Exciton <Emphasis Type="Italic">Cotton</Emphasis> Effects of Benzoates in the <Superscript>1</Superscript>B Transition Region. Demonstration and Applications

Summary. The benzoate ¹B region exciton Cotton effects, hitherto unexplored, were analyzed for their use in stereochemical assignments in both acyclic (conformationally flexible) and cyclic molecules. It was found that a strong, allowed ¹B_a transition, polarized longitudinally, dominates the ¹B region (185–210nm) both in the UV and the CD spectra. The exciton Cotton effects due to this transition have the same sign (but differing magnitude) as those due to the ¹L_a (CT) band. The other component of the nearly degenerate ¹B system, i.e. ¹B_b transition, polarized orthogonally to the ¹B_a transition, gives a Cotton effect in the case of di- and poly(4-chlorobenzoate) chromophoric system, the sign of which is sensitive to the configuration of di- or polyol. In rigid 5-steroid skeleton ¹B_b transition couplings appear responsible for strong exciton Cotton effects due to nearly parallelly oriented benzoate chromophores. Whereas ¹B_a transition excitation energy appears insensitive to the nature of a substituent in 4-position of the benzoate chromophore, substitution with a donor group (methoxy, dimethylamino) brings about a red shift of the ¹B_b band, although less pronounced than the red shift of the ¹L_a (CT) band.     

Exciton transitions in quasi-one-dimensional crystals. 9,10-dichloroanthracene

Polarized reflection spectra of the first singlet transition of the α-crystalline form of 9,10-dichloroanthracene are reported. Crystal faces (001), (011) and (010) were examined in spectral range 450 to 350 nm at two temperatures, 5 K and 300 K. Two systems of transitions were observed. The first system is assigned to neutral excitons. Spectral similarities with unsubstituted anthracene and arguments based on the one-dimensional stacking of molecules are used to construct a model of the exciten band structures. The M-polarized ππ* molecular transition gives rise to a four branch band with two allowed transitions. The 0-0_b (A_g → A_u) transition lies 50–100 cm^?1 above the bottom of the exciton band and the 0-0_c′ (A_g → B_u) transition lies at the top of the band. In the reflection spectrum the Davydov splitting c′b for transverse excitons is 210 cm^?1. The exciton band of the 00 molecular transition is not isolated but overlaps the two-particle manifold of the 0–1 vibronic transition. As a result of the 0–1_c′ transition is unexpectedly strong in the spectra of the (010) face. The second system is polarized along the stack-axis a and starts 2500 cm^?1 above the first system. It is tentatively assigned as |a(A_g → B_u) charge transfer exciton transition in agreement with earlier observations.     

Unambiguous Determination of the Absolute Configurations of Acetylene Alcohols by Combination of the <Emphasis Type="Italic">Sonogashira</Emphasis> Reaction and the CD Exciton Chirality Method – Exciton Coupling between Phenylacetylene and Benzoate Chromophores

Summary. The CD exciton chirality method was applied to various phenylacetylene alcohols to determine their absolute configurations; the long axis polarized –^* transition (_max=252nm) of the 4-methoxyphenylacetylene chromophore couples with the transition (_max=257nm) of the 4-methoxybenzoate group to generate intense exciton split CD Cotton effects, from the signs of which the absolute configurations of phenylacetylene alcohols were unambiguously determined. As an extension of the results, a new methodology for determining the absolute configurations of acetylene alcohols having the HCCCH(OH)-moiety by combination of the Sonogashira reaction and the CD exciton chirality method has been developed and applied. Since the –^* transition of acetylene triple bond is located below 180nm, it is difficult to observe ideal bisignate CD Cotton effects due to the exciton coupling between acetylene and benzoate chromophores. To observe the ideal exciton split Cotton effects necessary for the unambiguous determination of absolute configuration, the terminal acetylene group was converted, by the Sonogashira reaction, to the 4-methoxyphenylacetylene moiety, which exhibits an intense –^* absorption band polarized along the long axis of the chromophore at 252nm. As a partner of exciton coupling, 4-methoxybenzoate showing a –^* band at 257nm was introduced into the alcohol moiety, and the benzoates formed showed intense bisignate CD Cotton effects, from the signs of which the absolute configurations of original acetylene alcohols could be determined in an unambiguous manner.     

