Enhanced ion binding by the benzocrown receptor and a carbonyl of the aminonaphthalimide fluorophore in water-soluble logic gates

Andreas Diacono, Marie Claire Aquilina, Andrej Calleja, Godfrey Agius, Gabriel Gauci, Konrad Szaciłowski and David C. Magri*
Department of Chemistry, Faculty of Science, University of Malta, Msida, MSD 2080, Malta. E-mail: david.magri@um.edu.mt; Web: https://www.um.edu.mt/profile/davidmagri

Received 10th January 2020 , Accepted 24th March 2020

First published on 1st April 2020


Two fluorescent logic gates 1 and 2 were designed and synthesised with a ‘receptor1-spacer1-fluorophore-spacer2-receptor2’ format. The molecules comprise of an aminonaphthalimide fluorophore, methylpiperazine and either benzo-15-crown-5 or benzo-18-crown-6. Model 3, with a weakly binding 3,4-dimethoxyphenyl moiety, was also synthesised. The compounds were studied both in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (v/v) methanol/water and water by UV-visible absorption and steady-state fluorescence spectroscopy. The green fluorescence of 1–3 is modulated by photoinduced electron transfer (PET) and internal charge transfer (ICT) mechanisms, and by solvent polarity. In 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (v/v) methanol/water, logic gates 1 and 2 emit with Φf = 0.21 and 0.28, and bind with pβNa+ = 1.6 and pβK+ = 2.6, respectively, and pβH+ = 7.4 ± 0.1. In water, logic gates 1 and 2 emit with Φf = 0.14 and 0.26, and bind with pβNa+ = 0.86 and pβK+ = 1.6, respectively, and pβH+ = 8.1 ± 0.1. The measured pβNa+ are significantly lower than reported for analogous classic anthracene-based Na+, H+-driven AND logic gates indicating a stronger Na+ binding interaction, which is attributed to direct interaction with one carbonyl moiety within the aminonaphthalimide. Supporting evidence is provided by DFT calculations. Furthermore, we illustrate an example of logic function modulation by a change in solvent polarity. In 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (v/v) methanol/water, molecules 1 and 2 function as Na+, H+ and K+, H+-driven AND logic gates. In water, the molecules function as single input H+-driven YES logic gates, while consideration as two-input devices, 1 and 2 function as AND-INH-OR logic arrays.


Introduction

Molecular logic-based computation1 emerged in 1993 with the first molecular AND logic gate by de Silva.2 Purposely designed according to a ‘fluorophore-spacer1-receptor1-spacer2-receptor2’ format, an anthracene fluorophore was linked by methylene spacers to a proton-accepting aliphatic amine (receptor1) and a benzo-15-crown-5 (receptor2) for binding H+ and Na+, respectively. Four years later, an improved prototype embodying the same modular units, but arranged in a ‘receptor1-spacer1-fluorophore-spacer2-receptor2’ format (Fig. 1) gave a much brighter turn-on fluorescence.3 The breakthrough resulted from shortening the distance for photoinduced electron transfer (PET) between the fluorophore and the two receptors.4 The hydrophobic nature of anthracene, though, restrained these first demonstrations of molecular logic to alcoholic solutions.
image file: d0ob00059k-f1.tif
Fig. 1 The colour-coded design concept4 (top) and molecular structures (bottom) of the logic gates 1–3. The inputs are Na+ or K+ (receptor1) and H+ (receptor2). Spacer1 is a virtual C0-type spacer, while spacer2 is a diethylene (C2) spacer.

Now over a quarter of a century later, the field of molecular logic-based computation is focusing on two significant challenges. The first challenge is the development of tools for pragmatic uses.5 For example, crown-ether containing molecules,6 including M+/H+ logic gates, are applicable as smart fluorescent probes for investigating biological membrane interfaces, M+/H+ antiporters,7 protein interactions8 and theranostics.9 The second challenge is the development of molecular tools that function in water.5 Overcoming these issues requires the application of physical organic chemistry principles.10 For instance; the mechanism of fluorescence enhancement in boronic acid saccharide sensors has been a topic of debate.11 The original paradigm postulated for ortho-aminomethylphenylboronic acid chemosensors was a PET mechanism and a B–N bonding interaction upon sugar binding.12 The latest evidence with anthracene and 4-aminonaphthalimide models points to a solvent-induced effect via a vibrational-coupled excited-state relaxation mechanism.11a

While anthracene is an example of a fluorophore with a pure π–π* excited state, aminonaphthalimide is an example with an π–π* internal charge transfer (ICT) and a photoelectric field effect in the excited state.12 N-Aryl-aminonaphthalimides are also intriguing, because of the virtual C0-type spacer as a result of a frontier orbital node at the imide nitrogen atom.14 Typically PET systems (i.e. de Silva's first AND logic gates)2,4 are designed with methylene (C1) spacers, which allows for some degree of conformational mobility. However, the π molecular orbitals of the aminonaphthalimide fluorophore, and those of the N-aryl receptor, preferentially adopt an orthogonal geometry to prevent steric clash. Such systems are often described as possessing a non-emissive twisted internal charge transfer (TICT) excited state.15

Most naphthalimide-based chemosensors with crown ethers have been studied in organic solvents.16 Fedorova and co-workers have reported many examples of benzo-15-crown-5 and aza-crowns in acetonitrile.17 Studies in aqueous methanol or water, however, are limited to an azadithiacrown (for Hg2+),18 and N-phenylaza-dithia-15-crown-5 (for Ag+),19 an aza-15-crown-5 (for Hg2+)20 and benzo-15-crown-5 and benzo-18-crown-6 (for Na+ and K+).21

Our curiosity was stimulated by the Heagy study.21 Benzo-15-crown-5 and benzo-18-crown-6 were attached at the N-imide position of a 1,8-naphthalic anhydride while a sulfonate moiety was attached at the 4-position. Apparent binding constants of 1.12 mM (pβNa+ = 2.95) and 0.4 mM (pβK+ = 3.40) were measured in buffered water with the benzo-15-crown-5 and benzo-18-crown-6 chemosensors, respectively. The selectivity for the Na+ probe is admirable better than the N-(2-methoxyphenyl)aza-15-crown-5 receptor with its pendant methoxy moiety,22 which is used to monitor Na+ blood serum levels.23 We hypothesised that the enhanced selectivity and sensitivity for Na+ binding by the benzo-15-crown-5, as reported by Heagy,21 is due to Na+ coordinating with at least one naphthalimide carbonyl moiety.

