Proton Transfer in Crystalline Aromatic Schiff Bases and Their Electronic Properties

Abstract

Aromatic Schiff bases having extended II-electron systems and their charge transfer (CT) complexes have been prepared and subjected to structural, optical, and electrical conductivity studies. It is demonstrated by the X-ray crystal-lographic study that the hydroxyl protons form hydrogen bonds to the imino nitrogens, strength of which can be varied by chemical modification of the molecules. Intramolecular proton transfer takes place in solid state and is manifested in the thermochromic spectral changes.     

Excited‐State Intramolecular Proton Transfer: Photoswitching in Salicylidene Methylamine Derivatives

The effect of chemical substitutions on the photophysical properties of the salicylidene methylamine molecule (SMA) (J. Jankowska, M. F. Rode, J. Sadlej, A. L. Sobolewski, ChemPhysChem, 2012 , 13, 4287–4294) is studied with the aid of ab initio electronic structure methods. It is shown that combining π‐electron‐donating and π‐electron‐withdrawing substituents results in an electron‐density push‐and‐pull effect on the energetic landscape of the ground and the lowest excited ππ* and nπ* singlet states of the system. The presented search for the most appropriate SMA derivatives with respect to their photoswitching functionality offers an efficient prescreening tool for finding chemical structures before real synthetic realization.     

Photoinduced Excited State Intramolecular Proton Transfer of New Schiff Base Derivatives with Extended Conjugated Chromophores: A Comprehensive Theoretical Survey

This paper presented comprehensive theoretical investigation of excited state intramolecular proton transfer (ESIPT) of four new large Schiff base derivatives with extended conjugated chromophores. The properties of the ground state and the excited state of phototautomers of C1 to C4 [ C1 : 2‐(4′‐nitro‐stilbene‐4‐ylimino)methylphenol; C2 : 2‐(4′‐cyano‐stilbene‐4‐ylimino)methylphenol; C3 : 2‐(4′‐methoxyl‐stilbene‐4‐ylimino)methylphenol; C4 : 2‐(4′‐N,N‐diethylamino‐stilbene‐4‐ylimino)methylphenol], which included geometrical parameter, energy, rate constant, frontier orbit, Mulliken charge, dipole moment change, were studied by DFT (density functional theory), CIS (configuration interaction singles‐excitation), TDDFT (time‐dependent DFT) methods to analyze the effects of chromophore part on the occurrence of ESIPT and the role of substituent groups. The structural parameter calculation showed that the shorter R_{H? N} and larger R_{O? H} from enol to enol* form, and less twisted configuration in the excited state implied that these molecules could undergo ESIPT as excitation. Stable transition states and a low energy barrier were observed for C1 to C4 . This suggested that chromophore part increased some difficulty to undergo ESIPT for these molecules, while the possibility of occurrence of ESIPT was quite high. The negative ΔE^* (?9.808 and ?9.163 kJ/mol) of C1 and C2 and positive ΔE^* (0.599 and 1.029 kJ/mol) of C3 and C4 indicated that withdrawing substituent groups were favorable for the occurrence of ESIPT. The reaction rate constants of proton transfer of these compounds were calculated in the S₀ and S₁ states respectively, and the high rate constants of these compounds were observed at S₁ state. C1 even reached at 1.45×10¹⁵ s^?1 in the excited state, which is much closed to 2.05×10¹⁵ s^?1 of the parent moiety (salicylidene methylamine). Electron‐donating and electron‐withdrawing substituent groups had different effects on the electron density distribution of frontier orbits and Mulliken charges of the atoms, resulting in different dipole moment changes in enol*→keto* process. These differences in turn suggested that C1 and C2 had more ability to undergo ESIPT than C3 and C4 . The ultraviolet/visible absorption spectra, normal fluorescence emission spectra and ESIPT fluorescence emission spectra of these compounds were predicted in theory.     

Control over Excited State Intramolecular Proton Transfer and Photoinduced Tautomerization: Influence of the Hydrogen‐Bond Geometry

The influence of H‐bond geometry on the dynamics of excited state intramolecular proton transfer (ESIPT) and photoinduced tautomerization in a series of phenol‐quinoline compounds is investigated. Control over the proton donor–acceptor distance (d_DA) and dihedral angle between the proton donor–acceptor subunits is achieved by introducing methylene backbone straps of increasing lengths to link the phenol and quinoline. We demonstrate that a long d_DA correlates with a higher barrier for ESIPT, while a large dihedral angle opens highly efficient deactivation channels after ESIPT, preventing the formation of the fully relaxed tautomer photoproduct.     

