Effect of nonproximate atomic substitution on excited state intramolecular proton transfer

The central C atom of the OCCCO skeleton of the malonaldehyde molecule is replaced by N, and the effects upon the intramolecular H-bond and the proton transfer are monitored by ab initio calculations in the ground and excited electronic states. The H-bond is weakened in the singlet and triplet states arising from n→π* excitation in both molecules, which is accompanied by a heightened barrier to proton transfer.³ππ* behaves in the same manner, but the singlet ππ* state has a stronger H-bond and lower barrier. Replacement of the central C atom by N strengthens the intramolecular H-bond. Although the proton transfer barrier in the ground state of formimidol is lower than in malonaldehyde, the barriers in all four excited states are higher in the N-analog. The latter substitution also dampens the effect of the n→π* excitation upon the H-bond and increases the excitation energies of the various states, particularly ππ*. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 129–138, 1998     

Anion-(π)n-π Catalytic Micelles

Anion-π catalysis operates by stabilizing anionic transition states on π-acidic aromatic surfaces. In anion-(π)_n-π catalysis, π stacks add polarizability to strengthen interactions. In search of synthetic methods to extend π stacks beyond the limits of foldamers, the self-assembly of micelles from amphiphilic naphthalenediimides (NDIs) is introduced. To interface substrates and catalysts, charge-transfer complexes with dialkoxynaphthalenes (DANs), a classic in supramolecular chemistry, are installed. In π-stacked micelles, the rates of bioinspired ether cyclizations exceed rates on monomers in organic solvents by far. This is particularly impressive considering that anion-π catalysis in water has been elusive so far. Increasing rates with increasing π acidity of the micelles evince operational anion-(π)_n-π catalysis. At maximal π acidity, autocatalytic behavior emerges. Dependence on position and order in confined micellar space promises access to emergent properties. Anion-(π)_n-π catalytic micelles in water thus expand supramolecular systems catalysis accessible with anion-π interactions with an inspiring topic of general interest and great perspectives.     

Theoretical study of π-complexes Ti(P)L of titanium porphyrin with ligands containing multiple bonds C-C,C-N,and N-N

The structural parameters, energies, and spectroscopic characteristics of singlet and triplet titanium porphyrin π-complexes Ti(P)(π-L) (P = C₂₀H₁₂N₄) with the axial ligands L = C₂H₂, C₂H₄, N₂H₂, HCN, C₆H₆, N₂, and C₆₀ coordinated to the Ti atom through C-C, C-N, and N-N multiple bonds have been calculated by the density functional theory B3LYP method. The changes in the calculated properties of the π-complexes as compared with the properties of the isolated (uncoordinated) Ti(P) and L molecules have been examined. The activation of multiple bonds on coordination to the titanium atom is manifested in (i) their sharp weakening and elongation by 0.10–0.20 Å or more, (ii) a long-wavelength shift of their stretching modes by 300-500 cm^-1 or more, (iii) considerable electron density transfer from the porphyrin ring (P ring) to the ligand and the corresponding ligand distortion and polarization, (iv) a strong displacement (0.5-6 Å) of the Ti atom from the P ring plane toward the π-ligand and the dome distortion of the P ring. For the Ti(P)(π-L) systems, the addition of the second axial π-ligand to form six-coordinate ππ-complexes is not typical. In the Ti(P)(π-L)₂ with identical ligands, the Ti atom is strongly displaced out of the P ring plane toward one of the ligands and the second ligand is repulsed from the P ring and actually removed from the metal coordination sphere. In the Ti(P)(π-L) (π-L’) complexes with different ligands, according to the relative strength of the Ti–L and Ti-L’ bonds, which decreases in the series N₂H₂ > C₂H₂ > HCN > C₆₀ > C₂H₄ > C₆H₆ > N₂, the weaker ligand is forced out by the stronger ligand (acetylene is pushed out of the coordination sphere by diimine; ethylene, by acetylene and fullerene; fullerene, by hydrogen cyanide; etc.). In mixed πσ-complexes Ti(P)(π-L)(CO) in the singlet state, acetylene pushes out the CO group; conversely, in the triplet state, acetylene is pushed out by carbonyl. There is a trend in the behavior of the activation effects along the series of the above ligands and with a change in the electronic state multiplicity of the complexes.     

