Dissipative control of the fluorescence of a 1,3-dipyrenyl calix[4]arene in the cone conformation

Abstract

The temporal control (ON/OFF/ON) of the fluorescence of a dichloromethane/acetonitrile 1 : 1 solution of calixarene 3 decorated with two pyrenyl moieties at the upper rim is attained by the addition of CCl₃CO₂H used as a convenient chemical fuel.

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Introduction

Biochemical systems often exploit the energy derived from the consumption of a chemical fuel to accomplish their function under a dissipative regime.¹ For this reason, in recent years a great effort has been devoted to the design of stimuli-responsive artificial systems, which operate under dissipative conditions, achieving impressive results in the fields of self-assembly,² synthetic DNA nanotechnology,³ and fully abiotic molecular machines.⁴ Lately, a number of dissipative systems have been developed, whose operation was made possible by transient variations of the solution pH. Such examples involve (i) hydrolysis of activated lactones,⁵ (ii) biocatalytic enzymatic reaction networks,⁶ (iii) the thiosulfate/sulfite redox oscillator,⁷ (iv) irradiation of merocyanine-based metastable photoacids,⁸ and (v) decarboxylation of activated carboxylic acids, such as 2-cyano-2-phenylpropanoic acid⁹ and its derivatives,¹⁰ trichloroacetic acid,¹¹ and nitroacetic acid.¹² For instance, in 2020 we employed 2-cyano-2-phenylpropanoic acid 2 as a chemical fuel⁴ to control over time the “breathing” movement¹³ between the two pinched-cone conformations of 1,3-diaminocalix[4]arene 1 (Fig. 1).^10c In the presence of the fuel acid, monoprotonation of 1 forces the calixarene conformation in a “locked” geometry with the two aniline rings slightly convergent to share the proton (proton-bridged pinched-cone conformation). When the fuel is exhausted, the calixarene reverts to its original “unlocked” state. Here we show that trichloroacetic acid 4 (Fig. 1) can be conveniently used to control, in a dissipative manner, the geometry of 1,3-dipyrenyl-2,4-diaminocalix[4]arene 3, and, consequently, its fluorescence emission.

Fig. 1 Structures of fuel acids 2 and 4 used to dissipatively drive the “breathing” movement of 1,3-diaminocalix[4]arenes 1 (see ref. 10c) and 3.

The rationale behind our hypothesis is that when the calixarene conformation is “unlocked” (state A, Scheme 1), the two pyrene units are allowed to get close together and the fluorescence spectrum would be characterized by the typical excimer emission. In contrast, the addition of a fuel⁴ acid RCO₂H would trigger a first conformational change to the “locked” pinched-cone structure (state B′). In this state, the two chromophores are forced far apart in consequence of the convergence of the aniline rings to share a proton, and should display only the monomer fluorescence (quenching of the excimer emission). Then, the just formed carboxylate anion would slowly lose CO₂ to give the ion pair 3H⁺·R⁻ (state B′′). A final back proton transfer from protonated calix[4]arene 3H⁺ to carbanion R⁻ would restore the initial state A, leading to the “unlocked” calix[4]arene 3 and the waste product RH. Thus, the addition of the fuel with the consequent locking of the proton-bridged pinched-cone conformation would switch off the excimer fluorescence intensity of 3, due to the separation of the pyrenyl units. Only when the fuel is exhausted, the initial fluorescence intensity would be recovered.

Scheme 1 Expected conformational cycle of 3 controlled in a dissipative manner by means of activated carboxylic acids 2 and 4.

Results and discussion

Calix[4]arene 3 was synthesized in two steps from 5,17-diiodo-11,23-dinitro-25,26,27,28-tetrakis(2-ethoxyethoxy)calix[4]arene 5 ¹⁴ (Scheme 2a). First, the pyrenyl moieties were introduced by a Suzuki coupling of 5 with pyrene-1-boronic acid pinacol ester 6, obtaining 7 in 60% yield, and then the nitro groups were reduced to amines with hydrazine and Pd/C (80% yield).

Scheme 2 (a) Synthesis of 1,3-dipyrenyl-2,4-diaminocalix[4]arene 3. (b) Three views of the X-ray molecular structure of 1,3-dipyrenyl-2,4-dinitrocalix[4]arene 7.

