A fluorescent calix[4]arene with naphthalene units at the upper rim exhibits long fluorescence emission lifetime without fluorescence quenching

We synthesised a new compound with four naphthyl groups in the upper rims of calix[4]arene (1). Compared to the monomer unit, compound 1 has redshifted absorption and fluorescence, together with high fluorescence quantum yield and long fluorescence lifetime, which is extremely rare because long fluorescence lifetime emission tends to reduce the quantum yield. Single-crystal X-ray analysis and quantum calculations in the S1 state revealed π–π through-space interactions between naphthalene rings.

In 1954, Förster and Kasper rst reported dimerised aromatic compounds in the excited state, 1 which were termed 'excimers' by Stevens and Hutton. 2 Excimers have attracted much attention in various elds such as organic solar cells, [3][4][5] organic electronics, 6 chemical sensors, 7 and biotechnology [8][9][10][11][12] because of their unique photophysical properties. In particular, with the development of time-resolved uorescence imaging and stimulated emission depletion microscopy (STED) microscopes, there is a growing demand for uorescent dyes with both high brightness and long uorescence lifetime. [9][10][11][12] Excimers with long emission lifetime are promising candidates for nextgeneration imaging probes. 13 Many progresses have been made towards understanding the relationship between the molecular structure of organic dyes and their uorescence intensity. However, there is little knowledge on the relationship between the uorescence lifetime and molecular structure. Also, only a few uorescent probes have achieved both high intensity and long lifetime. [13][14][15] In recent years, it was reported that bright and long-lived uorescence can be obtained from the excimer state 13,[16][17][18][19] that was conventionally thought to be the cause of quenching due to the low-energy excimer trap states with forbidden radiative transition and activated non-radiative process. [20][21][22][23][24] However, most of these reports were in the solid state, where the molecular movement is suppressed and the luminescence occurs in a single crystal. [16][17][18][19] In contrast, there are few reports of dyes with long uorescent lifetime in solution systems with free molecular movement. The reported substances also suffer from low synthesis yields in the ring-forming reaction and difficulty in introducing functional groups (such as hydrophilic substituents). 13 In general, the yield of the ring formation step is extremely low in the synthesis of cyclophanes. 13,[25][26][27] Therefore, there remains the need for new molecules that can be easily synthesised and chemically modied.
In this study, we adopted the calixarene skeleton as the macrocyclic structure. Calixarenes have been used in supramolecular chemistry, 28 analytical chemistry, 29 biochemistry, 30 material science, 31 and catalysts 32 because of their easy molecular modication. Nevertheless, almost all studies introducing uorescence sites do so at the lower rim of calixarene, while few reports considered incorporating uorophores at the upper rim. Further, no researchers have investigated the uorescence lifetime, and there was also no reported computational investigation of the excited state. [33][34][35] Specically, we synthesised a uorescent molecule in which naphthyl group was introduced into the upper rim of calix [4] arene. The new molecule showed both a long uorescence wavelength and a high quantum yield. Aer measuring the conformation of tetranaphthylcalix [4]arene in a single crystal, structural optimisation of the ground state and excited state (S 1 ) was performed by time-dependent density functional theory (TD-DFT) at the DFT-D3-CAM-B3LYP/6-31G(d) level. The calculation takes into account effects such as dispersion force. [36][37][38][39] Molecular orbital calculation conrmed that the p orbitals of the naphthalene rings of tetranaphthylcalixarene have a binding interactions in LUMO of the S 1 state.
The synthesis of tetranaphthylcalix [4]arene (1) was carried out by deprotecting the tert-Bu groups of tetra-tert-butyl(tetrahydroxy)calix [4]arene, followed by introduction of substituents by Williamson ether synthesis of phenolic hydroxyl groups, and iodation by silver tetrauoroacetate and iodine. The iodination was followed by Suzuki-Miyaura cross-coupling reaction with naphthylboronic acid pinacol ether (ESI †).
We also synthesised the phenylnaphthalene derivative 2 (Fig. 1), which is the unit molecule of 1. To compare its photophysical properties with that of 1, rst we measured the absorption spectra. At 1 Â 10 À4 mol L À1 in chloroform solution, 1 and 2 have their maximum absorption wavelengths at 297 and 294 nm, and the molar absorption coefficients were 3.3 Â 10 5 and 1.0 Â 10 5 mol À1 L cm À1 , respectively (Fig. 2). Their uorescence spectra were measured in chloroform solution at 1 Â 10 À4 mol L À1 (Fig. 3). 1 has a broader uorescence peak with maximum intensity at 389 nm, which is redshied by 25 nm from that of 2. Furthermore, the uorescence spectrum of the powder aer grinding in a mortar was also measured and it was found that the uorescence wavelength was increased by 22 nm (Fig. S14, ESI †). We also measured the absorption and uorescence spectra at a lower concentration of 1 Â 10 À5 mol L À1 (Fig. S15, ESI †), and there was almost no change in the wavelength or shape of the peak. Therefore, the spectral changes in tetranaphthylcalix [4]arene from the unit model molecule are due to intramolecular rather than intermolecular interactions. Furthermore, the temperature dependence of uorescence was investigated by measuring uorescence by changing the temperature from 20 C to 80 C (Fig. S16 †). As the temperature rose from 20 C to 80 C, a blue shi of the uorescence wavelength of about 10 nm was observed, and the half-value width of the peak narrowed. These results indicate that as the temperature rises, the intramolecular interaction in the excited state weakens and the light emission becomes closer to that of 2.