Hence, we set out to develop molecular AND logic gates for biologically relevant analytes in water to exploit the concept of fluorophore-assisted binding. Herein we report the synthesis and photophysics of novel compounds 1–3 that function as Na+, H+ and K+, H+ logic gates in aqueous methanol and water (Fig. 1). Molecules 1 and 2 are designed with a receptor1-spacer1-fluorophore-spacer2-receptor2 format with an aminonaphthalimide fluorophore; a methylpiperazine H+ receptor; and either a benzo-15-crown-5 or benzo-18-crown-6 receptor, principally for binding Na+ and K+, respectively (Scheme 1). Compound 3, containing a 3,4-dimethoxyphenyl moiety, is included for comparison.


image file: d0ob00059k-s1.tif
Scheme 1 Synthetic protocols for the naphthalimide-based molecular logic gates 1–3.

Results and discussion

The syntheses of 1–3 are shown in Scheme 1. 4-Bromo-1,8-naphthalic anhydride or 4-chloro-1,8-naphthalic anhydride were reacted with 4′-aminobenzo-15-crown-5, 4′-aminobenzo-18-crown-6 or 3,4-dimethoxyaniline in 2-methoxyethanol (or acetic acid) at 110 °C resulting in N-(benzo-15-crown-5)-4-bromo-1,8-naphthalimide 5, N-(benzo-18-crown-6)-4-bromo-1,8-naphthalimide 7, and N-(3,4-dimethoxyphenyl)-4-bromo-1,8-naphthalimide 9 in 73%, 57% and 72% yields. The chloro-substituted analogues 4, 6 and 8 were synthesised in 62%, 61% and 85%. The intermediates 4–9 were subsequently reacted with methylpiperazine in hot DMF to obtain the target compounds 1–3. Products 1–3 were purified by column chromatography with dichloromethane/methanol and isolated as yellow, orange and yellow powders in 65%, 49% and 70% yield. Analytical spectroscopic data is provided in the Experimental section and the corresponding spectra are provided in the ESI (Fig. S1–S36).

Prior to performing the spectroscopic studies, we assessed the partition coefficients and PET thermodynamics of 1–3. The octanol-water partition coefficients (log[thin space (1/6-em)]P) of the logic gates were determined by the shake flask method.24 Sample UV-visible absorption spectra from the extraction are provided in the ESI (Fig. S37 and S38). The experimental log[thin space (1/6-em)]P values of 1–3 are 0.097, −0.216 and 0.273, which are in good agreement with the log[thin space (1/6-em)]P values of 0.15, 0.060 and 0.97 predicted for protonated 1–3 by ChemDraw Pro (version 12.19). The partition coefficients of the ionized species at pH 4.0 (log[thin space (1/6-em)]D) were calculated using eqn (1) based on a pKa of 8.1. The log[thin space (1/6-em)]D values of 1–3 are indeed substantially more negative at −4.00, −4.32 and −3.83. Hence to our satisfaction, 1–3 were predicted to be fully soluble in water under acidic conditions. Our experimental results confirmed that 1–3 are indeed hydrophilic and readily dissolve in water in contrast to our prior study of an aza-crown anthracene-based logic gate.25 We welcomed these findings as they provided quantitative evidence for shifting from 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (v/v) aqueous methanol to water.

 
log[thin space (1/6-em)]D = log[thin space (1/6-em)]P + log[1/(1 + 10(pKa-pH))] (1)

The thermodynamics for the PET driving forces were calculated using the Weller equation, eqn (2), based on electrochemical and photophysical data.4,26 The driving forces for PET from the tertiary amine and the 3,4-dimethoxyphenyl to the aminonaphthalimide fluorophore are 0.07 eV and 0.37 eV (6.7 and 35 kJ mol−1), respectively, in acetonitrile (electrochemical data in methanol or water are not readily available). These values are mildly endothermic and susceptible to the influence of solvent polarity, which is accounted for in the Coulombic term e2/εr, where ε is the solvent dielectric constant. The ion-paring term e2/εr was taken as 0.10 eV (10 kJ mol−1) in acetonitrile.27 The stabilization offered by the solvent to the radical ion pair after a PET process to a neutral molecule generally increases with solvent polarity.27 Therefore, a change in solvent polarity, in this study from 1[thin space (1/6-em)]:[thin space (1/6-em)]1 MeOH/H2O to water, could significantly alter the ΔGPET such that PET from both receptors becomes less endothermic (more exothermic), and consequently, the fluorescence quantum yield decreases. The situation with 4-aminonaphthalimides, however, is not so clear-cut as the ICT pathway near the piperazyl is assisted by a photoexcited electric field effect,28 while at the N-imide end, PET from the 3,4-dimethoxyphenyl to the excited fluorophore is hindered by a negative node at the imide nitrogen atom.

 
ΔGPET = EoxEredEse2/εr (2)

The UV-visible absorption spectra of 1–3 were first studied in both 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (v/v) MeOH/H2O and water as a function of pH with various cations including Na+ and K+ (Fig S39). Unprotonated 1–3 in water in the presence of 10−11 M H+ has a λmax at 400 nm (log[thin space (1/6-em)]ε = 3.91), which is not significantly affected in the presence of 200 mM Na+. Upon addition of 10−4 M H+, protonation of the piperazinyl nitrogen atom results in a hypsochromic (blue) and hyperchromic (increase) shift to a λmax of 391 nm (log[thin space (1/6-em)]ε = 3.96). Titrations with acid between 10−11 M and 10−2 M H+ reveals an isosbestic point at 412 nm (Fig. S40). The spectral shift is consistent with an excited state ICT due to charge repulsion between the protonated methylpiperazine and the positively charged pole at the 4-position. A summary of photophysical data is given in Table 1.