The ¹H NMR spectra at room temperature of 7 both in CDCl₃ and in toluene-d₈ are characterized by relatively sharp peaks for the calixarene protons and broad ones for the protons of the pyrenyl units, indicative of a hindered rotation of these groups around the calixarene–pyrene C–C bond. Moreover, the chemical shifts of the aromatic protons of the calixarene are diagnostic for the pinched-cone conformation of the scaffold,¹⁵ with the pyrene substituted phenol rings parallel and the nitrophenol rings diverging. Taken together, these spectral features suggest attractive π−π interactions between the pyrene groups and a molecular geometry presumably similar to the conformation adopted by 7 in the solid state (Scheme 2b, see the ESI† for a description of the X-ray structure). In the ¹H NMR spectra of diamino–dipyrenyl calixarene 3 in different solvents (CDCl₃, CD₂Cl₂, CD₃OD and toluene-d₈) the signals of the pyrene protons are broad as well, while the calixarene peaks remain sharp, pointing also for this compound to a π−π interaction of the pyrene units. The presence of a strong excimer-like§ band (Fig. S16†) in the fluorescence spectrum of 3 confirms that the two pyrene units are close together and that the calixarene conformation is in the unlocked state A (see Scheme 1). As in the case of 1,^10c the ¹H NMR titration of 2.0 mM 3 with TFA (Fig. 2a, CD₂Cl₂/CD₃CN 1 : 1, 25 °C) shows that the protonation of 3 initially causes an up-field shift of the signal related to the aromatic protons ortho to the NH₂ groups from 6.59 ppm to lower values, which is a characteristic feature of the “locked” pinched-cone conformation with the aniline rings slightly convergent compared to the pyrene-substituted ones, which are downfield shifted and divergent. This unusual up-field shift of the protons ortho to the NH₂ upon catching the proton is due to the shielding cone that the divergent aromatic rings exert on the convergent ones. In addition, such protonation dramatically affects the aromatic signals of the pyrenyl moieties, which are shifted to lower fields and become sharper, suggesting a break-up of the π–π interaction and a higher conformational freedom. Upon further addition of TFA (more than 2 mol equiv.), the signal of the NH₂-substituted aromatic rings tends to revert to the original chemical shift and becomes broader; the same, although much more mildly, holds for the pyrene-substituted aromatic rings signal. Also, the pyrene signals are again slightly shifted to higher fields and become broader. The latter pieces of evidence point to a second protonation of the calixarene structure to give 3H₂²⁺, with one proton on each of the two amino groups, whose electrostatic repulsion would cause a partial rapprochement of the pyrenyl moieties. Although less basic than 1 (two molar equiv. of TFA are needed to reach the highest concentration of mono-protonated 3H⁺, under the explored conditions, while mono-protonation of 1 was complete upon the addition of just one molar equivalent of TFA),^10c3 behaves similar to 1. Thus, in the mono-protonated form 3H⁺ the proton turns out to be shared between the two amino groups, which, consequently, are tilted inwards in the “locked” conformation.

Fig. 2 (a) Titration of 2.0 mM 3 with TFA (¹H NMR monitoring; CD₂Cl₂/CD₃CN 1 : 1, 25 °C). For the assignment of the marked signals see Fig. 1, other signals belong to pyrenyl moieties. (b) Titration of 0.15 mM 3 with TFA (fluorescence monitoring, λ_exc = 385 nm; CH₂Cl₂/CH₃CN 1 : 1, 25 °C). (c) Additional fluorescence spectra obtained after the addition of further TFA (7, 30 and 50 mol equiv.) with respect to (b). The excimer band again increases due to re-approaching of the pyrenyl groups.

Fluorescence titration of 0.15 mM 3 with TFA (Fig. 2b, λ_exc = 385 nm, CH₂Cl₂/CH₃CN 1 : 1, 25 °C) shows a decrease of the excimer emission band at 475 nm and a corresponding increase of the band at 408 nm due to the monomeric emission of the pyrenyl units, confirming the adoption of the pinched-cone conformation with the two pyrenyl units pointing far from one another in the mono-protonated form 3H⁺. Conversely, the addition of a larger excess of TFA (up to 50 mol equiv., see Fig. 2c), causes the excimer band at 475 nm to increase again. This is due to the protonation of both the amino- to ammonium groups, whose repulsion translates into a re-approaching of the pyrenyl groups.

