Theoretical studies on structures and spectroscopic properties of bis-cyclometalated iridium complexes [Ir(ppy)2X2]

The electronic structures and spectroscopic properties of a series of mixed bis-cyclometalated iridium(III) complexes [Ir(ppy)₂X₂]⁻ (X = CN, 1; X = NCS, 2; X = NCO, 3; ppy = 2-phenylpyridl) were investigated at the B3LYP/LANL2DZ and CIS/LANL2DZ levels. The calculated geometry parameters in the ground state are well consistent with the corresponding experimental values. The HOMO of 1 is dominantly localized on Ir atom and ppy ligand, but the HOMO of 2 and 3 have significant X ligand composition. Under the TD-DFT level with PCM model, the absorption and phosphorescence in CH₂Cl₂ media were calculated based on the optimized geometries in the ground and excited states, respectively. The lowest-lying absorption of 1 at 403 nm is attributed to {[d_x²-y²(Ir)+d_xy(Ir)+π(ppy)]→[π^∗(ppy)]} transition with metal-to-ligand and intraligand charge transfer (MLCT/ILCT) transition characters, whereas those of 2 (449 nm) and 3 (475 nm) are related to {[d_x²-y²(Ir)+d_xy(Ir)+π(ppy)+π(NCS/NCO)]→[π^∗(ppy)]} transition with MLCT/ILCT and ligand-to-ligand charge transfer (LLCT) transition characters. The phosphorescence of 1 at 466 nm can be described as originating from ³{[d_x²-y²(Ir)+d_xy(Ir)+π(ppy)][π^∗(ppy)]} excited state, while those of 2 (487 nm) and 3 (516 nm) originate from ³{[d_x²-y²(Ir)+d_xy(Ir)+π(ppy)+π(NCS/NCO)][π^∗(ppy)]} excited states. The calculated results showed that the transition character of the absorption and emission can be changed by adjusting the π electron-accepting abilities of the X ligands and the phosphorescent color can be tuned by altering the X ligands.     

Infrared study of some specificities of the dπpπ interaction in organogermanium compounds

The analysis is presented for the frequencies of stretching modes ν(GeH) in the IR spectra of organogermanium compounds R₂XGeH, RX₂GeH, RXYGeH, X₂YGeH and XYGeH₂ (where R is a substituent which does not make a d_πp_π bond with germanium, and X and Y are groups capable of d_πp_π interaction with germanium). It is shown that ν(GeH) in these compounds is dependent on both the I effect of R, X and Y, and the d_πp_π interaction in GeX and GeY bonds. If only one substituent capable of d_πp_π interaction with germanium is present, the value of such an effect is determined by itsσo_Rconstant. However, when germanium is bound to several substituents capable of d_πp_π interaction its magnitude depends on the effective charge at germanium which is determined by the inductive and mesomeric effects of X and Y. The data obtained are compared to the dependences observed in the IR spectra of similar organosilicon compounds.     

Fourier transform NMR investigations of 13C labeled silyl and stannyl acetylenes

The ¹³C NMR spectra of ¹³C labeled mono- and di-substituted silyl- and stannyl-acetylenes have been studied. It was found that the values of ¹J(CC) coupling constants between acetylenic carbons decrease very sharply in the series Alk₃SiCCH, Alk₃SnCCH, Alk₃SiCCSiAlk₃ and Alk₃SnCCSnAlk₃. These results and the observed changes in the geminal hetero-atom β-acetylenic carbon couplings suggest a very strong p_πd_π interaction between the π-electrons of the triple bond and the vacant d orbitals of silicon and tin.     

