Spectroscopic studies of vitreous 2-bromobenzophenone have been carried out over the respective domain of its stability. Glassy 2-bromobenzophenone samples were obtained by abrupt cooling of the melt by cold helium vapor. Quantum yield measurements allowed us to establish that the upper boundary of stable glass is slightly above 100K, while at about 220K the glass melts. Phosphorescence measurements at 4.2K showed that even at this low temperature the emission contains a strong excimer component. The energy position and shape (two bands) of the excimer emission are close to those observed in the crystal of 2-bromobenzophenone at higher temperatures. Contrary to findings in the crystal, the monomeric emission of the glass contains only one C=O stretch series, every band of which is substantially broader than in the crystal. As the temperature is raised, the monomeric emission intensity falls, disappearing completely above 70K.

Phosphorescence spectra of 2-bromobenzophenone (2Br-BP) crystals were measured from 70 K to room temperature. A capital crossover in the emission mechanism takes place within this temperature range, leaving at room temperature only a broad two-hump feature. Additional phosphorescence experiments, carried out to understand the nature of this spectrum, include measurements from 2Br-BP solutions in ethanol as well as time-resolved and varying-excitation-intensity measurements from crystals. Based on results of these experiments and on our own single-crystal X-ray structure data, we conclude that the above two-band spectrum recorded near room temperature is due to the emission of single-photon bi-molecular triplet excimer formed by the carbonyl groups of two neighbor 2-bromobenzophenone molecules. (c) 2006 Elsevier B.V. All rights reserved.

Phosphorescence spectra of crystalline ortho-bromobenzophenone (2-bromobenzophenone, 2BrBP) were measured from 1.6K to room temperature. A cardinal emission mechanism crossover occurs within this temperature range. At low temperature the phosphorescence spectrum is a superposition of two similar sets of equidistant bands spaced by the CO stretch frequency. We ascribe these two sets to the emission of two different conformers, one of which is metastable and does not manifest itself above approximately 70K. The presence of two conformers is explained by the fact that the 2BrBP molecule deforms considerably upon excitation. The emission from the stable conformer survives up to approximately 140K or even higher. At roughly 60K and higher, another type of emission reveals itself in the shape of two very broad, partly overlapping bands, which gains in intensity with increasing temperature. Above 150K only this two-hump feature is observed in phosphorescence spectra. Additional phosphorescence experiments were carried out to elucidate the nature of this spectrum, including phosphorescence measurements of 2BrBP in ethanol solutions as well as time-resolved and varying-excitation-intensity measurements from crystals. Based on results of all the experiments reported here and on our own single-crystal x-ray structure data we conclude that the above two-band spectrum recorded near room temperature is due to the emission of single-photon bimolecular triplet excimer formed by the carbonyl groups of two neighboring 2-bromobenzophenone molecules. This is the first time that a bimolecular excimer has been observed and reliably identified in a benzophenone derivative solid.

Details of the crystal structure and quantum chemistry calculations of the title mol­ecule, C 13 H 9 BrO, illustrate the effects of intermolecular interactions and the substitution of one of the two aromatic rings on the molecular conformation. The asymmetry of the mol­ecule is documented by the two C aryl —C aryl —C=O torsion angles of −68.3 (5) and −17.6 (6)°. A C—H·O hydrogen bond [H·O = 2.5 Å, C·O = 3.412 (5) Å and C—H·O = 174°] and a C—H·π contact involving the H atom at position 4 of the substituted ring and the π-system of the unsubstituted ring of an adjacent mol­ecule [H· Cg = 2.96 Å, C· Cg = 3.806 (5) Å and C—H· Cg = 153°; where Cg is the centroid of the unsubstituted ring] are observed in the crystal structure.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Details of the crystal structure and quantum chemistry calculations of the title molecule, C13H9BrO, illustrate the effects of intermolecular interactions and the substitution of one of the two aromatic rings on the molecular conformation. The asymmetry of the molecule is documented by the two C-aryl - C-aryl - C=O torsion angles of -68.3 ( 5) and - 17.6 ( 6)degrees. A C - H center dot center dot center dot O hydrogen bond [H center dot center dot center dot O = 2.5 angstrom, C center dot center dot center dot O = 3.412 ( 5) angstrom and C - H center dot center dot center dot O = 174 degrees] and a C - H center dot center dot center dot pi contact involving the H atom at position 4 of the substituted ring and the pi-system of the unsubstituted ring of an adjacent molecule [H center dot center dot center dot Cg = 2.96 angstrom, C center dot center dot center dot Cg = 3.806 ( 5) angstrom and C - H center dot center dot center dot Cg = 153 degrees; where Cg is the centroid of the unsubstituted ring] are observed in the crystal structure.

