Conventional vs. Microwave- or Mechanically-Assisted Synthesis of Dihomooxacalix[4]arene Phthalimides: NMR, X-ray and Photophysical Analysis ^§

Abstract

Direct O-alkylation of p-tert-butyldihomooxacalix[4]arene (1) with N-(bromopropyl)- or N-(bromoethyl)phthalimides and K₂CO₃ in acetonitrile was conducted under conventional heating (reflux) and using microwave irradiation and ball milling methodologies. The reactions afforded mono- and mainly distal di-substituted derivatives in the cone conformation, in a total of eight compounds. They were isolated by column chromatography, and their conformations and the substitution patterns were established by NMR spectroscopy (¹H, ¹³C, COSY and NOESY experiments). The X-ray structures of four dihomooxacalix[4]arene phthalimide derivatives (2a, 3a, 3b and 5a) are reported, as well as their photophysical properties. The microwave (MW)-assisted alkylations drastically reduced the reaction times (from days to less than 45 min) and produced higher yields of both 1,3-di-substituted phthalimides (3a and 6a) with higher selectivity. Ball milling did not reveal to be a good method for this kind of reaction.

1. Introduction

Calixarene-based molecules are amongst the most investigated frameworks in host-guest and supramolecular chemistry [1,2]. Their relatively easy functionalization at either the upper or lower rim, a pre-organized cavity available in different sizes and conformations, and ion-binding sites are the key for their great diversity of applications. The recognized importance of anions in biological, chemical and environmental processes continues to attract chemists for the design and use of new synthetic anion receptors [3,4]. Calixarenes possessing (thio)urea moieties have been developed as receptors, with anion interactions established by hydrogen bonding only.

Since the first articles on the use of domestic microwave ovens for organic synthesis in 1986 [5,6], this technology has become a very important tool in a wide range of chemical reactions [7,8,9,10]. The microwave (MW) technique provides uniform heating of the reagents throughout the reaction vessel, allowing a rapid and homogeneous heat transfer. MW heating drastically reduces reaction times compared to conventional heating (from hours or even days to some minutes), saving a huge amount of energy and time. Higher product yields, higher reaction selectivity and low waste are other advantages of this technique, as well as the use of less or even no solvent and catalyst. Ball milling (BM), another environmentally friendly methodology applied to organic synthesis under solvent-free conditions, has also been developed mainly in the last decade [11,12,13]. Both techniques have been employed in the synthesis of calixarenes and related macrocyclic hosts [14], although ball milling is much less extensive.

The syntheses of p-alkylcalix[n]arenes (R = t-Bu, n = 4 and 8; R = Me, n = 6) were the first calixarene reactions carried out under MW irradiation [15,16]. Other examples reported are the preparation of water-soluble azocalix[4]arenes [17], the copper(I)-catalysed syntheses of calix[4]arene glycoconjugates [18] and calix[4]arene tetraazide derivatives [19], as well as calix[4]arenes containing chiral unsymmetrical urea moieties at the lower rim [20]. The alkylation of p-tert-butylcalix[4 and 6]arenes in the presence of K₂CO₃ and MeCN or DMF are among the most investigated MW-assisted reactions [21,22,23]. Calix[6]arenes functionalized at the lower and upper rims were also successfully obtained [24]. All these reactions were shown to be faster and more efficient compared to conventional methods. The formation of p-benzylcalix[5 and 7]arenes from p-benzylcalix[6 or 8]arenes in the presence of KOH, formaldehyde and molecular sieves [25], and the synthesis of a p-nitrocalix[6]arene [26] are the only examples of ball milling methods employed in calixarene synthesis found in the literature.

In the course of our research on anion binding by ureido-dihomooxacalix[4]arenes [27,28,29,30,31], we were interested in studying the introduction of urea binding sites into the 1,3-positions of the calixarene lower rim, through shorter spacers (propyl or ethyl) compared to the ones obtained in our previous dihomooxa receptors. The two remaining phenolic OH groups will participate in the formation of hydrogen bonds, contributing to keep the macrocycle in the cone conformation. It is known that O-alkylation reactions of calixarenes by conventional methods can take a long time and produce low yield products. Thus, MW irradiation and ball milling were employed in this work to obtain the target 1,3-disubstituted phthalimides, which have been prepared as precursors to urea-terminated anion-binding macrocycles. Other differently substituted derivatives were also obtained. Their solution and solid state conformational analysis and the determination of some photophysical properties are also reported.

2. Results and Discussion

2.1. Conventional vs. MW- and Mechanically-Assisted Synthesis

With the aim of introducing urea groups at the 1,3-positions of the p-tert-butyldihomooxacalix[4]arene (1) via a propyl spacer, the parent compound 1 was treated with 2 equiv of N-(3-bromopropyl)phthalimide and 2 equiv of K₂CO₃ in refluxing acetonitrile for 4 days (Scheme 1). This reaction afforded, according to the proton NMR spectrum, a mixture composed of distal 1,3-diphthalimide 3a as the major product (65%), along with minor amounts of mono 2a (13%) and tetra-phthalimide 4 (12%), and a further 3% of proximal 3,4-diphthalimide 3b and 6% of unreacted 1 (

Table 1. Conditions and product percentages of reaction of 1 with Br(CH₂)₃Pht.

Entry	Method	Reaction Time	1:RX:Base Ratio	Reaction Mixture (%) 1
				1	2a	3a	3b	4
1	Reflux	4 days	1:2:2	6	13	65	3	12
2	Reflux	5 days	1:2:2	—	4	76	15	5
3	MW	20 min	1:2:2	—	10	86	2	2
4	MW	30 min	1:2:2	—	3	93	2	2
5	BM	2 h	1:2:2	70	30	—	—	—
6	BM	12 h	1:4:4	27	46	19	4	4

¹ Percentage of product formation in the reaction mixture determined by ¹H NMR.

, entry 1). A trace amount of mono 2b was also observed. When the reaction was conducted for 5 days, the percentage of formation of 3a increased (76%), as well as that of 3b (15%), while minor amounts of tetra 4, mono 2a and no starting material were obtained (Table 1, entry 2).

