Control of the site and potential of reduction and oxidation processes in pi-expanded quinoxalinoporphyrins

Quinoxalino[2,3-b]porphyrins are pi-expanded porphyrins, having a quinoxaline fused to a beta,beta-pyrrolic position of the porphyrin. They are used as components in systems proposed as molecular wires. Knowledge of their redox properties is of value in the design of electron- or hole-conduction systems. In particular, the location of the charge density in the radical anions of quinoxalinoporphyrins can be modulated by peripheral functionalization. New theoretical treatments of electrochemical potentials are developed that identify the site of reduction in both the anions and the dianions of 33 quinoxalinoporphyrins. These molecules include free-base and metallated macrocycles substituted on the quinoxaline with electron-withdrawing groups (NO2, Cl, Br) and/or electron-donating groups (NH2, OCH3). Spectroelectrochemistry, density-functional theory calculations, and substituent-parameter models are used to verify the analysis. Five distinct patterns are observed for the locations of the first and second reductions; some of these patterns involve delocalized charges. Nitroquinoxalinoporphyrins with the nitro groups at the 5- and 6-quinoxaline positions are found to have quite different properties owing to distortions caused by peri interactions that force the nitro group of the 5-nitro regioisomer out of conjugation. Charge localization on the nitroquinoxaline fragment is found for some molecules, and this is attributed to ion-pairing with the 0.1 M tetrabutylammonium perchlorate electrolyte used, leading to the verified prediction that electron-paramagnetic resonance spectra of these molecules taken without the electrolyte yield delocalized anions. These properties enable the control of conduction through molecular wires synthesised from quinoxalinoporphyrins.     

Control of the orbital delocalization and implications for molecular rectification in the radical anions of porphyrins with coplanar 90 degrees and 180 degrees beta,beta'-fused extensions

Through-porphyrin electronic communication is investigated using "linear-type" and "corner-type" bis(quinoxalino)porphyrins in free-base form and their ZnII, CuII, NiII, and PdII derivatives. These compounds are porphyrins with quinoxalines fused on opposite or adjacent beta,beta'-pyrrolic positions; they were synthesized from 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)-porphyrin-2,3,12,13- and -2,3,7,8-tetraone, respectively, by reaction with 1,2-phenylenediamine. The degree of electron spin delocalization into the fused rings in the pi-radical anions of the free-base and metal(II) bisquinoxalinoporphyrins was elucidated by electrochemistry, UV-vis absorption, and electron spin resonance (ESR) spectra of the singly reduced species and density functional theory calculations. Hyperfine splitting patterns in the ESR spectra of the pi-radical anions show that symmetric molecules have delocalized electron spin, indicating that significant inter-quinoxaline interactions are mediated through the central porphyrin unit, these interactions being sufficient to guarantee through-molecule conduction. However, when molecular symmetry is broken by tautomeric exchange of the inner nitrogen hydrogens in the free-base porphyrin with a corner-type quinoxaline substitution pattern, the pi-radical anion becomes confined so that one quinoxaline group is omitted from spin delocalization. This indicates the appearance of a unidirectional barrier to through-molecule conduction, suggesting a new motif for chemically controlled rectification.     

Androgynous porphyrins. Silver(II) quinoxalinoporphyrins act as both good electron donors and acceptors

