Self-trapping of excitons, violation of condon approximation, and efficient fluorescence in conjugated cycloparaphenylenes

Cycloparaphenylenes, the simplest structural unit of armchair carbon nanotubes, have unique optoelectronic properties counterintuitive in the class of conjugated organic materials. Our time-dependent density functional theory study and excited state dynamics simulations of cycloparaphenylene chromophores provide a simple and conceptually appealing physical picture explaining experimentally observed trends in optical properties in this family of molecules. Fully delocalized degenerate second and third excitonic states define linear absorption spectra. Self-trapping of the lowest excitonic state due to electron-phonon coupling leads to the formation of spatially localized excitation in large cycloparaphenylenes within 100 fs. This invalidates the commonly used Condon approximation and breaks optical selection rules, making these materials superior fluorophores. This process does not occur in the small molecules, which remain inefficient emitters. A complex interplay of symmetry, π-conjugation, conformational distortion and bending strain controls all photophysics of cycloparaphenylenes.Fil: Adamska, Lyudmyla. Los Alamos National Laboratory. Los Alamos; Estados UnidosFil: Nayyar, Iffat. Los Alamos National Laboratory. Los Alamos; Estados UnidosFil: Chen, Hang. Boston University; Estados UnidosFil: Swan, Anna K.. Boston University; Estados UnidosFil: Oldani, Andres Nicolas. Universidad Nacional de Quilmes; ArgentinaFil: Fernández Alberti, Sebastián. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Quilmes; ArgentinaFil: Golder, Matthew R.. University of Oregon; Estados UnidosFil: Jasti, Ramesh. University of Oregon; Estados UnidosFil: Doorn, Stephen K.. Los Alamos National Laboratory. Los Alamos; Estados UnidosFil: Tretiak, Sergei. Los Alamos National Laboratory. Los Alamos; Estados Unido

Self-Assembly of Charged Supramolecular Sandwiches Formed by Corannulene Tetraanions and Lithium Cations

The reduction of corannulene (C₂₀H₁₀) with excess lithium metal in a strong chelating O-donor solvent, diglyme, leads to the formation of the highly reduced C₂₀H₁₀^4– anion. However, in contrast to the formation of the sandwich-type supramolecular aggregate [Li₅(C₂₀H₁₀^4–)₂]^3– observed in THF, corannulene tetraanions and lithium counterions in diglyme form only contact ion pairs according to ⁷Li NMR spectroscopy. Furthermore, the slow dissociation of the premade sandwich [Li₅(C₂₀H₁₀^4–)₂]^3– in neat diglyme has been demonstrated by multinuclear NMR spectroscopy. In contrast, the [Li₅(C₂₀H₁₀^4–)₂]^3– sandwich can be crystallized from the THF/diglyme mixture as the new crystalline product [Li­(THF)₂(diglyme)]⁺­[Li₂(THF)­(diglyme)//Li₅(C₂₀H₁₀^4–)₂]⁻, showing a complex 1D hybrid architecture according to the single-crystal X-ray diffraction study