In view of the long reaction time, the alkylation of parent 1 was investigated under MW irradiation and ball milling methodologies. The reaction of 1 with 2 equiv of the previous alkylating agent and base in acetonitrile after 20 min of irradiation (160 °C) afforded 86% of 1,3-diphthalimide 3a, accompanied by 10% of mono 2a and minor amounts (<2%) of proximal di-phthalimide 3b and tetra 4 (Table 1, entry 3). The increase in the irradiation time to 30 min (Table 1, entry 4) produced an increase of 1,3-diphthalimide 3a (93%) and a decrease of mono 2a (3%). The use of solvent-free ball milling during 2 h with a rotation speed of 500 rpm, 200 stainless steel balls and keeping the same reagent ratio as before gave a mixture composed of 70% of unreacted 1 and 30% of mono 2a (Table 1, entry 5). Replacing stainless steel balls with zirconium oxide balls and increasing the rotation speed to 650 rpm led to a similar result. After several attempts to improve the reaction yield, we found that using 4 equiv of alkylating agent and base, and prolonging the reaction time to 12 h, raised the yield of mono 2a to 46%, although 27% of the starting material was still present (Table 1, entry 6). The formation of the previous distal and proximal diphthalimides 3a and 3b, as well as of the tetra derivative 4, was also observed, with distal 3a being obtained as the main compound (19%).

The separation of the reaction mixture into the pure compounds was achieved by column chromatography and by recrystallization. The tetra-phthalimide derivative 4 was not purified, as it had already been obtained by us before [29].

Concerning the synthesis of the 1,3-diethylphthalimide 6a (Scheme 2), a urea precursor with an even shorter spacer (only two carbon atoms), several difficulties were found in its preparation. The alkylation of parent 1 with N-(2-bromoethyl)phthalimide and K₂CO₃ was tried under different conditions. Firstly, the reaction was carried out using 2 equiv of the reactant and base for 3 days in acetonitrile under reflux. According to the proton NMR integration, a mixture composed mainly of unreacted 1 (61%) and mono-phthalimide 5a (21%), along with minor amounts of mono 5b (8%) and 1,3-diphthalimide 6a (11%), was obtained (

Table 2. Conditions and product percentages of reaction of 1 with Br(CH₂)₂Pht.

Entry	Method	Reaction Time	1:RX:Base:KI Ratio	Reaction Mixture (%) 1
				1	5a	5b	6a	6b
1	Reflux	3 days	1:2:2:0	61	21	6	11	1
2	Reflux	3 days	1:2.5:2:2	30	45	7	16	2
3	Reflux	3 days	1:4:4:2	19	44	4	30	3
4	Reflux	7 days	1:6:6:4	18	37	12	30	3
5	MW	6 + 12 min	1:4:2:2	16	58	—	26	—
6	MW	6 + 36 min	1:4:2:2	—	59	—	41	—
7	MW	12 + 36 min	1:4:2:2	—	38	—	62	—
8	MW	12 + 36 min	1:4:2:0	—	43	—	57	—
9	MW	18 + 36 min	1:4:4:0	—	—	—	100	—
10	MW	6 + 36 min	1:4:4:0	—	—	—	100	—
11	BM	12 h	1:4:4:0	82	18	—	—	—

¹ Percentage of product formation in the reaction mixture determined by ¹H NMR.

, entry 1). When the reaction was conducted for the same time, but in the presence of KI (to promote the halogen exchange of the electrophile in situ) and a 25% excess of alkylating agent, the percentage of unreacted 1 decreased (30%) and those of mono 5a (45%) and 1,3-di 6a increased (16%), although the latter only slightly (Table 2, entry 2). To improve the reactivity of the nucleophile, 1 was also previously heated with the base in MeCN for about 30 min. Four equiv of reactant and base were necessary to raise the percentage of 6a to 30% and to decrease that of 1 to 19% (Table 2, entry 3). A final attempt under extreme conditions (6 equiv of Br(CH₂)₂Pht and base for 7 days in refluxing MeCN) gave identical results (Table 2, entry 4). Substitution and elimination reactions are almost always in competition with each other. The peak assignments in the proton NMR spectrum of the reaction mixture compatible with bromo- and iodo-ethylphthalimides, as well as N-ethenylphthalimide in an approximately 40:60 ratio, indicate that elimination should predominate over substitution, explaining the low yields obtained even under extreme conditions [32].

The alkylation reaction with N-(2-bromoethyl)phthalimide was also studied by MW irradiation, under a range of conditions. The irradiation of parent 1 with 2 equiv of base in acetonitrile for 6 min at 110 °C, followed by the addition of 4 equiv of reagent and 2 equiv of KI and a further 12 min irradiation at 150 °C, afforded a mixture of mono 5a as the main product, along with 1,3-di 6a and unreacted 1 (Table 2, entry 5). An increase in the irradiation time at 150 °C (36 min) led to the disappearance of 1 and to an increase of 6a (Table 2, entry 6). The effect of KI on the reaction yield was investigated in two MW-assisted alkylations carried out with and without KI (Table 2, entries 7 and 8, respectively). Given that just a slight improvement (5%) of 6a yield was obtained in the presence of KI, the next assay was performed without KI, but with 4 equiv of base, an irradiation time of 18 min (110 °C), followed by a further 36 min (150 °C), resulting 6a as the unique product (Table 2, entry 9). The reduction of the time at 110 °C to 6 min provided 1,3-diphthalimide 6a as the unique product as well (Table 2, entry 10). As expected, the MW-assisted reactions showed several advantages compared to the conventional synthesis, mainly in the case of the alkylation with N-(bromoethyl)phthalimide. Beyond the removal of KI reactant and the reduction in solvent quantity, the MW irradiation provided drastically shorter reaction times. The alkylation of parent 1 was complete after 42 min under MW conditions, while under reflux for 7 days the conversion was still incomplete. Higher reaction selectivity was also observed, with distal 1,3-disubstituted phthalimide 6a being the only derivative obtained. Ball milling was likewise performed, but gave the poorest results (Table 2, entry 11).

It was not possible to isolate the mono-phthalimide 5b as a pure compound, despite the several columns and preparative chromatographies done. Recrystallization attempts in different solvents have also failed. Similarly, distal 1,3-diphthalimide 6a and proximal 3,4-diphthalimide 6b could not be separated from each other, as they have the same R_f value in all the solvents tested. A similar situation was reported before for distal and proximal di-substituted calix[4]arene derivatives [19].

Besides the NMR assignment of all the previous derivatives, the proton spectra of both reaction mixtures still present a set of small signals, which could not be identified. Since a total of nine possible O-alkylating compounds can be obtained [33], the formation of other differently substituted phthalimide derivatives cannot be excluded.

2.2. NMR Conformational Analysis

The conformation and the substitution pattern of the phthalimide derivatives were established by proton, carbon-13, COSY and NOESY NMR spectroscopy in chloroform at room temperature.