Computational Study of Proton Transfer in Tautomers of 3‐ and 5‐Hydroxypyrazole Assisted by Water

The tautomerism of 3‐ and 5‐hydroxypyrazole is studied at the B3LYP, CCSD and G3B3 computational levels, including the gas phase, PCM–water effects, and proton transfer assisted by water molecules. To understand the propensity of tautomerization, hydrogen‐bond acidity and basicity of neutral species is approached by means of correlations between donor/acceptor ability and H‐bond interaction energies. Tautomerism processes are highly dependent on the solvent environment, and a significant reduction of the transition barriers upon solvation is seen. In addition, the inclusion of a single water molecule to assist proton transfer decreases the barriers between tautomers. Although the second water molecule further reduces those barriers, its effect is less appreciable than the first one. Neutral species present more stable minima than anionic and cationic species, but relatively similar transition barriers to anionic tautomers.     

The Host/Guest Type of Excited‐State Proton Transfer; a General Review

Contemporary progress regarding guest/host types of excited‐state double proton transfer has been reviewed, among which are the biprotonic transfer within doubly H‐bonded host/guest complexes, the transfer through a solvent bridge relay, the intramolecular double proton transfer and solvation dynamics coupled proton transfer. Of particular emphases are the photophysical and photochemical properties of excited‐state double proton transfer (ESDPT) in 7‐azaindole and its corresponding analogues. From the chemical aspect, two types of ESDPT reaction, namely the catalytic and non‐catalytic types of ESDPT, have been classified and reviewed separately. For the case of static host/guest hydrogen‐bonded complexes both hydrogen‐bonding strength and configuration (i.e. geometry) play key roles in accounting for the reaction dynamics. In addition to the dynamical concern, excited‐state thermodynamics are of importance to fine‐tune the proton transfer reaction in the non‐catalytic host/guest type of ESDPT. The mechanisms of protic solvent assisted ESDPT, depending on host molecules and proton‐transfer models, have been reviewed where the plausible resolution is deduced. Particular attention has been given to the excited‐state proton transfer dynamics in pure water, aiming at its future perspective in biological applications. Finally, the differentiation in mechanism between solvent diffusive reorganization and solvent relaxation to affect the host/guest ESPT dynamics is made and discussed in de tail.     

Excited‐State Proton Transfer and Intramolecular Charge Transfer in 1,3‐Diketone Molecules

The photophysical signature of the tautomeric species of the asymmetric (N,N‐dimethylanilino)‐1,3‐diketone molecule are investigated using approaches rooted in density functional theory (DFT) and time‐dependent DFT (TD‐DFT). In particular, since this molecule, in the excited state, can undergo proton transfer reactions coupled to intramolecular charge transfer events, the different radiative and nonradiative channels are investigated by making use of different density‐based indexes. The use of these tools, together with the analysis of both singlet and triplet potential energy surfaces, provide new insights into excited‐state reactivity allowing one to rationalize the experimental findings including different behavior of the molecule as a function of solvent polarity.     

Amphiphilic Schiff Base Organogels: Metal‐Ion‐Mediated Chiral Twists and Chiral Recognition

New amphiphilic gelators that contained both Schiff base and L ‐glutamide moieties, abbreviated as o‐SLG and p‐SLG, were synthesized and their self‐assembly in various organic solvents in the absence and presence of metal ions was investigated. Gelation test revealed that o‐SLG formed a thermotropic gel in many organic solvents, whilst p‐SLG did not. When metal ions, such as Cu²⁺, Zn²⁺, Mg²⁺, Ni²⁺, were added, different behaviors were observed. The addition of Cu²⁺ induced p‐SLG to from an organogel. In the case of o‐SLG, the addition of Cu²⁺ and Mg²⁺ ions maintained the gelating ability of the compound, whilst Zn²⁺ and Ni²⁺ ions destroyed the gel. In addition, the introduction of Cu²⁺ ions caused the nanofiber gel to perform a chiral twist, whilst the Mg²⁺ ions enhanced the fluorescence of the gel. More interestingly, the Mg²⁺‐ion‐mediated organogel showed differences in the fluorescence quenching by D ‐ and L ‐tartaric acid, thus showing a chiral recognition ability.     

