Push–Pull Porphyrins via β-Pyrrole Functionalization: Evidence of Excited State Events Leading to High-Potential Charge-Separated States

A new set of free-base and zinc(II)-metallated, β-pyrrole-functionalized unsymmetrical push–pull porphyrins were designed and synthesized via β-mono- and dibrominated tetraphenylporphyrins using Sonogashira cross-coupling reactions. The ability of donors and acceptors on the push–pull porphyrins to produce high-potential charge separated states was investigated. The porphyrins were functionalized at the opposite β,β′-pyrrole positions of porphyrin ring bearing triphenylamine push groups and naphthalimide pull groups. Systematic studies involving optical absorption, steady-state and time-resolved emission revealed existence of intramolecular type interactions both in the ground and excited states. The push–pull nature of the molecular systems was supported by frontier orbitals generated on optimized structures, wherein delocalization of HOMO over the push group and LUMO over the pull group connecting the porphyrin π-system was witnessed. Electrochemical studies were performed to visualize the effect of push and pull groups on the overall redox potentials of the porphyrins. Spectroelectrochemical studies combined with frontier orbitals helped in characterizing the one-electron oxidized and reduced porphyrins. Finally, by performing transient absorption studies in polar benzonitrile, the ability of push–pull porphyrins to produce charge-separated states upon photoexcitation was confirmed and the measured rates were in the range of 10⁹ s⁻¹. The lifetime of the final charge separated state was around 5 ns. This study ascertains the importance of push–pull porphyrins in solar energy conversion and diverse optoelectronic applications, for which high-potential charge-separated states are warranted.     

Ab initio studies of push–pull systems

Twelve push–pull ethylene derivatives, NH₂CH=CHX, NH₂C≡CCH=CHX, and ⁻OCHX=CHX (with X=BH₂, C≡N, NO₂, and CH₂ ⁺) have been studied by ab initio calculations. The rotational barrier around the central double bond was chosen as a probe for push–pull effects, as push–pull effects would remove electron density from the central double bond. The amount of reduction of double bond character will increase with the contribution of the zwitterionic resonance hybrid structure. Complete geometry optimizations and calculations of vibrational frequencies were performed for all minima and transition state structures of these 12 systems. The calculations were carried out with the B3LYP and MP2 methods using the 6-311+G(d,p) and the 6-311++G(d,p) basis sets. All the systems investigated exhibited properties consistent with push–pull effects such as elongated C=C double bonds, dipolar electronic structures, and reduced barriers to internal rotation.     

A theoretical insight into the role of counter anions and their interactions in nitropentaamminecobalt(III) toward linkage isomerism-induced photochemical motion

Density functional theory (B3LYP, B3LYP-D3, wB97XD, M062X, and M06L) and ab initio methods (MP2 and CCSD(T)) in conjunction with 6-31+G(d,p) and LanL2DZ were employed to investigate the interaction energies between [Co(NH₃)₅NO₂]²⁺ linkage isomers and chloride and nitrate in both gas phase and solid state. The nature of the chemical bonding has been analyzed by means of the atoms in molecules, electron density shift, natural bond orbitals, symmetry adapted perturbation theory, and energy decomposition analysis. The electronic structures of the two lowest laying singlet states (S_o and S₁) of [Co(NH₃)₅NO₂](NO₃)Cl isomers were also investigated using CASSCF(6,6) with LanL2DZ and 6-31G(d) basis sets. Our results show that [Co(NH₃)₅NO₂]²⁺ linkage isomers interact more strongly with chloride than nitrate. The structures of [Co(NH₃)₅NO₂](NO₃)Cl linkage isomers and their relative stabilities were examined in gas phase and in solid state and confirmed the nitro-complex as the most stable following by a viable intermediate endo-complex. Study of the nitro-nitrito linkage isomerization in [Co(NH₃)₅NO₂](NO₃)Cl revealed that anions form strong electrostatic bonds with [Co(NH₃)₅NO₂]²⁺ leading to decrease in an activation energy compared to the [Co(NH₃)₅ONO]²⁺ isomers. A concerted action of ionic interactions and hydrogen bonds are suspected of regulating the isomerization in solid state. Assessment of various DFT methods with respect to CCSD(T) suggests M062X suitable method for [Co(NH₃)₅NO₂](NO₃)Cl linkage-isomerization study. Potential energy surface calculations at the CASSCF/6-31G(d) level of theory shows that the conical intersection (S₁/S_o) might play an important role in the photoisomerization of [Co(NH₃)₅NO₂](NO₃)Cl.     