The metal-centered and macrocycle-centered electron-transfer oxidations and reductions of silver(II) porphyrins were characterized in nonaqueous media by electrochemistry, UV-vis spectroelectrochemistry, EPR spectroscopy, and DFT calculations. The investigated compounds are {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrinato}silver(II), {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b']porphyrinato}silver(II), {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b':7,8-b']porphyrinato}silver(II), and {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b':12,13-b']porphyrinato}silver(II). The first one-electron oxidation and first one-electron reduction both occur at the metal center to produce stable compounds with Ag(III) or Ag(I) metal oxidation states, irrespective of the type of porphyrin ligand. The electrochemical HOMO-LUMO gap, determined by the difference in the first oxidation and first reduction potentials, decreases by introduction of quinoxaline groups fused to the Ag(II) porphyrin macrocycle. This provides a unique androgynous character to Ag(II) quinoxalinoporphyrins that enables them to act as both good electron donors and good electron acceptors, something not previously observed in other metalloporphyrin complexes. The second one-electron oxidation and second one-electron reduction of the compounds both occur at the porphyrin macrocycle to produce Ag(III) porphyrin pi-radical cations and Ag(I) porphyrin pi-radical anions, respectively. The macrocycle-centered oxidation potentials of each quinoxalinoporphyrin are shifted in a negative direction, while the macrocycle-centered reduction potentials are shifted in a positive direction as compared to the same electrode reactions of the porphyrin without the fused quinoxaline ring(s). Both potential shifts are due to a stabilization of the radical cations and radical anions by pi-extension of the porphyrin macrocycle after fusion of one or two quinoxaline moieties at the beta-pyrrolic positions of the macrocycle. Introduction of quinoxaline groups fused to the Ag(II) porphyrin macrocycle provides a unique androgynous character to Ag(II) quinoxalinoporphyrins that enables them to act as both good electron donors and good electron acceptors.     

Quinoxalino[2,3-b']porphyrins behave as pi-expanded porphyrins upon one-electron reduction: broad control of the degree of delocalization through substitution at the macrocycle periphery

The synthesis and redox properties of a series of free-base and metal(II) quinoxalino[2,3-b']porphyrins and their use in an investigation of the substituent effects on the degree of communication between the porphyrin and its beta,beta'-fused quinoxalino component are reported. ESR, thin-layer spectroelectrochemistry, and quantum chemical calculations of the resultant radical anions from one-electron reduction indicate that localization of the unpaired electron across both the porphyrin and the fused quinoxalino group can be controlled, the system as a whole behaving as a highly polarizable pi-expanded porphyrin radical anion. ESR studies on the radical anions of zinc(II) quinoxalino[2,3-b']porphyrin derivatives indicate that nitrogen-atom spin distribution changes as a function of chemical substitution: 27% quinoxaline character when the porphyrin ring bears a 7-nitro substituent, 34% quinoxaline character in the unsubstituted parent, and 51-61% nitroquinoxaline character when the quinoxalino unit has one or more nitro groups. Close analogies are found between the calculated and observed nitrogen-atom spin distributions, indicating that the calculations embody the key chemical effects. The calculations also indicate that the nitrogen-atom spin distributions closely parallel the important total porphyrin, quinoxaline, and nitro spin distributions, indicating that the observed quantities realistically depict the change in the nature of the delocalization of the radical anion as a function of chemical substitution. The profound effects observed indicate long-range communication of the type that is essential in molecular electronics applications.     

Effect of ortho substitution on the charge localization of dinitrobenzene radical anions

Optical and electron paramagnetic resonance spectroscopies were used to study the radical anions of several m-dinitrobenzenes and p-dinitrobenzenes with substituents on ortho positions relative to the nitro groups. 1,4-Dinitrobenzene, 1,4-dimethyl-2,5-dinitrobenzene, and 2,5-dinitrobenzene-1,4-diamine radical anions are delocalized (class III) mixed valence species, but in the dinitrodurene radical anion the nitro groups are forced out of the ring plane due to the steric hindrance, which results in localization of the charge. The radical anions m-dinitrobenzene, 2,6-dinitrotoluene, and dinitromesitylene are all localized (class II) mixed valence species, as is common for m-dinitrobenzenes, and the rate of intramolecular electron transfer reaction strongly decreases with the number of methyl substituents. The same mechanism of rotation of the nitro groups out of the ring plane due to steric hindrance caused by neighboring methyl groups is also responsible for slowing the reaction. However, 2,6-dinitroaniline radical anion and 2,6-dinitrophenoxide radical dianion are charge-delocalized because the strong electron releasing amino and oxido groups increase the conjugation between the two charge-bearing units.     

