Z. Ludmer

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.

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.

Structural influence on the excimer emission from a dimorphic crystalline stilbene

Abstract The compound 2,4-dichloro-3′-methyl- trans -stilbene is dimorphic. The two forms are assigned “pair” and “stack” crystal structures on the basis of their cell dimensions and of the structures of the photodimers derived from them. Excimer emission is observed from both forms, with that from the stack structure being of higher energy. This is in keeping with an earlier theoretical analysis and argues for the validity and generality of this analysis.

The spectroscopy and photochemistry of the excimer-emitting 9-cyanoanthracene crystal; The importance of multiphonon processes

Abstract A study of the spectroscopy and photochemistry of crystalline 9-cyanoanthracene is reported. This crystal gives rise to excimer emission; at 77 K the excimer has a lifetime of 160 ±5 ns and the emission spectrum is of gaussian shape. There is, prior to reaction, transfer of excitation energy by a process occurring at the excimer—exciton level and involving an activation energy of about 420 cm−1 . Radiationless deactivation of the excimer involves an activation energy of 660 cm −1 . These experimental observations are in good agreement with the predictions of theories based on the model of strong exciton—phonon coupling. The photoreaction occurs at specific sites which are believed to be head-to-tail defects.

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.

Electronic energy trapping in a crystal due, to a dopant of higher excitation energy than the host

Abstract An excimer emitting crystal (9-cyanoanthracene) doped with a guest molecule (9-methoxyanthracene) having its first singlet level ca. 2000 cm−1 above the host singlet exciton band exhibits efficient energy trapping as demonstrated by host sensitized, red-shifted emission and hetero-photodimerization. It is considered that the trapping is due to exciplex formation between host and guest molecules.

Exciplex and heteroexcimer emission from pyrene crystals doped with anthracene derivatives

Abstract Excited-state interactions in pyrene crystals doped with various anthracene derivatives are examined. Both donor-acceptor and heteroexcimer-type exciplexes, formed between host and guest molecules, are found. The exciplex ground state is found to be less repulsive than that of the pyrene excimer.

Excimer fluorescence in β-9,10-dichloroanthracene crystals

The fluorescence of single crystals of β-9,10-dichloroanthracene at 4.2 K consists solely of excimer emission (τ = 95 ± 5 ns). The absence of monomenc emission shows that excimer formation in this crystal is not a thermally activated process. This result is confirmed by excimer-excimer annihilation studies (γ(4.2 K) = 6 × 10−13, γ(298K) = 3 × 10−12 cm3 s−1).

Comment: Excimer formation and low-temperature fluorescence in 9,10-dichloroanthracene crystals

Abstract Low-temperature monomer emission detected from β-9,10-dichloroanthracene crystals can be greatly reduced by annealing. Thus excimer formation in this system is not a thermally activated process in contrast to the case of perylene.