Mendel D. Cohen

The absorption and fluorescence properties of pyrene crystal: a theoretical approach. I. harmonic approximation

Abstract On the basis of the structure of pyrene crystal we argue that excitation in the crystals localized at sites consisting of pairs of close-spaced molecules, i.e., that we are dealing with a Frenkel exciton. We postulate potentials for ground- and excited-state pairs in which both in-plane and out-of-plane vibrations play a role. Using these potentials and selected experimental data we are able to estimate the values of several features of the absorption and fluorescene spectra of, and the energy migration in, cyrstalline pyrene. Agreement with experiment is satisfactory.

Pair-states in α-perylene crystal. A theoretical study

Abstract The low- and high-temperature emissions from α-perylene are interpreted as originating in, respectively, loosely- and tightly-bound pair complexes. The change-over from one to the other results from thermal expansion and deformation of the lattice and the resulting change in contact distances between the complexing pair and the surrounding molecules. With this model the binding energies, Stokes shifts, lifetimes, and emissive bandwidths associated with the complexes are estimated, as is the activation energy for excimer-exciton migration; the values agree well with the available experimental data.

Excimers in Crystals

Abstract The concept of the reaction cavity is used to indicate what types of reaction are allowed in the crystal state. Application is made to the formation of excimers in the crystal. The lattice determines whether or not the excimer is formed. There two types of excimer-forming structures, stack and pair ones; it appears that the former forms cis-excimers, the latter trans- ones. In the anthracenes, and probably generally, the excimer is an intermediate on the route to photodimer. The crystal constraints which determine the geometry of the excimer determine the stereochemistry of the product. However, the geometry of the product and of the precursor excimer may differ between bulk phase and structural defects. Reactions in chiral crystals may yield optically active products. In an example of photodimerization the chirality of the dominant product appears to be determined by the chirality of the dominant excimer precursor.

The excimer as intermediate in the photodimerization of anthracene in fluid solution

Abstract A method for detecting non-emitting electronically excited species by energy transfer to a dye is described. This method is utilized to show that the photodimerization of anthracene in solution proceeds through the excimer; the lifetime of this non-emitting excimer is ca. 1–1.5 ns.

Excimer emission and photodimerization in a crystalline stilbene

Abstract Crystalline 2,4-dichloro- trans -stilbene (II) displays excimer emission and undergoes photoreaction to yield the mirror-symmetric dimer 1, cis -2-di(2, 4-dichlorophenyl)- trans -3, tarans -4-diphenlcyclobutane (III). In solution, under a variety of conditions. II shows only monomer emission and yields, in addition to III, the centric photodimer 1, trans -3-di(2,4-dichlorophenyl)- cis -2, trans -4-diphenylcyclcbutane (IV) and cis -monomer.

Chlorine atoms in molecular crystals: Potential energy calculations

Abstract Parameters for the Buckingham potential function for pairwise non-bonded interatomic interactions of chlorine atoms in chloroaromatic crystals have been obtained. In the optimization procedure, the observation has been taken into account that the electron distribution in the chlorine atom readily responds to the intra- and inter-molecular environment. The C⋯Cl function is derived by fitting to crystal structure and sublimation energy data of hexachlorobenzene extrapolated to 0 K; it has the form φ = − Ar −6 + B exp (−α r ) with A = 2980 kcal A 6 /mole, B = 4580 kcal/mole, and α = 2.262 A −‡ . The function is compared with other such functions for chloroaromatics available in the literature and can be satisfactorily transferred to (γ)-1,4-dichlorobenzene and to 1,3,5-trichlorobenzene. However, none of the functions examined can be satisfactorily transferred to chlorine (Cl 2 ). The discrepancy for this crystal may be due to appreciable electrostatic and zero-point vibrational contributions to the lattice energy. With a soft atom such as chlorine one would expect the limitations of the central-force approximation for interatomic interactions to be more apparent; the calculations tend to point up such limitations.

The absorption and fluorescence properties of pyrene crystal: a theoretical approach. III. Relaxation from highly excited vibrational levels

Abstract It is shown that the model based on vibrational relaxation to the bath, while invalid for highly excited vibrational levels at low temperatures, is valid at high temperature. Thus relaxation time of ∼10 −13 s previously estimated is acceptable. Using this time, known radiative lifetimes, and a reasonable value for relaxation times of the vibrations of the isolated molecules, a modified model for excimer absorption and emission is proposed.

Micromeasurements of exciton diffusion lengths in single crystals of anthracene

Abstract A micromethod is used to determine singlet exciton diffusion lengths in anthracene crystal normal to the ab plane. It is found that these lengths are significantly longer when measurements are performed on smooth, rather than non-smooth, areas of the surface.

Structural Factors in Some Solid State Photodimerizations

Abstract It has been reported previously that in the transcinnamic acids and some substituted butadienes the crystal structure determines whether or not photodimerization occurs and the molecular structure of the dimer, if any. Similar results have been obtained also with chalcones. However, in anthracene and its 9-substituted derivatives the results are qualitatively different. Thus anthracene undergoes dimerization, if slowly, even though potentially combining atoms are 6 A apart. 9-Cyano and 9-chloro-anthracenes, in structures with short axes of 4 A, react to give head-to-tail dimers, whereas if structural effects were dominant the head-to-head dimers would be formed. It is concluded that in the last three examples, at least, the reaction does not occur in the bulk (i.e., at ordered sites). Two alternatives are suggested and discussed; the first is that reaction occurs in the vapor phase, the second that reaction occurs at defect sites, some of which are present initially, whereas others are produced d...

The D. C. Conductivities of Some N-Salicylideneanilines

Abstract A good deal of spectroscopic and structural data are now available on crystalline N-salicylideneanilines. The crystals fall into two groups which are markedly different in both molecular and crystal structures, on the one hand, and in photochemical and spectroscopic properties, on the other. It was considered that these differences should be reflected in a difference in the conduction characteristics of the two groups. A number of members of each group were selected for this study. Conductivity measurements were performed on single crystals in different crystallographic directions. In general, all crystals studied were found to have similar specific resistivities and activation energies for conduction. However, in one case appreciably lower values of these properties were observed. This is believed to be due to suitable orientation of the molecules with respect to the crystal faces to which the electrodes were attached. Overlap calculations for this system are described in an Appendix, and the fa...