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@C 60 , 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 sp2, sp3, 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.[2] arene [2] pyrimidine scaffold. J. Org. Chem.. 2012 ">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 (C60) 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]pyrimidine-bis(Zn-porphyrin) selectively recognized C70, and several other oxacalixrenes did not recognize C70 or C60.[2] arene [2] pyrimidine-bis (Zn-porphyrin) tweezer as a selective receptor towards fullerene C. Tetrahedron Lett.. 2010 ">26] Therefore, we think that more oxacalixarenes should be synthesized and applied in supramolecular chemistry, particularly for interactions with fullerene C60. 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 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 C60 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 C60, which could be applied to supramolecular chemistry.

Scheme 1

Synthesis of the 4OA-N, 6OA-N, and 8OA-N

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. K2CO3 (anhydrous, granulated) was finely ground (with mortar and pestle) prior to use. Reaction solvents (DMF, petroleum ether, ethyl acetate, THF, and CH2Cl2; ≥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 = νabs – νem stands for the Stokes shift (νabs and νem 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, μg and μe are the ground-state dipole in the ground-state 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 1O2 scavenger. The 1O2 production was monitored by following the absorbance of DPBF at 414 nm. To determine the singlet oxygen quantum yield (ΦΔ), a relative method was used according to eq . Ru(bpy)3[PF6]2 was used as the standard (Φstd = 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.

Results and Discussion

Synthesis and Characterization

The synthesis of 4OA-N, 6OA-N, and 8OA-N is shown in Scheme . A convenient one-pot procedure for the synthesis of oxacalix[m]arene[n]pyrimidine was developed in a previous work, and we synthesized three precursors 4OA, 6OA, and 8OA using this method.[28] Then, target compounds 4OA-N, 6OA-N, and 8OA-N were obtained through a Liebeskind–Srögl coupling reaction[29,30] between the precursors and 1-naphthalene boric acid under N2. 4OA-N, 6OA-N, and 8OA-N were characterized by 1H NMR spectroscopy and HRMS and further confirmed by single-crystal X-ray diffraction analysis.

Crystallographic Data of 8OA and 8OA-N

Diffraction-grade single crystals of 8OA (CCDC 2070449) and 8OA-N (CCDC 2070448) were obtained for the slow evaporation of a CH2Cl2/EtOAc solution. Crystal structures are shown in Figure a 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.

Figure 1

The crystal structures of (a) 8OA and (b) 8OA-N. The red dashed lines represent the distance between adjacent pyrimidine carbons, and the black dashed lines represent the bond length between naphthalene and the linked pyrimidine.

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.

1H NMR Analysis

In CDCl3 and DMSO-d6, 8OA and 8OA-N give rise to well-resolved 1H 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 and Table 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 (ε318 nm= 5.87 × 104 M–1 cm–1) is over 13× that of 8OA (ε288 nm= 0.45 × 104 M–1 cm–1). 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 a).

Figure 2

(a) UV–vis absorption spectra of 4OA, 6OA, and 8OA. (b) UV–vis absorption spectra of 4OA-N, 6OA-N, and 8OA-N in toluene (c = 1.0 × 10–5 M, 25 °C).

Table 1

Photophysical properties of 4OA, 6OA, 8OA, 4OA-N, 6OA-N, and 8OA-N

compd	λexa (nm) sole	εb	λemc (nm) sole	Φd (%) sole	τFe (ns)	Φ△g (%)
4OA	274/282	0.17	295	–f	–f	–f
6OA	273/282	1.41	294	–f	–f	–f
8OA	279/288	0.45	293	–f	–f	–f
4OA-N	277/317	4.72	384	27.50	1.0252 (96.78%)	45.10
					19.0925 (3.22%)
6OA-N	282/317	4.28	414	28.93	1.1733 (31.15%)	56.60
					7.9397 (68.49%)
80A-N	286/318	5.87	405	24.31	1.0732 (41.17%) 8.2853(58.82%)	65.70

Absorption wavelength.

Molar extinction coefficient, ε = 104 M–1 cm–1.

Emission wavelength.

Fluorescence quantum yield acquired using a calibrated integrating sphere.

In toluene.

Not applicable or not observed.

Singlet oxygen quantum yields. Ru(bpy)3[PF6]2 was used as a standard compound (Φ△ = 57% in DCM)

Figure 8

(a) Calculated HOMO and LUMO molecular ground-state orbitals of naphthalene, 4OA, 6OA, 8OA, 4OA-N, 6OA-N, and 8OA-N. (b) Ground-state frontier molecular orbital distribution of 4OA-N@C.