2.2.1. Mono-Propylphthalimide 2a

The absence of symmetry in compound 2a is reflected by its proton and carbon-13 NMR spectra. The ¹H NMR spectrum (Figure 1a) displays four singlets for the tert-butyl groups, five AB quadruplets for the CH₂ bridge protons, four pairs of doublets for the aromatic protons and three singlets for the OH groups of the calixarene skeleton, besides several multiplets for the -OCH₂CH₂CH₂N- methylene protons and phthalimide group (two signals). The proton assignments were confirmed by cross-peak correlations in a COSY spectrum. The ¹³C NMR spectrum shows 28 downfield resonances arising from the aromatic carbon atoms and phthalimide group, three downfield resonances arising from the methylene carbon atoms of -OCH₂CH₂CH₂N- and CH₂OCH₂ groups, and 12 of the expected 13 upfield resonances arising from the quaternary (3 lines in a 1:1:2 ratio) and methyl carbon atoms (4 lines) of the tert-butyl groups, and the methylene carbon atoms of the ArCH₂Ar (3 lines) and -OCH₂CH₂CH₂N- (2 lines) groups. All resonances were assigned by DEPT experiments. The three ArCH₂Ar resonances appear at 30.1, 31.3, and 32.8 ppm, indicating a cone conformation for 2a [34].

The position of the phthalimide group was confirmed by proton-proton correlations observed in the NOESY spectrum. The preferential formation of 2a over 2b is expected, as described by us before [33]. The simultaneous observation of two NOE effects between the OH₁ proton at 9.08 ppm (position 29) and the two axial methylene protons at positions 10 and 16, as well as those observed between the OH₂ group at 8.47 ppm (position 30) and the axial CH₂ protons at positions 4 and 10, and between the OH₃ group at 7.73 ppm (position 27) and the axial CH₂ protons at positions 2, 22 and 4, are conclusive for the position of the substituent group (Figure 2). In addition to those effects, two other relevant NOEs were observed between OH₁ and -OCH₂CH₂CH₂N- groups and OH₁ and OH₂ groups.

Concerning mono-propylphthalimide 2b, it could be identified as a trace compound in a set of fractions obtained by column chromatography (Figure S1, Table S1).

2.2.2. 1,3-Dipropylphthalimide 3a

The ¹H NMR spectrum of 3a (Figure 1b) is also asymmetric and shows identical number and type of signals as 2a for the majority of the protons of the calixarene skeleton. Two singlets for the OH groups and four multiplets for the phthalimide aromatic protons can, in addition, be observed. The ¹³C NMR spectrum displays 32 downfield, four midfield and 13 of the expected 15 upfield resonances. The presence of the three ArCH₂Ar resonances in the range 29.8–32.9 ppm indicates a cone conformation for 3a.

The 1,3-substitution pattern was also confirmed by a NOESY experiment. The preferential formation of distal disubstituted compounds is expected, as previously reported for the formation of other dihomooxacalix[4]arenes [28,33]. Two simultaneous NOE effects between OH₁ proton at 8.04 ppm (position 29) and the two axial methylene protons at positions 10 and 16 were observed. Moreover, both OH groups (at 8.04 and 7.20 ppm) give NOE effects on both -OCH₂CH₂CH₂N- groups (positions 28 and 30), and OH₂ (position 27) also gives effects on the axial CH₂ protons at positions 2 and 22.

2.2.3. 3,4-Dipropylphthalimide 3b

Derivative 3b exhibits symmetric proton and carbon-13 NMR spectra, matching with a proximal disubstituted compound in the cone conformation. The ¹H NMR spectrum (Figure 1c) shows two singlets for the tert-butyl groups, three AB quadruplets in a 2:2:1 ratio for the CH₂ bridge protons, two pairs of doublets for the aromatic protons, one singlet for the OH groups, four multiplets for the -OCH₂CH₂CH₂N- and two multiplets for the phthalimide groups. The ¹³C NMR spectrum displays 16 downfield, two midfield and 8 upfield resonances. The ArCH₂Ar resonances appear at 30.8 (one carbon atom) and 31.6 (two carbon atoms), indicating a cone conformation for 3b.

The positions of the phthalimide groups were confirmed by two simultaneous NOE effects observed between the -OCH₂CH₂CH₂N- protons (position 28) and the two axial CH₂ protons at positions 16 and 22, as well as between the OH proton (position 27) and the axial CH₂ proton at position 2.

2.2.4. Mono-Ethylphthalimide 5a

Similarly to mono-propylphthalimide 2a, derivative 5a also presents asymmetric proton (Figure 3a) and carbon-13 spectra, being the lower number of multiplets for the CH₂ protons of the pendant group the only observed difference. The same 12 upfield carbon resonances are observed (as in mono 2a), as there is no overlapping of the quaternary carbon atoms (4 lines) of the tert-butyl groups.

Concerning the phthalimide group position, the NOESY spectrum of 5a shows identical effects to those exhibited for 2a. NOEs between each of the three OH groups (position 29, 30 and 27) and the two adjacent axial CH₂ protons (positions 10/16, 4/10 and 2/22, respectively) are observed. Furthermore, the -OCH₂CH₂N- group gives NOE effects on both OH₁ and OH₃ protons (positions 29 and 27), and each OH group gives NOE on its OH neighbor (OH₁ $\leftrightarrow$ OH₂ $\leftrightarrow$ OH₃).

A pure mixture of mono-phthalimides 5b and 5a was obtained by preparative chromatography (Figure S1, Table S1). However, further separation of mono 5b from the mixture was not possible to achieve.

2.2.5. 1,3- and 3,4-Diethylphthalimides 6a and 6b

As mentioned before, 1,3-diphthalimide 6a and 3,4-diphthalimide 6b could not be separated from each other, as they have the same R_f value. In the case of the MW-assisted reaction, a 100% conversion of parent 1 into 6a was obtained. However, after several purification attempts it was not possible to obtain pure 6a, as revealed by the ¹H spectrum (Figure 3b).

Both ¹H and ¹³C NMR spectra of 6a show spectral patterns compatible with an asymmetric molecule and very similar to those obtained for 3a. The presence of the three ArCH₂Ar carbon-13 resonances at 29.8, 30.0 and 32.7 ppm indicates a cone conformation for 6a. The 1,3-substitution was also corroborated by proton-proton correlations observed in the NOESY spectrum. Concerning derivative 6b, it was possible to clearly identify in the ¹H spectrum two singlets for the tert-butyl groups, two pairs of doublets for the aromatic protons and one singlet for the OH groups (Figure S1, Table S1). The overlapping of signals (with those of 6a) prevents the assignment of the methylenic region.

