Human brain-inspired chemical artificial intelligence tools for the analysis and prediction of the anion-sensing characteristics of an imidazole-based luminescent Os(II)-bipyridine complex

Neural network and decision tree-based soft computing techniques are implemented in this work for the thorough analysis of the multichannel anion-sensing characteristics of an Os(II)-bipyridine complex derived from imidazole-4,5-bis(benzimidazole) ligand. With the aid of three imidazole NH protons in its outer coordination sphere, a substantial change in the spectral response as well as Os^II/Os^III potential is made possible upon treating with anions of varying basicity. Initial hydrogen bonding between NH protons and anions and thereafter complete proton transfer from the complex backbone probably take place in the process. The deprotonation of the complex by specific anions and restoration to its original form by acid is also reversible. The responsiveness of the new compound is complex enough to imitate multiple sophisticated binary and ternary Boolean logic (BL) functions (NOT logic, combinational logic, traffic signal, set-reset flip-flop logic, and ternary NOR logic) by employing its spectral and redox outputs upon the action of suitable anions and acid in a proper sequence. Executing sensing investigations on altering the amount of the anions within a widespread range is often time-consuming and tedious. To overcome the lacuna, we implemented multiple soft computing techniques, viz., fuzzy logic (FL), artificial neural networks (ANNs), adaptive neuro-fuzzy inference system (ANFIS), and decision tree (DT) regression, for the thorough analysis and prediction of the experimentally observed results. The outcomes obtained from different techniques were compared among themselves as well as with the experimental data and utilized for the proper modeling of the anion-sensing behaviors of the complex.

Analysis and prediction of anion- and temperature responsive behaviours of luminescent Ru(II)-terpyridine complexes by using Boolean, fuzzy logic, artificial neural network and adapted neuro fuzzy inference models

Anion- and temperature responsive behaviors of three luminescent Ru(II)-terpyridine complexes are utilized here to demonstrate multiple Boolean (BL) and fuzzy logic (FL) operations. Taking advantage of the imidazole NH protons, anion-promoted alteration of the photophysical characteristics of the complexes was thoroughly investigated via absorption, and emission spectral and lifetime measurements. In their free state, the complexes display luminescence representing the "on-state", whereas quenching of luminescence is observed with anions demonstrating the "off-state". Likewise, lowering of temperature induces a substantial increase of luminescence and lifetime demonstrating the "on-state", while the increase of temperature induces a significant decrease of emission intensity and lifetime indicating the "off-state" and the process is reversible in both cases. The complexes thus can act as anion- and temperature-responsive molecular switches. The spectral signatures of the complexes under the influence of anions and temperature were employed to mimic multiple BL and FL functions. Performing very detailed sensing studies by varying the analyte concentration within a wide domain is very tedious, time-consuming and expensive. In order to overcome the lacuna, we implemented machine learning and soft computing tools such as artificial neural networks (ANNs), fuzzy-logic and adaptive neuro-fuzzy inference system (ANFIS) to predict the experimental anion sensing data of the complexes.

Stimuli-Responsive Molecular Switches and Logic Devices Based on Ru(II)-Terpyridyl-Imidazole Coordination Motif

We report herein the synthesis, photophysics, and electrochemistry of three Ru(II)-terpyridine complexes derived from a new terpyridyl-imidazole ligand (tpy-HImzPh₃F₂) and study their pH- and temperature-responsive behaviors toward the fabrication of molecular switches. The complexes emitted at room temperature (RT) have a lifetime within the 4.5-49.0 ns domain, depending on the auxiliary ligand and the solvent used. In the acidic region, the complexes exhibit emission, indicating the "on-state", while in the basic condition, the emission is totally quenched, indicating the "off-state". Similarly, when the temperature is lowered, the emission intensity and lifetime are enhanced, demonstrating the on-state, while increase of temperature leads to quenching of the emission intensity and lifetime, designated as the off-state. In both cases, the process is reversible. The bathochromic shift of the spectral band together with the emission quenching and lowering of the Ru³⁺/Ru²⁺ potential is also observed upon deprotonation at elevated pH. In addition, systematic variation of the absorption spectral behaviors upon variation of pH helps in evaluation of the pK_a's of the complexes. In essence, the complexes can act as switches emanated from a huge change in their absorption, emission, and redox behaviors as a function of their acidity/basicity (pH) and temperature. Moreover, their emission spectral responses as a function of pH and temperature were utilized for the fabrication of two-input binary logic gates. Density-functional theory (DFT) and time-dependent density-functional theory (TD-DFT) computations are performed for appropriate interpretation of the spectral bands.