Table 1 Various parameters for 1–3 in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (v/v) methanol/water and water determined by UV-visible absorption and fluorescence spectroscopya,b
Parameter 1 1 2 2 3 3
MeOH/H2O H2O MeOH/H2O H2O MeOH/H2O H2O
a 10−5 μM 1 excited at λisos.b High H+ level 10−4 M. Low H+ level 10−9 M by addition of 0.10 M HCl or 0.10 M Bu4NOH solution (25% wt in H2O).c High Na+ or K+ 200 mM NaCl or KCl. Low Na+ or K+ level with no salt added.d Molar absorptivity ε in L mol−1 cm−1.e Quantum yields measured with reference to quinine sulfate in 0.1 M H2SO4 water.f H+-induced fluorescence enhancement (FE) IFpH 4/IFpH 9.g Determined by log[(ImaxI)/(IImin)] = −log[M+] + log[thin space (1/6-em)]βH+ from emission spectra.h Value for 18-crown-6 in methanol. Ref. 6.
λAbs pH 9/nmc 408 400 400 398 413 403
log[thin space (1/6-em)]εpH 9d 3.99 3.70 3.99 3.91 4.00 3.94
λAbs pH 4/nmc 391 391 388 390 390 392
log[thin space (1/6-em)]εpH 4d 4.09 4.07 4.09 3.96 4.12 3.92
λisos/nm 400 412 395 408 401 413
λflu pH 4/nmc 533 540 532 540 536 541
Φfe 0.21 0.14 0.28 0.26 0.05 0.08
FEf 26 4 11 6 3 4
pβH+g 7.5 8.2 7.4 8.0 7.4 7.2
pβNa+g 1.6 0.86 4.4h 1.8 0.88 0.18
pβK+g 1.3 0.76 2.6 1.6


The fluorescence spectra of 1 in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (v/v) MeOH/H2O as a function of H+ and/or Na+ inputs are shown in Fig. 2. The peak maxima are observed at 525 nm, which is characteristic of a green coloured emission at high H+ and Na+ levels. Low threshold concentrations of H+ or Na+, or the absence of both H+ and Na+, results in little fluorescence at 10−9 M H+. At a lower proton concentration of 10−11 M H+, adjusted with 0.10 M Bu4NOH solution, no emission is observed. In contrast, in the presence of excess threshold levels of H+ and Na+, the fluorescence is substantially high as shown for 1 in Fig. 2. A similar outcome was observed for 2 at high H+ and K+ levels. This pattern of three low and one high emission states exemplifies AND logic. Truth tables for 1 and 2, including the quantum yields of fluorescence (Φf), are given in Table 2 and Table S1, respectively.


image file: d0ob00059k-f2.tif
Fig. 2 Fluorescence spectra of 7 μM 1 in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (v/v) MeOH/H2O at 10−9 M H+ or 10−4 M H+ and 200 mM M Na+. See Table 2 for specific details.
Table 2 Truth tables for logic gate 1 in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (v/v) methanol/water and watera
Input1 (H+)b Input2 (Na+)c Output ΦFd MeOH/H2O Output ΦFd H2O
a 7 μM 1 excited at 405 nm.b High H+ level 10−4 M. Low H+ level 10−9 M by addition of 0.10 M HCl or 0.10 M Bu4NOH solution (25% wt in H2O).c High Na+ level 200 mM NaCl. Low Na+ level no NaCl added.d Quantum yields measured with reference to quinine sulfate in 0.1 M H2SO4 water.
0 (low) 0 (low) 0 (low, 0.007) 0 (low, 0.020)
0 (low) 1 (high) 0 (low, 0.038) 0 (low, 0.066)
1 (high) 0 (low) 0 (low, 0.052) 1 (high, 0.11)
1 (high) 1 (high) 1 (high, 0.21) 1 (high, 0.14)


Spectrofluorimetric pH titrations were performed for 1–3 at a constant ionic strength of 0.20 M salt in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (v/v) MeOH/H2O and water. The binding constants were evaluated in the presence of excess amount of the other input (either 10−4 M H+ or 0.20 M salt) such that the crown ether receptor would be saturated or the methylpiperazine fully protonated. The ion binding constants were determined from IF-pM profiles13 according to eqn (3)

 
log[(IFmaxIF)/(IFIFmin)] = −log[M+] − log[thin space (1/6-em)]βM+ (3)
where pM = −log[M+] and M+ is either H+, Na+ or K+. From the IF-pM titrations, the binding constants (pβM+) were determined as the concentration at which half the receptor population is occupied by the specific analyte. Sigmoidal-shaped curves were observed over three logarithm units (Fig. S40). The data was fit to a linearised form of the Henderson–Hasselbalch, eqn (3). The pβNa+ for 1 are 1.6 and 0.86 in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (v/v) MeOH/H2O and water, and the pβK+ for 2 are 2.6 and 1.6 in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (v/v) MeOH/H2O and water, respectively. The pβH+ are 7.4 ± 0.1 and 8.1 ± 0.1 in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (v/v) MeOH/H2O and water, respectively. The data are tabulated in Table 1.

Table 3 summarises the fluorescence quantum yields (Φf), Na+ and K+ binding (pβNa+, pβK+) and H+ binding (pβH+) constants of logic gates 1 and 10–13 (Fig. 3). The data for 10–13 are from published literature in methanol or aqueous alcoholic solution.4,21,29,31,38 Molecules 10 and 11 are naphthalimide-based H+-driven and Na+-driven YES gates, and 12 and 13 are Na+, H+-driven two-input AND, and Na+, H+, Zn2+-driven three-input AND gates, respectively. Logic gate 1 is reminiscent of de Silva's AND logic gate 12 with its ‘receptor1-spacer1-fluorophore-spacer2-receptor2’ connectivity. However, while 12 has a pure ππ* excited state, 1 has a ππ* ICT excited state due to the induced dipole moment with the unsymmetrical fluorophore. Another design difference within 1 and 2 is the virtual C0-type and C2-diethylene spacers rather than the C1-methylene spacers in 12 and 13.


image file: d0ob00059k-f3.tif
Fig. 3 Naphthalimide and anthracene-based logic gates 10–13.
Table 3 Fluorescence quantum yields (ΦFmax) and binding constants (pβM+) of logic gates 10–13 and 1 in methanol, 50% methanol or water
Parameters 1 1 10a 11b 12c 13d 13d
ΦFmax in the presence of excess H+ and/or Na+ cations.a Ref. 31.b Ref. 21.c Ref. 4.d Ref. 38.e pβM+ = −log[thin space (1/6-em)]βM+ where M+ = Na+ or H+.
Solvent MeOH/H2O H2O 1[thin space (1/6-em)]:[thin space (1/6-em)]4 (v/v) MeOH/H2O H2O MeOH MeOH/H2O H2O
ΦFmax 0.21 0.14 0.58 0.22 0.070 0.020
pβNa+e 1.6 0.86 1.1 2.7 0.9 −0.3
pβH+e 7.5 8.2 7.4 8.9 7.8 7.8