The luminescent emission spectra of 4OA, 6OA, 8OA, 4OA-N, 6OA-N, and 8OA-N in toluene are shown in Figure a–c. 4OA, 6OA, and 8OA have little fluorescence compared with 4OA-N, 6OA-N, and 8OA-N. The maximum emission wavelengths of 4OA-N, 6OA-N, and 8OA-N are 384, 414, and 405 nm in toluene and the quantum yields are 27.50%, 28.93%, and 24.31%, respectively. Moreover, all three compounds exhibit similar emission behaviors (Figures d and S15d). In previous reports,[33,34] donor–acceptor (D–A) systems showed dual-fluorescence emission behavior, and the charge-transfer (CT) emission peak usually had bandwidth emission and an obvious red-shift relative to the maximum emission peak. In these naphthalene-oxacalix[m]arene analogues, there was no obvious shift in the emission peak of 4OA-N with the increase in the solvent polarity, and 4OA-N in the hexane solution exhibited a fine structure at 300–500 nm, which is characteristic of typical locally excited (LE) luminescence (Figure a).[35] To further assign the emission of the naphthalene-oxacalix[m]arene, the relationship between the Stokes shift △ν and the polarity of the solvent △f was analyzed using the Lippert–Mataga equation.[36,37] The Lippert–Mataga diagrams are shown in Figures S16–S21, and the data are summarized in Tables S3–S8. The slope of the approximate line prepared from the emission band near 380 nm was almost zero. These data indicate that the luminescent bands around 380 nm should be assigned to the LE states of 4OA-N, 6OA-N, and 8OA-N.

Figure 3

Fluorescence spectra of (a) 4OA and 4OA-N; (b) 6OA and 6OA-N; (c) 8OA and 8OA-N; and (d) 4OA-N, 6OA-N, and 8OA-N normalized in toluene (c = 1.0 × 10–5 M, 25 °C).

Figure 4

Fluorescence spectra of 4OA-N in (a) different solvents and (b) a toluene solution under different atmospheres (c = 1.0 × 10–5 M, 25 °C).

The fluorescence emission of 4OA-N under the oxygen environment is lower than that under the nitrogen environment and is blue-shifted (Figures b, S22, and S23). The triplet levels (from theoretical calculations) of the three dyes (E = 1.90 eV, E = 1.91 eV, and E = 1.88 eV) are lower than the spectral levels in nitrogen and air environments (E = 1240/λem). We believe that this change in the luminescence intensity is not due to phosphorus emission.

Fullerene Binding Studies

At present, C60 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 C60 due to their unique pore cavity structure.[39] Here, the interaction of O-calixarenes with fullerene C60 was investigated by means of fluorescence titration experiments (Figures S24–S26). We found that the three congeners complex well with fullerene C60 in toluene solutions. The complexation constants range from 4.10 × 104 to 6.68 × 104 M–1, and the complexation ratio is 1:1 (Table ), indicating that the prepared O-calixarenes can effectively form stable host–guest complexes with C60. 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 C60 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 C60.

Table 2

Complexation Parameters of 4OA-N, 6OA-N, and 8OA-N with C60 in a Toluene Solution at 298 K

compd	complexation constant (M–1)	complexation ratio
4OA-N	5.65 × 104	1:1
6OA-N	6.68 × 104	1:1
8OA-N	4.10 × 104	1:1

The UV–vis absorption intensities of the three compounds increase with the concentration in accordance with the Lambert–Beer law (Figures a, S27, and S28). The UV–vis absorption spectra obviously changed after the addition of C60, and isosbestic point for 8OA-N@C was at 316 nm (Figure b), indicating that there was an obvious interaction between the host 8OA-N and the guest C60.[40] In the UV–vis and fluorescence emission spectra (Figures c and d, S29, and S30), we can see the fluorescence was distinctly quenched after the addition of C60. This phenomenon is same as that in previous reports and indicates that these naphthalene-oxacalix[m]arene[n]pyrimidines interact strongly with fullerene C60.[41] We tried to use C60 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 C60 by noncovalent bonds.

Figure 5

(a) UV–vis absorption spectra of different concentrations of 8OA-N in toluene at 25 °C. The concentrations of 8OA-N (from bottom to top) are 2.13 × 10–6, 4.27 × 10–6, 6.40 × 10–6, 8.53 × 10–6, 10.67 × 10–6, 12.80 × 10–6, 14.93 × 10–6, 17.07 × 10–6, and 19.20 × 10–6 M. (b) UV–vis absorption spectra of 8OA-N in toluene the presence of different concentrations of C60 at 25 °C. The concentrations of C60 (from bottom to top) are 2.13 × 10–6, 4.27 × 10–6, 6.40 × 10–6, 8.53 × 10–6, 10.67 × 10–6, 12.80 × 10–6, 14.93 × 10–6, 17.07 × 10–6, 19.20 ×10–6 M. (c) UV–vis absorption spectra of 8OA-N, 8OA-N@C, and C60. (d) Emission spectra 8OA-N and 8OA-N@C in toluene at 25 °C. The concentration of 8OA-N and C60 is 1.1 × 10–5 M.