Synthesis of a New Cavitand. 2 C60 Complex

Abstract

A new cavitand 2 and its complexation with fullerene to afford complex 3 was prepared and characterized by spectroscopic methods. Macrocycle 2 was studied in solution by NMR, and in the solid state by ¹³C CP-MAS, NMR and X-ray diffraction. The macrocycle 2 can host 2 fullerene C₆₀ molecules in its structure. For the complex 3, π-π, CH-π and n-π interactions were observed by ¹³C CP-MAS and FTIR spectroscopy. MM and MD calculations were carried out.     

On the correlation between photoelectron and electronic spectra in trans- azomethane

A possibility of correlating electronic and photoelectron spectra is discussed, using trans-azomethane as an example. The Coulomb and exchange integrals required were obtained by three semi-empirical SCF-methods: MINDO/2, CNDO/2, and a modified CNDO method. The orbital energies were taken as minus the corresponding experimental ionization potentials. The sequence of the transition energies ΔE (n_s → π*) Δ E (n_a → π*) < ΔE (π → π*) is found to be different from the ionization potential sequence IP (n_s) < IP (π) < IP (n_a), in agreement with previous spectroscopic studies; the results support the latest view that the π → π* transition of the azo group occurs at around 12 eV.     

Molecular orbital theory of the hydrogen bond. 25. Water–uracil complexes in excited n → π states

Hydrogen bonding of uracil with water in excited n → π* states has been investigated by means of ab initio SCF -CI calculations on uracil and water–uracil complexes. Two low-energy excited states arise from n → π* transitions in uracil. The first is due to excitation of the C₄? O group, while the second is associated with excitation of the C₂? O group. In the first n → π* state, hydrogen bonds at O₄ are broken, so that the open water–uracil dimer at O₄ dissociates. The “wobble” dimer, in which a water molecule is essentially free to move between its position in an open structure at N₃? H and a cyclic structure at N₃? H and O₄ in the ground state, collapses to a different “wobble” dimer at N₃? H and O₂ in the excited state. The third dimer, a “wobble” dimer at N₁? H and O₂, remains intact, but is destabilized relative to the ground state. Although hydrogen bonds at O₂ are broken in the second n → π* state, the three water–uracil dimers remain bound. The “wobble” dimer at N₁? H and O₂ changes to an excited open dimer at N₁? H. The “wobble” dimer at N₃? H and O₄ remains intact, and the open dimer at O₄ is further stabilized upon excitation. Dimer blue shifts of n → π* bands are nearly additive in 2:1 and 3:1 water:uracil structures. The fates of the three 2:1 water:uracil trimers and the 3:1 water:uracil tetramer in the first and second n → π* states are determined by the fates of the corresponding excited dimers in these states.     

Constructing a novel nonlinear optical materials: substituents and heteroatoms in π-π systems effect on the first hyperpolarizability

By doping π-π systems with Li atom, a series of Li@sandwich configuration and Li@T-shaped configuration compounds have been theoretically designed and investigated using density functional theory. It is revealed that energy gaps (E _gap) between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of all compounds are in a range of 0.4–0.9 ev. When Li atom is introduced into different sandwich configuration π-π systems (C₆₀-toluene, C₆₀-fluorobenzene, C₆₀-phenol, C₆₀-benzonitrile), Li@C₆₀-benzonitrile exhibits considerable first hyperpolarizability as large as 19,759 au, which is larger by about 18,372–18,664 au than those of other compounds. When Li atom is introduced into different T-shaped configuration π-π systems (C₆₀-pyridine, C₆₀-pyrazine, C₆₀-1, 3, 5-triazine, C₆₀-pyridazine), Li@C₆₀-pyridazine is found to present largest first hyperpolarizability up to 67,945 au in all compounds. All compounds are transparency in the deep ultraviolet spectrum range. We hope that this study could provide a new idea for designing nonlinear optical materials using π-π systems as building blocks.     

