Selective Photoinduced Electron Transfer-Based Cyanoanthryl Fluorescent Chemosensors for Paramagnetic Metal Cations
Bulletin of the Korean Chemical Society (Print)
The fluorescent chemosensor of fluorophore-space-receptor type requires the suppression of the interaction between the fluorophore and the quenching metal cations so as to observe fluorescence changes on the metal binding. The crown ethers have been derivatized with anthryl groups and different spacer units in the "fluorophore-space-receptor". These systems have been designed to explore further understanding of PET process. The spacer units have been varied in order to observe the effect of
... e the effect of spacer on the flexibility of the fluorophore unit with respect to the receptor moiety, the orientation of the fluorophores with respect to each other depending on the structure of spacer, and the interaction between the fluorophore and the receptor unit. Since Czarnik, 1 and de Silva 2 successfully developed sensor systems to show chelation enhanced fluorescence (CHEF) and CHEQ in the presence of metal guests using the anthryl polyaza receptors which complex the metal guests in its cavity leading to the suppression of the quenching interaction, the development of PET sensors has been focused only on the electron-delocalized anthracence fluorophore without consideration of electron-localized system by electron donating or withdrawing functional group. Here we report the preparation of PET-based cyanoanthryl fluorescent chemosensors (3, 4) for the paramagnetic metal guest, which was designed to explore the influence of the structural factor by electron withdrawing group, rigidity on the efficiency of the signaling mechanism, and the photophysical behavior (Scheme 1). The cyanoanthryl fluorophores were synthesized via the synthesis of functionalized anthracene derivatives in introducing the azacrown ether substituents at 9-anthrylic position in order to generate the photoinduced electron transfer signal via the methylene (-CH2) bridge from the electron donor to electron acceptor. The absorption and fluorescence spectra were measured for the synthesized cyanoanthryl fluorophores in methanol at 5 × 10 −7 M, excited by 254 nm. All sample solutions show a maximum absorption at 254 nm, and a maximum emission at 426 nm. The maximum emission peaks of the π-π* transition show a red shift compare to those in anthryl derivatized fluorophores as shown in Figure 1 . 3 This shift of emission peak may be attributed to some conjugative effect by the electron withdrawing property of cyano group. The fluorescence quenching efficiency of cyanoanthryl fluorophores is dependent on the ring size and the number of donor atom in crown ethers as a receptor. The results indicate that signaling mechanism has to rely on long-range processes, which is quenched by the nonbonding electrons of benzylic nitrogen and oxygen atoms participating in nonradiative quenching process. Figure 2 shows the pHdependency of the fluorescence intensity of cyanoanthryl fluorophores at the 262 nm of excitation wavelength, showing a maximum at pH 3-5 in Tris-buffer solution. This dependency can be explained by the intramolecular amine quenching mechanism that has been previously described. 4 Protonation of amine group in anthrylic position results in the elimination of photoinduced electron transfer. The fact that two cyanoanthryl fluorophores show same maximum intensity at the same pH would be a major reason, indicationg the protonated species on the anthrylic nitrogen accounts for a majority of the observed intramolecular quenching. Also, this means that an electron transfer process from excited anthracene moiety to the partially positive charged amine take place, thus competing with radiative deactivation and partially quenching fluorescence. Fluorescence quenching is attributed to the interaction between the lone pair electron and π-orbital of the photo-induced anthracene fragment from the electron-rich anthrylic amine moiety in spacer unit to anthryl fluorophore. In the lower pH, the strong fluorescence intensity was observed reducing an electron withdrawing property of cyano group because of the formation of -CNH by protonation. But, the fluorescence intensity shows drastically quenching in higher pH owing to the localization of electron density by cyano group and lone-pair electrons in receptor. Among the 18 metal nitrates, which the fluorescence intensity of the cyanoanthryl fluorphores was measured for 0.1, 1, 10, 1000 µM of metal concentration at pH 9.2, only four paramagnetic metal ions of Mn 2+ (d 5 ), Fe 3+ (d 5 ), Co 2+ (d 7 ), and Cu 2+ (d 9 ) quenched the fluorescence drastically at higher concentration. Figure 3 shows the fluorescence quenching curves depending on the metal concentration and a linear function of Stern-Volmer plots indicating that fluorescence quenching is dynamic in nature in the linear part. The degree of quenching shows an order of Co 2+ > Cu 2+ > Mn 2+ > Fe 3+ .