Having demonstrated that the protonation is effective in producing the expected conformational variations of 3, we then started to explore the use of a chemical fuel to achieve a temporal control of the geometry of the calixarene. Initially, 2-cyano-2-phenylpropanoic acid was employed as the fuel. Unfortunately, while the ¹H NMR monitoring of the reaction corroborated our hypothesis (pages S8 and S9†), we were unable to observe the expected fluorescence quenching of the excimer emission during the reaction, probably due to an unexpected interposition of the aromatic portion of the fuel 2 between the two pyrenyl moieties of 3. We then resorted to trichloroacetic acid 4, although it was found less manageable due to its deliquescence. The addition of 2.3 mol equiv. of trichloroacetic acid to 2.0 mM 3 (Fig. 3a, trace b) results in a series of shifts that closely resemble those observed in the titration with TFA, i.e. a larger separation of the aromatic signals corresponding to the calixarene scaffold (red and green stripes, Fig. 3a), and the down-field shift of the signals of the pyrenyl moieties that become more resolved, proving the adoption of the “locked” pinched-cone conformation of 3 (state B′ in Scheme 1, compare traces a and b).¶ In the following 12–13 hours, no substantial variations can be detected in the ¹H NMR spectrum except for the building up of the signal at 7.49 ppm corresponding to CHCl₃ produced during the decarboxylation of 4 (traces c and d, blue stripe on Fig. 3a). From this point (t > 12 h), all the signals corresponding to the calix[4]arene scaffold begin to revert to the initial chemical shift values, and at the end, after 21 h, the ¹H NMR spectrum turns to be superimposable to the initial one (cfr. Fig. 3a traces f and a), demonstrating that the whole cycle A → B′ → B′′ → A has occurred. In other words, the “locked” pinched-cone structure persists as long as an excess fuel is present. Consequently, it is possible to extend the time in which the calix[4]arene scaffold remains in the “locked” state by increasing the amount of the added fuel, increasing from 13 h with 2.3 mol equiv. to 32 hours with 4.0 mol equiv. of added 4 (Fig. 3b, see pages S10 and S11†).

Fig. 3 (a) ¹H NMR monitoring of the reaction between 2.0 mM 3 (trace a) and 2.3 mol equiv. of 4. Spectra from b to f were recorded after 5 min, 7 h, 12.8 h, 18 h, and 21.2 h from the addition of 4, respectively (CD₂Cl₂/CD₃CN 1 : 1, 25 °C). The blue stripe highlights the CHCl₃ signal, which increases during the reaction, while red and green stripes highlight the signals related to the aromatic protons ortho to the pyrenyl units and to the NH₂ groups, respectively. (b) Percentage of 3 in its locked conformation in function of time for the reactions carried out in the presence of 2.3 mol equiv. (orange trace) and 4.0 mol equiv. (azure trace) of 4 (raw data at pages S10 and S11,† as detected by ¹H NMR spectroscopy).

Then, we monitored the reaction using a spectrofluorometer, recording over time the emission intensity at 475 nm (λ_exc = 385 nm) of a 0.15 mM 3 solution to which 5.0 mol equiv. of fuel 4 were added (Fig. 4a). After the addition, an immediate decrease of the intramolecular excimer fluorescence emission takes place, witnessing the unpairing of the pyrenyl moieties (Scheme 1) and the formation of the locked conformation. The reaction proceeds and, after approximately 3 h, the emission intensity at 475 nm is recovered since the cycle A → B′ → B′′ → A is over and the pyrenyl units are again spatially paired. The observation that the reaction rate is faster at these concentrations (0.15 mM of 3) than that observed at higher concentrations (2.00 mM of 3, ¹H NMR experiments), is likely due to a weaker ion pairing between 3H⁺ and RCO₂⁻ (see Scheme 1) at lower concentrations, which destabilizes the RCO₂⁻ anion and facilitates the decarboxylation step. As a matter of fact, decarboxylation does not proceed at any extents if dichloromethane alone is used as a solvent instead of dichloromethane : acetonitrile 1 : 1, both at high and low concentrations, suggesting an important role of the solvent that influences the character of ion pairs involved. Eventually, Fig. 4b shows that the system is robust and reversible. Four subsequent A → B′ → B′′ → A cycles have been achieved, with consequent loss and recovery of the fluorescence at 475 nm, by means of four successive shots of fuel 4.

