HETEROCYCLES

Note | Special issue | Vol. 82, No. 1, 2010, pp. 833-838
Received, 1st April, 2010, Accepted, 7th May, 2010, Published online, 10th May, 2010.
DOI: 10.3987/COM-10-S(E)17

■ Synthesis and Complexation Behavior of 4,10-bis(1-Pyrenylmethyl)-1,7-dioxa-4,10-diazacyclododecane

Kanji Kubo,* Hanae Komatsuzaki, Tadamitsu Sakurai, Tetsutaro Igarashi, Taisuke Matsumoto, Hajime Takahashi, and Haruko Takechi

School of Dentistry, Health Sciences University of Hokkaido, Ishikari-Tobetsu, Hokkaido 061-0293, Japan

Abstract

Fluorescent photoinduced electron transfer (PET) fluoroionophore (2c) that consists of diaza-12-crown-4 and two 1-pyrenylmethyl pendants shows fluorescent enhancement with various metal cations. The sensor (2c) exhibited Zn2+ selectivity and in the presence of this cation the host fluorescence was increased by a factor of 38.

Photoresponsive supramolecular systems are of great significance particularly for their potential application to nanoscale devices for cation sensor and switch.1-6 There are extensive investigations toward the characterization of fluoroionophores including crown ether, calixarene, and cyclodextrin derivatives with naphthalene, umbelliferone, anthracene, or pyrene fluorophore. Recently, a number of fluoroionophores has been designed for metal ions. Most of them operate by a photoinduced electron transfer (PET) mechanism. In a classic example from the de Silva group,7,8 the binding component of the sensor is N-(9-anthrylmethyl)-18-azacrown-6. The uncomplexed fluoroionophore is weak fluorescent, as the photoexcited fluorophore is quenched by the electron transfer from amine group. Following metal incorporation (Na+ and K+), the metal-ligand interaction decreases the amine oxidation potential drastically and prevents the electron transfer. As a consequence, the intense and characteristic anthracene fluorescence is largely restored. We have also reported that multi-donor-spacer-multi-acceptor systems such as diazacrown ethers carrying two fluorophores which have higher fluorescent switch-on ability for guest cation complexation than the corresponding monoazacrown derivatives,9-12 while the cation binding such as alkali metal cation exerted a strong effect on the ratio of the monomer/excimer fluorescence intensity of 7,16-bis(1-pyrenylmethyl)-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (1).12,13 As an approach to the manipulation of PET fluoroionophores, we now report the complexation and fluorescence behavior of the diaza-12-crown-4 ether (2c) with two 1-pyrenylmethy pendants in the presence of guest salts.

The pyrene-functionalized diaza-12-crown-4 ether (2c) was prepared by the N-alkylation of 1,7-diaza-12-crown-4 with 1-chloromethylpyrene in triethylamine-toluene-tetrahydrofuran solution. The structure of 2c was ascertained by NMR spectroscopy and HR FAB MS.
Figure 1 illustrates the fluorescence spectral behavior of 2c (1.0 x 10–6 M) and 1-methylpyrene (1-MP, 2.0 x 10–6 M) in MeOH-CHCl3 (9:1 v/v). Fluoroionophore 2c (when excited at 342 nm) gave a broad fluorescence band in a range from 450 to 580 nm in addition to monomer fluorescence (376,

396, and 416 nm). The formation of an intramolecular exciplex should be responsible for the appearance of the former fluorescence band. The latter fluorescence-band intensity of 2c was reduced to approximately one-52nd that of standard substance (1-MP). This indicates that the quenching of the excited pyrene chromophore by the azacrown unit proceeds in a mechanism similar to that for the classical pyrene-dialkylamine system.14,15 The quenching efficiency (I2c/I1-MP: 1.9 x 10-2) of 2c is lower than that (I1/I1-MP: 3.7 x 10-3)12,13 of 1. This means that the smaller macrocycle ring inhibits the PET occurring from the nitrogen atoms in the crown to excited fluorescent moieties.
In Figure 2 is illustrated the relative fluorescence spectral behavior of guest cation complexes 2c (1.0 x 10-6 M) in MeOH-CHCl3 (9:1 v/v) at room temperature. A dramatic change in the fluorescence intensity of 2c (I2c) was observed upon the addition of various amounts of guest cations (Li+, Na+, K+, Rb+, Cs+, Ca2+, Ba2+, Zn2+, Mg2+ and NH4+). When the guest cations were added (104 molar equivalents), the relative fluorescence intensity (Icomplex/I2c), being used as a measure of the molecular recognition sensing, changed from 1 to 38 depending on the nature of guest cations as shown in Figure 2 and Table 1.

