A novel water-soluble 1,8-naphthalimide as a fluorescent pH-probe and a molecular logic circuit

Journal of Luminescence 187 (2017) 383–391

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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

A novel water-soluble 1,8-naphthalimide as a fluorescent pH-probe and a molecular logic circuit Nikolai I. Georgiev, Margarita D. Dimitrova, Paoleta V. Krasteva, Vladimir B. Bojinov n Department of Organic Synthesis, University of Chemical Technology and Metallurgy, 8 Kliment Ohridsky Str., 1756 Sofia, Bulgaria

art ic l e i nf o

a b s t r a c t

Article history: Received 9 December 2016 Received in revised form 16 March 2017 Accepted 22 March 2017 Available online 23 March 2017

A novel highly water-soluble fluorescence sensing 1,8-naphthalimide is synthesized and investigated. The novel compound is designed on the “fluorophore-receptor1-spacer-receptor2” model as a molecular fluorescence probe for determination of ions in 100% aqueous media. The novel probe comprising hydrazide and N-methylpiperazine substituents is capable of operating simultaneously via ICT and PET signaling mechanism and of recognizing selectively protons and hydroxyl anions over the representative metal ions and anions. Due to the remarkable fluorescence changes as a function of pH the system is able to act as a three output combinatorial logic circuit with two chemical inputs. Two INHIBIT gates in fluorescence and absorption mode as well as an IMPLICATION logic gate are obtained. Because of the parallel action of both INHIBIT gates a magnitude digital comparator is achieved for the first time in this way. & 2017 Elsevier B.V. All rights reserved.

Keywords: 1,8-Naphthalimide-based pH probe 100% Aqueous media Internal charge transfer (ICT) Photoinduced electron transfer (PET) INHIBIT and IMPLICATION molecular logic gates Three output combinatorial logic circuit

1. Introduction The development of fluorescent molecular systems for sensing and reporting of chemical species have recently received considerable attention, based on their short response time, low cost instrumentation and simple manipulation [1–5]. Also they are excellent diagnostic tool in the medicine and biology since they enable real-time monitoring and imaging [6–12]. The extension of this field has been further exploited to create switchable molecular systems with two or more states that emulate familiar electronic devices such as logic gates. These molecules are capable of carrying out a variety of sensing functions simultaneously, and that computes a composite result autonomously, have been gaining wider interest [13–19]. The construction of molecular information processors is largely attributable to the development and miniaturization of modern electronic devices, in which the information processing is carried out by nanosized logic gates on the semiconducting transistors [20]. However, there are instances where the molecular device solves problems which the semiconductor electronic device cannot and becomes even more relevant when we recognize that molecule-based information processing occurs naturally in living cells. Due to their extremely small size molecular logic devices can penetrate and work in living organisms without threaten their life which makes them an integral part of n

Corresponding author. E-mail address: [email protected] (V.B. Bojinov).

http://dx.doi.org/10.1016/j.jlumin.2017.03.049 0022-2313/& 2017 Elsevier B.V. All rights reserved.

biomechatronics as they can provide communication between the living object and machine or device in real time. The molecular logic devices have a potential for real-life applications such as object coding and imaging, intelligent materials, drug delivery and activation [21–23]. Also, the Boolean logic processing of concentration information by a molecular device could be particularly useful for fast diagnostic of medical conditions where the analysis is not conducted by a medical doctor but by the molecular device itself [24]. Such molecular devices can save doctors’ time especially when health services are overstretched, e.g. in epidemic or bioterrorism contexts [25]. Many studies have been reported in the literature on the basic molecular logic gate operations and molecular logic devices that have more complicated functions [26–31], such as half-adder/subtractor [32,33], digital comparator [34–36], keypad locks [37] and encoder/decoder [38,39]. Also, simple games such as Tic-Tac-Toe and cryptography based on molecular logic have been presented [40,41]. Nevertheless, the physical integration of molecular logic gates in molecular logic circuit is still a major scientific task due to the rising complexity of the designed molecular logic architectures which require higher number of inputs and outputs [42–45]. In recent years, more and more attention has been paid to the development of fluorescent sensing systems for ions in aqueous environments under physiological conditions [46–48]. However the biggest disadvantage of most organic probes is their hydrophobicity and the work in the presence of organic solvents which is not environmental friendly and significantly restricts their practical applications, especially in living organisms. The