Fig. 4 (a) Time course monitoring of the fluorescence emission at 475 nm (λ_exc = 385 nm) before and after the addition of 5.0 mol equiv. of 4 (CH₂Cl₂/CH₃CN 1 : 1 at 25 °C). (b) Up to four dissipative conformational cycles were obtained by the subsequent additions of 5.0 mol equiv. of 4 (CH₂Cl₂/CH₃CN 1 : 1 at 25 °C).

Experimental section

Instrument and methods

All moisture sensitive reactions were carried out under a nitrogen or argon atmosphere, using previously degassed solvents. Most of the solvents and reagents were obtained from commercial sources and used without further purification. Analytical TLC was performed using prepared plates of silica gel (Merck 60 F 254 on aluminium) and then revealed with UV lights. Merck silica gel 60 was used for flash chromatography (40–63 μm). ¹H-NMR and ¹³C-NMR spectra were recorded on Bruker AV400 and AV300 spectrometers and partially deuterated solvents were used as internal standards to calculate the chemical shifts (δ values in ppm). All ¹³C-NMR spectra were recorded with proton decoupling. Electrospray ionization (ESI) mass analyses were performed with a Waters single-quadrupole spectrometer in the positive mode using MeOH or CH₃CN as solvents. High resolution mass spectra were recorded on a LTQ Orbitrap XL instrument in the positive mode using MeOH as solvent. Melting points were determined on an Electrothermal apparatus in closed capillaries. Absorption spectra were collected with a PerkinElmer Lambda 18 spectrophotometer in freshly prepared solutions at room temperature. Emission measurements were performed with a Fluoromax-3 Horiba Jobin–Yvon fluorometer (T = 25 °C). All collected data were corrected by means of a built-in program in order to counterbalance the decay in sensitivity in the near infrared region and divided by the corrected reference detector. Samples were prepared immediately before the fluorescence measurements in quartz cuvette with a 1 cm path-length.
Kinetic experiments. ¹H NMR spectra were recorded on a 300 MHz spectrometer. The spectra were internally referenced to the residual proton signal of the solvent at 5.32 ppm (CD₂Cl₂) and 1.94 ppm (CD₃CN). All the experiments were carried out in thermostated NMR tubes. Emission measurements were performed as described before, with both excitation and emission slit sets at 1 nm.

Synthesis

Synthesis of 5,17-(dipyren-1-yl)-11,23-dinitro-25,26,27,28-tetrakis(2-ethoxyethoxy)calix[4]arene (7). A mixture of 5,17-diiodo-11,23-dinitro-25,26,27,28-tetrakis(2-ethoxyethoxy)calix[4]arene (5) (0.233 g, 0.22 mmol,), 4,4,5,5-tetramethyl-2-(pyren-1-yl)-1,3,2-dioxoborolane (6) (0.160 g, 0.48 mmol) and [1,1′-bis(diphenylphosphino)ferrocene]dichloropalladium(II) (0.018 g, 0.022 mmol) was prepared under an argon atmosphere in a Schlenk tube. THF (5.2 mL) and 2 M Na₂CO₃ (1.35 mL) were added sequentially, and the mixture was stirred at 80 °C for 2 days. After cooling, the reaction was quenched with distilled water. The crude product was extracted with dichloromethane (3 × 50 mL) and the combined organic phases were dried over anhydrous Na₂SO₄ and evaporated under reduced pressure. Purification by column chromatography (silica gel, eluent: dichloromethane/ethyl acetate 95 : 5) yielded the pure product as a light-yellow solid (0.161 g, 60%). Mp 247.7–248.2 °C. ¹H NMR (400 MHz, CDCl₃) δ 8.21 (4H, bs, ArH), 7.68 (4H, br s, ArH(Py)), 7.50 (4H, br s, ArH(Py)), 7.25 (4H, br s, ArH(Py)), 6.94 (2H, br s, ArH(Py)), 6.84 (4H, br s, ArH), 6.56 (2H, br s, ArH(Py)), 5.98 (2H, br s, ArH(Py)), 4.87 (4H, d, ArCHH_eqAr, J = 13.3 Hz) 4.75 (4H, br s, OCH₂CH₂O), 4.15 (4H, br s, OCH₂CH₂O), 4.08 (4H, br s, OCH₂CH₂O), 3.91 (4H, br s, OCH₂CH₂O), 3.68 (4H, q, OCH₂CH₃J = 7.0 Hz), 3.59 (4H, q, OCH₂CH₃, J = 7.0 Hz), 3.53 (4H, d, ArCHH_axAr, J = 13.3 Hz), 1.36 (6H, t, OCH₂CH₃, J = 6.5 Hz) 1.25 (6H, t, OCH₂CH₃, J = 7.0 Hz). ¹³C NMR (100 MHz, CDCl₃) δ 163.57, 154.04, 142.73, 138.17, 135.76, 135.08, 132.15, 131.23, 130.57, 129.60, 129.07, 126.62, 126.03, 125.52, 124.86, 124.21, 124.02, 123.07, 122.80, 77.24, 74.79, 73.81, 70.15, 69.65, 66.64, 66.31, 31.22, 15.37. MS (ESI⁺-MS, m/z): 1225.64 ([M − Na]⁺, 100%), 1241.47 (40, [M − K]⁺), 1203.47 (10, M − H⁺).

