Photophysical Properties of Naphthalene-oxacalix[m]arene and Recognition of Fullerene C60

Three different pore sizes of oxacalix[m]arene[n]pyrimidines modified with a naphthalene substituent were synthesized and characterized by HRMS, 1H NMR, and single-crystal analysis (8OA and 8OA-N). Steady-state spectroscopy indicates these naphthalene-oxacalix[m]arenes exhibit good fluorescence properties, which isattributed to the locally excited (LE) state emission, and electrochemical results show that the photoinduced electron transfer (PET) process occurs from the naphthalene substituent to the linked pyrimidine. Nanosecond transient absorption spectra, singlet oxygen quantum yields (ΦΔ4OA-N = 45.1%, ΦΔ6OA-N = 56.6%, and ΦΔ8OA-N = 65.7%) and theoretical calculations demonstrate that the torsion angle between the donor (naphthalene) and the acceptor (pyrimidine) promotes intersystem crossing (ISC), and the lifetime of the triplet state reaches ca. 8 ms. Interestingly, all three host molecules (4OA-N, 6OA-N, and 8OA-N) showed a high affinity for fullerene C60, and significant binding constants in the range of 4.10–6.68 × 104 M–1 were obtained by fluorescence titration; in contrast, previous reports indicated that the similar oxacalix[m]arene[n]pyrimidine scaffold could not efficiently complex with C60. In the frontier molecular orbital theory calculations of the supramolecular system of 4OA-N@C60, the HOMO is distributed on 4OA-N and the LUMO is localized on fullerene. The calculation results further demonstrated that there are strong interactions between the host and the fullerene guest, which is consistent with the result of the experiments. The characteristic photophysical properties of these novel naphthyl-decorated oxacalix[m]arene[n]pyrimidines broaden their application field, and the stable host–guest system with fullerene can be applied to supramolecular chemistry.


■ INTRODUCTION
Macrocyclic compounds play an important role in supramolecular chemistry. Their preconstructed rigid structures can effectively overcome the unfavorable factors caused by the reduction of entropy in molecular recognition and assembly. In recent years, heteroatom bridge and heteroaromatic hybrid calixarenes have widely attracted attention 1,2 and have been used in conformation control, molecular recognition, sensor systems, optoelectronic devices, and advanced materials. 3−10 According to previous reports, nitrogen as a bridging atom has different hybrid modes of sp 2 , sp 3 , or between them and can form different degrees of conjugation with adjacent aromatics, which means heterocalixarenes have adjustable configurations and the potential for application in supramolecular chemistry. 11 Additionally, there are similar properties when oxygen atoms act as bridging atoms. At present, there are many reports on the synthesis and modification of oxacalix[m]arenes 12−14 and a few reports about the application of oxacalix[m]arenes in supramolecular recognition. Wim Dehaen et al. 15 reported the presence of the anion receptors on the oxacalix [2]arene [2]pyrimidine scaffold, and Chen et al. 16 reported that triptycene oxacalixarene derivatives were the hosts of fullerenes. Hitherto, the research on the host−guest chemistry of oxacalix[m]arenes is still insufficient, especially the introduction of specific luminescent substituents into supramolecular systems for energy or electron transfer.
Fullerenes (C 60 ) with unique structures and photoelectric properties have attracted much attention since their discovery in 1985. 17−20 Fullerenes can interact with the host molecules through the force of noncovalent bonds and are widely studied in the field of supramolecular chemistry. 21 The highly selective recognition of fullerenes through different hole sizes and geometries can not only separate and purify fullerene isomers but can also expand the application of fullerene-based materials and devices. 22−24 Oxacalix[m]arenes are seldom applied in the assembly of host−guest system in supramolecular chemistry, especially oxacalix[m]arenes hybridized with heteroaromatic rings. 25 Wim et al. reported that oxacalix [2]arene [2]pyrimidinebis(Zn-porphyrin) selectively recognized C 70 , and several other oxacalixrenes did not recognize C 70 or C 60 . 26 Therefore, we think that more oxacalixarenes should be synthesized and applied in supramolecular chemistry, particularly for interactions with fullerene C 60 . Hence, we synthesized three oxacalix[m]arene[n]pyrimidine homologues 4OA, 6OA, and 8OA 27 and modified them with a naphthyl group to give 4OA-N, 6OA-N, and 8OA-N, respectively (Scheme 1 and Figures S1−S4). The photoproperties of the target oxacalix[m]arenes were tested using spectroscopic methods, and the three homologues show similar fluorescence emission behaviors. Interestingly, when the interactions between the host oxacalix-[m]arene[n]pyrimidines and the guest fullerene C 60 were measured in a toluene solution, they could form a stable concave-convex complex with a ratio of 1:1 and exhibited a high association constant. The study of oxacalix[m]arenes opens the way for the selective and efficient recognition of C 60 , which could be applied to supramolecular chemistry.

