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

Abstract

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.

Structures, Metal Ion Affinities, and Fluorescence Properties of Soluble Derivatives of Tris((6-phenyl-2-pyridyl)methyl)amine

“…While we did not further investigate the nature of the new absorption band at ~ 340 nm in the Zn( 13 ) complex, we conclude that the red-shifted fluorescence emission band in the case of the Zn( 3 ) with a maximum at 457 nm (Figure 3B) is a result of the structure, impact and interactions between the three arms in the excited state of the Zn( 3 ) complex. While similar red-shifted absorption bands as in the Zn( 13 ) complexes have been observed in Zn 2+ complexes with other terpyridine6,7 and bipyridyl derivatives2, in most cases red-shifted fluorescence emission spectra were also observed. These red-shifted emission spectra were attributed to a planarization of the two pyrimidyl rings in the bipyridyl metal ion-bound fluorophore2, or to intramolecular charge transfer complex formation6,7.…”

“…While similar red-shifted absorption bands as in the Zn( 13 ) complexes have been observed in Zn 2+ complexes with other terpyridine6,7 and bipyridyl derivatives2, in most cases red-shifted fluorescence emission spectra were also observed. These red-shifted emission spectra were attributed to a planarization of the two pyrimidyl rings in the bipyridyl metal ion-bound fluorophore2, or to intramolecular charge transfer complex formation6,7.…”

“…The binding of Zn 2+ ions to a macrocyclic phenanthrolinophane derivative bearing a pendant arm containing a coordinating amine and an anthracene aromatic ring system, results in a red-shifted emission in ethanol solution due to exciplex formation between a phenanthroline and an anthracene moiety4. Both red-shifted absorption and fluorescence bands have been observed with other ligands and attributed to charge-transfer (CT) states upon Zn 2+ ion binding to a terpyridine electron acceptor and a diphenylaminophenyl electron donor moiety in ethanol5, or a substituted phenyl or biphenyl electron donor group in acetonitrile,6 and pyrene-terpyridine conjugates in buffered aqueous solution7. An analogous CT mechanism was invoked to account for the appearance of a broad, structureless, red-shifted fluorescence mechanism when an aromatic iminodiacetate electron acceptor, connected to an anthracene fluorophore by covalent methylene spacers was titrated with Zn 2+ ions in aqueous solutions.…”

Mini review: Fluorescent heteroditopic ligands of metal ions

“…Compound 28 was investigated by the Resch-Genger group ( Figure 19) (98). Recognising the need for fluorescent ditopic ligands for metal ion detection over large concentration ranges and technologies for simultaneous detection of multiple analytes, 28 was studied as a model system for evaluating the design principle of such ligands based on ICT type fluorophores.…”

Biphenyl-type sensing system in proton-rich environment by fluorescence and quantum-chemical calculations

“…This suggests that not only the polarity of the solvent but also solutesolvent specific interactions play an important role. The photophysics of biphenyl-based compounds is strongly affected by hydrogen bonding [7][8][9][10][11], but the effect is also dependent on particular derivatives. In contrast to the weak solvent dependence of the absorption spectrum, the fluorescence emission spectra are strongly solvent dependent, showing change in the character of the electronic state through the solvent relaxation in the excited state [12][13].…”