2.3. X-ray Structural Analysis

Small single crystals of the mono-substituted 2a and 5a and the di-substituted 3a and 3b were analyzed by X-ray diffraction at 100 K using synchrotron radiation. With the exclusion of the symmetric 3b, all the other molecules are inherently chiral due to the presence of the phthalimide substituents on the lower rim, which are not located symmetrically with respect to the oxa bridge inserted in the calixarene cone (Figure 4). As all their space groups are centrosymmetric (C2/c for 2a and 3a and P-1 for 5a), racemic mixtures of the two inherently chiral enantiomers are present in these crystals.

A different number of crystallographically independent molecules was observed for the analyzed structures: four for 2a, three for 3a, and one for 3b and 5a. In order to compare the different structures in a coherent manner, the asymmetric units with all the molecules having the same inherent chirality were selected. The same labeling scheme was applied to these molecules, namely, the phenyl ring labels A-D are ordered anti-clockwise, as viewed from above the calix cone, with A and B across the oxa bridge. In this scheme, ring C always has a phthalimide substituent and ring B always has a hydroxyl group, while rings A and D have a hydroxyl group (2a and 5a), except for the di-substituted derivatives, in which the second phthalimide substituent is present on ring A (3a) or ring D (3b) (Figure 5).

The X-ray structures show that all the dihomooxacalix[4]arenes adopt comparable cone conformations (

Table 3. Comparison of cone conformations: Dihedral angles (canting angles) between corresponding aryl planes of the calixarene cones (A, B, C and D) and the mean planes of the bridging methylene carbon atoms for various dihomooxacalix[4]arenes.

Molecule		A	B	C	D
2a *	(I)	138.4	154.7	105.0	126.1
	(II)	141.0	146.1	107.9	132.5
	(III)	138.6	149.5	102.3	122.4
	(IV)	128.0	148.8	98.5	129.9
3a **	(I)	137.02	145.72	100.30	118.38
	(II)	134.54	146.88	100.23	118.73
	(III)	135.17	147.47	101.34	116.39
3b		140.96	146.90	94.82	120.85
5a		144.25	149.98	106.82	118.39

* Four crystallographically independent molecules in the asymmetric unit; ** Three crystallographically independent molecules in the asymmetric unit. See Figure 5 for description of aryl rings A, B, C and D.

and Figure 6). All the canting angles, defined as the dihedral angles between the mean planes of the phenyl rings and the mean plane of the five methylene bridging carbon atoms, lean outwards from the center of the cone with similar values for corresponding phenyl rings, irrespective of the substituent pattern on the lower rim and the consequent differences in the intramolecular H-bond network (Figure 5). In particular, the two phenyl rings adjacent to the oxa bridge show large canting angles (mean values with standard deviation in parenthesis: A = 138(5)° and B = 148(3)°), while the other two are smaller (C = 102(4)° and D = 123(6)°) (see Table 3). This observation indicates that the conformation of the cone in these compounds is not determined by the different substitution pattern and different intramolecular H-bond networks. However, it should be noted that with respect to the cone conformation, all these structures have in common a bifurcated H-bond between the OH-donor group on ring B and acceptor oxygens atoms of the oxa bridge and the hydroxyl/alkoxy group on ring A (Figure 5, Table S2). This means that in all crystallographically independent molecules, the oxa bridge conformation has the oxygen atom oriented towards the center of the cone. The di-substituted derivatives show an additional intramolecular H-bond involving the second OH donor hydroxyl groups on ring A or D with O-acceptor atoms on ring D or C for 3a or 3b, respectively (Figure 5, Table S2). On the other hand, both of these H-bonds (A-D and D-C) are observed in the mono-substituted 2a and 5a, which have hydroxyl functions on the A and D rings. However, these are both bifurcated H-bonds, involving an additional O acceptor atom of a phthalimide carbonyl group. These bifurcated H-bonds are quite asymmetric, with the second, phthalimide group interaction characterized by long O•••O distances (Figure 5). Although these interactions are weak, they are important to determine the orientation of the phthalimide arms on the lower rims. For example, it is of interest to note that in 2a, the four crystallographically independent molecules all have the same orientation of the phthalimide arms (Figure 6a). The intramolecular H-bonds involving the phthalimide groups are weaker in 2a, in which the linker is longer, with respect to 5a (propyl vs. ethyl). Interestingly, when the linker is even longer, as in the case of a butyl derivative, this potential intramolecular H-bond interaction with the phthalimide group is completely lost (Figure 6b) [35].

The effect on the phthalimide arm orientation of the different linkers is also evidenced by the different α angles which the planes of the phthalimide groups make with the mean planes of the five calixarene methylene groups (Figure 6b and Table S3). For 2a it is almost orthogonal (mean value 77°), while for 5a it is more parallel (22°). With regard to the di-substituted derivatives 3a and 3b, the presence of the second phthalimide arm in place of a hydroxyl group reduces the number of H-bond donors, which reduces the overall strength of potential H-bond interactions with the phthalimide arms. The loss of these interactions increases the conformational freedom of the substituents on the lower rim (Figure 6c,d). In fact, for 3a, the three crystallographically independent molecules exhibit three different conformations (Figure 6c, Table S3). The presence of a second phthalimide group allows the possibility of intramolecular π-stacking interactions between these aromatic groups, as observed for 3b and for one crystallographically independent molecule of 3a (Figure 4 and Figure 6c). On the other hand, intermolecular π-stacking between two phthalimide groups is also observed for the other two molecules of 3a and for the mono-substituted 2a derivative (Figure 7). In 2a, for example, the four molecules can be described as two dimers formed by the intermolecular π-stacking of phthalimide groups.

The importance of the intramolecular H-bonds network on the open conformation of the calixarene cavity is evidenced by the comparison with the dihomooxa calixarene structures reported with four alkoxy groups on the low rim [28,30,31]. In all structures with four substituents on the lower rim, one phenyl group leans towards the center of the cavity, closing the aperture with the tert-butyl groups. In fact, no solvent guest molecules were observed in these crystal structures. On the contrary, in the present structure the presence of hydroxyl groups, responsible for the above described H-bond network, pre-organizes the cavity to host guest molecules. In fact, all the reported molecules contain a solvent guest molecule in the calixarene cavity (Figure 4).