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measurement of pH is very important in biological, chemical and industrial fields. Intracellular pH plays a critical role in many cellular events, including cell growth and apoptosis, ion transport and homeostasis and enzymatic activity [49]. Abnormal pH values are associated with inappropriate cell function, growth, and division and are observed in some common disease types [12]. For example, intratumoral pH was recently reported to range from 6.3 to 7.0 and lung airway pHs as low as 5.2 have been recorded in asthmatics [50]. Acid responsive probes may also find application for cellular imaging of acidic intracellular vesicles, such as endosomes, lysosomes, and phagosomes, where pH can range from 4.5 to 6.5. Hence the determination of pH has attracted increasing interests. Therefore, in this work, we pay attention to the design, synthesis, sensor activity and molecular logic in 100% aqueous media of a novel 1,8-naphthalimide pH-probe using simultaneously both PET and ICT processes. The optical chemosensing behavior of the synthesized compound was studied and sequential logic circuit on molecular level was designed. We chose the simultaneous action of PET and ICT in one molecule [51–54] due to the possibility of amalgamation of these pathways to provide multilevel logic systems with multiple logic functions [55–57].

2. Experimental 2.1. Materials Commercially available 4-chloro-1,8-naphthalic anhydride (ACROS ORGANIC), n-butylamine, hydrazine monohydrate, N-methylpiperazine and chloroacethyl chloride (Aldrich) were used without purification. All solvents (Aldrich, Fisher Chemical) were pure or of spectroscopy grade. Zn(NO3)2, Cu(NO3)2, Ni(NO3)2, Co (NO3)2, Pb(NO3)2, Fe(NO3)3, Hg(NO3)2, Al(NO3)3, Cr(NO3)3, Cd (NO3)2 salts were the sources for metal cations and KCl, NaNO3, Na2SO4, NaHSO4, Na2CO3, NaHCO3, CH3COONa, C7H5NaO3 (Sodium salicylate), KBr, KI, NaNO2, Na2S2O3, Na2S2O4, Na2S2O5, KNO3, Na2B4O7, NaH2PO4, K2HPO4, K3PO4, NaF, NaBrO3, KClO4 were the sources for anions (Aldrich salts at p.a. grade). 2.2. Methods FT-IR spectra were recorded on a Varian Scimitar 1000 spectrometer. The 1H NMR spectra (chemical shifts are given as δ in ppm) were recorded on a Bruker AV-600 spectrometer operating at 600.13 MHz. TLC was performed on silica gel, Fluka F60 254, 20
20, 0.2 mm. The UV–vis absorption spectra were recorded on a spectrophotometer Hewlett Packard 8452 A. The fluorescence spectra were taken on a Scinco FS-2 spectrofluorimeter. The fluorescence quantum yields (ΦF) were measured to Coumarin 6 (ΦF ¼ 0.78 in ethanol [58]) as a standard. All the experiments were performed at room temperature (25.0 °C). A 1
1 cm quartz cuvette was used for all spectroscopic analysis. The spectral data were collected using FluoroMaster Plus 1.3 and further processed by OriginPro 6.1 software. To adjust the pH, very small volumes of hydrochloric acid and sodium hydroxide were used. The effect of the metal cations and anions upon the fluorescence intensity was examined by adding portions of the cation stock solution to a known volume of the fluorophore solution (10 mL). The addition was limited to 100 μL so that dilution remains insignificant. 2.3. Synthesis of 4-hydrazino-N-butyl-1,8-naphthalimide (3) A suspension of 4-chloro-1,8-naphthalic anhydride 1 (1.00 g, 0.0043 mol) and n-butylamine (0.42 mL, 0.0043 mol) in 20 mL of ethanol was heated under reflux for 10 h. After cooling the precipitate was filtered off, washed with ethanol and dried to give