Ion Association and Solvation Behavior of Some 1-1 Electrolytes in 2-Ethoxyethanol Probed by a Conductometric Study

Precise measurements of the electrical conductances of solutions of potassium thiocyanate (KCNS), ammonium thiocyanate (NH₄CNS), sodium nitrate (NaNO₃) and ammonium nitrate (NH₄NO₃) in 2-ethoxyethanol (EE) at temperatures 35, 40, 45 and 50,^∘C are reported. The conductance data have been analyzed by the 1978 Fuoss conductance equation. A thermodynamic analysis of the ionic association processes has also been made and the Coulombic forces are found to play a major role in the association processes. The ionic contributions to the limiting equivalent conductances have been determined using the reference electrolyte method. Strong association was found for all these electrolytes in this solvent medium. The cations are found to be substantially solvated in 2-ethoxyethanol, whereas the anions appear to have only weak interaction with the solvent molecules.     

Carbon-13 nuclear magnetic resonance examination of benzonorbornene derivatives. Assignment of site of aromatic ring substitution in benzonorbornen-2-ones

¹³C nuclear magnetic resonance spectra were collected for a series of 5- and 6-monoaromatic ring-substituted benzonorbornadienes and 5-, 6-, 7-, and 8-monosubstituted benzonorbornen-2-ones. ¹³C chemical shifts values were found to be useful in the differentiation of the four monosubstituted aromatic ring isomers of benzonorbornen-2-one (1). We found that the ¹³C NMR spectrum of a substituted benzonorbornen-2-one (A) could be predicted with good accuracy (R² for each aromatic carbon ≥ 0.925) from a knowledge of the ¹³C NMR spectrum of 1 and the appropriately substituted benzene. The substituents studied were NO₂, NH₂, I, CF₃, CN, OCH₃, and H. Correlation analysis showed that the carbonyl in A was effectively insulated from the ring π-system.     

Fluorine Kα x-ray emission studies of fluorinated organic compounds

Fluorine K_α X-ray emission spectra have been measured and interpreted using UV photoelectron and X-ray photoelectron spectral data and the results of quantum-chemical calculations, for a series of fluorine-containing organic molecules: CH₃F, n-C₅F₁₂, polytetrafluoroethylene, tetrafluoroethylene, 4-XC₆H₄F (X = H, F, NH₂, NO₂), 1,3-difluorobenzene, 1,2,4,5-tetrafluorobenzene, 1,4-difluorobenzene, C₆F₅X (X = H, F, SCH₃, OCH₃, CN, NO₂, C₆F₅, P(OCH₃)₂), pentafluoropyridine, octafluoronaphthalene and 2,4-dinitrofluorobenzene, all in solid or gaseous states. It has been concluded that the fluorine 2pAO contribution to the highest occupied π-orbitals of the benzene ring and π-orbital of the ethylene bond is small: it is somewhat higher for a system of lower-lying π-orbitals and the highest for σ-orbitals. CH₃F is assumed to have hyperconjugation.     

Is Our Way of Thinking about Excited States Correct? Time‐Resolved Dispersive IR Study on p‐Nitroaniline

Low‐lying excited electronic states of an important class of molecules known as push–pull chromophores are central to understanding their potential nonlinear optical properties. Here we report that a combination of high‐sensitivity nanosecond time‐resolved dispersive IR spectroscopy and DFT calculations on p‐nitroaniline (PNA), a prototypical push–pull molecule, reveals that PNA in the lowest excited triplet state has a partial quinoid structure. In this structure, the quinoid configuration is restricted to a part of the phenyl ring adjacent to the NO₂ group. The partial quinoid structure of PNA cannot be explained by a commonly used hybrid of a neutral form and a zwitterionic charge‐transfer form. Our findings not only cast doubt on the general applicability of the classical way of looking at excited states, based exclusively on characteristic resonance structures, but also provide deeper insights into excited‐state structure of highly polarizable molecular systems.     