A reference point for our discussion is the dibutylated aminonaphthalimide dye. It exemplifies a PASS 1 logic gate – between pH 4–12 the Φf is constant at 0.23 in 1[thin space (1/6-em)]:[thin space (1/6-em)]4 (v/v) methanol/water.30 Dialkylamino substituents at the 4-amino position contribute towards an enhanced fluorescence on protonation with fluorescence quantum yields of ∼0.70 to 0.80, and in the case of 10, the Φf is comparable at 0.58.31 This value placed an upper limit on the expected Φf of 1–3. The excited state pβH+ of 7.4 is in agreement with 1 corroborating the modular design. Comparison to 11 provides insight into the contribution from the benzo-15-crown-5. The sulfonate enhances water-solubility, and contributes an inductive Hammett effect (σp (SO3) = 0.35)32 towards a favourable PET from the benzo-15-crown-5 to the fluorophore. The pβNa+ of 1.1 attests to the weaker binding in aqueous media. Two anthracene analogues of 11 with benzo-15-crown-5 have a pβNa+ of 3.0 and 2.7 in MeOH.33

We take the opportunity to compare 1 versus 13 in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (v/v) methanol/water and 1121 versus 13 in water in order to delineate the contribution from the naphthalimide carbonyl towards complexation of Na+ with the crown ether. In 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (v/v) methanol/water, the pβNa+ of 1 and 13 are 1.6 and 0.9, whilst in water the pβNa+ of 11 and 13 are 1.1 and −0.3, respectively. Hence, coordination of Na+ to the naphthalimide carbonyl contributes 0.7–1.4 log units towards the Na+ binding constant. The origin of this phenomenon was elucidated on the basis of DFT calculations (Fig. 4).


image file: d0ob00059k-f4.tif
Fig. 4 Geometries and distributions of electrostatic potential (mapped on total electron density) calculated at B3LYP/6-31+g(d,p) theory level in methanol modelled with the IEFPCM approach. Red patches indicate areas bearing negative charge, green are electrically neutral, and blue are positively charged. The identical full colour scale of ±0.05 elemental charge units are used for the sake of comparison.

The perpendicular orientation of the organic chromophores (naphthalimide and anthracene moieties) excludes any significant cation π interactions. Both carbonyls of the naphthalimide are highly negative, but one is closer to the benzocrown ether cavity, consequently, there is a slight asymmetry – so a negative charge ‘pocket’ is formed. Therefore, at least one of the carbonyl groups of the aminonaphthalimide moiety contribute towards the formation of a negatively charged cation-binding pocket in 1 and 2, whereas the anthracene-based sensor 12 does not show any chromophore-induced charge redistribution. Hence, the aminonaphthalimide is not acting in the traditional manner with respect to the signal analyte binding. In fact, there is a cooperative, synergistic contribution, increasing the negative charge at the receptor site, which enhances cation binding. This is a significant finding as benzocrown ethers are inherently poor at coordinating alkaline metal ions such Na+ and K+ in water. Ion-dipole interactions primarily occur at the two oxygen atoms of the 1,2-dimethoxybenzene, and not as much with the aliphatic oxygen atoms.34 Hence, the presence of a fluorophore carbonyl within the vicinity of the benzocrown increases cation affinity analogous to an object held between four fingers and a thumb (Fig. 4, Compound 1). Similar phenomena was observed by Valeur with coumarin C153 linked to dibenzo-16-crown-5 and tribenzo-19-crown-6, but in acetonitrile and ethanol, respectively.35

Generally, the binding constants and fluorescence emission quantum yields tend to be lower in water than in organic solvents because of differences in solvation energies. Specifically, the effect of solvent polarity on the Φf of aminonaphthalimide dyes is attributed to a non-radiative deactivation.36 Often times, the electron transfer pathway is never fully prevented, which undermines the quality of the Φf switching between the off and on states.37 For instance, the lower Φf of 13 is due to a residual PET from the benzo-15-crown-5 to the excited anthracene as the Na+ ion does not adequately lower the HOMO level of the benzo-15-crown-5 receptor. ‘Lab-on-a-molecule’ 13 required 0.50 M Na+ (pβNa+ = −0.30) in water to turn on compared to only 0.86 mM (pβNa+ = 3.07) and 1.12 mM (pβNa+ = 2.95) for 1 and 11 (Table 3).

The benzocrown derivatives 1 and 2 showed no significant selectivity in water (Fig. S41, Table 2 and Table S1). The Φf is high at 10−4 M H+ regardless of the presence of the second guest species, whilst at 10−9 M H+ the Φf is low and the green fluorescence turned off. This observation is due to the weak electron-donating capability of the benzocrown, as predicted by the Weller equation, and the weaker interaction between the benzocrown receptor and the guest cation. As eluded to earlier, benzocrowns bind cations in water rather weakly (the dibenzo-18-crown-6 equilibrium binding constant for K+ equals ca. 42 in water, 560 in 50% methanol and over 70[thin space (1/6-em)]000 in methanol).39

Therefore, the systems can be considered, in terms of Boolean logic, as H+-driven YES gates in water where the second input (metal ion) is neglected (Fig. 5). In contrast, the molecules function as Na+, H+ and K+, H+-driven two-input AND logic gates in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (v/v) MeOH/H2O. Hence, we have an example of logic function modulation by a change in solvent polarity.40 The effect is postulated to be a polarity-modulated interaction between the guest and host moieties.


image file: d0ob00059k-f5.tif
Fig. 5 Combinatorial logic circuit of 1 and 2 with input A = H+ and input B = Na+ modulated by solvent polarity in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (v/v) MeOH/H2O (output D) and H2O (output C).