Emission Switching in the Near-Infrared by Reversible Trans-Cis Photoisomerization of Styrylbenzene-Conjugated Osmium Terpyridine Complexes

A new array of homoleptic osmium(II) complexes based on styrylbenzene-conjugated terpyridine ligands (tpy-pvp-X) were synthesized and their photophysical, electrochemical, and photoisomerization behaviors thoroughly investigated in this work. Both electron-donating and -withdrawing substituents were incorporated onto a tpy-pvp-X (X = H, Me, Cl, NO₂, and Ph) moiety to tune the optical properties and also the rate of photoisomerization behaviors in the complexes. All complexes display strong spin-allowed singlet metal-to-ligand charge-transfer bands in the visible (495-506 nm) and weak singlet ground state to triplet metal-to-ligand charge-transfer (³MLCT) broad bands within the 600-700 nm range. The complexes also exhibit strong phosphorescence emission from their ³MLCT state in the near-infrared domain (737-752 nm) at room temperature with excited-state lifetimes spanning between 107 and 165 ns. Two styrylbenzene units promote reversible trans-trans to trans-cis/cis-cis isomerization induced by light. The rate constants and quantum yields of photoisomerization were found to vary linearly with the Hammett σ_p parameters of the substituents. The rate and quantum yields were also found to decrease with increasing polarity of the solvents. Considerable modulation of the optical behavior along with luminescence switching in the complexes has been achieved upon photoisomerization. Moreover, the optical outputs as a function of two photonic stimuli inputs were used to demonstrate the binary function of a two-input IMPLICATION logic gate. In conjunction with the experimental study, computational investigations were also carried out in all three conformations of the complexes (trans-trans, trans-cis, and cis-cis) to have a perception of their electronic structures and for correct assignment of their absorption and emission spectral bands.

pH-Responsive colorimetric, emission and redox switches based on Ru(ii)-terpyridine complexes

We have undertaken a thorough investigation on pH-responsive optical and redox switching behaviors of our recently reported trans form of bis-tridentate Ru(ii) luminophores, [(H₂pbbzim)Ru(tpy-pvp-X)]²⁺ where X = H, Me, Cl, NO₂, and Ph. The complexes possess two benzimidazole protons in their second coordination sphere, which became acidic upon coordinating influence of Ru²⁺ and could be successively deprotonated with the increase of pH. The effect of pH on photophysical and electrochemical behaviours of the complexes was thoroughly studied. Substantial quenching of emission together with the red-shift of both absorption (color change) and emission bands is noticed for all complexes upon dissociation of NH protons. Absorption vs. pH data were employed for determination of ground-state pK_a values, while excited-state pK_a (pK_a*) values were estimated by employing the Förster cycle based equation. The electronic nature of X induces a small but finite effect on the pK_a values and a linear correlation is found by plotting pK_avs. Hammett σ_p parameters of X. Proton-coupled electrochemical behaviours were investigated within the pH range of 1-10. From the E_1/2vs. pH plot, acid dissociation constants in different protonation states of the complexes were estimated in both Ru²⁺ and Ru³⁺ states. Compared with their protonated forms which exhibit reversible oxidation within 0.91-0.95 V, the oxidation potential of the doubly deprotonated forms shifted remarkably to the cathodic region (0.61-0.66 V). In essence, the present complexes act as potential pH-responsive colorimetric, emission and redox switches.