Topological analysis of the molecular charge density and impact sensitivy models of energetic molecules

Important explosives of practical use are composed of nitroaromatic molecules. In this work, we optimized geometries and calculated the electron density of 17 nitroaromatic molecules using the Density Functional Theory (DFT) method. From the DFT one-electron density matrix, we computed the molecular charge densities, thus the electron densities, which were then decomposed into electric multipoles located at the atomic sites of the molecules using the distributed multipole analysis (DMA). The multipoles, which have a direct chemical interpretation, were then used to analyze in details the ground state charge structure of the molecules and to seek for correlations between charge properties and sensitivity of the corresponding energetic material. The DMA multipole moments do not present large variations when the size of the Gaussian basis set is changed; the largest variations occurred in the range 10-15% for the dipole and quadrupole moments of oxygen atoms. The charges on the carbon atoms of the aromatic ring of each molecule become more positive when the number of nitro groups increases and saturate when there are five and six nitro groups. The magnitude and the direction of the dipole moments of the carbon atoms, indicators of site polarization, also depend on the nature of adjacent groups, with the largest dipole value being for C-H bonds. The total magnitude of the quadrupole moment of the aromatic ring carbon atoms indicates a decrease in the delocalized electron density due to an electron-withdrawing effect. Three models for sensitivity of the materials based on the DMA multipoles were proposed. Explosives with large delocalized electron densities in the aromatic ring of the component molecule, expressed by large quadrupole values on the ring carbon atoms, correspond to more insensitive materials. Furthermore, the charges on the nitro groups also influence the impact sensitivity.     

β‐Functionalized meso Tetrahalothien‐2‐ylporphyrins: Synthesis,Spectral, and Electrochemical Properties

Two types of β‐ functionalized (mono nitrated and perbrominated) meso tetrakis(5‐halothien‐;2‐yl)porphyrins, which can be used as precursors for the synthesis of other asymmetric and highly substituted porphyrins, have been synthesised and characterized. Introduction of a nitro group at the β‐ position shifted soret band 11–16 nm to the red region and redox potentials to > 170 mV for oxidation and > 250 mV for reduction anodically. Perbromination of halothienylporphyrins lead to enhanced bathochromically shifted uv‐visible spectral bands, but had only marginal influence on oxidation potentials. Effect of mono nitro group and eight bromo groups on the electronic properties of the porphyrins is attributed, respectively to, the electron deficiency created in the porphyrin π‐ system and the nonplanar conformation induced by the bulky bromo groups.     

Carboxylate–phenolate tautomerism in 5‐[(nitrophenyl)diazenyl]salicylate anions

Aryldiazenyl derivatives of salicylic acid and their salts are used as dyes. In these structures, the carboxylate groups are engaged in short contacts with the cations and in hydrogen bonds with water molecules, if present. If both O atoms of the carboxylate group take part in such interactions, the negative charge is delocalized over the two atoms. In the absence of hydrogen bonds and contacts with cations, the negative charge is localized on one of the O atoms. In the crystal structures of tetramethylammonium 2‐hydroxy‐5‐[(E)‐(4‐nitrophenyl)diazenyl]benzoate and tetramethylammonium 2‐hydroxy‐5‐[(E)‐(2‐nitrophenyl)diazenyl]benzoate, both C₄H₁₂N⁺·C₁₃H₈N₃O₅⁻, all the interactions between the cations and anions are weak, and their effect on the geometry of the anions is negligible. Under these conditions, the 2‐nitro‐substituted anion is an almost pure phenol–carboxylate tautomer, whereas in the 4‐nitro‐substituted anion, the phenolic H atom is shifted towards the carboxylate group, and thus the structure of this anion is intermediate between the phenol–carboxylate and phenolate–carboxylic acid tautomeric forms. The probable formation of such an intermediate form is supported by quantum chemical calculations. Being the characteristic feature of this form, a short distance between the phenolic and carboxylate O atoms is observed in the 4‐nitro‐substituted anion, as well as in the structures of some 3,5‐dinitrosalicylates reported in the literature.     