Water Activities, Osmotic and Activity Coefficients and Some Correlation of Aqueous Mixtures of Alkaline-Earth and Ammonium Nitrates, NH4NO3-Y(NO3)2-H2O with Y ≡ Ba2+, Mg2+ and Ca2+ at T = 25 °C

The thermodynamic properties of the mixed aqueous electrolyte of ammonium and alkaline earth metal nitrates have been studied using the hygrometric method at 25?°C. The water activities of these {yNH₄NO₃+(1?y)Y(NO₃)₂}(aq) systems with Y ≡ Ba²⁺, Mg²⁺ and Ca²⁺ were measured at total molalities ranging from 0.10 mol?kg^?1 to saturation for different NH₄NO₃ ionic-strength fractions of y=0.20, 0.50 and 0.80. These data allow the calculation of osmotic coefficients. From these measurements, the ionic mixing parameters are determined and used to calculate the solute activity coefficients in the mixtures at different ionic-strength fractions. The results of these ternary solution measurements are compared with those for binary solutions of the alkaline earth nitrates of magnesium, calcium and barium with ammonium nitrates. The behavior of the aqueous electrolyte solutions containing mixtures of barium or calcium or magnesium with ammonium nitrates are correlated and show that ionic interactions are more important for the system containing Mg²⁺ than for Ca²⁺ or Ba²⁺. The trends are mainly due to the effects of the ionic size, polarizability and the hydration of the ions in these solutions.     

Ethylene polymerization by sterically and electronically modulated Ni(II) α‐diimine complexes

A series of highly active ethylene polymerization catalysts based on bidendate α‐diimine ligands coordinated to nickel are reported. The ligands are prepared via the condensation of bulky ortho‐substituted anilines bearing remote push–pull substituents with acenaphthenequinone, and the precatalysts are prepared via coordination of these ligands to (DME)NiBr₂ (DME = 1,2‐dimethoxyethane) to form complexes having general formula [ZN = C(An)‐C(An) = NZ]NiBr₂ [Z = (4‐NH₂‐3,5‐C₆H₂R₂)₂CH(4‐C₆H₄Y); An, acenaphthene quinone; R, Me, Et, iPr; Y = H, NO₂, OCH₃]. When activated with methylaluminoxane (MAO) or common alkyl aluminiums such as ethyl aluminium sesquichloride (EAS) all catalysts polymerize ethylene with activities exceeding 10⁷ g‐PE/ mol‐Ni h atm at 30 °C and atmospheric pressure. Among the cocatalysts used EAS records the best activity. Effects of remote substituents on ethylene polymerization activity are also investigated. The change in potential of metal center induced by remote substituents, as evidenced by cyclic voltammetric measurements, influences the polymerization activity. UV–visible spectroscopic data have specified the important role of cocatalyst in the stabilization of nickel‐based active species. A tentative interpretation based on the formation of active and dormant species has been discussed. The resulting polyethylene was characterized by high molecular weight and relatively broad molecular weight distribution, and their microstructure varied with the structure of catalyst and cocatalyst. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1066–1082, 2008     

Synthesis,Characterization, and Crystal Structures of Cu,Ag, and Pd Dinitramide Salts

The literature known, but not fully characterized, silver dinitramide transfer reagents AgN(NO₂)₂ ( 1 ), [Ag(NCCH₃)][N(NO₂)₂] ( 2 ), and [Ag(py)₂][N(NO₂)₂] ( 3 ) have been investigated by ¹⁰⁹Ag, ¹⁴N NMR and vibrational spectroscopy (IR, Raman). In addition, the poorly understood [Cu(NH₃)₄][N(NO₂)₂)]₂ ( 4 ) and [Pd(NH₃)₄][N(NO₂)₂]₂, ( 5 ) have also been prepared and characterized by ¹⁴N NMR and vibrational spectroscopy (IR, Raman). The structures of 2 — 5 have also been determined by X‐ray diffraction.     