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 and Table ). 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 (△GCS) of PET was calculated using the Weller equation (eqs –6).[42] The values of ΔGCS 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. Because △GCS is less than 0, the intramolecular charge transfer (ICT) process is also present in solutions with different polarities, not just a single LE state.

Figure 6

Cyclic voltammograms of 4OA-N in (a) deaerated DCM and (b) deaerated ACN. Ferrocene (Fc) was used as the internal reference (set as 0 V in the cyclic voltammograms), and 0.10 M Bu4NPF6 was used as the supporting electrolyte. The scan rate was 100 mV s–1, c = 1.0 × 10–3 M, and data were recorded at 20 °C.

Table 3

Electrochemical Redox Potentials,a Gibbs Free Energy Changes of the Charge Separation (ΔGCS), and Charge-Separation-State Energy Levels (ECS) of 4OA-N at 298 K

compd	EOX (V)	ERED (V)
4OA-N	1.39	–1.02
	0.42	–2.40

Recorded with Bu4NPF6 as the electrolyte in CH2Cl2 (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/AgNO3 couple was used as the reference electrode.

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. For 4OA-N (Figure a), the excited-state absorption (ESA) of the triplet state is the naphthalenen portion and the naphthalene-pyrimidine diplexes.[43] For 4OA-N@C (Figure S31), ESA was observed not only for the host molecule but also for the guest molecule C60 in the range of 700–800 nm.[44] Interestingly, the triplet-state lifetime of 4OA-N (τT = 35.7234 μs (0.57%), 7817.8111 μs (99.43%)) was longer than that of 4OA-N@C (τT = 9.03 μs). We speculated that the triplet–triplet energy transfer (TTET) process occurred from 4OA-N to C60. Meanwhile, we calculated the attributions of the ESA bands (Figure S38) using time-dependent DFT (TD-DFT). For 4OA-N, the ESA band at 385 nm is attributed to the T1 →T28 transition, whereas the ESA band in the 450–550 nm region is attributed to T1 → T (n = 8–19) transitions. Furthermore, the spin density of the triplet state is localized on the D–A moiety in 4OA-N (Figure c), while that for 4OA-N@C is localized on C60 (Figure d). This indicates that TTET occurred between the host and the guest.

Figure 7

(a) Nanosecond transient absorption spectra of 4OA-N (λex = 350 nm) and (b) the τT values. Spin density surfaces of (c) 4OA-N and (d) 4OA-N@C.

DFT Calculations

The ground state (S0) and the single excited state (S1) 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 . The frontier molecular orbitals of the HOMO in 8OA (Figure S34a) are localized on the pyrimidine part, and those of the LUMO are distributed on the whole molecule. This means that the molecular orbitals of pyrimidine and benzene in 8OA are separated from each other. Considering its symmetry and large pore size, both the HOMO and the LUMO in 8OA-N (Figure S34b) are distributed over two opposite naphthalene and pyrimidine units. Different from 8OA, the 8OA-N molecular orbitals overlap each other. The result is further illustrated in Figure a. The HOMO – LUMO gap (△E) is 4.81 eV for naphthalene and 5.29 eV for 8OA in their ground states, while the △E of 8OA-N is 3.98 eV. This indicates that the reduction of △E is due to the overlap of the 8OA-N molecular orbitals. The same conclusion was reached for 4OA, 4OA-N, 6OA, and 6OA-N (Figure S32 and S33).

Meanwhile, we calculated the molecular orbital distribution of the 4OA-N@C supramolecular system (Figure b). Because the LUMO energy level of C60 is lower than that of the host molecule, the HOMO of 4OA-N@C is distributed on 4OA-N and the LUMO is localized on fullerene C60. 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 C60 after the construction of the 4OA-N@C supramolecular system.

From the theoretical calculation of the UV–vis absorption spectrum (Figure S37a and Table ), we can see that the S0 → S1 transition in 8OA-N is symmetrically forbidden and the absorption is mainly derived from HOMO – LUMO + 2 (S0 → S2 electronic transition), which leads to the decrease of the 8OA-N quantum yield and explains why it is lower than the other molecules (Table ). 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 ). The fluorescence calculations of 4OA-N, 6OA-N, and 8OA-N reveal that the S0 → S1 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).