Silicon in a Negatively Charged Shell: Anions of Spirosilabifluorene

Mono- and dianions of a polycyclic compound with a central sp³-hybridized silicon atom, spirosilabifluorene (C₂₄H₁₆Si, <b>1</b>), were prepared by reduction with alkali metals. The salts containing <b>1</b>^•– and <b>1</b>^2– anions were isolated and studied by single-crystal X-ray diffraction. The lithium salt of the C₂₄H₁₆Si^•– radical monoanion ([Li­(THF)₄⁺]­[<b>1</b>^•–], <b>2</b>) exists as a solvent-separated ion pair in the solid state. Substantially different geometrical parameters were found for each of the fluorene groups within the C₂₄H₁₆Si^•– anion of <b>2</b> due to asymmetric charge distribution. The C₂₄H₁₆Si^2– dianion was isolated in the form of its sodium ([{Na­(THF)₃⁺}­{Na­(THF)⁺(<b>1</b>^2–)], <b>3</b>) or potassium ([{K­(THF)⁺}₂(<b>1</b>^2–)], <b>4</b>) salt. The environment at the central silicon atom in the dianion is flattened in comparison to the monoanion and neutral compound, with the angle between the two fluorene planes measured at 55° in <b>1</b>^2– vs 89<b>°</b> in <b>1</b>^•– and 83° in <b>1</b>⁰. The aggregation of dianions and alkali-metal counterions leads to the formation of dimeric units and 1D polymeric chains in the solid sodium and potassium salts, respectively. The structure of the cesium salt <b>5</b>, containing both mono- and dianions in the crystal lattice, was also studied by X-ray diffraction. Complexes <b>2</b>–<b>5</b> were investigated by ESR and variable-temperature multinuclear NMR spectroscopy. Theoretical investigations at the PBE0, MP2, and multireference NEVPT2 levels of theory for the C₂₄H₁₆Si^<i>n</i>– (<i>n</i> = 0–2) species revealed the conjugation of two fluorene units over the central silicon atom and a singlet ground state for the dianion

Silicon in a Negatively Charged Shell: Anions of Spirosilabifluorene

Mono- and dianions of a polycyclic compound with a central sp³-hybridized silicon atom, spirosilabifluorene (C₂₄H₁₆Si, <b>1</b>), were prepared by reduction with alkali metals. The salts containing <b>1</b>^•– and <b>1</b>^2– anions were isolated and studied by single-crystal X-ray diffraction. The lithium salt of the C₂₄H₁₆Si^•– radical monoanion ([Li­(THF)₄⁺]­[<b>1</b>^•–], <b>2</b>) exists as a solvent-separated ion pair in the solid state. Substantially different geometrical parameters were found for each of the fluorene groups within the C₂₄H₁₆Si^•– anion of <b>2</b> due to asymmetric charge distribution. The C₂₄H₁₆Si^2– dianion was isolated in the form of its sodium ([{Na­(THF)₃⁺}­{Na­(THF)⁺(<b>1</b>^2–)], <b>3</b>) or potassium ([{K­(THF)⁺}₂(<b>1</b>^2–)], <b>4</b>) salt. The environment at the central silicon atom in the dianion is flattened in comparison to the monoanion and neutral compound, with the angle between the two fluorene planes measured at 55° in <b>1</b>^2– vs 89<b>°</b> in <b>1</b>^•– and 83° in <b>1</b>⁰. The aggregation of dianions and alkali-metal counterions leads to the formation of dimeric units and 1D polymeric chains in the solid sodium and potassium salts, respectively. The structure of the cesium salt <b>5</b>, containing both mono- and dianions in the crystal lattice, was also studied by X-ray diffraction. Complexes <b>2</b>–<b>5</b> were investigated by ESR and variable-temperature multinuclear NMR spectroscopy. Theoretical investigations at the PBE0, MP2, and multireference NEVPT2 levels of theory for the C₂₄H₁₆Si^<i>n</i>– (<i>n</i> = 0–2) species revealed the conjugation of two fluorene units over the central silicon atom and a singlet ground state for the dianion

Tuning the separation and coupling of corannulene trianion-radicals through sizable alkali metal belts

International audienceThe first heterobimetallic sandwich-type aggregate formed by bowl-shaped corannulene trianion-radicals,C20H10c3-, has been synthesized using mixed-metal reduction of C20H10. The product was crystallographically characterized to reveal the self-assembly of [Cs+//(C20H10.3-)/4K+/(C20H10 3-)//Cs+], in which two triply-charged corannulene decks encapsulate a rectangle of four potassium ions (the K/K separations are 4.212(4) and 5.185(4) °A), with the exterior concave bowl cavities being selectively filled by one cesium ion each. In order to provide insights into the geometrical features and electronic structure of this novel mixed-metal organometallic self-assembly, an in-depth theoretical investigation has been carried out..