Lastly, we examine the case in water where both chemical inputs are considered. According to Table 2 for 1 (and Table S1 for 2), we have a situation where a bright fluorescence is observed for two of the four input conditions. Fluorescence (output 1) is observed when both H+ and M+ are high (inputs 1, 1) and when H+ is high and M+ is low (inputs 1, 0). These results are characteristic of the output of individual AND and INHIBIT logic gates, respectively. Therefore, 1 and 2 provide a fluorescent signal according to the Boolean sum-of-products expression A·B + A·[B with combining overline]. In other words, in water, 1 and 2 operate as a combinatorial logic circuit (Fig. 6.) incorporating an AND gate (symbolized by A·B) and an INHIBIT gate (symbolized by A·[B with combining overline]) in parallel and connected sequentially to an OR logic gate (symbolized by +).41


image file: d0ob00059k-f6.tif
Fig. 6 A combinatorial logic circuit representation of 1 and 2 in water with integrated two-input AND, INHIBIT and OR functions. The output equals the sum-of-products expression O = A·B + A·[B with combining overline]. Accordingly, in Table 2: A = H+, B = Na+ and O = Φf.

Conclusions

Aminonaphthalimide-based molecular logic gates were synthesised with a ‘receptor1-spacer1-fluorophore-spacer2-receptor2’ format. The molecules incorporate virtual C0-type and diethylene (C2) spacers, and benzocrown and methylpiperazine receptors. We have shown that: (1) PET from the dimethoxyphenyl electron donor is disrupted on increasing solvent polarity, (2) enhanced crown ether binding occurs via Na+ or K+ interaction with an aminonaphthalimide carbonyl, and (3) YES and AND Boolean logic gate operations are modulated by Na+, K+ and H+ inputs, in water and methanol/water, respectively. These findings highlight an alternative approach to regulating the logic functions of fluorophores endowed with an ICT excited state and delineate the limitations of the PET/ICT design model. Future studies could dwell into the PET kinetics to better understand the activation barriers and electronic coupling factors.28 These prototypes hold promise as fluorescent sensors for probing the microenvironment of protein and membrane interfaces,7,8,42 and as building blocks for the development of more complex logic gates including ‘lab-on-a-molecule’ systems.38,43

Experimental

A list of chemicals and instrumentation are provided in the ESI. The syntheses of 1–3 are shown in Scheme 1. Synthetic procedures and spectroscopic analytical data are given below.

2-(2,3,5,6,8,9,11,12-Octahydrobenzo[b][1,4,7,10,13]pentaoxacyclo pentadecin-15-yl)-6-(piperazin-1-yl)-1H-benzo[de]isoquinoline-1,3 (2H)-dione 15-crown-5 (1)

Benzo-15-crown-5-4-bromo-1,8-naphthalimide (150 mg, 0.28 mmol) and excess methylpiperazine (0.050 mL, 0.45 mmol) were dissolved in 3 mL of DMF. The flask was warmed and stirred at 120 °C for 24 hours. On cooling two drops of tetrabutylammonium hydroxide were added. The solution was diluted with 30 mL of CH2Cl2, washed with water (3 × 10 mL) and the solvent removed by rotary evaporation to give a yellow solid, which was purified by column chromatography (102 mg, 65%). Rf (9[thin space (1/6-em)]:[thin space (1/6-em)]1 CH2Cl2/MeOH) = 0.45; m.p. 232 °C (dec.); 1H NMR (500 MHz, CDCl3, ppm): δH 8.61 (d, J = 6.3 Hz, 1H, Ar–H), 8.54 (d, J = 7.5 Hz, 1H, Ar–H), 8.45 (d, J = 7.4 Hz, 1H, Ar–H), 7.71 (m, 1H, Ar–H), 7.26 (s, 1H, Ar–H), 7.00 (d, J = 8.3 Hz, 1H, Ph–H), 6.85 (d, J = 8.2 Hz, 1H, Ph–H), 6.82 (s, 1H, Ph–H), 4.17 (m, 4H, –h(OC[H with combining low line]2CH2O–)2), 3.91 (m, 4H, –Ph(OCH2C[H with combining low line]2O–)2), 3.77 (m, 8H, (–OCH2CH2O–)2), 3.33 (s, 4H, –N(C[H with combining low line]2CH2)2NCH3), 2.76 (s, 4H, –N(CH2C[H with combining low line]2)2NCH3), 2.45 (s, 3H, –NCH3); 13C NMR (126 MHz, CDCl3, ppm): δC 46.11, 52.96 (2C), 55.12 (2C), 68.85, 69.04, 69.25, 69.34, 70.48 (2C), 70.91 (2C), 113.89, 114.32, 115.01, 116.68, 121.40, 123.37, 125.65, 126.24, 128.79, 130.23, 130.60, 131.46, 132.95, 148.88, 149.33, 156.20, 164.24, 164.75; IR νmax (NaCl, cm−1): 3072, 2935, 2875, 2853, 2793, 2743, 1694, 1654, 1587, 1517, 1510, 1452, 1373, 1247, 1141, 1079, 1054, 1009, 979, 928, 786, 754; HRMS (ESI TOF): m/z cal. C31H36N3O7 [M + H] 562.2553, found 562.2557.

2-(2,3,5,6,8,9,11,12,14,15-Decahydrobenzo[b][1,4,7,10,13,16]hexa oxacyclooctadecin-18-yl)-6-(4-methylpiperazin-1-yl)-1H-benzo[de] isoquinoline-1,3(2H)-dione 18-crown-6 (2)

