Interplay of structural dynamics and electronic effects in an engineered assembly of pentacene in a metal–organic framework

Charge carrier mobility is an important figure of merit to evaluate organic semiconductor (OSC) materials. In aggregated OSCs, this quantity is determined by inter-chromophoric electronic and vibrational coupling. These key parameters sensitively depend on structural properties, including the density of defects. We have employed a new type of crystalline assembly strategy to engineer the arrangement of the OSC pentacene in a structure not realized as crystals to date. Our approach is based on metal–organic frameworks (MOFs), in which suitably substituted pentacenes act as ditopic linkers and assemble into highly ordered π-stacks with long-range order. Layer-by-layer fabrication of the MOF yields arrays of electronically coupled pentacene chains, running parallel to the substrate surface. Detailed photophysical studies reveal strong, anisotropic inter-pentacene electronic coupling, leading to efficient charge delocalization. Despite a high degree of structural order and pronounced dispersion of the 1D-bands for the static arrangement, our experimental results demonstrate hopping-like charge transport with an activation energy of 64 meV dominating the band transport over a wide range of temperatures. A thorough combined quantum mechanical and molecular dynamics investigation identifies frustrated localized rotations of the pentacene cores as the reason for the breakdown of band transport and paves the way for a crystal engineering strategy of molecular OSCs that independently varies the arrangement of the molecular cores and their vibrational degrees of freedom.

Pentacene assembled into 1D arrays using a metal–organic framework (MOF) approach. This cofacial packing motif, which is not present in pentacene bulk, shows an interesting interplay of band-like and hopping-type transport.     

Water-soluble cruciforms: response to protons and selected metal ions

The syntheses of three water-soluble cruciform fluorophores (XF) carrying aniline- N, N-bisacetic acid, 2-hydroxyaniline- N, N, O-trisacetic acid, and 1,2-phenylenediamine- N, N, N', N'-tetrakisacetic acid are reported. The XF skeleton was synthesized by a Horner reaction to assemble the distyrylbenzene unit followed by a Sonogashira coupling to attach the phenyleneethynylene modules. The photophysics of both the sodium salts and the ethyl esters of the three carboxylated 1,4-bis(aminostyryl)-2,5-bis(phenylethynyl)benzenes were investigated in chloroform and in aqueous buffered solution at a pH of 7.0 and compared to that of 1,4-bis(dibutylaminostyryl)-2,5-bis(phenylethynyl)benzene (BDB). The attachment of the carboxylate units to the aniline nitrogens influenced the photophysics and the sensory responses of the XFs, as the combined effect of steric bulk and charge repulsion led to a blue-shifted absorption when compared to that of BDB. While the fluorescence of the water-soluble XFs is sensitive toward metal cations, the mode of sensing action is different from that of BDB, where direct complexation to the aniline nitrogen lowers the energy of the HOMO (but not of the LUMO), leading to a blue-shifted emission. In the case of the 2-hydroxyaniline- N, N, O-trisacetic acid and 1,2-phenylenediamine- N, N, N', N'-tetrakisacetic acid-functionalized XFs, interaction with metal cations in aqueous buffered solution is guided by a breakup of excimers that form in water at XF concentrations as low as 50 micromol x L (-1).     

Reduced fluorescence quenching of cyclodextrin-acetylene dye rotaxanes

An acetylene dye rotaxane with alpha-CD has been synthesized using the Heck-Cassar-Sonogashira-Hagihara-type reaction in aqueous solution. Free dye with tetracarboxylic acids is found to be highly sensitive to various metal ions, showing high Stern-Volmer constants, KSV. As CD encapsulation protects and stabilizes the threaded chromophore against an outside quencher, metal-insensitive biological tags are an obvious application for this class of molecules.     

Tetrabenzononacene: “Butterfly Wings” Stabilize the Core

In combination with bulky substituents at the core, fourfold benzannulation at the cata-positions stabilizes a nonacene sufficiently to allow its isolation and characterization by ¹H NMR and X-ray analysis. The four benzo units blueshift the absorption spectrum in comparison to a solely linear nonacene, but significantly increase the stability in the solid state.     