Phosphine-Manipulated p-π and π-π Synergy Enables Efficient Ultralong Organic Room-Temperature Phosphorescence

Organic room temperature phosphorescence (RTP) attracts extensive attentions, but still faces the challenge of achieving both high RTP efficiencies (η_RTP) and long lifetimes (τ_RTP), due to the intrinsic contradiction between triplet radiation and stabilization. In this work, we developed three carbazole-triphenylphosphine hybrids named xCzTPP, in which phosphine groups provide nonbonding electrons and steric hindrance to modulate intermolecular p-π and π-π interactions. With the rational orientations and spatial positions of functional groups, para-substituted pCzTPP achieves high η_RTP over 10 % and more than twofold increased τ_RTP (>600 ms), compared to ortho- and meta- isomers. Theoretical simulation and photophysical investigation indicate that the strongest intermolecular p-π and π-π electronic interplays of pCzTPP harmonize high transition probability of ³pπ state and triplet stability of ³ππ state, reflecting the p-π and π-π synergy in RTP process.     

Acetato(N‐phenylpyridine‐2‐carboxamidato‐κ2N,N)(N‐phenylpyridine‐2‐carboxamide‐κ2N1,O)copper(II)

The title complex, [Cu(C₁₂H₉N₂O)(C₂H₃O₂)(C₁₂H₁₀N₂O)], is a neutral Cu^II complex with a primary N₃O₂ coordination sphere. The Cu centre coordinates to both a deprotonated and a neutral molecule of N‐phenylpyridine‐2‐carboxamide and also to an acetate anion. The coordination around the metal centre is asymmetric, the deprotonated ligand providing two N donor atoms [Cu—N = 1.995 (2) and 2.013 (2) Å] and the neutral ligand providing one N and one O donor atom to the coordination environment [Cu—N = 2.042 (2) Å and Cu—O = 2.2557 (19) Å], the fifth donor being an O atom of the acetate ion [Cu—O = 1.9534 (19) Å]. The remaining O atom from the acetate ion can be considered as a weak donor atom [Cu—O = 2.789 (2) Å], conferring to the Cu complex an asymmetric octahedral geometry. The crystal structure is stabilized by intermolecular N—H...O, C—H...O and C—H...π interactions.     

Selective Population of the n, π* and π, π* Triplet States of 1-Indanones

The two components of the dual phosphorescence of 1-indanone ( 1 ) and six related ketones ( 2–7 ) possess different excitation spectra exhibiting the vibrational progression characteristic of the S₀ → S₁ (n, π*) transition (shorter-lived emission) and two bands of the S₀ → S_{2 and 3} (π,π*) 0–0 transitions, respectively. The most favorable intersystem crossing routes are S₁ (n, π*) → T (n, π*) and S_2,3 (π*) → T (π, π*). Internal conversion to S₁ competes more effectively with S (π, π*) → T (π, π*) intersystem crossing only from higher vibrational levels of the S₂ and S₃ states.     

Di­chloro­[N,N,N′,N′-tetrakis(2-pyridyl­methyl)­benzene-1,4-di­amine]­iron(II)

In the title complex, [FeCl₂(C₃₀H₂₈N₆)], the Fe atom is five-coordinated by two terminal chloride ligands and one end of the bis-tridentate ligand. The complexes display intermolecular C—H⃛π, π-stacking and C—H⃛X (X = N, Cl) interactions.     

Ab initio calculations on urea

Multiconfiguration wave functions constructed from contracted Gaussian-lobe functions have been found for the ground and valence-excited states of urea. ICSCF molecular orbitals of the excited states were used as the parent configurations for the CI calculations except for the ¹A₁(π → π*) state. The ¹A₁(π → π*) state used as its parent configuration an orthogonal linear combination of natural orbitals obtained from the second root of a three-configuration SCF calculation. The lowest excited states are predicted to be the n π → π* and π → π* triplet states. The lowest singlet state is predicted to be the n π → π* state with an energy in good agreement with the one known UV band at 7.2 eV. The π → π* singlet state is predicted to be about 1.9 eV higher, contrary to several previous assignments which assumed the lowest band was a π → π* amide resonance band. The predicted ionization energy of 9.0 eV makes this and higher states autoionizing.     