Unambiguous Determination of the Absolute Configurations of Acetylene Alcohols by Combination of the Sonogashira Reaction and the CD Exciton Chirality Method – Exciton Coupling between Phenylacetylene and Benzoate Chromophores

The CD exciton chirality method was applied to various phenylacetylene alcohols to determine their absolute configurations; the long axis polarized –^* transition (_max=252nm) of the 4-methoxyphenylacetylene chromophore couples with the transition (_max=257nm) of the 4-methoxybenzoate group to generate intense exciton split CD Cotton effects, from the signs of which the absolute configurations of phenylacetylene alcohols were unambiguously determined. As an extension of the results, a new methodology for determining the absolute configurations of acetylene alcohols having the HCCCH(OH)-moiety by combination of the Sonogashira reaction and the CD exciton chirality method has been developed and applied. Since the –^* transition of acetylene triple bond is located below 180nm, it is difficult to observe ideal bisignate CD Cotton effects due to the exciton coupling between acetylene and benzoate chromophores. To observe the ideal exciton split Cotton effects necessary for the unambiguous determination of absolute configuration, the terminal acetylene group was converted, by the Sonogashira reaction, to the 4-methoxyphenylacetylene moiety, which exhibits an intense –^* absorption band polarized along the long axis of the chromophore at 252nm. As a partner of exciton coupling, 4-methoxybenzoate showing a –^* band at 257nm was introduced into the alcohol moiety, and the benzoates formed showed intense bisignate CD Cotton effects, from the signs of which the absolute configurations of original acetylene alcohols could be determined in an unambiguous manner.     

Intensités des Transitions Électroniques de [PdCl4]2− Intensities of Electronic Transitions of [PdCl4]2−

The oscillator strengths of the symmetry-forbidden transitions of the ion [PdCl₄]^2– were calculated by a method based on the evaluation of the MO of the distorted ion. It is not very likely that the transition to¹ B _1g (a _1g (d _z ²)b _1g ) state, weak, mainlyz polarized, explains the band that appears as a shoulder observed towards 30 000 cm^–1 in the absorption spectrum. If this band is specific of the ion [PdCl₄]^2–, the assignment to the forbidden charge-transfer transition,¹ A _1g ¹ B _2g , is more plausible.

     

Synthesis and characterization of hetero-binuclear Co-Rh complexes [CpCoS2C2(B9H10)][Rh(COD)] and [CpCoSe2C2(B10H10)][Rh(COD)]

Two hetero-binuclear complexes [Cp^∗CoS₂C₂(B₉H₁₀)][Rh(COD)] (2a) and [Cp^∗CoSe₂C₂(B₁₀H₁₀)][Rh(COD)] (2b) [Cp^∗ = η⁵-pentamethylcyclopentadienyl, COD = cyclo-octa-1,5-diene (C₈H₁₂)] were synthesized by the reactions of half-sandwich complexes [Cp^∗CoE₂C₂(B₁₀H₁₀)] [E = S (1a), Se (1b)] with low valent transition metal complexes [Rh(COD)(OEt)]₂ and [Rh(COD)(OMe)]₂. Although the reaction conditions are the same, the structures of two products for dithiolato carborane and diselenolato carborane are different. The cage of the carborane in 2a was opened; However, the carborane cage in 2b was intact. Complexes 2a and 2b have been fully characterized by ¹H, ¹¹B NMR and IR spectroscopy, as well as by elemental analyses. The molecular structures of 2a and 2b have been determined by single-crystal X-ray diffraction analyses and strong metal-metal interactions between cobalt and rhodium atoms (2.6260 Å (2a) and 2.7057 Å (2b)) are existent.     

Synthesis and characterization of heterometallic Rh/Ru complexes supported by 1,2-dichalcogenolato-o-carborane ligands

The 16-electron half-sandwich complexes Cp^∗Rh[E₂C₂(B₁₀H₁₀)] (E = S, 1a; Se, 1b) react with [Ru(COD)Cl₂]_x under different conditions to give different types of heterometallic complexes. When the reactions were carried out in THF for 24 h, the binuclear Rh/Ru complexes [Cp^∗Rh(μ-Cl)₂(COD)Ru][E₂C₂(B₁₀H₁₀)] (E = S, 2a; Se, 2b) bridged by two Cl atoms and the binuclear Rh/Rh complexes (Cp^∗Rh)₂[E₂C₂(B₁₀H₁₀)] (E = S, 3a; Se, 3b) with direct Rh-Rh bond can be isolated in moderate yields. [Ru(COD)Cl₂] fragments in 2a and 2b have inserted into the Rh-E bond. If the [Ru(COD)Cl₂]_x was reacted with 1a in the presence of K₂CO₃ in methanol solution, the product [Cp^∗Rh(COD)]Ru[S₂C₂(B₁₀H₁₀]] (4a), K[(μ-Cl)(μ-OCH₃)Ru(COD)]₄ (5a) and 3a were obtained. The B(3)-H activation in complex 4a was found. However, when the reaction between 1b and [Ru(COD)Cl₂]_x was carried out in excessive NaHCO₃, the carborane cage opened products {Cp^∗Rh[S₂C₂(B₉H₁₀)]}Ru(COD) (6b), {Cp^∗Rh[S₂C₂(B₉H₉)]}Ru(COD)(OCH₃) (7b) and 3b were obtained. All complexes were fully characterized by their IR, ¹H NMR and elemental analyses. The molecular structures of 2a, 2b, 3b, 4a, 5a, and 7b have been determined by X-ray crystallography.     