Synthesis of 5,17-(dipyren-1-yl)-11,23-diamino-25,26,27,28-tetrakis(2-ethoxyethoxy) calix[4]arene (3). To a solution of 5,17-(dipyren-1-yl)-11,23-dinitro-25,26,27,28-tetrakis(2-ethoxyethoxy) calix[4]arene (7) (0.160 g, 0.133 mmol) in ethanol (15 mL), hydrazine hydrate (0.126 mL, 3.99 mmol) and a catalytic amount of Pd/C (10%) were added. The reaction mixture was refluxed for 24 h and quenched by catalyst filtration. The filtrate was evaporated under reduced pressure; the residue was dissolved with dichloromethane and washed with distilled water. The organic layer was separated, dried over Na₂SO₄ and evaporated to dryness under reduced pressure to obtain the pure product as a light-yellow solid (0.121 g, 80%). Mp 180.7–181.2 °C. ¹H NMR (400 MHz, CD₂Cl₂/CH₃CN) δ 7.66 (4H, br s, ArH(Py)), 7.53 (2H, br s, ArH(Py)), 7.41 (2H, br s, ArH(Py)), 7.25 (4H, br s, ArH(Py)), 6.90 (4H, br s, ArH(Py), 6.86 (4H, s, ArH,), 6.59 (4H, br s, ArH), 6.25 (2H, s, ArH(Py)), 4.61 (4H, d, ArCHH_eqAr, J = 13.0 Hz), 4.36 (4H, t, OCH₂CH₂O, J = 8.6 Hz), 4.11 (4H, t, OCH₂CH₂O J = 8.6 Hz), 4.03 (4H, t, OCH₂CH₂O J = 6.2 Hz), 3.88 (4H, t, OCH₂CH₃, J = 6.2 Hz,), 3.62 (8H, q, J = 9.3 Hz), 3.25 (4H, d, ArCHH_axAr, J = 13.0 Hz), 1.27 (6H, t, OCH₂CH₃, J = 9.3 Hz), 1.23 (6H, t, OCH₂CH₃, J = 9.3 Hz). ¹³C NMR (100 MHz, CD₂Cl₂) δ 154.16, 150.40, 141.72, 137.44, 136.54, 134.57, 133.43, 130.71, 130.54, 129.80, 128.70, 126.83, 126.14, 125.71, 125.70, 124.69, 124.18, 123.99, 123.84, 123.63, 123.37, 123.06, 115.57, 74.54, 72.93, 70.22, 69.83, 66.48, 66.30, 31.04, 15.34, 15.16. MS (HR-MS, m/z) calculated for C₇₆H₇₅N₂O₈ (M − H)⁺ 1143.55179, found 1143.55037.

Kinetic and titration experiments

NMR titrations of 3 with CF₃CO₂H. Calix[4]arene 3 (1.37 mg) was weighed in an NMR tube and diluted with 580 μL of CD₂Cl₂/CD₃CN 1/1 to give an 2.06 mM solution. At this point, subsequent aliquots of 5.0 μL of a 0.060 M stock solution of TFA were added up to 7 mol equiv. NMR spectra of the solution were recorded at each titrant addition after thermal equilibration.