Table 4

The UV–vis Absorption and Singlet-State (Fluorescence) Energy (Calculated at TDDFT/B3LYP/6-31G(d)) of 4OA-N, 6OA-N, and 8OA-N

		TDDFT/B3LYP/6-31G(d)
compd	electronic transitiona	excitation energy		fb	compositionc	CId
4OA-N	UV–vis	S0 → S1	3.5106 eV (353 nm)	0.6747	H – 1 → L	0.5453
		S0 → S2	3.5408 eV (350 nm)	0.3069	H → L	0.5586
	fluorescence	S0 → S1	3.1307 eV (396 nm)	0.5233	H → L	0.7003
		S0 → S3	3.6922 eV (336 nm)	0.4350	H – 1 → L + 1	0.6970
6OA-N	UV–vis	S0 → S1	3.5734 eV (347 nm)	0.6463	H – 2 → L	0.5161
		S0 → S2	3.5810 eV (346 nm)	0.7485	H → L + 2	0.4200
	fluorescence	S0 → S1	3.1064 eV (399 nm)	0.5404	H → L	0.7000
		S0 → S4	3.6480 eV (340 nm)	0.6938	H – 1 → L + 1	0.4582
8OA-N	UV–vis	S0 → S1	3.6329 eV (341 nm)	0.0000	H– 1 → L + 2	0.4167
		S0 → S2	3.6418 eV (340 nm)	0.2600	H → L + 2	0.3737
	fluorescence	S0 → S1	3.1192 eV (397 nm)	0.3091	H → L	0.6997
		S0 → S2	3.3525 eV (370 nm)	0.0004	H – 3 → L	0.7021

Only selected excited states were considered. Numbers in parentheses are the excitation energy in wavelength.

Oscillator strength.

H stands for the HOMO and L stands for the LUMO. Only the main configurations are presented.

Coefficient of the wave function for each excitation. CI coefficients are given in absolute values.

At the ground and excited states of the electrostatic potential (ESP) of 4OA-N, 6OA-N, and 8OA-N (Figures a 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 C60. Moreover, the electron-rich methyl group can also form a C–H···π interaction with C60. This ESP result also further confirms that naphthalene-oxacalix[m]arenes are suitable “nests” for fullerene C60.

Figure 9

(a) Inner view and (b) outer view of the electrostatic potential mapped onto the electron density isosurface of 8OA-N (red represents deficient electrons and blue represents rich electrons).

Interestingly, taking 4OA-N as an example (Figures , 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 contribute to the different ISC efficiencies of the compounds, although it is difficult to confirm this issue explicitly; in other words, a few factors may simultaneously contribute to the SOCT-ISC efficiency. The other two dyads show higher singlet oxygen quantum yields than 4OA-N (Table ), which is attributed to them having more D–A units.

Figure 10

Dihedral angle of of 4OA-N in (a) the ground state and (b) the excited state. Calculations were performed at the B3LYP/6-31G(d) level with Gaussian 09W.

Conclusion

In summary, novel oxacalix[m]arene[n]pyrimidine derivatives decorated with luminescent-emission naphthyl groups were prepared through a cross-coupling reaction. The structure of naphthlene-oxacalix[m]arene[n]pyrimidine (8OA-N) was confirmed by single-crystal analysis. Photoluminescence (PL) spectra and electrochemical studies certified that the emission belongs to the LE state and exhibits the PET process. Meanwhile, nanosecond transient absorption spectra and the spin density of the compounds indicate that the triplet lifetime of 4OA-N (τT = 35.7234 (0.57%), 7817.8111 μs (99.43%)) is longer than that of the supramolecular system (4OA-N@C, τT = 9.03 μs), which may be due to the TTET between the host and the guest. Dihedral angle optimization suggests that the degree of torsion between the donor and the acceptor promotes the ISC. The result of theoretical calculations is in good agreement with the experimental results. Contrary to previous reports that oxacalix[m]arene[n]pyrimidine could not recognize fullerene C60 very well, these naphthalene-oxacalix[m]arenes can form stable 1:1 host–guest complexes, and complexation constants from 4.10 × 104 to 6.68 × 104 M–1 were determined through fluorescence titration. These oxacalix[m]arene[n]pyrimidines with suitable pore sizes and concave conformations are potent “hunters” of fullerene (C60). The fluorescence emission is obviously quenched by the addition of fullerene C60, which means there is CT between the host oxacalix[m]arene and the fullerene guest. The strength of complexation is related not only to the pore size but also the molecular configuration. The experimental results were also verified through theoretical calculations, showing us that oxacalix[m]arene[n]pyrimidine can recognize fullerene C60 well. Taking into account that the oxacalix[m]arene[n]pyrimidines are easy to modify chemically, versatile oxacalix[m]arene derivatives could be synthesized by reasonable design and applied in supramolecular chemistry.