The 4000 Å transition of crystal anthracene

New low temperature determinations of ?₂(ω) for crystal anthracene locate the 0-0 transition origin at 25096 ± 6 cm^?1 for the b-polarization. There is no Stokes shift of the fluorescence origin. The exciton bands which are optically accessible by probing the (ab) crystal face with normal incidence light polarized either parallel or perpendicular to the b-axis have a temperature dependent splitting. The Davydov splitting at 77°K is 267 ± 12 cm^?1. The temperature dependence of the k = 0 polariton splitting and the exciton bandwidths (b-polarization and a-polarization) are also given and together with the Davydov splitting provide a test of the adequacy of theories of exciton interactions. The polarization ratio measured as the ratio of the oscillator strengths of the two transitions (f_b/f_a) is quite temperature dependent for both the region of the (0-0) transition and over the total vibronic transition. At 77°K, the value of f_b/f_a is 8.0 for the origin region close to that expected of an oriented gas but is 4.5 over the total transition. Some spectral structure observed only in the b-polarized reflectance spectrum at temperatures less than 77°K is related to the existence of defects localized near the surface.     

Exciton dynamics in molecular crystals from line shape analysis. Time response function of singlet excitons in crystal anthracene

From an accurate measure of the temperature dependence of the line shape function ω ?₂ (ω) information is obtained about the time behaviour of the response function of singlet excitons of small wavevector encompassed by the (0-0) band of the 4000 A Å transition in crystal anthracene. An apparatus to determine the reflection band profile with high accuracy needed to give correct ω ?₂ (ω) data is described. Although the data analysis is not without problems, there is strong evidence that the time behaviour of excitons in this transition is characterized by a stochastic collision time τ_c. The temperature dependence of τ_c is consistent with a model in which the intermolecular phonons are weakly coupled with the exciton created by either b or a polarized light. Phonon annihilation is predominant for the b-polarized transition but both phonon creation and annihilation are active for these polarized transition. The similar values of the exciton—phonon coupling function for both polarizations may indicate either the importance of higher multipole terms for that function or strong interband scattering. The relationships between τ_c and parameters from other experimental results on singlet excitons in crystal anthracene are considered. The results may allow for a better understanding of the mechanism of exciton—phonon coupling in crystals.     

Exciton dynamics in molecular crystals from line shape analysis. Spectral moment analysis of the origin region of the 4000 Å transition of crystal anthracene

A spectral moment analysis of the line shape function ω?₂(ω) in the region of the (0—0) band of the 4000 ŹB_2u ← ¹A_g transition in crystal anthracene at various temperatures is performed. The data are compared with the predictions of three coupling models, viz., weak exciton—photon with weak exciton—phonon coupling, strong exciton—photon with weak exciton—phonon coupling, weak exciton—photon with strong exciton—phonon coupling. The terms contributing to each spectral moment for each model are rendered explicit. The experimental data indicate that the exciton—intermolecular phonon coupling is primarily weak. The exciton interacts with optical phonons of about 90 cm^-1 frequency with a coupling strength of about 140 cm^-1 , a value near that predicted by a weak coupling model. The coupling strength is nearly the same irrespective of whether the exciton is created by b- or a-polarized light probably indicative of the importance of higher multipole contributions to the coupling although the existence of strong interband scattering could affect that suggestion. The coupling parameters g_op and g_ac are about 10^-1 and 10^-4 respectively.     

The effect of covalent contact between solvent molecules and the VO(DMSO) 5 2+ ion on the electronic structure of the ion

The dissolution of the vanadyl complex VO(DMSO)₅(CLO₄)₂ in alcohols ROH (R= Me, Et, or Prⁿ) results in the appearance of a new absorption band at 372 nm in the electronic absorption spectra of the vanadyl ion. The calculation of the OVO₅ ²⁺ ion by the X-scattering wave method is carried out to interpret the spectra. It is shown that the new band in the spectrum of the VO(DMSO)₄OHR²⁺ complex corresponds to the b₂a₁ transition in the OVO₅ ²⁺ ion. However, this transition is not a pure d-d transition due to strong covalent bonding and a significant contribution from the AO 2p of oxygen to the a₁ MO. The appearance of the new absorption band in the electronic spectra of the vanadyl complexes is caused by the participation of ligand orbitals in bond formation.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 699–702, April, 1995.     