0.89 g (72%) of intermediate 4-chloro-N-butyl-1,8-naphthalimide 2. Then the above prepared 4-chloro-N-butyl-1,8-naphthalimide 2 (0.89 g, 0.0031 mol) was dissolved in 5 mL of DMF and 4 mL of hydrazine monohydrate were added dropwise. The mixture was refluxed for 3 h and poured into water to give after filtration and drying 0.77 g (88%) of 4-hydrazino-N-butyl-1,8-naphthalimide 3. FT-IR (KBr) cm  1: 3382 and 3324 (ν NH2); 1677 (νas CQO); 1639 (νs CQO). 2.4. Synthesis of intermediate compound (4) A suspension of 0.77 g (0.0027 mol) 4-hydrazino-N-butyl-1,8naphthalimide 3 in 8 mL of dry dioxane was heated to 70 °C and 0.45 mL (0.0054 mol) of chloroacethyl chloride were added dropwise. The resulting mixture was stirred at the same temperature for 3 h. After cooling the precipitate was filtered off, washed with dioxane and dried to give 0.63 g of pure intermediate 4 as yellow crystals (0.63 g, 70%). FT-IR (KBr) cm  1: 3432 (νNH); 1701 (νasN– CQO); 1651 (νsN–CQO). 1H NMR (DMSO-d6, 600.13 MHz) δ ppm: 10.61 (s, 1 H, NH), 9.70 (s, 1 H, NH), 8.69 (dd, 1 H, J ¼8.6 Hz, J ¼0.9 Hz, Naphthalimide H-7), 8.47 (dd, 1 H, J ¼7.3 Hz, J ¼0.8 Hz, Naphthalimide H-5), 8.32 (d, 1 H, J ¼8.4 Hz, Naphthalimide H-2), 7.77 (dd, 1 H, J ¼8.4 Hz, J¼ 7.4 Hz, Naphthalimide H-6), 6.96 (d, 1 H, J ¼ 8.4 Hz, Naphthalimide H-3), 4.33 (s, 2 H, COCH2Cl), 4.03 (t, 2 H, J ¼7.4 Hz, (CO)2NCH2), 1.73-1.49 (m, 2 H, CH2CH2CH2CH3), 1.39-1.30 (m, 2 H, CH2CH2CH2CH3), 0.93 (t, 3 H, J ¼7.3 Hz, CH2CH2CH2CH3). 2.5. Synthesis of sensor (5) Intermediate compound 4 (0.63 g, 0.0017 mol) was dissolved in 7 mL of DMF and 0.4 mL of N-methylpiperazine (0.0034 mol) were added. The solution was heated under reflux for 4 h. Then the desired compound 5 was isolated after cooling by silica gel chromatography (n-hexane: acetone ¼1:1) as yellow solid (0.17 g, 23%). FT-IR (KBr) cm  1: 3425 (νNH); 1665 (νN–CQO). 1H NMR (CHCl3-d, 600.13 MHz,) δ ppm: 9.73 (s, 1 H, NH), 8.82 (s, 1 H, NH), 8.29 (d, 1 H, J ¼8.2 Hz, Naphthalimide H-7), 8.19 (d, 1 H, J ¼7.1 Hz, Naphthalimide H-5), 7.92 (d, 1 H, J ¼8.2 Hz, Naphthalimide H-2), 7.12 (t, 1 H, J ¼ 7.9 Hz, Naphthalimide H-6), 6.79 (d, 1 H, J ¼8.2 Hz, Naphthalimide H-3), 4.09 (t, 2 H, J ¼7.4 Hz, (CO)2NCH2), 3.26 (s, 2 H, COCH2N), 2.68 (m, 4 H, 2
CH2 piperazine), 2.28 (s, 3 H, NCH3 piperazine), 1.87 (m, 4 H, 2
CH2 piperazine), 1.70-1.63 (m, 2 H, CH2CH2CH2CH3), 1.43-1.35 (m, 2 H, CH2CH2CH2CH3), 0.93 (t, 3 H, J¼ 7.4 Hz, CH2CH2CH2CH3). Elemental analysis: calculated for C23H29N5O3 (MW423.51) C 65.23, H 6.90, N 16.54%; found C 64.88, H 6.73, N 16.72%.