Single-crystal X-ray crystal structure analysis of 1 revealed that two of the four naphthyl groups facing each other had an intramolecular stacking structure, with a distance of 3.54 A between them ( Fig. 4 and Table S3, ESI †).
The macrocyclic structure improved the uorescence quantum yield from 0.38 in 2 to 0.46 in 1 ( Table 1). The unit model molecule showed single-exponential uorescence decay with a uorescence lifetime of 2.0 ns. In contrast, 1 displayed non-single exponential uorescence decay. When the data were tted using a two-component exponential equation (Fig. S17 15 To investigate this characteristic property, the uorescence emission rate constant k f and the nonradiative decay rate constant k nr were determined by the following equation: where F f is the uorescence quantum yield and s f is the uorescence lifetime. The values are summarized in Table 1. k nr decreased by more than one order of magnitude from 0.31 ns À1 to 0.021 ns À1 for 1. The small nonradiative decay rate of 1 is probably due to the suppression of molecular motion by the rigid macrocyclic structure of 1. On the other hand, the origin of   the small k f has been not identied. Furthermore, the ground and excited electronic states of 1 were obtained using density functional theory (DFT) and time-dependent (TD)-DFT calculations based on the structure from single-crystal X-ray diffraction. All calculations were performed at the DFT-D3-CAM-B3LYP/6-31 (d) level of theory (see Computational details in the ESI †). First, the structure of the ground state was optimised. The distance of the naphthalene rings facing each other, which is the focus of this study, was slightly smaller in the crystalline state (3.59 A) compared to 3.49 A of the ground state structure. Similar calculation was carried out for the ground state structure of 2. Then, the absorption spectra of both compounds were predicted by TD-DFT calculations. The absorption wavelength of 1 is longer, which qualitatively agrees with the experimental results (Fig. 2, S18, Tables S1 and S2, ESI †). Thus, it was possible to explain the change in absorption spectrum by calculating the molecular orbitals. In particular, for the rst two excited states S 1 and S 2 , the main constituent orbitals are HOMO to LUMO+2 and HOMO to LUMO+3 (Table S1, ESI †). These results indicate that the redshi of absorption in 1 involves the orbital of the intramolecular naphthalene units facing the molecule (Fig. S19, ESI †). Furthermore, structural optimisation of the S 1 state was performed for compound 1 using the same level of theory and basis functions. The results further reduced the interplanar distance of naphthalene rings in the S 1 state to 3.18 A (Fig. 5a). In the frontier orbitals calculated for the S 1 geometry, there were clear binding interactions between the naphthalene rings according to the LUMO ( Fig. 5b and S20, ESI †) and a small oscillator strength value (0.0019) of the HOMO ) LUMO transition in agreement with the lower value of k f compared with that of 2. An oscillator strength value of HOMO ) LUMO transition of 2 with the optimized structure in S 1 state is 0.700. Therefore, the reason why 1 has both a high uorescence quantum yield and a long uorescence lifetime is that a decrease in k nr due to suppression of molecular motion by a rigid macrocyclic structure contributes more signicantly to the uorescence enhancement than the decrease in k f by intramolecular electronic interaction in the excited state. Subsequently, our group has been developing new imaging dyes using this molecular skeleton having uorescence sites that have a long conjugation length and therefore can be excited by visible light.
In summary, we synthesised tetranaphthylcalix [4]arene (1). Such a structure with naphthalene substituents on the upper rim has never been reported before. A unit model molecule of 1 was also synthesised. For the optical properties, it was found that 1 has a higher uorescence quantum yield and a uorescence lifetime at least 11 times longer than that of the unit model molecule. These differences were interpreted based on single-crystal X-ray structure analysis and molecular orbital calculations, which revealed binding interactions between the naphthalene rings in the S 1 state. These results will provide new insights into the molecular design of dyes with high uorescence quantum yields and long uorescence lifetimes.

Conflicts of interest
There are no conicts to declare. Fig. 4 (a) Side and (b) top views of the structure of 1 determined by single-crystal X-ray crystallographic analysis. The structures were drawn by ORTEP program with the thermal ellipsoids set at 50% probability.