Collision-induced emission of O2(b1Σ → a1Δg) in the gas phase

In flow tube studies of the quenching of O₂(b¹Σ), broad band emission of O₂(b):M collision complexes was found to appear under the discrete rotational lines of the 0–0 band of the b¹Σ → a¹Δ_g electric quadrupole transition at higher oxygen pressures and on addition of foreign gases. Bimolecular rate constants for the collision-induced emission processes have been derived from the ratio of the intensities of the discrete lines and the continuum as well as from low-resolution measurements of the relative intensities of the b → a and b → X bands as a function of O₂ and added gas pressure. They range from ≈10^?21 cm³ s^?1 for He to ≈4 × 10^?19 cm³ s^?1 for PCl₃ vapor.     

Synthesis and characterization of half-sandwich Rh, Ir complexes containing mono-thiolate-ortho-carborane ligand

Two binuclear complexes [Cp^∗M(Cl)Carb^S]₂ (Cp^∗ = η⁵-C₅Me₅, M = Rh (1a), Carb^S = SC₂(H)B₁₀H₁₀, Ir (1b)) were synthesized by the reaction of LiCarb^S with the dimeric metal complexes [Cp^∗MCl(μ-Cl)]₂ (M = Rh, Ir). Four mononuclear complexes Cp^∗M(Cl)(L)Carb^S (L = BuⁿPPh₂, M = Rh (2a), Ir (2b); L = PPh₃, M = Rh (4a), Ir (4b)) were synthesized by reactions of 1a or 1b with L (L = BuⁿPPh₂ (2); PPh₃ (4)) in moderate yields, respectively. Complexes 3a, 3b, 5a, 5b were obtained by treatment of 2a, 2b, 4a, 4b with AgPF₆ in high yields, respectively. All of these compounds were fully characterized by IR, NMR, and elemental analysis, and the crystal structures of 1a, 1b, 2a, 2b, 4a, 4b were also confirmed by X-ray crystallography. Their structures showed 3a, 3b and 5a, 5b could be expected as good candidates for heterolytic dihydrogen activation. Preliminary experiments on the dihydrogen activation driven by these half-sandwich Rh, Ir complexes were done under mild conditions.     

Pressure-induced phase transition and octahedral tilt system change of Ba2BiSbO6

High-resolution X-ray synchrotron powder diffraction studies under high-pressure conditions are reported for the ordered double perovskite Ba₂BiSbO₆. Near 4 GPa, the oxide undergoes a pressure-induced phase transition. The symmetry of the material changes during the phase transition from space group to space group I2/m, which is consistent with a change in the octahedral tilting distortion from an a⁻a⁻a⁻ type to a⁰b⁻b⁻ type using the Glazer notation. A fit of the volume-pressure data using the Birch-Murnagaham equation of state yielded a bulk modulus of 144(8) GPa for the rhombohedral phase.     

Simultaneous analysis of three avermectins in soils by high-performance liquid chromatography with fluorescence detection

A simple and sensitive method has been developed and validated for the determination of abamectin B_1a (ABA B_1a), emamectin B_1a (EMA B_1a) benzoate and ivermectin H₂B_1a (IVM H₂B_1a) in soils. The avermectins (AVMs) residues were extracted from soils with acetonitrile/water (9?:?1, v/v) and then were purified on C₁₈ solid-phase extraction (SPE) cartridge. After being derivatised by N-methylimidazole (N-MIM) and trifluoroacetic anhydride (TFAA), the residues of three AVMs were analysed by high-performance liquid chromatography with fluorescence detection (HPLC-FLD). The method was validated in terms of system suitability, linearity, selectivity, precision, recovery, specificity and stability. There was a good linear relationship (R ^2?>?0.99) for three AVMs ranged from 0.01 to 5?µg?mL^?1. The LOD and LOQs of ABA B_1a, EMA B_1a benzoate and IVM H₂B_1a for standard solutions were 1.1–1.7 and 3.6–5.7?µg?L^?1 respectively. The accuracy of AVMs in soils was from 83.7 to 115.5% with precision less than or equal to 12.4%. Using the developed method, 9 soil samples with 9.3–12806.3?µg?kg^?1 of AVMs residues had been detected.     