3. Results and discussion 3.1. Design and synthesis The novel probe 5 was configured on the “fluorophorereceptor1-spacer-receptor2” model (Scheme 1). The 1,8-naphthalimide-hydrazide moiety in 5 represents the “fluorophore-receptor1” architecture with a strong internal charge transfer (ICT) character, which results in a large dipole moment in the excited state. It is well known that the light absorption properties of the 1,8-naphthalimide derivatives are basically related to the polarization of their chromophoric system [59]. Light absorption in this molecule generates a charge transfer interaction between the substituent at a C-4 position and the imide carbonyl groups which strongly depends on the microenvironment of the fluorophore. Thus, recognition of guest affects the ICT efficiency that changes the energy between ground and excited state and results in shifting of

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Scheme 1. Design of probe 5 on the “fluorophore-receptor1-spacer-receptor2” model.

the fluorophore electronic spectra. The hydrazide receptor possessing labile hydrogen was chosen due to the widespread use of similar fragments for anion recognition through hydrogen-bonding or deprotonation interactions [60,61]. When an anion interacts with the hydrazide receptor, bounded in the C-4 position of the 1,8-naphthalimide fluorophore, its electron-donating ability increases due to the deprotonation which generates a strong electron density around C-4 position of the fluorophore. As a result the ICT efficiency in the 1,8-naphthalimide fluorophore increases and red shifting is expected [62]. Furthermore 5 is a typical PET system based on “fluorophorespacer-receptor” architecture for cation detection where the 1,8naphthalimide is the fluorophore, the piperazine moiety is the cation receptor (receptor2) and the methylene part between them is the spacer that covalently separates the both parts. Thus we expect that photoinduced electron transfer (PET) from piperazine nitrogen atoms will effectively quench the fluorescence of the yellow-green emitting fluorophore. Upon recognition of a cation, which binds to the receptor engaging its lone pairs of electrons, PET communication between the receptor and the fluorophore would be cut off and the fluorescence of the system will be recovered [63–66]. It should be pointed out that the novel probe could be presented as “fluorophore-receptor1-spacer1-receptor2-spacer2-receptor3” architecture due to the two possible sites of protonation on the methylpiperazine. However it is well known that the rates of PET processes depend exponentially on the distance of separation of the donor and acceptor component and spacers containing more than three consequently bonded atoms practically are useless [67]. The 1,8-naphthalimede fluorophore and the closer piperazine nitrogen are separated through three consequently bonded atoms and low effective but still very useful PET process is expectable [67,68]. Furthermore the piperazine methylamino nitrogen and 1,8-