2.4. Photophysical Properties

Parent calixarenes bear a mesitol-like intrinsic fluorophore. This opens the possibility of using its fluorescence for structural and dynamic information, and for sensing applications [30,31,36]. In the present case, compounds 2a, 3a, 3b and 5a contain not only mesitol-like and alkylated mesitol-like fluorophores, but also a second chromophore (phthalimide) that can engage in intramolecular electronic energy transfer, acting as acceptor towards the ring fluorophores, as will be shown. To evaluate the changes caused by the introduction of the phthalimide substituent, parent 1 and model compounds 1,3,5-tri-tert-butyl-2-methoxybenzene (A) and N-butylphthalimide (B) were also studied. The absorption and emission spectra of 1, of model compounds and of dihomooxa phthalimide derivatives in dichloromethane are shown in Figure 8. All of them present a well-defined absorption in the ultraviolet region, with phthalimide derivatives slightly red shifted compared to parent 1 and displaying a shoulder at the long wavelength side (Figure 8b). The absorption of 1 is in turn red-shifted with respect to that of A. These results indicate that the macrocycle dominates the absorption in the near UVC region (270–290 nm), while the phthalimide groups contribute to the absorption in the UVB region (290–320 nm). Dihomooxa compounds also display a well-defined emission in the UV region (Figure 8c) that extends into the visible owing to the (minor) phthalimide moiety contribution. Indeed, B exhibits a broad spectrum peaking at about 400 nm and extending from the UVA to the yellow region, in agreement with data reported for other N-phthalimides [37]. The weak 400 nm band is more evident for proximal di-phthalimide 3b, probably owing to the proximity of the substituent groups at the lower rim. The relevant photophysical properties of all compounds studied are collected in

Table 4. Photophysical properties of the studied compounds in CH₂Cl₂ at 293 K.

	λmax,abs (nm)	ε (M–1 cm–1)	λmax,em (nm)	Stokes Shift a (nm)	τf (ns)	ΦFb
A	271	5.0 × 102	288	17	4.50 c	7.5 × 10−3
B	294	8.2 × 102	403	109	0.26 d	1.8 × 10−3
1	286	3.6 × 104	306	20	1.50 c	1.3 × 10−1
2a	288	2.0 × 104	310	22	—	1.1 × 10−2
3a	290	1.9 × 104	314	22	1.32 d	1.7 × 10−3
3b	282	1.7 × 104	314	30	3.57 d	2.5 × 10−3 e
5a	288	2.9 × 104	310	22	—	1.4 × 10−3

^a Computed as ${\lambda }_{\max ,\mathrm{em}}-{\lambda }_{\max ,\mathrm{abs}}$; ^b Against quinine sulfate Φ_F = 0.546 in H₂SO₄ 0.5 M; ^c${\lambda }_{\mathrm{ex}}$ = 280 nm and ${\lambda }_{\mathrm{em}}$ = 306 nm; ^d ${\lambda }_{\mathrm{ex}}$ = 295 nm and ${\lambda }_{\mathrm{em}}$ = 400 nm; ^e After subtraction of the fluorescence of the phthalimide moiety (peaking at 400 nm, see Figure 8c).

.

The significant spectral overlap between the absorption and the fluorescence of the calixarene chromophores, on the one hand, and between the absorption of the phthalimides and the fluorescence of the calixarene chromophores, on the other hand (Figure S2), along with the close distance between chromophores, makes possible the existence of resonance energy transfer, both of the homotransfer (reversible) type and of the heterotransfer (irreversible) type (Figure 9).

The dominant energy transfer mechanism is likely Förster resonance energy transfer (FRET) type. The computed Förster radii are 1.4 nm for homotransfer and 1.3 nm for heterotransfer. Given that average distances between neighboring chromophores are of the order of 0.5 to 0.7 nm for homotransfer, but probably somewhat higher for heterotransfer (conformation and structure dependent, between 0.6 and 1.0 nm), it is expected that efficient and fast (a few picoseconds, on average) energy hopping takes place in the calixarene skeleton, before the decay of the substituted phenoxy chromophores occurs either by the intrinsic radiative and nonradiative channels, or by irreversible FRET to the phthalimide group(s) (Figure 9). Evidence for homotransfer is obtained from fluorescence anisotropy measurements of both 1 (model compound for the calixarene chromophore) and A in a rigid medium (Zeonex film). The respective excitation fluorescence anisotropy spectra are shown in Figure 10, as well as the ratio of the two anisotropies.

The anisotropy r₀(λ_exc) of the model compound A (Figure 10) attains the value of 0.40 at the S₁←S₀ absorption onset (280 nm), corresponding to collinear absorption and emission transition dipole moments [38] (¹L_b band), as expected. The excitation wavelength-dependence of the anisotropy, with a progressive decrease along the first band down to 0.24 at 260 nm, is compatible with a mixed polarisation, where the effect of the orthogonally-polarised ¹L_a band increases when decreasing the excitation wavelength, as observed by Weber [39] for both p-cresol and tyrosine (but unlike the results reported in [40]). The fluorescence anisotropy excitation spectrum of parent 1 (Figure 10) follows the same trend, albeit with lower values, starting from 0.12 at 280 nm. The drop in anisotropy observed for this calixarene can be entirely ascribed to fast energy migration (FRET homotransfer). Given that there are four equivalent chromophores, with nearly isotropic and uncorrelated orientations, the anisotropy is expected to reduce to r₀(λ_exc)/4 owing to FRET [41]. As seen in Figure 10, this is indeed borne out by the experiment, with a ratio of 0.30 ± 0.03 being obtained between 265 and 290 nm. The slight excess over 0.25 can be ascribed to a residual degree of orientational anisotropy and orientational correlation. The fluorescence quantum yield decrease (by one or two orders of magnitude) in the case of compounds 2a, 3a, 3b and 5a results mainly from long-range quenching by the phthalimides owing to irreversible FRET. The two compounds bearing two acceptor phthalimides, 3a and 3b, display similar but lower donor fluorescence quantum yields when compared to the monophthalimide compound 2a, as expected. The enhanced phthalimide emission in the 3b case may result from the existence of two acceptors both in close proximity of the calixarene fluorophores and in a non self-quenching geometry. The monophthalimide 5a differs from all the others by the number of carbon atoms of the spacer attaching the phthalimide (C₂ vs. C₃). The lower fluorescence quantum yield of 5a (in fact, the lowest of all) is in accordance with the shortest (C₂) chain, hence the shortest average donor-acceptor distance (down to 0.6 nm).

3. Material and Methods

3.1. Synthesis

All chemicals were reagent grade and were used without further purification. Microwave-assisted syntheses were performed with a CEM Discover SP microwave synthesizer system (2.45 GHz, 300 W) equipped with a non-contact infrared temperature sensor. The reaction temperature was controlled by variable microwave irradiation (0–150 W) and cooling by nitrogen current. The mechanosyntheses were performed on a PM100 Planetary Ball Mill (Retsch, Haan, Germany) using a 50 mL stainless steel or zirconium oxide reactor, and 200 balls with 5 mm of diameter each, of the corresponding materials. Chromatographic separations were performed on Merck silica gel 60 (particle size 40–63 μm, 230–400 mesh). Melting points were measured on a Stuart Scientific apparatus and are uncorrected. FTIR spectra were recorded on a Shimadzu Model IRaffinity-1 spectrophotometer (Kyoto, Japan). ¹H and ¹³C NMR spectra were recorded on a Bruker Avance III 500 MHz spectrometer (Bruker, Billerica, MA, USA), with TMS as internal reference. The conventional COSY 45 and the phase-sensitive NOESY experiments were collected as 256 × 2 K complex points. Elemental analysis was determined on a Fisons EA 1108 microanalyser (Milano, Italy).

