Substituted 1,5-diphenyl-3-benzothiazol-2-yl-Delta(2)-pyrazolines: Synthesis, X-ray structure, photophysics, and cation complexation properties

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 stu dies as a function of solvent polarity as well as temperature, for electron -rich amino donor substituents in polar solvents, deactivation of the stron gly emissive charge transfer (CT) state of the basic 1-phenyl-3-benzothiazo l-2-yl-Delta(2)-pyrazoline chromophore has to compete with a fast intramole cular electron transfer (ET) quenching reaction. In the case of the dimethy lamino derivative (R DMA), the rate constant of ET in acetonitrile was dete rmined to k(et) = 3 x 10(10) s(-1). This ET process can be utilized for met al ion sensing by introducing nitrogen containing aza crown ether receptor units to the if-position of the 5-phenyl group. The spectroscopically deter mined ET rates of the 5-(N-alkyl)amino substituents, a DMA, a tetrathia- (A T(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 i n the 4-R position, chelate formation in the 3-benzothiazol-2-yl-Delta(2)-p yrazoline moiety, and electrophotochemical detection. Furthermore, the rigi d "pseudo spiro" geometry of the molecules, which holds the three substitue nts of the central Delta(2)-pyrazoline ring in a fixed prearrangement, was confirmed by X-ray structure analysis.