Substituted 1,5-Diphenyl-3-benzothiazol-2-yl-Δ²-pyrazolines:  Synthesis, X-ray Structure, Photophysics, and Cation Complexation Properties

Abstract

The spectroscopic properties of 1-phenyl-3-benzothiazol-2-yl-5-(4-R-phenyl)-∆ 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-3benzothiazol-2-yl-∆ 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 × 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 4-position of the 5-phenyl group. The spectroscopically determined ET rates of the 5-(N-alkyl)anilino 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-∆ 2 -pyrazoline moiety, and electrophotochemical detection. Furthermore, the rigid "pseudo spiro" geometry of the molecules, which holds the three substituents of the central ∆ 2 -pyrazoline ring in a fixed prearrangement, was confirmed by X-ray structure analysis.

Towards simple and efficient molecular reporters: combining electron transfer and charge transfer in functional dyes of donor–acceptor–spacer–donor constitution

“…Solvent-dependent spectroscopic behavior in the absence of chemical stimuli In accordance with results obtained on related 3-benzothiazol-2-yl-substituted triaryl-∆ 2 -pyrazolines, 10 1, 2 and the reference compound 3 which lacks a potential binding site in its 1,3-chromophoric part show rather unstructured, broad and largely Stokes-shifted absorption and emission bands in apolar as well as polar solvents. As an example, the spectra of 2 in acetonitrile and diethylether are given in Figure 1.…”

“…Such a correlation of 3-acceptor strength and emission behavior as manifested in red-shifted fluorescence band position and reduced quantum yield in highly polar solvents has also been found in a comparative study of 3-benzothiazol-2-yl-and 3-phenylsubstituted triaryl-∆ 2 -pyrazolines and was ascribed to a competition of radiative deactivation of the S 1 state (via an allowed S 0 ← 1 CT transition) and electron transfer (ET) quenching from the electronically decoupled, remote and electron-rich 5-(4-dimethylaminophenyl) substituent. 10 Due to the sp 3 -hybridization of C (5) and the rigidity of the central five-membered ring the 5-substituent is held in a pseudo-spiro orientation, this donor-acceptor-spacer-donor constitution preventing pronounced electronic interaction of both molecular subunits in the ground state. 10 Only in the excited state, a long-range process such as ET can take place.…”

“…10 Due to the sp 3 -hybridization of C (5) and the rigidity of the central five-membered ring the 5-substituent is held in a pseudo-spiro orientation, this donor-acceptor-spacer-donor constitution preventing pronounced electronic interaction of both molecular subunits in the ground state. 10 Only in the excited state, a long-range process such as ET can take place. Analysis of the fluorescence lifetime data revealed that the reduced fluorescence quantum yields are accompanied by shorter fluorescence decay times thus indicating increased rate constants of nonradiative deactivation k nr and virtually unchanged rate constants of fluorescence k f .…”

“…26 The fluorescence decays were recorded with a time division of 5.2 ps channel -1 and details on detection, calibration and fitting procedures are given in ref. 10.…”

“…A simple constitution that allows to construct individually configurable bifunctional molecular reporters is the donor-acceptor-spacer-donor arrangement of 1,3,5-triaryl-∆ 2 -pyrazolines. 10 Despite being structurally the center of these dyes, the only partly unsaturated five-membered ring has a central position as it (i) is part of the basic chromophore, (ii) contains a nitrogen donor atom which is a potential coordination site for a metal ion, 11 and (iii) holds the 5-substituent in a non-conjugated, fixed and pseudo-spiro-oriented configuration, i.e. in a rather closely neighbored yet electronically decoupled position.…”