Synthesis, Dual Fluorescence, and Fluoroionophoric Behavior of Dipyridylaminomethylstilbenes

The synthesis, dual fluorescence, and fluoroionophoric behavior of two donor−σ 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).