desired 4-alkoxy-1,8-naphthalimides under
similar mild conditions.
The used in this work synthetic route offers short reaction time
at room temperature. No huge excess of alkylation agent or phase
transfer catalyst are needed and the products are obtained with
excellent yield. The target compounds were isolated very simply
in high yields and purity, and characterized by elemental analysis
data, UV–vis, fluorescence, FT-IR, 1H and 13C NMR spectroscopy.
ꢂ
ꢃ
ꢀ
ꢁꢀ
ꢁ
n2sample
n2ref
Ssample
Sref
Aref
˚
F
= ˚
(1)
ref
Asample
3.2. Photophysical characterization
Another interesting effect was observed in the study. The
Photophysical properties of the 1,8-naphthalimide derivates
are basically related to the polarization of their chromophoric
quantum yield of 4-phenoxy-1,8-naphthalimide 2 (not contain-
ing receptor unit) turned out to be strongly solvent dependent
O
O
N
N
N
N
H2N
O H
N
O
O
O
O2N
Ethanol, reflux
DMF, 20oC, KOH
O
O
O
O
1
5
O
N
O
NO2
O
H2N
O H
4
N
O
DMF, 20oC, KOH
Ethanol, reflux
O2N
2
6
O
N
O
H3C
O H
H3C
DMF, 20oC, KOH
O
3
Scheme 2.

N.V. Marinova et al. / Journal of Photochemistry and Photobiology A: Chemistry 254 (2013) 54–61
57
Table 1
Photophysical properties of naphthalimides 1, 2 and 3.
b
Solvent
ꢁA (nm)a
ꢁF (nm)
ФF
ꢀA − ꢀF (cm−1
)
Compound
1
2
3
1
2
3
1
2
3
1
2
3
Toluene
Dioxane
Chloroform
Acetone
Ethanol
Methanol
Acetonitrile
DMF
360
358
366
358
364
360
360
360
360
356
364
360
362
362
358
360
360
358
364
360
364
366
362
364
419
425
429
429
433
439
439
429
419
419
425
423
433
437
429
431
421
423
429
431
439
443
433
434
0.004
0.088
0.305
0.099
0.017
0.006
0.005
0.003
0.529
0.685
0.692
0.303
0.036
0.012
0.213
0.122
0.920
0.871
0.879
0.868
0.869
0.872
0.818
0.775
3929
4404
4007
4618
4362
4978
4978
4462
3929
4241
3943
4143
4514
4720
4618
4565
4030
4298
4157
4565
4673
4724
4514
4442
a
Molar extinction coefficient in chloroform ε: 12,512 l mol−1 cm−1 for compound 1; 13,077 l mol−1 cm−1 for compound 2 and 11,672 l mol−1 cm−1 for compound 3.
ФF measured relatively to the p-methoxybenzylidenephthalide (˚F = 0.14 in ethanol).
b
and decreased dramatically in polar and protic solvents (Table 1).
3.3. Sensing properties
This fact has been previously reported for 4-phenoxy substituted
nahthalimides [39] and might be related to the specific substituent
at C-4 position. In respect to the compound 2, naphthalimide
3 which lacks the phenoxy substituent shows significantly less
dependent on the solvent nature quantum yield, proving that
the sensitivity of 2 is due to the phenoxy group at a C-4 1,8-
naphthalimide position.
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 calculated by Eq. (2) Stoke’s shift values
(cm−1) were and correspond to the results for other blue emitting
1,8-naphthalimide derivatives [40,41].
Compound 1 was designed as PET based fluorescent sensor for
detection of metal cations and protons. The effect of metal cations
on the fluorescence intensity of 1 is presented in Fig. 1. The study
was performed in dioxane to avoid the quenching effect caused by
the aromatic C-4 substituent. As expected the presence of “guest”
metal ions in the solution of 1 was signalled by strong fluorescent
enhancement (FE) due to the coordination of the receptor with the
cations thus disallowing the PET quenching process. The FE = I/Io
is determined as the ratio between the maximum fluorescence
intensity (I – after metal ion addition) and the minimum fluo-
rescence intensity (Io – free of metal cations solution). The value
of FE depends on the nature of the metal cation. Under identi-
cal condition upon addition of 0.5 equiv. of Fe3+, Zn2+, Ni2+, Cu2+
and Co2+ the highest FE was observed in the presence of Fe3+
(FE = 194).
The competition experiments were also carried out for 1.