Compound 2 was prepared similar to 1. Yellow solid (40 mg, 49%); Rf (9[thin space (1/6-em)]:[thin space (1/6-em)]1 CH2Cl2/MeOH) = 0.60; m.p. 320 °C (dec.); 1H NMR (500 MHz, CDCl3, ppm): δH 8.64 (d, J = 6.2 Hz, 1H, Ar–H), 8.57 (d, J = 6.5 Hz, 1H, Ar–H), 8.47 (d, J = 8.9 Hz, 1H, Ar–H), 7.75 (t, J = 7.3 Hz, 1H, Ar–H), 7.27 (t, J = 9.8 Hz, 1H, Ar–H), 7.04 (d, J = 8.8 Hz, 1H, Ph–H), 6.87 (dd, J = 8.5 Hz, 2.4 Hz, 1H, Ph–H), 6.84 (s, 1H, Ph–H), 4.24 (m, 4H, –Ph(OC[H with combining low line]2CH2O–)2), 3.98 (m, 4H, –Ph(OCH2C[H with combining low line]2O)2–), 3.78–3.82 (m, 12H, (–OCH2CH2O–)3), 3.37 (s, 4H, –N(C[H with combining low line]2CH2)2NCH3), 2.79 (s, 4H, –N(CH2C[H with combining low line]2)2NCH3), 2.48 (s, 3H, –NCH3); 13C NMR (126 MHz, CDCl3, ppm): δC 45.97, 53.67 (2C), 55.14 (2C), 68.80, 68.96, 69.38, 69.47, 70.58, 70.61, 70.74 (2C), 70.77, 70.81, 113.71, 114.14, 115.07, 116.72, 121.23, 123.41, 125.68, 126.27, 128.65, 130.26, 130.61, 131.47, 132.94, 148.84, 149.30, 156.19, 164.15, 164.80; IR νmax (KBr, cm−1): 3087, 3035, 2990, 2879, 2766, 2710, 1695, 1660, 1585, 1520, 1516, 1380, 1245, 1127, 1091, 1002, 975, 790, 763; MS (ES-TOF, 2.81 mV) m/z (%): 312(13), 335(13), 524(32), 525(13), 606(20), 62(30), 628(100); MS (ES-TOF, 298 mV) m/z (%): 312(9), 314(9), 335(12), 337(12), 524(28), 525(11), 606(20); HRMS (ESI TOF): m/z cal. C33H40N3O8 [M + H] 606.2815, found 606.2830.

2-(3,4-Dimethoxyphenyl)-6-(4-methylpiperazin-1-yl)-1H-benzo[de] isoquinoline-1,3(2H)-dione (3)

3,4-Dimethoxyaniline-4-chloro-1,8-naphthalimide (100 mg, 0.27 mmol) and excess methylpiperazine (0.050 mL, 0.45 mmol) were dissolved in 20 mL of DMF and heated at 130 °C for 48 hours. Heptane was added to the DMF solution to form an azeotropic mixture, which was removed by rotary evaporation. The residue was dissolved in CH2Cl2, washed with water (3 × 10 mL) and the CH2Cl2 removed by rotary evaporator to yield a yellow solid, which was washed with 10 mL of water and dried under vacuum (82 mg, 70%); Rf (95[thin space (1/6-em)]:[thin space (1/6-em)]5 CH2Cl2/MeOH) = 0.94; m.p. 230 °C (dec.); 1H NMR (500 MHz, CDCl3, ppm): δH 8.63 (d, J = 7.2 Hz, 1H, Ar–H), 8.56 (d, J = 8.0 Hz, 1H, Ar–H), 8.46 (d, J = 8.4 Hz, 1H, Ar–H), 7.73 (t, J = 7.3 Hz, 1H, Ar–H), 7.25 (d, J = 8.0 Hz, 1H, Ar–H), 7.02 (d, J = 8.5 Hz, 1H, Ph–H), 6.88 (d, J = 8.4 Hz, 1H, Ph–H), 6.81 (s, 1H, Ph–H), 3.95 (s, 3H, –OCH3), 3.88 (s, 3H, –OCH3), 3.35 (s, 4H, –N(C[H with combining low line]2CH2)2N CH3), 2.78 (s, 4H, –N(CH2C[H with combining low line]2)2NCH3), 2.47 (s, 3H, –NCH3); 13C NMR (126 MHz, CDCl3, ppm): δC 46.16, 53.02, 55.16, 55.91, 56.04, 111.40, 111.84, 115.05, 116.72, 120.79, 123.42, 125.70, 126.29, 128.37, 130.30, 130.67, 131.52, 133.01, 149.08, 149.57, 156.28, 164.38, 164.90; IR νmax (KBr, cm−1): 3075, 2955, 2853, 2802, 1701, 1649, 1593, 1516, 1381, 1240, 1189, 1002, 783; MS (ES-TOF, 298 mV) m/z (%): 174(3), 432(57), 433(15), 473(32, M + MeCN), 473(65, M + MeCN + Na); HRMS (ESI TOF): m/z cal. C25H26N3O4 [M + H] 432.1923, found 432.1927.

6-Chloro-2-(2,3,5,6,8,9,11,12-octahydrobenzo[b][1,4,7,10,13]penta oxacyclopentadecin-15-yl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (4)

Compound 4 was prepared similar to 5. Light brown solid (360 mg, 62%); Rf (9[thin space (1/6-em)]:[thin space (1/6-em)]1 CH2Cl2/MeOH) = 0.40; m.p. 210–213 °C; 1H NMR (500 MHz, CDCl3, ppm): δH: 8.70 (d, J = 7.2 Hz, 1H, Ar–H), 8.66 (d, J = 8.5 Hz, 1H, Ar–H), 8.55 (d, J = 7.9 Hz, 1H, Ar–H), 7.89 (t, J = 8.0 Hz, 1H, Ar–H), 7.86 (d, J = 7.9 Hz, 1H, Ar–H), 7.01 (d, J = 8.5 Hz, 1H, Ph–H), 6.86 (dd, J = 8.5 Hz, 2.4 Hz, 1H, Ph–H), 6.81 (d, J = 2.4 Hz, 1H, Ph–H), 4.18 (m, 4H, –Ph(OC[H with combining low line]2CH2O–)2), 3.92 (m, 4H, –Ph(OCH2C[H with combining low line]2O–)2), 3.77 (m, 8H, (–OCH2CH2O–)2); 13C NMR (126 MHz, CDCl3, ppm): δC 68.98, 69.11, 69.42, 69.51, 70.53, 70.58, 71.04, 71.08, 113.91, 114.23, 121.23, 121.70, 123.21, 127.47, 127.94, 128.12, 129.35, 129.42, 130.93, 131.47, 132.38, 139.37, 149.29, 149.59, 163.74, 164.01; IR νmax (KBr, cm−1): 3080, 3068, 2930, 2874, 2816, 1701, 1663, 1589, 1574, 1516, 1506, 1371, 1242, 1128, 1055, 785, 765, 750; MS (ES-TOF, 2.82 mV) m/z (%): 177(100), 178(13), 224(9), 334(20), 498(M + H, 11), 515(35), 520 (M + Na, 85), 521(25), 522(32), 523(9); HRMS m/z cal. C26H24NO7NaCl [M + Na] 520.1139, found 520.1146.