3.1.1. Conventional Procedure for the Synthesis of 2a, 3a and 3b

A mixture of p-tert-butyldihomooxacalix[4]arene (1 g, 1.47 mmol), N-(3-bromopropyl)phthalimide (0.81 g, 2.95 mmol) and K₂CO₃ (0.41 g, 2.95 mmol) in CH₃CN (25 mL) was refluxed and stirred under N₂ for 5 days. After cooling, the solvent was evaporated under reduced pressure and the residue was dissolved in CH₂Cl₂ (50 mL) and washed with 1 M HCl (2 × 30 mL), NH₄Cl saturated solution (2 × 30 mL) and brine (30 mL). The organic layer was dried over Na₂SO₄, filtered and the solvent evaporated to dryness. The crude product was subjected to flash chromatography on silica gel (eluent CH₂Cl₂/MeOH, 99:1). According to TLC analysis, the fractions were combined into three sets, corresponding to derivatives 2a, 3a and 3b, respectively.

7,13,19,25-Tetra-tert-butyl-28-[(3-phthalimidopropyl)oxy]-27,29,30-tri-hydroxy-2,3-dihomo-3-oxacalix[4]arene (2a): This compound was obtained in ≈ 2% yield (25 mg). An analytically pure sample was obtained by recrystallization from CH₂Cl₂/n-hexane. mp 171–173 °C; IR (KBr) 1717 cm⁻¹ (CO), 3377 cm⁻¹ (OH); ¹H NMR (CDCl₃, 500 MHz) δ 1.13, 1.230, 1.233, 1.26 [4s, 36H, C(CH₃)], 2.45, 2.57 (2m, 2H, OCH₂CH₂CH₂N), 3.31, 4.48 (ABq, 2H, J = 12.9 Hz, ArCH₂Ar), 3.46, 4.31 (ABq, 2H, J = 13.8 Hz, ArCH₂Ar), 3.54, 4.11 (ABq, 2H, J = 13.8 Hz, ArCH₂Ar), 4.01, 4.17 (2m, 2H, OCH₂CH₂CH₂N), 4.14–4.21 (m, 2H, OCH₂CH₂CH₂N), 4.23, 4.94 (ABq, 2H, J = 9.3 Hz, CH₂OCH₂), 4.43, 4.70 (ABq, 2H, J = 10.0 Hz, CH₂OCH₂), 6.87, 6.95, 7.03, 7.07, 7.09, 7.13, 7.28, 7.30 (8d, 8H, ArH), 7.72, 7.87 (2m, 4H, ArH-Pht), 7.73, 8.47, 9.08 (3s, 3H, OH); ¹³C NMR (CDCl₃, 125.8 MHz) δ 29.4 (OCH₂CH₂CH₂N), 30.1, 31.3, 32.8 (ArCH₂Ar), 31.1, 31.4, 31.5, 31.6 [C(CH₃)₃], 33.8, 33.9, 34.2 [C(CH₃)₃], 35.2 (OCH₂CH₂CH₂N), 71.7, 72.2 (CH₂OCH₂), 74.2 (OCH₂CH₂CH₂N), 122.6, 122.7, 126.5, 126.7, 127.4, 128.4, 131.3, 132.2, 132.8, 141.5, 142.4, 143.5, 147.8, 147.9, 149.3, 151.3, 152.7 (Ar), 123.4, 133.9 (ArH-Pht), 123.7, 125.2, 125.5, 125.6, 125.9, 126.7, 127.4, 128.1 (ArH), 168.3 (CO); Anal. Calcd for C₅₆H₆₇O₇N: C, 77.66; H, 7.80; N, 1.62. Found: C, 77.18; H, 7.60; N, 1.38.

7,13,19,25-Tetra-tert-butyl-27,29-bis[(3-phthalimidopropyl)oxy]-28,30-di-hydroxy-2,3-dihomo-3-oxacalix[4]arene (3a): This compound was obtained in 54% yield (0.84 g). An analytically pure sample was obtained by recrystallization from CH₂Cl₂/MeOH. mp 172–174 °C; IR (KBr) 1709 cm⁻¹ (CO), 3400 cm⁻¹ (OH); ¹H NMR (CDCl₃, 500 MHz) δ 1.10, 1.19, 1.23, 1.24 [4s, 36H, C(CH₃)], 2.55 (m, 4H, OCH₂CH₂CH₂N), 3.25, 4.53 (ABq, 2H, J = 12.8 Hz, ArCH₂Ar), 3.27, 4.67 (ABq, 2H, J = 12.8 Hz, ArCH₂Ar), 3.47, 4.18 (ABq, 2H, J = 13.1 Hz, ArCH₂Ar), 3.96, 4.13 (2m, 2H, OCH₂CH₂CH₂N), 4.08, 4.43 (2m, 2H, OCH₂CH₂CH₂N), 4.04–4.15 (2m, 4H, OCH₂CH₂CH₂N), 4.23, 4.89 (ABq, 2H, J = 9.5 Hz, CH₂OCH₂), 4.37, 4.70 (ABq, 2H, J = 10.1 Hz, CH₂OCH₂), 6.81, 6.91, 6.96, 7.00, 7.04, 7.09, 7.23, 7.44 (8d, 8H, ArH), 7.20, 8.04 (2s, 2H, OH), 7.61, 7.68, 7.73, 7.83 (4m, 8H, ArH-Pht); ¹³C NMR (CDCl₃, 125.8 MHz) δ 29.3, 30.0 (OCH₂CH₂CH₂N), 29.8, 29.9, 32.9 (ArCH₂Ar), 31.2, 31.4, 31.5, 31.6 [C(CH₃)₃], 33.8, 34.1 [C(CH₃)₃], 35.4, 35.7 (OCH₂CH₂CH₂N), 71.9, 72.6 (CH₂OCH₂), 72.7, 74.2 (OCH₂CH₂CH₂N), 122.2, 127.6, 128.0, 128.8, 129.2, 132.2, 132.3, 132.5, 132.6, 135.3, 141.2, 141.9, 146.4, 147.0, 149.4, 150.0, 152.8, 154.3 (Ar), 123.1, 123.2, 133.6, 133.8 (ArH-Pht), 123.7, 124.7, 125.1, 125.2, 125.6, 126.0, 127.7, 129.5 (ArH), 168.3, 168.4 (CO). Anal. Calcd for C₆₇H₇₆O₉N₂: C, 76.40; H, 7.27; N, 2.66. Found: C, 76.01; H, 7.63; N, 2.67.