385

naphthalimide in probe 5 are separated through six consequently bonded atoms and the resulted spacer is too long for effective PET. Meanwhile there are some examples where the feasibility of PET increases in architectures with larger and flexible spacers that may facilitate a conformation that reduces the fluorophore-receptor distance. For instance in the PET based perylene diimide the ethylene spacers results in high efficient PET in comparison with the methylene spacers [69] and the work by Magri et al. reveals well pronounced PET process in naphthalimides from substituents at the C4-position over a distance of at least four atoms [23]. However such examples are “the exception rather than the norm” and we believe that the proposed model “fluorophore-receptor1-spacerreceptor2” for the investigated probe is sufficiently correct and reliable. The synthesis of the target sensor 5 is shown at Scheme 2. First, the intermediate 4-hydrazino-N-butyl-1,8-naphthalimide 3 was prepared by analogy with the previously reported work after condensation of 4-chloro-1,8-naphthalic anhydride and n-butylamine, and subsequent substitution of the chlorine at C-4 position with hydrazine [70]. Then compound 3 was converted into intermediate 4 after acetylation with chloroacetyl chloride. The final product 5 was obtained by reaction of 1,8-naphthalimide 4 with Nmethylpiperazine in DMF at reflux for 4 h. The structure and purity of the synthesized compounds were characterized and confirmed by conventional techniques – elemental analysis data, UV–Vis, fluorescence, FT-IR and 1H NMR spectroscopy. 3.2. Chemosensing properties of probe 5 The novel compound 5 was designed as a molecular fluorescence probe for determination of ions in 100% aqueous media. This was the reason to investigate its basic photophysical characteristics in water solution (C ¼10  5 mol L  1) at different pH values in the absence and in the presence of different ions. In strong acid media (pH ¼1.8) the novel probe 5 shows an absorption band in the range between 330 and 470 nm with a well pronounced maximum at 402 nm (Fig. 1) due to the internal charge transfer process in the 1,8-naphthalimide chromophore. Also, in the acid media compound 5 is protonated (Scheme 3), the PET quenching process is not feasible under these conditions and the probe exhibits strong yellow-green fluorescence in a spectral region of 425–650 nm with maximum at 508 nm (Fig. 2). The quantum yield of fluorescence ΦF ¼0.13 was calculated using Coumarin 6 (ΦF ¼0.78 in ethanol) [58] as a standard according to Eq. (1), 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 [71,72].

Scheme 2. Synthesis of 1,8-naphthalimide based probe 5.

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Scheme 3. Photophysical behavior of probe 5 as a function of pH.

Fig. 1. Absorption spectra of probe 5 in aqueous solution at different pHs.

⎞ ⎛S ⎞⎛ A ⎞⎛ n 2 sample ref ⎟⎟⎜⎜ ⎟⎟⎜⎜ sample ⎟⎟ ФF = Фref ⎜⎜ 2 ⎝ Sref ⎠⎝ Asample ⎠⎝ nref ⎠

(1)

The addition of NaOH to probe 5 in aqueous media from pH ¼1.8 to pH ¼8 converts the quaternary ammonium PET receptor in a neutral amine with strong electron donating ability which makes the PET quenching process feasible (Scheme 3). As a result the fluorescence intensity of probe 5 gradually decreases with the increase of pH (Fig. 2A) and the lower quantum yield of fluorescence (ΦF ¼0.03) for probe 5 at pH ¼8 was calculated. For the decrease of fluorescence intensity, the qualitative parameter fluorescence quenching FQ ¼ 3.2 have been obtained. The FQ ¼ Io/I is determined as the ratio between the maximum fluorescence intensity Io at pH ¼1.8 and the minimum fluorescence intensity I at pH ¼ 8. The observed fluorescence changes in a pH range 1.8– 8.0 (Fig. 2B) are analyzed according to Eq. (2) [73–75] and the value of pKa ¼6.42 70.14 was calculated.

log⎡⎣ (IF max − IF )/(IF − IF min )⎤⎦ = pH − pKa

(2)

When the pH was changed from pH ¼2 to pH ¼8 the observed absorption spectra do not show so significant pH-dependent

changes as the fluorescence, since the PET process does not affect the 1,8-naphthalimide chromophoric system. By the transition from pH ¼2 to pH ¼8 a slightly decrease of the absorption intensity was observed which was accompanied by red shifting of the absorption maximum from 402 nm to 410 nm (Fig. 1A). As Gunnlaugsson et al. comment for the 1,8-naphthalimide fluorophores [76,77] the reason for the red shift is twofold. First, the protonation of the amine receptor in acid media exerts some weak charge repulsion on the 4-hydrazido moiety of the fluorophore. On the other hand, in very acidic conditions the push-pull character of the ICT state is partially reduced due to the protonation of the aromatic NH moiety bounded at the 1,8-naphthalimide C-4 position. Further increase of pH from pH ¼8 to pH ¼11.5 results in additional fluorescence quenching of probe 5 (ΦF ¼ 0.02, FQ ¼1.6). Also, it was found that the quenching effect was accompanied with gradually red shifting of the absorption maximum of probe 5 from 410 nm to 504 nm (Fig. 1A). This phenomenon was attributed to the deprotonation of hydrazide aromatic N-H in probe 5 (Scheme 3) that enhanced the “push-pull” character of the ICT transition, causing a considerable bathochromic shift (94 nm) of the absorption band (Fig. 3). Additionally an increase of the absorption band centered at 302 nm was observed which was explained before by Gunnlaugsson et al. with the n-π* transition in the deprotonated 4-NH substituted 1,8-naphthalimide fluorophore [78]. The analysis of the fluorescence changes in a pH range of 8– 11.5 (Fig. 2C) according to Eq. (2) gives pKa value of 9.497 0.10 for the deprotonation of probe 5. According to Eq. (3) [79] a similar