6-Bromo-2-(2,3,5,6,8,9,11,12-octahydrobenzo[b][1,4,7,10,13] penta oxacyclopentadecin-15-yl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (5)

4-Aminobenzo-15-crown-5 (130 mg, 0.45 mmol) and 4-bromo-1,8-naphthalic anhydride (270 mg, 0.97 mmol) were dissolved in 5 mL of 2-methoxyethanol in a 100 mL round-bottom flask. The reaction mixture was stirred and heated at 110 °C for 20 hours. The solvent was removed by rotary evaporation. Washing with cold 2-methoxyethanol gave an off-white powder (180 mg, 73%). Rf (9[thin space (1/6-em)]:[thin space (1/6-em)]1 CH2Cl2/MeOH) = 0.41; m.p. 235–238 °C; 1H NMR (500 MHz, CDCl3, ppm): δH 8.70 (d, J = 7.2 Hz, 1H, Ar–H), 8.63 (d, J = 8.4 Hz, 1H, Ar–H), 8.45 (d, J = 7.8 Hz, 1H, Ar–H), 8.08 (d, J = 7.9 Hz, 1H, Ar–H), 7.88 (t, J = 7.9 Hz, 1H, Ar–H), 7.01 (d, J = 8.5 Hz, 1H, Ph–H), 6.86 (dd, J = 8.5 Hz, 2.2 Hz, 1H, Ph–H), 6.81 (d, J = 2.2 Hz, 1H, Ph–H), 4.18 (m, 4H, –Ph(OC[H with combining low line]2CH2O–)2), 3.92 (m, 4H, –Ph(OCH2C[H with combining low line]2O–)2), 3.78 (d, J = 7.3 Hz, 8H, (–OCH2CH2O–)2); 13C NMR (126 MHz, CDCl3, ppm): δC 69.16, 69.31, 69.56, 69.66, 70.63, 70.67, 71.27, 71.30, 114.08, 114.32, 121.20, 122.40, 123.27, 128.10, 128.18, 129.30, 130.61, 130.76, 131.21, 131.57, 132.42, 133.57, 149.43, 149.74, 163.86, 163.90; IR νmax (KBr, cm−1): 3080, 3067, 2930, 2872, 2816, 1701, 1662, 1587, 1518, 1506, 1367, 1240, 1144, 1047, 785, 766, 750; MS (ES-TOF, 2.81 mV) m/z (%): 177(100), 178(13), 224(9), 391(4), 542(M, 10), 544(11), 559(28), 561(29), 564(73), 565(21), 566(72), 567(21); HRMS m/z cal. C26H24NO7NaBr [M + Na] 564.0634, found 564.0657.

6-Chloro-2-(2,3,5,6,8,9,11,12,14,15-decahydrobenzo[b] [1,4,7,10, 13,16]hexaoxacyclooctadecin-18-yl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (6)

Compound 6 was prepared similar to 5. White solid (140 mg, 61%); Rf (9[thin space (1/6-em)]:[thin space (1/6-em)]1 CH2Cl2/MeOH) = 0.28; m.p. 202–206 °C; 1H NMR (500 MHz, CDCl3, ppm): δH 8.70 (d, J = 7.2 Hz, 1H, Ar–H), 8.67 (d, J = 8.5 Hz, 1H, Ar–H), 8.55 (d, J = 7.9 Hz, 1H, Ar–H), 7.89 (m, J = 7.7 Hz, 1H, Ar–H), 7.87 (d, J = 8.0 Hz, 1H, Ar–H), 7.02 (d, J = 8.5 Hz, 1H, Ph–H), 6.86 (dd, J = 2.3 Hz, 8.5 Hz, 1H, Ph–H), 6.81 (d, J = 2.3 Hz, 1H, Ph–H), 4.20 (m, 4H, –Ph(OC[H with combining low line]2CH2O–)2), 3.95 (m, 4H, –Ph(OCH2C[H with combining low line]2O)2–), 3.73–3.79 (m, 12H, (–OCH2CH2O–)3); 13C NMR (126 MHz, CDCl3, ppm): δC 69.03, 69.13, 69.43, 69.52, 70.65, 70.69, 70.81 (2C), 70.86, 70.93, 113.76, 114.05, 121.11, 121.75, 123.26, 127.48, 127.94, 128.03, 129.39, 129.45, 130.94, 131.50, 132.41, 139.36, 149.13, 149.43, 163.77, 164.03; IR νmax (KBr, cm−1): 3084, 3071, 2934, 2883, 2876, 1705, 1683, 1663, 1589, 1516, 1506, 1373, 1242, 1126, 1083, 959, 786; MS (ES-TOF, 0.395 mV) m/z (%): 177(5), 559(22), 560(21), 561(20), 564(M + Na, 100), 566(36), 567(11); HRMS m/z cal. C28H28NO8NaCl 564.1401, found 564.1405.

6-Bromo-2-(2,3,5,6,8,9,11,12,14,15-decahydrobenzo[b] [1,4,7,10, 13,16]hexaoxacyclooctadecin-18-yl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (7)

Compound 7 was prepared similar to 5. Off-white solid (100 mg, 57%); Rf (9[thin space (1/6-em)]:[thin space (1/6-em)]1 CH2Cl2/MeOH) = 0.35; m.p. 197 °C (dec.); 1H NMR (500 MHz, CDCl3, ppm): δH 8.71 (d, J = 7.1 Hz, 1H, Ar–H), 8.62 (d, J = 8.5 Hz, 1H, Ar–H), 8.46 (d, J = 7.8 Hz, 1H, Ar–H), 8.06 (d, J = 7.9 Hz, 1H, Ar–H), 7.88 (t, J = 8.0 Hz, 1H, Ar–H), 7.02 (d, J = 8.4 Hz, 1H, Ph–H), 6.87 (dd, J = 8.3 Hz, 2.3 Hz, 1H, Ph–H), 6.82 (d, J = 2.4 Hz, 1H, Ph–H), 4.23 (m, 4H, –Ph(OC[H with combining low line]2CH2O–)2), 3.92 (m, 4H, –Ph(OCH2C[H with combining low line]2O)2–), 3.70–3.79 (m, 12H, (–OCH2CH2O–)3); 13C NMR (126 MHz, CDCl3, ppm): δC 68.18, 69.29, 69.51, 69.60, 70.74, 70.78, 70.89(2C), 70.96, 70.99, 114.00, 114.23, 121.14, 122.42, 123.30, 128.06, 128.19, 129.34, 130.62, 130.80, 131.22, 131.70, 132.43, 133.59, 149.25, 149.56, 163.88, 163.91; IR νmax (KBr, cm−1): 3080, 3079, 2930, 2875, 1699, 1680, 1662, 1595, 1520, 1516, 1370, 1242, 1130, 1087, 950, 785; MS (ES-TOF, 2.81 mV) m/z (%): 162(7), 224(18), 338(13), 390(86), 391(28), 418(13), 482(37), 511(24), 529(37), 603(97), 605(100), 610(63), 611(25); HRMS m/z cal. C28H28NO8NaBr [M + Na] 608.0896, found 608.0904.

