Typical Jahn-Teller Distortion Research Articles

Syntheses, crystal structures, and magnetic behaviors of phenoxo-bridged manganese compounds based on tetradentate Schiff bases

Six Mn(II)/Mn(III) coordination compounds with tetradentate Schiff bases, [Mn(L1)(N3)]2 (1), [Mn(L2)(N3)]2 (2), [Mn(L1)(SCN)]2 (3), [Mn(L2)(SCN)]2[Mn(L2)(SCN)(C2H5OH)]2 (4), [Mn(L1)(CH3OH)]2 (5), and [Mn3(L2)2(CH3COO)2] (6) [H2L1 = N,N'-bis(5-chloro-salicylaidimine)-1,2-diaminop-ropane), H2L2 = N,N'-bis(5-chloro-salicylaidimine)-2,2-dimethylpropane-1,3-diami- ne], were structurally and magnetically characterized by X-ray single crystal diffraction, powder X-ray diffraction (PXRD), elemental analysis, infrared (IR) spectroscopy and magnetic susceptibility measurement. Structural analysis reveals that each manganese center displays a typical Jahn-Teller distortion. Compounds 1, 2, 3 and 5 are binuclear systems in which two central manganese atoms are connected by di-u2-phenoxo-bridging from two inverse tetradentate ligands. The asymmetric unit of compound 4 contains two moieties, which is composed of a dimeric unit [Mn(L1)(SCN)]2 and two monomeric units [Mn(L1)(SCN)(C2H5OH)]. Compound 6 is a linear homo-trinuclear structure. Dc variable-temperature magnetic susceptibility measurements show that compounds 1, 2, 4 and 5 exhibit paramagnetic behaviors with J = +0.202 cm−1 for 1, J = +0.203 cm−1 for 2, J = +1.03 cm−1 for 4 and + 0.152 cm−1 for 5, respectively, whereas compounds 3 and 6 show a strong antiferromagnetic interaction between the high-spin metal centers. In addition, we investigated the relationship between the structures and magnetic behaviour.

Doping dependent electronic and magnetic ordering in mixed-valent La1−x Sr x MnO3 thin films

We have investigated the collective electronic and magnetic orderings of a series of La1−x Sr x MnO3 thin films grown epitaxially strained to (001) oriented strontium titanate substrates as a function of doping, x, for 0 ≤ x ≤ 0.4. We find that the ground states of these crystalline thin films are, in general, consistent with that observed in bulk crystals and thin film samples synthesized under a multitude of techniques. Our systematic study, however, reveal subtle features in the temperature dependent electronic transport and magnetization measurements, which presumably arise due to Jahn-Teller type distortions in the lattice for particular doping levels. For the parent compound LaMnO3 (x = 0), we report evidence of a strain-induced ferromagnetic ordering in contrast to the antiferromagnetic ground state found in bulk crystals.

Assessment of dynamic structural instabilities across 24 cubic inorganic halide perovskites.

Metal halide perovskites are promising candidates for next-generation photovoltaic and optoelectronic applications. The flexible nature of the octahedral network introduces complexity when understanding their physical behavior. It has been shown that these materials are prone to decomposition and phase competition, and the local crystal structure often deviates from the average space group symmetry. To make stable phase-pure perovskites, understanding their structure-composition relations is of central importance. We demonstrate, from lattice dynamics calculations, that the 24 inorganic perovskites ABX3 (A = Cs, Rb; B = Ge, Sn, Pb; X = F, Cl, Br, I) exhibit instabilities in their cubic phase. These instabilities include cation displacements, octahedral tilting, and Jahn-Teller distortions. The magnitudes of the instabilities vary depending on the chemical identity and ionic radii of the composition. The tilting instabilities are energetically dominant and reduce as the tolerance factor increases, whereas cation displacements and Jahn-Teller type distortions depend on the interactions between the constituent ions. We further considered representative tetragonal, orthorhombic, and monoclinic perovskite phases to obtain phonon-stable structures for each composition. This work provides insights into the thermodynamic driving force of the instabilities and will help guide computer simulations and experimental synthesis in material screening.