When 0.5 equiv. of Fe3+ were added into the solution of 1 in
the presence of 0.5 equiv. of other metal ions, the emission spec-
tra displayed a similar pattern to that with only Fe3+ ions. The
binding constants 1.1 × 105 l mol−1 for Fe3+, 6.9 × 104 l mol−1 for
Co2+, 7.7 × 104 l mol−1 for Cu2+, 4.3 × 104 l mol−1 for Zn2+ and
6.9 × 104 l mol−1 for Ni2+ were calculated using Benesi–Hildebrand
plot [44]. These results indicate that compound 1 could serve as
a selective and highly sensitive sensor for Fe3+ ions. The stoi-
chiometry of the complex between Fe3+ cation and the ligands
was determined using the method of continuous variations (Job’s
ꢄ
ꢅ
1
ꢁA
1
ꢁF
(ꢀA − ꢀF) =
−
× 107
(2)
As the dipole moment of the molecule is enhanced upon excita-
tion due to electron density redistribution, the excited molecule is
better stabilized in polar solvents because of stronger interaction
with the solvent dipoles [42,43]. This effect causes the red shift in
the fluorescence maxima resulting in large-scale augmentation of
the Stoke’s shift. The Stokes shift values of the compounds under
study do not indicate remarkable changes in the geometry of the
first singlet excited state in comparison with other naphthalimide
fluorophores.
Fig. 1. Fluorescent enhancement of 1 (2 × 10−5 M) in the presence of 0.5 equiv. Fe3+
(blue bars), 0.5 equiv. Me2+ (green bars), 0.5 equiv. Fe3+ and 0.5 equiv. Me2+ (red
bars). (For interpretation of the references to colour in this figure legend, the reader
is referred to the web version of this article.)
Scheme 3.

58
N.V. Marinova et al. / Journal of Photochemistry and Photobiology A: Chemistry 254 (2013) 54–61
Fig. 4. Fluorescent changes of acidified dioxane solution of 1 after addition of water
(ꢁex = 360 nm).
fluorescent emission of the compound was strongly enhanced
(FE = 47) due to the protonation of the tertiary amine receptor that
blocks the PET process (Scheme 4).
Because of the sensitivity of naphthalimide 1 towards polar and
protic solvents it was of interest to investigate its behaviour in the
presence of water. To get a better idea for the water and polarity
sensing capacities of compound 1 a titration of the acidified probe
with distilled water was carried out in the range of 0–11% (v/v). As
expected the addition of water to the acidified probe led to signif-
icant decrease of the fluorescent emission of 1,8-naphthalimide 1.
The changes of the fluorescence emission upon addition of water
are plotted in Fig. 4. The highest fluorescence quenching (FQ) was
observed after addition of 11% (v/v) of water (FQ = 13). The FQ = Io/I
has been determined from the ratio between the maximum fluo-
rescence intensity (Io – water free solution) and the fluorescence
intensity in the presence of water (I).
Fig. 2. Changes of fluorescent intensity (a) and Job’s plot (b) of 1 in dioxane upon
addition of Fe3+
.
The titration of dioxane solution of the referent compounds 2
and 3 with water was also carried out. In the case of naphthalimide
2 the maximal fluorescent quenching was achieved after addition
of 30% (v/v) of water (Fig. 5). Fluorescence quenching of the same
magnitude was observed (FQ = 10.2).
method). Job plot analysis of the titrations revealed a maximum
at about 0.33 mol fraction, indicating 1:2 binding stoichiometry
(Fig. 2).
Compound 3, which lacks a 4-phenoxy substituent, showed neg-
ligible fluorescent quenching (FQ = 1.4) after addition of 30% (v/v)
water (Fig. 6).
Similar behaviour of compound 1 in dioxane solution was
observed when 1 N hydrochloric acid was added (Fig. 3). The
Fig. 3. Fluorescent changes of 1 (10 ml dioxane solution, 2 × 10−5 M) upon addition
of protons.
Fig. 5. Effect of water on the fluorescent intensity of 2 in dioxane (ꢁex = 360 nm).

N.V. Marinova et al. / Journal of Photochemistry and Photobiology A: Chemistry 254 (2013) 54–61
59
Scheme 4.
It is worth to state that the absorption properties of the com-
pounds under study do not undergo significant changes during the
titration with water suggesting no aggregation processes. In Fig.
7 are plotted the absorption spectra of naphthalimide 1 recorded
at different volume content of the water (1–10%, v/v) as a typical
example for the absorption behaviour of all the compounds under
study (Fig. 7).
The appreciable difference in the FQ values for naphthalim-
ides 1 and 2 as compared to that of compound 3 suggests the
distinction in the quenching mechanism for 4-phenoxy- and 4-
alkoxy-1,8-naphthalimides. Obviously the fluorescence quenching
of compounds 1 and 2 is caused by specific interaction of the
molecules with the solvent, which could be related to the pres-
ence of a 4-phenoxy substituent in their structure. The fluorescence
quenching and the red shifted fluorescent maximum of compound
3 are attributed to the stabilization of the ICT state in polar solvents
which is typical for the 1,8-naphthalimide derivatives.
was demonstrated by titration of its glacial acetic acid solution with
water (Fig. 8).