■ EXPERIMENTAL SECTION
Materials and Methods. NMR spectra were acquired on commercial instruments (VARIAN INOVA-400 MHz). Chemical shifts (δ) are reported in parts per million (ppm) and referenced to the signals of tetramethylsilane or the internal (NMR) solvent. High-resolution mass spectra (HRMS) were recorded on a Bruker Daltonics Micro TOF-Q II spectrometer. UV−vis spectra were obtained on a U-3900H instrument. Fluorescence spectra and binding constants with fluorescence titration were acquired using an F-4500 spectrofluorometer. Nanosecond transient absorption spectra were recorded on an LP980 laser flash-photolysis spectrometer (Edinburgh Instrument Ltd., UK) equipped with an OPO laser (Opolette, the wavelength is tunable in the range of 210−2400 nm; OPOTEK, U.S.); the typical laser energy was ca. 5 mJ per pulse. The data (kinetic decay curve and spectrum) were obtained with the L900 software. Chemicals received from commercial sources were used without further purification. K 2 CO 3 (anhydrous, granulated) was finely ground (with mortar and pestle) prior to use. Reaction solvents (DMF, petroleum ether, ethyl acetate, THF, and CH 2 Cl 2 ; ≥99.5%) were used as received from commercial sources and not dried further prior to use.
Calculations. Density functional theory (DFT) calculations were used to optimize the ground-state geometries at the B3LYP/6-31G level. All the calculations were performed with Gaussian 09.
Lippert−Mataga equation. The simplest consideration for the general solvent effect is the Lippert−Mataga equation, which assumes that same excited -state is involved in both absorption and emission and that the energy difference between the ground state and the excited state is only proportional to solvent orientation polarizability (△f).
where△ν = ν abs − ν em stands for the Stokes shift (ν abs and ν em are the absorption and emission (cm −1 ), respectively), h is Planck's constant, c is the velocity of light in a vacuum, a is the radius of the solvent cavity in which the fluorophore resides (the Onsager cavity radius), △f is the orientation polarizability, μ g and μ e are the ground-state dipole in the groundstate geometry and the excited dipole in the excited-state geometry, respectively, ε is the permittivity of the vacuum, and n is the refractive index.
Singlet Oxygen Quantum Yield. The singlet oxygen of the photosensitizer was measured using a 355 nm excitation lamp source and 1,3-diphenylisobenzofuran (DPBF) as an 1 O 2 scavenger. The 1 O 2 production was monitored by following the absorbance of DPBF at 414 nm. To determine the singlet oxygen quantum yield (Φ Δ ), a relative method was used according to eq 3. Ru(bpy) 3 [PF 6 ] 2 was used as the standard (Φ std = 0.57 in dichloromethane). In the above equation, "sam" and "std" represent the sample and the standard, respectively. Φ, A, m, and η represent the singlet oxygen quantum yield, the absorbance at the excitation wavelength, the change in the slope of the absorbance of DPBF over time, and the refractive index of the solvent used for measurement, respectively.  Figure 1a and b, and crystal data are shown in Tables S1 and S2. The distances between the naphthalene and the pyrimidine in 8OA-N vary between 1.487 and 1.501 Å. This indicates that the molecular orbitals of naphthalene and pyrimidine overlap, resulting in a significant decrease in the C−C spacing. In 8OA and 8OA-N, the distance between adjacent pyrimidines is 6.476 and 7.203 Å and 7.012 and 7.401 Å respectively, which means that the pore size of 8OA-N increases after the addition of the naphthalene substituents and the crystal density decreases ( Figure S5c and d). These structural features are attributed to the charge distribution changes and spatial repulsion effects caused by the introduction of naphthyl groups.