Pauladamsite, Cu4(SeO3)(SO4)(OH)4·2H2O, a new mineral from the Santa Rosa mine, Darwin district, California, USA

AbstractPauladamsite (IMA2015-005), Cu4(SeO3)(SO4)(OH)4·2H2O, is a new mineral from the Santa Rosa mine, Darwin district, Inyo County, California, USA, where it occurs as a secondary oxidation-zone mineral in association with brochantite, chalcanthite, gypsum, ktenasite, mimetite, schulenbergite and smithsonite on limonitic gossan. Pauladamsite forms green, multiply twinned blades up to 0.5 mm long grouped in radial sprays. The streak is pale green. Crystals are transparent and have vitreous to silky lustre. The Mohs hardness is ∼2, the tenacity is brittle, the fracture is irregular and crystals exhibit one perfect cleavage on [001]. The calculated density is 3.535 g/cm3. Electron microprobe analyses provided: CuO 48.96, ZnO 3.56, SeO2 18.82, SO3 13.90, H2O 13.29 (calc.), total 98.53 wt.%. The empirical formula (based on 13 O apfu) is: (Cu3.55Zn0.25)∑3.80Se0.98SO13H8.50. Pauladamsite is triclinic, P1, a = 6.0742(7), b = 8.4147(11), c = 10.7798 (15) Å, α = 103.665(7), β = 95.224(7), γ = 90.004(6)°, V = 533.03(12) Å3 and Z = 2. The eight strongest lines in the powder X-ray diffraction pattern are [dobs in Å(I)(hkl)]: 10.5(46)(011); 3.245(100)(001); 5.81(50)(011); 2.743(49)(112); 3.994(67)(012); 3.431(23)(1̄12,1̄2̄1,1̄20); 2.692(57)(03̄2,1̄22,2̄1̄2); and 2.485(39)(21̄2,1̄3̄2,02̄4). The structure of pauladamsite (R1 = 10.6% for 2086 Fo > 4σF) contains Cu2+O6 octahedra, SO4 tetrahedra and Se4+O3 pyramids. There are four different CuO6 octahedra, each of which exhibits typical Jahn-Teller distortion, with four short equatorial Cu–O bonds and two much longer apical Cu–O bonds. The CuO6 octahedra share edges to form five-octahedra-wide bands extending along [100]. Adjacent bands are connected in the [011̄] direction by bridging SO4 tetrahedra and in the [011] direction by bridging Se4+O3 pyramids, thereby forming a framework.

Enhanced charge ordering transition in dopedCaFeO3through steric templating

We report density functional theory (DFT) investigation of $B$-site doped CaFeO$_3$, a prototypical charge-ordered perovskite. At 290 K, CaFeO$_3$ undergoes a metal-insulator transition and a charge disproportionation reaction 2Fe$^{4+}$$\rightarrow$Fe$^{5+}$+Fe$^{3+}$. We observe that when Zr dopants occupy a (001) layer, the band gap of the resulting solid solution increases to 0.93 eV due to a 2D Jahn-Teller type distortion, where FeO$_6$ cages on the $xy$ plane elongate along $x$ and $y$ alternatively between neighboring Fe sites. Furthermore, we show that the rock-salt ordering of the Fe$^{5+}$ and Fe$^{3+}$ cations can be enhanced when the $B$-site dopants are arranged in a (111) plane due to a collective steric effect that facilitates the size discrepancy between the Fe$^{5+}$O$_6$ and Fe$^{3+}$O$_6$ octahedra and therefore gives rise to a larger band gap. The enhanced charge disproportionation in these solid solutions is verified by rigorously calculating the oxidation states of the Fe cations with different octahedral cage sizes. We therefore predict that the corresponding transition temperature will increase due to the enhanced charge ordering and larger band gap. The compositional, structural and electrical relationships exploited in this paper can be extended to a variety of perovskites and non-perovskite oxides providing guidance in structurally manipulating electrical properties of functional materials.

