Reversible isomerization of chromogenic dimers and the nature of azulene chromaticity

The spectra of azulene solutions in siloxane and heptadecane were studied. The VIS band (435–714 nm) associated with the color blue was shown to transform reversibly as a result of a temperature increase and to assume a likeness to the less intensive VIS band typical for solutions in polar ethanol and dibutyl phthalate. It was concluded that supramolecular dimers rather than individual molecules are the chromogens of azulene, just as in the case of phthalocyanine, triphenylmethane, xanthene, and thiazine dyes. Clar’s conclusions regarding the nature of the visible band of azulene absorption and the mechanism of reversible thermal discoloration of azulene solutions in high-boiling-point hydrocarbons are consequently wrong. It was established that the thermochromism of azulene solutions is actually not associated with the isomerization of azulene molecules into nonplanar nonaromatic molecules but with the reversible isomerization of dimeric structures, accompanied by a change in the positions of the aromatic molecules relative to one another. The corresponding equilibrium is somewhat endothermic: ΔH = 6.6 kJ/mol, ΔG ^o ≈ − 1.45 kJ/mol. Based on these results, it was concluded that light absorption changes the energetic state of the dimeric structures (transition S _D→ S _D^*) without leading to electron transfer in single molecules according to the scheme S ₀ →S ₁. That is, the traditional idea that Kasha’s law is violated when azulene fluoresces is incorrect, since the radiation comes from the S ₁ level and not from the S ₂ level, as is stated in the literature.     

Mechanism of the protonation of azulenes in aqueous solutions of acids

In aqueous solutions of strong acids (H₂SO₄, H₃PO₄, and HCl) containing azulene, the fast reversible protonation of azulene is accompanied by the slow formation of a disperse dark violet dye insoluble in acids, alcohol, and heptane. On the basis of the kinetic specifics of this reaction and the nonlinear (nearly reciprocal quadratic) dependence of the concentrations ratio of their cationic and neutral forms on the Hammett acidity function known for azulene and 14 of its derivatives, azulene is shown not to be a Hammett base. A mechanism for the reversible reactions of the azulenium cation is proposed that considers supramolecular dimers to be the basic state of azulene and its derivatives. The scheme includes reactions of the unstable intermediate π complexes formed from the dimers and hydrated hydrogen cations; the complexes quickly dissociate in the opposite direction and react with the hydrated protons to yield azulenium cations and unstable molecules that induce polymerization of the dimers.     

The structure of dimers and the nature of azulene chromaticity

The structure of supramolecular azulene dimers responsible for the blue coloration of its crystals and solutions has been discussed on the basis of result of optical spectroscopy and literature data. It is established that two types of these dimers (I and II) absorb the light in the red region of the visible (VIS) spectrum and differ by the mutual orientation of the molecules. Dimers I have a VIS band with a vibronic structure; the azulene molecules in dimers I match their own seven-membered rings in type (stacking structure), and five-membered rings of the molecules are separated so that the molecular C _2v axes form an obtuse angle. The spectra of dimers II do not have a vibronic structure in the VIS band. The dipole moments of the molecules in these dimers are oriented antiparallel (five-membered rings are located over (or under) seven-membered rings, forming a structure with a center of inversion). It is concluded that due to their structure, dimers I should have a certain dipole moment, while dimers II have no dipole moment.     

The nature of chromaticity of triphenylmethane,xanthene, phthalocyanine,and thiazine dyes

Works concerned with the origin of coloration of organic compounds are reviewed. Proofs are given that individual triphenylmethane, xanthene, phthalocyanine, and thiazine dye molecules do not absorb light in the visible range and are not chromogens, that is, do not determine compound chromaticity. Individual molecules of these dyes should be considered chromophoric particles, necessary but insufficient for coloration generation. Elementary chromogens of the dyes under consideration are dimers (supramolecular particles). The blue coloration of aromatic compound azulene has a similar origin.     

Generation and characterization of highly vibrationally excited molecular beam

A simple method to generate and characterize a pure highly vibrationally excited azulene molecular beam is demonstrated. Azulene molecules initially excited to the S4 state by 266-nm UV photons reach high vibrationally excited levels of the ground electronic state upon rapid internal conversion from the S4 electronically excited state. VUV laser beams at 157 and 118 nm, respectively, are used to characterize the relative concentrations of the highly vibrationally excited azulene and the rotationally and vibrationally cooled azulene in the molecular beam. With a laser intensity of 34 mJ/cm2, 75% of azulene molecules absorb a single 266-nm photon and become highly vibrationally excited molecules. The remaining ground-state azulene molecules absorb two or more UV photons, ending up either as molecular cations, which are repelled out of the beam by an electric field, or as dissociation fragments, which veer off the molecular-beam axis. No azulene without absorption of UV photons is left in the molecular beam. The molecular beam that contains only highly vibrationally excited molecules and carrier gas is useful in various experiments related to the studies of highly vibrationally excited molecules.     

