Exciton Diffusion Constant Research Articles

Quenching of the triplet state of poly(2-vinylnaphthalene) in solution: Effect of molecular weight

The rate of decay of triplet-triplet absorption of excited poly(2-vinylnaphthalene) (P2VN) and 2-ethylnaphthalene (2EN) in room temperature benzene solutions was studied as a function of an added triplet quencher, piperylene. It was found that the efficiency of triplet quenching decreases as the molecular weight of the polymer increases, which is interpreted as arising from the density of the polymer coil in solution. If triplet energy migration occurs along the polymer chain the exciton diffusion constant is not significantly larger than the physical diffusion of the monomer model compound, 2EN. The triplet state was produced in solution by a 4 MeV electron beam, because the S 1 → T 1 process was found to be very inefficient for P2VN under optical excitation conditions.

Mechanisms of singlet energy transfer in doped anthracene crystals at 5 K, studied by nanosecond spectrofluorimetry

Energy transfer at 5 K in crystalline anthracene doped with 2-methylanthracene (2MeA) has been studied by a combination of fluorescence spectroscopy and time-resolved spectrofluorimetry. The dopant induces two distinct traps, A and B, which can be assigned to specific orientations of the guest. Energy transfer to the A traps (which are 20 times more abundant) can be described in terms of first-order kinetics; the rate constant is very large, ≈ 10–7 cm3 s–1, corresponding to an exciton diffusion constant of 0.1 cm2 s–1. In unstrained crystals, with more than 100 p.p.m. of 2MeA, energy transfer from A to B sites occurs. This process is relatively slow, with a time-dependent rate constant, and it proceeds principally by long-range resonant exchange. Straining the crystals markedly increases energy transfer direct from host to B traps; this is attributed to the pinning of line dislocations at B sites, in which the guest molecule is more crowded, so creating an extended trapping region.

The kinetics of the sensitized flourescence of naphthalene single crystals doped with 2-methylnaphthalene and anthracene

The kinetics of the sensitized flourescence in naphthalene crystals doped with 2-methylnaphthalene (2-MN) and anthracene (A) was studied as a function of the temperature. These experiments yield singlet exciton diffusion constants of (2 ± 1) × 10 -4 cm 2 s -1 at 4.2 K and roughly 10 -5 cm 2 s -1 at 300 K. The exciton trapping radii can be estimated to be of the order of 60 Å for A and 10 Å for 2-MN. At temperatures below 40 K a partially coherent exciton motion can be inferred, above 40 K the exciton motion appears to be purely incoherent.

Kinematics of Exciton-Exciton Annihilation in Molecular Crystals

A theory is developed for calculating the kinematic part of the exciton-exciton annihilation rate in molecular crystals. The spatial and spin motion of the excitons, as well as the annihilation process itself, is treated phenomenologically. Exciton propagation is assumed to take place as in the hopping model. The importance of the dimensionality of the exciton motion is pointed out; in nearly one- or two-dimensional cases, certain lifetime processes control the exciton collision rate, in contrast to the three-dimensional case. These lifetime processes include motion out of the one- or two-dimensional subspace and, for excitons with spin, spin relaxation. The theory leads to a description of magnetic field effects on the annihilation rate of triplet excitons at room temperature. When applied to triplet excitons in anthracene, this description gives a satsfactory fit to the observed effects and leads to the determination of the nearest-neighbor exciton annihilation rate, the singlet-channel annihilation rate constant, and the exciton diffusion constant for motion perpendicular to the $\mathrm{ab}$ plane of anthracene.