Investigation of the Properties of Ba-Substituted La0.7Sr0.3−x Ba x MnO3 Perovskite Manganite Films for Resistive Switching Applications

La0.7Sr0.3−xBaxMnO3 (LSBMO: x = 0.09, 0.18, and 0.27) thin films were prepared on Pt-coated Si substrates using a radiofrequency magnetron sputtering technique at a substrate heating temperature of 450°C. The effects of varying the amount of substituted Ba2+ on the physical, chemical, and electrical properties of the perovskite manganite films were systematically investigated. X-Ray diffraction showed that the growth orientation and crystallinity of films were not affected by the amount of substituted Ba cations. Raman spectroscopy was used to determine the tilt of MnO6 octahedra and the Jahn–Teller-type distortion variation of the manganite films. The change in covalent characteristics of Mn–O bonds with increasing amounts of Ba2+ substituent was analyzed by x-ray photoelectron spectroscopy, specifically to examine the effects of bond characteristics on the resistive switching properties of LSBMO. The resistance of the LSBMO films increased with increasing Ba2+ content due to an increase in the covalent nature of Mn–O bonds. The resistive switching ratio increased with increasing Ba2+ amount, and relationships among resistive switching, Jahn–Teller distortion, and Mn–O bond character of LSBMO films were interpreted.

End-to-End Azide-Bridged Manganese(III) Chain Compounds: Field-Induced Magnetic Phase Transitions and Variation of TC to 38 K Depending on the Side Groups of the Schiff Bases

Three one-dimensional coordination polymers [Mn(L)(N(3))](n) [L = L1 (1), L2 (2), L3 (3); L1H(2) = N,N'-bis(5-chlorosalicylideneiminato)-1,3-diaminopentane, L2H(2) = N,N'-bis(5-bromosalicylideneiminato)-1,3-diaminopentane, L3H(2) = N,N'-bis(5-bromosalicylideneiminato)-1,3-diamino-2-dimethylpropane] bridged by end-to-end azides were prepared. The crystal systems differ according to the Schiff bases used. Each Mn atom adopts a typical Jahn-Teller distortion. The helicity of the chains occurs in a racemic manner only for 2. No noncovalent forces are relevant in 2, while π-π contacts are visible in 1 and 3. Magnetic measurements show the presence of apparent spin canting. Complexes 1 and 3 exhibit a field-induced metamagnetic transition from an antiferromagnetic state to a weak ferromagnetic phase, whereas 2 embraces a field-induced two-step magnetic phase transition. The critical temperature is observed at 38 K for 2, which is relatively higher than those for 1 (11 K) and 3 (10 K). The pronounced long-range order may contribute from intrachain exchange couplings and through-space dipolar interactions between adjacent chains.

Bis(μ-4-phenylpyridineN-oxide-κ2O:O)bis[bis(1,1,1,5,5,5-hexafluoropentane-2,4-dionato)copper(II)

The asymmetric unit of the title compound, [Cu2(C5HF6O2)4(C11H9NO)2], contains one half of the centrosymmetric dinuclear complex. The coordination geometry of the CuII atom is octa­hedral, exhibiting a typical Jahn–Teller distortion. One trifluoro­methyl group is rotationally disordered between two orientations in a 1:1 ratio.