Theoretical Studies of Azulene and Its Derivatives

The results of ab initio calculation with STO-3G and 6-31G basis set on the azulene and its derivatives (included azulenequinones, diazoazulenequinones and polyether bridged azulenes) are presented in accordance with considerations of structures and bonding. Azulene is a nonalternant aromatic compound with ten π electrons and it has either C_s or symmetry dgpend on the different carbon and carbon bonding. The calculation results indicate that C_s symmetry is the ground state structure of azulene. TTie simple MO and CIS calculations describe the excited state of azulene.     

First examples of functionalized triphenylene discotic dimers: molecular engineering of advanced materials

A series of novel functionalized triphenylene discotic dimers was synthesized starting from 2-hydroxy-3,6,7,10,11-pentaalkoxytriphenylene. Nitration of monohydroxypentaalkoxytriphenylene gave the mononitromonohydroxypentaalkoxytriphenylene which was alkylated with 2-bromoethanol. The resulting alcohol was coupled with various diacids. These compounds are unique in that they possess an electron withdrawing group (and consequently a large dipole moment) connected directly to the aromatic core. It is well known that connecting two discotic molecules together via a spacer (discotic dimers) stabilizes the columnar mesophase significantly and often leads to the formation of glassy materials. The introduction of functionality into LCs allows the variation of their properties on a wide scale and opens the route to new synthetic supramolecular systems for various device applications.     

Hydrophobic side-chain interactions in a family of dimeric amide foldamers-potential alpha-helix mimetics

A series of new alpha-helix mimetics based on a benzamide scaffold and potentially able to disrupt protein-protein interactions have been synthesized and characterized by X-ray analysis. Inspection of the solid state structures of aromatic amide dimers confirmed that the molecules adopt a curved conformation with intramolecular H-bonding between the amide NH and the alkoxy oxygen of the neighboring aromatic fragment (d_NH…_O ∼ 2 Å). Adjacent dimer molecules are prone to form supramolecular assemblies due to both hydrophobic alkyl side-chain/side-chain interactions and intermolecular H-bonding.     

First examples of functionalized triphenylene discotic dimers: molecular engineering of advanced materials

A series of novel functionalized triphenylene discotic dimers was synthesized starting from 2-hydroxy-3,6,7,10,11-pentaalkoxytriphenylene. Nitration of monohydroxypentaalkoxytriphenylene gave the mononitromonohydroxypentaalkoxytriphenylene which was alkylated with 2-bromoethanol. The resulting alcohol was coupled with various diacids. These compounds are unique in that they possess an electron withdrawing group (and consequently a large dipole moment) connected directly to the aromatic core. It is well known that connecting two discotic molecules together via a spacer (discotic dimers) stabilizes the columnar mesophase significantly and often leads to the formation of glassy materials. The introduction of functionality into LCs allows the variation of their properties on a wide scale and opens the route to new synthetic supramolecular systems for various device applications.     

Three-dimensional supramolecular architecture based on 4,4′-methylene-bis(benzenamine) and aromatic carboxylic acid guests: Synthons cooperation, robust motifs and structural diversity

The two-component solid forms involving 4,4??-methylene-bis(benzenamine) included both salts and co-crystals, while 4,4??-methylene-bis(benzenamine) crystallized exclusively as a salt, in agreement with the differences in the pK _a values. Many of the crystal structures displayed either the neutral or the ionic form of the carboxylic acid-amino heterosynthon, and the similarity in crystal structures between the neutral and the ionized molecules makes the visual distinction between a salt and co-crystal dependent on the experimental location of the acidic proton. A variety of supramolecular hydrogen bonded motifs involving interactions between the aza molecules and carboxylic acid groups are observed rather than just the O-H??N/O-H??O motif. The motifs are identical in all the two compounds analyzed showing the robustness of these supramolecular synthons. In all adducts, recognition between the constituents is established through either N-H??O and/or O-H??O/O-H??N pairwise hydrogen bonds. In all adducts, COOH functional groups available on 1 and 2 interact with the N-donor compounds. The COOH moieties in 1 forms only single N-H??O hydrogen bonds, whereas in 2, it forms pairwise O-H??N/N-H??O hydrogen bonds. The supramolecular architectures are elegant and simple, with stacking of networks in 2, but a rather complex network with a threefold interpenetration pattern was found in 1. Thermal stability of these compounds has been investigated by thermogravimetric analysis (TGA) of mass loss.     