A DFT study of Lp···π/halogen bond competition in complexes of perhalogenated alkenes with oxygen/nitrogen containing simple molecules

The possible noncovalent lone pair‐π/halogen bond (lp···π/HaB) complexes of perhalogenated unsaturated C₂Cl_nF_4?n (n = 0–4) molecules with four simple molecules containing oxygen or nitrogen as electron donor, formaldehyde (H₂CO), dimethyl ether (DME), NH₃, and trimethylamine (TMA), have been systematically examined at the M062X/aug‐cc‐pVTZ level. Natural bond orbital (NBO) analysis at the same level is used for understanding the electron density distributions of these complexes. The progressive introduction of Cl atom on C₂Cl_nF_4?n influences more on the lp···π complexes over the corresponding HaB ones. Within the scope of this study, gem‐C₂Cl₂F₂ is the best partner molecule for lp···π interaction with the simple molecules, coupled with the greatest interaction energy (IE) and second‐order orbital interaction [E(2) value], whereas C₂F₄ is the poorest one. The C₂Cl₃F·H₂CO and C₂Cl₄·H₂CO complexes exhibit reverse lp···π bonding, while the Z/E‐C₂Cl₂F₂·NH₃, C₂Cl₃F·NH₃ and C₂Cl₄·NH₃ complexes perform half‐lp···π bonding according to the NBO analysis. The lp···π interaction involving the oxygen/nitrogen and the π‐hole of C₂Cl_nF_4?n overwhelms the HaB involving the oxygen/nitrogen and the σ‐hole of the Cl atom. The electron‐donating methyl groups contribute significantly to the two competitive interactions, therefore, DME and TMA engage stronger in the partner molecules than H₂CO and NH₃. Our theoretical study would be useful for future experimental investigation on noncovalent complexes. © 2016 Wiley Periodicals, Inc.     

π* ← n and π* ← π absorption spectra of aminopyrazine

π* ← n and π* ← π absorption spectra of aminopyrazine have been recorded and analysed assuming C_s symmetry for the molecule.     

catena‐Poly[[silver(I)‐μ‐[(E)‐1,2‐bis(2‐pyridyl)ethylene‐N:N′]] nitrate]

In the title complex, {[Ag(C₁₂H₁₀N₂)]NO₃}_n, the Ag atom, which is in a linear AgN₂ geometry, is surrounded by two trans‐related N atoms of two bpe ligands [Ag—N = 2.173 (3) and 2.176 (3) Å; bpe is trans‐1,2‐bis(2‐pyridyl)­ethyl­ene]. The bpe ligands bridge neighbouring Ag atoms to form zigzag polymeric chains in the lattice. These adjacent one‐dimensional zigzag chains are extended into a three‐dimensional supramolecular array by strong interchain π?π interactions between the pyridyl rings of adjacent chains.     

Tricarbonyl(η5‐formylcyclopentadienyl)manganese(I) and tricarbonyl(η5‐formylcyclopentadienyl)rhenium(I) containing short π(CO)...π(CO) and π(CO)...π interactions

The structures of tricarbonyl(formylcyclopentadienyl)manganese(I), [Mn(C₆H₅O)(CO)₃], (I), and tricarbonyl(formylcyclopentadienyl)rhenium(I), [Re(C₆H₅O)(CO)₃], (II), were determined at 100 K. Compounds (I) and (II) both possess a carbonyl group in a trans position relative to the substituted C atom of the cyclopentadienyl ring, while the other two carbonyl groups are in almost eclipsed positions relative to their attached C atoms. Analysis of the intermolecular contacts reveals that the molecules in both compounds form stacks due to short attractive π(CO)...π(CO) and π(CO)...π interactions, along the crystallographic c axis for (I) and along the [201] direction for (II). Symmetry‐related stacks are bound to each other by weak intermolecular C—H...O hydrogen bonds, leading to the formation of the three‐dimensional network.     

Photoelectron spectra of the allyl amines

The Hel photoelectron spectra of the three allyl amines show low ionization potential bands arising from nitrogen lone-pair n electrons and ethylenic π-bond electrons. Analysis of the spectra using MINDO/3 calculations and comparisons with ionization data of related molecules shows that π-π interactions are considerable, but n-π interactions are small. The π-π splitting in triallyl amine is consistent with a near-planar C₃ symmetry structure for the gas phase molecule.     