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387

Fig. 2. Fluorescent spectra in aqueous solution at different pHs (A, B and C) and Cycle Index of probe 5 (D).

log⎡⎣ (A max − A)/(A − A min )⎤⎦ = pH − pKa

Fig. 3. Yellow-green emission at pH 1.8 and red emission at pH 11.5 of probe 5.

pKa value (pKa ¼9.72 7 0.13) was calculated from the absorption changes at 402 nm as a function of pH (Fig. 1B) which suggests that the both absorption and fluorescence changes are related to the same process (deprotonation of probe 5).

(3)

From a practical standpoint the reversibility of the pH sensors is of paramount importance. Fig. 2D demonstrates that the fluorescent properties of probe 5 persist even after more than several pH cycles. The above results showed that the novel 1,8-naphthalimide 5 extends the field of organic pH-chemosensing fluorophores growing the body of the environmental friendly water-soluble probes. The water-soluble 1,8-naphthalimide probes operating simultaneously via ICT and PET are still very rare. The most reported probes form this category are the PET-based dihydroimidazonaphthalimide derivatives which showed an “off-onoff” fluorescent pH-sensing response (Scheme 4) [19,74]. Compared to them probe 5 has a fluorescent signal within the similar spectral range but with a multi leveled fluorescent titration plot containing three states: “low”, “medium” and “high”. Furthermore the dihydroimidazonaphthalimides show colorimetric changes at low pHs while probe 5 has colorimetric response in alkaline media. In view of the detailed comparison of the properties of probe 5 and dihydroimidazonaphthalimides, Table 1 summarizes the basic photophysical parameters of some of the related compounds [19,74] and those of the compound under study.

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Scheme 4. 1,8-Naphthalimide and dihydroimidazonaphthalimide based probes 5–8.