Supramolecular organization and thermal properties of polypseudorotaxanes based on α-cyclodextrine and an ionogenic polymer

The structure, morphology, and thermal properties of polymeric complexes, initial components, and new ionic polypseudorotaxane based on poly(2-acrylamido-2-methylpropanesulfonic acid),-N, N-dimethyl-N??-(4-nitrophenyl)-decane-1,10-diamine-??-cyclodextrine containing photophysically active groups in side chains are studied by X-ray diffraction analysis, scanning electron microscopy, DSC, and TGA methods. It is shown that supramolecular inclusion complexes that give rise to a crystalline precipitate with the columnar structure and hexagonal packing of macrocycles in the base plane are formed. It is found that the structure of complexes is affected by temperature. The models of supramolecular ordering in the complexes that relate the structure and thermophysical properties of the studied systems are suggested.     

Transformations of methyl orange dimers in aqueous–acid solutions,according to UV–Vis spectroscopy data

The effect acidity has on the UV–Vis absorption spectra of azo dye methyl orange (MOD) in aqueous solutions of hydrochloric acid in the pH range of 1.7 to 7 and sulfuric acid in the 0.24 to 18 mol/L range of concentrations is investigated. The spectral transformations of MOD solutions are compared to the corresponding spectral transformations of solutions of dimethylaminoazobenzene (DAB), which is an azo dye akin to MOD. A close resemblance between the spectral transformations of MOD and dimers DAB₂ is revealed. It is concluded that the ground state of MOD, like the ground state of DAB, consists of not individual molecules but of supramolecular dimers MOD₂. It is found that dimers MOD₂ in aqueous low-acidic solutions are reversibly protonated with the formation of di- and triprotonated forms, which reversibly dissociate into diprotonated monomers upon an increase in acidity. The structural formulas of the chromogenic groups responsible for the spectral transformations, and the mechanisms of their reversible transformations, are given.     

para‐Quinodimethane‐Bridged Perylene Dimers and Pericondensed Quaterrylenes: The Effect of the Fusion Mode on the Ground States and Physical Properties

Polycyclic hydrocarbon compounds with a singlet biradical ground state show unique physical properties and promising material applications; therefore, it is important to understand the fundamental structure/biradical character/physical properties relationships. In this study, para‐quinodimethane (p‐QDM)‐bridged quinoidal perylene dimers 4 and 5 with different fusion modes and their corresponding aromatic counterparts, the pericondensed quaterrylenes 6 and 7 , were synthesized. Their ground‐state electronic structures and physical properties were studied by using various experiments assisted with DFT calculations. The proaromatic p‐QDM‐bridged perylene monoimide dimer 4 has a singlet biradical ground state with a small singlet/triplet energy gap (?2.97 kcal mol^?1), whereas the antiaromatic s‐indacene‐bridged N‐annulated perylene dimer 5 exists as a closed‐shell quinoid with an obvious intramolecular charge‐transfer character. Both of these dimers showed shorter singlet excited‐state lifetimes, larger two‐photon‐absorption cross sections, and smaller energy gaps than the corresponding aromatic quaterrylene derivatives 6 and 7 , respectively. Our studies revealed how the fusion mode and aromaticity affect the ground state and, consequently, the photophysical properties and electronic properties of a series of extended polycyclic hydrocarbon compounds.     

Revision of the dissociation energies of mercury chalcogenides--unusual types of mercury bonding.

Mercury chalcogenides HgE (E=O, S, Se, etc.) are described in the literature to possess rather stable bonds with bond dissociation energies between 53 and 30 kcal mol(-1), which is actually difficult to understand in view of the closed-shell electron configuration of the Hg atom in its ground state (...4f(14)5d(10)6s(2)). Based on relativistically corrected many body perturbation theory and coupled-cluster theory [IORAmm/MP4, Feenberg-scaled IORAmm/MP4, IORAmm/CCSD(T)] in connection with IORAmm/B3LYP theory and a [17s14p9d5f]/aug-cc-pVTZ basis set, it is shown that the covalent HgE bond is rather weak (2-7 kcal mol(-1)), the ground state of HgE is a triplet rather than a singlet state, and that the experimental bond dissociation energies have been obtained for dimers (or mixtures of monomers, dimers, and even trimers) Hg2E2 rather than true monomers. The dimers possess association energies of more than 100 kcal mol(-1) due to electrostatic forces between the monomer units. The covalent bond between Hg and E is in so far peculiar as it requires a charge transfer from Hg to E (depending on the electronegativity of E) for the creation of a single bond, which is supported by electrostatic forces. However, a bonding between Hg and E is reduced by strong lone pair-lone pair repulsion to a couple of kcal mol(-1). Since a triplet configuration possesses somewhat lower destabilizing lone pair energies, the triplet state is more stable. In the dimer, there is a Hg-Hg pi bond of bond order 0.66 without any a support. Weak covalent Hg-O interactions are supported by electrostatic bonding. The results for the mercury chalcogenides suggests that all experimental dissociation energies for group-12 chalcogenides have to be revised because of erroneous measurements.     