Increased activity of in situ catalysts for alkyne metathesis

[reaction: see text] Reaction of an enyne (1,1-diphenyl-pent-1-ene-3-yne) with a preheated mixture of Mo(CO)6/4-chlorophenol/3-hexyne at 130 degrees C furnished 1,1,6,6-tetraphenylhex-1,5-diene-3-yne in an 80% yield. If the starting material was heated with a mixture of Mo(CO)6 and 4-chlorophenol under identical conditions, no reaction was observed.     

Hydroxydialkylamino cruciforms: amphoteric materials with unique photophysical properties

Two amphoteric cruciforms 6 and 7 (XF; 4,4'-[(1E,1'E)-(2,5-bis{[4-(dibutylamino)phenyl]ethynyl}-1,4-phenylene)bis(ethene-2,1-diyl)]diphenol, 4,4'-[{2,5-bis[(E)-4-(dibutylamino)styryl]-1,4-phenylene}bis(ethyne-2,1-diyl)]diphenol) were prepared by a Horner reaction followed by a Sonogashira coupling and subsequent deprotection. The XFs display significant changes in absorption and emission when exposed to trifluoroacetic acid, tetrabutylammonium hydroxide, and metal triflates. The substitution pattern of 6 and 7 leads to spatial separation of the frontier molecular orbitals, which allows the HOMO or LUMO of the XF to be addressed independently by acidic or basic agents. XF 6, which has hydroxyl groups on the styryl axis, displays changes in emission color upon exposure to ten amines in eight different solvents. The change in fluorescence upon the addition of amines was analyzed by linear discriminant analysis. These XFs may have potential in sensor applications for metal cations and amines.     

A study of molybdenum catalysts in the polymerization of 2,5-didodecyl-1,4-dipropynylbenzene

The reaction of 2,5-didodecyl-1,4-dipropynylbenzene with different molybdenum sources (Mo(CO)₆, norbornadiene-Mo(CO)₄, cyclooctadiene-Mo(CO)₄, cycloheptatriene-Mo(CO)₃, (PhCCPh)₃Mo(CO), (acac)₂MoO₂/AlEt₃) was investigated in the presence of 4-chlorophenol or 2-fluorophenol. Upon heating to 105-130 °C, the formation of didodecyl-PPE resulted. The degree of polymerization of the PPE is dependent on the used phenol and to the utilized molybdenum precursor. The most active catalyst forms from (acac)₂MoO₂, AlEt₃ and 2-fluorophenol. This catalyst combination gives high molecular weight PPEs after 6 h at 105 °C.     

Linear and Star-Shaped Extended Di- and Tristyrylbenzenes: Synthesis,Characterization and Optical Response to Acid and Metal Ions

Two linear 1,4-distyrylbenzenes and five star-shaped 1,3,5-tristyrylbenzene derivatives ( L_2a and L_2b , Y₀–Y₃ and Y_NBu ) were synthesized and spectroscopically characterized. The photophysical properties, optical response to acid and metal ions were investigated. Upon backbone extension of linear distyrylbenzenes or the introduction of dibutylanilines, the electronic spectra are redshifted. Incorporation of electron-deficient pyridyl units does not significantly affect the optical properties. Variation of the number of pyridine rings and substitution pattern tune the fluorescence response to acids and metal ions. The novel arenes discriminate Al³⁺, Mn²⁺, Fe³⁺, Fe²⁺, Cd²⁺, Ag⁺ and Hg²⁺.     

Stress‐Induced Domain Wall Motion in a Ferroelastic Mn3+ Spin Crossover Complex

Domain wall motion is detected for the first time during the transition to a ferroelastic and spin state ordered phase of a spin crossover complex. Single‐crystal X‐ray diffraction and resonant ultrasound spectroscopy (RUS) revealed two distinct symmetry‐breaking phase transitions in the mononuclear Mn³⁺ compound [Mn(3,5‐diBr‐sal₂(323))]BPh₄, 1. The first at 250 K, involves the space group change Cc→Pc and is thermodynamically continuous, while the second, Pc→P1 at 85 K, is discontinuous and related to spin crossover and spin state ordering. Stress‐induced domain wall mobility was interpreted on the basis of a steep increase in acoustic loss immediately below the the Pc‐P1 transition