Structure and thermodynamic properties of nitroformic acid,nitroformyl nitrate,and ammonium nitroformate

As part of a series of investigations of molecules with large numbers of nitro groups, the structures and properties of nitroformic acid, NO₂CO₂H, nitroformyl nitrate, NO₂C(O)ONO₂, and the ion pair ammonium nitroformate, NH₄NO₂CO₂, have been determined using GX theories (X = 2, 3, 4). Minimum-energy structures, vibrational frequencies, selected bond energies, and enthalpies of formation were determined. The title substances may not have much application as high energy materials like other high-nitro compounds, but they may act as good nitrating agents since certain bonds in the molecules are of low enough energy to suggest high reactivity.     

AIM and NPA studies of the role of polarization in electronic structures

Atomic charges, as measured by Atoms in Molecule (AIM) or Natural Population Analyses (NPA), of the enolate anions of acetaldehyde and crotonaldehyde and of pentadienyl anion and cation show both charge transfer and polarization effects. In general, normal resonance structures and "curved arrow" symbolism give good representations of π-electron distributions, but back-polarizations in the σ-system complicate these electronic structures and can obscure the correspondence to resonance symbols. Twisting a vinyl group to orthogonality disrupts the π-system, but the vinyl group retains significant charge transfers and polarizations. The role of polarization is also demonstrated by the effect of external positive and negative charges on the electronic structure of ethylene. The π-electronic changes again are straightforward but are changed significantly by σ-polarizations. The polarizability of ethane is about half that of ethylene.     

Synthesis and characterization of push–pull organic semiconductors with various acceptors for solution-processed small molecule organic solar cells

New efficient push–pull organic semiconductors comprising of the bis(9,9-dimethyl-9H-fluoren-2-yl)aniline (bisDMFA) donor and the various acceptors such as NO₂, DCBP, and TCF, which were linked with bithiophene or vinyl bithiophene π-conjugation bridges, were synthesized, and their photovoltaic characteristics were investigated in solution-processed small molecule organic solar cells (SMOSCs). The intramolecular charge transfers of these materials were effectively appeared in between bisDMFA donor and acceptors, depending on the electron-withdrawing strength of acceptors. The organic semiconductors having NO₂ and DCBP acceptors exhibited the most efficient photovoltaic performance, showing power conversion efficiency (PCE) of 1.98% (±0.17) and 2.01% (±0.21), respectively. When the TiO_x thin layer was treated on photoactive layer, the organic semiconductor having NO₂ showed the best PCE of 2.70% with short circuit current of 8.19 mA/cm², fill factor of 0.40, and open circuit voltage of 0.83 V in SMOSC devices.     

Ceric ammonium nitrate (CAN) as oxidizing or nitrating reagent for organic reactions in ionic liquids

The reaction of ceric ammonium nitrate, (NH₄)₂[Ce(NO₃)₆] or CAN, with naphthalene and 2-methylnaphthalene in the ionic liquid 1-ethyl-3-methylimidazolium triflate showed that the reaction products are strongly dependent on the water content of the ionic liquid and that cerium(IV) in the ionic liquid can electrochemically be regenerated.     