As shown in Figure S5a and b, the intermolecular distance in 8OA is 3.640 Å and that in 8OA-N is 3.661 Å (red dashed line), respectively; both distances are less than 3.800 Å, showing the existence of intermolecular π−π interactions. 31 Moreover, the shortest C−H···O distances in 8OA and 8OA-N are 2.919 and 2.943 Å (green dashed line), respectively, showing formation of intermolecular hydrogen bonding. 32 As shown in Figure S5b, the distances between naphthalene units and the adjacent pyrimidine are 7.102 and 7.815 Å within 8OA-N (blue dashed line), indicating that there is no intramolecular interaction between naphthalene and the adjacent pyrimidine. 1 H NMR Analysis. In CDCl 3 and DMSO-d 6 , 8OA and 8OA-N give rise to well-resolved 1 H NMR spectra, as shown in Figures S8 and S11, respectively. For instance, the single peaks at 2.26 and 3.39 ppm represent the methyl groups on benzene  and methyl groups with sulfur, respectively. After 8OA was coupled with naphthyl, the peak of the methyl groups with sulfur disappeared and the signal of the naphthyl groups appeared in range of 7.20−8.60 ppm, indicating that the target molecules were prepared successfully.
UV−vis Absorption and Fluorescence Emission Spectra. Figure 2 and Table 1 are the UV−vis absorption spectra and photophysical parameters, respectively, of 4OA, 6OA, 8OA, 4OA-N, 6OA-N, and 8OA-N. 8OA and 8OA-N show absorption bands at 288 and 318 nm respectively, and the molar extinction coefficient (ε) of 8OA-N (ε 318 nm = 5.87 × . After the introduction of the naphthyl group, the absorption band of 8OA-N is obviously red-shifted compared with that of 8OA ( Figure S14). This could be attributed to the introduction of naphthyl as an electron-donating group reducing the HOMO and LUMO energy levels, which was also verified by theoretical calculations (Figure 8a    The fluorescence emission of 4OA-N under the oxygen environment is lower than that under the nitrogen environment and is blue-shifted (Figures 4b, S22, and S23). The triplet levels (from theoretical calculations) of the three dyes (E 4OA-N,T1 = 1.90 eV, E 6OA-N,T1 = 1.91 eV, and E 8OA-N,T1 = 1.88 eV) are lower than the spectral levels in nitrogen and air environments (E = 1240/λ em ). We believe that this change in the luminescence intensity is not due to phosphorus emission.
Fullerene Binding Studies. At present, C 60 has been developed as an electron receptor in host−guest systems. 38 Wang et al. designed and synthesized N-calixarenes that performed well in the subject−object recognition of C 60 due to their unique pore cavity structure. 39 Here, the interaction of Ocalixarenes with fullerene C 60 was investigated by means of fluorescence titration experiments (Figures S24−S26). We found that the three congeners complex well with fullerene C 60 in toluene solutions. The complexation constants range from 4.10 × 10 4 to 6.68 × 10 4 M −1 , and the complexation ratio is 1:1 ( Table 2), indicating that the prepared O-calixarenes can effectively form stable host−guest complexes with C 60 . The complexation constants have the order of magnitude but are not proportional to the pore sizes of the O-calixarenes, and the binding ability to C 60 follows the order 6OA-N > 4OA-N > 8OA-N. This indicates that the pore sizes of the O-calixarenes have effects on the capture of C 60 .
The UV−vis absorption intensities of the three compounds increase with the concentration in accordance with the Lambert−Beer law (Figures 5a, S27, and S28). The UV−vis absorption spectra obviously changed after the addition of C 60 , and isosbestic point for 8OA-N@C 60 was at 316 nm ( Figure  5b), indicating that there was an obvious interaction between the host 8OA-N and the guest C 60 . 40 In the UV−vis and fluorescence emission spectra (Figures 5c and d, S29, and S30), we can see the fluorescence was distinctly quenched after the addition of C 60 . This phenomenon is same as that in previous reports and indicates that these naphthalene-oxacalix-[m]arene[n]pyrimidines interact strongly with fullerene C 60 . 41 We tried to use C 60 as the supramolecular guest template to grow the crystals. However, a dark red needle solid was obtained after many attempts, and we failed to get good crystals for the collection of X-ray diffraction data. Regardless, the unique concave cavity structure of 4OA-N, 6OA-N, and 8OA-N can construct supramolecular systems with C 60 by noncovalent bonds.
