Trinuclear Copper(I) Complex Containing 3,4,9,10,15,16-Hexamethyl-1,6,7,12,13,18-hexaazatrinaphthylene: A Structural, Spectroscopic, and Computational Study

Reactions of polyaromatic molecules in crystals under electron beam of the transmission electron microscope

Reactivity of a series of related molecules under the 80 keV electron beam have been investigated and correlated with their structures and chemical composition. Hydrogenated and halogenated derivatives of hexaazatrinaphthylene, coronene, and phthalocyanine were prepared by sublimation in vacuum to form solventless crystals then deposited onto transmission electron microscopy (TEM) grids. The transformation of the molecules in the microcrystals were triggered by an 80 keV electron beam in the TEM and studied using correlated selected area electron diffraction, conventional bright field imaging, and energy dispersive X-ray spectroscopy. The critical fluence (ē nm^-2) required to cause a disappearance of the diffraction pattern was recorded and used as a measure of the reactivity of the molecules. The same electron flux (10² ē nm^-2 s^-1) was used throughout. Fully halogenated molecules were found to be the most stable and did not change significantly under our experimental conditions, followed by fully hydrogenated molecules with critical fluences of 10⁴ ē nm^-2. Surprisingly, semi-halogenated molecules that contained an equal number of hydrogen and halogen atoms were found to be the least stable, with critical fluences an order of magnitude lower at 10³ ē nm^-2. This is attributed to elimination of H-X (where X = F or Cl), followed by polymerisation of aryne / aryl radicals within the crystal. The critical fluence for the semi-fluorinated hexaazatrinaphthylene is the lowest as the presence of water molecules in its crystal lattice significantly decreased the stability of the organic molecules under the electron beam. Semi-halogenation reduces the beam stability of organic molecules compared to the parent hydrogenated molecule, thus providing the chemical guidance for design of electron beam stable materials. Understanding of molecular reactivity in the electron beam is necessary for advancement of molecular imaging and analysis methods by the TEM, molecular materials processing, and electron beam-driven synthesis of novel materials.

Acid Responsive Hydrogen-Bonded Organic Frameworks

A porous hydrogen-bonded organic framework (HOF) responsive to acid was constructed from a hexaazatrinaphthylene derivative with carboxyphenyl groups (CPHATN). Precise structures of both 1,2,4-trichlorobenzene solvate [CPHATN-1(TCB)] and activated HOF with permanent porosity (CPHATN-1a) were successfully determined by single-crystalline X-ray diffraction analysis. Permanent porosity of CPHATN-1a was evaluated by gas sorption experiments at low temperature. CPHATN-1a also shows significant thermal stability up to 633 K. Its crystals exhibit a rich photochemistry thanks to intramolecular charge-transfer and interunit proton-transfer reactions. Femtosecond (fs) experiments on crystals demonstrate that these events occur in ≤200 fs and 1.2 ps, respectively. Moreover, single-crystal fluorescence microscopy reveals a shift of the emission spectra most probably as a result of defects and a high anisotropic behavior, reflecting an ordered crystalline structure with a preferential orientation of the molecular dipole moments. Remarkably, CPHATN-1a, as a result of the protonation of pyradyl nitrogen atoms embedded in its π-conjugated core, shows reversible vapor acid-induced color changes from yellow to reddish-brown, which can be also followed by an ON/OFF of its emission. To the best of our knowledge, this is the first HOF that exhibits acid-responsive color changes. The present work provides new findings for developing stimuli responsive HOFs.

Polynuclear Cu(i) and Ag(i) phosphine complexes containing multi-dentate polytopic ligands: syntheses, crystal structures and photoluminescence properties

A series of polynuclear metal complexes, [Cu2(L1)(PPh3)4](ClO4)2 (1), [Cu3(L2)(PPh3)6](ClO4) (2), [Cu3(L3)(PPh3)6] (3), [Ag2(L1)(PPh3)4](BF4)2 (4), [Ag4(L2)2(PPh3)6] (5) and [Ag3(L3)(PPh3)5] (6), have been obtained from the reactions of the highly conjugated bridging ligands 2,3-bis(2-pyridyl)pyrazine (L1), 2,3-bis(2-tetrazoyl)pyrazine (H2L2) and 2,3-bis(2-tetrazoyl)imidazole (H3L3) with [Cu(MeCN)4]ClO4 and AgBF4, respectively. Their crystal structures have been determined by X-ray crystallography and their photophysical properties have been investigated in detail. Complexes 1 and 3 show photoluminescence in CH2Cl2 solution, while all the complexes exhibit obvious luminescence in the solid state; detailed photophysical studies and density functional theory calculations of these complexes have revealed that their lowest energy absorptions and emissions are predominantly derived from either metal-to-ligand charge-transfer (MLCT) or intraligand (IL) excited states.

Stretching the phenazine MO in dppz: the effect of phenyl and phenyl–ethynyl groups on the photophysics of Re(i) dppz complexes

Dppz complexes with extended conjugation show π,π* and MLCT excited states in solution. Raman spectral features for the short-lived π,π* state are observed in single-colour transient experiments.

Heteroleptic Cu(I) bis-diimine complexes of 6,6'-dimesityl-2,2'-bipyridine: a structural, theoretical and spectroscopic study

A series of heteroleptic Cu(I) complexes containing 6,6'-dimesityl-2,2'-bipyridine and phenanthroline-, bipyridine-, and biquinoline-based ligands is studied. The HETPHEN strategy is utilized to synthesize the heteroleptic complexes, which are stable in solution. The X-ray crystal structures of the complexes are presented; the solid-state four-coordinate Cu(I) geometries are quantified by using the τ4 parameter. A feature of the crystal structures is the intramolecular π-stacking between the mesityl ring(s) and the diimine ligand; the phen-based complexes exhibit stacking between the phen ligand and one of the mesityl rings, creating a "Pac-Man" motif. On the other hand, the bpy-based complexes show different types of packing interaction, with both mesityl rings "clamping down" on the bpy based ligand to give π-stacking. Cyclic voltammetry is used to examine the redox chemistry of the complexes. The most positive potentials for the oxidation process are observed for the complexes with bulky substituents ortho to the coordination nitrogens atoms, i.e., 2,9-dimethyl-1,10-phenanthroline and 6,6'-dibromo-2,2'-bipyridine. The Cu(I) MLCT transitions of the complexes are investigated by resonance Raman spectroscopy in concert with TD-DFT calculations. The resonance Raman spectra of complexes containing substituted biquinolines are straightforward, in that vibrational bands of the biquinoline-based ligand are selectively enhanced over bpy(Mes)2 bands. This is consistent with the purple color of the complexes, due to the lower energy of the biquinoline-based LUMO compared to the bpy(Mes)2 LUMO. All the phen- and bpy-based complexes show enhancement of bpy(Mes)2 bands.