Magnatic field induced phase transition and transport properties in phase separated Eu0.5Sr0.5MnO3 system

The structure，electrical and magnetic properties of the half doping phase separated Eu0.5Sr0.5MnO3 manganite have been systematically studied. The results show that Eu0.5Sr0.5MnO3 compound has the O′ orthorhombic structure and shows typical Jahn-Teller distortion. It is found that ferromagnetic phase turn up near 75 K and evident split of field cooling （FC） and zero field cooling （ZFC） under 4000 A/m is observed around 42 K. The ac susceptibility curve shows also a sharp peak around 42 K，indicating cluster-glass state rather than a spin glass state at 42 K. Meanwhile，throughout the measuring temperature range，the electric transport shows insulating behavior and there is no insulator-metal （I-M） transition. However，an applied magnetic field of 1.6×105 A/m can induce I-M transition. The conducting behavior of Eu0.5Sr0.5MnO3 is well fitted by the Mott variable range hopping （VRH） model. All these phenomena indicate that for the ground state of Eu0.5Sr0.5MnO3，there exists the competition mechanism of several complex magnetic interactions. The study provides abundant experimental information to underdtand the mechanism of the strongly electron correlated system.

X-ray study of the modulated structure in quenched Ga 2Te 3 with a defect zinc-blende lattice

Ga 2Te 3 crystallizes into the zinc-blende structure, where one-third of the cation sites are vacant. Single-crystal X-ray diffraction studies have been performed on quenched Ga 2Te 3 crystals. The diffraction maps show, apart from the main zinc-blende-type reflections, satellite reflections at q = (0.06 0.06 0.00) c, where the subscript c indicates the cubic sublattice. Analysis of the satellite reflections shows that the incommensurate structure is two-dimensional. The structure is ascribable to the amplitude-type modulation caused by Ga vacancies and the coupled displacive modulation of Te atoms, which has a polarization vector along the 〈0 0 1〉 c direction. The nature of the atomic modulations is also discussed and the origin of the modulation is attributed to Jahn–Teller-type distortions around Ga vacancies.

The role of orbital ordering in the tetragonal-to-cubic phase transition in CuCr 2O 4

Copper(II) chromite (CuCr 2O 4) undergoes a first-order structural transition from a tetragonal distorted spinel structure in space group ( I4 1/ amd) to a cubic spinel structure in Fd3¯ m, near 600 °C. The transition has been followed using synchrotron X-ray powder diffraction between room temperature and 750 °C. The structure changes as a consequence of a transition from an orbitally ordered to orbital disordered state associated with a Jahn–Teller-type distortion of the CuO 4 tetrahedron. The orbital melting results in a small increase in cell volume.

Mechanisms for biosorption of chromium(III), copper(II) and mercury(II) using water extracts of Moringa oleifera seed powder

In continuation of our work on heavy metal remediation using Moringa seed powder, this study examines the mechanisms of metal sorption on water extracts of Moringa oleifera (MO) seed powder using extended X-ray absorption fine structure (EXAFS). Chromium(III) is hydrolysed to form a mixture of [Cr3(OH)4(H2O)10]5+ and [Cr(OHx)2 ]n n
(2x-3)+ complexes, x = 1 or 2. The chromium(III) complexes are octahedral with mean Cr-O bond distance of 1.97(2) A. Copper (II) forms complexes with ligands with oxygen and/or nitrogen donor atoms, most likely amino and carboxylate groups. Copper(II) retain the typical Jahn-Teller distortion with Cu-O/N bond distances of 1.97(2) and 2.21(4) A. Furthermore, a Cu


C distance at 2.96 A, and a corresponding Cu-O-C 3-leg scattering path at 3.10 A are observed as well, strongly supporting that a large fraction of carboxylate groups are bound to the copper(II) ion on the equatorial plane. Mercury (II) forms mainly linear complexes with nitrogen donor ligands, d(Hg-N) =2.15(2) A, most probably from amino groups in amino acids or proteins. This shows that the softer metal ions, copper(II) and mercury(II), form complexes with oxygen and/or nitrogen donor ligands in the MO extracts, while the harder and more highly charged chromium(III) ion becomes hydrolysed. The study therefore suggests that the successful biosorption of heavy metals by Moringa, a potential heavy metal removing agent, is attributable to its oxygen and nitrogen donating carboxylate and amino groups.