Effect of electrolytes on the retention behavior of some benzenesulfonates in electrochemically modulated liquid chromatography

The effects of electrolytes on the retention behavior of some benzenesulfonates in electrochemically modulated liquid chromatography were studied. Both cations and anions were found to have considerable effects on retention. As cation size increases, retention decreases, while anions show more complicated effects were anionic size and charge distribution contribute to the overall behavior of anions. Large anions with a delocalized negative charge on the whole species result in lower retention times, and vice versa. Also, electrolyte concentration plays an important role in the retention behavior observed. Initially, as electrolyte concentration was increased retention increased due to electrostatic interactions of cations with the negatively charged stationary phase. However, retention starts to slightly decrease or increase after some specific electrolyte concentration depending on the nature of the electrolytic species. Finally, an interesting behavior of double peak appearance of a single solute was observed at low electrolyte concentrations and was attributed to the presence of other active sites on the carbon stationary phase.     

Characterization of dinitroporphyrin zinc complexes by electrospray ionization tandem mass spectrometry. Unusual fragmentations of beta-(1,3-dinitroalkyl) porphyrins

The zinc complexes of diaryl bis(p-nitrophenyl)porphyrins and beta-(1,3-dinitroalkyl)tetraphenylporphyrins were studied by electrospray ionization (ESI) tandem mass spectrometry (MS/MS). All porphyrins showed the protonated molecule under ESI conditions. The protonated molecules were induced to fragment and the corresponding ESI tandem mass spectra were analysed. Porphyrins with two p-nitrophenyl groups showed, as expected, characteristic fragmentations including either loss of one nitro group, as the major fragment of the tandem mass spectra, and loss of both nitro groups. In contrast, MS/MS of the beta-(1,3-dinitroalkyl)porphyrins provided interesting and unexpected results such as the absence (or in insignificant abundance) of the ions formed by loss of one nitro group. However, these porphyrins show an abundant fragment due to combined loss of the two nitro groups. Also, the typical beta-cleavage of the alkyl chain is not observed per se, only when combined with loss of HNO2 or *NO2. Instead, alpha-cleavage, with loss of the beta-pyrrolic substituent, is the most favourable process.     

Chemical stability and reactivity of deprotonated oligonucleotides (DNA) in the gas phase: protonation and solvation with hydrogen bromide

Selected deprotonated oligodeoxynucleotides generated by electrospray ionization were exposed to a variety of neutral molecules in the gas phase at room temperature in flowing helium gas at 0.35 Torr. Single-stranded [AGTCTG-nH]n- and single- and double-stranded [GCATGC-nH]n- anions were found to be remarkably unreactive with strong oxidants (O3, O2, N2O) and potential intercalators (benzene, pyridine, toluene, and quinoxaline). Hydration also was observed to be inefficient. However, [AGTCTG-nH]n- anions with n=2, 3, 4, and 5 were seen to be sequentially protonated and/or hydrobrominated with HBr (but not damaged) and displayed an interesting "end effect" against protonation. Measurements are provided for the rate coefficients of reaction and the efficiencies of protonation. These experimental results point toward the exciting prospect of measuring the intrinsic chemistry of other bare DNA-like anions, including double-stranded oligonucleotide anions in the gas phase at room temperature.     

Electron impact studies. CIII—Negative ion mass spectra of naturally occurring compounds: Nitrobenzoyl derivatives of amino esters

The negative ion mass spectra of p-nitrobenzoyl derivatives of amino esters show pronounced molecular anions and characteristic fragmentation patterns. Fragment anions observed in the spectra of o-nitrobenzoyl analogues arise by various interactions between the nitro and amino ester groups. No fragmentation of the amino ester residue is observed for the m-nitrobenzoyl derivatives, but the +E spectra of these systems can be used to provide definitive structural information.     

