Rare Example of TICT Based Optical Responses for the Specific Recognition of Cr³⁺ by a 2,2′:6′,2″-Terpyridine Derivative and Demonstration of Multiple Logic Operations

Chemosensor <b>L</b> showed a <i>nonlinear fluorescence</i> response on specific binding to Cr³⁺ ion in the presence of various alkali, alkaline-earth, transition, and lanthanide metal ions. A luminescence band with maxima at 512 nm for <b>L</b> was observed (λ_ext = 330 nm) for a twisted intramolecular charge transfer (TICT) transition following antienergy gap law behavior. However, normal energy gap law behavior prevailed on formation of a new nonluminescent charge transfer complex, M^<i>n</i>+·<b>L</b>. For paramagnetic metal ions, paramagnetism induced luminescence quenching could have also contributed to this. A new <i>switched on</i> fluorescence response at λ_ems of 356 nm (λ_ext of 330 nm) was observed due to Franck–Condon charge transfer (FC-CT) transition only on the formation of a complex, (Cr³⁺)₂·<b>L</b>. Spectral studies revealed a unique dynamic coordination behavior and migration of Cr³⁺ from the terpyridyl fragment to the N_NMe₂ center of <b>L</b> as a function of the varying concentration of another ion (Zn²⁺) and the subtle difference in the binding affinities of the terpyridyl moiety toward Cr³⁺ and Zn²⁺. Further, spectral responses of <b>L</b> toward Zn²⁺, different concentration of Cr³⁺, H⁺ and on subsequent addition of F^– as different ionic inputs could be correlated well for demonstrating various basic and combinatorial circuits

A Taco Complex Derived from a Bis-Crown Ether Capable of Executing Molecular Logic Operation through Reversible Complexation

As learned from natural systems, self-assembly and self-sorting help in interconnecting different molecular logic gates and thus achieve high-level logic functions. In this context, demonstration of important logic operations using changes in optical responses due to the formation of molecular assemblies is even more desirable for the construction of a molecular computer. Synthesis of an appropriate divalent as well as a luminescent crown ether based host <b>1</b> and paraquat derivatives, <b>2</b>(PF₆)₂ and <b>3</b>(PF₆)₂, as guests helped in demonstrating a reversible [3]­(taco complex) (<b>1</b>·{<b>2</b>(PF₆)₂}₂ or <b>1</b>·{<b>3</b>(PF₆)₂}₂) formation in nonpolar solvent. Detailed ¹H NMR studies revealed that two paraquat units were bound cooperatively by the two crown units in <b>1</b>. Because of preorganization, the flexible host molecule <b>1</b> adopts a folded conformation, where each of two paraquat units remain sandwiched between the two aromatic units of each folded crown ether moiety in <b>1</b>. Disassembly of the “taco” complex in the presence of KPF₆ and reassembly on subsequent addition of DB18C6 was initially demonstrated by ¹H NMR spectral studies, which were subsequently corroborated through luminescence spectral studies. Further, luminescence spectral responses as output signals with appropriate and two independent molecular inputs could be correlated to demonstrate basic logic operation like OR and YES gates, while the results of the three molecular inputs could be utilized to demonstrate important logic operation like an INHIBIT gate

Recognition of Hg²⁺ and Cr³⁺ in Physiological Conditions by a Rhodamine Derivative and Its Application as a Reagent for Cell-Imaging Studies

A new rhodamine-based receptor, derivatized with an additional fluorophore (quinoline), was synthesized for selective recognition of Hg²⁺ and Cr³⁺ in an acetonitrile/HEPES buffer medium of pH 7.3. This reagent could be used as a dual probe and allowed detection of these two ions by monitoring changes in absorption and the fluorescence spectral pattern. In both instances, the extent of the changes was significant enough to allow visual detection. More importantly, the receptor molecule could be used as an imaging reagent for detection of Hg²⁺ and Cr³⁺ uptake in live human cancer cells (MCF7) using laser confocal microscopic studies. Unlike Hg­(ClO₄)₂ or Hg­(NO₃)₂ salts, HgCl₂ or HgI₂ failed to induce any visually detectable change in color or fluorescence upon interaction with <b>L</b>_<b>1</b> under identical experimental conditions. Presumably, the higher covalent nature of Hg^II in HgCl₂ or HgI₂ accounts for its lower acidity and its inability to open up the spirolactam ring of the reagent <b>L</b>_<b>1</b>. The issue has been addressed on the basis of the single-crystal X-ray structures of <b>L</b>_<b>1</b>·HgX₂ (X^– = Cl^– or I^–) and results from other spectral studies

Recognition of Hg²⁺ and Cr³⁺ in Physiological Conditions by a Rhodamine Derivative and Its Application as a Reagent for Cell-Imaging Studies

A new rhodamine-based receptor, derivatized with an additional fluorophore (quinoline), was synthesized for selective recognition of Hg²⁺ and Cr³⁺ in an acetonitrile/HEPES buffer medium of pH 7.3. This reagent could be used as a dual probe and allowed detection of these two ions by monitoring changes in absorption and the fluorescence spectral pattern. In both instances, the extent of the changes was significant enough to allow visual detection. More importantly, the receptor molecule could be used as an imaging reagent for detection of Hg²⁺ and Cr³⁺ uptake in live human cancer cells (MCF7) using laser confocal microscopic studies. Unlike Hg­(ClO₄)₂ or Hg­(NO₃)₂ salts, HgCl₂ or HgI₂ failed to induce any visually detectable change in color or fluorescence upon interaction with <b>L</b>_<b>1</b> under identical experimental conditions. Presumably, the higher covalent nature of Hg^II in HgCl₂ or HgI₂ accounts for its lower acidity and its inability to open up the spirolactam ring of the reagent <b>L</b>_<b>1</b>. The issue has been addressed on the basis of the single-crystal X-ray structures of <b>L</b>_<b>1</b>·HgX₂ (X^– = Cl^– or I^–) and results from other spectral studies

Ratiometric Detection of Cr³⁺ and Hg²⁺ by a Naphthalimide-Rhodamine Based Fluorescent Probe

Newly synthesized rhodamine derivatives, <b>L</b>_<b>1</b> and <b>L</b>_<b>2</b>, are found to bind specifically to Hg²⁺ or Cr³⁺ in presence of large excess of other competing ions with associated changes in their optical and fluorescence spectral behavior. These spectral changes are significant enough in the visible region of the spectrum and thus, allow the visual detection. For <b>L</b>_<b>1</b>, the detection limit is even lower than the permissible [Cr³⁺] or [Hg²⁺] in drinking water as per standard U.S. EPA norms; while the receptor, <b>L2</b> could be used as a ratiometric sensor for detection of Cr³⁺ and Hg²⁺ based on the resonance energy transfer (RET) process involving the donor naphthalimide and the acceptor Cr³⁺/Hg²⁺-bound xanthene fragment. Studies reveal that these two reagents could be used for recognition and sensing of Hg²⁺/Cr³⁺. Further, confocal laser microscopic studies confirmed that the reagent <b>L</b>_<b>2</b> could also be used as an imaging probe for detection of uptake of these ions in A431 cells