Reentering spin glass behavior and charge ordering in phase separation Nd0.5Ca0.5MnO3 system

Half-doped manganese oxides are very important on clarifying physical mechanism of strong correlation electron system and colossal magnetoresistance (CMR) effec t due to their rich physical content. The structure, electrical and magnetic tra nsport properties of Nd0.5Ca0.5MnO3 manganite w as systemically studied. The results show that Nd0.5Ca0.5 MnO3 compound displays the O′-orthorhombic structure and indicate th e existence of the typical Jahn-Teller distortion under 300 K. The transport pro perties show that the system undergoes paramagnetic insulator-ferromagnetic meta l transition under a magnetic field of 8 T and is accompanied by CMR effect. Mea nwhile, it is found that the temperature of antiferromagnetic transition and cha rge ordering is around 150 and 240K, respectively. There appears a typical reent rant spin glass behavior around 41K and the negative magnetization is also obser ved in the compound. All these phenomena indicate that there exist several comp lex magnetic interactions in the ground state of this half-doped Nd0.5Ca0.5MnO3 compound at low temperature. The present result s provide experimental data for strongly electronic correlated system of manga nites.

Charge order phase and reentrant spin-glass behavior in half doped Sm0.5Ca0.5MnO3 manganite

The structure, transport and magnetic properties of half doped Sm0.5Ca0.5MnO3 manganite has been systamatically studied. The results show that Sm0.5Ca0.5MnO3 compound has the O′orthorhombic structure and shows typical Jahn-Teller distortion. Throughout the temperature range used, the electric transport shows semiconducting behavior and there are no I-M transition and CMR effect. Meanwhile, it is found that the charge-order and antiferromagnetic transition temperatures of the system are around 270K and 200K, respectively, whereas at lower temperature there appears a typical spin-glass state around 41K with reentrant behavior. The negative magnetization is also observed in this compound. All these phenomena indicate that for the ground state of Sm0.5Ca0.5MnO3, there exists the competition mechanism of several complex magnetic interactions. Present study may provide abundant experimental information for understandig the mechanism of the strongly electron correlated system.

Dimensional Crossover in Transport Property in Manganite Thin Films with Finely Controlled Lattice Distortion

We have investigated the effect of Jahn–Teller-type distortion in manganite thin films with finely controlled lattice distortion by exploiting epitaxial strain effect and tuning A -site ionic radius. The magnetic and transport properties of these films indicate the existence of the dimensional crossover in conduction as the lattice distortion changes. We also found a universal relationship between magnetization and resistivity irrespective of the sample compositions.

Evolution of the spin state of a 3d ion in a pyramidal complex

The evolution of the spin state of a metallic 3d ion in a pyramidal MeO5 environment is studied. Ions of metals with 3d6 (Fe2+,Co3+) and 3d4 configurations (Mn3+,Cr2+) are studied. The change in the spin state of the metal ion from S=0 to S=1 and S=2 with a variation of the effective charge Zeff of the metal ion and the displacements of the oxygen ions is investigated. The stability of the spin state as a function of the magnitude and symmetry of the displacements of the ligands is checked. Breathing-mode distortions, displacements of the 3d ion along the Z axis, Jahn-Teller distortions of the basal plane of the pyramid, and fluting distortions of the plane of the pyramid are studied. It is shown that different types of distortions produce different behaviors of the terms corresponding to a change in the spin states of all three types. It is found that the spin state of the coordination complex MeO5 with the 3d4 configuration of the metal ion is most sensitive to breathing-mode distortions and Jahn-Teller type distortions stabilize the state with spin S=1 for the 3d6 configuration.