Synthesis, molecular structure and BSA-binding properties of a new binuclear Cd(II) complex based on 2-(1H-tetrazol-1-methyl)-1H-imidazole-4,5-dicarboxylic acid

A new binuclear Cd(II) complex, [Cd2(H2tmidc)4(H2O)2]·6H2O (1) based on 2-(1H-tetrazol-1-methyl)-1H-imidazole-4,5-dicarboxylic acid (H3tmidc) has been synthesized and structurally characterized. The single-crystal X-ray diffraction analysis has revealed that there are two crystallographically distinct H2tmidc– anions in complex 1, one of which is coordinated to Cd(II) ion in a terminal fashion, while the other acts as a bis-connector linking two Cd(II) cations to form the dinuclear structure. The dimeric units are stabilized by intra-molecular O–H···O hydrogen bonds and π-π stacking interactions and are further connected into a three-dimensional supramolecular architecture through inter-molecular hydrogen bonds and π-π stacking interactions. The interactions of complex 1 with bovine serum albumin (BSA) were analyzed by fluorescence measurements under physiological conditions. The results have indicated that the fluorescence intensity of BSA was decreased considerably upon the addition of complex 1 through a static quenching mechanism. The synchronous fluorescence spectrum suggested that the interaction of complex 1 with BSA affects the conformations of tryptophan and tyrosine residues and thereby has an influence on the conformation of BSA.

Stimuli-Responsive Near-Infrared Emissive Os(II)-Terpyridine Complexes with a Sense of Logic

Two bis-tridentate Os(II) compounds based on a heteroditopic terpyridine-bipyridine-type ligand were synthesized, and their photophysical properties were thoroughly studied. The compounds exhibit strong spin-allowed ¹MLCT bands in the visible domain (489-521 nm) as well as weak ¹GS to ³MLCT bands within the 668-815 nm domain. The compounds display strong luminescence from the ³MLCT state in the near-infrared domain (728-780 nm) at room temperature having lifetimes in the range of 20.0-171.0 ns. After coordination of [Os(tpy-PhCH₃/H₂pbbzim)₂]²⁺ unit to the terpyridine site of tpy-Hbzim-dipy, the complexes offer vacant pyridine-imidazole motifs for interacting with cationic and anionic guests. Consequently, photophysical properties of the compounds were tuned to a great extent upon interaction with selected cations, anions, pH, as well as protons. Anion-induced alteration of the ground- and excited-state properties of the compound lead to recognition of specific anions in solution. Significant change in the optical spectral behaviors as well as switching of emission spectral properties of the compounds was done in the NIR region upon treating with anions, cations, protons, and solvents (dichloromethane, acetonitrile, methanol, dimethylsulfoxide, and water). Moreover, the optical outputs in response to external stimuli were used to demonstrate binary functions of two-input IMPLICATION, NOR, and XNOR logic gates.

A new two-dimensional managnese(II) coordination polymer constructed by 2,2'-(1,2-phenylene)bis(1H-imidazole-4,5-dicarboxylate)

In recent years, N-heterocyclic carboxylate ligands have attracted much interest in the preparation of new coordination polymers since they contain N-atom donors, as well as O-atom donors, and have a rich variety of coordination modes which can lead to polymers with intriguing structures and interesting properties. A new two-dimensional coordination polymer, namely poly[[μ₃-2,2'-(1,2-phenylene)bis(4-carboxy-1H-imidazole-5-carboxylato)-κ⁶O⁴,N³,N^3',O^4':O⁵:O^5']manganese(II)], [Mn(C₁₆H₈N₄O₈)]_n or [Mn(H₄Phbidc)]_n, has been synthesized by the reaction of Mn(OAc)₂·4H₂O (OAc is acetate) with 2,2'-(1,2-phenylene)bis(1H-imidazole-4,5-dicarboxylic acid) (H₆Phbidc) under solvothermal conditions. In the polymer, each Mn^II ion is six-coordinated by two N atoms from one H₄Phbidc^2- ligand and by four O atoms from three H₄Phbidc^2- ligands, forming a significantly distorted octahedral MnN₂O₄ coordination geometry. The Mn^II ions are linked by hexadentate H₄Phbidc^2- ligands, leading to a two-dimensional structure parallel to the ac plane. In the crystal, adjacent layers are further connected by N-H...O hydrogen bonds, forming a three-dimensional structure in the solid state.

