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The complexation behaviour of 2,2-bipyridyl-3,3-diol and 2,2-bipyridyl-3-ol, BP(OH)2 and BPOH, with various heavy and transition metal
ions has been investigated in aqueous, alcoholic, and acetonitrile solutions. Whereas the complexes with paramagnetic ions and HgII
are non-fluorescent, ZnII and CdII form highly fluorescent complexes, their coordination geometries depending on the
solvent proticity and hydrogen-bond donating ability. Through a comparative study with the corresponding chelates of 2,2-bipyridyl (bipy),
N,O-coordination in a six-membered ring chelate has been found to be the dominant binding mode in both the ZnII and CdII
complexes in the protic solvents water and ethanol. Only for ZnII and BPOH is exclusive N,N-chelation found in acetonitrile. NMR
measurements on BP(OH)2, BPOH, and bipy in the presence of ZnII and CdII in acetonitrile confirmed these
findings.
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-3-benzothiazol-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 ket = 3 × 1010 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- (AT415C5), and a tetraoxa-monoaza-15-crown-5 (A15C5) group, correlate with electrochemical data and increase in the order AT415C5 < 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.
The fluorescence quenching of 1,5-bis[4-(diethylamino)phenyl]penta-1,4-dien-3-one (1), 1-[4-(diethylamino)phenyl]-5-phenylpenta-1,4-dien-3-one (2) and chalcone 3 (1-[4-(diethylamino)phenyl]but-1-en-3-one) was studied in acetonitrile using ferrocene and three commercially available derivatives, 1,1?-diacetylferrocene, acetylferrocene and 1,1?-dimethylferrocene. The spectroscopic and electrochemical characteristics of all the compounds were investigated. The feasibility of the quenching process was calculated, showing in particular that electron transfer was thermodynamically possible in every case. The steady-state fluorescence of the three organic dyes was then measured in the presence and in the absence of the ferrocenyl derivatives and the data were corrected for inner-filter effects. 1,1?-Diacetylferrocene and acetylferrocene proved to be the most efficient quenchers, and among the three dyes investigated, chalcone 3 was the most sensitive to the presence of the ferrocenyl derivatives. However, the fluorescence decay times of the ketocyanines were not affected by the presence of the ferrocene derivatives, indicating that quenching was not diffusion-controlled but rather the consequence of the formation of a ground state complex.
The acceptor strength of the boron-dipyrromethene chromophore in directly linked donoracceptor compounds can be tuned by substituents in such a way that a fast excited state charge transfer takes place even for the comparatively weak benzo crown electron donor. This leads to strong fluorescence quenching. Upon binding of cations (Na+ and K+) to the benzo crown receptor, the donor properties of the latter are further reduced, partly suppressing charge transfer. Large fluorescence enhancement factors and cation-selective fluorescence decay times result which are the basis for improved analytical application of these dyes as highly sensitive fluorescent probes.
Real-time and real-space analysis of heavy and transition metal ions employing fluorescent sensor molecules has received much attention over the past few years. Since many of these cations possess intrinsic properties that usually quench the fluorescence of organic dye molecules, a lot of research has lately been devoted to designing fluorescent probes that show complexation-induced fluorescence enhancement. Such an analytical reaction would be highly desirable in terms of increased sensitivity and selectivity. However, in this particular field of sensor research, the photophysical and photochemical mechanisms involved as well as the chemical constitutions of the sensor molecules employed are rather diverse and up to now, very few attempts have been made to establish some general concepts for rational probe design. By analyzing various systems published by other researchers as well as own work, this contribution aims at an elucidation of some of the underlying principles of heavy and transition metal ion-enhanced emission.
A novel fluorescent host-guest material, molecular sieves of AlPO4-5-type doped with 2,2'-bipyridyl-3,3'-diol, was prepared and spectroscopically characterized. The composite crystals show a pronounced optical anisotropy, indicating a high degree of alignment of the guest molecules within the zeolitic pore system. A mean tilting angle of 22 was found for the orientation of the individual dye molecules in the straight channels. The corrected fluorescence emission spectra were determined, and time-resolved fluorescence studies revealed that the dye molecules are preferentially found in three different types of microenvironment. By invoking pH-dependent studies of the dye in aqueous solution, we could trace these spectroscopic features back to two main influences, coadsorbed water within AlPO4-5 pores and guest-host interactions with a few relatively weak Brnsted acid (defect) sites of the inorganic host network.
One of the key interests in the recent development of fluorescent molecular sensors and switches is the realization of systems that show strong signal changes as a response to the chemical trigger. Aiming at rational probe design, this article compiles and compares different promising strategies to extract those supramolecular and photophysical features that allow the construction of molecular devices suitable for efficient signaling. The examples comprise fluorescence OFFON as well as ONOFF operative systems and the mechanisms, properties, and limitations of the different design concepts are discussed.