Table 1 Photophysical properties of compounds 6–8 (Scheme 4). λabs nm Acid media

λabs nm

λabs nm

λfl nm

λfl nm

λfl nm

Neutral media

Alkaline media

Acid media

Neutral media

Alkaline media

402

410

504

508

514

535

6.4 9.5

6 [71] 476

423

434

573

513

520

5.3 8.3

7 [19] 472

430

436

567

510

526

8 [73] 430

440

450

535

535

550

5.7 7.6 6.3

5

pKa

It should be pointed out that the synthesized probe 5 also extends the previous work by Gunnlaugsson et al. [76] in which a very similar probe 8 (Table 1) for pH-detection in 100% aqueous media was reported. Probe 8 shows exactly the same pKa such as compound 5 due to the PET process but it lacks fluorescent or colorimetric changes at higher pHs. Probably the carbonyl group from 4-hydrazido moiety in 5 stabilizes the formed anion in strong alkaline media making 5 a very useful probe for pH determination at extremely high pH in respect of compound 8. The signaling fluorescent properties of compound 5 in the presence of cations (Co2 þ , Cu2 þ , Fe3 þ , Ni2 þ , Pb2 þ , Cd2 þ , Zn2 þ , Hg2 þ , Cr3 þ and Al3 þ ) and anions (Cl  , NO3  , SO42 , HSO4  , CO32 , HCO3  , CH3COO  , Sodium salicylate, Br  , I  , NO2  , S2O32 , S2O42 , S2O52 , B4O72 , H2PO4  , HPO42 , PO43 , F  and BrO3  ) were also investigated. The tested ions were added gradually up to 10 equivalents (10  4 mol L  1) to a solution of 5 with concentration 10  5 mol L  1. To maintain a constant pH values, the experiments have been performed in buffered aqueous solutions in the presence (10 mM) of ammonia/ammonium chloride buffer (pH 9), acetate buffer (pH 4.5) or Tris-HCl (pH 8). The above buffers were used because at these pHs probe 5 is in its anion, cation and neutral form, respectively. Furthermore the effect of the ions was observed using two methods. The first one was carried out by adding of the metal stock solutions to different buffered solutions of the examined probe 5, while the second one was performed by addition of buffer solutions to solutions of probe 5 in the presence of the metal cation. In both cases a very similar effect was observed. It was found that the addition of all ions gradually decrease the fluorescence intensity of probe 5 with the increase of the ion concentration. However, the observed effects were minor as the maximal fluorescence quenching FQ ¼1.4 was calculated in the presence of 10 eq. (c ¼1
10  4 mol L  1) of Cu2 þ as a typical representative example for the cation quenching effects on the fluorescent spectra of probe 5. Fluorescent changes of probe 5 at pH ¼ 8 after addition of 10 eq. of the tested ions are shown in Fig. 4. Based on the performed experiments and the results obtained it can be generalized that the novel probe 5 is able to recognize only protons and hydroxyl anions which makes it a selective indicator for pH determination in wide range.

Fig. 4. Effect of (A) metal cations (c ¼ 1
10  4 mol L  1) and (B) anions on the fluorescence of 5 (c ¼1
10  5 mol L  1) in aqueous solution buffered with 10 mM Tris-HCl (pH ¼8).

3.3. Probe 5 based molecular computing Starting from pH ¼8, by monitoring of the fluorescent changes at 508 nm and absorption changes at 410 nm and 504 nm of probe 5 in the presence of H þ (0.001 M HCl) and OH- (0.001 M NaOH) a combinatorial logic circuit with two chemical inputs and three optical outputs would be constructed. At the starting conditions probe 5 is in its neutral form and shows low fluorescence output due to the PET quenching process, which is coded as 0 in

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Table 3 The truth table for the operation of the 5-based digital comparator. Input 1 (H þ )a

Hþ Hþ Hþ Hþ a b

¼ OH  4 OH  o OH  ¼ OH 

0 1 0 1

Input 2 (OH  )b

0 0 1 1

Output 1 (Fl

508)

Output 2 (A 504)

INHIBIT (greater than)

INHIBIT (less than)

0 1 0 0

0 0 1 0

High input level 10  3 M HCl. Low input level in absence of added HCl. High input level 10  3 M NaOH. Low input level 10  10 M NaOH.

Scheme 5. Combinatorial logic circuit on the basis of probe 5.

Fig. 5. The fluorescence (A) and the absorption (B) changes of probe 5 with HCl (C ¼10  3 mol L  1) and NaOH (C ¼ 10  3 mol L  1) as chemical inputs in aqueous solution starting from pH ¼ 8.

Table 2 The truth table for the operation of compound 5.

A B C D a b

Input 1 (H þ )a

Input 2 (OH  )b

Output 1 (Fl508)

Output 2 (A504)

Output 3 (A410)

0 1 0 1

0 0 1 1

0 1 0 0

0 0 1 0

1 1 0 1

High input level 10  3 M HCl. Low input level in absence of added HCl. High input level 10  3 M NaOH. Low input level 10  10 M NaOH.

fluorescence binary mode (Fig. 5A). The addition of base as chemical input converts 5 in its nonfluorescent anion form with fluorescent binary signal 0. In the presence of acid as input the PET quenching is cutoff and probe 5 possesses a high fluorescence as output coded for binary 1. The simultaneous inputs of acid and base are annihilated each other and generated the initial low fluorescence coded as 0. This behavior mimicked the INHIBIT logic gate where the OH- input inhibits the action of the H þ input (Table 2). By monitoring of the absorption output at 504 nm a second INHIBIT gate at molecular level is achievable, but in this case the H þ input is the inhibitor. At the starting conditions the system has yellow-green color and does not absorbed at 504 nm.