As can be seen from the emission spectra depicted in Fig. 8, the
fluorescence intensity of the glacial acetic acid solution of com-
pound 1 was quenched more than 4 times after addition of small
portions of water. This indicates that 1,8-naphthalimide 1 could be
successfully used as a sensor for water content in acetic acid in the
range of 0–3% (v/v).
3.4. Molecular logic
Compound 1 contains a tertiary amine receptor, sensitive to pro-
tons and metal cations, and a 4-phenoxy substituent, sensitive to
water. The combination of two different receptors in one molecule
makes the compound suitable for execution of logic functions at
molecular level.
The behaviour of compound 1 upon acidification and subse-
quent water addition correlates very well with the INHIBIT logic
gate. Monitoring the fluorescent changes of 1 at 426 nm as an out-
put using HCl and water as two inputs, the function INHIBIT can
The valuable properties of naphthalimides 1 and 2 might be use-
ful for water sensing purposes. Further application of compound 1
Fig. 6. Effect of water on the fluorescent intensity of 3 in dioxane (ꢁex = 360 nm).
Fig. 7. Effect of water on the absorption properties of 1 in dioxane.

60
N.V. Marinova et al. / Journal of Photochemistry and Photobiology A: Chemistry 254 (2013) 54–61
restore the initial state of the molecular device, enabling the system
to execute new logic operations [5].
As acid is the first input of the novel INHIBIT logic gate, its action
could be reset by addition of hydroxide, which regenerates the
initial state of the system with water and salt as products. Unfortu-
nately, as the second input is water which removing is very difficult,
the RESET function was not achieved.
4. Conclusions
A
series of blue emitting 1,8-naphthalimide fluorophores
with 4-phenoxy or 4-alkoxy substituent have been synthesized
employing novel facile synthesis, allowing high yields under mild
conditions. The compounds were fully characterized, their photo-
physical properties were studied in solvents of different polarity
and discussed. One of the synthesized compounds (1), contain-
ing a tertiary amine receptor moiety, was designed according to
the “fluorophore-spacer-receptor” model. In the presence of pro-
tons and transition metal ions (Zn2+, Cu2+, Ni2+, Co2+, Fe3+) this
molecule sustained appreciable enhancement in its fluorescence
intensity with pronounced selectivity towards Fe3+ ions. These
changes can be attributed to coordination of the amine receptors
with the analyte, whereupon the receptor increase its oxidation
potential, and as such, thermodynamically disallow the electron
transfer and the emission is “switched on”. The quenching effect on
the fluorescence of the 4-phenoxy-1,8-naphthalimides after addi-
tion of small portions of water to their solutions was also observed.
The results obtained indicate that compound 1 can act as a sensitive
and selective sensor for Fe3+ as well as a potential detector for small
quantities of water. Also, the potential of compound 1 to execute
logic functions at molecular level was demonstrated. INHIBIT logic
gate was achieved which would be very useful for future application
of molecule 1 as a sensor for quick identification of concentrated
acids.
Fig. 8. Effect of water on fluorescence intensity of 1 (2 × 10−5 M) in acetic acid
(ꢁex = 360 nm).
be achieved starting with solution of 1 in dioxane and low emis-
sion coded for 0. With input acid 1, the emission is high coded for
binary 1 (Fig. 9). Input water 1 itself does not lead to fluorescence
enhancement and the output remains 0. Furthermore input water
1 can quench the fluorescence of the acidified probe (input acid 1)
resulting again in output 0.
The compounds acting as molecular logic gates with chemical
inputs are actually complex chemosensors. They are able to reveal
if the analyzed sample responses to predefined standard. It can be
expected that due to the possibility of sensor 1 to act as an INHIBIT
logic gate using H+ and water as inputs, 1 could be used as a sensor
for concentrated acids. If the fluorescence output of 1 is 1 then
the tested acid is concentrated and if the fluorescence output is 0
then the tested acid is diluted. The predefined standard for the acid
concentration can be adjusted by changing the threshold values.
One difference in running an electronic device and a molecular
one lies in the reset capacity after each logic operation. An elec-
tronic device recovers its initial state immediately once electricity
current is cut off. However, a molecular device retains its binding
state even when the input signals are cut off, and thus prevents
further operation, as the molecular system has been remaining in
thermodynamic equilibrium with the input introduced. Therefore,
the effect of the input signals must be removed or neutralized to
Acknowledgements
This work was supported by the National Science Foundation
of Bulgaria (project DDVU-02/97). Authors also acknowledge the
Science Foundation at the University of Chemical Technology and
Metallurgy (Sofia, Bulgaria).
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