Redox-Enabled, pH-Disabled Pyrazoline-Ferrocene INHIBIT Logic Gates

Metallocene-Naphthalimide Derivatives: The Effect of Geometry, DFT Methodology, and Transition Metals on Absorption Spectra

In the present paper, the photophysical properties of metallocene-4-amino-1,8-naphthalimide-piperazine molecules (1-M²⁺), as well as their oxidized and protonated derivatives (1-M³⁺, 1-M²⁺-H⁺, and 1-M³⁺-H⁺), where M = Fe, Co, and Ni, were studied via DFT and TD-DFT, employing three functionals, i.e., PBE0, TPSSh, and wB97XD. The effect of the substitution of the transition metal M on their oxidation state, and/or the protonation of the molecules, was investigated. The present calculated systems have not been investigated before and, except for the data regarding their photophysical properties, the present study provides important information regarding the effect of geometry and of DFT methodology on absorption spectra. It was found that small differences in geometry, specifically in the geometry of N atoms, reflect significant differences in absorption spectra. The common differences in spectra due to the use of different functionals can be significantly increased when the functionals predict minima even with small geometry differences. For most of the calculated molecules, the main absorption peaks in visible and near-UV areas correspond mainly to charge transfer excitations. The Fe complexes present larger oxidation energies at 5.4 eV, whereas Co and Ni complexes have smaller ones, at about 3.5 eV. There are many intense UV absorption peaks with excitation energies similar to their oxidation energies, showing that the emission from these excited states can be antagonistic to their oxidation. Regarding the use of functionals, the inclusion of dispersion corrections does not affect the geometry, and consequently the absorption spectra, of the present calculated molecular systems. For certain applications, where there is a need for a redox molecular system including metallocene, the oxidation energies could be lowered significantly, to about 40%, with the replacement of the iron with cobalt or nickel. Finally, the present molecular system, using cobalt as the transition metal, has the potential to be used as a sensor.

3-Input AND Molecular Logic Gate with Enhanced Fluorescence Output: The Key Atom for the Accurate Prediction of the Spectra

The development of artificial receptors for sensing and recognition of species, as well as for advanced logic functions, is a significant challenge in the field of molecular information technology. Here, we study theoretically, via DFT/TD-DFT calculations, the photophysical properties of a 3-input AND molecular logic gate which presents an enhanced fluorescence spectrum. It was found that the geometry conformation at an N atom of the piperazine group is the key factor for the correct calculation of the absorption spectra of the calculated structures. Its geometry is between tetrahedral and planar, while changes in the corresponding CNCC dihedral angle of about 10 degrees can cause significant shifts of the main peak of the absorption spectra up to 100 nm. Moreover, the unusually enhanced fluorescence of a molecular logic gate (MLG) is explained. Finally, we conclude that molecular systems having N atoms, whose geometry is between planar and tetrahedral, can be ideal molecules as sensors and molecular logic gates. Our calculated absorption and emission spectra are in excellent agreement with available experimental data.

Fluorescent Molecular Logic Gates and Pourbaix Sensors in Polyacrylamide Hydrogels

Polyacrylamide hydrogels formed by free radical polymerisation were formed by entrapping anthracene and 4-amino-1,8-naphthalimide fluorescent logic gates based on photoinduced electron transfer (PET) and/or internal charge transfer (ICT). The non-covalent immobilisation of the molecules in the hydrogels resulted in semi-solid YES, NOT, and AND logic gates. Two molecular AND gates, examples of Pourbaix sensors, were tested in acidic aqueous methanol with ammonium persulfate, a strong oxidant, and displayed greater fluorescence quantum yields than previously reported. The logic hydrogels were exposed to aqueous solutions with chemical inputs, and the fluorescence output response was viewed under 365 nm UV light. All of the molecular logic gates diffuse out of the hydrogels to some extent when placed in solution, particularly those with secondary basic amines. The study exemplifies an effort of taking molecular logic gates from homogeneous solutions into the realm of solid-solution environments. We demonstrate the use of Pourbaix sensors as pE-pH indicators for monitoring oxidative and acidic conditions, notably for excess ammonium persulfate, a reagent used in the polymerisation of SDS-polyacrylamide gels.

Molecular engineering of fluorescent bichromophore 1,3,5-triaryl-Δ2-pyrazoline and 4-amino-1,8-naphthalimide molecular logic gates

Emissive bichromophoric solvatochromatic molecules are introduced as a new platform for the development of fluorescent molecular logic gates.

Fluorescent Molecular Logic Gates Driven by Temperature and by Protons in Solution and on Solid

Temperature-driven fluorescent NOT logic is demonstrated by exploiting predissociation in a 1,3,5-trisubstituted Δ² -pyrazoline on its own and when grafted onto silica microparticles. Related Δ² -pyrazolines become proton-driven YES and NOT logic gates on the basis of fluorescent photoinduced electron transfer (PET) switches. Additional PASS 1 and YES+PASS 1 logic gates on silica are also demonstrated within the same family. Beside these small-molecule systems, a polymeric molecular thermometer based on a benzofurazan-derivatized N-isopropylacrylamide copolymer is attached to silica to produce temperature-driven fluorescent YES logic.

A fluorescent polymeric INHIBIT logic gate based on the natural product quinidine

The natural product quinidine was polymerised with acrylamide and demonstrated as a H⁺-enabled, Cl⁻-disabled INHIBIT logic gate in water.

Ionization Energy and Reduction Potential in Ferrocene Derivatives: Comparison of Hybrid and Pure DFT Functionals

Hybrid density functionals have been regularly applied in state-of-the-art computational models for predicting reduction potentials. Benchmark calculations of the absolute reduction potential of ferricenium/ferrocene couple, the IUPAC-proposed reference in nonaqueous solution, include the B3LYP/6-31G(d)/LanL2TZf protocol. We used this procedure to calculate ionization energies and reduction potentials for a comprehensive set of ferrocene derivatives. The protocol works very well for a number of derivatives. However, a significant discrepancy (>1 V) between experimental and calculated data was detected for selected cases. Three variables were assessed to detect an origin of the observed failure: density functional, basis set, and solvation model. It comes out that the Hartree-Fock exchange fraction in hybrid-DFT methods is the main source of the error. The accidental errors were observed for other hybrid models like PBE0, BHandHLYP, and M06-2X. Therefore, hybrid DFT methods should be used with caution, or pure functionals (BLYP or M06L) may be used instead.

Recent Progress on the Evolution of Pourbaix Sensors: Molecular Logic Gates for Protons and Oxidants

Recent progress in the area of molecular logic, in particular molecules capable of sensing for acidity and oxidizability, are gathered together in this short review. Originally proposed as AND logic gates that provide a high fluorescence output when simultaneously protonated and oxidized, the concept has been extended from two-input to three-input variants and to include molecules that function as INHIBIT logic gates. Photochemical concepts such as photoinduced electron transfer (PET) and internal charge transfer (ICT) are exploited as favorite design concepts. This review highlights the evolution of Pourbaix sensors with anthracene, pyrazoline, and naphthalimide fluorophores. Future applications abound in various disciplines from corrosion science, material science, geochemistry to cell imaging.

Covalent attachment of a fluorescent ‘Pourbaix sensor’ onto a polymer bead for sensing in water

A covalently immobilised 4-amino-1,8-naphthalimide AND logic gate, responsive to acidity and oxidisability, emits a green fluorescence on a Tentagel solid support.