Spectroscopic and electrochemical properties of mixed-ligand cycloplatinated complexes based on 2-(2-thienyl)pyridine and 2-phenylpyridine with 1,10-phenantroline and its 1,4-diazine derivatives

A comparative study of dipyrido-and dibenzo-substituted 1,4-diazines {dipyrido[f,h]quinoxaline (dpq), dipyrido[a,c]phenazine (dppz), 6,7-dicyanodipyrido[f,h]quinoxaline (dicnq), dibenzo[f,h]quinoxaline, dibenzo[a,c]phenazine, 6,7-dicyanodibenzo[f,h]-quinoxaline}, o-phenantroline (phen), and also of the complexes [Pt(N∧C)(N∧N)]⁺[(N∧C)^? are deproronated forms of 2-phenylpyridine and 2-(2-thienyl)pyridine; (N∧N) is ethylenediamine, phen, dpq, dppz, dicnq] was carried out by the methods of ¹H NMR, electronic absorption, and emission spectroscopy and by cyclic voltammetry. It was found that in frozen solutions of [Pt(N∧C)·(N∧N)]⁺ complexes the photoexcitation energy decay from two lowest in energy electronic excited states has isolated character and is localized on {Pt(N∧C)} and {Pt(N∧N)} metal-complex fragments: (d-π _N∧C ^* ) and (d-π _phen ^* ) [(N∧N) = phen, dpq, dicnq)] or (d-π _N∧C ^* ) and (π-π _diaz ^* ) [(N∧N) = dppz]. Thermal quenching of the luminescence from the (d-π _phen ^* ) and (π-π _diaz ^* ) states gives rise to luminescence of the complexes in liquid solutions at 293 K only from the (d-π _N∧C ^* ) state.     

Application des spectrographies RMN 13C et 1H a l'etude de derives cycliques du silicium et germanium,interactions pπdπ

The properties of 1-sila- and 1-gemma-cyclopent-2-(or -3-)enes with various substituents have been investigated by ¹³C and ¹H NMR spectroscopies. Values of chemical shifts and coupling constants in olefinic cyclic systems and in aromatic or ethylenic substituents attached to the hereto atom suggest the presence of a mesomeric p_πd_π interaction, particularly for the silicon atom. Studies at low temperatures given conformational data, which have been compared with data from the literature.     

Factor group splitting in the lowest triplet state of p-benzoquinone-d4 crystals

Polarized Stark-modulated Zeeman absorption experiments on p-benzoquinone-d₄ single crystals at 2 K show the factor group splitting in the origin of the lowest B_1g (nπ^*) triplet state at 18649 cm^?1 to be 0.62±0.06 cm^?1. The ordering of the crystal states is such that the orbital plus state lies at higher energy. The absence of a measurable factor group solitting in the ³A_u (nπ^*) state at 12.1 cm^?1 from the origin is taken as a further confirmation of the vibronic nature of this state. The ZFS parameter D of this level is found to be ?10±3 GHz.     

Assignment and analysis of the absorption and emission spectra of the lowest nπ* triplet state in 9,10-anthraquinone

Polarized Zeeman absorption experiments on 9,10-anthraquinone crystals show the lowest triplet state in this molecule to be a g nπ^* state. The gap between this state and the higher u nπ^* state is found to be 410 cm^?1. The phosphorescence spectrum of an isotopically mixed crystal of AQ-h₈ in AQ-d₈ is analyzed in detail and confirms the orbitally forbidden nature of the emitting state. The results are compared with those previously obtained for p-benzoquinone.     

The electronic structure of CrO3+ and MoO3+ complexes

The results of ab initio molecular orbital calculations for [CrOf_s]^2? and polarised single crystal electronic spectra of [MoOCl₃(Op(NMe₂)₃)₂] and Ph₄As[MoOCl₄(H₂O)] are presented. These data are consistent with the electronic transitions of the MO³⁺ moieties, O_2pπ → M_dxy and M_dxy → Md_dxy,dyz being the lowest energy transitions in the spectra of their respective complexes, both these transitions being of low intensity.     

Complexes of Rh(III), Ir(III), and Pt(IV) with metallated 2-(n-tolyl)pyridine,ethylendiamine, acetate,and diethyldithiocarbamate ligands

cis-C,C Isomers of the [M(ptpy)₂(L∧L)](PF₆)_Z complexes [M = Rh(III), Ir(III), Pt(IV); ptpy^? = deprotonated form of 2-(n-tolyl)pyridine, (L∧L) = acetate, trifluoroacetate, or diethyldithiocarbamate anions, or ethylenediamine; z = 0, 1, 2] were prepared and characterized by ¹H and ¹⁹F NMR, IR, electronic absorption and emissions spectroscopy, and by voltammetry methods. The highest occupied and the lowest unoccupied molecular orbitals were assigned to d _π and π*_ptpy orbitals of the metal and the metallated ligand. Luminescence of the complexes in the visible spectral region was assigned to the spin-forbidden optical transition from the lowest energy state of the mixed nature (π_ptpy-π*_ptpy/d _x -π*_ptpy).     