Synthesis and Characterization of some Unsymmetrical Schiff Base Ligands and their Nickel(II) Complexes Incorporating o‐Phenylenediimine Units

Six new nickel(II) complexes of the unsymmetrical Schiff base ligands derived from o‐phenylenediamine were synthesized. These complexes were prepared by template and non‐template reactions of the precursor 3‐acetyl‐4‐[N‐(2'‐aminophenyl)‐amino]‐3‐buten‐2‐one ( HL °) with appropriate o‐hydroxycarbonyl aromatic compounds, aromatic 1, 3‐oxo aldehydes and 1, 3‐diketones. The nickel(II) compounds were characterized by analytical and spectroscopic methods. Crystal structure of complex [3‐acetyl‐(6, 7)‐benzo‐8‐salicylidene‐5, 8‐diazahepta‐3‐ene‐2‐onato(2‐)]nickel(II) ( NiL ¹) has been determined by X‐ray powder diffraction method, revealed that the molecules are almost flat, and there are no forces other than van der Waals interactions between molecules. The structure was solved by global optimisation technique and refined by the Rietveld method, obtained R_F and R_wp are 11.6 and 17.4%, respectively. The synthesis of a new unsymmetrical nickel(II) tetraazamacrocyclic complex is also described.     

Schiff Base Structured Acid–Base Cooperative Dual Sites in an Ionic Solid Catalyst Lead to Efficient Heterogeneous Knoevenagel Condensations

An acid–base bifunctional ionic solid catalyst [PySaIm]₃PW was synthesized by the anion exchange of the ionic‐liquid (IL) precursor 1‐(2‐salicylaldimine)pyridinium bromide ([PySaIm]Br) with the Keggin‐structured sodium phosphotungstate (Na₃PW). The catalyst was characterized by FTIR, UV/Vis, XRD, SEM, Brunauer–Emmett–Teller (BET) theory, thermogravimetric analysis, ¹H NMR spectroscopy, ESI‐MS, elemental analysis, and melting points. Together with various counterparts, [PySaIm]₃PW was evaluated in Knoevenagel condensation under solvent and solvent‐free conditions. The Schiff base structure attached to the IL cation of [PySaIm]₃PW involves acidic salicyl hydroxyl and basic imine, and provides a controlled nearby position for the acid–base dual sites. The high melting and insoluble properties of [PySaIm]₃PW are relative to the large volume and high valence of PW anions, as well as the intermolecular hydrogen‐bonding networks among inorganic anions and IL cations. The ionic solid catalyst [PySaIm]₃PW leads to heterogeneous Knoevenagel condensations. In solvent‐free condensation of benzaldehyde with ethyl cyanoacetate, it exhibits a conversion of 95.8 % and a selectivity of 100 %; the conversion is even much higher than that (78.2 %) with ethanol as a solvent. The solid catalyst has a convenient recoverability with only a slight decrease in conversion following subsequent recyclings. Furthermore, the new catalyst is highly applicable to many substrates of aromatic aldehydes with activated methylene compounds. On the basis of the characterization and reaction results, a unique acid–base cooperative mechanism within a Schiff base structure is proposed and discussed, which thoroughly explains not only the highly efficient catalytic performance of [PySaIm]₃PW, but also the lower activities of various control catalysts.     

Ground‐ and Excited‐State Tautomerization Rates in Porphycenes

Reaction rates from polarized spectroscopy : A new method based on ultrafast pump–probe polarization spectroscopy enables the measurement of the ground‐ and excited‐state reaction rates for self‐exchange processes. The technique was used to study double hydrogen transfer in porphycene and its derivatives (see figure).