Nitro-functionalized oligothiophenes as a novel type of electroactive molecular material: spectroscopic,electrochemical, and computational study

A novel series of terthiophenes bearing electron-donor and electron-acceptor groups at the end alpha-positions has been prepared. The analysis of the UV-vis, infrared, and Raman spectra, performed with the aid of density functional theory calculations, shows that the asymmetrically substituted nitro compounds PhT(3)NO(2) and BrT(3)NO(2) behave as push-pull systems and present an intense photoinduced charge transfer in the visible spectrum. The symmetrically substituted dinitro compound NO(2)T(3)NO(2) displays a highly delocalized structure with a low single-double bond length alternation and also displays a low-energy absorption band in the visible region. The novel nitroterthiophenes possess attractive electrochemical properties since they generate stable species both upon oxidation and reduction. Oxidation mainly involves changes in the oligothiophene backbone and leads to the formation of stable cations even for NO(2)T(3)NO(2). Reduction is mainly nitro-centered but also affects the conjugated structure. Radical anions and dianions are formed for PhT(3)NO(2) and BrT(3)NO(2). Dianions, not radical anions, and trianions are obtained for NO(2)T(3)NO(2). Nitro-functionalized terthiophenes are shown to be very promising as electroactive molecular materials since they behave as push-pull systems, present a very intense photoinduced charge transfer in the visible region, and could act as both n- and p-channel conductors in organic electronic transistors.     

Influence of electronic and structural effects on the oxidative behavior of nickel porphyrins

With the aim of better understanding the electronic and structural factors which govern electron-transfer processes in porphyrins, the electrochemistry of 29 nickel(II) porphyrins has been examined in dichloromethane containing either 0.1 M tetra-n-butylammonium perchlorate (TBAP) or tetra-n-butylammonium hexafluorophosphate (TBAPF(6)) as supporting electrolyte. Half-wave potentials for the first oxidation and first reduction are only weakly dependent on the supporting electrolyte, but E(1/2) for the second oxidation varies considerably with the type of supporting electrolyte. E(1/2) values for the first reduction to give a porphyrin pi-anion radical are effected in large part by the electronic properties of the porphyrin macrocycle substituents, while half-wave potentials for the first oxidation to give a pi-cation radical are affected by the substituents as well as by nonplanar deformations of the porphyrin macrocycle. The potential difference between the first and second oxidations (Delta/Ox(2) - Ox(1)/) is highly variable among the 29 investigated compounds and ranges from 0 mV (two overlapped oxidations) to 460 mV depending on the macrocycle substituents and the anion of the supporting electrolyte. The magnitude of Delta/Ox(2) - Ox(1)/ is generally smaller for compounds with very electron-withdrawing substituents and when TBAP is used as the supporting electrolyte. This behavior is best explained in terms of differences in the binding strengths of anions from the supporting electrolyte (ClO(4)(-) or PF(6)(-)) to the doubly oxidized species. A closer analysis suggests two factors which are important in modulating Delta/Ox(2) - Ox(1)/ and thus the binding affinity of the anion to the porphyrin dication. One is the type of pi-cation radical (a proxy for the charge distribution in the dication), and the other is the conformation of the porphyrin macrocycle (either planar or nonplanar). These findings imply that the redox behavior of porphyrins can be selectively tuned to display separate or overlapped oxidation processes.     

Effect of structural and electronic properties of substituents on the metal porphyrin formation kinetics

5,10,15-trinitro-2,3,7,8,12,13,17,18-octaethylporphyrin was synthesized and the kinetics of its coordination to zinc acetate was studied in comparison with previously studied β-octamethylporphyrin, β-octaphenylporphyrin, 5,10,15-triphenyl-2,3,7,8,12,13,17,18-octamethylporphyrin, and 5,10,15,20-tetraphenyl-2,3,7,8,12,13,17,18-octamethylporphine. The effect of structural and electronic properties of substituents on the kinetics of formation of metal porphyrins is analyzed. It is shown that the properties of 5,10,15-trinitro-2,3,7,8,12,13,17,18-octaethylporphyrin are determined by both strong electron-withdrawing influence of three nitro groups and by the distortion of the planar structure of the tetrapyrrole macrocycle.     