Jacquesdietrichite, Cu2BO(OH)2(OH)3, a new mineral from the Tachgagalt mine, Morocco: Description and crystal structure

Jacquesdietrichite, Cu 2 [BO(OH) 2 ](OH) 3 , space group Pnma, a , = 9.455(2), b = 5.866(2), c = 8.668(2) A, V = 480.8 A 3 , Z = 4, is a new mineral from the Tachgagalt mine, Morocco. The strongest eight lines of the X-ray powder-diffraction pattern [ d in A (I) (hkl)] are: 4.734 (100) (200), 3.941 (90) (102), 3.192 (40) (202), 2.545 (45) (302), 2.489 (50) (220), 1.922 (50) (322), 1.838 (40) (420), 1.712 (40) (124). It occurs as transparent bright-blue blades, sometimes intergrown into broader tablets, and as aggregates of pale-blue scales. It is found in small cavities in gaudefroyite-calcite rock and as inclusions in calcite. Blades do not exceed 0.5 mm in length and 0.02 mm in thickness (parallel to {001}), and often taper somewhat in thickness from tip to base. The forms in order of prominence are {001}, {100} and {010}. The streak is pale blue, the luster is vitreous and the hardness is about 2. Blades are slightly flexible with irregular fracture and exhibit a perfect and very easy {100} cleavage and good {010} and {001} cleavages. The measured density (by sink-float) is 3.28(5) g/cm 3 ; the calculated density is 3.303 g/cm 3 . The mineral dissolves quickly in dilute HC1. Electron microprobe analyses yielded CuO 72.68, CaO 0.11, SiO 2 1.00, B 2 O 3 (calc.) [14.57], H 2 O (calc.) [18.85], total 107.21 wt%. CaO and SiO 2 are considered to be from impurities and the high total to be due to volatilization. Jacquesdietrichite is biaxial (−) with α = 1.627(2), β = 1.699(2), γ = 1.769(2), 2 V (calc.) = 86°, strong pleochroism: X (deep blue) > Y (medium blue) > Z (pale blue); orientation: X = a , Y = c , Z = b . The name honors Jacques Emile Dietrich, Ph.D., of Nice, France (born 1926), retired geologist/mineralogist (Chief Geologist - Autonomous Corps of Geologists of Overseas French Territories) who worked extensively in Morocco. Dr. Dietrich discovered the new mineral. The crystal structure of jacquesdietrichite was solved by direct methods and refined by full-matrix least-squares techniques to R = 6.57% for 295 observed reflections [Fo > 4σ(Fo)]. Difference maps allowed the approximate location of all hydrogens. CuO 6 octahedra form trans edge-sharing (rutile-like) chains parallel to [010]. The CuO 6 octahedra exhibit typical Jahn-Teller distortion with lengthened Cu-O bonds corresponding to the trans apical vertices that do not participate in the octahedral edge-sharing. The octahedral chains are joined into layers parallel to (100) by sharing the apical octahedral vertices in the [001] direction. Triangular BO(OH) 2 groups link the octahedral layers in the [100] direction yielding a framework structure. Jacquesdietrichite is the only known mineral containing the BO(OH) 2 group and its structure is not closely related to any other known borate mineral. In the structural hierarchy of copper-oxysalt minerals, jacquesdietrichite has an M=M-T framework structure, but the structure is not topologically similar to that of any known copper-oxysalt mineral.

Cu(II) in liquid ammonia: an approach by hybrid quantum-mechanical/molecular-mechanical molecular dynamics simulation.

To investigate the solvation structure of the Cu(II) ion in liquid ammonia, ab initio quantum-mechanical/molecular-mechanical (QM/MM) molecular dynamics (MD) simulations were carried out at Hartree Fock (HF) and hybrid density functional theory (B3 LYP) levels. A sixfold-coordinated species was found to be predominant in the HF case whereas five- and sixfold-coordinated complexes were obtained in a ratio 2:1 from the B3 LYP simulation. In contrast to hydrated Cu(II), which exhibits a typical Jahn-Teller distortion, the geometrical arrangement of ligand molecules in the case of ammonia can be described as a [2 + 4] ([2 + 3]) configuration with 4 (3) elongated copper-nitrogen bonds. First shell solvent exchange reactions at picosecond rate took place in both HF and B3 LYP simulations, again in contrast to the more stable sixfold-coordinated hydrate. NH3 ligands apparently lead to strongly accelerated dynamics of the Cu(II) solvate due to the "inverse" [2 + 4] structure with its larger number of elongated copper-ligand bonds. Several dynamical properties, such as mean ligand residence times or ion-ligand stretching frequencies, prove the high lability of the solvated complex.