Spectroelectrochemical studies of a ruthenium complex containing the pH sensitive 4,4'-dihydroxy-2,2'-bipyridine ligand

Attaining high oxidation states at the metal center of transition metal complexes is a key design principle for many catalytic processes. One way to support high oxidation state chemistry is to utilize ligands that are electron-donating in nature. Understanding the structural and electronic changes of metal complexes as higher oxidation states are reached is critical towards designing more robust catalysts that are able to turn over at high rates without decomposing. To this end, we report herein the changes in structural and electronic properties as [Ru(bpy)₂(44'bpy(OH)₂)]²⁺ is oxidized to [Ru(bpy)₂(44'bpy(OH)₂)]³⁺ (bpy = 2,2'-bipyridine; 44'bpy(OH)₂ = 4,4'-dihydroxy-2,2'-bipyridine). The 44'bpy(OH)₂ ligand is a pH-dependent ligand where deprotonation of the hydroxyl groups leads to significant electronic donation to the metal center. A Pourbaix Diagram of the complex reveals a pH independent reduction potential below pH = 2.0 for the Ru^3+/2+ process at 0.91 V vs. Ag/AgCl. Above pH = 2.0, pH dependence is observed with a decrease in reduction potential until pH = 6.8 where the complex is completely deprotonated, resulting in a reduction potential of 0.62 V vs. Ag/AgCl. Spectroelectrochemical studies as a function of pH reveal the disappearance of the Metal to Ligand Charge Transfer (MLCT) or Mixed Metal-Ligand to Charge Transfer bands upon oxidation and the appearance of a new low energy band. DFT calculations for this low energy band were carried out using both B3LYP and M06-L functionals for all protonation states and suggest that numerous new transition types occur upon oxidation to Ru³⁺.

Ru-Os dyads based on a mixed bipyridine-terpyridine bridging ligand: modulation of the rate of energy transfer and pH-induced luminescence switching in the infrared domain

A series of heterobimetallic complexes of compositions [(bpy/phen)₂Ru(dipy-Hbzim-tpy)Os (tpy-PhCH₃/H₂pbbzim)]⁴⁺ (bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline, tpy-PhCH₃ = 4'-(4-methylphenyl)-2,2':6',2''-terpyridine and H₂pbbzim = 2,6-bis(benzimidazole-2-yl)pyridine)), derived from a heteroditopic bpy-tpy bridging ligand, were synthesized and thoroughly characterized in this work. The heterometallic complexes exhibit two successive one-electron reversible metal-centered oxidations corresponding to Os^II/Os^III at lower potential and Ru^II/Ru^III at higher potential. All the four dyads exhibit very intense, ligand centered absorption bands in the UV region and moderately intense MLCT bands in the visible region. The dyads also show intense infrared emission with the emission maximum spanning between 734 nm and 775 nm with reasonably long room temperature lifetimes varying between 30 ns and 104 ns. Both steady state and time resolved luminescence spectroscopic investigations indicate that efficient and fast intramolecular energy transfer from the ³MLCT state of the Ru(ii) center to the Os-center takes place in all the four dyads. In addition, the rate of energy transfer was found to depend on the terminal ligand on the Os-site. Due to the presence of a number of imidazole NH protons in the dyads, significant modulation of both the ground and excited state properties of the complexes was made possible by varying the pH of the solution. By varying the terminal ligand, pH-induced "on-off", "off-off-on" and "on-off-on" emission switching of the complexes was nicely demonstrated in the infrared region.

A pyrene-benzimidazole composed effective fluoride sensor: potential mimicking of a Boolean logic gate

A highly selective fluoride sensor based on a pyrene benzimidazole unit was developed and studied for recyclable memory function.