NMR decarboxylation experiment of fuel 2 in the presence of 2.0 mM calix[4]arene 3. In a typical NMR decarboxylation experiment, 1.37 mg of calix[4]arene 3 were weighed in an NMR tube and diluted with CD₂Cl₂ giving a solution of ∼2.0 mM. The t = 0 ¹H NMR spectrum was recorded and then a small aliquot of a stock solution of 2 was added so that the final volume of the solution was 600 μL. Thus, the concentration of calix[4]arene 3 was 2.0 mM, that of fuel 2 was 4.0 mM or higher, for the experiments with excess of fuel.

NMR decarboxylation experiment of fuel 4 in the presence of 2.0 mM calix[4]arene 3. In a typical NMR decarboxylation experiment, 1.37 mg of calix[4]arene 3 were weighed in an NMR tube and diluted with CD₂Cl₂/CD₃CN giving a solution of ∼2.0 mM. The t = 0 ¹H NMR spectrum was recorded and then a small aliquot of a stock solution of 4 in CD₂Cl₂ was added so that 600 μL of a solution in CD₂Cl₂/CD₃CN 1/1 were obtained. Thus, the concentration of calix[4]arene 3 was 2.0 mM, that of fuel 4 was 4.6 mM or higher, for the experiments with excess of fuel.

Fluorescence titrations of 3 with CF₃CO₂H. Calix[4]arene 3 (0.72 mg) was weighed in a vial and diluted with 500 μL of CD₂Cl₂/CD₃CN 1/1 to give a 1.8 mM solution. The stock solution of 3 was then diluted in a 1.0 cm path length fluorescence cuvette giving a solution of 0.15 mM. At this point, subsequent aliquots of 5.0 μL of a 0.090 M stock solution of TFA were added up to 7 mol equiv. Fluorescence spectra of the solution were recorded at each titrant addition exciting at 385 nm.

Fluorescence decarboxylation experiment of fuel 4 in the presence of 0.15 mM calix[4]arene 3. In fluorescence decarboxylation experiments, 0.72 mg of calix[4]arene 3 were weighed in a vial and diluted with CD₂Cl₂/CD₃CN 1/1 giving a solution of 1.8 mM. A small aliquot of the stock solution of 3 was then diluted in a 1.0 cm path length quartz cuvette giving a solution of ∼0.15 mM. Emission at 475 nm (λ_exc = 385 nm) was monitored for a few seconds before addition of the fuel 4. Then a small aliquot of a stock solution of 4 in CH₂Cl₂ was added so that the final volume of the solution was 3.0 mL. Thus, the concentration of calix[4]arene 3 was 0.15 mM, that of fuel 4 was 0.75 mM. After the recovery of the fluorescence intensity, the same amounts of the stock solution of 4 were added in the fluorescence cuvette in order to obtain the subsequent conformational cycles.

Conclusions

In conclusion, we have demonstrated that trichloroacetic acid 4 can be used for driving the temporal control of the A → B′ → B′′ → A conformational cycle of calix[4]arene 3 in a dissipative fashion. Given the presence of the two pyrenyl units in the opposite positions of the upper rim of calix[4]arene 3 such control translates into the dissipative time-modulation of the solution fluorescence.¹⁶

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

FR, AC, LM and LB acknowledge the Centro Interdipartimentale Misure “G. Casnati” of Parma University, the COMP-HUB Initiative ‘Departments of Excellence’ (MIUR, 2018–2022), BacHounds project (MIUR-2017E44A9P) and Chiesi Farmaceutici SpA for the support of the D8 Venture X-ray equipment. SDS thanks the University of Rome La Sapienza (Grandi Progetti di Ricerca, Ateneo 2018).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Crystallographic data of 7, fluorescence and UV-Vis spectra, and NMR data. CCDC 2108932. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1ob02096j

‡ These authors contributed equally.

§ Given the evidence of an attractive interaction between the pyrenes in their ground-states, this band can be considered as originating from a “static excimer”, see the ESI, page S17† for a detailed explanation.

¶ Fig. S15 in the ESI† shows the comparison of the ¹H NMR spectra of the solutions obtained (i) after the addition of 2 mol equiv. of TFA to 2.0 mM 3 and (ii) immediately after the addition of 2.3 mol equiv. of 4 to 2.0 mM 3 (CD₂Cl₂/CD₃CN 1 : 1). The spectra, which correspond to the third trace from the bottom in Fig. 2a and trace b in Fig. 3a, respectively, are very similar indicating very similar conformation and protonation states of calixarene 3 in the two cases.

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