That is why the absorption output at 504 nm is coded in binary as 0 (Table 2). The addition of acid as input does not change the absorption of the system and output at 504 nm is still 0 (Fig. 5B). However, the addition of base as input changes the system's color in red due to the deprotonation of 5 which results in high absorption band at 504 nm and give the binary code 1 of output at 504 nm. In the presence of the both inputs the acid neutralizes the action of the base input and the color of the system remains unchanged which gives 0 as binary code at 504 nm absorption output. Also due to the ICT chemosensing properties of the probe in absorption mode – additional IMPLICATION gate at molecular level could be constructed. The IMPLICATION gate is related inversely (negative logic) to INHIBIT similarly to the both signaling outputs in the ICT chemosensing systems. As can been seen (Table 2) the signal output at 410 nm represents a logical inversion of the signal output at 504 nm – when output at 410 nm is 1 then the output at 504 nm is 0 and on opposite when output at 410 nm is 0 then the output at 504 nm is 1. As a result by the monitoring of the absorption output at 410 nm with acid and base as inputs IMPLICATION gate at molecular scale would be obtained. Furthermore, based on the parallel action of the both INHIBIT gates of compound 5 a magnitude digital comparator at the molecular scale is achievable. Herein we pay attention on the possibility of 5 to act as magnitude digital comparator because it works using two INHIBIT gates, while the previously reported molecular magnitude comparators were obtained by parallel action of XNOR and INHIBIT gates [35,36]. The truth table for the operation of the 5-based digital comparator was presented in Table 3. As can be seen the obtained INHIBIT gate in fluorescence mode (Fl508) is 1 only when the Input 1 (H þ ) is higher than Input 2 (OH-) that is why it plays the role of a mathematical symbol “greater than”. The absorption INHIBIT gate which output is 1 only when the Input 1 (H þ ) is less than Input 2 (OH-) works as mathematical symbol “less than”. When the both inputs are equal then the both outputs are 0 simultaneously. On the basis of the present study and the results obtained above the molecular computing based on 5 could be summarized with the electronic representation in Scheme 5.

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4. Conclusions A novel highly water-soluble 1,8-naphthalimide derivative with fluorescence sensing properties based on PET and ICT was synthesized and its photophysical behavior as a function of pH and in the presence of representative metal ions and anions was studied. The novel probe was designed on the “fluorophore-receptor1spacer-receptor2” model comprising C-4 hydrazide (“receptor1”) and N-methylpiperazine (“receptor2”) substituents and as such is able to operate simultaneously via ICT and PET signaling mechanism. The synthesized compound shows selective signaling properties towards protons and hydroxyl anions over the representative metal ions and anions. In an acid media due to the protonation of the tertiary amine (“receptor2”) the PET quenching process is not feasible and probe 5 exhibits strong yellow-green fluorescence, while in an environment near to the neutral the due to the PET quenching fluorescence intensity of probe 5 decreases. In an alkaline media the fluorescence of probe 5 is additionally quenched and due to deprotonation of the hydrazide unit (“receptor1”) that enhanced the “push-pull” character of the ICT transition, the absorption maximum of the probe is red shifted (94 nm). In the presence of representative metal ions and anions the observed quenching effect was negligible. Due to the remarkable fluorescence changes in the presence of protons and hydroxyl ions the novel system is able to act as a three output combinatorial logic circuit with two chemical inputs and to execute two INHIBIT (in fluorescence and absorption mode) and IMPLICATION logic gates. Furthermore, due to the parallel action of both INHIBIT logic gates, for the first time a magnitude digital comparator at a molecular level is achieved in this way.

Acknowledgements This work was supported by the Science Foundation at the University of Chemical Technology and Metallurgy (Sofia, Bulgaria) – project No 11611/2016 and project No 11614/2016.

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