7,13,19,25-Tetra-tert-butyl-28,29-bis[(3-phthalimidopropyl)oxy]-27,30-di-hydroxy-2,3-dihomo-3-oxacalix[4]arene (3b): This compound was obtained in 7% yield (0.11 g). An analytically pure sample was obtained by recrystallization from CH₂Cl₂/MeOH. mp 145–147 °C; IR (KBr) 1715 cm⁻¹ (CO), 3397 cm⁻¹ (OH); ¹H NMR (CDCl₃, 500 MHz) δ 1.06, 1.25 [2s, 36H, C(CH₃)], 2.46, 2.52 (2m, 4H, OCH₂CH₂CH₂N), 3.38, 4.29 (ABq, 4H, J = 13.7 Hz, ArCH₂Ar), 3.39, 4.56 (ABq, 2H, J = 13.0 Hz, ArCH₂Ar), 3.93–4.05 (m, 4H, OCH₂CH₂CH₂N), 4.03, 4.25 (2m, 4H, OCH₂CH₂CH₂N), 4.25, 4.71 (ABq, 4H, J = 9.7 Hz, CH₂OCH₂), 6.84, 6.90, 6.96, 7.21 (4d, 8H, ArH), 7.66, 7.78 (2m, 8H, ArH-Pht), 7.68 (s, 2H, OH); ¹³C NMR (CDCl₃, 125.8 MHz) δ 29.4 (OCH₂CH₂CH₂N), 30.8, 31.6 (ArCH₂Ar), 31.2, 31.6 [C(CH₃)₃], 33.8, 34.0 [C(CH₃)₃], 35.4 (OCH₂CH₂CH₂N), 71.7 (CH₂OCH₂), 73.1 (OCH₂CH₂CH₂N), 123.17, 133.8 (ArH-Pht), 123.22, 128.2, 132.2, 132.6, 133.4, 141.7, 146.4, 151.5, 152.1 (Ar), 124.7, 125.2, 125.8, 128.0 (ArH), 168.3 (CO); Anal. Calcd for C₆₇H₇₆O₉N₂: C, 76.40; H, 7.27; N, 2.66. Found: C, 76.34; H, 7.62; N, 2.63.

3.1.2. Conventional Procedure for the Synthesis of 5a and 6a

A mixture of 1 (1 g, 1.47 mmol) and K₂CO₃ (0.81 g, 5.9 mmol) in MeCN (25 mL) was stirred and gently warmed under N₂ for 30 min. After cooling, N-(2-bromoethyl)-phthalimide (1.58 g, 5.9 mmol) and KI (0.49 g, 2.95 mmol) were added, and the reaction mixture was refluxed and stirred under N₂ for 3 days. By the same workup described above, the crude product obtained was subjected to flash chromatography on silica gel (eluent gradient from n-hexane/ethyl acetate, 90:10 to 70:30). According to TLC analysis, the fractions were combined into two sets, corresponding mainly to derivatives 5a and 6a, respectively.

7,13,19,25-Tetra-tert-butyl-28-[(2-phthalimidoethyl)oxy]-27,29,30-tri-hydroxy-2,3-dihomo-3-oxacalix[4]arene (5a): This compound was obtained in 24% yield (0.30 g). An analytically pure sample was obtained by recrystallization from CH₂Cl₂/MeOH. mp 176–178 °C; IR (KBr) 1717 cm⁻¹ (CO), 3375 cm⁻¹ (OH); ¹H NMR (CDCl₃, 500 MHz) δ 1.11, 1.21, 1.222, 1.224 [4s, 36H, C(CH₃)], 3.24, 4.25 (ABq, 2H, J = 12.8 Hz, ArCH₂Ar), 3.38, 4.15 (ABq, 2H, J = 13.7 Hz, ArCH₂Ar), 3.52, 4.09 (ABq, 2H, J = 13.6 Hz, ArCH₂Ar), 4.20, 4.86 (ABq, 2H, J = 9.1 Hz, CH₂OCH₂), 4.28, 4.40 (ABq, 2H, J = 10.0 Hz, CH₂OCH₂), 4.29, 4.49 (2m, 2H, OCH₂CH₂N), 4.39 (t, 2H, OCH₂CH₂N), 6.78, 6.94, 6.99, 7.06, 7.07, 7.08, 7.19, 7.26 (8d, 8H, ArH), 7.28, 8.25, 8.60 (3s, 3H, OH), 7.68, 7.91 (2m, 4H, ArH-Pht); ¹³C NMR (CDCl₃, 125.8 MHz) δ 29.8, 31.2, 32.2 (ArCH₂Ar), 31.1, 31.4, 31.5, 31.6 [C(CH₃)₃], 33.77, 33.81, 33.9, 34.2 [C(CH₃)₃], 37.5 (OCH₂CH₂N), 71.6, 71.9 (CH₂OCH₂), 72.0 (OCH₂CH₂N), 122.2, 122.7, 126.4, 126.7, 127.3, 127.9, 131.5, 132.5, 132.5, 141.2, 142.2, 143.3, 147.8, 147.9, 148.6, 151.3, 152.5 (Ar), 123.4, 133.7 (ArH-Pht), 123.7, 125.2, 125.3, 125.5, 125.7, 126.7, 127.3, 128.2 (ArH), 168.5 (CO); Anal. Calcd for C₅₅H₆₅O₇N: C, 77.52; H, 7.69; N, 1.64. Found: C, 77.12; H, 8.01; N, 1.65. Although this compound had been synthesised before by a slightly different procedure [34], neither the mp nor the ¹H and ¹³C NMR chemical shifts are totally correct.

