16 research outputs found
Structure and photophysics of indigoids for singlet fission: Cibalackrot
We report an investigation of structure and photophysics of thin layers of cibalackrot, a sturdy dye derived from indigo by double annulation at the central double bond. Evaporated layers contain up to three phases, two crystalline and one amorphous. Relative amounts of all three have been determined by a combination of X-ray diffraction and FT-IR reflectance spectroscopy. Initially, excited singlet state rapidly produces a high yield of a transient intermediate whose spectral properties are compatible with charge-transfer nature. This intermediate more slowly converts to a significant yield of triplet, which, however, does not exceed 100% and may well be produced by intersystem crossing rather than singlet fission. The yields were determined by transient absorption spectroscopy and corrected for effects of partial sample alignment by a simple generally applicable procedure. Formation of excimers was also observed. In order to obtain guidance for improving molecular packing by a minor structural modification, calculations by a simplified frontier orbital method were used to find all local maxima of singlet fission rate as a function of geometry of a molecular pair. The method was tested at 48 maxima by comparison with the ab initio Frenkel-Davydov exciton model. Published under license by AIP Publishing
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Structure of a monolayer of molecular rotors on aqueous subphase from grazing-incidence X-ray diffraction.
In situ grazing-incidence X-ray scattering shows that a monolayer of artificial rod-shaped dipolar molecular rotors produced on the surface of an aqueous subphase in a Langmuir trough has a structure conducive to a 2D ferroelectric phase. The axes of the rotors stand an average of 0.83 nm apart in a triangular grid, perpendicular to the surface within experimental error. They carry 2,3-dichlorophenylene rotators near rod centers, between two decks of interlocked triptycenes installed axially on the rotor axle. The analysis is based first on simultaneous fitting of observed Bragg rods and second on fitting the reflectivity curve with only three adjustable parameters and the calculated rotor electron density, which also revealed the presence of about seven molecules of water near each rotator. Dependent on preparation conditions, a minor and variable amount of a different crystal phase may also be present in the monolayer
Production of Hydrated Metal Ions by Fast Ion or Atom Beam Sputtering. Collision-Induced Dissociation and Successive Hydration Energies of Gaseous Cu+ with 1-4 Water Molecules
Low-temperature sputtering of frozen aqueous solutions of metal salts, of hydrated crystalline transition-metal salts, of frosted metal surfaces, and of frosted metal salts with kiloelectronvolt energy rare gas atoms or ions produces copious amounts of cluster ions, among which M+(H2O)n and/or M+OH(H2O)n frequently dominate. Variable-energy collision-induced dissociation of these ions in a triple quadrupole mass spectrometer yields the successive gas-phase solvation energies. Several known hydration and bond energies have been reproduced, and the first and second hydration energies of the Cu+ ion have been determined as 35 ± 3 and 39 ± 3 kcal/mol, respectively. It is concluded that gaseous Cu+ prefers dicoordination.
Porphene and Porphite as Porphyrin Analogs of Graphene and Graphite
Two-dimensional materials have unusual properties and promise applications in nanoelectronics, spintronics, photonics, (electro)catalysis, separations, and elsewhere. Most are inorganic and their properties are difficult to tune. Here we report the preparation of Zn porphene, a member of a new organic metalloporphene family. Similar to graphene, these also are fully conjugated two-dimensional polymers, but are composed of fused metalloporphyrin rings. Zn porphene is synthesized on water surface by two-dimensional oxidative polymerization of a Langmuir bilayer of Zn porphyrin with K2IrCl6, reminiscent of known one-dimensional polymerization of pyrroles. It is transferable to other substrates and bridges μm-sized pits. Contrary to previous theoretical predictions of metallic conductivity, it is a p-type semiconductor due to a Peierls distortion of its unit cell from square to rectangular, analogous to the appearance of bond-length alternation in antiaromatic molecules. The observed reversible insertion of various metal ions, possibly carrying a fifth or sixth ligand, promises tunability and even patterning of circuits on an atomic canvas without removing any π centers from conjugation
The Scope of Direct Alkylation of Gold Surface with Solutions of C<sub>1</sub>–C<sub>4</sub> <i>n</i>‑Alkylstannanes
Treatment
of cleaned gold surfaces with dilute tetrahydrofuran
or chloroform solutions of tetraalkylstannanes (alkyl = methyl, ethyl, <i>n</i>-propyl, <i>n</i>-butyl) or di-<i>n</i>-butylmethylstannyl tosylate under ambient conditions causes a self-limited
growth of disordered monolayers consisting of alkyls and tin oxide.
Extensive use of deuterium labeling showed that the alkyls originate
from the stannane and not from ambient impurities, and that trialkylstannyl
groups are absent in the monolayers, contrary to previous proposals.
Methyl groups attached to the Sn atom are not transferred to the surface.
Ethyl groups are transferred slowly, and propyl and butyl rapidly.
In all cases, tin oxide is codeposited in submonolayer amounts. The
monolayers were characterized by ellipsometry, contact angle goniometry,
polarization modulated IR reflection absorption spectroscopy, X-ray
photoelectron spectroscopy, and electrochemical impedance spectroscopy
with ferrocyanide/ferricyanide, which revealed a very low charge-transfer
resistance. The thermal stability of the monolayers and their resistance
to solvents are comparable with those of an <i>n</i>-octadecanethiol
monolayer. A preliminary examination of the kinetics of monolayer
deposition from a THF solution of tetra-<i>n</i>-butylstannane
revealed an approximately half-order dependence on the bulk solution
concentration of the stannane, hinting that more than one alkyl can
be transferred from a single stannane molecule. A detailed structure
of the attachment of the alkyl groups is not known, and it is proposed
that it involves direct single or multiple bonding of one or more
C atoms to one or more Au atoms
Challenges in the Structure Determination of Self-Assembled Metallacages: What Do Cage Cavities Contain, Internal Vapor Bubbles or Solvent and/or Counterions?
Proving
the structures of charged metallacages obtained by metal
ion coordination-driven solution self-assembly is challenging, and
the common use of routine NMR spectroscopy and mass spectrometry is
unreliable. Carefully determined diffusion coefficients from diffusion-ordered
proton magnetic resonance (DOSY NMR) for six cages of widely differing
sizes lead us to propose a structural reassignment of two molecular
cages from a previously favored trimer to a pentamer or hexamer, and
another from a trimer to a much higher oligomer, possibly an intriguing
tetradecamer. In the former case, strong support for the reassignment
to a larger cage is provided by an observation of a slow reversible
transformation of the initially formed cage into a smaller but spectrally
very similar one upon dilution. In the latter case, freeze-fracture
transmission electron micrographs demonstrate that at least some of
the solutions are colloidal, and high-resolution electron transmission
and atomic force microscopy images are compatible with a tetradecamer
but not a trimer. Comparison of solute partial molar volumes deduced
from measurement of solution density with volumes anticipated from
molecular models argues strongly against the presence of large voids
(solvent vapor bubbles) in cages dissolved in nitromethane. The presence
of bubbles was previously proposed in an attempt to account for the
bilinear nature of the Eyring plot of the rate constant for pyridine
ligand edge exchange reaction in one of the cages and for the unusual
activation parameters in the high-temperature regime. An alternative
interpretation is proposed now