     

Solid‐State Emission of the Anthracene‐o‐Carborane Dyad from the Twisted‐Intramolecular Charge Transfer in the Crystalline State

The emission process of the o ‐carborane dyad with anthracene originating from the twisted intramolecular charge transfer (TICT) state in the crystalline state is described. The anthracene‐o ‐carborane dyad was synthesized and its optical properties were investigated. Initially, the dyad had aggregation‐ and crystallization‐induced emission enhancement (AIEE and CIEE) properties via the intramolecular charge transfer (ICT) state. Interestingly, the dyad presented the dual‐emissions assigned to both locally excited (LE) and ICT states in solution. From the mechanistic studies and computer calculations, it was indicated that the emission band from the ICT should be attributable to the TICT emission. Surprisingly, even in the crystalline state, the TICT emission was observed. It was proposed from that the compact sphere shape of o ‐carborane would allow for rotation even in the condensed state.     

Structuring Redox Polymers Poly[M(Schiff)] (M = Ni,Pd; Schiff = Schiff Bases) on a Molecular Level: Methods and Results of an Investigation

This work is devoted to a consideration of methods of electrochemical synthesis of redox polymers poly[M(Schiff)], where “M” is Ni, or Pd and “Schiff” stands for a four-dentate Schiff base, with a controlled distribution of structural elements (stacks) over the electrode's surface for the optimization of the transport of charge-compensating ions, which is the stage that determines the charge transfer rate in the system under consideration. It is shown that the structuring of these polymers on a molecular level by means of a purposeful selection of the composition of the initial compounds and the conditions of synthesis—the potential of formation and the nature of the solvent and supporting electrolyte—makes it possible to a considerable extent accelerate the charge transfer process in the polymers and improve stability of their electrochemical characteristics.     

Excited‐State Double Proton Transfer in Model Base Pairs: The Stepwise Reaction on the Heterodimer of 7‐Azaindole Analogues

A four fused‐ring system 11‐propyl‐6H‐indolo[2,3‐b]quinoline ( 6 HIQ ) is strategically designed and synthesized; it possesses a central moiety of 7‐azaindole ( 7AI ) and undergoes excited‐state double proton transfer (ESDPT). Despite a barrierless type of ESDPT in the 6 HIQ dimer, femtosecond dynamics and a kinetic isotope effect provide indications for a stepwise ESDPT process in the 6 HIQ/7AI heterodimer, in which 6 HIQ (deuterated 6 HIQ ) delivers the pyrrolyl proton (deuteron) to 7AI (deuterated 7AI ) in less than 150 fs, forming an intermediate with a charge‐transfer‐like ion pair, followed by the transfer of a pyrrolyl proton (deuteron) from cation‐like 7AI (deuterated 7AI ) to the pyridinyl nitrogen of the anion‐like 6 HIQ (deuterated 6 HIQ ) in ～1.5±0.3 ps (3.5±0.3 ps). The barrier of second proton transfer is estimated to be 2.86 kcal mol^?1 for the 6 HIQ/7AI heterodimer.     

Catalytic Asymmetric Phase‐Transfer Michael Reaction and Mannich‐Type Reaction of Glycine Schiff Bases with Tartrate‐Derived Diammonium Salts

Catalytic asymmetric Michael and Mannich‐type reactions of glycine Schiff bases with chiral two‐center organocatalysts, tartrate‐derived diammonium salts (TaDiASs), are described. On the basis of conformational studies, optimized TaDiASs with a 2,6‐disubstituted cyclohexane spiroacetal were newly designed. These TaDiASs catalyzed the asymmetric Michael and Mannich‐type reactions of glycine Schiff bases with higher enantioselectivity than previous catalysts. In the Mannich‐type reaction, aromatic N‐Boc‐protected imines (Boc=tert‐butoxycarbonyl) as well as enolizable alkyl imines were applicable. As a synthetic application of the catalytic asymmetric Mannich‐type reaction with the optimized TaDiASs, we developed a catalytic asymmetric total synthesis of (+)‐nemonapride, which is an antipsychotic agent.     