Density Functional Theory Studies on the Adsorption of NH2NO2 on Al13 Cluster

Density functional theory method with full geometry optimization was used to study the adsorption of nitroamine (NH₂NO₂) on Al₁₃ cluster. Both dissociative and nondissociative adsorption structures were predicted with different NH₂NO₂ molecule orientations on Al₁₃ cluster surfaces. In dissociative chemisorption, the main decomposition products of NH₂NO₂ are O atom(s) and NH₂NO or NH₂N species. The O atoms being ruptured from the N?CO bond form strong Al?CO bonds with the neighboring Al around the adsorbed sites. In addition, the species obtained as a result of O atom elimination remains bonded to the surface. The largest adsorption energy is ?737.66?kJ/mol when the NH₂NO₂ molecule decomposes into two O atoms and a NH₂N fragment. For nondissociative adsorption, the seriously deformed nitroamine forms various N?CO?CAl bonding configurations with Al. The significant charge transfer occurs for all adsorption configurations. The most charge transfer is 2.068 e from the Al cluster surface to the fragments of the decomposed NH₂NO₂. The change of the electronic structures is obvious due to the adsorption or dissociation of NH₂NO₂ molecule. Nitroamine readily oxidizes the aluminum surface of the Al₁₃ cluster.     

Synthesis and characterization of [Co(NH3)5NO2][M(NO2)4] (M = Pt, Pd) compounds and their thermolysis products

Compounds [Co(NH₃)₅NO₂][Pd(NO₂)₄] (I) and [Co(NH₃)₅NO₂][Pt(NO₂)₄] · 1.5H₂O (II) have been crystallized from solution. Their crystal structures have been solved, and thermolysis under various conditions studied. The thermolysis products are Co_0.5M_0.5 ordered solid solutions.     

Coordination Chemistry of Acrylamide 4. Crystal Structures and IR Spectroscopic Properties of Acrylamide Complexes with CoII,NiII, and ZnII nitrates

Acrylamide complexes of metal nitrates: [M(O‐OC(NH₂)CHCH₂)_n(H₂O)_m][NO₃]₂ (M = Co( 1 ), Ni( 2 ) (n = 6 and m = 0) and Zn( 3 ) (n = 4 and m = 2)) have been determined by using single crystal X‐ray diffraction analysis. All complexes crystallize in the triclinic space group . The structures of 1 and 2 represent octahedral species [M(AAm)₆]²⁺ (AAm = O‐OC(NH₂)CHCH₂ and M = Co or Ni) and uncoordinated nitrate ions. The structure of 3 involves the octahedral cation [Zn(AAm)₄(H₂O)₂]²⁺ in which the Zn²⁺ environment includes oxygen atoms of four acrylamide and two water molecules that are stabilized using ionic nitrate ions. The observations of the solid‐state IR spectroscopic vibrational frequencies of these acrylamide complexes are in agreement with the crystal structures.     

N‐Méthyl succinimide

The molecular structure of 1‐methylpyrrolidine‐2,5‐dione, C₅H₇NO₂, corresponds to the dicarbonyl tautomer with an envelope ring conformation. The packing is stabilized by weak intermolecular hydrogen bonds and presents push–pull nucleophile–electrophile interactions of the carbonyl groups.     

A DICD (dispersion-induced circular dichroism) and normal absorption study of the n → π* carbonyl transition for the isoelectronic substituent series −CH3, −NH2, −OH

An experimental DICD (dispersion-induced circular dichroism) and parallel normal absorption study of the lowest n → π* transition of the carbonyl chromophore in simple carbonyls of the form R₁R₂CO for the isoelectronic substituent series R_i = −CH₃, −OH and −NH₂ is presented. The results indicate that, contrary to conventional expectations, the energetic position of the transition is progressively shifted to lower energy for the above substituent ordering. The weakness of the absorption bands in acetic acid and urea provides a rationale for why these bands have not been previously reported. The results suggest that the relative shift is ascribable to resonance (through space) coupling between the substituent and the carbonyl π-system.     

Crystal structures of [Pd(NH3)3(NO2)][Rh(NH3)2(NO2)4] and [PdEn2][Rh(NH3)(NO2)5]·0.75H2O

The structure of double complex salts [Pd(NH₃)₃(NO₂)][Rh(NH₃)₂(NO₂)₄] and [PdEn₂][Rh(NH₃)(NO₂)₅]·0.75H₂O is determined by single crystal X-ray diffraction. In the structures, the main structural moieties are identified.