Cyclometalated Ir(iii) complexes incorporating a photoactive anthracene-based ligand: syntheses, crystal structures and luminescence switching by light irradiation

Compound aip and its Ir(iii) complexes [Ir(dfppy)₂(aip)](PF₆) (1) and [Ir(ppy)₂(aip)](PF₆) (2) show light-irradiation-induced luminescence switching behaviors.

Photoacidic and Photobasic Behavior of Transition Metal Compounds with Carboxylic Acid Group(s)

Excited state proton transfer studies of six Ru polypyridyl compounds with carboxylic acid/carboxylate group(s) revealed that some were photoacids and some were photobases. The compounds [Ru(II)(btfmb)2(LL)](2+), [Ru(II)(dtb)2(LL)](2+), and [Ru(II)(bpy)2(LL)](2+), where bpy is 2,2'-bipyridine, btfmb is 4,4'-(CF3)2-bpy, and dtb is 4,4'-((CH3)3C)2-bpy, and LL is either dcb = 4,4'-(CO2H)2-bpy or mcb = 4-(CO2H),4'-(CO2Et)-2,2'-bpy, were synthesized and characterized. The compounds exhibited intense metal-to-ligand charge-transfer (MLCT) absorption bands in the visible region and room temperature photoluminescence (PL) with long τ > 100 ns excited state lifetimes. The mcb compounds had very similar ground state pKa's of 2.31 ± 0.07, and their characterization enabled accurate determination of the two pKa values for the commonly utilized dcb ligand, pKa1 = 2.1 ± 0.1 and pKa2 = 3.0 ± 0.2. Compounds with the btfmb ligand were photoacidic, and the other compounds were photobasic. Transient absorption spectra indicated that btfmb compounds displayed a [Ru(III)(btfmb(-))L2](2+)* localized excited state and a [Ru(III)(dcb(-))L2](2+)* formulation for all the other excited states. Time dependent PL spectral shifts provided the first kinetic data for excited state proton transfer in a transition metal compound. PL titrations, thermochemical cycles, and kinetic analysis (for the mcb compounds) provided self-consistent pKa* values. The ability to make a single ionizable group photobasic or photoacidic through ligand design was unprecedented and was understood based on the orientation of the lowest-lying MLCT excited state dipole relative to the ligand that contained the carboxylic acid group(s).

Potential anion sensing properties by a redox and substitution series of [Ru(bpy)3−n(Hdpa)n]2+, n = 1–3; Hdpa = 2,2′-dipyridylamine: selective recognition and stoichiometric binding with cyanide and fluoride ions

Efficient anion sensors [Ru(bpy)_3−n(Hdpa)_n]²⁺, n = 1–3 have been developed for selective recognition of cyanide and fluoride ions with remarkable stoichiometric binding.

Cyclometalated Osmium-Amine Electronic Communication through the p-Oligophenylene Wire

A series of bis-tridentate cyclometalated osmium complexes with a redox-active triarylamine substituent have been prepared, where the amine substituent is separated from the osmium ion by a p-oligophenylene wire of various lengths. X-ray crystallographic data of complexes 3(PF6) and 4(PF6) with three or four repeating phenyl units between the osmium ion and the amine substituent are presented. These complexes show two consecutive anodic redox couples between +0.1 and +0.9 V vs Ag/AgCl, with the potential splitting in the range of 300-390 mV. A combined experimental and theoretical study suggests that, in the one-electron-oxidized state, the odd electron is delocalized for short congeners and localized on the osmium component for long congeners. The electronic coupling parameter (Vab) was estimated by the Marcus-Hush analysis. The distance dependence plot of ln(Vab) versus the osmium-amine geometrical distance (Rab) gives a negative linear relationship with a decay slope of -0.19 Å(-1), which is slightly steeper with respect to the previously reported ruthenium-amine series with the same molecular wire. DFT calculations with the long-range-corrected UCAM-B3LYP functional gave more reasonable results for the osmium complexes with respect to those with UB3LYP.