Electrochemical Studies: Cyclic Voltammograms and the Gibbs Free Energy Changes of the Photoinduced Electron Transfer (PET). The redox properties of the dyads were studied by cyclic voltammetry (Figure 6 and Table 3). 4OA-N exhibited irreversible oxidation waves at +1.39 (in DCM) and +0.42 V (in ACN) and irreversible reduction waves at −1.02 (in DCM) and −2.40 V (in ACN). From the redox potentials as described above, the free energy (△G CS ) of PET was calculated using the Weller equation (eqs 4−6). 42 The values of ΔG CS are negative in all solutions and become more negative as the solution polarity increases. This result suggests that charge separation (CS) can also occur in n-hexane.
Nanosecond Transient Absorption Spectroscopy. To confirm the populations of the triplet excited states of the dyads upon photoexcitation, nanosecond time-resolved transient difference absorption spectroscopy was performed for 4OA-N and 4OA-N@C 60 . For 4OA-N (Figure 7a), the excited-state absorption (ESA) of the triplet state is the naphthalenen portion and the naphthalene-pyrimidine diplexes. 43 For 4OA-N@C 60 ( Figure S31), ESA was observed not only for the host molecule but also for the guest molecule C 60 in the range of 700−800 nm. 44 Interestingly, the tripletstate lifetime of 4OA-N (τ T = 35.7234 μs (0.57%), 7817.8111 μs (99.43%)) was longer than that of 4OA-N@C 60 (τ T = 9.03 μs). We speculated that the triplet−triplet energy transfer (TTET) process occurred from 4OA-N to C 60 . Meanwhile, we calculated the attributions of the ESA bands ( Figure S38 (Figure 7d). This indicates that TTET occurred between the host and the guest.
DFT Calculations. The ground state (S 0 ) and the single excited state (S 1 ) of the complex were calculated with DFT and TD-DFT using the Gaussian 09 software package. 45−47 The ground-state frontier molecular orbitals of the compound are shown in Figure 8. The frontier molecular orbitals of the Recorded with Bu 4 NPF 6 as the electrolyte in CH 2 Cl 2 (0.1 M) at room temperature. A Pt electrode was used as the counter electrode, a glassy carbon electrode was used as the working electrode, ferrocene (Fc/Fc + ) was used as the internal reference, and the Ag/AgNO 3 couple was used as the reference electrode.    Figure S32 and S33). Meanwhile, we calculated the molecular orbital distribution of the 4OA-N@C 60 supramolecular system (Figure 8b). Because the LUMO energy level of C 60 is lower than that of the host molecule, the HOMO of 4OA-N@C 60 is distributed on 4OA-N and the LUMO is localized on fullerene C 60 . This indicates that the host−guest molecular orbitals in supramolecular systems are separated from each other, which is consistent with experimental results showing that the UV−vis absorption peak of the host molecule changed to C 60 after the construction of the 4OA-N@C 60 supramolecular system.
From the theoretical calculation of the UV−vis absorption spectrum ( Figure S37a and Table 4), we can see that the S 0 → S 1 transition in 8OA-N is symmetrically forbidden and the absorption is mainly derived from HOMO − LUMO + 2 (S 0 → S 2 electronic transition), which leads to the decrease of the 8OA-N quantum yield and explains why it is lower than the other molecules (Table 1). Compared with 4OA-N and 6OA-N, the distorted conformation of 8OA-N may cause the decrease of the fluorescence quantum yield. The distorted conformation also impacts the binding of host and guest because the complexation constant of 8OA-N is also the lowest ( Table 2). The fluorescence calculations of 4OA-N, 6OA-N, and 8OA-N reveal that the S 0 → S 1 electronic transition is mainly derived from HOMO − LUMO and partly overlapped, which indicates that the lowest excited transition state is a LE state. The result of the theoretical calculations is in agreement with the experimental results (Figures S35−S37 and S39− S41).
At the ground and excited states of the electrostatic potential (ESP) of 4OA-N, 6OA-N, and 8OA-N (Figures 9a and b, S42, and S43), the electron-deficient portions of the three compounds are concentrated on the pyrimidine and bridging oxygen atoms and can interact with fullerene C 60 . Moreover, the electron-rich methyl group can also form a C−H···π interaction with C 60 . This ESP result also further confirms that naphthalene-oxacalix[m]arenes are suitable "nests" for fullerene C 60 .
Interestingly, taking 4OA-N as an example (Figures 10, S44, and S45), the dihedral angle between naphthlene and pyrimidine is about 3°at the ground state, which is smaller than that in the excited state (near 27°). According to previous studies, an orthogonal geometry is beneficial for spin−orbit charge-transfer intersystem crossing (SOCT-ISC). 48 These different dihedral angles between the π-conjugation planes may