Electronically driven structural distortions in lithium intercalates of the n = 2 Ruddlesden-Popper-type host Y2Ti2O5S2: synthesis, structure, and properties of LixY2Ti2O5S2 (0 < x < 2).

Lithium intercalation into the oxide slabs of the cation-deficient n = 2 Ruddlesden-Popper oxysulfide Y(2)Ti(2)O(5)S(2) to produce Li(x)Y(2)Ti(2)O(5)S(2) (0 < x < 2) is described. Neutron powder diffraction measurements reveal that at low levels of lithium intercalation into Y(2)Ti(2)O(5)S(2), the tetragonal symmetry of the host is retained: Li(0.30(5))Y(2)Ti(2)O(5)S(2), I4/mmm, a = 3.80002(2) A, c = 22.6396(2) A, Z = 2. The lithium ion occupies a site coordinated by four oxide ions in an approximately square planar geometry in the perovskite-like oxide slabs of the structure. At higher levels of lithium intercalation, the symmetry of the cell is lowered to orthorhombic: Li(0.99(5))Y(2)Ti(2)O(5)S(2), Immm, a = 3.82697(3) A, b = 3.91378(3) A, c = 22.2718(2) A, Z = 2, with ordering of Li(+) ions over two inequivalent sites. At still higher levels of lithium intercalation, tetragonal symmetry is regained: Li(1.52(5))Y(2)Ti(2)O(5)S(2), I4/mmm, a = 3.91443(4) A, c = 22.0669(3) A, Z = 2. A phase gap exists close to the transition from the tetragonal to orthorhombic structures (0.6 < x < 0.8). The changes in symmetry of the system with electron count may be considered analogous to a cooperative electronically driven Jahn-Teller type distortion. Magnetic susceptibility and resistivity measurements are consistent with metallic properties for x > 1, and the two-phase region is identified as coincident with an insulator to metal transition.

Photophysics of trioxatriangulenium ion. electrophilic reactivity in the ground state and excited singlet state.

Trioxatriangulenium (TOTA+. 4,8,12-trioxa-4,8,12,12c-tetrahydro-dibenzo[cd,mn]-pyrenylium) is a closed shell carbenium ion, which is stable in non-nucleophilic polar solvents at ambient temperatures In alcohols, small quantities of the leuco ether are formed in a reversible reaction. The physical and chemical properties of the excited singlet state of the trioxatriangulenium (TOTA+) carbenium ion are investigated by experimental and computational means The degeneracy of the lowest excited states is counteracted by Jahn-Teller-type distortion, which leads to vibronic broadening of the long wavelength absorption band. A strong fluorescence is observed at 520 nm (tau(fl) = 14.6 ns, phi(fl) = 0.12 in deaerated acetonitrile). The fluorescence is quenched by 10 aromatic electron donors predominantly via a dynamic charge transfer mechanism, but ground state complexation is shown to contribute in varying degrees Quenching is also observed in the presence of halide ions Quenching rate constants are derived from lifetime measurements while charge transfer (CT) complex formation constants follow from the steady-state Stern-Volmer plots. CT-complex formation with three discogenic triphenylenes is studied separately. Phosphorescence spectra, triplet lifetimes. and triplet-triplet absorption spectra are provided. In the discussion, TOTA+ is compared to the unsubstituted xanthenium ion and its 9-phenyl derivative with respect to the excited state properties.