The problem of σ- and π-bonding in the tin substituted tetracarbonyl cobalt compounds RlXmSn {Co(CO)4n}, R = alkyl,phenyl, X = halogen, l + m + n = 4

The results of each type of spectroscopic research on organometallics, such as r_ix_m Sn- [Co(CO)₄]_n (R = Alkyl, Phenyl, X = Halogen, l + m + n = 4) provide information clarifying the bonding in these heteronuclear metal—metal bonded molecules. This report is a concise survey comparing the conclusions drawn from nuclear magnetic, nuclear quadrupole and nuclear -γ resonance spectroscopy, together with the polar characteristics investigated by the dipole moments method and the vibrational data obtained from the far infrared region. The experimental data are critically discussed in terms of (1) metal-metal σ bonding changes, (2) d_π-d_π contributions which may occur between a group IV-B metal and a transition metal and (3) p_π-d_π overlap resulting from a back-donation of the unshared halogen p-electrons into symmetry-proper vacant d-orbitals of the tin atom.     

High resolution NMR studies of allylic derivatives of group IVB elements

The high resolution NMR spectra of certain Group IVB allyl compounds, FC₃M(CH₂)_nCHCH₂R′ (where R = Me, Et, Ph; R′ = H, Ph; M = Sn, Si, Ge; n = 1 or 2), have been studied in an attempt to detect manifestations of ground state p_πd_π overlap. Analyses, using sub-spectral techniques for ABMX₂ and ABX₂ systems, were confirmed using computer iteration methods. The emphasis has been placed here on the sensitivity of the coupling constants of the allyl group towards anticipated steric and electronic perturbation or towards the interaction between the Group IVB atom d orbitals and the olefinic p_π orbitals. Some ¹¹⁹Sn chemical shifts have also been recorded. The conclusions reached do not support the existence of ground state d_πp_π overlap in these compounds.     

Nitric oxide—heme binding model for nitrosylhemoglobin from its magnetic properties

A simple orbital model of the binding of NO to the heme iron in nitrosylhemoglobin is presented that fits the experimentally measured g factors and spin distribution. The NO π^☆ orbitals are split by approximately 4000 cm^?1 X hc such that the lower singly occupied member lies 8000 cm^?1 X hc above the filled Fe d orbitals. Binding of Fe to the N of NO is primarily of two comparable in-plane (xz) types, dσ—pσ with 23% d_z² character and dπ—pπ with 25% d_xz character. There is relatively less of the out-of-plane dπ—pπ and it involves almost no d_yz character.     

Excited state geometry of and radiationless processes in the lowest B3u(nπ*) singlet state of s-tetrazine

High resolution absorption and laser induced emission spectra of the lowest B_3u(nπ^*) singlet state of s-tetrazine-h₂ and -d₂ in a benzene crystal at 1.8 K are presented and discussed. The absorption spectrum with origin at 17231 cm^?1 (h₂) is dominated by a progression in ν_6a and a Herzberg-Teller origin which has been assigned as ν₁. The absence of ν₁ in the emission spectrum is explained as being due to a destructive vibronic interference effect. The Franck-Condon envelope of the unique ν_6a progression in emission is used for a determination of the excited state structure and the limitations of this procedure are examined. Direct lifetime measurements using a dye laser and single photon counting techniques show the fluorescence lifetime of s-tetrazine-h₂ and -d₂ to be shorter than 1.5 ns. From a deconvolution of the emission pulse of dimethyl s-tetrazine its fluorescence lifetime in the gas phase is found to be 6.0 ± 0.3 ns. Through a comparison of the fluorescence quantum yield of s-tetrazine-h₂ and dimethyl s-tetrazine we calculate for s-tetrazine-h₂ a fluorescence lifetime of 1.5 ± 0.2 ns and a fluoresence quantum yield of 1.8 × 10^?3. The ratio of the emissive lifetimes of s-tetrazine-d₂ and -h₂ was measured from relative fluorescence yields and found to be 1.18 ± 0.05. Photodissociation quantum yield studies on s-tetrazine-h₂, -d₂ and dimethyl for excitation into the origin of the ¹B_3u(nπ^*) state show this yield to be in the range of 1.3 ± 0.3, and this could explain the low fluorescence yields of the s-tetrazines. The fluorescence quantum yields in the gas phase are found to vary among the vibronic levels of the ¹B_3u state. This finding is in agreement with earlier measurements by Vemulapalli and Cassen, but the report by these authors that such quantum yield variations also occurred in the rovibronic structure is not confirmed.     