6-Chloro-2-(3,4-dimethoxyphenyl)-1H-benzo[de]isoquinoline-1,3 (2H)-dione (8)

4-Chloro-1,8-naphthalimide anhydride (1.7 g, 7.3 mmol) and 3,4-dimethoxyaniline (2.6 g, 11 mmol) were dissolved in 3 mL of glacial acetic acid and heated at 130 °C for 48 hours. After cooling, hexane was added and the azeotropic mixture removed by rotary evaporation. The product was washed with acetone and dried by suction filtration to yield a white solid (2.3 g, 85%), Rf = 0.85 (9[thin space (1/6-em)]:[thin space (1/6-em)]1 CH2Cl2/CH3OH), m.p. = 287.5–289.0 °C (dec.); 1H NMR (500 MHz, CDCl3, ppm): δH 8.71 (dd, 1H, J = 7.3 Hz, 1.1 Hz, Ar–H), 8.66 (dd, 1H, J = 8.5 Hz, 1.1 Hz, Ar–H), 8.55 (d, 1H, J = 7.8 Hz, Ar–H), 7.89 (dd, 1H, J = 8.5 Hz, 7.4 Hz, Ar–H), 7.86 (d, 1H, J = 7.8 Hz, Ar–H), 7.03 (d, 1H, J = 8.4 Hz, Ph–H), 6.88 (dd, 1H, J = 8.6 Hz, 2.4 Hz, Ph–H), 6.81 (d, 1H, J = 2.6 Hz, Ph–H), 3.95 (s, 3H, –OCH3), 3.88 (s, 3H, –OCH3); 13C NMR (126 MHz, CDCl3, ppm): δC 55.92, 56.05, 111.37, 111.65, 120.65, 121.68, 123.20, 127.72, 127.90, 129.34, 129.40, 130.92, 131.46, 132.37, 139.36, 149.25, 149.61, 163.79, 164.05; IR νmax (KBr, cm−1): 3076, 2937, 2836, 1713, 1658, 1591, 1514, 1370, 1241, 1138, 1024, 781, 747; MS (ES-TOF) m/z (%): 205(15), 243(10), 299(15), 368(100), 369(22), 370(34); HRMS m/z cal. C20H15NO4Cl 368.0690, found 368.0691.

6-Bromo-2-(3,4-dimethoxyphenyl)-1H-benzo[de]isoquinoline-1,3 (2H)-dione (9)

Compound 9 was prepared similar to 8. Off-white solid (534 mg, 72%); Rf = 0.90 (9[thin space (1/6-em)]:[thin space (1/6-em)]1 CH2Cl2/CH3OH), m.p. = 280 °C (dec.); 1H NMR (500 MHz, CDCl3, ppm): δH 8.71 (dd, 1H, J = 7.0 Hz, 0.9 Hz, Ar–H), 8.64 (dd, 1H, J = 8.6 Hz, 0.7 Hz, Ar–H), 8.46 (d, 1H, J = 7.8 Hz, Ar–H), 8.08 (d, 1H, J = 7.9 Hz, Ar–H), 7.89 (dd, 1H, J = 8.3 Hz, 7.5 Hz, Ar–H), 7.03 (d, 1H, J = 8.5 Hz, Ph–H), 6.89 (dd, 1H, J = 8.4 Hz, 2.3 Hz, Ph–H), 6.81 (d, 1H, J = 2.3 Hz, Ph–H), 3.95 (s, 3H, –OCH3), 3.88 (s, 3H, –OCH3); 13C NMR: (126 MHz, CDCl3, ppm): δC 55.97, 56.07, 111.43, 111.67, 120.69, 122.04, 123.28, 127.75, 128.20, 129.35, 130.69, 130.81, 131.24, 131.62, 132.48, 133.65, 149.31, 163.84, 163.99, 164.03; IR νmax (KBr, cm−1): 3075, 2963, 2835, 1706, 1659, 1513, 1368, 1239, 1139, 1028, 780; MS (ES-TOF) m/z (%): 414(100), 412(97), 240(10), HRMS cal. C20H15NO4Br 412.0184, found 412.0176.

DFT calculations were performed using Gaussian 09 Revision E.01 software package using standard B3LYP functional and 6-31+g(d,p) basis.44 The Polarizable Continuum Model using the integral equation formalism variant (IEFPCM) was used to account for solvent effects, with methanol as a solvent.45 Chemical names were obtained using Chemdraw Ultra version 12.0.2.1076.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

Financial support is gratefully acknowledged from the University of Malta and the Strategic Educational Pathways Scholarship (Malta) program part-financed by the European Social Fund (ESF) under Operational Programme II – Cohesion Policy 2007-2013. KS acknowledges financing from the National Science Centre (Poland) under contract UMO-2015/17/B/ST8/01783. DFT calculations were performed at the PL-Grid Infrastructure (Academic Computing Centre CYFRONET AGH) under computational grant GRAPHENE5. Prof. Robert M. Borg is appreciatively thanked for NMR training and assistance.

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Footnotes

This paper is a contribution on The Mechanics of Supramolecular Chemistry to commemorate the 60th birthday of Eric Anslyn.
Electronic supplementary information (ESI) available: Synthetic and experimental details, UV-visible absorption, fluorescence, 1H & 13C NMR, IR, mass spectra and truth Table S1. See DOI: 10.1039/d0ob00059k

This journal is © The Royal Society of Chemistry 2020