Unusual multi-step sequential Au(III)/Au(II) processes of gold(III) quinoxalinoporphyrins in acidic non-aqueous media

The electrochemistry of gold(III) mono- and bis-quinoxalinoporphyrins was examined in CH(2)Cl(2) or PhCN containing 0.1 M tetra-n-butylammonium perchlorate (TBAP) before and after the addition of trifluoroacetic acid to solution. The investigated porphyrins are represented as Au(PQ)PF(6) and Au(QPQ)PF(6), where P is the dianion of the 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin and Q is a quinoxaline group fused to a β,β'-pyrrolic position of the porphyrin macrocycle; in Au(QPQ)PF(6) there is a linear arrangement where the quinoxalines are fused to pyrrolic positions that are opposite each other. The porphyrin without the fused quinoxaline groups, Au(P)PF(6), was also investigated under the same solution conditions. In the absence of acid, all three gold(III) porphyrins undergo a single reversible Au(III)/Au(II) process leading to the formation of a Au(II) porphyrin which can be further reduced at more negative potentials to give stepwise the Au(II) porphyrin π-anion radical and dianion, respectively. However, in the presence of acid, the initial Au(III)/Au(II) processes of Au(PQ)PF(6) and Au(QPQ)PF(6) are followed by an internal electron transfer and protonation to regenerate new Au(III) porphyrins assigned as Au(III)(PQH)(+) and Au(III)(QPQH)(+). Both protonated gold(III) quinoxalinoporphyrins then undergo a second Au(III)/Au(II) process at more negative potentials. The electrogenerated monoprotonated monoquinoxalinoporphyrin, Au(II)(PQH), is then further reduced to its π-anion radical and dianion forms, but this is not the case for the monoprotonated bis-quinoxalinoporphyrin, Au(II)(QPQH), which accepts a second proton and is rapidly converted to Au(III)(HQPQH)(+) before undergoing a third Au(III)/Au(II) process to produce Au(II)(HQPQH) as a final product. Thus, Au(P)PF(6) undergoes one metal-centered reduction while Au(PQ)PF(6) and Au(QPQ)PF(6) exhibit two and three Au(III)/Au(II) processes, respectively. These unusual multistep sequential Au(III)/Au(II) processes were monitored by thin-layer spectroelectrochemistry and a reduction/oxidation mechanism for Au(PQ)PF(6) and Au(QPQ)PF(6) in acidic media is proposed.     

Photo-, solvent-, and ion-controlled multichromism of imidazolium-substituted diarylethenes

Cationic diarylethenes with an imidazolium ring are synthesized for the first time. The imidazolium cationic moiety is connected directly to the ethene unit as one of the aryl units that take part in the photoinduced pericyclization reaction. The imidazolium-substituted diarylethenes undergo reversible photochromic reactions in a variety of organic media, including ionic liquids, even though they have a delocalized cationic charge in one of the five-membered aromatic rings. The closed-ring isomer shows solvatochromism depending on the solvent donor numbers. Addition of some tetraalkylammonium salts, such as tetrabutyl ammonium nitrate, into the colored organic solution of diarylethene also causes a color change, indicating its ionochromic property. These solvato- and ionochromic properties are considered in connection with the shift of chemical equilibrium between the closed-ring isomers, one with an extended pi-conjugation system and one with limited pi-conjugation due to the strong interaction with solvent molecules and anions with high donor number.     