Optically detected electron—nuclear double resonance and electron—nuclear—nuclear triple resonance study of 17O hyperfine coupling in the lowest nπ* triplet state of benzil

Optically detected ENDOR and electron—nuclear—nuclear triple resonance of ¹⁷O were measured via phosphorescence from ³(nπ^*) benzil in benzophenone-d₁₀ crystals at high magnetic field. The n and π^* spin densities on the oxygen atom are 0.201 and 0.092, respectively, the angle between the two CO bonds being 150°.     

Syntheses, crystal structures and luminescent properties of two new 1D d coordination polymers constructed from 2,2′-bibenzimidazole and 1,4-benzenedicarboxylate

Two novel interesting d¹⁰ metal coordination polymers, [Zn(H₂bibzim)(BDC)]_n (1) and [Cd(H₂bibzim)(BDC)]_n (2) [H₂bibzim=2,2′-bibenzimidazole, BDC=1,4-benzenedicarboxylate] have been synthesized under solvothermal conditions and structurally characterized. Both 1 and 2 are constructed from infinite neutral zigzag-like one-dimensional (1D) chains. The π-π interactions and interchain hydrogen-bonding interactions further extend the 1D arrangement to generate a 3D supramolecular architecture for 1 and 2. Both complexes have high thermal stability and display strong blue fluorescent emissions in the solid state upon photo-excitation at 365 nm at room temperature. They are the first two examples that 2,2′-bibenzimidazole has been introduced into the d¹⁰ coordination polymeric framework.     

Structure and dynamics of the lowest triplet state in p-benzoquinone: I. An isotope effect study on the optical absorption,emission and ODMR spectra

The results of detailed spectroscopic experiments on the lowest nπ* triplet state of p-benzoquinone-h₄, -dh₃, 2,6-d₂h₂, -d₄ and -CH₃ in mixed and isotopic mixed crystals are presented and analyzed. The origin of the lowest B_1g (nπ*) singlet-triplet transition in p-benzoquinone-h₄ (PBQ-h₄) is shown to be induced by asymmetric isotopic substitution and the oscillator strength of this origin is seen to be accounted for by a corresponding decrease in intensity of a level 16.9 cm^?1 higher in energy in the pure PBQ-h₄ crystal. The combined oscillator strength of these close lying levels is measured and found to be almost independent of deuteration.These results are discussed in reference to the previously proposed double minimum potential model for the lowest nπ* triplet state in PBQ-h₄ and the applicability for this model is critically examined.Optical absorption experiments on heavily doped isotopic mixed crystals of PBQ-h₄ in PBQ-d₄ show hydrogen (deuterium) bounding effects between translational inequivalent molecules to be primarily responsible for the observed cluster states. These hydrogen bounding effects also induce the electronic origin of the B_1g (nπ*) triplet state in case of a translational inequivalent dimer.A detailed vibrational analysis of the phosphorescence spectrum of PBQ-h₄ in a PBQ-d₄ host crystal at 1.8 K is presented and it is shown that the unobserved origin of the B_1g (nπ*) triplet state of PBQ-h₄ is located at 18609 ± 1 cm^?1 and that the inversion splitting in this lowest excited state amounts to 21 ± 1 cm^?1 in this mixed crystal system. An isotope effect is study on the vibronic structure in the emission spectrum further indicates that the excited state structure of PBQ is isotope dependent.The observed large isotope effect on the ZFS parameters of the lowest triplet state of PBQ-h₄ is demonstrated to be an intramolecular phenomenon and explained as an isotope dependent spin-orbit contribution to the ZI-S parameters, induced by localization of the nπ* excitation on oxygen.Finally the dynamics of energy migration in the dilute PBQ-h₄ in PBQ-d₄ isotopic mixed crystal is probed by concentration and temperature dependent phosphorescence intensity measurements and it is suggested that trap-exciton band communication effects are of importance in this system.     