The results of a study of the low-temperature (5–90 K) photoluminescence of thin films of C60 obtained by vacuum deposition on heated mica substrates are reported. The structure of the films is analyzed by the method of high-energy electron diffraction. The features of the luminescence of structural traps (X traps), which arise on account of mechanical stresses created by bending of C60 films of different structure on mica substrates, are investigated for the first time. The temperature behavior of the photoluminescence bands due to defects of this kind is investigated. The processes of trapping and transport of electronic excitations in the low-temperature phase of C60 are discussed.

The "additional" wide photoluminescence band is observed first in highly stable and finely dispersed colloidal solutions of fullerenes C60 in water. Unusual dependence on exciting light wavelength for this band is found. The position of the band maximum exhibits long-wavelength shift following the excited light wavelength, when last one becomes longer then 440 nm. A very simple "additional" luminescence band is found in the interface of water and fullerite C60 thin films also.

$^{a}$S. Ebbinghaus, D. Abeln, M. Epple, Z. Kristallogr. 212, 339 (1997). $^{b}$S. Dym, R. M. Hochstrasser, J. Chem. Phys. 52, 2458 (1969).

Author Institution: Verkin Institute for Low Temperature Physics and Engineering; Department of Mechanical Engineering, Northwestern University

Powder x-ray diffractometry was employed to study infusion of He into C60 fullerite. It has been shown that the intercalation at a pressure of 0.1 MPa is a two-stage process, the first stage being the saturation of the octahedral voids, virtually complete after 55 hr. Photoluminescence spectra have been measured at 5 K on C60 with virtually completely saturated octahedral voids. Helium in the lattice voids is shown to reduce that part of the luminescent emission which is due to 0-0 transitions around 1.69 eV from the so-called deep traps, or according to existing notions, covalently bound pairs of C60 molecules. The effect of intercalation with helium on dimer formation in fullerite C60 is ascribed to a combined action of the following factors: intercalation-related changes of the pentagon to hexagon configuration ratio, formation of a bonded state of He in the C60 lattice, and moderation by helium of the orientational phase transition near 260 K, the last factor resulting in a reduced number of dislocations (which also promotes dimer formation).

As inferred from our x-ray single-crystal diffraction data, the microscopic mechanism of the structural phase transition at 190 K in 4,4'-dichlorobenzophenone is such that certain structural imperfections must be inevitably present in the low-temperature phase. These defects can be the physical cause behind the shallow X-traps for triplet excitons. as observed in luminescence spectra.

We present the results of investigation of low-temperature (5 K) photoluminescence of thin fullerite C60 films of various structures obtained on monocrystalline NaCl substrates by vacuum deposition at temperatures in the interval 290–400 K. Depending on the conditions of deposition, the film structure, which was controlled by the standard transmission high-energy electron diffraction (THEED) technique, varies from a disoriented structure to an oriented structure with different average sizes of microcrystals. The effect of the film structure on the photoluminescence spectral band of fullerite, associated with the luminescence of structural defects (X-traps), is attributed to the peculiarities of transport and capture of coherent singlet excitons in single crystals of various sizes.

In this paper results of a study of fullerite C-60 vacuum deposited thin films low temperature photoluminescence (PL) are provided. The two kinds of the films on a NaCl substrate were selected for the investigation: the nonoriented polycrystalline films with the ultimate dispersibility degree and with the microcrystals average size about 5 nm and the films with good structural ordering and with the average microcrystals size about 30 - 40 nm. For the first time the PL spectra of the free thin films of the second kind were studied. The PL spectra were measured at 5 K covering the spectral range from 1.5 to 1.85 eV. All PL spectra were fitted to four Gaussians peaks at 1.78; 1.71; 1.6 and 1.52 eV. Probable versions of the crystal excited states are considered. It is likely that the PL peak at 1.71 eV corresponds to the radiative recombination of a self-trapped Frenkel exciton with a contribution of a charge-transfer character and the PL peak at 1.78 eV corresponds to the radiative recombination of the excited X-traps. The nature of the X-traps is discussed. The X-trap FL peak at 1.78 eV is considerably larger for oriented films than for nonoriented ones. It is assumed that this phenomenon is stipulated by peculiarities of exciton transport and localization within the microcrystals of films.

The excited triplet states of shallow X-traps in neat single crystals of 4,4-dichlorobenzophenone were studied at liquid helium temperature by luminescence spectroscopy and magnetic resonance with optical detection. The vibrational phosphorescence bands, the polarization of phosphorescence spectra, and the triplet zero-field splitting parameters of these traps have been analyzed. A specific model of the structure defect (a molecule is turned by 180° on its long axis) has been considered in organic crystals of benzophenone type which consist of molecules whose dipole moments compensate each other in a unit cell. The calculated depth of such a dipole X-trap for triplet excitons in 4,4-dichlorobenzophenone crystals at the account of a defect molecule relaxation upon its slewing was found to be comparable with the experimental one.