Structure Formation and Molecular Mobility in Water and in Aqueous Solutions

The study of the structure of water and of aqueous solutions has recently received new impetus from the efforts at commercial desalineation of sea water and from developments in molecular biology. The current view that, apart from single molecules, water contains only one type of structural element, namely “flickering” network structures with tetrahedrally hydrogen-bonded water molecules (two-states model) is proving inadequate in the interpretation of new experimental data and in the calculation of thermodynamic functions. After a critical discussion of the basis of this model and of the concept of hydrogen bonds, a second kind of structural element, i.e. a third state, is suggested: small aggregates of molecules containing mainly non-tetrahedral hydrogen bonds as well as some tetrahedral ones, and packed more densely than allowed by the lattice-like structure. These aggregates – dimers to hexamers – can be regarded as the primary products of disruption of the network structures, and displace the latter as structural components in water with increasing temperature or concentration of solutes. This “combined” model allows a consistent interpretation of the properties of water and of the various effects of dissolved substances.     

Azaarene Dimers

Binaphthyl-3,3′,4,4′-tetraone was prepared and coupled to different bis(TIPS-ethynyl)-substituted (TIPS=triisopropyl silane) aromatic diamines, resulting in the formation of dimeric benzo-fused azaacenes, centrally connected by a single bond. The two halves of the molecules are highly twisted with respect to each other and showed limited electronic interaction in the ground state because their absorption spectra remained very similar to those of the constituting monomers. The dimers displayed greatly reduced fluorescence when compared to the monomers, suggesting that there is a significant interaction of the two azarene units in the excited state. Preliminary investigations showed that the dimers are attractive for application as acceptors in organic photovoltaic because they significantly outperform their monomeric counterparts.     

Topological analysis of the electron density and of the electron localization function of pyrene and its radicals

The topological properties of the charge distribution of pyrene and the three derived monoradicals in their ground state and of didehydrogenated pyrenes in the lowest singlet and triplet electronic states are discussed in detail by means of the quantum theory of atoms in molecules (TAIM) and by the electron localization function (ELF). The non-equivalence of the fused aromatic rings of pyrene prevents one from anticipating the stability and reactivity of these species from the chemistry of didehydrogenated species derived from benzene only. Whereas some of these didehydrogenated molecules were found to display a diradical character in the singlet ground state, the topological analysis reveals that others correspond to normal closed shells. Using these theoretical tools, the energetic and geometric details of o-, m- and p-benzyne-like pyrene derivatives are explained.     

Fluorescence from aromatic compounds isolated in the solid state by double intercalation using layered polymer crystals as the host solid

Poly(muconic acid)s, stereoregular polymer crystals obtained by topochemical polymerization using supramolecular control, function as the layered host solids for organic intercalation, in which alkylamines as the guest species are reversibly inserted into them through an acid-base interaction. We now report a double-intercalation method using alkylamine and pyrene as the guests to control the fluorescence property in the solid state. An aromatic compound can be separately introduced into the hydrophobic layers of the ammonium polymer crystals. The aromatic molecules, which are sandwiched between two alkyl layers, show fluorescence emission from the single molecule but not the excimer. This method can be applied to various organic photofunctional materials showing unique fluorescence properties.     

Theoretical approach to the out-of-plane deformation of 1,3-disubstituted azulenes

Computational calculations at B3LYP/6-311++G(d,p) and MP2/6-311++G(d,p) levels were employed to analyze the structure and conformation of 1,3-bis(4-bromophenyl)azulene (1), 1,3-bis(2-thienyl)azulene (2), and 1,3-bis(2-pyrrollyl)azulene (3) in order to rationalize the out-of-plane deformation found in the azulene cores of 1 and 2 in the crystalline state, whereas compound 3 shows a totally planar azulene moiety. Our results indicate that 1,3-disubstituted azulenes possess two almost equally stable and easily convertible minimum energy conformers, which differ in the relative orientation of the substituent groups and in the planarity degree of the azulene core. An absolute planarity index (P) is introduced to quantify the out-of-plane distortion found in the azulenes under study. The aromaticity of minimum energy conformers was evaluated by means of geometric (HOMA), magnetic (NICS), and energetic (the frequency of the lowest out-of-plane vibration, ν_min) aromaticity indicators, which suggest that compound 3 possesses the most aromatic azulene core within the group. Calculated molecular dipole moments suggest that the conformation of 1,3-disubstituted azulenes in the crystalline state can be explained in terms of electrostatic intermolecular interactions rather than relative stability of planar and non-planar conformers.