Tunable Electronic Properties of a Proton‐Responsive N,N‐Dimethylaminoazobenzene Fullerene (C60) Dyad

A basic N,N‐dimethylaminoazobenzene–fullerene (C₆₀) dyad molecular skeleton is modelled and synthesized. In spite of the myriad use of azobenzene as a photo‐ and electrochromic moiety, the idea presented herein is to adopt a conceptually different path by using it as a bridge in a donor–bridge–acceptor single‐molecular skeleton, connecting the electron acceptor N‐methylfulleropyrrolidine with an electron donor N,N‐dimethylaniline. Addition of trifluoroacetic acid (TFA) results in a drastic colour change of the dyad from yellow to pink in dichloromethane (DCM). The structure of the protonated species are established from electronic spectroscopy and time‐dependent density functional theory (TD‐DFT) calculations. UV/Vis spectroscopic investigations reveal the disappearance of the 409 nm ¹(π→π*) transition with appearance of new features at 520 and 540 nm, attributed to protonated β and α nitrogens, respectively, along with a finite weight of the C₆₀ pyrrolidinic nitrogen. Calculations reveal intermixing of n_(N?N)→π*_(N?N) and charge transfer (CT) transitions in the neutral dyad, whereas, the n_(N?N)→π*_(N?N) transition in the protonated dyad is buried under the dominant ¹(π →π*) feature and is red‐shifted upon Gaussian deconvolution. The experimental binding constants involved in the protonation of N,N‐dimethylanilineazobenzene and the dyad imply an almost equal probability of existence of both α‐ and β‐protonated forms. Larger binding constants for the protonated dyads imply more stable dyad complexes than for the donor counterparts. One of the most significant findings upon protonation resulted in frontier molecular orbital (FMO) switching with the dyad LUMO located on the donor part, evidenced from electrochemical investigations. The appearance of a new peak, prior to the first reduction potential of N‐methylfulleropyrrolidine, clearly indicates location of the first incoming electron on the donor‐centred LUMO of the dyad, corroborated by unrestricted DFT calculations performed on the monoanions of the protonated dyad. The protonation of the basic azo nitrogens thus enables a rational control over the energetics and location of the FMOs, indispensable for electron transport across molecular junctions in realizing futuristic current switching devices.     

Hydrothermal synthesis, crystal structure and photoluminescence of [HgCl2(C6NO2H5)]nn[HgCl2]n(C6NO2H5)

The first example of isonicotinic acid compounds with infinite mercury halide chains, [HgCl₂(C₆NO₂H₅)]_n n[HgCl2]n(C₆NO₂H₅) (1), was synthesized through hydrothermal reactions and structurally characterized by X-ray single crystal diffraction. Compound 1 features a one-dimensional (1-D) motif, based on infinite 1-D [HgCl₂(C₆NO₂H₅)]_n chains, neutral HgCl₂ moieties and isolated isonicotinic acid molecules. The [HgCl₂(C₆NO₂H₅)]_n chains, HgCl₂ moieties and isonicotinic acid molecules are interlinked by hydrogen bonds and π-π interactions to give a two-dimensional supramolecular layer. Photoluminescent investigation reveals that the title compound exhibits a strong emission in blue region. The emission band is identified as the π -π* transitions of the isonicotinic acid moieties.     

Structural aspects of two α‐dihydrazones displaying a complete survey of intermolecular interactions

The compounds (2′E,2′E)‐2,2′‐(propane‐1,2‐diylidene)bis[1‐(2‐nitrophenyl)hydrazine], C₁₅H₁₄N₆O₄, (I), and (2Z,3Z)‐ethyl 3‐[2‐(2‐nitrophenyl)hydrazinylidene]‐2‐[2‐(4‐nitrophenyl)hydrazinylidene]butanoate tetrahydrofuran hemisolvate, C₁₈H₁₈N₆O₆·0.5C₄H₈O, (II), are puzzling outliers deviating from a general synthetic route aimed at the preparation of substituted pyrazoles. Possible reasons for this outcome, which is exceptional in an otherwise firmly established synthetic procedure, are analyzed. Compound (I) is unsolvated, while compound (II) crystallizes with a tetrahydrofuran solvent molecule lying on an inversion centre. The ethoxycarbonyl chain of (II), in turn, appears disordered into two equally populated (50%) moieties. In both structures, a plethora of different commonly occurring weak intermolecular interactions [viz. π(phenyl)...π(phenyl), π(C=N)...π(C=N), π(phenyl)...π(C=N), N—H...O and C—H...O] appear responsible for the crystal stability. Much less common are the short O(nitro)...O(nitro) contacts which are observed in the structure of (I), an example of unusual `electron donor–acceptor' (EDA) interactions.