Intramolecular exciplex formation and complexing behavior of 1-(2-naphthalenecarboxy)-n-(p-substituted benzenecarboxy)oxaalkanes as fluorescent chemosensors for calcium and barium ions

The "Off-On" Fluorescence Detection of Alkaline Earth Metal Ions by Using 1-Naphthylacetanilide Derivatives: New Cation Sensors Based on Oligo-ethylene Oxides Connecting 1-Naphthylacetanilide at Its Terminals

New fluorescent ionophores containing the 1-naphthylacetanilide moiety based on oligo-ethylene oxide (1(4) and 1(5)) have been synthesized, and their photochemical behaviors have been studied. In the absence of a metal ion, the 1-naphthylacetanilide moiety showed a weak fluorescence emission (fluorescence "off state"). However, complexation with Ca(2+) induced a large enhancement effect on the fluorescence intensity of 1(4) and 1(5) from the naphthalene ring (fluorescence "on" state). This behavior can be explained in terms of a similar twisted intramolecular charge-transfer relaxation mechanism. Such "off-on" fluorescence responses of 1(4) and 1(5) provide the digital detection of alkaline earth metal ions.

Unprecedented chemiluminescence behaviour during peroxyoxalate chemiluminescence of oxalates with fluorescent or electron-donating aryloxy groups

A series of diaryl and bis(4-styrylphenyl) oxalates with electron-donating substituents or fluorescent moieties were subjected to the peroxyoxalate chemiluminescence (PO-CL) reaction, some of which were found to behave in a unprecedented manner. The reaction of bis(p-methyoxyphenyl) oxalate, as a representative example, emits light due not only to the emission from the externally added excited fluorophore, but also from the presumable excimer of p-methoxyphenol. Also, during the reaction of the bis(4-styrylphenyl) oxalates, the emission based on the fluorescence as well as the excimer of the eliminating group were observed. These experimental results suggest that such emitting species would be formed by an intra- and intermolecular electronic interaction with a high-energy intermediate, such as a dioxetanone.

Synthesis, Dual Fluorescence, and Fluoroionophoric Behavior of Dipyridylaminomethylstilbenes

The synthesis, dual fluorescence, and fluoroionophoric behavior of two donor-sigma spacer-acceptor (D-s-A) compounds, trans-4-(N,N-bis(2-pyridyl)amino)methylstilbene (1H) and trans-4-(N,N-bis(2-pyridyl)amino)methyl-4'-cyanostilbene (1CN), are reported and compared to that of trans-4-(N,N-bis(2-pyridyl)amino)methyl-4'-(N,N-dimethylamino)stilbene (1DPA). To gain insights into the dual fluorescence properties for 1H and 1CN in polar but not in nonpolar solvents, model compounds resulting from a replacement of the stilbene group by alkyl (2R) or xylyl (2X) groups or from a replacement of the dipyridylamino (dpa) group by dianisoleamino (3AA), diethylamino (3EE), methylanilino (3MP), or diphenylamino (3PP) groups also have been investigated. In addition to 1H and 1CN, all four compounds of 3 display dual fluorescence. The locally excited (LE) fluorescence mainly results from the stilbene group and the ICT fluorescence from the through-bond interactions between the amino donor and the stilbene acceptors. In the presence of transition metal ions such as Zn(II), Ni(II), Cu(II), and Cd(II), the ICT processes are switched from dpa (D) --> stilbene (A) in 1H and 1CN to stilbene (D) --> dpa/metal ion (A) in their complexes. Whereas the ICT states for the complexes are generally nonfluorescent, an exception was found for the case of 1H/Zn(II). As a result, substituent-dependent fluoroionophoric behavior has been demonstrated by 1H, 1CN, and 1DPA in response to Zn(II).

Selective Calcium Ion Sensing with a Bichromophoric Squaraine Foldamer

Several squaraine tethered bichromophoric podand systems 1a-d and a monochromophoric analogue 2 were prepared and characterized. Among these, the bichromophore, 1b, containing five oxygen atoms in the flexible podand moiety was found to specifically bind Ca(2+) in the presence of other metal ions such as K(+), Na(+), and Mg(2+). The selective binding of Ca(2+) is clear from the absorption and emission spectral changes as well as by the visual color change of 1b from light-blue to an intense purple-blue. Benesi-Hildebrand and Job plots confirmed a 1:1 binding between 1b and Ca(2+). Signaling of the binding event is achieved by the cation-induced folding of the bichromophore and the resultant exciton coupling between the squaraine chromophores. The monochromophoric squaraine dye 2 failed to give optical signals upon Ca(2+) binding, due to the absence of exciton interaction in the bound complex. Titration of the folded complex 9 with EDTA released the metal ion from the complex, thereby regaining the original absorption and emission properties of the bichromophore. The squaraine foldamer 1b reported here is the first example of a selective chromogenic Ca(2+) sensor, which works on the principle of exciton interaction in the folded Ca(2+) complex of a bichromophore, the optical properties of which are similar to those of the "H"-type aggregates of analogous squaraine dyes.