Interestingly, the fluorescence intensity ratio (Icomplex/I2c) was different among bound guest cations and decreased in the following order: Zn2+ (38) > NH4+ (4.4) > Ba2+ (3.4) > Ca2+, Mg2+ (3.2) > Na+ (2.5) > K+ (1.6) > Cs+ (1.4) > Li+ (1.3) > Rb+ (1.0). The order of Icomplex/I2c is similar to that of Icomplex/I2a10,12 and Icomplex/I2b.11,12 In Table 1 is illustrated the fluorescence intensity ratio (Icomplex/I1-MP) of guest cation complexes for 1-MP, as a measure of the guest cation-induced

fluorescence recovery. The fluorescence intensity ratio (I2c-zinc cation complex/I1-MP) of 2c was 0.74. Zinc cation binding can then cause high fluorescence recovery. This recovery is due to coordination from the nitrogen atoms of the diazacrown to the Zn2+. The strength of this binding interaction modulates the PET from the amine to pyrene. The order of Zn2+-induced fluorescence recovery (I2-zinc cation complex/Istandard substance) is 2b > 2c > 2a. This means that the 9-anthrylmethyl derivative (2b) is more excellent PET fluoroionophore for Zn2+.
Guest concentration dependence of the fluorescence intensity allowed us to determine the association constants (K) by the non-linear curve-fitting method13,16 (Table 2). The K values of diaza-12-crown-4 derivative (2c) for various guest cations were larger than those of 1. The order of K values for Zn2+ is 2a > 2c > 2b. This means the two larger aromatic rings may block the incorporation of guest cations in host. The diazacrown derivative (2c) showed the following cation selectivity: Zn2+ > Ca2+ > Mg2+ > NH4+ > Na+ > Ba2+. Comparison of the selectivity order for 1 and 2c confirms that the size and electronic property of ionophore attached with aromatic pendants. The pendants may control the selectivity of the host toward guest cations in a delicate manner.
In conclusion, the cation binding to the pyrene-attached diaza-12-crown-4 (2c) did not show the excimer fluorescence enhancement. However the diaza-12-crown-4 (2c) exhibited high Zn2+ selectivity and fluorescence selectivity. The diaza-12-crown-4 having two 1-pyrenylmethyl pendants may be utilized as a PET fluorescent sensor for Zn2+.

EXPERIMENTAL
Melting points were obtained with a Mitamura Riken Kogyo 7-15 Micro Melting Point Apparatus and are uncorrected. NMR spectra were measured on a JEOL JNM-ECA-500 Model spectrometer in CDCl3; the chemical shifts are expressed by an δ unit using tetramethylsilane as an internal standard. The mass spectra were measured with a JEOL JMS-700 spectrometer. IR spectra were recorded on a Shimadzu Prestige-21 infrared spectrophotometer. UV spectra were measured using a Shimadzu Model UV-3150 spectrophotometer. Fluorescence spectra were measured with a JASCO Model FP-6500 spectrofluorimeter. The stationary phase for the column chromatography was supplied by Merck and the eluent was a mixture of ethyl acetate, chloroform, and hexane. The association constants (K) were determined by the same procedure in the previous study.13,16
Synthesis of 4,10-bis(1-pyrenylmethyl)-1,7-dioxa-4,10-diazacyclododecane (2c)
A THF-toluene solution (10 mL, 1:1 v/v) of 1,7-diaza-12-crown-4 (34.8 mg, 0.20 mmol), triethylamine (0.5 mL, 3.6 mmol), and 1-chloromethylpyrene (250 mg, 1.00 mmol) was heated at 80 ˚C for 8 h. The mixture was then diluted with 1.0 M NH3 solution (30 mL), extracted with CHCl3. The solvent was evaporated and the residue was purified by column chromatography over silica gel using EtOAc and CHCl3 (1:1 v/v) as the eluent. The physical and spectral data of 2c (112 mg, 93%) are given below.
2c: pale yellow crystals, mp 152.0-153.0 ˚C, 1H NMR (CDCl3) δ = 2.83 (8H, t, J = 4.6 Hz), 3.45 (8H, t, J = 4.6 Hz), 4.25 (4H, s), 7.93- 8.01 (8H, m), 8.03 (2H, d, J = 7.4 Hz), 8.04 (2H, d, J = 9.2 Hz), 8.10 (2H, d, J = 8.6 Hz), 8.12 (2H, d, J = 8.6 Hz), and 8.56 (2H, d, J = 9.2 Hz). 13C NMR (CDCl3) δ = 55.5 (4C), 59.2 (2C), 69.4 (4C), 124.4 (2C), 124.5 (2C), 124.7 (2C), 124.8 (2C), 124.9 (4C), 125.7 (2C), 126.9 (2C), 127.0 (2C), 127.4 (2C), 128.1 (2C), 129.8 (2C), 130.6 (2C), 130.9 (2C), 131.3 (2C), and 133.2 (2C). IR (KBr) ν 1036, 1045, 1057, 1123, 1445, 1458, 1685, and 1601 cm-1. UV (CHCl3) λ 242 (ε = 126000), 265 (52000), 276 (82400), 313 (25600), 327 (53400), and 343 (81200) nm. HR FAB MS. Found: 603.3009. Calcd for C42H39N2O2: 603.3012.

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