Bifunctional charge transfer operated fluorescent probes with acceptor and donor receptors. 2. Bifunctional cation coordination behavior of biphenyl-type sensor molecules incorporating 2,2':6',2' '-terpyridine acceptors.

Based on donor (D)-acceptor (A) biphenyl (b) type molecules, a family of fluorescent reporters with integrated acceptor receptors and noncoordinating and coordinating donor substituents of varying strength has been designed for ratiometric emission sensing and multimodal signaling of metal ions and protons. In part 2 of this series on such charge transfer (CT) operated mono- and bifunctional fluorescent devices, the cation coordination behavior of the sensor molecules bpb-R equipped with a proton- and cation-responsive 2,2':6',2' '-terpyridine (bp) acceptor and either amino-type donor receptors (R = DMA, A15C5 = monoaza-15-crown-5) or nonbinding substituents (R = CF(3), H, OMe) is investigated employing the representative metal ions Na(I), Ca(II), Zn(II), Hg(II), and Cu(II) and steady-state and time-resolved fluorometry. The bpb-R molecules, the spectroscopic behavior and protonation behavior of which have been detailed in part 1 of this series, present rare examples for CT-operated bifunctional fluorescent probes that can undergo consecutive and/or simultaneous analyte recognition. The analyte-mediated change of the probes' intramolecular CT processes yields complexation site- and analyte-specific outputs, i.e., absorption and fluorescence modulations in energy, intensity, and lifetime. As revealed by the photophysical studies of the cation complexes of these fluoroionophores and the comparison to other neutral and charged D-A biphenyls, the spectroscopic properties of the acceptor chelates of bpb-R and A- and D-coordinated bpb-R are governed by CT control of an excited-state barrier toward formation of a forbidden charge transfer state, by the switching between analytically favorable anti-energy and common energy gap law type behavior, and by the electronic nature of the ligated metal ion. This accounts for the astonishingly high fluorescence quantum yields of the acceptor chelates of bpb-R equipped with weak or medium-sized donors and the red emission of D- and A-coordinated bpb-R observed for nonquenching metal ions.

CT-operated bifunctional fluorescent probe based on a pretwisted donor-donor-biphenyl.

Taking into account the structural requirements for TICT-type sensor molecules, a general synthetic route to derive pH and cation-responsive pretwisted donor (D)-donor (D) biphenyls (b) equipped with donor receptors is developed and a first model compound containing a mono aza-15-crown-5 and a DMA receptor is synthesized, see Scheme 1. The spectroscopic properties of this new bifunctional D-D biphenyl are studied in the non-polar and polar solvents cyclohexane, acetonitrile, and methanol. Protonation as well as complexation studies are performed with the representative metal ions Na(I), K(I), Ca(II), Ag(I), Zn(II), Cd(II), Hg(II), and Pb(II) to reveal the potential of this molecule for communication of whether none, only one, or both binding sites are engaged in analyte coordination by spectroscopically distinguishable outputs. The results are compared to those obtained with closely related donor (D)-acceptor (A) substituted biphenyl-type sensor molecules and are discussed within the framework of neutral and ionic D-A biphenyls.

Traceability in fluorometry: Part II. Spectral fluorescence standards.

The need for the traceable characterization of fluorescence instruments is emphasized from a chemist's point of view, focusing on spectral fluorescence standards for the determination of the wavelength- and polarization-dependent relative spectral responsivity and relative spectral irradiance of fluorescence measuring systems, respectively. In a first step, major sources of error of fluorescence measurements and instrument calibration are revealed to underline the importance of this issue and to illustrate advantages and disadvantages of physical and chemical transfer standards for generation of spectral correction curves. Secondly, examples for sets of traceable chemical emission and excitation standards are shown that cover a broad spectral region and simple procedures for the determination of corrected emission spectra with acceptable uncertainties are presented. With proper consideration of the respective measurement principle and geometry, these dye-based characterization procedures can be not only applied to spectrofluorometers but also to other types of fluorescence measuring systems and even to Raman spectrometers.