Effects of Hofmeister anions on the phase transition temperature of elastin-like polypeptides

The modulation of the lower critical solution temperature (LCST) of two elastin-like polypeptides (ELPs) was investigated in the presence of 11 sodium salts that span the Hofmeister series for anions. It was found that the hydrophobic collapse/aggregation of these ELPs generally followed the series. Specifically, kosmotropic anions decreased the LCST by polarizing interfacial water molecules involved in hydrating amide groups on the ELPs. On the other hand, chaotropic anions lowered the LCST through a surface tension effect. Additionally, chaotropic anions showed salting-in properties at low salt concentrations that were related to the saturation binding of anions with the biopolymers. These overall mechanistic effects were similar to those previously found for the hydrophobic collapse and aggregation of poly(N-isopropylacrylamide), PNIPAM. There is, however, a crucial difference between PNIPAM and ELPs. Namely, PNIPAM undergoes a two-step collapse process as a function of temperature in the presence of sufficient concentrations of kosmotropic salts. By contrast, ELPs undergo collapse in a single step in all cases studied herein. This suggests that the removal of water molecules from around the amide moieties triggers the removal of hydrophobic hydration waters in a highly coupled process. There are also some key differences between the LCST behavior of the two ELPs. Specifically, the more hydrophilic ELP V5A2G(3)-120 construct displays collapse/aggregation behavior that is consistent with a higher concentration of anions partitioning to polymer/aqueous interface as compared to the more hydrophobic ELP V(5)-120. It was also found that larger anions could bind with ELP V5A2G(3)-120 more readily in comparison with ELP V(5)-120. These latter results were interpreted in terms of relative binding site accessibility of the anion for the ELP.     

Density functional theory for charge transfer: the nature of the N-bands of porphyrins and chlorophylls revealed through CAM-B3LYP, CASPT2, and SAC-CI calculations

While density functional theory (DFT) has been proven to be extremely useful for the prediction of thermodynamic and spectroscopic properties of molecules, to date most functionals used in common implementations of DFT display a systematic failure to predict the properties of charge-transfer processes. While this is explicitly manifest in Rydberg transitions of atoms and molecules and in molecular charge-transfer spectroscopy, it also becomes critical for systems containing extended conjugation such as polyenes and other conducting polymers, porphyrins, chlorophylls, etc. A new density functional, a Coulomb-attenuated hybrid exchange-correlation functional (CAM-B3LYP), has recently been developed specifically to overcome these limitations, and it has been shown to properly predict molecular charge-transfer spectra. Here, we demonstrate that it predicts qualitatively reasonable spectra for porphyrin, some oligoporphyrins, and chlorophyll. However, alternate density functionals developed to overcome the same limitations such as current-density functional theory are shown, in their present implementation, to remain inadequate. The CAM-B3LYP results are shown to be in excellent agreement with complete-active-space plus second-order M?ller-Plesset perturbation theory and symmetry-adapted cluster configuration interaction calculations: These depict the N and higher bands of porphyrins and chlorophylls as being charge-transfer bands associated with localization of molecular orbitals on individual pyrrole rings. The validity of the basic Gouterman model for the spectra of porphyrins and chlorophylls is confirmed, rejecting modern suggestions that non-Gouterman transitions lie close in energy to the Q-bands of chlorophylls. As porphyrins and chlorophylls provide useful paradigms for problems involving extended conjugation, the results obtained suggest that many significant areas of nanotechnology and biotechnology may now be realistically treated by cost-effective density-functional-based computational methods.     

Basicity of nitropyrazoles and their simultaneous spectrophotometric determination

The protonation constants of some 1-, 3(5)-, 3-, 4- and 5-nitropyrazoles have been determined, and compared with those of nitroimidazoles. The effect of the position of the nitro group in the pyrazole and imidazole ring is discussed. The ortho effects of the nitro group in pyrazole and imidazole are compared and found to have identical values. The effect of the nitro group on protonation constants is greater when the nitro group is close to the pyridine nitrogen atom. This, together with the ability of N-unsubstituted nitropyrazoles to dissociate to give nitropyrazole anions, with an accompanying shift of the spectra to longer wavelengths, permits the simultaneous spectrophotometric determination of nitropyrazoles.