7,13,19,25-Tetra-tert-butyl-27,29-bis[(3-phthalimidoethyl)oxy]-28,30-di-hydroxy-2,3-dihomo-3-oxacalix[4]arene (6a): This compound was obtained in 18% yield (0.27 g). ¹H NMR (CDCl₃, 500 MHz) δ 1.06, 1.20, 1.21, 1.25 [4s, 36H, C(CH₃)], 3.17, 4.37 (ABq, 2H, J = 13.0 Hz, ArCH₂Ar), 3.30, 4.67 (ABq, 2H, J = 13.0 Hz, ArCH₂Ar), 3.48, 4.18 (ABq, 2H, J = 13.3 Hz, ArCH₂Ar), 4.21, 4.92 (ABq, 2H, J = 9.5 Hz, CH₂OCH₂), 4.25–4.34 (m, 2H, OCH₂CH₂N),4.32, 4.77 (2m, 2H, OCH₂CH₂N), 4.38, 4.74 (ABq, 2H, J = 10.3 Hz, CH₂OCH₂), 4.42, 4.55 (2m, 2H, OCH₂CH₂N), 4.51 (m, 2H, OCH₂CH₂N), 6.78, 6.82, 6.95, 7.00, 7.09, 7.16, 7.41 (7d, 8H, ArH), 7.14, 7.75 (2s, 2H, OH), 7.61, 7.69, 7.82, 7.85 (4m, 8H, ArH-Pht); ¹³C NMR (CDCl₃, 125.8 MHz) δ 29.8, 30.0, 32.7 (ArCH₂Ar), 31.1, 31.4, 31.5, 31.6 [C(CH₃)₃], 33.77, 33.79, 34.1 [C(CH₃)₃], 37.4, 38.4 (OCH₂CH₂N), 70.1, 71.7, 72.8 (CH₂OCH₂ and OCH₂CH₂N), 122.0, 127.4, 127.8, 128.6, 129.0, 132.1, 132.3, 132.4, 134.7, 141.0, 141.9, 146.4, 147.1, 149.5, 149.7, 152.5, 154.0 (Ar), 123.1, 123.3, 133.6, 133.9 (ArH-Pht), 123.3, 124.8, 125.2, 125.3, 125.6, 126.0, 127.5, 129.6 (ArH), 168.1, 168.5 (CO).

3.1.3. Microwave- and Mechanically-Assisted Syntheses

MW procedure for the synthesis of 2a, 3a and 3b: A mixture of 1 (0.30 g, 0.44 mmol), N-(3-bromopropyl)phthalimide (0.24 g, 0.89 mmol) and K₂CO₃ (0.12 g, 0.89 mmol) was suspended in CH₃CN (3 mL) and irradiated under stirring for 30 min at 160 °C. The products were isolated and purified by the same workup and purification steps described in 3.1.1.

MW procedure for the synthesis of 6a: A mixture of 1 (0.30 g, 0.44 mmol) and K₂CO₃ (0.24 g, 1.77 mmol) was suspended in CH₃CN (3 mL) and irradiated under stirring for 6 min at 110 °C. After cooling, N-(2-bromoethyl)phthalimide (0.45 g, 1.77 mmol) was added and the reaction mixture irradiated for a further 36 min at 150 °C. The product was obtained by the same steps described in 3.1.2.

Mechanochemical reaction protocol: 50 mg of parent 1, N-(bromoalkyl)phthalimide and K₂CO₃ were loaded into the selected reactor containing the corresponding 200 balls. The reactor was then adjusted into the planetary ball mill and the mixture was grinded at 500 rpm, with rotation inversion cycles of 30 min (2.5 min. pause between inversion cycles). Different reagent ratios and reaction times were tested, as mentioned before (see 2.1.). The products were obtained by the same workup described previously.

3.2. Determination of the Crystallographic Structures of 2a, 3a, 3b and 5a

Single crystals suitable for X-ray investigation were obtained by slow evaporation of acetonitrile solutions containing compound 2a, 3a or 3b, while dichloromethane/ethanol solvent mixtures were used for 5a. Data collection was carried out at the XRD1 beamline of the Elettra synchrotron (Trieste, Italy), employing the rotating-crystal method with a Dectris Pilatus 2M area detector. Single crystals were dipped in paratone cryoprotectant, mounted on a loop and flash-frozen under a nitrogen stream at 100 K. Diffraction data were indexed and integrated using the XDS package [42], while scaling was carried out with XSCALE [43]. Structures were solved using the SHELXT program [44] and the refinement was performed with SHELXL-14 [45], operating through the WinGX GUI [46], by full-matrix least-squares (FMLS) methods on F². Non-hydrogen atoms were anisotropically refined with the exception of non-hydrogen atoms with a low occupancy factor, which were refined isotropically. Hydrogen atoms, located on the difference Fourier maps, were added at the calculated positions and refined using the riding model. Crystallographic data and refinement details are reported in Table S4.

3.3. Fluorescence Studies

Absorption and fluorescence studies were performed using an UV-3101PC UV-Vis-NIR spectrophotometer (Shimadzu, Kyoto, Japan) and a Fluorolog F112A fluorimeter (Spex Industries, Edison, NJ, USA) in right-angle configuration, respectively. The absorption spectra were recorded between 240 and 340 nm and the emission ones between 285 and 600 nm, and using quartz cells with an optical path length of 1 cm. Details concerning the photophysical properties determination are given in the Supplementary Materials.

4. Conclusions

The alkylation of p-tert-butyldihomooxacalix[4]arene (1) with N-(bromopropyl)- or N-(bromoethyl)phthalimides and K₂CO₃ in MeCN was carried out by conventional heating and by microwave irradiation and ball milling methodologies. Eight compounds were identified and five isolated (2a, 3a, 3b, 5a and 6a), including mono- and mainly 1,3-disubstituted derivatives in the cone conformation. A microwave-assisted procedure was developed, affording both 1,3-disubstituted phthalimides (3a and 6a) as major/unique products. The reaction times were drastically reduced (30–42 min) and the selectivity increased when compared to refluxing conditions (several days for completion). Minor amounts of solvent and less reactants used were other advantages of this environmentally friendly technique. The results obtained open new perspectives for using this MW-assisted protocol in the synthesis of new calixarenes. Mechanically-assisted synthesis was not shown to be a good method for these alkylation reactions. The conformation and the substitution patterns were confirmed by NMR spectroscopy (¹H, ¹³C, COSY and NOESY) and X-ray diffraction. The X-ray structural analysis of 2a, 3a, 3b and 5a reveals a common open conformation of the calixarene cone, determined by an intramolecular H-bond which involves the oxygen acceptor of the oxa bridge oriented towards the center of the macrocycle. The lower rim substitution patterns, which determine the intramolecular H-bond network, strongly influence the orientation and π-stacking interactions of the phthalimide arms. The photophysical properties of 2a, 3a, 3b and 5a were also determined. In particular, it is found that upon excitation of the arene fluorophores, fast energy migration (FRET homotransfer) within the ring ensues, followed by irreversible energy transfer (FRET heterotransfer) to the phthalimide chromophores. These phthalimide derivatives will be used as precursors to urea-terminated anion-binding macrocycles. The ureido-dihomooxacalix[4]arenes bearing propyl or ethyl spacers are expected to be better receptors than those with butyl spacers, mainly for ion pairs, as they will be in a closer proximity. This study is currently under investigation.