Iron Oxide Nanoparticles Labeled with an Excited‐State Intramolecular Proton Transfer Dye

Excited‐state intramolecular proton transfer (ESIPT) is a particularly well known reaction that has been very little studied in magnetic environments. In this work, we report on the photophysical behavior of a known ESIPT dye of the benzothiazole class, when in solution with uncoated superparamagnetic iron oxide nanoparticles, and when grafted to silica‐coated iron oxide nanoparticles. Uncoated iron oxide nanoparticles promoted the fluorescence quenching of the ESIPT dye, resulting from collisions during the lifetime of the excited state. The assembly of iron oxide nanoparticles with a shell of silica provided recovery of the ESIPT emission, due to the isolation promoted by the silica shell. The silica network gives protection against the fluorescence quenching of the dye, allowing the nanoparticles to act as a bimodal (optical and magnetic) imaging contrast agent with a large Stokes shift.     

Syntheses,Crystal Structures,and Antibacterial Activities of Four Schiff Base Complexes of Copper and Zinc

Four Schiff base complexes, [Cu₂(L1)₂(μ‐NCS)₂] ( 1 ), [Cu₂(L2)₂(μ‐N₃)₂] ( 2 ), Cu[Cu(CH₃COO)(L3)]₂ ( 3 ), and [Zn{Zn(C₃H₄N₂)(L3)}₂(NO₃)](NO₃) ( 4 ) (where L1 = 2‐[(pyridin‐2‐ylmethylimino)methyl]phenol, L2 = 1‐[(pyridin‐2‐ylmethylimino)methyl]naphthalen‐2‐ol, and L3 = bis(salicylidene)‐1, 3‐propanediamine), were synthesized and characterized by elemental analyses, infrared spectroscopy, and single crystal X‐ray determinations. Both 1 and 2 are structurally similar di‐nuclear complexes, which are located at crystallographic inversion centers (with the center of the central Cu₂N₂ ring). In 1 , each copper atom has a slightly distorted square pyramidal configuration, coordinated by two nitrogen atoms and one oxygen atom from L1 and another two terminal nitrogen atoms from two bridging thiocyanate anions. The Cu···Cu separation is 3.466(3) Å. The structure of 2 is similar to that of 1 , with Cu···Cu separation of 3.368(2) Å. Both 3 and 4 are linear tri‐nuclear complexes. In 3 , the central Cu²⁺ ion is located on an inversion centre and has a distorted octahedral coordination involving four bridging O atoms from two Schiff base ligands (L3) in the equatorial plane and one O atom from each bridging acetate group in the axial positions. The coordination around the terminal Cu²⁺ ions is irregular‐square pyramidal, with two O and two N atoms of L3 in the basal plane and one O atom from an acetate group in the apical position. The acetate bridges linking the central and terminal Cu²⁺ ions are mutually trans. The Cu···Cu separation is 3.009(3) Å. In 4 , the coordination configuration of the central and the terminal zinc atoms are similar to that of the 3 , with Zn···Zn separation of 3.153(4) Å. The three Schiff bases and the corresponding three copper complexes exhibit good antibacterial properties, while the zinc complex 4 has nearly no.     

Excited‐State Proton Transfer Can Tune the Color of Protein Fluorescent Markers

We show by quantum mechanical/molecular mechanical (QM/MM) simulations that phenylbenzothiazoles undergoing an excited‐state proton transfer (ESPT) can be used to probe protein binding sites. For 2‐(2′‐hydroxy‐4′‐aminophenyl)benzothiazole (HABT) bound to a tyrosine kinase, the absolute and relative intensities of the fluorescence bands arising from the enol and keto forms are found to be strongly dependent on the active‐site conformation. The emission properties are tuned by hydrogen‐bonding interactions of HABT with the neighboring amino acid T766 and with active‐site water. The use of ESPT tuners opens the possibility of creating two‐color fluorescent markers for protein binding sites, with potential applications in the detection of mutations in cancer cell lines.     

Synthesis of One‐Dimensional Schiff Base Polymers that Contain an Oligothiophene Building Block on the Graphite Surface

Surface‐mediated Schiff base coupling reactions between oligothiophenes equipped with an aldehyde group and aromatic diamines were investigated on highly oriented pyrolytic graphite (HOPG) by means of scanning tunneling microscopy (STM) under ambient conditions. To investigate the evolution process from monomers to resultant polymers and the mechanism of reactions, we controlled the ratio of precursors and the reactive temperature, and we obtained high‐resolution STM images of different stages of the surface reaction. The results suggest that preferential adsorption of one kind of monomer has a great influence on the on‐ surface Schiff base reaction.