Étude lcao-mo-cndo/2 des liaisons po et no dans des composés X3PO ET X3NO

The authors have studied the electronic structure of X₃PO and X₃NO compounds (with X = F, Cl, CH₃), using the semi-empirical CNDO/2 method. All the calculations have been made with and without 3d functions on the phosphorus atom. The comparison between the calculated and experimental values, especially in the case of bond length, dipole moment, and orbital level order, shows the influence of the 3d orbitals in the PO bond, which contains a sigma donation P → O and a pπ(O)-dπ(P) back bonding. The NO bond has sigma character in trimethylamine oxide, but is partially a double bond in trifluoramine oxide.     

Energies of optical transitions in metal and semiconductor nanotubes

The energies of low-energy optical transitions were calculated, using the linearized attached cylindrical waves (LACW) method, as a function of inverse diameter d ^?1 for (n, n) metal nanotubes with n ranging from 3 to 12 and for (n, 0) semiconductor nanotubes with n ranging from 10 to 25. The calculations show that E ₁₁ in the metal nanotubes is higher than in the semiconductor nanotubes. Significant violations of the E ₁₁ ～ d ^?1 relationship are observed. For metal nanotubes, the situation is more complex because of the close energies of the π-π*-and σ-π* vertical transitions and because of the intersection of these characteristics in the range of 0.7 nm^?1 < d ^?1 < 1.0 nm^?1 (n = 8, 9, 10). For the semiconductor nanotubes, the E ₁₁ versus d ^?1 relationship is not linear, rather, it is oscillating; the E ₁₁(d ^?1) function alternates between two curves that refer to the (n, 0) nanotubes for which division of n by 3 gives 1 or 2 in the residue. On one hand, these features hamper the use of optical measurements in structure determination for the nanotubes; on the other, new criteria for nanotube classification appear.     

Nmr study of the VixSiX4−x compounds with X = Cl,Br; x = 0, 1, 2, 3

¹H, ¹³C and ²⁹Si NMR data for the compounds Vi_xSiX_4?x are reported. While the ¹H and ¹³C resonances from the π system are indicative of the electron-withdrawing inductive effect (-I) of the halogens, the ²⁹Si chemical shift data reveal not only a shift contribution originating from this inductive effect but also the important influence of a {d, σ^*-π} hyperconjugation [1]. This back-donation originates from the vinyl π system and not from the halogens. The chemical shift data and the coupling constants also show an important influence from steric interaction and even from an electric field effect caused by polarization of the silicon—halogen bond.     

High-resolution absorption spectra of diphenylnmethylenes

High-resolution absorption spectra of the following diphenylmethylenes (DPM_s) dispersed in benzophenone crystals at liquid-helium temperatures are presented: DPM-h₁₀, DPM-d₁₀, 4-chloro-DPM, and 4-bromo-DPM. The substituent effects concerning the electronic structure, transition energy and intensity are discussed. From polarization measurements, the electronic configurations of the ground and the first excited triplet states of these DPM_s are assigned as (pπ)¹(pσ)¹ and (pσ)¹(π*)¹, respectively. Further studies reveal a second excited triplet state, designated as (pπ)¹(π*)¹, which lies less than 1000 cm^-1 above the first excited triplet state of DPM. Diffuse broad bands appear as common features in all the spectra. Such diffuseness is discussed in terms of electron-phonon coupling of the low-lying excited states.