The spectroscopic properties of 1-phenyl-3-benzothiazol-2-yl-5-(4-R-phenyl)-Delta(2)-pyrazolines are strongly dependent on both the electronic nature of the substituent R and solvent polarity. As revealed by spectroscopic studies as a function of solvent polarity as well as temperature, for electron-rich amino donor substituents in polar solvents, deactivation of the strongly emissive charge transfer (CT) state of the basic 1-phenyl-3-benzothiazol-2-yl-Delta(2)-pyrazoline chromophore has to compete with a fast intramolecular electron transfer (ET) quenching reaction. In the case of the dimethylamino derivative (R DMA), the rate constant of ET in acetonitrile was determined to k(et) = 3 x 10(10) s(-1). This ET process can be utilized for metal ion sensing by introducing nitrogen containing aza crown ether receptor units to the if-position of the 5-phenyl group. The spectroscopically determined ET rates of the 5-(N-alkyl)amino substituents, a DMA, a tetrathia- (AT(4)15C5), and a tetraoxa-monoaza-15-crown-5 (A15C5) group, correlate with electrochemical data and increase in the order AT(4)15C5 < A15C5 < DMA. The metal ion sensing abilities of the two crowned derivatives are presented, and the different signaling mechanisms include binding to the crown ether in the 4-R position, chelate formation in the 3-benzothiazol-2-yl-Delta(2)-pyrazoline moiety, and electrophotochemical detection. Furthermore, the rigid "pseudo spiro" geometry of the molecules, which holds the three substituents of the central Delta(2)-pyrazoline ring in a fixed prearrangement, was confirmed by X-ray structure analysis.