Die Elektronenzustände von Perimeter-π-Systemen. II. Das Elektronenspektrum des trans-15, 16-Dimethyl-dihydropyrens

The electronic spectrum and the polarisation of the transitions have been determined in the region from 15000 to 50000 cm^?1 for the 2-acetamino derivative of trans-15, 16-dimethyl-dihydropyrene, whose spectrum resembles closely that of the parent compound and its di-ethyl analogue. It is shown that the sequence of states is ¹L_b, ¹L_a, ¹B_b, ¹B_a, in agreement with theoretical predictions which were deduced from a configuration interaction model for the D_2h-π-perimeter of these systems. The influence of inductive and hyperconjugative effects on the band positions and the band intensities has been discussed.     

The electronic structures and spectra of proflavine,acridine orange,and their DNA complexes

The electronic structure of the proflavine cation is studied by the SCF –ASMO –CI method using the Pariser–Parr–Pople approximations. It is shown that the band at 445 mμ may be assigned to the ¹A₁ → ¹B₁, transition polarized along the long axis of the molecule. The bands in the neighbourhood of 260 mμ, which are composed of three absorption bands, are tentatively assigned to the ¹A₁ → ¹B₁, ¹A₁ → ¹B₁, and ¹A₁ → ¹A₁ transitions, respectively, in order of decreasing wavelength. The spectrum of the acridine orange cation may be understood to have the same assignment as that of the proflavine cation. The acridine dye cations are well known for their dimerization with concentration. The intermolecular distances in these dimers are estimated from the band shifts due to the formation of dimers, using the exciton theory. The main contribution to the molecular interaction is shown to be the electrostatic dipole–dipole interaction. Since the first excitation band of the dye molecule which exhibits a remarkable change due to the formation of the DNA–acridine dye complex, is suggested to be polarized along the long axis, preference of the outside stacking or the intercalation model is qualitatively discussed from the spectral shift of the acridine dye bound to the DNA, assuming simple models.     

Anisotropy of the T-T absorption spectra of naphthalene and phenanthrene

The naphthalene absorption in the spectral region of 3500–4100 Å consists of two perpendicularly polarized band systems. This can be shown by polarization measurements. The arguments given do not allow a decision as to whether or not the short axis polarized band is the ³A_g⁻ ← ³B_iu⁺ transition or an induced vibronic band of the ³B_3g⁻ ← ³B_iu⁺ transition generated by a b_3g vibration mode. The T-T polarization of phenanthrene as a function of the excitation wavenumber is quantitatively compared with the equivalent S₁ ← S₀ fluorescence polarization. There is a good agreement between the calculated and the measured curve.     

Intrinsic symmetry of the AABb-type distribution

By taking an A_aB_b-type polymerisation as an example

1 Each unit has a functional groups of type A and b of type B.

, an intrinsic symmetry of the equilibrium number fraction distribution of species is revealed. The symmetry shows that the distribution is invariant under transformation between the variables which describe the total system and the variables which describe the sol. It results that, when the polymer quantities for the pre-gel regime are given, the corresponding quantities for the post-gel regime can easily be obtained. As an illustration, the k th radius of an A_aB_b-type polymerisation is discussed.     

Spectroscopic Studies of Cyclometallated Pd(II) and Pt(II) Complexes: Optical absorption and emission of single crystals of [Pd(thpy)2] and [Pt(thpy)2]

The polarized optical absorption and emission (spectra, decay times) of single crystals of [Pd(thpy)₂] and [Pt(thpy)₂] (thpy ≡ C(3′)-deprotonated form of 2-(2-thienyl)pyridine) at temperatures 1.9 K ? T ? 80 K are reported. The emission of [Pt(thpy)₂] can be influenced strongly by applied magnetic fields (0 ? H ? 6 T). Depending on the central ions Pd and Pt, the lowest excited electronic states of the single complexes are ligand-centered (LC) states and metal-to-ligand charge transfer (MLCT) states, respectively. This difference leads to distinctly dissimilar properties of the emission of both compounds. The experimental data show that the emission of single crystals of [Pd(thpy)₂] and [Pt(thpy)₂] at T ? 30 K originates from several types of traps (defect states of symmetry ³B₂?stabilized below the exciton band) with LC and MLCT character, respectively. In the Pt compound, the ³B₂ is split by spin-orbit coupling into three states. The states B and A, which determine the emission properties, are separated by Δv ～ 13 cm^?1. Both states can mix under the influence of an applied magnetic field yielding an increase of the emission intensity by a factor of ～ 1.5 at H = 6 T.